Evagelos K. Athanassiou

Nanoparticle cytotoxicity depends on intracellular solubility: comparison of stabilized copper metal and degradable copper oxide nanoparticles

Metal nanoparticles have distinctly different chemical and physical properties than currently investigated oxides. Since pure metallic nanoparticles are igniting at air, carbon stabilized copper nanoparticles were used as representative material for this class. Using copper as a representative example, we compare the cytotoxicity of copper metal nanoparticles stabilized by a carbon layer to copper oxide nanoparticles using two different cell lines. Keeping the copper exposure dose constant, the two forms of copper showed a distinctly different response. Whilst copper oxide had already been reported to be highly cytotoxic, carbon-coated copper nanoparticles were much less cytotoxic and more tolerated. Measuring the two materials intra- and extracellular solubility in model buffers explained this difference on the basis of altered copper release when supplying copper metal or the corresponding oxide particles to the cells. Control experiments using pure carbon nanoparticles were used to exclude significant surface effects. Reference experiments with ionic copper solutions confirmed a similar response of cultures if exposed to copper oxide nanoparticles or ionic copper. These observations are in line with a Trojan horse-type mechanism and illustrate the dominating influence of physico-chemical parameters on the cytotoxicity of a given metal.

Crosslinking Metal Nanoparticles into the Polymer Backbone of Hydrogels Enables Preparation of Soft, Magnetic Field‐Driven Actuators with Muscle‐Like Flexibility

The combination of force and flexibility is at the core of biomechanics and enables virtually all body movements in living organisms. In sharp contrast, presently used machines are based on rigid, linear (cylinders) or circular (rotator in an electrical engine) geometries. As a potential bioinspired alternative, magnetic elastomers can be realized through dispersion of micro- or nanoparticles in polymer matrices and have attracted significant interest as soft actuators in artificial organs, implants, and devices for controlled drug delivery. At present, magnetic particle loss and limited actuator strength have restricted the use of such materials to niche applications. We describe the direct incorporation of metal nanoparticles into the backbone of a hydrogel and application as an ultra-flexible, yet strong magnetic actuator. Covalent bonding of the particles prevents metal loss or leaching. Since metals have a far higher saturation magnetization and higher density than oxides, the resulting increased force/volume ratio afforded significantly stronger magnetic actuators with high mechanical stability, elasticity, and shape memory effect.

Permanent Pattern‐Resolved Adjustment of the Surface Potential of Graphene‐Like Carbon through Chemical Functionalization

The exceptional electronic and optical properties of graphene have caught the attention of physicists and materials scientists since the first effective preparation of this two-dimensional form of carbon by Novoselov et al. in 2004. Much effort is currently being invested in the large-scale production of graphene surfaces 3] and in the investigation of its peculiar quantum effects. Graphene is viewed as a potential alternative to silicon as a material for the construction of nanoscale electronic circuits. The use of graphene in this way would require control of its electronic band structure and the withdrawal or injection of electron density to adjust or tilt the Fermi level in a graphene sheet. Such pattern-resolved control of the energy level is the two-dimensional equivalent of n or p doping in classical semiconductors. In contrast to silicon, graphene has a continuous band structure with zero band gap. Thus, single adsorbed molecules modify the band structure and affect the electronic properties of graphene significantly, 10] which makes graphene difficult to handle. Device fabrication requires reliable and permanent control over the different electronic states and the Fermi energy of an air-stable material. The adsorption of organic molecules can result in p-type doping through a sandwichlike p-stacking arrangement on graphene. The injection of electrons is possible through n-type doping with potassium; however, such materials are highly sensitive to air and water. In the search for a robust and highly precise doping method, we investigated well-established protocols from organic radical chemistry to attach an air-stable dopant covalently and thus permanently alter the electronic structure of graphene sheets. The relative surface charge levels were measured by Kelvin force microscopy (KFM). The application of the linear free-enthalpy relationship for substituted aromatic compounds enabled the direct prediction of the charge-withdrawing or charge-injecting effect of graphene modification. We therefore concluded that this approach should enable direct control of the surface potential, Y, of modified graphene. Furthermore, the Hammett concept enabled a precise correlation between the observed change in the surface potential, DY, and the structure of the covalently bonded reagents. This concept was confirmed experimentally by using strongly electron withdrawing (p-nitrophenyl, s = 0.78) and electron donating substituents (p-methoxyphenyl, s = 0.23). For our experiments, we used the top graphene layer of highly ordered pyrolytic graphite (HOPG) as a model material. From a physical point of view, this model is not representative for detailed investigations on band structure or electronic effects. However, from a chemical point of view, the reactivity of the graphene stacks of HOPG is comparable to that of single-walled carbon nanotubes, which can be considered as rolls of graphene. For additional experimental validation, we also carried out the graphene modifications described herein on carbon-coated nanoparticles (two or three layers of graphene on copper). Detailed structural evidence was then provided by diffuse reflectance FTIR to characterize the products and confirm the direct covalent attachment of the modifying groups to the top graphene layer. This functionalization approach extends p and n doping based on adsorbed molecules or ions to make it a systematic and robust method with which molecular electronics elements can be attached perpendicular to the graphene plane in a third dimension. The experimental approach to covalent graphene modification is shown in Figure 1. The model material (top layer of a monocrystalline graphene stack) was first patterned by lithography, so that a plain (unfunctionalized) graphene surface would be preserved below the photoresist. The unmasked areas were functionalized by exposure to highly diluted diazonium reagents (see the Supporting Information). After removal of the photoresist, the graphene surface was investigated by scanning electron microscopy (SEM) and Kelvin force microscopy (KFM) in tapping mode to image the relative surface-potential levels of modified and native areas of the graphene surface. The chemical derivatization depends [*] MSc Chem. Eng. F. M. Koehler, MSc Mat. Sci. N. A. Luechinger, Dipl.-Chem.-Ing. E. K. Athanassiou, Dr. R. N. Grass, Prof. Dr. W. J. Stark Institute for Chemical and Bioengineering Department of Chemistry and Applied Biosciences, ETH Zurich Wolfgang-Pauli-Strasse 10, 8093 Zurich (Switzerland) Fax: (+ 41)44-633-1083 E-mail: wendelin.stark@chem.ethz.ch Homepage: http://www.fml.ethz.ch

Chemical Aerosol Engineering as a Novel Tool for Material Science: From Oxides to Salt and Metal Nanoparticles

Aerosol nanotechnology has rapidly evolved in the past years. This fascinating technology has resulted in the development of functional nanomaterials providing novel solutions in industrial applications. The extensive research on the physical understanding of gas phase processes has strongly contributed to the present industrial use of single and mixed oxides and the design of industrial aerosol reactors. Recent advances have shown that chemical aerosol engineering can be established on the interface between classical aerosol science and chemical engineering. The emerging new methods give access to a much broader class of functional materials including salt and metal nanoparticles. The latter implies that aerosol production units can now be considered as chemical reactors. The incorporation of thermodynamic considerations and chemical kinetics in the modelling of gas phase processes will further boost the development of aerosol engineering and will provide deeper understanding of the fundamentals of particle formation mechanisms. This will ultimately enable access to new multi-component materials with various structures or morphologies and the development of more sustainable, energy efficient gas phase processes.

Immobilized β-Cyclodextrin on Surface-Modified Carbon-Coated Cobalt Nanomagnets: Reversible Organic Contaminant Adsorption and Enrichment from Water

Surface-modified magnetic nanoparticles can be used in extraction processes as they readily disperse in common solvents and combine high saturation magnetization with excellent accessibility. Reversible and recyclable adsorption and desorption through solvent changes and magnetic separation provide technically attractive alternatives to classical solvent extraction. Thin polymer layered carbon-coated cobalt nanoparticles were tagged with β-cyclodextrin. The resulting material reversibly adsorbed organic contaminants in water within minutes. Isolation of the immobilized inclusion complex was easily carried out within seconds by magnetic separation due to the strong magnetization of the nanomagnets (metal core instead of hitherto used iron oxide). The trapped molecules were fully and rapidly recovered by filling the cyclodextrin cavity with a microbiologically well accepted substitute, e.g., benzyl alcohol. Phenolphthalein was used as a model compound for organic contaminants such as polychlorinated dibenzodioxins (PCDDs) or bisphenol A (BPA). Fast regeneration of nanomagnets (compared to similar cyclodextrin-based systems) under mild conditions resulted in 16 repetitive cycles (adsorption/desorption) at full efficiency. The high removal and regeneration efficiency was examined by UV-vis measurements at chemical equilibrium conditions and under rapid cycling (5 min). Experiments at ultralow concentrations (160 ppb) underline the high potential of cyclodextrin modified nanomagnets as a fast, recyclable extraction method for organic contaminants in large water streams or as an enrichment tool for analytics.

Gold adsorption on the carbon surface of C/Co nanoparticles allows magnetic extraction from extremely diluted aqueous solutions

The elusive chemistry of gold has made refining from ores a difficult task and often involves handling of large volumes of water at low pH values with associated high environmental burden. As a result, the broader use of gold in environmental catalysis, organic synthesis and in electronics is still limited in spite of its most attractive chemistry. Present gold extraction suffers from metal loss in the form of gold adsorbed on active carbon particles that are washed out of the extraction process. Here, we investigate the use of magnetic carbon in the form of carbon-coated metal nanomagnets for ionic gold recovery. In contrast to acid-labile iron oxide nanoparticles, the carbon/cobalt nanomagnets resisted dissolution in acidic refining/recycling waters. Repetitive extraction runs demonstrated the possibility to recycle the magnetic reagent. A series of dilution studies showed a high affinity of the ionic gold to the carbon surfaces of the nanomagnets which enabled gold extraction down to the part per billion level (microgram per litre). Detailed investigations on the morphology of the Au-loaded nanomagnets after use suggest a mechanism based on the selective reduction of ionic gold on the C/Co surface and transfer of cobalt through the carbon shell. The resulting irreversible deposition of metallic gold correlated with the release of oxidized (ionic) cobalt into the aqueous phase.

Iron core/shell nanoparticles as magnetic drug carriers: possible interactions with the vascular compartment

AIMS Nanomagnets with metal cores have recently been shown to be promising candidates for magnetic drug delivery due to higher magnetic moments compared with commonly used metal oxides. Successful application strongly relies on a safe implementation that goes along with detailed knowledge of interactions and effects that nanomagnets might impart once entering the body. MATERIALS & METHODS In this work, we put a particular focus on the interactions of ultra-strong metal nanomagnets (≥ three-times higher in magnetization compared with oxide nanoparticles) within the vascular compartment. Individual aspects of possible effects are addressed, including interactions with the coagulation cascade, the complement system, phagocytes and toxic or inflammatory reactions both by blood and endothelial cells in response to nanomagnet exposure. RESULTS We show that carbon-coated metal nanomagnets are well-tolerated by cells of the vascular compartment and have only minor effects on blood coagulation. CONCLUSION These findings provide the fundament to initiate successful first in vivo evaluations opening metal nanomagnets with improved magnetic properties to fascinating applications in nanomedicine.

Physical Defect Formation in Few Layer Graphene-like Carbon on Metals: Influence of Temperature, Acidity, and Chemical Functionalization

A systematical examination of the chemical stability of cobalt metal nanomagnets with a graphene-like carbon coating is used to study the otherwise rather elusive formation of nanometer-sized physical defects in few layer graphene as a result of acid treatments. We therefore first exposed the core-shell nanomaterial to well-controlled solutions of altering acidity and temperature. The release of cobalt into these solutions over time offered a simple tool to monitor the progress of particle degradation. The results suggested that the oxidative damage of the graphene-like coatings was the rate-limiting step during particle degradation since only fully intact or entirely emptied carbon shells were found after the experiments. If ionic noble metal species were additionally present in the acidic solutions, the noble metal was found to reduce on the surface of specific, defective particles. The altered electrochemical gradients across the carbon shells were however not found to lead to a faster release of cobalt from the particles. The suggested mechanistic insight was further confirmed by the covalent chemical functionalization of the particle surface with chemically inert aryl species, which leads to an additional thickening of the shells. This leads to reduced cobalt release rates as well as slower noble metal reduction rates depending on the augmentation of the shell thickness.

Sintering of core–shell Ag/glass nanoparticles: metal percolation at the glass transition temperature yields metal/glass/ceramic composites

In order to investigate the sintering evolution of core/shell metal/glass nanoparticles, two model compounds were synthesized and their structural evolution was investigated. Silica glass coated Ag nanoparticles were synthesized by flame spray pyrolysis, pressed into bulk pills and subsequently sintered at different temperatures finally resulting in composites with a highly conductive percolated silver network embedded in a ceramic matrix. By synthesizing two glass silver nanocomposites differing in their glass composition and corresponding glass transition temperature, the direct influence of the glass matrix on the percolation network formation and the conductive properties could be investigated. The analysis of the two systems by X-ray diffraction, scanning and transmission electron microscopy and by energy-dispersive X-ray detection clearly showed that the formation of the percolated network is initiated at the glass transition temperature of the matrix.