Control over the Vertical Growth of Single Calcitic Crystals in Biomineralized Structures

Acidic biomacromolecules frequently incorporate into biomineralized structures to control the morphology and extent of crystal growth. The study of such processes has been hindered by the scarcity of a model system that mimics the influence of acidic biomacromolecules on mineral crystal growth. A carbonic anhydrase-assisted system was developed to model CaCO3 deposition at an air/solution interface. Textured CaCO3 crystals were found to grow in a direction orthogonal (vertical) to the air/solution interface. The crystal growth anisotropy became more pronounced upon addition of an anionic polymer, and an amorphous morphology was found at sufficiently high polymer concentrations. X-ray diffraction and high-resolution transmission electron microscopy studies showed that most calcite crystals grew along the (01̅4) and (001) planes vertically, whereas the (012) and (110) planes were oriented in the lateral direction. The added acidic polymers adsorbed predominantly onto the (012) or (110) faces of the growing crystals, contributing to epitaxy and crystal growth anisotropy in the vertical direction by inhibiting crystal growth at specific lateral faces that interacted with the acidic polymer. This alignment is characteristic of crystal growth in biomineralized calcites. These observations suggest that the presence of the acidic biomacromolecules induce crystals to grow with specific longitudinal and lateral orientations.