Brigid R. Heywood

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.

Controlled crystallization of CaCO3 under stearic acid monolayers

A fundamental concept in the study of biomineralization concerns the molecular recognition of inorganic materials at organized organic macromolecular substrates1. Here we investigate this concept through the use of stearic acid monolayers in the controlled crystallization of CaCO3 from supersaturated solutions. Whereas crystallization in the absence of a monolayer results in rhombohedral calcite crystals, the presence of an organized monolayer gives rise to oriented vaterite formation. The vaterite nuclei are aligned with their (0001) face parallel to the plane of the organic substrate and develop initially in the form of disk-shaped single crystals. The degree of compression of the monolayer dictates the homogeneity of vaterite nucleation. In particular, partially compressed films are optimal for controlled crystallization, suggesting that the mobility of organic surfaces may be of general importance. Our results can be explained by electrostatic and stereochemical interactions at the inorganic–organic interface and these observations support current theories of biomineralization, as well as being of potential significance in the crystal engineering of microscopic inorganic assemblies2.

Electron Microscopy Study of Magnetosomes in a Cultured Coccoid Magnetotactic Bacterium

Intracellular magnetite (Fe3O4) crystals produced by the magnetotactic bacterium MC-1 were analysed by transmission electron microscopy (TEM). Strain MC-1 represents the first-reported isolation of a coccoid magnetotactic bacterium in axenic culture. The magnetosomes of this bacterium displayed a pseudo-hexagonal prismatic habit, were elongated along <111> the crystallographic direction, and were truncated by {111}, {100} and {110} faces. The presence of {111} truncations represents a modification of the magnetosome morphology previously determined for those in other coccoid bacteria. Study of immature crystals produced by strain MC-1 showed that the crystal morphology was controlled even at early stages of development. Changes in the culture media affected both the number and shape of the bacterial magnetite crystals. Cells grown in an acetate-containing medium contained on average more crystals than those in cells grown in a sulphide-containing medium. Crystals synthesized in the acetate-grown cells tended to be less truncated than those in the sulphide-grown cells. No iron sulphide minerals, such as greigite, were observed in cells grown in the presence of sulphide.

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.

Controlled biosynthesis of greigite (Fe 3 S 4 ) in magnetotactic bacteria

We report the presence of intracellular greigite crystals in two different types of rod-shaped single-celled magnetotactic bacteria collected from sulfide-rich sites

Influence of monosaccharides and related molecules on the morphology of hydroxyapatite

The influence of monosaccharides and related molecules on the crystal morphology of hydroxyapatite (HAP) precipitated from chloride-containing supersaturated solutions has been studied by transmission electron microscopy. Pulse addition of aliquots of CaCl2 to Na2HPO4 solutions resulted in the formation of plate-like HAP crystals. In contrast, the presence of the monosaccharides D-glucose, D-galactose, D-mannose, glucuronic acid, N-acetyl glucosamine and D-fructose (phosphate:additive = 6:1) induced the precipitation of needle-shaped HAP crystals elongated along the c-axis. Other molecules, such as sorbitol and 1,2-butanediol, also showed this morphological effect. The order of decreasing aspect ratios was; sorbitol = 1,2-butanediol (25:1)> fructose (20:1)> glucose = galactose = mannose = glucuronic acid = N-acetyl glucosamine (10:1). Expression of the needle-like morphology is discussed in terms of kinetic factors involving nucleation and growth. The former is dependent on the exclusion of Cl- from HAP nuclei due to additive interactions with an amorphous calcium phosphate precursor phase. The latter is related to the strength of additive binding with the subsequent development of {100} HAP crystal faces.

Oriented crystallization of CaCo3 under compressed monolayers. Part 2.—Morphology, structure and growth of immature crystals

Transmission electron microscopy and electron diffraction have been used to determine the structure, orientation and morphology of CaCO3 crystals at the early stages of growth under compressed Langmuir monolayers of stearic acid [CH3(CH2)16CO2H] and octadecylamine [CH3(CH2)17NH+3]. At [Ca]≈ 9 mmol dm–3, elongated plate-like calcite single crystals of narrow particle size distribution were nucleated under fully compressed stearate monolayers. The crystals were elongated along the c axis and oriented with a {1text-decoration:overline10} face parallel to the monolayer/solution interface. Subsequent development of the crystals resulted in rhombohedral outgrowth into the bulk solution. Calcite crystals nucleated under partially compressed monolayers showed the same crystal chemical properties but had a more uniform size population. Nucleation under octadecylamine monolayers resulted in disc-shaped vaterite single crystals of narrow particle size distribution. Two types of disc were observed: (a) circular/hexagonal crystals oriented with their c axis perpendicular to the monolayer, and (b) oval-shaped crystals oriented with their a axis perpendicular to the monolayer. Changes in monolayer surface pressure did not influence these properties. The preferred orientation of calcite can be rationalized in terms of geometric and stereochemical matching between the carboxylate headgroups and the ions in the {1text-decoration:overline10} crystal face. There is no geometric match for vaterite but a stereochemical relationship exists on stearate monolayers with respect to the (00.1) face. For octadecylamine monolayers, bidentate binding of HCO–3 may be important in determining the preferential orientation of nuclei formed at the organic surface.

Oriented crystallization of CaCo3 under compressed monolayers. Part 1.—Morphological studies of mature crystals

The crystallization of CaCO3 under compressed Langmuir monolayers of stearic acid [CH3(CH2)16CO2H], octadecylamine [CH3(CH2)17NH2], octadecanol [CH3(CH2)17OH] and cholesterol (C27H45OH) has been studied over a range of supersaturation conditions by optical and scanning electron microscopy and X-ray diffraction. At total [Ca]= 9 mmol dm–3, negatively charged stearate monolayes induced the oriented nucleation of calcite. The crystals were of two related morphological types, one of which was formed through the physical realignment of the oriented crystals at the organic surface during growth. Reducing the total [Ca] to 4.5 mmol dm–3, resulted in oriented vaterite nucleation on the stearate monolayers. Vaterite was the major product on positively charged octadecylamine films, independent of [Ca]. Unlike the vaterite crystals of stearate films, these crystals were of two distinct morphological forms which represented two specific nucleation orientations. Neutral monolayers of octadecanol inhibited crystallization and those of cholesterol gave random non-oriented calcite deposition analogous to the control experiments. Morphological analyses indicated that the crystals were oriented with their [1text-decoration:overline1.0](calcite) and [00.1] and [11.0](vaterite) axes perpendicular to the monolayer surface.

Control over calcium carbonate phase formation by dendrimer/surfactant templates.

Poly(propylene imine) dendrimers that are modified with long alkyl chains self-assemble to form well-defined aggregates. The geometry and surface chemistry of the dendrimer assemblies can be varied through the addition of surfactants. These dendrimer/surfactant aggregates can be tuned to template the formation of the different phases of calcium carbonate. The use of octadecylamine results in the formation of polyhedral aggregates that become embedded within an amorphous calcium carbonate phase that persists in competition with the thermodynamic product, calcite. In combination with hexadecyltrimethylammonium bromide, small spherical aggregates are formed that induce the formation of vaterite. The use of the negatively charged surfactant SDS results in growth retardation by the Ca(2+)-induced agglomeration of dendrimer/surfactant aggregates into giant spherical particles. Eventually these particles become overgrown by rhombohedral calcite.

Structural and stereochemical relationships between Langmuir monolayers and calcium carbonate nucleation

The structural and stereochemical factors involved in the oriented nucleation of CaCO3 under compressed Langmuir monolayers have been studied by optical and electron microscopy, and X-ray and electron diffraction. The effect of headgroup charge, stereochemistry and changes in supersaturation have been investigated. At total (Ca)=9 mM, monolayers with carboxylate headgroups induce the oriented nucleation of calcite with the (11.0) axis perpendicular to the organic surface. At (Ca)=4.5 mM, oriented vaterite with the (00.1) axis normal to the monolayer, is deposited. The common features of these interactions are: Ca accumulation at the stearate headgroups, geometric matching of lattice distances (calcite only) and stereochemical correspondence between carboxylate and carbonate groups at the monolayer/crystal interface. In contrast, nucleation under positively charged octadecylamine monolayers was independent of (Ca) and gave vaterite aligned either along the (00.1) or (11.0) crystallographic axes. Although Ca binding is absent on these films, there is the possibility of stereochemical matching between bidentate binding of bicarbonate and the corresponding anions in the (00.1) and (11.0) crystal surfaces. Crystals (calcite and vaterite) grown under liquid-phase stearate monolayers were of increased size and uniformity in particle size distribution compared with those nucleated under fully compressed films. CaCO3 crystallization under octadecanol monolayers was inhibited and crystals grown in the presence of cholesterol films were the same as in the control experiments.