Scientific publications

Porous carbon electrodes have significant potential for energy-efficient water desalination using a promising technology called Capacitive Deionization (CDI). In CDI, salt ions are removed from brackish water upon applying an electrical voltage difference between two porous electrodes, in which the ions will be temporarily immobilized. These electrodes are made of porous carbons optimized for salt storage capacity and ion and electron transport. We review the science and technology of CDI and describe the range of possible electrode materials and the various approaches to the testing of materials and devices. We summarize the range of options for CDI-designs and possible operational modes, and describe the various theoretical–conceptual approaches to understand the phenomenon of CDI.

Self oscillations in homogeneous chemical systems have been studied extensively in diverse array of systems ranging from flow and non-flow, stirred and unstirred, for non-linear chemical reactions coupled with or without the effect of diffusion and multiple systems exhibiting synchronized oscillations. There are however not many examples in which specific geometric features of the reaction chamber turns an otherwise stable system oscillatory, importantly at very low Reynolds number regime. Here we describe such a phenomenon in regard to a microfluidic fuel cell for which the cell potential turns oscillatory when its smooth wall is replaced by one patterned with microscopic features. In essence, we have a membraneless fuel cell in which fuel and oxidant streams flow in laminar contact, side by side, forming a sharp interface. For smooth surface of the channel wall, with steady flow of fuel and oxidant, the cell generates a constant open circuit potential (OCP). However, when the surface of the channel is patterned with parallel triangular ridges, although the flow continues to remain temporally steady, time invariant oscillating OCP ensues within a small window of very low Reynolds number. We have presented a mechanism for occurrence of these oscillations.

Polymers capable of dynamic bonding/debonding reactions are of great interest in modern day research. Potential applications can be found in the fields of self-healing materials or printable networks. Since temperature is often used as a stimulus for triggering reversible bonding reactions, an analysis operating at elevated temperatures is very useful for the in situ investigation of the reaction mechanism, as unwanted side effects can be minimized when performing the analyses at the same temperature at which the reactions occur. A temperature-dependent size exclusion chromatographic system (TD SEC) has been optimized for investigating the kinetics of retro Diels−Alder-based depolymerization of Diels−Alder polymers. The changing molecular weight distribution of the analyzed polymers during depolymerization gives valuable quantitative information on the kinetics of the reactions. Adequate data interpretation methods were developed for the correct evaluation of the chromatograms. The results are confirmed by high-temperature dynamic light scattering, thermogravimetric analysis, and time-resolved nuclear magnetic resonance spectroscopy at high temperatures. In addition, the SEC system and column material stability under application conditions were assessed using thermoanalysis methods, infrared spectroscopy, nitrogen physisorption, and scanning electron microscopy. The findings demonstrate that the system is stable and, thus, we can reliably characterize such dynamically bonding/debonding systems with TD SEC.

Force-displacement measurements were performed on single macroscopic polydimethylsiloxane pillars. The pillars had diameters of 400 μm, aspect ratios between 1 and 5, and spherical, flat, and mushroom shaped tips. We present a method to derive numerous parameters, e.g. stress, strain, elastic modulus, and work of separation from these force-displacement measurements. We also show that the deformation of the pillar, their tips and the backing layers can be separately identified from the same experiments. This method can help to better characterize the mechanical properties of polymeric microstructures using a simple experimental approach.

We tested the adhesive response of polymer surfaces structured with arrays of cylindrical fibrils having diameters of 10-20 µm and aspect ratios 1-2.4. Fibrils had two different tip shapes of end-flaps and round edges. A preload-induced mechanical buckling instability of the fibrils was used to switch between the states of adhesion and non-adhesion. Non-adhesion in fibrils with round edges was reached at preloads that caused fibril buckling, whereas fibrils with end-flaps showed adhesion loss only at very high preloads. The round edge acted as a circumferential flaw prohibiting smooth tip contact recovery leading to an adhesion loss. In situ observations showed that, after reversal of buckling, the end-flaps unfold and re-form contact under prevailing compressive stress, retaining adhesion in spite of buckling. At very high preloads, however, end-flaps are unable to re-form contact resulting in adhesion loss. Additionally, the end-flaps showed varying contact adaptability as a function of the fibril-probe alignment, which further affects the preload for adhesion loss. The combined influence of preload, tip shape and alignment on adhesion can be used to switch adhesion in bioinspired fibrillar arrays.

Das INM – Leibniz-Institut für Neue Materialien in Saarbrücken erforscht und entwickelt neue Materialien vom Molekül bis zur Pilotfertigung. Nach einer externen Evaluierung im Jahre 2005 wurde das Institut als modernes, interdisziplinäres Forschungszentrum neu gestaltet, wobei auch strukturell neue Wege gegangen wurden. Der Artikel beschreibt diese Konzepte und deren erfolgreiche Umsetzung in den letzten Jahren. Entscheidend waren die internationale Rekrutierung von wissenschaftlichem Personal, die stärkere Einbeziehung junger Forscherinnen und Forscher, die Schaffung einer flachen Hierarchie sowie die Umsetzung wirksamer Kommunikationsstrategien. Die Balance zwischen hervorragender Grundlagenforschung und indus trieller Anwendung ist einem dauernden Optimierungsprozess unterworfen. Als in der Rechtsform einer gemeinnützigen GmbH geführte, selbstständige Einrichtung verfügt das INM über einen kaufmännischen Bereich, dessen Strukturen und Prozesse zur Unterstützung der Forschung, der Projektakquise sowie der Verwertung ebenfalls neu aufgestellt wurden und sich damit an den Anforderungen des heutigen Wissenschaftsmanagements orientiert.

Nanoparticles of heavy materials such as gold can be used as markers in quantitative electron microscopic studies of protein distributions in cells with nanometer spatial resolution. Studying nanoparticles within the context of cells is also relevant for nanotoxicological research. Here, we report a method to quantify the locations and the number of nanoparticles, and of clusters of nanoparticles inside whole eukaryotic cells in three dimensions using scanning transmission electron microscopy (STEM) tomography. Whole-mount fixed cellular samples were prepared, avoiding sectioning or slicing. The level of membrane staining was kept much lower than is common practice in transmission electron microscopy (TEM), such that the nanoparticles could be detected throughout the entire cellular thickness. Tilt-series were recorded with a limited tilt-range of 80° thereby preventing excessive beam broadening occurring at higher tilt angles. The 3D locations of the nanoparticles were nevertheless determined with high precision using computation. The obtained information differed from that obtained with conventional TEM tomography data since the nanoparticles were highlighted while only faint contrast was obtained on the cellular material. Similar as in fluorescence microscopy, a particular set of labels can be studied. This method was applied to study the fate of sequentially up-taken low-density lipoprotein (LDL) conjugated to gold nanoparticles in macrophages. Analysis of a 3D reconstruction revealed that newly up-taken LDL-gold was delivered to lysosomes containing previously up-taken LDL-gold thereby forming onion-like clusters.

Transmission electron microscopy (TEM) in combination with electron tomography is widely used to obtain nanometer scale three-dimensional (3D) structural information about biological samples. However, studies of whole eukaryotic cells are limited in resolution and/or contrast on account of the effect of chromatic aberration of the TEM objective lens on electrons that have been scattered inelastically in the specimen. As a result, 3D information is usually obtained from sections and not from whole cells. Here, we use chromatic aberration-corrected TEM to record bright-field TEM images of nanoparticles in a whole mount macrophage cell. Tilt series of images are used to generate electron tomograms, which are analyzed to assess the spatial resolution that can be achieved for different vertical positions in the specimen. The uptake of gold nanoparticles coated with low-density lipoprotein (LDL) is studied. The LDL is found to assemble in clusters. The clusters contain nanoparticles taken up on different days, which are joined without mixing their nanoparticle cargo.

Imaging single epidermal growth factor receptors (EGFR) in intact cells is presently limited by the available microscopy methods. Environmental scanning electron microscopy (ESEM) of whole cells in hydrated state in combination with specific labeling with gold nanoparticles was used to localize activated EGFRs in the plasma membranes of COS7 and A549 cells. The use of a scanning transmission electron microscopy (STEM) detector yielded a spatial resolution of 3 nm, sufficient to identify the locations of individual EGFR dimer subunits. The sizes and distribution of dimers and higher order clusters of EGFRs were determined. The distance between labels bound to dimers amounted to 19 nm, consistent with a molecular model. A fraction of the EGFRs was found in higher order clusters with sizes ranging from 32-56 nm. ESEM can be used for quantitative whole cell screening studies of membrane receptors, and for the study of nanoparticle-cell interactions in general.