Scientific publications

Optimization of the microstructure of porous electrodes plays an important role in the enhancement of the performance of solid oxide fuel cells. For this, microstructural models based on percolation theory have proven useful for the estimation of the effective material properties of the electrode material, assumed to consist of a binary mixture of spherical electron and ion conducting particles. In this work, we propose an extension of prior approaches for calculating the effective size of the three-phase boundary, which we judge to be physically more sound and, in particular, well suited for characterizing mixtures of particles of different sizes. This approach is then employed in a one-dimensional cell level model encompassing the entire set of processes of gas transport, electronic and ionic conduction as well as the electrochemical reactions. The impact of the electron and ion conducting particle sizes, their volume fraction and their size ratio on the performance of the fuel cell are investigated in a parametric study. Under certain conditions, cathode microstructures having electronic conducting particles of size different from that of the ionic conducting particles become preferable and yield a higher maximum power density when compared to the best possible configuration of monodisperse particles.

The incorporation of rhodamine dyes in the cell wall of diatoms Coscinodiscus granii and Coscinodiscus wailesii for the production of luminescent hybrid nanostructures is investigated. By systematic variation of the substitution pattern of the rhodamine core, we found that carbonic acids are considerably better suited than esters because of their physiological compatibility. The amino substitution pattern that controls the optical properties of the chromophore has no critical influence on dye uptake and incorporation, thus a variety of biocomposites with different emission maxima can be prepared. Applications in biomineralization studies as well as in materials science are envisioned.

For about a decade, superresolution fluorescence microscopy has been advancing steadily, maturing from the proof-of-principle stage to routine application. Of the various techniques, STED (stimulated emission depletion) microscopy was the first to break the diffraction barrier. Today, it is a prominent and versatile form of superresolution light microscopy. STED microscopy has shed a sharper light on numerous topics in cell biology, but also in material sciences. Both disciplines extend into the nanometer range, making detailed studies of structural and functional relationships difficult or even impossible to achieve using diffraction-limited microscopy. With recent advancements like spectral multiplexing or live-cell imaging, STED microscopy makes nanoscale materials and components of the cell accessible for fluorescence-based investigations. With multicolor superresolution imaging, even the interactions between biological and engineered nanostructures can be studied in detail. This review gives an introduction into the working principle of STED microscopy, provides a detailed overview of recent advancements and new techniques implemented for use with STED microscopy and shows how these have been applied in the life sciences and nanotechnologies.

The interactions of nanoparticles with human cells are of large interest in the context of nanomaterial safety. Here, we use live cell imaging and image-based fluorescence correlation methods to determine colocalization of 88 nm and 32 nm silica nanoparticles with endocytotic vesicles derived from the cytoplasmic membrane and lysosomes, as well as to quantify intracellular mobility of internalized particles, in contrast to particle number quantification by counting techniques. In our study, A549 cells are used as a model for human type II alveolar epithelial cells. We present data supporting endocytotic uptake of the particles and subsequent active transport to the perinuclear region. The presence of particles in lamellar bodies is proposed as a potential exocytosis route. Live cell imaging and image-based fluorescence correlation methods were used to quantify the intracellular mobility and interactions of 32 and 88 nm silica nanoparticles in A549 cells as model for human type II alveolar epithelial cells. Our data support uptake by endocytosis and active transport to the perinuclear region.

In cardiac myocytes, cytochalasin D (CytoD) was reported to act as an actin disruptor and mechanical uncoupler. Using confocal and super-resolution STED microscopy, we show that CytoD preserves the actin filament architecture of adult rat ventricular myocytes in culture. Five hundred nanomolar CytoD was the optimal concentration to achieve both preservation of the T-tubular structure during culture periods of 3days and conservation of major functional characteristics such as action potentials, calcium transients and, importantly, the contractile properties of single myocytes. Therefore, we conclude that the addition of CytoD to the culture of adult cardiac myocytes can indeed be used to generate a solid single-cell model that preserves both morphology and function of freshly isolated cells. Moreover, we reveal a putative link between cytoskeletal and T-tubular remodeling. In the absence of CytoD, we observed a loss of T-tubules that led to significant dyssynchronous Ca2+-induced Ca2+ release (CICR), while in the presence of 0.5μM CytoD, T-tubules and homogeneous CICR were majorly preserved. Such data suggested a possible link between the actin cytoskeleton, T-tubules and synchronous, reliable excitation-contraction-coupling. Thus, T-tubular re-organization in cell culture sheds some additional light onto similar processes found during many cardiac diseases and might link cytoskeletal alterations to changes in subcellular Ca2+ signaling revealed under such pathophysiological conditions.

In the present work the influence of nano-scaled zirconia particles with different structures on the structural and electrical properties of polyurethane (PU) insulating coatings was investigated. PU matrix insulating coatings containing non-doped and 4 mole % yttrium doped nano-scaled zirconia with average particle size between 12 and 13 nm were prepared by wet chemical methods. High resolution transmission electron microscopy analysis of thermally cured composites showed that the nano dispersed zirconia particles were uniformly distributed within matrix. Incorporation of 5 vol. % nanoparticles into the PU matrix resulted in a slight increase of the micro hardness and elastic modulus. The effect of zirconia filling (non-doped and doped) on the electrical breakdown strength was not observed. But, a strong influence of the nanoparticle addition on the partial discharge stability was detected. Partial discharge breakdown time for non-filled PU matrix was 2.5 h, for PU filled with non-doped zirconia 65 h and for PU filled with yttrium doped zirconia >; 100 h, respectively. The additional benefit of doped zirconia may be explained by its different electronic structure and polarisation behaviour.

The corrosion protective ability of hybrid oxy silane nano-composite coatings deposited on AA2024 by sol-gel technique was studied. The coatings are developed as an environmentally friendly alternative of the toxic chromium containing coatings on aluminium. A cerium salt, Ce2(SO4)3, was used as inhibitor of the corrosion process. Two methods were applied to introduce the salt in the hybrid matrix: directly in the matrix, or by porous Al2O3 nano-particles preliminary loaded by the salt. Atomic force microscopy (AFM) was used to evaluate the superficial morphology of the coatings, while their layer structure was studied by means of scanning electron microscopy (SEM). Linear voltammetry (LVA) and electrochemical impedance spectroscopy (EIS) were used for assessment of the barrier ability. The hybrid matrix was found to possess remarkable barrier ability which was preserved even after prolonged exposure of the coatings to a model corrosive medium of 0.05 M NaCl. In all cases, the cerium salt involved either directly or by Al2O3 nano-particles proved to deteriorate the protective properties of the coatings and to accelerate pitting nucleation. The experimental results have shown that cerium sulphate, introduced in the by the both manners in the hybrid matrix did not efficiently inhibit the corrosion of AA2024, unlike the reported inhibiting properties of other cerium salts.

In this study, the environmental stress cracking (ESC) resistance of nano-SiO2 particle-filled polycarbonate (PC) was investigated at room temperature by using a fracture mechanical test method. Two PC-based nanocomposites with nano-SiO2 content of 1 and 4 vol% were compounded using a twin-screw extruder. The incorporation of nanoparticles into PC leads to an improvement in ESC resistance, which increases with increased filler loading up to 4 vol%. Fractography analyses suggest that the formation of cavities/debonding and localized microdeformation of matrix induced by nanoparticles as a result of energy dissipation are responsible for the improvement of ESC resistance.

Nanoscale zinc oxide (ZnO) powder with Brunauer-Emmelt-Teller surface area of 43 m2 g-1 has been synthesized by soft chemistry at low temperature via reaction of zinc acetate dehydrate (Zn(CH3COO)2.2H2O) and sodium hydroxide (NaOH). The influence of the pH value of the sol on the structure and morphology of ZnO powder have been investigated by X-Ray Diffraction, Scanning Electron Microscopy and High Resolution Transmission Electron Microcopy. Their thermal properties have been determined by simultaneous Differential Thermal Analysis and Thermogravimetry coupled to Mass Spectroscopy analysis. The nanoparticles are single crystals with (101) preferred orientation but agglomerated. Their crystallite size can be adjusted from 15 nm to 35 nm by controlling the pH value between 7 and 13. Thick porous crystalline coatings have been obtained by doctor blade coating on conducting SnO2:F glass substrates using pastes prepared by wetting the crystalline powders with polyethylene glycol and water. After sintering at 400 °C and Ruthenium 535 dye sensitization, the coatings have been tested in a three electrode electrochemical cell containing an appropriate electrolyte in the dark and under 450 W Xenon lamp illumination. The influence of the electrolyte iodine concentration, the film thickness and the light intensity on the current density are presented and discussed. Such coatings appeared promising for the realization of dye sensitized solar cells.

As ferroelectric ceramic/polymer composites may offer the possibility to lower the operating voltage of state of the art polymer ferroelectric memories, the retention behavior of these composites needs to be studied. This work presents for the first time the polarization decay of a ceramic/polymer composite memory and discusses some influencing parameters. The existence of a depolarization field in such particulate structures, which is responsible for the polarization loss is demonstrated by finite element simulations, and first theoretical predictions are generated how to reduce this depolarizing field. Interestingly, first results from the experimental approach of varying the ionic concentration in the surrounding medium of the particles contradict these theoretical assumptions.