Joachim Bill

Sponge spicules as blueprints for the biofabrication of inorganic–organic composites and biomaterials

While most forms of multicellular life have developed a calcium-based skeleton, a few specialized organisms complement their body plan with silica. However, of all recent animals, only sponges (phylum Porifera) are able to polymerize silica enzymatically mediated in order to generate massive siliceous skeletal elements (spicules) during a unique reaction, at ambient temperature and pressure. During this biomineralization process (i.e., biosilicification) hydrated, amorphous silica is deposited within highly specialized sponge cells, ultimately resulting in structures that range in size from micrometers to meters. Spicules lend structural stability to the sponge body, deter predators, and transmit light similar to optic fibers. This peculiar phenomenon has been comprehensively studied in recent years and in several approaches, the molecular background was explored to create tools that might be employed for novel bioinspired biotechnological and biomedical applications. Thus, it was discovered that spiculogenesis is mediated by the enzyme silicatein and starts intracellularly. The resulting silica nanoparticles fuse and subsequently form concentric lamellar layers around a central protein filament, consisting of silicatein and the scaffold protein silintaphin-1. Once the growing spicule is extruded into the extracellular space, it obtains final size and shape. Again, this process is mediated by silicatein and silintaphin-1, in combination with other molecules such as galectin and collagen. The molecular toolbox generated so far allows the fabrication of novel micro- and nanostructured composites, contributing to the economical and sustainable synthesis of biomaterials with unique characteristics. In this context, first bioinspired approaches implement recombinant silicatein and silintaphin-1 for applications in the field of biomedicine (biosilica-mediated regeneration of tooth and bone defects) or micro-optics (in vitro synthesis of light waveguides) with promising results.

Structural investigations of Si/C/N-ceramics from polysilazane precursors by nuclear magnetic resonance

Abstract Polymeric precursors to Si/C/N-ceramics can be obtained by thermal crosslinking of polysilazanes. The precursor is then pyrolysed at a temperature of 1000 °C into the amorphous ceramic. Between 1000 and 2000 °C this amorphous state is transformed into the thermodynamically stable phases. The processes during pyrolysis and crystallization are connected with microstructural conversion. Simultaneously, changes of chemical and physical properties take place. The successive conversion of the atomic coordination during the transformation from the polymer into the amorphous ceramic and finally into the crystalline ceramic was studied using solid-state NMR. 29 Si-, 13 C- and 1 H-spectra were recorded. The result is a detailed report of the reactions and changes in structure occuring during ceramization of a polyhydridomethylsilazane and crystallization of its ceramic residue. Furthermore, the amorphous state of this material was compared with a polyvinylsilazane-derived amorphous ceramic.

Structure analysis and properties of Si-C-N ceramics derived from polysilazanes

Pyrolysis of different polysilazanes has been used to prepare novel covalent amorphous ceramics composed of silicon, carbon, and nitrogen. The formation and structure of the as-pyrolyzed amorphous state and its devitrification into stable crystalline phases have been investigated by means of nuclear magnetic resonance spectroscopy, infrared spectroscopy, thermogravimetry, mass spectroscopy, X-ray and neutron diffraction, and by transmission electron microscopy. Additionally, the electrical conductivity and the compression creep behaviour of the as-received materials have been analyzed.

Correlation of boron content and high temperature stability in Si-B-C-N ceramics II

Abstract This study treats the preparation of polyborosilazanes which are obtained by hydroboration of oligovinylsilazane [–(H 2 CCH)SiHNH–] n with different amounts of H 3 B . SMe 2 . Pyrolysis of these precursors yields amorphous ceramic materials, which only differ in their relative boron content whereas their Si/C/N ratio is very similar. High temperature thermogravimetric analysis reveals that with increasing boron content the materials thermal degradation is shifted toward higher temperatures. Furthermore, the observed mass loss due to nitrogen evaporation during the materials decomposition is noticeably smaller in boron-rich materials than thermodynamically calculated. X-ray diffraction (XRD) experiments support the presence of crystalline silicon nitride in heat treated samples with a minimum boron content of 5 at.%. The decomposition temperature of this phase strongly depends on the amount of boron and can exceed 2000°C. However, high temperature stable materials are not only characterized by a definite B/N ratio, but also by the ability to develop appropriate microstructures.

Precursor-derived Si-B-C-N ceramics

In recent years, the pyrolysis of preceramic organometallic compounds became of increased interest for the synthesis of new inorganic materials. The main objective of this process route is to build up novel materials from molecular units in order to design the material on an atomic scale. According to this process, preceramic polymers are synthesized from monomer units. After cross-linking of such precursors, the obtained preceramic networks will be transformed by pyrolysis into amorphous materials. Further increase of the temperature yields thermodynamically metastable and/or stable crystalline phases. Because of the fraction of covalent bonding providing high thermally, chemically and mechanically stable materials, materials based on the Si-B-C-N system are of special interest. Bulk materials, coatings, and fibers of such materials reveal quite interesting high temperatus properties.

Virus-templated synthesis of ZnO nanostructures and formation of field-effect transistors.

The search for novel methods for the synthesis of nanostructured materials is an important step towards the miniaturization of multifunctional devices, which requires careful and appropriate integration of various materials into a single unit. However, most of the conventional synthesis methods for multicomponent systems involve harsh reaction conditions and thereby introduce limitations in the choice of materials to be combined. For instance, in ceramic synthesis methods, extreme heating and/or pressure are often used, which may be inapplicable to certain components of a device structure. Further factors critical to the miniaturization are the size of the obtained powder particles and their tendency to agglomerate. Hence, the integration of different materials is still a challenging goal and can hardly be achieved by conventional processing. Biomineralization is a process used by organisms to generate composite materials composed of organic and inorganic phases, which often exhibit exceptional properties. [ 1 ] Organic molecules, such as peptides, proteins, or polysaccharides, guide the crystal growth at ambient conditions that eventually determine the morphology and the functional properties of the materials. [ 2 ] The integration of biomolecules as templates or structure-directing agents, on the other hand, offers the opportunity to explore alternative low-temperature methods in the synthesis of bioinorganic hybrid materials with novel tailored functionalities. [ 3 , 4 ] For some applications, however, the adaptation of bionic mineralization approaches to the synthesis of artifi cial composite materials is not possible, since no interactions between the inorganic phase

Interface characterization of nanosized B-doped Si3N4/SiC ceramics

Recent results on the crystallization behaviour of nanosized boron-doped silicon carbonitride ceramics as revealed by structural characterization with various transmission electron microscopy methods are described. Ceramic composites in the quaternary system Si-C-N-B were produced by polymer pyrolysis of doped polysilazanes without the presence of oxide sintering additives. Annealing at elevated temperatures induces the crystallization of the initially amorphous materials, resulting in a microstructure consisting of nanocrystalline Si 3 N 4 , SiC and turbostratic BN(C). Attention is focused on the BN(C) interface layers and the lattice structure of the small-scaled crystallites in these nanocrystalline ceramics.

Toughening through nature-adapted nanoscale design

The extraordinary combination of strength and toughness attained by natures highly sophisticated structural design in nacre has inspired the synthesis of novel nanocomposites. In this context, the organic-inorganic hierarchical design of nacre has been mimicked. However, two key features of nacre, namely the scaling of the structural components and the low content of the organic phase, have not been replicated yet. Here, we present thin nanocomposite films with properly adjusted thicknesses of the organic and inorganic layers, as well as a microstructure that closely resembles that of nacre. These films, which are obtained by the combination of low-temperature chemical bath deposition of titania with layer-by-layer assembly of polyelectrolytes, exhibit enhancement in a fracture toughness by a factor of 4, combined with notable increase in hardness, while the Youngs modulus is largely preserved in comparison to the single titania layer. Our findings highlight the significance of the 10:1 inorganic/organic layer thickness ratio evolved by nature, and provide novel perspectives for the future development of efficient bioinspired thin films.

Boron-containing polysilylcarbodi-imides: A new class of molecular precursors for Si-B-C-N ceramics

Abstract The synthesis of boron-containing polysilylcarbodi-imides of general type {B[C 2 H 4 (R)SiNCN] 3 } n [ 6a R = CH 3 , 6b R = H, 6c R = (NCN) 0.5 ] by different reactions and their thermal behaviour are discussed. The title compounds 6a–6c can be obtained by a hydroboration reaction of BH 3 *S(CH 3 ) 2 ( 5 ) with the vinyl-substituted polysilylcarbodi-imides [(H 2 C=CH)(R)SiNCN] n [ 3a R = CH 3 , 3b R = H, 3c R = (NCN) 0.5 ], which themselves are accessible from the reaction of the vinyl-substituted chlorosilanes (H 2 C=CH)(R)SiCl 2 ( 1a R = CH 3 , lb R = H, 1c R = Cl) with stoichiometric amounts of H 2 N-C≡N ( 2 ). Furthermore, a method for the synthesis of compounds 6a–6c is given by treatment of compounds 1a–1c with (H 3 C) 3 SiN=C=NSi(CH 3 ) 3 ( 4 ). An alternative reaction sequence, which finally leads to compounds 6a–6c is the hydroboration of the monomeric vinyl-substituted chlorosilanes 1a–1c with BH 3 * S(CH 3 ) 2 ( 5 ) to yield the chlorosilylethylboranes B[C 2 H 4 (R)SiCl 2 ] 3 ( 7a R = CH 3 , 7b R = H, 7c R = Cl) at first. Treatment of compounds 7a–7c with (H 3 C) 3 SiN=C=NSi(CH 3 ) 3 ( 4 ), even with or without solvent, stoichiometrically or with excess 4 , produces the hydroborated polysilylcarbodi-imides 6a–6c . The thermogravimetric behaviour of the polysilycarbodi-imides 3a–3c as well as the boron-containing polymers 6a–6c have been determined by simultaneous TGA (20–1100°C, argon, heating rate 2 Kmin −1 ). The ceramic yields of the polymers are in the range of 53–73%, depending on the structure and the composition of the polymers applied.

Isolation of ZnO-binding 12-mer peptides and determination of their binding epitopes by NMR spectroscopy.

Inorganic-binding peptides are in the focus of research fields such as materials science, nanotechnology, and biotechnology. Applications concern surface functionalization by the specific coupling to inorganic target substrates, the binding of soluble molecules for sensing applications, or biomineralization approaches for the controlled formation of inorganic materials. The specific molecular recognition of inorganic surfaces by peptides is of major importance for such applications. Zinc oxide (ZnO) is an important semiconductor material which is applied in various devices. In this study the molecular fundamentals for a ZnO-binding epitope was determined. 12-mer peptides, which specifically bind to the zinc- or/and the oxygen-terminated sides of single-crystalline ZnO (0001) and (000-1) substrates, were selected from a random peptide library using the phage display technique. For two ZnO-binding peptides the mandatory amino acid residues, which are of crucial importance for the specific binding were determined with a label-free nuclear magnetic resonance (NMR) approach. NMR spectroscopy allows the identification of pH dependent interaction sites on the atomic level of 12-mer peptides and ZnO nanoparticles. Here, ionic and polar interaction forces were determined. For the oxygen-terminated side the consensus peptide-binding sequence (HSXXH) was predicted in silico and confirmed by the NMR approach.