Will fluoride toughen or weaken our teeth? Understandings based on nucleation, morphology, and structural assembly.

Fluoride exerts a substantial impact on the biominearlization of hydroxyapatite (HAP). In this study, the effect of fluoride on the nucleation kinetics of HAP was examined quantitatively in a simulated body fluid. It was found for the first time that fluoride promotes the nucleation of HAP. Furthermore, both the concentration of fluoride and the supersaturation of HAP play a key role in the formation of an ordered assembly of rod-like HAP crystallites. At relatively low concentrations of fluoride, an ordered assembly of rod-like crystallites similar to the structure of enamel could be obtained in the simulated body fluid, which could be explained by the self-epitaxial nucleation-mediated assembly. At high concentrations, fluoride may lead to the deterioration of the ordered assembled structure of HAP crystallites. That result is caused by the fast nucleation and growth of HAP crystallites when the supply of nucleation seeds is limited, resulting in the formation of spherulites. Our results illustrate how and why fluoride influences the self-assembled structures of HAP crystallites at low and high concentrations of F(-). Thus, they will substantially advance our understanding on the impact of fluoride ions on the mineralization of HAP and consequently on the dental and bone fluorosis.