Scientific Output

Over 10.000 scientific papers have been published by members of the Materials Chain since the foundation of the University Alliance Ruhr in 2010. This tremendous output is proof of the excellent environment the Ruhr Area provides for research in the field of materials science and technology.

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  • 2020 • 209 Vibration-Driven Self-Doping of Dangling-Bond Wires on Si(553)-Au Surfaces
    Braun, C. and Neufeld, S. and Gerstmann, U. and Sanna, S. and Plaickner, J. and Speiser, E. and Esser, N. and Schmidt, W.G.
    Physical Review Letters 124 (2020)
    Density-functional theory is used to explore the Si(553)-Au surface dynamics. Our study (i) reveals a complex two-stage order-disorder phase transition where with rising temperature first the ×3 order along the Si step edges and, subsequently, the ×2 order of the Au chains is lost, (ii) identifies the transient modification of the electron chemical potential during soft Au chain vibrations as instrumental for disorder at the step edge, and (iii) shows that the transition leads to a self-doping of the Si dangling-bond wire at the step edge. The calculations are corroborated by Raman measurements of surface phonon modes and explain previous electron diffraction, scanning tunneling microscopy, and surface transport data. © 2020 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the "" Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.
    view abstractdoi: 10.1103/PhysRevLett.124.146802
  • 2020 • 208 Diffusion of Gold Nanoparticles in Inverse Opals Probed by Heterodyne Dynamic Light Scattering
    Giraudet, C. and Knoll, M.S.G. and Galvan, Y. and Süß, S. and Segets, D. and Vogel, N. and Rausch, M.H. and Fröba, A.P.
    Transport in Porous Media 131 723-737 (2020)
    The diffusive behavior of nanoparticles inside porous materials is attracting a lot of interest in the context of understanding, modeling, and optimization of many technical processes. A very powerful technique for characterizing the diffusive behavior of particles in free media is dynamic light scattering (DLS). The applicability of the method in porous media is considered, however, to be rather difficult due to the presence of multiple sources of scattering. In contrast to most of the previous approaches, the DLS method was applied without ensuring matching refractive indices of solvent and porous matrix in the present study. To test the capabilities of the method, the diffusion of spherical gold nanoparticles within the interconnected, periodic nanopores of inverse opals was analyzed. Despite the complexity of this system, which involves many interfaces and different refractive indices, a clear signal related to the motion of particles inside the porous media was obtained. As expected, the diffusive process inside the porous sample slowed down compared to the particle diffusion in free media. The obtained effective diffusion coefficients were found to be wave vector-dependent. They increased linearly with increasing spatial extension of the probed particle concentration fluctuations. On average, the slowing-down factor measured in this work agrees within combined uncertainties with literature data. © 2019, Springer Nature B.V.
    view abstractdoi: 10.1007/s11242-019-01364-1
  • 2020 • 207 Unravelling the nucleation, growth, and faceting of magnetite-gold nanohybrids
    Nalench, Y.A. and Shchetinin, I.V. and Skorikov, A.S. and Mogilnikov, P.S. and Farle, M. and Savchenko, A.G. and Majouga, A.G. and Abakumov, M.A. and Wiedwald, U.
    Journal of Materials Chemistry B 8 3886-3895 (2020)
    The chemical synthesis of nanoparticles with a preassigned size and shape is important for an optimized performance in any application. Therefore, systematic monitoring of the synthesis is required for the control and detailed understanding of the nucleation and growth of the nanoparticles. Here, we study Fe3O4-Au hybrid nanoparticles in detail using probes of the reaction mixture during synthesis and their thorough characterization. The proposed approach eliminates the problem of repeatability and reproducibility of the chemical synthesis and was carried out using laboratory equipment (standard transmission electron microscopy, X-ray diffraction, and magnetometry) for typically 10 μL samples instead of, for example, a dedicated synthesis and inspection at a synchrotron radiation facility. From the three independent experimental techniques we extract the nanoparticle size at 12 stages of the synthesis. These diameters show identical trends and good quantitative agreement. Two consecutive processes occur during the synthesis of Fe3O4-Au nanoparticles, the nucleation and the growth of spherical Fe3O4nanoparticles on the surface of Au seeds during the heating stage and their faceting towards octahedral shape during reflux. The final nanoparticles with sizes of 15 nm Fe3O4and 4 nm Au exhibit superparamagnetic behavior at ambient temperature. These are high-quality, close to stoichiometric Fe3O4nanocrystals with nearly volumetric magnetic behavior as confirmed by the presence of the Verwey transition. Understanding the processes occurring during the synthesis allows the nanoparticle size and shape to be adjusted, improving their capabilities in biomedical applications. © The Royal Society of Chemistry 2020.
    view abstractdoi: 10.1039/c9tb02721a
  • 2020 • 206 Superconducting NbN and CaFe0.88Co0.12AsF studied by point-contact spectroscopy with a nanoparticle Au array
    Wu, Y.F. and Yu, A.B. and Lei, L.B. and Zhang, C. and Wang, T. and Ma, Y.H. and Huang, Z. and Chen, L.X. and Liu, Y.S. and Schneider, C.M. and Mu, G. and Xiao, H. and Hu, T.
    Physical Review B 101 (2020)
    The point-contact-spectroscopy measurement is a powerful method to detect the superconducting gap and the spin polarization of materials. However, it is difficult to get a stable and clean point contact by conventional techniques. In this work, we fabricate multiple point contacts by depositing Au nanoparticle arrays on the surface of a superconductor through an anodic aluminum oxide patterned shadow mask. We obtained the superconducting gaps of niobium nitride thin film (NbN, Tc=16 K) and iron superconductors CaFe0.88Co0.12AsF single crystals (Ca-1111, Tc=21.3 K) by fitting the point-contact spectroscopy with the Blonder-Tinkham-Klapwijk theory. We found that NbN's gap (Δ) exhibits the BCS-like temperature dependence with Δ≈2.88 meV at 0 K and 2Δ/kBTc≈4.22 in agreement with previous reports. By contrast, Ca-1111 has a multigap structure with Δ1≈1.99 meV and Δ2≈5.01 meV at 0 K, and the ratio between the superconducting gap and Tc is 2Δ1/kBTc=2.2 and 2Δ2/kBTc=5.5, suggesting an unconventional paring mechanism of Ca-1111 also in agreement with previous reports on other Fe-based superconductors. Our multiple point-contacts method thus provides an alternative way to perform measurements of the superconducting gap. © 2020 American Physical Society. ©2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.101.174502
  • 2019 • 205 Comparison of the productivity and ablation efficiency of different laser classes for laser ablation of gold in water and air
    Dittrich, S. and Streubel, R. and McDonnell, C. and Huber, H.P. and Barcikowski, S. and Gökce, B.
    Applied Physics A: Materials Science and Processing 125 (2019)
    In this study, we compare different laser systems used for the synthesis of nanoparticles. The productivity and ablation efficiency of laser ablation of gold in water and in air are determined for three pulsed laser systems with comparable pulse energy but different pulse duration and repetition rate. All experiments are performed in a fluence range of up to 20J/cm2. The highest productivity among the considered lasers is found for a high-power picosecond laser, which shows 12 times higher ablation rate for the ablation in air compared to ablation in liquid. Further, we find that the threshold fluence for ablation in air is up to 1.9 times higher than for ablation in water. The highest ablation efficiency, which is defined as an energy specific ablation volume by the ablation rate divided by the laser power, is found for the low power, compact nanosecond laser system. © 2019, Springer-Verlag GmbH Germany, part of Springer Nature.
    view abstractdoi: 10.1007/s00339-019-2704-8
  • 2019 • 204 Initiation and stagnation of room temperature grain coarsening in cyclically strained gold films
    Glushko, O. and Dehm, G.
    Acta Materialia 169 99-108 (2019)
    Despite the large number of experiments demonstrating that grains in a metallic material can grow at room temperature due to applied mechanical load, the mechanisms and the driving forces responsible for mechanically induced grain coarsening are still not understood. Here we present a systematic study of room temperature grain coarsening induced by cyclic strain in thin polymer-supported gold films. By means of detailed electron backscatter diffraction analysis we were able to capture both the growth of individual grains and the evolution of the whole microstructure on the basis of statistical data over thousands of grains. The experimental data are reported for three film thicknesses with slightly different microstructures and three different amplitudes of cyclic mechanical loading. Although different kinds of grain size evolution with increasing cycle number are observed depending on film thickness and strain amplitude, a single model based on a thermodynamic driving force is shown to be capable to explain initiation and stagnation of grain coarsening in all cases. The main implication of the model is that the grains having lower individual yield stress are coarsening preferentially. Besides, it is demonstrated that the existence of local shear stresses imposed on a grain boundary is not a necessary requirement for room-temperature grain coarsening. © 2019 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2019.03.004
  • 2019 • 203 Pulsed electron gun for electron diffraction at surfaces with femtosecond temporal resolution and high coherence length
    Hafke, B. and Witte, T. and Brand, C. and Duden, T. and Horn-Von Hoegen, M.
    Review of Scientific Instruments 90 (2019)
    We present a newly designed 30 kV pulsed electron gun for ultrafast electron diffraction suited for pump-probe setups driven by femtosecond laser pulses. The electron gun can be operated both in transmission and reflection geometry. A robust design with a back illuminated Au photocathode, extraction fields of 7.5 kV/mm, and a magnetic focus lens ensures reliable daily use. Magnetic deflectors allow for beam alignment and characterization. Focusing of the UV pulse on the photocathode results in a small source size of photoemitted electrons and thus a high transverse coherence length of more than 50 nm in diffraction. A low difference of ΔE = 0.1 eV between the work function of the 10 nm Au photocathode and photon energy of the frequency tripled UV laser pulses results in an instrumental temporal resolution of 330 fs full width at half maximum. This resolution is discussed with respect to the number of electrons per pulse. © 2019 Author(s).
    view abstractdoi: 10.1063/1.5086124
  • 2019 • 202 Synthesis of Mixed AuZn Nanoparticles by Spark Discharge Technique
    Kala, S. and Kruis, F.E.
    MRS Advances 4 1621-1629 (2019)
    In this study, feasibility of spark discharge technique to generate mixed metal nanoparticles is demonstrated. Two immiscible metals Au and Zn are selected to prepare AuZn mixed nanoparticles. Ignition of spark between Au and Zn electrodes under normal pressure, in the presence of carrier gas, leads to formation of mixed nanoparticles by condensation and nucleation. Online particle size-distribution is monitored by a scanning mobility particle sizer on changing carrier gas flow rate and capacitor charging current during co-sparking between Au and Zn electrodes. The technique provides flexibility to generate binary mixture of AuZn nanoparticles in the size range of 10-80 nm. Distribution of Au and Zn in the prepared mixed nanoparticles is mapped by scanning electron microscopy and high resolution electron microscopy. © Materials Research Society 2019.
    view abstractdoi: 10.1557/adv.2019.221
  • 2019 • 201 Templated Dealloying: Designing Ultrastructures by Memory Effect
    Kamp, M. and Tymoczko, A. and Schürmann, U. and Jakobi, J. and Rehbock, C. and Barcikowski, S. and Kienle, L.
    Crystal Growth and Design 19 4957-4963 (2019)
    Tailoring the morphology of nanoporous structures widens the scope of applications in catalysis and sensing. The synthesis of versatile nanoporous morphologies with the spatial distribution of porosity is permitted by the dealloying of unique, metastable Au-Fe alloy template nanoparticles generated by laser ablation in liquids. This approach opens the door to a novel process, which involves a special transformation mechanism, including oxidation and Kirkendall effect, which is decisive for the stabilization of hollow structures with the spatial distribution of porosity and represents a memory effect of morphology. Within this work, nanoporous Au particles, hollow nanoporous Au shells with the spatial distribution of porosity, and yolk-shell-like Au nanoparticles encapsulated in ultrathin Au shells are synthesized. A distinct variation of crystallinity and an increased lattice strain is observed, which implies an improved catalytic activity for oxidation reactions. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.cgd.9b00175
  • 2019 • 200 Fast and easy fabrication methodology of Fresnel zone plates for the extreme ultraviolet and soft x-ray regions
    Schümmer, A. and Mertins, H.-C.H. and Schneider, C.M. and Adam, R. and Trellenkamp, S. and Borowski, R. and Juschkin, L. and Berges, U.
    Applied Optics 58 1057-1063 (2019)
    Zone plate design and efficient methods for the fabrication of zone plates for extreme ultraviolet (EUV) and soft x-ray applications in a newly developed scanning reflection microscope are presented. Based on e-beam lithography, three types of transmission zone plates with focal lengths between 6 and 15 mm are reported: (i) phase-shifting zone plates made by 190 nm thick PMMA rings on Si 3 N 4 membranes, (ii) absorbing zone plates made by 75 nm thick Au ring structures on Si 3 N 4 , and (iii) freestanding Au rings of 50 nm thickness and increased transmission in the EUV range. Experiments at the DELTA synchrotron facility reveal a minimum spot size and resulting spatial resolution of 9 3 μm, which is the theoretical limit resulting from the synchrotron beam parameters at 60 eV photon energy. Images of a Ti/Si chessboard test pattern are recorded exploiting the energy dependence of the element-specific reflectance. © 2019 Optical Society of America.
    view abstractdoi: 10.1364/AO.58.001057
  • 2018 • 199 Probing magnetic coupling between LnPc2 (Ln = Tb, Er) molecules and the graphene/Ni (111) substrate with and without Au-intercalation: Role of the dipolar field
    Corradini, V. and Candini, A. and Klar, D. and Biagi, R. and De Renzi, V. and Lodi Rizzini, A. and Cavani, N. and Del Pennino, U. and Klyatskaya, S. and Ruben, M. and Velez-Fort, E. and Kummer, K. and Brookes, N.B. and Gargiani, P...
    Nanoscale 10 277-283 (2018)
    Lanthanides (Ln) bis-phthalocyanine (Pc), the so-called LnPc2double decker, are a promising class of molecules with a well-defined magnetic anisotropy. In this work, we investigate the magnetic properties of LnPc2 molecules UHV-deposited on a graphene/Ni(111) substrate and how they modify when an Au layer is intercalated between Ni and graphene. X-ray absorption spectroscopy (XAS), and linear and magnetic circular dichroism (XLD and XMCD) were used to characterize the systems and probe the magnetic coupling between LnPc2 molecules and the Ni substrate through graphene, both gold-intercalated and not. Two types of LnPc2 molecules (Ln = Tb, Er) with a different magnetic anisotropy (easy-axis for Tb, easy-plane for Er) were considered. XMCD shows an antiferromagnetic coupling between Ln and Ni(111) even in the presence of the graphene interlayer. Au intercalation causes the vanishing of the interaction between Tb and Ni(111). In contrast, in the case of ErPc2, we found that the gold intercalation does not perturb the magnetic coupling. These results, combined with the magnetic anisotropy of the systems, suggest the possible importance of the magnetic dipolar field contribution for determining the magnetic behaviour. © 2017 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c7nr06610d
  • 2018 • 198 Coherently strained [Fe-Co(C)/Au-Cu]n multilayers: A path to induce magnetic anisotropy in Fe-Co films over large thicknesses
    Giannopoulos, G. and Salikhov, R. and Varvaro, G. and Psycharis, V. and Testa, A.M. and Farle, M. and Niarchos, D.
    Journal of Physics D: Applied Physics 51 (2018)
    Among novel critical-element-free materials for permanent magnets, the nearly equiatomic Fe-Co alloy has recently attracted a great deal of attention as a large magneto-crystalline anisotropy can be induced by straining the Fe-Co unit cell. In thin film systems, the use of a suitable underlayer allows a tetragonal reconstruction of the Fe-Co to be triggered up to a critical thickness of few nanometers, above which the crystal structure relaxes to the magnetically soft cubic phase. Scaling-up the thickness of the metastable tetragonal Fe-Co phase is of crucial significance for different nanoscale applications, such as magnetic micro- and nano-electromechanical systems. To suppress the strain relaxation occurring at high thicknesses, we explored a novel approach based on Fe-Co(C)/Au-Cu multilayer films, where both Au-Cu interlayers and carbon (C) doping were used to stabilize the strained Fe-Co tetragonal phase over large thicknesses. Both doped and un-doped multilayer structures show a coherently strained regime, persisting up to a thickness of 60 nm, which leads, possibly in combination with the surface anisotropy induced at the Au-Cu interfaces, to the appearance of a large out-of-plane anisotropy (up to 0.4 MJ m-3), thus suggesting the potential of such an approach to develop critical-element-free thin film permanent magnets for a variety of nanoscale applications. © 2018 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/aaa41c
  • 2018 • 197 An in situ XPS study of L-cysteine co-adsorbed with water on polycrystalline copper and gold
    Jürgensen, A. and Raschke, H. and Esser, N. and Hergenröder, R.
    Applied Surface Science 435 870-879 (2018)
    The interactions of biomolecules with metal surfaces are important because an adsorbed layer of such molecules introduces complex reactive functionality to the substrate. However, studying these interactions is challenging: they usually take place in an aqueous environment, and the structure of the first few monolayers on the surface is of particular interest, as these layers determine most interfacial properties. Ideally, this requires surface sensitive analysis methods that are operated under ambient conditions, for example ambient pressure x-ray photoelectron spectroscopy (AP-XPS). This paper focuses on an AP-XPS study of the interaction of water vapour and l-Cysteine on polycrystalline copper and gold surfaces. Thin films of l-Cysteine were characterized with XPS in UHV and in a water vapour atmosphere (P ≤ 1 mbar): the structure of the adsorbed l-Cysteine layer depended on substrate material and deposition method, and exposure of the surface to water vapour led to the formation of hydrogen bonds between H2O molecules and the COO− and NH2 groups of adsorbed l-Cysteine zwitterions and neutral molecules, respectively. This study also proved that it is possible to investigate monolayers of biomolecules in a gas atmosphere with AP-XPS using a conventional laboratory Al-Kα x-ray source. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2017.11.150
  • 2018 • 196 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 21718-21723 (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 abstractdoi: 10.1021/acs.jpcc.8b05407
  • 2018 • 195 Determination of pore size gradients of virus filtration membranes using gold nanoparticles and their relation to fouling with protein containing feed streams
    Kosiol, P. and Müller, M.T. and Schneider, B. and Hansmann, B. and Thom, V. and Ulbricht, M.
    Journal of Membrane Science 548 598-608 (2018)
    Virus filtration membranes contribute to the virus safety of biopharmaceutical drugs due to their capability to retain virus particles mainly based on size-exclusion mechanisms. Typical product molecules like monoclonal antibodies with 9–12 nm in hydrodynamic diameter have to be transmitted by >95% while small viruses, e.g. parvoviridae (B19, MVM, PPV) with a diameter of 18–26 nm, have to be retained by at least 99.99%. Therefore, membrane fouling caused by product aggregates, which are similar in size compared to the viruses that have to be retained, is a common observation. Minimal membrane fouling is a requirement for economical processes and is influenced by both the membrane surface chemistry and the membrane structure, particularly with regard to the pore size gradient (PSG). In this work, virus filtration membranes were challenged with gold nanoparticles (GNPs) in order to determine PSGs for a wide range of different commercial and non-commercial parvovirus retentive membranes differing in structure, material and surface chemistry. GNP adsorption to the membrane material was suppressed by the use of an anionic surfactant, allowing to gain insights into size-exclusion properties of the membranes. Membrane performance with regard to fouling was further investigated by determination of protein mass throughputs up-to a defined membrane flux decay using solutions containing intravenous immunoglobulin (IVIG) as model protein. Additionally, the fouling mechanism of IVIG was investigated and confirmed to be caused by trace amounts of species larger than IVIG monomers and dimers, which were already present in the feed. The fouling results are discussed in relationship to the determined PSGs, since the porous support structure of virus filtration membranes can act as a depth pre-filter protecting the separation-active layer from particulate foulants. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.memsci.2017.11.048
  • 2018 • 194 Rapid and sensitive SERS detection of the cytokine tumor necrosis factor alpha (tnf-α) in a magnetic bead pull-down assay with purified and highly Raman-active gold nanoparticle clusters
    Lai, Y. and Schlücker, S. and Wang, Y.
    Analytical and Bioanalytical Chemistry 410 5993-6000 (2018)
    Tumor necrosis factor alpha (TNF-α) is a cytokine with significance in early diagnosis of cardiovascular diseases, obesity and insulin resistance. We demonstrate the proof of concept for a rapid and sensitive detection of TNF-α using a magnetic bead pull-down assay in combination with surface-enhanced Raman scattering (SERS). The use of purified and highly SERS-active small clusters of gold nanoparticles (AuNP) provides the high sensitivity of the assay with a limit of detection of ca. 1 pg/mL. Continuous density gradient centrifugation was employed for separating the very bright silica-encapsulated AuNP dimers and trimers from the significantly weaker AuNP monomers. Negative control experiments with other cytokines (IL-6, IL-8) and bovine serum albumin (BSA) confirm the high specificity of the assay, but indicate also space for future improvements by further reducing non-specific binding between proteins and the SERS nanotags. The multiplexing potential of this SERS-based detection scheme is exemplarily demonstrated by using a set of three spectrally distinct and highly SERS-active AuNP clusters with unique spectral barcodes. [Figure not available: see fulltext.]. © 2018, Springer-Verlag GmbH Germany, part of Springer Nature.
    view abstractdoi: 10.1007/s00216-018-1218-0
  • 2018 • 193 Retention mechanisms of 1.7 nm ZnS quantum dots and sub-20 nm Au nanoparticles in ultrafiltration membranes
    Lee, H. and Segets, D. and Süß, S. and Peukert, W. and Chen, S.-C. and Pui, D.Y.H.
    Journal of Membrane Science 567 58-67 (2018)
    Membrane processes are considered to be a very effective and promising method for drinking water and wastewater treatments. However, particle removal mechanisms have not been fully elucidated due to complex surface interactions between colloids and membranes, especially for very small colloidal particles. In this study, a series of systematic filtration tests for eight different types of membrane filters, having nominal pore sizes from 0.005 to 0.1 µm, against 1.7 nm ZnS quantum dots (QDs) and 5, 10 and 20 nm Au nanoparticles (NPs) was performed to understand their retention mechanisms, including rejection in front of the filter surface and adsorption inside the filter. By comparing rejection, adsorption and recovery, it was found that the predominant retention mechanisms for retaining small NPs varied from filter to filter. For instance, electrostatic repulsion played a significant role for the rejection of NPs, i.e. impeding them entering the membrane pores in most membranes. In comparison, the Nylon membrane had a significant adsorption retention ability for Au NPs due to electrostatic attraction. Besides, it was found that filtration flow rate, or flux, was also an important parameter for the final retention because the enhanced hydrodynamic drag could trigger the detachment of deposited NPs or press NPs flowing through the superficial entrance leading to penetration. Tests of 10 nm Au NP retention using five different membranes with the same nominal pore size of 0.1 µm showed large variation of NP retention efficiencies demonstrating that pore size should not be used as the only criterion for rating filter performance, especially for small NPs. Our results provide not only detailed insights into the retention mechanisms of various membranes but also suggestions on how to select membrane filters for different filtration purposes. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.memsci.2018.09.033
  • 2018 • 192 Primary particle diameter differentiation and bimodality identification by five analytical methods using gold nanoparticle size distributions synthesized by pulsed laser ablation in liquids
    Letzel, A. and Gökce, B. and Menzel, A. and Plech, A. and Barcikowski, S.
    Applied Surface Science 435 743-751 (2018)
    For a known material, the size distribution of a nanoparticle colloid is a crucial parameter that defines its properties. However, measured size distributions are not easy to interpret as one has to consider weighting (e.g. by light absorption, scattering intensity, volume, surface, number) and the way size information was gained. The radius of a suspended nanoparticle can be given as e.g. sphere equivalent, hydrodynamic, Feret or radius of gyration. In this study, gold nanoparticles in water are synthesized by pulsed-laser ablation (LAL) and fragmentation (LFL) in liquids and characterized by various techniques (scanning transmission electron microscopy (STEM), small-angle X-ray scattering (SAXS), analytical disc centrifugation (ADC), dynamic light scattering (DLS) and UV–vis spectroscopy with Mie-Gans Theory) to study the comparability of different analytical techniques and determine the method that is preferable for a given task related to laser-generated nanoparticles. In particular, laser-generated colloids are known to be bimodal and/or polydisperse, but bimodality is sometimes not analytically resolved in literature. In addition, frequently reported small size shifts of the primary particle mode around 10 nm needs evaluation of its statistical significance related to the analytical method. Closely related to earlier studies on SAXS, different colloids in defined proportions are mixed and their size as a function of the nominal mixing ratio is analyzed. It is found that the derived particle size is independent of the nominal mixing ratio if the colloid size fractions do not overlap considerably. Conversely, the obtained size for colloids with overlapping size fractions strongly depends on the nominal mixing ratio since most methods cannot distinguish between such fractions. Overall, SAXS and ADC are very accurate methods for particle size analysis. Further, the ability of different methods to determine the nominal mixing ratio of sizes fractions is studied experimentally. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2017.11.130
  • 2018 • 191 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 812 194-198 (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 abstractdoi: 10.1016/j.jelechem.2017.12.023
  • 2018 • 190 Heterogeneous to homogeneous melting transition visualized with ultrafast electron diffraction
    Mo, M.Z. and Chen, Z. and Li, R.K. and Dunning, M. and Witte, B.B.L. and Baldwin, J.K. and Fletcher, L.B. and Kim, J.B. and Ng, A. and Redmer, R. and Reid, A.H. and Shekhar, P. and Shen, X.Z. and Shen, M. and Sokolowski-Tinten, K....
    Science 360 1451-1455 (2018)
    The ultrafast laser excitation of matters leads to nonequilibrium states with complex solid-liquid phase-transition dynamics. We used electron diffraction at mega–electron volt energies to visualize the ultrafast melting of gold on the atomic scale length. For energy densities approaching the irreversible melting regime, we first observed heterogeneous melting on time scales of 100 to 1000 picoseconds, transitioning to homogeneous melting that occurs catastrophically within 10 to 20 picoseconds at higher energy densities. We showed evidence for the heterogeneous coexistence of solid and liquid. We determined the ion and electron temperature evolution and found superheated conditions. Our results constrain the electron-ion coupling rate, determine the Debye temperature, and reveal the melting sensitivity to nucleation seeds. © 2017 The Authors
    view abstractdoi: 10.1126/science.aar2058
  • 2018 • 189 Crystallographic characterization of laser-generated, polymer-stabilized 4 nm silver-gold alloyed nanoparticles
    Prymak, O. and Jakobi, J. and Rehbock, C. and Epple, M. and Barcikowski, S.
    Materials Chemistry and Physics 207 442-450 (2018)
    Monometallic silver and gold nanoparticles and bimetallic silver-gold (AgAu) nanoparticles were prepared by laser ablation in liquids in the atomic composition range of Ag:Au from 0:100 to 100:0 with steps of 10 at% and colloidally stabilized with poly(N-vinylpyrrolidone) (PVP). As metallic bulk targets for laser ablation, pure silver, pure gold, and alloyed AgAu foils with the desired composition were used. Size separation by centrifugation and freeze-drying gave monodisperse spherical nanoparticles with a diameter of 4 nm as determined by differential centrifugal sedimentation (DCS) and transmission electron microscopy (TEM). A crystallographic characterization of the nanoparticles was carried out by X-ray powder diffraction (XRD) and Rietveld refinement, leading to highly precise cubic lattice parameters (fcc crystal system) and crystallite sizes. For comparison, the same analysis including the determination of the microstrain was carried out for the bulk target materials (AgAu alloys in the full concentration range). Both nanoparticles and bulk target materials obeyed Vegard's rule, with only slight deviations. The fact that the crystallite size as determined by XRD was identical to the hydrodynamic diameter by DCS and the Feret diameter by TEM indicates that the particles consist of only one domain, i.e. they are single crystals. The combination of UV-vis spectroscopy with energy-dispersive X-ray spectroscopy (EDX) as line scan along the nanoparticle showed a homogenous distribution of the gold and silver inside the nanoparticles, indicating solid solution alloys, in contrast to what was observed earlier for chemically prepared AgAu nanoparticles by reduction of metal ions in water. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.matchemphys.2017.12.080
  • 2018 • 188 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 1066-1071 (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 abstractdoi: 10.1038/s41565-018-0232-x
  • 2018 • 187 Effect of Pt and Au current collector in LiMn2O4 thin film for micro-batteries
    Trócoli, R. and Dushina, A. and Borhani-Haghighi, S. and Ludwig, Al. and La Mantia, F.
    Nanotechnology 29 (2018)
    The crystal orientation and morphology of sputtered LiMn2O4 thin films is strongly affected by the current collector. By substituting Pt with Au, it is possible to observe in the x-ray diffraction pattern of LiMn2O4 a change in the preferential orientation of the grains from (111) to (400). In addition, LiMn2O4 thin films deposited on Au show a higher porosity than films deposited on Pt. These structural differences cause an improvement in the electrochemical performances of the thin films deposited on Au, with up to 50% more specific charge. Aqueous cells using thin film based on LiMn2O4 sputtered on Au or Pt as the cathode electrode present a similar retention of specific charge, delivering 85% and 100%, respectively, of the initial values after 100 cycles. The critical role of the nature of the substrate used in the morphology and electrochemical behaviour observed could permit the exploration of similar effects for other lithium intercalation electrodes. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6528/aa9e33
  • 2018 • 186 Simultaneous Rayleigh/Mie and Raman/Fluorescence Characterization of Molecularly Functionalized Colloids by Correlative Single-Particle Real-Time Imaging in Suspension
    Wissler, J. and Wehmeyer, M. and Bäcker, S. and Knauer, S. and Schlücker, S.
    Analytical Chemistry 90 723-728 (2018)
    Many applications of nano- and microparticles require molecular functionalization. Assessing the heterogeneity of a colloidal sample in terms of its molecular functionalization is highly desirable but not accessible by conventional ensemble experiments. Retrieving this information necessitates single-particle experiments which simultaneously detect both functionalized and nonfunctionalized particles via two separate imaging channels. In this contribution, we present an optical setup for performing correlative single-particle imaging using laser light-sheet illumination: the first detection channel records elastic light scattering (Rayleigh/Mie), while the second channel detects inelastic light scattering (Raman) or fluorescence. The instrument is tested with Raman reporter-functionalized SERS-active metal nanoparticles (core/satellite silver nanoparticles, dimers and monomers of gold nanoparticles) and fluorophore-functionalized colloids (fluorescent polymer microparticles, dye-labeled protein on gold nanoparticles). © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.analchem.7b02528
  • 2018 • 185 Ideal Dimers of Gold Nanospheres for Precision Plasmonics: Synthesis and Characterization at the Single-Particle Level for Identification of Higher Order Modes
    Yoon, J.H. and Selbach, F. and Langolf, L. and Schlücker, S.
    Small 14 (2018)
    Ideal dimers comprising gold nanoparticles with a smooth surface and high sphericity are synthesized by a substrate-based assembly strategy with efficient cetyltrimethylammonium bromide removal. An unprecedented structural and plasmonic uniformity at the single-particle level is observed since inhomogeneities resulting from variations in gap morphology are eliminated. Single ideal dimers are analyzed by polarization-resolved dark-field scattering spectroscopy. Contributions from transverse as well as quadrupolar and octupolar longitudinal plasmon coupling modes can be discriminated because of their orthogonal polarization behavior. The assignment of these higher order coupling modes is supported by computer simulations. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/smll.201702754
  • 2017 • 184 Efïicient and low-complexity space time code for massive MIMO RFID systems
    Abouzeid, M.S. and Lopacinski, L. and Grass, E. and Kaiser, T. and Kraemer, R.
    Iberian Conference on Information Systems and Technologies, CISTI (2017)
    In this paper, a Space-Time Block Code (STBC) based on the Golden number, Golden code is proposed for a massive MIMO-RFID systems. Based on channel modelling for massive MIMO-RFID system, the proposed space-time code is applied to the tag side. Simulation results show that the proposed code for massive MIMO-RFID systems outperforms Alamouti code while simplifying the receiver's complexity. The Bit Error Rate (BER) performance of the proposed technique demonstrates that high diversity gain for the tag is achieved leading to a highly reliable and more robust RFID range. Furthermore, the link capacity between the tagged item and the reader can be increased. The proposed RFID technique provides superior performance against the state-of-the-art RFID techniques. © 2017 AISTI.
    view abstractdoi: 10.23919/CISTI.2017.7976046
  • 2017 • 183 Femtosecond Spin Current Pulses Generated by the Nonthermal Spin-Dependent Seebeck Effect and Interacting with Ferromagnets in Spin Valves
    Alekhin, A. and Razdolski, I. and Ilin, N. and Meyburg, J.P. and Diesing, D. and Roddatis, V. and Rungger, I. and Stamenova, M. and Sanvito, S. and Bovensiepen, U. and Melnikov, A.
    Physical Review Letters 119 (2017)
    Using the sensitivity of optical second harmonic generation to currents, we demonstrate the generation of 250-fs long spin current pulses in Fe/Au/Fe/MgO(001) spin valves. The temporal profile of these pulses indicates ballistic transport of hot electrons across a sub-100 nm Au layer. The pulse duration is primarily determined by the thermalization time of laser-excited hot carriers in Fe. Considering the calculated spin-dependent Fe/Au interface transmittance we conclude that a nonthermal spin-dependent Seebeck effect is responsible for the generation of ultrashort spin current pulses. The demonstrated rotation of spin polarization of hot electrons upon interaction with noncollinear magnetization at Au/Fe interfaces holds high potential for future spintronic devices. © 2017 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.119.017202
  • 2017 • 182 Surface-Enhanced Raman Spectroscopy and Density Functional Theory Calculations of a Rationally Designed Rhodamine with Thiol Groups at the Xanthene Ring
    Brem, S. and Schlücker, S.
    Journal of Physical Chemistry C 121 15310-15317 (2017)
    Rhodamines are widely used dyes in fluorescence and surface-enhanced Raman spectroscopy (SERS). The latter requires adsorption of the dye onto the surface of plasmonic nanostructures, a process which requires attractive molecule-surface interactions. Here, we report an experimental SERS and computational density functional theory (DFT) study investigating the role of thiol functionalization at the xanthene ring of the rhodamine in the adsorption onto gold nanoparticles. For this purpose, a new bisthiolated rhodamine derivative was rationally designed and synthesized via a PPh3/I2 reduction route. The introduction of two thiol moieties directly at the xanthene ring provides the shortest possible distance between the molecular π-system and the metal surface for maximum SERS enhancement combined with the strong Au-S interaction for chemisorption. The comparison of experimental SERS spectra obtained from gold nanostars and a film of gold nanoparticles with results from DFT calculations (molecular electrostatic potential, normal modes) suggests adsorption via the thiol groups at the xanthene moiety. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.7b01504
  • 2017 • 181 Compositional fingerprint of soy sauces via hydrophobic surface interaction
    Jakobi, V. and Salmen, P. and Paulus, M. and Tolan, M. and Rosenhahn, A.
    Food Chemistry 218 256-260 (2017)
    In this work, the interaction of soy sauces with hydrophobic surfaces has been analyzed. Hydrophobic self-assembled monolayers on gold or silicon dioxide were used to harvest conditioning layers from soy sauce products with varying amounts of additives. The data was compared to adsorption of soy protein and glutamic acid as common ingredients. Spectral ellipsometry revealed that all tested sauces led to the formation of thin overlayers on hydrophobic surfaces. Products with less additives yielded adlayers in the same thickness range as pure soy protein. In contrast, sauces with more ingredients create distinctly thicker films. Using water contact angle goniometry, it is shown that all adlayers render the substrate more hydrophilic. Infrared spectroscopy provided a deeper insight into the adlayer chemistry and revealed that the adlayer composition is dominated by protein rich components. X-ray reflectivity on selected films provided further insight into the density profiles within the adlayers on the molecular scale. © 2016 Elsevier Ltd
    view abstractdoi: 10.1016/j.foodchem.2016.09.045
  • 2017 • 180 Efficient Gene Transfection through Inhibition of β-Sheet (Amyloid Fiber) Formation of a Short Amphiphilic Peptide by Gold Nanoparticles
    Jana, P. and Samanta, K. and Bäcker, S. and Zellermann, E. and Knauer, S. and Schmuck, C.
    Angewandte Chemie - International Edition (2017)
    The effect of citrate-stabilized gold nanoparticles (AuNPs) on the secondary structure of an artificial β-sheet-forming cationic peptide has been studied. The AuNPs inhibited β-sheet formation and led to fragmented fibrils and spherical oligomers with assembled AuNPs on their surface. Besides this structural change, the functional properties of the peptide are also different. Whereas the peptide was unable to act as a vector for gene delivery, formation of a complex with AuNPs allowed successful gene delivery into cells. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201700713
  • 2017 • 179 Colloidal Stability and Surface Chemistry Are Key Factors for the Composition of the Protein Corona of Inorganic Gold Nanoparticles
    Johnston, B.D. and Kreyling, W.G. and Pfeiffer, C. and Schäffler, M. and Sarioglu, H. and Ristig, S. and Hirn, S. and Haberl, N. and Thalhammer, S. and Hauck, S.M. and Semmler-Behnke, M. and Epple, M. and Hühn, J. and Del Pino, ...
    Advanced Functional Materials 27 (2017)
    To study the influence of colloidal stability on protein corona formation, gold nanoparticles are synthesized with five distinct surface modifications: coating with citric acid, bis(p-sulfonatophenyl)phenylphosphine dihydrate dipotassium salt, thiol-terminated methoxy-polyethylene glycol, dodecylamine-grafted poly(isobutylene-alt-maleic anhydride), and dodecylamine-grafted poly(isobutylene-alt-maleic anhydride) conjugated with polyethylene glycol. The nanoparticles are incubated with serum or bronchoalveolar lavage fluid from C57BL/6 mice (15 min or 24 h) to assess the effect of differential nanoparticle surface presentation on protein corona formation in the air–blood barrier exposure pathway. Proteomic quantification and nanoparticle size measurements are used to assess protein corona formation. We show that surface modification has a clear effect on the size and the composition of the protein corona that is related to the colloidal stability of the studied nanoparticles. Additionally, differences in the composition and size of the protein corona are shown between biological media and duration of exposure, indicating evolution of the corona through this exposure pathway. Consequently, a major determinant of protein corona formation is the colloidal stability of nanoparticles in biological media and chemical or environmental modification of the nanoparticles alters the surface presentation of the functional epitope in vivo. Therefore, the colloidal stability of nanoparticles has a decisive influence on nano–bio interactions. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adfm.201701956
  • 2017 • 178 Effect of titania surface modification of mesoporous silica SBA-15 supported Au catalysts: Activity and stability in the CO oxidation reaction
    Kučerová, G. and Strunk, J. and Muhler, M. and Behm, R.J.
    Journal of Catalysis 356 214-228 (2017)
    As part of an ongoing effort to understand the deactivation and improve the stability of metal oxide-supported Au catalysts in the low-temperature CO oxidation reaction while maintaining their high activity, we have investigated the influence of a mesoporous silica SBA-15 substrate on the activity and stability of Au/TiO2 catalysts, which consist of a SBA-15 support surface modified by a monolayer of TiOx with Au nanoparticles on top. The extent of the TiOx surface modification was systematically increased, while the Au loading and the Au particle sizes were largely kept constant. Employing kinetic measurements at three different temperatures (30 °C, 80 °C, 180 °C) and a number of ex situ methods as well as in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) for catalyst characterization, we found that the activity of these catalysts increases significantly with the Ti concentration and with reaction temperature. The tendency for deactivation remains essentially unchanged. Detailed in situ DRIFTS measurements reveal that the Au nanoparticles are largely formed on the TiOx surface-modified areas of the SBA-15 support and that the tendency for surface carbonate formation is very low. The observed deactivation may at least partly be related to the accumulation of molecularly adsorbed H2O species, in particular at low temperatures (30 °C). These are likely to be formed from surface hydroxyl groups, they may affect the reaction either by blocking of active sites or by blocking the adsorption of reactants on the substrate. Other effects, such as reaction induced changes in the titania layer, must however, play a role as well, both at 80 °C and in particular at 180 °C, where accumulation of adsorbed species is negligible. The mechanistic ideas are supported by reactivation tests subsequent to calcination at 400 °C, which were found to fully restore the initial activity. © 2017 Elsevier Inc.
    view abstractdoi: 10.1016/j.jcat.2017.09.017
  • 2017 • 177 Thermally induced substitutional reaction of Fe into Mo2GaC thin films
    Lai, C.-C. and Petruhins, A. and Lu, J. and Farle, M. and Hultman, L. and Eklund, P. and Rosen, J.
    Materials Research Letters 1-7 (2017)
    The first Fe-based MAX phase is realized by solid-state substitution reaction of an Fe/Au/Mo2GaC thin-film diffusion couple, as determined by X-ray diffraction and scanning transmission electron microscopy. Chemical analysis together with elemental mapping reveals that as much as 50 at.% Fe on the A site can be obtained by thermally induced Au and Fe substitution for Ga atomic layers in Mo2GaC. One-sixth of the original Ga is also replaced by Au atoms. When annealing Mo2GaC thin films covered with Fe only, the Mo2GaC phase remains intact, that is, Au acts as a catalyst for the substitution reaction. IMPACT STATEMENT The first direct evidence showing Fe-containing MAX phase, Mo2AC, with Fe ∼50 at.% on the A sites is presented, synthesized by thermally induced Fe and Au substitution reaction catalyzed by Au. © 2017 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group
    view abstractdoi: 10.1080/21663831.2017.1343207
  • 2017 • 176 Direct Integration of Laser-Generated Nanoparticles into Transparent Nail Polish: The Plasmonic "goldfinger"
    Lau, M. and Waag, F. and Barcikowski, S.
    Industrial and Engineering Chemistry Research 56 3291-3296 (2017)
    A transparent nail varnish can be colored simply and directly with laser-generated nanoparticles. This does not only enable coloring of the varnish for cosmetic purposes, but also gives direct access to nanodoped varnishes to be used on any solid surface. Therefore, nanoparticle properties such as plasmonic properties or antibacterial effects can be easily adapted to surfaces for medical or optical purposes. The presented method for integration of metal (gold, platinum, silver, and alloy) nanoparticles into varnishes is straightforward and gives access to nanodoped polishes with optical properties, difficult to be achieved by dispersing powder pigments in the high-viscosity liquids. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.iecr.7b00039
  • 2017 • 175 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 67-72 (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 abstractdoi: 10.1039/C6TA08088J
  • 2017 • 174 Water-based, surfactant-free cytocompatible nanoparticle-microgel-composite biomaterials-rational design by laser synthesis, processing into fiber pads and impact on cell proliferation
    Million, N. and Coger, V. and Wilke, P. and Rehbock, C. and Vogt, P.M. and Pich, A. and Barcikowski, S.
    BioNanoMaterials 18 (2017)
    This work highlights the laser-based aqueous synthesis and processing of nanocomposites, composed of zinc or iron nanoparticles embedded in a N-Vinylcaprolactam microgel matrix, with potential applicability as ion releasing fiber pads for wound healing. An in situ laser process for microgel synthesis is developed and optimized for high embedded nanoparticle yields, evaluating influences of laser repetition rate and monomer concentration. The impact of the nanoparticles on polymerization was increased by embedded zinc oxide nanoparticles, and reduced in the presence of iron oxide. Furthermore, TEM images verified that the nanoparticles were homogeneously embedded into the polymer matrix. The nanoparticle-loaded microgels were thermally stable up to 429 °C, which ensures that the composites maintain their integrity after heat sterilization and during rapid prototyping by thermal polymer processing. The general suitability of the hydrogels as active biomaterial for wound healing was assessed in toxicity, cell proliferation and migration assays using human dermal fibroblasts and keratinocytes, where cytocompatibility was verified, while the proliferation was affected by the gel alone as well as the embedded nanoparticles. The hydrogels were processed to suit their use as a biomaterial for wound coverages via electrospinning resulting in a centimeter scale fully cytocompatible fiber pad with the nanoparticle-filled microgel capsules supported on the fiber's surface. © 2017 Walter de Gruyter GmbH, Berlin/Boston.
    view abstractdoi: 10.1515/bnm-2017-0004
  • 2017 • 173 Size-dependent reactivity of gold-copper bimetallic nanoparticles during CO2 electroreduction
    Mistry, H. and Reske, R. and Strasser, P. and Roldan Cuenya, B.
    Catalysis Today 288 30-36 (2017)
    New catalysts are needed to achieve lower overpotentials and higher faradaic efficiency for desirable products during the electroreduction of CO2. In this study, we explore the size-dependence of monodisperse gold-copper alloy nanoparticles (NPs) synthesized by inverse micelle encapsulation as catalysts for CO2 electroreduction. X-ray spectroscopy revealed that gold-copper alloys were formed and were heavily oxidized in their initial as prepared state. Current density was found to increase significantly for smaller NPs due to the increasing population of strongly binding low coordinated sites on NPs below 5nm. Product analysis showed formation of H2, CO, and CH4, with faradaic selectivity showing a minor dependence on size. The selectivity trends observed are assigned to reaction-induced segregation of gold atoms to the particle surface and altered electronic or geometric properties due to alloying. © 2016.
    view abstractdoi: 10.1016/j.cattod.2016.09.017
  • 2017 • 172 Gold-Palladium Bimetallic Catalyst Stability: Consequences for Hydrogen Peroxide Selectivity
    Pizzutilo, E. and Freakley, S.J. and Cherevko, S. and Venkatesan, S. and Hutchings, G.J. and Liebscher, C.H. and Dehm, G. and Mayrhofer, K.J.J.
    ACS Catalysis 7 5699-5705 (2017)
    During application, electrocatalysts are exposed to harsh electrochemical conditions, which can induce degradation. This work addresses the degradation of AuPd bimetallic catalysts used for the electrocatalytic production of hydrogen peroxide (H2O2) by the oxygen reduction reaction (ORR). Potential-dependent changes in the AuPd surface composition occur because the two metals have different dissolution onset potentials, resulting in catalyst dealloying. Using a scanning flow cell (SFC) with an inductively coupled plasma mass spectrometer (ICP-MS), simultaneous Pd and/or Au dissolution can be observed. Thereafter, three accelerated degradation protocols (ADPs), simulating different dissolution regimes, are employed to study the catalyst structure degradation on the nanoscale with identical location (IL) TEM. When only Pd or both Au and Pd dissolve, the composition changes rapidly and the surface becomes enriched with Au, as observed by cyclic voltammetry and elemental mapping. Such changes are mirrored by the evolution of electrocatalytic performances toward H2O2 production. Our experimental findings are finally summarized in a dissolution/structure/selectivity mechanism, providing a clear picture of the degradation of bimetallic catalyst used for H2O2 synthesis. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.7b01447
  • 2017 • 171 Probing Oxide Reduction and Phase Transformations at the Au-TiO2 Interface by Vibrational Spectroscopy
    Pougin, A. and Lüken, A. and Klinkhammer, C. and Hiltrop, D. and Kauer, M. and Tölle, K. and Havenith-Newen, M. and Morgenstern, K. and Grünert, W. and Muhler, M. and Strunk, J.
    Topics in Catalysis 60 1744-1753 (2017)
    By a combination of FT-NIR Raman spectroscopy, infrared spectroscopy of CO adsorption under ultrahigh vacuum conditions (UHV-IR) and Raman spectroscopy in the line scanning mode the formation of a reduced titania phase in a commercial Au/TiO2 catalyst and in freshly prepared Au/anatase catalysts was detected. The reduced phase, formed at the Au-TiO2 interface, can serve as nucleation point for the formation of stoichiometric rutile. TinO2n−1 Magnéli phases, structurally resembling the rutile phase, might be involved in this process. The formation of the reduced phase and the rutilization process is clearly linked to the presence of gold nanoparticles and it does not proceed under similar conditions with the pure titania sample. Phase transformations might be both thermally or light induced, however, the colloidal deposition synthesis of the Au/TiO2 catalysts is clearly ruled out as cause for the formation of the reduced phase. © 2017, The Author(s).
    view abstractdoi: 10.1007/s11244-017-0851-8
  • 2017 • 170 Morphological analysis of cerium oxide stabilized nanoporous gold catalysts by soft X-ray ASAXS
    Rumancev, C. and Von Gundlach, A.R. and Baier, S. and Wittstock, A. and Shi, J. and Benzi, F. and Senkbeil, T. and Stuhr, S. and Garamusx, V.M. and Grunwaldt, J.-D. and Rosenhahn, A.
    RSC Advances 7 45344-45350 (2017)
    Nanoporous (np) gold is a promising catalyst material for selective oxidation reactions. Especially the addition of oxide deposits like ceria (CeO2) promises enhanced morphological stability for high temperature applications. Describing such temperature induced morphological changes in porous materials is challenging. Here, X-ray nanoanalysis is particularly promising due to the high penetration depth that allows studying of the bulk properties with high spatial sensitivity. We applied soft X-ray small angle scattering (SAXS) to determine temperature induced structural changes in nanoporous gold catalysts. The results show that CeO2 deposits enhance the temperature stability of the nanoporous gold catalyst. Moreover, we demonstrate the ability of soft X-rays to selectively provide structural information on the stabilizing cerium oxide deposits via resonant, anomalous SAXS (ASAXS) measurements at the cerium M-edge, revealing no growth of the ceria particles. © 2017 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c7ra05396g
  • 2017 • 169 Enhanced spin-orbit coupling in tetragonally strained Fe-Co-B films
    Salikhov, R. and Reichel, L. and Zingsem, B. and Abrudan, R. and Edström, A. and Thonig, D. and Rusz, J. and Eriksson, O. and Schultz, L. and Fähler, S. and Farle, M. and Wiedwald, U.
    Journal of Physics Condensed Matter 29 (2017)
    Tetragonally strained interstitial Fe-Co-B alloys were synthesized as epitaxial films grown on a 20 nm thick Au0.55Cu0.45 buffer layer. Different ratios of the perpendicular to in-plane lattice constant c/a = 1.013, 1.034 and 1.02 were stabilized by adding interstitial boron with different concentrations 0, 4, and 10 at.%, respectively. Using ferromagnetic resonance (FMR) and x-ray magnetic circular dichroism (XMCD) we found that the total orbital magnetic moment significantly increases with increasing c/a ratio, indicating that reduced crystal symmetry and interstitial B leads to a noticeable enhancement of the effect of spin-orbit coupling (SOC) in the Fe-Co-B alloys. First-principles calculations reveal that the increase in orbital magnetic moment mainly originates from B impurities in octahedral position and the reduced symmetry around B atoms. These findings offer the possibility to enhance SOC phenomena - namely the magnetocrystalline anisotropy and the orbital moment - by stabilizing anisotropic strain by doping 4 at.% B. Results on the influence of B doping on the Fe-Co film microstructure, their coercive field and magnetic relaxation are also presented. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-648X/aa7498
  • 2017 • 168 Laser-Induced Functionalization of Organo/Carbon Interfaces for Selective Adsorption of Au Nanoparticles in Microsized Domains
    Schade, M. and Franzka, S. and Hartmann, N.
    Langmuir 33 8686-8692 (2017)
    Laser microprocessing of highly oriented pyrolytic graphite (HOPG) in conjunction with chemical functionalization routines is used to fabricate functional microsized domains. Infrared and Auger electron spectroscopy, contact angle measurements, and electron microscopy are used for characterization of laser-fabricated structures. HOPG samples are coated with alkylsiloxane monolayers. Laser-induced bromination of coated HOPG samples in gaseous bromine is carried out using a microfocused laser beam at a wavelength of 514 nm and 1/e2 laser spot diameter of about 2 μm. Subsequent azidation and amination results in functional domains with sizes in the range of 1.2 to 40 μm and more. At low laser powers and irradiation times fully functionalized circular-shaped structures are formed. At high laser powers and irradiation times laser processing results in decomposition of the organic monolayer and substrate in the center of the structures yielding donut-shaped structures. After laser processing and chemical transformation Au nanoparticles are selectively adsorbed onto the functional domains. This provides an opportunity to build up functional nanoparticle microarrays on carbon-based materials, e.g., for applications in sensing and electrocatalysis. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.langmuir.7b00695
  • 2017 • 167 Advanced SERS Sensor Based on Capillarity-Assisted Preconcentration through Gold Nanoparticle-Decorated Porous Nanorods
    Xue, L. and Xie, W. and Driessen, L. and Domke, K.F. and Wang, Y. and Schlücker, S. and Gorb, S.N. and Steinhart, M.
    Small (2017)
    A preconcentrating surface-enhanced Raman scattering (SERS) sensor for the analysis of liquid-soaked tissue, tiny liquid droplets and thin liquid films without the necessity to collect the analyte is reported. The SERS sensor is based on a block-copolymer membrane containing a spongy-continuous pore system. The sensor's upper side is an array of porous nanorods having tips functionalized with Au nanoparticles. Capillarity in combination with directional evaporation drives the analyte solution in contact with the flat yet nanoporous underside of the SERS sensor through the continuous nanopore system toward the nanorod tips where non-volatile components of the analyte solution precipitate at the Au nanoparticles. The nanorod architecture increases the sensor surface in the detection volume and facilitates analyte preconcentration driven by directional solvent evaporation. The model analyte 5,5'-dithiobis(2-nitrobenzoic acid) can be detected in a 1 × 10-3m solution ≈300 ms after the sensor is brought into contact with the solution. Moreover, a sensitivity of 0.1 ppm for the detection of the dissolved model analyte is achieved. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/smll.201603947
  • 2016 • 166 Ultrafiltration membrane-based purification of bioconjugated gold nanoparticle dispersions
    Alele, N. and Streubel, R. and Gamrad, L. and Barcikowski, S. and Ulbricht, M.
    Separation and Purification Technology 157 120-130 (2016)
    Functionalization of nanoparticles (NP) with biomolecules to form bioconjugated systems has received large attention in biomedical applications. However, purification of these nanoparticle bioconjugates from unbound free biofunctional ligands (e.g., peptides) remains a significant challenge in the production of well-defined materials. The conventional separation methods often compromise the product's properties and recovery. In this work, removal of excess of unbound peptides after the bioconjugation step to yield functionalized gold nanoparticles (AuNP) was achieved by exploiting the sieving properties of commercial regenerated cellulose (RC) ultrafiltration (UF) membranes. The RC membrane with nominal molecular weight cut-off (NMWCO) of 30 kDa precisely fractionated the mixtures and purified gold nanoparticle-peptide bioconjugates in a pressure driven semi-continuous diafiltration process. The RC 30 kDa membrane showed absolute rejection of the bioconjugated AuNP and the recovery of AuNP-peptide bioconjugate in the retentate was >87% relative to the initial amount in the mixture. In addition, the separation efficiency and throughput results were much better compared to the centrifugal membrane filtration method using an analogous membrane. All results indicate that by choice of an appropriate membrane type and barrier pore size, and with optimized solution chemistry and filtration parameters, ultrafiltration membranes, and in particular RC membranes, can be very well suited for the purification of bioconjugated nanoparticle dispersions, and the diafiltration mode is very well suited for upscaling. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.seppur.2015.11.033
  • 2016 • 165 Laser-based in situ embedding of metal nanoparticles into bioextruded alginate hydrogel tubes enhances human endothelial cell adhesion
    Blaeser, A. and Million, N. and Campos, D.F.D. and Gamrad, L. and Köpf, M. and Rehbock, C. and Nachev, M. and Sures, B. and Barcikowski, S. and Fischer, H.
    Nano Research 9 3407-3427 (2016)
    Alginate is a widely used hydrogel in tissue engineering owing to its simple and non-cytotoxic gelation process, ease of use, and abundance. However, unlike hydrogels derived from mammalian sources such as collagen, alginate does not contain cell adhesion ligands. Here, we present a novel laser ablation technique for the in situ embedding of gold and iron nanoparticles into hydrogels. We hypothesized that integration of metal nanoparticles in alginate could serve as an alternative material because of its chemical biofunctionalization ability (coupling of RGD ligands) to favor cell adhesion. Cytocompatibility and biofunctionality of the gels were assessed by cell culture experiments using fibroblasts and endothelial cells. Nanoparticles with an average particle size of 3 nm (gold) and 6 nm (iron) were generated and stably maintained in alginate for up to 6 months. Using an extrusion system, several centimeter-long alginate tubes with an outer diameter of approximately 3 mm and a wall thickness of approximately 150 μm were manufactured. Confocal microscopy revealed homogeneously distributed nanoparticle agglomerates over the entire tube volume. Endothelial cells seeded on iron-loaded gels showed significantly higher viability and an increased degree of spreading, and the number of attached cells was also elevated in comparison to the control and gold-loaded alginates. We conclude that laser-based in situ integration of iron nanoparticles (≤ 0.01 wt.%) in alginate is a straightforward method to generate composite materials that favor the adhesion of endothelial cells. In addition, we show that nanoparticle integration does not impair the alginate’s gelation and 3D biofabrication properties. [Figure not available: see fulltext.] © 2016, Tsinghua University Press and Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s12274-016-1218-3
  • 2016 • 164 Probing the Dynamic Structure and Chemical State of Au Nanocatalysts during the Electrochemical Oxidation of 2-Propanol
    Choi, Y. and Sinev, I. and Mistry, H. and Zegkinoglou, I. and Roldan Cuenya, B.
    ACS Catalysis 6 3396-3403 (2016)
    A size-dependent trend was observed for the electrochemical total oxidation of 2-propanol to CO2 over Au nanoparticles (NPs), with increasing activity (increased current density and lower overpotential) for decreasing NP size. Furthermore, an enhanced stability against poisoning by the unreacted acetone intermediate was also obtained for NPs smaller than ∼2 nm. Operando X-ray absorption fine structure (XAFS) measurements provided insight into the dynamic evolution of the NP structure and chemical state under reaction conditions, shedding light on the nature of the most catalytically active species and catalyst deactivation phenomena via chemically driven sintering. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.6b00057
  • 2016 • 163 Plasmonic Au/TiO2 nanostructures for glycerol oxidation
    Dodekatos, G. and Tüysüz, H.
    Catalysis Science and Technology 6 7307-7315 (2016)
    Au nanoparticles supported on P25 TiO2 (Au/TiO2) were prepared by a facile deposition-precipitation method with urea and investigated for surface plasmon-assisted glycerol oxidation under base-free conditions. Au/TiO2 samples were characterized in detail by X-ray diffraction, UV-vis spectroscopy, transmission electron microscopy and energy-dispersive X-ray spectroscopy. The adopted synthetic methodology permits deposition of Au nanoparticles with similar mean particle sizes up to 12.5 wt% loading that allows for the evaluation of the influence of the Au amount (without changing the particle size) on its photocatalytic performance for glycerol oxidation. The reaction conditions were optimized by carrying out a systematic study with different Au loadings on TiO2, reaction times, temperatures, catalyst amounts, O2 pressures and Au particle sizes for photocatalytic reactions as well as traditional heterogeneous catalysis. It has been shown that visible light irradiation during the reaction has a beneficial effect on the conversion of glycerol where the best catalytic results were observed for 7.5 wt% Au loading with an average particle size of around 3 nm. The main product observed, with selectivities up to 63%, was high-value dihydroxyacetone that has important industrial applications, particularly in the cosmetic industry. © 2016 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c6cy01192f
  • 2016 • 162 Gold on Different Manganese Oxides: Ultra-Low-Temperature CO Oxidation over Colloidal Gold Supported on Bulk-MnO2 Nanomaterials
    Gu, D. and Tseng, J.-C. and Weidenthaler, C. and Bongard, H.-J. and Spliethoff, B. and Schmidt, W. and Soulimani, F. and Weckhuysen, B.M. and Schüth, F.
    Journal of the American Chemical Society 138 9572-9580 (2016)
    Nanoscopic gold particles have gained very high interest because of their promising catalytic activity for various chemicals reactions. Among these reactions, low-temperature CO oxidation is the most extensively studied one due to its practical relevance in environmental applications and the fundamental problems associated with its very high activity at low temperatures. Gold nanoparticles supported on manganese oxide belong to the most active gold catalysts for CO oxidation. Among a variety of manganese oxides, Mn2O3 is considered to be the most favorable support for gold nanoparticles with respect to catalytic activity. Gold on MnO2 has been shown to be significantly less active than gold on Mn2O3 in previous work. In contrast to these previous studies, in a comprehensive study of gold nanoparticles on different manganese oxides, we developed a gold catalyst on MnO2 nanostructures with extremely high activity. Nanosized gold particles (2-3 nm) were supported on α-MnO2 nanowires and mesoporous β-MnO2 nanowire arrays. The materials were extremely active at very low temperature (-80 °C) and also highly stable at 25 °C (70 h) under normal conditions for CO oxidation. The specific reaction rate of 2.8 molCO·h-1·gAu -1 at a temperature as low as -85 °C is almost 30 times higher than that of the most active Au/Mn2O3 catalyst. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/jacs.6b04251
  • 2016 • 161 Epitaxial growth of thermally stable cobalt films on Au(111)
    Haag, N. and Laux, M. and Stöckl, J. and Kollamana, J. and Seidel, J. and Großmann, N. and Fetzer, R. and Kelly, L.L. and Wei, Z. and Stadtmüller, B. and Cinchetti, M. and Aeschlimann, M.
    18 (2016)
    Ferromagnetic thin films play a fundamental role in spintronic applications as a source for spin polarized carriers and in fundamental studies as ferromagnetic substrates. However, it is challenging to produce such metallic films with high structural quality and chemical purity on single crystalline substrates since the diffusion barrier across the metal-metal interface is usually smaller than the thermal activation energy necessary for smooth surface morphologies. Here, we introduce epitaxial thin Co films grown on an Au(111) single crystal surface as a thermally stable ferromagnetic thin film. Our structural investigations reveal an identical growth of thin Co/Au(111) films compared to Co bulk single crystals with large monoatomic Co terraces with an average width of 500 Å, formed after thermal annealing at 575 K. Combining our results from photoemission and Auger electron spectroscopy, we provide evidence that no significant diffusion of Au into the near surface region of the Co film takes place for this temperature and that no Au capping layer is formed on top of Co films. Furthermore, we show that the electronic valence band is dominated by a strong spectral contribution from a Co 3d band and a Co derived surface resonance in the minority band. Both states lead to an overall negative spin polarization at the Fermi energy. © 2016 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/18/10/103054
  • 2016 • 160 Plasmon assisted 3D microstructuring of gold nanoparticle-doped polymers
    Jonušauskas, L. and Lau, M. and Gruber, P. and Gökce, B. and Barcikowski, S. and Malinauskas, M. and Ovsianikov, A.
    Nanotechnology 27 (2016)
    3D laser lithography of a negative photopolymer (zirconium/silicon hybrid solgel SZ2080) doped with gold nanoparticles (Au NPs) is performed with a 515 nm and 300 fs laser system and the effect of doping is explored. By varying the laser-generated Au NP doping concentration from 4.8 • 10-6 wt% to 9.8 • 10-3 wt% we find that the fabricated line widths are enlarged by up to 14.8% compared to structures achieved in pure SZ2080. While implicating a positive effect on the photosensitivity, the doping has no adverse impact on the mechanical quality of intricate 3D microstructures produced from the doped nanocompound. Additionally, we found that SZ2080 increases the long term (∼months) colloidal stability of Au NPs in isopropanol. By discussing the nanoparticle-light interaction in the 3D polymer structures we provide implications that our findings might have on other fields, such as biomedicine and photonics. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/27/15/154001
  • 2016 • 159 Metal-semiconductor pair nanoparticles by a physical route based on bipolar mixing
    Kala, S. and Theissmann, R. and Rouenhoff, M. and Kruis, F.E.
    Nanotechnology 27 (2016)
    In this report a methodology is described and demonstrated for preparing Au-Ge pair nanoparticles with known compositions by extending and modifying the basic steps normally used to synthesize nanoparticles in carrier gas. For the formation of pair nanoparticles by bipolar mixing, two oppositely charged aerosols of nanoparticles having the desired size are produced with the help of two differential mobility analyzers. Then both are allowed to pass through a tube, which provides sufficient residence time to result in nanoparticle pair formation due to Brownian collisions influenced by Coulomb forces. The effect of residence time on the formation of nanoparticle pairs as well as the influence of diffusion and discharging is described. Subsequently, necessary modifications to the experimental setup are demonstrated systematically. The kinetics of nanoparticles pair formation in a carrier gas is also calculated and compared with measurements made with the help of an online aerosol size analysis technique. This synthesis of nanoparticle pairs can be seen as a possible route towards Janus-type nanoparticles. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/27/12/125604
  • 2016 • 158 Optical and electron microscopy study of laser-based intracellular molecule delivery using peptide-conjugated photodispersible gold nanoparticle agglomerates
    Krawinkel, J. and Richter, U. and Torres-Mapa, M.L. and Westermann, M. and Gamrad, L. and Rehbock, C. and Barcikowski, S. and Heisterkamp, A.
    Journal of Nanobiotechnology 14 (2016)
    Background: Cell-penetrating peptides (CPPs) can act as carriers for therapeutic molecules such as drugs and genetic constructs for medical applications. The triggered release of the molecule into the cytoplasm can be crucial to its effective delivery. Hence, we implemented and characterized laser interaction with defined gold nanoparticle agglomerates conjugated to CPPs which enables efficient endosomal rupture and intracellular release of molecules transported. Results: Gold nanoparticles generated by pulsed laser ablation in liquid were conjugated with CPPs forming agglomerates and the intracellular release of molecules was triggered via pulsed laser irradiation (λ = 532 nm, τpulse = 1 ns). The CPPs enhance the uptake of the agglomerates along with the cargo which can be co-incubated with the agglomerates. The interaction of incident laser light with gold nanoparticle agglomerates leads to heat deposition and field enhancement in the vicinity of the particles. This highly precise effect deagglomerates the nanoparticles and disrupts the enclosing endosomal membrane. Transmission electron microscopy images confirmed this rupture for radiant exposures of 25 mJ/cm2 and above. Successful intracellular release was shown using the fluorescent dye calcein. For a radiant exposure of 35 mJ/cm2 we found calcein delivery in 81 % of the treated cells while maintaining a high percentage of cell viability. Furthermore, cell proliferation and metabolic activity were not reduced 72 h after the treatment. Conclusion: CPPs trigger the uptake of the gold nanoparticle agglomerates via endocytosis and co-resident molecules in the endosomes are released by applying laser irradiation, preventing their intraendosomal degradation. Due to the highly localized effect, the cell membrane integrity is not affected. Therefore, this technique can be an efficient tool for spatially and temporally confined intracellular release. The utilization of specifically designed photodispersible gold nanoparticle agglomerates (65 nm) can open novel avenues in imaging and molecule delivery. Due to the induced deagglomeration the primary, small particles (~5 nm) are more likely to be removed from the body. © 2016 Krawinkel et al.
    view abstractdoi: 10.1186/s12951-015-0155-8
  • 2016 • 157 Crystal plasticity study of monocrystalline stochastic honeycombs under in-plane compression
    Ma, D. and Eisenlohr, P. and Epler, E. and Volkert, C.A. and Shanthraj, P. and Diehl, M. and Roters, F. and Raabe, D.
    Acta Materialia 103 796-808 (2016)
    We present a study on the plastic deformation of single crystalline stochastic honeycombs under in-plane compression using a crystal plasticity constitutive description for face-centered cubic (fcc) materials, focusing on the very early stage of plastic deformation, and identifying the interplay between the crystallographic orientation and the cellular structure during plastic deformation. We observe that despite the stochastic structure, surprisingly, the slip system activations in the honeycombs are almost identical to their corresponding bulk single crystals at the early stage of the plastic deformation. On the other hand, however, the yield stresses of the honeycombs are nearly independent of their crystallographic orientations. Similar mechanical response is found in compression testing of nanoporous gold micro-pillars aligned with various crystallographic orientations. The macroscopic stress tensors of the honeycombs show the same anisotropy as their respective bulk single crystals. Locally, however, there is an appreciable fluctuation in the local stresses, which are even larger than for polycrystals. This explains why the Taylor/Schmid factor associated with the crystallographic orientation is less useful to estimate the yield stresses of the honeycombs than the bulk single crystals and polycrystals, and why the plastic deformation occurs at smaller strains in the honeycombs than their corresponding bulk single crystals. Besides these findings, the observations of the crystallographic reorientation suggest that conventional orientation analysis tools, such as inverse pole figure and related tools, would in general fail to study the plastic deformation mechanism of monocrystalline cellular materials. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.11.016
  • 2016 • 156 Single-shot mega-electronvolt ultrafast electron diffraction for structure dynamic studies of warm dense matter
    Mo, M.Z. and Shen, X. and Chen, Z. and Li, R.K. and Dunning, M. and Sokolowski-Tinten, K. and Zheng, Q. and Weathersby, S.P. and Reid, A.H. and Coffee, R. and Makasyuk, I. and Edstrom, S. and McCormick, D. and Jobe, K. and Hast, C...
    Review of Scientific Instruments 87 (2016)
    We have developed a single-shot mega-electronvolt ultrafast-electron-diffraction system to measure the structural dynamics of warm dense matter. The electron probe in this system is featured by a kinetic energy of 3.2 MeV and a total charge of 20 fC, with the FWHM pulse duration and spot size at sample of 350 fs and 120 μm respectively. We demonstrate its unique capability by visualizing the atomic structural changes of warm dense gold formed from a laser-excited 35-nm freestanding single-crystal gold foil. The temporal evolution of the Bragg peak intensity and of the liquid signal during solid-liquid phase transition are quantitatively determined. This experimental capability opens up an exciting opportunity to unravel the atomic dynamics of structural phase transitions in warm dense matter regime. © 2016 Author(s).
    view abstractdoi: 10.1063/1.4960070
  • 2016 • 155 Resonant laser processing of nanoparticulate Au/TiO2 films on glass supports: Photothermal modification of a photocatalytic nanomaterial
    Schade, L. and Franzka, S. and Thomas, M. and Hagemann, U. and Hartmann, N.
    Surface Science 650 57-63 (2016)
    Resonant laser processing at λ = 532 nm is used to modify thin Au/TiO2 nanoparticle films on soda lime glass plates. A microfocused continuous-wave laser is employed for local patterning at distinct laser powers. In conjunction with microscopic techniques this approach allows for reproducible high-throughput screening of laser-induced material modifications. Optical microscopy and microspectroscopy reveal laser darkening, i.e. a significantly increased optical absorbance. Scanning electron microscopy and X-ray photoelectron spectroscopy show laser-induced film growth and roughening along with the integration of SiO2 from the glass supports. Raman spectroscopy displays a phase transition from anatase to rutile. Au evaporation and/or integration only takes place at high laser powers. All these modifications provide promising perspectives in view of photocatalytic applications. Data from complementary laser experiments with unblended pure TiO2 coatings at λ = 532 nm and λ = 355 nm point to a photothermal process, in which the optical energy is selectively deposited in the Au nanoparticles and transformed into heat. As a result, thermally activated modifications take place. General prospects of laser processing in targeted modification of nanomaterials for photocatalysis are emphasized. © 2016 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.susc.2016.01.006
  • 2016 • 154 Surface-enhanced Raman spectroscopy on laser-engineered ruthenium dye-functionalized nanoporous gold
    Schade, L. and Franzka, S. and Biener, M. and Biener, J. and Hartmann, N.
    Applied Surface Science 374 19-22 (2016)
    Photothermal processing of nanoporous gold with a microfocused continuous-wave laser at λ = 532 nm provides a facile means in order engineer the pore and ligament size of nanoporous gold. In this report we take advantage of this approach in order to investigate the size-dependence of enhancement effects in surface-enhanced Raman spectroscopy (SERS). Surface structures with laterally varying pore sizes from 25 nm to ≥200 nm are characterized using scanning electron microscopy and then functionalized with N719, a commercial ruthenium complex, which is widely used in dye-sensitized solar cells. Raman spectroscopy reveals the characteristic spectral features of N719. Peak intensities strongly depend on the pore size. Highest intensities are observed on the native support, i.e. on nanoporous gold with pore sizes around 25 nm. These results demonstrate the particular perspectives of laser-fabricated nanoporous gold structures in fundamental SERS studies. In particular, it is emphasized that laser-engineered porous gold substrates represent a very well defined platform in order to study size-dependent effects with high reproducibility and precision and resolve conflicting results in previous studies. ©2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.apsusc.2015.08.168
  • 2016 • 153 Conjugation of thiol-terminated molecules to ultrasmall 2 nm-gold nanoparticles leads to remarkably complex 1H-NMR spectra
    Schuetze, B. and Mayer, C. and Loza, K. and Gocyla, M. and Heggen, M. and Epple, M.
    Journal of Materials Chemistry B 4 2179-2189 (2016)
    Gold nanoparticles, functionalized by aliphatic and aromatic mercapto-functionalized carboxylic acids and by two small peptides (CG and CGGRGD), respectively, were synthesized by the reduction of HAuCl4 with NaBH4 in the presence of the above ligands. After purification by centrifugation or filtration and redispersion, the dispersed nanoparticles were analysed by differential centrifugal sedimentation (DCS), high-resolution transmission electron microscopy (HRTEM), and a variety of NMR spectroscopic techniques: 1H-NMR, 1H,1H-COSY and 1H-DOSY. The hydrodynamic diameter of the particles was between 1.8 and 4.4 nm, as determined by DOSY, in good agreement with the DCS and HRTEM results. Diffusion ordered spectroscopy (DOSY) turned out to be a valuable and non-destructive tool to determine the hydrodynamic diameter of dispersed nanoparticles and to control the purity of the final particles. The coordination of the organic molecules to the gold nanoparticles resulted in distinct and complex changes in the 1H-NMR spectra. These were only partially explainable but clearly caused by the vicinity of the molecules to the gold nanoparticle. © The Royal Society of Chemistry 2016.
    view abstractdoi: 10.1039/c5tb02443a
  • 2016 • 152 Gold-manganese oxide core-shell nanoparticles produced by pulsed laser ablation in water
    Simao, T. and Chevrier, D.M. and Jakobi, J. and Korinek, A. and Goupil, G. and Lau, M. and Garbarino, S. and Zhang, P. and Barcikowski, S. and Fortin, M.-A. and Guay, D.
    Journal of Physical Chemistry C 120 22635-22645 (2016)
    A single-step procedure for the preparation of Au-MnOx NPs was achieved through pulsed laser ablation of a gold-manganese metal target made of a pressed metal powder mixture. First, a 248 nm nanosecond laser at 66.7 J cm-2 was used to synthesize Au-MnOx NPs from a gold-manganese metal target immersed in an aqueous solution at pH 11 (NaOH). It is demonstrated that the Au-MnOx NPs are made of a small Au core (around 5 nm in diameter) surrounded by a very thin manganese oxide layer (0.3-1.3 nm) as characterized by TEM, HAADF HR-STEM, and EELS. The superficial MnOx layer has a local structure that bears a close resemblance to that of Mn2O3 and MnO2 as revealed by EXAFS and XANES measurements. Comparative studies were also performed with a 1064 nm nanosecond laser at 1.4 J cm-2. In that case, the resulting colloids are mainly made of a mixture of Au NPs and MnOx NPs, with few Au-MnOx NPs, thereby suggesting the impact of the laser wavelength and fluence on the synthesis process. The mechanisms responsible for the production of Au-MnOx core-shell NPs are discussed. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.6b05838
  • 2016 • 151 Laser-synthesized ligand-free Au nanoparticles for contrast agent applications in computed tomography and magnetic resonance imaging
    Simão, T. and Chevallier, P. and Lagueux, J. and Côté, M.-F. and Rehbock, C. and Barcikowski, S. and Fortin, M.-A. and Guay, D.
    Journal of Materials Chemistry B 4 6413-6427 (2016)
    In recent years, pulsed laser ablation in liquids (PLAL) has emerged as a new green chemistry method to produce different types of nanoparticles (NPs). It does not require the use of reducing or stabilizing agents, therefore enabling the synthesis of NPs with highly-pure surfaces. In this study, pure Au NPs were produced by PLAL in aqueous solutions, sterically stabilized using minimal PEG excess, and functionalized with manganese chelates to produce a dual CT/MRI contrast agent. The small hydrodynamic size (36.5 nm), low polydispersity (0.2) and colloidal stability of Au NPs@PEG-Mn2+ were demonstrated by DLS. The particles were further characterized by TEM, XPS, FTIR and 1H NMR to confirm the purity of the Au surfaces (i.e. free from the common residual chemicals found after NP synthesis) and the presence of the different surface molecules. The potential of these particles as contrast agents for CT/MRI was assessed in vivo (e.g. chicken embryo). Au NPs@PEG-Mn2+ also demonstrated strong blood retention for at least 90 minutes following intravenous injection in mouse models. The promising performance of PEGylated PLAL-synthesized Au NPs containing manganese chelates could open new possibilities for the production of purer dual imaging contrast agents based on Au colloids. © 2016 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c6tb01162d
  • 2016 • 150 Characterizing the Effect of Multivalent Conjugates Composed of Aβ-Specific Ligands and Metal Nanoparticles on Neurotoxic Fibrillar Aggregation
    Streich, C. and Akkari, L. and Decker, C. and Bormann, J. and Rehbock, C. and Müller-Schiffmann, A. and Niemeyer, F.C. and Nagel-Steger, L. and Willbold, D. and Saccà, B. and Korth, C. and Schrader, T. and Barcikowski, S.
    ACS Nano 10 7582-7597 (2016)
    Therapeutically active small molecules represent promising nonimmunogenic alternatives to antibodies for specifically targeting disease-relevant receptors. However, a potential drawback compared to antibody-antigen interactions may be the lower affinity of small molecules toward receptors. Here, we overcome this low-affinity problem by coating the surface of nanoparticles (NPs) with multiple ligands. Specifically, we explored the use of gold and platinum nanoparticles to increase the binding affinity of Aβ-specific small molecules to inhibit Aβ peptide aggregation into fibrils in vitro. The interactions of bare NPs, free ligands, and NP-bound ligands with Aβ are comprehensively studied via physicochemical methods (spectroscopy, microscopy, immunologic tests) and cell assays. Reduction of thioflavin T fluorescence, as an indicator for β-sheet content, and inhibition of cellular Aβ excretion are even more effective with NP-bound ligands than with the free ligands. The results from this study may have implications in the development of therapeutics for treating Alzheimer's disease. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acsnano.6b02627
  • 2015 • 149 Bioconjugated gold nanoparticles penetrate into spermatozoa depending on plasma membrane status
    Barchanski, A. and Taylor, U. and Sajti, C.L. and Gamrad, L. and Kues, W.A. and Rath, D. and Barcikowski, S.
    Journal of Biomedical Nanotechnology 11 1597-1607 (2015)
    Spermatozoa are not only essential for animal reproduction they also represent important tools for the manipulation of animal genetics. For instance, the genetic labeling and analysis of spermatozoa could provide a prospective complementation of pre-fertilization diagnosis and could help to prevent the inheritance of defective alleles during artificial insemination or to select beneficial traits in livestock. Spermatozoa feature extremely specialized membrane organization and restricted transport mechanisms making the labeling of genetically interesting DNA-sequences, e.g., with gold nanoparticles, a particular challenge. Here, we present a systematic study on the size-related internalization of ligand-free, monovalent and bivalent polydisperse gold nanoparticles, depending on spermatozoa membrane status. While monovalent conjugates were coupled solely to either negatively-charged oligonucleotides or positively-charged cell-penetrating peptides, bivalent conjugates were functionalized with both molecules simultaneously. The results clearly indicate that the cell membrane of acrosome-intact, bovine spermatozoa was neither permeable to ligand-free or oligonucleotide-conjugated nanoparticles, nor responsive to the mechanisms of cell-penetrating peptides. Interestingly, after acrosome reaction, which comprises major changes in sperm membrane composition, fluidity and charge, high numbers of monovalent and bivalent nanoparticles were found in the postequatorial segment, depicting a close and complex correlation between particle internalization and membrane organization. Additionally, depending on the applied peptide and for nanoparticle sizes <10 nm even a successive nuclear penetration was observed, making the bivalent conjugates promising for future genetic delivery and sorting issues. Copyright © 2015 American Scientific Publishers All rights reserved.
    view abstractdoi: 10.1166/jbn.2015.2094
  • 2015 • 148 An approach for transparent and electrically conducting coatings: A transparent plastic varnish with nanoparticulate magnetic additives
    Beck, G. and Barcikowski, S. and Chakravadhanula, V.S.K. and Comesaña-Hermo, M. and Deng, M. and Farle, M. and Hilgendorff, M. and Jakobi, J. and Janek, J. and Kienle, L. and Mogwitz, B. and Schubert, T. and Stiemke, F.
    Thin Solid Films 595 96-107 (2015)
    For the purpose of preparing TCCs (= transparent and electrical conducting coatings), metallic and ferromagnetic nano-additives were dispersed into a transparent varnish and the obtained dispersions were coated on transparent plastic substrates. During hardening of the dispersion the magnetic nano-additives were aligned by a magnetic field. The resulting coatings have electrical pathways along lines of nano-additive chains and are highly transparent in the areas between the lines. Therefore, the electrical conductivity is anisotropic, and it depends on the alignment of the nano-additives (i.e. on the distance between the nano-additives within the chains and the length of the lines) as well as on the thickness of an oxide and/or solvent shell around the nano-additives. The transparency depends also on the alignment and here especially on the thickness and the distance between the formed lines. The quality of the alignment in turn, depends on the magnetic properties and on the size of the particles. We used commercial plastic varnishes, which form electrically isolating (≥ 10− 12 S/m) and transparent (about 90% transparency) coatings, and the following magnetic additives: Co-, Fe-, CoPt3, CoPt3@Au- and Fe@Au-nanoparticles as well as CoNi-nanowires. Coatings with Fe@Au-nanoparticles show the best results in terms of the electrical conductivity (10− 5 S/m–10− 6 S/m) at transparencies above 70%. Furthermore, in addition to the magnetic nano-additives, transparent additives (Al2O3-particles) and non-magnetic, but better conducting additives (carbon-nanotubes) were added to the varnish to increase the transparency and the electrical conductivity, respectively. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2015.10.059
  • 2015 • 147 New Au-Cu-Al thin film shape memory alloys with tunable functional properties and high thermal stability
    Buenconsejo, P.J.S. and Ludwig, Al.
    Acta Materialia 85 378-386 (2015)
    An Au-Cu-Al thin film materials library prepared by combinatorial sputter-deposition was characterized by high-throughput experimentation in order to identify and assess new shape memory alloys (SMAs) in this alloy system. Automated resistance measurements during thermal cycling between -20 and 250 °C revealed a wide composition range that undergoes reversible phase transformations with martensite transformation start temperatures, reverse transformation finish temperatures and transformation hysteresis ranging from -15 to 149 °C, 5 to 185 °C and 8 to 60 K, respectively. High-throughput X-ray diffraction analysis of the materials library confirmed that the phase-transforming compositions can be attributed to the existence of the β-AuCuAl parent phase and its martensite product. The formation of large amount of phases based on face-centered cubic (Au-Cu), Al-Cu and Al-Au is responsible for limiting the range of phase-transforming compositions. Selected alloys in this system show excellent thermal cyclic stability of the phase transformation. The functional properties of these alloys, combined with the inherent properties of Au-based alloys, i.e. aesthetic value, oxidation and corrosion resistance, makes them attractive as smart materials for a wide range of applications, including applications as SMAs for elevated temperatures in harsh environment. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.11.035
  • 2015 • 146 The effect of the Au loading on the liquid-phase aerobic oxidation of ethanol over Au/TiO2 catalysts prepared by pulsed laser ablation
    Dong, W. and Reichenberger, S. and Chu, S. and Weide, P. and Ruland, H. and Barcikowski, S. and Wagener, P. and Muhler, M.
    Journal of Catalysis 330 497-506 (2015)
    Gold nanoparticles (NPs) synthesized by pulsed laser ablation of a gold target in water were efficiently deposited on TiO<inf>2</inf> (P25) without any post-treatment yielding catalysts with Au loadings up to 10 wt%. Regardless of the loading, the Au NPs had a mean diameter of 8 nm before and after deposition. The ligand-free Au NPs strongly bind to TiO<inf>2</inf> surface oxygen vacancies and maintain a homogeneous distribution with loadings up to 4 wt%, while a further increase in Au content up to 10 wt% results in additional weakly adsorbed Au NPs. The catalytic tests of the Au/TiO<inf>2</inf> samples in the selective oxidation of ethanol in the liquid phase identified an optimal loading of 4 wt% resulting in the highest yield of acetic acid, which is ascribed to the homogeneous Au distribution and the adequate occupation of surface oxygen vacancies by strongly bound Au NPs without significant Au sintering during reaction. © 2015 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.jcat.2015.07.033
  • 2015 • 145 Conformational Equilibria of Organic Adsorbates on Nanostructures in Aqueous Solution: MD Simulations
    Giri, A.K. and Spohr, E.
    Journal of Physical Chemistry C 119 25566-25575 (2015)
    We have performed atomistic molecular dynamics (MD) simulations of gold nanoparticles (GNPs) in aqueous NaCl solution. Alkanethiol chain-covered GNPs at grafting densities between approximately one-third and full coverage were studied with nonpolar CH3 and charged COO- and NH3 terminations. Special attention was given to the penetration depth of water and ions into the diffuse shell of the functionalized alkanethiol chains and its dependence on grafting density and functionalization. Solutions with polar terminations were neutralized by an excess of Na+ and Cl- ions. The penetration of water and ions into the hydration shell increases with decreasing grafting density irrespective of termination. High grafting densities lead to more extended hydrocarbon chains. Charged functionalized GNPs produce nonmonotonous counter charge distributions with reduced ion mobility. Partial replacement of first shell solvation water by the charged groups leads to a drastic increase in torsional relaxation times of the chain termini. Due to the large curvature of the GNPs with a diameter of 2 nm, gold cores remain accessible to both ions and water even at the highest studied grafting densities of about 5 chains/nm2. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.5b06249
  • 2015 • 144 Ligand-free Gold Nanoparticles as a Reference Material for Kinetic Modelling of Catalytic Reduction of 4-Nitrophenol
    Gu, S. and Kaiser, J. and Marzun, G. and Ott, A. and Lu, Y. and Ballauff, M. and Zaccone, A. and Barcikowski, S. and Wagener, P.
    Catalysis Letters 145 1105-1112 (2015)
    The reduction of 4-nitrophenol by sodium borohydride is a common model reaction to test the catalytic activity of metal nanoparticles. As all reaction steps proceed solely on the surface of the metal nanoparticles (Langmuir-Hinshelwood model), ligand-coverage of metal nanoparticles impedes the merging of theory and experiment. Therefore we analyzed the catalytic activity of bare gold nanoparticles prepared by laser ablation in liquid without any stabilizers or ligands. The catalytic reaction is characterized by a full kinetic analysis including 4-hydroxylaminophenol as an intermediate species. Excellent agreement between theory and experiment is found. Moreover, the suspension of the nanoparticles remains stable. Hence, ligand-free nanoparticles can be used as a reference material for mechanistic studies of catalytic reactions. In addition, the analysis shows that gold nanoparticles synthesized by laser ablation are among the most active catalysts for this reaction. (Graph Presented). © 2015 Springer Science+Business Media.
    view abstractdoi: 10.1007/s10562-015-1514-7
  • 2015 • 143 Uniform 2 nm gold nanoparticles supported on iron oxides as active catalysts for CO oxidation reaction: Structure-activity relationship
    Guo, Y. and Gu, D. and Jin, Z. and Du, P.-P. and Si, R. and Tao, J. and Xu, W.-Q. and Huang, Y.-Y. and Senanayake, S. and Song, Q.-S. and Jia, C.-J. and Schüth, F.
    Nanoscale 7 4920-4928 (2015)
    Uniform Au nanoparticles (∼2 nm) with narrow size-distribution (standard deviation: 0.5-0.6 nm) supported on both hydroxylated (Fe-OH) and dehydrated iron oxide (Fe-O) have been prepared by either deposition-precipitation (DP) or colloidal-deposition (CD) methods. Different structural and textural characterizations were applied to the dried, calcined and used gold-iron oxide samples. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) showed high homogeneity in the supported Au nanoparticles. The ex situ and in situ X-ray absorption fine structure (XAFS) characterization monitored the electronic and short-range local structure of active gold species. The synchrotron-based in situ X-ray diffraction (XRD), together with the corresponding temperature-programmed reduction by hydrogen (H<inf>2</inf>-TPR), indicated a structural evolution of the iron-oxide supports, correlating to their reducibility. An inverse order of catalytic activity between DP (Au/Fe-OH < Au/Fe-O) and CD (Au/Fe-OH > Au/Fe-O) was observed. Effective gold-support interaction results in a high activity for gold nanoparticles, locally generated by the sintering of dispersed Au atoms on the oxide support in the DP synthesis, while a hydroxylated surface favors the reactivity of externally introduced Au nanoparticles on Fe-OH support for the CD approach. This work reveals why differences in the synthetic protocol translate to differences in the catalytic performance of Au/FeO<inf>x</inf> catalysts with very similar structural characteristics in CO oxidation. © The Royal Society of Chemistry 2015.
    view abstractdoi: 10.1039/c4nr06967f
  • 2015 • 142 Elemental composition and radical formation potency of PM10 at an urban background station in Germany in relation to origin of air masses
    Hellack, B. and Quass, U. and Beuck, H. and Wick, G. and Kuttler, W. and Schins, R.P.F. and Kuhlbusch, T.A.J.
    Atmospheric Environment 105 1-6 (2015)
    At an urban background station in Mülheim-Styrum, North Rhine Westphalia, Germany, a set of 75 PM10 samples was collected over a one year period, followed by analyses for mass, chemical composition and hydroxyl radical (OH. ) formation potency. Additionally, the origin of air masses for the sampling days was calculated by 48-hbackward trajectories, subdivided into the four cardinal sectors. Significant lower PM10 mass concentrations were observed for summertime air masses from the west compared to the other seasons and cardinal sectors. For the OH. formation potency higher values were detected if air masses originate from east and south, thus predominantly being of continental origin. From the elevated OH. formation potencies in fall and winter a seasonal trend with low potencies in summers is assumed. Furthermore, source apportionment was performed by a positive matrix factor analysis, separating seven plausible factors which could be attributed to mineral dust, secondary nitrate, industry, non-exhaust traffic, fossil fuel combustion, marine aerosol and secondary aerosol factors. The intrinsic OH. formation potency was found to be associated mainly with the fossil fuel combustion factor (45%) and industry factor (22%). © 2015 Elsevier Ltd.
    view abstractdoi: 10.1016/j.atmosenv.2015.01.033
  • 2015 • 141 Metal-to-Insulator Transition in Au Chains on Si(111)-5×2-Au by Band Filling: Infrared Plasmonic Signal and Ab Initio Band Structure Calculation
    Hötzel, F. and Seino, K. and Chandola, S. and Speiser, E. and Esser, N. and Bechstedt, F. and Pucci, A.
    Journal of Physical Chemistry Letters 6 3615-3620 (2015)
    The Si(111)-5×2-Au surface is increasingly of interest because it is one of the rare atomic chain systems with quasi-one-dimensional properties. For the deposition of 0.7 monolayers of Au, these chains are metallic. Upon the evaporation of an additional submonolayer amount of gold, the surface becomes insulating but keeps the 5×2 symmetry. This metal-to-insulator transition was in situ monitored based on the infrared plasmonic signal change with coverage. The phase transition is theoretically explained by total-energy and band-structure calculations. Accordingly, it can be understood in terms of the occupation of the originally half-filled one-dimensional band at the Fermi level. By annealing the system, the additional gold is removed from the surface and the plasmonic signal is recovered, which underlines the stability of the metallic structure. So, recent results on the infrared plasmonic signals of the Si(111)-5 × 2-Au surface are supported. The understanding of potential one-dimensional electrical interconnects is improved. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpclett.5b01530
  • 2015 • 140 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 15064-15068 (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 abstractdoi: 10.1002/anie.201506672
  • 2015 • 139 Antibacterial activity of microstructured Ag/Au sacrificial anode thin films
    Köller, M. and Sengstock, C. and Motemani, Y. and Khare, C. and Buenconsejo, P.J.S. and Geukes, J. and Schildhauer, T.A. and Ludwig, Al.
    Materials Science and Engineering C 46 276-280 (2015)
    Ten different Ag dot arrays (16 to 625 microstructured dots per square mm) were fabricated on a continuous Au thin film and for comparison also on Ti film by sputter deposition and photolithographic patterning. To analyze the antibacterial activity of these microstructured films Escherichia coli and Staphylococcus aureus were placed onto the array surfaces and cultivated overnight. To analyze the viability of planktonic as well as surface adherent bacteria, the applied bacterial fluid was subsequently aspirated, plated on blood agar plates and adherent bacteria were detected by fluorescence microscopy. A particular antibacterial effect towards both bacterial strains was induced by Ag dot arrays on fabricated Au thin film (sacrificial anode system for Ag), due to the release of Ag ions from dissolution of Ag dots in contrast to Ag dot arrays fabricated on the Ti thin films (non-sacrificial anode system for Ag) which remained intact to the original dot shape. The required number of Ag dots on gold film to achieve complete bactericidal effects for both bacterial strains was seven times lower than that observed with Ag dot arrays on Ti film. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.msec.2014.10.058
  • 2015 • 138 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 8176-8183 (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 abstractdoi: 10.1021/acs.langmuir.5b01155
  • 2015 • 137 Ultrafast non-local spin dynamics in metallic bilayers by linear and non-linear magneto-optics
    Melnikov, A. and Alekhin, A. and Bürstel, D. and Diesing, D. and Wehling, T.O. and Rungger, I. and Stamenova, M. and Sanvito, S. and Bovensiepen, U.
    Springer Proceedings in Physics 159 34-36 (2015)
    We make a step towards the understanding of spin dynamics induced by spin-polarized hot carriers in metals. Exciting the Fe layer of Au/Fe/MgO(001) structures with femtosecond laser pulses, we demonstrate the ultrafast spin transport from Fe into Au using time-resolved MOKE and mSHG for depthsensitive detection of the transient magnetization. © Springer International Publishing Switzerland 2015.
    view abstractdoi: 10.1007/978-3-319-07743-7_12
  • 2015 • 136 Cysteine-containing oligopeptide β-sheets as redispersants for agglomerated metal nanoparticles
    Mizutaru, T. and Sakuraba, T. and Nakayama, T. and Marzun, G. and Wagener, P. and Rehbock, C. and Barcikowski, S. and Murakami, K. and Fujita, J. and Ishii, N. and Yamamoto, Y.
    Journal of Materials Chemistry A 3 17612-17619 (2015)
    Oligopeptide β-sheets comprising a fluorenyl methoxy carbonyl (Fmoc) group on its N-terminus and five amino acid residues of cysteine, lysine and valine displays redispersive properties with respect to agglomerated metal nanoparticles (MNPs, M = Au, Cu, Pt and Pd). The ligand-free MNPs prepared by a laser ablation technique in liquid maintain a high dispersion state due to the inherent surface charges delivered by anionic species present in solution, but may agglomerate after the preparation depending on concentration or salinity. We show how the agglomerated MNPs can be returned to the dispersed state by adding the Fmoc-oligopeptide β-sheets in methanol, as characterized by photoabsorption spectroscopy and transmission electron microscopy. Systematic studies in which we vary the concentration, the amino acid sequences and the secondary structures of a series of the oligopeptides clarify that the β-sheet structure is essential for the redispersion of the MNPs, where metal-binding thiol groups are integrated on one side and positively charged amino groups are located on the other side of the β-sheet. A possible mechanism for the redispersion may be that the agglomerated MNPs are subsequently enwrapped by the flexible β-sheets and gradually separated due to the reconstruction of peptide β-sheets under the assembly/disassembly equilibrium. © The Royal Society of Chemistry 2015.
    view abstractdoi: 10.1039/c5ta02098k
  • 2015 • 135 Inhibition of interfacial oxidative degradation during SiOx plasma polymer barrier film deposition on model organic substrates
    Ozkaya, B. and Mitschker, F. and Ozcan, O. and Awakowicz, P. and Grundmeier, G.
    Plasma Processes and Polymers 12 392-397 (2015)
    Interfacial processes during the initial stages of SiO<inf>x</inf>-like plasma-polymer barrier coating deposition were investigated by means of polarization modulation infrared reflection-absorption spectroscopy, and the resulting effect on defect densities were studied by cyclic voltammetry. Octadecanethiol self-assembled monolayers on Au-film coated wafers served as sensor layers to investigate interface chemistry during the plasma deposition. Both the spectroscopic and electrochemical data revealed that a thin SiOCH interlayer could reduce oxidative degradation of the SAM during subsequent deposition of the SiO<inf>x</inf> barrier film from an oxygen-rich plasma phase. The present electrochemical investigation confirmed effective inhibition of interfacial oxidative degradation processes of an aliphatic polymer in the presence of a SiOCH interfacial layer. Interfacial processes during the initial stages of SiO<inf>x</inf>-like plasma barrier-coating deposition are investigated. Self-assembled monolayers on Au-film coated wafers serve as sensor layers. SiOCH-type organic interlayers are found to oxidize and reduce surface degradation processes upon subsequent barrier film deposition from oxygen-rich gas mixture. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/ppap.201400105
  • 2015 • 134 The fate of nano-silver in aqueous media
    Plowman, B.J. and Tschulik, K. and Walport, E. and Young, N.P. and Compton, R.G.
    Nanoscale 7 12361-12364 (2015)
    Silver nanoparticles offer highly attractive properties for many applications, however concern has been raised over the possible toxicity of this material in environmental systems. While it is thought that the release of Ag+ can play a crucial role in this toxicity, the mechanism by which the oxidative dissolution of nano-silver occurs is not yet understood. Here we address this through the electrochemical analysis of gold-core silver-shell nanoparticles in various solutions. This novel method allows the direct quantification of silver dissolution by normalisation to the gold core signal. This is shown to be highly effective at discriminating between silver dissolution and the loss of nanoparticles from the electrode surface. We evidence through this rigorous approach that the reduction of O<inf>2</inf> drives the dissolution of nano-silver, while in the presence of Cl- this dissolution is greatly inhibited. This work is extended to the single nanoparticle level using nano-impact experiments. © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c5nr02995c
  • 2015 • 133 Nanostructure of wet-chemically prepared, polymer-stabilized silver-gold nanoalloys (6 nm) over the entire composition range
    Ristig, S. and Prymak, O. and Loza, K. and Gocyla, M. and Meyer-Zaika, W. and Heggen, M. and Raabe, D. and Epple, M.
    Journal of Materials Chemistry B 3 4654-4662 (2015)
    Bimetallic silver-gold nanoparticles were prepared by co-reduction using citrate and tannic acid in aqueous solution and colloidally stabilized with poly(N-vinylpyrrolidone) (PVP). The full composition range of silver:gold from 0:100 to 100:0 (n:n) was prepared with steps of 10 mol%. The nanoparticles were spherical, monodispersed, and had a diameter of ∼6 nm, except for Ag:Au 90:10 nanoparticles and pure Ag nanoparticles which were slightly larger. The size of the nanoalloys was determined by differential centrifugal sedimentation (DCS) and transmission electron microscopy (TEM). By means of X-ray powder diffraction (XRD) together with Rietveld refinement, precise lattice parameters, crystallite size and microstrain were determined. Scanning transmission electron microscopy (STEM) combined with energy-dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS) showed that the particles consisted of a gold-rich core and a silver-rich shell. XRD and DCS indicated that the nanoparticles were not twinned, except for pure Ag and Ag:Au 90:10, although different domains were visible in the TEM. A remarkable negative deviation from Vegard's linear rule of alloy mixtures was observed (isotropic contraction of the cubic unit cell with a minimum at a 50:50 composition). This effect was also found for Ag:Au bulk alloys, but it was much more pronounced for the nanoalloys. Notably, it was much less pronounced for pure silver and gold nanoparticles. The microstrain was increased along with the contraction of the unit cell with a broad maximum at a 50:50 composition. The synthesis is based on aqueous solvents and can be easily scaled up to a yield of several mg of a well dispersed nanoalloy with application potential due to its tuneable antibacterial action (silver) and its optical properties for bioimaging. © The Royal Society of Chemistry 2015.
    view abstractdoi: 10.1039/c5tb00644a
  • 2015 • 132 Synthesis, characterization and in vitro effects of 7 nm alloyed silver-gold nanoparticles
    Ristig, S. and Chernousova, S. and Meyer-Zaika, W. and Epple, M.
    Beilstein Journal of Nanotechnology 6 1212-1220 (2015)
    Alloyed silver-gold nanoparticles were prepared in nine different metal compositions with silver/gold molar ratios of ranging from 90:10 to 10:90. The one-pot synthesis in aqueous medium can easily be modified to gain control over the final particle diameter and the stabilizing agents. The purification of the particles to remove synthesis by-products (which is an important factor for subsequent in vitro experiments) was carried out by multiple ultracentrifugation steps. Characterization by transmission electron microscopy (TEM), differential centrifugal sedimentation (DCS), dynamic light scattering (DLS), UV-vis spectroscopy and atomic absorption spectroscopy (AAS) showed spherical, monodisperse, colloidally stable silver-gold nanoparticles of ≈7 nm diameter with measured molar metal compositions very close to the theoretical values. The examination of the nanoparticle cytotoxicity towards HeLa cells and human mesenchymal stem cells (hMSCs) showed that the toxicity is not proportional to the silver content. Nanoparticles with a silver/gold molar composition of 80:20 showed the highest toxicity. © 2015 Ristig et al.
    view abstractdoi: 10.3762/bjnano.6.124
  • 2015 • 131 Resonant photothermal laser processing of hybrid gold/titania nanoparticle films
    Schade, L. and Franzka, S. and Dzialkowski, K. and Hardt, S. and Wiggers, H. and Reichenberger, S. and Wagener, P. and Hartmann, N.
    Applied Surface Science 336 48-52 (2015)
    Photothermal processing of thin anatase TiO2 and hybrid Au/anatase TiO2 nanoparticle films on glass supports is investigated using continuous-wave microfocused lasers at λ = 355 nm and λ = 532 nm. UV/Vis spectroscopy, Raman spectroscopy, optical microscopy, atomic force microscopy and scanning electron microscopy are used for characterization. Processing of TiO2 nanoparticle films is feasible at λ = 355 nm only. In contrast, the addition of Au nanoparticles enhances the overall absorbance of the material in the visible range and enables processing at both wavelengths, i.e. at λ = 355 nm and λ = 532 nm. Generally, laser heating induces a transition from anatase to rutile. The modification degree increases with increasing laser power and laser irradiation time. Resonant laser processing of hybrid Au/TiO2-mesoporous films provide promising perspectives in various applications, e.g. in photovoltaics, where embedded nanoparticulate Au could be exploited to enhance light trapping. © 2014 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2014.09.118
  • 2015 • 130 Mesoporous Silica Supported Au and AuCu Nanoparticles for Surface Plasmon Driven Glycerol Oxidation
    Schünemann, S. and Dodekatos, G. and Tüysüz, H.
    Chemistry of Materials 27 7743-7750 (2015)
    Herein, we report for the first time the visible-light-assisted rate enhancement for glycerol oxidation using direct plasmonic photocatalysis. Au nanoparticles were loaded on various mesoporous SiO2 supports, and the catalytic performance was investigated with and without visible-light illumination. Monodispersed mesoporous silica spheres loaded with Au nanoparticles demonstrated a superior photoassisted catalytic rate enhancement compared to Au loaded ordered mesoporous silica (SBA-15, KIT-6, and MCM-41). The enhancement is attributed to the particle size of the Au nanoparticles and better light interaction resulting from the small SiO2 domains. Au loaded monodispersed mesoporous silica spheres exhibit a constant and remarkably small particle diameter of 2 nm at Au loadings of up to 15 wt % as a result of the support's small domain size and efficient pore confinement. The performance of the Au catalyst could be further improved by preparing bimetallic AuCu nanoparticles. Synergistic effects between Au and Cu improved the glycerol conversion by a factor of 2.5 and the dihydroxyacetone selectivity from 80% to 90% compared to monometallic Au catalysts. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.chemmater.5b03520
  • 2015 • 129 Influence of gold, silver and gold-silver alloy nanoparticles on germ cell function and embryo development
    Taylor, U. and Tiedemann, D. and Rehbock, C. and Kues, W.A. and Barcikowski, S. and Rath, D.
    Beilstein Journal of Nanotechnology 6 651-664 (2015)
    The use of engineered nanoparticles has risen exponentially over the last decade. Applications are manifold and include utilisation in industrial goods as well as medical and consumer products. Gold and silver nanoparticles play an important role in the current increase of nanoparticle usage. However, our understanding concerning possible side effects of this increased exposure to particles, which are frequently in the same size regime as medium sized biomolecules and accessorily possess highly active surfaces, is still incomplete. That particularly applies to reproductive aspects, were defects can be passed onto following generations. This review gives a brief overview of the most recent findings concerning reprotoxicological effects. The here presented data elucidate how composition, size and surface modification of nanoparticles influence viablility and functionality of reproduction relevant cells derived from various animal models. While in vitro cultured embryos displayed no toxic effects after the microinjection of gold and silver nanoparticles, sperm fertility parameters deteriorated after co-incubation with ligand free gold nanoparticles. However, the effect could be alleviated by bio-coating the nanoparticles, which even applies to silver and silver-rich alloy nanoparticles. The most sensitive test system appeared to be in vitro oocyte maturation showing a dose-dependent response towards protein (BSA) coated gold-silver alloy and silver nanoparticles leading up to complete arrest of maturation. Recent biodistribution studies confirmed that nanoparticles gain access to the ovaries and also penetrate the blood-testis and placental barrier. Thus, the design of nanoparticles with increased biosafety is highly relevant for biomedical applications. © 2015 Taylor et al.
    view abstractdoi: 10.3762/bjnano.6.66
  • 2015 • 128 Simultaneous Analysis of Hydrodynamic and Optical Properties Using Analytical Ultracentrifugation Equipped with Multiwavelength Detection
    Walter, J. and Sherwood, P.J. and Lin, W. and Segets, D. and Stafford, W.F. and Peukert, W.
    Analytical Chemistry 87 3396-3403 (2015)
    Analytical ultracentrifugation (AUC) has proven to be a powerful tool for the study of particle size distributions, particle shapes, and interactions with high accuracy and unrevealed resolution. In this work we show how the analysis of sedimentation velocity data from the AUC equipped with a multiwavelength detector (MWL) can be used to gain an even deeper understanding of colloidal and macromolecular mixtures. New data evaluation routines have been integrated in the software SEDANAL to allow for the handling of MWL data. This opens up a variety of new possibilities because spectroscopic information becomes available for individual components in mixtures at the same time using MWL-AUC. For systems of known optical properties information on the hydrodynamic properties of the individual components in a mixture becomes accessible. For the first time, the determination of individual extinction spectra of components in mixtures is demonstrated via MWL evaluation of sedimentation velocity data. In our paper we first provide the informational background for the data analysis and expose the accessible parameters of our methodology. We further demonstrate the data evaluation by means of simulated data. Finally, we give two examples which are highly relevant in the field of nanotechnology using colored silica and gold nanoparticles of different size and extinction properties. (Figure Presented). © 2015 American Chemical Society.
    view abstractdoi: 10.1021/ac504649c
  • 2015 • 127 Atomic layer-by-layer construction of Pd on nanoporous gold via underpotential deposition and displacement reaction
    Yan, X. and Xiong, H. and Bai, Q. and Frenzel, J. and Si, C. and Chen, X. and Eggeler, G. and Zhang, Z.
    RSC Advances 5 19409-19417 (2015)
    Atomic layer-by-layer construction of Pd on nanoporous gold (NPG) has been investigated through the combination of underpotential deposition (UPD) with displacement reaction. It has been found that the UPD of Cu on NPG is sensitive to the applied potential and the deposition time. The optimum deposition potential and time were determined through potential- and time-sensitive stripping experiments. The NPG-Pd electrode shows a different voltammetric behavior in comparison to the bare NPG electrode, and the deposition potential was determined through the integrated charge control for the monolayer UPD of Cu on the NPG-Pd electrode. Five layers of Pd were constructed on NPG through the layer-by-layer deposition. In addition, the microstructure of the NPG-Pdx (x = 1, 2, 3, 4 and 5) films was probed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). The microstructural observation demonstrates that the atomic layers of Pd form on the ligament surface of NPG through epitaxial growth, and have no effect on the nanoporous structure of NPG. In addition, the hydrogen storage properties of the NPG-Pdx electrodes have also been addressed. This journal is © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c4ra17014h
  • 2015 • 126 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 19-26 (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 abstractdoi: 10.1016/j.talanta.2015.04.041
  • 2014 • 125 Structural and electronic properties of micellar Au nanoparticles: Size and ligand effects
    Behafarid, F. and Matos, J. and Hong, S. and Zhang, L. and Rahman, T.S. and Roldan Cuenya, B.
    ACS Nano 8 6671-6681 (2014)
    Gaining experimental insight into the intrinsic properties of nanoparticles (NPs) represents a scientific challenge due to the difficulty of deconvoluting these properties from various environmental effects such as the presence of adsorbates or a support. A synergistic combination of experimental and theoretical tools, including X-ray absorption fine-structure spectroscopy, scanning transmission electron microscopy, atomic force microscopy, and density functional theory was used in this study to investigate the structure and electronic properties of small (∼1-4 nm) Au NPs synthesized by an inverse micelle encapsulation method. Metallic Au NPs encapsulated by polystyrene 2-vinylpiridine (PS-P2VP) were studied in the solution phase (dispersed in toluene) as well as after deposition on γ-Al2O3. Our experimental data revealed a size-dependent contraction of the interatomic distances of the ligand-protected NPs with decreasing NP size. These findings are in good agreement with the results from DFT calculations of unsupported Au NPs surrounded by P2VP, as well as those obtained for pure (ligand-free) Au clusters of analogous sizes. A comparison of the experimental and theoretical results supports the conclusion that the P2VP ligands employed to stabilize the gold NPs do not lead to strong distortions in the average interatomic spacing. The changes in the electronic structure of the Au-P2VP NPs were found to originate mainly from finite size effects and not from charge transfer between the NPs and their environment (e.g., Au-ligand interactions). In addition, the isolated ligand-protected experimental NPs only display a weak interaction with the support, making them an ideal model system for the investigation of size-dependent physical and chemical properties of structurally well-defined nanomaterials. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/nn406568b
  • 2014 • 124 Composition-structure-function diagrams of Ti-Ni-Au thin film shape memory alloys
    Buenconsejo, P.J.S. and Ludwig, Al.
    ACS Combinatorial Science 16 678-685 (2014)
    Ti-Ni-Au thin film materials libraries were prepared from multilayer precursors by combinatorial sputtering. The materials libraries were annealed at 500, 600, and 700 °C for 1 h and then characterized by high-throughput methods to investigate the relations between composition, structure and functional properties. The identified relations were visualized in functional phase diagrams. The goal is to identify composition regions that are suitable as high temperature shape memory alloys. Phase transforming compositions were identified by electrical resistance measured during thermal cycles in the range of -20 and 250 °C. Three phase transformation paths were confirmed: (1) B2-R, (2) B2-R-B19', and (3) B2-B19. For the materials library annealed at 500 °C only the B2-R transformation was observed. For the materials libraries annealed at 600 and 700 °C, all transformation paths were observed. High transformation temperatures (Ms ≈100 °C) were only obtained by annealing at 600 or 700 °C, and with compositions of Ti ≈ 50 at. % and Au &gt; 20 at. %. This is the composition range that undergoes B2-B19 transformation. The phase transformation behaviors were explained according to the compositional and annealing temperature dependence of phase/structure formation, as revealed by X-ray diffraction analysis of the materials libraries. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/co5000745
  • 2014 • 123 Carbon-based yolk-shell materials for fuel cell applications
    Galeano, C. and Baldizzone, C. and Bongard, H. and Spliethoff, B. and Weidenthaler, C. and Meier, J.C. and Mayrhofer, K.J.J. and Schüth, F.
    Advanced Functional Materials 24 220-232 (2014)
    The synthesis of yolk-shell catalysts, consisting of platinum or gold-platinum cores and graphitic carbon shells, and their electrocatalytic stabilities are described. Different encapsulation pathways for the metal nanoparticles are explored and optimized. Electrochemical studies of the optimized AuPt, @C catalyst revealed a high stability of the encapsulated metal particles. However, in order to reach full activity, several thousand potential cycles are required. After the electrochemical surface area is fully developed, the catalysts show exceptionally high stability, with almost no degradation over approximately 30 000 potential cycles between 0.4 and 1.4 VRHE. Encapsulation of noble metals in graphitic hollow shells by hard templating is explored as a means for stabilizing fuel cell catalysts. Small platinum particles can be encapsulated, but the achievable loading is too small. Encapsulation of Au-Pt yolk-shell particles allows higher loading, and with such cores, stable catalysts could be produced. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adfm.201302239
  • 2014 • 122 Charge balancing of model gold-nanoparticle-peptide conjugates controlled by the peptide's net charge and the ligand to nanoparticle ratio
    Gamrad, L. and Rehbock, C. and Krawinkel, J. and Tumursukh, B. and Heisterkamp, A. and Barcikowski, S.
    Journal of Physical Chemistry C 118 10302-10313 (2014)
    Gold nanoparticles (AuNPs) covalently bound to biomolecules, termed bioconjugates,1 are highly relevant for biological applications like drug targeting or bioimaging. Here, the net charge of the bioconjugate is one key parameter affecting biocompatibility and cell membrane interaction. However, when negatively charged AuNPs are conjugated to positively charged biomolecules, resulting charge compensation compromises the stability of the conjugates. In this work, laser-generated negatively charged AuNPs exhibiting a bare surface were used as a model and separately conjugated to cell penetrating peptides (CPPs) carrying different positive net charges. Occurring charge compensation leads to two regimes where stable bioconjugates are obtained on both sides of the bioconjugate's isoelectric point. These regimes can be controlled by the peptide's net charge. Generally, increasing the peptide's net charges yields stable positively charged bioconjugates with decreased surface coverages. To demonstrate the compatibility of the bioconjugates in bioapplications, long-term stability measurements were performed. Furthermore, the uptake by live mammalian cells was investigated with multiphoton microscopy using the luminescence of the AuNP-peptide conjugates. The results for our model system of laser-generated AuNPs and CPPs show that a precise tuning of conjugate properties is possible. They can be transferred to other oppositely charged nanoparticle-ligand systems, avoiding occurrence of charge compensation with defined ligand load. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/jp501489t
  • 2014 • 121 Alloying colloidal silver nanoparticles with gold disproportionally controls antibacterial and toxic effects
    Grade, S. and Eberhard, J. and Jakobi, J. and Winkel, A. and Stiesch, M. and Barcikowski, S.
    Gold Bulletin 47 83-93 (2014)
    Elemental silver nanoparticles are an effective antibacterial substance and are found as additive in various medical applications. Gold nanoparticles are used due to their optical properties in microscopy and cancer therapy. These advantages might be combined within alloyed nanoparticles of both elements and thereby open new fields of interest in research and medical treatment. In this context, laser ablation of solid alloys in liquid gives access to colloidal silver-gold alloy nanoparticles with a homogeneous ultrastructure. Elemental and alloy silver-gold nanoparticles with increasing molar fractions of silver (50, 80, and 100 %) were produced and stabilized with citrate or albumin (BSA). Particles were embedded in agar at concentrations of 3-100 μg cm-3 and tested on clinical relevant Staphylococcus aureus regarding their antibacterial properties. Cytotoxic effects were measured within the same particle concentration range using human gingival fibroblasts (HGFib). As expected, a reduced fraction of silver in the nanoalloys decreased the antibacterial effect on S. aureus according to the evaluated minimal inhibitory concentrations. However, this decrease turned out stronger than expected by its relative mass per particle, due to the electrochemical, disproportionally high effect of gold on the bioresponse to silver within silver-gold nanoalloy particles. BSA was able to stabilize all colloids and maintain antibacterial activity, whereas sodium citrate reduced antibacterial effects and cytotoxicity even at high nanoparticle concentrations. The alloying of silver with gold by laser ablation in liquid produced nanoparticles with both reduced antibacterial and cytotoxic properties in comparison to silver nanoparticles but still retains the application spectrum of both elements combined in one colloid. In particular, alloying with gold may render silver nanoparticles more biocompatible, and allows bioconjugation via established thiol chemistry. © 2013 The Author(s).
    view abstractdoi: 10.1007/s13404-013-0125-6
  • 2014 • 120 Low-temperature oxidation of carbon monoxide with gold(III) ions supported on titanium oxide
    Grünert, W. and Großmann, D. and Noei, H. and Pohl, M.-M. and Sinev, I. and De Toni, A. and Wang, Y. and Muhler, M.
    Angewandte Chemie - International Edition 53 3245-3249 (2014)
    Au/TiO2 catalysts prepared by a deposition-precipitation process and used for CO oxidation without previous calcination exhibited high, largely temperature-independent conversions at low temperatures, with apparent activation energies of about zero. Thermal treatments, such as He at 623 K, changed the conversion-temperature characteristics to the well-known S-shape, with activation energies slightly below 30 kJ mol-1. Sample characterization by XAFS and electron microscopy and a low-temperature IR study of CO adsorption and oxidation showed that CO can be oxidized by gas-phase O2 at 90 K already over the freeze-dried catalyst in the initial state that contained Au exclusively in the +3 oxidation state. CO conversion after activation in the feed at 303 K is due to AuIII-containing sites at low temperatures, while Au0 dominates conversion at higher temperatures. After thermal treatments, CO conversion in the whole investigated temperature range results from sites containing exclusively Au0. Ionic or metallic: Au3+ ions on TiO2 (see HAADF-STEM image of a freshly prepared sample) can catalyze the oxidation of CO at low temperatures. The reaction rates at Au3+-containing centers are similar to those found at metallic gold clusters. However, the apparent activation energies are very low, which is probably due to the opposing influence of the true activation energy and the adsorption enthalpy of CO on Au3+ centers. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201308206
  • 2014 • 119 How different characterization techniques elucidate the nature of the gold species in a polycrystalline Au/TiO2 catalyst
    Grünert, W. and Großmann, D. and Noei, H. and Pohl, M.-M. and Sinev, I. and De Toni, A. and Wang, Y. and Muhler, M.
    Chemie-Ingenieur-Technik 86 1883-1889 (2014)
    TiO2-supported gold species were prepared via the deposition-precipitation route, with conservation of the initial speciation by freeze-drying. The structural and electronic properties of the Au species were investigated by X-ray absorption spectroscopy, electron microscopy, and IR spectroscopy of adsorbed CO in four states. Exclusively AuIII was deposited on the TiO2 surface in patches ranging from isolated Au ions to three-dimensional clusters. This paper illustrates in detail the unique contributions of all characterization techniques to this structural model. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/cite.201400039
  • 2014 • 118 Using the first steps of hydration for the determination of molecular conformation of a single molecule
    Henzl, J. and Boom, K. and Morgenstern, K.
    Journal of the American Chemical Society 136 13341-13347 (2014)
    Determination of the exact structure of individual molecules is the ultimate goal of high-resolution microscopy. However, the resolution of scanning tunneling microscopy (STM) is intrinsically limited to the extent of molecular orbitals, which frequently do not differ for small changes in the molecular conformation. Here we use the position of water molecules during the first hydration steps of an azobenzene derivative on Au(111) to determine not only the orientation of the end groups with respect to the phenyl rings but also the orientation of the two phenyl rings with respect to the azo group. We investigate the co-adsorption of 4,4'-hydroxy-azobenzene and water molecules on Au(111) by low-temperature STM. The water molecules are attached exclusively to the hydroxyl end groups of the azobenzene derivatives. Predominantly the trans-azobenzene molecule with the two hydroxyl groups pointing into opposite directions is adsorbed. As corroborated by the attachment of a single water molecule to 4-anilino-4?-nitro azobenzene on the same inert surface, the method is generally applicable for structure determination of molecules with appropriate end groups. Our study thus gives unprecedented information about the intramolecular orientation based on the first real space observation of the hydration of a functional molecule. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/ja506762t
  • 2014 • 117 Dose-dependent surface endothelialization and biocompatibility of polyurethane noble metal nanocomposites
    Hess, C. and Schwenke, A. and Wagener, P. and Franzka, S. and Laszlo Sajti, C. and Pflaum, M. and Wiegmann, B. and Haverich, A. and Barcikowski, S.
    Journal of Biomedical Materials Research - Part A 102 1909-1920 (2014)
    Surface pre-endothelialization is a promising approach to improve the hemocompatibility of implants, medical devices, and artificial organs. To promote the adhesive property of thermoplastic polyurethane (TPU) for endothelial cells (ECs), up to 1 wt % of gold (Au) or platinum (Pt) nanoparticles, fabricated by pulsed laser ablation in polymer solution, were embedded into the polymer matrix. The analysis of these nanocomposites showed a homogenous dispersion of the nanoparticles, with average diameters of 7 nm for Au or 9 nm for Pt. A dose-dependent effect was found when ECs were seeded onto nanocomposites comprising different nanoparticle concentrations, resulting in a fivefold improvement of proliferation at 0.1 wt % nanoparticle load. This effect was associated with a nanoparticle concentration-dependent hydrophilicity and negative charge of the nanocomposite. In dynamic flow tests, nanocomposites containing 0.1 wt % Au or Pt nanoparticles allowed for the generation of a confluent and resistant EC layer. Real-time polymerase chain reaction quantification of specific markers for EC activation indicated that ECs cultivated on nanocomposites remain in an inactivated, nonthrombogenic and noninflammatory state; however, maintain the ability to trigger an inflammatory response upon stimulation. These findings were confirmed by a platelet and leukocyte adhesion assay. The results of this study suggest the possible applicability of TPU nanocomposites, containing 0.1 wt % Au or Pt nanoparticles, for the generation of pre-endothelialized surfaces of medical devices. © 2013 Wiley Periodicals, Inc.
    view abstractdoi: 10.1002/jbm.a.34860
  • 2014 • 116 High-throughput fabrication of Au-Cu nanoparticle libraries by combinatorial sputtering in ionic liquids
    König, D. and Richter, K. and Siegel, A. and Mudring, A.-V. and Ludwig, Al.
    Advanced Functional Materials 24 2049-2056 (2014)
    Materials libraries of binary alloy nanoparticles (NPs) are synthesized by combinatorial co-sputter deposition of Cu and Au into the ionic liquid (IL) 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C 1C4im][Tf2N]), which is contained in a micromachined cavity array substrate. The resulting NPs and NP-suspensions are investigated by transmission electron microscopy (TEM), X-ray diffraction (XRD), UV-Vis measurements (UV-Vis), and attenuated total reflection Fourier transformed infrared (ATR-FTIR) spectroscopy. Whereas the NPs can be directly observed in the IL using TEM, for XRD measurements the NP concentration is too low to lead to satisfactory results. Thus, a new NP isolation process involving capping agents is developed which enables separation of NPs from the IL without changing their size, morphology, composition, and state of aggregation. The results of the NP characterization show that next to the unary Cu and Au NPs, both stoichiometric and non-stoichiometric Cu-Au NPs smaller than 7 nm can be readily obtained. Whereas the size and shape of the alloy NPs change with alloy composition, for a fixed composition the NPs have a small size distribution. The measured lattice constants of all capped NPs show unexpected increased values, which could be related to the NP/surfactant interactions. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adfm.201303140
  • 2014 • 115 In vitro and in vivo interactions of selected nanoparticles with rodent serum proteins and their consequences in biokinetics
    Kreyling, W.G. and Fertsch-Gapp, S. and Schäffler, M. and Johnston, B.D. and Haberl, N. and Pfeiffer, C. and Diendorf, J. and Schleh, C. and Hirn, S. and Semmler-Behnke, M. and Epple, M. and Parak, W.J.
    Beilstein Journal of Nanotechnology 5 1699-1711 (2014)
    When particles incorporated within a mammalian organism come into contact with body fluids they will bind to soluble proteins or those within cellular membranes forming what is called a protein corona. This binding process is very complex and highly dynamic due to the plethora of proteins with different affinities and fractions in different body fluids and the large variation of compounds and structures of the particle surface. Interestingly, in the case of nanoparticles (NP) this protein corona is well suited to provide a guiding vehicle of translocation within body fluids and across membranes. This NP translocation may subsequently lead to accumulation in various organs and tissues and their respective cell types that are not expected to accumulate such tiny foreign bodies. Because of this unprecedented NP accumulation, potentially adverse biological responses in tissues and cells cannot be neglected a priori but require thorough investigations. Therefore, we studied the interactions and protein binding kinetics of blood serum proteins with a number of engineered NP as a function of their physicochemical properties. Here we show by in vitro incubation tests that the binding capacity of different engineered NP (polystyrene, elemental carbon) for selected serum proteins depends strongly on the NP size and the properties of engineered surface modifications. In the following attempt, we studied systematically the effect of the size (5, 15, 80 nm) of gold spheres (AuNP), surface-modified with the same ionic ligand; as well as 5 nm AuNP with five different surface modifications on the binding to serum proteins by using proteomics analyses. We found that the binding of numerous serum proteins depended strongly on the physicochemical properties of the AuNP. These in vitro results helped us substantially in the interpretation of our numerous in vivo biokinetics studies performed in rodents using the same NP. These had shown that not only the physicochemical properties determined the AuNP translocation from the organ of intake towards blood circulation and subsequent accumulation in secondary organs and tissues but also the the transport across organ membranes depended on the route of AuNP application. Our in vitro protein binding studies support the notion that the observed differences in in vivo biokinetics are mediated by the NP protein corona and its dynamical change during AuNP translocation in fluids and across membranes within the organism. © 2014 Kreyling et al.
    view abstractdoi: 10.3762/bjnano.5.180
  • 2014 • 114 Probing the electronic transport on the reconstructed Au/Ge(001) surface
    Krok, F. and Kaspers, M.R. and Bernhart, A.M. and Nikiel, M. and Jany, B.R. and Indyka, P. and Wojtaszek, M. and Möller, R. and Bobisch, C.A.
    Beilstein Journal of Nanotechnology 5 1463-1471 (2014)
    By using scanning tunnelling potentiometry we characterized the lateral variation of the electrochemical potential μec on the goldinduced Ge(001)-c(8 × 2)-Au surface reconstruction while a lateral current flows through the sample. On the reconstruction and across domain boundaries we find that μec shows a constant gradient as a function of the position between the contacts. In addition, nanoscale Au clusters on the surface do not show an electronic coupling to the gold-induced surface reconstruction. In combination with high resolution scanning electron microscopy and transmission electron microscopy, we conclude that an additional transport channel buried about 2 nm underneath the surface represents a major transport channel for electrons. © 2014 Krok et al.
    view abstractdoi: 10.3762/bjnano.5.159
  • 2014 • 113 Ligand-free gold atom clusters adsorbed on graphene nano sheets generated by oxidative laser fragmentation in water
    Lau, M. and Haxhiaj, I. and Wagener, P. and Intartaglia, R. and Brandi, F. and Nakamura, J. and Barcikowski, S.
    Chemical Physics Letters 610-611 256-260 (2014)
    Over three decades after the first synthesis of stabilized Au 55-clusters many scientific questions about gold cluster properties are still unsolved and ligand-free colloidal clusters are difficult to fabricate. Here we present a novel route to produce ultra-small gold particles by using a green technique, the laser ablation and fragmentation in water, without using reductive or stabilizing agents at any step of the synthesis. For fabrication only a pulsed laser, a gold-target, pure water, sodium hydroxide and hydrogen peroxide are deployed. The particles are exemplarily hybridized to graphene supports showing that these carbon-free colloidal clusters might serve as versatile building blocks. © 2014 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.cplett.2014.07.047
  • 2014 • 112 Near-field-enhanced, off-resonant laser sintering of semiconductor particles for additive manufacturing of dispersed Au-ZnO-micro/nano hybrid structures
    Lau, M. and Niemann, R.G. and Bartsch, M. and O'Neill, W. and Barcikowski, S.
    Applied Physics A: Materials Science and Processing 114 1023-1030 (2014)
    Off-resonant near-field enhancement by gold nanoparticles adsorbed on crystalline zinc oxide significantly increases the energy efficiency of infrared laser sintering. In detail, ten different gold mass loads on zinc oxide were exposed to 1,064 nm cw-laser radiation. Variation of scan speed, laser power, and spot size showed that the energy threshold required for sintering decreases and sintering process window widens compared to laser sintering of pure zinc oxide powder. Transmission electron microscope analysis after focused ion beam cross sectioning of the sintered particles reveals that supported gold nanoparticles homogenously resolidify in the sintered semiconductor matrix. The enhanced sintering process with ligand-free gold nanoparticles gives access to metal-semiconductor hybrid materials with potential application in light harvesting or energy conversion. © 2014 Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00339-014-8270-1
  • 2014 • 111 A critical evaluation of the interpretation of electrocatalytic nanoimpacts
    Ly, L.S.Y. and Batchelor-Mcauley, C. and Tschulik, K. and Kätelhön, E. and Compton, R.G.
    Journal of Physical Chemistry C 118 17756-17763 (2014)
    The kinetics of the proton reduction reaction is studied on a variety of gold surfaces including both macro (r0 = 1.0 mm) and micro (r 0 = 4.6 μm) electrodes, as well as gold nanoparticles (r NP = ∼10 nm). For the gold nanoparticles, two complementary methodologies of study are used. First the particles are investigated as part of an ensemble response in an array (k0 ∼ 7 × 10-8 m s-1). Second, the rate is recorded stochastically at individually impacting nanoparticles (k0 ∼2 × 10-9 m s -1). This apparent decrease in reaction rates on transitioning from the ensemble to individual nanoparticles is understood in terms of the differing connectivity of the nanoparticles to the electrode surface. During the course of the individual catalytic impacts, or "pulses", the recorded current is found to be highly variable; this variability is interpreted as originating from the nanoscopic motion of the particle above the electrode interface. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/jp504968j
  • 2014 • 110 Micro-patterning of self-assembled organic monolayers by using tunable ultrafast laser pulses
    Maragkaki, S. and Aumann, A. and Schulz, F. and Schröter, A. and Schöps, B. and Franzka, S. and Hartmann, N. and Ostendorf, A.
    Proceedings of SPIE - The International Society for Optical Engineering 8972 (2014)
    We study the application of tunable ultrafast laser pulses in micropatterning self- assembled organic monolayer (SAMs) employing non collinear optical parametric amplification (NOPA). SAMs are ultrathin organic monolayers, which can be used in a variety of ways to assemble functionalized surface structures. In our study, we investigate the characteristics of SAMs as monomolecular resists during etching of gold. NOPA is a versatile method which provides the generation of ultrafast laser pulses, with a tunable wavelength in the visible and near infrared range. Due to the noncollinear geometry, a broadened spectral range can be amplified. The NOPA delivers wavelengths in the range of 480 nm to 950 nm at laser pulse lengths in the sub- 30 femtosecond range using a prism compressor after the nonlinear conversion. The ultrashort laser technology together with the advantages of the NOPA system guarantee high precision and allows us to determine the optimum conditions of sub-wavelength patterning by studying the effects of the fluence and the wavelength. At the same time, single-pulse processing allows us to selectively remove the ultrathin organic coating, while it ensures short processing time. In our study we used thiol-based SAMs as ultrathin layers on gold-coated glass substrates with a film thickness of 1-2 nm and 40 nm respectively. © 2014 SPIE.
    view abstractdoi: 10.1117/12.2037716
  • 2014 • 109 In situ non-DLVO stabilization of surfactant-free, plasmonic gold nanoparticles: Effect of Hofmeister's anions
    Merk, V. and Rehbock, C. and Becker, F. and Hagemann, U. and Nienhaus, H. and Barcikowski, S.
    Langmuir 30 4213-4222 (2014)
    Specific ion effects ranking in the Hofmeister sequence are ubiquitous in biochemical, industrial, and atmospheric processes. In this experimental study specific ion effects inexplicable by the classical DLVO theory have been investigated at curved water-metal interfaces of gold nanoparticles synthesized by a laser ablation process in liquid in the absence of any organic stabilizers. Notably, ion-specific differences in colloidal stability occurred in the Hückel regime at extraordinarily low salinities below 50 μM, and indications of a direct influence of ion-specific effects on the nanoparticle formation process are found. UV-vis, zeta potential, and XPS measurements help to elucidate coagulation properties, electrokinetic potential, and the oxidation state of pristine gold nanoparticles. The results clearly demonstrate that stabilization of ligand-free gold nanoparticles scales proportionally with polarizability and antiproportionally with hydration of anions located at defined positions in a direct Hofmeister sequence of anions. These specific ion effects might be due to the adsorption of chaotropic anions (Br-, SCN-, or I-) at the gold/water interface, leading to repulsive interactions between the partially oxidized gold particles during the nanoparticle formation process. On the other hand, kosmotropic anions (F - or SO4 2-) seem to destabilize the gold colloid, whereas Cl- and NO3 - give rise to an intermediate stability. Quantification of surface charge density indicated that particle stabilization is dominated by ion adsorption and not by surface oxidation. Fundamental insights into specific ion effects on ligand-free aqueous gold nanoparticles beyond purely electrostatic interactions are of paramount importance in biomedical or catalytic applications, since colloidal stability appears to depend greatly on the type of salt rather than on the amount. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/la404556a
  • 2014 • 108 Detection of the magnetocrystalline anisotropy in x-ray magnetic linear dichroism reflection spectra across the Fe 3p and 2p edges
    Mertins, H.-C. and Legut, D. and Tesch, M. and Jansing, C. and Gilbert, M. and Gaupp, A. and Oppeneer, P.M. and Bürgler, D.E. and Schneider, C. M. and Berges, U.
    IEEE Transactions on Magnetics 50 (2014)
    The magnetocrystalline anisotropy of X-ray magnetic linear dichroism (XMLD) reflection spectra measured on single-crystalline bcc Fe films across the 3p and 2p edges are presented. The XMLD spectra were obtained from a series of reflection spectra by aligning the electric field vector of linearly polarized undulator radiation with respect to the crystal axes. Our results show the presence of a huge magnetocrystalline anisotropy in the XMLD reflection spectra. The XMLD signal is further investigated as a function of the Fe film thickness in Au/Fe/Ag/GaAs layered systems. Simulations of the reflection spectra reveal the influences of interference effects, which can enhance or diminish the XMLD signals. The measured spectra are in good agreement with ab initio calculated spectra. © 1965-2012 IEEE.
    view abstractdoi: 10.1109/TMAG.2014.2321632
  • 2014 • 107 Oxidation of bioethanol using zeolite-encapsulated gold nanoparticles
    Mielby, J. and Abildstrøm, J.O. and Wang, F. and Kasama, T. and Weidenthaler, C. and Kegnæs, S.
    Angewandte Chemie - International Edition 53 12513-12516 (2014)
    With the ongoing developments in biomass conversion, the oxidation of bioethanol to acetaldehyde may become a favorable and green alternative to the preparation from ethylene. Here, a simple and effective method to encapsulate gold nanoparticles in zeolite silicalite-1 is reported and their high activity and selectivity for the catalytic gas-phase oxidation of ethanol are demonstrated. The zeolites are modified by a recrystallization process, which creates intraparticle voids and mesopores that facilitate the formation of small and disperse nanoparticles upon simple impregnation. The individual zeolite crystals comprise a broad range of mesopores and contain up to several hundred gold nanoparticles with a diameter of 2-3 nm that are distributed inside the zeolites rather than on the outer surface. The encapsulated nanoparticles have good stability and result in 50%conversion of ethanol with 98% selectivity toward acetaldehyde at 200°C, which (under the given reaction conditions) corresponds to 606 mol acetaldehyde/mol Au hour-1. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA Weinheim.
    view abstractdoi: 10.1002/anie.201406354
  • 2014 • 106 Exceptional size-dependent activity enhancement in the electroreduction of CO2 over Au nanoparticles
    Mistry, H. and Reske, R. and Zeng, Z. and Zhao, Z.-J. and Greeley, J. and Strasser, P. and Cuenya, B.R.
    Journal of the American Chemical Society 136 16473-16476 (2014)
    The electrocatalytic reduction of CO2 to industrial chemicals and fuels is a promising pathway to sustainable electrical energy storage and to an artificial carbon cycle, but it is currently hindered by the low energy efficiency and low activity displayed by traditional electrode materials. We report here the size-dependent catalytic activity of micelle-synthesized Au nanoparticles (NPs) in the size range of ∼1-8 nm for the electroreduction of CO2 to CO in 0.1 M KHCO3. A drastic increase in current density was observed with decreasing NP size, along with a decrease in Faradaic selectivity toward CO. Density functional theory calculations showed that these trends are related to the increase in the number of low-coordinated sites on small NPs, which favor the evolution of H2 over CO2 reduction to CO. We show here that the H2/CO product ratio can be specifically tailored for different industrial processes by tuning the size of the catalyst particles. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/ja508879j
  • 2014 • 105 Magnesiothermic conversion of the silicamineralizing golden algae Mallomonas caudata and Synura petersenii to elemental silicon with high geometric precision
    Petrack, J. and Jost, S. and Boenigk, J. and Epple, M.
    Beilstein Journal of Nanotechnology 5 554-560 (2014)
    Chrysophyceae, also known as golden algae, contain characteristic, three-dimensional biomineralized silica structures. Their chemical composition and microscopic structure was studied. By high-temperature conversion of the skeleton of Mallomonas caudata and Synura petersenii into elementary silicon by magnesium vapour, nanostructured defined replicates were produced which were clearly seen after removal of the formed magnesium oxide with acid. © 2014 Petrack et al; licensee Beilstein-Institut.
    view abstractdoi: 10.3762/bjnano.5.65
  • 2014 • 104 Biochemical component identification by plasmonic improved whispering gallery mode optical resonance based sensor
    Saetchnikov, V.A. and Tcherniavskaia, E.A. and Saetchnikov, A.V. and Schweiger, G. and Ostendorf, A.
    Proceedings of SPIE - The International Society for Optical Engineering 9126 (2014)
    Experimental data on detection and identification of variety of biochemical agents, such as proteins, microelements, antibiotic of different generation etc. in both single and multi component solutions under varied in wide range concentration analyzed on the light scattering parameters of whispering gallery mode optical resonance based sensor are represented. Multiplexing on parameters and components has been realized using developed fluidic sensor cell with fixed in adhesive layer dielectric microspheres and data processing. Biochemical component identification has been performed by developed network analysis techniques. Developed approach is demonstrated to be applicable both for single agent and for multi component biochemical analysis. Novel technique based on optical resonance on microring structures, plasmon resonance and identification tools has been developed. To improve a sensitivity of microring structures microspheres fixed by adhesive had been treated previously by gold nanoparticle solution. Another technique used thin film gold layers deposited on the substrate below adhesive. Both biomolecule and nanoparticle injections caused considerable changes of optical resonance spectra. Plasmonic gold layers under optimized thickness also improve parameters of optical resonance spectra. Biochemical component identification has been also performed by developed network analysis techniques both for single and for multi component solution. So advantages of plasmon enhancing optical microcavity resonance with multiparameter identification tools is used for development of a new platform for ultra sensitive label-free biomedical sensor. © 2014 SPIE.
    view abstractdoi: 10.1117/12.2051486
  • 2014 • 103 Plasmonic improvement of microcavity biomedical sensor spectroscopic characteristics
    Saetchnikov, V.A. and Tcherniavskaia, E.A. and Saetchnikov, A.V. and Schweiger, G. and Ostendorf, A. and Ghadiri, R.
    Progress in Biomedical Optics and Imaging - Proceedings of SPIE 8957 (2014)
    New opportunity to improve a sensetivity of a label-free biomolecule detection in sensing systems based on microcavity evanescent wave optical sensors has been recently found and is being under intensive development. Novel technique based on combination of optical resonance on microring structures with plasmon resonance. Recently developed tools based on neural network data processing can realize real-Time identification of biological agents. So combining advantages of plasmon enhancing optical microcavity resonance with identification tools can give a new platform for ulta sensitive label-free biomedical sensor. Our developed technique used standard glass and polymer microspheres as sensetive elements. They are fixed in the solution flow by adhesive layer on the surface being in the field of evanescence wave. Sensitive layer have been treated by gold nanoparticel (GN) solution. Another technique used thin film gold layers deposited on the substrate below adhesive. The light from a tuneable diode laser is coupled into the microsphere through a prism and was sharply focussed on the single microsphere. Images were recorded by CMOS camera. Normalized by free spectral range resonance shift of whispering gallery mode (WGM) and a relative efficiency of their excitation were used as input data for biomolecule classification. Both biomolecules and NP injection was obtained caused WGM spectra modification. But after NP treatment spectral shift and intensity of WGM resonances in biomolecule solutions increased. WGM resonances in microspheres fixed on substrate with gold layer with optimized layer thickness in biomolecule solutions also had higher intensity and spectra modification then without gold layer. © 2014 SPIE.
    view abstractdoi: 10.1117/12.2039049
  • 2014 • 102 Two-color SERS microscopy for protein co-localization in prostate tissue with primary antibody-protein A/G-gold nanocluster conjugates
    Salehi, M. and Schneider, L. and Ströbel, P. and Marx, A. and Packeisen, J. and Schlücker, S.
    Nanoscale 6 2361-2367 (2014)
    SERS microscopy is a novel staining technique in immunohistochemistry, which is based on antibodies labeled with functionalized noble metal colloids called SERS labels or nanotags for optical detection. Conventional covalent bioconjugation of these SERS labels cannot prevent blocking of the antigen recognition sites of the antibody. We present a rational chemical design for SERS label-antibody conjugates which addresses this issue. Highly sensitive, silica-coated gold nanoparticle clusters as SERS labels are non-covalently conjugated to primary antibodies by using the chimeric protein A/G, which selectively recognizes the Fc part of antibodies and therefore prevents blocking of the antigen recognition sites. In proof-of-concept two-color imaging experiments for the co-localization of p63 and PSA on non-neoplastic prostate tissue FFPE specimens, we demonstrate the specificity and signal brightness of these rationally designed primary antibody-protein A/G-gold nanocluster conjugates. © 2014 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c3nr05890e
  • 2014 • 101 Control of molecular orientation and morphology in organic bilayer solar cells: Copper phthalocyanine on gold nanodots
    Sasaki, T. and Tabata, K. and Tsukagoshi, K. and Beckel, A. and Lorke, A. and Yamamoto, Y.
    Thin Solid Films 562 467-470 (2014)
    Molecular orientation, morphology of donor (D)/acceptor (A) interface and photoabsorptivity in organic bilayer solar cells were controlled using Au nanodots with an ∼ 20 nm diameter inserted between the bottom electrode and the organic layer. Copper phthalocyanine (CuPc) molecules deposited onto the Au nanodot-coated electrode were mostly oriented face-on with large surface roughness, which is beneficial for photoabsorption, charge separation and transport. Furthermore, Au nanodots exhibit blue-shifted plasmon bands so that CuPc absorbs light more efficiently than that on thin Au layer. Bilayer C 60/CuPc solar cells containing Au nanodots exhibited 1.4 times higher photoelectric conversion efficiency than those without Au nanodots. Factors for the enhanced efficiency are (i) improvement of the optical absorption characteristics by face-on orientation of CuPc and (ii) increase of the D/A heterointerface area. In addition, the shift of the plasmon absorption band of Au by the formation of nanodots makes absorption of the CuPc layer much more efficiently, resulting in better photovoltaic output. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2014.03.090
  • 2014 • 100 Photothermal laser microsintering of nanoporous gold
    Schade, L. and Franzka, S. and Mathieu, M. and Biener, M.M. and Biener, J. and Hartmann, N.
    Langmuir 30 7190-7197 (2014)
    Photothermal processing of nanoporous gold using a microfocused continuous-wave laser at a wavelength of 532 nm and a 1/e2 spot diameter of 2.9 μm has been studied. In addition, complementary experiments have been carried out via conventional annealing. Scanning electron microscopy has been used for characterization. Local laser irradiation at distinct laser powers and pulse lengths results in coarsening of the porous gold structures. During laser processing the pore size of the native nanoporous gold increases to maximum values in the range of 0.25-3 μm. The affected areas exhibit lateral dimensions in the range of 2-10 μm. Overall two regions are distinguished. An inner region, where large pores and ligaments are formed and an outer region, where the pore size and ligament size gradually change and approach the feature sizes of the native material. A qualitative thermokinetic model allows one to reproduce the experimentally observed dependence of the laser-induced morphologies on the laser parameters. On the basis of this model the underlying processes are attributed to sintering and melting of the gold structures. The presented results demonstrate the prospects of photothermal laser processing in engineering porous gold with spatially varying porosities on micrometer to nanometer length scales. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/la5011192
  • 2014 • 99 Gold electrodes from recordable CDs for the sensitive, semi-quantitative detection of commercial silver nanoparticles in seawater media
    Stuart, E.J.E. and Tschulik, K. and Lowinsohn, D. and Cullen, J.T. and Compton, R.G.
    Sensors and Actuators, B: Chemical 195 223-229 (2014)
    We report the use of homemade disposable gold electrodes fabricated from commercial recordable CDs for the detection and quantification of silver nanoparticles from a consumer product in a seawater sample. The "CDtrode" is immersed in a seawater sample containing silver nanoparticles for a certain amount of time during which the silver nanoparticles adsorb onto the CDtrode surface under open circuit conditions. The CDtrode is then transferred to an aqueous electrolyte and oxidative stripping is used to determine the amount of silver nanoparticles that have become stuck to the electrode surface. Depending on immersion time and silver nanoparticle concentration, up to a full monolayer coverage of silver nanoparticles on the CDtrode surface has been achieved. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.snb.2014.01.040
  • 2014 • 98 Gold nanoparticles interfere with sperm functionality by membrane adsorption without penetration
    Taylor, U. and Barchanski, A. and Petersen, S. and Kues, W.A. and Baulain, U. and Gamrad, L. and Sajti, L. and Barcikowski, S. and Rath, D.
    Nanotoxicology 8 118-127 (2014)
    To examine gold nanoparticle reprotoxicity, bovine spermatozoa were challenged with ligand-free or oligonucleotide-conjugated gold nanoparticles synthesized purely without any surfactants by laser ablation. Sperm motility declined at nanoparticle mass dose of 10 μg/ml (corresponding to ∼14 000 nanoparticles per sperm cell) regardless of surface modification. Sperm morphology and viability remained unimpaired at all concentrations. Transmission electron microscopy showed an modification dependant attachment of nanoparticles to the cell membrane of spermatozoa, but provided no evidence for nanoparticle entrance into sperm cells. A molecular examination revealed a reduction of free thiol residues on the cell membrane after nanoparticle exposure, which could explain the decrease in sperm motility. Sperm fertilising ability decreased after exposure to 10 μg/ml of ligand-free nanoparticles indicating that agglomerated ligand-free nanoparticles interfere with membrane properties necessary for fertilisation. In conclusion, nanoparticles may impair key sperm functions solely by interacting with the sperm surface membrane. © 2014 Informa UK Ltd. All rights reserved.
    view abstractdoi: 10.3109/17435390.2013.859321
  • 2014 • 97 Injection of ligand-free gold and silver nanoparticles into murine embryos does not impact pre-implantation development
    Taylor, U. and Garrels, W. and Barchanski, A. and Peterson, S. and Sajti, L. and Lucas-Hahn, A. and Gamrad, L. and Baulain, U. and Klein, S. and Kues, W.A. and Barcikowski, S. and Rath, D.
    Beilstein Journal of Nanotechnology 5 677-688 (2014)
    Intended exposure to gold and silver nanoparticles has increased exponentially over the last decade and will continue to rise due to their use in biomedical applications. In particular, reprotoxicological aspects of these particles still need to be addressed so that the potential impacts of this development on human health can be reliably estimated. Therefore, in this study the toxicity of gold and silver nanoparticles on mammalian preimplantation development was assessed by injecting nanoparticles into one blastomere of murine 2 cell-embryos, while the sister blastomere served as an internal control. After treatment, embryos were cultured and embryo development up to the blastocyst stage was assessed. Development rates did not differ between microinjected and control groups (gold nanoparticles: 67.3%, silver nanoparticles: 61.5%, sham: 66.2%, handling control: 79.4%). Real-time PCR analysis of six developmentally important genes (BAX, BCL2L2, TP53, OCT4, NANOG, DNMT3A) did not reveal an influence on gene expression in blastocysts. Contrary to silver nanoparticles, exposure to comparable Ag+-ion concentrations resulted in an immediate arrest of embryo development. In conclusion, the results do not indicate any detrimental effect of colloidal gold or silver nanoparticles on the development of murine embryos. © 2014 Taylor et al; licensee Beilstein-Institut.
    view abstractdoi: 10.3762/bjnano.5.80
  • 2014 • 96 Rational design of gold nanoparticle toxicology assays: A question of exposure scenario, dose and experimental setup
    Taylor, U. and Rehbock, C. and Streich, C. and Rath, D. and Barcikowski, S.
    Nanomedicine 9 1971-1989 (2014)
    Many studies have evaluated the toxicity of gold nanoparticles, although reliable predictions based on these results are rare. In order to overcome this problem, this article highlights strategies to improve comparability and standardization of nanotoxicological studies. To this end, it is proposed that we should adapt the nanomaterial to the addressed exposure scenario, using ligand-free nanoparticle references in order to differentiate ligand effects from size effects. Furthermore, surface-weighted particle dosing referenced to the biologically relevant parameter (e.g., cell number or organ mass) is proposed as the gold standard. In addition, it is recommended that we should shift the focus of toxicological experiments from 'live-dead' assays to the assessment of cell function, as this strategy allows observation of bioresponses at lower doses that are more relevant for in vivo scenarios. © 2014 Future Medicine Ltd.
    view abstractdoi: 10.2217/nnm.14.139
  • 2014 • 95 Reprotoxicity of gold, silver, and gold-silver alloy nanoparticles on mammalian gametes
    Tiedemann, D. and Taylor, U. and Rehbock, C. and Jakobi, J. and Klein, S. and Kues, W.A. and Barcikowski, S. and Rath, D.
    Analyst 139 931-942 (2014)
    Metal and alloy nanoparticles are increasingly developed for biomedical applications, while a firm understanding of their biocompatibility is still missing. Various properties have been reported to influence the toxic potential of nanoparticles. This study aimed to assess the impact of nanoparticle size, surface ligands and chemical composition of gold, silver or gold-silver alloy nanoparticles on mammalian gametes. An in vitro assay for porcine gametes was developed, since these are delicate primary cells, for which well-established culture systems exist and functional parameters are defined. During coincubation with oocytes for 46 h neither any of the tested gold nanoparticles nor the gold-silver alloy particles with a silver molar fraction of up to 50% showed any impact on oocyte maturation. Alloy nanoparticles with 80% silver molar fraction and pure silver nanoparticles inhibited cumulus-oocyte maturation. Confocal microscopy revealed a selective uptake of gold nanoparticles by oocytes, while silver and alloy particles mainly accumulated in the cumulus cell layer surrounding the oocyte. Interestingly sperm vitality parameters (motility, membrane integrity and morphology) were not affected by any of the tested nanoparticles. Only sporadic association of nanoparticles with the sperm plasma membrane was found by transmission electron microscopy. In conclusion, mammalian oocytes were sensitive to silver containing nanoparticles. Likely, the delicate process of completing meiosis in maternal gametes features high vulnerability towards nanomaterial derived toxicity. The results imply that released Ag +-ions are responsible for the observed toxicity, but the compounding into an alloy seemed to alleviate the toxic effects to a certain extent. This journal is © The Royal Society of Chemistry 2014.
    view abstractdoi: 10.1039/c3an01463k
  • 2014 • 94 Femtogram detection of cytokines in a direct dot-blot assay using SERS microspectroscopy and hydrophilically stabilized Au-Ag nanoshells
    Wang, Y. and Salehi, M. and Schütz, M. and Schlücker, S.
    Chemical Communications 50 2711-2714 (2014)
    Rapid parallel detection of two cytokines (IL-6 and IL-8) with femtogram sensitivity in a simple direct dot-blot assay is demonstrated. The microspectroscopic SERS acquisition scheme employs rationally designed, hydrophilically stabilized Au-Ag nanoshells as SERS labels, which are optimized for signal enhancement upon red laser excitation. © 2014 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c3cc48633h
  • 2014 • 93 Evaluation of pulsed laser ablation in liquids generated gold nanoparticles as novel transfection tools: Efficiency and cytotoxicity
    Willenbrock, S. and Durán, M.C. and Barchanski, A. and Barcikowski, S. and Feige, K. and Nolte, I. and Murua Escobar, H.
    Proceedings of SPIE - The International Society for Optical Engineering 8972 (2014)
    Varying transfection efficiencies and cytotoxicity are crucial aspects in cell manipulation. The utilization of gold nanoparticles (AuNP) has lately attracted special interest to enhance transfection efficiency. Conventional AuNP are usually generated by chemical reactions or gas pyrolysis requiring often cell-toxic stabilizers or coatings to conserve their characteristics. Alternatively, stabilizer- and coating-free, highly pure, colloidal AuNP can be generated by pulsed laser ablation in liquids (PLAL). Mammalian cells were transfected efficiently by addition of PLAL-AuNP, but data systematically evaluating the cell-toxic potential are lacking. Herein, the transfection efficiency and cytotoxicity of PLAL AuNP was evaluated by transfection of a mammalian cell line with a recombinant HMGB1/GFP DNA expression vector. Different methods were compared using two sizes of PLAL-AuNP, commercialized AuNP, two magnetic NP-based protocols and a conventional transfection reagent (FuGENE HD; FHD). PLAL-AuNP were generated using a Spitfire Pro femtosecond laser system delivering 120 fs laser pulses at a wavelength of 800 nm focusing the fs-laser beam on a 99.99% pure gold target placed in ddH2O. Transfection efficiencies were analyzed after 24h using fluorescence microscopy and flow cytometry. Toxicity was assessed measuring cell proliferation and percentage of necrotic, propidium iodide positive cells (PI %). The addition of PLAL-AuNP significantly enhanced transfection efficiencies (FHD: 31 %; PLAL-AuNP size-1: 46 %; size-2: 50 %) with increased PI% but no reduced cell proliferation. Commercial AuNP-transfection showed significantly lower efficiency (23 %), slightly increased PI % and reduced cell proliferation. Magnetic NP based methods were less effective but showing also lowest cytotoxicity. In conclusion, addition of PLAL-AuNP provides a novel tool for transfection efficiency enhancement with acceptable cytotoxic side-effects. © 2014 SPIE.
    view abstractdoi: 10.1117/12.2038453
  • 2014 • 92 Gold and silver nanoparticle monomers are non-SERS-active: A negative experimental study with silica-encapsulated Raman-reporter-coated metal colloids
    Zhang, Y. and Walkenfort, B. and Yoon, J.H. and Schlücker, S. and Xie, W.
    Physical Chemistry Chemical Physics 17 21120-21126 (2014)
    Noble metal nanoparticles (NPs) are the most commonly employed plasmonic substrates in surface-enhanced Raman scattering (SERS) experiments. Computer simulations show that monomers of Ag and Au nanocrystals ("spherical" NPs) do not exhibit a notable plasmonic enhancement, i.e., they are essentially non-SERS-active. However, in experiments, SERS enhanced by spherical NP colloids has been frequently reported. This implies that the monomers do not have strong SERS activity, but detectable enhancement should more or less be there. Because of the gap between theory and practice, it is important to demonstrate experimentally how SERS-active the metal colloid actually is and, in case a SERS signal is observed, where it originates from. In particular the aggregation of the colloid, induced by high centrifugal forces in washing steps or due to a harsh ionic environment of the suspension medium, should be controlled since it is the very high SERS activity of NP clusters which dominates the overall SERS signal of the colloid. We report here the experimental evaluation of the SERS activity of 80 nm Au and Ag NP monomers. Instead of showing fancy nanostructures and super SERS enhancement, we present the method on how to obtain negative experimental data. In this approach, no SERS signal was obtained from the colloid with a Raman reporter on the metal surface when the NPs were encapsulated carefully within a thick silica shell. Without silica encapsulation, if a very low centrifugation speed is used for the washing steps, only a negligible SERS signal can be detected even at very high NP concentrations. In contrast, strong SERS signals can be detected when the NPs are suspended in acidic solutions. These results indicate that Au and Ag NP monomers essentially exhibit no SERS activity of practical relevance. © the Owner Societies 2015.
    view abstractdoi: 10.1039/c4cp05073h
  • 2013 • 91 Towards the understanding of sintering phenomena at the nanoscale: Geometric and environmental effects
    Behafarid, F. and Roldan Cuenya, B.
    Topics in Catalysis 56 1542-1559 (2013)
    One of the technologically most important requirements for the application of supported metal nanoparticles (NPs) to the field of heterogeneous catalysis is the achievement of thermally and chemically stable systems under reaction conditions. For this purpose, a thorough understanding of the different pathways underlying coarsening phenomena is needed. In particular, in depth knowledge must be achieved on the role of the NP synthesis method, geometrical features of the NPs (size and shape), initial NP dispersion on the support (interparticle distance), support pre-treatment (affecting its morphology and chemical state), and reaction environment (gaseous or liquid medium, pressure, temperature). This study provides examples of the stability and sintering behavior of nanoscale systems monitored ex situ, in situ, and under operando conditions via transmission electron microscopy, atomic force microscopy, scanning tunneling microscopy, and X-ray absorption fine-structure spectroscopy. Experimental data corresponding to physical-vapor-deposited and micelle-synthesized metal (Pt, Au) NPs supported on TiO<inf>2</inf>, SiO<inf>2</inf> and Al<inf>2</inf>O <inf>3</inf> will be used to illustrate Ostwald-ripening and diffusion coalescence processes. In addition, the role of the annealing environment (H<inf>2</inf>, O<inf>2</inf>, water vapor) on the stability of NPs will be discussed. © 2013 Springer Science+Business Media New York.
    view abstractdoi: 10.1007/s11244-013-0149-4
  • 2013 • 90 Hard x-ray photoemission spectroscopy on the trilayer system MgO/Au/Fe using standing-wave excitation
    Döring, S. and Schönbohm, F. and Berges, U. and Bürgler, D.E. and Schneider, C.M. and Gorgoi, M. and Schäfers, F. and Westphal, C.
    Journal of Physics D: Applied Physics 46 (2013)
    The trilayer system MgO/Au monolayer/Fe was investigated by hard x-ray photoemission experiments in combination with the standing-wave technique. The insertion of the Au layer into the Fe/MgO tunnel junction provides an additional handle to influence the properties of the interface. The recently explored method of standing-wave excited hard x-ray photoemission was used to investigate both the structural properties and chemical states of the interfacial layers in one experiment. The results show that the Au monolayer does not grow as a closed layer, but intermixes strongly with the Fe below. This behaviour results in a very sharp interface between the Au/Fe and the MgO layer on top. However, the XPS spectra show no hint for a formation of FeO at the interface. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/46/37/375001
  • 2013 • 89 Size control of gold nanoparticles during laser ablation in liquids with different functional molecules
    Essaidi, A. and Chakif, M. and Schöps, B. and Aumman, A. and Xiao, S. and Esen, C. and Ostendorf, A.
    Journal of Laser Micro Nanoengineering 8 131-136 (2013)
    Over the last decade many papers dedicated to the generation of nanoparticles in liquids using la-ser ablation have been published. They have shown that the nanoparticles size distribution is de-pendent of the laser fluence and shifts towards smaller sizes when the surfactant concentration is in-creased. In this paper an alternative approach is presented that permits the control of the size distri-bution of gold nanoparticles during laser ablation in aqueous solutions containing different surfactant molecules with different shapes and size.
    view abstractdoi: 10.2961/jlmn.2013.02.0003
  • 2013 • 88 Molecular understanding of reactivity and selectivity for methanol oxidation at the Au/TiO2 interface
    Farnesicamellone, M. and Zhao, J. and Jin, L. and Wang, Y. and Muhler, M. and Marx, D.
    Angewandte Chemie - International Edition 52 5780-5784 (2013)
    Gold catalysis: Experimental and theoretical data demonstrated consistently that the interfacial sites on a Au/TiO2 catalyst show both high reactivity and selectivity for low-temperature methanol oxidation with O 2 to give formaldehyde. The microscopic mechanism of this complex reaction has been unraveled in full molecular detail (see picture, gold cluster on TiO2 surface). Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201301868
  • 2013 • 87 Metal-supported catalysts encapsulated in mesoporous solids: Challenges and opportunities of a model concept
    Grünert, W. and Gies, H. and Muhler, M. and Polarz, S. and Lehmann, C.W. and Großmann, D. and van den Berg, M. and Tkachenko, O.P. and De Toni, A. and Sinev, I. and Bandyopadhyay, M. and Narkhede, V. and Dreier, A. and Klementie...
    Physica Status Solidi (B) Basic Research 250 1081-1093 (2013)
    An overview of work with model systems designed to study metal-support interactions in heterogeneous catalysts is given. In these models, metal and support are both miniaturized by introduction as guests into a mesoporous host. The use of such models is demonstrated with Au-TiO2 clusters encaged in MCM-48, and Cu-ZnO clusters encapsulated in siliceous mesopore systems and in carbon nanotubes. The models promise a better opportunity to track changes in the support component during catalyst activation and catalysis, including the action of poisons that may at first be trapped on the support surface. Challenges to be met are the stabilization of the mesoporous matrix during synthesis and catalysis, possible reactivity of the matrix surface towards any of the catalyst components, as well as clustering and segregation of the latter from the matrix. The challenges were encountered as pore damage during preparation of Au-TiO2/MCM-48 catalysts, as deactivating interactions of siliceous walls with zinc ions during deposition of zinc species from aqueous media, and as clustering of the Cu component during calcination and reduction. Among the conclusions drawn from the studies are the irrelevance of order at the Au-TiO2 interface (and, hence, of epitaxy and of crystal strain in gold) for high activity of Au/TiO2 catalysts in CO oxidation. In the models for Cu-ZnO methanol synthesis catalysts, two different types of Cu-Zn interaction could be observed: a direct contact between Zn2+ and Cu(0) under strong reducing conditions, and the formation of alloy nanoparticles (nano-brass). A discussion of the relevance of these interactions for the methanol synthesis reaction is given. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssb.201248454
  • 2013 • 86 Trends in spin and orbital magnetism of free and encapsulated FePt nanoparticles
    Gruner, M.E.
    Physica Status Solidi (A) Applications and Materials Science 210 1282-1297 (2013)
    Owed to the large magneto-crystalline anisotropy (MCA) of the bulk FePt alloys, nanostructures with a few nm in diameter are considered for ultra-high density recording applications. First principles calculations in the framework of density functional theory (DFT) permit insight into the close interrelation between particle composition, morphology, and magnetism with access to the electronic level. The present survey will systematically highlight the impact of an additional encapsulation with Cu, Au, Al, and further main group elements on spin- and orbital magnetism and MCA with special emphasis on the role of the interface. Site resolved orbital moment anisotropy (OMA) of an uncovered 147 atom FePt nanoparticle. Large-scale first principles calculations in the framework of density functional theory offer detailed insight into the close interrelation between particle composition, morphology and magnetism with electronic resolution. Exploiting the power of contemporary supercomputers, one can identify systematic trends in spin and orbital magnetism of nanometer-sized hard magnetic particles related to their structure or chemical environment. This Feature Article concentrates on Fe-Pt nanoparticles, which are considered as promising candidates for ultra-high density recording media. Special emphasis is made on the role of the surfaces and the impact of a protective encapsulation with Cu, Au, Al or further main group elements on the hard magnetic properties. The anisotropy of the orbital moments turns out to be a valuable quantity characterizing the particular contribution of surfaces and interfaces on the atomic scale. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.201329048
  • 2013 • 85 Synthesis of Au microwires by selective oxidation of Au-W thin-film composition spreads
    Hamann, S. and Brunken, H. and Salomon, S. and Meyer, R. and Savan, A. and Ludwig, Al.
    Science and Technology of Advanced Materials 14 (2013)
    We report on the stress-induced growth of Au microwires out of a surrounding Au-W matrix by selective oxidation, in view of a possible application as 'micro-Velcro'. The Au wires are extruded due to the high compressive stress in the tungsten oxide formed by oxidation of elemental W. The samples were fabricated as a thin-film materials library using combinatorial sputter deposition followed by thermal oxidation. Sizes and shapes of the Au microwires were investigated as a function of the W to Au ratio. The coherence length and stress state of the Au microwires were related to their shape and plastic deformation. Depending on the composition of the Au-W precursor, the oxidized samples showed regions with differently shaped Au microwires. The Au48W52 composition yielded wires with the maximum length to diameter ratio due to the high compressive stress in the tungsten oxide matrix. The values of wire length (35 μm) and diameter (2 μm) achieved at the Au48W52 composition are suitable for micro-Velcro applications. © 2013 National Institute for Materials Science.
    view abstractdoi: 10.1088/1468-6996/14/1/015003
  • 2013 • 84 Facile synthesis of polymer core@silver shell hybrid nanoparticles with super surface enhanced Raman scattering capability
    Huo, D. and He, J. and Yang, S. and Zhou, Z. and Hu, Y. and Epple, M.
    Journal of Colloid and Interface Science 393 119-125 (2013)
    Silver nano-shells (SNSs) were synthesized via a two-step seeds-mediated method. Polymer cores were composed of ultrafine gold nanoparticles (NPs) modified chitosan-poly(acrylic acid) nanoparticles (CS-PAA NPs). Then, deposition of silver upon gold nucleus leads to the seed enlargement and finally forms silver shell on the surface of CS-PAA NPs to get SNSs. Transmission electron microscope (TEM) showed SNSs had a discrete silver shell plus some pores and gaps, which could acted as "hot spots" and provided the great potential of these SNSs to be used as SERS substrates with wavelength ranging from visible to infrared region (700-1000. nm) by tuning shell coverage of silver. SERS experiments with dibenzyl disulphide (DBDS) as the indicator showed that the resulting SNSs allowed the production of highly consistent enhancement of the Raman signals down to nM concentrations of DBDS. Considering the excellent biocompatibility of polymer core and their small size, these SNSs are highly desirable candidates as the enhancers for high performance SERS analysis and as SERS optical labels in biomedical imaging. © 2012 Elsevier Inc.
    view abstractdoi: 10.1016/j.jcis.2012.11.003
  • 2013 • 83 Methanol oxidation as probe reaction for active sites in Au/ZnO and Au/TiO2 catalysts
    Kähler, K. and Holz, M.C. and Rohe, M. and Van Veen, A.C. and Muhler, M.
    Journal of Catalysis 299 162-170 (2013)
    Methanol oxidation was used as test reaction to investigate the influence of the metal, of the support, and of metal-support interactions in Au/ZnO and Au/TiO2 catalysts. Catalytic measurements as well as infrared spectroscopy were applied under continuous flow conditions in fixed-bed reactors. A strong effect of the Au loading ranging from 0.6 wt.% to 1.9 wt.% was found for both Au/ZnO and Au/TiO2 catalysts with Au particle sizes in the range from 3 to 7 nm. Methanol combustion yielding H2O and CO2 was the main reaction path, but also reactions such as partial oxidation of methanol, steam reforming of methanol, methanol decomposition as well as the selective oxidation of methanol to methyl formate, formaldehyde, or dimethoxymethane were found to occur. Smaller Au particles and a higher amount of small Au particles had a beneficial effect on the activity. Infrared spectroscopy identified methoxy species adsorbed on the metal oxides as intermediates in methanol oxidation. The product distribution was found to depend on the oxide used as support due to the different Lewis acidities. On Au/TiO2, strongly bound formates acted as reversible catalyst poison. The catalytic activity was found to be correlated with the number of Au atoms at the perimeter of the Au nanoparticles. Correspondingly, oxygen activation is assumed to occur at their perimeter, and the oxide provides methoxy species reacting at the interface. © 2012 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.jcat.2012.12.001
  • 2013 • 82 Generation of AuGe nanocomposites by co-sparking technique and their photoluminescence properties
    Kala, S. and Theissmann, R. and Kruis, F.E.
    Journal of Nanoparticle Research 15 (2013)
    The feasibility of spark discharge technique for preparing metal-semiconductor nanocomposites is demonstrated. In the AuGe system, Au shows only 10-3 atomic percent solid solubility in Ge, whereas 3.1 at.% Ge is soluble in Au. During the co-sparking, Au is used as anode material; the cathode is composed of Ge. The relative atomic percent of Au and Ge in the initially generated mixture can be changed by changing the charging current to the capacitor used to trigger the sparking. Depending upon the atomic ratio of Au and Ge in the initial mixture, AuGe agglomerates form AuGe composite nanoparticles on subsequent sintering, in which AuGe alloy nanoparticles are found dispersed in a Ge matrix. The size of the dispersed AuGe alloy nanoparticles depend on the relative atomic concentration of Au and Ge in the initial mixture as well as on the sintering temperature. AuGe alloy nanoparticles dispersed in the Ge matrix are observed to exhibit an intense photoluminescence between 550 and 600 nm. © 2013 Springer Science+Business Media.
    view abstractdoi: 10.1007/s11051-013-1963-0
  • 2013 • 81 Evidence for metal-support interactions in Au modified TiO x/SBA-15 materials prepared by photodeposition
    Mei, B. and Wiktor, C. and Turner, S. and Pougin, A. and Van Tendeloo, G. and Fischer, R.A. and Muhler, M. and Strunk, J.
    ACS Catalysis 3 3041-3049 (2013)
    Gold nanoparticles have been efficiently photodeposited onto titanate-loaded SBA-15 (Ti(x)/SBA-15) with different titania coordination. Transmission electron microscopy shows that relatively large Au nanoparticles are photodeposited on the outer surface of the Ti(x)/SBA-15 materials and that TiOx tends to form agglomerates in close proximity to the Au nanoparticles, often forming core-shell Au/TiOx structures. This behavior resembles typical processes observed due to strong-metal support interactions. In the presence of gold, the formation of hydrogen on Ti(x)/SBA-15 during the photodeposition process and the performance in the hydroxylation of terephthalic acid is greatly enhanced. The activity of the Au/Ti(x)/SBA-15 materials is found to depend on the TiOx loading, increasing with a larger amount of initially isolated TiO4 tetrahedra. Samples with initially clustered TiOx species show lower photocatalytic activities. When isolated zinc oxide (ZnOx) species are present on Ti(x)/SBA-15, gold nanoparticles are smaller and well dispersed within the pores. Agglomeration of TiOx species and the formation of Au/TiO x structures is negligible. The dispersion of gold and the formation of Au/TiOx in the SBA-15 matrix seem to depend on the mobility of the TiOx species. The mobility is determined by the initial degree of agglomeration of TiOx. Effective hydrogen evolution requires Au/TiOx core-shell composites as in Au/Ti(x)/SBA-15, whereas hydroxylation of terephthalic acid can also be performed with Au/ZnO x/TiOx/SBA-15 materials. However, isolated TiOx species have to be grafted onto the support prior to the zinc oxide species, providing strong evidence for the necessity of Ti-O-Si bridges for high photocatalytic activity in terephthalic acid hydroxylation. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/cs400964k
  • 2013 • 80 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 7897-7903 (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 abstractdoi: 10.1021/ac401317y
  • 2013 • 79 Nano-gold diggers: Au-Assisted SiO2-decomposition and desorption in supported nanocatalysts
    Ono, L.K. and Behafarid, F. and Cuenya, B.R.
    ACS Nano 7 10327-10334 (2013)
    An investigation of the thermal stability of size-selected Au nanoparticles (NPs) synthesized via inverse micelle encapsulation and deposited on SiO 2(4 nm)/Si(100) is presented. The size and mobility of individual Au NPs after annealing at elevated temperatures in ultrahigh vacuum (UHV) was monitored via atomic force microscopy (AFM). An enhanced thermal stability against coarsening and lack of NP mobility was observed up to 1343 K. In addition, a drastic decrease in the average NP height was detected with increasing annealing temperature, which was not accompanied by the sublimation of Au atoms/clusters in UHV. The apparent decrease in the Au NP height observed is assigned to their ability to dig vertical channels in the underlying SiO 2 support. More specifically, a progressive reduction in the thickness of the SiO2 support underneath and in the immediate vicinity of the NPs was evidenced, leading to NPs partially sinking into the SiO2 substrate. The complete removal of silicon oxide in small patches was observed to take place around the Au NPs after annealing at 1343 K in UHV. These results reveal a Au-assisted oxygen desorption from the support via reverse oxygen spillover to the NPs. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/nn404744b
  • 2013 • 78 Universal frozen spectra after time-dependent symmetry restoring phase transitions
    Queisser, F. and Navez, P. and Schützhold, R.
    Journal of Physics Condensed Matter 25 (2013)
    For a general O(N) model, we study the time-dependent phase transition from a state with broken symmetry to the symmetric phase . During this non-equilibrium process, the primordial quantum (or thermal) fluctuations of the initial Goldstone modes are frozen and result in a deviation from the final ground (or thermal) state. For very slow transitions, we find that these fluctuations display a universal scaling behaviour. Their spectra are universal functions of a single parameter, which combines the initial frequency of the Goldstone modes and the sweep rate. As a result, the final two-point function is not exponentially suppressed at large distances Δr = r - r′ (as it would be in the ground state) but decays polynomially in 1/|Δr|. Finally, we exemplify this universal behaviour for the transition from the super-fluid phase to the Mott state in the Bose-Hubbard model. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/25/40/404215
  • 2013 • 77 Size control of laser-fabricated surfactant-free gold nanoparticles with highly diluted electrolytes and their subsequent bioconjugation
    Rehbock, C. and Merk, V. and Gamrad, L. and Streubel, R. and Barcikowski, S.
    Physical Chemistry Chemical Physics 15 3057-3067 (2013)
    Size control of laser-fabricated surfactant-free gold nanoparticles is a challenging endeavor. In this work, we show that size control can be achieved by adding ions with low salinity during synthesis. In addition, this approach offers the opportunity to fundamentally study ion interactions with bare nanoparticle surfaces and can help to elucidate the nanoparticle formation mechanism. The studies were carried out in a flow-through reactor and in the presence of NaCl, NaBr and sodium phosphate buffer at minimal ionic strengths. A significant size quenching effect at ionic strengths from 1-50 μM was found, which allowed surfactant-free nanoparticle size control with average diameters of 6-30 nm. This effect went along with low polydispersity and minimal aggregation tendencies and was confirmed by UV-vis spectroscopy, TEM, SEM and analytical disk centrifugation. Our findings indicate that size quenching originates from an anionic electrostatic stabilization depending on the nanoparticle surface area, which may be caused by specific ion adsorption. By subsequent delayed bioconjugation in liquid-flow using bovine serum albumin as a stabilizing agent, nano-bioconjugates with good stability in cell culture media were obtained, which are applicable in toxicology and cell biology. This journal is © 2013 the Owner Societies.
    view abstractdoi: 10.1039/c2cp42641b
  • 2013 • 76 Fluid pumping cell of photonic - Plasmonic microcavity sensor for biomedical application
    Saetchnikov, V.A. and Tcherniavskaia, E.A. and Saetchnikov, A.V. and Schweiger, G. and Ostendorf, A.
    Proceedings of the International Conference on Advanced Optoelectronics and Lasers, CAOL 199-201 (2013)
    Fluid pumping cell for plasmonic - photonic microcavity sensor for label-free biomolecule detection and identification has been developed and tested with drug and gold nanoparticle solutions including additional gold layer. Resonant spectra parameters have being analyzed. © 2013 IEEE.
    view abstractdoi: 10.1109/CAOL.2013.6657577
  • 2013 • 75 Rapid immuno-SERS microscopy for tissue imaging with single-nanoparticle sensitivity
    Salehi, M. and Steinigeweg, D. and Ströbel, P. and Marx, A. and Packeisen, J. and Schlücker, S.
    Journal of Biophotonics 6 785-792 (2013)
    Immuno-SERS microscopy is a novel imaging technique in nano-biophotonics, which employs antibodies labeled with SERS-active nanoparticles in conjunction with Raman microscopy. Rapid data acquisition is of central importance for screening large areas of tissue specimens. Here, we first discuss the role of SERS labels with single-particle sensitivity in immuno-SERS microscopy, in particular with respect to false-negative results. In combined single-particle experiments (SERS microscopy/dark-field microscopy/HR-SEM), we then demonstrate that small glass-coated clusters (dimers and trimers) of gold nanospheres exhibit the desired single-particle SERS sensitivity, even at acquisition times as short as 30 msec per pixel, while monomers do not. The proof-of-concept for rapid immuno-SERS microscopy with 30 msec acquisition time per pixel for selective imaging of the p53 family member p63 in prostate tissue sections is demonstrated. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/jbio.201200148
  • 2013 • 74 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 15-20 (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 abstractdoi: 10.1016/j.bioelechem.2012.11.001
  • 2013 • 73 Single gold trimers and 3D superstructures exhibit a polarization- independent SERS response
    Steinigeweg, D. and Schütz, M. and Schlücker, S.
    Nanoscale 5 110-113 (2013)
    Dimers of metal nanospheres are well-known for their characteristic anisotropic optical response. Here, we demonstrate in single-particle SERS experiments that individual gold trimers and 3D superstructures exhibit a polarization-independent SERS response. This optical behavior of single particle clusters provides constant SERS signals, independent of the mutual orientation of the incident laser polarization and the plasmonic nanostructure, which is desired or even required in many SERS applications. © 2013 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c2nr31982a
  • 2013 • 72 A kinetic study of oxygen reduction reaction and characterization on electrodeposited gold nanoparticles of diameter between 17 nm and 40 nm in 0.5 M sulfuric acid
    Wang, Y. and Laborda, E. and Ward, K.R. and Tschulik, K. and Compton, R.G.
    Nanoscale 5 9699-9708 (2013)
    Kinetic and mechanistic studies of the oxygen reduction reaction (ORR) in oxygen saturated 0.5 M sulfuric acid at 298 K at a gold macroelectrode and at an electrodeposited gold nanoparticle-modified glassy carbon electrode are reported. The conditions of electrodeposition are optimized to obtain small nanoparticles of diameter from 17 nm to 40 nm. The mechanism and kinetics of ORR on the gold macroelectrode are investigated and compared with those obtained for nanoparticle-modified electrodes. The mechanism for this system includes two electron and two proton transfers and hydrogen peroxide as the final product. The first electron transfer step corresponding to the reduction of O2 to O2 - is defined as the rate determining step. No significant changes are found for the nanoparticles here employed: electron transfer rate constant (k0) is k0,bulk = 0.30 cm s -1 on the bulk material and k0,nano = 0.21 cm s -1 on nanoparticles; transfer coefficient (α) changes from αbulk = 0.45 on macro-scale to αnano = 0.37 at the nano-scale. © The Royal Society of Chemistry 2013.
    view abstractdoi: 10.1039/c3nr02340k
  • 2013 • 71 Label-free SERS monitoring of chemical reactions catalyzed by small gold nanoparticles using 3D plasmonic superstructures
    Xie, W. and Walkenfort, B. and Schlücker, S.
    Journal of the American Chemical Society 135 1657-1660 (2013)
    Label-free in situ surface-enhanced Raman scattering (SERS) monitoring of reactions catalyzed by small gold nanoparticles using rationally designed plasmonic superstructures is presented. Catalytic and SERS activities are integrated into a single bifunctional 3D superstructure comprising small gold satellites self-assembled onto a large shell-isolated gold core, which eliminates photocatalytic side reactions. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/ja309074a
  • 2012 • 70 Impact of spacer and strand length on oligonucleotide conjugation to the surface of ligand-free laser-generated gold nanoparticles
    Barchanski, A. and Hashimoto, N. and Petersen, S. and Sajti, C.L. and Barcikowski, S.
    Bioconjugate Chemistry 23 908-915 (2012)
    Gold nanoparticles conjugated to nucleic acids are widely used for biomedical targeting and sensing applications; however, little is known about the conjugation chemistry covering the impact of steric dimension and strand orientation of single-stranded oligonucleotides (ssO) on the conjugation process and binding efficiencies. In this context, we present an extensive investigation concerning the attachment of thiolated ssO to the surface of laser-generated gold nanoparticles, altering both strand length and binding orientation by the insertion of different spacer types at either the 3′ or 5′ ssO terminus. A significant reduction of conjugation efficiency of about 30-50% is determined for spacer-prolonged bionanoconjugates due to coiling effects of the flexible ssO strand on the particle surface which increases deflection angle of oligonucleotides and limits the number of biomolecules attached to the nanoparticles. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/bc200462b
  • 2012 • 69 Understanding properties of electrified interfaces as a prerequisite for label-free DNA hybridization detection
    Gebala, M. and Schuhmann, W.
    Physical Chemistry Chemical Physics 14 14933-14942 (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 abstractdoi: 10.1039/c2cp42382k
  • 2012 • 68 Imaging of waveguiding and scattering interferences in individual GaAs nanowires via second-harmonic generation
    Grange, R. and Brönstrup, G. and Sergeyev, A. and Richter, J. and Pertsch, T. and Tünnermann, A. and Christiansen, S. and Leiterer, C. and Fritzsche, W. and Gutsche, C. and Lysov, A. and Prost, W. and Tegude, F.-J.
    Proceedings of SPIE - The International Society for Optical Engineering 8424 (2012)
    We use the nonlinear optical property of GaAs to directly visualize the path of the near infrared incident laser light coupled into individual nanowires. We fully illuminate with near infrared pulse laser untapered and tapered GaAs nanowires grown via the Au-assisted vapor-liquid-solid mechanism. We record second-harmonic generation (SHG) signals in the visible spectrum. In some nanowires, an interference pattern is observed and investigated in terms of distances between the maxima of the SHG signal taking into account the effective refractive index in such sub wavelength structures with radius below 90 nm. We propose a model to explain the periodicity of the maxima in the SHG interference pattern. The theoretical model includes the waveguiding and the Mie scattering theories for obtaining the 2π periodicity fitting well the experiments. Moreover, we also measure interferences in tapererd nanowires with a radius down to 76 nm. The possible effect of the gold in non radiative recombination and the presence of the gold particle at the tip of some nanowires are also discussed. © 2012 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).
    view abstractdoi: 10.1117/12.921758
  • 2012 • 67 Au, @ZrO 2 yolk-shell catalysts for CO oxidation: Study of particle size effect by ex-post size control of Au cores
    Güttel, R. and Paul, M. and Galeano, C. and Schüth, F.
    Journal of Catalysis 289 100-104 (2012)
    Gold nanoparticles supported on transition metal oxides are found to exhibit a pronounced particle size effect in CO oxidation. However, the preparation of comparable supported gold nanoparticles with different sizes remains challenging, since the catalytic behavior of these materials is very sensitive to the preparation conditions. To overcome this difficulty, Au, @ZrO 2 catalysts with gold core sizes between 5 and 15 nm were prepared by partial leaching of gold in an ex-post manner. The material obtained offers a unique comparability for particle size effect studies in CO oxidation. No effect of gold particle size was observed in the studied size range. © 2012 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.jcat.2012.01.021
  • 2012 • 66 Direct determination of minority carrier diffusion lengths at axial GaAs nanowire p-n junctions
    Gutsche, C. and Niepelt, R. and Gnauck, M. and Lysov, A. and Prost, W. and Ronning, C. and Tegude, F.-J.
    Nano Letters 12 1453-1458 (2012)
    Axial GaAs nanowire p-n diodes, possibly one of the core elements of future nanowire solar cells and light emitters, were grown via the Au-assisted vapor-liquid-solid mode, contacted by electron beam lithography, and investigated using electron beam induced current measurements. The minority carrier diffusion lengths and dynamics of both, electrons and holes, were determined directly at the vicinity of the p-n junction. The generated photocurrent shows an exponential decay on both sides of the junction and the extracted diffusion lengths are about 1 order of magnitude lower compared to bulk material due to surface recombination. Moreover, the observed strong diameter-dependence is well in line with the surface-to-volume ratio of semiconductor nanowires. Estimating the surface recombination velocities clearly indicates a nonabrupt p-n junction, which is in essential agreement with the model of delayed dopant incorporation in the Au-assisted vapor-liquid-solid mechanism. Surface passivation using ammonium sulfide effectively reduces the surface recombination and thus leads to higher minority carrier diffusion lengths. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/nl204126n
  • 2012 • 65 Conventional and microwave-assisted synthesis of hyperbranched and highly branched polylysine towards amphiphilic core-shell nanocontainers for metal nanoparticles
    Ho, C.H. and Thiel, M. and Celik, S. and Odermatt, E.K. and Berndt, I. and Thomann, R. and Tiller, J.C.
    Polymer (United Kingdom) 53 4623-4630 (2012)
    Hyperbranched amphiphilic polymeric systems with core-shell architecture can be used as versatile nanocontainers and templates with great potential in application fields ranging from medicine to organic coatings. In order to explore an alternative to the already widely used and established synthetic macromolecules, we synthesized new polymers based on hyperbranched polylysine. Polylysine was prepared with classical heating and microwave-assisted heating, respectively. While, the synthesis at 160 °C resulted in hyperbranched polylysine with degrees of branching (DB) between 0.50 and 0.54, the microwave-assisted heating at 200 °C resulted in highly branched polymers with DB values of 0.30-0.32. The molecular weight M n could be controlled in a range of 5000-12,000 g/mol. The hyperbranched polylysine was hydrophobized via polymer-analogue reactions using a mixture of stearoyl/palmitoyl chloride and glycidyl hexadecyl ether, respectively. These reactions yielded in high degrees of modification (80% and 90%, respectively). The synthesized polymers are soluble in non-polar organic solvents, such as toluene and chloroform, and take up metal salts to up to 25 wt.%. They support the formation of Ag, Au, and Pd nanoparticles and nanocrystals in organic solvents and stabilize them. Thus, the here presented macromolecules are a promising readily achievable alternative to existing core-shell systems. © 2012 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.polymer.2012.08.032
  • 2012 • 64 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 13 131-139 (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 abstractdoi: 10.1002/cphc.201100578
  • 2012 • 63 Catalytic role of gold nanoparticle in GaAs nanowire growth: A density functional theory study
    Kratzer, P. and Sakong, S. and Pankoke, V.
    Nano Letters 12 943-948 (2012)
    The energetics of Ga, As, and GaAs species on the Au(111) surface (employed as a model for Au nanoparticles) is investigated by means of density functional calculations. Apart from formation of the compound Au 7Ga 2, Ga is found to form a surface alloy with gold with comparable ΔH ∼ -0.5 eV for both processes. Dissociative adsorption of As 2 is found to be exothermic by more than 2 eV on both clean Au(111) and AuGa surface alloys. The As-Ga species formed by reaction of As with the surface alloy is sufficiently stable to cover the surface of an Au particle in vacuo in contact with a GaAs substrate. The results of the calculations are interpreted in the context of Au-catalyzed growth of GaAs nanowires. We argue that arsenic is supplied to the growth zone of the nanowire mainly by impingement of molecules on the gold particle and identify a regime of temperatures and As 2 partial pressures suitable for Au-catalyzed nanowire growth in molecular beam epitaxy. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/nl204004p
  • 2012 • 62 Silver, gold, and alloyed silver-gold nanoparticles: Characterization and comparative cell-biologic action
    Mahl, D. and Diendorf, J. and Ristig, S. and Greulich, C. and Li, Z.A. and Farle, M. and Köller, M. and Epple, M.
    Journal of Nanoparticle Research 14 (2012)
    Silver, gold, and silver-gold-alloy nanoparticles were prepared by citrate reduction modified by the addition of tannin during the synthesis, leading to a reduction in particle size by a factor of three. Nanoparticles can be prepared by this easy waterbased synthesis and subsequently functionalized by the addition of either tris(3-sulfonatophenyl)phosphine or poly(N-vinylpyrrolidone). The resulting nanoparticles of silver (diameter 15-25 nm), gold (5-6 nm), and silver-gold (50:50; 10-12 nm) were easily dispersable in water and also in cell culture media (RPMI + 10 % fetal calf serum), as shown by nanoparticle tracking analysis and differential centrifugal sedimentation. High-resolution transmission electron microscopy showed a polycrystalline nature of all nanoparticles. EDX on single silver-gold nanoparticles indicated that the concentration of gold is higher inside a nanoparticle. The biologic action of the nanoparticles toward human mesenchymal stem cells (hMSC) was different: Silver nanoparticles showed a significant concentration-dependent influence on the viability of hMSC. Gold nanoparticles showed only a small effect on the viability of hMSC after 7 days. Surprisingly, silver-gold nanoparticles had no significant influence on the viability of hMSC despite the silver content. Silver nanoparticles and silver-gold nanoparticles in the concentration range of 5-20 μg mL -1 induced the activation of hMSC as indicated by the release of IL-8. In contrast, gold nanoparticles led to a reduction of the release of IL-6 and IL-8. © Springer Science+Business Media B.V. 2012.
    view abstractdoi: 10.1007/s11051-012-1153-5
  • 2012 • 61 Microgradient-heaters as tools for high-throughput experimentation
    Meyer, R. and Hamann, S. and Ehmann, M. and Thienhaus, S. and Jaeger, S. and Thiede, T. and Devi, A. and Fischer, R.A. and Ludwig, Al.
    ACS Combinatorial Science 14 531-536 (2012)
    A microgradient-heater (MGH) was developed, and its feasibility as a tool for high-throughput materials science experimentation was tested. The MGH is derived from microhot plate (MHP) systems and allows combinatorial thermal processing on the micronano scale. The temperature gradient is adjustable by the substrate material. For an Au-coated MGH membrane a temperature drop from 605 to 100 °C was measured over a distance of 965 μm, resulting in an average temperature change of 0.52 K/μm. As a proof of principle, we demonstrate the feasibility of MGHs on the example of a chemical vapor deposition (CVD) process. The achieved results show discontinuous changes in surface morphology within a continuous TiO 2 film. Furthermore the MGH can be used to get insights into the energetic relations of film growth processes, giving it the potential for microcalorimetry measurements. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/co3000488
  • 2012 • 60 Interface of nanoparticle-coated electropolished stents
    Neumeister, A. and Bartke, D. and Bärsch, N. and Weingärtner, T. and Guetaz, L. and Montani, A. and Compagnini, G. and Barcikowski, S.
    Langmuir 28 12060-12066 (2012)
    Nanostructures entail a high potential for improving implant surfaces, for instance, in stent applications. The electrophoretic deposition of laser-generated colloidal nanoparticles is an appropriate tool for creating large-area nanostructures on surfaces. Until now, the bonding and characteristics of the interface between deposited nanoparticles and the substrate surface has not been known. It is investigated using X-ray photoelectron spectroscopy, Auger electron spectroscopy, and transmission electron microscopy to characterize an electropolished NiTi stent surface coated by laser-generated Au and Ti nanoparticles. The deposition of elemental Au and Ti nanoparticles is observed on the total 3D surface. Ti-coated samples are composed of Ti oxide and Ti carbide because of nanoparticle fabrication and the coating process carried out in 2-propanol. The interface between nanoparticles and the electropolished surface consists of a smooth, monotone elemental depth profile. The interface depth is higher for the Ti nanoparticle coating than for the Au nanoparticle coating. This smooth depth gradient of Ti across the coating-substrate intersection and the thicker interface layer indicate the hard bonding of Ti-based nanoparticles on the surface. Accordingly, electron microscopy reveals nanoparticles adsorbed on the surface without any sorption-blocking intermediate layer. The physicomechanical stability of the bond may benefit from such smooth depth gradients and direct, ligand-free contact. This would potentially increase the coating stability during stent application. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/la300308w
  • 2012 • 59 Probing the mechanism of low-temperature CO oxidation on Au/ZnO catalysts by vibrational spectroscopy
    Noei, H. and Birkner, A. and Merz, K. and Muhler, M. and Wang, Y.
    Journal of Physical Chemistry C 116 11181-11188 (2012)
    Adsorption and oxidation of CO on Au/ZnO catalysts were studied by Fourier transform infrared (FTIR) spectroscopy using a novel ultra-high-vacuum (UHV) system. The high-quality UHV-FTIRS data provide detailed insight into the catalytic mechanism of low-temperature CO oxidation on differently pretreated Au/ZnO catalysts. For the samples without O 2 pretreatment, negatively charged Au nanoparticles are identified which exhibit high reactivity to CO oxidation at 110 K, yielding CO 2 as well as carbonate species bound to various ZnO facets. O 2 pretreatment leads to formation of neutral Au nanoparticles where CO is activated on the low-coordinated Au sites at the interface. Activation of impinging O 2 occurs at the Au/ZnO interface and is promoted by preadsorbed CO forming an OC-O 2 intermediate complex, accompanied by charge transfer from Au/ZnO substrate to O 2. The CO molecules adsorbed on ZnO serve as a reservoir for reactants and are mobile enough at 110 K to reach the Au/ZnO interface where they react with activated oxygen yielding CO 2. Different carbonate species are further produced via interaction of formed CO 2 with surface oxygen atoms on ZnO. It was found that the active interface sites are slowly blocked at 110 K by the inert carbonate species, thus causing a gradual decrease of the catalytic activity. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/jp302723r
  • 2012 • 58 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 92-95 (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 abstractdoi: 10.1016/j.elecom.2011.12.007
  • 2012 • 57 Substrate-mediated effects in photothermal patterning of alkanethiol self-assembled monolayers with microfocused continuous-wave lasers
    Schröter, A. and Kalus, M. and Hartmann, N.
    Beilstein Journal of Nanotechnology 3 65-74 (2012)
    In recent years, self-assembled monolayers (SAMs) have been demonstrated to provide promising new approaches to nonlinear laser processing. Most notably, because of their ultrathin nature, indirect excitation mechanisms can be exploited in order to fabricate subwavelength structures. In photothermal processing, for example, microfocused lasers are used to locally heat the substrate surface and initiate desorption or decomposition of the coating. Because of the strongly temperature-dependent desorption kinetics, the overall process is highly nonlinear in the applied laser power. For this reason, subwavelength patterning is feasible employing ordinary continuous-wave lasers. The lateral resolution, generally, depends on both the type of the organic monolayer and the nature of the substrate. In previous studies we reported on photothermal patterning of distinct types of SAMs on Si supports. In this contribution, a systematic study on the impact of the substrate is presented. Alkanethiol SAMs on Au-coated glass and silicon substrates were patterned by using a microfocused laser beam at a wavelength of 532 nm. Temperature calculations and thermokinetic simulations were carried out in order to clarify the processes that determine the performance of the patterning technique. Because of the strongly temperature-dependent thermal conductivity of Si, surface-temperature profiles on Au/Si substrates are very narrow ensuring a particularly high lateral resolution. At a 1/e spot diameter of 2 μm, fabrication of subwavelength structures with diameters of 300-400 nm is feasible. Rapid heat dissipation, though, requires high laser powers. In contrast, patterning of SAMs on Au/glass substrates is strongly affected by the largely distinct heat conduction within the Au film and in the glass support. This results in broad surface temperature profiles. Hence, minimum structure sizes are larger when compared with respective values on Au/Si substrates. The required laser powers, though, are more than one order of magnitude lower. Also, the laser power needed for patterning decreases with decreasing Au layer thickness. These results demonstrate the impact of the substrate on the overall patterning process and provide new perspectives in photothermal laser patterning of ultrathin organic coatings. © 2012 Schröter et al.
    view abstractdoi: 10.3762/bjnano.3.8
  • 2012 • 56 Toxicity of gold nanoparticles on somatic and reproductive cells
    Taylor, U. and Barchanski, A. and Garrels, W. and Klein, S. and Kues, W. and Barcikowski, S. and Rath, D.
    Advances in Experimental Medicine and Biology 733 125-133 (2012)
    Along with the number of potential applications for gold nanoparticles (AuNP) especially for medical and scientific purposes, the interest in possible toxic effects of such particles is rising. The general perception views nanosized gold colloids as relatively inert towards biological systems. However, a closer analysis of pertinent studies reveals a more complex picture. While the chemical compound of which the nanoparticles consists plays an important role, further biocompatibility determining aspects have been made out. The vast majority of trials concerning AuNP-toxicity were performed using somatic cell culture lines. The results show a considerable dependency of toxic effects on size, zeta potential and surface functionalisation. In vivo studies on this subject are still rare. Based on the existing data it can be assumed, that a dosage of under <400 μg Au/kg showed no untoward effects. If higher amounts were applied toxicity depended on route of administration and particle size. Since nanoparticles have been shown to cross reproduction-relevant biological barriers such as the blood-testicle and the placental barrier the question of their reprotoxicity arises. Yet data concerning this subject is far from adequate. Regarding gametes, recent experiments showed a dose-dependent sensitivity of spermatozoa towards AuNP. Oocytes have not yet been tested in that respect. Interestingly, so far no effects were detected on embryos after gold nanoparticle exposure. In conclusion, the biocompatibility of gold nanoparticles depends on a range of particle specific aspects as well as the choice of target tissue. Further clarification of such matters are subject to ongoing research. © 2012 Springer Science+Business Media B.V.
    view abstractdoi: 10.1007/978-94-007-2555-3_12
  • 2012 • 55 Quantifying the influence of polymer coatings on the serum albumin corona formation around silver and gold nanoparticles
    Treuel, L. and Malissek, M. and Grass, S. and Diendorf, J. and Mahl, D. and Meyer-Zaika, W. and Epple, M.
    Journal of Nanoparticle Research 14 (2012)
    When nanoparticles (NPs) come into contact with biological fluids, proteins, and other biomolecules interact with their surface. Upon exposure to biological fluids a layer of proteins adsorbs onto their surface, the so-called protein corona, and interactions of biological systems with NPs are therefore mediated by this corona. Here, interactions of serum albumin with silver and gold NPs were quantitatively investigated using circular dichroism spectroscopy. Moreover, surface enhanced Raman spectroscopy was used for further elucidation of protein binding to silver surfaces. The decisive role of poly(vinylpyrrolidone), coatings on the protein adsorption was quantitatively described for the first time and the influential role of the polymer coatings is discussed. Research in nanotoxicology may benefit from such molecular scale data as well as scientific approaches seeking to improve nanomedical applications by using a wide range of polymer surface coatings to optimize biological transport and medical action of NPs. © Springer Science+Business Media B.V. 2012.
    view abstractdoi: 10.1007/s11051-012-1102-3
  • 2012 • 54 How citrate ligands affect nanoparticle adsorption to microparticle supports
    Wagener, P. and Schwenke, A. and Barcikowski, S.
    Langmuir 28 6132-6140 (2012)
    Residual ligands from colloidal synthesis of nanoparticles influence adsorption of nanoparticles to supports and may complicate fabrication of nanoparticle-decorated microparticles. In this work, we studied the adsorption of completely ligand-free metal nanoparticles and controlled ligand-functionalized nanoparticles to chemically inert microparticle supports. Adsorption of ligand-free silver nanoparticles to barium sulfate microparticle supports is a quantitative, nonreversible process following Freundlich adsorption isotherm. However, adsorption efficiency is very sensitive to ligand concentration applied during laser-based synthesis of silver nanoparticles: exceeding a specific threshold concentration of 50 μmol/L citrate equal to a nanoparticle ligand surface coverage of about 50%, results in an almost complete prevention of nanoparticle adsorption because of electrosteric repulsion by ligand shell. Laser-based synthesis of nanoparticle-decorated microparticles is demonstrated with a variety of metal nanoparticles (Ag, Au, Pt, Fe) and supporting microparticles (calcium phosphate, titanium dioxide, barium sulfate) with application potential in heterogeneous catalysis or biomedicine where ligand control offers extra value, like enhanced catalytic activity or biocompatibility. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/la204839m
  • 2012 • 53 Tuning the properties of magnetic thin films by interaction with periodic nanostructures
    Wiedwald, U. and Haering, F. and Nau, S. and Schulze, C. and Schletter, H. and Makarov, D. and Plettl, A. and Kuepper, K. and Albrecht, M. and Boneberg, J. and Ziemann, P.
    Beilstein Journal of Nanotechnology 3 831-842 (2012)
    The most important limitation for a significant increase of the areal storage density in magnetic recording is the superparamagnetic effect. Below a critical grain size of the used CoCrPt exchange-decoupled granular films the information cannot be stored for a reasonable time (typically ten years) due to thermal fluctuations arbitrary flipping of the magnetization direction. An alternative approach that may provide higher storage densities is the use of so-called percolated media, in which defect structures are imprinted in an exchange-coupled magnetic film. Such percolated magnetic films are investigated in the present work. We employ preparation routes that are based on (i) self-assembly of Au nanoparticles and (ii) homogeneous size-reduction of self-assembled polystyrene particles. On such non-close-packed nanostructures thin Fe films or Co/Pt multilayers are grown with in-plane and outof-plane easy axis of magnetization. The impact of the particles on the magnetic switching behavior is measured by both integral magnetometry and magnetic microscopy techniques. We observe enhanced coercive fields while the switching field distribution is broadened compared to thin-film reference samples. It appears possible to tailor the magnetic domain sizes down to the width of an unperturbed domain wall in a continuous film, and moreover, we observe pinning and nucleation at or close to the imprinted defect structures. © 2012 Wiedwald et al.
    view abstractdoi: 10.3762/bjnano.3.93
  • 2011 • 52 Gold catalyst initiated growth of GaN nanowires by MOCVD
    Ahl, J.-P. and Behmenburg, H. and Giesen, C. and Regolin, I. and Prost, W. and Tegude, F.J. and Radnoczi, G.Z. and Pécz, B. and Kalisch, H. and Jansen, R.H. and Heuken, M.
    Physica Status Solidi (C) Current Topics in Solid State Physics 8 2315-2317 (2011)
    Our study shows the impact of the process parameters V/III ratio, pressure and temperature on growth and morphology of GaN nanowires (NWs) synthesized by an Au-initiated vapour-liquid-solid mechanism on a sapphire substrate. We confined a temperature window for successful GaN NW growth and show how the variation of reactor pressure changes the NW morphology. Using a very low V/III ratio, NW tapering, which was observed for higher V/III ratios, could be avoided. The optimization of these process parameters led to non-tapered GaN NWs, aligned perpendicular to the substrate. Further evaluation by scanning electron microscopy showed a high density (~3·109/cm2) of hexagonal c-plane GaN NWs having diameters of 60 ± 9 nm. Transmission electron microscopy revealed single-crystalline NWs without threading dislocations but some stacking faults. The use of a very low V/III ratio was found to be important for the successful selective growth and, most interestingly, led to a difference in NW and gold catalyst droplet diameter. For chemical analysis of the NW and its catalyst droplet, electron energy loss spectroscopy was employed confirming gold as the catalyst material. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssc.201000992
  • 2011 • 51 Intercalation in layered metal-organic frameworks: Reversible inclusion of an extended π-system
    Arslan, H.K. and Shekhah, O. and Wieland, D.C.F. and Paulus, M. and Sternemann, C. and Schroer, M.A. and Tiemeyer, S. and Tolan, M. and Fischer, R.A. and Wöll, C.
    Journal of the American Chemical Society 133 8158-8161 (2011)
    We report the synthesis of layered [Zn 2(bdc) 2(H 2O) 2] and [Cu 2(bdc) 2(H 2O) 2] (bdc = benzdicarboxylate) metal-organic frameworks (MOF) carried out using the liquid-phase epitaxy approach employing self-assembled monolayer (SAM) modified Au-substrates. We obtain Cu and Zn MOF-2 structures, which have not yet been obtained using conventional, solvothermal synthesis methods. The 2D Cu 2+ dimer paddle wheel planes characteristic for the MOF are found to be strictly planar, with the planes oriented perpendicular to the substrate. Intercalation of an organic dye, DXP, leads to a reversible tilting of the planes, demonstrating the huge potential of these surface-anchored MOFs for the intercalation of large, planar molecules. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/ja2037996
  • 2011 • 50 Golden Perspective: Application of Laser-Generated Gold Nanoparticle Conjugates in Reproductive Biology
    Barchanski, A. and Taylor, U. and Klein, S. and Petersen, S. and Rath, D. and Barcikowski, S.
    Reproduction in Domestic Animals 46 42-52 (2011)
    Contents: The current demand for female calves has grown rapidly and controlling the sex of offspring provides an economically flexible management for the livestock producer. The only functioning method of efficiently producing separate populations of X and Y sperm in mammals is based on relative DNA differentiation by high-speed flow cytometry. In this context, gold nanoparticles conjugated to sex chromosome-specific moieties display promising application as novel fluorophor-alternative for the high-throughput screening, since they feature no photo bleaching, high quantum yield, good biocompatibility and the possibility of non-destructive membrane penetration. Especially, gold nanoparticles fabricated by pulsed laser ablation are in the recent focus of interest, due to excellent biocompatibility, fabrication-dependent, tuneable particle size as well as surface charge and ease of (bio)-functionalization with a remarkably strong ligand binding. For the purpose of our studies functionalized gold nanoparticles may be used as novel markers for sex-sorting of mammalian sperm and, depending on the selected probe, also for the selection of sperm with heritable DNA-sequences interesting for animal breeding. © 2011 Blackwell Verlag GmbH.
    view abstractdoi: 10.1111/j.1439-0531.2011.01844.x
  • 2011 • 49 Design of Bi-functional bioconjugated gold nanoparticles by pulsed laser ablation with minimized degradation
    Barchanski, A. and Sajti, C.L. and Sehring, C. and Petersen, S. and Barcikowski, S.
    Journal of Laser Micro Nanoengineering 6 124-130 (2011)
    Femtosecond-pulsed laser ablation of gold is investigated in electron donor moiety containing biomolecule solutions, resulting in the controlled design of biocompatible, partially charged and electrostatic-stabilized gold nanoconjugates by in situ surface functionalization. The cell-penetrating peptide penetratin and a targeting miniStrep aptamer sequence are conjugated simultaneously to a single nanoparticle for the first time, producing highly stable gold nanoparticle bi-conjugates with defined degrees of surface coverages for each biomolecule. Ablating gold by 100 μJ laser pulses in presence of 1 μM penetratin and 4.5 μM miniStrep yields an average bioconjugate size of 10 nm and results in 61 pmol cm-2 surface coverage for the aptamer and 30 pmol cm-2 for the penetratin. Whereat, degree of surface coverage shows direct correlation to process parameters as well as to biomolecule size, concentration and steric dimension of the biomolecules used. To investigate the influence of time-delayed bioconjugation in view of more efficient up-scaling process, a fast ex situ conjugation is presented in a continuous flow chamber secondary, allowing highly controlled bioconjugation without irradiating the photo- and thermal-sensitive biomolecules and permitting the use of nanosecond laser pulses.
    view abstractdoi: 10.2961/jlmn.2011.02.0006
  • 2011 • 48 Light driven reactions of single physisorbed azobenzenes
    Bazarnik, M. and Henzl, J. and Czajka, R. and Morgenstern, K.
    Chemical Communications 47 7764-7766 (2011)
    We present a successful attempt of decoupling a dye molecule from a metallic surface via physisorption for enabling direct photoisomerization. Effective switching between the isomers is possible by exposure to UV light via the rotation pathway. © 2011 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c1cc11578b
  • 2011 • 47 Nanoepitaxy using micellar nanoparticles
    Behafarid, F. and Roldan Cuenya, B.
    Nano Letters 11 5290-5296 (2011)
    The shape of platinum and gold nanoparticles (NPs) synthesized via inverse micelle encapsulation and supported on TiO 2(110) has been resolved by scanning tunneling microscopy. Annealing these systems at high temperature (∼1000 °C) and subsequent cooling to room temperature produced ordered arrays of well-separated three-dimensional faceted NPs in their equilibrium state. The observed shapes differ from the kinetically limited shapes of conventional physical vapor deposited NPs, which normally form two-dimensional flat islands upon annealing at elevated temperatures. The initial NP volume was found to provide a means to control the final NP shape. Despite the liquid-phase ex situ synthesis of the micellar particles, the in situ removal of the encapsulating ligands and subsequent annealing consistently lead to the development of a well-defined epitaxial relationship of the metal NPs with the oxide support. The observed epitaxial relationships could be explained in terms of the best overlap between the interfacial Pt (or Au) and TiO 2 lattices. In most cases, the ratio of {100}/{111} facets obtained for the NP shapes resolved clearly deviates from that of conventional bulklike Wulff structures. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/nl2027525
  • 2011 • 46 Comparison of nanoparticle-mediated transfection methods for DNA expression plasmids: Efficiency and cytotoxicity
    Durán, M.C. and Willenbrock, S. and Barchanski, A. and Müller, J.-M.V. and Maiolini, A. and Soller, J.T. and Barcikowski, S. and Nolte, I. and Feige, K. and Murua Escobar, H.
    Journal of Nanobiotechnology 9 (2011)
    Background: Reproducibly high transfection rates with low methodology-induced cytotoxic side effects are essential to attain the required effect on targeted cells when exogenous DNA is transfected. Different approaches and modifications such as the use of nanoparticles (NPs) are being evaluated to increase transfection efficiencies. Several studies have focused on the attained transfection efficiency after NP-mediated approaches. However, data comparing toxicity of these novel approaches with conventional methods is still rare.Transfection efficiency and methodology-induced cytotoxicity were analysed after transfection with different NP-mediated and conventional approaches. Two eukaryotic DNA-expression-plasmids were used to transfect the mammalian cell line MTH53A applying six different transfection protocols: conventional transfection reagent (FuGENE HD, FHD), FHD in combination with two different sizes of stabilizer-free laser-generated AuNPs (PLAL-AuNPs_S1,_S2), FHD and commercially available AuNPs (Plano-AuNP), and two magnetic transfection protocols. 24 h post transfection efficiency of each protocol was analysed using fluorescence microscopy and GFP-based flow cytometry. Toxicity was assessed measuring cell proliferation and percentage of propidium iodide (PI%) positive cells. Expression of the respective recombinant proteins was evaluated by immunofluorescence.Results: The addition of AuNPs to the transfection protocols significantly increased transfection efficiency in the pIRES-hrGFPII-eIL-12 transfections (FHD: 16%; AuNPs mean: 28%), whereas the magnet-assisted protocols did not increase efficiency. Ligand-free PLAL-AuNPs had no significant cytotoxic effect, while the ligand-stabilized Plano-AuNPs induced a significant increase in the PI% and lower cell proliferation. For pIRES-hrGFPII-rHMGB1 transfections significantly higher transfection efficiency was observed with PLAL-AuNPs (FHD: 31%; PLAL-AuNPs_S1: 46%; PLAL-AuNPs_S2: 50%), while the magnet-assisted transfection led to significantly lower efficiencies than the FHD protocol. With PLAL-AuNPs_S1 and _S2 the PI% was significantly higher, yet no consistent effect of these NPs on cell proliferation was observed. The magnet-assisted protocols were least effective, but did result in the lowest cytotoxic effect.Conclusions: This study demonstrated that transfection efficiency of DNA-expression-plasmids was significantly improved by the addition of AuNPs. In some combinations the respective cytotoxicity was increased depending on the type of the applied AuNPs and the transfected DNA construct. Consequently, our results indicate that for routine use of these AuNPs the specific nanoparticle formulation and DNA construct combination has to be considered. © 2011 Durán et al; licensee BioMed Central Ltd.
    view abstractdoi: 10.1186/1477-3155-9-47
  • 2011 • 45 Yolk-shell gold nanoparticles as model materials for support-effect studies in heterogeneous catalysis: Au, @C and Au, @ZrO2 for CO oxidation as an example
    Galeano, C. and Güttel, R. and Paul, M. and Arnal, P. and Lu, A.-H. and Schüth, F.
    Chemistry - A European Journal 17 8434-8439 (2011)
    The use of nanostructured yolk-shell materials offers a way to discriminate support and particle-size effects for mechanistic studies in heterogeneous catalysis. Herein, gold yolk-shell materials have been synthesized and used as model catalysts for the investigation of support effects in CO oxidation. Carbon has been selected as catalytically inert support to study the intrinsic activity of the gold nanoparticles, and for comparison, zirconia has been used as oxidic support. Au, @C materials have been synthesized through nanocasting using two different nonporous-core@mesoporous-shell exotemplates: Au@SiO 2@ZrO2 and Au@SiO2@m-SiO2. The catalytic activity of Au, @C with a gold core of about 14nm has been evaluated and compared with Au, @ZrO2 of the same gold core size. The strong positive effect of metal oxide as support material on the activity of gold has been proved. Additionally, size effects were investigated using carbon as support to determine only the contribution of the nanoparticle size on the catalytic activity of gold. Therefore, Au, @C with a gold core of about 7nm was studied showing a less pronounced positive effect on the activity than the metal oxide support effect. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201100318
  • 2011 • 44 Real-space imaging of inelastic Friedel-like surface oscillations emerging from molecular adsorbates
    Gawronski, H. and Fransson, J. and Morgenstern, K.
    Nano Letters 11 2720-2724 (2011)
    We report real space imaging measurements of inelastic Friedel oscillations. The inelastic electron tunneling spectroscopy, using scanning tunneling microscopy, around dimers of dichlorobenze adsorbates on Au(111) surface display clear spatial modulations that we attribute to inelastic scattering at the molecular sites caused by molecular vibrations. Due to local interactions between the adsorbate and the surface states, the molecular vibrations generate a redistribution of the charge density at energies in a narrow range around the inelastic mode. Our experimental findings are supported by theoretical arguments. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/nl201076g
  • 2011 • 43 3D self-assembled plasmonic superstructures of gold nanospheres: Synthesis and characterization at the single-particle level
    Gellner, M. and Steinigeweg, D. and Ichilmann, S. and Salehi, M. and Schütz, M. and Kömpe, K. and Haase, M. and Schlücker, S.
    Small 7 3445-3451 (2011)
    The synthesis of 3D self-assembled plasmonic superstructures of gold nanospheres as well as the characterization of their structural and optical properties at the single-particle level is presented. This experimental work is complemented by FEM (finite element method) simulations of elastic scattering spectra and the spatial |E| 4 distribution for establishing structure-activity correlations in these complex 3D nanoclusters. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/smll.201102009
  • 2011 • 42 Plasmonically active micron-sized beads for integrated solid-phase synthesis and label-free SERS analysis
    Gellner, M. and Niebling, S. and Kuchelmeister, H.Y. and Schmuck, C. and Schlücker, S.
    Chemical Communications 47 12762-12764 (2011)
    Self-assembly of gold nanospheres with a very thin glass shell onto the surface of beads yields a plasmonically active micron-sized substrate for integrated solid-phase synthesis and label-free SERS analysis. The proof-of-principle of this approach is demonstrated by the vibrational spectroscopic discrimination of three distinct amino acids and a dipeptide. © 2011 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c1cc13562g
  • 2011 • 41 Ohmic contacts to n-GaAs nanowires
    Gutsche, C. and Lysov, A. and Regolin, I. and Brodt, A. and Liborius, L. and Frohleiks, J. and Prost, W. and Tegude, F.-J.
    Journal of Applied Physics 110 (2011)
    We report on the technology and the electrical properties of two different contact systems on n-GaAs nanowires. Annealed Ge/Ni/Ge/Au and Pd/Ge/Au multilayer metallization were investigated. Rapid thermal annealing at temperatures common for identical contact systems on n-GaAs layers is found to be crucial due to an enhanced out-diffusion of the Ga component into the Au contact layer. The maximum annealing temperatures ensuring intact nanowires are 320°C for Ge/Ni/Ge/Au and 280 °C for Pd/Ge/Au. The fabricated Pd/Ge/Au contacts reveal a specific contacts resistance of 2.77×10-7 ωcm2, which is about one order of magnitude lower compared to the values of Ge/Ni/Ge/Au and also lower than Pd/Ge/Au contacts on bulk material (1.2 ×10-6 ωcm2). © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3603041
  • 2011 • 40 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 9678-9690 (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 abstractdoi: 10.1002/chem.201003764
  • 2011 • 39 Transient (000)-order attenuation effects in ultrafast transmission electron diffraction
    Ligges, M. and Rajkovi, I. and Streubhr, C. and Brazda, T. and Zhou, P. and Posth, O. and Hassel, C. and Dumpich, G. and Von Der Linde, D.
    Journal of Applied Physics 109 (2011)
    We discuss the observation of a transient (000)-order attenuation in time-resolved transmission electron diffraction experiments. It is shown that this effect causes a decrease of the diffraction intensity of all higher diffraction orders. This effect is not unique to specific materials as it was observed in thin Au, Ag and Cu films. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3554405
  • 2011 • 38 Anodic repassivation of low energy Au-implanted ultra-thin anodic Al 2O 3
    Mardare, A.I. and Melnikov, A. and Wieck, A.D. and Hassel, A.W.
    Physica Status Solidi (A) Applications and Materials Science 208 1270-1274 (2011)
    Ultrathin anodic alumina with a film thickness of 11nm was implanted by Au atoms with low energy of 2, 5 or 10keV. Stopping range simulations yielded three essentially different geometries ranging from surface near implantation over well penetrated oxide to near oxide metal interface implantation, covering the entire range of possible implantation modifications. This work aims at demonstrating how to perform band gap engineering in alumina not only on an energetic level but also targeting a certain geometrical position of the doping atoms by means of the implantation parameters. Beside the intended implantation the oxide destruction in the implantation path and its possible repair was of interest. The repassivation behaviour was considerably different showing a significant redox contribution of the gold nanoclusters on top of the simple oxide repassivation. Near surface implanted Au remained electrochemically active for low repassivation potentials. Higher repassivation potentials always buried the implanted Au atoms under anodic alumina. The repassivation charge determined allowed determining the volume destructed by the implantation. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.201001212
  • 2011 • 37 Hydrodynamic size distribution of gold nanoparticles controlled by repetition rate during pulsed laser ablation in water
    Menéndez-Manjón, A. and Barcikowski, S.
    Applied Surface Science 257 4285-4290 (2011)
    Most investigations on the laser generation and fragmentation of nanoparticles focus on Feret particle size, although the hydrodynamic size of nanoparticles is of great importance, for example in biotechnology for diffusion in living cells, or in engineering, for a tuned rheology of suspensions. In this sense, the formation and fragmentation of gold colloidal nanoparticles using femtosecond laser ablation at variable pulse repetition rates (100-5000 Hz) in deionized water were investigated through their plasmon resonance and hydrodynamic diameter, measured by Dynamic Light Scattering. The increment of the repetition rate does not influence the ablation efficiency, but produces a decrease of the hydrodynamic diameter and blue-shift of the plasmon resonance of the generated gold nanoparticles. Fragmentation, induced by inter-pulse irradiation of the colloids was measured online, showing to be more effective low repetition rates. The pulse repetition rate is shown to be an appropriate laser parameter for hydrodynamic size control of nanoparticles without further influence on the production efficiency. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2010.12.037
  • 2011 • 36 Structure and flow of droplets on solid surfaces
    Müller-Buschbaum, P. and Magerl, D. and Hengstler, R. and Moulin, J.-F. and Körstgens, V. and Diethert, A. and Perlich, J. and Roth, S.V. and Burghammer, M. and Riekel, C. and Gross, M. and Varnik, F. and Uhlmann, P. and Stamm, ...
    Journal of Physics Condensed Matter 23 (2011)
    The structure and flow of droplets on solid surfaces is investigated with imaging and scattering techniques and compared to simulations. To access nanostructures at the liquid-solid interface advanced scattering techniques such as grazing incidence small-angle x-ray scattering (GISAXS) with micro-and nanometer-sized beams, GISAXS and insitu imaging ellipsometry and GISAXS tomography are used. Using gold nanoparticle suspensions, structures observed in the wetting area due to deposition are probed insitu during the drying of the droplets. After drying, nanostructures in the wetting area and inside the dried droplets are monitored. In addition to drying, a macroscopic movement of droplets is caused by body forces acting on an inclined substrate. The complexity of the solid surfaces is increased from simple silicon substrates to binary polymer brushes, which undergo a switching due to the liquid in the droplet. Nanostructures introduced in the polymer brush due to the movement of droplets are observed. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/23/18/184111
  • 2011 • 35 Evolution of crystallinity of free gold agglomerates and shape transformation
    Nanda, K.K. and Maisels, A. and Kruis, F.E.
    RSC Advances 1 568-572 (2011)
    We report the shape evolution of free gold agglomerates with different morphologies that transform to ellipsoidal and then to spherical shapes during the heating cycle. The shape transformation is associated with a structural transition from polycrystalline to single crystalline. The structural transition temperature is shown to be dependent on the final size of the particles and not on the initial morphologies of the agglomerates. It is also shown that the transition occurs well below the melting temperature which is in contrast with the melt-freeze process reported in the literature. © The Royal Society of Chemistry 2011.
    view abstractdoi: 10.1039/c1ra00208b
  • 2011 • 34 Chemical composition of surface-functionalized gold nanoparticles
    Rostek, A. and Mahl, D. and Epple, M.
    Journal of Nanoparticle Research 13 4809-4814 (2011)
    The composition of surface-functionalized gold nanoparticles (diameter of the metallic core: 17-20 nm) was determined by elemental analysis (C, H, N, S, Au, Na) after preparation of a larger batch. Gold nanoparticles were prepared and functionalized with citrate according to the classical Turkevich method. The citrate-functionalized nanoparticles contained about 3.1 wt% of organic material (135 ng cm-2 or 3.1 molecules nm-2). A partial exchange of citrate was accomplished by tris(sodium-m-sulfonato-phenyl)phosphine (TPPTS) which led to 2.1 wt% of citrate (90 ng cm-2 or 2.1 molecules nm-2) and 1.4 wt% TPPTS (61 ng cm-2 or 0.6 molecules nm-2). The citrate coating was quantitatively exchanged by poly(N-vinyl pyrrolidone) (PVP) after immersion in solutions with concentrations of 33, 66 and 128 mg L-1, respectively, leading to contents of 4 to 6 wt% of PVP (171-271 ng cm-2 or 9-15 PVP monomer units nm -2). © 2011 Springer Science+Business Media B.V.
    view abstractdoi: 10.1007/s11051-011-0456-2
  • 2011 • 33 Delay time and concentration effects during bioconjugation of nanosecond laser-generated nanoparticles in a liquid flow
    Sajti, C.L. and Barchanski, A. and Wagener, P. and Klein, S. and Barcikowski, S.
    Journal of Physical Chemistry C 115 5094-5101 (2011)
    Fast ex situ functionalization of gold nanoparticles with fluorophore-labeled cell-penetrating peptides is investigated with a novel liquid flow cascade injection system. Successful conjugation is proved by various methods, such as UV-vis spectrometry and electron microscopy, whereas nanoparticle size-quenching is clearly observed. By variation of the peptide concentration introduced promptly after particle generation, gold nanoparticle bioconjugates with different degrees of cluster formation and/or aggregation and different peptide surface coverage values are obtained. The sizes of synthesized inorganic-organic gold nanoparticle bioconjugate show obvious correlation with time-delayed conjugation, giving evidence that laser-generated nanoparticles continue growing outside the cavitation bubble in the multisecond time scale until achieving their final size. Introducing 6.6 μM bioactive ligands, the highest conjugation efficiency of 93% and ? potential of 27.5 mV is reached at the shortest delay time (200 ms), resulting in 20 nm average sized bioconjugates. Finally, in a preliminary biological application, laser scanning confocal microscopy clearly revealed an amplified cellular uptake using HIV-1 transactivator peptide-conjugated gold nanoparticles compared with nonconjugated entities within embryonic fibroblasts after a short coincubation time of 1 h. The generation of high amounts of highly pure cell-penetrating nanomarkers by the nanosecond laser-assisted fast ex situ conjugation is thus a promising method to probe biological activities such as nanodrug internalization mechanisms. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/jp1093405
  • 2011 • 32 Electronic excitations induced by hydrogen surface chemical reactions on gold
    Schindler, B. and Diesing, D. and Hasselbrink, E.
    Journal of Chemical Physics 134 (2011)
    Associated with chemical reactions at surfaces energy may be dissipated exciting surface electronic degrees of freedom. These excitations are detected using metal-insulator-metal (MIM) heterostructures (Ta-TaOx-Au) and the reactions of H with and on a Au surface are probed. A current corresponding to 510-5 electrons per adsorbing H atom and a marked isotope effect are observed under steady-state conditions. Analysis of the current trace when the H atom flux is intermitted suggests that predominantly the recombination reaction creates electronic excitations. Biasing the front versus the back electrode of the MIM structure provides insights into the spectrum of electronic excitations. The observed spectra differ for the two isotopes H and D and are asymmetric when comparing negative and positive bias voltages. Modeling indicates that the excited electrons and the concurrently created holes differ in their energy distributions. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3523647
  • 2011 • 31 Immuno-surface-enhanced coherent anti-Stokes Raman scattering microscopy: Immunohistochemistry with target-specific metallic nanoprobes and nonlinear Raman microscopy
    Schlücker, S. and Salehi, M. and Bergner, G. and Schütz, M. and Ströbel, P. and Marx, A. and Petersen, I. and Dietzek, B. and Popp, J.
    Analytical Chemistry 83 7081-7085 (2011)
    Immunohistochemistry (IHC) is one of the most widely used staining techniques for diagnostic purposes. The selective localization of target proteins in tissue specimens by conventional IHC is achieved with dye- or enzyme-labeled antibodies in combination with light microscopy. In this contribution, we demonstrate the proof-of-principle for IHC based on surface-enhanced coherent Raman scattering for contrast generation. Specifically, antibody-labeled metallic nanoshells in conjunction with surface-enhanced coherent anti-Stokes Raman scattering (SECARS) microscopy are employed for the selective, sensitive, and rapid localization of the basal cell protein p63 in normal prostate tissue. Negative control experiments were performed in order to confirm the selective binding of the target-specific metal nanoprobes and to disentangle the role of plasmonic (metal) and molecular (Raman reporter) resonances in this plasmon-assisted four-wave mixing technique. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/ac201284d
  • 2011 • 30 Hydrophilically stabilized gold nanostars as SERS labels for tissue imaging of the tumor suppressor p63 by immuno-SERS microscopy
    Schütz, M. and Steinigeweg, D. and Salehi, M. and Kömpe, K. and Schlücker, S.
    Chemical Communications 47 4216-4218 (2011)
    A biocompatible, seed-mediated synthesis of monodisperse ∼60 nm gold nanostars, followed by hydrophilic stabilization with ethylene glycol-modified Raman reporter molecules, is presented. Their application as SERS labels for imaging of the tumor suppressor p63 in prostate biopsies by immuno-SERS microscopy is demonstrated. © 2011 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c0cc05229a
  • 2011 • 29 Fast and cost-effective purification of gold nanoparticles in the 20-250 nm size range by continuous density gradient centrifugation
    Steinigeweg, D. and Schütz, M. and Salehi, M. and Schlücker, S.
    Small 7 2443-2448 (2011)
    A multilayer quasi-continuous density gradient centrifugation method for separating 20-250 nm metal colloids with high size resolution while maintaining particle stability is presented. Colloidal mixtures containing monodisperse gold nanospheres and clusters thereof, in particular, gold dimers, are purified with yields up to 94%. The rapid method uses standard laboratory equipment. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/smll.201100663
  • 2011 • 28 Amphiphilic gold nanoparticles: Synthesis, characterization and adsorption to PEGylated polymer surfaces
    Tarnawski, R. and Ulbricht, M.
    Colloids and Surfaces A: Physicochemical and Engineering Aspects 374 13-21 (2011)
    The direct synthesis of water-soluble gold nanoparticles with a mixed shell of two different thiols, 1-mercaptoundec-11-yl-hexa(ethylene glycol) (EG6) and dodecanethiol (C12), and their characterization are reported. Data from IR spectroscopy and contact angle (CA) measurements as well as the solubility of the nanoparticles in water support that the composition of the shell is in the range of the thiol ratio used for synthesis (EG6:C12 = 72:28). Results of transmission electron microscopy and atomic force microscopy (AFM) for deposited particles as well as the UV-vis spectrum in solution are in line with a size of ≤10. nm. Self-assembled monolayers (SAMs) as model surfaces were prepared from mixtures of EG6 and C12 on planar gold films. Polystyrene (PSt) spin-coated films on silicon wafers and on gold-coated surface plasmon resonance (SPR) sensor disks were used as substrates for surface functionalization via adsorption/self-assembly of a polystyrene poly(ethylene glycol) diblock copolymer (PSt- b-PEG) from aqueous solutions. CA and AFM results revealed pronounced differences of the hydrophilicity/hydrophobicity and topography of the surface as a function of PSt- b-PEG concentration used for the modification. The adsorption of myoglobin and the novel gold nanoparticles to the PSt- b-PEGylated surfaces was analyzed by SPR. A control of adsorbed amounts by the degree of surface PEGylation, i.e. a reduction by up to 55% for the highest degree of modification, could be confirmed for both kinds of colloids. Adsorption of the novel gold nanoparticles to the mixed SAM surfaces as analyzed by SPR showed an even stronger dependency of surface composition. All experiments demonstrate that amphiphilic, water-soluble gold-based nanoparticles can be used as model colloids for the investigation of interactions with polymer surfaces of varied structure and architecture, and that they could be further developed for analytical or biological applications. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.colsurfa.2010.10.027
  • 2011 • 27 Synthesis of bifunctional Au/Pt/Au core/shell nanoraspberries for in situ SERS monitoring of platinum-catalyzed reactions
    Xie, W. and Herrmann, C. and Kömpe, K. and Haase, M. and Schlücker, S.
    Journal of the American Chemical Society 133 19302-19305 (2011)
    The synthesis of bifunctional Au/Pt/Au nanoraspberries for use in quantitative in situ monitoring of platinum-catalyzed reactions by surface-enhanced Raman scattering (SERS) is presented. Highly convolved SERS spectra of reaction mixtures can be decomposed into the contributions of distinct molecular species by multivariate data analysis. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/ja208298q
  • 2010 • 26 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 2617-2623 (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 abstractdoi: 10.1007/s00216-010-3926-y
  • 2010 • 25 Penetration of thin C 60 films by metal nanoparticles
    Duffe, S. and Grönhagen, N. and Patryarcha, L. and Sieben, B. and Yin, C. and Von Issendorff, B. and Moseler, M. and Hövel, H.
    Nature Nanotechnology 5 335-339 (2010)
    Metal nanoparticles supported by thin films are important in the fields of molecular electronics, biotechnology and catalysis, among others. Penetration of these nanoparticles through their supporting films can be undesirable in some circumstances but desirable in others, and is often considered to be a diffusive process. Here, we demonstrate a mechanism for the penetration of thin films and other nanoscopic barriers that is different from simple diffusion. Silver clusters that are soft-landed onto a monolayer of C 60 supported by gold sink through the monolayer in a matter of hours. However, the clusters are stable when landed onto two monolayers of C 60 supported on gold, or on one monolayer of C 60 supported on graphite. With backing from atomistic calculations, these results demonstrate that a metallic substrate exerts attractive forces on metallic nanoparticles that are separated from the substrate by a single monolayer. © 2010 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/nnano.2010.45
  • 2010 • 24 Planar Au/TiO2 model catalysts: Fabrication, characterization and catalytic activity
    Eyrich, M. and Kielbassa, S. and Diemant, T. and Biskupek, J. and Kaiser, U. and Wiedwald, U. and Ziemann, P. and Bansmann, J.
    ChemPhysChem 11 1430-1437 (2010)
    Different types of planar Au/TiO2 model catalysts are produced on TiO2(110) single-crystal substrates and thin TiO2 films on Ru(0001) by physical vapor deposition of gold under ultrahigh-vacuum (UHV) conditions or by micelle-based chemical routes. Both the Au nanoparticles and the support are characterized by surface-science-based methods (such as atomic force microscopy and X-ray photoelectron spectroscopy) as well as by transmission electron microscopy. Finally, the activity of the model catalysts in the CO oxidation reaction is analyzed in a microflow reactor. Au/TiO2(110) model catalysts with a stoichiometric TiO2(110) support exhibit only a low catalytic activity compared to those with a reduced crystal and Au/TiO2 model catalysts with thin TiO2 films on Ru(0001) as a substrate. The possible influence of Ti interstitials in the reduced TiO2(110) substrates on the CO oxidation activity is discussed.© 2010 Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cphc.200900942
  • 2010 • 23 Ex-post size control of high-temperature-stable yolk-shell Au,@ZrO 2 catalysts
    Güttel, R. and Paul, M. and Schüth, F.
    Chemical Communications 46 895-897 (2010)
    Yolk-shell catalysts have attracted interest in both academia and industry, since they combine high-temperature stability with a reduced complexity for kinetic and mechanistic investigations. This contribution presents a possibility to adjust the size of an active gold core inside a porous zirconia shell via an ex-post-modification approach. © The Royal Society of Chemistry 2010.
    view abstractdoi: 10.1039/b921792d
  • 2010 • 22 Fabrication of two-dimensional Au@FePt core-shell nanoparticle arrays by photochemical metal deposition
    Härtling, T. and Uhlig, T. and Seidenstücker, A. and Bigall, N.C. and Olk, P. and Wiedwald, U. and Han, L. and Eychmüller, A. and Plettl, A. and Ziemann, P. and Eng, L.M.
    Applied Physics Letters 96 (2010)
    In this report, we experimentally demonstrate that single platinum nanoparticles exhibit the necessary catalytic activity for the optically induced reduction of H [AuCl4] complexes to elemental gold. This finding is exploited for the parallel Au encapsulation of FePt nanoparticles arranged in a self-assembled two-dimensional array. Magnetic force microscopy reveals that the thin gold layer formed on the FePt particles leads to a strongly increased long-term stability of their magnetization under ambient conditions. © 2010 American Institute of Physics.
    view abstractdoi: 10.1063/1.3425670
  • 2010 • 21 Small gold particles supported on MgFe2O4 nanocrystals as novel catalyst for CO oxidation
    Jia, C.-J. and Liu, Y. and Schwickardi, M. and Weidenthaler, C. and Spliethoff, B. and Schmidt, W. and Schüth, F.
    Applied Catalysis A: General 386 94-100 (2010)
    We present the study on the catalytic performance of gold particles supported on spinel type MgFe2O4 nanocrystals (Au/MgFe2O4) which exhibit high activity for low temperature CO oxidation. Using XRD, TEM, XPS and CO titration techniques, we investigated the effect of the pretreatment atmosphere on the structure and catalytic properties of the Au/MgFe2O4 catalyst in CO oxidation. TEM, XPS and XRD showed that the pretreatment atmosphere had a negligible effect on the particle size distribution, chemical states of the gold, and the structure of the support. Among the various pretreated catalysts, O2-Au/MgFe2O4 exhibits superior activity, indicating that pretreatment in oxidative atmosphere induced the high capability of the catalyst to activate CO and supply active oxygen for CO oxidation as confirmed by CO titration experiments. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.apcata.2010.07.036
  • 2010 • 20 Very low temperature CO oxidation over colloidally deposited gold nanoparticles on Mg(OH)2 and MgO
    Jia, C.-N. and Liu, Y. and Bongard, H. and Schüth, F.
    Journal of the American Chemical Society 132 1520-1522 (2010)
    (Figure Presented) The colloidal deposition method was used to prepare Au/Mg(OH)2 (0.7 wt % gold) catalysts with gold particle sizes between 1.5 to 5 nm which exhibited very high activity for CO oxidation with specific rates higher than 3.7 molCO·h-1·g Au-1 even at temperatures as low as -89° C. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/ja909351e
  • 2010 • 19 Probing the reactivity of ZnO and Au/ZnO nanoparticles by methanol adsorption: A TPD and DRIFTS study
    Kähler, K. and Holz, M.C. and Rohe, M. and Strunk, J. and Muhler, M.
    ChemPhysChem 11 2521-2529 (2010)
    The adsorption of methanol on pure ZnO and A--u-decorated ZnO nanoparticles and its thermal decomposition monitored by temperature-programmed desorption (TPD) experiments and by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), both applied under continuous flow conditions in fixed bed reactors, is reported. Two distinguishable methoxy species are formed during methanol adsorption on ZnO differing in the C-O stretching bands. During the subsequent TPD experiments two different H2peaks are observed, indicating the conversion of methoxy into formate species. By applying different heating rates, activation energies of 109 kJmol-1 and 127 kJmol-1 for the selective oxidation of the two methoxy species are derived. Correspondingly, the methoxy decomposition results in two distinguishable formate species, which are identified by the asymmetric and symmetric OCO stretching bands on pure ZnO and Au/ZnO. Based on the decreased intensities of the OH bands during methanol adsorption, which are specific for the various ZnO single crystal surfaces, on the different reactivities of these surfaces, and on the formate FTIR bands observed on ZnO single crystal surfaces, the two methoxy and the corresponding formate species are identified to be adsorbed on the exposed less reactive non-polar ZnO(101̄0) surface and on the highly reactive polar ZnO(0001̄) surface. The simultaneous formation of H2, CO, and CO2 at about 550-600 K during the TPD experiments indicate the decomposition of adsorbed formate species. The CO/CO2 ratio decreases with increasing Au loading, and a broad band due to electronic transitions from donor sites to the conduction band is observed in the DRIFT spectra for the Au-decorated ZnO nanoparticles. Thus, the presence of the Au nanoparticles results in an enhanced reducibility of ZnO facilitating the generation of oxygen vacancies. © 2010 Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cphc.201000282
  • 2010 • 18 Quantification of colloidal and intracellular gold nanomarkers down to the single particle level using confocal microscopy
    Klein, S. and Petersen, S. and Taylor, U. and Rath, D. and Barcikowski, S.
    Progress in Biomedical Optics and Imaging - Proceedings of SPIE 7573 (2010)
    The high quantum yield and exclusively photo-stable excitation of gold nanoparticles combined with their bio-inert characteristics make them ideal cellular markers. The aim of the study was to visualise gold nanoparticles size-dependently as colloid and in cells after co-incubation. We show the quantification of colloidal gold particles by standard confocal microscopy down to the single particle level. A calibration is demonstrated for pixel numbers in dilution series of uncoated gold nanoparticles. We give implications for practical use of advanced cellular imaging in cultured cells. © 2010 Copyright SPIE - The International Society for Optical Engineering.
    view abstractdoi: 10.1117/12.840985
  • 2010 • 17 Quantitative visualization of colloidal and intracellular gold nanoparticles by confocal microscopy
    Klein, S. and Petersen, S. and Taylor, U. and Rath, D. and Barcikowski, S.
    Journal of Biomedical Optics 15 (2010)
    Gold nanoparticles (AuNPs) have the potential to become a versatile biomarker. For further use of AuNPs labeled with functionalized molecules, their visualization in biological systems by routine laboratory tools such as light microscopy is crucial. However, the size far below the diffraction limit affords specialized parameters for microscopical detection, which stimulated the current study, aimed to determine from which size onward AuNPs, either in dispersion or cell-associated, can be reliably detected by standard confocal microscopy. First, gold colloids of size-restricted fractions are examined in dispersion. At a minimum particle size of 60 nm, detection appears to be reliable. Particle counts in dilution series confirm these results by revealing single particle detection of 60-nm colloids. Second, AuNPs are visualized and quantified in cells, which interestingly cause a phase shift in the reflection of AuNPs. Gold mass spectroscopy confirms the number of AuNPs counted microscopically inside cells. Furthermore, it demonstrates for the first time a very high diffusion rate of 15-nm particles into the cells. In conclusion, the results back the suitability of confocal microscopy for the quantitative tracking of colloidal and intracellular gold nanoparticles sized 60 nm. © 2010 Society of Photo-Optical Instrumentation Engineers.
    view abstractdoi: 10.1117/1.3461170
  • 2010 • 16 Combined ellipsometry and X-ray related techniques for studies of ultrathin organic nanocomposite films
    Krämer, M. and Roodenko, K. and Pollakowski, B. and Hinrichs, K. and Rappich, J. and Esser, N. and Von Bohlen, A. and Hergenröder, R.
    Thin Solid Films 518 5509-5514 (2010)
    Ultrathin nanocomposite films of nitrobenzene on silicon were analyzed by Infrared Spectroscopic Ellipsometry (IRSE), X-ray reflectivity (XRR) and X-ray standing waves (XSW) before and after evaporation of gold. Infrared Spectroscopic Ellipsometry measurements were performed for identification of adsorbates and for investigation of the molecular orientation. Results for film thickness were correlated with XRR measurements. Further, XSW measurements of elements incorporated in nitrobenzene (C, N, and O) were performed with soft X-rays. The combination of the different methods allowed to confirm a model for the electrochemically deposited nitrobenzene films before and after gold evaporation. The characterization by XRR and XSW scans using hard X-rays showed that gold had penetrated into the nitrobenzene film and thus changed density and optical properties of this layer significantly. A depth profile correlated to the electron density is deduced from the XRR measurements. This profile allows to localize-in vertical direction-gold islands within the composite film. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2010.04.033
  • 2010 • 15 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 2854-2861 (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 abstractdoi: 10.1002/cphc.200900881
  • 2010 • 14 Substrate size-selective catalysis with zeolite-encapsulated gold nanoparticles
    Laursen, A.B. and Højholt, K.T. and Lundegaard, L.F. and Simonsen, S.B. and Helveg, S. and Schüth, F. and Paul, M. and Grunwaldt, J.-D. and Kegnœs, S. and Christensen, C.H. and Egeblad, K.
    Angewandte Chemie - International Edition 49 3504-3507 (2010)
    The Dark Crystal: A hybrid material is reported that is comprised of 1-2 nm sized gold nanoparticles, accessible only through zeolite micropores in a silicalite-1 crystal, as shown by three-dimensional TEM tomography (see picture). Calcination experiments indicate that the embedded nanoparticles are highly stable towards sintering. Figure Equation Present. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/anie.200906977
  • 2010 • 13 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 12 939-943 (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 abstractdoi: 10.1016/j.elecom.2010.04.026
  • 2010 • 12 From glycerol to allyl alcohol: Iron oxide catalyzed dehydration and consecutive hydrogen transfer
    Liu, Y. and Tüysüz, H. and Jia, C.-J. and Schwickardi, M. and Rinaldi, R. and Lu, A.-H. and Schmidt, W. and Schüth, F.
    Chemical Communications 46 1238-1240 (2010)
    Using iron oxide as catalyst, glycerol can be converted to allyl alcohol through a dehydration and consecutive hydrogen transfer. © 2010 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/b921648k
  • 2010 • 11 Highly active iron oxide supported gold catalysts for CO oxidation: How small must the gold nanoparticles be?
    Liu, Y. and Jia, C.-N. and Yamasaki, J. and Terasaki, O. and Schüth, F.
    Angewandte Chemie - International Edition 49 5771-5775 (2010)
    (Figure Presented) The shape of gold: The title catalyst has been prepared through a colloidal deposition method. Scanning transmission electron microscopy studies confirmed that for the catalyst, gold clusters with a bilayer structure and a diameter of about 0.5 nm are not mandatory to achieve the high activity (see image). © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201000452
  • 2010 • 10 Gold nanoparticles: Dispersibility in biological media and cell-biological effect
    Mahl, D. and Greulich, C. and Meyer-Zaika, W. and Köller, M. and Epple, M.
    Journal of Materials Chemistry 20 6176-6181 (2010)
    Spherical gold nanoparticles with a hydrodynamic diameter between 25 and 37 nm were prepared and stabilised with poly(N-vinylpyrrolidone) (PVP) or tris(sodium-m-sulfonatophenyl)phosphine (TPPTS). They were subjected to different cell culture media, e.g. pure RPMI, RPMI containing up to 10% of fetal calf serum (FCS), and RPMI containing up to 10% of bovine serum albumin (BSA), and the rate of agglomeration was studied by dynamic light scattering. In pure RPMI, a strong agglomeration was observed whereas in the RPMI-FCS and RPMI-BSA mixtures the particles remained well dispersed above 1 wt% protein concentration. The effect of PVP-stabilised gold nanoparticles on human mesenchymal stem cells (hMSC) was studied as well. No significant influence on the viability and chemotaxis was observed after incubation of hMSC with gold nanoparticles. However, gold nanoparticles induced the activation of hMSC as indicated by the release of IL-6 and IL-8. © 2010 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c0jm01071e
  • 2010 • 9 Sub-wavelength patterning of organic monolayers via nonlinear processing with continuous-wave lasers
    Mathieu, M. and Hartmann, N.
    New Journal of Physics 12 (2010)
    In recent years, nonlinear processing with continuous-wave lasers has been demonstrated to be a facile means of rapid nanopatterning of organic monolayers down to the sub-100 nm range. In this study, we report on laser patterning of thiol-based organic monolayers with sub-wavelength resolution. Au-coated silicon substrates are functionalized with 1-hexadecanethiol. Irradiation with a focused beam of an Ar+ laser operating at λ = 514 nm allows one to locally remove the monolayer. Subsequently, the patterns are transferred into the Au film via selective etching in a ferri-/ferrocyanide solution. Despite a 1/e2 spot diameter of about 2.8 μm, structures with lateral dimensions down to 250 nm are fabricated. The underlying nonlinear dependence of the patterning process on laser intensity is traced back to the interplay between the laser-induced transient local temperature rise and the thermally activated desorption of the thiol molecules. A simple thermokinetic analysis of the data allows us to determine the effective kinetic parameters. These results complement our previous work on photothermal laser patterning of ultrathin organic coatings, such as silane-based organic monolayers, organo/silicon interfaces and supported membranes. A general introduction to nonlinear laser processing of organic monolayers is presented. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/12/12/125017
  • 2010 • 8 Influence of water temperature on the hydrodynamic diameter of gold nanoparticles from laser ablation
    Menéndez-Manjón, A. and Chichkov, B.N. and Barcikowski, S.
    Journal of Physical Chemistry C 114 2499-2504 (2010)
    Defined hydrodynamic properties of nanoparticle colloids are required for applications in dosimetry, rheology, or biosensing studies. During the generation of nanoparticles by laser ablation of a solid target in liquids, the temperature of the liquid increases, which may effect cavitation bubble and particle formation. We demonstrate that this temperature variation influences the hydrodynamic diameter of the resulting colloidal nanoparticles when a gold target is ablated by an IR femtosecond laser in water at different stabilized liquid temperatures in the range of 283-353 K. The maximum hydrodynamic diameter was observed at 330 K, the temperature at which the compressibility of water reaches its minimum. The formation of particles by condensation of ablated species in the liquid matrix or inside the confined cavitation bubble is discussed, as well as the influence of the physical properties of the liquid that vary with temperature, such as viscosity and compressibility. The reduction of the hydrodynamic particle diameter at the higher compressible state of water indicates that a lower number of agglomerates are dispersed in the liquid, reducing the polydispersity index of the gold colloid. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/jp909897v
  • 2010 • 7 Influence of the microstructure of gold-zirconia yolk-shell catalysts on the CO oxidation activity
    Pandey, A.D. and Güttel, R. and Leoni, M. and Schüth, F. and Weidenthaler, C.
    Journal of Physical Chemistry C 114 19386-19394 (2010)
    The gold-zirconia yolk-shell system is an interesting catalyst for CO oxidation. The size distribution of the gold nanoparticles is very narrow, and they are well separated from each other also after treatment at high temperature, which is due to their encapsulation in crystalline zirconia hollow spheres. Because this allows thermal and chemical treatment without affecting the size distribution, different defect structures of the gold nanoparticles can be induced, and the effect on catalytic activity can be investigated. Line profile analysis of the powder diffraction data based on the whole powder pattern modeling approach was used to determine the domain size distribution and lattice defects present in this two-phase system. The influence of different diffractometer setups on the results of the line profile analysis was also investigated. Variation of the chemical and thermal treatment procedures allowed altering the microstructure of the system. The resulting catalysts showed substantial variation in the activity for CO oxidation. Lower dislocation densities and less stacking faults result in decreased catalytic activity. These contributions to activity could be studied without any superimposed size effect due to the constant gold particle sizes. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/jp106436h
  • 2010 • 6 In-situ bioconjugation in stationary media and in liquid flow by femtosecond laser ablation
    Sajti, C.L. and Petersen, S. and Menéndez-Manjón, A. and Barcikowski, S.
    Applied Physics A: Materials Science and Processing 101 259-264 (2010)
    In-situ functionalization of gold nanoparticles with fluorophore-tagged oligonucleotides is studied by comparing femtosecond laser ablation in stationary liquid and in biomolecule flow. Femtosecond laser pulses induce significant degradation to sensitive biomolecules when ablating gold in a stationary solution of oligonucleotides. Contrary, in-situ conjugation of nanoparticles in biomolecule flow considerably reduces the degree of degradation studied by gel electrophoresis and UV-Vis spectrometry. Ablating gold with 100 μJ femtosecond laser pulses DNA sequence does not degrade, while the degree of fluorophore tag degradation was 84% in stationary solution compared to 5% for 1 mL/min liquid flow. It is concluded that femtosecond laser-induced degradation of biomolecules is triggered by absorption of nanoparticle conjugates suspended in the colloid and not by ablation of the target. Quenching of nanoparticle size appears from 0.5 μM biomolecule concentration for 0.3 μg/s nanoparticle productivity indicating the successful surface functionalization. Finally, increasing the liquid flow rate from stationary to 450 mL/min enhances nanoparticle productivity from 0.2 μg/s to 1.5 μg/s, as increasing liquid flow allows removal of light absorbing nanoparticles from the ablation zone, avoiding attenuation of subsequent laser photons. © 2010 The Author(s).
    view abstractdoi: 10.1007/s00339-010-5813-y
  • 2010 • 5 Micro- and nanopatterning of functional organic monolayers on oxide-free silicon by laser-induced photothermal desorption
    Scheres, L. and Klingebiel, B. and Ter Maat, J. and Giesbers, M. and De Jong, H. and Hartmann, N. and Zuilhof, H.
    Small 6 1918-1926 (2010)
    The photothermal laser patterning of functional organic monolayers, prepared on oxide-free hydrogen-terminated silicon, and subsequent backfi lling of the laser-written lines with a second organic monolayer that differs in its terminal functionality, is described. Since the thermal monolayer decomposition process is highly nonlinear in the applied laser power density, subwavelength patterning of the organic monolayers is feasible. After photothermal laser patterning of hexadecenyl monolayers, the lines freed up by the laser are backfi lled with functional acid fl uoride monolayers. Coupling of cysteamine to the acid fl uoride groups and subsequent attachment of Au nanoparticles allows easy characterization of the functional lines by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Depending on the laser power and writing speed, functional lines with widths between 1.1 μm and 250 nm can be created. In addition, trifl uoroethyl-terminated (TFE) monolayers are also patterned. Subsequently, the decomposed lines are backfi lled with a nonfunctional hexadecenyl monolayer, the TFE stripes are converted into thiol stripes, and then finally covered with Au nanoparticles. By reducing the lateral distance between the laser lines, Au-nanoparticle stripes with widths close to 100 nm are obtained. Finally, in view of the great potential of this type of monolayer in the fi eld of biosensing, the ease of fabricating biofunctional patterns is demonstrated by covalent binding of fl uorescently labeled oligo-DNA to acidfl uoride-backfi lled laser lines, which-as shown by fl uorescence microscopy-is accessible for hybridization.Copyright © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/smll.201000189
  • 2010 • 4 Nonendosomal cellular uptake of ligand-free, positively charged gold nanoparticles
    Taylor, U. and Klein, S. and Petersen, S. and Kues, W. and Barcikowski, S. and Rath, D.
    Cytometry Part A 77 439-446 (2010)
    Gold nanoparticles (GNPs) have interesting optical properties, such as exceptionally high quantum yields and virtually limitless photostability. Therefore, they show the potential for applications as biomarkers especially suitable for in vivo and long-term studies. The generation of GNPs using pulsed laser light rather than chemical means provides nanoparticles, which are remarkably stable in a variety of media without the need of stabilizing agents or ligands. This stabilization is achieved by partial oxidation of the gold surface resulting in positively charged GNPs. However, little is known about cellular uptake of such ligand-free nanoparticles, their intracellular fate, or cell viability after nanoparticle contact. The current work is aimed to explore the response of a bovine cell line to GNP exposure mainly using laser scanning confocal microscopy (LSCM) supported by other techniques. Cultured bovine immortalized cells (GM7373) were coincubated with GNP (average diameter 15 nm, 50 μM Au) for 2, 24, and 48 h. The detection of GNP-associated light scattering by the LSCM facilitated a clear distinction between GNP-containing cells and the negative controls. After 48 h, 75% of cells had visibly incorporated nanoparticles. No colocalization was detected with either Rab5a or Lamp1-positive structures, i.e., endosomes or lysosomes, respectivley. However, transmission electron microscope analysis of GNP-coincubated cells indicated the nanoparticles to be positioned within electron-dense structures. Coincubation at 4°C did not inhibit nanoparticle uptake, suggesting diffusion as possible entrance mechanism. Although the assessment of cell morphology, membrane integrity, and apoptosis revealed no GNP-related loss of cell viability at a gold concentration of 25 μM or below, a cytotoxic effect was observed in a proliferation assay after exposing low cell numbers to 50 μM Au and above. In conclusion, this study confirmed the cellular uptake of ligand-free gold nanoparticles during coincubation apparently without using endocytic pathways. © 2010 International Society for Advancement of Cytometry.
    view abstractdoi: 10.1002/cyto.a.20846
  • 2010 • 3 Optical response of metal-insulator-metal heterostructures and their application for the detection of chemicurrents
    Thissen, P. and Schindler, B. and Diesing, D. and Hasselbrink, E.
    New Journal of Physics 12 (2010)
    The optical response of thin-film metal-insulator-metal (MIM) systems of tantalum-tantalum oxide-Au type is studied by recording the macroscopic current across the device resulting from the low-energy electron-hole pairs excited in the metals by red and near-infrared (NIR) light (hν < 2 eV). It is observed that current flows from the top Au to the back Ta electrode, although a larger number of photons is absorbed in the latter. This directional preference is attributed to the built-in electric field across the oxide layer. The yield per photon increases strongly as photon energy becomes comparable to the barrier height. Current exhibits a strong dependence on bias voltages applied across the oxide layer. Photoyields induced by NIR light (hν ∼ 1.5 eV) were found to be comparable to recently observed chemicurrents arising from exposure of a MIM sensor to atomic hydrogen, when compared on a current per photon to current per impinging hydrogen atom basis. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/12/11/113014
  • 2010 • 2 Laser ablation-based one-step generation and bio-functionalization of gold nanoparticles conjugated with aptamers
    Walter, J.G. and Petersen, S. and Stahl, F. and Scheper, T. and Barcikowski, S.
    Journal of Nanobiotechnology 8 (2010)
    Background: Bio-conjugated nanoparticles are important analytical tools with emerging biological and medical applications. In this context, in situ conjugation of nanoparticles with biomolecules via laser ablation in an aqueous media is a highly promising one-step method for the production of functional nanoparticles resulting in highly efficient conjugation. Increased yields are required, particularly considering the conjugation of cost-intensive biomolecules like RNA aptamers.Results: Using a DNA aptamer directed against streptavidin, in situ conjugation results in nanoparticles with diameters of approximately 9 nm exhibiting a high aptamer surface density (98 aptamers per nanoparticle) and a maximal conjugation efficiency of 40.3%. We have demonstrated the functionality of the aptamer-conjugated nanoparticles using three independent analytical methods, including an agglomeration-based colorimetric assay, and solid-phase assays proving high aptamer activity. To demonstrate the general applicability of the in situ conjugation of gold nanoparticles with aptamers, we have transferred the method to an RNA aptamer directed against prostate-specific membrane antigen (PSMA). Successful detection of PSMA in human prostate cancer tissue was achieved utilizing tissue microarrays.Conclusions: In comparison to the conventional generation of bio-conjugated gold nanoparticles using chemical synthesis and subsequent bio-functionalization, the laser-ablation-based in situ conjugation is a rapid, one-step production method. Due to high conjugation efficiency and productivity, in situ conjugation can be easily used for high throughput generation of gold nanoparticles conjugated with valuable biomolecules like aptamers. © 2010 Walter et al; licensee BioMed Central Ltd.
    view abstractdoi: 10.1186/1477-3155-8-21
  • 2010 • 1 Preparation and characterization of supported magnetic nanoparticles prepared by reverse micelles
    Wiedwald, U. and Han, L. and Biskupek, J. and Kaiser, U. and Ziemann, P.
    Beilstein Journal of Nanotechnology 1 24-47 (2010)
    Monatomic (Fe, Co) and bimetallic (FePt and CoPt) nanoparticles were prepared by exploiting the self-organization of precursor loaded reverse micelles. Achievements and limitations of the preparation approach are critically discussed. We show that selfassembled metallic nanoparticles can be prepared with diameters d = 2-12 nm and interparticle distances D = 20-140 nm on various substrates. Structural, electronic and magnetic properties of the particle arrays were characterized by several techniques to give a comprehensive view of the high quality of the method. For Co nanoparticles, it is demonstrated that magnetostatic interactions can be neglected for distances which are at least 6 times larger than the particle diameter. Focus is placed on FePt alloy nanoparticles which show a huge magnetic anisotropy in the L10 phase, however, this is still less by a factor of 3-4 when compared to the anisotropy of the bulk counterpart. A similar observation was also found for CoPt nanoparticles (NPs). These results are related to imperfect crystal structures as revealed by HRTEM as well as to compositional distributions of the prepared particles. Interestingly, the results demonstrate that the averaged effective magnetic anisotropy of FePt nanoparticles does not strongly depend on size. Consequently, magnetization stability should scale linearly with the volume of the NPs and give rise to a critical value for stability at ambient temperature. Indeed, for diameters above 6 nm such stability is observed for the current FePt and CoPt NPs. Finally, the long-term conservation of nanoparticles by Au photoseeding is presented. © 2010 Wiedwald et al; licensee Beilstein-Institut.
    view abstractdoi: 10.3762/bjnano.1.5