Biologically Inspired Engineering of CaCO3 Crystals

Several studies have been directed to the phenomenon of heteroepitaxy in CaCO3 biomineralization. Seminal contributions have come from the group of Mann and co-workers who studied the influence of negatively charged surfactants on crystal nucleation. The group made use of a Langmuir film balance that allows for spreading surfactant molecules as a mono-layer at the air-water interface with the charged head groups pointing toward the aqueous subphase. Nucleation of calcite single crystals was observed from monolayers of aliphatic monocarboxylic acids, sulfates or phosphonates. The crystals that grew underneath the monolayers in general showed a significantly narrower size distribution and reduced nucleation time, as compared to calcite crystals precipitated spontaneously from supersaturated solutions. Moreover, the crystals grew crystallographi-cally oriented relative to the monolayer. Calcite single crystals nucleated preferentially from the {10.0} face underneath compressed monolayers of amphiphilic carboxylic acids, while monolayers of alkylsulfates and -phosphonates led to calcite single crystals that nucleated from the (0 0 1) face. Detailed schemes were proposed to rationalize the different modes of interaction between the different head groups of amphiphiles in the monolayers and the corresponding faces from which the crystals were nucleated.

To gain more precise controls over the relative positions of coordinating residues, patterned self-assembled monolayers of bifunctional ro-terminated alkanethiols (HS(CH2)nX, X = CO3", SO3, PO3~ and OH) have been produced very recently on different supporting metals (Au, Ag) by means of microcontact printing. Depending on the appropriate combination of functional groups, length of alkyl chains, metal substrate and nucleating area, the selective nucleation of calcite single crystals has been achieved for a huge variety of crystallographic orientations. As a unique feature, the self-assembled monolayers allow one to grow isolated and oriented single crystals with a defined separation which is encoded in the tiling of the nucleating areas. The technique has been further employed to grow crystalline CaCO3 layers that consist of alternating domains of differently oriented calcite single crystals.

The few cited examples demonstrate how model studies may focus on aspects of induced nucleation at organized organic surfaces. Single structural parameters of the organic matrix can be varied systematically and analysed for their influence on crystal nucleation and growth. Results gleaned from these experiments provide important informations that could help to interpret the observed, often highly complex, structures of biominerals. The models studies, furthermore, indicate as to how biologically inspired design and novel technical approaches combine into innovative synthetic strategies for engineering artificial crystalline architectures.

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