Biomimetic formation of fluorapatite nanorods in confinement and the opposite effects of additives on the crystallization kinetics

Abstract

Enameloid is the most highly mineralized tissue of shark teeth with excellent mechanical properties, which consists of fluorapatite (FAP) crystallites in the form of an intricate hierarchical arrangement. However, the crystallization process of FAP within biological confined volumes is still unknown. Here, we introduce track-etched membranes with cylindrical channels as an available alternative to synthesize biomimetic FAP nanorods in confinement. We show that amorphous calcium phosphate (ACP) nanoparticles first precipitate and aggregate within the channels and then transform into crystalline FAP nanorods through a solid-state transformation pathway. During the crystallization process, crystal domains with different orientations are observed, which further fuse into large crystalline grains oriented along the [002] direction. More interestingly, we find that all additives, such as Mg²⁺, Sr²⁺ and poly(acrylic acid sodium salt) (PAAs), could accelerate the crystallization process of FAP nanorods at low concentrations while significantly stabilizing amorphous nanorods at high concentrations. Overall, these results demonstrate the important roles of confinement and additives in controlling the crystallization kinetics and morphology of inorganic crystals and provide inspiration for the rapid synthesis of biomimetic materials with outstanding properties.