Prof. Dr. Wolfgang Schuhmann

Analytical Chemistry
Ruhr-Universität Bochum

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  • A Tandem Solar Biofuel Cell: Harnessing Energy from Light and Biofuels
    Riedel, M. and Höfs, S. and Ruff, A. and Schuhmann, W. and Lisdat, F.
    Angewandte Chemie - International Edition 60 (2021)
    We report on a photobioelectrochemical fuel cell consisting of a glucose-oxidase-modified BiFeO3 photobiocathode and a quantum-dot-sensitized inverse opal TiO2 photobioanode linked to FAD glucose dehydrogenase via a redox polymer. Both photobioelectrodes are driven by enzymatic glucose conversion. Whereas the photobioanode can collect electrons from sugar oxidation at rather low potential, the photobiocathode shows reduction currents at rather high potential. The electrodes can be arranged in a sandwich-like manner due to the semi-transparent nature of BiFeO3, which also guarantees a simultaneous excitation of the photobioanode when illuminated via the cathode side. This tandem cell can generate electricity under illumination and in the presence of glucose and provides an exceptionally high OCV of about 1 V. The developed semi-artificial system has significant implications for the integration of biocatalysts in photoactive entities for bioenergetic purposes, and it opens up a new path toward generation of electricity from sunlight and (bio)fuels. © 2020 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
    view abstract10.1002/anie.202012089
  • Closing the Gap for Electronic Short-Circuiting: Photosystem I Mixed Monolayers Enable Improved Anisotropic Electron Flow in Biophotovoltaic Devices
    Wang, P. and Frank, A. and Zhao, F. and Szczesny, J. and Junqueira, J.R.C. and Zacarias, S. and Ruff, A. and Nowaczyk, M.M. and Pereira, I.A.C. and Rögner, M. and Conzuelo, F. and Schuhmann, W.
    Angewandte Chemie - International Edition 60 (2021)
    Well-defined assemblies of photosynthetic protein complexes are required for an optimal performance of semi-artificial energy conversion devices, capable of providing unidirectional electron flow when light-harvesting proteins are interfaced with electrode surfaces. We present mixed photosystem I (PSI) monolayers constituted of native cyanobacterial PSI trimers in combination with isolated PSI monomers from the same organism. The resulting compact arrangement ensures a high density of photoactive protein complexes per unit area, providing the basis to effectively minimize short-circuiting processes that typically limit the performance of PSI-based bioelectrodes. The PSI film is further interfaced with redox polymers for optimal electron transfer, enabling highly efficient light-induced photocurrent generation. Coupling of the photocathode with a [NiFeSe]-hydrogenase confirms the possibility to realize light-induced H2 evolution. © 2020 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
    view abstract10.1002/anie.202008958
  • Comparing the Activity of Complex Solid Solution Electrocatalysts Using Inflection Points of Voltammetric Activity Curves as Activity Descriptors
    Löffler, T. and Waag, F. and Gökce, B. and Ludwig, Al. and Barcikowski, S. and Schuhmann, W.
    ACS Catalysis 11 (2021)
    Complex solid solution (CSS) (often denoted as high-entropy alloy) electrocatalysts enable access to unique possibilities for tailoring active sites while overcoming ever-existing limitations in electrocatalysis by unique interactions of various elements in direct neighborhood. The challenge lies in the development of strategies, which allow for systematic design of element combination and composition optimization in the multinary composition space. This challenge is accompanied by a lack of a suitable analysis method of experimental activity measurements, which can cope with the complex surface structure of this catalyst class. In this work, we propose the advantageous use of inflection points of voltammetric activity curves as activity descriptors enabling to correlate the potential of individual surface site groups to the respective peaks in the adsorption energy distribution pattern. This concept allows to methodologically gather information about the importance of each element in a CSS with respect to activity and stability of the relevant active sites and provides the basis for a guideline for systematic composition optimization. Further, the effect of phase stability on specific surface site groups as induced by degradation of the CSS phase or oxidation can be monitored. These concepts are experimentally evaluated using Cr-Mn-Fe-Co-Ni as a model system. Nanoparticles are synthesized with systematically varied compositions by means of scalable laser ablation synthesis using a multinary target. The composition is optimized with respect to the electrocatalytic activity for the oxygen reduction reaction (ORR) by varying its Mn content via laser ablation synthesis in ethanol. Subsequently, the concept is applied using rotating disk electrodes for ORR analysis in alkaline media. © 2021 American Chemical Society. All rights reserved.
    view abstract10.1021/acscatal.0c03313
  • Complex-Solid-Solution Electrocatalyst Discovery by Computational Prediction and High-Throughput Experimentation**
    Batchelor, T.A.A. and Löffler, T. and Xiao, B. and Krysiak, O.A. and Strotkötter, V. and Pedersen, J.K. and Clausen, C.M. and Savan, A. and Li, Y. and Schuhmann, W. and Rossmeisl, J. and Ludwig, Al.
    Angewandte Chemie - International Edition (2021)
    Complex solid solutions (“high entropy alloys”), comprising five or more principal elements, promise a paradigm change in electrocatalysis due to the availability of millions of different active sites with unique arrangements of multiple elements directly neighbouring a binding site. Thus, strong electronic and geometric effects are induced, which are known as effective tools to tune activity. With the example of the oxygen reduction reaction, we show that by utilising a data-driven discovery cycle, the multidimensionality challenge raised by this catalyst class can be mastered. Iteratively refined computational models predict activity trends around which continuous composition-spread thin-film libraries are synthesised. High-throughput characterisation datasets are then used as input for refinement of the model. The refined model correctly predicts activity maxima of the exemplary model system Ag-Ir-Pd-Pt-Ru. The method can identify optimal complex-solid-solution materials for electrocatalytic reactions in an unprecedented manner. © 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
    view abstract10.1002/anie.202014374
  • Electrocatalysis in confined space
    Andronescu, C. and Masa, J. and Tilley, R.D. and Gooding, J.J. and Schuhmann, W.
    Current Opinion in Electrochemistry 25 (2021)
    The complex interplay of restricted mass transport leading to local accumulation or depletion of educts, intermediates, products, counterions and co-ions influences the reactions at the active sites of electrocatalysts when electrodes are rough, three-dimensionally mesoporous or nanoporous. This influence is important with regard to activity, and even more to selectivity, of electrocatalytic reactions. The underlying principles are discussed based on the growing awareness of these considerations over recent years. © 2020 Elsevier B.V.
    view abstract10.1016/j.coelec.2020.100644
  • Importance of catalyst–photoabsorber interface design configuration on the performance of Mo-doped BiVO4 water splitting photoanodes
    Krysiak, O.A. and Junqueira, J.R.C. and Conzuelo, F. and Bobrowski, T. and Masa, J. and Wysmolek, A. and Schuhmann, W.
    Journal of Solid State Electrochemistry 25 (2021)
    Photoelectrochemical water splitting is mostly impeded by the slow kinetics of the oxygen evolution reaction. The construction of photoanodes that appreciably enhance the efficiency of this process is of vital technological importance towards solar fuel synthesis. In this work, Mo-modified BiVO4 (Mo:BiVO4), a promising water splitting photoanode, was modified with various oxygen evolution catalysts in two distinct configurations, with the catalysts either deposited on the surface of Mo:BiVO4 or embedded inside a Mo:BiVO4 film. The investigated catalysts included monometallic, bimetallic, and trimetallic oxides with spinel and layered structures, and nickel boride (NixB). In order to follow the influence of the incorporated catalysts and their respective properties, as well as the photoanode architecture on photoelectrochemical water oxidation, the fabricated photoanodes were characterised for their optical, morphological, and structural properties, photoelectrocatalytic activity with respect to evolved oxygen, and recombination rates of the photogenerated charge carriers. The architecture of the catalyst-modified Mo:BiVO4 photoanode was found to play a more decisive role than the nature of the catalyst on the performance of the photoanode in photoelectrocatalytic water oxidation. Differences in the photoelectrocatalytic activity of the various catalyst-modified Mo:BiVO4 photoanodes are attributed to the electronic structure of the materials revealed through differences in the Fermi energy levels. This work thus expands on the current knowledge towards the design of future practical photoanodes for photoelectrocatalytic water oxidation. © 2020, The Author(s).
    view abstract10.1007/s10008-020-04636-9
  • Recovering activity of anodically challenged oxygen reduction electrocatalysts by means of reductive potential pulses
    Medina, D. and Löffler, T. and Morales, D.M. and Masa, J. and Bobrowski, T. and Barwe, S. and Andronescu, C. and Schuhmann, W.
    Electrochemistry Communications 124 (2021)
    The stability of electrocatalysts is of great importance to ensure their applicability, but stability is generally only considered for catalysts polarised to a constant potential or current density. This excludes stability evaluation under start/stop conditions in a fuel cell or in reversible batteries in which the catalyst is alternately polarised to high opposite potentials. For example, the poor cyclability of metal-air batteries is mainly due to the decrease in the oxygen reduction activity of electrocatalysts during the high applied potentials for the oxygen evolution reaction during battery charging. To investigate and at least partially mitigate the loss of electrocatalytic activity for the oxygen reduction reaction, we employed reductive pulses with the aim of restoring the catalytic activity of the active sites for the oxygen reduction reaction. Optimisation of the reductive pulse parameters makes it possible to substantially prolong the oxygen reduction activity of a Fe-Nx-doped carbon-based oxygen reduction electrocatalyst. © 2021 The Author(s)
    view abstract10.1016/j.elecom.2021.106960
  • Single-Entity Electrocatalysis of Individual “Picked-and-Dropped” Co3O4 Nanoparticles on the Tip of a Carbon Nanoelectrode
    Quast, T. and Aiyappa, H.B. and Saddeler, S. and Wilde, P. and Chen, Y.-T. and Schulz, S. and Schuhmann, W.
    Angewandte Chemie - International Edition 60 (2021)
    Nano-electrochemical tools to assess individual catalyst entities are critical to comprehend single-entity measurements. The intrinsic electrocatalytic activity of an individual well-defined Co3O4 nanoparticle supported on a carbon-based nanoelectrode is determined by employing an efficient SEM-controlled robotic technique for picking and placing a single catalyst particle onto a modified carbon nanoelectrode surface. The stable nanoassembly is microscopically investigated and subsequently electrochemically characterized. The hexagonal-shaped Co3O4 nanoparticles demonstrate size-dependent electrochemical activity and exhibit very high catalytic activity with a current density of up to 11.5 A cm−2 at 1.92 V (vs. RHE), and a turnover frequency of 532±100 s−1 at 1.92 V (vs. RHE) towards catalyzing the oxygen evolution reaction. © 2020 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
    view abstract10.1002/anie.202014384
  • A Universal Nano-capillary Based Method of Catalyst Immobilization for Liquid-Cell Transmission Electron Microscopy
    Tarnev, T. and Cychy, S. and Andronescu, C. and Muhler, M. and Schuhmann, W. and Chen, Y.-T.
    Angewandte Chemie - International Edition 59 (2020)
    A universal nano-capillary based method for sample deposition on the silicon nitride membrane of liquid-cell transmission electron microscopy (LCTEM) chips is demonstrated. It is applicable to all substances which can be dispersed in a solvent and are suitable for drop casting, including catalysts, biological samples, and polymers. Most importantly, this method overcomes limitations concerning sample immobilization due to the fragility of the ultra-thin silicon nitride membrane required for electron transmission. Thus, a straightforward way is presented to widen the research area of LCTEM to encompass any sample which can be externally deposited beforehand. Using this method, NixB nanoparticles are deposited on the μm-scale working electrode of the LCTEM chip and in situ observation of single catalyst particles during ethanol oxidation is for the first time successfully monitored by means of TEM movies. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/anie.201916419
  • Bioelectrocatalysis as the basis for the design of enzyme-based biofuel cells and semi-artificial biophotoelectrodes
    Ruff, A. and Conzuelo, F. and Schuhmann, W.
    Nature Catalysis 3 (2020)
    Bioelectrocatalysis provides access to sustainable and highly efficient technological applications. However, several limitations related either to the intrinsic properties of the biocatalyst or to technical difficulties still hamper or even prevent the integration of such devices into technologically relevant large-scale processes. In this Review, we challenge the common viewpoint suggesting biology-based catalytic systems as a promising approach for the provision of sustainable stored energy and discuss the status of bioelectrocatalytic devices developed for energy conversion. In particular, we focus on two major research areas in the field, that is, H2-powered hydrogenase-based biofuel cells and biophotoelectrodes for solar energy harvesting. We identify the main limitations that have to be addressed to gain access to applied large-scale bio-based and bio-inspired advanced energy conversion systems. Moreover, we show recent examples and milestones that are paving the way towards potential realization of these technologies by overcoming existing limiting factors. © 2019, Springer Nature Limited.
    view abstract10.1038/s41929-019-0381-9
  • Breaking scaling relations in electrocatalysis
    Masa, J. and Schuhmann, W.
    Journal of Solid State Electrochemistry 24 (2020)
    view abstract10.1007/s10008-020-04757-1
  • CoFe-OH Double Hydroxide Films Electrodeposited on Ni-Foam as Electrocatalyst for the Oxygen Evolution Reaction
    Sliozberg, K. and Aniskevich, Y. and Kayran, U. and Masa, J. and Schuhmann, W.
    Zeitschrift fur Physikalische Chemie 234 (2020)
    Cobalt-iron double hydroxide (CoFe-OH) films were electrochemically deposited on 3D Ni foam electrodes for the oxygen evolution reaction (OER). The dependence of the OER activity on film composition and thickness was evaluated, which revealed an optimal Fe:Co ratio of about 1:2.33. The composition of the catalyst film was observed to vary with film thickness. The electrodeposition parameters were carefully controlled to yield microstructured Ni-foam decorated with CoFe-OH films of controlled thickness and composition. The most active electrode exhibited an overpotential as low as 360 mV OER at an industrial scale current density of 400 mA cm-2 that remained stable for at least 320 h. This work contributes towards the fabrication of practical electrodes with the focus on the development of stable electrodes for electrocatalytic oxygen evolution at high current densities. © 2020 Wolfgang Schuhmann et al., published by De Gruyter, Berlin/Boston 2020.
    view abstract10.1515/zpch-2019-1466
  • Combinatorial Search for New Solar Water Splitting Photoanode Materials in the Thin-Film System Fe-Ti-W-O
    Kumari, S. and Khare, C. and Xi, F. and Nowak, M. and Sliozberg, K. and Gutkowski, R. and Bassi, P.S. and Fiechter, S. and Schuhmann, W. and Ludwig, Al .
    Zeitschrift fur Physikalische Chemie 234 (2020)
    In order to identify new solar water splitting photoanodes, Fe-Ti-W-O materials libraries were fabricated by combinatorial reactive co-sputtering and investigated by high-throughput characterization methods to elucidate compositional, thickness, and structural properties. In addition, photoelectrochemical measurements such as potentiodynamic photocurrent determination and open circuit potential measurements were performed using an automated scanning droplet cell. In the thin-film library, a quaternary photoactive region Fe30-49Ti29-55W13-22Ox was identified as a hit composition region, comprising binary and ternary phases. The identified region shows a distinct surface morphology with larger grains (∼200 nm) being embedded into a matrix of smaller grains (∼80-100 nm). A maximum photocurrent density of 117 μA/cm2 at a bias potential of 1.45 V vs. RHE in NaClO4 as an electrolyte under standard solar simulating conditions was recorded. Additional samples with compositions from the hit region were fabricated by reactive co-sputtering and spin coating followed by annealing. Synchrotron X-ray diffraction of sputtered Fe32Ti52W16Ox thin-films, annealed in air (600 °C, 700 °C, 800 °C) revealed the presence of the phases FeTiO3 and Ti0.54W0.46O2. The composition Fe48Ti30W22Ox from the hit region was fabricated by spin coating and subsequent annealing for a detailed investigation of its structure and photoactivity. After annealing the spin-coated sample at 650 °C for 6 h, X-ray diffraction results showed a dominant pattern with narrow diffraction lines belonging to a distorted FeWO4 (ferberite) phase along with broad diffraction lines addressed as Fe2TiO5 and in a small fraction also, Fe1.7Ti0.23O3. In hematite, Fe can be substituted by Ti, therefore we suggest that in the newfound ferberite-type phase, Ti partially substitutes for Fe leading to a small lattice distortion and a doubling of the monoclinic unit cell. In addition, Na from the substrate stabilizes the new phase: its tentative chemical formula is NaxFe0.33Ti0.67W2O8. A maximum photocurrent density of around 0.43 mA/cm2 at 1.45 V vs. RHE in 1M NaOH (pH ∼13.6) as an electrolyte was measured. Different aspects of the dependence of annealing and precursor solution concentration on phase transformation and photoactivity are discussed. © 2020 Alfred Ludwig et al., published by De Gruyter, Berlin/Boston 2020.
    view abstract10.1515/zpch-2019-1462
  • Controlling the Number of Branches and Surface Facets of Pd-Core Ru-Branched Nanoparticles to Make Highly Active Oxygen Evolution Reaction Electrocatalysts
    Myekhlai, M. and Benedetti, T.M. and Gloag, L. and Poerwoprajitno, A.R. and Cheong, S. and Schuhmann, W. and Gooding, J.J. and Tilley, R.D.
    Chemistry - A European Journal 26 (2020)
    Producing stable but active materials is one of the enduring challenges in electrocatalysis and other types of catalysis. Producing branched nanoparticles is one potential solution. Controlling the number of branches and branch size of faceted branched nanoparticles is one of the major synthetic challenges to achieve highly active and stable nanocatalysts. Herein, we use a cubic-core hexagonal-branch mechanism to synthesize branched Ru nanoparticles with control over the size and number of branches. This structural control is the key to achieving high exposure of active {10–11} facets and optimum number of Ru branches that enables improved catalytic activity for oxygen evolution reaction while maintaining high stability. © 2020 Wiley-VCH GmbH
    view abstract10.1002/chem.202003561
  • Coupling electrochemistry with a fluorescence reporting reaction enabled by bipolar electrochemistry
    Stefano, J.S. and Conzuelo, F. and Masa, J. and Munoz, R.A.A. and Schuhmann, W.
    Journal of Electroanalytical Chemistry 872 (2020)
    A bipolar electrochemistry setup for the sensitive indirect detection of redox active analytes by means of a fluorescence signal generated by the oxidation of dihydroresorufin is proposed. The redox conversion leads to the in situ and real time formation of the oxidized form resorufin, a highly fluorescent molecule. A photomultiplier tube is used for the detection of the emitted fluorescence light. The system was first characterized using the electrochemical reduction of [Fe(CN)6]3− as model analyte at the cathodic bipolar pole, promoting an increase in the fluorescence signal which is proportional to the concentration of [Fe(CN)6]3− in solution. Indirect quantification is enabled with a linear range between 10 μM and 50 μM and a limit of detection down to 0.2 μM. The system was successfully applied for the detection of glucose and hydrogen peroxide using enzyme modified electrodes at the detection pole. The use of a closed bipolar system allows translating the electrochemical redox process for analyte detection into a fluorescence reporting reaction, providing (bio)sensing capabilities with adequate sensitivity and the possibility for optically monitoring non-fluorogenic redox reactions. © 2020 Elsevier B.V.
    view abstract10.1016/j.jelechem.2020.113921
  • Design of Complex Solid-Solution Electrocatalysts by Correlating Configuration, Adsorption Energy Distribution Patterns, and Activity Curves
    Löffler, T. and Savan, A. and Meyer, H. and Meischein, M. and Strotkötter, V. and Ludwig, Al. and Schuhmann, W.
    Angewandte Chemie - International Edition 59 (2020)
    Complex solid-solution electrocatalysts (also referred to as high-entropy alloy) are gaining increasing interest owing to their promising properties which were only recently discovered. With the capability of forming complex single-phase solid solutions from five or more constituents, they offer unique capabilities of fine-tuning adsorption energies. However, the elemental complexity within the crystal structure and its effect on electrocatalytic properties is poorly understood. We discuss how addition or replacement of elements affect the adsorption energy distribution pattern and how this impacts the shape and activity of catalytic response curves. We highlight the implications of these conceptual findings on improved screening of new catalyst configurations and illustrate this strategy based on the discovery and experimental evaluation of several highly active complex solid solution nanoparticle catalysts for the oxygen reduction reaction in alkaline media. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/anie.201914666
  • Differentiation between Carbon Corrosion and Oxygen Evolution Catalyzed by NixB/C Hybrid Electrocatalysts in Alkaline Solution using Differential Electrochemical Mass Spectrometry
    Möller, S. and Barwe, S. and Dieckhöfer, S. and Masa, J. and Andronescu, C. and Schuhmann, W.
    ChemElectroChem 7 (2020)
    Carbon is a frequently used electrode material and an important additive in catalyst films. Its corrosion is often reported during electrocatalysis at high anodic potentials, especially in acidic electrolyte. Investigation of the carbon corrosion in alkaline environment is difficult due to the CO2/CO32− equilibrium. We report the on-line determination of electrolysis products generated on NixB/C hybrid electrocatalysts in alkaline electrolyte at anodic potentials using differential electrochemical mass spectrometry (DEMS). NixB/C catalyst films were obtained from mixtures containing different ratios of NiXB and benzoxazine monomers followed by polymerization and pyrolysis. The impact of the composition of the electrocatalyst on the dominant electrolysis process allows to distinguish between the oxygen evolution reaction and carbon corrosion using DEMS results as well as the catalyst surface composition evaluated from X-ray photoelectron spectra. At the imposed highly oxidative conditions, an increasing amount of NixB in the electrocatalyst leads to a suppression of carbon corrosion. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/celc.202000697
  • Drug Release from Polymer Thin Films and Gel Pellets: Insights from Programmed Microplate Electroanalysis
    Ruff, A. and Jaikaew, W. and Khunkaewla, P. and Schuhmann, W. and Schulte, A.
    ChemPlusChem 85 (2020)
    Robotic electroanalysis in 24-well microplates was used to determine Paracetamol (PCT) release from thin films of chitosan and two pH-sensitive synthetic polymers as well as blends of the polymers with each other and with agarose. Square-wave voltammograms were recorded automatically in a potential window of 0.35 V–0.85 V vs. Ag/AgCl/0.1 M KCl and their evaluation revealed time-dependent PCT release into acidic and basic media. Comparison of the release profiles showed that pure chitosan layers released PCT quickly in a single-phase process while liberation from synthetic polymer thin films was slower with a sigmoidal shape at pH 1.2 and pH 8.0 with a maximum release of PCT after approximately 150 and 140 min, respectively. The release profile from thicker agarose films was between those of the thin films. Agarose blended with chitosan or synthetic polymers formed films with biphasic release behavior. Chitosan linearized the initial section of the release profile in chitosan/polymer blends. The automated procedure for release testing offers the advantage of low-cost, labor-effective and error-free data acquisition. The procedure has been validated as a useful microplate assay option for release profile testing. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/cplu.202000129
  • Electrocatalysis as the Nexus for Sustainable Renewable Energy: The Gordian Knot of Activity, Stability, and Selectivity
    Masa, J. and Andronescu, C. and Schuhmann, W.
    Angewandte Chemie - International Edition 59 (2020)
    The use of renewable energy by means of electrochemical techniques by converting H2O, CO2 and N2 into chemical energy sources and raw materials, is the basis for securing a future sustainable “green” energy supply. Some weaknesses and inconsistencies in the practice of determining the electrocatalytic performance, which prevents a rational bottom-up catalyst design, are discussed. Large discrepancies in material properties as well as in electrocatalytic activity and stability become obvious when materials are tested under the conditions of their intended use as opposed to the usual laboratory conditions. They advocate for uniform activity/stability correlations under application-relevant conditions, and the need for a clear representation of electrocatalytic performance by contextualization in terms of functional investigation or progress towards application is emphasized. © 2020 The Authors. Published by Wiley-VCH GmbH
    view abstract10.1002/anie.202007672
  • Electroenzymatic CO2 Fixation Using Redox Polymer/Enzyme-Modified Gas Diffusion Electrodes
    Szczesny, J. and Ruff, A. and Oliveira, A.R. and Pita, M. and Pereira, I.A.C. and De Lacey, A.L. and Schuhmann, W.
    ACS Energy Letters 5 (2020)
    We describe the fabrication of gas diffusion electrodes modified with polymer/enzyme layers for electroenzymatic CO2 fixation. For this, a metal-free organic low-potential viologen-modified polymer has been synthesized that reveals a redox potential of around-0.39 V vs SHE and is thus able to electrically wire W-dependent formate dehydrogenase from Desulfovibrio vulgaris Hildenborough, which reversibly catalyzes the conversion of CO2 to formate. The use of gas diffusion electrodes eliminates limitations arising from slow mass transport when solid carbonate is used as CO2 source. The electrodes showed satisfactory stability that allowed for their long-term electrolysis application for electroenzymatic formate production. Copyright © 2019 American Chemical Society.
    view abstract10.1021/acsenergylett.9b02436
  • Electroenzymatic Nitrogen Fixation Using a MoFe Protein System Immobilized in an Organic Redox Polymer
    Lee, Y.S. and Ruff, A. and Cai, R. and Lim, K. and Schuhmann, W. and Minteer, S.D.
    Angewandte Chemie - International Edition 59 (2020)
    We report an organic redox-polymer-based electroenzymatic nitrogen fixation system using a metal-free redox polymer, namely neutral-red-modified poly(glycidyl methacrylate-co-methylmethacrylate-co-poly(ethyleneglycol)methacrylate) with a low redox potential of −0.58 V vs. SCE. The stable and efficient electric wiring of nitrogenase within the redox polymer matrix enables mediated bioelectrocatalysis of N3−, NO2− and N2 to NH3 catalyzed by the MoFe protein via the polymer-bound redox moieties distributed in the polymer matrix in the absence of the Fe protein. Bulk bioelectrosynthetic experiments produced 209±30 nmol NH3 nmol MoFe−1 h−1 from N2 reduction. 15N2 labeling experiments and NMR analysis were performed to confirm biosynthetic N2 reduction to NH3. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA
    view abstract10.1002/anie.202007198
  • Enhancing Electrogenerated Chemiluminescence on Platinum Electrodes through Surface Modification
    Fiorani, A. and Eßmann, V. and Santos, C.S. and Schuhmann, W.
    ChemElectroChem 7 (2020)
    Increasing the light emission of electrogenerated chemiluminescence is an important goal for enhancing the sensitivity for potential practical applications. Electrogenerated chemiluminescence is primarily triggered by a heterogeneous electron transfer reaction, for which the electrode material plays a pivotal role. We investigated how a platinum electrode, one of the most used but poorly efficient noble metal electrode materials in electrogenerated chemiluminescence, can be modified to enhance light emission. A polypyrrole layer was deposited on the platinum electrode through electrochemically induced polymerization, and subsequently pyrolyzed with the formation of a carbonaceous film. Electrochemiluminescence of the [Ru(bpy)3]2+/tri-n-propylamine system on such carbon film electrodes showed an enhancement of up to a 4 times increase, as compared with the bare platinum electrode. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/celc.202000103
  • Faceted Branched Nickel Nanoparticles with Tunable Branch Length for High-Activity Electrocatalytic Oxidation of Biomass
    Poerwoprajitno, A.R. and Gloag, L. and Watt, J. and Cychy, S. and Cheong, S. and Kumar, P.V. and Benedetti, T.M. and Deng, C. and Wu, K.-H. and Marjo, C.E. and Huber, D.L. and Muhler, M. and Gooding, J.J. and Schuhmann, W. and Wang, D.-W. and Tilley, R.D.
    Angewandte Chemie - International Edition 59 (2020)
    Controlling the formation of nanosized branched nanoparticles with high uniformity is one of the major challenges in synthesizing nanocatalysts with improved activity and stability. Using a cubic-core hexagonal-branch mechanism to form highly monodisperse branched nanoparticles, we vary the length of the nickel branches. Lengthening the nickel branches, with their high coverage of active facets, is shown to improve activity for electrocatalytic oxidation of 5-hydroxymethylfurfural (HMF), as an example for biomass conversion. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA
    view abstract10.1002/anie.202005489
  • Factors Governing the Activity of α-MnO2 Catalysts in the Oxygen Evolution Reaction: Conductivity versus Exposed Surface Area of Cryptomelane
    Heese-Gärtlein, J. and Morales, D.M. and Rabe, A. and Bredow, T. and Schuhmann, W. and Behrens, M.
    Chemistry - A European Journal 26 (2020)
    Cryptomelane (α-(K)MnO2) powders were synthesized by different methods leading to only slight differences in their bulk crystal structure and chemical composition, while the BET surface area and the crystallite size differed significantly. Their performance in the oxygen evolution reaction (OER) covered a wide range and their sequence of increasing activity differed when electrocatalysis in alkaline electrolyte and chemical water oxidation using Ce4+ were compared. The decisive factors that explain this difference were identified in the catalysts’ microstructure. Chemical water oxidation activity is substantially governed by the exposed surface area, while the electrocatalytic activity is determined largely by the electric conductivity, which was found to correlate with the particle morphology in terms of needle length and aspect ratio in this sample series. This correlation is rather explained by an improved conductivity due to longer needles than by structure sensitivity as was supported by reference experiments using H2O2 decomposition and carbon black as additive. The most active catalyst R-cryptomelane reached a current density of 10 mA cm−2 at a potential 1.73 V without, and at 1.71 V in the presence of carbon black. The improvement was significantly higher for the catalyst with lower initial activity. However, the materials showed a disappointing catalytic stability during alkaline electrochemical OER, whereas the crystal structure was found to be stable at working conditions. © 2020 The Authors. Published by Wiley-VCH GmbH
    view abstract10.1002/chem.201905090
  • Glutamate detection at the cellular level by means of polymer/enzyme multilayer modified carbon nanoelectrodes
    Marquitan, M. and Mark, M.D. and Ernst, A. and Muhs, A. and Herlitze, S. and Ruff, A. and Schuhmann, W.
    Journal of Materials Chemistry B 8 (2020)
    Carbon nanoelectrodes in the sub-micron range were modified with an enzyme cascade immobilized in a spatially separated polymer double layer system for the detection of glutamate at the cellular level. The enzyme cascade consists of glutamate oxidase (GlutOx) that was immobilized in a hydrophilic redox silent polymer on top of a horseradish peroxidase (HRP)/redox polymer layer. In the presence of O2, glutamate was oxidized under concomitant reduction of O2to H2O2at GlutOx. H2O2is further reduced to water by means of HRP and electrons are shuttledviathe redox polymer matrix that wires the HRP to the electrode surface, hence delivering a current response proportional to the glutamate concentration. The nanometer-sized sensors could be successfully used to measure glutamate release from primary mouse astrocytes in 10 mM HEPES buffer. © The Royal Society of Chemistry 2020.
    view abstract10.1039/c9tb02461a
  • High-throughput characterization of Ag–V–O nanostructured thin-film materials libraries for photoelectrochemical solar water splitting
    Kumari, S. and Helt, L. and Junqueira, J.R.C. and Kostka, A. and Zhang, S. and Sarker, S. and Mehta, A. and Scheu, C. and Schuhmann, W. and Ludwig, Al.
    International Journal of Hydrogen Energy 45 (2020)
    Ag–V–O thin-film materials libraries, with both composition (Ag22-77V23-78Ox) and thickness (123–714 nm) gradients were fabricated using combinatorial reactive magnetron co-sputtering aiming on establishing relations between composition, structure, and functional properties. As-deposited libraries were annealed in air at 300 °C for 10 h. High-throughput characterization methods of composition, structure and functional properties were used to identify photoelectrochemically active regions. The phases AgV6O15, Ag2V4O11, AgVO3, and Ag4V2O7 were observed throughout the composition gradient. The photoelectrochemical properties of Ag–V–O films are dependent on composition and morphology. An enhanced photocurrent density (~300–554 μA/cm2) was obtained at 30 to 45 at.% Ag along the thickness gradient. Thin films of these compositions show a nanowire morphology, which is an important factor for the enhancement of photoelectrochemical performance. The photoelectrochemically active regions were further investigated by high-throughput synchrotron-X-ray diffraction and transmission electron microscopy (Ag32V68Ox) which confirmed the presence of Ag2V4O11 as the dominating phase along with the minor phases AgV6O15 and AgVO3. This enhanced photoactive region shows bandgap values of ~2.30 eV for the direct and ~1.87 eV for the indirect bandgap energies. The porous nanostructured films improve charge transport and are hence of interest for photoelectrochemical water splitting. © 2020 Hydrogen Energy Publications LLC
    view abstract10.1016/j.ijhydene.2020.02.154
  • High-Throughput Characterization of Structural and Photoelectrochemical Properties of a Bi-Mo-W-O Thin-Film Materials Library
    Nowak, M. and Gutkowski, R. and Junqueira, J. and Schuhmann, W. and Ludwig, Al
    Zeitschrift fur Physikalische Chemie 234 (2020)
    A Bi-W-Mo-O thin-film materials library was fabricated by combinatorial reactive magnetron sputtering. The composition spread was investigated using high-throughput methods to determine crystalline phases, composition, morphology, optical properties, and photoelectrochemical performance. The aurivillius phase (Bi2O2)2+ (BiM(W1-NMoN)M-1O3M+1)2- is the predominantly observed crystal structure, indicating that the thin films in the library are solid solutions. With increasing amounts of Mo ≙ 7-22% the diffraction peak at 2θ = 28° ≙ [131] shifts due to lattice distortion, the photoelectrochemical activity is increasing up to a wavelength of 460 nm with an incident photon to current efficiency (IPCE) of 4.5%, and the bandgap decreases. A maximum photocurrent density of 31 μA/cm2 was measured for Bi31W62Mo7Oz at a bias potential of 1.23 V vs. RHE (0.1 M Na2SO4). © 2020 Wolfgang Schuhmann, Alfred Ludwig et al., published by De Gruyter, Berlin/Boston 2020.
    view abstract10.1515/zpch-2019-1439
  • High-Throughput Exploration of Metal Vanadate Thin-Film Systems (M-V-O, M = Cu, Ag, W, Cr, Co, Fe) for Solar Water Splitting: Composition, Structure, Stability, and Photoelectrochemical Properties
    Kumari, S. and Junqueira, J.R.C. and Schuhmann, W. and Ludwig, Al.
    ACS Combinatorial Science 22 (2020)
    Combinatorial synthesis and high-throughput characterization of thin-film materials libraries enable to efficiently identify both photoelectrochemically active and inactive, as well as stable and instable systems for solar water splitting. This is shown on six ternary metal vanadate (M-V-O, M = Cu, Ag, W, Cr, Co, Fe) thin-film materials libraries, fabricated using combinatorial reactive magnetron cosputtering with subsequent annealing in air. By means of high-throughput characterization of these libraries correlations between composition, crystal structure, photocurrent density, and stability of the M-V-O systems in different electrolytes such as acidic, neutral and alkaline media were identified. The systems Cu-V-O and Ag-V-O are stable in alkaline electrolyte and exhibited photocurrents of 170 and 554 μA/cm2, respectively, whereas the systems W-V-O, Cr-V-O, and Co-V-O are not stable in alkaline electrolyte. However, the Cr-V-O and Co-V-O systems showed an enlarged photoactive region in acidic electrolyte, albeit with very low photocurrents (<10 μA/cm2). Complete data sets obtained from these different screening sets, including information on nonpromising systems, lays groundwork for their use to predict new systems for solar water splitting, for example, by machine learning. ©
    view abstract10.1021/acscombsci.0c00150
  • Improved quantum efficiency in an engineered light harvesting/photosystem II super-complex for high current density biophotoanodes
    Hartmann, V. and Harris, D. and Bobrowski, T. and Ruff, A. and Frank, A. and Günther Pomorski, T. and Rögner, M. and Schuhmann, W. and Adir, N. and Nowaczyk, M.M.
    Journal of Materials Chemistry A 8 (2020)
    Photosystem II (PSII) is the only enzyme that catalyzes light-induced water oxidation, the basis for its application as a biophotoanode in various bio-photovoltaics and photo-bioelectrochemical cells. However, the absorption spectrum of PSII limits the quantum efficiency in the range of visible light, due to a gap in the green absorption region of chlorophylls (500-600 nm). To overcome this limitation, we have stabilized the interaction between PSII and Phycobilisomes (PBSs)-the cyanobacterial light harvesting complex, in vitro. The PBS of three different cyanobacteria (Acaryochloris marina, Am, Mastigocladus laminosus, ML, and Synechocystis sp. PCC 6803, Syn) are analyzed for their ability to transfer energy to Thermosynechococcus elongatus (Te) PSII by fluorescence spill-over and photo-current action spectra. Integration of the PBS-PSII super-complexes within an Os-complex-modified hydrogel on macro-porous indium tin oxide electrodes (MP-ITO) resulted in notably improved, wavelength dependent, incident photon-to-electron conversion efficiencies (IPCE). IPCE values in the green gap were doubled from 3% to 6% compared to PSII electrodes without PBS and a maximum IPCE up to 10.9% at 670 nm was achieved. © 2020 The Royal Society of Chemistry.
    view abstract10.1039/d0ta03444d
  • Insight into Electron Transfer from a Redox Polymer to a Photoactive Protein
    Białek, R. and Thakur, K. and Ruff, A. and Jones, M.R. and Schuhmann, W. and Ramanan, C. and Gibasiewicz, K.
    Journal of Physical Chemistry B 124 (2020)
    Biohybrid photoelectrochemical systems in photovoltaic or biosensor applications have gained considerable attention in recent years. While the photoactive proteins engaged in such systems usually maintain an internal charge separation quantum yield of nearly 100%, the subsequent steps of electron and hole transfer beyond the protein often limit the overall system efficiency and their kinetics remain largely uncharacterized. To reveal the dynamics of one of such charge-transfer reactions, we report on the reduction of Rhodobacter sphaeroides reaction centers (RCs) by Os-complex-modified redox polymers (P-Os) characterized using transient absorption spectroscopy. RCs and P-Os were mixed in buffered solution in different molar ratios in the presence of a water-soluble quinone as an electron acceptor. Electron transfer from P-Os to the photoexcited RCs could be described by a three-exponential function, the fastest lifetime of which was on the order of a few microseconds, which is a few orders of magnitude faster than the internal charge recombination of RCs with fully separated charge. This was similar to the lifetime for the reduction of RCs by their natural electron donor, cytochrome c2. The rate of electron donation increased with increasing ratio of polymer to protein concentrations. It is proposed that P-Os and RCs engage in electrostatic interactions to form complexes, the sizes of which depend on the polymer-to-protein ratio. Our findings throw light on the processes within hydrogel-based biophotovoltaic devices and will inform the future design of materials optimally suited for this application. ©
    view abstract10.1021/acs.jpcb.0c08714
  • Insights into the Formation, Chemical Stability, and Activity of Transient NiyP@NiO x Core-Shell Heterostructures for the Oxygen Evolution Reaction
    Wilde, P. and Dieckhöfer, S. and Quast, T. and Xiang, W. and Bhatt, A. and Chen, Y.-T. and Seisel, S. and Barwe, S. and Andronescu, C. and Li, T. and Schuhmann, W. and Masa, J.
    ACS Applied Energy Materials 3 (2020)
    NiyP emerged as a highly active precatalyst for the alkaline oxygen evolution reaction where structural changes play a crucial role for its catalytic performance. We probed the chemical stability of NiyP in 1 M KOH at 80 °C and examined how exposure up to 168 h affects its structure and catalytic performance. We observed selective P-leaching and formation of NiyP/NiOx core-shell heterostructures, where shell thickness increases with ageing time, which is detrimental for the activity. By tuning the particle size, we demonstrate that prevention of complete catalyst oxidation is essential to preserve the outstanding electrochemical performance of NiyP in alkaline media. © 2020 American Chemical Society.
    view abstract10.1021/acsaem.9b02481
  • Light-controlled imaging of biocatalytic reactionsviascanning photoelectrochemical microscopy for multiplexed sensing
    Riedel, M. and Ruff, A. and Schuhmann, W. and Lisdat, F. and Conzuelo, F.
    Chemical Communications 56 (2020)
    A light-controlled multiplexing platform has been developed on the basis of a quantum dot-sensitized inverse opal TiO2electrode with integrated biocatalytic reactions. Spatially resolved illumination enables multiplexed sensing and imaging of enzymatic oxidation reactions at relatively negative applied potentials. © The Royal Society of Chemistry 2020.
    view abstract10.1039/d0cc00777c
  • Needle-type organic electrochemical transistor for spatially resolved detection of dopamine
    Mariani, F. and Quast, T. and Andronescu, C. and Gualandi, I. and Fraboni, B. and Tonelli, D. and Scavetta, E. and Schuhmann, W.
    Microchimica Acta 187 (2020)
    In this work, the advantages of carbon nanoelectrodes (CNEs) and orgonic electrochemical transistors (OECTs) were merged to realise nanometre-sized, spearhead OECTs based on single- and double-barrel CNEs functionalised with a conducting polymer film. The needle-type OECT shows a high aspect ratio that allows its precise positioning by means of a macroscopic handle and its size is compatible with single-cell analysis. The device was characterised with respect to its electrolyte-gated behaviour and was employed as electrochemical sensor for the proof-of-concept detection of dopamine (DA) over a wide concentration range (10−12—10−6 M). Upon application of fixed drain and gate voltages (Vd = − 0.3 V, Vg = − 0.9 V, respectively), the nano-sized needle-type OECT sensor exhibited a linear response in the low pM range and from 0.002 to 7 μM DA, with a detection limit of 1 × 10−12 M. [Figure not available: see fulltext.]. © 2020, The Author(s).
    view abstract10.1007/s00604-020-04352-1
  • Online Monitoring of Electrochemical Carbon Corrosion in Alkaline Electrolytes by Differential Electrochemical Mass Spectrometry
    Möller, S. and Barwe, S. and Masa, J. and Wintrich, D. and Seisel, S. and Baltruschat, H. and Schuhmann, W.
    Angewandte Chemie - International Edition 59 (2020)
    Carbon corrosion at high anodic potentials is a major source of instability, especially in acidic electrolytes and impairs the long-term functionality of electrodes. In-depth investigation of carbon corrosion in alkaline environment by means of differential electrochemical mass spectrometry (DEMS) is prevented by the conversion of CO2 into CO3 2−. We report the adaptation of a DEMS system for online CO2 detection as the product of carbon corrosion in alkaline electrolytes. A new cell design allows for in situ acidification of the electrolyte to release initially dissolved CO3 2− as CO2 in front of the DEMS membrane and its subsequent detection by mass spectrometry. DEMS studies of a carbon-supported nickel boride (NixB/C) catalyst and Vulcan XC 72 at high anodic potentials suggest protection of carbon in the presence of highly active oxygen evolution electrocatalysts. Most importantly, carbon corrosion is decreased in alkaline solution. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/anie.201909475
  • Polymer-Based Batteries—Flexible and Thin Energy Storage Systems
    Hager, M.D. and Esser, B. and Feng, X. and Schuhmann, W. and Theato, P. and Schubert, U.S.
    Advanced Materials 32 (2020)
    Batteries have become an integral part of everyday life—from small coin cells to batteries for mobile phones, as well as batteries for electric vehicles and an increasing number of stationary energy storage applications. There is a large variety of standardized battery sizes (e.g., the familiar AA-battery or AAA-battery). Interestingly, all these battery systems are based on a huge number of different cell chemistries depending on the application and the corresponding requirements. There is not one single battery type fulfilling all demands for all imaginable applications. One battery class that has been gaining significant interest in recent years is polymer-based batteries. These batteries utilize organic materials as the active parts within the electrodes without utilizing metals (and their compounds) as the redox-active materials. Such polymer-based batteries feature a number of interesting properties, like high power densities and flexible batteries fabrication, among many more. © 2020 The Authors. Published by Wiley-VCH GmbH
    view abstract10.1002/adma.202000587
  • Polymer/enzyme-modified HF-etched carbon nanoelectrodes for single-cell analysis
    Marquitan, M. and Ruff, A. and Bramini, M. and Herlitze, S. and Mark, M.D. and Schuhmann, W.
    Bioelectrochemistry 133 (2020)
    Carbon-based nanoelectrodes fabricated by means of pyrolysis of an alkane precursor gas purged through a glass capillary and subsequently etched with HF were modified with redox polymer/enzyme films for the detection of glucose at the single-cell level. Glucose oxidase (GOx) was immobilized and electrically wired by means of an Os-complex-modified redox polymer in a sequential dip coating process. For the synthesis of the redox polymer matrix, a poly(1-vinylimidazole-co-acrylamide)-based backbone was used that was first modified with the electron transfer mediator [Os(bpy)2Cl]+ (bpy = 2,2′-bipyridine) followed by the conversion of the amide groups within the acrylamide monomer into hydrazide groups in a polymer-analogue reaction. The hydrazide groups react readily with bifunctional epoxide-based crosslinkers ensuring high film stability. Insertion of the nanometre-sized polymer/enzyme modified electrodes into adherently growing single NG108-15 cells resulted in a positive current response correlating with the intracellular glucose concentration. Moreover, the nanosensors showed a stable current output without significant loss in performance after intracellular measurements. © 2020
    view abstract10.1016/j.bioelechem.2020.107487
  • Reassessing the rationale behind herbicide biosensors: The case of a photosystem II/redox polymer-based bioelectrodefs
    Wang, P. and Zhao, F. and Hartmann, V. and Nowaczyk, M.M. and Ruff, A. and Schuhmann, W. and Conzuelo, F.
    Bioelectrochemistry 136 (2020)
    Interfacing photosynthetic protein complexes with electrodes is frequently used for the identification of electron transfer mechanisms and the fabrication of biosensors. Binding of herbicide compounds to the terminal plastoquinone QB at photosystem II (PSII) causes disruption of electron flow that is associated with a diminished performance of the associated biodevice. Thus, the principle of electron transport inhibition at PSII can be used for herbicide detection and has inspired the fabrication of several biosensors for this purpose. However, the biosensor performance may reveal a more complex behavior than generally expected. As we present here for a photobioelectrode constituted by PSII embedded in a redox polymer matrix, the effect caused by inhibitors does not only impact the electron transfer from PSII but also the properties of the polymer film used for immobilization and electrical wiring of the protein complexes. Incorporation of phenolic inhibitors into the polymer film surprisingly translates into enhanced photocurrents and, in particular cases, in a higher stability of the overall electrode architecture. The achieved results stress the importance to evaluate first the possible influence of analytes of interest on the biosensor architecture as a whole and provide important insights for consideration in future design of bioelectrochemical devices. © 2020
    view abstract10.1016/j.bioelechem.2020.107597
  • Redox-Polymer-Based High-Current-Density Gas-Diffusion H2-Oxidation Bioanode Using [FeFe] Hydrogenase from Desulfovibrio desulfuricans in a Membrane-free Biofuel Cell
    Szczesny, J. and Birrell, J.A. and Conzuelo, F. and Lubitz, W. and Ruff, A. and Schuhmann, W.
    Angewandte Chemie - International Edition 59 (2020)
    The incorporation of highly active but also highly sensitive catalysts (e.g. the [FeFe] hydrogenase from Desulfovibrio desulfuricans) in biofuel cells is still one of the major challenges in sustainable energy conversion. We report the fabrication of a dual-gas diffusion electrode H2/O2 biofuel cell equipped with a [FeFe] hydrogenase/redox polymer-based high-current-density H2-oxidation bioanode. The bioanodes show benchmark current densities of around 14 mA cm−2 and the corresponding fuel cell tests exhibit a benchmark for a hydrogenase/redox polymer-based biofuel cell with outstanding power densities of 5.4 mW cm−2 at 0.7 V cell voltage. Furthermore, the highly sensitive [FeFe] hydrogenase is protected against oxygen damage by the redox polymer and can function under 5 % O2. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA
    view abstract10.1002/anie.202006824
  • Redox-Polymer-Wired [NiFeSe] Hydrogenase Variants with Enhanced O2 Stability for Triple-Protected High-Current-Density H2-Oxidation Bioanodes
    Ruff, A. and Szczesny, J. and Vega, M. and Zacarias, S. and Matias, P.M. and Gounel, S. and Mano, N. and Pereira, I.A.C. and Schuhmann, W.
    ChemSusChem 13 (2020)
    Variants of the highly active [NiFeSe] hydrogenase from D. vulgaris Hildenborough that exhibit enhanced O2 tolerance were used as H2-oxidation catalysts in H2/O2 biofuel cells. Two [NiFeSe] variants were electrically wired by means of low-potential viologen-modified redox polymers and evaluated with respect to H2-oxidation and stability against O2 in the immobilized state. The two variants showed maximum current densities of (450±84) μA cm−2 for G491A and (476±172) μA cm−2 for variant G941S on glassy carbon electrodes and a higher O2 tolerance than the wild type. In addition, the polymer protected the enzyme from O2 damage and high-potential inactivation, establishing a triple protection for the bioanode. The use of gas-diffusion bioanodes provided current densities for H2-oxidation of up to 6.3 mA cm−2. Combination of the gas-diffusion bioanode with a bilirubin oxidase-based gas-diffusion O2-reducing biocathode in a membrane-free biofuel cell under anode-limiting conditions showed unprecedented benchmark power densities of 4.4 mW cm−2 at 0.7 V and an open-circuit voltage of 1.14 V even at moderate catalyst loadings, outperforming the previously reported system obtained with the [NiFeSe] wild type and the [NiFe] hydrogenase from D. vulgaris Miyazaki F. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/cssc.202000999
  • Scalable Fabrication of Biophotoelectrodes by Means of Automated Airbrush Spray-Coating
    Bobrowski, T. and Conzuelo, F. and Ruff, A. and Hartmann, V. and Frank, A. and Erichsen, T. and Nowaczyk, M.M. and Schuhmann, W.
    ChemPlusChem 85 (2020)
    The fabrication and electrochemical evaluation of transparent photoelectrodes consisting of Photosystem I (PSI) or Photosystem II (PSII) is described, which are embedded and electrically wired by a redox polymer. The fabrication process is performed by an automated airbrush-type spray coating system, which ensures controlled and scalable electrode preparation. As proof of concept, electrodes with a surface area of up to 25 cm2 were prepared. The macro-porous structure of the indium tin oxide electrodes allows a high loading of the photoactive protein complexes leading to enhanced photocurrents, which are essential for potentially technologically relevant solar-powered devices. In addition, we show that unpurified crude PSII extracts, which can be provided in comparatively high yields for electrode modification, are suitable for photoelectrode fabrication with comparable photocurrent densities. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/cplu.202000291
  • Spray-Flame-Prepared LaCo1–xFexO3 Perovskite Nanoparticles as Active OER Catalysts: Influence of Fe Content and Low-Temperature Heating
    Alkan, B. and Medina, D. and Landers, J. and Heidelmann, M. and Hagemann, U. and Salamon, S. and Andronescu, C. and Wende, H. and Schulz, C. and Schuhmann, W. and Wiggers, H.
    ChemElectroChem 7 (2020)
    Spray-flame synthesis was used to produce high-surface-area perovskite electrocatalysts with high phase purity, minimum surface contamination, and high electrochemical stability. In this study, as-prepared LaCo1–xFexO3 perovskite nanoparticles (x=0.2, 0.3, and 0.4) were found to contain a high degree of combustion residuals, and mostly consist of both, stoichiometric and oxygen-deficient perovskite phases. Heating them at moderate temperature (250 °C) in oxygen could remove combustion residuals and increases the content of stoichiometric perovskite while preventing particle growth. A higher surface crystallinity was observed with increasing iron content coming along with a rise in oxygen deficient phases. With heat treatment, OER activity and stability of perovskites improved at 30 and 40 at.% Fe while deteriorating at 20 at.% Fe. This study highlights spray-flame synthesis as a promising technique to synthesize highly active nanoscale perovskite catalysts with improved OER activity. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/celc.201902051
  • Sputter deposition of highly active complex solid solution electrocatalysts into an ionic liquid library: Effect of structure and composition on oxygen reduction activity
    Manjón, A.G. and Löffler, T. and Meischein, M. and Meyer, H. and Lim, J. and Strotkötter, V. and Schuhmann, W. and Ludwig, Al. and Scheu, C.
    Nanoscale 12 (2020)
    Complex solid solution electrocatalysts (often called high-entropy alloys) present a new catalyst class with highly promising features due to the interplay of multi-element active sites. One hurdle is the limited knowledge about structure-activity correlations needed for targeted catalyst design. We prepared Cr-Mn-Fe-Co-Ni nanoparticles by magnetron sputtering a high entropy Cantor alloy target simultaneously into an ionic liquid library. The synthesized nanoparticles have a narrow size distribution but different sizes (from 1.3 ± 0.1 nm up to 2.6 ± 0.3 nm), different crystallinity (amorphous, face-centered cubic or body-centered cubic) and composition (i.e. high Mn versus low Mn content). The Cr-Mn-Fe-Co-Ni complex solid solution nanoparticles possess an unprecedented intrinsic electrocatalytic activity for the oxygen reduction reaction in alkaline media, some of them even surpassing that of Pt. The highest intrinsic activity was obtained for body-centered cubic nanoparticles with a low Mn and Fe content which were synthesized using the ionic liquid 1-etyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [Emimi][(Tf)2N]. This journal is © The Royal Society of Chemistry.
    view abstract10.1039/d0nr07632e
  • Structural and photoelectrochemical properties in the thin film system Cu-Fe-V-O and its ternary subsystems Fe-V-O and Cu-V-O
    Kumari, S. and Junqueira, J.R.C. and Sarker, S. and Mehta, A. and Schuhmann, W. and Ludwig, Al.
    Journal of Chemical Physics 153 (2020)
    Thin-film material libraries in the ternary and quaternary metal oxide systems Fe-V-O, Cu-V-O, and Cu-Fe-V-O were synthesized using combinatorial reactive co-sputtering with subsequent annealing in air. Their compositional, structural, and functional properties were assessed using high-throughput characterization methods. Prior to the investigation of the quaternary system Cu-Fe-V-O, the compositions (Fe61V39)Ox and (Cu52V48)Ox with promising photoactivity were identified from their ternary subsystems Fe-V-O and Cu-V-O, respectively. Two Cu-Fe-V-O material libraries with (Cu29-72Fe4-27V22-57)Ox and (Cu11-55Fe27-73V12-34)Ox composition spread were investigated. Seven mixed ternary and quaternary phase regions were identified: I (α-Cu3FeV6O26/FeVO4), II (Cu5V2O10/FeVO4/α-Cu3Fe4V6O26), III (Cu5V2O10), IV (Cu5V2O10/FeVO4, V (FeVO4/γ-Cu2V2O7/α-Cu3Fe4V6O26), VI (β-Cu2V2O7/α-Cu3Fe4V6O26/FeVO4), and VII (β-Cu3Fe4V6O26/FeVO4). In the investigated composition range, two photoactive regions, (Cu53Fe7V40)Ox and (Cu45Fe21V34)Ox, were identified, exhibiting 103 μA/cm2 and 108 μA/cm2 photocurrent density for the oxygen evolution reaction at 1.63 V vs reversible hydrogen electrode, respectively. The highest photoactive region (Cu45Fe21V34)Ox comprises the dominant α-Cu3Fe4V6O24 phase and minor FeVO4 phase. This photoactive region corresponds to having an indirect bandgap of 1.87 eV and a direct bandgap of 2.58 eV with an incident photon-to-current efficiency of 30% at a wavelength of 310 nm. © 2020 Author(s).
    view abstract10.1063/5.0009512
  • Surface Properties of Battery Materials Elucidated Using Scanning Electrochemical Microscopy: The Case of Type I Silicon Clathrate
    Tarnev, T. and Wilde, P. and Dopilka, A. and Schuhmann, W. and Chan, C.K. and Ventosa, E.
    ChemElectroChem 7 (2020)
    Silicon clathrates have attracted interest as potential anodes for lithium-ion batteries with unique framework structures. However, very little is known about the surface reactivity and solid electrolyte interphase (SEI) properties of clathrates. In this study, operando scanning electrochemical microscopy (SECM) is used to investigate the effect of pre-treatment on the formation dynamics and intrinsic properties of the SEI in electrodes prepared from type I Ba8Al16Si30 silicon clathrates. Although X-ray photoelectron spectroscopy (XPS) analysis does not reveal large changes in SEI composition, it is found through SECM measurements that ball-milling combined with chemical acid/base etching of the clathrates lead to a more stable and rapidly formed SEI as compared to purely ball-milled samples, resulting in enhanced coulombic efficiency. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/celc.201901688
  • Synergistic Effect of Molybdenum and Tungsten in Highly Mixed Carbide Nanoparticles as Effective Catalysts in the Hydrogen Evolution Reaction under Alkaline and Acidic Conditions
    Fu, Q. and Peng, B. and Masa, J. and Chen, Y.-T. and Xia, W. and Schuhmann, W. and Muhler, M.
    ChemElectroChem 7 (2020)
    Monometallic Mo and W carbides as well as highly mixed (Mo,W) carbides with various Mo/W ratios were synthesized directly on oxygen-functionalized carbon nanotubes (OCNTs), and used as noble-metal-free electrocatalysts in the hydrogen evolution reaction (HER) under both acidic and alkaline conditions. A purely orthorhombic structure was found in both monometallic and mixed carbide samples by X-ray diffraction. Transmission electron microscopy images showed that the carbide particles were highly dispersed on the OCNTs with well-controlled particle size. The homogeneous distribution of Mo and W in the carbides was confirmed by elemental mapping. (Mo,W)2C/OCNT with a Mo/W ratio of 3 : 1 showed the lowest overpotential to reach a current density of 10 mA/cm2 (87 mV in 0.1 M KOH and 92 mV in 0.5 M H2SO4), and the smallest Tafel slope of 34 mV/dec. Long-term stability under both alkaline and acidic conditions was demonstrated for 24 h. Our results revealed that an optimal amount of W in the mixed carbide can significantly improve its performance in the HER following the Tafel reaction pathway, most likely due to the weakened Mo−Hads bond. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/celc.202000047
  • The importance of nanoscale confinement to electrocatalytic performance
    Wordsworth, J. and Benedetti, T.M. and Alinezhad, A. and Tilley, R.D. and Edwards, M.A. and Schuhmann, W. and Gooding, J.J.
    Chemical Science 11 (2020)
    Electrocatalytic nanoparticles that mimic the three-dimensional geometric architecture of enzymes where the reaction occurs down a substrate channel isolated from bulk solution, referred to herein as nanozymes, were used to explore the impact of nano-confinement on electrocatalytic reactions. Surfactant covered Pt-Ni nanozyme nanoparticles, with Ni etched from the nanoparticles, possess a nanoscale channel in which the active sites for electrocatalysis of oxygen reduction are located. Different particle compositions and etching parameters allowed synthesis of nanoparticles with different average substrate channel diameters that have varying amounts of nano-confinement. The results showed that in the kinetically limited regime at low overpotentials, the smaller the substrate channels the higher the specific activity of the electrocatalyst. This is attributed to higher concentrations of protons, relative to bulk solution, required to balance the potential inside the nano-confined channel. However, at higher overpotentials where limitation by mass transport of oxygen becomes important, the nanozymes with larger substrate channels showed higher electrocatalytic activity. A reaction-diffusion model revealed that the higher electrocatalytic activity at low overpotentials with smaller substrate channels can be explained by the higher concentration of protons. The model suggests that the dominant mode of mass transport to achieve these high concentrations is by migration, exemplifying how nano-confinement can be used to enhance reaction rates. Experimental and theoretical data show that under mass transport limiting potentials, the nano-confinement has no effect and the reaction only occurs at the entrance of the substrate channel at the nanoparticle surface. © The Royal Society of Chemistry.
    view abstract10.1039/c9sc05611d
  • The sum is more than its parts: stability of MnFe oxide nanoparticles supported on oxygen-functionalized multi-walled carbon nanotubes at alternating oxygen reduction reaction and oxygen evolution reaction conditions
    Morales, D.M. and Kazakova, M.A. and Purcel, M. and Masa, J. and Schuhmann, W.
    Journal of Solid State Electrochemistry 24 (2020)
    Successful design of reversible oxygen electrocatalysts does not only require to consider their activity towards the oxygen reduction (ORR) and the oxygen evolution reactions (OER), but also their electrochemical stability at alternating ORR and OER operating conditions, which is important for potential applications in reversible electrolyzers/fuel cells or metal/air batteries. We show that the combination of catalyst materials containing stable ORR active sites with those containing stable OER active sites may result in a stable ORR/OER catalyst if each of the active components can satisfy the current demand of their respective reaction. We compare the ORR/OER performances of oxides of Mn (stable ORR active sites), Fe (stable OER active sites), and bimetallic Mn0.5Fe0.5 (reversible ORR/OER catalyst) supported on oxidized multi-walled carbon nanotubes. Despite the instability of Mn and Fe oxide for the OER and the ORR, respectively, Mn0.5Fe0.5 exhibits high stability for both reactions. © 2020, The Author(s).
    view abstract10.1007/s10008-020-04667-2
  • Trimetallic Mn-Fe-Ni Oxide Nanoparticles Supported on Multi-Walled Carbon Nanotubes as High-Performance Bifunctional ORR/OER Electrocatalyst in Alkaline Media
    Morales, D.M. and Kazakova, M.A. and Dieckhöfer, S. and Selyutin, A.G. and Golubtsov, G.V. and Schuhmann, W. and Masa, J.
    Advanced Functional Materials 30 (2020)
    Discovering precious metal-free electrocatalysts exhibiting high activity and stability toward both the oxygen reduction (ORR) and the oxygen evolution (OER) reactions remains one of the main challenges for the development of reversible oxygen electrodes in rechargeable metal–air batteries and reversible electrolyzer/fuel cell systems. Herein, a highly active OER catalyst, Fe0.3Ni0.7OX supported on oxygen-functionalized multi-walled carbon nanotubes, is substantially activated into a bifunctional ORR/OER catalyst by means of additional incorporation of MnOX. The carbon nanotube-supported trimetallic (Mn-Ni-Fe) oxide catalyst achieves remarkably low ORR and OER overpotentials with a low reversible ORR/OER overvoltage of only 0.73 V, as well as selective reduction of O2 predominantly to OH−. It is shown by means of rotating disk electrode and rotating ring disk electrode voltammetry that the combination of earth-abundant transition metal oxides leads to strong synergistic interactions modulating catalytic activity. The applicability of the prepared catalyst for reversible ORR/OER electrocatalysis is evaluated by means of a four-electrode configuration cell assembly comprising an integrated two-layer bifunctional ORR/OER electrode system with the individual layers dedicated for the ORR and the OER to prevent deactivation of the ORR activity as commonly observed in single-layer bifunctional ORR/OER electrodes after OER polarization. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/adfm.201905992
  • Trivalent iron rich CoFe layered oxyhydroxides for electrochemical water oxidation
    Weiß, S. and Ertl, M. and Varhade, S.D. and Radha, A.V. and Schuhmann, W. and Breu, J. and Andronescu, C.
    Electrochimica Acta 350 (2020)
    Layered double hydroxides (LDHs) are presently among the best-performing oxygen evolution reaction (OER) electrocatalysts in alkaline media. The high activity of LDHs is due to synergistic effects between two transition metals as well as the layered structure which facilitates electron transfer. Because of a perfect match with the size of interlayer carbonate a ratio of 2:1 for the di- and tri-valent octahedral cations is energetically preferred. Here we present a strategy, where first mixed valent (Co2+ 1-zFe2+ z)4 Fe3+ 2 - LDHs, with z values between 0 and 0.75 are synthesized, which are subsequently oxidized to Co2+Fe3+ LDH-type layered (oxy)hydroxides with an unusual high trivalent Fe content. Characterization of the chemically oxidized materials using bulk and surface techniques demonstrated the successful synthesis of LDH-like trivalent iron rich (Co2+)4-4z (Fe3+)2+4z (oxy)hydroxides with a final Fe content ranging from 33.3 to 83.3%. Current densities of up to 200 mA cm−2 were obtained at potentials lower than 1.7 V vs. RHE for (Co2+)4-4z (Fe3+)2+4z (oxy)hydroxides containing a maximum of 80% Fe. © 2020 Elsevier Ltd
    view abstract10.1016/j.electacta.2020.136256
  • Tuning Light-Driven Water Oxidation Efficiency of Molybdenum-Doped BiVO4 by Means of Multicomposite Catalysts Containing Nickel, Iron, and Chromium Oxides
    Krysiak, O.A. and Junqueira, J.R.C. and Conzuelo, F. and Bobrowski, T. and Wilde, P. and Wysmolek, A. and Schuhmann, W.
    ChemPlusChem 85 (2020)
    Mo-doped BiVO4 has emerged as a promising material for photoelectrodes for photoelectrochemical water splitting, however, still shows a limited efficiency for light-driven water oxidation. We present the influence of an oxygen-evolution catalyst composed of Ni, Fe, and Cr oxides on the activity of Mo:BiVO4 photoanodes. The photoanodes are prepared by spray-coating, enabling compositional and thickness gradients of the incorporated catalyst. Two different configurations are evaluated, namely with the catalyst embedded into the Mo:BiVO4 film or deposited on top of it. Both configurations provide a significantly different impact on the photoelectrocatalytic efficiency. Structural characterisation of the materials by means of SEM, TEM and XRD as well as the photoelectrocatalytic activity investigated by means of an optical scanning droplet cell and in situ detection of oxygen using scanning photoelectrochemical microscopy are presented. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/cplu.201900701
  • A Combinatorial Approach for Optimization of Oxygen Evolution Catalyst Loading on Mo-doped BiVO4 Photoanodes
    Gutkowski, R. and Masa, J. and Schuhmann, W.
    Electroanalysis 31 (2019)
    The incident photon to current efficiency (IPCE) of a photoactive surface strongly depends on the loading and thickness of the active materials. We present a combinatorial approach based on an optical scanning droplet cell for simultaneous deposition and systematic characterization of co-catalysts for the oxygen evolution reaction (OER) on Mo-doped BiVO4 (Mo−BiVO4) photoanodes electrochemically pre-deposited on transparent conductive FTO substrates. The loading and photoelectrochemical characterization of 10 different OER co-catalysts deposited by three different deposition techniques on FTO-supported Mo−BiVO4 were investigated aiming at determination of the suitable deposition parameters affording the highest enhancement of photoelectrochemical oxygen evolution for the different OER/Mo−BiVO4 films. A comparison of the photoelectrochemical performance of films of various OER co-catalyst deposited on FTO-supported Mo−BiVO4 by electrodeposition, photo-assisted electrodeposition and photodeposition revealed the necessity of a material specific optimization with respect to co-catalyst loading and deposition technique to achieve optimal IPCE for each co-catalysts. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/elan.201900147
  • A light-driven Nernstian biosupercapacitor
    Zhao, F. and Bobrowski, T. and Ruff, A. and Hartmann, V. and Nowaczyk, M.M. and Rögner, M. and Conzuelo, F. and Schuhmann, W.
    Electrochimica Acta 306 (2019)
    Following inspiration by natural photosynthesis, the design and fabrication of semi-artificial biophotoelectrochemical devices able to harvest solar energy and aiming on the implementation of green and sustainable energy conversion systems is presently an important field of research. Here we present the development of a fully light-driven biosupercapacitor fabricated by incorporation of isolated photosystem 2 and photosystem 1 protein complexes embedded within the same Os-complex modified redox polymer. By this, light energy is stored at both electrodes within the polymer-based pseudocapacitive matrix in the form of Os 3+ centers at the photosystem1-based biocathode and in the form of Os 2+ centers at the photosystem 2-based bioanode. The stored energy can be released on demand into bursts of electricity. Due to the purely light-driven self-charging process, the biosupercapacitor provided a power output of 1.0 μW cm −2 after 200 s charging time. Moreover, the use of different electrode materials and their implication on the performance of the implemented biodevice is evaluated. © 2019 Elsevier Ltd
    view abstract10.1016/j.electacta.2019.03.168
  • A photosystem i monolayer with anisotropic electron flow enables Z-scheme like photosynthetic water splitting
    Zhao, F. and Wang, P. and Ruff, A. and Hartmann, V. and Zacarias, S. and Pereira, I.A.C. and Nowaczyk, M.M. and Rögner, M. and Conzuelo, F. and Schuhmann, W.
    Energy and Environmental Science 12 (2019)
    Photosynthetic protein complexes are attractive building blocks for the fabrication of semi-artificial energy conversion devices. However, limitations in the efficiency of the implemented biophotovoltaic systems prevent their use in practical applications. In particular, the effective minimization of recombination processes in photosystem I (PSI) modified bioelectrodes is crucial for enabling a unidirectional electron flow allowing the true potential of the large charge separation at PSI being exploited. Here, we present controlled immobilization of PSI monolayers with a predefined preferential orientation that translates into anisotropic electron flow upon irradiation. Further interface of the oriented PSI monolayer with redox polymers allows an efficient electron transfer and minimization of possible short-circuiting pathways. To complete the functional biophotocathode, the PSI monolayer is coupled to a hydrogenase (H2ase) to realize light-induced H2 evolution. The PSI/H2ase biocathode is then combined with a redox polymer/photosystem II-based bioanode demonstrating a fully light-driven Z-scheme mimic biophotovoltaic cell for bias-free water splitting. © 2019 The Royal Society of Chemistry.
    view abstract10.1039/c9ee01901d
  • A sensitive and selective graphene/cobalt tetrasulfonated phthalocyanine sensor for detection of dopamine
    Diab, N. and Morales, D.M. and Andronescu, C. and Masoud, M. and Schuhmann, W.
    Sensors and Actuators, B: Chemical 285 (2019)
    Electrochemical dopamine-sensing surfaces were fabricated by deposition of graphene sheets modified with cobalt tetrasulfonated phthalocyanine on glassy carbon electrodes (CoTSPc/Gr-GC) using differential pulse amperometry. Differential pulse stripping voltammetry was used to detect dopamine (DA) and the influence of pH value, scan rate, accumulation potential and time as well as dopamine concentration on the performance of CoTSPc/Gr-GC electrodes was investigated. The modified electrodes were successfully used as sensors for the selective and high sensitive determination of DA in presence of high concentrations of ascorbic acid (AA) and uric acid (UA) with a detection limit of 0.87 nM over the dynamic linear range of 20 nM to 220 nM. © 2019 Elsevier B.V.
    view abstract10.1016/j.snb.2019.01.022
  • A Z-Scheme-Inspired Photobioelectrochemical H 2 O/O 2 Cell with a 1 V Open-Circuit Voltage Combining Photosystem II and PbS Quantum Dots
    Riedel, M. and Wersig, J. and Ruff, A. and Schuhmann, W. and Zouni, A. and Lisdat, F.
    Angewandte Chemie - International Edition 58 (2019)
    A biohybrid photobioanode mimicking the Z-scheme has been developed by functional integration of photosystem II (PSII) and PbS quantum dots (QDs) within an inverse opal TiO 2 architecture giving rise to a rather negative water oxidation potential of about −0.55 V vs. Ag/AgCl, 1 m KCl at neutral pH. The electrical linkage between both light-sensitive entities has been established through an Os-complex-modified redox polymer (P Os ), which allows the formation of a multi-step electron-transfer chain under illumination starting with the photo-activated water oxidation at PSII followed by an electron transfer from PSII through P Os to the photo-excited QDs and finally to the TiO 2 electrode. The photobioanode was coupled to a novel, transparent, inverse-opal ATO cathode modified with an O 2 -reducing bilirubin oxidase for the construction of a H 2 O/O 2 photobioelectrochemical cell reaching a high open-circuit voltage of about 1 V under illumination. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/anie.201811172
  • A Z-Scheme-Inspired Photobioelectrochemical H2O/O2 Cell with a 1 V Open-Circuit Voltage Combining Photosystem II and PbS Quantum Dots
    Riedel, M. and Wersig, J. and Ruff, A. and Schuhmann, W. and Zouni, A. and Lisdat, F.
    Angewandte Chemie - International Edition 58 (2019)
    A biohybrid photobioanode mimicking the Z-scheme has been developed by functional integration of photosystem II (PSII) and PbS quantum dots (QDs) within an inverse opal TiO2 architecture giving rise to a rather negative water oxidation potential of about −0.55 V vs. Ag/AgCl, 1 m KCl at neutral pH. The electrical linkage between both light-sensitive entities has been established through an Os-complex-modified redox polymer (POs), which allows the formation of a multi-step electron-transfer chain under illumination starting with the photo-activated water oxidation at PSII followed by an electron transfer from PSII through POs to the photo-excited QDs and finally to the TiO2 electrode. The photobioanode was coupled to a novel, transparent, inverse-opal ATO cathode modified with an O2-reducing bilirubin oxidase for the construction of a H2O/O2 photobioelectrochemical cell reaching a high open-circuit voltage of about 1 V under illumination. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/anie.201811172
  • Amperometric Detection of the Urinary Disease Biomarker p-HPA by Allosteric Modulation of a Redox Polymer-Embedded Bacterial Reductase
    Teanphonkrang, S. and Ernst, A. and Janke, S. and Chaiyen, P. and Sucharitakul, J. and Suginta, W. and Khunkaewla, P. and Schuhmann, W. and Schulte, A. and Ruff, A.
    ACS Sensors 4 (2019)
    We report an amperometric biosensor for the urinary disease biomarker para-hydroxyphenylacetate (p-HPA) in which the allosteric reductase component of a bacterial hydroxylase, C1-hpah, is electrically wired to glassy carbon electrodes through incorporation into a low-potential Os-complex modified redox polymer. The proposed biosensing strategy depends on allosteric modulation of C1-hpah by the binding of the enzyme activator and analyte p-HPA, stimulating oxidation of the cofactor NADH. The pronounced concentration-dependence of allosteric C1-hpah modulation in the presence of a constant concentration of NADH allowed sensitive quantification of the target, p-HPA. The specific design of the immobilizing redox polymer with suitably low working potential allowed biosensor operation without the risk of co-oxidation of potentially interfering substances, such as uric acid or ascorbic acid. Optimized sensors were successfully applied for p-HPA determination in artificial urine, with good recovery rates and reproducibility and sub-micromolar detection limits. The proposed application of the allosteric enzyme C1-hpah for p-HPA trace electroanalysis is the first successful example of simple amperometric redox enzyme/redox polymer biosensing in which the analyte acts as an effector, modulating the activity of an immobilized biocatalyst. A general adVantage of the concept of allosterically modulated biosensing is its ability to broaden the range of approachable analytes, through the move from substrate to effector detection. © 2019 American Chemical Society.
    view abstract10.1021/acssensors.9b00144
  • Cascade Reactions in Nanozymes: Spatially Separated Active Sites inside Ag-Core-Porous-Cu-Shell Nanoparticles for Multistep Carbon Dioxide Reduction to Higher Organic Molecules
    O'Mara, P.B. and Wilde, P. and Benedetti, T.M. and Andronescu, C. and Cheong, S. and Gooding, J.J. and Tilley, R.D. and Schuhmann, W.
    Journal of the American Chemical Society 141 (2019)
    Enzymes can perform complex multistep cascade reactions by linking multiple distinct catalytic sites via substrate channeling. We mimic this feature in a generalized approach with an electrocatalytic nanoparticle for the carbon dioxide reduction reaction comprising a Ag core surrounded by a porous Cu shell, providing different active sites in nanoconfined volumes. The architecture of the nanozyme provides the basis for a cascade reaction, which promotes C-C coupling reactions. The first step occurs on the Ag core, and the subsequent steps on the porous copper shell, where a sufficiently high CO concentration due to the nanoconfinement facilitates C-C bond formation. The architecture yields the formation of n-propanol and propionaldehyde at potentials as low as-0.6 V vs RHE. Copyright © 2019 American Chemical Society.
    view abstract10.1021/jacs.9b07310
  • Catalytic Reactivation of Industrial Oxygen Depolarized Cathodes by in situ Generation of Atomic Hydrogen
    Öhl, D. and Franzen, D. and Paulisch, M. and Dieckhöfer, S. and Barwe, S. and Andronescu, C. and Manke, I. and Turek, T. and Schuhmann, W.
    ChemSusChem 12 (2019)
    Electrocatalytically active materials on the industrial as well as on the laboratory scale may suffer from chemical instability during operation, air exposure, or storage in the electrolyte. A strategy to recover the loss of electrocatalytic activity is presented. Oxygen-depolarized cathodes (ODC), analogous to those that are utilized in industrial brine electrolysis, are analyzed: the catalytic activity of the electrodes upon storage (4 weeks) under industrial process conditions (30 wt % NaOH, without operation) diminishes. This phenomenon occurs as a consequence of surface oxidation and pore blockage, as revealed by scanning electron microscopy, focused ion beam milling, X-ray photoelectron spectroscopy, and Raman spectroscopy. Potentiodynamic cycling of the oxidized electrodes to highly reductive potentials and the formation of “nascent” hydrogen re-reduces the electrode material, ultimately recovering the former catalytic activity. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/cssc.201900628
  • Cobalt metalloid and polybenzoxazine derived composites for bifunctional oxygen electrocatalysis
    Barwe, S. and Andronescu, C. and Engels, R. and Conzuelo, F. and Seisel, S. and Wilde, P. and Chen, Y.-T. and Masa, J. and Schuhmann, W.
    Electrochimica Acta 297 (2019)
    The development of bifunctional oxygen electrodes is a key factor for the envisaged application of rechargeable metal-air batteries. In this work, we present a simple procedure based on pyrolysis of polybenzoxazine/metal metalloid nanoparticles composites into efficient bifunctional oxygen reduction and oxygen evolution electrocatalysts. This procedure generates nitrogen-doped carbon with embedded metal metalloid nanoparticles exhibiting high activity towards both, oxygen reduction and oxygen evolution, in 0.1 M KOH with a roundtrip voltage of as low as 0.81 V. Koutecký-Levich analysis coupled with scanning electrochemical microscopy reveals that oxygen is preferentially reduced in a 4e− transfer pathway to hydroxide rather than to hydrogen peroxide. Furthermore, the polybenzoxazine derived carbon matrix allows for stable catalyst fixation on the electrode surface, resulting in unattenuated activity during continuous alternate polarisation between oxygen evolution at 10 mA cm−2 and oxygen reduction at −1.0 mA cm−2. © 2018 Elsevier Ltd
    view abstract10.1016/j.electacta.2018.12.047
  • Controlling DNA/Surface Interactions for Potential Pulse-Assisted Preparation of Multi-Probe DNA Microarrays
    Jambrec, D. and Kayran, Y.U. and Schuhmann, W.
    Electroanalysis 31 (2019)
    The possibility of selectively modifying microarray electrodes with different DNA sequences in a controlled way without the need for local positioning of solutions or local modification of array surfaces is demonstrated. Potential pulse sequences are employed to perform sequential surface modification of a 32-gold-electrode array with two different thiolated DNA capture sequences, surface passivation and regeneration of selected microarray electrodes, all by adjusting the potential intensities of the same potential pulse-assisted method. We achieve reproducible and controlled DNA immobilization together with minimization of false signals originating from unspecific adsorption or undesired co-immobilization. This methodology is not limited to DNA chips and it is potentially suitable for a wide range of applications employing Au−S chemistry. It can be employed in laboratory conditions for localizing different reactive chemistries onto predefined electrodes of an array without the need for complex and expensive apparatus and special conditions. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/elan.201900233
  • Direct Growth of Highly Strained Pt Islands on Branched Ni Nanoparticles for Improved Hydrogen Evolution Reaction Activity
    Alinezhad, A. and Gloag, L. and Benedetti, T.M. and Cheong, S. and Webster, R.F. and Roelsgaard, M. and Iversen, B.B. and Schuhmann, W. and Gooding, J.J. and Tilley, R.D.
    Journal of the American Chemical Society 141 (2019)
    The direct growth of Pt islands on lattice mismatched Ni nanoparticles is a major synthetic challenge and a promising strategy to create highly strained Pt atoms for electrocatalysis. By using very mild reaction conditions, Pt islands with tunable strain were formed directly on Ni branched particles. The highly strained 1.9 nm Pt-island on branched Ni nanoparticles exhibited high specific activity and the highest mass activity for hydrogen evolution (HER) in a pH 13 electrolyte. These results show the ability to synthetically tune the size of the Pt islands to control the strain to give higher HER activity. Copyright © 2019 American Chemical Society.
    view abstract10.1021/jacs.9b07659
  • Enhanced sensitivity of scanning bipolar electrochemical microscopy for O2 detection
    Santos, C.S. and Conzuelo, F. and Eßmann, V. and Bertotti, M. and Schuhmann, W.
    Analytica Chimica Acta 1087 (2019)
    The Scanning Bipolar Electrochemical Microscope (SBECM) allows precise positioning of an electrochemical micro-probe serving as bipolar electrode that can be wirelessly interrogated by coupling the electrochemical detection reaction with an electrochemiluminescent reporting process. As a result, the spatially heterogeneous concentrations of an analyte of interest can be converted in real time into a map of sample reactivity. However, this can only be achieved upon optimization of the analytical performance ensuring adequate sensitivity. Here, we present the evaluation and optimized operation of the SBECM for the detection of small changes in local O2 concentrations. Parameters for achieving an improved sensitivity as well as possibilities for improving the signal-to-noise ratio in the optical signal readout are evaluated. The capability of the SBECM for O2 detection is shown at controlled conditions by recording the topography of a patterned sample and monitoring O2 evolution from a photoelectrocatalyst material. © 2019 Elsevier B.V.
    view abstract10.1016/j.aca.2019.08.049
  • Enhancing Electrocatalytic Activity through Liquid-Phase Exfoliation of NiFe Layered Double Hydroxide Intercalated with Metal Phthalocyanines in the Presence of Graphene
    Morales, D.M. and Barwe, S. and Vasile, E. and Andronescu, C. and Schuhmann, W.
    ChemPhysChem 20 (2019)
    Earth-abundant transition-metal-based catalysts are attractive for alkaline water electrolysis. However, their catalytic properties are often limited by their poor electrical conductivity. Here, we present a strategy for enhancing the electrical conductivity of NiFe layered double hydroxide (LDH) in order to further improve its properties as an electrocatalyst for the oxygen evolution reaction (OER) in alkaline media. We show that NiFe LDH containing metal tetrasulfonate phthalocyanine in the interlayers between the NiFe oxide galleries can be coupled with graphene during liquid-phase exfoliation by taking advantage of their π-π stacking capabilities. A substantial enhancement in the electrocatalytic activity of NiFe LDH with respect to the OER was observed. Moreover, the activity and selectivity of the catalyst materials towards the oxygen reduction reaction were investigated, demonstrating that both the metal hydroxide layer and the interlayer species contribute to the electrocatalytic performance of the composite material. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/cphc.201900577
  • Enhancing the Selectivity between Oxygen and Chlorine towards Chlorine during the Anodic Chlorine Evolution Reaction on a Dimensionally Stable Anode
    Wintrich, D. and Öhl, D. and Barwe, S. and Ganassin, A. and Möller, S. and Tarnev, T. and Botz, A. and Ruff, A. and Clausmeyer, J. and Masa, J. and Schuhmann, W.
    ChemElectroChem 6 (2019)
    The selectivity of the chlorine evolution reaction over the oxygen evolution reaction during the electrolysis of aqueous NaCl is, despite being very high, still insufficient to prevent expensive separation of the formed Cl2 and O2 by means of liquefaction. We hypothesize that, by decreasing the local activity of H2O near the anode surface by substantially increasing the ionic strength of the electrolyte, the oxygen evolution reaction would be suppressed, leading concomitantly to a higher selectivity of Cl2 over O2 formation. Hence, the influence of the ionic strength on the competition between electrochemical evolution of O2 and Cl2 at dimensionally stable anodes (DSAs) was investigated. Addition of a high concentration of NaNO3, an inert electrolyte additive, increases the selectivity for chlorine at high current density, as determined by means of online electrochemical mass spectrometry and UV-vis spectroscopy. We propose conditions in which free water is suppressed, owing to under-coordination of the solvation shells of ions, as a general concept to modulate the selectivity of competing electrochemical reactions. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/celc.201900784
  • Enhancing the water splitting performance of cryptomelane-type α-(K)MnO2
    Antoni, H. and Morales, D.M. and Bitzer, J. and Fu, Q. and Chen, Y.-T. and Masa, J. and Kleist, W. and Schuhmann, W. and Muhler, M.
    Journal of Catalysis 374 (2019)
    Sustainable energy storage systems require the development of non-precious metal catalysts for water splitting. Cryptomelane-type α-(K)MnO2 is one of the few oxides of manganese with promising electrocatalytic activity for the oxygen evolution reaction (OER). We report a strategy to boost the performance of cryptomelane-type α-(K)MnO2 in OER electrocatalysis in alkaline electrolyte by thermal treatments in specific gas atmospheres. The thermal treatment of α-(K)MnO2 at 300 °C in He, H2O/He and air can lower the potential required to reach a current density of 10 mA cm−2 for the OER by up to 60 mV. We discuss the structural changes on the atomic level, by combining X-ray diffraction, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, transmission electron microscopy and electrochemical impedance spectroscopy. The simultaneous presence of three structural features for MnOx-based water oxidation catalysts is considered essential: (i) Mn3+ sites, (ii) structural defects, and (iii) mono- and di-µ-oxo bridges, which can be tailored on the surface and in the bulk. Notably, extended X-ray absorption fine structure measurements suggest that more Mn3+ sites are detected in the corner-sharing positions, which are suggested to be the favorable sites for the electrocatalytic water oxidation. © 2019
    view abstract10.1016/j.jcat.2019.05.010
  • Erratum: Mechanism of Protection of Catalysts Supported in Redox Hydrogel Films (Journal of the American Chemical Society (2015) 137 (5494-5505) DOI: 10.1021/jacs.5b01194)
    Fourmond, V. and Stapf, S. and Li, H. and Buesen, D. and Birrell, J. and Rüdiger, O. and Lubitz, W. and Schuhmann, W. and Plumeré, N. and Léger, C.
    Journal of the American Chemical Society 141 (2019)
    We have noted the following typographical errors and mistakes in our 2015 Article. We apologize for the confusion they may have caused. The conclusions of the paper are not impacted. Page 5496. The value of dO in Table 1 is incorrect. It should be equal to 5 × 103 (as indicated in the main text, on page 5498, right column, line 31), not 4 × 103. The correct value was used in the simulations. (Formula Presented). © 2019 American Chemical Society.
    view abstract10.1021/jacs.9b11594
  • Extended Operational Lifetime of a Photosystem-Based Bioelectrode
    Zhao, F. and Ruff, A. and Rögner, M. and Schuhmann, W. and Conzuelo, F.
    Journal of the American Chemical Society 141 (2019)
    The development of bioelectrochemical assemblies for sustainable energy transformation constitutes an increasingly important field of research. Significant progress has been made in the development of semiartificial devices for conversion of light into electrical energy by integration of photosynthetic biomolecules on electrodes. However, sufficient long-term stability of such biophotoelectrodes has been compromised by reactive species generated under aerobic operation. Therefore, meeting the requirements of practical applications still remains unsolved. We present the operation of a photosystem I-based photocathode using an electron acceptor that enables photocurrent generation under anaerobic conditions as the basis for a biodevice with substantially improved stability. A continuous operation lifetime considerably superior to previous reports and at higher light intensities is paving the way toward the potential application of semiartificial energy conversion devices. © 2019 American Chemical Society.
    view abstract10.1021/jacs.8b13869
  • Fe/Co/Ni mixed oxide nanoparticles supported on oxidized multi-walled carbon nanotubes as electrocatalysts for the oxygen reduction and the oxygen evolution reactions in alkaline media
    Kazakova, M.A. and Morales, D.M. and Andronescu, C. and Elumeeva, K. and Selyutin, A.G. and Ishchenko, A.V. and Golubtsov, G.V. and Dieckhöfer, S. and Schuhmann, W. and Masa, J.
    Catalysis Today (2019)
    Fabrication of efficient and cost-effective bifunctional oxygen electrocatalysts for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) remains a challenge for the development of rechargeable metal-air batteries and unitized regenerative fuel cells technologies. Herein, we report high-performance bifunctional ORR/OER electrocatalysts consisting of mixed transition metal (Fe, Co, Ni) oxide nanoparticles supported on oxidized multi-walled carbon nanotubes (MWCNT). Investigation of the ORR and OER activity of samples with different metal compositions showed that trimetallic/MWCNT composites having Fe:Ni:Co = x:x:(1-2x) ratios, with 0.25 ≤ x ≤ 0.4, exhibit highest bifunctional activity in terms of the reversible ORR/OER overvoltage at a given current density. Moreover, the trimetallic catalysts exhibited improved selectivity with respect to the reduction of O 2 to OH − compared to the bimetallic Fe-Ni, Fe-Co and Co-Ni catalysts, thus revealing synergistic interactions among the metal oxide components. Correlation of the electrocatalytic activity with the structure of the composites is discussed for the most representative cases. © 2019 Elsevier B.V.
    view abstract10.1016/j.cattod.2019.02.047
  • High spatial resolution electrochemical biosensing using reflected light microscopy
    Munteanu, R.-E. and Ye, R. and Polonschii, C. and Ruff, A. and Gheorghiu, M. and Gheorghiu, E. and Boukherroub, R. and Schuhmann, W. and Melinte, S. and Gáspár, S.
    Scientific Reports 9 (2019)
    If the analyte does not only change the electrochemical but also the optical properties of the electrode/solution interface, the spatial resolution of an electrochemical sensor can be substantially enhanced by combining the electrochemical sensor with optical microscopy. In order to demonstrate this, electrochemical biosensors for the detection of hydrogen peroxide and glucose were developed by drop casting enzyme and redox polymer mixtures onto planar, optically transparent electrodes. These biosensors generate current signals proportional to the analyte concentration via a reaction sequence which ultimately changes the oxidation state of the redox polymer. Images of the interface of these biosensors were acquired using bright field reflected light microscopy (BFRLM). Analysis showed that the intensity of these images is higher when the redox polymer is oxidized than when it is reduced. It also revealed that the time needed for the redox polymer to change oxidation state can be assayed optically and is dependent on the concentration of the analyte. By combining the biosensor for hydrogen peroxide detection with BFRLM, it was possible to determine hydrogen peroxide in concentrations as low as 12.5 µM with a spatial resolution of 12 µm × 12 µm, without the need for the fabrication of microelectrodes of these dimensions. © 2019, The Author(s).
    view abstract10.1038/s41598-019-50949-9
  • Influence of the Nature of Boron-Doped Diamond Anodes on the Dehydrogenative Phenol-Phenol Cross-Coupling
    Gleede, B. and Yamamoto, T. and Nakahara, K. and Botz, A. and Graßl, T. and Neuber, R. and Matthée, T. and Einaga, Y. and Schuhmann, W. and Waldvogel, S.R.
    ChemElectroChem 6 (2019)
    Boron-doped diamond (BDD) represents a powerful and innovative electrode material. In particular, in combination with fluorinated solvents such as 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), the system exhibits the largest known electrochemical window of approximately 5 V in protic media. Furthermore, the anodic treatment allows the direct formation of oxyl radicals, which are known to exhibit specific reactivity. The electrochemical dehydrogenative phenol-phenol cross-coupling is a versatile and useful transformation to non-symmetric biphenols. This electro-organic conversion can be divided into two regimes: initial oxidation at the anode and the electrolyte-controlled follow-up reaction. This work intends to provide an answer about the influence of BDD electrodes on oxidation reactions in electrosynthesis. Depending on the electro-organic transformation, the support material of BDD, its boron content, and its fabrication method have a significant influence on the electrosynthetic efficiency. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/celc.201900225
  • Introducing Pseudocapacitive Bioelectrodes into a Biofuel Cell/Biosupercapacitor Hybrid Device for Optimized Open Circuit Voltage
    Alsaoub, S. and Conzuelo, F. and Gounel, S. and Mano, N. and Schuhmann, W. and Ruff, A.
    ChemElectroChem 6 (2019)
    We report the fabrication of a polymer/enzyme-based biosupercapacitor (BSC)/biofuel cell (BFC) hybrid device with an optimized cell voltage that can be switched on demand from energy conversion to energy storage mode. The redox polymer matrices used for the immobilization of the biocatalyst at the bioanode and biocathode act simultaneously as electron relays between the integrated redox enzymes and the electrode surface (BFC) and as pseudocapacitive charge storing elements (BSC). Moreover, owing to the self-charging effect based on the continuously proceeding enzymatic reaction, a Nernstian shift in the pseudocapacitive elements, that is, in the redox polymers, at the individual bioelectrodes leads to a maximized open circuit voltage of the device in both operating modes. Comparison with a conventional fuel cell design, that is, using redox mediators with redox potentials that are close to the potentials of the used redox proteins, indicates that the novel hybrid device shows a similar voltage output. Moreover, our results demonstrate that the conventional design criteria commonly used for the development of redox polymers for the use in biofuel cells have to be extended by considering the effect of a Nernstian shift towards the potentials of the used biocatalysts in those pseudocapacitive elements. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/celc.201900256
  • Microscopic Determination of Carrier Density and Mobility in Working Organic Electrochemical Transistors
    Mariani, F. and Conzuelo, F. and Cramer, T. and Gualandi, I. and Possanzini, L. and Tessarolo, M. and Fraboni, B. and Schuhmann, W. and Scavetta, E.
    Small 15 (2019)
    A comprehensive understanding of electrochemical and physical phenomena originating the response of electrolyte-gated transistors is crucial for improved handling and design of these devices. However, the lack of suitable tools for direct investigation of microscale effects has hindered the possibility to bridge the gap between experiments and theoretical models. In this contribution, a scanning probe setup is used to explore the operation mechanisms of organic electrochemical transistors by probing the local electrochemical potential of the organic film composing the device channel. Moreover, an interpretative model is developed in order to highlight the meaning of electrochemical doping and to show how the experimental data can give direct access to fundamental device parameters, such as local charge carrier concentration and mobility. This approach is versatile and provides insight into the organic semiconductor/electrolyte interface and useful information for materials characterization, device scaling, and sensing optimization. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/smll.201902534
  • MOFs for Electrocatalysis: From Serendipity to Design Strategies
    Aiyappa, H.B. and Masa, J. and Andronescu, C. and Muhler, M. and Fischer, R.A. and Schuhmann, W.
    Small Methods 3 (2019)
    The rapid upsurge of metal–organic frameworks (MOFs) as well as MOF-derived materials has stimulated profound interest to capitalize on their many potential untapped benefits in electrocatalysis for energy applications. The possibility of tuning the metal–ligand junctions of the MOF architecture opens new avenues to design robust, extended heterostructures for addressing the present-day energy challenges. Interestingly, despite having detailed crystallographic information, it is often difficult to envisage the interplay of charge transport (electrons and ions), mass transport (pore system) together with the specific effects of the molecularly defined reaction center of MOFs for a given electrocatalytic reaction. Here, guidelines are offered for judiciously engineering the electronic structure of MOFs to deliver targeted electrocatalytic function. Some of the pivotal works on MOF-based materials for electrocatalysis are discussed, which can be correlated to the biological models in terms of their structural resemblance and an instructive insight is provided about the “new chemistry” that can be explored based on the lessons learned from nature in combination with the theoretical understanding of the energetics of the reactions. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/smtd.201800415
  • Ni-Metalloid (B, Si, P, As, and Te) Alloys as Water Oxidation Electrocatalysts
    Masa, J. and Piontek, S. and Wilde, P. and Antoni, H. and Eckhard, T. and Chen, Y.-T. and Muhler, M. and Apfel, U.-P. and Schuhmann, W.
    Advanced Energy Materials 9 (2019)
    Breakthroughs toward effective water-splitting electrocatalysts for mass hydrogen production will necessitate material design strategies based on unexplored material chemistries. Herein, Ni-metalloid (B, Si, P, As, Te) alloys are reported as an emergent class of highly promising electrocatalysts for the oxygen evolution reaction (OER) and insight is offered into the origin of activity enhancement on the premise of the surface electronic structure, the OER activation energy, influence of the guest metalloid elements on the lattice structure of the host metal (Ni), and surface-oxidized metalloid oxoanions. The metalloids modify the lattice structure of Ni, causing changes in the nearest Ni–Ni interatomic distance (dNi–Ni). The activation energy Ea scales with dNi–Ni indicating an apparent dependence of the OER activity on lattice properties. During the OER, surface Ni atoms are oxidized to nickel oxyhydroxide, which is the active state of the catalyst, meanwhile, the surface metalloids are oxidized to the corresponding oxoanions that affect the interfacial electrode/electrolyte properties and hence the adsorption/desorption interaction energies of the reacting species. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/aenm.201900796
  • Nitrogen-Doped Metal-Free Carbon Materials Derived from Cellulose as Electrocatalysts for the Oxygen Reduction Reaction
    Wütscher, A. and Eckhard, T. and Hiltrop, D. and Lotz, K. and Schuhmann, W. and Andronescu, C. and Muhler, M.
    ChemElectroChem 6 (2019)
    Development of metal-free carbon-based electrocatalysts for reducing oxygen to water (ORR), preferentially following a 4 electron transfer pathway, is of high importance. We present a two-step synthesis of N-doped carbon-based ORR electrocatalysts by using an efficient thermal treatment of hydrothermally carbonized cellulose in ammonia combining devolatilization, reduction and nitrogen doping. The influence of the synthesis temperature as well as of the ammonia concentration used during the synthesis on the electrocatalytic ORR activity was analyzed using bulk- and surface-sensitive techniques. Correlation of electrocatalytic activity with structural features of the catalysts provided deeper mechanistic understanding and enabled us to optimize the synthesis conditions. The nitrogen-doped metal-free catalyst originating from the treatment in 100 % NH3 at 800 °C achieved a current density of −1 mA cm−2 at 0.83 V vs. RHE positioning it among the most active noble-metal free and biomass-based ORR catalysts reported so far. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/celc.201801217
  • On the Theory of Electrolytic Dissociation, the Greenhouse Effect, and Activation Energy in (Electro)Catalysis: A Tribute to Svante Augustus Arrhenius
    Masa, J. and Barwe, S. and Andronescu, C. and Schuhmann, W.
    Chemistry - A European Journal 25 (2019)
    Svante Augustus Arrhenius (1859, Vik - 1927, Stockholm) received the Nobel Prize for Chemistry in 1903 “in recognition of the extraordinary services he rendered to the advancement of chemistry by his electrolytic theory of dissociation”. Arrhenius was a physicist, and he received his PhD from the University of Uppsala, where he later became a professor for phyiscal chemistry, the first in the country for this subject. He was offered several positions as professor abroad, but decided to remain in Sweden and to build a Nobel Institute for physical chemistry using the Nobel funds. He remained director of the Institute until his death. There are powerful lessons to take from Svante August Arrhenius’ journey leading to a Nobel laureate as there are from his tremendous contributions to chemistry and science in general, including climate science, immunochemistry and cosmology. The theory of electrolytic dissociation for which Arrhenius received the 1903 Nobel Prize in Chemistry has had a profound impact on our understanding of the chemistry of solutions, chemical reactivity, mechanisms underlying chemical transformations as well as physiological processes. As a tribute to Arrhenius, we present a brief historical perspective and present status of the theory of electrolytic dissociation, its relevance and role to the development of electrochemistry, as well as some perspectives on the possible role of the theory to future advancements in electroanalysis, electrocatalysis and electrochemical energy storage. The review briefly highlights Arrhenius’ contribution to climate science owing to his studies on the potential effects of increased anthropogenic CO2 emissions on the global climate. These studies were far ahead of their time and revealed a daunting global dilemma, global warming, that we are faced with today. Efforts to abate or reverse CO2 accumulation constitute one of the most pressing scientific problems of our time, “man's urgent strive to save self from the adverse effects of his self-orchestrated change on the climate”. Finally, we review the application of the Arrhenius equation that correlates reaction rate constants (k) and temperature (T); (Formula presented.), in determining reaction barriers in catalysis with a particular focus on recent modifications of the equation to account for reactions exhibiting non-linear Arrhenius behavior with concave curvature due to prevalence of quantum mechanical tunneling, as well as infrequent convexity of Arrhenius plots due to decrease of the microcanonical rate coefficient with energy as observed for some enzyme catalyzed reactions. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/chem.201805264
  • Operando Thin-Layer ATR-FTIR Spectroelectrochemical Radial Flow Cell with Tilt Correction and Borehole Electrode
    Cychy, S. and Hiltrop, D. and Andronescu, C. and Muhler, M. and Schuhmann, W.
    Analytical Chemistry (2019)
    A novel spectroelectrochemical ATR-FTIR thin-film cell was designed and applied to elucidate the intermediates during electrocatalytic alcohol oxidation. In the novel cell design, the working electrode is positioned coplanar above the internal reflection element (IRE) to ensure uniform electrolyte film thickness at reaction conditions. The depletion of the reactant (i.e., ethanol or ethylene glycol in the case of electrocatalytic alcohol oxidation) is decreased by a specifically designed flow-through glassy carbon borehole electrode embedded in PEEK. The electrolyte can be pumped through the disk-shaped gap between the ring working electrode and the IRE into the borehole via an external peristaltic pump. To ensure a radially uniform electrolyte flow, the working electrode and the internal reflection element need to be aligned in parallel at a well-controlled distance, which was achieved by a three-microelectrode-assisted tilt correction. Tilt correction of this four-electrode ensemble and the IRE was performed by three step-motor-driven micrometer screws that allow adjustment of the electrode orientation. The effect of electrolyte pumping through the borehole electrode was analyzed by performing anodic ethanol oxidation using nickel boride as electrocatalyst. The applicability, reliability, and functionality of the cell was further assessed by oxidizing ethylene glycol and determining the reaction products as a function of the electrolyte flow rate. It is found to be essential to induce forced electrolyte convection into the thin electrolyte layer to achieve well-defined steady-state conditions, as mass transport by diffusion is by far insufficient, resulting in reactant depletion, product accumulation, and local pH changes. © 2019 American Chemical Society.
    view abstract10.1021/acs.analchem.9b02734
  • Optimizing the synthesis of Co/Co–Fe nanoparticles/N-doped carbon composite materials as bifunctional oxygen electrocatalysts
    Medina, D. and Barwe, S. and Masa, J. and Seisel, S. and Schuhmann, W. and Andronescu, C.
    Electrochimica Acta 318 (2019)
    A future widespread application of electrochemical energy conversion and storage technologies strongly depends on the substitution of precious metal-based electrocatalysts for the high-overpotential oxygen reduction and oxygen evolution reactions. We report a novel Co/Co–Fe nanoparticles/N-doped carbon composite electrocatalyst (Co/CoxFey/NC) obtained by pyrolysis of CoFe layered double hydroxide (CoFe LDH) embedded in a film of a bisphenol A and tetraethylenepentamine-based polybenzoxazine poly(BA-tepa). During pyrolysis poly(BA-tepa) forms a highly conductive nitrogen-doped carbon matrix encapsulating Co/Co–Fe nanoparticles, thereby circumventing the need of any additional binder material and conductive additives. Optimization with respect to pyrolysis temperature, the CoFe LDH/BA-tepa ratio, as well as of the gas atmosphere used during the thermal treatment was performed. The optimized Co/CoxFey/NC composite material catalyst exhibits remarkable bifunctional activity towards oxygen reduction (ORR) and oxygen evolution (OER) reactions in 0.1 M KOH represented by a potential difference of only 0.77 V between the potentials at which current densities of −1 mA cm−2 for the ORR and 10 mA cm−2 for the OER were recorded. Moreover, the Co/CoxFey/NC composite material pyrolyzed in ammonia atmosphere exhibits promising stability during both the ORR and the OER. © 2019 Elsevier Ltd
    view abstract10.1016/j.electacta.2019.06.048
  • Oxygen Evolution Electrocatalysis of a Single MOF-Derived Composite Nanoparticle on the Tip of a Nanoelectrode
    Aiyappa, H.B. and Wilde, P. and Quast, T. and Masa, J. and Andronescu, C. and Chen, Y.-T. and Muhler, M. and Fischer, R.A. and Schuhmann, W.
    Angewandte Chemie - International Edition 58 (2019)
    Determination of the intrinsic electrocatalytic activity of nanomaterials by means of macroelectrode techniques is compromised by ensemble and film effects. Here, a unique “particle on a stick” approach is used to grow a single metal–organic framework (MOF; ZIF-67) nanoparticle on a nanoelectrode surface which is pyrolyzed to generate a cobalt/nitrogen-doped carbon (CoN/C) composite nanoparticle that exhibits very high catalytic activity towards the oxygen evolution reaction (OER) with a current density of up to 230 mA cm−2 at 1.77 V (vs. RHE), and a high turnover frequency (TOF) of 29.7 s−1 at 540 mV overpotential. Identical location transmission electron microscopy (IL-TEM) analysis substantiates the “self-sacrificial” template nature of the MOF, while post-electrocatalysis studies reveal agglomeration of Co centers within the CoN/C composite during the OER. “Single-entity” electrochemical analysis allows for deriving the intrinsic electrocatalytic activity and furnishes insight into the transient behavior of the electrocatalyst under reaction conditions. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/anie.201903283
  • Plasmon Enhanced Two-Photon Probing with Gold and Silver Nanovoid Structures
    Madzharova, F. and Öhl, D. and Junqueira, J. and Schuhmann, W. and Kneipp, J.
    Advanced Optical Materials 7 (2019)
    Nonlinear optical signals benefit greatly from the enhanced local optical fields in the vicinity of plasmonic nanostructures. Gold and silver nanovoid arrays of varying size and thickness, fabricated by electrochemical deposition are shown here to act as stable plasmonic nanostructures and to enhance the weak, incoherent two-photon excited process of surface-enhanced hyper Raman scattering (SEHRS) with high microscopic homogeneity and reproducibility that typical SEHRS experiments have not been addressing so far. Silver nanovoids yield stronger enhancement than gold voids, but gold nanovoid arrays show improved stability at high laser excitation intensities. Combined screening experiments using SEHRS and second-harmonic generation (SHG) reveal a dependence of the enhancement of both signals on void structural parameters and similar optimum geometries for both two-photon processes. The results confirm the suggested important role for the enhancement of the near-infrared excitation field in SEHRS and suggest SHG as a fast screening tool to identify nanostructures that can support high SEHRS enhancement. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/adom.201900650
  • Polymer-Bound DuBois-Type Molecular H 2 Oxidation Ni Catalysts Are Protected by Redox Polymer Matrices
    Ruff, A. and Janke, S. and Szczesny, J. and Alsaoub, S. and Ruff, I. and Lubitz, W. and Schuhmann, W.
    ACS Applied Energy Materials 2 (2019)
    The immobilization, protection, and electrical wiring of sensitive catalysts by specifically designed supporting matrixes are of particular importance for technological relevant applications. Here, we describe the protection of a DuBois-type H 2 oxidation catalyst, which was covalently bound to an inert polymer matrix, against molecular O 2 by forming blends together with an O 2 -reducing redox polymer matrix. This matrix simultaneously acts as an electron relay for shuttling electrons between the catalyst and the electrode. © 2019 American Chemical Society.
    view abstract10.1021/acsaem.9b00269
  • Polymer-Bound DuBois-Type Molecular H2 Oxidation Ni Catalysts Are Protected by Redox Polymer Matrices
    Ruff, A. and Janke, S. and Szczesny, J. and Alsaoub, S. and Ruff, I. and Lubitz, W. and Schuhmann, W.
    ACS Applied Energy Materials 2 (2019)
    The immobilization, protection, and electrical wiring of sensitive catalysts by specifically designed supporting matrixes are of particular importance for technological relevant applications. Here, we describe the protection of a DuBois-type H2 oxidation catalyst, which was covalently bound to an inert polymer matrix, against molecular O2 by forming blends together with an O2-reducing redox polymer matrix. This matrix simultaneously acts as an electron relay for shuttling electrons between the catalyst and the electrode. © 2019 American Chemical Society.
    view abstract10.1021/acsaem.9b00269
  • Role of Boron and Phosphorus in Enhanced Electrocatalytic Oxygen Evolution by Nickel Borides and Nickel Phosphides
    Masa, J. and Andronescu, C. and Antoni, H. and Sinev, I. and Seisel, S. and Elumeeva, K. and Barwe, S. and Marti-Sanchez, S. and Arbiol, J. and Roldan Cuenya, B. and Muhler, M. and Schuhmann, W.
    ChemElectroChem 6 (2019)
    The modification of nickel with boron or phosphorus leads to significant enhancement of its electrocatalytic activity for the oxygen evolution reaction (OER). However, the precise role of the guest elements, B and P, in enhancing the OER of the host element (Ni) remains unclear. Herein, we present insight into the role of B and P in enhancing electrocatalysis of oxygen evolution by nickel borides and nickel phosphides. The apparent activation energy, Ea*, of electrocatalytic oxygen evolution on Ni2P was 78.4 kJ/mol, on Ni2B 65.4 kJ/mol, and on Ni nanoparticles 94.0 kJ/mol, thus revealing that both B and P affect the intrinsic activity of nickel. XPS data revealed shifts of −0.30 and 0.40 eV in the binding energy of the Ni 2p3/2 peak of Ni2B and Ni2P, respectively, with respect to that of pure Ni at 852.60 eV, thus indicating that B and P induce opposite electronic effects on the surface electronic structure of Ni. The origin of enhanced activity for oxygen evolution cannot, therefore, be attributed to such electronic modification or ligand effect. Severe changes induced on the nickel lattice, specifically, the Ni-Ni atomic order and interatomic distances (strain effect), by the presence of the guest atoms seem to be the dominant factors responsible for enhanced activity of oxygen evolution in nickel borides and nickel phosphides. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/celc.201800669
  • Scanning Electrochemical Cell Microscopy Investigation of Single ZIF-Derived Nanocomposite Particles as Electrocatalysts for Oxygen Evolution in Alkaline Media
    Tarnev, T. and Aiyappa, H.B. and Botz, A. and Erichsen, T. and Ernst, A. and Andronescu, C. and Schuhmann, W.
    Angewandte Chemie - International Edition 58 (2019)
    “Single entity” measurements are central for an improved understanding of the function of nanoparticle-based electrocatalysts without interference arising from mass transfer limitations and local changes of educt concentration or the pH value. We report a scanning electrochemical cell microscopy (SECCM) investigation of zeolitic imidazolate framework (ZIF-67)-derived Co−N-doped C composite particles with respect to the oxygen evolution reaction (OER). Surmounting the surface wetting issues as well as the potential drift through the use of a non-interfering Os complex as free-diffusing internal redox potential standard, SECCM could be successfully applied in alkaline media. SECCM mapping reveals activity differences relative to the number of particles in the wetted area of the droplet landing zone. The turnover frequency (TOF) is 0.25 to 1.5 s−1 at potentials between 1.7 and 1.8 V vs. RHE, respectively, based on the number of Co atoms in each particle. Consistent values at locations with varying number of particles demonstrates OER performance devoid of macroscopic film effects. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/anie.201908021
  • Simultaneous Anodic Adsorptive Stripping Voltammetric Determination of Luteolin and 3-Hydroxyflavone in Biological Fluids Using Renewable Pencil Graphite Electrodes
    Temerk, Y. and Ibrahim, H. and Schuhmann, W.
    Electroanalysis 31 (2019)
    Simultaneous anodic adsorptive stripping voltammetry was applied for selective and sensitive electrochemical determination of the flavones luteolin (LU) and the basic flavone core 3-hydroxyflavone (3HF) using a renewable pencil graphite electrode (PGE). The increased separation of the anodic peak potential of LU and 3HF on a PGE surface together with the increased sensitivity renders their simultaneous determination feasible by square wave anodic adsorptive stripping voltammetry (SWAASV). The electrochemical parameters such as surface concentration (Γ), electron transfer coefficient (α), and the standard rate constant (ks) of both LU and 3HF at a PGE were calculated. For simultaneous detection of both compounds by synchronous change of the concentration of LU and 3HF, the detection limits were 1.34 nM and 5.15 nM, respectively. The proposed procedure was successfully applied for the simultaneous detection of LU and 3HF in human serum and urine samples with satisfactory results. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/elan.201900066
  • Single entity electrochemistry for the elucidation of lithiation kinetics of TiO 2 particles in non-aqueous batteries
    Löffler, T. and Clausmeyer, J. and Wilde, P. and Tschulik, K. and Schuhmann, W. and Ventosa, E.
    Nano Energy 57 (2019)
    In battery research, the development of analytical techniques is of key importance for determining intrinsic properties of active materials ultimately dictating the battery performance. We report the application of nano-impact electrochemistry to gain insight into the intrinsic properties of commercial battery materials i.e. TiO 2 particles in non-aqueous media. Potentiostatic lithiation measurements do not only provide qualitative information about the rate-limiting step in the lithiation process, but also demonstrate that nano-impact electrochemistry is a suitable technique in non-aqueous media in complete absence of oxygen and water. Our results reveal that the intrinsic lithiation rate of individual TiO 2 particles is not – as generally assumed – determined by interfacial ion transfer kinetics, mobility of ion and/or electrons in the bulk of the particle, but by the solid-solid electron transfer. These findings have important implications for future studies of fundamental properties of battery materials considering that charge transfer in battery electrodes does not always obey Butler-Volmer kinetics. © 2018 Elsevier Ltd
    view abstract10.1016/j.nanoen.2018.12.064
  • Single entity electrochemistry for the elucidation of lithiation kinetics of TiO2 particles in non-aqueous batteries
    Löffler, T. and Clausmeyer, J. and Wilde, P. and Tschulik, K. and Schuhmann, W. and Ventosa, E.
    Nano Energy 57 (2019)
    In battery research, the development of analytical techniques is of key importance for determining intrinsic properties of active materials ultimately dictating the battery performance. We report the application of nano-impact electrochemistry to gain insight into the intrinsic properties of commercial battery materials i.e. TiO2 particles in non-aqueous media. Potentiostatic lithiation measurements do not only provide qualitative information about the rate-limiting step in the lithiation process, but also demonstrate that nano-impact electrochemistry is a suitable technique in non-aqueous media in complete absence of oxygen and water. Our results reveal that the intrinsic lithiation rate of individual TiO2 particles is not – as generally assumed – determined by interfacial ion transfer kinetics, mobility of ion and/or electrons in the bulk of the particle, but by the solid-solid electron transfer. These findings have important implications for future studies of fundamental properties of battery materials considering that charge transfer in battery electrodes does not always obey Butler-Volmer kinetics. © 2018 Elsevier Ltd
    view abstract10.1016/j.nanoen.2018.12.064
  • Special Collection on Bioelectrochemistry
    Matsue, T. and Pingarrón, J. and Schuhmann, W.
    ChemElectroChem 6 (2019)
    view abstract10.1002/celc.201901722
  • Spray-Flame-Synthesized LaCo1−xFexO3 Perovskite Nanoparticles as Electrocatalysts for Water and Ethanol Oxidation
    Alkan, B. and Cychy, S. and Varhade, S. and Muhler, M. and Schulz, C. and Schuhmann, W. and Wiggers, H. and Andronescu, C.
    ChemElectroChem 6 (2019)
    Coupling electrochemical generation of hydrogen with the concomitant formation of an industrially valuable product at the anode instead of oxygen can balance the high costs usually associated with water electrolysis. We report the synthesis of a variety of nanoparticulate LaCo1−xFexO3 perovskite materials through a specifically optimized spray-flame nanoparticle synthesis method, using different ratios of La, Co, and Fe precursor compounds. Structural characterization of the resulting materials by XRD, TEM, FTIR, and XPS analysis revealed the formation of mainly perovskite-type materials. The electrocatalytic performance of the formed perovskite-type materials towards the oxygen evolution reaction and the ethanol oxidation reaction was investigated by using rotating disk electrode voltammetry. An increased Fe content in the precursor mixture leads to a decrease in the electrocatalytic activity of the nanoparticles. The selectivity towards alcohol oxidation in alkaline media was assessed by using the ethanol oxidation reaction as a model reaction. Operando electrochemistry/ATR-IR spectroscopy results reveal that acetate and acetaldehyde are the final products, depending on the catalyst composition as well as on the applied potential. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/celc.201900168
  • The Key Role of Water Activity for the Operating Behavior and Dynamics of Oxygen Depolarized Cathodes
    Röhe, M. and Botz, A. and Franzen, D. and Kubannek, F. and Ellendorff, B. and Öhl, D. and Schuhmann, W. and Turek, T. and Krewer, U.
    ChemElectroChem 6 (2019)
    Advanced chlor-alkali electrolysis with oxygen depolarized cathodes (ODC) requires 30 % less electrical energy than conventional hydrogen-evolution-based technology. Herein, we confirm that the activities of hydroxide and water govern the ODC performance and its dynamics. Experimental characterization of ODC under varying mass transfer conditions on the liquid side reveals large differences in the polarization curves as well as in potential step responses of the electrodes. Under convective transport in the liquid electrolyte, the ODC is not limited by mass transfer in its current density at j&gt;3.9 kA m−2, whereas transport limitations are already reached at j≈1.3 kA m−2 with a stagnant electrolyte. Since gas phase conditions do not differ significantly between the measurements, these results are in contrast the common assumption that oxygen supply determines ODC performance. A dynamic model reveals the strong influence of the electrolyte mass transfer conditions on oxygen availability and thus performance. Dynamic responses of the current density to step-wise potential changes are dominated by the mass transport of water and hydroxide ions, which is by orders of magnitude faster with convective electrolyte flow. Without convective liquid electrolyte transport, a high accumulation of hydroxide ions significantly lowers the oxygen solubility. Thus, a fast mass transport of water and hydroxide is essential for high ODC performance and needs to be ensured for technical applications. The predicted accumulation of ions is furthermore validated experimentally by means of scanning electrochemical microscopy. We also show how the outlined processes can explain the distinctively different potential step responses with and without electrolyte convection. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/celc.201901224
  • The Role of Non-Metallic and Metalloid Elements on the Electrocatalytic Activity of Cobalt and Nickel Catalysts for the Oxygen Evolution Reaction
    Masa, J. and Schuhmann, W.
    ChemCatChem 11 (2019)
    Compounds and alloys of cobalt and nickel with some nonmetals (N, P, S, Se) and metalloids (C, B, C, As and Te) have emerged as very promising noble metal-free pre-catalysts for the oxygen evolution reaction (OER) in alkaline electrolytes. However, the exact role played by the non-metals and metalloids in promoting the OER is not well understood. A holistic understanding of the origin of the OER activity enhancement in these compounds is vital for their exploitation as models to inspire knowledge-guided design of improved OER catalysts. In this review, we elucidate the factors that govern the activity and stability of OER catalysts derived from MX compounds (M=Co or Ni, and X=nonmetal or metalloid), including the impact of surface electronic structure, M : X stoichiometry, material composition, structure and crystallinity, as well as the role of oxoanions on the properties of the electrochemical double layer and interaction energies of the reaction intermediates. Finally, we outline a few perspectives and research directions towards a deeper understanding of the role of the nonmetal and metalloid elements and design of improved OER catalysts. ©2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/cctc.201901151
  • Time-resolved ATP measurements during vesicle respiration
    Lin, J. and Weixler, D. and Daboss, S. and Seibold, G.M. and Andronescu, C. and Schuhmann, W. and Kranz, C.
    Talanta 205 (2019)
    In vitro synthesis of ATP catalyzed by the ATP-synthase requires membrane vesicles, in which the ATP-synthase is present within the bilayer membrane. Inverted vesicle prepared from Gram negative cells (e.g., Escherichia coli or Pseudomonas putida) can be readily obtained and used for in vitro ATP-synthesis. Up to now, quantification of ATP synthesized by membrane vesicles has been mostly analyzed via bioluminescence-based assays. Alternatively, vesicle respiration and the associated ATP level can be determined using biosensors, which not only provide high selectivity, but allow ATP measurements without the sample being illuminated. Here, we present a microbiosensor for ATP in combination with scanning electrochemical microscopy (SECM) using an innovative two-compartment electrochemical cell for the determination of ATP levels at E.coli or P. putida inverted vesicles. For a protein concentration of 22 mg/ml, a total amount of 0.29 ± 0.03 μM/μl ATP per vesicle was determined in case of E.coli; in turn, P. putida derived vesicles yielded 0.48 ± 0.02 μM/μl ATP per vesicle at a total protein concentration of 25.2 mg/ml. Inhibition experiments with Venturicidin A clearly revealed that the respiratory chain enzyme complex responsible for ATP generation is effectively involved. © 2019 Elsevier B.V.
    view abstract10.1016/j.talanta.2019.06.083
  • Toward a Paradigm Shift in Electrocatalysis Using Complex Solid Solution Nanoparticles
    Löffler, T. and Savan, A. and Garzón-Manjón, A. and Meischein, M. and Scheu, C. and Ludwig, Al. and Schuhmann, W.
    ACS Energy Letters 4 (2019)
    Complex solid solution (CSS) nanoparticles were recently discovered as efficient electrocatalysts for a variety of reactions. As one of many advantages, they exhibit the potential to replace noble-metal catalysts with multinary combinations of transition metals because they offer formation of new unique and tailorable active sites of multiple elements located next to each other. This Perspective reports on the current state and on challenges of the (combinatorial) synthesis of multinary nanoparticles and advanced electron microscopy characterization techniques for revealing structure-activity correlations on an atomic scale. We discuss what distinguishes this material class from common catalysts to highlight their potential to act as electrocatalysts and rationalize their nontypical electrochemical behavior. We provide an overview about challenges in synthesis, characterization, and electrochemical evaluation and propose guidelines for future design of CSS catalysts to achieve further progress in this research field, which is still in its infancy. © 2019 American Chemical Society.
    view abstract10.1021/acsenergylett.9b00531
  • Tuning Light-Driven Water Splitting Efficiency of Mo-Doped BiVO4: Optimised Preparation and Impact of Oxygen Evolution Electrocatalysts
    Junqueira, J.R.C. and Bobrowski, T. and Krysiak, O.A. and Gutkowski, R. and Schuhmann, W.
    ChemCatChem 11 (2019)
    We present airbrush spray-coating as a reproducible method for the preparation of Mo-doped BiVO4 (Mo : BiVO4) as photoabsorber with different layer thicknesses and Mo content. Optimisation of layer thickness is aiming on diminishing limitations by the electronic conductivity within the photoabsorber, thus increasing the incident photon to current efficiency (IPCE) of the samples. Furthermore, the Mo to V ratio leading to the highest photocurrent density was determined, and the optimised Mo : BiVO4 samples were decorated with a variety of oxygen evolution reaction (OER) electrocatalysts such as cobalt phosphate and layered double hydroxides. A mass loading gradient of Ni−Fe LDH was sprayed on top of the Mo : BiVO4 photoanode for optimisation of the OER catalyst loading. The photocurrent density was enhanced by up to 5.8 times at 0.8 V vs. RHE in comparison with the pristine Mo : BiVO4 sample in absence of any OER electrocatalyst. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/cctc.201901646
  • A fully protected hydrogenase/polymer-based bioanode for high-performance hydrogen/glucose biofuel cells
    Ruff, A. and Szczesny, J. and Marković, N. and Conzuelo, F. and Zacarias, S. and Pereira, I.A.C. and Lubitz, W. and Schuhmann, W.
    Nature Communications 9 (2018)
    Hydrogenases with Ni- and/or Fe-based active sites are highly active hydrogen oxidation catalysts with activities similar to those of noble metal catalysts. However, the activity is connected to a sensitivity towards high-potential deactivation and oxygen damage. Here we report a fully protected polymer multilayer/hydrogenase-based bioanode in which the sensitive hydrogen oxidation catalyst is protected from high-potential deactivation and from oxygen damage by using a polymer multilayer architecture. The active catalyst is embedded in a low-potential polymer (protection from high-potential deactivation) and covered with a polymer-supported bienzymatic oxygen removal system. In contrast to previously reported polymer-based protection systems, the proposed strategy fully decouples the hydrogenase reaction form the protection process. Incorporation of the bioanode into a hydrogen/glucose biofuel cell provides a benchmark open circuit voltage of 1.15 V and power densities of up to 530 µW cm−2 at 0.85 V. © 2018, The Author(s).
    view abstract10.1038/s41467-018-06106-3
  • A gas breathing hydrogen/air biofuel cell comprising a redox polymer/hydrogenase-based bioanode
    Szczesny, J. and Marković, N. and Conzuelo, F. and Zacarias, S. and Pereira, I.A.C. and Lubitz, W. and Plumeré, N. and Schuhmann, W. and Ruff, A.
    Nature Communications 9 (2018)
    Hydrogen is one of the most promising alternatives for fossil fuels. However, the power output of hydrogen/oxygen fuel cells is often restricted by mass transport limitations of the substrate. Here, we present a dual-gas breathing H2/air biofuel cell that overcomes these limitations. The cell is equipped with a hydrogen-oxidizing redox polymer/hydrogenase gas-breathing bioanode and an oxygen-reducing bilirubin oxidase gas-breathing biocathode (operated in a direct electron transfer regime). The bioanode consists of a two layer system with a redox polymer-based adhesion layer and an active, redox polymer/hydrogenase top layer. The redox polymers protect the biocatalyst from high potentials and oxygen damage. The bioanodes show remarkable current densities of up to 8 mA cm-2. A maximum power density of 3.6 mW cm-2 at 0.7 V and an open circuit voltage of up to 1.13 V were achieved in biofuel cell tests, representing outstanding values for a device that is based on a redox polymer-based hydrogenase bioanode. © 2018, The Author(s).
    view abstract10.1038/s41467-018-07137-6
  • An Air-breathing Carbon Cloth-based Screen-printed Electrode for Applications in Enzymatic Biofuel Cells
    Marković, N. and Conzuelo, F. and Szczesny, J. and González García, M.B. and Hernández Santos, D. and Ruff, A. and Schuhmann, W.
    Electroanalysis (2018)
    We report a prototype air-breathing carbon cloth-based electrode that was fabricated starting from a commercially available screen-printed electrode equipped with a transparent ITO working electrode (DropSens, ref. ITO10). The fabrication of the air-breathing electrodes is straightforward, shows satisfactory reproducibility and a good electrochemical response as evaluated by means of [Fe(CN)6]3−/4− voltammetry. The gas-diffusion electrodes were successfully modified with the O2 reducing enzyme bilirubin oxidase from Myrothecium verrucaria in a direct electron transfer regime. The enzyme modified electrodes showed a remarkable high current density for O2 reduction in passive air-breathing mode of up to 5 mA cm−2. Moreover, the enzyme modified electrodes were applied as O2 reducing biocathodes in a glucose/air enzymatic biofuel cell in combination with a high current density glucose oxidase/redox polymer bioanode. The biofuel cell provides a high maximum power density of (0.34±0.02) mW cm−2 at 0.25 V. The straightforward design, low cost and the high reproducibility of these electrodes are considered as basis for standardized measurements under gas-breathing conditions and for high throughput screening of gas converting (bio-)catalysts. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/elan.201800462
  • An O2 Tolerant Polymer/Glucose Oxidase Based Bioanode as Basis for a Self-Powered Glucose Sensor
    Lopez, F. and Zerria, S. and Ruff, A. and Schuhmann, W.
    Electroanalysis (2018)
    The development of O2 tolerant glucose sensors based on the highly active and robust enzyme glucose oxidase is still a major challenge because of the competition between the natural electron acceptor O2 and free-diffusing or polymer-bound artificial electron acceptors. We report the fabrication of a glucose oxidase based bioanode that operates under ambient conditions. Combination of this bioanode with a bilirubin oxidase based biocathode enabled the fabrication of a glucose/O2 powered biofuel cell as integrated power source for a self-powered device. Glucose oxidase at the anode was electrically wired via a low-potential redox polymer, i.e. a Toluidine Blue-modified poly(methacrylate) based polymer, that ensures a high open-circuit voltage of the biofuel cell but also catalytically reduces O2 and hence requires a protection shield for measurements under ambient conditions. The sensing layer was deposited by means of potential pulse-assisted co-deposition of glucose oxidase within the redox polymer and was protected from O2 by a newly proposed lactate oxidase/catalase based O2 removal layer that was immobilized within a hydrophilic redox-silent polymer on top of the sensing layer. The protection layer was powered by lactate, a natural component in human blood. The biofuel cell exhibited an OCV of ca. 650mV and the power output was dependent on the glucose concentration without any interference from oxygen providing that lactate was available in the analyte solution. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/elan.201700785
  • Analysis of photosystem II electron transfer with natural PsbA-variants by redox polymer/protein biophotoelectrochemistry
    Hartmann, V. and Ruff, A. and Schuhmann, W. and Rögner, M. and Nowaczyk, M.M.
    Photosynthetica (2018)
    Redox polymer/protein biophotoelectrochemistry was used to analyse forward electron transfer of isolated PSII complexes with natural PsbA-variants. PsbA1- or PsbA3-PSII was embedded in a redox hydrogel that allows diffusion-free electron transfer to the electrode surface and thus measurement of an immediate photocurrent response. The initial photocurrent density of the electrode is up to ~2-fold higher with PsbA1-PSII under all tested light conditions, the most prominent under high-light [2,300 μmol(photon) m–2 s–1] illumination with 5 μA cm–2 for PsbA3-PSII and 9.5 μA cm–2 for PsbA1-PSII. This indicates more efficient electron transfer in low-light-adapted PsbA1-PSII. In contrast, the photocurrent decays faster in PsbA1-PSII under all tested light conditions, which suggests increased stability of high-light-adapted PsbA3-PSII. These results confirm and extend previous observations that PsbA3-PSII has increased P680+·/QA –· charge recombination and thus less efficient photon-to-charge conversion, whereas PsbA1-PSII is optimised for efficient electron transfer with limited stability. © 2018 The Institute of Experimental Botany
    view abstract10.1007/s11099-018-0775-y
  • Bifunctional Oxygen Reduction/Oxygen Evolution Activity of Mixed Fe/Co Oxide Nanoparticles with Variable Fe/Co Ratios Supported on Multiwalled Carbon Nanotubes
    Elumeeva, K. and Kazakova, M.A. and Morales, D.M. and Medina, D. and Selyutin, A. and Golubtsov, G. and Ivanov, Y. and Kuznetzov, V. and Chuvilin, A. and Antoni, H. and Muhler, M. and Schuhmann, W. and Masa, J.
    ChemSusChem 11 (2018)
    A facile strategy is reported for the synthesis of Fe/Co mixed metal oxide nanoparticles supported on, and embedded inside, high purity oxidized multiwalled carbon nanotubes (MWCNTs) of narrow diameter distribution as effective bifunctional catalysts able to reversibly drive the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) in alkaline solutions. Variation of the Fe/Co ratio resulted in a pronounced trend in the bifunctional ORR/OER activity. Controlled synthesis and in-depth characterization enabled the identification of an optimal Fe/Co composition, which afforded a low OER/OER reversible overvoltage of only 0.831 V, taking the OER at 10 mA cm−2 and the ORR at −1 mA cm−2. Importantly, the optimal catalyst with a Fe/Co ratio of 2:3 exhibited very promising long-term stability with no evident change in the potential for both the ORR and the OER after 400 charge/discharge (OER/ORR) cycles at 15 mA cm−2 in 6 m KOH. Moreover, detailed investigation of the structure, size, and phase composition of the mixed Fe/Co oxide nanoparticles, as well as their localization (inside of or on the surface of the MWCNTs) revealed insight of the possible contribution of the individual catalyst components and their synergistic interaction in the catalysis. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/cssc.201702381
  • Bioelectrocatalytic and electrochemical cascade for phosphate sensing with up to 6 electrons per analyte molecule
    Kopiec, G. and Starzec, K. and Kochana, J. and Kinnunen-Skidmore, T.P. and Schuhmann, W. and Campbell, W.H. and Ruff, A. and Plumeré, N.
    Biosensors and Bioelectronics 117 (2018)
    Despite the availability of numerous electroanalytical methods for phosphate quantification, practical implementation in point-of-use sensing remains virtually nonexistent because of interferences from sample matrices or from atmospheric O2. In this work, phosphate determination is achieved by the purine nucleoside phosphorylase (PNP) catalyzed reaction of inosine and phosphate to produce hypoxanthine which is subsequently oxidized by xanthine oxidase (XOx), first to xanthine and then to uric acid. Both PNP and XOx are integrated in a redox active Os-complex modified polymer, which not only acts as supporting matrix for the bienzymatic system but also shuttles electrons from the hypoxanthine oxidation reaction to the electrode. The bienzymatic cascade in this second generation phosphate biosensor selectively delivers four electrons for each phosphate molecule present. We introduced an additional electrochemical process involving uric acid oxidation at the underlying electrode. This further enhances the anodic current (signal amplification) by two additional electrons per analyte molecule which mitigates the influence of electrochemical interferences from the sample matrix. Moreover, while the XOx catalyzed reaction is sensitive to O2, the uric acid production and therefore the delivery of electrons through the subsequent electrochemical process are independent of the presence of O2. Consequently, the electrochemical process counterbalances the O2 interferences, especially at low phosphate concentrations. Importantly, the electrochemical uric acid oxidation specifically reports on phosphate concentration since it originates from the product of the bienzymatic reactions. These advantageous properties make this bioelectrochemical-electrochemical cascade particularly promising for point-of-use phosphate measurements. © 2018 Elsevier B.V.
    view abstract10.1016/j.bios.2018.06.047
  • Biological imaging with scanning electrochemical microscopy
    Conzuelo, F. and Schulte, A. and Schuhmann, W.
    Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences 474 (2018)
    Scanning electrochemical microscopy (SECM) is a powerful and versatile technique for visualizing the local electrochemical activity of a surface as an ultramicroelectrode tip is moved towards or over a sample of interest using precise positioning systems. In comparison with other scanning probe techniques, SECM not only enables topographical surface mapping but also gathers chemical information with high spatial resolution. Considerable progress has been made in the analysis of biological samples, including living cells and immobilized biomacromolecules such as enzymes, antibodies and DNA fragments. Moreover, combinations of SECM with complementary analytical tools broadened its applicability and facilitated multi-functional analysis with extended life science capabilities. The aim of this review is to present a brief topical overview on recent applications of biological SECM, with particular emphasis on important technical improvements of this surface imaging technique, recommended applications and future trends. © 2018 The Author(s) Published by the Royal Society. All rights reserved.
    view abstract10.1098/rspa.2018.0409
  • Cobalt-metalloid alloys for electrochemical oxidation of 5-hydroxymethylfurfural as an alternative anode reaction in lieu of oxygen evolution during water splitting
    Weidner, J. and Barwe, S. and Sliozberg, K. and Piontek, S. and Masa, J. and Apfel, U.-P. and Schuhmann, W.
    Beilstein Journal of Organic Chemistry 14 (2018)
    The electrochemical water splitting commonly involves the cathodic hydrogen and anodic oxygen evolution reactions (OER). The oxygen evolution reaction is more energetically demanding and kinetically sluggish and represents the bottleneck for a commercial competitiveness of electrochemical hydrogen production from water. Moreover, oxygen is essentially a waste product of low commercial value since the primary interest is to convert electrical energy into hydrogen as a storable energy carrier. We report on the anodic oxidation of 5-hydroxymethylfurfural (HMF) to afford the more valuable product 2,5-furandicarboxylic acid (FDCA) as a suitable alternative to the oxygen evolution reaction. Notably, HMF oxidation is thermodynamically more favorable than water oxidation and hence leads to an overall improved energy efficiency for H2 production. In addition, contrary to the “waste product O2”, FDCA can be further utilized, e.g., for production of polyethylene 2,5-furandicarboxylate (PEF), a sustainable polymer analog to polyethylene terephthalate (PET) and thus represents a valuable product for the chemical industry with potential large scale use. Various cobalt-metalloid alloys (CoX; X = B, Si, P, Te, As) were investigated as potential catalysts for HMF oxidation. In this series, CoB required 180 mV less overpotential to reach a current density of 55 mA cm−2 relative to OER with the same electrode. Electrolysis of HMF using a CoB modified nickel foam electrode at 1.45 V vs RHE achieved close to 100% selective conversion of HMF to FDCA at 100% faradaic efficiency. © 2018 Weidner et al.
    view abstract10.3762/bjoc.14.121
  • Combinatorial Synthesis and High-Throughput Characterization of Fe-V-O Thin-Film Materials Libraries for Solar Water Splitting
    Kumari, S. and Gutkowski, R. and Junqueira, J.R.C. and Kostka, A. and Hengge, K. and Scheu, C. and Schuhmann, W. and Ludwig, Al.
    ACS Combinatorial Science 20 (2018)
    The search for suitable materials for solar water splitting is addressed with combinatorial material science methods. Thin film Fe-V-O materials libraries were synthesized using combinatorial reactive magnetron cosputtering and subsequent annealing in air. The design of the libraries comprises a combination of large compositional gradients (from Fe10V90Ox to Fe79V21Ox) and thickness gradients (from 140 to 425 nm). These material libraries were investigated by high-throughput characterization techniques in terms of composition, structure, optical, and photoelectrochemical properties to establish correlations between composition, thickness, crystallinity, microstructure, and photocurrent density. Results show the presence of the Fe2V4O13 phase from ∼11 to 42 at. % Fe (toward low-Fe region) and the FeVO4 phase from ∼37 to 79 at. % Fe (toward Fe-rich region). However, as a third phase, Fe2O3 is present throughout the compositional gradients (from low-Fe to Fe-rich region). Material compositions with increasing crystallinity of the FeVO4 phase show enhanced photocurrent densities (∼160 to 190 μA/cm2) throughout the thickness gradients whereas compositions with the Fe2V4O13 phase show comparatively low photocurrent densities (∼28 μA/cm2). The band gap energies of Fe-V-O films were inferred from Tauc plots. The highest photocurrent density of ∼190 μA/cm2 was obtained for films with ∼54 to 66 at. % Fe for the FeVO4 phase with ∼2.04 eV for the indirect and ∼2.80 eV for the direct band gap energies. © 2018 American Chemical Society.
    view abstract10.1021/acscombsci.8b00030
  • Comparative studies on the interaction of anticancer drug irinotecan with dsDNA and ssDNA
    Temerk, Y. and Ibrahim, M. and Ibrahim, H. and Schuhmann, W.
    RSC Advances 8 (2018)
    A systematic comparative study on the binding of anticancer drug irinotecan (Irino) with dsDNA and ssDNA was investigated in phosphate buffer solutions using voltammetric and spectroscopic methods. The voltammetric results show that the Irino molecule, acting as an intercalator, is inserted into the base stacking domain of the DNA double helix and the strength of interaction is independent of the ionic strength. The hyperchromic effect observed in the UV-visible spectra of Irino in the presence of dsDNA provided the evidence for the intercalation of the drug chromophore with dsDNA base. The interaction mode of Irino molecules with ssDNA is electrostatic attraction via negative phosphate on the exterior of the ssDNA with Irino. The binding constants, stoichiometric coefficients and thermodynamic parameters of Irino-dsDNA and Irino-ssDNA complexes were evaluated. The magnitude of changes in ΔGo, ΔHo and ΔSo indicated that the binding process of Irino with ssDNA was more affected than that with dsDNA. The decrease of the peak current of Irino was proportional to DNA concentration, which was applied for determination of dsDNA and ssDNA concentration. The achieved limits of detection of dsDNA and ssDNA were 5.49 × 10-7 and 1.87 × 10-7 M, respectively. © The Royal Society of Chemistry 2018.
    view abstract10.1039/c8ra03231a
  • Controlling the amorphous and crystalline state of multinary alloy nanoparticles in an ionic liquid
    Garzón-Manjón, A. and Meyer, H. and Grochla, D. and Löffler, T. and Schuhmann, W. and Ludwig, Al. and Scheu, C.
    Nanomaterials 8 (2018)
    Controlling the amorphous or crystalline state of multinary Cr-Mn-Fe-Co-Ni alloy nanoparticles with sizes in the range between ~1.7 and ~4.8 nm is achieved using three processing routes. Direct current sputtering from an alloy target in the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide leads to amorphous nanoparticles as observed by high-resolution transmission electron microscopy. Crystalline nanoparticles can be achieved in situ in a transmission electron microscope by exposure to an electron beam, ex situ by heating in vacuum, or directly during synthesis by using a high-power impulse magnetron sputtering process. Growth of the nanoparticles with respect to the amorphous particles was observed. Furthermore, the crystal structure can be manipulated by the processing conditions. For example, a body-centered cubic structure is formed during in situ electron beam crystallization while longer ex situ annealing induces a face-centered cubic structure. © 2018 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/nano8110903
  • Discovery of a Multinary Noble Metal–Free Oxygen Reduction Catalyst
    Löffler, T. and Meyer, H. and Savan, A. and Wilde, P. and Garzón Manjón, A. and Chen, Y.-T. and Ventosa, E. and Scheu, C. and Ludwig, Al. and Schuhmann, W.
    Advanced Energy Materials 8 (2018)
    In the endeavor of discovering new noble metal–free electrocatalysts for the oxygen reduction reaction, noble metal–free multinary transition metal nanoparticle libraries are investigated. The complexity of such multiple principal element alloys provides access to a large variety of different elemental compositions, each with potentially unique properties. The strategy for efficient identification of novel electrocatalytically active systems comprises combinatorial co-sputtering into an ionic liquid followed by potential-assisted immobilization of the formed nanoparticles at a microelectrode which allows the evaluation of their intrinsic electrocatalytic activity in alkaline media. A surprisingly high intrinsic activity is found for the system Cr–Mn–Fe–Co–Ni, which is at least comparable to Pt under the same conditions, an unexpected result based on the typical properties of its constituents. Systematic removal of each element from the quinary alloy system yields a significant drop in activity for all quaternary alloys, indicating the importance of the synergistic combination of all five elements, likely due to formation of a single solid solution phase with altered properties which enables the limitations of the single elements to be overcome. Multinary transition metal alloys as a novel material class in electrocatalysis with basically unlimited possibilities for catalyst design, targeting the replacement of noble metal–based materials, are suggested. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/aenm.201802269
  • Dual properties of a hydrogen oxidation Ni-catalyst entrapped within a polymer promote self-defense against oxygen /639/638/77/886 /639/638/161/893 /639/638/675 /120 /128 /140/131 article
    Oughli, A.A. and Ruff, A. and Boralugodage, N.P. and Rodríguez-Maciá, P. and Plumeré, N. and Lubitz, W. and Shaw, W.J. and Schuhmann, W. and Rüdiger, O.
    Nature Communications 9 (2018)
    The Ni(P2N2)2 catalysts are among the most efficient non-noble-metal based molecular catalysts for H2 cycling. However, these catalysts are O2 sensitive and lack long term stability under operating conditions. Here, we show that in a redox silent polymer matrix the catalyst is dispersed into two functionally different reaction layers. Close to the electrode surface is the "active" layer where the catalyst oxidizes H2 and exchanges electrons with the electrode generating a current. At the outer film boundary, insulation of the catalyst from the electrode forms a "protection" layer in which H2 is used by the catalyst to convert O2 to H2O, thereby providing the "active" layer with a barrier against O2. This simple but efficient polymer-based electrode design solves one of the biggest limitations of these otherwise very efficient catalysts enhancing its stability for catalytic H2 oxidation as well as O2 tolerance. © 2018 The Author(s).
    view abstract10.1038/s41467-018-03011-7
  • Dynamics of nanointerfaces: General discussion
    Alzahrani, H. and Bentley, C. and Burrows, R. and Cao, C. and Cai, Q. and Chikere, C. and Crooks, R.M. and Dunevall, J. and Edwards, M. and Ewing, A. and Gao, R. and Hillman, R. and Kahram, M. and Kanoufi, F. and Kranz, C. and Lemineur, J.-F. and Long, Y. and McKelvey, K. and Mirkin, M. and Moore, S. and Nogala, W. and Ren, H. and Schuhmann, W. and Unwin, P. and Vezzoli, A. and White, H. and Willets, K. and Yang, Z. and Ying, Y.
    Faraday Discussions 210 (2018)
    view abstract10.1039/C8FD90026D
  • Electrocatalytic Nanoparticles That Mimic the Three-Dimensional Geometric Architecture of Enzymes: Nanozymes
    Benedetti, T.M. and Andronescu, C. and Cheong, S. and Wilde, P. and Wordsworth, J. and Kientz, M. and Tilley, R.D. and Schuhmann, W. and Gooding, J.J.
    Journal of the American Chemical Society 140 (2018)
    Enzymes are characterized by an active site that is typically embedded deeply within the protein shell thus creating a nanoconfined reaction volume in which high turnover rates occur. We propose nanoparticles with etched substrate channels as a simplified enzyme mimic, denominated nanozymes, for electrocatalysis. We demonstrate increased electrocatalytic activity for the oxygen reduction reaction using PtNi nanoparticles with isolated substrate channels. The PtNi nanoparticles comprise an oleylamine capping layer that blocks the external surface of the nanoparticles participating in the catalytic reaction. Oxygen reduction mainly occurs within the etched channels providing a nanoconfined reaction volume different from the bulk electrolyte conditions. The oxygen reduction reaction activity normalized by the electrochemically active surface area is enhanced by a factor of 3.3 for the nanozymes compared to the unetched nanoparticles and a factor of 2.1 compared to mesoporous PtNi nanoparticles that possess interconnecting pores. © Copyright 2018 American Chemical Society.
    view abstract10.1021/jacs.8b08664
  • Electrocatalytic Oxidation of 5-(Hydroxymethyl)furfural Using High-Surface-Area Nickel Boride
    Barwe, S. and Weidner, J. and Cychy, S. and Morales, D.M. and Dieckhöfer, S. and Hiltrop, D. and Masa, J. and Muhler, M. and Schuhmann, W.
    Angewandte Chemie - International Edition 57 (2018)
    The electrochemical oxidation of the biorefinery product 5-(hydroxymethyl)furfural (HMF) to 2,5-furandicarboxylic acid (FDCA), an important platform chemical for the polymer industry, is receiving increasing interest. FDCA-based polymers such as polyethylene 2,5-furandicarboxylate (PEF) are sustainable candidates for replacing polyethylene terephthalate (PET). Herein, we report the highly efficient electrocatalytic oxidation of HMF to FDCA, using Ni foam modified with high-surface-area nickel boride (NixB) as the electrode. Constant potential electrolysis in combination with HPLC revealed a high faradaic efficiency of close to 100 % towards the production of FDCA with a yield of 98.5 %. Operando electrochemistry coupled to ATR-IR spectroscopy indicated that HMF is oxidized preferentially via 5-hydroxymethyl-2-furancarboxylic acid rather than via 2,5-diformylfuran, which is in agreement with HPLC results. This study not only reports a low-cost active electrocatalyst material for the electrochemical oxidation of HMF to FDCA, but additionally provides insight into the reaction pathway. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/anie.201806298
  • Energy conversion at nanointerfaces: General discussion
    Alzahrani, H. and Bentley, C. and Bohn, P.W. and Chikere, C. and Commandeur, D. and Crooks, R.M. and Ehi-Eromosele, C. and Ewing, A. and Galeyeva, A. and Hersbach, T. and Hillman, R. and Kanoufi, F. and Koper, M. and Kranz, C. and Löffler, T. and Long, Y. and Macpherson, J. and McKelvey, K. and Minteer, S. and Mirkin, M. and Nichols, R. and Nogala, W. and Öhl, D. and Pelta, J. and Ren, H. and Rudd, J. and Schuhmann, W. and Tian, Z. and Unwin, P. and Vezzoli, A. and Willets, K. and Wu, Y. and Yang, Z. and Zhan, D. and Zhao, C.
    Faraday Discussions 210 (2018)
    view abstract10.1039/c8fd90025f
  • Evaluation of the intrinsic catalytic activity of nanoparticles without prior knowledge of the mass loading
    Löffler, T. and Wilde, P. and Öhl, D. and Chen, Y.-T. and Tschulik, K. and Schuhmann, W.
    Faraday Discussions 210 (2018)
    The quantitative characterisation of electrocatalytic properties of nanoparticle catalyst materials is so far only performed for layers typically comprising additionally conducting additives and binders. We propose a method enabling the evaluation of intrinsic catalytic activity of nanoparticles based on the diffusion-limited steady-state current. In a step-after-step process, the influence of coverage on kinetic and diffusion limited current is evaluated to highlight the challenges of sub-monolayer electroanalysis. Conclusions are used to point out strategies and their limitations for qualitative and quantitative comparison of intrinsic catalytic properties. Particularly, the impact of coverage, electrode geometry, altered diffusion profile for nanoparticles and the catalyst activity and selectivity are discussed. Fundamental information about electrochemical sub-monolayer nanoparticle analysis is provided. © 2018 The Royal Society of Chemistry.
    view abstract10.1039/c8fd00029h
  • Exceeding 6500 cycles for LiFePO4/Li metal batteries through understanding pulsed charging protocols
    García, G. and Dieckhöfer, S. and Schuhmann, W. and Ventosa, E.
    Journal of Materials Chemistry A 6 (2018)
    Improving the performance of Li metal anodes is of key importance for the next generation high energy-density batteries. Here, we study an easily implementable strategy for prolonging the cycle stability of Li metal anodes that is based on the application of pulsed charging protocols. Introducing short periods of relaxation without current flow allows the concentration of Li+ ions to be replenished in front of the electrode surface promoting a uniform and efficient plating of Li metal. We demonstrate that the cycle life of LiFePO4/Li metal batteries is prolonged from 700 to more than 6500 cycles at high charge-rates. In contrast to the assumed failure due to Li dendrite formation, we show that the proposed potential pulse protocols mitigate the growth of a porous film within the Li metal electrode which appears to be responsible for the battery failure. © 2018 The Royal Society of Chemistry.
    view abstract10.1039/c8ta00962g
  • H2 quantification based on selective pre-concentration and oxidative stripping at Pd modified microelectrodes
    Koster, D. and Gutkowski, R. and Masa, J. and Schuhmann, W.
    Journal of Electroanalytical Chemistry 812 (2018)
    A concept for amperometric detection and quantification of H2 employing a palladium microsensor is demonstrated. Resistive sensors exploit the outstanding affinity of Pd toward H2 absorption leading to a detectable modulation in resistivity, while amperometric sensors are generally non-Pd-based and detect the electrochemical oxidation of H2 at the electrode surface. The latter method requires the use of a porous membrane in order to ensure that the electrode reaction is limited by the diffusion of H2 from the gas phase to the sensing electrode. We introduce direct quantification of dissolved H2 in aqueous electrolytes that relies on a pre-concentration mechanism at Pd modified microelectrodes and the subsequent amperometric or coulometric oxidation of H2 from bulk Pd. Due to the straightforward data analysis, this method allows for precise determination of H2 concentration in solution, with the maximum sensitivity obtained by adjusting the pre-concentration time. © 2017 Elsevier B.V.
    view abstract10.1016/j.jelechem.2017.12.030
  • High resolution, binder-free investigation of the intrinsic activity of immobilized NiFe LDH nanoparticles on etched carbon nanoelectrodes
    Wilde, P. and Barwe, S. and Andronescu, C. and Schuhmann, W. and Ventosa, E.
    Nano Research 11 (2018)
    The determination of the intrinsic properties of nanomaterials is essential for their optimization as electrocatalysts, however it poses great challenges from the standpoint of analytical tools and methods. Herein, we report a novel methodology that allows for a binder-free investigation of electrocatalyst nanoparticles. The potential-assisted immobilization of a non-noble metal catalyst, i.e., nickel-iron layered double hydroxide (NiFe LDH) nanoparticles, was employed to directly attach small nanoparticle ensembles from a suspension to the surface of etched carbon nanoelectrodes. The dimensions of this type of electrodes allowed for the immobilization of the catalyst material below the picogram scale and resulted in a high resolution towards the faradaic current response. In addition the effect of the electrochemical aging on the intrinsic activity of the catalyst was investigated in alkaline media by means of continuous cyclic voltammetry. A change in the material properties could be observed, which was accompanied by a substantial decrease in its intrinsic activity. [Figure not available: see fulltext.] © 2018, Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature.
    view abstract10.1007/s12274-018-2119-4
  • Influence of Temperature and Electrolyte Concentration on the Structure and Catalytic Oxygen Evolution Activity of Nickel–Iron Layered Double Hydroxide
    Andronescu, C. and Seisel, S. and Wilde, P. and Barwe, S. and Masa, J. and Chen, Y.-T. and Ventosa, E. and Schuhmann, W.
    Chemistry - A European Journal 24 (2018)
    NiFe layered double hydroxide (LDH) is inarguably the most active contemporary catalyst for the oxygen evolution reaction under alkaline conditions. However, the ability to sustain unattenuated performance under challenging industrial conditions entailing high corrosivity of the electrolyte (≈30 wt. % KOH), high temperature (&gt;80 °C) and high current densities (&gt;500 mA cm−2) is the ultimate criterion for practical viability. This work evaluates the chemical and structural stability of NiFe LDH at conditions akin to practical electrolysis, in 30 % KOH at 80 °C, however, without electrochemical polarization, and the resulting impact on the OER performance of the catalyst. Post-analysis of the catalyst by means of XRD, TEM, FT-IR, and Raman spectroscopy after its immersion into 7.5 m KOH at 80 °C for 60 h revealed a transformation of the structure from NiFe LDH to a mixture of crystalline β-Ni(OH)2 and discrete predominantly amorphous FeOOH containing minor non-homogeneously distributed crystalline domains. These structural and compositional changes led to a drastic loss of the OER activity. It is therefore recommended to study catalyst stability at industrially relevant conditions. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/chem.201803165
  • Influence of the Fe:Ni Ratio and Reaction Temperature on the Efficiency of (FexNi1-x)9S8 Electrocatalysts Applied in the Hydrogen Evolution Reaction
    Piontek, S. and Andronescu, C. and Zaichenko, A. and Konkena, B. and Junge Puring, K. and Marler, B. and Antoni, H. and Sinev, I. and Muhler, M. and Mollenhauer, D. and Roldan Cuenya, B. and Schuhmann, W. and Apfel, U.-P.
    ACS Catalysis 8 (2018)
    Inspired by our recent finding that Fe4.5Ni4.5S8 rock is a highly active electrocatalyst for HER, we set out to explore the influence of the Fe:Ni ratio on the performance of the catalyst. We herein describe the synthesis of (FexNi1-x)9S8 (x = 0-1) along with a detailed elemental composition analysis. Furthermore, using linear sweep voltammetry, we show that the increase in the iron or nickel content, respectively, lowers the activity of the electrocatalyst toward HER. Electrochemical surface area analysis (ECSA) clearly indicates the highest amount of active sites for a Fe:Ni ratio of 1:1 on the electrode surface pointing at an altered surface composition of iron and nickel for the other materials. Specific metal-metal interactions seem to be of key importance for the high electrocatalytic HER activity, which is supported by DFT calculations of several surface structures using the surface energy as a descriptor of catalytic activity. In addition, we show that a temperature increase leads to a significant decrease of the overpotential and gain in HER activity. Thus, we showcase the necessity to investigate the material structure, composition and reaction conditions when evaluating electrocatalysts. © 2017 American Chemical Society.
    view abstract10.1021/acscatal.7b02617
  • Light as Trigger for Biocatalysis: Photonic Wiring of Flavin Adenine Dinucleotide-Dependent Glucose Dehydrogenase to Quantum Dot-Sensitized Inverse Opal TiO2 Architectures via Redox Polymers
    Riedel, M. and Parak, W.J. and Ruff, A. and Schuhmann, W. and Lisdat, F.
    ACS Catalysis 8 (2018)
    The functional coupling of photoactive nanostructures with enzymes creates a strategy for the design of light-triggered biocatalysts. This study highlights the efficient wiring of flavin adenine dinucleotide (FAD)-dependent glucose dehydrogenase (FAD-GDH) to PbS quantum dot (QD)-sensitized inverse opal TiO2 electrodes (IO-TiO2) by means of an Os-complex-containing redox polymer for the light-driven glucose oxidation. For the construction of IO-TiO2 scaffolds, a template approach has been developed, enabling the tunability of the surface area and a high loading capacity for the integration of QDs, redox polymer, and enzyme. The biohybrid signal chain can be switched on with light, generating charge carriers within the QDs, triggering a multistep electron-transfer cascade from the enzyme toward the redox polymer via the QDs and finally to the IO-TiO2 electrode. The resulting anodic photocurrent can be modulated by the potential, the excitation intensity, and the glucose concentration, providing a new degree of freedom for the control of biocatalyic reactions at electrode interfaces. Maximum photocurrents of 207 μA cm-2 have been achieved in the presence of glucose, and a first gain of electrons from the biocatalytic reaction is found at -540 mV vs Ag/AgCl, 1 M KCl, which lowers the working potential by &gt;500 mV as compared to light-insensitive electrodes. The biohybrid system combines the advantages of a high surface area of IO films, an efficient charge-carrier generation and separation at the QDs/TiO2 interface, and an efficient wiring of FAD-GDH to the QDs via a redox polymer, resulting in photo(bio)anodes of high performance for sensing and power supply. © 2018 American Chemical Society.
    view abstract10.1021/acscatal.8b00951
  • Light-induced formation of partially reduced oxygen species limits the lifetime of photosystem 1-based biocathodes
    Zhao, F. and Hardt, S. and Hartmann, V. and Zhang, H. and Nowaczyk, M.M. and Rögner, M. and Plumeré, N. and Schuhmann, W. and Conzuelo, F.
    Nature Communications 9 (2018)
    Interfacing photosynthetic proteins specifically photosystem 1 (PS1) with electrodes enables light-induced charge separation processes for powering semiartificial photobiodevices with, however, limited long-term stability. Here, we present the in-depth evaluation of a PS1/Os-complex-modified redox polymer-based biocathode by means of scanning photoelectrochemical microscopy. Focalized local illumination of the bioelectrode and concomitant collection of H2O2 at the closely positioned microelectrode provide evidence for the formation of partially reduced oxygen species under light conditions. Long-term evaluation of the photocathode at different O2 concentrations as well as after incorporating catalase and superoxide dismutase reveals the particularly challenging issue of avoiding the generation of reactive species. Moreover, the evaluation of films prepared with inactivated PS1 and free chlorophyll points out additional possible pathways for the generation of oxygen radicals. To avoid degradation of PS1 during illumination and hence to enhance the long-term stability, the operation of biophotocathodes under anaerobic conditions is indispensable. © 2018 The Author(s).
    view abstract10.1038/s41467-018-04433-z
  • Live cyanobacteria produce photocurrent and hydrogen using both the respiratory and photosynthetic systems
    Saper, G. and Kallmann, D. and Conzuelo, F. and Zhao, F. and Tóth, T.N. and Liveanu, V. and Meir, S. and Szymanski, J. and Aharoni, A. and Schuhmann, W. and Rothschild, A. and Schuster, G. and Adir, N.
    Nature Communications 9 (2018)
    Oxygenic photosynthetic organisms perform solar energy conversion of water and CO2 to O2 and sugar at a broad range of wavelengths and light intensities. These cells also metabolize sugars using a respiratory system that functionally overlaps the photosynthetic apparatus. In this study, we describe the harvesting of photocurrent used for hydrogen production from live cyanobacteria. A non-harmful gentle physical treatment of the cyanobacterial cells enables light-driven electron transfer by an endogenous mediator to a graphite electrode in a bio-photoelectrochemical cell, without the addition of sacrificial electron donors or acceptors. We show that the photocurrent is derived from photosystem I and that the electrons originate from carbohydrates digested by the respiratory system. Finally, the current is utilized for hydrogen evolution on the cathode at a bias of 0.65 V. Taken together, we present a bio-photoelectrochemical system where live cyanobacteria produce stable photocurrent that can generate hydrogen. © 2018 The Author(s).
    view abstract10.1038/s41467-018-04613-x
  • Local Activities of Hydroxide and Water Determine the Operation of Silver-Based Oxygen Depolarized Cathodes
    Botz, A. and Clausmeyer, J. and Öhl, D. and Tarnev, T. and Franzen, D. and Turek, T. and Schuhmann, W.
    Angewandte Chemie - International Edition 57 (2018)
    Local ion activity changes in close proximity to the surface of an oxygen depolarized cathode (ODC) were measured by scanning electrochemical microscopy (SECM). While the operating ODC produces OH− ions and consumes O2 and H2O through the electrocatalytic oxygen reduction reaction (ORR), local changes in the activity of OH− ions and H2O are detected by means of a positioned Pt microelectrode serving as an SECM tip. Sensing at the Pt tip is based on the pH-dependent reduction of PtO and obviates the need for prior electrode modification steps. It can be used to evaluate the coordination numbers of OH− ions and H2O, and the method was exploited as a novel approach of catalyst activity assessment. We show that the electrochemical reaction on highly active catalysts can have a drastic influence on the reaction environment. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/anie.201807798
  • Local Surface Modifications Investigated by Combining Scanning Electrochemical Microscopy and Surface-Enhanced Raman Scattering
    Clausmeyer, J. and Nebel, M. and Grützke, S. and Kayran, Y.U. and Schuhmann, W.
    ChemPlusChem 83 (2018)
    Scanning electrochemical microscopy (SECM) is coupled with surface-enhanced Raman scattering (SERS) microscopy to characterize local surface modifications. SECM is utilized for the surface patterning of para-nitrothiophenol self-assembled monolayers (SAMs) in the direct mode of SECM with a subsequent readout of the chemical patterns by means of combined SECM-SERS. The SECM-SERS combination allows monitoring of local electrochemical processes and provides simultaneous complementary spectroscopic information. The reaction products upon SAM reduction, specifically p-aminothiophenol groups, are distinguished from the pristine SAM and locally ruptured areas. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/cplu.201800031
  • Local Surface Structure and Composition Control the Hydrogen Evolution Reaction on Iron Nickel Sulfides
    Bentley, C.L. and Andronescu, C. and Smialkowski, M. and Kang, M. and Tarnev, T. and Marler, B. and Unwin, P.R. and Apfel, U.-P. and Schuhmann, W.
    Angewandte Chemie - International Edition 57 (2018)
    In order to design more powerful electrocatalysts, developing our understanding of the role of the surface structure and composition of widely abundant bulk materials is crucial. This is particularly true in the search for alternative hydrogen evolution reaction (HER) catalysts to replace platinum. We report scanning electrochemical cell microscopy (SECCM) measurements of the (111)-crystal planes of Fe4.5Ni4.5S8, a highly active HER catalyst. In combination with structural characterization methods, we show that this technique can reveal differences in activity arising from even the slightest compositional changes. By probing electrochemical properties at the nanoscale, in conjunction with complementary structural information, novel design principles are revealed for application to rational material synthesis. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/anie.201712679
  • Long-term implantable glucose biosensors
    Bobrowski, T. and Schuhmann, W.
    Current Opinion in Electrochemistry 10 (2018)
    Long-term implantable glucose sensors with a demonstrated lifetime of at least two weeks play a key role in continuous glucose monitoring (CGM) and the development of closed-loop artificial pancreas systems for the emerging number of diabetes patients. This short review summarises the current state of the art of CGM systems and points out specific limitations of amperometric glucose biosensors regarding their inherent sensor long-term and signal stability that are mainly caused by the inevitable foreign body response after sensor implantation. © 2018 Elsevier B.V.
    view abstract10.1016/j.coelec.2018.05.004
  • Monitoring Cobalt-Oxide Single Particle Electrochemistry with Subdiffraction Accuracy
    Brasiliense, V. and Clausmeyer, J. and Berto, P. and Tessier, G. and Combellas, C. and Schuhmann, W. and Kanoufi, F.
    Analytical Chemistry 90 (2018)
    By partially overcoming the diffraction limit, superlocalization techniques have extended the applicability of optical techniques down to the nanometer size-range. Herein, cobalt oxide-based nanoparticles are electrochemically grown onto carbon nanoelectrodes and their individual catalytic properties are evaluated through a combined electrochemical-optical approach. Using dark-field white light illumination, edges superlocalization techniques are applied to quantify changes in particle size during electrochemical activation with down to 20 nm precision. It allows the monitoring of (i) the anodic electrodeposition of cobalt hydroxide material and (ii) the large and reversible volume expansion experienced by the cobalt hydroxide particle during its oxidation. Meanwhile, the particle light scattering provides chemical information such as the Co redox state transformation, which complements both the particle size and the recorded electrochemical current and provides in operando mechanistic information on particle electrocatalytic properties. © 2018 American Chemical Society.
    view abstract10.1021/acs.analchem.8b00649
  • Monitoring Potential-Induced DNA Dehybridization Kinetics for Single Nucleotide Polymorphism Detection by using In Situ Surface Enhanced Raman Scattering
    Kayran, Y.U. and Cinar, N. and Jambrec, D. and Schuhmann, W.
    ChemElectroChem (2018)
    Changes in temperature, ionic strength or electrical field are generally employed to dehybridize double-stranded DNA (dsDNA). In contrast, we propose potential-induced dsDNA dehybridization to distinguish fully matched target DNA (tDNA) from tDNA with a single nucleotide polymorphism (SNP) by following their dehybridization kinetics through insitu surface enhanced Raman scattering (SERS). The determination of the potential that evokes notable dehybridization of dsDNA without causing any desorption of the immobilized probe DNA (pDNA) from the electrode surface was performed by investigating the desorption kinetics of labelled single-stranded DNA (ssDNA) and dehybridization kinetics of dsDNA with labelled tDNA. This comparatively simple approach allows for SNP detection within minutes. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/celc.201701220
  • Nanostructured DNA Microarrays for Dual SERS and Electrochemical Read-Out
    Kayran, Y.U. and Jambrec, D. and Schuhmann, W.
    Electroanalysis (2018)
    We present a strategy to fabricate nanostructured microarrays ready to perform a dual read-out, namely electrochemical (EC) as well as surface-enhanced Raman spectroscopy (SERS) based detection of DNA hydridization. A polystyrene nanobeads monolayer assembly, obtained by means of a Langmuir Blodgett type technique, followed by electrochemical Au deposition, was employed to construct homogeneous nanostructures in the form of inverse-opal nanovoids on a 32-electrode Au microarray chip. Characterization of the obtained nanostructured electrodes of the array by means of cyclic voltammetry demonstrated high reproducibility of the surface modification process. The performance of the obtained array platform was investigated by modifying the microarray electrodes with three different oligonucleotide capture probes using a previously developed potential-assisted surface modification protocol. Two ferrocene-labeled target DNA sequences and one target RNA sequence with a Texas red label were detected electrochemically and via SERS, respectively. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/elan.201800579
  • Nucleic acid hybridization on an electrically reconfigurable network of gold-coated magnetic nanoparticles enables microRNA detection in blood
    Tavallaie, R. and McCarroll, J. and Le Grand, M. and Ariotti, N. and Schuhmann, W. and Bakker, E. and Tilley, R.D. and Hibbert, D.B. and Kavallaris, M. and Gooding, J.J.
    Nature Nanotechnology 13 (2018)
    There is intense interest in quantifying the levels of microRNA because of its importance as a blood-borne biomarker. The challenge has been to develop methods that can monitor microRNA expression both over broad concentration ranges and in ultralow amounts directly in a patient’s blood. Here, we show that, through electric-field-induced reconfiguration of a network of gold-coated magnetic nanoparticles modified by probe DNA (DNA–Au@MNPs), it is possible to create a highly sensitive sensor for direct analysis of nucleic acids in samples as complex as whole blood. The sensor is the first to be able to detect concentrations of microRNA from 10 aM to 1 nM in unprocessed blood samples. It can distinguish small variations in microRNA concentrations in blood samples of mice with growing tumours. The ultrasensitive and direct detection of microRNA using an electrically reconfigurable DNA–Au@MNPs network makes the reported device a promising tool for cancer diagnostics. © 2018, The Author(s).
    view abstract10.1038/s41565-018-0232-x
  • Optimized Ag Nanovoid Structures for Probing Electrocatalytic Carbon Dioxide Reduction Using Operando Surface-Enhanced Raman Spectroscopy
    Öhl, D. and Kayran, Y.U. and Junqueira, J.R.C. and Eßmann, V. and Bobrowski, T. and Schuhmann, W.
    Langmuir 34 (2018)
    Surface-enhanced Raman spectroscopy is a powerful analytical tool and a strongly surface structure-dependent process. Importantly, it can be coupled with electrochemistry to simultaneously record vibrational spectroscopic information during electrocatalytic reactions. Highest Raman enhancements are obtained using precisely tuned nanostructures. The fabrication and evaluation of a high number of different nanostructures with slightly different properties is time-consuming. We present a strategy to systematically determine optimal nanostructure properties of electrochemically generated Ag void structures in order to find the void size providing highest signal enhancement for Raman spectroscopy. Ag-coated Si wafers were decorated with a monolayer of differently sized polymer nanospheres using a Langmuir-Blodgett approach. Subsequently, bipolar electrochemistry was used to electrodeposit a gradient of differently sized void structures. The gradient structures were locally evaluated using Raman spectroscopy of a surface-adsorbed Raman probe, and the surface regions exhibiting the highest Raman enhancement were characterized by means of scanning electron microscopy. High-throughput scanning droplet cell experiments were utilized to determine suitable conditions for the electrodeposition of the found highly active structure in a three-electrode electrochemical cell. This structure was subsequently employed as the working electrode in operando surface-enhanced Raman measurements to verify its viability as the signal amplifier and to spectroscopically rationalize the complex electrochemical reduction of carbon dioxide. © 2018 American Chemical Society.
    view abstract10.1021/acs.langmuir.8b02501
  • Overcoming cathode poisoning from electrolyte impurities in alkaline electrolysis by means of self-healing electrocatalyst films
    Barwe, S. and Mei, B. and Masa, J. and Schuhmann, W. and Ventosa, E.
    Nano Energy 53 (2018)
    The performance of electrolyzers for hydrogen production is strongly influenced by electrolyte impurities having either a positive or negative impact on the activity of electrocatalysts. We show that cathode deactivation by zinc impurities present in the electrolyte can be overcome by employing catalyst immobilization based on self-assembled and self-healing films. During electrolysis zinc impurities deposit as dendritic films on the cathode electrode increasing the overpotential for the hydrogen evolution reaction (HER), however, continuous self-assembling and self-healing of HER catalyst films subsequently mask the zinc dendrites restoring the advantageous HER overpotential. Zn electrolyte impurities are turned from having a negative to a positive impact leading to an enhanced performance of the cathode due to the increase in surface area caused by the growth of the Zn dendrites. © 2018 Elsevier Ltd
    view abstract10.1016/j.nanoen.2018.09.045
  • Oxidative Deposition of Manganese Oxide Nanosheets on Nitrogen-Functionalized Carbon Nanotubes Applied in the Alkaline Oxygen Evolution Reaction
    Antoni, H. and Morales, D.M. and Fu, Q. and Chen, Y.-T. and Masa, J. and Schuhmann, W. and Muhler, M.
    ACS Omega 3 (2018)
    The development of nonprecious catalysts for water splitting into hydrogen and oxygen is one of the major challenges to meet future sustainable fuel demand. Herein, thin layers of manganese oxide nanosheets supported on nitrogen-functionalized carbon nanotubes (NCNTs) were formed by the treatment of NCNTs dispersed in aqueous solutions of KMnO4 or CsMnO4 under reflux or under hydrothermal (HT) conditions and used as electrocatalysts for the oxygen evolution reaction (OER) in alkaline media. The samples were characterized by X-ray photoelectron spectroscopy, X-ray diffraction, transmission electron microscopy, and Raman spectroscopy. Our results show that the NCNTs treated under reflux were covered by partly amorphous and birnessite-type manganese oxides, while predominantly crystalline birnessite manganese oxide was observed for the hydrothermally treated samples. The latter showed, depending on the temperature during synthesis, an electrocatalytically favorable reduction from birnessite-type MnO2 to γ-MnOOH. OER activity measurements revealed a decrease of the overpotential for the OER at a current density of 10 mA cm-2 from 1.70 VRHE for the bare NCNTs to 1.64 VRHE for the samples treated under reflux in the presence of KMnO4. The hydrothermally treated samples afforded the same current density at a lower potential of 1.60 VRHE and a Tafel slope of 75 mV dec-1, suggesting that the higher OER activity is due to γ-MnOOH formation. Oxidative deposition under reflux conditions using CsMnO4 along with mild HT treatment using KMnO4, and low manganese loadings in both cases, were identified as the most suitable synthetic routes to obtain highly active MnOx/NCNT catalysts for electrochemical water oxidation. © 2018 American Chemical Society.
    view abstract10.1021/acsomega.8b01433
  • Oxygen Evolution Catalysis with Mössbauerite—A Trivalent Iron-Only Layered Double Hydroxide
    Ertl, M. and Andronescu, C. and Moir, J. and Zobel, M. and Wagner, F.E. and Barwe, S. and Ozin, G. and Schuhmann, W. and Breu, J.
    Chemistry - A European Journal 24 (2018)
    Mössbauerite is investigated for the first time as an “iron-only” mineral for the electrocatalytic oxygen evolution reaction in alkaline media. The synthesis proceeds via intermediate mixed-valence green rust that is rapidly oxidized in situ while conserving the layered double hydroxide structure. The material catalyzes the oxygen evolution reaction on a glassy carbon electrode with a current density of 10 mA cm−2 at 1.63 V versus the reversible hydrogen electrode. Stability measurements, as well as post-electrolysis characterization are presented. This work demonstrates the applicability of iron-only layered double hydroxides as earth-abundant oxygen evolution electrocatalysts. Mössbauerite is of fundamental importance since as an all Fe3+ material its performance has no contributions from unknown synergistic effects as encountered for mixed valence Co/Ni/Fe LDH. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/chem.201801938
  • Oxygen Reduction Activity and Reversible Deactivation of Single Silver Nanoparticles during Particle Adsorption Events
    Öhl, D. and Clausmeyer, J. and Barwe, S. and Botz, A. and Schuhmann, W.
    ChemElectroChem 5 (2018)
    The activity towards the oxygen reduction reaction (ORR) of single silver nanoparticles (AgNP) was quantified by using AgNP impacts on dual-bore carbon nanoelectrodes in highly alkaline media. We found suitable conditions for the particles to adhere sufficiently stably for detailed electrochemical characterization of a single particle. The special electrode design opens the possibility to dose gaseous oxygen to the nanoparticle under study. Deactivation of the catalytic activity of the AgNP upon excessive exposure to oxygen as well as the recovery of catalytic activity under reducing conditions is presumably attributed to hydrogen evolution at the applied low potentials. The proposed approach allows mechanistic parameters for the ORR to be extracted at a single AgNP in highly alkaline media in the absence of any binder materials and under exclusion of averaging ensemble effects. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/celc.201800094
  • Potential pulse-assisted immobilization of Myrothecium verrucaria bilirubin oxidase at planar and nanoporous gold electrodes
    Lopez, F. and Siepenkoetter, T. and Xiao, X. and Magner, E. and Schuhmann, W. and Salaj-Kosla, U.
    Journal of Electroanalytical Chemistry 812 (2018)
    A potential pulse-assisted approach was used to immobilize Myrothecium verrucaria bilirubin oxidase at planar and nanoporous gold electrodes (NPG) containing pores of ca. 20 nm and ca. 40 nm in diameter. An increase in the current due to the bioelectrocatalytic reduction of oxygen by MvBOD-modified gold electrodes obtained from a 20 μL drop by the proposed pulse-assisted approach was observed when compared to the response obtained with electrodes modified by drop-casting. This increase likely arises from a preferential orientation of MvBOD molecules at the planar gold surface obtained by fast switching of the potential pulses between opposite charges. The concomitant ion stirring effect induces the attraction of the enzymes to the charged gold surface and forces access to the internal pore volume of the NPG. Immobilization of MvBOD using the potential pulse-assisted approach significantly increases current densities by facilitating the electron transfer between the enzyme and the electrode surface. © 2017 Elsevier B.V.
    view abstract10.1016/j.jelechem.2017.12.023
  • Potential-pulse assisted thiol chemisorption minimizes non-specific adsorptions in DNA assays
    Jambrec, D. and Conzuelo, F. and Zhao, B. and Schuhmann, W.
    Electrochimica Acta 276 (2018)
    Effective suppression of non-specific adsorption is crucial for high-sensitive DNA hybridization sensors. Thiol-based passivation layers as generally used in DNA sensors often exhibit insufficient surface blocking and hence additional blocking agents are used to fully protect the surface from non-specific adsorption. We demonstrate that potential pulse-assisted thiol self-assembly on gold electrodes as surface passivation step in DNA assays provides minimized non-specific adsorption as evidenced by chronoamperometric and SECM measurements. Fast and highly efficient surface blocking is achieved avoiding the necessity of additional blocking steps. © 2018
    view abstract10.1016/j.electacta.2018.04.180
  • Processes at nanoelectrodes: General discussion
    Alzahrani, H. and Antoine, C. and Aoki, K. and Baker, L. and Balme, S. and Bentley, C. and Bhattacharya, G. and Bohn, P.W. and Cai, Q. and Cao, C. and Commandeur, D. and Crooks, R.M. and Edwards, M. and Ewing, A. and Fu, K. and Galeyeva, A. and Gao, R. and Hersbach, T. and Hillman, R. and Hu, Y.-X. and Jiang, L. and Kanoufi, F. and Kranz, C. and Liu, S. and Löffler, T. and Long, Y. and Macpherson, J. and McKelvey, K. and Minteer, S. and Mirkin, M. and Mount, A. and Nichols, R. and Nogala, W. and Öhl, D. and Qiu, K. and Ren, H. and Rudd, J. and Schuhmann, W. and Siwy, Z. and Tian, Z. and Unwin, P. and Wang, Y. and Wilde, P. and Wu, Y. and Yang, Z. and Ying, Y.
    Faraday Discussions 210 (2018)
    view abstract10.1039/C8FD90024H
  • Processes at nanopores and bio-nanointerfaces: General discussion
    Alzahrani, H. and Antoine, C. and Baker, L. and Balme, S. and Bhattacharya, G. and Bohn, P.W. and Cai, Q. and Chikere, C. and Crooks, R.M. and Das, N. and Edwards, M. and Ehi-Eromosele, C. and Ermann, N. and Jiang, L. and Kanoufi, F. and Kranz, C. and Long, Y. and Macpherson, J. and McKelvey, K. and Mirkin, M. and Nichols, R. and Nogala, W. and Pelta, J. and Ren, H. and Rudd, J. and Schuhmann, W. and Siwy, Z. and Tian, Z. and Unwin, P. and Wen, L. and White, H. and Willets, K. and Wu, Y. and Ying, Y.
    Faraday Discussions 210 (2018)
    view abstract10.1039/C8FD90023J
  • Rechargeable, flexible and mediator-free biosupercapacitor based on transparent ITO nanoparticle modified electrodes acting in µM glucose containing buffers
    Bobrowski, T. and González Arribas, E. and Ludwig, R. and Toscano, M.D. and Shleev, S. and Schuhmann, W.
    Biosensors and Bioelectronics 101 (2018)
    We present a transparent and flexible self-charging biosupercapacitor based on an optimised mediator- and membrane-free enzymatic glucose/oxygen biofuel cell. Indium tin oxide (ITO) nanoparticles were spray-coated on transparent conducting ITO supports resulting in a flocculent, porous and nanostructured electrode surface. By this, high capacitive currents caused by an increased electrochemical double layer as well as enhanced catalytic currents due to a higher number of immobilised enzyme molecules were obtained. After a chemical pre-treatment with a silane derivative, bilirubin oxidase from Myrothecium verrucaria was immobilized onto the ITO nanostructured electrode surface under formation of a biocathode, while bioanodes were obtained by either immobilisation of cellobiose dehydrogenase from Corynascus thermophilus or soluble PQQ-dependent glucose dehydrogenase from Acinetobacter calcoaceticus. The latter showed a lower apparent KM value for glucose conversion and higher catalytic currents at µM glucose concentrations. Applying the optimised device as a biosupercapacitor in a discontinuous charge/discharge mode led to a generated power output of 0.030 mW/cm2 at 50 µM glucose, simulating the glucose concentration in human tears. This represents an enhancement by a factor of 350 compared to the power density obtained from the continuously operating biofuel cell with a maximum power output of 0.086 µW/cm2 under the same conditions. After 17 h of charging/discharging cycles a remarkable current enhancement was still measured. The entire device was transferred to flexible materials and applied for powering a flexible display showing its potential applicability as an intermittent power source in smart contact lenses. © 2017 Elsevier B.V.
    view abstract10.1016/j.bios.2017.10.016
  • Robotic microplate voltammetry for real-time hydrogel drug release testing
    Jaikaew, W. and Ruff, A. and Khunkaewla, P. and Erichsen, T. and Schuhmann, W. and Schulte, A.
    Analytica Chimica Acta 1041 (2018)
    Robotic square wave voltammetry (SVW) in 24-well microtiter plates has been developed as a reliable non-manual procedure for quantifying drug release from pharmaceutical hydrogels. The assay was established using 1% agarose disks containing Paracetamol® (PCT) as a model preparation. Computerized buffer delivery and SVW in calibration and hydrogel sample wells were performed by a three-electrode arrangement combined with a thin plastic tube. For the glassy carbon working electrode of the assembly the upper limit of the linear response and the lower detection limit of sequential ‘in-well’ PCT-SVW were 1000 and 0.5 μM, respectively. During non-stop runs through plate wells with equal drug titers the voltammetric PCT signal was stable for at least 6 h. For the construction of drug-release curves with triplicate data points PCT-SVW was performed sequentially on three identical hydrogel samples in neighboring plate wells, preceded and followed by sensor calibrations for response validation. The results showed bi-phasic PCT release profiles exhibiting an initial rapid loss of the drug near the surface of the gel, followed by slowly decelerating release of more deeply buried drug and the dissipation of the concentration gradient that drives diffusion. The proposed automation of voltammetric testing generates reliable hydrogel drug release profiles without the need for operator intervention, avoiding human errors from monotonous manual electroanalysis and releasing skilled staff for other work. This approach is therefore suggested as an economic option for hydrogel dissolution testing in academic or industrial R&amp;D, particularly when the required multi-parameter optimization creates many samples. © 2018 Elsevier B.V.
    view abstract10.1016/j.aca.2018.08.033
  • Scanning Bipolar Electrochemical Microscopy
    Eßmann, V. and Santana Santos, C. and Tarnev, T. and Bertotti, M. and Schuhmann, W.
    Analytical Chemistry 90 (2018)
    Electrochemical techniques offer high temporal resolution for studying the dynamics of electroactive species at samples of interest. To monitor fastest concentration changes, a micro- or nanoelectrode is accurately positioned in the vicinity of a sample surface. Using a microelectrode array, it is even possible to investigate several sites simultaneously and to obtain an instantaneous image of local dynamics. However, the spatial resolution is limited by the minimal electrode size required in order to contact the electrodes. To provide a remedy, we introduce the concept of scanning bipolar electrochemical microscopy and the corresponding experimental system. This technique allows precise positioning of a wireless scanning bipolar electrode to convert spatially heterogeneous concentrations of the analyte of interest into an electrochemiluminescence map of the sample reactivity. After elucidating the working principle by recording bipolar line and array scans, a bipolar electrode array is positioned at the site of interest to record an electrochemical image of the localized release of analyte molecules. © 2018 American Chemical Society.
    view abstract10.1021/acs.analchem.8b00928
  • Self-powered bioelectrochemical devices
    Conzuelo, F. and Ruff, A. and Schuhmann, W.
    Current Opinion in Electrochemistry 12 (2018)
    Autonomous bioelectrochemical devices have been described since the beginning of the 21st century. The properties and broad potential applications of such devices encouraged the development of a plethora of examples. Self-powered biodevices have been mainly used for sensing applications with different strategies reported for the detection of various important analytes. Moreover, these devices had also inspired the design and fabrication of logic- and biocomputing-based systems with further applications in logic-activated drug release for the development of sense-act-treat systems. In addition, their use as self-sustained systems for energy supply has been recently reported. This review summarizes the development and progress of self-powered biodevices with particular attention to latest advances and novel applications. © 2018 Elsevier B.V.
    view abstract10.1016/j.coelec.2018.05.010
  • Spectroelectrochemical studies on the effect of cations in the alkaline glycerol oxidation reaction over carbon nanotube-supported Pd nanoparticles
    Hiltrop, D. and Cychy, S. and Elumeeva, K. and Schuhmann, W. and Muhler, M.
    Beilstein Journal of Organic Chemistry 14 (2018)
    The effects of the alkali cations Na+ and K+ were investigated in the alkaline electrochemical oxidation of glycerol over Pd nanoparticles (NPs) deposited on functionalized carbon nanotubes (CNTs). The electrocatalytic activity was assessed by cyclic voltammetry revealing a lower overpotential of glycerol oxidation for nitrogen-functionalized Pd/NCNTs compared with oxygen-functionalized Pd/OCNTs. Whereas significantly lower current densities were observed for Pd/OCNT in NaOH than in KOH in agreement with stronger non-covalent interactions on the Pd surface, Pd/NCNT achieved an approximately three-times higher current density in NaOH than in KOH. In situ electrochemistry/IR spectroscopy was applied to unravel the product distribution as a function of the applied potential in NaOH and KOH. The IR spectra exhibited strongly changing band patterns upon varying the potential between 0.77 and 1.17 V vs RHE: at low potentials oxidized C3 species such as mesoxalate and tartronate were formed predominantly, and with increasing potentials C2 and C1 species originating from C-C bond cleavage were identified. The tendency to produce carbonate was found to be less pronounced in KOH. The less favored formation of highly oxidized C3 species and of carbonate is deduced to be the origin of the lower current densities in the cyclic voltammograms (CVs) for Pd/NCNT in KOH. The enhanced current densities in NaOH are rationalized by the presence of Na+ ions bound to the basic nitrogen species in the NCNT support. Adsorbed Na+ ions can form complexes with the organic molecules, presumably enhanced by the chelate effect. In this way, the organic molecules are assumed to be bound more tightly to the NCNT support in close proximity to the Pd NPs facilitating their oxidation. © 2018 Hiltrop et al.
    view abstract10.3762/bjoc.14.120
  • The Open Circuit Voltage in Biofuel Cells: Nernstian Shift in Pseudocapacitive Electrodes
    Conzuelo, F. and Marković, N. and Ruff, A. and Schuhmann, W.
    Angewandte Chemie - International Edition 57 (2018)
    In the development of biofuel cells great effort is dedicated to achieving outstanding figures of merit, such as high stability, maximum power output, and a large open circuit voltage. Biofuel cells with immobilized redox mediators, such as redox polymers with integrated enzymes, show experimentally a substantially higher open circuit voltage than the thermodynamically expected value. Although this phenomenon is widely reported in the literature, there is no comprehensive understanding of the potential shift, the high open circuit voltages have not been discussed in detail, and hence they are only accepted as an inherent property of the investigated systems. We demonstrate that this effect is the result of a Nernstian shift of the electrode potential when catalytic conversion takes place in the absence or at very low current flow. Experimental evidence confirms that the immobilization of redox centers on the electrode surface results in the assembled biofuel cell delivering a higher power output because of charge storage upon catalytic conversion. Our findings have direct implications for the design and evaluation of (bio)fuel cells with pseudocapacitive elements. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/anie.201808450
  • Three-Dimensional Branched and Faceted Gold–Ruthenium Nanoparticles: Using Nanostructure to Improve Stability in Oxygen Evolution Electrocatalysis
    Gloag, L. and Benedetti, T.M. and Cheong, S. and Li, Y. and Chan, X.-H. and Lacroix, L.-M. and Chang, S.L.Y. and Arenal, R. and Florea, I. and Barron, H. and Barnard, A.S. and Henning, A.M. and Zhao, C. and Schuhmann, W. and Gooding, J.J. and Tilley, R.D.
    Angewandte Chemie - International Edition 57 (2018)
    Achieving stability with highly active Ru nanoparticles for electrocatalysis is a major challenge for the oxygen evolution reaction. As improved stability of Ru catalysts has been shown for bulk surfaces with low-index facets, there is an opportunity to incorporate these stable facets into Ru nanoparticles. Now, a new solution synthesis is presented in which hexagonal close-packed structured Ru is grown on Au to form nanoparticles with 3D branches. Exposing low-index facets on these 3D branches creates stable reaction kinetics to achieve high activity and the highest stability observed for Ru nanoparticle oxygen evolution reaction catalysts. These design principles provide a synthetic strategy to achieve stable and active electrocatalysts. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/anie.201806300
  • Towards Reproducible Fabrication of Nanometre-Sized Carbon Electrodes: Optimisation of Automated Nanoelectrode Fabrication by Means of Transmission Electron Microscopy
    Wilde, P. and Quast, T. and Aiyappa, H.B. and Chen, Y.-T. and Botz, A. and Tarnev, T. and Marquitan, M. and Feldhege, S. and Lindner, A. and Andronescu, C. and Schuhmann, W.
    ChemElectroChem 5 (2018)
    The reproducible fabrication of nanometre-sized carbon electrodes poses great challenges. Especially, the field of single entity electrochemistry has strict requirements regarding the geometry of these electrochemical probes. Herein, an automated setup for the fabrication of carbon nanoelectrodes based on the pyrolysis of a propane/butane gas mixture within pulled quartz capillaries by means of a moving heating coil is presented. It is shown that mere electrochemical characterisation with conventional redox mediators does not allow for a reliable assessment of the electrode's geometry and quality. Therefore, high-throughput transmission electron microscopy is used in parallel to evaluate and optimise preparation parameters. Control of the latter gives access to three different electrode types: nanopipettes, nanosamplers and nanodisks. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/celc.201800600
  • Tuned Amperometric Detection of Reduced β-Nicotinamide Adenine Dinucleotide by Allosteric Modulation of the Reductase Component of the p -Hydroxyphenylacetate Hydroxylase Immobilized within a Redox Polymer
    Teanphonkrang, S. and Janke, S. and Chaiyen, P. and Sucharitakul, J. and Suginta, W. and Khunkaewla, P. and Schuhmann, W. and Ruff, A. and Schulte, A.
    Analytical Chemistry 90 (2018)
    We report the fabrication of an amperometric NADH biosensor system that employs an allosterically modulated bacterial reductase in an adapted osmium(III)-complex-modified redox polymer film for analyte quantification. Chains of complexed Os(III) centers along matrix polymer strings make electrical connection between the immobilized redox protein and a graphite electrode disc, transducing enzymatic oxidation of NADH into a biosensor current. Sustainable anodic signaling required (1) a redox polymer with a formal potential that matched the redox switch of the embedded reductase and avoided interfering redox interactions and (2) formation of a cross-linked enzyme/polymer film for stable biocatalyst entrapment. The activity of the chosen reductase is enhanced upon binding of an effector, i.e. p-hydroxy-phenylacetic acid (p-HPA), allowing the acceleration of the substrate conversion rate on the sensor surface by in situ addition or preincubation with p-HPA. Acceleration of NADH oxidation amplified the response of the biosensor, with a 1.5-fold increase in the sensitivity of analyte detection, compared to operation without the allosteric modulator. Repetitive quantitative testing of solutions of known NADH concentration verified the performance in terms of reliability and analyte recovery. We herewith established the use of allosteric enzyme modulation and redox polymer-based enzyme electrode wiring for substrate biosensing, a concept that may be applicable to other allosteric enzymes. © 2018 American Chemical Society.
    view abstract10.1021/acs.analchem.7b05467
  • Ultrathin 2D Cobalt Zeolite-Imidazole Framework Nanosheets for Electrocatalytic Oxygen Evolution
    Jayaramulu, K. and Masa, J. and Morales, D.M. and Tomanec, O. and Ranc, V. and Petr, M. and Wilde, P. and Chen, Y.-T. and Zboril, R. and Schuhmann, W. and Fischer, R.A.
    Advanced Science 5 (2018)
    2D layered materials, including metal-di-chalcogenides and transition metal layered double hydroxides, among others, are intensively studied because of new properties that emerge from their 2D confinement, which are attractive for advanced applications. Herein, 2D cobalt ion (Co2+) and benzimidazole (bIm) based zeolite-imidazole framework nanosheets, ZIF-9(III), are reported as exceptionally efficient electrocatalysts for the oxygen evolution reaction (OER). Specifically, liquid-phase ultrasonication is applied to exfoliate a [Co4(bIm)16] zeolite-imidazole framework (ZIF), named as ZIF-9(III) phase, into nanoscale sheets. ZIF-9(III) is selectively prepared through simple mechanical grinding of cobalt nitrate and benzimidazole in the presence of a small amount of ethanol. The resultant exfoliated nanosheets exhibit significantly higher OER activity in alkaline conditions than the corresponding bulk phases ZIF-9 and ZIF-9(III). The electrochemical and physicochemical characterization data support the assignment of the OER activity of the exfoliated nanosheet derived material to nitrogen coordinated cobalt oxyhydroxide N4CoOOH sites, following a mechanism known for Co-porphyrin and related systems. Thus, exfoliated 2D nanosheets hold promise as potential alternatives to commercial noble metal electrocatalysts for the OER. © 2018 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/advs.201801029
  • Understanding the Effect of Au in Au-Pd Bimetallic Nanocrystals on the Electrocatalysis of the Methanol Oxidation Reaction
    Kelly, C.H.W. and Benedetti, T.M. and Alinezhad, A. and Schuhmann, W. and Gooding, J.J. and Tilley, R.D.
    Journal of Physical Chemistry C 122 (2018)
    Pd or Pt alloyed with a secondary metal are the typical catalysts at the anode for the direct oxidation of methanol. The secondary metal is employed to diminish deactivation commonly ascribed to CO poisoning. Here we investigate the origin of the improved performance of Au-Pd core-shell and alloy nanocrystals as electrocatalysts for the methanol oxidation reaction (MOR), relative to Pd alone. Monodisperse Au-Pd core-shell nanocrystals were synthesized using H2 as a mild reducing agent followed by annealing under a 5% H2 atmosphere to produce the Au-Pd alloys. The nanocrystals were characterized using high-resolution electron microscopy to confirm their structures. The core-shell and alloy nanocrystals showed an improvement in specific activity with respect to pure Pd nanocrystals. Importantly, the stability was also improved by the inclusion of Au for both nanocrystals, being 2.7× higher for the alloy than for the core-shell after 30 min, while the activity is completely lost for the Pd nanocrystals within 10 min. We show that there is no evidence of CO formation for any of the Pd-based catalysts in an alkaline environment. The origin of the improvement in terms of both activity and stability results from positive shifts in the PdO formation/reduction potential caused by the presence of Au, which results in more Pd sites available for the MOR. © 2018 American Chemical Society.
    view abstract10.1021/acs.jpcc.8b05407
  • Unravelling electron transfer processes at photosystem 2 embedded in an Os-complex modified redox polymer
    Zhao, F. and Hartmann, V. and Ruff, A. and Nowaczyk, M.M. and Rögner, M. and Schuhmann, W. and Conzuelo, F.
    Electrochimica Acta 290 (2018)
    In the development of semi-artificial biophotovoltaic assemblies, deeper understanding of electrochemical processes is required to achieve functional and efficient devices. Evaluation of photosystem 2 embedded in an Os-complex modified redox polymer using scanning photoelectrochemical microscopy (SPECM) provides insight into the intricate electrochemical processes of the immobilized protein complex and its electrical communication pathways with the redox tethers of the polymer matrix. The use of local irradiation during an SPECM array scan prevents sample inactivation prior to analysis. Moreover, the simultaneously possible collection of partially reduced oxygen species in the form of hydrogen peroxide confirms the presence of competing charge transfer pathways involved in the reduction of oxygen at the chlorophyll pigments upon irradiation of the sample. In addition, evaluation of photocurrent in the presence of an inhibitor that blocks the terminal plastoquinone QB binding site of the photosystem reveals electrochemical communication between the intermediate plastoquinone QA and the redox polymer. The obtained information proves to be relevant for further design and optimization of devices for technological applications. © 2018 Elsevier Ltd
    view abstract10.1016/j.electacta.2018.09.093
  • Utilization of the catalyst layer of dimensionally stable anodes. Part 2: Impact of spatial current distribution on electrocatalytic performance
    Zeradjanin, A.R. and Ventosa, E. and Masa, J. and Schuhmann, W.
    Journal of Electroanalytical Chemistry 828 (2018)
    An improved general understanding of electrocatalytic gas evolution reactions may - besides contributing to fundamental knowledge - play a crucial role in efforts directed to energy savings during large-scale industrial electrocatalytic processes. Aside from the search for new electrocatalytically active materials, in-depth understanding of electrode surface properties may contribute to more efficient utilization of known electrocatalysts. SECM is applied in the sample generation/tip collection mode to determine the spatial distribution of electrocatalytic activity during Cl2 evolution at the solid/liquid interface of dimensionally stable anodes. Statistical interpretation of the local microscopic distribution of the electrocatalytic activity of dimensionally stable anodes was used as a basis for improved understanding of their overall electrocatalytic performance. © 2018
    view abstract10.1016/j.jelechem.2018.09.034
  • Viologen-modified electrodes for protection of hydrogenases from high potential inactivation while performing H2 oxidation at low overpotential
    Oughli, A.A. and Vélez, M. and Birrell, J.A. and Schuhmann, W. and Lubitz, W. and Plumeré, N. and Rüdiger, O.
    Dalton Transactions 47 (2018)
    In this work we present a viologen-modified electrode providing protection for hydrogenases against high potential inactivation. Hydrogenases, including O2-tolerant classes, suffer from reversible inactivation upon applying high potentials, which limits their use in biofuel cells to certain conditions. Our previously reported protection strategy based on the integration of hydrogenase into redox matrices enabled the use of these biocatalysts in biofuel cells even under anode limiting conditions. However, mediated catalysis required application of an overpotential to drive the reaction, and this translates into a power loss in a biofuel cell. In the present work, the enzyme is adsorbed on top of a covalently-attached viologen layer which leads to mixed, direct and mediated, electron transfer processes; at low overpotentials, the direct electron transfer process generates a catalytic current, while the mediated electron transfer through the viologens at higher potentials generates a redox buffer that prevents oxidative inactivation of the enzyme. Consequently, the enzyme starts the catalysis at no overpotential with viologen self-activated protection at high potentials. © The Royal Society of Chemistry.
    view abstract10.1039/c8dt00955d
  • A novel versatile microbiosensor for local hydrogen detection by means of scanning photoelectrochemical microscopy
    Zhao, F. and Conzuelo, F. and Hartmann, V. and Li, H. and Stapf, S. and Nowaczyk, M.M. and Rögner, M. and Plumeré, N. and Lubitz, W. and Schuhmann, W.
    Biosensors and Bioelectronics 94 (2017)
    The development of a versatile microbiosensor for hydrogen detection is reported. Carbon-based microelectrodes were modified with a [NiFe]-hydrogenase embedded in a viologen-modified redox hydrogel for the fabrication of a sensitive hydrogen biosensor By integrating the microbiosensor in a scanning photoelectrochemical microscope, it was capable of serving simultaneously as local light source to initiate photo(bio)electrochemical reactions while acting as sensitive biosensor for the detection of hydrogen. A hydrogen evolution biocatalyst based on photosystem 1-platinum nanoparticle biocomplexes embedded into a specifically designed redox polymer was used as a model for proving the capability of the developed hydrogen biosensor for the detection of hydrogen upon localized illumination. The versatility and sensitivity of the proposed microbiosensor as probe tip allows simplification of the set-up used for the evaluation of complex electrochemical processes and the rapid investigation of local photoelectrocatalytic activity of biocatalysts towards light-induced hydrogen evolution. © 2017 Elsevier B.V.
    view abstract10.1016/j.bios.2017.03.037
  • A Polymer Multilayer Based Amperometric Biosensor for the Detection of Lactose in the Presence of High Concentrations of Glucose
    Lopez, F. and Ma, S. and Ludwig, R. and Schuhmann, W. and Ruff, A.
    Electroanalysis 29 (2017)
    We report on the development of an amperometric polymer multilayer-based biosensor for the determination of lactose in the presence of high concentrations of glucose. The sensor platform consists of a two-layer system with the first layer (sensing layer) comprising cellobiose dehydrogenase (CDH) bound by electrostatic interactions in a favourite orientation for efficient direct electron transfer. The second layer on top of the sensing element consists of a specifically designed hydrophilic polymer which co-entraps glucose oxidase (GOx) and catalase (CAT). This bi-enzymatic system is able to remove glucose (up to a concentration of at least 140 mM) in the outer layer of the modified electrode and thus prevents the interfering analyte from reaching the active CDH layer. Moreover, the concomitantly generated H2O2 is efficiently removed by means of CAT. The linear range for the detection of lactose was from 10 to 100 μM. The sensor is able to detect lactose at low concentrations in lactose-free milk samples. Sample pre-treatment consist of a simple dilution step. Quantification of the lactose content in lactose-free dairy products showed that the amount of lactose is below the threshold given by the manufacturer. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/elan.201600575
  • A Self-Powered Ethanol Biosensor
    Ruff, A. and Pinyou, P. and Nolten, M. and Conzuelo, F. and Schuhmann, W.
    ChemElectroChem 4 (2017)
    We describe the fabrication of a self-powered ethanol biosensor comprising a β-NAD+-dependent alcohol dehydrogenase (ADH) bioanode and a bienzymatic alcohol oxidase (AOx) and horseradish peroxidase (HRP) biocathode. β-NAD+ is regenerated by means of a specifically designed phenothiazine dye (i.e. toluidine blue, TB) modified redox polymer in which TB was covalently anchored to a hexanoic acid tethered poly(4-vinylpyridine) backbone. The redox polymer acts as an immobilization matrix for ADH. Using a carefully chosen anchoring strategy through the formation of amide bonds, the potential of the TB-based mediator is shifted to more positive potentials, thus preventing undesired O2 reduction. To counterbalance the rather high potential of the TB-modified polymer, and thus the bioanode, a high-potential AOx/HRP-based biocathode is suggested. HRP is immobilized in a direct-electron-transfer regime on screen-printed graphite electrodes functionalized with multi-walled carbon nanotubes. The nanostructured cathode ensures the wiring of the iron-oxo complex within oxidized HRP, and thus a high potential for the reduction of H2O2 of about +550mV versus Ag/AgCl/3M KCl. The proposed biofuel cell exhibits an open-circuit voltage (OCV) of approximately 660mV and was used as self-powered device for the determination of the ethanol content in liquor. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/celc.201600864
  • Alternating current-bipolar electrochemistry
    Eßmann, V. and Clausmeyer, J. and Schuhmann, W.
    Electrochemistry Communications 75 (2017)
    Rotation of a bipolar electrode in a constant electric field between feeder electrodes causes an alternating bipolar current at an AC frequency that depends on the rotation rate. The corresponding oscillation of the feeder current is evaluated by means of a lock-in amplifier. This innovative approach allows the current flowing through the non-wired bipolar electrode in an open bipolar system to be extracted without relying on electrochemical reporter reactions. © 2017 Elsevier B.V.
    view abstract10.1016/j.elecom.2017.01.006
  • Amperometric Detection of dsDNA using an Acridine-Orange-Modified Glucose Oxidase
    Jambrec, D. and Lammers, K. and Bobrowski, T. and Pöller, S. and Schuhmann, W. and Ruff, A.
    ChemPlusChem 82 (2017)
    Invited for this month's cover is the group of Prof. Dr. Wolfgang Schuhmann, Dr. Daliborka Jambrec and Dr. Adrian Ruff at Ruhr-Universität in Bochum, Germany. The cover picture shows a novel procedure for the preferential post-hybridization labeling of double-stranded DNA based on the intercalating compound acridine orange, which was covalently bound to glucose oxidase. Labeling with a highly active biocatalyst allows for a simple and sequence-independent amplification of the signal proportional to the amount of hybridized DNA that may be coupled with other amplification strategies. Read the full text of the article at 10.1002/cplu.201700279. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/cplu.201700373
  • An Intrinsic Self-Charging Biosupercapacitor Comprised of a High-Potential Bioanode and a Low-Potential Biocathode
    Alsaoub, S. and Ruff, A. and Conzuelo, F. and Ventosa, E. and Ludwig, R. and Shleev, S. and Schuhmann, W.
    ChemPlusChem (2017)
    An intrinsic self-charging biosupercapacitor built on a unique concept for the fabrication of biodevices based on redox polymers is presented. The biosupercapacitor consists of a high-potential redox polymer based bioanode and a low-potential redox polymer based biocathode in which the potentials of the electrodes in the discharged state show an apparent potential mismatch Eanode&gt;Ecathode and prevent the use of the device as a conventional biofuel cell. Upon charging, the potentials of the electrodes are shifted to more positive (cathode) and more negative (anode) values because of a change in the aox-to-ared ratio within the redox polymer matrix. Hence, a potential inversion occurs in the charged state (Eanode&lt;Ecathode) and an open circuit voltage of &gt;0.4V is achieved and the biodevice acts as a true biosupercapacitor. The bioanode consists of a novel specifically designed high-potential Os complex modified polymer for the efficient immobilization and electrical wiring of glucose converting enzymes, such as glucose oxidase and flavin adenine dinucleotide (FAD)-dependent glucose dehydrogenase. The cathodic side is constructed from a low-potential Os complex modified polymer integrating the O2 reducing enzyme, bilirubin oxidase. The large potential differences between the redox polymers and the prosthetic groups of the biocatalysts ensure fast and efficient charging of the biodevice. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cplu.201700114
  • Biofest: Bioinspired Chemistry, Biomaterials and Bioelectrochemistry
    De Cola, L. and Schuhmann, W.
    ChemPlusChem 82 (2017)
    Bio, bio, bio! This issue features the latest advances in bioinspired chemistry, biomaterials and bioelectrochemistry. Containing both original research and informative review articles, it is a must read for those committed to a multidisciplinary approach to these bio-orientated fields/topics. The idea behind this special issue was to showcase how a multidisciplinary approach can lead to important scientific insights in three separate bio-related themes at the interface between biology and chemistry
    view abstract10.1002/cplu.201700109
  • Catalytic Oxidation of Soot Spray-Coated Lithium Zirconate in a Plate Reactor
    Emmerich, T. and Lotz, K. and Sliozberg, K. and Schuhmann, W. and Muhler, M.
    Chemie-Ingenieur-Technik 89 (2017)
    A plate reactor was designed to investigate the catalytic soot oxidation applying glass ceramic plates coated with lithium zirconate. The results are compared to the corresponding powder catalysts in thermogravimetric experiments. The deposition of soot by spray coating resulted in an intimate contact mode equivalent to the mortaring preparation of the tight contact powder samples. In the presence of lithium ions the soot oxidation temperature was decreased significantly both in the thermobalance and in the plate reactor. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cite.201600118
  • Co3O4@Co/NCNT Nanostructure Derived from a Dicyanamide-Based Metal-Organic Framework as an Efficient Bi-functional Electrocatalyst for Oxygen Reduction and Evolution Reactions
    Sikdar, N. and Konkena, B. and Masa, J. and Schuhmann, W. and Maji, T.K.
    Chemistry - A European Journal 23 (2017)
    There has been growing interest in the synthesis of efficient reversible oxygen electrodes for both the oxygen reduction reaction (ORR) and the oxygen evolution reactions (OER), for their potential use in a variety of renewable energy technologies, such as regenerative fuel cells and metal-air batteries. Here, a bi-functional electrocatalyst, derived from a novel dicyanamide based nitrogen rich MOF {[Co(bpe)2(N(CN)2)]⋅(N(CN)2)⋅(5 H2O)}n [Co-MOF-1, bpe=1,2-bis(4-pyridyl)ethane, N(CN)2 −=dicyanamide] under different pyrolysis conditions is reported. Pyrolysis of the Co-MOF-1 under Ar atmosphere (at 800 °C) yielded a Co nanoparticle-embedded N-doped carbon nanotube matrix (Co/NCNT-Ar) while pyrolysis under a reductive H2/Ar atmosphere (at 800 °C) and further mild calcination yielded Co3O4@Co core–shell nanoparticle-encapsulated N-doped carbon nanotubes (Co3O4@Co/NCNT). Both catalysts show bi-functional activity towards ORR and OER, however, the core–shell Co3O4@Co/NCNT nanostructure exhibited superior electrocatalytic activity for both the ORR with a potential of 0.88 V at a current density of −1 mA cm−2 and the OER with a potential of 1.61 V at 10 mA cm−2, which is competitive with the most active bi-functional catalysts reported previously. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/chem.201704211
  • Cobalt boride modified with N-doped carbon nanotubes as a high-performance bifunctional oxygen electrocatalyst
    Elumeeva, K. and Masa, J. and Medina, D. and Ventosa, E. and Seisel, S. and Kayran, Y.U. and Genç, A. and Bobrowski, T. and Weide, P. and Arbiol, J. and Muhler, M. and Schuhmann, W.
    Journal of Materials Chemistry A 5 (2017)
    The development of reversible oxygen electrodes, able to drive both the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR), is still a great challenge. We describe a very efficient and stable bifunctional electrocatalytic system for reversible oxygen electrodes obtained by direct CVD growth of nitrogen-doped carbon nanotubes (NCNTs) on the surface of cobalt boride (CoB) nanoparticles. A detailed investigation of the crystalline structure and elemental distribution of CoB before and after NCNT growth reveals that the NCNTs grow on small CoB nanoparticles formed in the CVD process. The resultant CoB/NCNT system exhibited outstanding activity in catalyzing both the OER and the ORR in 0.1 M KOH with an overvoltage difference of only 0.73 V between the ORR at -1 mA cm-2 and the OER at +10 mA cm-2. The proposed CoB/NCNT catalyst showed stable performance during 50 h of OER stability assessment in 0.1 M KOH. Moreover, CoB/NCNT spray-coated on a gas diffusion layer as an air-breathing electrode proved its high durability during 170 galvanostatic charge-discharge (OER/ORR) test cycles (around 30 h) at ±10 mA cm-2 in 6 M KOH, making it an excellent bifunctional catalyst for potential Zn-air battery application. © 2017 The Royal Society of Chemistry.
    view abstract10.1039/c7ta06995b
  • Combinatorial screening of Pd-based quaternary electrocatalysts for oxygen reduction reaction in alkaline media
    Li, J. and Stein, H.S. and Sliozberg, K. and Liu, J. and Liu, Y. and Sertic, G. and Scanley, E. and Ludwig, Al. and Schroers, J. and Schuhmann, W. and Taylor, A.D.
    Journal of Materials Chemistry A 5 (2017)
    The implementation of electrochemical systems such as fuel cells has been hindered by the slow development of low cost high activity catalysts. Here we examine the oxygen reduction reaction performance of a combinatorial Pd-Au-Ag-Ti thin film library using high-throughput screening and correlate the electrochemical behavior to the crystallographic properties. We find compositions of ca. 40-60 at% Pd and 30-35 at% Au exhibit both a low overpotential of close to the value of pure Pt as well as high current density. We also observe a volcano-like relationship between the overpotential and the solid formation strain. This study provides compositional guidance towards the future synthesis of nanostructured quaternary Pd-Au-Ag-Ti alloys and suggests the potential for broader application of high-throughput electrochemical characterization by means of an automatic scanning droplet cell. © The Royal Society of Chemistry.
    view abstract10.1039/C6TA08088J
  • Combinatorial synthesis and high-throughput characterization of structural and photoelectrochemical properties of Fe:WO3 nanostructured libraries
    Khare, C. and Sliozberg, K. and Stepanovich, A. and Schuhmann, W. and Ludwig, Al.
    Nanotechnology 28 (2017)
    Porous and photoelectrochemically active Fe-doped WO3 nanostructures were obtained by a combinatorial dealloying method. Two types of precursor materials libraries, exhibiting dense and nano-columnar morphology were fabricated by using two distinct magnetron sputter deposition geometries. Both libraries were subjected to combinatorial dealloying enabling preparation and screening of a large quantity of compositions having different nanostructures. This approach allows identifying materials with interesting photoelectrochemical characteristics. The dealloying process selectively dissolved Fe from the composition gradient precursor W-Fe materials library, resulting in formation of monoclinic single crystalline nanoblade-like structures over the entire surface. Photoelectrochemical properties of nanostructured Fe:WO3 films were found to be composition-dependent. The measurement region doped with ∼1.7 at % Fe and a film thickness of ∼ 900-1100 nm displayed highly porous WO3 nanostructures and exhibited the highest photocurrent density of ∼ 72 μA cm-2. This enhanced photocurrent density is attributed to the decreased bandgap values, suppressed recombination of electron-hole pairs, improved light absorption as well as efficient charge transport in the highly porous Fe-doped film with single crystalline WO3 nanoblades. © 2017 IOP Publishing Ltd.
    view abstract10.1088/1361-6528/aa6964
  • Complete Prevention of Dendrite Formation in Zn Metal Anodes by Means of Pulsed Charging Protocols
    Garcia, G. and Ventosa, E. and Schuhmann, W.
    ACS Applied Materials and Interfaces 9 (2017)
    Zn metal as anode in rechargeable batteries, such as Zn/air or Zn/Ni, suffers from poor cyclability. The formation of Zn dendrites upon cycling is the key limiting step. We report a systematic study of the influence of pulsed electroplating protocols on the formation of Zn dendrites and in turn on strategies to completely prevent Zn dendrite formation. Because of the large number of variables in electroplating protocols, a scanning droplet cell technique was adapted as a high-throughput methodology in which a descriptor of the surface roughness can be in situ derived by means of electrochemical impedance spectroscopy. Upon optimizing the electroplating protocol by controlling nucleation, zincate ion depletion, and zincate ion diffusion, scanning electron microscopy and atomic force microscopy confirmed the growth of uniform and homogenous Zn deposits with a complete prevention of dendrite growth. The implementation of pulsed electroplating as the charging protocol for commercially available Ni-Zn batteries leads to substantially prolonged cyclability demonstrating the benefits of pulsed charging in Zn metal-based batteries. © 2017 American Chemical Society.
    view abstract10.1021/acsami.7b01705
  • Correlating Oxygen Evolution Catalysts Activity and Electronic Structure by a High-Throughput Investigation of Ni 1-y-z Fe y Cr z O x
    Schwanke, C. and Stein, H.S. and Xi, L. and Sliozberg, K. and Schuhmann, W. and Ludwig, Al. and Lange, K.M.
    Scientific Reports 7 (2017)
    High-throughput characterization by soft X-ray absorption spectroscopy (XAS) and electrochemical characterization is used to establish a correlation between electronic structure and catalytic activity of oxygen evolution reaction (OER) catalysts. As a model system a quasi-ternary materials library of Ni 1-y-zFe y Cr z O x was synthesized by combinatorial reactive magnetron sputtering, characterized by XAS, and an automated scanning droplet cell. The presence of Cr was found to increase the OER activity in the investigated compositional range. The electronic structure of Ni II and Cr III remains unchanged over the investigated composition spread. At the Fe L-edge a linear combination of two spectra was observed. These spectra were assigned to Fe III in O h symmetry and Fe III in T d symmetry. The ratio of Fe III O h to Fe III T d increases with the amount of Cr and a correlation between the presence of the Fe III O h and a high OER activity is found.
    view abstract10.1038/srep44192
  • From the Precursor to the Active State: Monitoring Metamorphosis of Electrocatalysts During Water Oxidation by In Situ Spectroscopy
    Hollmann, D. and Rockstroh, N. and Grabow, K. and Bentrup, U. and Rabeah, J. and Polyakov, M. and Surkus, A.-E. and Schuhmann, W. and Hoch, S. and Brückner, A.
    ChemElectroChem 4 (2017)
    In situ Raman and in situ EPR spectroscopy in combination with electrochemistry have been used to investigate the behavior of mixed cobalt nickel and cobalt copper oxides in the oxygen evolution reaction (OER). All experiments were carried out in homemade electrochemical cells using 0.1 M KOH as the electrolyte. The OER activities vary depending on the annealing conditions of the catalyst precursors, also reflected by different behaviours during the in situ spectroscopic experiments. The different activity of the Co/Ni oxides is most likely related to the formation of either γ- or β-NiO(OH), characterized by distinct features in the Raman spectra. Thus, a higher percentage of β-NiO(OH) is present in the more active catalyst. A different behaviour of Co/Cu catalysts has been shown by in situ Raman spectroscopy too, but the active phase could not be identified because of missing spectral features. However, in situ EPR spectroscopy revealed the partial dissolution of Cu(II), suggesting the formation of a Co-enriched oxide/hydroxide surface. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/celc.201700142
  • High-resolution analysis of photoanodes for water splitting by means of scanning photoelectrochemical microscopy
    Conzuelo, F. and Sliozberg, K. and Gutkowski, R. and Grutzke, S. and Nebe, M. and Schuhmann, W.
    Analytical Chemistry 89 (2017)
    In pursuance of efficient tools for the local analysis and characterization of novel photoelectrocatalytic materials, several SECM-based techniques have been developed, aiming on the combined benefit of a local irradiation of the analyzed sample and a microelectrode probe for the localized electrochemical analysis of the surface. We present the development and application of scanning photoelectrochemical microscopy (SPECM) for the laterally resolved characterization of photoelectrocatalytic materials. Particularly, the system was developed for the photoelectrochemical characterization of n-type semiconductor- based photoanodes for water splitting. By using the tip microelectrode simultaneously for local irradiation and as an electrochemical probe, SPECM was capable to simultaneously provide information about the local photocurrent generated at the sample under irradiation and to detect the photoelectrocatalytically evolved oxygen at the microelectrode. In combination with a novel means of irradiation of the interrogated sample, local analysis of semiconductor materials for light-induced water splitting with improved lateral resolution is achieved. © 2016 American Chemical Society.
    view abstract10.1021/acs.analchem.6b03706
  • In Operando Investigation of Electrical Coupling of Photosystem 1 and Photosystem 2 by Means of Bipolar Electrochemistry
    Eßmann, V. and Zhao, F. and Hartmann, V. and Nowaczyk, M.M. and Schuhmann, W. and Conzuelo, F.
    Analytical Chemistry 89 (2017)
    Electrochemical communication between two photobioelectrochemical half-cells based on photosystem 1 and photosystem 2 is investigated in operando. The driving force for the electron-transfer reactions is applied in a wireless mode using bipolar electrochemistry with the actual electrode potentials being self-regulated by the redox processes. Four parameters are assessed to understand the overall performance and elucidate the limiting reactions of the photobioelectrochemical cell. In addition to the potential differences for oxidation and reduction reactions, the current flowing between the half-cells as well as in situ collection of locally evolved O2 by photosystem 2 using a positioned scanning electrochemical microscopy tip are evaluated. In this way, changes in the enzymatic performances as a result of inactivation of either of the protein complexes or variations in the external conditions are monitored. © 2017 American Chemical Society.
    view abstract10.1021/acs.analchem.7b01222
  • Influence of Ni to Co ratio in mixed Co and Ni phosphides on their electrocatalytic oxygen evolution activity
    Barwe, S. and Andronescu, C. and Vasile, E. and Masa, J. and Schuhmann, W.
    Electrochemistry Communications 79 (2017)
    Prompted by the impact of Ni-based support materials on the intrinsic activity of electrocatalysts, we investigated the influence of partial Co substitution by Ni during the reductive thermal synthesis of cobalt-cobalt phosphide nanoparticles from triphenylphosphine complexes. The obtained catalysts were characterised by X-ray diffraction and electrochemistry. Increasing the amount of Ni in the precursor complexes leads to materials with lower overpotential for the OER at low current densities, and lower Tafel slopes. Co nanoparticles, which are only formed in materials with low Ni content, increase the intrinsic material conductivity and reduce the OER overpotential at high current densities. © 2017
    view abstract10.1016/j.elecom.2017.04.014
  • Interrogation of a PS1-Based Photocathode by Means of Scanning Photoelectrochemical Microscopy
    Zhao, F. and Plumeré, N. and Nowaczyk, M.M. and Ruff, A. and Schuhmann, W. and Conzuelo, F.
    Small 13 (2017)
    In the development of photosystem-based energy conversion devices, the in-depth understanding of electron transfer processes involved in photocurrent generation and possible charge recombination is essential as a basis for the development of photo-bioelectrochemical architectures with increased efficiency. The evaluation of a bio-photocathode based on photosystem 1 (PS1) integrated within a redox hydrogel by means of scanning photoelectrochemical microscopy (SPECM) is reported. The redox polymer acts as a conducting matrix for the transfer of electrons from the electrode surface to the photo-oxidized P700 centers within PS1, while methyl viologen is used as charge carrier for the collection of electrons at the reduced FB site of PS1. The analysis of the modified surfaces by SPECM enables the evaluation of electron-transfer processes by simultaneously monitoring photocurrent generation at the bio-photoelectrode and the associated generation of reduced charge carriers. The possibility to visualize charge recombination processes is illustrated by using two different electrode materials, namely Au and p-doped Si, exhibiting substantially different electron transfer kinetics for the reoxidation of the methyl viologen radical cation used as freely diffusing charge carrier. In the case of p-doped Si, a slower recombination kinetics allows visualization of methyl viologen radical cation concentration profiles from SPECM approach curves. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/smll.201604093
  • Measurements of lithium-ion concentration equilibration processes inside graphite electrodes
    Kindermann, F.M. and Osswald, P.J. and Klink, S. and Ehlert, G. and Schuster, J. and Noel, A. and Erhard, S.V. and Schuhmann, W. and Jossen, A.
    Journal of Power Sources 342 (2017)
    Methods for estimating inner states in a lithium-ion cell require steady state conditions or accurate models of the dynamic processes. Besides often used inner states such as state-of-charge, state-of-health or state-of-function, relaxation processes strongly influence the mentioned states. Inhomogeneous utilization of electrodes and consequent limitations in the operating conditions have recently been brought to attention. Relaxation measurements after an inhomogeneous current distribution through the thickness of an electrode have not been addressed so far. By using a previously developed laboratory cell, we are able to show an inhomogeneous retrieval of lithium-ions from a graphite electrode through the layer with spatial resolution. After this inhomogeneity caused by a constant current operation, equilibration processes are recorded and can be assigned to two different effects. One effect is an equilibration inside the particles (intra-particle) from surface to bulk and vice versa. The other effect is an assimilation between the particles (inter-particle) to reach a homogeneous state-of-charge in each particle throughout the electrode layer. While intra-particle relaxation is observed to be finished within 4 h, inter-particle relaxation through the layer takes more than 40 h. The overall time for both equilibration processes shows to be in the order of 48 h. © 2016 Elsevier B.V.
    view abstract10.1016/j.jpowsour.2016.12.093
  • Metallic NiPS3@NiOOH Core-Shell Heterostructures as Highly Efficient and Stable Electrocatalyst for the Oxygen Evolution Reaction
    Konkena, B. and Masa, J. and Botz, A. J. R. and Sinev, I. and Xia, W. and Kossmann, J. and Drautz, R. and Muhler, M. and Schuhmann, W.
    ACS Catalysis 7 (2017)
    We report metallic NiPS3@NiOOH core shell heterostructures as an efficient and durable electrocatalyst for the oxygen evolution reaction, exhibiting a low onset potential of 1.48 V (vs RHE) and stable performance for over 160 h. The atomically thin NiPS3 nanosheets are obtained by exfoliation of bulk NiPS3 in the presence of an ionic surfactant. The OER mechanism was studied by a combination of SECM, in situ Raman spectroscopy, SEM, and XPS measurements, which enabled direct observation of the formation of a NiPS3@NiOOH core shell heterostructure at the electrode interface. Hence, the active form of the catalyst is represented as NiPS3@NiOOH core shell structure. Moreover, DFT calculations indicate an intrinsic metallic character of the NiPS3 nanosheets with densities of states (DOS) similar to the bulk material. The high OER activity of the NiPS3 nanosheets is attributed to a high density of accessible active metallic-edge and defect sites due to structural disorder, a unique NiPS3@NiOOH core shell heterostructure, where the presence of P and S modulates the rface electronic structure of Ni in NiPS3, thus providing excellent conductive pathway for efficient electron-transport to the NiOOH shell. These findings suggest that good size control during liquid exfoliation may be advantageously used for the formation of electrically conductive NiPS3@ NiOOH core shell electrode materials for the electrochemical water oxidation.
    view abstract10.1021/acscatal.6b02203
  • Metal–Organic Framework Derived Carbon Nanotube Grafted Cobalt/Carbon Polyhedra Grown on Nickel Foam: An Efficient 3D Electrode for Full Water Splitting
    Aijaz, A. and Masa, J. and Rösler, C. and Xia, W. and Weide, P. and Fischer, R.A. and Schuhmann, W. and Muhler, M.
    ChemElectroChem 4 (2017)
    The growth of metal–organic framework (ZIF-67) nanocrystals on nickel foam (NF), followed by carbonization in diluted H2, leads to a nitrogen-doped carbon-nanotube-grafted cobalt/carbon polyhedra film on NF. The obtained material serves as a highly active binder-free electrocatalyst for both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER), enabling high-performance alkaline (0.1 m KOH) water electrolysis with potentials of 1.62 and 0.24 V, respectively, at OER and HER current densities of 10 mA cm−2. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/celc.201600452
  • Micrometer-Precise Determination of the Thin Electrolyte Layer of a Spectroelectrochemical Cell by Microelectrode Approach Curves
    Hiltrop, D. and Masa, J. and Botz, A.J.R. and Lindner, A. and Schuhmann, W. and Muhler, M.
    Analytical Chemistry 89 (2017)
    A spectroelectrochemical cell is presented that allows investigations of electrochemical reactions by means of attenuated total reflection infrared (ATR-IR) spectroscopy. The electrode holder for the working (WE), counter and reference electrode as mounted in the IR spectrometer cause the formation of a thin electrolyte layer between the internal reflection element (IRE) and the surface of the WE. The thickness of this thin electrolyte layer (dTL) was estimated by performing a scanning electrochemical microscopy-(SECM) like approach of a Pt microelectrode (ME), which was leveled with the WE toward the IRE surface. The precise lowering of the ME/WE plane toward the IRE was enabled by a micrometer screw. The approach curve was recorded in negative feedback mode of SECM and revealed the contact point of the ME and WE on the IRE, which was used as reference point to perform the electro-oxidation of ethanol over a drop-casted Pd/NCNT catalyst on the WE at different thin-layer thicknesses by cyclic voltammetry. The reaction products were detected in the liquid electrolyte by IR spectroscopy, and the effect of variations in dTL on the current densities and IR spectra were analyzed and discussed. The obtained data identify dTL as an important variable in thin-layer experiments with electrochemical reactions and FTIR readout. © 2017 American Chemical Society.
    view abstract10.1021/acs.analchem.6b03732
  • MOF-Templated Assembly Approach for Fe3C Nanoparticles Encapsulated in Bamboo-Like N-Doped CNTs: Highly Efficient Oxygen Reduction under Acidic and Basic Conditions
    Aijaz, A. and Masa, J. and Rösler, C. and Antoni, H. and Fischer, R.A. and Schuhmann, W. and Muhler, M.
    Chemistry - A European Journal (2017)
    Developing high-performance non-precious metal catalysts (NPMCs) for the oxygen-reduction reaction (ORR) is of critical importance for sustainable energy conversion. We report a novel NPMC consisting of iron carbide (Fe3C) nanoparticles encapsulated in N-doped bamboo-like carbon nanotubes (b-NCNTs), synthesized by a new metal-organic framework (MOF)-templated assembly approach. The electrocatalyst exhibits excellent ORR activity in 0.1m KOH (0.89V at -1mAcm-2) and in 0.5m H2SO4 (0.73V at -1mAcm-2) with a hydrogen peroxide yield of below 1% in both electrolytes. Due to encapsulation of the Fe3C nanoparticles inside porous b-NCNTs, the reported NPMC retains its high ORR activity after around 70hours in both alkaline and acidic media. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/chem.201701389
  • Nanoporous Nitrogen-Doped Graphene Oxide/Nickel Sulfide Composite Sheets Derived from a Metal-Organic Framework as an Efficient Electrocatalyst for Hydrogen and Oxygen Evolution
    Jayaramulu, K. and Masa, J. and Tomanec, O. and Peeters, D. and Ranc, V. and Schneemann, A. and Zboril, R. and Schuhmann, W. and Fischer, R.A.
    Advanced Functional Materials (2017)
    Engineering of controlled hybrid nanocomposites creates one of the most exciting applications in the fields of energy materials and environmental science. The rational design and in situ synthesis of hierarchical porous nanocomposite sheets of nitrogen-doped graphene oxide (NGO) and nickel sulfide (Ni7S6) derived from a hybrid of a well-known nickel-based metal-organic framework (NiMOF-74) using thiourea as a sulfur source are reported here. The nanoporous NGO/MOF composite is prepared through a solvothermal process in which Ni(II) metal centers of the MOF structure are chelated with nitrogen and oxygen functional groups of NGO. NGO/Ni7S6 exhibits bifunctional activity, capable of catalyzing both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) with excellent stability in alkaline electrolytes, due to its high surface area, high pore volume, and tailored reaction interface enabling the availability of active nickel sites, mass transport, and gas release. Depending on the nitrogen doping level, the properties of graphene oxide can be tuned toward, e.g., enhanced stability of the composite compared to commonly used RuO2 under OER conditions. Hence, this work opens the door for the development of effective OER/HER electrocatalysts based on hierarchical porous graphene oxide composites with metal chalcogenides, which may replace expensive commercial catalysts such as RuO2 and IrO2. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adfm.201700451
  • NH3 Post-Treatment Induces High Activity of Co-Based Electrocatalysts Supported on Carbon Nanotubes for the Oxygen Evolution Reaction
    Yang, F. and Xia, W. and Maljusch, A. and Masa, J. and Hollmann, D. and Sinev, I. and Cuenya, B.R. and Schuhmann, W. and Muhler, M.
    ChemElectroChem 4 (2017)
    Cobalt oxide nanoparticles were deposited on nitrogen-doped carbon nanotubes (NCNTs) through impregnation by using cobalt nitrate as a precursor and subsequent drying and calcination. Co loadings were prepared in the range from 4 to 40 wt%, and hydrogen and ammonia were applied in the thermal post-treatment of the CoOx/NCNT samples. The Co3O4 spinel structure was detected in all samples, while the thermal treatment in ammonia and hydrogen led to the formation of CoO and metallic Co in addition. Treatment in ammonia resulted in the partial reduction of Co3O4 to CoO and nitrogen doping of the oxides, leading to excellent electrocatalytic activity in the oxygen evolution reaction (OER) and stability despite of the lower Co oxidation states compared with the sample calcined in air. In contrast, the sample reduced in hydrogen showed a lower activity and stability in the OER. The high activity of the ammonia-treated sample can be assigned to improved conductivity, favorable surface properties with surface nitrogen improving the hydrophilicity of the catalysts, and the more facile transformation to the OER-active layered cobalt oxyhydroxide phase under anodic conditions. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/celc.201700109
  • On the pH Dependence of the Potential of Maximum Entropy of Ir(111) Electrodes
    Ganassin, A. and Sebastian, P. and Climent, V. and Schuhmann, W. and Bandarenka, A.S. and Feliu, J.
    Scientific Reports 7 (2017)
    Studies over the entropy of components forming the electrode/electrolyte interface can give fundamental insights into the properties of electrified interphases. In particular, the potential where the entropy of formation of the double layer is maximal (potential of maximum entropy, PME) is an important parameter for the characterization of electrochemical systems. Indeed, this parameter determines the majority of electrode processes. In this work, we determine PMEs for Ir(111) electrodes. The latter currently play an important role to understand electrocatalysis for energy provision; and at the same time, iridium is one of the most stable metals against corrosion. For the experiments, we used a combination of the laser induced potential transient to determine the PME, and CO charge-displacement to determine the potentials of zero total charge, (EPZTC). Both PME and EPZTC were assessed for perchlorate solutions in the pH range from 1 to 4. Surprisingly, we found that those are located in the potential region where the adsorption of hydrogen and hydroxyl species takes place, respectively. The PMEs demonstrated a shift by ∼30 mV per a pH unit (in the RHE scale). Connections between the PME and electrocatalytic properties of the electrode surface are discussed. © 2017 The Author(s).
    view abstract10.1038/s41598-017-01295-1
  • Opto-electrochemical In Situ Monitoring of the Cathodic Formation of Single Cobalt Nanoparticles
    Brasiliense, V. and Clausmeyer, J. and Dauphin, A.L. and Noël, J.-M. and Berto, P. and Tessier, G. and Schuhmann, W. and Kanoufi, F.
    Angewandte Chemie - International Edition 56 (2017)
    Single-particle electrochemistry at a nanoelectrode is explored by dark-field optical microscopy. The analysis of the scattered light allows in situ dynamic monitoring of the electrodeposition of single cobalt nanoparticles down to a radius of 65 nm. Larger sub-micrometer particles are directly sized optically by super-localization of the edges and the scattered light contains complementary information concerning the particle redox chemistry. This opto-electrochemical approach is used to derive mechanistic insights about electrocatalysis that are not accessible from single-particle electrochemistry. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/anie.201704394
  • Overcoming the Instability of Nanoparticle-Based Catalyst Films in Alkaline Electrolyzers by using Self-Assembling and Self-Healing Films
    Barwe, S. and Masa, J. and Andronescu, C. and Mei, B. and Schuhmann, W. and Ventosa, E.
    Angewandte Chemie - International Edition 56 (2017)
    Engineering stable electrodes using highly active catalyst nanopowders for electrochemical water splitting remains a challenge. We report an innovative and general approach for attaining highly stable catalyst films with self-healing capability based on the in situ self-assembly of catalyst particles during electrolysis. The catalyst particles are added to the electrolyte forming a suspension that is pumped through the electrolyzer. Particles with negatively charged surfaces stick onto the anode, while particles with positively charged surfaces stick to the cathode. The self-assembled catalyst films have self-healing properties as long as sufficient catalyst particles are present in the electrolyte. The proof-of-concept was demonstrated in a non-zero gap alkaline electrolyzer using NiFe-LDH and NixB catalyst nanopowders for anode and cathode, respectively. Steady cell voltages were maintained for at least three weeks during continuous electrolysis at 50–100 mA cm−2. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/anie.201703963
  • Poly(benzoxazine) as an Immobilization Matrix for Miniaturized ATP and Glucose Biosensors
    Ziller, C. and Lin, J. and Knittel, P. and Friedrich, L. and Andronescu, C. and Pöller, S. and Schuhmann, W. and Kranz, C.
    ChemElectroChem 4 (2017)
    The reproducible immobilization of enzymes represents a key requirement in developing sensitive and fast-responding amperometric microbiosensors. A microbiosensor for the respective detection of glucose and adenosine-5′-triphosphate (ATP) is presented by using a poly(benzoxazine) derivative for the entrapment of the enzymes glucose oxidase (GOD) and hexokinase (HEX) at platinum (Pt) microelectrodes (MEs). For glucose, a sensitivity of 123.05±10.78 pA/mM (n=5) was obtained, which shows twice as high sensitivity compared to microbiosensors that use electrophoretic paints as the immobilization matrix for the same size ME (radius: 25 μm). For the determination of ATP, a sensitivity of 48.47±5.12 pA/μM and a signal-to-noise ratio of 40 at physiological pH values were obtained. Apart from their enhanced sensitivity, a significant improvement of these sensors is related to their improved mechanical stability. The applicability of these poly(benzoxine)-based microbiosensors for ATP detection was demonstrated with measurements at receptor protein tyrosine phosphatase zeta (PTPRζ) osteoblastic cells during mechanical stimulation. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/celc.201600765
  • Polybenzoxazine-Derived N-doped Carbon as Matrix for Powder-Based Electrocatalysts
    Barwe, S. and Andronescu, C. and Masa, J. and Ventosa, E. and Klink, S. and Genç, A. and Arbiol, J. and Schuhmann, W.
    ChemSusChem 10 (2017)
    In addition to catalytic activity, intrinsic stability, tight immobilization on a suitable electrode surface, and sufficient electronic conductivity are fundamental prerequisites for the long-term operation of particle- and especially powder-based electrocatalysts. We present a novel approach to concurrently address these challenges by using the unique properties of polybenzoxazine (pBO) polymers, namely near-zero shrinkage and high residual-char yield even after pyrolysis at high temperatures. Pyrolysis of a nanocubic prussian blue analogue precursor (KmMnx[Co(CN)6]y⋅n H2O) embedded in a bisphenol A and aniline-based pBO led to the formation of a N-doped carbon matrix modified with MnxCoyOz nanocubes. The obtained electrocatalyst exhibits high efficiency toward the oxygen evolution reaction (OER) and more importantly a stable performance for at least 65 h. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/cssc.201700593
  • Powder Catalyst Fixation for Post-Electrolysis Structural Characterization of NiFe Layered Double Hydroxide Based Oxygen Evolution Reaction Electrocatalysts
    Andronescu, C. and Barwe, S. and Ventosa, E. and Masa, J. and Vasile, E. and Konkena, B. and Möller, S. and Schuhmann, W.
    Angewandte Chemie - International Edition 56 (2017)
    Highly active electrocatalysts for the oxygen evolution (OER) reaction are in most cases powder nanomaterials, which undergo substantial changes upon applying the high potentials required for high-current-density oxygen evolution. Owing to the vigorous gas evolution, the durability under OER conditions is disappointingly low for most powder electrocatalysts as there are no strategies to securely fix powder catalysts onto electrode surfaces. Thus reliable studies of catalysts during or after the OER are often impaired. Herein, we propose the use of composites made from precursors of polybenzoxazines and organophilically modified NiFe layered double hydroxides (LDHs) to form a stable and highly conducting catalyst layer, which allows the study of the catalyst before and after electrocatalysis. Characterization of the material by XRD, SEM, and TEM before and after 100 h electrolysis in 5 m KOH at 60 °C and a current density of 200 mA cm−2 revealed previously not observed structural changes. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/anie.201705385
  • Promotional Effect of Fe Impurities in Graphene Precursors on the Activity of MnOX/Graphene Electrocatalysts for the Oxygen Evolution and Oxygen Reduction Reactions
    Morales, D.M. and Masa, J. and Andronescu, C. and Schuhmann, W.
    ChemElectroChem 4 (2017)
    Bifunctional oxygen electrocatalysts were fabricated following a three-step synthesis method, which consisted of i) liquid-phase exfoliation of graphite in the presence of nitrogen-containing manganese macrocyclic complexes, using DMF as the dispersion medium under formation of few-layer graphene sheets. Subsequently, ii) solvent removal by vacuum filtration and drying, and iii) pyrolysis of the resulting composites under an inert gas atmosphere with subsequent mild calcination yielded manganese oxides embedded within a graphitic carbon matrix (MnOX/G). We further demonstrate that traces of Fe impurities in the used graphite result in enhanced electrocatalytic activity of the MnOX/G towards both the oxygen reduction and the oxygen evolution reactions, owing to synergistic interaction of the iron impurities with the species formed upon thermal decomposition of Mn macrocyclic complexes. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/celc.201700496
  • Protection and Reactivation of the [NiFeSe] Hydrogenase from Desulfovibrio vulgaris Hildenborough under Oxidative Conditions
    Ruff, A. and Szczesny, J. and Zacarias, S. and Pereira, I.A.C. and Plumeré, N. and Schuhmann, W.
    ACS Energy Letters 2 (2017)
    We report on the fabrication of bioanodes for H2 oxidation based on [NiFeSe] hydrogenase. The enzyme was electrically wired by means of a specifically designed low-potential viologen-modified polymer, which delivers benchmark H2 oxidizing currents even under deactivating conditions owing to efficient protection against O2 combined with a viologen-induced reactivation of the O2 inhibited enzyme. Moreover, the viologen-modified polymer allows for electrochemical co-deposition of polymer and biocatalyst and, by this, for control of the film thickness. Protection and reactivation of the enzyme was demonstrated in thick and thin reaction layers. © 2017 American Chemical Society.
    view abstract10.1021/acsenergylett.7b00167
  • Prussian Blue Analogues: A Versatile Framework for Solid-Contact Ion-Selective Electrodes with Tunable Potentials
    Klink, S. and Ishige, Y. and Schuhmann, W.
    ChemElectroChem 4 (2017)
    The development of solid-contact ion-selective electrodes (SC-ISEs) (e.g. for point-of-care sensors) requires simple inner reference electrodes (iREs) with predictable and reproducible potentials. Intercalation compounds fulfill these requirements, as they respond to target ions present in the ion-selective membrane. Their applicability, however, is limited by the availability of intercalation frameworks capable to intercalate the target ion of interest. We report that Prussian Blue analogues (PBAs) can serve as versatile iREs for a range of target ions of clinical interest, such as Na+, K+, or Ca2+. Combining target-ion intercalated PBAs with ion-selective membranes results in a family of all-solid SC-ISEs, which are capable as ISEs with an inner filling, yet cheap and suitable for mass-production. The SC-ISEs′ standard potential is predictable and can be tuned by altering the PBAs′ redox-active transition metal or by changing its state of charge. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/celc.201700091
  • Revealing the electronic character of the positive electrode/electrolyte interface in lithium-ion batteries
    Zampardi, G. and Trocoli, R. and Schuhmann, W. and La Mantia, F.
    Physical Chemistry Chemical Physics 19 (2017)
    High voltage operating active materials are among the most promising components for positive electrodes of future high energy lithium-ion batteries. However, the operating potential range of such materials often exceeds anodically the thermodynamic stability window of the electrolyte. A surface layer is therefore formed, which is supposed to be one of the reasons for the high irreversible charge loss of these electrodes. The electronic character of such a surface layer formed at the electrode/electrolyte interface of LiNi0.5Mn1.5O4 (LNM), stoichiometric (x = 0) and overlithiated (x = 0.1) Li1+x(Ni1/3Mn1/3Co1/3)1-xO2 (NMC) based paste electrodes was investigated in situ using feedback-mode scanning electrochemical microscopy (SECM). The role in the formation of an electronically insulating layer of a conductive carbon additive-based electrode and of the Al current collector was explored as well. The surface layers formed on all oxide based paste electrodes and on conductive carbon additive based electrodes showed unexpectedly an electronic conducting behavior, while the Al current collector formed an electronically insulating layer which was found to be influenced by the electrolyte. © the Owner Societies 2017.
    view abstract10.1039/c7cp05453j
  • Solar biosupercapacitor
    González-Arribas, E. and Aleksejeva, O. and Bobrowski, T. and Toscano, M.D. and Gorton, L. and Schuhmann, W. and Shleev, S.
    Electrochemistry Communications 74 (2017)
    Here we report on an entirely new kind of bioelectronic device – a solar biosupercapacitor, which is built from a dual-feature photobioanode combined with a double-function enzymatic cathode. The self-charging biodevice, based on transparent nanostructured indium tin oxide electrodes modified with biological catalysts, i.e. thylakoid membranes and bilirubin oxidase, is able to capacitively store electricity produced by direct conversion of radiant energy into electric energy. When self-charged during 10 min, using ambient light only, the biosupercapacitor provided a maximum of 6 mW m− 2 at 0.20 V. © 2016 Elsevier B.V.
    view abstract10.1016/j.elecom.2016.11.009
  • Solid electrolyte interphase (SEI) at TiO2 electrodes in li-ion batteries: Defining apparent and effective SEI based on evidence from X-ay photoemission spectroscopy and scanning electrochemical microscopy
    Ventosa, E. and Madej, E. and Zampardi, G. and Mei, B. and Weide, P. and Antoni, H. and La Mantia, F. and Muhler, M. and Schuhmann, W.
    ACS Applied Materials and Interfaces 9 (2017)
    The high (de)lithiation potential of TiO2 (ca. 1.7 V vs Li/ Li+ in 1 M Li+) decreases the voltage and, thus, the energy density of a corresponding Li-ion battery. On the other hand, it offers several advantages such as the (de)lithiation potential far from lithium deposition or absence of a solid electrolyte interphase (SEI). The latter is currently under controversial debate as several studies reported the presence of a SEI when operating TiO2 electrodes at potentials above 1.0 V vs Li/Li+. We investigate the formation of a SEI at anatase TiO2 electrodes by means of X-ray photoemission spectroscopy (XPS) and scanning electrochemical microscopy (SECM). The investigations were performed in different potential ranges, namely, during storage (without external polarization), between 3.0-2.0 V and 3.0-1.0 V vs Li/Li+, respectively. No SEI is formed when a completely dried and residues-free TiO2 electrode is cycled between 3.0 and 2.0 V vs Li/Li+. A SEI is detected by XPS in the case of samples stored for 6 weeks or cycled between 3.0 and 1.0 V vs Li/Li+. With use of SECM, it is verified that this SEI does not possess the electrically insulating character as expected for a "classic" SEI. Therefore, we propose the term apparent SEI for TiO2 electrodes to differentiate it from the protecting and ef fective SEI formed at graphite electrodes. © 2016 American Chemical Society.
    view abstract10.1021/acsami.6b13306
  • Synergistic Effect of Cobalt and Iron in Layered Double Hydroxide Catalysts for the Oxygen Evolution Reaction
    Yang, F. and Sliozberg, K. and Sinev, I. and Antoni, H. and Bähr, A. and Ollegott, K. and Xia, W. and Masa, J. and Grünert, W. and Cuenya, B.R. and Schuhmann, W. and Muhler, M.
    ChemSusChem 10 (2017)
    Co-based layered double hydroxide (LDH) catalysts with Fe and Al contents in the range of 15 to 45 at % were synthesized by an efficient coprecipitation method. In these catalysts, Fe3+ or Al3+ ions play an essential role as trivalent species to stabilize the LDH structure. The obtained catalysts were characterized by a comprehensive combination of surface- and bulk-sensitive techniques and were evaluated for the oxygen evolution reaction (OER) on rotating disk electrodes. The OER activity decreased upon increasing the Al content for the Co- and Al-based LDH catalysts, whereas a synergistic effect in Co- and Fe-based LDHs was observed, which resulted in an optimal Fe content of 35 at %. This catalyst was spray-coated on Ni foam electrodes and showed very good stability in a flow-through cell with a potential of approximately 1.53 V at 10 mA cm−2 in 1 m KOH for at least 48 h. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/cssc.201601272
  • Topotactic Synthesis of Porous Cobalt Ferrite Platelets from a Layered Double Hydroxide Precursor and Their Application in Oxidation Catalysis
    Ortega, K.F. and Anke, S. and Salamon, S. and Özcan, F. and Heese, J. and Andronescu, C. and Landers, J. and Wende, H. and Schuhmann, W. and Muhler, M. and Lunkenbein, T. and Behrens, M.
    Chemistry - A European Journal (2017)
    Monocrystalline, yet porous mosaic platelets of cobalt ferrite, CoFe2O4, can be synthesized from a layered double hydroxide (LDH) precursor by thermal decomposition. Using an equimolar mixture of Fe2+, Co2+, and Fe3+ during co-precipitation, a mixture of LDH, (FeIICoII)2/3FeIII 1/3(OH)2(CO3)1/6mH2O, and the target spinel CoFe2O4 can be obtained in the precursor. During calcination, the remaining FeII fraction of the LDH is oxidized to FeIII leading to an overall Co2+:Fe3+ ratio of 1:2 as required for spinel crystallization. This pre-adjustment of the spinel composition in the LDH precursor suggests a topotactic crystallization of cobalt ferrite and yields phase pure spinel in unusual anisotropic platelet morphology. The preferred topotactic relationship in most particles is [111]Spinel∥[001]LDH. Due to the anion decomposition, holes are formed throughout the quasi monocrystalline platelets. This synthesis approach can be used for different ferrites and the unique microstructure leads to unusual chemical properties as shown by the application of the ex-LDH cobalt ferrite as catalyst in the selective oxidation of 2-propanol. Compared to commercial cobalt ferrite, which mainly catalyzes the oxidative dehydrogenation to acetone, the main reaction over the novel ex-LDH cobalt is dehydration to propene. Moreover, the oxygen evolution reaction (OER) activity of the ex-LDH catalyst was markedly higher compared to the commercial material. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/chem.201702248
  • Transparent, mediator- and membrane-free enzymatic fuel cell based on nanostructured chemically modified indium tin oxide electrodes
    González-Arribas, E. and Bobrowski, T. and Di Bari, C. and Sliozberg, K. and Ludwig, R. and Toscano, M.D. and De Lacey, A.L. and Pita, M. and Schuhmann, W. and Shleev, S.
    Biosensors and Bioelectronics 97 (2017)
    We detail a mediator- and membrane-free enzymatic glucose/oxygen biofuel cell based on transparent and nanostructured conducting supports. Chemically modified indium tin oxide nanoparticle modified electrodes were used to substantially increase the active surface area without significantly compromising transparency. Two different procedures for surface nanostructuring were employed, viz. spray-coating and drop-coating. The spray-coated biodevice showed superior characteristics as compared to the drop-coated enzymatic fuel cell, as a result of the higher nanostructured surface area as confirmed by electrochemical characterisation, as well as scanning electron and atomic force microscopy. Subsequent chemical modification with silanes, followed by the immobilisation of either cellobiose dehydrogenase from Corynascus thermophiles or bilirubin oxidase from Myrothecium verrucaria, were performed to obtain the bioanodes and biocathodes, respectively. The optimised biodevice exhibited an OCV of 0.67 V and power output of up to 1.4 µW/cm2 at an operating voltage of 0.35 V. This is considered a significant step forward in the field of glucose/oxygen membrane- and mediator-free, transparent enzymatic fuel cells. © 2017 Elsevier B.V.
    view abstract10.1016/j.bios.2017.05.040
  • Tuning the oxidation state of manganese oxide nanoparticles on oxygen- and nitrogen-functionalized carbon nanotubes for the electrocatalytic oxygen evolution reaction
    Antoni, H. and Xia, W. and Masa, J. and Schuhmann, W. and Muhler, M.
    Physical Chemistry Chemical Physics 19 (2017)
    Manganese oxides are promising electrocatalysts for the oxygen evolution reaction due to their versatile redox properties. Manganese oxide (MnOx) nanoparticles were synthesized on oxygen- and nitrogen-functionalized carbon nanotubes (OCNTs and NCNTs) by calcination in air of Mn-impregnated CNTs with a loading of 10 wt% Mn. The calcined samples were exposed to reducing conditions by thermal treatment in H2 or NH3, and to strongly oxidizing conditions using HNO3 vapor, which enabled us to flexibly tune the oxidation state of Mn from 2+ in MnO to 4+ in MnO2. The samples were characterized by X-ray photoelectron spectroscopy, X-ray diffraction, transmission electron microscopy and temperature-programmed reduction. The oxidation state of Mn was more easily changed in the MnOx/NCNTs samples compared with the MnOx/OCNTs samples. Furthermore, the reduction of MnO2 to MnO occurred in one-step on NCNTs, whereas Mn2O3 intermediate states were observed for OCNTs. STEM and TEM images revealed a smaller and uniform dispersion of the MnOx nanoparticles on NCNTs as compared to OCNTs. Electrocatalytic oxygen evolution tests in 0.1 M KOH showed that Mn in high oxidation states, specifically 4+ as in MnO2 generated by HNO3 vapor treatment, is more active than Mn in lower oxidation states, using the potential at 10 mA cm-2 and the Tafel slopes as the performance metrics. © the Owner Societies 2017.
    view abstract10.1039/c7cp02717f
  • Ultrathin High Surface Area Nickel Boride (NixB) Nanosheets as Highly Efficient Electrocatalyst for Oxygen Evolution
    Masa, J. and Sinev, I. and Mistry, H. and Ventosa, E. and de la Mata, M. and Arbiol, J. and Muhler, M. and Roldan Cuenya, B. and Schuhmann, W.
    Advanced Energy Materials (2017)
    The overriding obstacle to mass production of hydrogen from water as the premium fuel for powering our planet is the frustratingly slow kinetics of the oxygen evolution reaction (OER). Additionally, inadequate understanding of the key barriers of the OER is a hindrance to insightful design of advanced OER catalysts. This study presents ultrathin amorphous high-surface area nickel boride (NixB) nanosheets as a low-cost, very efficient and stable catalyst for the OER for electrochemical water splitting. The catalyst affords 10 mA cm-2 at 0.38 V overpotential during OER in 1.0 m KOH, reducing to only 0.28 V at 20 mA cm-2 when supported on nickel foam, which ranks it among the best reported nonprecious catalysts for oxygen evolution. Operando X-ray absorption fine-structure spectroscopy measurements reveal prevalence of NiOOH, as well as Ni-B under OER conditions, owing to a Ni-B core at nickel oxyhydroxide shell (Ni-B at NiOxH) structure, and increase in disorder of the NiOxH layer, thus revealing important insight into the transient states of the catalyst during oxygen evolution. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/aenm.201700381
  • Unraveling compositional effects on the light-induced oxygen evolution in Bi(V-Mo-X)O4 material libraries
    Gutkowski, R. and Khare, C. and Conzuelo, F. and Kayran, Y.U. and Ludwig, Al. and Schuhmann, W.
    Energy and Environmental Science 10 (2017)
    The influence of co-deposited transition metals X (X = Ta, W, Nb) with various relative concentrations on the photoelectrochemical performance of BiVO4 is investigated. Thin film material libraries with well-defined composition gradients of Bi, V and two transition metals are fabricated by combinatorial sputter co-deposition. Materials with the highest photoelectrochemical performance are identified by high-throughput characterization of the Bi(V-Mo-X)O4 material libraries using an optical scanning droplet cell. Bi(V-Mo-W)O4 and Bi(V-Mo-Nb)O4 material libraries show the highest improvement in the photocurrent, with ten times higher photocurrents of up to 1 mA cm-2 compared to a BiVO4 reference material library. Deviations from the V:Bi equiatomic ratio lead to a decrease in the photocurrent for pristine monoclinic BiVO4. By the addition of transition metals this effect is minimized and no significant decrease in the photocurrent occurs up to 10 at% variation from the equiatomic V:Bi ratio. Excellent photoelectrochemical performance is reached under these conditions in regions with a V:Bi atomic ratio of 70:30 and co-deposited Nb concentrations of &gt;10 at%. Scanning photoelectrochemical microscopy allows the evaluation of the correlation between the generated oxygen at a photoanode and the measured photocurrent. © 2017 The Royal Society of Chemistry.
    view abstract10.1039/c7ee00287d
  • Wireless light-emitting electrochemical rotors
    Eßmann, V. and Voci, S. and Loget, G. and Sojic, N. and Schuhmann, W. and Kuhn, A.
    Journal of Physical Chemistry Letters 8 (2017)
    Bipolar electrochemistry has been shown to enable and control various kinds of propulsion of nonwired conducting objects: translation, rotation, and levitation. There is a very rapid development in the field of controlled motion combined with other functionalities. Here we integrate two different concepts in one system to generate wireless electrochemical motion of a specifically designed rotor and track its polarization simultaneously by electrochemical light emission. Locally produced hydrogen bubbles at the cathodic pole of the bipolar rotor are the driving force of the motion, whereas [Ru(bpy)3]Cl2 and tripropylamine react at the anodic extremity, thus generating an electrochemiluminescence signal with an intensity directly correlated with the orientation of the rotor arms. This allows in a straightforward way the qualitative visualization of the changing interfacial potential differences during rotation and shows for the first time that light emission can be coupled to autonomously rotating bipolar electrodes. © 2017 American Chemical Society.
    view abstract10.1021/acs.jpclett.7b01899
  • A Nernstian Biosupercapacitor
    Pankratov, D. and Conzuelo, F. and Pinyou, P. and Alsaoub, S. and Schuhmann, W. and Shleev, S.
    Angewandte Chemie - International Edition 55 (2016)
    We propose the very first “Nernstian biosupercapacitor”, a biodevice based on only one redox polymer: poly(vinyl imidazole-co-allylamine)[Os(bpy)2Cl], and two biocatalysts. At the bioanode PQQ-dependent glucose dehydrogenase reduces the Os3+ moieties at the polymer to Os2+ shifting the Nernst potential of the Os3+/Os2+ redox couple to negative values. Concomitantly, at the biocathode the reduction of O2 by means of bilirubin oxidase embedded in the same redox polymer leads to the oxidation of Os2+ to Os3+ shifting the Nernst potential to higher values. Despite the use of just one redox polymer an open circuit voltage of more than 0.45 V was obtained during charging and the charge is stored in the redox polymer at both the bioanode and the biocathode. By connecting both electrodes via a predefined resistor a high power density is obtained for a short time exceeding the steady state power of a corresponding biofuel cell by a factor of 8. © 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/anie.201607144
  • A Simple Approach towards High-Performance Perovskite-Based Bifunctional Oxygen Electrocatalysts
    Elumeeva, K. and Masa, J. and Tietz, F. and Yang, F. and Xia, W. and Muhler, M. and Schuhmann, W.
    ChemElectroChem 3 (2016)
    To accelerate the large-scale commercialization of electrochemical energy storage and conversion technologies through water splitting and regeneration in reversible fuel cells, cost-effective, highly efficient, and durable reversible oxygen electrodes are required. We report a comparatively simple approach to modify a group of oxygen-evolving perovskites based on lanthanum cobaltite into effective bifunctional systems through partial atom substitution, which, upon intermixing with nitrogen-doped carbon nanotubes, achieve remarkably low round-trip overvoltage of <850mV in the electrocatalysis of oxygen reduction and oxygen evolution in an alkaline electrolyte, KOH (0.1m). Besides the bifunctional electrocatalytic performance, the composite systems with a low Fe content possessed promising long-term stability. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/celc.201500353
  • A Three-Electrode, Battery-Type Swagelok Cell for the Evaluation of Secondary Alkaline Batteries: The Case of the Ni-Zn Battery
    Garcia, G. and Schuhmann, W. and Ventosa, E.
    ChemElectroChem 3 (2016)
    Three-electrode cells are essential in understanding battery materials under operando conditions. A three-electrode, battery-type Swagelok cell for electrochemical studies of secondary alkaline batteries, in particular Ni-Zn batteries, is presented. The relevance of the three-electrode battery-type cell (i.e. sealed and non-flooded) configuration is demonstrated as analytical tool with three observations: 1)The Ni electrode is shown to limit the system in the first cycles, while the Zn electrode becomes limiting in subsequent cycles. 2)Non-woven separators (NWSs) clearly improve the performance of the battery. Besides the known fact of hindering the dendritic growth of Zn, NWSs inhibit the evolution of oxygen and hydrogen at the positive and negative electrodes. 3)The kinetics of the Ni electrode is much slower than that of the Zn electrode, as derived from the charge-transfer resistance of the Ni electrode, which is substantially larger than that of the Zn electrode. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/celc.201500474
  • Advanced Evaluation of the Long-Term Stability of Oxygen Evolution Electrocatalysts
    Maljusch, A. and Conradi, O. and Hoch, S. and Blug, M. and Schuhmann, W.
    Analytical Chemistry 88 (2016)
    Evaluation of the long-term stability of electrocatalysts is typically performed using galvanostatic polarization at a predefined current density. A stable or insignificant increase in the applied potential is usually interpreted as high long-term stability of the tested catalyst. However, effects such as (i) electrochemical degradation of a catalyst due to its oxidation, (ii) blocking of the catalyst surface by evolved gas bubbles, and (iii) detachment of the catalyst from the electrode surface may lead to a decrease of the catalyst's active surface area being exposed to the electrolyte. In order to separate these effects and to evaluate the true electrochemical degradation of electrocatalysts, an advanced evaluation protocol based on subsequently performed electrochemical impedance, double layer capacitance, cyclic voltammetry, and galvanostatic polarization measurements was developed and used to evaluate the degradation of IrO2 particles drop-coated on glassy carbon rotating disk electrode using Nafion as a binder. A flow-through electrochemical cell was developed enabling circulation of the electrolyte leading to an efficient removal of evolved oxygen bubbles even at high current densities of up to 250 mA/cm2. The degradation rate of IrO2 was evaluated over 225 test cycles (0.733 ± 0.022 mV/h) with a total duration of galvanostatic polarization measurements of over 55 h. © 2016 American Chemical Society.
    view abstract10.1021/acs.analchem.6b01289
  • Amorphous Cobalt Boride (Co2B) as a Highly Efficient Nonprecious Catalyst for Electrochemical Water Splitting: Oxygen and Hydrogen Evolution
    Masa, J. and Weide, P. and Peeters, D. and Sinev, I. and Xia, W. and Sun, Z. Y. and Somsen, C. and Muhler, M. and Schuhmann, W.
    Advanced Energy Materials 6 (2016)
    It is demonstrated that amorphous cobalt boride (Co2B) prepared by the chemical reduction of CoCl2 using NaBH4 is an exceptionally efficient electrocatalyst for the oxygen evolution reaction (OER) in alkaline electrolytes and is simultaneously active for catalyzing the hydrogen evolution reaction (HER). The catalyst achieves a current density of 10 mA cm(-2) at 1.61 V on an inert support and at 1.59 V when impregnated with nitrogen-doped graphene. Stable performance is maintained at 10 mA cm(-2) for at least 60 h. The optimized catalyst, Co2B annealed at 500 degrees C (Co2B-500) evolves oxygen more efficiently than RuO2 and IrO2, and its performance matches the best cobalt-based catalysts reported to date. Co2B is irreversibly oxidized at OER conditions to form a CoOOH surface layer. The active form of the catalyst is therefore represented as CoOOH/Co2B. EXAFS observations indicate that boron induces lattice strain in the crystal structure of the metal, which potentially diminishes the thermodynamic and kinetic barrier of the hydroxylation reaction, formation of the OOH* intermediate, a key limiting step in the OER.
    view abstract10.1002/aenm.201502313
  • Application of scanning electrochemical microscopy (SECM) to study electrocatalysis of oxygen reduction by MN4-macrocyclic complexes
    Masa, J. and Ventosa, E. and Schuhmann, W.
    Electrochemistry of N4 Macrocyclic Metal Complexes: Volume 1: Energy, Second Edition (2016)
    N4-macrocyclic complexes are among the most widely investigated molecular materials for the oxygen reduction reaction (ORR). These complexes are attractive because they inherently provide well-defined structural models for describing the ORR not only in nature, for example, in cytochrome c oxidases, but also for electrocatalysts of industrial importance. The development of more efficient N4-macrocyclic complexes as electrocatalysts for the ORR requires in-depth understanding of the most crucial properties that govern their functionality. This goal necessitates employing advanced techniques and methods to accurately probe electrocatalytic behavior. This chapter covers a brief introduction of scanning electrochemical microscopy (SECM) and discusses its application for evaluation of the electrocatalytic behavior of materials, with particular focus on the ORR. A general overview of the benefits of using SECM as an alternative or as a complimentary technique to rotating-ring disk electrode (RRDE) voltammetry in studying the kinetics of the ORR is provided, with examples of this application dedicated to catalysts derived from MN4-macrocyclic complexes. The chapter also covers examples of the application of SECM as a semi-combinatorial and high-throughput tool for catalyst screening and development, and the evaluation of electrocatalysts at temperatures of industrial relevance. Discussion of some recent developments of the application of SECM, or SECM coupled to other auxiliary techniques, in electrocatalysis, for example, in probing electrolysis of individual nanoparticles, and a forecast of its potential future applications in both fundamental and applied science are included at the end of this chapter. © Springer International Publishing Switzerland 2016.
    view abstract10.1007/978-3-319-31172-2_4
  • Benchmarking the Performance of Thin-Film Oxide Electrocatalysts for Gas Evolution Reactions at High Current Densities
    Ganassin, A. and Maljusch, A. and Colic, V. and Spanier, L. and Brandl, K. and Schuhmann, W. and Bandarenka, A.
    ACS Catalysis 6 (2016)
    Oxide materials are among the state-of-the-art heterogeneous electrocatalysts for many important large-scale industrial processes, including O2 and Cl2 evolution reactions. However, benchmarking their performance is challenging in many cases, especially at high current densities, which are relevant for industrial applications. Serious complications arise particularly due to (i) the formation of a nonconducting gas phase which blocks the surface during the reactions, (ii) problems in determination of the real electroactive electrode area, and (iii) the large influence of surface morphology alterations (stability issues) under reaction conditions, among others. In this work, an approach overcoming many of these challenges is presented, with a focus on electrochemically formed thin-film oxide electrocatalysts. The approach is based on benefits provided by the use of microelectrodes, and it gives comprehensive information about the surface roughness, catalyst activity, and stability. The key advantages of the proposed method are the possibility of characterization of the whole microelectrode surface by means of atomic force microscopy and an accurate assessment of the specific activity (and subsequently stability) of the catalyst, even at very high current densities. Electrochemically deposited CoOx thin films have been used in this study as model catalysts. © 2016 American Chemical Society.
    view abstract10.1021/acscatal.6b00455
  • Bipolar Electrochemistry for Concurrently Evaluating the Stability of Anode and Cathode Electrocatalysts and the Overall Cell Performance during Long-Term Water Electrolysis
    Eßmann, V. and Barwe, S. and Masa, J. and Schuhmann, W.
    Analytical Chemistry 88 (2016)
    Electrochemical efficiency and stability are among the most important characteristics of electrocatalysts. These parameters are usually evaluated separately for the anodic and cathodic half-cell reactions in a three-electrode system or by measuring the overall cell voltage between the anode and cathode as a function of current or time. Here, we demonstrate how bipolar electrochemistry can be exploited to evaluate the efficiency of electrocatalysts for full electrochemical water splitting while simultaneously and independently monitoring the individual performance and stability of the half-cell electrocatalysts. Using a closed bipolar electrochemistry setup, all important parameters such as overvoltage, half-cell potential, and catalyst stability can be derived from a single galvanostatic experiment. In the proposed experiment, none of the half-reactions is limiting on the other, making it possible to precisely monitor the contribution of the individual half-cell reactions on the durability of the cell performance. The proposed approach was successfully employed to investigate the long-term performance of a bifunctional water splitting catalyst, specifically amorphous cobalt boride (Co2B), and the durability of the electrocatalyst at the anode and cathode during water electrolysis. Additionally, by periodically alternating the polarization applied to the bipolar electrode (BE) modified with a bifunctional oxygen electrocatalyst, it was possible to explicitly follow the contributions of the oxygen reduction (ORR) and the oxygen evolution (OER) half-reactions on the overall long-term durability of the bifunctional OER/ORR electrocatalyst. © 2016 American Chemical Society.
    view abstract10.1021/acs.analchem.6b02393
  • Characterisation of bifunctional electrocatalysts for oxygen reduction and evolution by means of SECM
    Chen, X. and Botz, A.J.R. and Masa, J. and Schuhmann, W.
    Journal of Solid State Electrochemistry 20 (2016)
    Electrocatalysts that can reversibly reduce oxygen and oxidise water are of prime importance for the advancement of new emerging electrochemical energy storage and conversion systems. We present in this work the application of scanning electrochemical microscopy (SECM) for characterisation of bifunctional catalysts. By using model bifunctional catalysts based on oxides of cobalt (CoxOy) and nickel (NixOy) embedded in nitrogen-doped carbon (NC), we specifically show the unique ability of using SECM to determine a range of the important electrocatalytic parameters including the selectivity of the oxygen reduction reaction (ORR), the initial mechanistic steps during the oxygen evolution reaction (OER), and the onset potential for both ORR and OER in a single experiment. We were able to observe directly that prior to oxygen evolution, local depletion of oxygen occurs at the SECM tip during redox transition accompanying most likely metal oxyhydroxide formation thus enabling direct in situ observation of the initial mechanistic steps of the OER. © 2015, Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s10008-015-3028-z
  • Co@Co3O4 Encapsulated in Carbon Nanotube-Grafted Nitrogen-Doped Carbon Polyhedra as an Advanced Bifunctional Oxygen Electrode
    Aijaz, A. and Masa, J. and Rösler, C. and Xia, W. and Weide, P. and Botz, A.J.R. and Fischer, R.A. and Schuhmann, W. and Muhler, M.
    Angewandte Chemie - International Edition 55 (2016)
    Efficient reversible oxygen electrodes for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are vitally important for various energy conversion devices, such as regenerative fuel cells and metal-air batteries. However, realization of such electrodes is impeded by insufficient activity and instability of electrocatalysts for both water splitting and oxygen reduction. We report highly active bifunctional electrocatalysts for oxygen electrodes comprising core-shell Co@Co3O4 nanoparticles embedded in CNT-grafted N-doped carbon-polyhedra obtained by the pyrolysis of cobalt metal-organic framework (ZIF-67) in a reductive H2 atmosphere and subsequent controlled oxidative calcination. The catalysts afford 0.85 V reversible overvoltage in 0.1 m KOH, surpassing Pt/C, IrO2, and RuO2 and thus ranking them among one of the best non-precious-metal electrocatalysts for reversible oxygen electrodes. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/anie.201509382
  • Demonstrating the steady performance of iron oxide composites over 2000 cycles at fast charge-rates for Li-ion batteries
    Sun, Z. and Madej, E. and Genç, A. and Muhler, M. and Arbiol, J. and Schuhmann, W. and Ventosa, E.
    Chemical Communications 52 (2016)
    The feasibility of using iron oxides as negative electrode materials for safe high-power Li-ion batteries is demonstrated by the carbon-coated FeOx/CNT composite synthesized by controlled pyrolysis of ferrocene, which delivered a specific capacity retention of 84% (445 mA h g-1) after 2000 cycles at 2000 mA g-1 (4C). © 2016 The Royal Society of Chemistry.
    view abstract10.1039/c6cc00168h
  • Design of an Os Complex-Modified Hydrogel with Optimized Redox Potential for Biosensors and Biofuel Cells
    Pinyou, P. and Ruff, A. and Pöller, S. and Ma, S. and Ludwig, R. and Schuhmann, W.
    Chemistry - A European Journal 22 (2016)
    Multistep synthesis and electrochemical characterization of an Os complex-modified redox hydrogel exhibiting a redox potential ≈+30 mV (vs. Ag/AgCl 3 m KCl) is demonstrated. The careful selection of bipyridine-based ligands bearing N,N-dimethylamino moieties and an amino-linker for the covalent attachment to the polymer backbone ensures the formation of a stable redox polymer with an envisaged redox potential close to 0 V. Most importantly, the formation of an octahedral N6-coordination sphere around the Os central atoms provides improved stability concomitantly with the low formal potential, a low reorganization energy during the Os3+/2+ redox conversion and a negligible impact on oxygen reduction. By wiring a variety of enzymes such as pyrroloquinoline quinone (PQQ)-dependent glucose dehydrogenase, flavin adenine dinucleotide (FAD)-dependent glucose dehydrogenase and the FAD-dependent dehydrogenase domain of cellobiose dehydrogenase, low-potential glucose biosensors could be obtained with negligible co-oxidation of common interfering compounds such as uric acid or ascorbic acid. In combination with a bilirubin oxidase-based biocathode, enzymatic biofuel cells with open-circuit voltages of up to 0.54 V were obtained. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/chem.201504591
  • Detection of individual nanoparticle impacts using etched carbon nanoelectrodes
    Clausmeyer, J. and Wilde, P. and Löffler, T. and Ventosa, E. and Tschulik, K. and Schuhmann, W.
    Electrochemistry Communications 73 (2016)
    A rapid and reliable nanofabrication route produces electrodes with beneficial properties for electrochemistry based on stochastic nanoparticle collision events. Carbon nanoelectrodes are etched to expose conical carbon tips which present an increased surface area for the detection of nanoparticle impacts. The tuneable electrode size as well as the conical geometry allow to increase the observed particle impact frequency while maintaining low background noise. Moreover, anodic particle coulometry for the sizing of silver nanoparticles shows that the detected impacts are representative of the polydisperse particle population. © 2016
    view abstract10.1016/j.elecom.2016.11.003
  • Differentiation between Single- and Double-Stranded DNA through Local Capacitance Measurements
    Estrada-Vargas, A. and Jambrec, D. and Kayran, Y.U. and Kuznetsov, V. and Schuhmann, W.
    ChemElectroChem 3 (2016)
    Local differentiation of single-stranded (ss) and double-stranded (ds) DNA in microarrays by using electrochemical techniques has gained increasing interest. We propose a method for distinguishing areas on gold electrodes modified with ssDNA and dsDNA based on the difference in their local capacitance. The local capacitance is visualized by means of scanning electrochemical impedance microscopy and alternating-current scanning electrochemical microscopy. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/celc.201600075
  • DNA Intercalators for Detection of DNA Hybridisation: SCS(MI)–MP2 Calculations and Electrochemical Impedance Spectroscopy
    Jambrec, D. and Haddad, R. and Lauks, A. and Gebala, M. and Schuhmann, W. and Kokoschka, M.
    ChemPlusChem 81 (2016)
    Quantum mechanical SCS(MI)–MP2/cc-pVTZ calculations predict the strength of proflavine, ellipticine and 1-pyrenemethylamine intercalation into single-stranded (ss) and double-stranded (ds) DNA. The results were compared with experimental results obtained from electrochemical impedance spectroscopy (EIS). Similar interaction energies of ellipticine with the guanine–cytosine base pair compared to the individual nucleobases guanine and cytosine suggested non-specific binding also to ssDNA. Accordingly, EIS identified ellipticine as being non-selective and therefore unsuitable for the detection of DNA hybridisation. The interaction energy of proflavine is significantly higher than the minimum required energy for a single intercalation site, and substantially lower with respect to the minimum energy needed for binding with ssDNA. In EIS studies, proflavine did not show any change in the charge-transfer resistance with respect to ssDNA and a decrease with respect to dsDNA. Calculations showed that 1-pyrenemethylamine has sufficiently high interaction energy to intercalate into dsDNA, however, the interaction energy towards ssDNA is close to the minimum required value, suggesting a weak interaction with ssDNA. EIS measurements support the calculations. A method for the calculation of interaction energies is provided, which can be used to characterise the interaction strength between new intercalators and DNA before being synthesised. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/cplu.201600173
  • Electrocatalysis and bioelectrocatalysis – Distinction without a difference
    Masa, J. and Schuhmann, W.
    Nano Energy 29 (2016)
    Nature's subtle systems drive essential reactions responsible for sustenance of our existence “reactions of life” through sophisticated mechanisms of charge transfer, energy harvest and conversion. The interconnectedness between living nature and technologically relevant electrochemical reactions, for example, oxygen reduction and evolution catalyzed by cytochrome c oxidases and photosystem II respectively, and hydrogen oxidation and evolution catalyzed by hydrogenases, does not only intrigue but also inspires us. To what extent therefore can our present understanding of electrocatalysis guide us to decipher nature's sophistication, or rather, can bioinspired electrocatalysis succeed to replicate and supersede nature's perfection “the exemplar paragon”? Herein, we present a harmonized perspective of the principle factors which govern electrocatalysis and bioelectrocatalysis featuring examples of technologically important electrochemical reactions catalyzed by both enzymes and inorganic electrocatalysts. Sound knowledge of the inter-relationships linking electrocatalysis and bioelectrocatalysis is essential for enabling a deeper understanding of nature's bioelectrochemical reactions, and for insightful design of functional catalysts inspired by models from living nature. © 2016 Elsevier Ltd
    view abstract10.1016/j.nanoen.2016.04.007
  • Electrochemically induced sol-gel deposition of ZnO films on Pt-nanoparticle modified FTO surfaces for enhanced photoelectrocatalytic energy conversion
    Gutkowski, R. and Schuhmann, W.
    Physical Chemistry Chemical Physics 18 (2016)
    The low conductivity of transparent conductive oxides such as fluorine-doped tin oxides (FTO) has a high impact on the electrochemically induced deposition of semiconductor films for photoelectrocatalytic investigations. Furthermore, the often high recombination rate of photogenerated electron-hole pairs influences the photoelectrochemical performance of semiconductor films. In order to improve the semiconductor deposition process as well as to decrease electron-hole pair recombination, we propose modification of FTO by electrochemically induced deposition of Pt nanoparticles. The deposited Pt nanoparticles improve on the one hand the conductivity of the FTO and on the other hand they create nuclei at which the sol-gel semiconductor deposition starts. We use ZnO as a well-characterised material to evaluate the effect of the influencing parameters during electrochemically induced sol-gel deposition with respect to the incident photon-to-current efficiency (IPCE) derived from wavelength dependent photocurrent spectroscopy. Using optimised deposition parameters a substantially decreased recombination rate of photogenerated charge carriers is demonstrated, if Pt-nanoparticles are first deposited on the FTO surface. By improving the diffusion of photogenerated electrons to the Pt nanoparticles and hence to the back contact the photoelectrochemical performance of the deposited ZnO films is substantially increased. © the Owner Societies 2016.
    view abstract10.1039/c5cp07678a
  • Electrochemistry of single nanoparticles: general discussion
    Albrecht, T. and MacPherson, J. and Magnussen, O. and Fermin, D. and Crooks, R. and Gooding, J. and Hersbach, T. and Kanoufi, F. and Schuhmann, W. and Bentley, C. and Tao, N. and Mitra, S. and Krischer, K. and Tschulik, K. and Faez, S. and Nogala, W. and Unwin, P. and Long, Y. and Koper, M. and Tian, Z. and Alpuche-Aviles, M.A. and White, H. and Brasiliense, V. and Kranz, C. and Schmickler, W. and Stevenson, K. and Jing, C. and Edwards, M.
    Faraday Discussions 193 (2016)
    view abstract10.1039/c6fd90068b
  • Erratum: Amorphous Cobalt Boride (Co2B) as a Highly Efficient Nonprecious Catalyst for Electrochemical Water Splitting: Oxygen and Hydrogen Evolution
    Masa, J. and Weide, P. and Peeters, D. and Sinev, I. and Xia, W. and Sun, Z. and Somsen, C. and Muhler, M. and Schuhmann, W.
    Advanced Energy Materials 6 (2016)
    There is an error in the reference list in the PDF version of the originally published article. References [8] and [9] were inadvertently merged during production and are both incorrectly contained within reference [8]. These references are correct as follows: [8] Y. Jiao, Y. Zheng, M. Jaroniec, S. Z. Qiao, Chem. Soc. Rev. 2015, 44, 2060. [9] a) K. L. Nardi, N. Yang, C. F. Dickens, A. L. Strickler, S. F. Bent, Adv. Energy Mater. 2015, 5, 1500412; b) Z. Peng, D. Jia, A. M. Al-Enizi, A. A. Elzatahry, G. Zheng, Adv. Energy Mater. 2015, 5, 1402031; c) A. Zhao, J. Masa, W. Xia, A. Maljusch, M.-G. Willinger, G. Clavel, K. Xie, R. Schlögl, W. Schuhmann, M. Muhler, J. Am. Chem. Soc 2014, 136, 7551; d) J. Masa, W. Xia, I. Sinev, A. Zhao, Z. Sun, S. Grützke, P. Weide, M. Muhler, W. Schuhmann, Angew. Chem. Int. Ed. 2014, 53, 8508; e) R. Frydendal, E. A. Paoli, I. Chorkendorff, J. Rossmeisl, I. E. L. Stephens, Adv. Energy Mater. 2015, 1500991; f) M. Gong, Y. Li, H. Wang, Y. Liang, J. Z. Wu, J. Zhou, J. Wang, T. Regier, F. Wei, H. Dai, J. Am. Chem. Soc 2013, 135, 8452; g) S. Cobo, J. Heidkamp, P.-A. Jacques, J. Fize, V. Fourmond, L. Guetaz, B. Jousselme, V. Ivanova, H. Dau, S. Palacin et al., Nat. Mater. 2012, 11, 80; h) F. Cheng, J. Shen, B. Peng, Y. Pan, Z. Tao, J. Chen, Nature Chem 2010, 3, 79; i) J. Nai, H. Yin, T. You, L. Zheng, J. Zhang, P. Wang, Z. Jin, Y. Tian, J. Liu, Z. Tang, L. Guo, Adv. Energy Mater. 2015, 5, 1401880. As a result of this error, beginning with ref. [10], all citations in the published PDF version refer to the reference before the citation number. For example: Citation [10] refers to ref. [9], citation [11] refers to ref. [10], etc. The editorial office apologizes for this error and any confusion caused. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/aenm.201600980
  • Evaluation of kinetic constants on porous, non-noble catalyst layers for oxygen reduction - A comparative study between SECM and hydrodynamic methods
    Dobrzeniecka, A. and Zeradjanin, A.R. and Masa, J. and Blicharska, M. and Wintrich, D. and Kulesza, P.J. and Schuhmann, W.
    Catalysis Today 262 (2016)
    An advanced approach based on scanning electrochemical microscopy (SECM) was used to investigate the kinetics of the oxygen reduction reaction (ORR) on multiwalled carbon nanotubes (MWCNTs) and a composite of MWCNTs and cobalt (IX) protoporphyrin (MWCNTs/CoP). The amount of hydrogen peroxide produced during ORR was studied as a function of catalyst loading in an electrolyte of pH 7. Additionally, a Pt ultra microelectrode (UME) was used to determine changes in interfacial oxygen concentration from which intrinsic rate constants of heterogeneous electron transfer during the ORR were calculated. The amount of hydrogen peroxide produced and the number of electrons exchanged during oxygen reduction, and the heterogeneous electron transfer rate constants determined using SECM were compared with the corresponding values obtained using methods based on forced convection, namely RRDE and RDE. It was found that SECM offers some advantages compared to RDE or RRDE with regard to accuracy in determining the number of electrons transferred during the ORR, particularly in the case of thick and porous catalyst films. However, the heterogeneous electron transfer rate constants were similar for both methods, indicating that the determination of the surface concentration of reactants using RC-SECM suffers from some drawbacks. © 2015 Elsevier B.V.
    view abstract10.1016/j.cattod.2015.07.043
  • Few-layer graphene modified with nitrogen-rich metallo-macrocyclic complexes as precursor for bifunctional oxygen electrocatalysts
    Morales, D.M. and Masa, J. and Andronescu, C. and Kayran, Y.U. and Sun, Z. and Schuhmann, W.
    Electrochimica Acta 222 (2016)
    We propose a method for the formation of highly active bifunctional oxygen electrocatalysts, by exploiting the unique features of nitrogen-rich metallo-macrocyclic complexes and the structural and electronic properties of few-layer graphene. The precursors of the electrocatalysts were synthesized by sonication of graphite in DMF leading to exfoliation and the formation of few-layer graphene sheets in the presence of a suitable transition metal macrocyclic complex. After pyrolysis and subsequent mild calcination metal oxide nanoparticles as well as metal-nitrogen (MNx) moieties embedded within a N-doped graphitic carbon matrix are obtained. The formation, in-depth characterization and electrochemical performance of two different catalysts derived from Co and Ni containing precursor complexes are demonstrated. © 2016 Elsevier Ltd
    view abstract10.1016/j.electacta.2016.11.092
  • From single cells to single molecules: general discussion
    Gooding, J. and Magnussen, O. and Fermin, D. and Crooks, R. and Kanoufi, F. and Schuhmann, W. and Nichols, R. and Schmickler, W. and Tao, N. and Chen, S. and Actis, P. and Page, A. and Tschulik, K. and Faez, S. and Edwards, M. and Johnson, R. and Nogala, W. and Kranz, C. and Eikerling, M. and Unwin, P. and Thomas, B. and Prabhakaran, V. and Clausmeyer, J. and Vincent, K. and Koper, M. and Tian, Z. and Mount, A. and Alpuche-Aviles, M.A. and White, H. and Ewing, A. and Higgins, S. and Baker, L. and Zhan, D. and Ulstrup, J. and Bohn, P.W. and Lemay, S.
    Faraday Discussions 193 (2016)
    view abstract10.1039/c6fd90066f
  • Improved photoelectrochemical performance of electrodeposited metal-doped BiVO4 on Pt-nanoparticle modified FTO surfaces
    Gutkowski, R. and Peeters, D. and Schuhmann, W.
    Journal of Materials Chemistry A 4 (2016)
    The recombination of photogenerated electron-hole pairs is one of the main limiting factors of photoelectrocatalysts absorbing in the visible part of the solar spectrum. Especially for BiVO4 the slow electron transport to the back contact facilitates charge recombination. Hence, thin layers have to be used to obtain higher photocurrents which are concomitantly only allow low absorption of the incident light. To address this limitation we have modified FTO substrates with Pt-nanoparticles before electrodepositing BiVO4. The Pt-nanoparticles decrease the overpotential for the electrodeposition of BiVO4, but more importantly they provide the basis for decreased charge recombination. Electrodeposited Mo-doped BiVO4 on Pt-nanoparticle modified FTO exhibits a substantially decreased recombination of photogenerated charge carriers during frontside illumination. Simultaneous co-doping of BiVO4 with two different metals leads to a substantial enhancement of the incident-photon-to-current efficiency (IPCE) during light driven oxygen evolution reaction. Highest IPCE (&gt;30% at 1.2 V vs. RHE) values were obtained for Mo/Zn- and Mo/B-doped BiVO4. © 2016 The Royal Society of Chemistry.
    view abstract10.1039/c6ta01340f
  • In Situ Characterization of Ultrathin Films by Scanning Electrochemical Impedance Microscopy
    Estrada-Vargas, A. and Bandarenka, A. and Kuznetsov, V. and Schuhmann, W.
    Analytical Chemistry 88 (2016)
    Control over the properties of ultrathin films plays a crucial role in many fields of science and technology. Although nondestructive optical and electrical methods have multiple advantages for local surface characterization, their applicability is very limited if the surface is in contact with an electrolyte solution. Local electrochemical methods, e.g., scanning electrochemical microscopy (SECM), cannot be used as a robust alternative yet because their methodological aspects are not sufficiently developed with respect to these systems. The recently proposed scanning electrochemical impedance microscopy (SEIM) can efficiently elucidate many key properties of the solid/liquid interface such as charge transfer resistance or interfacial capacitance. However, many fundamental aspects related to SEIM application still remain unclear. In this work, a methodology for the interpretation of SEIM data of "charge blocking systems" has been elaborated with the help of finite element simulations in combination with experimental results. As a proof of concept, the local film thickness has been visualized using model systems at various tip-to-sample separations. Namely, anodized aluminum oxide (Al2O3, 2-20 nm) and self-assembled monolayers based on 11-mercapto-1-undecanol and 16-mercapto-1-hexadecanethiol (2.1 and 2.9 nm, respectively) were used as model systems. (Figure Presented). © 2016 American Chemical Society.
    view abstract10.1021/acs.analchem.6b00011
  • Intercalation Compounds as Inner Reference Electrodes for Reproducible and Robust Solid-Contact Ion-Selective Electrodes
    Ishige, Y. and Klink, S. and Schuhmann, W.
    Angewandte Chemie - International Edition 55 (2016)
    With billions of assays performed every year, ion-selective electrodes (ISEs) provide a simple and fast technique for clinical analysis of blood electrolytes. The development of cheap, miniaturized solid-contact (SC-)ISEs for integrated systems, however, remains a difficult balancing act between size, robustness, and reproducibility, because the defined interface potentials between the ion-selective membrane and the inner reference electrode (iRE) are often compromised. We demonstrate that target cation-sensitive intercalation compounds, such as partially charged lithium iron phosphate (LFP), can be applied as iREs of the quasi-first kind for ISEs. The symmetrical response of the interface potentials towards target cations ultimately results in ISEs with high robustness towards the inner filling (ca. 5 mV dec-1 conc.) as well as robust and miniaturized SC-ISEs. They have a predictable and stable potential derived from the LiFePO4/FePO4 redox couple (97.0±1.5 mV after 42 days). © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/anie.201600111
  • Interrogation of immunoassay platforms by SERS and SECM after enzyme-catalyzed deposition of silver nanoparticles
    Conzuelo, F. and Grützke, S. and Stratmann, L. and Pingarrón, J.M. and Schuhmann, W.
    Microchimica Acta 183 (2016)
    The manuscript demonstrates the interrogation of immunoassay platforms after enzyme-catalyzed deposition of silver nanoparticles (AgNPs) using scanning electrochemical microscopy (SECM) and surface-enhanced Raman scattering (SERS). We have selected an immunoassay model platform for the determination of residues of the antibiotic sulfapyridine in milk. The assay involves a direct competitive approach with an HRP-labeled antigen analog and selective capture antibodies immobilized on the surface of glassy carbon plates modified with protein G. Interrogation of the modified substrate by SECM and SERS was carried out after enzyme-catalyzed in-situ deposition of AgNPs. The enhanced Raman scattering for proflavine in the presence of AgNPs was used as signalling system. Hexacyanoferrate(III) was applied as the redox probe in SECM; the visualization of the deposited AgNP spots was enabled by using a competition between the SECM tip microelectrode and the modified carbon plate for the oxidation of ferrocyanide. This SECM strategy proved to be an efficient tool for the interrogation of sensing surfaces that were amplified by enzyme-catalyzed silver deposition. [Figure not available: see fulltext.] © 2015, Springer-Verlag Wien.
    view abstract10.1007/s00604-015-1654-x
  • Intracellular Hydrogen Peroxide Detection with Functionalised Nanoelectrodes
    Marquitan, M. and Clausmeyer, J. and Actis, P. and Córdoba, A.L. and Korchev, Y. and Mark, M.D. and Herlitze, S. and Schuhmann, W.
    ChemElectroChem 3 (2016)
    Hydrogen peroxide (H2O2) is one of the most important reactive oxygen species, and it is involved in a number of cellular processes ranging from signal transduction to immune defence and oxidative stress. It is of great interest to intracellularly quantify H2O2 to improve the understanding of its role in disease processes. In this study, we present an amperometric nanosensor for the quantification of H2O2 at the single-cell level. Deposition of the electrocatalyst Prussian Blue on carbon nanoelectrodes enables selective H2O2 reduction at mild potentials. Owing to their small size and needle-type shape, these nanoelectrodes can penetrate the membrane of single living cells, causing only minimal perturbation. The nanosensors allow for the monitoring of penetration-induced oxidative outbursts as well as the uptake of H2O2 from the extracellular environment in single murine macrophages. © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/celc.201600390
  • Kinetic passivation effect of localized differential aeration on brass
    Kuznetsov, V. and Estrada-Vargas, A. and Maljusch, A. and Berkes, B.B. and Bandarenka, A.S. and Souto, R.M. and Schuhmann, W.
    ChemPlusChem 81 (2016)
    The formation of a localized differential aeration cell on metals, susceptible to both anodic and cathodic corrosion, is a serious threat because of multiple degradation processes commencing with the passivation layer destruction. By using local electrochemical and X-ray dispersive techniques, it has been demonstrated that the differential aeration cell formed on high brass (α-brass, Cu65-Zn35) in the presence of 1H-benzotriazole or 5-methyl-1H-benzotriazole plays both corrosion-inhibiting and accelerating roles, depending on the inhibitor exposure time. Alternating-current scanning electrochemical microscopy was used to image local electrochemical activity, whereas energy-dispersive X-ray spectroscopy provided evidence for the mechanism of the observed phenomena. Short-term exposure to the inhibitor (5 min) promotes the formation of a passivation layer in the waterline region. In contrast, after prolonged exposure (45 min), a deficient passivation layer develops for both inhibitors. An excess of zinc(II)-inhibitor complexes in the passivation layer is accountable for the corrosion resistance of the region with high differential aeration. Rapid dezincification and local alkalinization facilitate the initial rapid formation of a passivation layer in the area under differential aeration to preserve its composition upon further modification. Waterline passivation: A differential aeration cell formed at the meniscus between air and electrolyte invokes local corrosion of brass. In the presence of inhibitors, a short-term passivation effect takes place that can be analyzed by spectroscopic and local electrochemical techniques (see figure). Simultaneous rapid cathodic alkalinization and dezincification lead to the rapid formation of a barrier layer that is temporarily superior to the bulk layer. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cplu.201500398
  • Making the hydrogen evolution reaction in polymer electrolyte membrane electrolysers even faster
    Tymoczko, J. and Calle-Vallejo, F. and Schuhmann, W. and Bandarenka, A.S.
    Nature Communications 7 (2016)
    Although the hydrogen evolution reaction (HER) is one of the fastest electrocatalytic reactions, modern polymer electrolyte membrane (PEM) electrolysers require larger platinum loadings (∼0.5-1.0 mg cm-2) than those in PEM fuel cell anodes and cathodes altogether (∼0.5 mg cm-2). Thus, catalyst optimization would help in substantially reducing the costs for hydrogen production using this technology. Here we show that the activity of platinum(111) electrodes towards HER is significantly enhanced with just monolayer amounts of copper. Positioning copper atoms into the subsurface layer of platinum weakens the surface binding of adsorbed H-intermediates and provides a twofold activity increase, surpassing the highest specific HER activities reported for acidic media under similar conditions, to the best of our knowledge. These improvements are rationalized using a simple model based on structure-sensitive hydrogen adsorption at platinum and copper-modified platinum surfaces. This model also solves a long-lasting puzzle in electrocatalysis, namely why polycrystalline platinum electrodes are more active than platinum(111) for the HER.
    view abstract10.1038/ncomms10990
  • Mesoporous nitrogen containing carbon materials for the simultaneous detection of ascorbic acid, dopamine and uric acid
    Joshi, A. and Schuhmann, W. and Nagaiah, T.C.
    Sensors and Actuators, B: Chemical 230 (2016)
    Mesoporous nitrogen rich carbonaceous (MNC) materials have been synthesized by pyrolyzing the polymerized ethylenediamine nanocasted into a SBA-15 hard template at 600 and 800 °C and explored for simultaneous determination of ascorbic acid (AA), dopamine (DA) and uric acid (UA). The electrocatalytic activity of these materials for the oxidation of analyte molecules was examined by means of redox-competition mode of scanning electrochemical microscopy (SECM), voltammetric, chronoamperometric and rotating disc electrode (RDE) measurements. MNC material exhibits a superior sensitivity towards the oxidation of AA, DA, and UA with a lowest detection limit of 0.01 μM, 0.001 μM and 0.01 μM respectively without any substantial interferences including glucose at physiologically relevant concentrations. © 2016 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.snb.2016.02.050
  • MoSSe@reduced graphene oxide nanocomposite heterostructures as efficient and stable electrocatalysts for the hydrogen evolution reaction
    Konkena, B. and Masa, J. and Xia, W. and Muhler, M. and Schuhmann, W.
    Nano Energy 29 (2016)
    Non-noble metal based materials efficiently catalyzing the hydrogen evolution reaction (HER) are reported based on a novel strategy where electrocatalytically active ultrathin molybdenum sulphoselenide sheets are incorporated into electrically conducting reduced graphene oxide sheets via a self-assembly approach. By taking advantage of the electrostatic attraction between the two oppositely charged nanosheets, MoSSe@rGO composite materials are obtained exhibiting superior electrocatalytic activity and stability for the HER allowing a current density of 5 mA cm−2 at a low overpotential of only 135 mV. These findings pave the way to novel electrocatalysts based on composites of MoSSe and reduced graphene oxide towards the design of ultra-light, mechanically robust and electrically conductive electrode materials for electrocatalytic water splitting. © 2016 Elsevier Ltd
    view abstract10.1016/j.nanoen.2016.04.018
  • Nanoelectrodes reveal the electrochemistry of single nickelhydroxide nanoparticles
    Clausmeyer, J. and Masa, J. and Ventosa, E. and Öhl, D. and Schuhmann, W.
    Chemical Communications 52 (2016)
    Individual Ni(OH)2 nanoparticles deposited on carbon nanoelectrodes are investigated in non-ensemble measurements with respect to their energy storage properties and electrocatalysis for the oxygen evolution reaction (OER). Charging by oxidation of Ni(OH)2 is limited by the diffusion of protons into the particle bulk and the OER activity is independent of the particle size. © 2016 The Royal Society of Chemistry.
    view abstract10.1039/c5cc08796a
  • Nanoelectrodes: Applications in electrocatalysis, single-cell analysis and high-resolution electrochemical imaging
    Clausmeyer, J. and Schuhmann, W.
    TrAC - Trends in Analytical Chemistry 79 (2016)
    High sensitivity and high spatial resolution in localized electrochemical measurements are the key advantages of electroanalysis using nanometer-sized electrodes. Due to recent progress in nanoelectrode fabrication and electrochemical instrument development, nanoelectrochemical methods are becoming more widespread. We summarize different protocols for the fabrication of needle-type nanoelectrodes and discuss their properties with regard to various applications. We discuss the limits of conventional theory to describe electrochemistry at the nanoscale and point out technical aspects for characterization and handling of nanometric electrodes. Different applications are highlighted: i) Nanoelectrodes are powerful tools for non-ensemble studies of electrocatalysis at single nanoparticles at high mass transport rates. ii) Electrochemical nanosensors are employed for highly localized non-invasive analysis of single living cells and intracellular detection of neurotransmitters and metabolites. iii) Used in scanning electrochemical probe techniques, nanoprobes afford topographical and truly chemical imaging of samples with high spatial resolution. © 2016 Published by Elsevier B.V.
    view abstract10.1016/j.trac.2016.01.018
  • New materials for the light-induced hydrogen evolution reaction from the Cu-Si-Ti-O system
    Stein, H.S. and Gutkowski, R. and Siegel, A. and Schuhmann, W. and Ludwig, Al.
    Journal of Materials Chemistry A 4 (2016)
    Cu-containing photocathodes are generally limited by fast photocorrosion under working conditions. Hence stabilization of these materials is a key factor in their potential application for the light-induced hydrogen evolution reaction (HER). In order to identify new materials, oxidized Cu-Si-Ti metallic thin film precursor materials libraries were evaluated using a combinatorial approach. High-throughput photoelectrochemical characterization using an automated optical scanning droplet cell was performed on a material library to analyze doping and alloying effects on the light-induced HER. The results revealed that compositions near Ti-doped CuSiO3 (dioptase and copper-polysilicate) and Si-doped Cu3TiOx act as comparatively stable and highly active materials for HER. © 2016 The Royal Society of Chemistry.
    view abstract10.1039/c5ta10186g
  • Pd deposited on functionalized carbon nanotubes for the electrooxidation of ethanol in alkaline media
    Hiltrop, D. and Masa, J. and Maljusch, A. and Xia, W. and Schuhmann, W. and Muhler, M.
    Electrochemistry Communications 63 (2016)
    Large scale commercialization of direct ethanol fuel cells is hampered by the high cost and scarcity of noble metal electrocatalysts employed at both the anode and cathode. We demonstrate improved utilization of palladium as anode catalyst for ethanol oxidation by exploiting the strong interaction between Pd nanoparticles and nitrogen-doped carbon nanotubes (NCNTs) as support. 0.85 wt% Pd supported on NCNTs achieved a specific current density of 517 A gPd - 1 compared with 421 A gPd - 1 for 0.86 wt% Pd on oxygen-functionalized carbon nanotubes. The electrocatalytic performance deteriorated only gradually and catalysis was sustained for at least 80 h. © 2015 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.elecom.2015.11.010
  • Pentlandite rocks as sustainable and stable efficient electrocatalysts for hydrogen generation
    Konkena, B. and Puring, K.J. and Sinev, I. and Piontek, S. and Khavryuchenko, O. and Dürholt, J.P. and Schmid, R. and Tüysüz, H. and Muhler, M. and Schuhmann, W. and Apfel, U.-P.
    Nature Communications 7 (2016)
    The need for sustainable catalysts for an efficient hydrogen evolution reaction is of significant interest for modern society. Inspired by comparable structural properties of [FeNi]-hydrogenase, here we present the natural ore pentlandite (Fe 4.5 Ni 4.5 S 8) as a direct rock' electrode material for hydrogen evolution under acidic conditions with an overpotential of 280 mV at 10 mA cm -2. Furthermore, it reaches a value as low as 190 mV after 96 h of electrolysis due to surface sulfur depletion, which may change the electronic structure of the catalytically active nickel-iron centres. The rock' material shows an unexpected catalytic activity with comparable overpotential and Tafel slope to some well-developed metallic or nanostructured catalysts. Notably, the rock' material offers high current densities (≤650 mA cm -2) without any loss in activity for approximately 170 h. The superior hydrogen evolution performance of pentlandites as rock' electrode labels this ore as a promising electrocatalyst for future hydrogen-based economy.
    view abstract10.1038/ncomms12269
  • Perovskite-based bifunctional electrocatalysts for oxygen evolution and oxygen reduction in alkaline electrolytes
    Elumeeva, K. and Masa, J. and Sierau, J. and Tietz, F. and Muhler, M. and Schuhmann, W.
    Electrochimica Acta 208 (2016)
    Due to the high cost of precious metal-based electrocatalysts for oxygen reduction and oxygen evolution, the development of alternative low cost and efficient catalysts is of high importance for energy storage and conversion technologies. Although non-precious catalysts that can efficiently catalyze oxygen reduction and oxygen evolution have been developed, electrocatalysts with high bifunctional activity for both oxygen evolution and reduction are needed. Perovskites based on modified lanthanum cobaltite possess significant activity for the oxygen evolution reaction. We describe the synthesis of a bifunctional oxygen electrode with simultaneous activity for the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) in alkaline media by direct growth of nitrogen-doped carbon nanotubes on the surface of a perovskite containing Co and Fe by means of chemical vapor deposition. The difference in the overvoltage between ORR (at 1 mA/cm2) and OER (at 10 mA/cm2) was below 880 mV in 0.1 M KOH. The formation of H2O2 during the ORR was reduced by at least three fold when using the bifunctional catalyst as compared to the non-modified perovskite. Long-term durability tests indicate stable performance for at least 37 h during the OER and 23 h during the ORR. © 2016 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.electacta.2016.05.010
  • Potential-Pulse-Assisted Formation of Thiol Monolayers within Minutes for Fast and Controlled Electrode Surface Modification
    Jambrec, D. and Conzuelo, F. and Estrada-Vargas, A. and Schuhmann, W.
    ChemElectroChem 3 (2016)
    We propose a potential-pulse-assisted method for the formation of highly compact thiol self-assembled monolayers (SAMs), ensuring fully covered surfaces within minutes. By pulsing between potentials that are more positive and more negative with respect to the potential of zero charge, kinetics of SAM formation is substantially enhanced. The formation of the SAM is followed by using real-time impedance measurements by superimposing the applied potential-pulse profile with a high-frequency AC signal that allows for calculation of the interfacial capacitance and provides information about the compactness of the formed layers. A systematic study of the influence of the pulse potential intensity, the pulse duration, and the nature of the thiol derivative on the potential-pulse-assisted SAM formation is performed. We show that compact thiol monolayers are obtained much faster with the suggested technique, as compared to SAM formation performed at the open-circuit potential or by applying a constant potential. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/celc.201600308
  • Promoting effect of nitrogen doping on carbon nanotube-supported RuO2 applied in the electrocatalytic oxygen evolution reaction
    Xie, K. and Xia, W. and Masa, J. and Yang, F. and Weide, P. and Schuhmann, W. and Muhler, M.
    Journal of Energy Chemistry 25 (2016)
    RuO2 nanoparticles supported on multi-walled carbon nanotubes (CNTs) functionalized with oxygen (OCNTs) and nitrogen (NCNTs) were employed for the oxygen evolution reaction (OER) in 0.1 M KOH. The catalysts were synthesized by metal-organic chemical vapor deposition using ruthenium carbonyl (Ru3(CO)12) as Ru precursor. The obtained RuO2/OCNT and RuO2/NCNT composites were characterized using TEM, H2-TPR, XRD and XPS in order probe structure-activity correlations, particularly, the effect of the different surface functional groups on the electrochemical OER performance. The electrocatalytic activity and stability of the catalysts with mean RuO2 particle sizes of 13-14 nm was evaluated by linear sweep voltammetry, cyclic voltammetry, and chronopotentiometry, showing that the generation of nitrogen-containing functional groups on CNTs was beneficial for both OER activity and stability. In the presence of RuO2, carbon corrosion was found to be significantly less severe. © 2016 Science Press and Dalian Institute of Chemical Physics. All rights reserved.
    view abstract10.1016/j.jechem.2016.01.023
  • Reactions at the nanoscale: general discussion
    Hersbach, T. and MacPherson, J. and Magnussen, O. and Crooks, R. and Higgins, S. and Fermin, D. and Kanoufi, F. and Schuhmann, W. and Nichols, R. and Mitra, S. and Schmickler, W. and Tschulik, K. and Bartlett, P. and Faez, S. and Nogala, W. and Eikerling, M. and Kranz, C. and Unwin, P. and Koper, M. and Lemay, S. and Mount, A. and Ewing, A. and Tian, Z. and White, H. and Chen, S. and Clausmeyer, J. and Krischer, K.
    Faraday Discussions 193 (2016)
    view abstract10.1039/C6FD90067D
  • Scanning electrochemical microscopy: Visualization of local electrocatalytic activity of transition metals hexacyanoferrates
    Komkova, M.A. and Maljusch, A. and Sliozberg, K. and Schuhmann, W. and Karyakin, A.A.
    Russian Journal of Electrochemistry 52 (2016)
    The redox competition mode of scanning electrochemical microscopy (SECM) was used to visualize differences in local electrocatalytic activity of Fe and Ni hexacyanoferrates (HCFs) in hydrogen peroxide reduction. The uniform round-shaped spots of electrocatalysts for the SECM measurements were electrochemically deposited using a scanning droplet cell. A negligible activity of NiHCF towards H2O2 reduction compared to Prussian Blue (PB) was observed. The dependence of local Prussian Blue activity on the applied potential was investigated. The proposed strategy explores the potential application of SECM as a rapid screening tool for HCF film activity within a single experiment. © 2016, Pleiades Publishing, Ltd.
    view abstract10.1134/S1023193516120065
  • Selection of Highly SERS-Active Nanostructures from a Size Gradient of Au Nanovoids on a Single Bipolar Electrode
    Kayran, Y.U. and Eßmann, V. and Grützke, S. and Schuhmann, W.
    ChemElectroChem 3 (2016)
    As surface-enhanced Raman scattering (SERS) crucially depends on the morphology of nanostructured metal surfaces, we developed a convenient approach to produce a size gradient of truncated spherical Au nanovoids on a single bipolar electrode. The continuous potential drop in solution implies a linearly changing interfacial potential difference at the wireless electrode, leading to a linearly changing rate of Au electrodeposition. Such a structural gradient enables fast and reproducible screening for those structures, evoking high SERS intensity in a particular experiment. The optimal Au deposition potential with respect to the highest SERS amplification was determined and applied for the fabrication of highly active SERS substrates. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/celc.201500423
  • Simultaneous measurements of photocurrents and H2O2 evolution from solvent exposed photosystem 2 complexes
    Vöpel, T. and Saw, E.N. and Hartmann, V. and Williams, R. and Müller, F. and Schuhmann, W. and Plumeré, N. and Nowaczyk, M. and Ebbinghaus, S. and Rögner, M.
    Biointerphases 11 (2016)
    In plants, algae, and cyanobacteria, photosystem 2 (PS2) catalyzes the light driven oxidation of water. The main products of this reaction are protons and molecular oxygen. In vitro, however, it was demonstrated that reactive oxygen species like hydrogen peroxide are obtained as partially reduced side products. The transition from oxygen to hydrogen peroxide evolution might be induced by light triggered degradation of PS2’s active center. Herein, the authors propose an analytical approach to investigate light induced bioelectrocatalytic processes such as PS2 catalyzed water splitting. By combining chronoamperometry and fluorescence microscopy, the authors can simultaneously monitor the photocurrent and the hydrogen peroxide evolution of light activated, solvent exposed PS2 complexes, which have been immobilized on a functionalized gold electrode. The authors show that under limited electron mediation PS2 displays a lower photostability that correlates with an enhanced H2O2 generation as a side product of the light induced water oxidation. © 2015 American Vacuum Society.
    view abstract10.1116/1.4938090
  • Spearhead Nanometric Field-Effect Transistor Sensors for Single-Cell Analysis
    Zhang, Y. and Clausmeyer, J. and Babakinejad, B. and López Córdoba, A. and Ali, T. and Shevchuk, A. and Takahashi, Y. and Novak, P. and Edwards, C. and Lab, M. and Gopal, S. and Chiappini, C. and Anand, U. and Magnani, L. and Coombes, R.C. and Gorelik, J. and Matsue, T. and Schuhmann, W. and Klenerman, D. and Sviderskaya, E.V. and Korchev, Y.
    ACS Nano 10 (2016)
    Nanometric field-effect-transistor (FET) sensors are made on the tip of spear-shaped dual carbon nanoelectrodes derived from carbon deposition inside double-barrel nanopipettes. The easy fabrication route allows deposition of semiconductors or conducting polymers to comprise the transistor channel. A channel from electrodeposited poly pyrrole (PPy) exhibits high sensitivity toward pH changes. This property is exploited by immobilizing hexokinase on PPy nano-FETs to give rise to a selective ATP biosensor. Extracellular pH and ATP gradients are key biochemical constituents in the microenvironment of living cells; we monitor their real-time changes in relation to cancer cells and cardiomyocytes. The highly localized detection is possible because of the high aspect ratio and the spear-like design of the nano-FET probes. The accurately positioned nano-FET sensors can detect concentration gradients in three-dimensional space, identify biochemical properties of a single living cell, and after cell membrane penetration perform intracellular measurements. © 2016 American Chemical Society.
    view abstract10.1021/acsnano.5b05211
  • Square Wave Cathodic Adsorptive Stripping Voltammetric Determination of the Anticancer Drugs Flutamide and Irinotecan in Biological Fluids Using Renewable Pencil Graphite Electrodes
    Temerk, Y.M. and Ibrahim, H. and Schuhmann, W.
    Electroanalysis 28 (2016)
    A sensitive electrochemical method based on square wave cathodic adsorptive stripping voltammetry (SWCASV) using pencil graphite electrodes (PGE) was developed for the individual and simultaneous determination of the anticancer drugs flutamide (Flu) and irinotecan (Irino) in biological fluids. Calibration curves showed an excellent linear response with limits of detection of 1.68×10-9and 1.55×10-8M Irino and Flu, respectively. The statistical evaluation of within-day repeatability (n=5) and day to day precision (n=5) showed satisfactory accuracy and precision. SWCASV using a PGE for individual and simultaneous determination of both drugs in bulk form, human urine and serum samples was demonstrated. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/elan.201500329
  • Surface Structure and Photocatalytic Properties of Bi2WO6 Nanoplatelets Modified by Molybdena Islands from Chemical Vapor Deposition
    Dittmer, A. and Menze, J. and Mei, B. and Strunk, J. and Luftman, H.S. and Gutkowski, R. and Wachs, I.E. and Schuhmann, W. and Muhler, M.
    Journal of Physical Chemistry C 120 (2016)
    We report on a novel route of preparing molybdena-modified bismuth tungstates and their successful application in the photocatalytic oxygen evolution reaction and the oxidation of glycerol. Hierarchically assembled monocrystalline Bi2WO6 nanoplatelets with a specific surface area of 10 m2/g were obtained applying a hydrothermal synthesis method using Na2WO4 and Bi(NO3)3 as precursors, followed by a solvent-free chemical vapor deposition method using Mo(CO)6, resulting in highly dispersed molybdena species. Extensive characterization using X-ray photoelectron spectroscopy, low-energy ion scattering, and Raman spectroscopy showed that microcrystalline MoO3 islands were formed on the bismuth tungstate surface that grew in height and lateral dimension with increasing loading. Correspondingly, the molybdena-modified materials were found to have favorable photocatalytic and photoelectrochemical properties in the oxygen evolution reaction and the selective oxidation of glycerol. © 2016 American Chemical Society.
    view abstract10.1021/acs.jpcc.6b07007
  • The oxygen reduction reaction at the three-phase boundary: nanoelectrodes modified with Ag nanoclusters
    Clausmeyer, J. and Botz, A. and Öhl, D. and Schuhmann, W.
    Faraday Discussions 193 (2016)
    Silver nanoclusters are deposited on bifunctional Θ-shaped nanoelectrodes consisting of a carbon nanoelectrode combined with a hollow nanopipette. The Θ-nanoelectrodes are used as model systems to study interfacial mass transport in gas diffusion electrodes and in particular oxygen-depolarized cathodes (ODC) for the oxygen reduction reaction (ORR) in chlor-alkali electrolysers. By local delivery of O2 gas to the electroactive Ag nanoclusters through the adjacent nanopipette, enhanced currents for the ORR at the Ag nanoparticles are recorded which are not accountable when considering the low solubility and slow diffusion of O2 in highly alkaline media. Instead, local oversaturation of O2 leads to current enhancement at the Ag nanoclusters. Due to the intrinsic high mass transport rates at the nanometric electrodes accompanied by local delivery of reactants, the method generally allows to study electrochemical reactions at single nanoparticles beyond the limitations induced by slow diffusion and low reactant concentration. Kinetic and mechanistic information, for instance derived from Tafel slopes, can be obtained from kinetic regimes not accessible to standard techniques. © The Royal Society of Chemistry.
    view abstract10.1039/c6fd00101g
  • Thermoresponsive amperometric glucose biosensor
    Pinyou, P. and Ruff, A. and Pöller, S. and Barwe, S. and Nebel, M. and Alburquerque, N.G. and Wischerhoff, E. and Laschewsky, A. and Schmaderer, S. and Szeponik, J. and Plumeré, N. and Schuhmann, W.
    Biointerphases 11 (2016)
    The authors report on the fabrication of a thermoresponsive biosensor for the amperometric detection of glucose. Screen printed electrodes with heatable gold working electrodes were modified by a thermoresponsive statistical copolymer [polymer I: poly(ω-ethoxytriethylenglycol methacrylate-co-3-(N,N-dimethyl-N-2-methacryloyloxyethyl ammonio) propanesulfonate-co-ω-butoxydiethylenglycol methacrylate-co-2-(4-benzoyl-phenoxy)ethyl methacrylate)] with a lower critical solution temperature of around 28 °C in aqueous solution via electrochemically induced codeposition with a pH-responsive redox-polymer [polymer II: poly(glycidyl methacrylate-co-allyl methacrylate-co-poly(ethylene glycol)methacrylate-co-butyl acrylate-co-2-(dimethylamino)ethyl methacrylate)-[Os(bpy)2(4-(((2-(2-(2-aminoethoxy)ethoxy)ethyl)amino)methyl)-N,N-dimethylpicolinamide)](2+)] and pyrroloquinoline quinone-soluble glucose dehydrogenase acting as biological recognition element. Polymer II bears covalently bound Os-complexes that act as redox mediators for shuttling electrons between the enzyme and the electrode surface. Polymer I acts as a temperature triggered immobilization matrix. Probing the catalytic current as a function of the working electrode temperature shows that the activity of the biosensor is dramatically reduced above the phase transition temperature of polymer I. Thus, the local modulation of the temperature at the interphase between the electrode and the bioactive layer allows switching the biosensor from an on- to an off-state without heating of the surrounding analyte solution.
    view abstract10.1116/1.4938382
  • Traditional earth-abundant coal as new energy materials to catalyze the oxygen reduction reaction in alkaline solution
    Chen, X. and Huang, X. and Wang, T. and Barwe, S. and Xie, K. and Kayran, Y.U. and Wintrich, D. and Schuhmann, W. and Masa, J.
    Electrochimica Acta 211 (2016)
    Coal is an earth-abundant energy resource, however, its direct combustion results in serious environmental pollution. Therefore, it becomes important to design value-added products from coal and to maximize its value chain. Herein, brown coal was used to develop non-precious metal catalysts for the oxygen reduction reaction (ORR) in fuel cells as green energy conversion systems. The brown coal was first pretreated with different acids, followed by N-doping at 800 °C in a stream of NH3. A trace amount of Fe was further added to improve the electrocatalytic performance of the prepared catalyst towards ORR. The prepared coal-derived N-doped carbon further modified with 0.5% Fe exhibited onset potential of 0.92 V vs. RHE at a current density of -0.1 mA cm-2 and a predominantly 4-electron transfer pathway of oxygen to water in 0.1 M NaOH, which was evaluated by RDE and RRDE. The prepared electrocatalysts were further characterized by elemental analysis, XRD, Raman and XPS. The results suggest that the coal-derived ORR catalyst have convoluted graphitic and amorphous carbon structures. The N-content increased after acid-pretreatment and subsequent functionalization with nitrogen, while it slightly decreased after Fe incorporation apparently due to coordination of Fe with N. ORR activity enhancement after the incorporation of Fe is expected to mainly arise from a synergetic effect involving the interaction of Fe with N groups distributed in the carbon matrix. © 2016 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.electacta.2016.05.137
  • Understanding memory effects in Li-ion batteries: Evidence of a kinetic origin in TiO2 upon hydrogen annealing
    Ventosa, E. and Löffler, T. and La Mantia, F. and Schuhmann, W.
    Chemical Communications 52 (2016)
    Memory effects in Li-ion battery materials have been explained on the basis of the thermodynamics of many-particles body, however the role of the (de-)intercalation kinetics is not yet clear. We demonstrate that kinetic aspects, specifically Li-ion mobility, are determining the magnitude of the memory effect in TiO2 by studying samples with different levels of oxygen vacancies. © The Royal Society of Chemistry 2016.
    view abstract10.1039/c6cc06070f
  • Understanding surface reactivity of Si electrodes in Li-ion batteries by: In operando scanning electrochemical microscopy
    Ventosa, E. and Wilde, P. and Zinn, A.-H. and Trautmann, M. and Ludwig, Al. and Schuhmann, W.
    Chemical Communications 52 (2016)
    In operando SECM is employed to monitor the evolution of the electrically insulating character of a Si electrode surface during (de-)lithiation. The solid-electrolyte interface (SEI) formed on Si electrodes is shown to be intrinsically electrically insulating. However, volume changes upon (de-)lithiation lead to the loss of the protecting character of the initially formed SEI. © The Royal Society of Chemistry 2016.
    view abstract10.1039/c6cc02493a
  • Wiring of the aldehyde oxidoreductase PaoABC to electrode surfaces via entrapment in low potential phenothiazine-modified redox polymers
    Pinyou, P. and Ruff, A. and Pöller, S. and Alsaoub, S. and Leimkühler, S. and Wollenberger, U. and Schuhmann, W.
    Bioelectrochemistry 109 (2016)
    Phenothiazine-modified redox hydrogels were synthesized and used for the wiring of the aldehyde oxidoreductase PaoABC to electrode surfaces. The effects of the pH value and electrode surface modification on the biocatalytic activity of the layers were studied in the presence of vanillin as the substrate. The enzyme electrodes were successfully employed as bioanodes in vanillin/O2 biofuel cells in combination with a high potential bilirubin oxidase biocathode. Open circuit voltages of around 700mV could be obtained in a two compartment biofuel cell setup. Moreover, the use of a rather hydrophobic polymer with a high degree of crosslinking sites ensures the formation of stable polymer/enzyme films which were successfully used as bioanode in membrane-less biofuel cells. © 2015 Elsevier B.V.
    view abstract10.1016/j.bioelechem.2015.12.005
  • A combinatorial study of photoelectrochemical properties of Fe-W-O thin films
    Sliozberg, K. and Schäfer, D. and Meyer, R. and Ludwig, Al. and Schuhmann, W.
    ChemPlusChem 80 (2015)
    A continuous mixed Fe-W-O thin-films materials library was fabricated by means of reactive co-sputtering from elemental iron and tungsten targets in argon/oxygen. The materials library was screened for its local photoelectrochemical properties by using an automated optical scanning droplet cell. Local scanning electron microscopy (SEM) and electron-dispersive X-ray spectroscopy (EDX) measurements of the materials library were performed to correlate the composition, morphology, and photocurrents. The iron content was varied in the range from Fe32W68Ox to Fe81W19Ox. A strong dependence of the film morphology and the measured photocurrents on the composition was observed with a maximal photocurrent from a measurement area containing 55 at% iron. The most photoactive area showed a porous structure with a high surface area. The bandgap values of these materials were assessed by photocurrent spectroscopy and showed a systematic variation of the bandgap values with the composition. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/cplu.201402277
  • A pH Responsive Redox Hydrogel for Electrochemical Detection of Redox Silent Biocatalytic Processes. Control of Hydrogel Solvation
    Contin, A. and Frasca, S. and Vivekananthan, J. and Leimkühler, S. and Wollenberger, U. and Plumeré, N. and Schuhmann, W.
    Electroanalysis 27 (2015)
    The control of bioelectrocatalytic processes by external stimuli for the indirect detection of non-redox active species was achieved using an esterase and a redox enzyme both integrated within a redox hydrogel. The poly(vinyl)imidazole Os(bpy)<inf>2</inf>Cl hydrogel displays pH-responsive properties. The esterase catalysed reaction leads to a local pH decrease causing protonation of imidazole moieties thus increasing hydrogel solvation and mobility of the tethered Os-complexes. This is the key step to enable improved electron transfer between an aldehyde oxidoreductase and the polymer-bound Os-complexes. The off-on switch is further integrated in a biofuel cell system for self-powered signal generation. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/elan.201400621
  • A Redox Hydrogel Protects the O2-Sensitive [FeFe]-Hydrogenase from Chlamydomonas reinhardtii from Oxidative Damage
    Oughli, A.A. and Conzuelo, F. and Winkler, M. and Happe, T. and Lubitz, W. and Schuhmann, W. and Rüdiger, O. and Plumeré, N.
    Angewandte Chemie - International Edition 54 (2015)
    The integration of sensitive catalysts in redox matrices opens up the possibility for their protection from deactivating molecules such as O2. [FeFe]-hydrogenases are enzymes catalyzing H2 oxidation/production which are irreversibly deactivated by O2. Therefore, their use under aerobic conditions has never been achieved. Integration of such hydrogenases in viologen-modified hydrogel films allows the enzyme to maintain catalytic current for H2 oxidation in the presence of O2, demonstrating a protection mechanism independent of reactivation processes. Within the hydrogel, electrons from the hydrogenase-catalyzed H2 oxidation are shuttled to the hydrogel-solution interface for O2 reduction. Hence, the harmful O2 molecules do not reach the hydrogenase. We illustrate the potential applications of this protection concept with a biofuel cell under H2/O2 mixed feed. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/anie.201502776
  • A structure zone diagram obtained by simultaneous deposition on a novel step heater: A case study for Cu2O thin films
    Stein, H. and Naujoks, D. and Grochla, D. and Khare, C. and Gutkowski, R. and Grützke, S. and Schuhmann, W. and Ludwig, Al.
    Physica Status Solidi (A) Applications and Materials Science 212 (2015)
    In thin film deposition processes, the deposition temperature is one of the crucial process parameters for obtaining films with desired properties. Usually the optimum deposition temperature is found by conducting several depositions sequentially in a time consuming process. This paper demonstrates a facile and rapid route of the simultaneous thin film deposition at six different deposition temperatures ranging from 100 to 1000 °C. Cuprite (Cu2O) was chosen for the study as this material is of interest for energy applications. The thin films are assessed for their crystallographic, microstructural, Raman scattering, and photoelectrochemical properties. The results show that the utilization of a step heater leads to the rapid optimization of thin film microstructures of an absorber material used in photoelectrochemistry. This results in a structure zone diagram for Cu2O. For a substrate temperature of 600 °C, an optimum between crystallinity and morphology occurs. © 2015 Wiley-VCH Verlag GmbH and Co. KGaA, Weinheim.
    view abstract10.1002/pssa.201532384
  • Assembling Paramagnetic Ceruloplasmin at Electrode Surfaces Covered with Ferromagnetic Nanoparticles. Scanning Electrochemical Microscopy in the Presence of a Magnetic Field
    Matysiak, E. and Botz, A.J.R. and Clausmeyer, J. and Wagner, B. and Schuhmann, W. and Stojek, Z. and Nowicka, A.M.
    Langmuir 31 (2015)
    Adsorption of ceruloplasmin (Cp) at a gold electrode modified with ferromagnetic iron nanoparticles encapsulated in carbon (Fe@C Nps) leads to a successful immobilization of the enzyme in its electroactive form. The proper placement of Cp at the electrode surface on top of the nanocapsules containing an iron core allowed a preorientation of the enzyme, hence allowing direct electron transfer between the electrode and the enzyme. Laser ablation coupled with inductively coupled plasma mass spectrometry indicated that Cp was predominantly located at the paramagnetic nanoparticles. Scanning electrochemical microscopy measurements in the sample-generation/tip-collection mode proved that Cp was ferrooxidative inactive if it was immobilized on the bare gold surface and reached the highest activity if it was adsorbed on Fe@C Nps in the presence of a magnetic field. © 2015 American Chemical Society.
    view abstract10.1021/acs.langmuir.5b01155
  • Co3O4-MnO2-CNT Hybrids Synthesized by HNO3 Vapor Oxidation of Catalytically Grown CNTs as OER Electrocatalysts
    Xie, K. and Masa, J. and Madej, E. and Yang, F. and Weide, P. and Dong, W. and Muhler, M. and Schuhmann, W. and Xia, W.
    ChemCatChem 7 (2015)
    An efficient two-step gas-phase method was developed for the synthesis of Co<inf>3</inf>O<inf>4</inf>-MnO<inf>2</inf>-CNT hybrids used as electrocatalysts in the oxygen evolution reaction (OER). Spinel Co-Mn oxide was used for the catalytic growth of multiwalled carbon nanotubes (CNTs) and the amount of metal species remaining in the CNTs was adjusted by varying the growth time. Gas-phase treatment in HNO<inf>3</inf> vapor at 200 °C was performed to 1)open the CNTs, 2)oxidize encapsulated Co nanoparticles to Co<inf>3</inf>O<inf>4</inf> as well as MnO nanoparticles to MnO<inf>2</inf>, and 3)to create oxygen functional groups on carbon. The hybrid demonstrated excellent OER activity and stability up to 37.5h under alkaline conditions, with longer exposure to HNO<inf>3</inf> vapor up to 72h beneficial for improved electrocatalytic properties. The excellent OER performance can be assigned to the high oxidation states of the oxide nanoparticles, the strong electrical coupling between these oxides and the CNTs as well as favorable surface properties rendering the hybrids a promising alternative to noble metal based OER catalysts. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cctc.201500469
  • Codeposited Poly(benzoxazine) and Os-Complex Modified Polymethacrylate Layers as Immobilization Matrix for Glucose Biosensors
    Barwe, S. and Andronescu, C. and Pöller, S. and Schuhmann, W.
    Electroanalysis 27 (2015)
    Benzoxazine oligomers synthesized by a Mannich type reaction of bisphenol A, tetraethylenepentamine and formaldehyde were electrochemically crosslinked in presence of both an Os-complex modified poly(methacrylate) polymer and glucose oxidase. The crosslinking led to the formation of a biocatalytically active layer on an electrode surface exhibiting a swelling process after immersion in an electrolyte solution containing glucose most likely due to the local decrease of the pH value upon glucose oxidation. Optimization of the poly(benzoxazine) to Os-complex modified poly(methacrylate) ratio was performed leading to a reagentless glucose biosensor with improved stability. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/elan.201500131
  • Combined AFM/SECM Investigation of the Solid Electrolyte Interphase in Li-Ion Batteries
    Zampardi, G. and Klink, S. and Kuznetsov, V. and Erichsen, T. and Maljusch, A. and LaMantia, F. and Schuhmann, W. and Ventosa, E.
    ChemElectroChem 2 (2015)
    The solid electrolyte interphase (SEI) is an electronically insulating film formed from the decomposition of the organic electrolyte at the surface of the negative electrodes in Li-ion batteries (LIBs). This film is of vital importance in the performance and safety of LIBs. Atomic force microscopy (AFM) and scanning electrochemical microscopy (SECM) are combined in one platform for the consecutive insitu investigation of surface reactions in LIBs inside an Ar-filled glovebox. As proof of concept, the formation and the electrochemical properties of the SEI formed on glassy carbon electrodes are investigated. Changes in topography during film formation of the SEI are studied via AFM. The AFM tip is then used to partially remove a small area (50×50μm2) of the SEI, which is subsequently probed using SECM in feedback mode. The AFM-scratched spot is clearly visualized in the SECM image, demonstrating the strength of the AFM/SECM combination for the investigation in the field of LIBs. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/celc.201500085
  • Controlling the charge of pH-responsive redox hydrogels by means of redox-silent biocatalytic processes. A biocatalytic off/on switch
    Contin, A. and Plumeré, N. and Schuhmann, W.
    Electrochemistry Communications 51 (2015)
    Coupling of redox-silent biocatalytic processes for analyte detection with enzyme-catalyzed redox reactions for signal generation is proposed by the modulation of electrostatic interactions between a pH-responsive polymer and a redox enzyme to control the off-on transition for electrochemical signal generation. Glassy carbon electrodes are modified with a poly(vinyl)imidazole Os(bipyridine)2Cl redox hydrogel film entrapping urease and PQQ-dependent glucose dehydrogenase, while glucose is present in the solution. The off-on transition is based on the detection of urea as model analyte which is hydrolyzed to ammonia by urease within the hydrogel film concomitantly increasing the local pH value thus invoking deprotonation of the imidazole groups at the polymer backbone. The decrease of positive charges at the polymer decreases electrostatic repulsion between the polymer and the positively charged PQQ-dependent glucose dehydrogenase. Hence, electron transfer rates between polymer-bound Os complexes and PQQ inside the enzyme are enhanced activating electrocatalytic oxidation of glucose. This process generates the electrochemical signal for urea detection. © 2014 Elsevier B.V.
    view abstract10.1016/j.elecom.2014.12.001
  • Coupling of an enzymatic biofuel cell to an electrochemical cell for self-powered glucose sensing with optical readout
    Pinyou, P. and Conzuelo, F. and Sliozberg, K. and Vivekananthan, J. and Contin, A. and Pöller, S. and Plumeré, N. and Schuhmann, W.
    Bioelectrochemistry 106 (2015)
    A miniaturized biofuel cell (BFC) is powering an electrolyser invoking a glucose concentration dependent formation of a dye which can be determined spectrophotometrically. This strategy enables instrument free analyte detection using the analyte-dependent BFC current for triggering an optical read-out system. A screen-printed electrode (SPE) was used for the immobilization of the enzymes glucose dehydrogenase (GDH) and bilirubin oxidase (BOD) for the biocatalytic oxidation of glucose and reduction of molecular oxygen, respectively. The miniaturized BFC was switched-on using small sample volumes (ca. 60μL) leading to an open-circuit voltage of 567mV and a maximal power density of (6.8±0.6) μWcm-2. The BFC power was proportional to the glucose concentration in a range from 0.1 to 1.0mM (R2=0.991). In order to verify the potential instrument-free analyte detection the BFC was directly connected to an electrochemical cell comprised of an optically-transparent SPE modified with methylene green (MG). The reduction of the electrochromic reporter compound invoked by the voltage and current flow applied by the BFC let to MG discoloration, thus allowing the detection of glucose. © 2015 Elsevier B.V..
    view abstract10.1016/j.bioelechem.2015.04.003
  • Detection of 2D phase transitions at the electrode/electrolyte interface using electrochemical impedance spectroscopy
    Tymoczko, J. and Colic, V. and Bandarenka, A.S. and Schuhmann, W.
    Surface Science 631 (2015)
    The capacitance of the electric double layer, CDL, formed at the electrode/electrolyte interface is generally determined by electrochemical impedance spectroscopy (EIS). However, CDL values obtained using EIS data often depend on the ac frequency of the potential perturbation used in EIS. The reasons for the observed frequency dispersions can be various, and hence extracting valuable information about the status of the electrified interface is not possible with the required certainty. In this work, using well-understood electrochemical systems, namely Pt(111) electrodes in contact with a series of acidic sulfate ions containing electrolytes, we provide strong evidence that 2D phase transitions in the adsorbate layers and, in general, structural effects at the electrode/electrolyte interface are in many cases responsible for the frequency dispersion of the double layer capacitance. These empirical findings open new opportunities for the detection and evaluation of 2D phase transition processes and other structural effects using EIS, even in presence of simultaneously occurring electrochemical processes. However, further theoretical elaboration of this effect is necessary. © 2014 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.susc.2014.04.014
  • Determination of the formation and range of stability of the SEI on glassy carbon by local electrochemistry
    Zampardi, G. and La Mantia, F. and Schuhmann, W.
    RSC Advances 5 (2015)
    The solid electrolyte interphase (SEI) is an electronic insulating and ionic conducting layer that is of main importance in lithium-ions batteries, since it critically affects the final performance of the battery system. The formation of this electronic insulating layer was determined in operando on a glassy carbon electrode by means of a microelectrode positioned in close proximity to its surface using scanning electrochemical microscopy (SECM). Glassy carbon was chosen as an ideal model system for carbonaceous materials, since it forms a SEI similar in composition to the one on graphite but concomitantly shows negligible intercalation of lithium ions. Moreover, the stability of the SEI was analysed depending on different potential ranges and the role of the cations on the insulating character of the SEI was investigated. © 2015 The Royal Society of Chemistry.
    view abstract10.1039/c5ra02940f
  • Doping Level of Boron-Doped Diamond Electrodes Controls the Grafting Density of Functional Groups for DNA Assays
    Švorc, L. and Jambrec, D. and Vojs, M. and Barwe, S. and Clausmeyer, J. and Michniak, P. and Marton, M. and Schuhmann, W.
    ACS Applied Materials and Interfaces 7 (2015)
    The impact of different doping levels of boron-doped diamond on the surface functionalization was investigated by means of electrochemical reduction of aryldiazonium salts. The grafting efficiency of 4-nitrophenyl groups increased with the boron levels (B/C ratio from 0 to 20 000 ppm). Controlled grafting of nitrophenyldiazonium was used to adjust the amount of immobilized single-stranded DNA strands at the surface and further on the hybridization yield in dependence on the boron doping level. The grafted nitro functions were electrochemically reduced to the amine moieties. Subsequent functionalization with a succinic acid introduced carboxyl groups for subsequent binding of an amino-terminated DNA probe. DNA hybridization significantly depends on the probe density which is in turn dependent on the boron doping level. The proposed approach opens new insights for the design and control of doped diamond surface functionalization for the construction of DNA hybridization assays. © 2015 American Chemical Society.
    view abstract10.1021/acsami.5b06394
  • Effect of the specific surface area on thermodynamic and kinetic properties of nanoparticle anatase TiO2 in lithium-ion batteries
    Madej, E. and Klink, S. and Schuhmann, W. and Ventosa, E. and La Mantia, F.
    Journal of Power Sources 297 (2015)
    Anatase TiO<inf>2</inf> nanoparticles with a specific surface area of 100 m2 g-1 and 300 m2 g-1 have been investigated as negative insertion electrode material for lithium-ion batteries. Galvanostatic intermittent titration (GITT) and electrochemical impedance spectroscopy (EIS) were used to investigate the effect of the specific surface area on the performance of the material. GITT was performed at C/10 rate, followed by an EIS measurement after each relaxation step. Separation of kinetic and thermodynamic contributions to the overpotential of the phase transformation on Li+ (de-)insertion allowed revealing a dependency of both terms on the specific surface area. The material with higher surface area undergoes intrinsic transformation during the initial cycles affecting the thermodynamics of (de-)insertion while the sample with lower surface area shows large and asymmetric kinetic hindrances. For the material with 15 nm particles, Li+ de-insertion appears to have a higher resistance than lithium insertion. © 2015, Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jpowsour.2015.07.079
  • Efficient Deposition of Semiconductor Powders for Photoelectrocatalysis by Airbrush Spraying
    Gutkowski, R. and Schäfer, D. and Nagaiah, T.C. and Heras, J.E.Y. and Busser, W. and Muhler, M. and Schuhmann, W.
    Electroanalysis 27 (2015)
    Powder catalysts were deposited as thin films on transparent conductive oxides (TCO) by means of an airbrush spray coating technique. Photoelectrocatalytic properties of the powder catalysts were characterized using photocurrent spectroscopy at different wavelengths demonstrating on the one hand the stability of the films and on the other hand the electrical connection with the electrode surface. The morphology and thickness of the deposited powder catalyst films on TCO were characterized using scanning electron microscopy. Aiming at photocatalytic water splitting, semiconductor powders like gallium oxide (Ga<inf>2</inf>O<inf>3</inf>) and zinc oxide (ZnO) were used as test samples to optimize the deposition technique resulting in thin homogeneous layers and good adhesion on the conductive substrate. The proposed airbrush deposition technique of powder catalysts allows closing an experimental gap between microheterogeneous systems and modified electrodes for finding suitable materials for photoelectrochemical water splitting. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/elan.201400363
  • Electrochemical communication between electrodes and rhodobacter capsulatus grown in different metabolic modes
    Hasan, K. and Reddy, K.V.R. and Eßmann, V. and Górecki, K. and Conghaile, P.O. and Schuhmann, W. and Leech, D. and Hägerhäll, C. and Gorton, L.
    Electroanalysis 27 (2015)
    The majority of efforts on microbial and photosynthetic microbial fuel cells are both curiosity driven and made to possibly meet the future growing demand for sustainable energy. The most metabolically versatile purple bacteria Rhodobacter capsulatus is a potential candidate for this purpose. However, utilizing bacteria in such systems requires efficient electronic transfer communication between the microbial cells and the electrodes, which is one of the greatest challenges. Previous studies demonstrated that osmium redox polymers (ORPs) could be used for extracellular electron transfer between the cells and electrodes. Recently, heterotrophically grown R. capsulatus has been wired with ORP modified electrodes. Here in this communication, we report electron transfer from R. capsulatus grown under heterotrophic as well as under photoheterotrophic conditions to electrodes. The cells, immobilized on bare graphite and ORP modified graphite electrodes, were excited with visible light and subsequent photosynthetic electron transfer was recorded using cyclic voltammetric and chronoamperometric measurements. Photoheterotrophically grown R. capsulatus cells on bare graphite generate a significant photocurrent density of 3.46μAcm-2, whereas on an ORP modified electrode the current density increases to 8.46μAcm-2. Furthermore, when 1mM p-benzoquinone is added to the electrolyte the photocurrent density reaches 12.25μAcm-2. Our results could have significant implications in photosynthetic energy conversion and in development of photobioelectrochemical devices. © 2015 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim.
    view abstract10.1002/elan.201400456
  • Electrochemical detection of synthetic DNA and native 16S rRNA fragments on a microarray using a biotinylated intercalator as coupling site for an enzyme label
    Zimdars, A. and Gebala, M. and Hartwich, G. and Neugebauer, S. and Schuhmann, W.
    Talanta 143 (2015)
    Abstract The direct electrochemical detection of synthetic DNA and native 16S rRNA fragments isolated from Escherichia coli is described. Oligonucleotides are detected via selective post-labeling of double stranded DNA and DNA-RNA duplexes with a biotinylated intercalator that enables high-specific binding of a streptavidin/alkaline phosphatase conjugate. The alkaline phosphatase catalyzes formation of p-aminophenol that is subsequently oxidized at the underlying gold electrode and hence enables the detection of complementary hybridization of the DNA capture strands due to the enzymatic signal amplification. The hybridization assay was performed on microarrays consisting of 32 individually addressable gold microelectrodes. Synthetic DNA strands with sequences representing six different pathogens which are important for the diagnosis of urinary tract infections could be detected at concentrations of 60 nM. Native 16S rRNA isolated from the different pathogens could be detected at a concentration of 30 fM. Optimization of the sensing surface is described and influences on the assay performance are discussed. © 2015 Elsevier B.V.
    view abstract10.1016/j.talanta.2015.04.041
  • Electrochemical sensor for nitric oxide using layered films composed of a polycationic dendrimer and nickel(II) phthalocyaninetetrasulfonate deposited on a carbon fiber electrode
    Cancino, J. and Borgmann, S. and Machado, S.A.S. and Zucolotto, V. and Schuhmann, W. and Masa, J.
    Microchimica Acta 182 (2015)
    We have developed an electrochemical sensor for nitric oxide that is based on multi-layers of nickel(II) phthalocyaninetetrasulfonate and a polyamidoamine dendrimer assembled on the surface of a carbon-fiber microelectrode. This sensor responds to nitric oxide at a working potential of 800 mV with a sensitivity of 5.54 pA∙μM‾1 which, however, depends on the dendrimer layer position deposited on the microelectrode. The limit of detection is as low as 5.5 μM at a signal-to-noise ratio of 3. The electrode exhibits good selectivity for nitric oxide over common interferents including dopamine, nitrite, hydrogen peroxide, norepinephrine, epinephrine and ascorbic acid. [Figure not available: see fulltext.] © 2014, European Union.
    view abstract10.1007/s00604-014-1425-0
  • Evaluation of the Electrochemical Stability of Model Cu-Pt(111) Near-Surface Alloy Catalysts
    Tymoczko, J. and Calle-Vallejo, F. and Čolić, V. and Schuhmann, W. and Bandarenka, A.S.
    Electrochimica Acta 179 (2015)
    Better understanding of the factors responsible for the long-term stability of electrocatalysts is of increasing importance for the development of new generations of efficient electrode materials relevant for sustainable energy provision. Therefore, experiments with model, often single-crystal catalytic surfaces are of significance for fundamental electrochemistry and technological applications. Among model electrocatalysts, near-surface alloys (NSAs) of Pt with Cu, Ni and other metals formed via electrochemical deposition and thermal annealing have shown remarkable properties, demonstrating high activity towards a number of important reactions, including the oxygen reduction reaction (ORR) and CO oxidation. However, relatively little is known about the electrochemical stability and mechanisms of degradation of model NSAs. In this work, we employ a simple electrochemical approach, supported by density functional theory calculations, to evaluate the stability of Cu-Pt(111) NSAs in 0.1 M HClO4. Our results show that ∼30% of the Cu atoms initially incorporated into the second atomic layer of Pt are lost within the first 2000 cycles performed between 0.05 V and 1.0 V (RHE). After 5000 cycles, ca. half of the Cu atoms initially placed in the second atomic layer still remained in the subsurface region. The dissolution of Cu has a substantial impact on the measured shift in the average OH-binding energy for the catalyst surface and, consequently, on the ORR activity. Interestingly, after dissolution of Cu from NSAs, voltammetric features, which are characteristic to the Pt(111) facets, are partially restored suggesting the formation of NSA and Pt(111) domains in the resulting surface. © 2015 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.electacta.2015.02.110
  • Experimental Aspects in Benchmarking of the Electrocatalytic Activity
    Čolić, V. and Tymoczko, J. and Maljusch, A. and Ganassin, A. and Schuhmann, W., Prof. and Bandarenka, A.S.
    ChemElectroChem 2 (2015)
    With the high interest in improving the performance of electrocatalysts for technologically significant reactions, great efforts are directed at the assessment of the activities of various catalytic materials. For this purpose, it is important to compare the catalytic activities measured using different methods and under different conditions. To achieve this, it is of utmost importance to avoid certain methodological and instrumental issues that can severely affect the obtained experimental results. Using well-defined systems, we demonstrate the importance of experimental conditions in the assessment and benchmarking of the activity of catalytic processes for various reactions. Particularly, we demonstrate that the correction of the uncompensated ohmic resistance using impedance spectroscopy measurements requires particular attention and additional procedures which are normally ignored. Additionally, we demonstrate how the uncompensated resistance changes with the potential if a non-conducting gas phase is accumulated in the system, hence influencing the activity measurement. It is further shown that a correct choice for surface-limited reactions for the determination of the real surface area of catalytic electrodes plays a key role in ensuring more meaningful activity assessment. Not as easy as it seems: Benchmarking of the electrocatalytic activity can be unexpectedly very demanding due to experimental issues which are often underestimated. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/celc.201402295
  • Fe-cr-al containing oxide semiconductors as potential solar water-splitting materials
    Sliozberg, K. and Stein, H.S. and Khare, C. and Parkinson, B.A. and Ludwig, Al. and Schuhmann, W.
    ACS Applied Materials and Interfaces 7 (2015)
    A high-throughput thin film materials library for Fe-Cr-Al-O was obtained by reactive magnetron cosputtering and analyzed with automated EDX and XRD to elucidate compositional and structural properties. An automated optical scanning droplet cell was then used to perform photoelectrochemical measurements of 289 compositions on the library, including electrochemical stability, potentiodynamic photocurrents and photocurrent spectroscopy. The photocurrent onset and open circuit potentials of two semiconductor compositions (n-type semiconducting: Fe51Cr47Al2Ox, p-type semiconducting Fe36.5Cr55.5Al8Ox) are favorable for water splitting. Cathodic photocurrents are observed at 1.0 V vs RHE for the p-type material exhibiting an open circuit potential of 0.85 V vs RHE. The n-type material shows an onset of photocurrents at 0.75 V and an open circuit potential of 0.6 V. The p-type material showed a bandgap of 1.55 eV, while the n-type material showed a bandgap of 1.97 eV. © 2015 American Chemical Society.
    view abstract10.1021/am508946e
  • Finding optimal surface sites on heterogeneous catalysts by counting nearest neighbors
    Calle-Vallejo, F. and Tymoczko, J. and Colic, V. and Vu, Q.H. and Pohl, M.D. and Morgenstern, K. and Loffreda, D. and Sautet, P. and Schuhmann, W. and Bandarenka, A.S.
    Science 350 (2015)
    A good heterogeneous catalyst for a given chemical reaction very often has only one specific type of surface site that is catalytically active. Widespread methodologies such as Sabatier-type activity plots determine optimal adsorption energies to maximize catalytic activity, but these are difficult to use as guidelines to devise new catalysts. We introduce "coordination-activity plots" that predict the geometric structure of optimal active sites. The method is illustrated on the oxygen reduction reaction catalyzed by platinum. Sites with the same number of first-nearest neighbors as (111) terraces but with an increased number of second-nearest neighbors are predicted to have superior catalytic activity. We used this rationale to create highly active sites on platinum (111), without alloying and using three different affordable experimental methods.
    view abstract10.1126/science.aab3501
  • High-quality functionalized few-layer graphene: Facile fabrication and doping with nitrogen as a metal-free catalyst for the oxygen reduction reaction
    Sun, Z. and Masa, J. and Weide, P. and Fairclough, S.M. and Robertson, A.W. and Ebbinghaus, P. and Warner, J.H. and Tsang, S.C.E. and Muhler, M. and Schuhmann, W.
    Journal of Materials Chemistry A 3 (2015)
    Functionalization of graphene is fundamental to facilitating its processing and offers a wide scope for advanced applications. Here we demonstrate a facile, highly efficient and mild covalent functionalization of graphene using HNO<inf>3</inf> vapour. This results in functionalized few-layer graphene (FLG) that is high in both quantity and quality. We fully characterized the structure and defect level of functionalized FLG by X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy and Raman spectroscopy. The results from this analysis show the tunability of the surface oxygen functionalities of FLG achieved through controlling the oxidation temperature without affecting the major intrinsic properties of graphene. This allows for further doping for applications, for example with nitrogen as a metal-free catalyst in the oxygen reduction reaction. © 2015 The Royal Society of Chemistry.
    view abstract10.1039/c5ta02248g
  • High-Throughput Screening of Thin-Film Semiconductor Material Libraries I: System Development and Case Study for Ti-W-O
    Sliozberg, K. and Schäfer, D. and Erichsen, T. and Meyer, R. and Khare, C. and Ludwig, Al. and Schuhmann, W.
    ChemSusChem 8 (2015)
    An automated optical scanning droplet cell (OSDC) enables high-throughput quantitative characterization of thin-film semiconductor material libraries. Photoelectrochemical data on small selected measurement areas are recorded including intensity-dependent photopotentials and -currents, potentiodynamic and potentiostatic photocurrents, as well as photocurrent (action) spectra. The OSDC contains integrated counter and double-junction reference electrodes and is fixed on a precise positioning system. A Xe lamp with a monochromator is coupled to the cell through a thin poly(methyl methacrylate) (PMMA) optical fiber. A specifically designed polytetrafluoroethylene (PTFE) capillary tip is pressed on the sample surface and defines through its diameter the homogeneously illuminated measurement area. The overall and wavelength-resolved irradiation intensities and the cell surface area are precisely determined and calibrated. System development and its performance are demonstrated by means of screening of a Ti-W-O thin film. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/cssc.201402917
  • High-throughput screening of thin-film semiconductor material libraries II: Characterization of Fe-W-O libraries
    Meyer, R. and Sliozberg, K. and Khare, C. and Schuhmann, W. and Ludwig, Al.
    ChemSusChem 8 (2015)
    Metal oxides are promising materials for solar water splitting. To identify suitable materials within the ternary system Fe-W-O, thin-film material libraries with combined thickness and compositional gradients were synthesized by combinatorial reactive magnetron sputtering. These libraries (>1000 different samples) were investigated by means of structural and functional high-throughput characterization techniques to establish correlations between composition, crystallinity, morphology, thickness, and photocurrent density in the compositional range between (Fe<inf>6</inf>W<inf>94</inf>)O<inf>x</inf> and (Fe<inf>61</inf>W<inf>39</inf>)O<inf>x</inf>. In addition to the well-known phase WO<inf>3</inf>, the binary phase W<inf>5</inf>O<inf>14</inf> and the ternary phase Fe<inf>2</inf>O<inf>6</inf>W show enhanced photoelectrochemical activity. The highest photocurrent density of 65 μA cm-2 was achieved for the composition (Fe<inf>15</inf>W<inf>85</inf>)O<inf>x</inf>, which contains the W<inf>5</inf>O<inf>14</inf> phase and has a thickness of 1060 nm. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cssc.201402918
  • In-operando evaluation of the effect of vinylene carbonate on the insulating character of the solid electrolyte interphase
    Zampardi, G. and La Mantia, F. and Schuhmann, W.
    Electrochemistry Communications 58 (2015)
    The solid electrolyte interphase (SEI) is an electronic insulating layer which highly affects the performance of lithium-ion batteries, especially when electrodes with low (de-)intercalation potentials such as graphite are employed. The formation of the SEI was investigated in-operando on graphite when vinylene carbonate (VC) was present as an additive in solution using feedback-mode SECM. The potential at which the surface started to become insulating was at 0.8 V vs. Li/Li+ in VC-free electrolytes, while it was at 1.3 V in VC-containing electrolytes. Nevertheless, potentials more cathodic than 0.8 V have to be reached to form a homogeneous SEI. No influence in the electronic properties of the formed SEI with different concentrations of VC was observed. (C) 2015 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.elecom.2015.05.013
  • Influence of the alkali metal cations on the activity of Pt(1 1 1) towards model electrocatalytic reactions in acidic sulfuric media
    Tymoczko, J. and Colic, V. and Ganassin, A. and Schuhmann, W. and Bandarenka, A.S.
    Catalysis Today 244 (2015)
    The impact of the alkali metal cations (Li+, Na+, K+, Rb+, Cs+) on the catalytic activity of Pt(1 1 1) electrodes towards model reactions (oxygen reduction, oxygen evolution, hydrogen evolution and hydrogen oxidation) in sulfuric acid has been evaluated. In contrast to essentially monotonic activity trends (i.e. from Li+ to Cs+) reported in the literature for alkaline media, the nature of the cations influences the activity of the Pt electrodes largely non-monotonously in the presence of SO4 2- ions. This is in certain cases due to the specifically adsorbing (bi)sulfate anions which make interactions between electrolyte components and reaction intermediates very complex. Surprisingly, the activity of the Pt(1 1 1) electrodes towards all investigated electrocatalytic reactions was substantially higher in Rb+ ions containing electrolytes. © 2014 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.cattod.2014.07.007
  • Intercalation of Proflavine in ssDNA Aptamers: Effect on Binding of the Specific Target Chloramphenicol
    Pilehvar, S. and Jambrec, D. and Gebala, M. and Schuhmann, W. and De Wael, K.
    Electroanalysis 27 (2015)
    The structural modification of ssDNA-based aptamers upon specific binding of its target molecule leads to changes of the charge-transfer resistance (R<inf>ct</inf>) of a negatively-charged free-diffusing redox probe. The aptamer adopts a structure due to self-hybridization which is stabilized using profalvine as intercalator. The pre-organized aptamer structure is used to detect chloramphenicol (CAP) requiring a substantial change of the aptamer structure indicated by a CAP concentration dependent increase in the R<inf>ct</inf> values. Pre-incubation of the aptamer-modified electrode with an intercalator allows for the modulation of the aptamer/target interaction and hence for a modulation of the CAP-dependent variation of the R<inf>ct</inf> values. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/elan.201500192
  • Light Induced H2 Evolution from a Biophotocathode Based on Photosystem 1 - Pt Nanoparticles Complexes Integrated in Solvated Redox Polymers Films
    Zhao, F. and Conzuelo, F. and Hartmann, V. and Li, H. and Nowaczyk, M.M. and Plumeré, N. and Rögner, M. and Schuhmann, W.
    Journal of Physical Chemistry B 119 (2015)
    We report on a biophotocathode based on photosystem 1 (PS1)-Pt nanoparticle complexes integrated in a redox hydrogel for photoelectrocatalytic H2 evolution at low overpotential. A poly(vinyl)imidazole Os(bispyridine)2Cl polymer serves as conducting matrix to shuttle the electrons from the electrode to the PS1-Pt complexes embedded within the hydrogel. Light induced charge separation at the PS1-Pt complexes results in the generation of photocurrents (4.8 ± 0.4 μA cm-2) when the biophotocathodes are exposed to anaerobic buffer solutions. Under these conditions, the protons are the sole possible electron acceptors, suggesting that the photocurrent generation is associated with H2 evolution. Direct evidence for the latter process is provided by monitoring the H2 production with a Pt microelectrode in scanning electrochemical microscopy configuration over the redox hydrogel film containing the PS1-Pt complexes under illumination. © 2015 American Chemical Society.
    view abstract10.1021/acs.jpcb.5b03511
  • Mechanism of protection of catalysts supported in redox hydrogel films
    Fourmond, V. and Stapf, S. and Li, H. and Buesen, D. and Birrell, J. and Rüdiger, O. and Lubitz, W. and Schuhmann, W. and Plumeré, N. and Léger, C.
    Journal of the American Chemical Society 137 (2015)
    The use of synthetic inorganic complexes as supported catalysts is a key route in energy production and in industrial synthesis. However, their intrinsic oxygen sensitivity is sometimes an issue. Some of us have recently demonstrated that hydrogenases, the fragile but very efficient biological catalysts of H<inf>2</inf> oxidation, can be protected from O<inf>2</inf> damage upon integration into a film of a specifically designed redox polymer. Catalytic oxidation of H<inf>2</inf> produces electrons which reduce oxygen near the film/solution interface, thus providing a self-activated protection from oxygen [Plumeré et al., Nat Chem. 2014, 6, 822-827]. Here, we rationalize this protection mechanism by examining the time-dependent distribution of species in the hydrogenase/polymer film, using measured or estimated values of all relevant parameters and the numerical and analytical solutions of a realistic reaction-diffusion scheme. Our investigation sets the stage for optimizing the design of hydrogenase-polymer films, and for expanding this strategy to other fragile catalysts. © 2015 American Chemical Society.
    view abstract10.1021/jacs.5b01194
  • Nitrogen-doped carbon cloth as a stable self-supported cathode catalyst for air/H2-breathing alkaline fuel cells
    Vivekananthan, J. and Masa, J. and Chen, P. and Xie, K. and Muhler, M. and Schuhmann, W.
    Electrochimica Acta 182 (2015)
    The power output of a fuel cell is limited by among others, the intrinsic activity of the active matrix and the mass transport of the products and reactants. Of equally crucial importance is the long-term durability of the cell components including the electrocatalysts. Herein, carbon cloth (CC) was functionalized with nitrogen-containing groups by treatment with NH<inf>3</inf> at 400 °C or by pyrolysis of a composite of polypyrrole on CC at 800 °C. The resulting N-doped CC (NCC) was employed as an air-breathing cathode in a custom-made air/H<inf>2</inf> alkaline fuel cell, serving as the current collector as well as catalytic matrix with enhanced oxygen transport. The cell exhibited high operational durability with only 2% loss in activity after 25 days and delivered a maximum power density of 120 mW m-2 at a voltage of 0.35 V. The concept of a self-supported highly stable metal-free catalyst and the breathing H<inf>2</inf>/air cell design provide platforms for the design and investigation of catalysts. Moreover, a higher cell voltage can be realized if the cell is operated under pressurized conditions or by replacing air with O<inf>2.</inf> © 2015 Published by Elsevier Ltd.
    view abstract10.1016/j.electacta.2015.09.064
  • Non-aqueous semi-solid flow battery based on Na-ion chemistry. P2-type NaxNi0.22Co0.11Mn0.66O2-NaTi2(PO4)3
    Ventosa, E. and Buchholz, D. and Klink, S. and Flox, C. and Chagas, L.G. and Vaalma, C. and Schuhmann, W. and Passerini, S. and Morante, J.R.
    Chemical Communications 51 (2015)
    We report the first proof of concept for a non-aqueous semi-solid flow battery (SSFB) based on Na-ion chemistry using P2-type Na<inf>x</inf>Ni<inf>0.22</inf>Co<inf>0.11</inf>Mn<inf>0.66</inf>O<inf>2</inf> and NaTi<inf>2</inf>(PO<inf>4</inf>)<inf>3</inf> as positive and negative electrodes, respectively. This concept opens the door for developing a new low-cost type of non-aqueous semi-solid flow batteries based on the rich chemistry of Na-ion intercalating compounds. © The Royal Society of Chemistry 2015.
    view abstract10.1039/c4cc09597a
  • Non-covalent interactions in water electrolysis: Influence on the activity of Pt(111) and iridium oxide catalysts in acidic media
    Ganassin, A. and Colic, V. and Tymoczko, J. and Bandarenka, A.S. and Schuhmann, W.
    Physical Chemistry Chemical Physics 17 (2015)
    Electrolyte components, which are typically not considered to be directly involved in catalytic processes at solid-liquid electrified interfaces, often demonstrate a significant or even drastic influence on the activity, stability and selectivity of electrocatalysts. While there has been certain progress in the understanding of these electrolyte effects, lack of experimental data for various important systems frequently complicates the rational design of new active materials. Modern proton-exchange membrane (PEM) electrolyzers utilize Pt- and Ir-based electrocatalysts, which are among the very few materials that are both active and stable under the extreme conditions of water splitting. We use model Pt(111) and Ir-oxide films grown on Ir(111) electrodes and explore the effect of alkali metal cations and sulfate-anions on the hydrogen evolution and the oxygen evolution reactions in acidic media. We demonstrate that sulfate anions decrease the activity of Ir-oxide towards the oxygen evolution reaction while Rb+ drastically promotes hydrogen evolution reaction at the Pt(111) electrodes as compared to the reference HClO4 electrolytes. Issues related to the activity benchmarking for these catalysts are discussed. This journal is © the Owner Societies.
    view abstract10.1039/c4cp04791e
  • Non-destructive Patterning of Carbon Electrodes by Using the Direct Mode of Scanning Electrochemical Microscopy
    Stratmann, L. and Clausmeyer, J. and Schuhmann, W.
    ChemPhysChem 16 (2015)
    Patterning of glassy carbon surfaces grafted with a layer of nitrophenyl moieties was achieved by using the direct mode of scanning electrochemical microscopy (SECM) to locally reduce the nitro groups to hydroxylamine and amino functionalities. SECM and atomic force microscopy (AFM) revealed that potentiostatic pulses applied to the working electrode lead to local destruction of the glassy carbon surface, most likely caused by etchants generated at the positioned SECM tip used as the counter electrode. By applying galvanostatic pulses, and thus, limiting the current during structuring, corrosion of the carbon surface was substantially suppressed. After galvanostatic patterning, unambiguous proof of the formation of the anticipated amino moieties was possible by modulation of the pH value during the feedback mode of SECM imaging. This patterning strategy is suitable for the further bio-modification of microstructured surfaces. Alkaline phosphatase, as a model enzyme, was locally bound to the modified areas, thus showing that the technique can be used for the development of protein microarrays. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cphc.201500585
  • On the Role of Metals in Nitrogen-Doped Carbon Electrocatalysts for Oxygen Reduction
    Masa, J. and Xia, W. and Muhler, M. and Schuhmann, W.
    Angewandte Chemie - International Edition 54 (2015)
    The notion of metal-free catalysts is used to refer to carbon materials modified with nonmetallic elements. However, some claimed metal-free catalysts are prepared using metal-containing precursors. It is highly contested that metal residues in nitrogen-doped carbon (NC) catalysts play a crucial role in the oxygen reduction reaction (ORR). In an attempt to reconcile divergent views, a definition for truly metal-free catalysts is proposed and the differences between NC and M-N<inf>x</inf>/C catalysts are discussed. Metal impurities at levels usually undetectable by techniques such as XPS, XRD, and EDX significantly promote the ORR. Poisoning tests to mask the metal ions reveal the involvement of metal residues as active sites or as modifiers of the electronic structure of the active sites in NC. The unique merits of both M-N<inf>x</inf>/C and NC catalysts are discussed to inspire the development of more advanced nonprecious-metal catalysts for the ORR. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/anie.201500569
  • One-Pot Synthesis of Carbon-Coated Nanostructured Iron Oxide on Few-Layer Graphene for Lithium-Ion Batteries
    Sun, Z. and Madej, E. and Wiktor, C. and Sinev, I. and Fischer, R.A. and Van Tendeloo, G. and Muhler, M. and Schuhmann, W. and Ventosa, E.
    Chemistry - A European Journal 21 (2015)
    Nanostructure engineering has been demonstrated to improve the electrochemical performance of iron oxide based electrodes in Li-ion batteries (LIBs). However, the synthesis of advanced functional materials often requires multiple steps. Herein, we present a facile one-pot synthesis of carbon-coated nanostructured iron oxide on few-layer graphene through high-pressure pyrolysis of ferrocene in the presence of pristine graphene. The ferrocene precursor supplies both iron and carbon to form the carbon-coated iron oxide, while the graphene acts as a high-surface-area anchor to achieve small metal oxide nanoparticles. When evaluated as a negative-electrode material for LIBs, our composite showed improved electrochemical performance compared to commercial iron oxide nanopowders, especially at fast charge/discharge rates. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/chem.201501935
  • Onset potential determination at gas-evolving catalysts by means of constant-distance mode positioning of nanoelectrodes
    Botz, A.J.R. and Nebel, M. and Rincón, R.A. and Ventosa, E. and Schuhmann, W.
    Electrochimica Acta 179 (2015)
    The onset potential of an electrocatalytic reaction is frequently used as an indicator to compare the catalytic performance of electrocatalysts. However, in addition to the fact that the onset potential is an undefined physico-chemical value which is dependent on the sensitivity of the used potentiostat its determination using voltammetry at the catalyst-modified electrode surface may be superimposed by additional Faradaic reactions e.g. from redox conversions of the catalyst material or corrosion processes. Gas-evolving electrodes suffer additionally from the dynamics of gas bubble formation and departure leading to inherent limitations of voltammetric studies directly performed at the catalyst-modified electrode. Nanometer-sized electrodes accurately positioned by means of shearforce-based constant-distance mode SECM are proposed for the highly sensitive determination of the onset potential of microcavity electrodes filled with different perovskites as oxygen evolution catalysts. Double barrel microcavity electrodes are additionally suggested for the simultaneous investigation of two catalysts. They enable direct referencing of a catalyst with a benchmark catalyst material in a single experiment. © 2015 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.electacta.2015.04.145
  • Optimization of Os-complex modified redox polymers for improving biocatalysis of PQQ-sGDH based electrodes
    Pinyou, P. and Pöller, S. and Chen, X. and Schuhmann, W.
    Electroanalysis 27 (2015)
    A variety of Os-complex modified redox polymers were synthesized and optimized for an improved bioelectrochemical communication with PQQ-sGDH. The properties of the polymers were varied by changing the monomers composition leading to improved immobilization and increased enzyme loading. Three Os-complexes were designed exhibiting different ligands but similar redox potentials adapted for efficient electron transfer with PQQ-sGDH. Effects of the ligand sphere, polymer backbone and length of the tether chain between Os-complex and polymer backbone as well as the impact of different bifunctional crosslinkers on the catalytic current were evaluated. The optimized bioelectrodes exhibited significantly improved catalytic currents and stability. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/elan.201400436
  • Oxygen-plasma-functionalized carbon nanotubes as supports for platinum-ruthenium catalysts applied in electrochemical methanol oxidation
    Chetty, R. and Maniam, K.K. and Schuhmann, W. and Muhler, M.
    ChemPlusChem 80 (2015)
    Multiwalled carbon nanotubes (CNTs) functionalized by oxygen plasma were used as a support for platinum-ruthenium nanoparticles for electrochemical methanol oxidation. The influence of plasma treatment time on the electrocatalytic activity was investigated by cyclic voltammetry, CO stripping voltammetry, and chronoamperometry. The electrocatalysts were characterized by Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The results showed that oxygen plasma treatment led to the formation of -CO and -COO groups on the CNT surface. Platinum-ruthenium nanoparticles dispersed with an optimum plasma treatment time of 30 min exhibited the maximum catalytic activity towards methanol oxidation. The rationale for the high catalytic activity is discussed. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/cplu.201402192
  • Phenolated Oleic Acid Based Polybenzoxazine Derivatives as Corrosion Protection Layers
    Bələnucə, B. and Raicopol, M. and Maljusch, A. and Garea, S. and Hanganu, A. and Schuhmann, W. and Andronescu, C.
    ChemPlusChem 80 (2015)
    Benzoxazine derivatives were synthesized using phenolated methyl oleate and either aniline, 1,6-diaminohexane, or 4,4′-diaminodiphenylmethane, respectively, as amine components. Polymerization of the benzoxazine derivatives led to the formation of hydrophobic and dense coatings on Zn-Mg-Al alloy coated steel sheets for passive corrosion protection. The polybenzoxazine coatings which are formed by crosslinking during a heat-treatment step invoked a substantial anodic shift of the open-circuit potential as well as the breakthrough potential in potentiodynamic measurements. The proposed polybenzoxazine derivatives pave the way for a new type of passive polymer protection system based on sustainably obtained precursor components. Copyright © 2015 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/cplu.201500092
  • Poly(benzoxazine)s Modified with Osmium Complexes as a Class of Redox Polymers for Wiring of Enzymes to Electrode Surfaces
    Alsaoub, S. and Barwe, S. and Andronescu, C. and Pöller, S. and Ruff, A. and Schuhmann, W.
    ChemPlusChem 80 (2015)
    Benzoxazine-based redox polymers bearing Os complexes are synthesized and used as an immobilization matrix for glucose oxidase (GOx) as a model system for a reagentless biosensor. The polymers are formed by electrochemically induced anodic polymerization of the corresponding benzoxazine monomers modified with Os complexes. The precursors are synthesized in a Mannich-type reaction between bisphenol A, formaldehyde, and the corresponding Os complexes or ligands, which contain free amino groups. The Os complexes are redox active within the polymer films, and thus, can be used as redox relays for the electron transfer between the electrode surface and the prosthetic group within the enzyme. Entrapment of GOx within the poly(benzoxazine) film is achieved successfully, as shown by the biocatalytic activity of the poly(benzoxazine)/GOx films upon the addition of glucose. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cplu.201500135
  • Potential-Assisted DNA Immobilization as a Prerequisite for Fast and Controlled Formation of DNA Monolayers
    Jambrec, D. and Gebala, M. and La Mantia, F. and Schuhmann, W.
    Angewandte Chemie - International Edition 54 (2015)
    Highly reproducible and fast potential-assisted immobilization of single-stranded (ss)DNA on gold surfaces is achieved by applying a pulse-type potential modulation. The desired DNA coverage can be obtained in a highly reproducible way within minutes. Understanding the underlying processes occurring during potential-assisted ssDNA immobilization is crucial. We propose a model that considers the role of ions surrounding the DNA strands, the distance dependence of the applied potentials within the electrolyte solution, and most importantly the shift of the potential of zero charge during the immobilization due to the surface modification with DNA. The control of the surface coverage of ssDNA as well as the achieved speed and high reproducibility are seen as prerequisites for improved DNA-based bioassays. Stir it up: The desired DNA coverage can be reached within minutes by stirring the DNA in front of an electrode by means of a pulse-type potential modulation. The mechanism of potential-assisted immobilization can be understood by considering the role of ions surrounding the DNA, the distance over which applied potentials have an impact on DNA, and the shift of the potential of zero charge (pzc) during the immobilization due to DNA immobilization. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/anie.201506672
  • Scanning Electrochemical Microscopy Applied to the Investigation of Lithium (De-)Insertion in TiO2
    Zampardi, G. and Ventosa, E. and La Mantia, F. and Schuhmann, W.
    Electroanalysis 27 (2015)
    The electrochemical insertion of lithium ions in anatase TiO<inf>2</inf> nanoparticles was investigated by scanning electrochemical microscopy in the feedback mode of operation using ferrocene as reporting redox couple at different potentials in presence and absence of lithium ions. By fixing the position of the tip while performing cyclic voltammetry at the TiO<inf>2</inf> paste electrode, the formation of the solid electrolyte interphase (SEI) on the surface of titania was followed in situ. Different scan rates have been used to study the kinetics of SEI formation. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/elan.201400613
  • Scanning electrochemical microscopy of Li-ion batteries
    Ventosa, E. and Schuhmann, W.
    Physical Chemistry Chemical Physics 17 (2015)
    Li-ion batteries (LIBs) are receiving increasing attention over the past decade due to their high energy density. This energy storage technology is expected to continue improving the performance, especially for its large-scale deployment in plug-in hybrid electric vehicles (PHEVs) and full electric vehicles (EVs). Such improvement requires having a large variety of analytical techniques at scientists' disposal in order to understand and address the multiple mechanisms and processes occurring simultaneously in this complex system. This perspective article aims to highlight the strength and potential of scanning electrochemical microscopy (SECM) in this field. After a brief description of a LIB system and the most commonly used techniques in this field, the unique information provided by SECM is illustrated by discussing several recent examples from the literature. © 2015 the Owner Societies.
    view abstract10.1039/c5cp02268a
  • Semi-artificial photosynthetic z-scheme for hydrogen production from water
    Kothe, T. and Schuhmann, W. and Rögner, M. and Plumeré, N.
    Biohydrogen (2015)
    view abstract10.1515/9783110336733.189
  • Solid electrolyte interphase in semi-solid flow batteries: A Wolf in sheep's clothing
    Ventosa, E. and Zampardi, G. and Flox, C. and La Mantia, F. and Schuhmann, W. and Morante, J.R.
    Chemical Communications 51 (2015)
    The formation of the alkyl carbonate-derived solid electrolyte interphase (SEI) enables the use of active materials operating at very cathodic potentials in Li-ion batteries. However, the SEI in semi-solid flow batteries results in a hindered electron transfer between a fluid electrode and the current collector restricting the operating potentials to ca. 0.8 V vs. Li/Li+ for EC-based electrolytes. © The Royal Society of Chemistry 2015.
    view abstract10.1039/c5cc04767f
  • Synthesis and electrochromic properties of conducting polymers based on highly planar 2,7-disubstituted xanthene derivatives
    Olech, K. and Gutkowski, R. and Kuznetsov, V., Dr. and Roszak, S., Prof. Dr. and Sołoducho, J. and Schuhmann, W.
    ChemPlusChem 80 (2015)
    On the basis of preliminary DFT calculations, p-type semiconducting polymers based on 2,7-substituted xanthene building blocks that show a high degree of planarity were designed. The synthesis, electrochemical characterization, and theoretical modeling of 2,7-bis(thiophen-2-yl)-9,9-dimethylxanthene (1) and 2,7-bis(3-hexylthiophen-2-yl)-9,9-dimethylxanthene (2) is described. The synthetic procedure is based on the incorporation of thiophene rings by means of Pd-catalyzed cross-coupling reactions, which lead to monomers 1 and 2. Copolymers P1 and P2 obtained by means of anodic polymerization have been characterized by spectroscopic and electrochemical methods. Electrochromism was observed for both polymers. The experimental data supported by density functional theory modeling explain the influence of alkyl chain substitution on the properties of the investigated copolymers. Copyright © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cplu.201402349
  • Temperature-Induced Modulation of the Sample Position in Scanning Electrochemical Microscopy
    Clausmeyer, J. and Schäfer, D. and Nebel, M. and Schuhmann, W.
    ChemElectroChem 2 (2015)
    Feedback-mode scanning electrochemical microscopy (SECM) was conducted with insitu control of the sample temperature using a Peltier element. The intrinsic heat pulsing of the temperature control feedback loop causes the sample position to oscillate periodically. By synchronizing tip current acquisition with the oscillatory temperature modulation of the sample, tip current values at different tip-to-sample separations are recorded and used to estimate the slope of the distance-current relationship (z-approach curve). In-phase amplification of only the distance-dependent current signal is achieved using the approximated slope and hence the relative imaging contrast is enhanced. Moreover, sample-position modulated SECM allows distinguishing between electrochemically active and inactive domains on the sample in a single area scan without an apriori knowledge of the sample topography. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/celc.201500087
  • Using cavity microelectrodes for electrochemical noise studies of oxygen-evolving catalysts
    Rincón, R.A. and Battistel, A. and Ventosa, E. and Chen, X. and Nebel, M. and Schuhmann, W.
    ChemSusChem 8 (2015)
    Cavity microelectrodes were used as a binder-free platform to evaluate oxygen evolution reaction (OER) electrocatalysts with respect to gas bubble formation and departure. Electrochemical noise measurements were performed by using RuO2 as a benchmark catalyst and the perovskite La0.58Sr0.4Fe0.8Co0.2O3 as a non-noble metal OER catalyst with lower intrinsic conductivity. Changes in the current during the OER originate from variations in electrolyte resistance during the formation of the gas phase and partial coverage of the active area. Fluctuations observed in current and conductance transients were used to establish the contribution from the ohmic overpotential and to determine the characteristic frequency of oxygen evolution. The proposed quantitative determination of gas bubble growth and departure opens up the route for a rational interface design by considering gas bubble growth and departure as a main contributing factor to the overall electrocatalytic activity at high current densities. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cssc.201402855
  • Vegetable oil-based polybenzoxazine derivatives coatings on Zn-Mg-Al alloy coated steel
    Raicopol, M. and Bălănucă, B. and Sliozberg, K. and Schlüter, B. and Gârea, S.A. and Chira, N. and Schuhmann, W. and Andronescu, C.
    Corrosion Science 100 (2015)
    Based on environmentally friendly (bio-based) precursor materials a new class of benzoxazine derivatives was synthesized using phenolated high oleic sunflower oil as phenol component and either aniline or 1,6-diaminohexane as amine components. Hydrophobic and dense poly(benzoxazine) coatings on Zn-Mg-Al alloy coated steel were obtained after spin-coating or air-brush type spray coating by crosslinking during a heat treatment step. The poly(benzoxazine)-coated ZM-steel samples showed an anodic shift of the open circuit potential as well as the break-through potential. Using an automatic scanning droplet cell the impact of the polymer film thickness on corrosion protection was evaluated. © 2015 Elsevier Ltd.
    view abstract10.1016/j.corsci.2015.08.018
  • Wet Nanoindentation of the Solid Electrolyte Interphase on Thin Film Si Electrodes
    Kuznetsov, V. and Zinn, A.-H. and Zampardi, G. and Borhani-Haghighi, S. and La Mantia, F. and Ludwig, Al. and Schuhmann, W. and Ventosa, E.
    ACS Applied Materials and Interfaces 7 (2015)
    The solid electrolyte interphase (SEI) film formed at the surface of negative electrodes strongly affects the performance of a Li-ion battery. The mechanical properties of the SEI are of special importance for Si electrodes due to the large volumetric changes of Si upon (de)insertion of Li ions. This manuscript reports the careful determination of the Young's modulus of the SEI formed on a sputtered Si electrode using wet atomic force microscopy (AFM)-nanoindentation. Several key parameters in the determination of the Young's modulus are considered and discussed, e.g., wetness and roughness-thickness ratio of the film and the shape of a nanoindenter. The values of the Young's modulus were determined to be 0.5-10 MPa under the investigated conditions which are in the lower range of those previously reported, i.e., 1 MPa to 10 GPa, pointing out the importance of the conditions of its determination. After multiple electrochemical cycles, the polymeric deposits formed on the surface of the SEI are revealed, by force-volume mapping in liquid using colloidal probes, to extend up to 300 nm into bulk solution. © 2015 American Chemical Society.
    view abstract10.1021/acsami.5b06700
  • A carbon-coated TiO2(B) nanosheet composite for lithium ion batteries
    Sun, Z. and Huang, X. and Muhler, M. and Schuhmann, W. and Ventosa, E.
    Chemical Communications 50 (2014)
    The carbon-coated TiO2(B) nanosheet composite synthesized by one-step hydrolysis of TiCl3 followed by vacuum annealing and air annealing delivers outstanding electrochemical performance as a negative electrode for Li-ion batteries, i.e. reversible capacity above 150 mA h g -1 at 30 C (10 A g-1). This journal is © the Partner Organisations 2014.
    view abstract10.1039/c4cc01888e
  • A miniaturized voltammetric pH sensor based on optimized redox polymers
    Pöller, S. and Schuhmann, W.
    Electrochimica Acta 140 (2014)
    A miniaturized pH sensor based on the voltammetric determination of the potential difference between a pH dependent toluidine blue O modified redox polymer and a pH independent osmium complex modified redox polymer was developed. Gold micro electrodes of various dimensions were modified with the two redox polymers by non-manual deposition procedures. Square wave voltammetry was used to determine the oxidation potentials of the polymer bound redox species allowing the precise determination of the pH value of the electrolyte solution without the need of a stable reference electrode. © 2014 Elsevier Ltd.
    view abstract10.1016/j.electacta.2014.03.116
  • A redox hydrogel protects hydrogenase from high-potential deactivation and oxygen damage
    Plumeré, N. and Rüdiger, O. and Oughli, A.A. and Williams, R. and Vivekananthan, J. and Pöller, S. and Schuhmann, W. and Lubitz, W.
    Nature Chemistry 6 (2014)
    Hydrogenases are nature's efficient catalysts for both the generation of energy via oxidation of molecular hydrogen and the production of hydrogen via the reduction of protons. However, their O2 sensitivity and deactivation at high potential limit their applications in practical devices, such as fuel cells. Here, we show that the integration of an O2 -sensitive hydrogenase into a specifically designed viologen-based redox polymer protects the enzyme from O2 damage and high-potential deactivation. Electron transfer between the polymer-bound viologen moieties controls the potential applied to the active site of the hydrogenase and thus insulates the enzyme from excessive oxidative stress. Under catalytic turnover, electrons provided from the hydrogen oxidation reaction induce viologen-catalysed O 2 reduction at the polymer surface, thus providing self-activated protection from O2. The advantages of this tandem protection are demonstrated using a single-compartment biofuel cell based on an O2 -sensitive hydrogenase and H2/O2 mixed feed under anode-limiting conditions.
    view abstract10.1038/nchem.2022
  • Activation of oxygen evolving perovskites for oxygen reduction by functionalization with Fe-Nx/C groups
    Rincón, R.A. and Masa, J. and Mehrpour, S. and Tietz, F. and Schuhmann, W.
    Chemical Communications 50 (2014)
    The incorporation of Fe-Nx/C moieties into perovskites remarkably activates them for the oxygen reduction reaction (ORR) and also leads to notable improvement of their activity towards the oxygen evolution reaction (OER) thus presenting a new route for realizing high performance, low cost bifunctional catalysts for reversible oxygen electrodes. This journal is © the Partner Organisations 2014.
    view abstract10.1039/c4cc06446a
  • Aging effects of anatase TiO2 nanoparticles in Li-ion batteries
    Madej, E. and Ventosa, E. and Klink, S. and Schuhmann, W. and La Mantia, F.
    Physical Chemistry Chemical Physics 16 (2014)
    Anatase TiO2 nanoparticles with a diameter of 5 nm have been investigated as a negative intercalation electrode material for Li-ion batteries. The focus was on the stability upon cycling within four different potential ranges, namely from 1.5, 1.2, 1.0 and 0.7 V vs. Li/Li+ as the lower potential limit to 3.0 V vs. Li/Li+ as the upper potential limit. While a lower cut-off potential allows for a higher amount of charge stored, the irreversible processes induce a faster fading of the specific charge. Galvanostatic cycling (GC), electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM) experiments suggest that SEI formation has a negligible contribution to the irreversible processes. It appears more plausible that an irreversible degradation of the bulk phase occurs, leading to a decrease in the amount of active sites. Moreover, it has been observed that this degradation appears as an anodic shift of the thermodynamic potential of (de-)intercalation of Li-ions in the TiO2 structure. The shift is caused by a change in the activity of Li-ions in the solid phase, which is driven by changes in the ionic atmosphere of the crystal. © 2014 the Partner Organisations.
    view abstract10.1039/c4cp00630e
  • Amine-based solvents for exfoliating graphite to graphene outperform the dispersing capacity of N-methyl-pyrrolidone and surfactants
    Sun, Z. and Huang, X. and Liu, F. and Yang, X. and Rösler, C. and Fischer, R.A. and Muhler, M. and Schuhmann, W.
    Chemical Communications 50 (2014)
    Four organic amine-based solvents were discovered which enable direct exfoliation of graphite to produce high-quality and oxygen-free graphene nanosheets. These solvents outperform previously used solvents and additives such as N-methyl-pyrrolidone and surfactants in terms of their dispersing capacity. The resulting dispersions allow the facile fabrication of zeolitic imidazolate framework (ZIF)-graphene nanocomposites with remarkable CO 2 storage capability. This journal is © the Partner Organisations 2014.
    view abstract10.1039/c4cc03923h
  • Application of PEDOT-CNT Microelectrodes for Neurotransmitter Sensing
    Samba, R. and Fuchsberger, K. and Matiychyn, I. and Epple, S. and Kiesel, L. and Stett, A. and Schuhmann, W. and Stelzle, M.
    Electroanalysis 26 (2014)
    In this work, composite microelectrodes from poly(3,4-ethylenedioxythiophene) (PEDOT) and carbon nanotubes (CNT) are characterized as electrochemical sensing material for neurotransmitters. Dopamine can be detected using square wave voltammetry at these microelectrodes. The CNTs improve the sensitivity by a factor of two. In addition, the selectivity towards dopamine in the presence of ascorbic acid and uric acid was examined. While both electrodes, PEDOT and PEDOT-CNT are able to detect all measured concentrations of dopamine in the presence of uric acid, small concentrations of dopamine and ascorbic acid are only distinguishable at PEDOT-CNT electrodes. Changing the pH has a strong influence on the selectivity. Moreover, it is possible to detect concentrations as low as 1μM dopamine in complex cell culture medium. Finally, other catecholamines like serotonin, epinephrine, norepinephrine and L-dopa are also electrochemically detectable at PEDOT-CNT microelectrodes. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/elan.201300547
  • Biofuel-Cell Cathodes Based on Bilirubin Oxidase Immobilized through Organic Linkers on 3D Hierarchically Structured Carbon Electrodes
    Vivekananthan, J. and Rincón, R.A. and Kuznetsov, V. and Pöller, S. and Schuhmann, W.
    ChemElectroChem 1 (2014)
    Different modification procedures to stabilize and control the orientation of Myrothecium verrucaria bilirubin oxidase (MvBOD) on 3D carbon nanotube/carbon microfiber-modified graphite electrode surfaces were evaluated for the development of biofuel-cell cathodes. The surface properties of different linkers for covalent binding of BOD were investigated by using atomic force microscopy-based techniques. For all immobilization strategies, the maximal current response was obtained at a pH value of 6.5 with temperatures between 20 and 35°C. The biocathode based on MvBOD immobilized through an imino bond to the electrode showed the highest current density (1600μAcm-2) and was resistant to the presence of chloride ions. A biofuel cell was constructed, and it exhibited a maximal power of 54μWcm-2 at 350mV with an open-circuit voltage of about 600mV by using a cellobiose dehydrogenase based bioanode and glucose as the fuel. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/celc.201402099
  • Cellobiose dehydrogenase entrapped within specifically designed Os-complex modified electrodeposition polymers as potential anodes for biofuel cells
    Shao, M. and Guschin, D.A. and Kawah, Z. and Beyl, Y. and Stoica, L. and Ludwig, R. and Schuhmann, W. and Chen, X.
    Electrochimica Acta 128 (2014)
    Electron-transfer pathways between cellobiose dehydrogenase from Myriococcum thermophilum (MtCDH) and the related flavodehydrogenase domain (FAD-MtCDH) and electrodes were evaluated using specifically designed Os-complex modified electrodeposition paints (EDPs). The properties of the Os-complex modified EDPs were varied by variation of the monomer composition, the coordination sphere of the polymer-bound Os-complexes, and the length and flexibility of the spacer chain between Os complex and polymer backbone. The MtCDH-to-EDP weight ratio, the pH value, as well as the operational temperature have been optimized. © 2013 Elsevier Ltd.
    view abstract10.1016/j.electacta.2013.11.019
  • Characterisation of localised corrosion processes using scanning electrochemical impedance microscopy
    Kuznetsov, V. and Maljusch, A. and Souto, R.M. and Bandarenka, A.S. and Schuhmann, W.
    Electrochemistry Communications 44 (2014)
    The concept of scanning electrochemical impedance microscopy (SEIM) is described for the studies of localised corrosion processes at metallic surfaces under conditions which simulate those present in the environment. SEIM is based on acquisition and comprehensive analysis (with modelling and spectra fitting) of arrays of localised impedance spectra obtained in a single area scan. This gives accurate quantitative information about important corrosion properties of non-uniform metal surfaces. It is demonstrated that SEIM-approach is advantageous in visualisation of surface domains with similar topography and morphology but different electrochemical properties as well as enables extracting parameters important in assessing local corrosion currents. © 2014 Elsevier B.V.
    view abstract10.1016/j.elecom.2014.04.011
  • Characterisation of non-uniform functional surfaces: Towards linking basic surface properties with electrocatalytic activity
    Maljusch, A. and Henry, J.B. and Tymoczko, J. and Bandarenka, A.S. and Schuhmann, W.
    RSC Advances 4 (2014)
    Functional materials, particularly heterogeneous catalysts, are often non-uniform at a microscopic level making their detailed characterisation extremely complex. This complexity inhibits the design and implementation of novel functional materials as such characterisation is a key to understanding interfaces for heterogeneous catalysis. We demonstrate that a combination of Scanning Kelvin Probe (SKP) and Scanning Electrochemical Microscopy (SECM) experiments made over the same sample surface using an integrated SKP-SECM system provides a powerful and robust tool to link basic surface properties with the observed electrocatalytic activity. As the SKP-response can be accurately assessed using modern quantum chemical approaches to benchmark analytical signals for different surface structures with varying compositions, application of an integrated SKP-SECM system can offer valuable insight into the origin of the observed electrocatalytic activity. As model objects, we used Pt(111)-like thin films modified with sub-monolayer and monolayer amounts of Cu atoms located at the electrode surface and in the sub-surface region. The exact position of the Cu atoms relative to the topmost Pt layer greatly affects basic surface properties and governs the electrocatalytic activity of the surface towards various reactions, i.e. the oxygen reduction reaction. SKP-SECM appeared to be a very sensitive tool to monitor those changes as a function of the spatial coordinates. © 2014 The Royal Society of Chemistry.
    view abstract10.1039/c3ra45845h
  • Characterization of Ta-Ti Thin Films by using a Scanning Droplet Cell in Combination with AC Linear Sweep Voltammetry
    Fan, M. and Sliozberg, K. and La Mantia, F. and Miyashita, N. and Hagymási, M. and Schnitter, C. and Ludwig, Al. and Schuhmann, W.
    ChemElectroChem 1 (2014)
    A binary Ta-Ti thin film composition-spread materials library is prepared through magnetron sputter co-deposition. An automated microelectrochemical investigation on selected surface areas, corresponding to a concentration gradient of Ti varying from 0.5 to 36at%, is achieved by using a scanning droplet cell. Simultaneously, during the anodic oxide growth, a small alternating current (AC) voltage is superimposed on the increasing direct current (DC) potential in order to record the capacitance of the mixed-metal oxide by using alternating current linear sweep voltammetry (AC-LSV). Valve metal behavior, with the current stabilizing after an initial rapid increase, is found for all investigated compositions. AC-LSV allows the ratio of the formation factor to the relative permittivity for different compositions to be calculated. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/celc.201300153
  • Constant-distance mode SECM as a tool to Visualize local electrocatalytic activity of oxygen reduction catalysts
    Nebel, M. and Erichsen, T. and Schuhmann, W.
    Beilstein Journal of Nanotechnology 5 (2014)
    Multidimensional shearforce-based constant-distance mode scanning electrochemical microscopy (4D SF/CD-SECM) was utilized for the investigation of the activity distribution of oxygen reduction catalysts. Carbon-supported Pt model catalyst powders have been immobilized in recessed microelectrodes and compared to a spot preparation technique. Microcavities serve as platform for the binder-free catalyst sample preparation exhibiting beneficial properties for constant-distance mode SECM imaging concerning modified surface area and catalyst loading. The integration of the redox competition mode of SECM into the detection scheme of the 4D SF/CD mode is demonstrated for specifically adapting high-resolution SECM experiments to powder-based catalyst preparations. © 2014 Nebel et al.
    view abstract10.3762/bjnano.5.14
  • D-lactate-selective amperometric biosensor based on the cell debris of the recombinant yeast Hansenula polymorpha
    Smutok, O.V. and Dmytruk, K.V. and Karkovska, M.I. and Schuhmann, W. and Gonchar, M.V. and Sibirny, A.A.
    Talanta 125 (2014)
    A d-lactate-selective biosensor has been developed using cellsdebris of recombinant thermotolerant methylotrophic yeast Hansenula polymorpha, overproducing d-lactate: cytochrome c-oxidoreductase (EC 1.1.2.4, d-lactate dehydrogenase (cytochrome), DlDH). The H. polymorpha DlDH-producer was constructed in two steps. First, the gene CYB2 was deleted on the background of the C-105 (gcr1 catX) strain of H. polymorpha impaired in glucose repression and devoid of catalase activity to avoid specific l-lactate-cytochrome c oxidoreductase activity. Second, the homologous gene DLD1 coding for DlDH was overexpressed under the control of the strong H. polymorpha alcohol oxidase promoter in the frame of a plasmid for multicopy integration in the Δcyb2 strain. The selected recombinant strain possesses 6-fold increased DlDH activity as compared to the initial strain. The cellsdebris was used as a biorecognition element of a biosensor, since DlDH is strongly bound to mitochondrial membranes. The cellsdebris, prepared by mechanic disintegration of recombinant cells, was immobilized on a graphite working electrode in an electrochemically generated layer using an Os-complex modified cathodic electrodeposition polymer. Cytochrome c was used as additional native electron mediator to improve electron transfer from reduced DlDH to the working electrode. The constructed d-lactate-selective biosensors are characterized by a high sensitivity (46.3-61.6 A M-1 m-2), high selectivity and sufficient storage stability. © 2014 Elsevier B.V.
    view abstract10.1016/j.talanta.2014.02.041
  • Detection and Quantification of Sulfonamide Antibiotic Residues in Milk Using Scanning Electrochemical Microscopy
    Conzuelo, F. and Stratmann, L. and Grützke, S. and Pingarrón, J.M. and Schuhmann, W.
    Electroanalysis 26 (2014)
    The use of scanning electrochemical microscopy (SECM) for the qualitative and quantitative determination of sulfapyridine (SPY) in milk is described. A direct competitive immunoassay was performed involving an antibiotic horseradish peroxidase (HRP)-labeled analog and using selective capture antibodies immobilized on the surface of Protein G-modified glassy carbon plates. SECM detection was accomplished by means of the sample generator/tip collector (GC) mode involving the reduction of benzoquinone (BQ) generated upon the HRP-catalyzed oxidation of hydroquinone (HQ) at the modified substrate surface in the presence of H2O2. The detection limit for SPY in milk samples was as low as 0.13ngmL-1. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/elan.201300577
  • Electrical potential-assisted DNA hybridization. How to mitigate electrostatics for surface DNA hybridization
    Tymoczko, J. and Schuhmann, W. and Gebala, M.
    ACS Applied Materials and Interfaces 6 (2014)
    Surface-confined DNA hybridization reactions are sensitive to the number and identity of DNA capture probes and experimental conditions such as the nature and the ionic strength of the electrolyte solution. When the surface probe density is high or the concentration of bulk ions is much lower than the concentration of ions within the DNA layer, hybridization is significantly slowed down or does not proceed at all. However, high-density DNA monolayers are attractive for designing high-sensitivity DNA sensors. Thus, circumventing sluggish DNA hybridization on such interfaces allows a high surface concentration of target DNA and improved signal/noise ratio. We present potential-assisted hybridization as a strategy in which an external voltage is applied to the ssDNA-modified interface during the hybridization process. Results show that a significant enhancement of hybridization can be achieved using this approach. © 2014 American Chemical Society.
    view abstract10.1021/am5027902
  • Electrochemical nanoprobes for single-cell analysis
    Actis, P. and Tokar, S. and Clausmeyer, J. and Babakinejad, B. and Mikhaleva, S. and Cornut, R. and Takahashi, Y. and López Córdoba, A. and Novak, P. and Shevchuck, A.I. and Dougan, J.A. and Kazarian, S.G. and Gorelkin, P.V. and Erofeev, A.S. and Yaminsky, I.V. and Unwin, P.R. and Schuhmann, W. and Klenerman, D. and Rusakov, D.A. and Sviderskaya, E.V. and Korchev, Y.E.
    ACS Nano 8 (2014)
    The measurement of key molecules in individual cells with minimal disruption to the biological milieu is the next frontier in single-cell analyses. Nanoscale devices are ideal analytical tools because of their small size and their potential for high spatial and temporal resolution recordings. Here, we report the fabrication of disk-shaped carbon nanoelectrodes whose radius can be precisely tuned within the range 5-200 nm. The functionalization of the nanoelectrode with platinum allowed the monitoring of oxygen consumption outside and inside a brain slice. Furthermore, we show that nanoelectrodes of this type can be used to impale individual cells to perform electrochemical measurements within the cell with minimal disruption to cell function. These nanoelectrodes can be fabricated combined with scanning ion conductance microscopy probes, which should allow high resolution electrochemical mapping of species on or in living cells. © 2013 American Chemical Society.
    view abstract10.1021/nn405612q
  • Electrochemical patterning as a tool for fabricating biomolecule microarrays
    Clausmeyer, J. and Schuhmann, W. and Plumeré, N.
    TrAC - Trends in Analytical Chemistry 58 (2014)
    High-density biomolecule arrays are powerful tools for the screening of pharmaceuticals, investigation of biomolecule interactions and patient diagnostics. Surfaces modified with electrochemically addressable films combined with electrochemical surface patterning techniques allow local triggering of DNA and protein immobilization. After a brief overview of classical patterning methods, such as printing, dip-pen nanolithography (DPN) and photolithography, we critically assess electrochemical strategies for local surface modification, such as the use of electrode arrays, electro-DPN and scanning electrochemical microscopy regarding their potential for fabrication and read-out of bioarrays. Capillary-based scanning probe methods are especially promising tools for truly chemoselective microarray and nanoarray generation due to their high patterning resolution and the possibility for directly probing the surface chemistry. © 2014 Elsevier Ltd.
    view abstract10.1016/j.trac.2014.03.004
  • Electrochemically induced deposition of poly(benzoxazine) precursors as immobilization matrix for enzymes
    Andronescu, C. and Pöller, S. and Schuhmann, W.
    Electrochemistry Communications 41 (2014)
    Water-soluble benzoxazine oligomers were synthesized by reacting bisphenol A, tetraethylenepentamine and formaldehyde. The pre-formed benzoxazine oligomers can be further electropolymerized from aqueous suspensions under formation of poly(benzoxazine)-based films on electrode surfaces. Integration of glucose oxidase in a poly(benzoxazine) film let to highly reproducible and stable biosensors. Poly(benzoxazines) are proposed as a new family of stable polymers for the design of enzyme electrodes. © 2014 Elsevier B.V.
    view abstract10.1016/j.elecom.2014.01.015
  • Engineered electron-transfer chain in photosystem 1 based photocathodes outperforms electron-transfer rates in natural photosynthesis
    Kothe, T. and Pöller, S. and Zhao, F. and Fortgang, P. and Rögner, M. and Schuhmann, W. and Plumeré, N.
    Chemistry (Weinheim an der Bergstrasse, Germany) 20 (2014)
    Photosystem 1 (PS1) triggers the most energetic light-induced charge-separation step in nature and the in vivo electron-transfer rates approach 50 e(-)  s(-1)  PS1(-1). Photoelectrochemical devices based on this building block have to date underperformed with respect to their semiconductor counterparts or to natural photosynthesis in terms of electron-transfer rates. We present a rational design of a redox hydrogel film to contact PS1 to an electrode for photocurrent generation. We exploit the pH-dependent properties of a poly(vinyl)imidazole Os(bispyridine)2Cl polymer to tune the redox hydrogel film for maximum electron-transfer rates under optimal conditions for PS1 activity. The PS1-containing redox hydrogel film displays electron-transfer rates of up to 335±14 e(-)  s(-1)  PS1(-1), which considerably exceeds the rates observed in natural photosynthesis or in other semiartificial systems. Under O2 supersaturation, photocurrents of 322±19 μA cm(-2) were achieved. The photocurrents are only limited by mass transport of the terminal electron acceptor (O2). This implies that even higher electron-transfer rates may be achieved with PS1-based systems in general. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/chem.201402585
  • Evaluation of Perovskites as Electrocatalysts for the Oxygen Evolution Reaction
    Rincõn, R.A. and Ventosa, E. and Tietz, F. and Masa, J. and Seisel, S. and Kuznetsov, V. and Schuhmann, W.
    ChemPhysChem 15 (2014)
    The oxygen evolution reaction (OER) is an enabling process for technologies in the area of energy conversion and storage, but its slow kinetics limits its efficiency. We performed an electrochemical evaluation of 14 different perovskites of variable composition and stoichiometry as OER electrocatalysts in alkaline media. We particularly focused on improved methods for a reliable comparison of catalyst activity. From initial electrochemical results we selected the most active samples for further optimization of electrode preparation and testing. An inverted cell configuration facilitated gas bubble detachment and thus minimized blockage of the active surface area. We describe parameters, such as the presence of specific cations, stoichiometry, and conductivity, that are important for obtaining electroactive perovskites for OER. Conductive additives enhanced the current and decreased the apparent overpotential of OER for one of the most active samples (La0.58Sr0.4Fe0.8Co0.2O3). Low-cost electrocatalysts: A family of perovskites has been studied as new electrocatalysts for the oxygen evolution reaction (OER) in alkaline media. Electrochemical characterization demonstrates the promise of this type of materials for the OER, and the use of conductive additives proves useful for obtaining higher current outputs. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cphc.201402137
  • High-concentration graphene dispersions with minimal stabilizer: a scaffold for enzyme immobilization for glucose oxidation
    Sun, Z. and Vivekananthan, J. and Guschin, D.A. and Huang, X. and Kuznetsov, V. and Ebbinghaus, P. and Sarfraz, A. and Muhler, M. and Schuhmann, W.
    Chemistry (Weinheim an der Bergstrasse, Germany) 20 (2014)
    Modified acrylate polymers are able to effectively exfoliate and stabilize pristine graphene nanosheets in aqueous media. Starting with pre-exfoliated graphite greatly promotes the exfoliation level. The graphene concentration is significantly increased up to 11 mg mL(-1) by vacuum evaporation of the solvent from the dispersions under ambient temperature. TEM shows that 75 % of the flakes have fewer than five layers with about 18 % of the flakes consisting of monolayers. Importantly, a successive centrifugation and redispersion strategy is developed to enable the formation of dispersions with exceptionally high graphene-to-stabilizer ratio. Characterization by high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and Raman spectroscopy shows the flakes to be of high quality with very low levels of defects. These dispersions can act as a scaffold for the immobilization of enzymes applied, for example, in glucose oxidation. The electrochemical current density was significantly enhanced to be approximately six times higher than an electrode in the absence of graphene, thus showing potential applications in enzymatic biofuel cells. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/chem.201400098
  • Hollow and yolk-shell iron oxide nanostructures on few-layer graphene in li-ion batteries
    Sun, Z. and Xie, K. and Li, Z.A. and Sinev, I. and Ebbinghaus, P. and Erbe, A. and Farle, M. and Schuhmann, W. and Muhler, M. and Ventosa, E.
    Chemistry - A European Journal 20 (2014)
    We report a simple and template-free strategy for the synthesis of hollow and yolk-shell iron oxide (FeOx) nanostructures sandwiched between few-layer graphene (FLG) sheets. The morphology and microstructure of this material are characterized in detail by X-ray diffraction, X-ray absorption near-edge structure, X-ray photoelectron spectroscopy, Raman spectroscopy, scanning and transmission electron microscopy. Its properties are evaluated as negative electrode material for Li-ion batteries and compared with those of solid FeOx/FLG and two commercial iron oxides. In all cases, the content of carbon in the electrode has a great influence on the performance. The use of pristine FLG improves the capacity retention and further enhancement is achieved with the hollow structure. For a low carbon loading of 18wt. %, the presence of metallic iron in the hollow and yolk-shell FeOx/FLG composite significantly enhances the capacity retention, albeit with a relatively lower initial reversible capacity, retaining above 97 % after 120cycles at 1000mA g-1 in the voltage range of 0.1-3.0V. © 2014 Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim.
    view abstract10.1002/chem.201303723
  • Immunologically Controlled Biofuel Cell as a Self-Powered Biosensor for Antibiotic Residue Determination
    Conzuelo, F. and Vivekananthan, J. and Pöller, S. and Pingarrón, J.M. and Schuhmann, W.
    ChemElectroChem 1 (2014)
    A biofuel cell consisting of a self-powered sulfonamide immunosensor as biocathode and a cellobiose dehydrogenase (CDH)-based bioanode was developed for the determination of sulfonamide antibiotics in milk. A graphite-rod electrode was modified with proteinG for the immobilization of selective capture antibodies. A direct competitive immunoassay with a horseradish-peroxidase-labeled analog of the antibiotic and the 2,2′-azino-bis(3-ethyl benzothiazoline-6-sulfonic acid) diammonium salt-mediated reduction of H2O2 allows quantification of antibiotic residues. CDH was co-immobilized with a toluidine-blue-modified redox polymer on a graphite electrode for the biocatalytic oxidation of lactose in milk. An open-circuit voltage of 676mV and a maximal power density of 6.9μWcm-2 were obtained. The power densities measured at 550mV (vs. the anode) as a function of antibiotic concentration in milk samples allowed the construction of a calibration curve with a detection limit for sulfapyridine as low as 2.4ngmL-1. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/celc.201402098
  • Impact of the specific surface area on the memory effect in Li-ion batteries: The case of anatase TiO2
    Madej, E. and Mantia, F.L. and Schuhmann, W. and Ventosa, E.
    Advanced Energy Materials 4 (2014)
    Until recently, the memory effect was believed to be absent in Li-ion battery materials. Here, the memory effect is clearly observed in anatase TiO2 nanoparticles when they are used as the negative electrode material in Li-ion batteries. Additionally, the memory effect strongly decreases with increasing specific surface area of the TiO2 sample. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/aenm.201400829
  • Koutecky-Levich analysis applied to nanoparticle modified rotating disk electrodes: Electrocatalysis or misinterpretation
    Masa, J. and Batchelor-McAuley, C. and Schuhmann, W. and Compton, R.G.
    Nano Research 7 (2014)
    The application of naive Koutecky-Levich analysis to micro- and nano-particle modified rotating disk electrodes of partially covered and non-planar geometry is critically analysed. Assuming strong overlap of the diffusion fields of the particles such that transport to the entire surface is time-independent and one-dimensional, the observed voltammetric response reflects an apparent electrochemical rate constant kapp o, equal to the true rate constant k o describing the redox reaction of interest on the surface of the nanoparticles and the ratio, ψ, of the total electroactive surface area to the geometric area of the rotating disk surface. It is demonstrated that Koutecky-Levich analysis is applicable and yields the expected plots of I -1 versus ω -1 where I is the current and ω is the rotation speed but that the values of the electrochemical rate constants inferred are thereof kapp o, not k o. Thus, for ψ &gt; 1 apparent electrocatalysis might be naively but wrongly inferred whereas for ψ < 1 the deduced electrochemical rate constant will be less than k o. Moreover, the effect of ψ on the observed rotating disk electrode voltammograms is significant, signalling the need for care in the overly simplistic application of Koutecky-Levich analysis to modified rotating electrodes, as is commonly applied for example in the analysis of possible oxygen reduction catalysts. [Figure not available: see fulltext.] © 2014 Tsinghua University Press and Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s12274-013-0372-0
  • Linking glucose oxidation to luminol-based electrochemiluminescence using bipolar electrochemistry
    Eßmann, V. and Jambrec, D. and Kuhn, A. and Schuhmann, W.
    Electrochemistry Communications 50 (2014)
    The range of potential analytes for bipolar electrochemistry can be significantly extended by modification of bipolar electrodes with enzymatic biosensing layers. In this study, we employed a Prussian blue-based glucose detection system involving electrochemical reduction of enzymatically generated hydrogen peroxide at the cathodic pole. The concentration of glucose in solution can be correlated with oxidative luminol electrochemiluminescence at the opposite anodic pole, which was recorded with a photomultiplier tube. This opens a route for novel analytical systems using glucose oxidase as an amplification element for the reporter reaction. © 2014 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.elecom.2014.11.015
  • Local visualization of catalytic activity at gas evolving electrodes using frequency-dependent scanning electrochemical microscopy
    Chen, X. and Maljusch, A. and Rincón, R.A. and Battistel, A. and Bandarenka, A.S. and Schuhmann, W.
    Chemical Communications 50 (2014)
    A new concept for the localized characterization of gas evolving electrodes based on scanning electrochemical microscopy (SECM) is suggested. It offers information about the spatial distribution of the predominant locations, which represent the most active catalytic sites, and dynamic characteristics of gas-bubble departure. The knowledge about gas-bubble departure is critical for the assessment and development of new electrode materials for energy applications. This journal is © the Partner Organisations 2014.
    view abstract10.1039/c4cc06100d
  • Localized impedance measurements for electrochemical surface science
    Bandarenka, A.S. and Maljusch, A. and Kuznetsov, V. and Eckhard, K. and Schuhmann, W.
    Journal of Physical Chemistry C 118 (2014)
    An approach for in-depth characterization of complex electrode/electrolyte interfaces based on localized impedance measurements is described in detail. The local ac probing of the interface is performed at different frequencies by means of scanning electrochemical microscopy (SECM) using ultramicroelectrodes (SECM tips) which enables visualization of dependences of the localized impedance spectra as a function of spatial coordinates. Subsequent fitting of these spectra to physical models visualize the local distribution of parameters describing the electrochemical interface, such as the electric double layer capacitance and the charge transfer resistance. Three model examples are analyzed dealing with typical situations, when the measurements are either affected or not by specific adsorption of anions at the SECM-tips. It is demonstrated that the approach holds promise for electrochemical surface science, particularly for better understanding of corrosion processes taking place at metal surfaces in aggressive, particularly aqueous electrolytes. © 2014 American Chemical Society.
    view abstract10.1021/jp412505p
  • Low temperature Hydrogen Reduction of High Surface Area Anatase and Anatase/β-TiO2 for High-Charging-Rate Batteries
    Ventosa, E. and Tymoczko, A. and Xie, K. and Xia, W. and Muhler, M. and Schuhmann, W.
    ChemSusChem 7 (2014)
    There are several strategies to improve the electrochemical performance of TiO2 as negative electrode material for Li-ion batteries. Introducing oxygen vacancies through hydrogen reduction leads to an enhancement in electrical conductivity. However, this strategy does not improve the low lithium-ion mobility. Herein, we show that by decreasing the temperature of hydrogen annealing the improved lithium-ion mobility of high-surface-area TiO2 and β-TiO2 can be combined with the enhanced electrical conductivity of oxygen deficiencies. Annealing at only 275–300 °C in pure hydrogen atmosphere successfully creates oxygen vacancies in TiO2, as confirmed by UV/Vis spectroscopy, whereas the temperature is low enough to maintain a high specific surface area and prevent β-to-anatase phase transformation. The hydrogen reduction of high-surface-area anatase or anatase/β-TiO2 at these temperatures leads to improvements in the performance, achieving charge capacities of 142 or 152 mAh g−1 at 10C, respectively. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/cssc.201402279
  • Mechanical properties of SiLix thin films at different stages of electrochemical Li insertion
    Zinn, A.-H. and Borhani-Haghighi, S. and Ventosa, E. and Pfetzing-Micklich, J. and Wieczorek, N. and Schuhmann, W. and Ludwig, Al.
    Physica Status Solidi (A) Applications and Materials Science 211 (2014)
    The mechanical properties of amorphous Si thin films, lithiated electrochemically to different Si£Li compositions are studied by ex situ nanoindentation. The compositions of the films are adjusted using an electrochemical routine that corrects for the Li consumed by SEI layer growth during initial lithiation. The mechanical properties such as Young's modulus and hardness are derived from nanoindentation. For compositions between Si and SiLi<inf>2.5</inf> the Young's modulus decreases with increasing Li content from ∼160 GPa to ∼8 GPa and the hardness decreases from ∼14 GPa to ∼0.1 GPa. The yield strength values, as deduced from hardness measurements, decrease from ∼5 GPa to 0.05 GPa. AFM imaging is used on the electrochemically cycled films to assess the SEIs impact on the nanomechanical measurements. XPS depth-profiling of the electrochemically cycled sample indicated a Li concentration gradient across the film thickness. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssa.201431303
  • Metal-free catalysts for oxygen reduction in alkaline electrolytes: Influence of the presence of Co, Fe, Mn and Ni inclusions
    Masa, J. and Zhao, A. and Wei, X. and Muhler, M. and Schuhmann, W.
    Electrochimica Acta 128 (2014)
    Metal-free nitrogen modified carbon catalysts (NC) are very closely related to MNC catalysts which contain a transition metal(s) (M), usually Fe or Co as an essential constituent. We investigated the influence of metal inclusions on the activity of nitrogen-doped carbon black in the electrocatalysis of the oxygen reduction reaction (ORR). A reference metal-free NC catalyst was prepared by pyrolysis of a polypyrrole/Vulcan XC72 composite at 800 °C for 2 h under helium. Controlled amounts of Co, Fe, Mn and Ni in low concentrations were then introduced into NC by impregnating it with the corresponding meso-tetra(4-pyridyl) porphyrin metal complex followed by further pyrolysis at 650 °C for 2 h under helium. The resulting catalysts were investigated for ORR using rotating disk electrode and rotating-ring disk electrode voltammetry in 0.1 M KOH. Additionally, the rate of decomposition of hydrogen peroxide by the different catalysts was determined in order to probe the influence of the metal inclusions on the mechanism and selectivity of the ORR. The results show that Fe, Co and Mn inclusions cause a substantial decrease of the overpotential of the reaction and enhance the catalytic current, whereas the presence of Ni has a poisoning effect on ORR. In the presence of Fe, the catalysts apparently reduce oxygen selectively to OH- in a direct four electron transfer process as opposed to the two-step, two electron pathway involving hydrogen peroxide as an intermediate for the case of the NC catalyst. © 2013 Elsevier Ltd.
    view abstract10.1016/j.electacta.2013.11.026
  • Microelectrochemical visualization of oxygen consumption of single living cells
    Nebel, M. and Grützke, S. and Diab, N. and Schulte, A. and Schuhmann, W.
    Faraday Discussions 164 (2014)
    The detection of cellular respiration activity is important for the assessment of the status of a biological cell. Due to its non-invasive character and high spatial resolution scanning electrochemical microscopy (SECM) is a powerful tool for single cell measurements. Common limitations of respiration studies performed by SECM are discussed and strategies provided to further adapt SECM detection schemes to the specific requirements for the investigation of single cell respiration. In particular the combination of a potential pulse technique in the redox competition mode of SECM with a shearforce-based constant-distance positioning of the SECM tip is proposed for characterising the impact of the tip reaction during SECM imaging. The adjustment of the driving force of the tip reaction and the selection of the time for data acquisition after applying the potential pulse allowed a successful visualization of cell respiration activity. © 2013 The Royal Society of Chemistry.
    view abstract10.1039/c3fd00011g
  • MnxOy/NC and CoxOy/NC nanoparticles embedded in a nitrogen-doped carbon matrix for high-performance bifunctional oxygen electrodes
    Masa, J. and Xia, W. and Sinev, I. and Zhao, A. and Sun, Z. and Grützke, S. and Weide, P. and Muhler, M. and Schuhmann, W.
    Angewandte Chemie - International Edition 53 (2014)
    Reversible interconversion of water into H2 and O2, and the recombination of H2 and O2 to H2O thereby harnessing the energy of the reaction provides a completely green cycle for sustainable energy conversion and storage. The realization of this goal is however hampered by the lack of efficient catalysts for water splitting and oxygen reduction. We report exceptionally active bifunctional catalysts for oxygen electrodes comprising Mn3O4 and Co 3O4 nanoparticles embedded in nitrogen-doped carbon, obtained by selective pyrolysis and subsequent mild calcination of manganese and cobalt N4 macrocyclic complexes. Intimate interaction was observed between the metals and nitrogen suggesting residual M-Nx coordination in the catalysts. The catalysts afford remarkably lower reversible overpotentials in KOH (0.1M) than those for RuO2, IrO2, Pt, NiO, Mn3O4, and Co3O4, thus placing them among the best non-precious-metal catalysts for reversible oxygen electrodes reported to date. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/anie.201402710
  • Nanosensors for the detection of hydrogen peroxide
    Clausmeyer, J. and Actis, P. and López Córdoba, A. and Korchev, Y. and Schuhmann, W.
    Electrochemistry Communications 40 (2014)
    Prussian Blue (PB) deposited on a nanoelectrode is the basis for an amperometric hydrogen peroxide sensor. Carbon nanoelectrodes, fabricated from pyrolytic decomposition of butane within a quartz nanopipette, were electrochemically etched and PB was deposited in the formed nanocavity. This procedure significantly increased the stability of PB films while maintaining a high mean sensitivity of 50 A mol- 1 l cm- 2 for H 2O2 detection at - 50 mV vs. Ag/AgCl (0.1 M Cl -) at neutral pH value. Hydrogen peroxide was selectively quantified in the concentration range from 10 μM to 3 mM. We envision the application of these nanosensors to the intracellular monitoring of oxidative stress in living cells. © 2013 Elsevier B.V.
    view abstract10.1016/j.elecom.2013.12.015
  • New insights into SEI formation in lithium ion batteries: Inhomogeneous distribution of irreversible charge losses across graphite electrodes
    Klink, S. and Weide, P. and Ventosa, E. and Muhler, M. and Schuhmann, W. and La Mantia, F.
    ECS Transactions 62 (2014)
    A vertical split electrode (VSE) with three layers was developed to investigate the formation of the solid electrolyte interphase (SEI) during first charge of graphite electrodes in lithium ion batteries. Ex-situ X-ray photoelectron spectroscopy (XPS) on each layer revealed that the first layer showed distinctively different signal patterns in the O 1s and C 1s regions. It was concluded that the SEI formed in the first layer closest to the counter electrode is thicker as well as different in chemical nature as compared to the SEI in the electrode bulk. © The Electrochemical Society.
    view abstract10.1149/06201.0265ecst
  • On the faradaic selectivity and the role of surface inhomogeneity during the chlorine evolution reaction on ternary Ti-Ru-Ir mixed metal oxide electrocatalysts
    Zeradjanin, A.R. and Menzel, N. and Schuhmann, W. and Strasser, P.
    Physical Chemistry Chemical Physics 16 (2014)
    The faradaic selectivity of the chlorine evolution reaction (CER) and oxygen evolution reaction (OER) on the industrially important Ti-Ru-Ir mixed metal oxide is discussed. Absolute evolution rates as well as volume fractions of Cl2 and O2 were quantified using differential electrochemical mass spectrometry (DEMS), while the catalyst surface redox behavior was analyzed using cyclic voltammetry. The spatial inhomogeneity of the surface catalytic reaction rate was probed using Scanning Electrochemical Microscopy (SECM). Although the nature of the competition between electrochemical discharging of chloride ions and water molecules remains elusive on a molecular scale, new insights into the spatial reactivity distribution of the CER and OER were obtained. Oxidation of water is the initial step in corrosion and concomitant deactivation of the oxide electrodes; however, at the same time the nature of interaction between the oxide surface and water is used as a rational indicator of selectivity and catalytic activity. An experimental procedure was established that would allow the study of selectivity of a variety of different catalyst materials using polycrystalline electrode surfaces. This journal is © the Partner Organisations 2014.
    view abstract10.1039/c4cp00896k
  • Optimization of primary printed batteries based on Zn/MnO2
    Madej, E. and Espig, M. and Baumann, R.R. and Schuhmann, W. and La Mantia, F.
    Journal of Power Sources 261 (2014)
    Thin-film batteries based on zinc/manganese dioxide chemistry with gel ZnCl2 electrolyte were manufactured as single (1.5 V) and double (3.0 V) cells from electrodes printed on paper substrates covered with different polymeric insulating coatings. Their properties were evaluated by means of electrochemical impedance spectroscopy and chronopotentiometry. Best performing cells achieved capacities in the range of 3 mAh cm-2 during discharge with 100 &mu;A current, corresponding approximately to C/100 discharge rate. The influence of the cell elements on the overvoltage was examined and suggestions for the optimization of their performance were postulated. In particular, it was observed that limitations in the delivered power were governed by the poor conductivity of the carbon current collector. An optimized cell was built and showed a 4-fold improvement in the power delivered at 1 mA. © 2014 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jpowsour.2014.03.103
  • Oxygen reduction at a Cu-modified Pt(111) model electrocatalyst in contact with nafion polymer
    Tymoczko, J. and Calle-Vallejo, F. and Colic, V. and Koper, M.T.M. and Schuhmann, W. and Bandarenka, A.S.
    ACS Catalysis 4 (2014)
    The effect of Nafion on the performance of a model Cu-modified Pt(111) electrocatalyst has been investigated using electrochemical techniques and density functional theory calculations. In this work, we demonstrate that Cu subsurface alloying not only increases the activity of model Pt(111) electrodes toward the oxygen reduction reaction (ORR) but also largely prevents catalyst poisoning by electrolyte components relevant for polymer electrolyte membrane fuel cell applications. Our results indicate that specific adsorption of (bi)sulfates and sulfonates (present in Nafion membranes) on the Cu-modified Pt(111) electrocatalyst is gradually suppressed, which implies that the ORR activity in 0.05 M H2SO4 electrolyte drastically increases, with a change in the corresponding pseudo-half-wave potential of ∼93 mV. Importantly, the Cu-modified Pt(111) electrocatalyst in contact with Nafion polymer shows an activity as high as that in the absence of this polymer in perchloric acid media. © 2014 American Chemical Society.
    view abstract10.1021/cs501037y
  • Photodeposition of copper and chromia on gallium oxide: The role of co-catalysts in photocatalytic water splitting
    Busser, G.W. and Mei, B. and Pougin, A. and Strunk, J. and Gutkowski, R. and Schuhmann, W. and Willinger, M.-G. and Schlögl, R. and Muhler, M.
    ChemSusChem 7 (2014)
    Split second: The photocatalytic activity of gallium oxide (β-Ga 2O3) depends strongly on the co-catalysts CuOx and chromia, which can be efficiently deposited in a stepwise manner by photoreduction of Cu2+ and CrO42-. The water-splitting activity can be tuned by varying the Cu loading in the range 0.025-1.5 wt %, whereas the Cr loading is not affecting the rate as long as small amounts (such as 0.05 wt %) are present. Chromia is identified as highly efficient co-catalyst in the presence of CuOx: it is essential for the oxidation of water. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cssc.201301065
  • Position of Cu atoms at the Pt(111) electrode surfaces controls electrosorption of (H)SO4 (2)- from H2SO4 electrolytes
    Tymoczko, J. and Schuhmann, W. and Bandarenka, A.S.
    ChemElectroChem 1 (2014)
    Selective positioning of monolayer amounts of foreign atoms at the surface and subsurface regions of metal electrodes is a promising way to fine-tune the properties of the electrode/ electrolyte interface. The latter is critical as it largely governs the adsorption of electrolyte components and reaction intermediates and, therefore, controls many key electrocatalytic processes. Using model Pt(111) single-crystal electrodes, we demonstrate how the relative position of Cu atoms at the surface drastically changes the adsorption energies for (bi)sulfate anions. Our measurements involve pseudomorphic overlayers of Cu on Pt(111) as well as Cu-Pt(111) surface and sub-surface alloys, where Cu atoms were located either in the first or in the second atomic layers of Pt, respectively. In the case of Cu- Pt(111) surface alloys, specific adsorption of the anions starts earlier compared to the unmodified Pt(111) surface. In contrast, placing Cu atoms into the second atomic layer weakens the binding between the surface and the anions. Surprisingly, Cu pseudomorphic overlayers do not reveal any specific adsorption of (bi)sulfates (within the region of the overlayer stability). Taking into account that electrified interfaces between Pt(111) electrodes and sulfate-containing electrolytes often play the role of benchmark systems in fundamental physico-chemical and, particularly, electrocatalytic studies, our findings demonstrate a promising and relatively easy route of tuning the properties of these interfaces. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/celc.201300107
  • PQQ-sGDH bioelectrodes based on os-complex modified electrodeposition polymers and carbon nanotubes
    Chen, X. and Shao, M. and Pöller, S. and Guschin, D. and Pinyou, P. and Schuhmann, W.
    Journal of the Electrochemical Society 161 (2014)
    Graphite electrodes were modified with specifically designed Os-complex modified electrodeposition polymers exhibiting a formal potential of the polymer-bound complex of about 0 to 20 mV (vs. Ag/AgCl/3MKCl) which is only about 100 mV anodic of the formal potential of pyrroloquinoline quinone (PQQ) in PQQ-dependent glucose dehydrogenase (PQQ-GDH). The efficiency of wiring the polymer-entrapped PQQ-GDH was dependent on the nature of the polymer backbone, the crosslinking with bifunctional crosslinkers and the co-entrapment of multi-walled carbon nanotubes. Due to the limited long-term stability a new polymer synthesis strategy was adapted using the same Os-complex but providing enhanced crosslinking capabilities by introducing epoxide functions at the polymer backbone. Related bioelectrodes showed enhanced glucose-dependent current and a stability of at least 3 days of continuous operation. © The Author(s) 2014.
    view abstract10.1149/2.0111413jes
  • Rational design of the electrode morphology for oxygen evolution-enhancing the performance for catalytic water oxidation
    Zeradjanin, A.R. and Topalov, A.A. and Van Overmeere, Q. and Cherevko, S. and Chen, X. and Ventosa, E. and Schuhmann, W. and Mayrhofer, K.J.J.
    RSC Advances 4 (2014)
    The fundamental understanding of the electrode/electrolyte interface is of pivotal importance for the efficient electrochemical conversion and storage of electrical energy. However, the reasons for the low rate of electrocatalytic oxygen evolution and issues of long-term material stability, which are central constraints for attaining desirable efficiency for sustainable technologies like water electrolysis or electrochemical CO2 reduction, are still not completely resolved. While a lot of attention has been directed towards the search for new materials with unique (electro)catalytic properties, experimental results accumulated during the last four decades and prediction from models suggest that RuO2 possesses superior activity for oxygen evolution under acidic conditions. Considering that RuO2 is a material of choice, we show that tailoring the surface morphology on the meso- and macroscale has great potential for the improvement of the efficiency of this gas evolving reaction. Advanced analytical tools have been utilized for the combined investigation of both activity and stability. Namely, the potential dependent frequencies of gas-bubble evolution, an indicator for the activity of the electrode, were acquired by scanning electrochemical microscopy (SECM), while the dissolution of RuO2 was monitored using a micro electrochemical scanning flow cell combined with an inductively coupled plasma mass spectrometer (SFC-ICP-MS). The obtained fundamental insights will aid improving the design and thus performance of electrode materials for water oxidation. © 2014 The Royal Society of Chemistry.
    view abstract10.1039/c3ra45998e
  • Redox hydrogels with adjusted redox potential for improved efficiency in Z-scheme inspired biophotovoltaic cells
    Hartmann, V. and Kothe, T. and Pöller, S. and El-Mohsnawy, E. and Nowaczyk, M.M. and Plumeré, N. and Schuhmann, W. and Rögner, M.
    Physical Chemistry Chemical Physics 16 (2014)
    The improvement of Z-scheme inspired biophotovoltaics is achieved by fine tuning the properties of redox hydrogels applied as immobilization and electron conducting matrices for the photosystem-protein complexes. The formal potentials of the redox hydrogels are adjusted to the respective redox sites in the photosystems for optimized electron transfer without substantial voltage loss. The anode is based on photosystem 2 (PS2) integrated in a phenothiazine modified redox hydrogel with a formal potential of -1 mV vs. SHE, which is 59 mV more positive than the QB acceptor site in PS2. The cathode is based on photosystem 1 (PS1) contacted via an Os-complex based redox hydrogel with a formal potential of 395 mV vs. SHE, i.e. 28 mV more negative than the primary P700 electron acceptor of PS1. The potential difference between the two redox hydrogels is 396 mV. An open circuit voltage (VOC) of 372.5 ± 2.1 mV could be achieved for the biophotovoltaic cell. The maximum power output is 1.91 ± 0.56 μW cm-2 and the conversion efficiency (η) is 4.5 × 10-5, representing a 125-fold improvement in comparison to the previously proposed device exploiting the photosynthetic Z-scheme for electrical energy production. © 2014 the Partner Organisations.
    view abstract10.1039/c4cp00380b
  • Reliable benchmark material for anatase TiO2 in Li-ion batteries: On the role of dehydration of commercial TiO2
    Madej, E. and La Mantia, F. and Mei, B. and Klink, S. and Muhler, M. and Schuhmann, W. and Ventosa, E.
    Journal of Power Sources 266 (2014)
    Commercially available anatase TiO2 nanoparticles (ca. 15-20 nm particle size) were investigated as negative electrode material for Li-ion batteries. Despite the high initial specific charge of 200 mAh g-1 at 0.5C, the pristine commercial TiO2 failed to retain the reversible capacity upon cycling, keeping only 23% of the initial value after 80 cycles. X-ray photoelectron spectroscopy (XPS) results together with electrochemical data suggest that the failure in cyclability is of kinetic nature as the loss in specific charge is not completely irreversible. Thermogravimetry analysis revealed that the pristine TiO2 contained a significant amount of TiO(OH)2 (ca. 8%) which can be easily removed by dehydration when annealing in air above 250 °C. Air-annealing of TiO2 at 300 °C resulted in a remarkable improvement in cyclability retaining 83% of initial specific charge after 80 cycles at 0.5C. No further improvement in cyclability was observed for TiO2 annealed at 450 °C suggesting that the dehydration of TiO(OH)2 was the primary source of the improvement. Knowing the role of dehydration of TiO2 allows obtaining a reliable benchmark material via simple air-annealing and becomes a key factor when developing advanced materials from commercial TiO2. © 2014 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jpowsour.2014.05.018
  • Revealing onset potentials using electrochemical microscopy to assess the catalytic activity of gas-evolving electrodes
    Maljusch, A. and Ventosa, E. and Rincón, R.A. and Bandarenka, A.S. and Schuhmann, W.
    Electrochemistry Communications 38 (2014)
    Determination of the so-called onset potentials, i.e. the lowest (for the anodic reactions) or the highest (for the cathodic reactions) potentials at which a reaction product is formed at a given electrode and at defined conditions, is very important for the evaluation of the catalytic activity and even more for the comparison of different catalysts. We present an approach for the determination of the onset potentials based on scanning electrochemical microscopy (SECM) using the "substrate generation-tip collection" mode. In the proposed method, the potential applied to the catalyst sample is changed stepwise. A micro-electrode serving as SECM tip is positioned in known close proximity to the catalyst surface and is used to detect the onset of the formation of the product of the catalytic reaction, specifically gas generation at the sample surface. The oxygen evolution reaction (OER) at model RuO 2 and perovskite catalyst surfaces is used to evaluate the approach. The suggested method is supposed to provide a clearer and sensitive means for the detection of the onset potentials of electrolytic gas evolution reactions as compared to conventional procedures which mainly use cyclic voltammetry on stationary or rotating (ring) disk electrodes. Moreover, the detection of the reaction product at the SECM tip allows distinguishing between parasitic reactions at the catalyst surface and the true formation of the anticipated reaction product. © 2013 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.elecom.2013.11.024
  • Scanning droplet cell for chemoselective patterning through local electroactivation of protected quinone monolayers
    Clausmeyer, J. and Henig, J. and Schuhmann, W. and Plumeré, N.
    ChemPhysChem 15 (2014)
    A reagentless strategy for template-free patterning of uniformly inert surfaces is suggested. A layer of p-hydroquinone (HQ) protected by the tert-butyldimethylsilyl (TBDMS) group is electrografted onto glassy carbon electrodes. Chemoselective activation is performed through electrochemically controlled cleavage of the TBDMS group, which yields the redox-active surface-confined quinone moieties. The latter are shown to undergo electrochemically induced Michael addition, which serves for subsequent functionalization of the electrode surface. Patterning of the TBDMS-quinone-modified surface is accomplished by using selective localized cleavage of the protecting group. State-of-the-art direct-mode scanning electrochemical microscopy (SECM) patterning fails to yield the anticipated interfacial reaction; however, the electrochemical scanning droplet cell (SDC) is capable of conducting the localized chemoselective reaction. In a small area, dictated by the dimensions of the droplet, electrochemically induced cleavage of the protecting group can be performed locally to give rise to arrays of active quinone spots. Upon deprotection, the redox signals, attributed to the hydroquinone/benzoquinone couple, provide the first direct evidence for chemoselective electrochemical patterning of sensitive functionalities. Subsequent SECM studies of the resulting modified areas demonstrate spatial control of the proposed patterning technique. Ink-free writing: Site-selective and chemoselective electrochemical patterning, starting from uniform surfaces modified with a layer of protected p-hydroquinone, is demonstrated with a scanning droplet cell. The anodic deprotection and activation is restricted to the region exposed to the electrolyte and opens the possibility for further derivatization through interfacial Michael addition. Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cphc.201300937
  • Self-powered wireless carbohydrate/oxygen sensitive biodevice based on radio signal transmission
    Falk, M. and Alcalde, M. and Bartlett, P.N. and De Lacey, A.L. and Gorton, L. and Gutierrez-Sanchez, C. and Haddad, R. and Kilburn, J. and Leech, D. and Ludwig, R. and Magner, E. and Mate, D.M. and Conghaile, P.Ó. and Ortiz, R. and Pita, M. and Pöller, S. and Ruzgas, T. and Salaj-Kosla, U. and Schuhmann, W. and Sebelius, F. and Shao, M. and Stoica, L. and Sygmund, C. and Tilly, J. and Toscano, M.D. and Vivekananthan, J. and Wright, E. and Shleev, S.
    PLoS ONE 9 (2014)
    Here for the first time, we detail self-contained (wireless and self-powered) biodevices with wireless signal transmission. Specifically, we demonstrate the operation of self-sustained carbohydrate and oxygen sensitive biodevices, consisting of a wireless electronic unit, radio transmitter and separate sensing bioelectrodes, supplied with electrical energy from a combined multi-enzyme fuel cell generating sufficient current at required voltage to power the electronics. A carbohydrate/oxygen enzymatic fuel cell was assembled by comparing the performance of a range of different bioelectrodes followed by selection of the most suitable, stable combination. Carbohydrates (viz. lactose for the demonstration) and oxygen were also chosen as bioanalytes, being important biomarkers, to demonstrate the operation of the self-contained biosensing device, employing enzyme-modified bioelectrodes to enable the actual sensing. A wireless electronic unit, consisting of a micropotentiostat, an energy harvesting module (voltage amplifier together with a capacitor), and a radio microchip, were designed to enable the biofuel cell to be used as a power supply for managing the sensing devices and for wireless data transmission. The electronic system used required current and voltages greater than 44 μA and 0.57 V, respectively to operate; which the biofuel cell was capable of providing, when placed in a carbohydrate and oxygen containing buffer. In addition, a USB based receiver and computer software were employed for proof-of concept tests of the developed biodevices. Operation of bench-top prototypes was demonstrated in buffers containing different concentrations of the analytes, showcasing that the variation in response of both carbohydrate and oxygen biosensors could be monitored wirelessly in real-time as analyte concentrations in buffers were changed, using only an enzymatic fuel cell as a power supply. © 2014 Falk et al.
    view abstract10.1371/journal.pone.0109104
  • Spinel Mn-Co oxide in N-doped carbon nanotubes as a bifunctional electrocatalyst synthesized by oxidative cutting
    Zhao, A. and Masa, J. and Xia, W. and Maljusch, A. and Willinger, M.-G. and Clavel, G. and Xie, K. and Schlögl, R. and Schuhmann, W. and Muhler, M.
    Journal of the American Chemical Society 136 (2014)
    The notorious instability of non-precious-metal catalysts for oxygen reduction and evolution is by far the single unresolved impediment for their practical applications. We have designed highly stable and active bifunctional catalysts for reversible oxygen electrodes by oxidative thermal scission, where we concurrently rupture nitrogen-doped carbon nanotubes and oxidize Co and Mn nanoparticles buried inside them to form spinel Mn-Co oxide nanoparticles partially embedded in the nanotubes. Impressively high dual activity for oxygen reduction and evolution is achieved using these catalysts, surpassing those of Pt/C, RuO2, and IrO2 and thus raising the prospect of functional low-cost, non-precious-metal bifunctional catalysts in metal-air batteries and reversible fuel cells, among others, for a sustainable and green energy future. © 2014 American Chemical Society.
    view abstract10.1021/ja502532y
  • Techniques and methodologies in modern electrocatalysis: Evaluation of activity, selectivity and stability of catalytic materials
    Bandarenka, A.S. and Ventosa, E. and Maljusch, A. and Masa, J. and Schuhmann, W.
    Analyst 139 (2014)
    The development and optimisation of materials that promote electrochemical reactions have recently attracted attention mainly due to the challenge of sustainable provision of renewable energy in the future. The need for better understanding and control of electrode-electrolyte interfaces where these reactions take place, however, implies the continuous need for development of efficient analytical techniques and methodologies capable of providing detailed information about the performance of electrocatalysts, especially in situ, under real operational conditions of electrochemical systems. During the past decade, significant efforts in the fields of electrocatalysis and (electro)analytical chemistry have resulted in the evolution of new powerful methods and approaches providing ever deeper and unique insight into complex and dynamic catalytic systems. The combination of various electrochemical and non-electrochemical methods as well as the application of quantum chemistry calculations has become a viable modern approach in the field. The focus of this critical review is primarily set on discussion of the most recent cutting-edge achievements in the development of analytical techniques and methodologies designed to evaluate three key constituents of the performance of electrocatalysts, namely, activity, selectivity and stability. Possible directions and future challenges in the design and elaboration of analytical methods for electrocatalytic research are outlined. © 2014 The Royal Society of Chemistry.
    view abstract10.1039/c3an01647a
  • The role of hydrophobicity of Os-complex-modified polymers for photosystem 1 based photocathodes
    Zhao, F. and Sliozberg, K. and Rögner, M. and Plumeré, N. and Schuhmann, W.
    Journal of the Electrochemical Society 161 (2014)
    The integration of photosystem 1 in redox hydrogels based on Os-complexes modified redox polymers on electrodes yields efficient photocathodes. The generation of high photocurrent relies on high loading in PS1 and fast electron transfer rates from the electrode to PS1. The interaction between the redox polymer and PS1 influences both the loading in protein and the electron transfer rates. Since PS1 exhibits extended hydrophobic regions, polymers with similar properties may favor attractive interactions. Here we investigate three approaches to confer hydrophobicity to the redox polymer. We demonstrate that the pyridine functionality enables to switch, via basic pH values, the polymer properties from hydrophilic to hydrophobic. The transition triggers a hydrogel collapse which allows for efficient entrapment of PS1. In addition the hydrophobic-hydrophilic balance was tuned by the addition of hydrophobic group in i) the polymer backbone and ii) as substituents at the Os-complex. The increased hydrophobicity of the backbone results in higher photocurrents from PS1 integrated in the corresponding hydrogel. On the other hand, further increasing hydrophobicity of the redox relay decreases the photocurrent due to either lower mobility of the Os-complexes or poor interaction with the hydrophilic site where the redox center of PS1 is located. © The Author(s) 2014. Published by ECS.
    view abstract10.1149/2.0081413jes
  • Vertical distribution of overpotentials and irreversible charge losses in lithium ion battery electrodes
    Klink, S. and Schuhmann, W. and La Mantia, F.
    ChemSusChem 7 (2014)
    Porous lithium ion battery electrodes are characterized using a vertical distribution of cross-currents. In an appropriate simplification, this distribution can be described by a transmission line model (TLM) consisting of infinitely thin electrode layers. To investigate the vertical distribution of currents, overpotentials, and irreversible charge losses in a porous graphite electrode in situ, a multi-layered working electrode (MWE) was developed as the experimental analogue of a TLM. In this MWE, each layer is in ionic contact but electrically insulated from the other layers by a porous separator. It was found that the negative graphite electrodes get lithiated and delithiated stage-by-stage and layer-by-layer. Several mass-transport- as well as non-mass-transport-limited processes could be identified. Local current densities can reach double the average, especially on the outermost layer at the beginning of each intercalation stage. Furthermore, graphite particles close to the counter electrode act as "electrochemical sieve" reducing the impurities present in the electrolyte such as water. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cssc.201400056
  • Zr-based conversion layer on Zn-Al-Mg alloy coated steel sheets: Insights into the formation mechanism
    Lostak, T. and Maljusch, A. and Klink, B. and Krebs, S. and Kimpel, M. and Flock, J. and Schulz, S. and Schuhmann, W.
    Electrochimica Acta 137 (2014)
    Zr-based conversion layers are considered as environmentally friendly alternatives replacing trication phosphatation in the automotive industry. Based on excellent electronic barrier properties they provide an effective corrosion protection of the metallic substrate. In this work, thin protective layers were grown on novel Zn-Al-Mg alloy coated steel sheets by increasing the local pH-value at the sample surface leading to deposition of a Zr-based conversion layer. For this purpose Zn-Al-Mg alloy (ZM) coated steel sheets were treated in an aqueous model conversion solution containing well-defined amounts of hexafluorozirconic acid (H2ZrF6) and characterized after different immersion times with SKPFM and field emission SEM (FE-SEM)/EDX techniques. A deposition mechanism of Zr-based conversion coatings on microstructural heterogeneous Zn-Al-Mg alloy surfaces was proposed. © 2014 Elsevier Ltd.
    view abstract10.1016/j.electacta.2014.05.163
  • A chemical lift-off process: Removing non-specific adsorption in an electrochemical Epstein-Barr virus immunoassay
    Stratmann, L. and Gebala, M. and Schuhmann, W.
    ChemPhysChem 14 (2013)
    Upon contact of sensor surfaces with complex biological samples containing a variety of different proteins, non-specific adsorption hampers the high-sensitive detection of the analyte in question. To substantially decrease the impact of non-specific adsorption at thiol-based self-assembled monolayers, a chemical lift-off process is introduced. A sequence of local hydrolysis of isooctyl 3-mercaptopropionate, covalent binding of an antigen against the Epstein-Barr virus (EBV), stepwise incubation with a serum sample possibly containing the EBV antibody and an enzyme-labeled anti-human antibody is completed with a lift-off by integral hydrolysis of the remaining ester groups at the self-assembled monolayer. The cleavage of the ester removes any non-specifically bound protein during a following stringent washing step. The substantial improvement of the detection limit of an electrochemical immunoassay against EBV using native recombinant antigens, their immobilization after local deprotection using a scanning electrochemical microscope (SECM) and the local read-out using the generator-collector mode of SECM with redox cycling amplification demonstrates the successful application of the proposed lift-off procedure. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cphc.201300029
  • A low-potential glucose biofuel cell anode based on a toluidine blue modified redox polymer and the flavodehydrogenase domain of cellobiose dehydrogenase
    Shao, M. and Pöller, S. and Sygmund, C. and Ludwig, R. and Schuhmann, W.
    Electrochemistry Communications 29 (2013)
    A pH-dependent, two-electron transfer redox polymer was synthesized based on a methacrylate backbone with tunable properties covalently modified with toluidine blue (P017-TB). Entrapment of the isolated flavodehydrogenase domain of cellobiose dehydrogenase from Myriococcum thermophilum (FAD-MtCDH) within P017-TB is the basis for low-potential bioanode with an open circuit potential of - 240 mV vs. Ag/AgCl and a maximum current density of 60 μA cm - 2 at - 150 mV. Together with a bilirubin oxidase biocathode a biofuel cell with an open circuit voltage of 720 mV and a power density of 6.1 μW cm- 2 was obtained. © 2013 Elsevier B.V.
    view abstract10.1016/j.elecom.2013.01.016
  • A versatile electrochemical cell for the preparation and characterisation of model electrocatalytic systems
    Tymoczko, J. and Schuhmann, W. and Bandarenka, A.S.
    Physical Chemistry Chemical Physics 15 (2013)
    An electrochemical cell for the controllable modification and comprehensive electrochemical characterisation of model electro-catalytic surfaces has been developed. In-depth electrochemical characterisation of stationary electrodes as well as rotating disc electrode (RDE) measurements in hanging meniscus configuration becomes possible. Additionally, the temperature of the electrodes in contact with electrolytes can be accurately controlled between room temperature and 70-80 °C. It is of particular importance for model electro-catalytic studies that in one experimental set-up (i) electrochemical metal and non-metal deposition to adjust the amount of the foreign atoms at the surface, (ii) controllable thermal treatment to vary the position of these atoms at the surface and subsurface regions, and (iii) state-of-the-art techniques common in electrocatalysis to characterise the resulting samples are possible. The deposition and annealing procedures under various atmospheres allow accurate control over the position of the foreign atoms at the electrode surface as overlayers, surface alloys and sub-surface (or near-surface) alloys, where the solute element is preferentially located in the second atomic layer of the host metal. The cell enables us to perform all operations without exposing the samples to the laboratory atmosphere at any of the experimental stages. To demonstrate the performance and advantages of the developed cell, we use model experiments with Pt(111) single crystal electrodes and Pt(111) surfaces modified with (sub)monolayer amounts of copper. © the Owner Societies 2013.
    view abstract10.1039/c3cp51998h
  • Activation and stabilization of nitrogen-doped carbon nanotubes as electrocatalysts in the oxygen reduction reaction at strongly alkaline conditions
    Zhao, A. and Masa, J. and Schuhmann, W. and Xia, W.
    Journal of Physical Chemistry C 117 (2013)
    Nitrogen-doped carbon nanotubes (NCNTs) are highly active electrocatalysts in the oxygen reduction reaction (ORR) at alkaline conditions. However, the initial activation and stabilization of NCNTs have rarely been investigated at industrially relevant conditions. Three types of NCNTs were synthesized by catalytic growth (NCNT-growth) or posttreatment of oxygen-functionalized CNTs with NH3 (NCNT-NH3) or aniline (NCNT-aniline). The obtained NCNTs were treated in 10 M KOH at 80 C for 5 h, and the formation of oxygen groups by alkaline treatment and their interaction with existing nitrogen groups was analyzed. X-ray photoelectron spectroscopy showed that the concentrations of pyridinic and quaternary nitrogen increased in NCNT-growth due to the KOH treatment accompanied by the decrease of pyrrolic nitrogen, whereas the nitrogen groups changed differently in NCNT-NH3 and NCNT-aniline. NCNT-NH3 showed the highest ORR activity before alkaline treatment. After the treatment, the activity of NCNT-growth was higher, whereas those of NCNT-NH3 and NCNT-aniline were lower. These results were found to be correlated with changes in the nitrogen groups caused by alkaline treatment. Furthermore, NCNTs showed different C=O/C-O ratios after alkaline treatment as compared to a strong increase of C-O in CNTs, indicating that the presence of nitrogen in NCNTs influences the formation of oxygen groups on carbon and surface oxidation. © 2013 American Chemical Society.
    view abstract10.1021/jp4059438
  • Ag-stabilized few-layer graphene dispersions in low boiling point solvents for versatile nonlinear optical applications
    Sun, Z. and Dong, N. and Wang, K. and König, D. and Nagaiah, T.C. and Sánchez, M.D. and Ludwig, Al. and Cheng, X. and Schuhmann, W. and Wang, J. and Muhler, M.
    Carbon 62 (2013)
    A solution stabilization strategy that uses an easily removable media is critical to graphene (G) applications. Here, we demonstrate that highly stable graphene dispersions in low boiling point solvents such as isopropanol can be readily achieved by the uniform deposition of Ag nanoparticles (NPs) on the surface of graphene. Optimizing the synthesis parameters such as ultrasonic intensity, feeding strategy, loading content and precursor concentration allowed us to tune the particle size and, in this way, the stabilizing effects of the NPs on the dispersions. The as-obtained Ag/G/i-PrOH dispersions exhibit versatile nonlinear optical properties suggesting a great potential in nanophotonic applications such as absorber for ultrafast lasers and eye protection. © 2013 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.carbon.2013.06.010
  • Ammonia-annealed TiO2 as a negative electrode material in Li-Ion batteries: N doping or oxygen deficiency?
    Ventosa, E. and Xia, W. and Klink, S. and Mantia, F.L. and Mei, B. and Muhler, M. and Schuhmann, W.
    Chemistry - A European Journal 19 (2013)
    Improving the chemical diffusion of Li ions in anatase TiO2 is essential to enhance its rate capability as a negative electrode for Li-ion batteries. Ammonia annealing has been used to improve the rate capability of Li4Ti5O12. Similarly, ammonia annealing improves the Li-ion storage performance of anatase TiO2 in terms of the stability upon cycling and the Crate capability. In order to distinguish whether N doping or oxygen deficiencies, both introduced upon ammonia annealing, are more relevant for the observed improvement, a systematic electrochemical study was performed. The results suggest that the creation of oxygen vacancies upon ammonia annealing is the main reason for the improvement of the stability and C-rate capability. © 2013 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/chem.201302306
  • Amperometric sensing - Bioelectroanalysis
    Seeber, R. and Schuhmann, W. and Terzi, F. and Zanardi, C. and Plumere, N. and Gebala, M.
    Analytical and Bioanalytical Chemistry 405 (2013)
    view abstract10.1007/s00216-013-6813-5
  • Analysis of the interaction of the molybdenum hydroxylase PaoABC from Escherichia coli with positively and negatively charged metal complexes
    Badalyan, A. and Yoga, E.G. and Schwuchow, V. and Pöller, S. and Schuhmann, W. and Leimkühler, S. and Wollenberger, U.
    Electrochemistry Communications 37 (2013)
    An unusual behavior of the periplasmic aldehyde oxidoreductase (PaoABC) from Escherichia coli has been observed from electrochemical investigations of the enzyme catalyzed oxidation of aromatic aldehydes with different mediators under different conditions of ionic strength. The enzyme has similarity to other molybdoenzymes of the xanthine oxidase family, but the catalytic behavior turned out to be very different. Under steady state conditions the turnover of PaoABC is maximal at pH 4 for the negatively charged ferricyanide and at pH 9 for a positively charged osmium complex. Stopped-flow kinetic measurements of the catalytic half reaction showed that oxidation of benzaldehyde proceeds also above pH 7. Thus, benzaldehyde oxidation can proceed under acidic and basic conditions using this enzyme, a property which has not been described before for molybdenum hydroxylases. It is also suggested that the electron transfer with artificial electron acceptors and PaoABC can proceed at different protein sites and depends on the nature of the electron acceptor in addition to the ionic strength. © 2013 Elsevier B.V.
    view abstract10.1016/j.elecom.2013.09.017
  • Application of SECM in tracing of hydrogen peroxide at multicomponent non-noble electrocatalyst films for the oxygen reduction reaction
    Dobrzeniecka, A. and Zeradjanin, A. and Masa, J. and Puschhof, A. and Stroka, J. and Kulesza, P.J. and Schuhmann, W.
    Catalysis Today 202 (2013)
    The redox competition mode of scanning electrochemical microscopy (RC-SECM) was used to study the electrocatalytic activity of three different non-noble metal O2 reduction catalysts at a pH value of 7.4, namely; multi-walled carbon nanotubes (MWCNTs), cobalt protoporphyrin (CoP) and a composite of MWCNTs/CoP. The collection efficiency of a scanning electrochemical microscopy (SECM) tip for the H2O2 generated by the reduction of O2 at the catalyst layer was almost 100%. Consequently, SECM experiments in a combined redox competition and generator/collector mode could be applied for the determination of the number of electrons exchanged during O2 reduction, leading to improved understanding of the intrinsic features of catalyst activity. This approach avoids the typical limitations encountered with rotating ring disk electrode (RRDE) voltammetry, notably, the variation of the quantity of H2O2 in the proximity of the electrode with the speed of electrode rotation or the chemical decomposition of reaction intermediates on the Pt ring, which often introduce inconsistencies and errors in the measured values of the number of exchanged electrons. It is commonly assumed that the O2 reduction reaction on most non-noble metal catalysts proceeds via formation of H2O 2 as an intermediate. The follow-up reaction of H2O 2, typically chemical decomposition or electrochemical reduction, influences the overall number of electrons exchanged during O2 reduction. In this study, we have confirmed by comparing the rate of electrochemical reduction of H2O2 using rotating disk electrode (RDE) measurements with its rate of chemical decomposition studied using a positioned SECM tip, that for the MWCNTs/CoP catalyst, chemical decomposition is predominantly determining the overall number of exchanged electrons per O2 molecule. © 2012 Elsevier B.V.
    view abstract10.1016/j.cattod.2012.03.060
  • At-line measurement of lactose in dairy-processing plants
    Glithero, N. and Clark, C. and Gorton, L. and Schuhmann, W. and Pasco, N.
    Analytical and Bioanalytical Chemistry 405 (2013)
    Environmental and process control applications have needs for sensors that operate continuously or repeatedly, making them applicable to batch measurement and flowing product stream measurement. Additionally, for lactose monitoring in dairy-processing plants, the sensors must have sufficient flexibility to handle a wide range of substrate concentration and be resilient to withstand wide pH excursions brought about by frequent exposure to clean-in-place chemicals that happen without any warning. This paper describes the development and trialling of an at-line lactose biosensor that meets the needs of the dairy industry for loss monitoring of lactose in dairy-processing plants by the combination of a third-generation enzyme biosensor with a sequential injection analyser. Results, both from grab sample analysis and an at-line factory prototype, are shown from their operation when installed at a Fonterra dairy factory (New Zealand) during the 2011-2012 season. Previous sensor fabrication methods were converted to a single-step process, and the flow-through cell was adapted to bubble-free operation. The lactose concentration in wastewater-processing streams was successfully monitored by taking and analysing samples every 2-3 min, semi-continuously, for 3 months by an unskilled operator. The Fonterra site flushes approximately 100-300,000 L of wastewater per hour from its lactose plant. In the 2011-2012 season, the daily mean lactose content of this wastewater varied significantly, from 0.0 to 8.0 % w/v (0-233,712 μM) and equated to substantial total losses of lactose over a 6-month period. These lactose losses represent lost saleable or useable product. © 2012 Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s00216-012-6598-y
  • Carbon Cloth/Carbon Nanotube Electrodes for Biofuel Cells Development
    Haddad, R. and Xia, W. and Guschin, D.A. and Pöller, S. and Shao, M. and Vivekananthan, J. and Muhler, M. and Schuhmann, W.
    Electroanalysis 25 (2013)
    Carbon nanotubes (CNTs) grown on carbon cloth substantially increased the surface area of the electrodes. Carbon cloths were pretreated with HNO3 vapor before CNTs growth and electrochemically oxidized afterwards. The CNT-modified carbon cloths were characterized using scanning electron microscopy, Raman spectroscopy, cyclic voltammetry and electrochemical impedance spectroscopy. Biofuel cells based on these CNT-modified electrode materials using Laccase from Trametes hirsuta and cellobiose dehydrogenase from Myriococcum thermophilium entrapped in specifically designed Os-complex modified redox polymers showed a power density of 5.87μW/cm2 which is 125 fold enhanced as compared with electrodes prepared on untreated carbon cloth. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/elan.201200444
  • Combination of a photosystem 1-based photocathode and a photosystem 2-based photoanode to a z-scheme mimic for biophotovoltaic applications
    Kothe, T. and Plumeré, N. and Badura, A. and Nowaczyk, M.M. and Guschin, D.A. and Rögner, M. and Schuhmann, W.
    Angewandte Chemie - International Edition 52 (2013)
    Z-Scheme on wires: The two photosystems of the natural photosynthetic Z-scheme have been connected by immobilizing them within redox hydrogels on individual electrodes. Upon irradiation, this biophotovoltaic device produced photocurrents as a closed and autonomous system. The open-circuit voltage of the cell corresponds to the potential difference between the two redox hydrogels and indicates the coupling of the two charge separation steps. © 2013 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. This is an open access article under the terms of the Creative Commons Attribution Non-Commercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.
    view abstract10.1002/anie.201303671
  • Coupling osmium complexes to epoxy-functionalised polymers to provide mediated enzyme electrodes for glucose oxidation
    Ó Conghaile, P. and Pöller, S. and MacAodha, D. and Schuhmann, W. and Leech, D.
    Biosensors and Bioelectronics 43 (2013)
    Newly synthesised osmium complex-modified redox polymers were tested for potential application as mediators in glucose oxidising enzyme electrodes for application to biosensors or biofuel cells. Coupling of osmium complexes containing amine functional groups to epoxy-functionalised polymers of variable composition provides a range of redox polymers with variation possible in redox potential and physicochemical properties. Properties of the redox polymers as mediators for glucose oxidation were investigated by co-immobilisation onto graphite with glucose oxidase or FAD-dependent glucose dehydrogenase using a range of crosslinkers and in the presence and absence of multiwalled carbon nanotubes. Electrodes prepared by immobilising [P20-Os(2,2'-bipyridine)2(4-aminomethylpyridine)Cl].PF6, carbon nanotubes and glucose oxidase exhibit glucose oxidation current densities as high as 560μAcm-2 for PBS containing 100mM glucose at 0.45V vs. Ag/AgCl. Films prepared by crosslinking [P20-Os(4,4'-dimethoxy-2,2'-bipyridine)2(4-aminomethylpyridin e)Cl].PF6, an FAD-dependent glucose dehydrogenase, and carbon nanotubes achieve current densities of 215μAcm-2 in 5mM glucose at 0.2V vs. Ag/AgCl, showing some promise for application to glucose oxidising biosensors or biofuel cells. © 2012 Elsevier B.V.
    view abstract10.1016/j.bios.2012.11.036
  • Determination of temperature gradients with micrometric resolution by local open circuit potential measurements at a scanning microelectrode
    Sode, A. and Nebel, M. and Pinyou, P. and Schmaderer, S. and Szeponik, J. and Plumeré, N. and Schuhmann, W.
    Electroanalysis 25 (2013)
    A method to determine localized temperature profiles using a scanning electrochemical microscopy (SECM) setup and potentiometry is presented. A Pt microelectrode was first calibrated to correlate the open circuit potential (OCP) with temperature in an electrolyte containing ferri/ferrocyanide. Using the calibration graph, the temperature at a given position and a time could be derived. For dynamic measurements, the thermal expansion of the surface was initially determined using shear force mode SECM. Following the OCP at the microelectrode static as well as dynamic temperature gradients above the heated surface were successfully probed and visualized with vertical micrometric resolution and with precision in temperature determination below 1°C. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/elan.201300258
  • Direct electron transfer of Trametes hirsuta laccase adsorbed at unmodified nanoporous gold electrodes
    Salaj-Kosla, U. and Pöller, S. and Schuhmann, W. and Shleev, S. and Magner, E.
    Bioelectrochemistry 91 (2013)
    The enzyme Trametes hirsuta laccase undergoes direct electron transfer at unmodified nanoporous gold electrodes, displaying a current density of 28μA/cm2. The response indicates that ThLc was immobilised at the surface of the nanopores in a manner which promoted direct electron transfer, in contrast to the absence of a response at unmodified polycrystalline gold electrodes. The bioelectrocatalytic activity of ThLc modified nanoporous gold electrodes was strongly dependent on the presence of halide ions. Fluoride completely inhibited the enzymatic response, whereas in the presence of 150mM Cl-, the current was reduced to 50% of the response in the absence of Cl-. The current increased by 40% when the temperature was increased from 20°C to 37°C. The response is limited by enzymatic and/or enzyme electrode kinetics and is 30% of that observed for ThLc co-immobilised with an osmium redox polymer. © 2012 Elsevier B.V.
    view abstract10.1016/j.bioelechem.2012.11.001
  • Electrochemical formation and surface characterisation of Cu 2-xTe thin films with adjustable content of Cu
    Huang, M. and Maljusch, A. and Calle-Vallejo, F. and Henry, J.B. and Koper, M.T.M. and Schuhmann, W. and Bandarenka, A.S.
    RSC Advances 3 (2013)
    Electrochemically driven "intercalation" of Cu into Te was used to prepare Cu<inf>2-x</inf>Te (0.2 < x ≤ 2) thin films and accurately control the composition of the resulting samples. A thorough theoretical analysis of the system using density functional theory (DFT) calculations showed that in the absence of external electric fields the driving forces for Cu atoms to move into the subsurface layers of the Te electrodes depend on the surface coverage of copper atoms. The Cu atoms tend to preferentially occupy the subsurface layers in the telluride films. The effective electric charge on Cu atoms inside the Te-electrodes is positive. These effective charge differences with respect to pure Cu and pure Te are only 0.2 e-. Scanning Kelvin probe (SKP), atomic force microscopy (AFM) and electrochemical techniques were used to characterise the surface status of the obtained samples. Both, DFT-calculated work function differences and the SKP-measured contact potential differences (CPD) change non-linearly with the variation of the film composition. Interfacial (solid/liquid) properties evaluated using electrochemical impedance spectroscopy depend on the nominal composition of the samples and display an abrupt change that correlates with a large change in the work function and CPD. While the proposed electrochemical synthetic route can efficiently and accurately control the composition of the Cu<inf>2-x</inf>Te thin films, SKP-measurements performed under close to ambient conditions in combination with DFT calculations can provide a promising tool to link fundamental surface properties and parameters which define the interface between solids and liquids. © The Royal Society of Chemistry 2013.
    view abstract10.1039/c3ra42504e
  • Electrochemically deposited Pd-Pt and Pd-Au codeposits on graphite electrodes for electrocatalytic H2O2 reduction
    Nagaiah, T.C. and Schäfer, D. and Schuhmann, W. and Dimcheva, N.
    Analytical Chemistry 85 (2013)
    Improved electrocatalytic activity and selectivity for the reduction of H2O2 were obtained by electrodepositing Pd-Pt and Pd-Au on spectrographic graphite from solutions containing salts of the two metals at varying ratio. The electrocatalytic activity of the resulting binary codeposits for H2O2 reduction was evaluated by means of the redox-competition mode of scanning electrochemical microscopy (SECM) and voltammetric methods. In a potential range from 0 to-600 mV (vs. Ag/AgCl/3 M KCl) at pH 7.0 in 0.1 M phosphate citrate buffer, the electrocatalytic activity of both Pd-Pt and Pd-Au codeposits was substantially improved as compared with the identically deposited single metals suggesting an electrocatalytic synergy of the codeposits. Pd-Pt and Pd-Au codeposits were characterized by X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM). Codepositing with Au caused a change of hedgehog-like shaped Pd nanoparticles into cauliflower-like nanoparticles with the particle size decreasing with increasing Au concentration. Codepositing Pd with Pt caused the formation of oblong structures with the size initially increasing with increasing Pt content. However, the particle size decreases with further increase in Pt concentration. The improved electrocatalytic capability for H2O2 reduction of the Pd-Pt electrodeposits on graphite was further demonstrated by immobilizing glucose oxidase as a basis for the development of an interference-free amperometric glucose biosensor. © 2013 American Chemical Society.
    view abstract10.1021/ac401317y
  • FEM modelling of a coaxial three-electrode test cell for electrochemical impedance spectroscopy in lithium ion batteries
    Klink, S. and Höche, D. and La Mantia, F. and Schuhmann, W.
    Journal of Power Sources 240 (2013)
    Electrochemical impedance spectroscopy for lithium ion batteries has recently gained increasing attention due to its ability of non-invasive evaluation of important electrochemical parameters. Commonly used three-electrode test cells, however, proved unreliable due to asymmetric current line distributions, causing severe distortions of impedance spectra. Finite element method (FEM) simulations can visualize these current lines at different frequencies and simulate impedance spectra at given geometries. By applying FEM simulations to a recently developed coaxial impedance test cell, limiting conditions for reliable impedance measurements could be identified. Using a reference electrode in coaxial position yields sufficiently reliable results as long as the electrode misalignment is small compared to the electrolyte thickness and edge effects are prevented. © 2013 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jpowsour.2013.03.186
  • Fundamental studies on the electrocatalytic properties of metal macrocyclics and other complexes for the electroreduction of O2
    Masa, J. and Ozoemena, K.I. and Schuhmann, W. and Zagal, J.H.
    Lecture Notes in Energy 9 (2013)
    The high prospects of exploiting the oxygen reduction reaction (ORR) for lucrative technologies, for example, in the fuel cells industry, chlor-alkali electrolysis, and metal-air batteries, to name but a few, have prompted enormous research interest in the search for cost-effective and abundant catalysts for the electrocatalytic reduction of oxygen. This chapter describes and discusses the electrocatalysis of oxygen reduction by metallomacrocyclic complexes and the prospect of their potential to be used in fuel cells. Since the main interest of most researchers in this field is to design catalysts which can achieve facile reduction of O2 at a high thermodynamic efficiency, this chapter aims to bring to light the research frontiers uncovering important milestones towards the synthesis and design of promising metallomacrocyclic catalysts which can accomplish the four-electron reduction of O2 at low overpotential and to draw attention to the fundamental requirements for synthesis of improved catalysts. Particular attention has been paid to discussion of the common properties which cut across these complexes and how they may be aptly manipulated for tailored catalyst synthesis. Therefore, besides discussion of the progress attained with regard to synthesis and design of catalysts with high selectivity towards the four-electron reduction of O2, a major part of this chapter highlights quantitative structure-activity relationships (QSAR) which govern the activity and stability of these complexes, which when well understood, refined, and carefully implemented should lead to rational design of better catalysts. A brief discussion about nonmacrocyclic copper (I) complexes, particularly Cu(I) phenanthrolines, and those with a laccase-like structure which exhibit promising activity for ORR has been included in a separate section at the end. © Springer-Verlag London 2013.
    view abstract10.1007/978-1-4471-4911-8_7
  • High-yield exfoliation of graphite in acrylate polymers: A stable few-layer graphene nanofluid with enhanced thermal conductivity
    Sun, Z. and Pöller, S. and Huang, X. and Guschin, D. and Taetz, C. and Ebbinghaus, P. and Masa, J. and Erbe, A. and Kilzer, A. and Schuhmann, W. and Muhler, M.
    Carbon 64 (2013)
    High-yield exfoliation of pristine graphite in low boiling point alcohols was achieved using a set of acrylate polymers resulting in few-layer graphene concentrations of up to ∼4 mg mL-1. The polymer showed superior dispersing capabilities for graphene compared to the best reported dispersants, including the solvent N-methyl-pyrrolidone, the surfactants sodium cholate and sodium taurodeoxycholate, and the polymer polyvinylpyrrolidone. The dispersions were stable regardless of freezing (-26 C) or heating (70 C) for 24 h, or dilution with water up to 80% volume ratio over 160 h. The as-obtained nanofluid exhibited an enhancement in thermal conductivity suggesting a great potential in coolant applications. © 2013 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.carbon.2013.07.063
  • Improving the current density and the coulombic efficiency by a cascade reaction of glucose oxidizing enzymes
    Zafar, M.N. and Shao, M. and Ludwig, R. and Leech, D. and Schuhmann, W. and Gorton, L.
    ECS Transactions 53 (2013)
    Improvements in current density and coulombic efficiency of a glucose oxidizing electrode were realized by a combination of pyranose dehydrogenase from Agaricus meleagris (AmPDH) with glucose dehydrogenase from Glomerella cingulata (GcGDH). The mixed enzyme electrode oxidizes glucose in several combinations at the C-1, C-2 and C-3 positions of the pyranose ring. This concerted action of enzymes increases (i) the coulombic efficiency by extracting more than 2e- per substrate molecule and (ii) the current density of the electrode when the mass-transfer of substrates becomes rate limiting. The electrodes were investigated with flow injection analysis (FIA) using different substrates under physiological conditions (pH 7.4). These investigations showed that the product of one enzyme can be used as substrate for the other enzyme and maximally 6e- can be gained from the oxidation of one glucose molecule using mixed enzyme electrode AmPDH/GcGDH/Os-polymer. We propose a bioanode for use in biofuel cells with an increased current density and coulombic efficiency obtained by a cascade reaction catalyzed by redox enzymes with a different site-specificity for glucose. © The Electrochemical Society.
    view abstract10.1149/05302.0131ecst
  • In situ visualization of Li-ion intercalation and formation of the solid electrolyte interphase on TiO2 based paste electrodes using scanning electrochemical microscopy
    Zampardi, G. and Ventosa, E. and La Mantia, F. and Schuhmann, W.
    Chemical Communications 49 (2013)
    Scanning electrochemical microscopy (SECM) inside a glove box was used for the in situ visualization of solid electrolyte interphase (SEI) formation as well as Li-ion intercalation and de-intercalation on anatase TiO2 based paste electrodes. © 2013 The Royal Society of Chemistry.
    view abstract10.1039/c3cc44576c
  • Interaction of antitumor flavonoids with dsDNA in the absence and presence of Cu(II)
    Temerk, Y.M. and Ibrahim, M.S. and Kotb, M. and Schuhmann, W.
    Analytical and Bioanalytical Chemistry 405 (2013)
    The binding of antitumor flavonoids, namely 3-hydroxyflavone (3HF) and hesperidin (Hesp) with dsDNA was investigated in the absence and presence of Cu(II) using cyclic voltammetry and square wave voltammetry at the hanging mercury drop electrode. The reduction currents of 3HF, 3HF-Cu complex, and the 3HF-β-cyclodextrin inclusion complex decreased after intercalation into dsDNA. The intercalation of Hesp into dsDNA is weak. dsDNA is reduced at a potential of -1.48 V overlaying the reduction of Hesp. In contrast, in the presence of Cu(II), the interaction of Hesp with dsDNA leads to a much stronger intercalation. The binding constants of the flavonoid-Cu complex with dsDNA were evaluated and calibration graphs for the determination of dsDNA were obtained from the decrease in the peak current in the cyclic voltammograms of 3HF in the presence of dsDNA. The proposed method exhibited good recovery and reproducibility for indirect determination of dsDNA. © 2013 Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s00216-012-6675-2
  • Intrinsic nitrogen-doped CVD-grown TiO2 thin films from all-N-coordinated Ti precursors for photoelectrochemical applications
    Kim, S.J. and Xu, K. and Parala, H. and Beranek, R. and Bledowski, M. and Sliozberg, K. and Becker, H.-W. and Rogalla, D. and Barreca, D. and Maccato, C. and Sada, C. and Schuhmann, W. and Fischer, R.A. and Devi, A.
    Chemical Vapor Deposition 19 (2013)
    N-doped titanium dioxide (TiO2) thin films are grown on Si(100) and indium tin oxide (ITO)-coated borosilicate glass substrates by metal-organic (MO)CVD. The intrinsic doping of TiO2 thin films is achieved using all-nitrogen-coordinated Ti precursors in the presence of oxygen. The titanium amide-guanidinate complex, [Ti(NMe2)3(guan)] (guan = N,N′-diisopropyl-2-dimethylamidoguanidinato) has been developed to compensate for the thermal instability of the parent alkylamide [Ti(NMe 2)4]. Both of these amide-based compounds are tested and compared as precursors for intrinsically N-doped TiO2 at various deposition temperatures in the absence of additional N sources. The structure and morphology of TiO2 thin films are characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). Rutherford back scattering (RBS), nuclear reaction analysis (NRA), and secondary ion mass spectrometry (SIMS) analyses are performed to determine N content and distribution in the films. The optical and photoelectrochemical properties of TiO2 thin films on ITO substrates are also examined. N-doped TiO2 thin films, grown from [Ti(NMe 2)3(guan)] at 600 °C, exhibit the lowest optical absorption edge (3.0 eV) and the highest visible light photocurrent response. When compared to undoped TiO2, while in UV light photoconversion efficiency decreases significantly, the intrinsically N-doped TiO2 shows enhanced photocurrents under visible light irradiation. The intrinsic doping of TiO2 thin films with nitrogen by MOCVD and the investigation of the photo-electrochemical properties of the films are reported. N-doped anatase phase TiO2 thin films are grown on Si(100) and ITO substrates under specific processing conditions, using [Ti(NMe2) 4] (1) and [Ti(NMe2)3(guan)] (2) (guan = N,N′-diisopropyl-2-dimethylamidoguanidinato) as precursors. The films grown from [Ti(NMe2)3(guan)] at 600 °C show relatively large surface roughness and lower bandgap related with high N content. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cvde.201206996
  • Kinetic and thermodynamic hysteresis imposed by intercalation of proflavine in ferrocene-modified double-stranded DNA
    Gebala, M. and La Mantia, F. and Schuhmann, W.
    ChemPhysChem 14 (2013)
    Surface-confined immobilized redox species often do not show the expected zero peak separation in slow-scan cyclic voltammograms. This phenomenon is frequently associated to experimental drawbacks and hence neglected. However, a nonzero peak separation, which is common to many electrochemical systems with high structural flexibility, can be rationally assigned to a thermodynamic hysteresis. To study this phenomenon, a surface-confined redox species was used. Specifically, a DNA strand which is tagged with ferrocene (Fc) moieties at its 5′ end and its complementary capture probe is thiolated at the 3′ end was self-assembled in a monolayer at a Au electrode with the Fc moieties being located at the bottom plane of the double-stranded DNA (dsDNA). The DNA-bound Fc undergoes rapid electron transfer with the electrode surface as evaluated by fast scan cyclic voltammetry. The electron transfer is sensitive to the ion transport along the DNA strands, a phenomenon which is modulated upon specific intercalation of proflavine into surface-bound dsDNA. The electron transfer rate of the Fc0/+ redox process is influenced by the cationic permselectivity of the DNA monolayer. In addition to the kinetic hindrance, a thermodynamic effect correlated with changes in the activity coefficients of the Fc0/+ moieties near the gold-dsDNA interface is observed and discussed as source of the observed hysteresis causing the non-zero peak separation in the voltammograms. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cphc.201300045
  • Layered WO3/TiO2 nanostructures with enhanced photocurrent densities
    Khare, C. and Sliozberg, K. and Meyer, R. and Savan, A. and Schuhmann, W. and Ludwig, Al.
    International Journal of Hydrogen Energy 38 (2013)
    Layered WO3/TiO2 nanostructures, fabricated by magnetron sputtering, demonstrate significantly enhanced photocurrent densities compared to individual TiO2 and WO3 layers. First, a large quantity of compositions having different microstructures and thicknesses were fabricated by a combinatorial approach: diverse WO3 microstructures were obtained by adjusting sputtering pressures and depositing the films in form of wedges; later layers of TiO2 nanocolumns were fabricated thereon by the oblique angle deposition. The obtained photocurrent densities of individual WO3 and TiO2 films show thickness and microstructure dependence. Among individual WO3 layers, porous films exhibit increased photocurrent densities as compared to the dense layer. TiO2 nanocolumns show length-dependent characteristics, where the photocurrent increases with increasing film thickness. However, by combining a WO3-wedge type layer with a layer of TiO2 nanocolumns, PEC properties strikingly improve, by about two orders of magnitude as compared to individual WO3 layers. The highest photocurrent that is measured in the combinatorial library of porous WO3/TiO2 films is as high as 0.11 mA/cm2. Efficient charge-separation and charge carrier transfer processes increase the photoconversion efficiency for such films. © 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights.
    view abstract10.1016/j.ijhydene.2013.09.142
  • Localized electrochemical impedance spectroscopy: Visualization of spatial distributions of the key parameters describing solid/liquid interfaces
    Bandarenka, A.S. and Eckhard, K. and Maljusch, A. and Schuhmann, W.
    Analytical Chemistry 85 (2013)
    Acquisition of localized electrochemical impedance spectra as a function of spatial coordinates combined with novel approaches of data analysis brings a key for visualization of two-dimensional distributions of important parameters describing solid/liquid interfaces. They include the capacitance of the electric double layer, the resistance of the interfacial charge transfer, capacitances of adsorption, or other parameters depending on the properties of the system. Additionally, the proposed approach eliminates many common methodological problems of localized electrochemical impedance microscopies related to the frequency dependence of the actual pictures and difficulties with raw data interpretation. Thus, it offers a unique insight into the localized processes at the interface which is not possible to achieve using classical techniques. © 2013 American Chemical Society.
    view abstract10.1021/ac303490t
  • Low potential biofuel cell anodes based on redox polymers with covalently bound phenothiazine derivatives for wiring flavin adenine dinucleotide-dependent enzymes
    Pöller, S. and Shao, M. and Sygmund, C. and Ludwig, R. and Schuhmann, W.
    Electrochimica Acta 110 (2013)
    The design of biofuel cell anodes with substantially decreased potential is a prerequisite for the development of biofuel cells with large open-circuit voltage and power density. Redox polymers with covalently attached phenothiazine derivatives such of thionine acetate, toluidine blue, azure B simultaneously providing epoxide functions for covalent binding to suitably modified electrode surfaces and crosslinking were synthesized and evaluated for their ability to transfer electrons from the FAD cofactor of the flavodehydrogenase domain of cellobiose dehydrogenase from Myriococcum thermophilum (FAD-MtCDH), the flavodehydrogenase domain of cellobiose dehydrogenase from Corynascus thermophilus (FAD-CtCDH), or glucose oxidase from Aspergillus niger (GOx). Polymer/enzyme films were covalently bound via polymer bound epoxy groups to terminal amino functions introduced to graphite electrode surfaces by electrochemically induced grafting of diaminoheptane or Boc-protected ethylene diamine (EDA). The electrodes were optimized for biocatalytic glucose oxidation with respect to the hydrophilicity of the polymer backbone, the nature of the phenothiazine derivative, the pH value, as well as the relative amount of enzyme, polymer and crosslinker. Biofuel cells based on toluidine blue-modified redox polymers with integrated FAD-MtCDH, FAD-CtCDH, or GOx in combination with a bilirubin oxidase based biocathode exhibited open-circuit voltages of more than 0.7 V and maximum power densities in the range of 4 to 6 μW cm -2 at a pH value of 7.8. © 2013 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.electacta.2013.02.083
  • Mutual enhancement of the current density and the coulombic efficiency for a bioanode by entrapping bi-enzymes with Os-complex modified electrodeposition paints
    Shao, M. and Nadeem Zafar, M. and Sygmund, C. and Guschin, D.A. and Ludwig, R. and Peterbauer, C.K. and Schuhmann, W. and Gorton, L.
    Biosensors and Bioelectronics 40 (2013)
    A bioanode with high current density and coulombic efficiency was developed by co-immobilization of pyranose dehydrogenase from Agaricus meleagris (AmPDH) with the dehydrogenase domain of cellobiose dehydrogenase from Corynascus thermophiles (recDH. CtCDH) expressed recombinantly in Escherichia coli. The two enzymes were entrapped in Os-complex modified electrodeposition polymers (Os-EDPs) with specifically adapted redox potential by means of chemical co-deposition. AmPDH oxidizes glucose at both the C2 and C3 positions whereas recDH. CtCDH oxidizes glucose only at the C1 position. Electrochemical measurements reveal that maximally 6 electrons can be harvested from one glucose molecule at the two-enzyme anode via a cascade reaction, as AmPDH oxidizes the products formed from of the recDH. CtCDH catalyzed substrate oxidation and vice versa. Furthermore, a significant increase in current density can be obtained by combining AmPDH and recDH. CtCDH in a single modified electrode. We propose the use of this bioanode in biofuel cells with increased current density and coulombic efficiency. © 2012 Elsevier B.V.
    view abstract10.1016/j.bios.2012.07.069
  • N-doped carbon synthesized from N-containing polymers as metal-free catalysts for the oxygen reduction under alkaline conditions
    Zhao, A. and Masa, J. and Muhler, M. and Schuhmann, W. and Xia, W.
    Electrochimica Acta 98 (2013)
    Nitrogen-doped carbon materials were synthesized and used as metal-free electrocatalysts for the oxygen reduction reaction (ORR) under alkaline conditions. The synthesis was achieved by thermal treatment of nitrogen-containing polymers diluted in different carbon materials. Polypyrrole, polyaniline and polyacrylonitrile were used as N precursors. Carbon black and two types of commercial carbon nanotubes were used as carbon matrices. The obtained N contents were in the range of 1-1.8 wt.%. Different N species including pyridinic, pyrrolic and quaternary N were quantitatively determined by X-ray photoelectron spectroscopy. The ORR activities were evaluated in 0.1 M KOH. Rotating disc electrode studies revealed the presence of multiple active centers in all the samples. The sample obtained using polypyrrole and small diameter nanotubes (ca. 15 nm) had the highest onset potential at -0.07 V vs. Ag/AgCl/3 M KCl, which also showed a significantly higher electrochemical stability than the sample from carbon black and polypyrrole. The ORR activity was not correlated to the total nitrogen amount, but to the amount of pyridinic and quaternary N species. For the onset potential and the (Npyridinic + Nquaternary)/Ntotal ratio a quasi-linear relation was found, which points to the substantial role of pyridinic- and quaternary-N species in ORR catalysis. © 2013 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.electacta.2013.03.043
  • Nanostructured few-layer graphene with superior optical limiting properties fabricated by a catalytic steam etching process
    Sun, Z. and Dong, N. and Xie, K. and Xia, W. and König, D. and Nagaiah, T.C. and Sánchez, M.D. and Ebbinghaus, P. and Erbe, A. and Zhang, X. and Ludwig, Al. and Schuhmann, W. and Wang, J. and Muhler, M.
    Journal of Physical Chemistry C 117 (2013)
    Tailoring the morphology and structure of graphene can result in novel properties for advanced applications. Here, we demonstrate the fabrication of nanostructured few-layer graphene through a mild etching process via catalytic steam gasification of carbon by Fe nanoparticles (NPs). Controlling the reaction temperature, steam concentration, and the loading density of the NPs enables the fine-tuning of the etching level of graphene. Well-defined nanotrenches with a width of less than 25 nm were formed by channeling of the catalytic NPs. Etching caves and quasi-semicircular etched edges were observed as well. The nonlinear optical properties of the resulting nanostructured graphene were studied under a 532 nm nanosecond pulse laser through an open-aperture apparatus. At the same level of the linear extinction coefficient, it exhibits superior optical limiting performance in comparison with pristine graphene and C60, showing a large potential in nanophotonic devices. This enhancement is ascribed to the defects formed by etching resulting in a finite band gap in nanostructured graphene. © 2013 American Chemical Society.
    view abstract10.1021/jp401736n
  • Optimization of a membraneless glucose/oxygen enzymatic fuel cell based on a bioanode with high coulombic efficiency and current density
    Shao, M. and Zafar, M.N. and Falk, M. and Ludwig, R. and Sygmund, C. and Peterbauer, C.K. and Guschin, D.A. and MacAodha, D. and Conghaile, P.Ó. and Leech, D. and Toscano, M.D. and Shleev, S. and Schuhmann, W. and Gorton, L.
    ChemPhysChem 14 (2013)
    After initial testing and optimization of anode biocatalysts, a membraneless glucose/oxygen enzymatic biofuel cell possessing high coulombic efficiency and power output was fabricated and characterized. Two sugar oxidizing enzymes, namely, pyranose dehydrogenase from Agaricus meleagris (AmPDH) and flavodehydrogenase domains of various cellobiose dehydrogenases (DHCDH) were tested during the pre-screening. The enzymes were mixed, "wired" and entrapped in a low-potential Os-complex-modified redox-polymer hydrogel immobilized on graphite. This anode was used in combination with a cathode based on bilirubin oxidase from Myrothecium verrucaria adsorbed on graphite. Optimization showed that the current density for the mixed enzyme electrode could be further improved by using a genetically engineered variant of the non-glycosylated flavodehydrogenase domain of cellobiose dehydrogenase from Corynascus thermophilus expressed in E. coli (ngDHCtCDHC310Y) with a high glucose-turnover rate in combination with an Os-complex-modified redox polymer with a high concentration of Os complexes as well as a low-density graphite electrode. The optimized biofuel cell with the AmPDH/ngDHCtCDHC310Y anode showed not only a similar maximum voltage as with the biofuel cell based only on the ngDH CtCDHC310Y anode (0.55 V) but also a substantially improved maximum power output (20 μW cm-2) at 300 mV cell voltage in air-saturated physiological buffer. Most importantly, the estimated half-life of the mixed biofuel cell can reach up to 12 h, which is apparently longer than that of a biofuel cell in which the bioanode is based on only one single enzyme. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cphc.201300046
  • Preparation of thin film Cu-Pt(1 1 1) near-surface alloys: One small step towards up-scaling model single crystal surfaces
    Henry, J.B. and Maljusch, A. and Tymoczko, J. and Schuhmann, W. and Bandarenka, A.S.
    Electrochimica Acta 112 (2013)
    A method for the preparation of Pt(1 1 1) like thin films and thin film Cu-Pt(1 1 1) near-surface alloys (where Cu is preferentially located in the subsurface region) is reported in detail. Cyclic voltammograms of the resultant Pt(1 1 1)-like thin films in 0.1 M HClO4 demonstrate characteristic "butterfly" peaks attributed to the disorder/order phase transition in the adsorbed*OH layer, typical for a large Pt(1 1 1) crystals. Modification of the film surface with a monolayer of Cu and subsequent annealing in a reducing Ar/H2(5%) atmosphere, the voltammograms resemble those obtained for Cu-Pt(1 1 1) near-surface alloys prepared on commercial bulk single crystals. This method shows promise for the fabrication of extended (1 1 1)-type alloy surfaces of Pt and its alloys and can additionally be used to up-scale model objects for wider industrial and laboratory applications. © 2012 Elsevier Ltd.
    view abstract10.1016/j.electacta.2012.11.139
  • Renewable pencil electrodes for highly sensitive anodic stripping voltammetric determination of 3-hydroxyflavone and morin in bulk form and in biological fluids
    Temerk, Y.M. and Ibrahim, M.S. and Kotb, M. and Schuhmann, W.
    Electroanalysis 25 (2013)
    An electrochemical anodic adsorptive stripping procedure for ultra-trace assay of 3-hydroxyflavone (3HF) and Morin at a renewable pencil electrode (PGE) in bulk form and in biological fluids is described. The nature of the oxidation process of 3HF and Morin taking place at the PGE was characterized by cyclic voltammetry. The results show that the determination of the oxidation peak current is the basis of a simple, accurate and rapid method for quantification of 3HF by square-wave anodic stripping voltammetry. Determination of Morin was achieved by square-wave anodic adsorptive stripping voltammetry of the formed Morin-Cu(II) complex at a PGE. Factors influencing the trace measurements of 3HF and the Morin-Cu (II) complex at a PGE are assessed. The limits of detection and quantitation for the determination of 3HF and Morin in bulk form and in biological fluids were determined. The statistical analysis and the calibration curve data for trace determination of 3HF and Morin are reported. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/elan.201300002
  • Ring-disk microelectrodes for simultaneous constant-distance and constant-current mode scanning electrochemical microscopy
    Nebel, M. and Neugebauer, S. and Eckhard, K. and Schuhmann, W.
    Electrochemistry Communications 27 (2013)
    Ring-disk microelectrodes are proposed to be applied in a double constant-current mode using simultaneously an irreversible and a reversible reaction at the SECM tip. This allows an independent determination of the tip-to-sample distance concomitantly with the visualization of the lateral electrochemical reactivity of the investigated sample surface. The principle feasibility is demonstrated using a large and topographically as well as electrochemically challenging model sample. © 2012 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.elecom.2012.11.028
  • Robotic heavy metal anodic stripping voltammetry: Ease and efficacy for trace lead and cadmium electroanalysis
    Intarakamhang, S. and Schuhmann, W. and Schulte, A.
    Journal of Solid State Electrochemistry 17 (2013)
    A novel strategy for the automation of trace lead (Pb2+) and cadmium (Cd2+) anodic stripping voltammetry (ASV) is described. This was achieved using an electrode assembly comprising a small standard reference electrode, a Pt wire counter electrode, and an in situ bismuth-plated pencil lead working electrode for ASV in a robotic device adapted for measurements in a 24-well microtiter plate format. The movement of the electrode assembly through individual wells was by computer-controlled micropositioning, and each microtiter plate run included a sequence of electrode pretreatment, water rinsing, and simultaneous Pb2+ and Cd2+ ASV measurements. Analyte concentrations down to 2 μg/L (Pb2+) and 20 μg/L (Cd2+) could be measured in drinking and tap water, a wastewater reference material and a soil sample, with an accuracy and standard deviation typical of stripping analysis. This robotic electrochemical strategy offers automated trace metal analysis with simple instrumentation and is suggested as an option for routine use in analytical laboratories such as those providing environmental heavy metal testing services. © 2013 Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s10008-013-2018-2
  • Scanning electrochemical microscopy at variable temperatures
    Schäfer, D. and Puschhof, A. and Schuhmann, W.
    Physical Chemistry Chemical Physics 15 (2013)
    All chemical reactions are influenced by temperature, however, temperature is usually not considered an important parameter which has to be varied or at least controlled in SECM measurements. A precise temperature-control unit was designed and integrated into a SECM setup which allows setting the temperature of the sample and the adjacent electrolyte in a range between 0 and 100 °C without causing convection. Data acquisition was synchronized with the current pulses through the Peltier element to decrease the noise and keep the tip-to-sample distance constant during imaging. SECM images in the feedback mode, generator collector mode and the redox competition mode for model samples such as an enzyme entrapped within a polymer spot or oxygen reduction catalysts demonstrate the importance of controlling temperature as well as performing SECM experiments at predefined and constant increased temperature. © 2013 The Owner Societies.
    view abstract10.1039/c3cp43520b
  • Stabilizing redox polymer films by electrochemically induced crosslinking
    Pöller, S. and Koster, D. and Schuhmann, W.
    Electrochemistry Communications 34 (2013)
    Electrochemically induced crosslinking is suggested to stabilize electrodeposition polymer/enzyme films selectively on an electrode surface. 4 different protected diamine or dithiol based bi-functional crosslinkers have been synthesized, which can be activated by a pH-shift invoked by electrochemical water oxidation or proton reduction. Deprotection occurs either simultaneously or sequentially to the deposition of specifically designed redox electrodeposition polymers. The stability of the resulting polymer films was substantially enhanced as evaluated using continuous potentiodynamic cycling alternated by difference pulse voltammetry. Electrochemically induced crosslinking is compatible with biological recognition elements using Trametes hirsuta laccase or glucose oxidase entrapped within specifically adapted Os-complex modified or phenothiazine-modified redox polymers. © 2013 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.elecom.2013.07.033
  • Systematic selection of metalloporphyrin-based catalysts for oxygen reduction by modulation of the donor-acceptor intermolecular hardness
    Masa, J. and Schuhmann, W.
    Chemistry - A European Journal 19 (2013)
    Incisive modulation of the intermolecular hardness between metalloporphyrins and O2 can lead to the identification of promising catalysts for oxygen reduction. The dependency of the electrocatalytic reduction of O2 by metalloporphyrins on the nature of the central metal yields a volcano-type curve, which is rationalized to be in accordance with the Sabatier principle by using an approximation of the electrophilicity of the complexes. By using electrochemical and UV/Vis data, the influence of a selection of meso-substituents on the change in the energy for the π→π* excitation of manganese porphyrins was evaluated allowing one to quantitatively correlate the influence of the various ligands on the electrocatalysis of O2 reduction by the complexes. A manganese porphyrin was identified that electrocatalyzes the reduction of oxygen at low overpotentials without generating hydrogen peroxide. The activity of the complex became remarkably enhanced upon its pyrolysis at 650 °C. Finding the strength: Incisive modulation of the intermolecular hardness between metalloporphyrins and O2 can lead to the identification of promising catalysts for the oxygen reduction reaction (see figure). The feasibility of this principle is demonstrated in the selection and design of a manganese metalloporphyrin with promising high activity for electrocatalytic oxygen reduction. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/chem.201203846
  • The constant phase element reveals 2D phase transitions in adsorbate layers at the electrode/electrolyte interfaces
    Tymoczko, J. and Schuhmann, W. and Bandarenka, A.S.
    Electrochemistry Communications 27 (2013)
    Using one of the most understood and well-characterized electrochemical systems, Pt(111) surface in contact with H2SO4, we provide evidences that specific adsorption, 2D phase transitions in the adsorbate layers and, in general, structural effects in the double layer are largely responsible for the so-called frequency dispersion of the double layer. The results also show promise that parameters of the constant phase element (which is used in impedance spectroscopy to account for the frequency dispersion) obtained as a function of the electrode potential can be reasonably used to detect 2D phase transitions at the electrode/electrolyte interfaces. This would provide a better insight into the interface, increasing the impact of measurements made by electrochemical impedance spectroscopy. © 2012 Elsevier B.V. All rights reserved. All rights reserved.
    view abstract10.1016/j.elecom.2012.11.001
  • TiO2(B)/anatase composites synthesized by spray drying as high performance negative electrode material in Li-ion batteries
    Ventosa, E. and Mei, B. and Xia, W. and Muhler, M. and Schuhmann, W.
    ChemSusChem 6 (2013)
    The power of spray-dried TiO2 in LIBs: TiO2(B)/ anatase is synthesized by spray drying and investigated as negative electrode material in Li-ion batteries. It exhibits excellent Li-ion storage performances, especially at high charge/discharge rates. The presence of the β phase of TiO2 improves Li-ion diffusivity. Additionally, the scalable synthesis method also allows for Nb-doping, which assists in the maintenance of the electronic conductivity as the thickness of film increases. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cssc.201300439
  • Trace metal residues promote the activity of supposedly metal-free nitrogen-modified carbon catalysts for the oxygen reduction reaction
    Masa, J. and Zhao, A. and Xia, W. and Sun, Z. and Mei, B. and Muhler, M. and Schuhmann, W.
    Electrochemistry Communications 34 (2013)
    We show in this study that the presence of trace metal residues in some supposedly metal-free catalysts for oxygen reduction, at concentrations which are difficult to detect using conventional methods such as XPS and EDX, can profoundly promote the ORR activity of the catalysts. © 2013 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.elecom.2013.05.032
  • Visualization of oxygen consumption of single living cells by scanning electrochemical microscopy: The influence of the faradaic tip reaction
    Nebel, M. and Grützke, S. and Diab, N. and Schulte, A. and Schuhmann, W.
    Angewandte Chemie - International Edition 52 (2013)
    The influence of the reaction rate at the SECM tip on the overall imaging result is often neglected during respiration studies performed by SECM. The effect of the driving force of the tip reaction is elucidated using a potential pulse profile implemented into a constant-distance mode. Time-dependent data acquisition allows visualization of the transition between a tip behaving as a passive observer and a tip actively inducing transmembrane diffusion of oxygen. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/anie.201301098
  • 65th birthday of Professor Wolfbeis
    Valentin, K. and Diaz-Garcia, M. E. and Stulik, K. and Gluckwunsch, H. and Fabry, L. and Trojanowicz, M. and Bakker, E. and Buchberger, W. and Niessner, R. and Broekaert, J. A. C. and Schuhmann, W. and da Costa-Lima, J. L. F. and Soukka, T. and Karst, U. and Lev, O.
    Microchimica Acta 178 (2012)
    view abstract10.1007/s00604-012-0845-y
  • A new synthesis route for Os-complex modified redox polymers for potential biofuel cell applications
    Pöller, S. and Beyl, Y. and Vivekananthan, J. and Guschin, D.A. and Schuhmann, W.
    Bioelectrochemistry 87 (2012)
    A new synthesis route for Os-complex modified redox polymers was developed. Instead of ligand exchange reactions for coordinative binding of suitable precursor Os-complexes at the polymer, Os-complexes already exhibiting the final ligand shell containing a suitable functional group were bound to the polymer via an epoxide opening reaction. By separation of the polymer synthesis from the ligand exchange reaction at the Os-complex, the modification of the same polymer backbone with different Os-complexes or the binding of the same Os-complex to a number of different polymer backbones becomes feasible. In addition, the Os-complex can be purified and characterized prior to its binding to the polymer. In order to further understand and optimize suitable enzyme/redox polymer systems concerning their potential application in biosensors or biofuel cells, a series of redox polymers was synthesized and used as immobilization matrix for Trametes hirsuta laccase. The properties of the obtained biofuel cell cathodes were compared with similar biocatalytic interfaces derived from redox polymers obtained via ligand exchange reaction of the parent Os-complex with a ligand integrated into the polymer backbone during the polymer synthesis. © 2011 Elsevier B.V.
    view abstract10.1016/j.bioelechem.2011.11.015
  • A quick method for the preparation of Pt(111)-like thin films
    Maljusch, A. and Henry, J.B. and Schuhmann, W. and Bondarenko, A.S.
    Electrochemistry Communications 16 (2012)
    A simple and quick method for forming Pt(111)-like thin films on Si/Ti substrates for electrochemical and/or electrocatalytic experiments is reported. This method involves physical vapour deposition followed by flame annealing, electrochemical cleaning and a short heat treatment under a controlled atmosphere. Careful selection of the substrate, surface preparation and cooling atmosphere allows production of Pt thin films which show voltammetry features typical of large Pt(111) single crystal electrodes in 0.1 M HClO 4. This technique promises a method for the production of Pt(111) type surfaces on a larger scale. © 2011 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.elecom.2011.12.004
  • CNTs grown on oxygen-deficient anatase TiO 2-δ as high-rate composite electrode material for lithium ion batteries
    Ventosa, E. and Chen, P. and Schuhmann, W. and Xia, W.
    Electrochemistry Communications 25 (2012)
    A CNTs-TiO 2-δ composite consisting of carbon nanotubes (CNTs) grown by catalytic chemical vapor deposition on oxygen-deficient titanium dioxide (TiO 2-δ) nanoparticles was synthesized and investigated as high-rate negative electrode material for Li-ion batteries. An initial reversible capacity of 185 mAhg - 1 was obtained at C/2, with an initial irreversible loss of 15%. The composite showed a high stability upon cycling, with 92% retention of the capacity after 37 cycles, and good high rate capability, with a capacity of 102 mAhg - 1 at 10C. The performance of the CNTs-TiO 2-δ composite was compared to that of pristine commercial TiO 2 and to that of oxygen-deficient TiO 2 - δwith the aim of identifying the source of the improvement. Both TiO 2-δ and CNTs network were found to contribute to the enhanced electrochemical performance of CNTs-TiO 2-δ composite. © 2012 Elsevier B.V.
    view abstract10.1016/j.elecom.2012.09.031
  • Combinatorial development of nanoporous WO 3 thin film photoelectrodes for solar water splitting by dealloying of binary alloys
    Stepanovich, A. and Sliozberg, K. and Schuhmann, W. and Ludwig, Al.
    International Journal of Hydrogen Energy 37 (2012)
    A combinatorial materials approach is suggested for the development of nanoporous thin film oxides for photoelectrochemical solar water splitting. As a precursor for nanoporous WO 3 films, metallic nanoporous W films were synthesized by dealloying sputtered W 1-xAl x and W 1-xFe x (0.06 < x < 0.67) thin film materials libraries in aqueous HNO 3 solutions with different concentrations for 24 h under open circuit conditions. The variation of the etchant concentration provided different film nanostructures. The films were then transformed into nanoporous WO 3 by controlled thermal oxidation at 500 °C in air. Screening of the photoelectrochemical properties of nanoporous WO 3 films shows a strong porosity- and thickness-dependence of the photocurrent. At the same time the photocurrent density does not depend on precursor composition, because dealloying in acid solutions of certain concentration leads to formation of identical nanostructures in a broad range of precursor compositions. ©, 2012 Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.ijhydene.2012.05.039
  • Combined high resolution Scanning Kelvin probe - Scanning electrochemical microscopy investigations for the visualization of local corrosion processes
    Maljusch, A. and Senöz, C. and Rohwerder, M. and Schuhmann, W.
    Electrochimica Acta 82 (2012)
    An integrated SKP-SECM system was successfully optimised with respect to improved lateral resolution. An aluminum alloy was synthesised by solidification of a liquid melt of pure Al, Cu and Mg metal powders in order to visualize single S-phase intermetallic particles (IMPs) using a newly proposed "glass free" SKP-SECM tip. The obtained IMPs were randomly distributed in the solid solution matrix of the alloy and their average chemical composition was in agreement with that of S-phase IMPs in commercially available AA2024-T351 alloys. The S-phase IMPs were localized in the SKP mode of the SKP-SECM system. The increased electrochemical activity of the S-phase IMPs was visualized using the feedback mode of SECM and the in situ consumption of O 2 on the surface of a single S-phase IMP was visualized in the redox-competition mode of the SECM using the same tip. Thus, the local Volta potential difference obtained in the SKP mode could be overlaid with the local electrochemical activity for O 2 reduction. © 2012 Elsevier Ltd.
    view abstract10.1016/j.electacta.2012.05.134
  • Copper nanoparticles stabilized on nitrogen-doped carbon nanotubes as efficient and recyclable catalysts for alkyne/aldehyde/cyclic amine A 3-type coupling reactions
    Ramu, V.G. and Bordoloi, A. and Nagaiah, T.C. and Schuhmann, W. and Muhler, M. and Cabrele, C.
    Applied Catalysis A: General 431-432 (2012)
    Metallic copper nanoparticles have been efficiently dispersed and stabilized on nitrogen-doped carbon nanotubes. They are about 8-10 nm in diameter and highly resistant against bulk oxidation. Their catalytic activity and recyclability have been investigated in A 3-type coupling reactions for the synthesis of propargylamines. It was easily possible to prepare diastereomerically pure derivatives of proline and to efficiently recover and reuse the supported catalyst several times. © 2012 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.apcata.2012.04.019
  • Dehydrogenase-Based Reagentless Biosensors: Electrochemically Assisted Deposition of Sol-Gel Thin Films on Functionalized Carbon Nanotubes
    Wang, Z. and Etienne, M. and Pöller, S. and Schuhmann, W. and Kohring, G.-W. and Mamane, V. and Walcarius, A.
    Electroanalysis 24 (2012)
    Multiwalled carbon nanotubes (MWCNT) have been functionalized, for the electrocatalytic detection of NADH, by microwave treatment, electrochemical deposition of poly(methylene green) or wrapping with an Os-complex modified polymer. Sol-gel thin films have been then electrodeposited on the carbon nanotube layers for co-immobilization of D-sorbitol dehydrogenase and diaphorase when necessary and NAD + via covalent linkage using glycidoxypropyltrimethoxysilane. The comparison of these systems shows that the electrodeposited sol-gel matrix can significantly affect the operational behavior of functionalized MWCNT. Only MWCNT wrapped with the Os-complex modified polymer and covered with a sol-gel biocomposite allowed the electrochemical detection of D-sorbitol in a reagentless configuration. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/elan.201100574
  • Detection of DNA hybridization using electrochemical impedance spectroscopy and surface enhanced Raman scattering
    Grützke, S. and Abdali, S. and Schuhmann, W. and Gebala, M.
    Electrochemistry Communications 19 (2012)
    The formation of double-stranded DNA (dsDNA) at gold electrodes decorated with a monolayer of gold nanoparticles bound through a self-assembled dithiol monolayer is detected via specific intercalation of proflavine. Hybridization as well as sequential built-up of the electrode architecture is monitored using Faradaic electrochemical impedance spectroscopy (EIS) as well as surface enhanced Raman scattering (SERS). The adsorption of secondary gold nanoparticles allow for amplified detection of the dsDNA integrated intercalator in a vertical gap mode configuration. The experimental design thus allows probing presence of the intercalator inside the dsDNA. © 2012 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.elecom.2012.03.026
  • Direct electron transfer of bilirubin oxidase (Myrothecium verrucaria) at an unmodified nanoporous gold biocathode
    Salaj-Kosla, U. and Pöller, S. and Beyl, Y. and Scanlon, M.D. and Beloshapkin, S. and Shleev, S. and Schuhmann, W. and Magner, E.
    Electrochemistry Communications 16 (2012)
    Well defined mediatorless bioelectrocatalytic reduction of oxygen with high current densities of 0.8 mA cm - 2 was obtained on nanoporous gold electrodes modified with Myrothecium verrucaria bilirubin oxidase. A stable faradaic response was observed when the enzyme modified electrode was coated with a specifically designed electrodeposition polymer layer. The response of the enzyme electrode was only slightly inhibited by the addition of F -. © 2011 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.elecom.2011.12.007
  • Direct evidence of early blister formation in polymer-coated metals from exposure to chloride-containing electrolytes by alternating-current scanning electrochemical microscopy
    Santana, J.J. and Pähler, M. and Souto, R.M. and Schuhmann, W.
    Electrochimica Acta 77 (2012)
    An early specific effect of chloride ions on the heterogeneous swelling at the metal-polymer interface in non-defective coated metals is elucidated using frequency-dependent alternating-current scanning electrochemical microscopy (AC-SECM). Spatially-resolved differences in the topography of the coated sample are visualized upon prolonged incubation with chloride-containing electrolyte, thus allowing to assume a direct evidence of chloride ion permeation through the polymer matrix simultaneous to water uptake. The implications of this finding are particularly relevant towards the development of a new mechanistic model for blister initiation in coated metals. © 2012 Elsevier Ltd All rights reserved.
    view abstract10.1016/j.electacta.2012.05.062
  • Electrochemical synthesis of metal-polypyrrole composites and their activation for electrocatalytic reduction of oxygen by thermal treatment
    Masa, J. and Schilling, T. and Bron, M. and Schuhmann, W.
    Electrochimica Acta 60 (2012)
    This work presents a new approach for synthesis of oxygen reduction catalysts constituted of a transition metal, nitrogen and carbon, by thermal treatment of electrochemically synthesized metal-polypyrrole (M-PPy) composites on glassy carbon electrodes. The synthesis procedure involves immobilization of PPy on glassy carbon followed by dosing of metal (M = Mn, Fe and Co) particles, alternately, by electropolymerization and electrochemical reduction respectively. Electrochemical characterization by cyclic voltammetry (CV) and hydrodynamic rotating disk electrode (RDE) measurements show that the M-PPy composites inherently catalyse the electroreduction of oxygen under acidic conditions. The activity of the composites is significantly augmented when they are heat treated at high temperatures (450-850 °C) under a continuous flow of nitrogen. The presence of metallic entities within the M-PPy composite structures and in the structures ensuing after heat treatment was confirmed by energy dispersive X-ray (EDX) analysis. © 2011 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.electacta.2011.11.076
  • Electrodeposition of Catechol on Glassy Carbon Electrode and Its Electrocatalytic Activity Toward NADH Oxidation
    Maleki, A. and Nematollahi, D. and Clausmeyer, J. and Henig, J. and Plumeré, N. and Schuhmann, W.
    Electroanalysis 24 (2012)
    Catechol can be oxidized electrochemically to its corresponding o-benzoquinone. The electrogenerated quinone can be deposited by cycling the potential at the surface of glassy carbon electrodes. We have studied the electrochemical features of films derived from catechol by cyclic voltammetry. The electrodeposited film shows stable reversible redox response, dependent on pH as anticipated for quinone/catechol functionalities. Glassy carbon electrodes covered with a film derived from catechol exhibit catalytic activity in the electrooxidation of NADH at a low potential. The catalytic current is proportional to the concentration of NADH over the range 0.02-0.34mM. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/elan.201200251
  • Enhanced direct electron transfer between laccase and hierarchical carbon microfibers/carbon nanotubes composite electrodes. Comparison of three enzyme immobilization methods
    Gutiérrez-Sánchez, C. and Jia, W. and Beyl, Y. and Pita, M. and Schuhmann, W. and De Lacey, A.L. and Stoica, L.
    Electrochimica Acta 82 (2012)
    Three immobilization protocols were investigated with respect to direct electron transfer between hierarchical carbon microfibers/carbon nanotubes composite material on graphite rod electrodes and Trametes hirsuta laccase. Immobilization was done by covalent binding of laccase to aminophenyl-modified electrodes via amide-bond formation with carboxylic acid residues or imino-bond formation with aldehyde groups introduced by oxidation of sugar residues of the enzyme's glycosylation shell. Moreover, immobilization was achieved by adsorbing laccase to electrodes hydrophilized with pyrene-hexanoic acid. High current densities for biocatalytic oxygen reduction were obtained for all immobilization strategies. The formation of the imino bonds let to the binding of laccase in close to 100% direct electron transfer configuration and consequently to the highest oxygen reduction currents. © 2012 Elsevier Ltd.
    view abstract10.1016/j.electacta.2011.12.134
  • Enhanced electrocatalytic stability of platinum nanoparticles supported on a nitrogen-doped composite of carbon nanotubes and mesoporous titania under oxygen reduction conditions
    Masa, J. and Bordoloi, A. and Muhler, M. and Schuhmann, W. and Xia, W.
    ChemSusChem 5 (2012)
    Cheers for titania: An N-doped composite of carbon nanotubes (CNTs) and mesoporous TiO 2 is used as support for Pt nanoparticles applied in the oxygen reduction reaction. The composite Pt/N-TiO 2-CNT shows a higher stability than Pt particles on carbon black or N-doped CNTs, as indicated by accelerated stress tests of up to 2000 cycles. The enhanced stability is attributed to strong interactions between TiO 2 and Pt and a higher corrosion resistance of TiO 2 as well as CNTs. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cssc.201100643
  • Enzymatic fuel cells: Recent progress
    Leech, D. and Kavanagh, P. and Schuhmann, W.
    Electrochimica Acta 84 (2012)
    There is an increasing interest in replacing non-selective metal catalysts, currently used in low temperature fuel cells, with enzymes as catalysts. Specific oxidation of fuel and oxidant by enzymes as catalysts yields enzymatic fuel cells. If the catalysts can be immobilised at otherwise inert anode and cathode materials, this specificity of catalysis obviates the requirement for fuel cell casings and membranes permitting fuel cell configurations amenable to miniaturisation to be adopted. Such configurations have been proposed for application to niche areas of power generation: powering remotely located portable electronic devices, or implanted biomedical devices, for example. We focus in this review on recent efforts to improve electron transfer between the enzymes and electrodes, in the presence or absence of mediators, with most attention on research aimed at implantable or semi-implantable enzymatic fuel cells that harvest the body's own fuel, glucose, coupled to oxygen reduction, to provide power to biomedical devices. This ambitious goal is still at an early stage, with device power output and stability representing major challenges. A comparison of performance of enzymatic fuel cell electrodes and assembled fuel cells is attempted in this review, but is hampered in general by lack of availability of, and conformity to, standardised testing and reporting protocols for electrodes and cells. We therefore highlight reports that focus on this requirement. Ultimately, insight gained from enzymatic fuel cell research will lead to improved biomimetics of enzyme catalysts for fuel cell electrodes. These biomimetics will mimic enzyme catalytic sites and the structural flexibility of the protein assembly surrounding the catalytic site. © 2012 Elsevier Ltd.
    view abstract10.1016/j.electacta.2012.02.087
  • Evaluation of the catalytic performance of gas-evolving electrodes using local electrochemical noise measurements
    Zeradjanin, A.R. and Ventosa, E. and Bondarenko, A.S. and Schuhmann, W.
    ChemSusChem 5 (2012)
    Characterization of gas evolution reactions at the electrode/electrolyte boundary is often difficult due to the dynamic behavior of interfacial processes. Electrochemical noise measurements determined by scanning electrochemical microscopy were used to characterize Cl 2 evolution at gas-evolving electrodes (GEEs). Analysis of the electrochemical noise is a powerful method to evaluate the efficiency of the catalyst layer at a GEE. The high sensitivity of the developed measurement system enabled accurate monitoring of the current fluctuations caused by gas-bubble detachment from the electrode surface. Fourier transform analysis of the obtained current responses allows extraction of the characteristic frequency, which is the main parameter of the macrokinetics of GEEs. The characteristic frequency was used as part of a methodology to evaluate the catalyst performance and, in particular, to estimate the fraction of the catalyst layer that is active during the gas evolution reaction. Tip of the iceberg: Positioned scanning electrochemical microscopy tips are used to determine the characteristic frequency of gas-bubble detachment from ruthenium-based dimensionally stable anodes at different applied potentials (see picture). Geometrical factors and optimized microstructures of the electrode surface are essential for improving the overall catalytic activity for industrial applications. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cssc.201200262
  • Glucose oxidase/horseradish peroxidase Co-immobilized at a CNT-modified graphite electrode: Towards potentially implantable biocathodes
    Jia, W. and Jin, C. and Xia, W. and Muhler, M. and Schuhmann, W. and Stoica, L.
    Chemistry - A European Journal 18 (2012)
    Concerted efforts: A high-potential biocathode based on co-immobilization of glucose oxidase (GOx) and horseradish peroxidase (HRP) onto a carbon nanotube/carbon microfiber modified graphite rod electrode (CNT/CMF/GR) is described (see figure). The GOx/HRP biocathode shows a remarkable biocatalytic activity in the presence of glucose and oxygen. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/chem.201102921
  • Highly concentrated aqueous dispersions of graphene exfoliated by sodium taurodeoxycholate: Dispersion behavior and potential application as a catalyst support for the oxygen-reduction reaction
    Sun, Z. and Masa, J. and Liu, Z. and Schuhmann, W. and Muhler, M.
    Chemistry - A European Journal 18 (2012)
    A high-yielding exfoliation of graphene at high concentrations in aqueous solutions is critical for both fundamental study and future applications. Herein, we demonstrate the formation of stable aqueous dispersions of pristine graphene by using the surfactant sodium taurodeoxycholate under tip sonication at concentrations of up to 7.1 mg mL -1. TEM showed that about 8 % of the graphene flakes consisted of monolayers and 82 % of the flakes consisted of less than five layers. The dispersions were stable regardless of freezing (-20 °C) or heat treatment (80 °C) for 24 h. The concentration could be significantly improved to about 12 mg mL -1 by vacuum-evaporation of the dispersions at ambient temperature. The as-prepared graphene dispersions were readily cast into conductive films and were also processed to prepare Pt/graphene nanocomposites that were used as highly active electrocatalysts for the oxygen-reduction reaction. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/chem.201103253
  • Impact of single basepair mismatches on electron-transfer processes at Fc-PNA·DNA modified gold surfaces
    Hüsken, N. and Gȩbala, M. and Battistel, A. and La Mantia, F. and Schuhmann, W. and Metzler-Nolte, N.
    ChemPhysChem 13 (2012)
    Gold-surface grafted peptide nucleic acid (PNA) strands, which carry a redox-active ferrocene tag, present unique tools to electrochemically investigate their mechanical bending elasticity based on the kinetics of electron-transfer (ET) processes. A comparative study of the mechanical bending properties and the thermodynamic stability of a series of 12-mer Fc-PNA·DNA duplexes was carried out. A single basepair mismatch was integrated at all possible strand positions to provide nanoscopic insights into the physicochemical changes provoked by the presence of a single basepair mismatch with regard to its position within the strand. The ET processes at single mismatch Fc-PNA·DNA modified surfaces were found to proceed with increasing diffusion limitation and decreasing standard ET rate constants k 0 when the single basepair mismatch was dislocated along the strand towards its free-dangling Fc-modified end. The observed ET characteristics are considered to be due to a punctual increase in the strand elasticity at the mismatch position. The kinetic mismatch discrimination with respect to the fully-complementary duplex presents a basis for an electrochemical DNA sensing strategy based on the Fc-PNA·DNA bending dynamics for loosely packed monolayers. In a general sense, the strand elasticity presents a further physicochemical property which is affected by a single basepair mismatch which may possibly be used as a basis for future DNA sensing concepts for the specific detection of single basepair mismatches. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cphc.201100578
  • Influence of Cs + and Na + on specific adsorption of *oH, *o, and *h at platinum in acidic sulfuric media
    Berkes, B.B. and Inzelt, G. and Schuhmann, W. and Bondarenko, A.S.
    Journal of Physical Chemistry C 116 (2012)
    The influence of Cs + and Na + on the adsorption of *H, *OH, and *O (where * denotes adsorbed species) at polycrystalline Pt in acidic sulfuric media has been investigated. Electrochemical impedance spectroscopy (EIS), cyclic voltammetry, and electrochemical nanogravimetry were used (i) to elucidate the models of the interface between polycrystalline Pt and the electrolytes in a wide range of electrode potentials and (ii) to resolve contributions originating from adsorbed *H, (bi)sulfate, *OH, and *O as well as the cations to the overall interface status. Using impedance analysis it was possible to separate at least two adsorption processes: (bi)sulfate and hydrogen adsorption. The nanogravimetry additionally resolves the contribution from Cs +. Specific adsorption of Cs + at Pt surface significantly affects hydrogen adsorption, while it has almost no effect on the dynamics of SO 4 2- adsorption. Specifically adsorbed alkali cations, however, are desorbed by the onset of *OH(*O) adsorption at the Pt surface. Nevertheless, the cations likely remain in the close proximity to the surface, probably in the second H 2O layer, and largely contribute to the formation of the *OH and *O adsorbed species originating from the surface water. © 2012 American Chemical Society.
    view abstract10.1021/jp300863z
  • Influence of surface functional groups on lithium ion intercalation of carbon cloth
    Ventosa, E. and Xia, W. and Klink, S. and La Mantia, F. and Muhler, M. and Schuhmann, W.
    Electrochimica Acta 65 (2012)
    Commercial carbon cloth made of PAN-based carbon fibres was used as free-standing anode for lithium intercalation. The role of surface functional groups on the specific irreversible charge loss and reversible charge during the intercalation and de-intercalation of lithium ions into carbon cloth has been investigated. Oxygen groups have been introduced by nitric acid vapour treatment and subsequently gradually removed by thermal treatment at different temperatures in He or H 2 atmosphere as confirmed by X-ray photoelectron spectroscopy. A clear correlation between the amount of surface-bound oxygen groups and the irreversible specific charge was observed. Three irreversible processes were distinguished during the first cathodic scan: (i) reduction of oxygen groups, (ii) formation of the solid electrolyte interphase (SEI) and (iii) presumably exfoliation. The latter one was only observed for samples with low surface oxygen concentration, and its contribution to the irreversible capacity was small due to the low graphitization degree of the samples. An increased specific reversible charge upon increasing the amount of oxygen-containing groups was observed with the main improvement above 1.5 V. © 2012 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.electacta.2011.12.128
  • Intestinal aspartate proteases TiCatD and TiCatD2 of the haematophagous bug Triatoma infestans (Reduviidae): Sequence characterisation, expression pattern and characterisation of proteolytic activity
    Balczun, C. and Siemanowski, J. and Pausch, J.K. and Helling, S. and Marcus, K. and Stephan, C. and Meyer, H.E. and Schneider, T. and Cizmowski, C. and Oldenburg, M. and Höhn, S. and Meiser, C.K. and Schuhmann, W. and Schaub, G.A.
    Insect Biochemistry and Molecular Biology 42 (2012)
    Two aspartate protease encoding complementary deoxyribonucleic acids (cDNA) were characterised from the small intestine (posterior midgut) of Triatoma infestans and the corresponding genes were named TiCatD and TiCatD2. The deduced 390 and 393 amino acid sequences of both enzymes contain two regions characteristic for cathepsin D proteases and the conserved catalytic aspartate residues forming the catalytic dyad, but only TiCatD2 possesses an entire C-terminal proline loop. The amino acid sequences of TiCatD and TiCatD2 show 51-58% similarity to other insect cathepsin D-like proteases and, respectively, 88 and 58% similarity to the aspartate protease ASP25 from T. infestans available in the GenBank database. In phylogenetic analysis, TiCatD and ASP25 clearly separate from cathepsin D-like sequences of other insects, TiCatD2 groups with cathepsin D-like proteases with proline loop. The activity of purified TiCatD and TiCatD2 was highest between pH 2 and 4, respectively, and hence, deviate from the pH values of the lumen of the small intestine, which varied in correlation with the time after feeding between pH 5.2 and 6.7 as determined by means of micro pH electrodes. Both cathepsins, TiCatD and TiCatD2, were purified from the lumen of the small intestine using pepstatin affinity chromatography and identified by nanoLC-ESI-MS/MS analysis as those encoded by the cDNAs. The proteolytic activity of the purified enzymes is highest at pH 3 and the respective genes are expressed in the both regions of the midgut, stomach (anterior midgut) and small intestine, not in the rectum, salivary glands, Malpighian tubules or haemocytes. The temporal expression pattern of both genes in the small intestine after feeding revealed a feeding dependent regulation for TiCatD but not for TiCatD2. © 2011 Elsevier Ltd.
    view abstract10.1016/j.ibmb.2011.12.006
  • Investigation of copper corrosion inhibition with frequency-dependent alternating-current scanning electrochemical microscopy
    Santana, J.J. and Pähler, M. and Schuhmann, W. and Souto, R.M.
    ChemPlusChem 77 (2012)
    Alternating current scanning electrochemical microscopy (ACSECM) is used to investigate the inhibition of copper corrosion by four potential inhibitors, namely benzotriazole (BTAH), 5-methyl-benzotriazole (MBTAH), 2-mercaptobenzimidazole (MBI), and ethyl xanthate (EX). It is shown that the formation of inhibitor films on the metal can be followed from the decrease of the substrate's local electrochemical activity associated with the formation of inhibitor-containing surface layers. Sensitive imaging can be performed that accounts for changes in electrochemical activity of the modified surfaces, as well as for their corrosive attack from the environment. AC-SECM is shown to be a powerful technique for the investigation of corrosion processes. © 2012 Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim.
    view abstract10.1002/cplu.201200091
  • Investigation of early degradation processes at coated metals by AC-scanning electrochemical microscopy
    Santana, J.J. and Souto, R.M. and González, S. and Pähler, M. and Schuhmann, W.
    ECS Transactions 41 (2012)
    Alternating-current scanning electrochemical microscopy (AC-SECM) is employed to characterize the early stages of the degradation reactions occurring in metal-coating systems upon exposure to an aqueous environment. The spatial resolution of the technique results from the measurement of changes in the resistance of the thin electrolyte layer comprised between the tip and the surface of the coating due to variations in the tip-substrate distance. Resistance measurements are conducted at various frequencies of the AC perturbation signal effectively allowing topographic changes to be monitored as a function of time. Furthermore, AC-SECM can be used to determine the tip-substrate distance without the addition of redox mediators to the electrolyte, which might affect the chemical properties of the system. In this way, the effect of chloride ions from the aqueous phase to induce either the heterogeneous absorption of water by the coating, or its accumulation at the metal-substrate interface, has been imaged. ©The Electrochemical Society.
    view abstract10.1149/1.3696868
  • Mass transport controlled oxygen reduction at anthraquinone modified 3D-CNT electrodes with immobilized Trametes hirsuta laccase
    Sosna, M. and Stoica, L. and Wright, E. and Kilburn, J.D. and Schuhmann, W. and Bartlett, P.N.
    Physical Chemistry Chemical Physics 14 (2012)
    Carbon nanotubes covalently modified with anthraquinone were used as an electrode for the immobilization of Trametes hirsuta laccase. The adsorbed laccase is capable of oxygen reduction at a mass transport controlled rate (up to 3.5 mA cm-2) in the absence of a soluble mediator. The storage and operational stability of the electrode are excellent. This journal is © 2012 the Owner Societies.
    view abstract10.1039/c2cp41588g
  • Mesoporous nitrogen-rich carbon materials as catalysts for the oxygen reduction reaction in alkaline solution
    Nagaiah, T.C. and Bordoloi, A. and Sánchez, M.D. and Muhler, M. and Schuhmann, W.
    ChemSusChem 5 (2012)
    ORR MNC, FTW! Mesoporous nitrogen-rich carbon (MNC) materials are synthesized by using polymer-loaded SBA-15 pyrolyzed at different temperatures. The activity and stability of the catalysts in the oxygen reduction reaction (ORR) are investigated by using cyclic voltammetry and rotating-disk electrode measurements. The MNC material pyrolyzed at 800 °C exhibits a high electrocatalytic activity towards the ORR in alkaline medium. © 2012 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim.
    view abstract10.1002/cssc.201100284
  • Microstructural impact of anodic coatings on the electrochemical chlorine evolution reaction
    Chen, R. and Trieu, V. and Zeradjanin, A.R. and Natter, H. and Teschner, D. and Kintrup, J. and Bulan, A. and Schuhmann, W. and Hempelmann, R.
    Physical Chemistry Chemical Physics 14 (2012)
    Sol-gel Ru 0.3Sn 0.7O 2 electrode coatings with crack-free and mud-crack surface morphology deposited onto a Ti-substrate are prepared for a comparative investigation of the microstructural effect on the electrochemical activity for Cl 2 production and the Cl 2 bubble evolution behaviour. For comparison, a state-of-the-art mud-crack commercial Ru 0.3Ti 0.7O 2 coating is used. The compact coating is potentially durable over a long term compared to the mud-crack coating due to the reduced penetration of the electrolyte. Ti L-edge X-ray absorption spectroscopy confirms that a TiO x interlayer is formed between the mud-crack Ru 0.3Sn 0.7O 2 coating and the underlying Ti-substrate due to the attack of the electrolyte. Meanwhile, the compact coating shows enhanced activity in comparison to the commercial coating, benefiting from the nanoparticle-nanoporosity architecture. The dependence of the overall electrode polarization behaviour on the local activity and the bubble evolution behaviour for the Ru 0.3Sn 0.7O 2 coatings with different surface microstructure are evaluated by means of scanning electrochemical microscopy and microscopic bubble imaging. © 2012 the Owner Societies.
    view abstract10.1039/c2cp41163f
  • NADH oxidation using modified electrodes based on lactate and glucose dehydrogenase entrapped between an electrocatalyst film and redox catalyst-modified polymers
    Al-Jawadi, E. and Pöller, S. and Haddad, R. and Schuhmann, W.
    Microchimica Acta 177 (2012)
    Electrocatalytic NADH oxidation was investigated at an electrode architecture involving an electropolymerized layer of poly(methylene blue) (pMB) or poly(methylene green) (pMG) in combination with specifically designed toluidine blue or nile blue modified methacrylate-based electrodeposition polymers. Either NAD +-dependent lactate dehydrogenase or NAD +-dependent glucose dehydrogenase were entrapped between the primary electropolymerized layer of pMB or pMG and the methacrylate-based redox polymer. The composition of the polymer backbone and the polymer-bound redox dye was evaluated and it could be demonstrated that the combination of the electropolymerized pMB or pMG layer together with the dye modified methacrylate-based redox polymer shows superior properties as compared with either of the components alone. NADH was oxidized at an applied potential of 0 mV vs Ag/AgCl/KCl 3 M and current densities of 17 μA·cm -2 and 28 μA·cm -2 were obtained for modified electrodes based on lactate dehydrogenase and glucose dehydrogenase, respectively, at substrate saturation. © 2012 Springer-Verlag.
    view abstract10.1007/s00604-012-0797-2
  • Oxygen reduction reaction using N 4-metallomacrocyclic catalysts: Fundamentals on rational catalyst design
    Masa, J. and Ozoemena, K. and Schuhmann, W. and Zagal, J.H.
    Journal of Porphyrins and Phthalocyanines 16 (2012)
    In this review, we describe and discuss the developments in the use of metalloporphyrins and metallophthalocyanines as catalysts for oxygen reduction in aqueous electrolytes. The main goal of most researchers in this field has been to design catalysts which can achieve facile reduction of oxygen by the four-electron transfer pathway at the lowest overpotential possible. With this in mind, the primary objective of this review was to bring to light the research frontiers uncovering important milestones towards the synthesis and design of promising N 4-metallomacrocyclic catalysts which accomplish the four-electron reduction of oxygen, and, based on literature, to draw attention to the fundamental requirements for synthesis of improved catalysts operating at low overpotentials. Our emphasis was not to make parallel comparisons between individual classes of N 4-metallomacrocyclic complexes with respect to their activity, but rather to focus on the commonalities of the fundamental properties that govern their reactivities and how these may be aptly manipulated to develop better catalysts. Therefore, besides discussion of the progress attained with regard to synthesis and design of catalysts with high selectivity towards four-electron reduction of O 2, a major part of the review highlights quantitative structure-activity relationships (QSAR) which govern the activity and stability of these complexes, which when well understood, refined and carefully implemented should constitute a fundamental gateway for rational design of better catalysts. Copyright © 2012 World Scientific Publishing Company.
    view abstract10.1142/S1088424612300091
  • PEDOT-CNT composite microelectrodes for recording and electrostimulation applications: Fabrication, morphology, and electrical properties
    Gerwig, R. and Fuchsberger, K. and Schroeppel, B. and Link, G.S. and Heusel, G. and Kraushaar, U. and Schuhmann, W. and Stett, A. and Stelzle, M.
    Frontiers in Neuroengineering (2012)
    Composites of carbon nanotubes and poly(3,4-ethylenedioxythiophene, PEDOT) and layers of PEDOT are deposited onto microelectrodes by electropolymerization of ethylene-dioxythiophene in the presence of a suspension of carbon nanotubes and polystyrene sulfonate. Analysis by FIB and SEM demonstrates that CNT-PEDOT composites exhibit a porous morphology whereas PEDOT layers are more compact. Accordingly, capacitance and charge injection capacity of the composite material exceed those of pure PEDOT layers. In vitro cell culture experiments reveal excellent biocompatibility and adhesion of both PEDOT and PEDOT-CNT electrodes. Signals recorded from heart muscle cells demonstrate the high S/N ratio achievable with these electrodes. Long-term pulsing experiments confirm stability of charge injection capacity. In conclusion, a robust fabrication procedure for composite PEDOT-CNT electrodesisdemonstrated and results show that these electrodes are well suited for stimulation and recording in cardiac and neurophysiological research. Copyright © 2012 Gerwig, Fuchsberger, Schroeppel, Link, Heusel, Kraushaar, Schuhmann, Stett and Stelzle.
    view abstract10.3389/fneng.2012.00008
  • Probing electrode/electrolyte interface during intercalation of Cu into Te
    Huang, M. and Maljusch, A. and Henry, J.B. and Schuhmann, W. and Bondarenko, A.S.
    Electrochemistry Communications 20 (2012)
    Electrochemically driven intercalation is among the most important processes for future energetic applications. However, real-time electrochemical characterization and control remain a challenge. Here we demonstrate how the intercalation can be characterized in-situ to provide a better understanding and control over the entire process and be used to synthesize some chalcogenide semiconductor thin films which are important for photovoltaic applications. Cu intercalation into Te is used as an example. © 2012 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.elecom.2012.04.006
  • Rapid and surfactant-free synthesis of bimetallic Pt-Cu nanoparticles simply via ultrasound-assisted redox replacement
    Sun, Z. and Masa, J. and Xia, W. and König, D. and Ludwig, Al. and Li, Z.-A. and Farle, M. and Schuhmann, W. and Muhler, M.
    ACS Catalysis 2 (2012)
    The synthesis of bimetallic nanoparticles (NPs) with well-defined morphology and a size of <5 nm remains an ongoing challenge. Here, we developed a facile and efficient approach to the design of bimetallic nanostructures by the galvanic replacement reaction facilitated by high-intensity ultrasound (100 W, 20 kHz) at low temperatures. As a model system, Pt-Cu NPs deposited on nitrogen-doped carbon nanotubes (NCNTs) were synthesized and characterized by spectroscopic and microscopic techniques. Transmission electron microscopy (TEM) inspection shows that the mean diameter of Pt-Cu NPs can be as low as ≈2.8 nm, regardless of the much larger initial Cu particle size, and that a significant increase in particle number density by a factor of 35 had occurred during the replacement process. The concentration of the Pt precursor solution as well as of the size of the seed particles were found to control the size of the bimetallic NPs. Energy dispersive X-ray spectroscopy performed in the scanning TEM mode confirmed the alloyed nature of the Pt-Cu NPs. Electrochemical oxygen reduction measurements demonstrated that the resulting Pt-Cu/NCNT catalysts exhibit an approximately 2-fold enhancement in both mass- and area-related activities compared with a commercial Pt/C catalyst. © 2012 American Chemical Society.
    view abstract10.1021/cs300187z
  • Role of Water in the Chlorine Evolution Reaction at RuO 2-Based Electrodesa-Understanding Electrocatalysis as a Resonance Phenomenon
    Zeradjanin, A.R. and Menzel, N. and Strasser, P. and Schuhmann, W.
    ChemSusChem 5 (2012)
    The reaction path of the Cl 2 evolution reaction (CER) was investigated by combining electrochemical and spectroscopic methods. It is shown that oxidation and reconstruction of the catalyst surface during CER is a consequence of the interaction between RuO 2 and water. The state of the RuO 2 surface during the electrochemical reaction was analyzed in situ by using Raman spectroscopy to monitor vibrations of the crystal lattice of RuO 2 and changes in the surface concentration of the adsorbed species as a function of the electrode potential. The role of the solvent was recognized as being crucial in the formation of an oxygen-containing hydrophilic layer, which is a key prerequisite for electrocatalytic Cl 2 formation. Water (more precisely the OH adlayer) is understood not just as a medium that allows adsorption of intermediates, but also as an integral part of the intermediate formed during the electrochemical reaction. New insights into the general understanding of electrocatalysis were obtained by utilizing the vibration frequencies of the crystal lattice as a dynamic catalytic descriptor instead of thermodynamic descriptors, such as the adsorption energy of intermediates. Interpretation of the derived "volcano" curve suggests that electrocatalysis is governed by a resonance phenomenon. Water powered! The reaction path of the Cl 2 evolution reaction (CER) is investigated by combining electrochemical and spectroscopic methods. Oxidation and reconstruction of the catalyst surface during CER is a consequence of the interaction between RuO 2 and water. Interpretation of the derived volcano curve suggests that electrocatalysis is governed by a resonance phenomenon (see picture). © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cssc.201200193
  • Scanning electrochemical microscopy (SECM) in proton exchange membrane fuel cell research and development
    Schuhmann, W. and Bron, M.
    Polymer Electrolyte Membrane and Direct Methanol Fuel Cell Technology: In Situ Characterization Techniques for Low Temperature Fuel Cells (2012)
    Scanning electrochemical microscopy (SECM) has been established as a powerful technique in fuel cell catalysis research and development. This chapter presents the principles of SECM with a focus on the application of the various SECM modes to the investigation of fuel cell electrocatalysts. For the two most important reactions, namely hydrogen oxidation and oxygen reduction, an overview is given of kinetic studies as well as efforts in catalyst development and testing. Other reactions such as methanol oxidation and hydrogen peroxide formation and the application of SECM to fuel cell electrodes are also discussed. © 2012 Woodhead Publishing Limited All rights reserved.
    view abstract10.1016/B978-1-84569-774-7.50013-X
  • SECM and SKPFM Studies of the Local Corrosion Mechanism of Al Alloys - A Pathway to an Integrated SKP-SECM System
    Senöz, C. and Maljusch, A. and Rohwerder, M. and Schuhmann, W.
    Electroanalysis 24 (2012)
    Scanning Kelvin Probe Force Microscopy and Scanning Electrochemical Microscopy were applied for the investigation of localized corrosion on heterogeneous aiming on the investigation of the possible correlation between the local surface potential differences, measured by the Kelvin probe technique in ambient conditions, and corrosion during immersion in a corrosive electrolyte. A model sample mimicking the interaction of Al and Cu in Al alloys was chosen to demonstrate the complementary nature of the information received from SKPFM and SECM. The necessary prerequisites for a future integration of SKP and SECM into a single set-up are discussed. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/elan.201100609
  • Single live cell topography and activity imaging with the shear-force-based constant-distance scanning electrochemical microscope
    Schulte, A. and Nebel, M. and Schuhmann, W.
    Methods in Enzymology 504 (2012)
    In recent years, scanning electrochemical microscopy (SECM) has become an important tool in topography and activity studies on single live cells. The used analytical probes ("SECM tips") are voltammetric micro- or nanoelectrodes. The tips may be tracked across a live cell in constant-height or constant-distance mode, while kept at potentials that enable tracing of the spatiotemporal dynamics of functional chemical species in the immediate environment. Depending on the type of single live cells studied, cellular processes addressable by SECM range from the membrane transport of metabolites to the stimulated release of hormones and neurotransmitters and processes such as cell respiration or cell death and differentiation. In this chapter, we provide the key practical details of the constant-distance mode of SECM, explaining the establishment, and operation of the tailored distance control unit that maintains a stable tip-to-cell separation during scanning. The continuously maintained tip positioning of the system takes advantage of the decreasing impact of very short-range hydrodynamic tip-to-surface shear-forces on the vibrational amplitude of an oscillating SECM tip, as the input for a computer-controlled feedback loop regulation. Suitable microelectrode probes that are nondestructive to soft cells are a prerequisite for the success of this methodology and their fabrication and successful application are the other topics covered. © 2012 Elsevier Inc.
    view abstract10.1016/B978-0-12-391857-4.00012-4
  • Synthesis of an improved hierarchical carbon-fiber composite as a catalyst support for platinum and its application in electrocatalysis
    Kundu, S. and Nagaiah, T.C. and Chen, X. and Xia, W. and Bron, M. and Schuhmann, W. and Muhler, M.
    Carbon 50 (2012)
    A hierarchical carbon-fiber composite was synthesized based on carbon cloth (CC) modified with primary carbon microfibers (CMF) and subsequently secondary carbon nanotubes (CNT), thus forming a three-dimensional hierarchical structure with high BET surface area. The primary CMFs and the secondary CNTs are grown with electrodeposited iron nanoparticles as catalysts from methane and ethylene, respectively. After deposition of Pt nanoparticles by chemical vapor deposition from (trimethyl)cyclopentadienylplatinum, the resulting hierarchical composite was used as catalyst in the electrocatalytic oxygen reduction (oxygen reduction reaction, ORR) as specific test reaction. The modification of the CC with CMFs and CNTs improved the electrochemical properties of the carbon composite as revealed by electrochemical impedance measurements evidencing a low charge transfer resistance for redox mediators at the modified CC. X-ray photoelectron spectroscopy measurements were carried out to identify the chemical state and the surface atomic concentration of the Pt catalysts deposited on the hierarchical carbon composites. The ORR activity of Pt supported on different composites was investigated using rotating disk electrode measurements and scanning electrochemical microscopy. These electrochemical studies revealed that the obtained structured catalyst support is very promising for electrochemical applications, e.g. fuel cells. © 2012 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.carbon.2012.05.037
  • Tailoring of CNT surface oxygen groups by gas-phase oxidation and its implications for lithium ion batteries
    Klink, S. and Ventosa, E. and Xia, W. and La Mantia, F. and Muhler, M. and Schuhmann, W.
    Electrochemistry Communications 15 (2012)
    Multi-walled CNT were oxidised with nitric acid in liquid and gas-phase. By splitting the capacity and initial charge loss during lithium intercalation into different potential regions, it was possible to relate these values to the CNT surface oxygen groups as determined by XPS. Gas-phase oxidised CNT show a significantly lower amount of initial charge loss (172 mAh/g) compared to liquid-phase oxidised CNT (283 mAh/g). This decrease originates from less pronounced exfoliation likely caused by an increase of surface carbonyl groups. © 2011 Elsevier B.V.
    view abstract10.1016/j.elecom.2011.11.012
  • The importance of cell geometry for electrochemical impedance spectroscopy in three-electrode lithium ion battery test cells
    Klink, S. and Madej, E. and Ventosa, E. and Lindner, A. and Schuhmann, W. and La Mantia, F.
    Electrochemistry Communications 22 (2012)
    The influence of geometric and electrochemical asymmetries on the impedance spectra recorded in three-electrode test cells for lithium ion batteries was investigated. These asymmetries lead to distortions such as e.g. scaling effects which appear in common Swagelok cells. Moving the reference electrode to a coaxial position in combination with a precise alignment of the electrode stack optimized the geometry of current lines, leading to reliable impedance spectra up to frequencies of 50 kHz regardless of the electrode configuration. © 2012 Elsevier B.V.
    view abstract10.1016/j.elecom.2012.06.010
  • Thin-film Cu-Pt(111) near-surface alloys: Active electrocatalysts for the oxygen reduction reaction
    Henry, J.B. and Maljusch, A. and Huang, M. and Schuhmann, W. and Bondarenko, A.S.
    ACS Catalysis 2 (2012)
    A simple method is presented for the formation of thin films of Cu-Pt(111) near-surface alloys (NSA). In these thin films, the solute metal (Cu) is preferentially located in the second platinum layer and protected by a Pt surface layer. The NSA-films act as active and fairly stable electrocatalysts for the reduction of oxygen with the activity and stability which approach those for bulk single crystalline Pt-alloy surfaces and ∼5 times more active than state-of-the-art Pt thin films. © 2012 American Chemical Society.
    view abstract10.1021/cs300165t
  • Towards a detailed in situ characterization of non-stationary electrocatalytic systems
    Huang, M. and Henry, J.B. and Berkes, B.B. and Maljusch, A. and Schuhmann, W. and Bondarenko, A.S.
    Analyst 137 (2012)
    A complementary combination of cyclic voltammetry, impedance spectroscopy and quartz crystal microbalance techniques was used to: (i) control the assembly of a model electrocatalytic system consisting of monolayer and sub-monolayer amounts of Ag and Pb on a Au electrode, (ii) evaluate the system performance for the reduction of NO 3 - and (iii) study the disassembly of the electrocatalytic system to explore any changes which occurred during the assembly and/or catalytic stages. Physical models of the electrochemical interface (described in terms of equivalent electric circuits) at all stages are found to be considerably different but consistent with each other. Deposition of the Ag atomic layer on Au is accompanied by spontaneous surface alloying and specific adsorption of anions. In the following, deposition of the Pb atomic layer triggers further alloying in the Ag ad/Au layer while perchlorate-ions leave the surface. Approximately 1/3 of the Pb atomic layer on Ag ad/Au was found to demonstrate the best activity towards nitrate reduction. The developed experimental approach shows promise for the in situ characterization and control of all the non-stationary stages which are usually of particular importance in electrocatalytic research. © 2012 The Royal Society of Chemistry.
    view abstract10.1039/c1an15671c
  • Understanding properties of electrified interfaces as a prerequisite for label-free DNA hybridization detection
    Gebala, M. and Schuhmann, W.
    Physical Chemistry Chemical Physics 14 (2012)
    Label-free electrochemical detection of DNA hybridization with high selectivity and sensitivity is only achievable if the properties of DNA at an electrified interface are understood in depth. After a short summary of concepts of electrochemical DNA detection as well as initial attempts towards label-free DNA assays the review discusses the physico-chemical properties and differences between single-stranded and double-stranded DNA immobilized at electrode surfaces in the light of their persistence lengths, structural conformation, impact of the charge screening by ion condensation and the electric field generated upon polarization of the electrode. Electrochemical impedance spectroscopy as a tool for label-free elucidation of DNA hybridization is reviewed and the necessity for an in-depth understanding of the interfacial properties is highlighted. Our major aim is to demonstrate the advantageous application of specifically designed intercalating compounds for the design of label-free detection of DNA hybridization. This journal is © 2012 the Owner Societies.
    view abstract10.1039/c2cp42382k
  • Utilization of the catalyst layer of dimensionally stable anodes - Interplay of morphology and active surface area
    Zeradjanin, A.R. and La Mantia, F. and Masa, J. and Schuhmann, W.
    Electrochimica Acta 82 (2012)
    The activities of four different samples of dimensionally stable anodes (DSA) for the Cl 2 evolution reaction (CER) were analysed and compared with respect to their geometric properties. The samples were made from the same catalyst material, namely mixed oxides of TiO 2 and RuO 2 supported on Ti, following a preparation method in which variations in morphological features of the obtained electrodes were caused by the tensile stress imposed on each sample. The study revealed intriguing correlations between activity, surface area and characteristic morphological features. It is demonstrated that a large number of active sites facilitates high catalytic performance only conditionally, while the overall activity being highly dependent on the accessibility of the active sites to effective transport of the electrolyte, where contribution of surface morphology is crucial for product escape during gas evolution. © 2012 Elsevier Ltd.
    view abstract10.1016/j.electacta.2012.04.101
  • A chloride resistant high potential oxygen reducing biocathode based on a fungal laccase incorporated into an optimized Os-complex modified redox hydrogel
    Beyl, Y. and Guschin, D.A. and Shleev, S. and Schuhmann, W.
    Electrochemistry Communications 13 (2011)
    A chloride-resistant high-potential biocathode based on Trametes hirsuta laccase incorporated into an optimized Os-complex modified redox hydrogel (80 mV potential difference to the T1 Cu) is described. The bioelectrocatalytic activity towards O 2 reduction is due to an intimate access of the polymer-bound Os-complex to the T1 Cu site. The chloride resistance of the biocathode is due to the tight binding of the polymer-bound Os-complex to the T1 Cu site. © 2011 Elsevier B.V.
    view abstract10.1016/j.elecom.2011.02.024
  • A new AC-SECM mode: On the way to high-resolution local impedance measurements in SECM
    Gȩbala, M. and Schuhmann, W. and La Mantia, F.
    Electrochemistry Communications 13 (2011)
    Scanning electrochemical microscopy (SECM) is frequently employed to detect local chemical activity of inhomogeneous samples. Here, a modified AC-SECM mode is proposed for detecting perturbations of concentration gradients of a redox couple and electric fields imposed by the local electrochemical activity of the sample while simultaneously recording the overall impedance of the sample. The method is based on an alternating current-perturbation of the sample while recording the local alternate potential of the positioned SECM tip. The low frequency part is predominantly determined by the spatial changes in the electrochemical activity of the sample. The proposed method is intrinsically not limited by the tip size and is hence able to substantially increase the resolution of local impedance measurements. © 2011 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.elecom.2011.04.010
  • A novel automated electrochemical ascorbic acid assay in the 24-well microtiter plate format
    Intarakamhang, S. and Leson, C. and Schuhmann, W. and Schulte, A.
    Analytica Chimica Acta 687 (2011)
    Automatic ascorbic acid (AA) voltammetry was established in 24-well microtiter plates. The assay used a movable assembly of a pencil rod working, an Ag/AgCl reference and a Pt counter electrode with differential pulse voltammetry (DPV) for concentration-dependent current generation. A computer was in command of electrode (z) and microtiter plate (x, y) positioning and timed potentiostat operation. Synchronization of these actions supported sequential approach of all wells and subsequent execution of electrode treatment procedures or AA voltammetry at defined intervals in a measuring cycle. DPV in well solutions offered a linear current/concentration range between 0.1 and 8.0mM, a sensitivity of about 1μAmM-1 AA, and a detection limit of 50μM. When used with a calibration curve or standard addition, automated voltammetry of samples with added known amounts of AA demonstrated good recovery rates. Also, the assay achieved the accurate determination of the AA content of vitamin C tablets, a fruit juice and an herbal tea extract. Robotic AA voltammetry has the advantage of conveniently handling multiple samples in a single measuring run without the continuous attention of laboratory personnel. It is a good option when the goal is cost-effective AA screening of sample libraries and has potential for applications in health care and the food processing, cosmetic and pharmaceutical industries. © 2010 Elsevier B.V.
    view abstract10.1016/j.aca.2010.11.023
  • Activation/inhibition effects during the coelectrodeposition of PtAg nanoparticles: Application for ORR in alkaline media
    Schwamborn, S. and Stoica, L. and Schuhmann, W.
    ChemPhysChem 12 (2011)
    PtAg bimetallic nanoparticles for oxygen reduction reaction (ORR) in alkaline media were prepared by pulse electrodeposition (PED). During PED the reduction of Ag + ions predominates, thus an increased Ag content in the co-deposit is accomplished. The mechanism for this anomalous co-deposition was elucidated by potential pulse experiments, which revealed that nuclei formation mainly occurs via the reduction of Pt 2+ ions. The growth of the particles is diffusion controlled leading to the formation of a Ag shell covering a PtAg alloyed region. However, the shell is not growing homogeneously on the PtAg alloy. Hence, regions of the PtAg alloy are exposed, which exhibit an enhanced ORR activity compared to a pure Ag surface. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cphc.201100029
  • Amplified detection of DNA hybridization using post-labelling with a biotin-modified intercalator
    Gbala, M. and Hartwich, G. and Schuhmann, W.
    Faraday Discussions 149 (2011)
    A 32-electrode microelectrode array modified with a self-assembled monolayer of a thiolated DNA capture strand and 11-mercapto-1-undecanol was used for the detection of multi-resistant Staphylococcus aureus (MRSA) upon hybridization of the complementary target DNA. In the proposed assay strategy the obtained double-stranded DNA (dsDNA) is at first non-covalently labeled by intercalation of a proflavine derivative which is functionalized via a flexible spacer with biotin moieties. Subsequent to this post-labelling a avidin/alkaline phosphatase conjugate is bound to the biotin moieties thus introducing a reporter group at sites bearing dsDNA. Hybridization and hence the presence of MRSA DNA is detected via oxidation of p-aminophenol enzymatically generated from p-aminophenylphosphate. The assay strategy was carefully evaluated using ferrocene-modified target strands. An increase in sensitivity of the proposed label-free DNA assays based on a careful design of the sensing interface and the implemented enzymatic amplification was achieved. © 2011 The Royal Society of Chemistry.
    view abstract10.1039/c005365a
  • Application of AC-SECM in corrosion science: Local visualisation of inhibitor films on active metals for corrosion protection
    Pähler, M. and Santana, J.J. and Schuhmann, W. and Souto, R.M.
    Chemistry - A European Journal 17 (2011)
    The suitability of frequency-dependent alternating-current scanning electrochemical microscopy (4D AC-SECM) for investigation of thin passivating layers covering the surface of corrosion-inhibited metals has been demonstrated. Inhibition of copper corrosion by benzotriazole (BTAH) and methylbenzotriazole (MBTAH), which are effective inhibitors for this metal under many environmental conditions, was investigated. Strong dependencies were found for the AC z-approach curves with both the duration of the inhibitor treatment and the frequency of the AC excitation signal applied in AC-SECM. Both negative and positive feedback behaviours were observed in the AC approach curves for untreated copper and for Cu/BTAH and Cu/MBTAH samples. Negative feedback behaviour occurred in the low-frequency range, whereas a positive feedback effect was observed at higher frequencies. A threshold frequency related to the passage from negative to positive regimes could be determined in each case. The threshold frequency for inhibitor-modified samples was found always to be significantly higher than for the untreated metal, because the inhibitor film provides electrical insulation for the surface. Moreover, the threshold frequency increased with increasing surface coverage by the inhibitor. 4D AC-SECM was successfully applied to visualizing spatially resolved differences in local electrochemical activity between inhibitor-free and inhibitor-covered areas of the sample. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/chem.201000689
  • Construction of uricase-overproducing strains of Hansenula polymorpha and its application as biological recognition element in microbial urate biosensor
    Dmytruk, K.V. and Smutok, O.V. and Dmytruk, O.V. and Schuhmann, W. and Sibirny, A.A.
    BMC Biotechnology 11 (2011)
    Background: The detection and quantification of uric acid in human physiological fluids is of great importance in the diagnosis and therapy of patients suffering from a range of disorders associated with altered purine metabolism, most notably gout and hyperuricaemia. The fabrication of cheap and reliable urate-selective amperometric biosensors is a challenging task.Results: A urate-selective microbial biosensor was developed using cells of the recombinant thermotolerant methylotrophic yeast Hansenula polymorpha as biorecognition element. The construction of uricase (UOX) producing yeast by over-expression of the uricase gene of H. polymorpha is described. Following a preliminary screening of the transformants with increased UOX activity in permeabilized yeast cells the optimal cultivation conditions for maximal UOX yield namely a 40-fold increase in UOX activity were determined.The UOX producing cells were coupled to horseradish peroxidase and immobilized on graphite electrodes by physical entrapment behind a dialysis membrane. A high urate selectivity with a detection limit of about 8 μM was found.Conclusion: A strain of H. polymorpha overproducing UOX was constructed. A cheap urate selective microbial biosensor was developed. © 2011 Dmytruk et al; licensee BioMed Central Ltd.
    view abstract10.1186/1472-6750-11-58
  • Electrochemical behaviour of the anticancer dacarbazine-cu2+complex and its analytical applications
    Temerk, Y.M. and Kamal, M.M. and Ibrahim, M.S. and Ibrahim, H. and Schuhmann, W.
    Electroanalysis 23 (2011)
    Electrochemical reduction of the dacarbazine-Cu2+complex was investigated using cyclic voltammetry and square wave voltammetry at a hanging mercury drop electrode. The reduction of the dacarbazine-Cu2+complex is irreversible. A reduction mechanism is proposed comprising a one-electron reduction of the Cu2+directly within the complex. The sharp peak of the adsorbed dacarbazine-Cu2+complex associated with an effective interfacial accumulation facilitates the determination of the anticancer drug dacarbazine in pharmaceutical formulations and biological fluids. Detection limits for dacarbazine of 6.12×10-10M, 1.57×10-10M and 1.97×10-9M were achieved for the determination of the drug in vial, human urine and serum, respectively. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/elan.201100038
  • Electron transfer between genetically modified Hansenula polymorpha yeast cells and electrode surfaces via os-complex modified redox polymers
    Shkil, H. and Schulte, A. and Guschin, D.A. and Schuhmann, W.
    ChemPhysChem 12 (2011)
    Graphite electrodes modified with redox-polymer-entrapped yeast cells were investigated with respect to possible electron-transfer pathways between cytosolic redox enzymes and the electrode surface. Either wild-type or genetically modified Hansenula polymorpha yeast cells over-expressing flavocytochrome b2 (FC b2) were integrated into Os-complex modified electrodeposition polymers. Upon increasing the L-lactate concentration, an increase in the current was only detected in the case of the genetically modified cells. The overexpression of FC b2 and the related amplification of the FC b2/L-lactate reaction cycle was found to be necessary to provide sufficient charge to the electron-exchange network in order to facilitate sufficient electrochemical coupling between the cells, via the redox polymer, to the electrode. The close contact of the Os-complex modified polymer to the cell wall appeared to be a prerequisite for electrically wiring the cytosolic FC b2/L-lactate redox activity and suggests the critical involvement of a plasma membrane redox system. Insights in the functioning of whole-cell-based bioelectrochemical systems have to be considered for the successful design of whole-cell biosensors or microbial biofuel cells. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cphc.201000889
  • Enhanced photoelectrochemical properties of WO3 thin films fabricated by reactive magnetron sputtering
    Vidyarthi, V.S. and Hofmann, M. and Savan, A. and Sliozberg, K. and König, D. and Beranek, R. and Schuhmann, W. and Ludwig, Al.
    International Journal of Hydrogen Energy 36 (2011)
    Polycrystalline WO3 thin films were fabricated by reactive magnetron sputtering at a substrate temperature of 350 °C under different Ar/O2 gas pressures. In order to study the thickness dependence of photoelectrochemical (PEC) behavior of WO3, the thickness-gradient films were fabricated and patterned using a micro-machined Si-shadow mask during the deposition process. The variation of the sputter pressure leads to the evolution of different microstructures of the thin films. The films fabricated at 2 mTorr sputter pressure are dense and show diminished PEC properties, while the films fabricated at 20 mTorr and 30 mTorr are less dense and exhibit enhanced water photooxidation efficiency. The enhanced photooxidation is attributed to the coexistence of porous microstructure and space charge region enabling improved charge carrier transfer to the electrolyte and back contact. A steady-state photocurrent as high as 2.5 mA cm-2 at 1 V vs. an Ag/AgCl (3 M KCl) reference electrode was observed. For WO3 films fabricated at 20 mTorr and 30 mTorr, the photocurrent increases continuously up to a thickness of 600 nm. © 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.ijhydene.2011.01.087
  • High-throughput characterization of Pt supported on thin film oxide material libraries applied in the oxygen reduction reaction
    Schäfer, D. and Mardare, C. and Savan, A. and Sanchez, M.D. and Mei, B. and Xia, W. and Muhler, M. and Ludwig, Al. and Schuhmann, W.
    Analytical Chemistry 83 (2011)
    Thin film metal oxide material libraries were prepared by sputter deposition of nanoscale Ti/Nb precursor multilayers followed by ex situ oxidation. The metal composition was varied from 6 at.% Nb to 27 at.% Nb. Additionally, thin wedge-type layers of Pt with a nominal thickness gradient from 0 to 5 nm were sputter-deposited on top of the oxides. The materials libraries were characterized with respect to metallic film composition, oxide thickness, phases, electrical conductivity, Pt thickness, and electrochemical activity for the oxygen reduction reaction (ORR). Electrochemical investigations were carried out by cyclic voltammetry using an automated scanning droplet cell. For a nominal Pt thickness >1 nm, no significant dependence of the ORR activity on the Pt thickness or the substrate composition was observed. However, below that critical thickness, a strong decrease of the surface-normalized activity in terms of reduction currents and potentials was observed. For such thin Pt layers, the conductivity of the substrate seems to have a substantial impact on the catalytic activity. Results from X-ray photoelectron spectroscopy (XPS) measurements suggest that the critical Pt thickness coincides with the transition from a continuous Pt film into isolated particles at decreasing nominal Pt thickness. In the case of isolated Pt particles, the activity of Pt decisively depends on its ability to exchange electrons with the oxide layer, and hence, a dependence on the substrate conductivity is rationalized. © 2011 American Chemical Society.
    view abstract10.1021/ac102303u
  • Highly active metal-free nitrogen-containing carbon catalysts for oxygen reduction synthesized by thermal treatment of polypyridine-carbon black mixtures
    Xia, W. and Masa, J. and Bron, M. and Schuhmann, W. and Muhler, M.
    Electrochemistry Communications 13 (2011)
    A straight-forward method for the synthesis of metal-free catalysts for oxygen reduction by thermal treatment of a mixture of poly(3,5-pyridine) with carbon black in helium is reported. The catalyst was characterized by X-ray diffraction and photoelectron spectroscopy, cyclic voltammetry and rotating disk electrode measurements. The new catalyst exhibited remarkable activity similar to Pt-based catalysts in alkaline media. © 2011 Elsevier B.V. All Rights Reserved.
    view abstract10.1016/j.elecom.2011.03.018
  • Impedimetric detection of hairpin ribozyme activity
    Piekielska, K. and Gebala, M. and Gwiazda, S. and Müller, S. and Schuhmann, W.
    Electroanalysis 23 (2011)
    Electrochemical impedance spectroscopy is successfully utilized for label-free monitoring of the cleavage reaction of a RNA substrate by a complementary hairpin ribozyme. The formation of the RNA substrate/ribozyme complex is increasing the negative charge at the interface and is modulating the kinetic of the redox conversion of a negatively charged redox mediator (e.g. [Fe(CN)6]3-/4-) hence leading to an increase in the charge transfer resistance. Upon addition of bivalent cations such as Mg2+ ions the conformation of the ribozyme is changed and the catalytic cleavage of the RNA is monitored by a decrease of the charge transfer resistance. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/elan.201000640
  • Integrated scanning kelvin probe-scanning electrochemical microscope system: Development and first applications
    Maljusch, A. and Schönberger, B. and Lindner, A. and Stratmann, M. and Rohwerder, M. and Schuhmann, W.
    Analytical Chemistry 83 (2011)
    The integration of a scanning Kelvin probe (SKP) and a scanning electrochemical microscope (SECM) into a single SKP-SECM setup, the concept of the proposed system, its technical realization, and first applications are presented and discussed in detail. A preloaded piezo actuator placed in a grounded stainless steel case was used as the driving mechanism for oscillation of a Pt disk electrode as conventionally used in SECM when the system was operated in the SKP mode. Thus, the same tip is recording the contact potential difference (CPD) during SKP scanning and is used as a working electrode for SECM imaging in the redox-competition mode (RC-SECM). The detection of the local CPD is established by amplification of the displacement current at an ultralow noise operational amplifier and its compensation by application of a variable backing potential (V b) in the external circuit. The control of the tip-to-sample distance is performed by applying an additional alternating voltage with a much lower frequency than the oscillation frequency of the Kelvin probe. The main advantage of the SKP-SECM system is that it allows constant distance measurements of the CPD in air under ambient conditions and in the redox-competition mode of the SECM in the electrolyte of choice over the same sample area without replacement of the sample or exchange of the working electrode. The performance of the system was evaluated using a test sample made by sputtering thin Pt and W films on an oxidized silicon wafer. The obtained values of the CPD correlate well with known data, and the electrochemical activity for oxygen reduction is as expected higher over Pt than W. © 2011 American Chemical Society.
    view abstract10.1021/ac200953b
  • Local electrocatalytic induction of sol-gel deposition at Pt nanoparticles
    Schwamborn, S. and Etienne, M. and Schuhmann, W.
    Electrochemistry Communications 13 (2011)
    Electrochemically-assisted deposition of sol-gel materials can be locally confined at Pt nanoparticles. Pt nanoparticles have been locally deposited on glassy carbon surfaces by pulse electrodeposition using a droplet cell. Upon applying a potential for electrochemically-assisted deposition, the formed sol-gel film mirrors the region of the glassy carbon surface previously modified with Pt nanoparticles. © 2011 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.elecom.2011.02.030
  • Mechanistic studies of Fc-PNA(·DNA) surface dynamics based on the kinetics of electron-transfer processes
    Hüsken, N. and Gȩbala, M. and La Mantia, F. and Schuhmann, W. and Metzler-Nolte, N.
    Chemistry - A European Journal 17 (2011)
    N-Terminally ferrocenylated and C-terminally gold-surface-grafted peptide nucleic acid (PNA) strands were exploited as unique tools for the electrochemical investigation of the strand dynamics of short PNA(·DNA) duplexes. On the basis of the quantitative analysis of the kinetics and the diffusional characteristics of the electron-transfer process, a nanoscopic view of the Fc-PNA(·DNA) surface dynamics was obtained. Loosely packed, surface-confined Fc-PNA single strands were found to render the charge-transfer process of the tethered Fc moiety diffusion-limited, whereas surfaces modified with Fc-PNA·DNA duplexes exhibited a charge-transfer process with characteristics between the two extremes of diffusion and surface limitation. The interplay between the inherent strand elasticity and effects exerted by the electric field are supposed to dictate the probability of a sufficient approach of the Fc head group to the electrode surface, as reflected in the measured values of the electron-transfer rate constant, k 0. An in-depth understanding of the dynamics of surface-bound PNA and PNA·DNA strands is of utmost importance for the development of DNA biosensors using (Fc-)PNA recognition layers. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/chem.201003764
  • Photocurrent generation by photosystem 1 integrated in crosslinked redox hydrogels
    Badura, A. and Guschin, D. and Kothe, T. and Kopczak, M.J. and Schuhmann, W. and Rögner, M.
    Energy and Environmental Science 4 (2011)
    Photosystem 1 (PS1) catalyzes the light driven translocation of electrons in the process of oxygenic photosynthesis. Isolated PS1 was immobilised on a gold electrode surface via an Os complex containing redox polymer hydrogel which simultaneously is used as immobilisation matrix and as electron donor for PS1. On addition of methyl viologen as sacrificial electron acceptor, a catalytic photocurrent with densities of up to 29 μA cm -2 at a light intensity of 1.8 mW cm -2 was observed upon illumination - equivalent to an incident photon to carrier efficiency (IPCE) of 3.1%. The strong dependence of the catalytic reaction on the light intensity and the dissolved oxygen concentration indicates that a significant photocurrent from excited PS1 to the electrode can only be realized in the presence of oxygen. © 2011 The Royal Society of Chemistry.
    view abstract10.1039/c1ee01126j
  • Polythiophene-assisted vapor phase synthesis of carbon nanotube-supported rhodium sulfide as oxygen reduction catalyst for HCl electrolysis
    Jin, C. and Nagaiah, T.C. and Xia, W. and Bron, M. and Schuhmann, W. and Muhler, M.
    ChemSusChem 4 (2011)
    Rhodium Drive: Carbon nanotube-supported rhodium sulfide electrocatalysts are prepared by sequential chemical vapor deposition of iron, controlled vapor phase polymerization of thiophene, and finally impregnation of the rhodium precursor and pyrolysis. The electrocatalysts are applied in the oxygen reduction reaction under HCl electrolysis conditions. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cssc.201000315
  • Probing the Pt Surface for Oxygen Reduction by Insertion of Ag
    Schwamborn, S. and Bron, M. and Schuhmann, W.
    Electroanalysis 23 (2011)
    We report on the probing of the Pt surface for oxygen reduction reaction (ORR) by insertion of Ag. Therefore, PtAg bimetallic nanoparticles were prepared by pulse electrodeposition. In a second step, Ag was electro-dissolved in acidic media from the particles under formation of Pt skeleton. The ORR activity of these Pt skeleton depends on two factors: (1) on the surface properties of the Pt-shell and (2) on the electronic as well as geometric influences of the remaining Ag in the particle core. By varying the conditioning procedure prior to measuring the ORR activity, we were able to differentiate between these two effects. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/elan.201000510
  • Scanning electrochemical microscopy for investigation of multicomponent bioelectrocatalytic films
    Dobrzeniecka, A. and Zeradjanin, A. and Masa, J. and Stroka, J. and Goral, M. and Schuhmann, W. and Kulesza, P.J.
    ECS Transactions 35 (2011)
    Scanning electrochemical microscopy in the redox competition mode (RC-SECM) is proposed as an useful technique for local investigation of the electrocatalytic activity of different catalysts towards the oxygen reduction reaction (ORR) with simultaneous detection of the produced undesirable intermediate hydrogen peroxide. We have used cobalt porphyrin dispersed in a multiwalled carbon nanotubes (MWCNTs) matrix as a model electrocatalyst for the predominant two-electron reduction of oxygen to hydrogen peroxide. Furthermore, Prussian Blue or horseradish peroxidase were used as catalysts for the further reduction of hydrogen peroxide to water. The properties of each component of the film were examined along with their potential interactions with the other components. As a result an efficient electrocatalyst for oxygen reduction at physiological pH could be obtained. ©The Electrochemical Society.
    view abstract10.1149/1.3646486
  • Simultaneous acquisition of impedance and gravimetric data in a cyclic potential scan for the characterization of nonstationary electrode/electrolyte interfaces
    Berkes, B.B. and Maljusch, A. and Schuhmann, W. and Bondarenko, A.S.
    Journal of Physical Chemistry C 115 (2011)
    Simultaneous acquisition of electrochemical impedance spectroscopy and quartz crystal microbalance (EIS-EQCM) data in cyclic electrode potential scans was used to characterize nonstationary underpotential deposition (UPD) of atomic layers of Ag on Au and Cu on Pt. Both EIS and EQCM data sets complemented each other in the elucidation of interface models and the investigation of different aspects of the interfacial dynamics. EIS-EQCM provided an opportunity to monitor coadsorption and competitive adsorption of anions during the Ag and Cu UPD using (i) the electrode mass change, (ii) adsorption capacitances, and (iii) double-layer capacitances. Kinetic information is available in the EIS-EQCM through the charge transfer resistances and apparent rate coefficients. The latter expresses the rate of UPD into the partially covered electrode surface. The apparent rate coefficients for the Ag UPD were determined to vary from 0.15 to 0.45 cm/s which is between the standard constant rates k0 of Ag bulk deposition on Ag reported previously for different Ag surfaces. Cu UPD on Pt and Ag UPD on Au contributed differently into a resonance resistance ?R(E) available from the EQCM data sets. Spontaneous surface alloying between Ag and Au during the Ag UPD continuously increased the ?R, while the Cu overlayer formation on Pt as well as experiments without Ag+ and Cu 2+ in the solution did not change this parameter significantly. The EIS-EQCM appeared to be a promising tool for an improved characterization and understanding of nonstationary electrochemical interfaces. © 2011 American Chemical Society.
    view abstract10.1021/jp200755p
  • Simultaneous visualization of surface topography and concentration field by means of scanning electrochemical microscopy using a single electrochemical probe and impedance spectroscopy
    Pähler, M. and Schuhmann, W. and Gratzl, M.
    ChemPhysChem 12 (2011)
    Scanning electrochemical microscopy visualizes concentration profiles. To determine the location of the probe relative to topographical features of the substrate, knowledge of the probe-to-sample distance at each probe position is required. The use of electrochemical impedance spectroscopy for obtaining information on the substrate-to-probe distance and on the concentration of interest using the electrochemical probe alone is suggested. By tuning the frequencies of interrogation, the probe-to-substrate distance can be derived followed by interrogation of processes that carry information on concentration at lower frequencies. These processes may include charge-transfer relaxation, diffusional relaxation at the electrode, and open-circuit potential at zero frequency. A potentiometric chloride sensing microprobe is used herein to reconstruct both topography and the concentration field at a microscopic diffusional source of chloride. Electrochemical impedance spectroscopy is used for simultaneously obtaining information on the substrate-to-probe distance and on the concentration of Cl - ions. By tuning the frequencies of interrogation to fast migrational relaxations in the solution, the substrate-to-probe distance can be derived followed by interrogation of processes that carry information on concentration at lower frequencies (see picture). Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cphc.201100428
  • The Thermophilic CotA Laccase from Bacillus subtilis: Bioelectrocatalytic Evaluation of O2 Reduction in the Direct and Mediated Electron Transfer Regime
    Beneyton, T. and Beyl, Y. and Guschin, D.A. and Griffiths, A.D. and Taly, V. and Schuhmann, W.
    Electroanalysis 23 (2011)
    The thermophilic bacterial laccase CotA from Bacillus subtilis adsorbed on graphite electrodes enables monitoring of the temperature dependent direct biocatalytic O2 reduction. Its entrapment in two different Os-complex modified redox hydrogels is the basis for mediated bioelectroreduction of O2. Besides the temperature and pH dependence of the bioelectrocatalytic response chloride and fluoride inhibition studies demonstrate that the Os-complexes are bound to the T1 copper centre of the enzyme. The interaction between CotA and the Os-complex modified polymers prevents inhibition by chloride ions. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/elan.201100054
  • Visualization of chlorine evolution at dimensionally stable anodes by means of scanning electrochemical microscopy
    Zeradjanin, A.R. and Schilling, T. and Seisel, S. and Bron, M. and Schuhmann, W.
    Analytical Chemistry 83 (2011)
    Scanning electrochemical microscopy (SECM) has been used to detect and visualize the local electrocatalytic activity of dimensionally stable anodes (DSA) for Cl2 evolution from brine. The sample generation-tip collection (SG-TC) mode of SECM shows limitations arising from complications connected with the reduction of Cl2 at the SECM tip due to the presence of a significant amount of nondissolved Cl2 gas. Because only dissolved Cl2 can be electrochemically reduced at the tip, a large amount of the Cl2 gas which is produced at active spots of the DSA is not detected. Additionally, a decrease of the cathodic current at the tip may occur owing to the adhesion of gas bubbles and blocking of the electrode surface. To overcome this limitation, the redox competition mode of SECM was extended and applied to the local visualization of Cl2 evolution from highly concentrated brine solutions. High concentrations of Cl2 produced at the sample can cause inhibition of the same reaction at the tip by accumulation of Cl2 in the proximity of the SECM tip. In this way the tip current is decreased, which can be used as a measure for the catalytic activity of the sample underneath the tip. © 2011 American Chemical Society.
    view abstract10.1021/ac200677g
  • Wiring photosynthetic enzymes to electrodes
    Badura, A. and Kothe, T. and Schuhmann, W. and Rögner, M.
    Energy and Environmental Science 4 (2011)
    The efficient electron transfer between redox enzymes and electrode surfaces can be obtained by wiring redox enzymes using, for instance, polymer-bound redox relays as has been demonstrated as a basis for the design of amperometric biosensors, logic gates or sensor arrays and more general as a central aspect of "bioelectrochemistry". Related devices allow exploiting the unique catalytic properties of enzymes, among which photosynthetic enzymes are especially attractive due to the possibility to trigger the redox reactions upon irradiation with light. Photocatalytic properties such as the light-driven water splitting by photosystem 2 make them unique candidates for the development of semiartificial devices which convert light energy into stable chemical products, like hydrogen. This review summarizes recent concepts for the integration of photosystem 1 and photosystem 2 into bioelectrochemical devices with special focus on strategies for the design of electron transfer pathways between redox enzymes and conductive supports. © 2011 The Royal Society of Chemistry.
    view abstract10.1039/c1ee01285a
  • 4D shearforce-based constant-distance mode scanning electrochemical microscopy
    Nebel, M. and Eckhard, K. and Erichsen, T. and Schulte, A. and Schuhmann, W.
    Analytical Chemistry 82 (2010)
    4D shearforce-based constant-distance mode scanning electrochemical microscopy (4D SF/CD-SECM) is designed to assess SECM tip currents at several but constant distances to the sample topography at each point of the x,y-scanning grid. The distance dependent signal is achieved by a shearforce interaction between the in-resonance vibrating SECM tip and the sample surface. A 4D SF/CD-SECM measuring cycle at each grid point involves a shearforce controlled SECM tip z-approach to a point of closest distance and subsequent stepwise tip retractions. At the point of closest approach and during the retraction steps, pairs of tip current (I) and position are acquired for various distances above the sample surface. Such a sequence provides x,y,I maps, that can be compiled and displayed for each selected data acquisition distance. Thus, multiple SECM images are obtained at known and constant distances above the sample topography. 4D SF/CD-SECM supports distance-controlled tip operation while continuous scanning of the SECM tip in the shear-force distance is avoided. In this way, constant-distance mode SECM imaging can be performed at user-defined, large tip-to-sample distances. The feasibility and the potential of the proposed 4D SF/CD-SECM imaging is demonstrated using on the one hand amperometric feedback mode imaging of a Pt band electrode array and on the other hand the visualization of the diffusion zone of a redox active species above a microelectrode in a generator/collector arrangement. © 2010 American Chemical Society.
    view abstract10.1021/ac1008805
  • A biotinylated intercalator for selective post-labeling of double-stranded DNA as a basis for high-sensitive DNA assays
    Gebala, M. and Stoica, L. and Guschin, D. and Stratmann, L. and Hartwich, G. and Schuhmann, W.
    Electrochemistry Communications 12 (2010)
    For increasing the sensitivity of label-free DNA assays an amplification strategy is proposed based on the synthesis of a proflavine derivative which on the one hand retains its high affinity for double-stranded DNA (dsDNA) intercalation and on the other hand is functionalized via a flexible spacer with biotin moieties. By this, subsequent to the post-labeling of areas with dsDNA, reporter systems such as streptavidin/enzyme conjugates can be bound. Amplified DNA hybridization detection using an oligonucleotide model system, a biotinylated proflavine as intercalator and streptavidin/alkaline phosphatase is demonstrated. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.elecom.2010.03.006
  • A microelectrochemical sensing system for the determination of Epstein-Barr virus antibodies
    Bandilla, M. and Zimdars, A. and Neugebauer, S. and Motz, M. and Schuhmann, W. and Hartwich, G.
    Analytical and Bioanalytical Chemistry 398 (2010)
    An electrochemical method for the detection of Epstein-Barr virus (EBV) infections is described. The method relies on an immunoassay with electrochemical read-outs based on recombinant antigens. The antigens are immobilised on an Au electrode surface and used to complementarily bind antibodies from serum samples found during different stages of infection with EBV. Thiol chemistry under formation of self-assembled monolayers functions as a means to immobilise the antigens at the Au electrodes. A reporter system consisting of a secondary antibody labelled with alkaline phosphatase is used for electrochemical detection. The feasibility of the assay design is demonstrated and the assay performance is tested against the current gold standard in EBV detection. Close correlation is obtained for the results found for the developed electrochemical immunoassay and a standard line assay. Moreover, the electrochemical immunoassay is combined with a nanoporous electrode system allowing signal amplification by means of redox recycling. An amplification factor of 24 could be achieved. © Springer-Verlag 2010.
    view abstract10.1007/s00216-010-3926-y
  • A single-electrode, dual-potential ferrocene-PNA biosensor for the detection of DNA
    Hüsken, N. and Gȩbala, M. and Schuhmann, W. and Metzler-Nolte, N.
    ChemBioChem 11 (2010)
    A Fc-PNA biosensor (Fc: ferrocenyl, C10H9Fe) was designed by using two electrochemically distinguishable recognition elements with different molecular information at a single electrode. Two Fc-PNA capture probes were therefore synthesized by N-terminal labeling different dodecamer PNA sequences with different ferrocene derivatives by click chemistry. Each of the two strands was thereby tethered with one specific ferrocene derivative. The two capture probes revealed quasi-reversible redox processes of the Fc 0/+ redox couple with a significant difference in their electrochemical half-wave potentials of ΔE1/2=160 mV. A carefully designed biosensor interface, consisting of a ternary self-assembled monolayer (SAM) of the two C-terminal cysteine-tethered Fc-PNA capture probes and 6-mercaptohexanol, was electrochemically investigated by square wave (SWV) and cyclic voltammetry (CV). The biosensor properties of this interface were analyzed by studying the interaction with DNA sequences that were complementary to either of the two capture probes by SWV. Based on distinct changes in both peak current and potential, a parallel identification of these two DNA sequences was successful with one interface design. Moreover, the primary electrochemical response could be converted by a simple mathematical analysis into a clear-cut electrochemical signal about the hybridization event. The discrimination of single-nucleotide polymorphism (SNP) was proven with a chosen single-mismatch DNA sequence. Furthermore, experiments with crude bacterial RNA confirm the principal suitability of this dual-potential sensor under real-life conditions. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/cbic.200900748
  • Carbon nanotube-supported sulfided Rh catalysts for the oxygen reduction reaction
    Jin, C. and Xia, W. and Guo, J. and Nagaiah, T.C. and Bron, M. and Schuhmann, W. and Muhler, M.
    Studies in Surface Science and Catalysis 175 (2010)
    Carbon nanotube (CNT) supported sulfided Rh catalysts were prepared applying three different routes: deposition-precipitation (DP), grafting of colloidal Rh nanoparticles, and polythiophene-assisted synthesis. The catalysts (1.4-1.8 wt%) prepared by DP were synthesized on CNTs from RhCl3 using hydrogen peroxide and subsequent exposure to on-line generated H 2S followed by heat treatment. The Rh particles were found to be highly dispersed on the CNT surface. Alternatively, RhSx/Rh nanoparticles with four different loadings (4.3-21.9 wt%) grafted on carbon nanotubes were prepared through a functionalization of CNTs with short chain thiols and subsequent binding of colloidal Rh nanoparticles onto the thiolated CNTs. All steps of the synthesis were monitored by XPS. Finally, polythiophene/CNT composites were prepared and employed in the preparation of Rh17S15/Rh nanoparticles supported on CNTs. The CNTs with the highest polythiophene loading yielded the highest amount of Rh 17S15 after Rh deposition and thermal treatment. The activity and stability of the prepared catalysts were studied towards the oxygen reduction reaction. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/S0167-2991(10)75020-5
  • Carbon nanotubes modified with electrodeposited metal porphyrins and phenanthrolines for electrocatalytic applications
    Schilling, T. and Okunola, A. and Masa, J. and Schuhmann, W. and Bron, M.
    Electrochimica Acta 55 (2010)
    Composites consisting of multi-walled carbon nanotubes (MWCNTs) and iron-nitrogen containing compounds as catalysts for the electroreduction of oxygen in acidic media were directly prepared on a glassy carbon (GC) electrode in a bottom-up synthesis. In a first step, MWCNTs were drop-coated in form of an ink onto the electrode. Afterwards the nanotubes were modified with catalytically active films of iron porphyrin (FeTMPP-Cl) or iron phenanthroline (Fe(phen)3) through a pulsed potential deposition technique. Finally the prepared electrodes were heat-treated in an inert gas atmosphere. By employing cyclic voltammetry and rotating disc electrode measurements it is shown that the activity for the oxygen reduction reaction (ORR) at such composites increases progressively with every applied synthesis step showing the possibility for direct synthesis of a catalyst on an electrode. The activities of FeTMPP-Cl/MWCNT and Fe(phen)3/MWCNT composites prepared by this technique are higher than that of similar electrocatalysts prepared by wet impregnation and heat treatment. The presented approach opens possibilities for systematic tuning of electrode structures, for example by stepwise build-up of gas diffusion electrodes. © 2009 Elsevier Ltd.
    view abstract10.1016/j.electacta.2009.11.092
  • Controlled orientation of DNA in a binary SAM as a key for the successful determination of DNA hybridization by means of electrochemical impedance spectroscopy
    Gebala, M. and Schuhmann, W.
    ChemPhysChem 11 (2010)
    Determination of DNA hybridization at electrode surfaces modified with thiol-tethered single-stranded DNA (ssDNA) capture probes and co-assembled with short-chain thiol derivatives using electrochemical impedance spectroscopy requires a careful design of the electrode/electrolyte interface as well as an in-depth understanding of the processes at the interface during DNA hybridization. The influence of the electrode potential, the ssDNA coverage, the ionic strength, the nature of the thiol derivative and especially the Debye length are shown to have a significant impact on the impedance spectra. A mixed monolayer comprising-in addition to the ssDNA capture probe-both mercaptohexanol (MCH) and mercaptopropionic acid (MPA) is suggested as an interface design which allows a high efficiency of the DNA hybridization concomitantly with a reliable modulation of the charge-transfer resistance of the electrode upon hybridization. © 2010 Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim.
    view abstract10.1002/cphc.201000210
  • Design of a bioelectrocatalytic electrode interface for oxygen reduction in biofuel cells based on a specifically adapted Os-complex containing redox polymer with entrapped Trametes hirsuta laccase
    Ackermann, Y. and Guschin, D.A. and Eckhard, K. and Shleev, S. and Schuhmann, W.
    Electrochemistry Communications 12 (2010)
    The design of the coordination shell of an Os-complex and its integration within an electrodeposition polymer enables fast electron transfer between an electrode and a polymer entrapped high-potential laccase from the basidiomycete Trametes hirsuta. The redox potential of the Os3+/2+-centre tethered to the polymer backbone (+ 720 mV vs. NHE) is perfectly matching the potential of the enzyme (+ 780 mV vs. NHE at pH 6.5). The laccase and the Os-complex modified anodic electrodeposition polymer were simultaneously precipitated on the surface of a glassy carbon electrode by means of a pH-shift to 2.5. The modified electrode was investigated with respect to biocatalytic O2 reduction to H2O. The proposed modified electrode has potential applications as biofuel cell cathode. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.elecom.2010.02.019
  • Discharge cavitation during microwave electrochemistry at micrometre-sized electrodes
    Rassaei, L. and Nebel, M. and Rees, N.V. and Compton, R.G. and Schuhmann, W. and Marken, F.
    Chemical Communications 46 (2010)
    Microwave induced activation of electrochemical processes at microelectrodes (ca. 0.8 m diameter) immersed in aqueous electrolyte media is shown to be driven by (i) continuous stable cavitation (giving rise to Faradaic current enhancements by up to three orders of magnitude) and (ii) transient discharge cavitation on the s timescale (giving rise to cathodic plasma current spikes and more violent surface erosion effects). © 2010 The Royal Society of Chemistry.
    view abstract10.1039/b920154h
  • Electrochemical synthesis of core-shell catalysts for electrocatalytic applications
    Kulp, C. and Chen, X. and Puschhof, A. and Schwamborn, S. and Somsen, C. and Schuhmann, W. and Bron, M.
    ChemPhysChem 11 (2010)
    A novel electrochemical method to prepare platinum shells around carbon-supported metal nanoparticles (Ru and Au) by pulsed electrodeposition from solutions containing Pt ions is presented. Shell formation is confirmed by characteristic changes in the cyclic voltammograms, and is further evidenced by monitoring particle growth by transmission electron microscopy as well as by energy-dispersive analysis of X rays (EDX). Scanning electrochemical microscopy and EDX measurements indicate a selective Pt deposition on the metal/carbon catalyst, but not on the glassy carbon substrate. The thus prepared carbon-supported core-shell nanoparticles are investigated with regard to their activity in electrocatalytic oxygen reduction, which demonstrates the applicability of these materials in electrocatalysis or sensors. © 2010 Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim.
    view abstract10.1002/cphc.200900881
  • Electrochemically induced formation of surface-attached temperature-responsive hydrogels. amperometric glucose sensors with tunable sensor characteristics
    Bünsow, J. and Enzenberg, A. and Pohl, K. and Schuhmann, W. and Johannsmann, D.
    Electroanalysis 22 (2010)
    Employing thermally responsive hydrogels, the design of an amperometric glucose sensor is proposed. The properties of the biosensor can be modulated upon changing the temperature. Homo-and copolymers of N-isopropylacrylamide (NIPAm) and oligo(ethylene glycol) methacrylate (OEGMA) were prepared by electrochemically induced polymer-ization thus yielding surface-attached hydrogels. The growth of the films as well as the change in the film thickness in dependence from the temperature were investigated by means of an electrochemical quartz crystal microbalance (EQCM). The layer thickness in the dry state ranged from 20 to 120 nm. The lower critical solution temperature (LCST) of the hydrogel increases with increasing content of the more hydrophilic OEGMA. Hence, the swelling in aqueous electrolyte is composition dependent and can be adjusted by selecting a specific NIPAm to OEGMA ratio. All homo-and copolymer films showed good biocompatibility and no fouling could be observed during exposing the surfaces to human serum albumin. For amperometric glucose detection, glucose oxidase was entrapped in the films during electrochemically-induced polymerization. Both the apparent Michaelis constant (Kapp M) and the apparent maximum current (iapp max) as determined by amperometry could be adjusted both by the film composition as well as the operation temperature. © 2010 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim.
    view abstract10.1002/elan.200900478
  • Ethylenediamine-anchored gold nanoparticles on multi-walled carbon nanotubes: Synthesis and characterization
    Li, N. and Xu, Q. and Zhou, M. and Xia, W. and Chen, X. and Bron, M. and Schuhmann, W. and Muhler, M.
    Electrochemistry Communications 12 (2010)
    Binding of gold nanoparticles (Au-NP) at amine-functionalised multi-walled carbon nanotubes (MWNTs) is proposed. The MWNTs are functionalised with acylchloride groups, which further react with ethylenediamine to form amine-functionalised MWCNTs. These amines are able to bind preformed colloidal Au-NPs. The Au/MWNT composite material facilitates electron-transfer reactions with free-diffusing redox compounds. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.elecom.2010.04.026
  • Local reactivity of diamond-like carbon modified PTFE membranes used in SO2 sensors
    Nebel, M. and Neugebauer, S. and Kiesele, H. and Schuhmann, W.
    Electrochimica Acta 55 (2010)
    The local electrochemical activity of polytetrafluorethylene (PTFE) membranes coated with diamond-like carbon (DLC) was investigated using scanning electrochemical microscopy (SECM). During z-approach curves in the feedback mode of SECM unexpected local variations in the electron-transfer rate of [Fe(CN)6]3-/4- and [Ru(NH3)6] 3+/2+ were observed. This local heterogeneity of the electrochemical activity was further evaluated in a system adapted from SO2 gas sensors. In this case, the Cu2+/+ couple is used as dissolved reversible redox system. Reaction of SO2 with Cu2+ yields Cu+ which is re-oxidized at the DLC-coated PTFE membrane. Gas permeation/tip-collection mode SECM experiments allowed visualizing the local pore distribution as sites where the SO2 is permeating through the membrane and hence formation of Cu+ takes place. © 2010 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.electacta.2010.02.031
  • Metal-free and electrocatalytically active nitrogen-doped carbon nanotubes synthesized by coating with polyaniline
    Jin, C. and Nagaiah, T.C. and Xia, W. and Spliethoff, B. and Wang, S. and Bron, M. and Schuhmann, W. and Muhler, M.
    Nanoscale 2 (2010)
    Nitrogen doping of multi-walled carbon nanotubes (CNTs) was achieved by the carbonization of a polyaniline (PANI) coating. First, the CNTs were partially oxidized with KMnO4 to obtain oxygen-containing functional groups. Depending on the KMnO4 loading, thin layers of birnessite-type MnO2 (10 wt% and 30 wt%) were obtained by subsequent thermal decomposition. CNT-supported MnO2 was then used for the oxidative polymerization of aniline in acidic solution, and the resulting PANI-coated CNTs were finally heated at 550 °C and 850 °C in inert gas. The samples were characterized by transmission electron microscopy and X-ray photoelectron spectroscopy. A thin layer of carbonized PANI was observed on the CNT surface, and the surface nitrogen concentration of samples prepared from 30% MnO 2 was found to amount to 7.6 at% and 3.8 at% after carbonization at 550 °C and 850 °C, respectively. These CNTs with nitrogen-containing shell were further studied by electrochemical impedance spectroscopy and used as catalysts for the oxygen reduction reaction. The sample synthesized from 30 wt% MnO2 followed by carbonization at 850 °C showed the best electrochemical performance indicating efficient nitrogen doping. © 2010 The Royal Society of Chemistry.
    view abstract10.1039/b9nr00405j
  • Nitrogen-doped carbon nanotubes as a cathode catalyst for the oxygen reduction reaction in alkaline medium
    Nagaiah, T.C. and Kundu, S. and Bron, M. and Muhler, M. and Schuhmann, W.
    Electrochemistry Communications 12 (2010)
    A new approach to synthesize nitrogen-doped carbon nanotubes (NCNTs) as catalysts for oxygen reduction by treating oxidized CNTs with ammonia is presented. The surface properties and oxygen reduction activities were characterized by cyclic voltammetry, rotating disk electrode and X-ray photoelectron spectroscopy. NCNTs treated at 800 °C show improved electrocatalytic activity for oxygen reduction as compared with commercially available Pt/C catalysts. © 2009 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.elecom.2009.12.021
  • Patterned CNT arrays for the evaluation of oxygen reduction activity by SECM
    Schwamborn, S. and Stoica, L. and Chen, X. and Xia, W. and Kundu, S. and Muhler, M. and Schuhmann, W.
    ChemPhysChem 11 (2010)
    view abstract10.1002/cphc.200900744
  • Pt-Ag catalysts as cathode material for oxygen-depolarized electrodes in hydrochloric acid electrolysis
    Maljusch, A. and Nagaiah, T.C. and Schwamborn, S. and Bron, M. and Schuhmann, W.
    Analytical Chemistry 82 (2010)
    Pt-Ag nanoparticles were prepared on a glassy carbon (GC) surface by pulsed electrodeposition and tested using cyclic voltammetry and scanning electrochemical microscopy (SECM) with respect to their possible use as catalyst material for oxygen reduction in 400 mM HCl solution. For comparison, a Pt catalyst was investigated under similar conditions. The redox competition mode of scanning electrochemical microscopy (RC-SECM) was adapted to the specific conditions caused by the presence of Cl ions and used to visualize the local catalytic activity of the Pt-Ag deposits. Similarly prepared Pt deposits were shown to dissolve underneath the SECM tip. Pt-Ag composites showed improved long-term stability toward oxygen reduction as compared with Pt even under multiple switching off to open-circuit potential in 400 mM HCl. © 2010 American Chemical Society.
    view abstract10.1021/ac902620g
  • Redox electrodeposition polymers: Adaptation of the redox potential of polymer-bound Os complexes for bioanalytical applications
    Guschin, D.A. and Castillo, J. and Dimcheva, N. and Schuhmann, W.
    Analytical and Bioanalytical Chemistry 398 (2010)
    The design of polymers carrying suitable ligands for coordinating Os complexes in ligand exchange reactions against labile chloro ligands is a strategy for the synthesis of redox polymers with bound Os centers which exhibit a wide variation in their redox potential. This strategy is applied to polymers with an additional variation of the properties of the polymer backbone with respect to pH-dependent solubility, monomer composition, hydrophilicity etc. A library of Os-complex-modified electrodeposition polymers was synthesized and initially tested with respect to their electron-transfer ability in combination with enzymes such as glucose oxidase, cellobiose dehydrogenase, and PQQ-dependent glucose dehydrogenase entrapped during the pH-induced deposition process. The different polymer-bound Os complexes in a library containing 50 different redox polymers allowed the statistical evaluation of the impact of an individual ligand to the overall redox potential of an Os complex. Using a simple linear regression algorithm prediction of the redox potential of Os complexes becomes feasible. Thus, a redox polymer can now be designed to optimally interact in electron-transfer reactions with a selected enzyme. © 2010 Springer-Verlag.
    view abstract10.1007/s00216-010-3982-3
  • Rh-RhSx nanoparticles grafted on functionalized carbon nanotubes as catalyst for the oxygen reduction reaction
    Jin, C. and Xia, W. and Nagaiah, T.C. and Guo, J. and Chen, X. and Li, N. and Bron, M. and Schuhmann, W. and Muhler, M.
    Journal of Materials Chemistry 20 (2010)
    Rhodium-rhodium sulfide nanoparticles supported on multi-walled carbon nanotubes (CNTs) were synthesized via a multi-step colloid route. The CNTs were first exposed to nitric acid to generate oxygen-containing functional groups, and then treated with thionyl chloride to generate acyl chloride groups. The grafting of thiol groups was subsequently carried out by reaction with 4-aminothiophenol. Colloidal rhodium nanoparticles were synthesized using rhodium chloride as metal source, sodium citrate as stabilizer, and sodium borohydride as reducing agent. The immobilization of the generated colloidal rhodium nanoparticles was achieved by adding the thiolated CNTs to the colloidal suspension. All these steps were monitored by X-ray photoelectron spectroscopy, which disclosed the presence of rhodium sulfide, whereas metallic rhodium was detected by X-ray diffraction, suggesting that the nanoparticles probably consist of a metallic Rh core covered by a sulfide layer. Scanning and transmission electron microscopy studies showed that the diameter of the catalyst particles was about 7 nm even at high Rh loadings. Rotating disc electrode measurements and cyclic voltammetry were employed to test the electrocatalytic activity in the oxygen reduction reaction in hydrochloric acid. Among all the synthesized catalysts with different rhodium loadings (4.3-21.9%), the 16.1% rhodium catalyst was found to be the most active catalyst. In comparison to the commercial E-TEK Pt/C catalyst, the 16.1% catalyst displayed a higher electrochemical stability in the highly corrosive electrolyte, as determined by stability tests with frequent current interruptions. © 2010 The Royal Society of Chemistry.
    view abstract10.1039/b916192a
  • Scanning electrochemical microscopy in neuroscience
    Schulte, A. and Nebel, M. and Schuhmann, W.
    Annual Review of Analytical Chemistry 3 (2010)
    This article reviews recent work involving the application of scanning electrochemical microscopy (SECM) to the study of individual cultured living cells, with an emphasis on topographical and functional imaging of neuronal and secretory cells of the nervous and endocrine system. The basic principles of biological SECM and associated negative amperometric-feedback and generator/collector-mode SECM imaging are discussed, and successful use of the methodology for screening soft and fragile membranous objects is outlined. The drawbacks of the constant-height mode of probe movement and the benefits of the constant-distance mode of SECM operation are described. Finally, representative examples of constant-height and constant-distance mode SECM on a variety of live cells are highlighted to demonstrate the current status of single-cell SECM in general and of SECM in neuroscience in particular. Copyright © 2010 by Annual Reviews. All rights reserved.
    view abstract10.1146/annurev.anchem.111808.073651
  • Shearforce-based constant-distance scanning electrochemical microscopy as fabrication tool for needle-type carbon-fiber nanoelectrodes
    Hussien, E.M. and Schuhmann, W. and Schulte, A.
    Analytical Chemistry 82 (2010)
    Carbon fiber nanoelectrodes with nanometer radii tip curvatures were fabricated using a shearforce-based constant-distance scanning electrochemical microscope and electrochemically induced polymer deposition. A simple DC etching procedure in alkaline solution provided conically sharpened single carbon fibers with well-formed nanocones at their bottom. Coating the stems but not the end of the tips of the tapered structures with anodic electrodeposition paint was the strategy for limiting the bare carbon to the foremost end and restricting a feasible voltammetry current response to exactly this section. The electrodeposition of the polymer was prevented at the foremost end of the tip using a shearforce-based tip-to-sample distance control that allowed approaching the etched tips carefully in just touching distance to a film of a silicone elastomer. Analysis of the steady-state cyclic voltammograms in presence of a reversible redox compound revealed effective radii for the obtained needle-type carbon-fiber nanoelectrodes down to as small as 46 nm. The method offers an alternative pathway toward the fabrication of highly miniaturized carbon electrodes. © 2010 American Chemical Society.
    view abstract10.1021/ac100738b
  • Towards a high potential biocathode based on direct bioelectrochemistry between horseradish peroxidase and hierarchically structured carbon nanotubes
    Jia, W. and Schwamborn, S. and Jin, C. and Xia, W. and Muhler, M. and Schuhmann, W. and Stoica, L.
    Physical Chemistry Chemical Physics 12 (2010)
    Adsorption of horseradish peroxidase (HRP) on graphite rod electrodes sequentially modified with carbon microfibers (CMF) carrying carbon nanotubes in a hierarchically structured arrangement and finally pyrene hexanoic acid (PHA) for improving hydrophilicity of the electrode surface is the basis for the direct bioelectrocatalytic reduction of H 2O 2 at potentials as high as about +600 mV. The high-potential direct bioelectrocatalytic reduction of H 2O 2 is implying a direct bioelectrochemical communication between the Fe IVO,P + redox state known as compound I. The HRP loading was optimized leading to a current of 800 μA at a potential of 300 mV. © 2010 the Owner Societies.
    view abstract10.1039/c0cp00349b
  • catalysis

  • corrosion

  • electrocatalysis

  • electrochemistry

  • nanoparticles

  • scanning electrochemical microscopy

  • scanning electron microscopy

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