Jon M. Didymus

Crystallization at Inorganic-organic Interfaces: Biominerals and Biomimetic Synthesis

Crystallization is an important process in a wide range of scientific disciplines including chemistry, physics, biology, geology, and materials science. Recent investigations of biomineralization indicate that specific molecular interactions at inorganic-organic interfaces can result in the controlled nucleation and growth of inorganic crystals. Synthetic systems have highlighted the importance of electrostatic binding or association, geometric matching (epitaxis), and stereochemical correspondence in these recognition processes. Similarly, organic molecules in solution can influence the morphology of inorganic crystals if there is molecular complementarity at the crystal-additive interface. A biomimetic approach based on these principles could lead to the development of new strategies in the controlled synthesis of inorganic nanophases, the crystal engineering of bulk solids, and the assembly of organized composite and ceramic materials.

Morphological influence of functionalized and non-functionalized α,ω-dicarboxylates on calcite crystallization

The influence of a range of α,ω-dicarboxylates on the morphology of calcite crystals grown from supersaturated bicarbonate solutions was studied by optical and scanning electron microscopy. At Ca/malonate ≈ 3, spindle-shaped crystals elongated along the c axis and with curved {1text-decoration:overline10} prismatic faces were formed. This effect was reduced with increasing chain length. The unsaturated derivative, maleate, was intermediate in potency compared with the saturated malonate and succinate compounds. In contrast, the trans isomer, fumarate, had minimal morphological effect. Functionalization of the lower chain acids had a marked influence on crystal morphology. Crystals grown in the presence of aspartate (α-aminosuccinate) exhibited well defined {1text-decoration:overline10} prismatic faces at Ca/additive = 17, whilst γ-carboxyglutamate had a pronounced effect at ratios as high as 85. The stabilization of the {1text-decoration:overline10} faces of calcite by αω-dicarboxylate binding is described in terms of electrostatic, geometric and stereochemical recognition at the crystal/additive interface.

Influence of low-molecular-weight and macromolecular organic additives on the morphology of calcium carbonate

The influence of a range of soluble biological and related molecules on the crystallization of CaCO3 from aqueous supersaturated solution has been studied by optical and scanning electron microscopy and X-ray diffraction. The efficacy of monofunctional additives to induce morphological changes increased with overall anionic charge. For anions of the same charge, the effect was reduced with decreasing partial charge density on the oxygen atoms of the ligand. Additional factors, such as the distance between ligands and conformation were important for multifunctional molecules. Orthophosphate, sulfate, various phosphonates, a polysaccharide associated with coccoliths of Emiliania huxleyi and alginate interacted specifically or pseudo-specifically with crystal faces approximately parallel to the c axis indicative of a bidentate binding motif. Phenyl phosphonate stabilized {01text-decoration:overline12} faces, inferring a tridentate interaction due to steric constraints. The bone protein, osteocalcin was found to be a non-specific inhibitor whereas a bone proteoglycan monomer and polygalacturonate had minimal morphological effect. A carboxylated hyperbranched polymer gave oriented nucleation owing to partial segregation of the macromolecule at the air/water interface. Fish anti-freeze glycopeptides and polyvinyl alcohol induced the precipitation of vaterite possibly by affecting the kinetics of cation dehydration. Multifunctional additives such as diphosphonates, the coccolith polysaccharide and alginate were also effective at promoting crystal aggregation.

Atomic Force Microscopy of the Nacreous Layer in Mollusc Shells

We present atomic force microscopy (AFM) observations of the aragonite tablets of mature nacre in two types of mollusc, a bivalve (Atrina sp.) and a gastropod (Haliotis rufescens). By imaging in liquids it was possible to dissolve away the nacre layer by layer to reveal both the structure of a single tablet and its relation to vertically adjacent tablets. Atrina tablets (inner face) had a concave appearance; the central depression was surrounded by elongate rings that mimicked the orientation and aspect ratio of the unit cell viewed along the c axis. The tablet surfaces had a rough texture, and flat (001) planes of aragonite were rarely observed. Unit cell orientations were generally aligned both vertically and laterally between tablets of Atrina. Etching tablets with HCI initially removed the elongate rings and produced etch rows parallel to the a axis. Further etching of bleached Atrina nacre lifted off individual tablets to reveal underlying nacreous layers, showing no morphological registry between vertically adjacent tablets. The nacreous structure of Haliotis differed from Atrina in three ways: (i) the tablets were flatter and showed no elongate rings; (ii) the positions of the central depressions approximately repeated between nacreous layers, showing that the (presumed) nucleation sites line up along a given stack; and (iii) the unit cell orientations were not preserved between laterally adjacent tablets but were approximately aligned between vertically adjacent tablets.

Construction and Morphogenesis of the Chiral Ultrastructure of Coccoliths from the Marine Alga Emiliania huxleyi

Electron microscopy was used to study the developmental stages in coccolith biomineralization for the marine alga Emiliania huxleyi, with particular reference to the formation of the chiral ultrastructure. Transmission electron microscopy showed that the earliest stages in coccolith formation are characterized by an elliptical proto-coccolith ring of discrete single crystals of calcite. These initial crystals appear to be deposited with alternating radial (R) and vertical (V) orientations (the V/R nucleation model). Scanning electron microscopy showed that the R-crystals initially develop in the vertical direction to form rhombic plates, possibly with rhombohedral (1̄018} faces, and a Z-shaped cross section. When viewed from above the cell, further growth involves an anticlockwise tangential extension from each R-crystal which overlaps the neighbouring R-crystal on the inner rim and generates the morphological handedness. The result is an apparent inner and outer tube cycle of R-crystals with opposite imbrication. Subsequent growth of the plate-like extension occurs radially inwards, towards the foci of the elliptical ring, whereas the external surface develops outwards, in a direction perpendicular to the {1ˉ018} face, to form the shield elements. Tracings of elements for mature coccoliths revealed a c-axis distribution that was best modelled by an offset of this axis by approximately 20° to the normal for the local tangent. The chiral ultrastructure suggests that the calcite crystals are enclosed within vesicles which are themselves laid down in a chiral arrangement. Implications for the V /R model are discussed.

Habit modification in synthetic crystals of aragonite and vaterite

The addition of Li+ to supersaturated calcium hydrogen carbonate solutions results in the preferential expression of the (001) faces of aragonite whereas addition of poly α,β-aspartate induces vaterite nucleation and the formation of helicoid morphologies.

Fullerates: interaction of divalent metal ions with Langmuir monolayers and multilayers of mono-substituted C60–malonic acid

Stable Langmuir monolayers of C60[C(CO2H)2] are formed on pure water and on solutions containing Ca2+ or Cd2+ ions; catioN–Headgroup interactions result in expanded monolayers and the transfer of Ca2+–fullerene multilayers onto quartz substrates.

Modelling Biomineralization: Studies on the Morphology of Synthetic Calcite

Many organisms possess the remarkable ability to deposit single crystals of calcite (CaCCO3) with morphologies not normally observed in the inorganic world [1]. Whilst it is true that single geological crystals of calcite can exhibit an enormous range of different habits, all these forms have common interfacial angles and symmetry as described by the R3c space group. By contrast, the external forms of some biological single crystals of calcite have symmetries that are non-crystallographic. The coccolith segments deposited by the unicellular marine alga Emiliania huxleyi illustrate this phenomenon particularly well:- Current theories of biomineralization suggest that calcifying organisms have adopted strategies for controlling morphology based on the deployment of functional organic molecules. For example, proteins rich in aspartate and glutamate residues and also phosphoserine, are common for molluscs [3] whilst coccoliths of E. huxleyi are deposited along with sulphated and carboxylated polysaccharides [4]. Thus, carboxylate groups and, to a lesser extent, sulphates and phosphates play an important role in the biomineralization of calcite. Open image in new window Fig. 1. Single crystal segment of E. huxleyi [2] (left) compared to a similarly oriented geological calcite crystal (right)