Scientific publications

The synthesis, structural characterization, electrochemistry and luminescence properties of a series of new yttrium and europium(+3) alkoxides bearing thiophene moieties are presented. The yttrium compounds were obtained by the reaction between Y[N(SiMe3)2]3 and the tertiary alcohols HO–C(C16H13S) (2), HO–C(C17H15S) (3), HO–C(C14H11S2) (4) and HO–C(C16H13S) (5) in thf or in a mixture of toluene and pyridine. The X-ray crystal diffraction measurements show a five-coordinated yttrium atom in distorted trigonal-bipyramidal geometry. The metal centres are surrounded by threemethoxido ligands in equatorial positions and two tetrahydrofuran [for {Y[OC(C16H13S)]3(thf)2}·toluene (8), {Y[OC(C17H15S)]3(thf)2}·toluene (10) and {Y[OC(C14H11S2)]3(thf)2}·1/2 toluene (12)] or two pyridine [for {Y[OC(C16H13S)]3(py)2}·toluene (9) and {Y[OC(C17H15S)]3(py)2}·toluene (11)] molecules in axial positions. The compounds Y[OC(C14H11S2)]3(py)2 (13) and Y[OC(C16H13S)]3(py)2 (14) were identified by NMR spectroscopy. In addition, a novel europium(+3) alkoxide {Eu[OC(C4H3S)3]3(thf)3}·thf (15) was synthesized by the reaction between Eu[N(SiMe3)2]3 and the tertiary alcohol HO–C(C4H3S)3 (1) in thf. The molecular structure of this compound reveals an approximately octahedral coordination sphere around the europium(+3) metal centre with three methoxido ligands and three facially arranged tetrahydrofuran molecules. The cyclic voltammograms of the yttrium alkoxides indicate that the electrochemical properties are essentially dominated by the organic ligands. The electrochemical properties of {Eu[OC(C4H3S)3]3(thf)3}·thf (15) are dominated by the oxidation of the thienyl moieties and in addition a reduction wave due to the reduction of Eu3+ to Eu2+ is visible. In comparison to the carbinols, the oxidation peak potentials of the thienyl units for the yttrium and europium(3+) alkoxides are marginally shifted towards higher values. The emission spectra of the carbinols and their derived yttrium compounds display broad bands attributed to the π*→π transitions of the aromatic ligands. Luminescence studies performed on compound 15 reveal the typical f–f transitions of the Eu3+ ions and suggest that an energy transfer from the ligand to the metal atom operates.

Insolubility is one of the possible functions of proteins involved in biomineralization, which often limits their native purification. This becomes a major problem especially when recombinant expression systems are required to obtain larger amounts. For example, the mollusc shell provides a rich source of unconventional proteins, which can interfere in manifold ways with different mineral phases and interfaces. Therefore, the relevance of such proteins for biotechnological processes is still in its infancy. Here we report a simple and reproducible purification procedure for a GFP-tagged lectin involved in biomineralization, originally isolated from mother-of-pearl in abalone shells. An optimization of E. coli host cell culture conditions was the key to obtain reasonable yields and high degrees of purity by using simple one-step affinity chromatography. We identified a dual functional role for the GFP domain when it became part of a mineralizing system in vitro. First, the GFP domain improved the solubility of an otherwise insoluble protein, in this case recombinant perlucin derivatives. Second, GFP inhibited calcium carbonate precipitation in a concentration dependent manner. This was demonstrated here using a simple bulk assay over a time period of 400 seconds. At concentrations of 2 mg/ml and higher, the inhibitory effect was observed predominantly for HCO3−as the first ionic interaction partner, but not necessarily for
Ca2+. The interference of GFP-tagged perlucin derivatives with the precipitation of calcium carbonate generated different types of GFP-fluorescent composite calcite crystals. GFP-tagging offers therefore a genetically tunable tool to gently modify mechanical and optical properties of synthetic biocomposite minerals.

The size, morphology and species-specific texture of mollusc shell biominerals is one of the unresolved questions in nature. In search of molecular control principles, chitin has been identified by Weiner and Traub (FEBS Lett. 1980, 111 : 311-316) as one of the organic compounds with a defined co-organization with mineral phases. Chitin fibers can be aligned with certain mineralogical axes of crystalline calcium carbonate in a speciesspecific manner. These original observations motivated the functional characterization of chitin forming enzymes in molluscs. The full-length cDNA cloning of mollusc chitin synthases identified unique myosin domains as part of the biological control system. The potential impact of molecular motors and other conserved domains of these complex transmembrane enzymes on the evolution of shell biomineralization is investigated and discussed in this article.

High power electrochemical double layer capacitors (also called supercapacitors) rely on highly conductive electrode materials with a large specific surface area, which is easily accessible for ions. Exohedral materials with a large ion-accessible outer surface and little or no porosity within the particles are particularly attractive for supercapacitor electrodes because they decrease mass transport limitations and enable very high charge/discharge rates. This study focuses on the investigation of charge induced expansion effects of spherical exohedral carbons, that is, onion-like carbons (OLC, diameter: 5–7 nm) and carbon black (diameter: ≈40 nm). Employing electrochemical in-situ dilatometry we studied the expansion behavior within ±1 V potential window versus carbon in an organic electrolyte (tetraethylammonium-tetrafluoroborate in acetonitrile). The expansion had a very small amplitude (<0.2% at ±0.08 C·m-2 accumulated charge; i.e., approximately ±1 V versus carbon) and was fully reversible. This was explained by ion adsorption on the exohedral carbons. Molecular dynamics (MD) simulations were employed to calculate the solvation shell of the respective cation and anion and the results were used to further evaluate the measured expansion. In summary, the experiments and simulations revealed that ion intercalation or ion sieving, which are commonly found in microporous (endohedral) carbons, were absent. Finally, low sweep rates resulted in a slight electrode compaction on a cycle-by-cycle basis, which can be explained by charge-induced restructuring of the nanoparticle network.

Sierpinski carbon: Macroporous carbide-derived carbon monoliths (DUT-38) were synthesized starting from SiC-PolyHIPEs, resulting in a hierarchical micro-, meso-, and macroporous structure. The high specific surface area and high macropore volume renders PolyHIPE-CDC an excellent adsorbent combining high storage capacity with excellent adsorption rates in gas storage and air filtration.

The nanoscale interactions of room temperature ionic liquids (RTILs) at uncharged (graphene) and charged (muscovite mica) solid surfaces were evaluated with high resolution X-ray interface scattering and fully atomistic molecular dynamics simulations. At uncharged graphene surfaces, the imidazolium-based RTIL ([bmim+][Tf2N-]) exhibits a mixed cation/anion layering with a strong interfacial densification of the first RTIL layer. The first layer density observed via experiment is larger than that predicted by simulation and the apparent discrepancy can be understood with the inclusion of, dominantly, image charge and π-stacking interactions between the RTIL and the graphene sheet. In contrast, the RTIL structure adjacent to the charged mica surface exhibits an alternating cation-anion layering extending 3.5 nm into the bulk fluid. The associated charge density profile demonstrates a pronounced charge overscreening (i.e., excess first-layer counterions with respect to the adjacent surface charge), highlighting the critical role of charge-induced nanoscale correlations of the RTIL. These observations confirm key aspects of a predicted electric double layer structure from an analytical Landau-Ginzburg-type continuum theory incorporating ion correlation effects, and provide a new baseline for understanding the fundamental nanoscale response of RTILs at charged interfaces.

While pressure sensitive adhesives in general consist of a layer of viscoelastic glue sandwiched between two adherents, we explore here the design of an adhesive embedded with microchannels which remain either open to atmosphere or pressurized to different positive and negative pressures. We subject these layers to indentation by a rigid cylinder such that in addition to adhesion between the indenter and the adhesive surface, the inner walls of the channels too self-adhere; during retraction of the indenter, these surfaces debond, but at a different load, thus resulting in hysteresis. When these channels are pressurized to different extents, the contact areas of various interfaces vary, so also the resultant hysteresis. For experiments with constant depth of indentation, the hysteresis increases and attains maxima at an intermediate value of the internal pressure inside the channels. The hysteresis increases also with the skin thickness of the adhesive over the channels. These results show that subsurface channels in an adhesive allow active manipulation of adhesion over a large range via coupled effect of geometry of channels, their surface characteristics, and the pressure inside.

In this work, the adhesion of biomimetic polydimethylsiloxane (PDMS) pillar arrays with mushroom-shaped tips was studied on nano- and micro-rough surfaces and compared to unpatterned controls. The adhesion strength on nano-rough surfaces invariably decreased with increasing roughness, but pillar arrays retained higher adhesion strengths than unpatterned controls in all cases. The results were analysed with a model that focuses on the effect on adhesion of depressions in a rough surface. The model fits the data very well suggesting that the pull-off strength for patterned PDMS is controlled by the deepest dimple-like feature on the rough surface. The lower pull-off strength for unpatterned PDMS may be explained by the initiation of the pull-off process at the edge of the probe, where significant stress concentrates. To micro-rough surfaces, pillar arrays showed a maximum in adhesion for a certain intermediate roughness, while unpatterned controls did not show any measurable adhesion. This effect can be explained by the inability of micropatterned surfaces to conform to very fine and very large surface asperities.