Rüdiger Kniep

Biomimetic Morphogenesis of Fluorapatite-Gelatin Composites: Fractal Growth, the Question of Intrinsic Electric Fields, Core/Shell Assemblies, Hollow Spheres and Reorganization of Denatured Collagen

The biomimetic growth of fluorapatite in gelatin matrices at ambient temperature (double-diffusion technique) starts with elongated hexagonal-prismatic seeds followed by self-similar branching (fractal growth) and ends up with anisotropic spherical aggregates. The chemical system fluorapatite/gelatin is closely related to in vivo conditions for bone or tooth formation and is well suited to a detailed investigation of the formation of an inorganic solid with complex morphology (morphogenesis). The fractal stage of the morphogenesis leads to the formation of closed spheres with diameters of up to 150 μm. The self-assembled hierarchical growth thereby shows immediate parallels to the topological branching criteria of the macromolecular starburst dendrimers. A second growth stage around the closed spheres of the first stage is characterized by the formation of concentric shells consisting of elongated prismatic fluorapatite units with nearly parallel orientation (maximum diameter of the complete core/shell spheres of 1 mm). The specific structure of the core/shell assembly is similar to the dentin/enamel structure in teeth. Together with the idea of the biological significance of electric fields (pyro-, piezoelectricity) during apatite formation under in vivo or biomimetic conditions the present paper considers the composite character of the material and the mechanisms of fractal growth (branching criteria and architecture, the influence of intrinsic electric fields etc.).

Ternary and quaternary metal nitrides: A new challenge for solid state chemistry

The crystal chemistry of ternary and quaternary phases of the systems A/AE - TE - N (A = alkali metal; AE = alkaline earth metal; TE = transition element) is pre- dominantly characterized by the presence and formation of complex nitridometalate anions. A suitable classification of the nitrido-compound s is based on the correlation between coordination-num bers (TE), oxidation s tates (TE) and the specific structural c haracteristics of the a nionic partial structures. Introduction and preparative aspects Compared with the l arge number of well-known and well- investigated oxometalates the crystal chemistry and physics of nitridometalates is rather young and just at the beginning. The particular i nterest in nitridometalate compounds is mainly raised by the h igh polarizability o f the nitride ion which is expected to cause specific properties in structure and bonding relations.

Li3FeN2, a ternary nitride with ∞1[FeN4/23-]-chains: Crystal structure and magnetic properties

Abstract Li3FeN2 was prepared from the melt under nitrogen and the crystal structure was determined by single-crystal diffraction (Ibam; a = 487.2(1) pm, b = 964.1(2) pm, c = 479.2(1) pm, Z = 4). The crystal structure can be described in terms of a fluorite superstructure which contains infinite chains 1 ∞ [FeN 4 2 3−] of edge-sharing FeN4 tetrahedra. Measurements of the magnetic susceptibilities vs. temperature reveal Curie-Weiss behaviour. Magnetic ordering occurs below 10 K. The calculated magnetic moment of 1.7(1) μB indicates a low spin state of the Fe3+ ion.

Fluorapatite-Gelatine-Nanocomposites: Self-Organized Morphogenesis, Real Structure and Relations to Natural Hard Materials

The biomimetic system fluorapatite-gelatine (in aqueous solutions) is perfectly suited for the study of biomimetic steps closely related to steps in osteo- and dentinogenesis. Although representing a relatively low level of complexity, the biomimetic system still includes all aspects of complexity, such as metastability, self assembly, self-similarity, fractals, pattern-formation, hierarchy, and others. The present review is focused on the morphogenesis and real structure of fluorapatite-gelatine-nanocomposites and is structured in a sequence from macroscopic/bulk-properties to mesoscopic and finally microscopic observations, in part also supported by atomistic simulations. The field encompasses a large variety of components reaching from basic science to applications.

An atomistic simulation scheme for modeling crystal formation from solution.

We present an atomistic simulation scheme for investigating crystal growth from solution. Molecular-dynamics simulation studies of such processes typically suffer from considerable limitations concerning both system size and simulation times. In our method this time-length scale problem is circumvented by an iterative scheme which combines a Monte Carlo-type approach for the identification of ion adsorption sites and, after each growth step, structural optimization of the ion cluster and the solvent by means of molecular-dynamics simulation runs. An important approximation of our method is based on assuming full structural relaxation of the aggregates between each of the growth steps. This concept only holds for compounds of low solubility. To illustrate our method we studied CaF2 aggregate growth from aqueous solution, which may be taken as prototypes for compounds of very low solubility. The limitations of our simulation scheme are illustrated by the example of NaCl aggregation from aqueous solution, which corresponds to a solute/solvent combination of very high salt solubility.

Hierarchical architecture and real structure in a biomimetic nano-composite of fluorapatite with gelatine: a model system for steps in dentino- and osteogenesis?

The morphogenesis of hierarchically ordered, spherical aggregates of fluorapatite–gelatine-nanocomposites starts with elongated hexagonal prismatic seeds, followed by fractal branchings and the development of growing dumbbell states. The general principles of this dramatic process of self-organisation are not yet known but should be laid out already in the real structure of the central seed (P. Simon, W. Carrillo-Cabrera, P. Formanek, C. Gobel, D. Geiger, R. Ramlau, H. Tlatlik, J. Buder and R. Kniep, J. Mater. Chem., 2004, 14, 2218, ). Here we describe the structure and inner architecture of the fluorapatite composite seeds which consist of self-similar nano-subunits nucleated by gelatine macromelecules. Although grown without any cell activities, the biomimetic composite closely resembles natural biomaterials based on interactions between collagen and apatite.

Preparation of various titanium suboxide powders by reduction of TiO2 with silicon

Various titanium suboxide powders have been produced by solid-state reaction between TiO2 powders or TiO2 coated mica and silicon as reducing agent at temperatures below 1000°C in an inert atmosphere. The process leads to blue-grey powders with colour characteristics depending on the composition and the structure of the materials. Titanium suboxides like Ti2O3, γ-Ti3O5, Ti4O7, Ti7O13, Ti8O15 or Ti9O17 are formed during the reduction processes. Calcium chloride as an additive, type of atmosphere as well as temperature have an important influence on the composition and on the colour of the powders. The titanium suboxide powders were characterized by XRD.

“Hidden” Hierarchy of Microfibrils within 3D‐Periodic Fluorapatite–Gelatine Nanocomposites: Development of Complexity and Form in a Biomimetic System

The living world creates organic–inorganic composites in the form of biominerals that act as functional materials, and which frequently also show fascinating forms and patterns on various length scales. Mimicking the processes of biomineralization to gain deeper insight into these complex events constitutes a great challenge, not only for basic research but also for materials science and applications. Since 1996 our specific interest in biomimetic processes and hybrid materials has been focused on (fluor-)apatite–gelatine nanocomposites bearing a strong resemblance to the biosystem hydroxyapatite–collagen, which plays a decisive role in the human body as a functional material in the form of bone and teeth. The biomimetic system apatite–gelatine is investigated by a double-diffusion arrangement in which the ions migrate into a gelatine gel from opposite reservoirs containing aqueous solutions of calcium and phosphate/fluoride, respectively. This system is perfectly suited to obtaining information on processes of self-organization, and may help in gaining insight into the essentials of the formation of organic–inorganic nanocomposites of biological relevance. Herein, we report a recent observation concerning the cooperative interplay between gelatine microfibrils and the nano-apatite–gelatine composite, which creates a new quality at an increased level of complexity that may be considered as a key scenario in the development of biological/biomimetic patterns of organic–inorganic nanocomposites. In particular, we would like to contribute to the general question of dumbbell formation during the fractal morphogenesis 7,8] of the fluorapatite–gelatine nanocomposite. The fractal morphogenesis of this nanocomposite (containing about 2.3 wt% gelatine) starts with an elongated hexagonal prism, which develops through outgrowth areas at both ends to form the first dumbbell state. This development is clearly seen in Figure 1, in which scanning electron microscopy (SEM) images of different growth states are superposed and differently colored: the central “young” seed grows into a “mature” one (violet areas in Figure 1) and subsequently splits up to form the first dumbbell state (yellow-green area in Figure 1). It has already been shown by X-ray diffraction (synchrotron radiation source) that the young seed exhibits scattering properties representative of a single crystal. The same is true for the mature seed (see Figure 4), although contour steps on the prism faces near the basal planes of the individual nanocrystals are clearly indicated. The X-ray pattern of the first dumbbell state is then characterized by sickle-shaped diffraction maxima with tendencies to form diffraction rings, which indicate the change in particle orientation. Until now, the fractal growth mechanism was predominantly described (and simulated) as a splitting procedure that develops directly from the basal planes of the (young) seed. This model was certainly oversimplified, as can immediately be seen from Figure 1, and it was speculated that the splitting scenario is more or less an outgrowth rather than an upgrowth phenomenon. This, however, would imply that even the young seed bears the intrinsic conception for its future shape development. No experimental evidence for such a kind of “equipment/talent” has been obtained so far, apart from the fact that the young seed builds up an intrinsic electrical dipole field, which, however, is not a sufficient condition for the shape development alone. An additional condition should be expected to explain the observation of controlled outgrowth from the inner seed volume in more detail. The real structure of the young seeds has already been investigated by high-resolution TEM (HRTEM) methFigure 1. Superposition of SEM images of the initial growth stages of (fractal) fluorapatite–gelatine nanocomposite aggregates.

Kristallstrukturell von Verbindungen A2X2 (A=S, Se; X=Cl, Br) / Crystal Structures of Compounds A2X2 (A=S, Se; X=Cl, Br)

Abstract The crystal structures of compounds A2X2 (A=S, Se; X=Cl, Br) contain molecules X-A-A-X with dihedral angles between 83.9° and 87.4°. Three different types of molecular packing are realized: S2CI2, S2Br2 (α-Se2Br2) and β-Se2Br2(Se2Cl2). Details of molecular geometries as well as crystal structures are discussed.

NaZn(H2O)2[BP2O8] · H2O: A Novel Open-Framework Borophosphate and its Reversible Dehydration to Microporous Sodium Zincoborophosphate Na[ZnBP2O8] · H2O with CZP-Topology

Crystals of NaZn(H2O)2[BP2O8].H2O were grown under mild hydrothermal conditions at 170 degrees C. The crystal structure (solved by X-ray single-crystal methods: hexagonal, P6(1)22 (no. 178), a = 946.2(2), c= 1583.5(1) pm, V= 1227.8(4).10(6) pm3, Z = 6) exhibits a chiral octahedral-tetrahedral framework related to the CZP topology and contains helical ribbons of corner-linked borate and phosphate tetrahedra. Investigation of the thermal behavior up to 180 degrees C shows a (reversible) dehydration process; this leads to the microporous compound Na[ZnBP2O8].H2O, which has the CZP topology. The crystal structure of Na[ZnBP2O8].H2O was determined by X-ray powder diffraction by using a combination of simulated annealing, lattice-energy minimization, and Rietveld refinement procedures (hexagonal, P6(1)22 (no. 178), a = 954.04(2), c = 1477.80(3) pm, V= 164.88(5).10(6) pm3, Z = 6). The essential structural difference caused by the dehydration concerns the coordination of Zn2- changing from octahedral to tetrahedral arrangement.