Silke Behrens

Synthesis and Characterization

As compared to bulk materials, magnetic nanoparticles possess distinct magnetic properties and attempts have been made to exploit their beneficial properties for technical and biomedical applications, e.g. for magnetic fluids, high-density magnetic recording, or biomedical diagnosis and therapy. Early magnetic fluids (MFs) were produced by grinding magnetite with heptane or long chain hydrocarbon and a grinding agent, e.g. oleic acid [152]. Later procedures for MFs precipitated Fe 3+/Fe 2+ of an aqueous solution with a base, coated the particles by oleic acid, and dispersed them in carrier liquid [161]. However, besides the elemental composition and crystal structure of the applied magnetic particles, particle size and particle size distribution determine the properties of the resulting MF. Many methods for nanoparticle synthesis including the preparation of metallic magnetic particles have been described in the literature. However, there still remain important questions, e.g. concerning control of particle size, shape, and monodispersity as well as their stability towards oxidation. Moreover, peptization by suitable surfactants or polymers into stable MFs is an important issue since each application in engineering or biomedicine needs special MFs with properties adjusted to the requirements of the system.

Synthesis of inorganic nanomaterials mediated by protein assemblies

Biological materials naturally display a variety of functional structures highly organized at the nanoscale. Various bioassemblies have been shown to template complex, multidimensional inorganic nanoarchitectures that are typically not available by conventional synthetic methodologies. In addition to the various naturally occurring templates, the powerful techniques developed by life sciences are an interesting tool for engineering approaches towards material science. Besides a structural and morphological control during synthesis, biotemplating procedures may add another dimension to inorganic materials such as biofunctionality (e.g., motility functions). This overview is focussed on recent advances in the fabrication of inorganic nanomaterials taking advantage of protein assemblies. It tries to elucidate chemical methodologies and reviews examples of templates based on protein and peptide building blocks that have been successfully employed to manufacture inorganic nanostructures.

Surface engineering of Co and FeCo nanoparticles for biomedical application

Monodisperse Co, Fe, and FeCo nanoparticles are prepared via thermal decomposition of metal carbonyls in the presence of aluminium alkyls, yielding air-stable magnetic metal nanoparticles after surface passivation. The particles are characterized by electron microscopy (SEM, TEM, ESI), electron spectroscopy (MIES, UPS, and XPS) and x-ray absorption spectroscopy (EXAFS). The particles are peptized by surfactants to form stable magnetic fluids in various organic media and water, exhibiting a high volume concentration and a high saturation magnetization. In view of potential biomedical applications of the particles, several procedures for surface modification are presented, including peptization by functional organic molecules, silanization, and in situ polymerization.

Highly Active Bimetallic Nickel–Palladium Alloy Nanoparticle Catalyzed Suzuki–Miyaura Reactions

A bimetallic Ni–Pd alloy nanoparticle catalyst with a low palladium content (Ni0.90Pd0.10 nanocatalyst) was prepared and its catalytic performance in Suzuki–Miyaura reactions was evaluated along with that of other Ni–Pd nanocatalysts (with varying Ni/Pd molar ratios in the range of ≈0.25–0.75) and the corresponding monometallic, Ni and Pd, analogues. Notably, the bimetallic Ni0.90Pd0.10 alloy nanocatalyst performed exceptionally well for the synthesis of biaryls by employing a wide range of substituted aryl halides and arylboronic acids having electron‐donating and electron‐withdrawing groups, and they exhibited high recyclability in water/ethanol solution at moderate reaction temperatures. Catalyst poisoning tests and leaching experiments inferred the heterogeneous nature of the Ni0.90Pd0.10 nanocatalysts. The significant synergistic interactions between Ni and Pd account for the observed high catalytic efficacy of the Ni0.90Pd0.10 nanocatalyst.

Air-stable Co-, Fe-, and Fe/Co-nanoparticles and ferrofluids

Summary Air-stable Co, Fe, and Fe/Co nanoparticles are accessible by thermolysis of the metal carbonyl precursors in the presence of aluminium alkyls and subsequent “smooth oxidation”. The structure of the particles was investigated by transmission electron microscopy (TEM, HRTEM), X-ray absorption spectroscopy (XAS), X-ray and ultraviolet photoelectron spectroscopy (XPS, UPS), metastable impact electron spectroscopy (MIES), and small-angle neutron scattering (SANS). The peptization of the nanoparticles with suitable surfactants (oleic and lauric acid, sodium dioctylsulfosuccinate (AOT), LP-4 (a fatty acid condensation polymer), and KorantinSH (N-oleyl sarcosine)) yields magnetic fluids dispersed in carrier liquids such as toluene, kerosene, vacuum and mineral oils which are remarkably stable in air under ambient conditions. The resulting magnetic fluids show good magnetic properties. Several methods for the preparation of water-based MF are presented, e.g., formation of surfactant bilayers, using phase transfer agents, or surface modification with L-cysteine ethyl ester. Water-based metallic magnetic fluids have a high potential for a number of technical and biomedical applications. Technical applications of the Co-based ferrofluids in the field of positioning systems and magnetohydrostatic bearings were investigated. The results emphasize the scope of nanoparticulate ferrofluids having a metallic core.

POTENTIAL OF GOLD-BOUND MICROTUBULES AS A NEW ULTRASOUND CONTRAST AGENT

Abstract Contrast agents based on gas-filled microspheres share the problem of time limited opacification due to low stability of microbubbles. The aim of this study was to test if gold-bound microtubules provide backscattering that allows microtubules to be potentially useful as an ultrasound (US) contrast agent. Gold colloids were immobilized on protein microtubule walls. Latex balloons were filled with gold-bound microtubules or conventional left heart contrast agent and were ultrasonographically imaged in fundamental and harmonic modes. Feasibility of anti-β-tubulin antibody conjugation to gold-bound microtubules was confirmed using immune fluorescence analysis. Gold particles were successfully bound to microtubules. Contrast intensities in latex balloons filled with gold-bound microtubules (141 ± 35) were comparable to those with Levovist™ (180 ± 35) and did not decrease significantly during continuous US imaging for 20 min (135 ± 34 vs. Levovist™ 5.0 ± 2.0). Anti-β-tubulin antibodies were successfully conjugated to gold-bound microtubules. Gold-bound microtubules provide a persistent contrast effect, suggesting their use as an ultrasonic contrast agent with the feasibility of antibody conjugation. (E-mail:helmut_kuecherer@med.uni-heidelberg.de)

Access to highly active Ni–Pd bimetallic nanoparticle catalysts for C–C coupling reactions

Bimetallic Ni–Pd alloy nanoparticles with high Ni to Pd atomic ratios (99:1 or 95:5) were prepared, and the catalytic performances of these nanoparticle catalysts were explored for C–C coupling reactions (Suzuki–Miyaura, Heck and Sonogashira reactions) under moderate reaction conditions. In contrast to their monometallic counterparts, significantly enhanced catalytic activity was achieved with the studied Ni–Pd nanoparticle catalysts for the C–C coupling reactions, and products were obtained in moderate to high yields. The turnover number (TON) increases with the increase in the Ni to Pd atomic ratio for Ni–Pd nanoparticle catalysts and can reach 3.6 × 103 for Ni0.99Pd0.01 nanoparticle catalysed Suzuki–Miyaura reaction of aryl bromides with arylboronic acid at 50 °C. Advantageously, such Ni–Pd nanoparticle catalysts with high Ni to Pd atomic ratios not only show significantly enhanced catalytic activity but are also stable (ICP-AES analysis showed only marginal or no Pd leaching) and retain their catalytic activities for several catalytic runs (>90% conversion even at the 7th catalytic run). Experimental and relevant theoretical calculations (net charge localization using first principles calculations) suggested a substantial Ni to Pd charge transfer which resulted in a highly negatively charged Pd centre, a favourable site for facile oxidative addition of aryl halides, and hence enhanced catalytic activity for Ni–Pd nanoparticle catalysts.

Bifunctional hybrid catalysts derived from Cu/Zn-based nanoparticles for single-step dimethyl ether synthesis

Well-defined colloidal Cu/Zn-based nanoparticles were synthesized and employed as precursors for the methanol active component in bifunctional syngas-to-dimethyl ether (STD) catalysts. The experiments were conducted using simulated biomass-derived, CO-rich syngas (H2 : CO ratio of 1 : 1) in a single continuous-flow reactor by combining the two catalytically active components (i.e., Cu/ZnO for methanol synthesis and γ-Al2O3 for its subsequent dehydration). Two different synthetic pathways were developed for synthesizing the colloidal Cu/Zn-based nanoparticles, while ensuring close contacts between the Cu nanoparticles and the Zn phase. Pure Cu nanoparticles were used as a reference. A series of bifunctional STD catalysts was prepared, where the nanoparticles were either directly supported on the dehydration catalyst or integrated into the STD catalyst by physical mixing. With this approach, active catalysts for the STD reaction with high DME selectivity were obtained.

Ruthenium Nanoparticles in High‐Throughput Studies of Chemoselective Carbonyl Hydrogenation Reactions

Small (≤1.4 nm) and very active Ru nanoparticles, stabilized in a polysiloxane matrix, were prepared and studied in hydrogenation reactions by the integration of catalysis and analysis. We used our strategy to combine catalytic activity and separation selectivity in a capillary microreactor, installed in a GC–MS instrument, to develop a fast and reliable screening tool for catalysis over Ru nanoparticles. A high conversion using a low catalyst loading of 0.3 mol % and temperature and long‐term stability of the catalytically active column were observed for the hydrogenation of various carbonyl compounds, which included aldehydes, ketones, and pyruvates. Additionally, we observed a high chemoselectivity for aromatic carbonyl systems. Comprehensive measurements were performed in this high‐throughput experimental setup to gain important insights into the kinetics of hydrogenation reactions at the interface between heterogeneous and homogeneous catalysis.

Investigation of the Hydrogenation of 5-Methylfurfural by Noble Metal Nanoparticles in a Microcapillary Reactor

On-column reaction gas chromatography (ocRGC) was successfully utilized as high-throughput platform for monitoring of the conversion and selectivity of hydrogenation of 5-methylfurfural catalyzed by polymer-stabilized Ru and Pd nanoparticles. We were able to elucidate the effect of various reaction conditions, mainly together with the catalyst loading on the conversion rate and the selectivity of the reaction. Our strategy yields significant improvements in reaction analysis times and cost effectiveness in comparison to standard methods. We are able to demonstrate that ocRGC approach provides valuable information about the reaction system that gives scientists a tool to design suitable catalytic systems for enhanced sustainable chemistry in the future.