Two competitive nucleation mechanisms of calcium carbonate biomineralization in response to surface functionality in low calcium ion concentration solution

Four self-assembled monolayer surfaces terminated with –COOH, –OH, –NH(2) and –CH(3) functional groups are used to direct the biomineralization processes of calcium carbonate (CaCO(3)) in low Ca(2+) concentration, and the mechanism of nucleation and initial crystallization within 12 h was further explored. On −COOH surface, nucleation occurs mainly via ion aggregation mechanism while prenucleation ions clusters may be also involved. On −OH and −NH(2) surfaces, however, nucleation forms via calcium carbonate clusters, which aggregate in solution and then are adsorbed onto surfaces following with nucleation of amorphous calcium carbonate (ACC). Furthermore, strongly negative-charged −COOH surface facilitates the direct formation of calcites, and the −OH and −NH(2) surfaces determine the formation of vaterites with preferred crystalline orientations. Neither ACC nor crystalline CaCO(3) is observed on −CH(3) surface. Our findings present a valuable model to understand the CaCO(3) biomineralization pathway in natural system where functional groups composition plays a determining role during calcium carbonate crystallization.