Nanoscale Ion Dynamics Control on Amorphous Calcium Carbonate Crystallization: Precise Control of Calcite Crystal Sizes

Amorphous calcium carbonate (ACC) is an essential component present during the early stages of biomineralization of many calcifying organisms, which is used as a precursor of crystalline calcium carbonate phases. Here, we performed X-ray photon correlation spectroscopy experiments on ACC which show that the amount of adsorbed water has a strong control over its diffusive dynamics. Results of crystallization experiments under conditions of controlled humidity indicate that the adsorbed water is enough to spark crystallization of calcite. A direct proportionality is found between the relative humidity of the environment and the final size of the calcite crystals. Different hypotheses are made to explain this result, with the confinement within the amorphous matrix being the main suspect of controlling the size of the crystallites. Control of water activity and water content is, therefore, a possible way to regulate biomineral crystallinity both in nature and for the design of functional biomimetic materials.