Combined effect of ion concentration and functional groups on surface chemistry modulated CaCO3 crystallization

The effects of self-assembled monolayer templates terminated with carboxyl, hydroxyl, amino and methyl groups (-COOH, -OH, -NH2 and -CH3) on CaCO3 crystallization are compared in aqueous solutions of low, medium and high Ca2+ concentrations. On the -COOH surface, only calcite rhombohedra are crystallized in the solutions at three different concentrations, while size, shape and orientation of the formed calcites are distinct in various Ca2+ concentration solutions. In low Ca2+ concentration solution, vaterites (111) and (200) are formed on -OH and -NH2 surfaces respectively while no crystals are observed on -CH3 surface. In high Ca2+ concentration solution, calcite (104) is formed on the -OH surface. Calcites and small amounts of vaterites and aragonites are formed on -NH2 and -CH3 surfaces. In medium concentration solutions, the results are similar to the case of high concentration with a lower density of crystals. From the view of the two well-known mechanisms for biomineralization, ion absorption and particle-based crystallization, the results in our study are discussed. Our findings can advance the understanding of CaCO3 biomineralization in nature and also supply guidance for the design of advanced biomaterials.