Calcium phosphates are one of the most important biogenic minerals found in living organisms. Recent studies have suggested a non-classical mineralization pathway, which emphasizes the amorphous calcium phosphate (ACP) as a precursor phase during biomineral crystallization. However, its precise mechanism in mineralization is still poorly understood. Here, we found the aggregation state of ACP affected the nucleation of hydroxyapatite (HAP), which was controlled by collagen-I fibrils in simulated body fluids (SBFs). Our experiment reveals that at low supersaturation, collagen-I fibrils can prevent the self-aggregation of ACP precursor nanoparticles, thus promoting HAP heterogeneous nucleation by increasing the effective surface area of ACP. However, at high supersaturation, the aggregation state of ACP didn't change, and the nucleation rate of HAP kept almost the same. This finding suggests that the aggregation of ACP plays an important role in controlling HAP nucleation kinetics, which follows a new strategy to promote the biomineralization process.

In this work, dual roles of silicate in the crystallization of HAP in simulated body fluids were revealed. At lower concentrations of silicate (0.05–0.5 mM), it promoted the nucleation of HAP, and the lower the amount of silicate, the faster the HAP nucleation. In contrast, at higher silicate concentrations (3–8 mM), it inhibited the nucleation of HAP.

Aggregation-based crystal growth is distinct from the classical understanding of solution crystallization. In this study, we reveal that N-stearoyl-l-glutamic acid (C18-Glu, an amphiphile that mimics a biomineralization-relevant biomolecule) can switch calcite crystallization from a classical ion-by-ion growth to a non-classical particle-by-particle pathway, which combines the classical and non-classical crystallization in one system. This growth mechanism change is controlled by the concentration ratio of [C18-Glu]/[Ca2+] in solution. The high [C18-Glu]/[Ca2+] can stabilize precursor nanoparticles to provide building blocks for aggregation-based crystallization, in which the interaction between C18-Glu and the nanoprecursor phase rather than that of C18-Glu on calcite steps is highlighted. Our finding emphasizes the enrollment of organic additives on metastable nano building blocks, which provides an alternative understanding about organic control in inorganic crystallization.

The regulation of citrate on amorphous calcium phosphate (ACP)-mediated crystallization of hydroxyapatite (HAP) is revealed in this work. The surface associated citrate on ACP plays the key role in controlling the nucleation of HAP by inhibiting the reaction of surface nucleation, and the effect of embedded citrate inside ACP or citrate in solution is weak.

Magnesium ions (Mg2+) are widely present in biological fluids, and they are suggested as the vital factors that inhibit spontaneous hydroxyapatite (HAP) precipitation in nature. However, the regulation mechanisms of Mg2+ on HAP crystallization are still under intensive debates. We find that a typical precipitation of HAP from supersaturated solutions should include five stages: s1, formation of ion clusters and amorphous calcium phosphate (ACP); s2, stabilization of ACP; s3, transformation from ACP to HAP via dissolution and crystallization; s4, classical crystal growth of HAP; s5, HAP aging under a near equilibrium state. Actually, Mg2+ ions exhibit different inhibitory effects on these stages. The ions have a negligible influence on the kinetics of initial ACP formation in solutions (s1) and final aging of crystals (s5). Rather, the ions can either adsorb onto or incorporate into the ACP precursor particles during s1. In s2, the lifetime of ACP in solution is extended significantly by both two types of Mg2+ ions. However, the absorbed ones are more effective than the incorporated ones on the inhibition of the phase transformation from ACP to HAP in this s2. In stages of s3 and s4, it is only surface Mg2+ ions that retard crystal growth of HAP and the incorporated Mg2+ become inert. These detailed findings reveal how Mg2+ ions affect the crystallization process of HAP mineralization from supersaturation. Such a detailed understanding can provide a chemical strategy for precise regulation of HAP formation kinetics in intermediate phase mediated crystallization.

Faster nucleation of hydroxyapatite (HAP) at lower pH (with lower supersaturation) contradicts classical understanding. We find that the residue calcium ion in the mother liquor is the key to trigger ACP phase transformation, which gives an understanding of nonclassical nucleation kinetics of ACP-mediated crystallization and sheds light on biomineralization.

It is interesting to note that the demineralization of natural enamel does not happen as readily as that of the synthesized hydroxyapatite (HAP), although they share a similar chemical composition. We suggest that the hierarchical structure of enamel is an important factor in the preservation of the natural material against dissolution. The anisotropic demineralization of HAP is revealed experimentally, and this phenomenon is understood by the different interfacial structures of HAP−water at the atomic level. It is found that HAP {001} facets can be more resistant against dissolution than {100} under acidic conditions. Although {100} is the largest surface of the typical HAP crystal, it is {001}, the smallest habit face, that is chosen by the living organisms to build the outer surface of enamel by an oriented assembly of the rodlike crystals. We reveal that such a biological construction can confer on enamel protections against erosion, since {001} is relatively dissolution-insensitive. Thus, the spontaneous dissolution of enamel surface can be retarded in biological milieu by such a smart construction. The current study demonstrates the importance of hierarchical structures in the functional biomaterials.

Faster nucleation of hydroxyapatite (HAP) at lower pH (with lower supersaturation) contradicts classical understanding. We find that the residue calcium ion in the mother liquor is the key to trigger ACP phase transformation, which gives an understanding of nonclassical nucleation kinetics of ACP-mediated crystallization and sheds light on biomineralization.