Self-Assembly of ZnO: From Nanodots to Nanorods

Many mineralizing organisms selectively form either calcite or aragonite, two polymorphs of calcium carbonate with very similar crystalline structures. Understanding how these organisms achieve this control has represented a major challenge in the field of biomineralization. Macromolecules extracted from the aragonitic shell layers of some mollusks induced aragonite formation in vitro when preadsorbed on a substrate of β-chitin and silk fibroin. Macromolecules from calcitic shell layers induced mainly calcite formation under the same conditions. The results suggest that these macromolecules are responsible for the precipitation of either aragonite or calcite in vivo.

https://science.sciencemag.org/content/sci/271/5245/67.full.pdf

Control of Aragonite or Calcite Polymorphism by Mollusk Shell Macromolecules

Supramolecular nanotube architectures based on amphiphilic molecules.

The systematic fabrication of advanced materials will require the construction of architectures over scales ranging from the molecular to the macroscopic. The basic constructional processes of biomineralization—supramolecular pre-organization, interfacial molecular recognition (templating) and cellular processing—can provide useful archetypes for molecular-scale building, or ‘molecular tectonics’, in inorganic materials chemistry.

Molecular tectonics in biomineralization and biomimetic materials chemistry

Organisms have been producing mineralized skeletons for the past
550
million years. They have evolved many different strategies for
improving
these materials at almost all hierarchical levels from
Angstroms to
millimetres. Key components of biological materials are the
macromolecules, which are intimately involved in controlling
nucleation,
growth, shaping and adapting mechanical properties of the mineral
phase to
function. One interesting tendency that we have noted is that
organisms
have developed several strategies to produce materials that have
more
isotropic properties. Much can still be learned from studying the
principles of structure–function relations of biological
materials.
Some of this information may also provide new ideas for improved
design of
synthetic materials.

Design strategies in mineralized biological materials

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.

Controlled crystallization of CaCO 3 under stearic acid monolayers

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 2.—Morphology, structure and growth of immature 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.

Oriented crystallization of CaCo3under compressed monolayers. Part 1.—Morphological studies of mature crystals

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.

http://iopscience.iop.org/article/10.1088/0022-3727/24/2/011/pdf

Structural and stereochemical relationships between Langmuir monolayers and calcium carbonate nucleation

Rhombohedral calcite crystals grown from supersaturated solutions undergo an unusual and specific morphological change in the presence of Li; the doped crystals are structurally invariant but exhibit well-developed basal (001) faces indicating that these relatively high energy crystal surfaces are stabilised by the incorporation of Li into surface interstitial sites.

Sundara Rajam

Papers

Controlled crystallization of CaCO 3 under stearic acid monolayers

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

Oriented crystallization of CaCo3under compressed monolayers. Part 1.—Morphological studies of mature crystals

Structural and stereochemical relationships between Langmuir monolayers and calcium carbonate nucleation

Selective stabilization of the (001) face of calcite in the presence of lithium