Structure and acid resistance of fluoride-treated hydroxyapatite particles

Topical fluoride is used in clinical dentistry to prevent dental caries. The primary reaction product formed on the tooth surface after topical fluoride application is calcium fluoride (CaF2). In this study, we investigated the effect of deposited CaF2 on the acid resistance of fluoride-treated hydroxyapatite (HAp). Commercial HAp particles were treated with two types of fluoride-containing acetic acid-sodium acetate (Ac-AcNa) buffer solutions, with and without the addition of phosphate ions. X-ray diffraction (XRD) and nuclear magnetic resonance (NMR) analyses confirmed the formation of fluorine-substituted hydroxyapatite (F-HAp), along with a hydration layer containing calcium phosphate and CaF2. Phase composition analysis revealed that the presence of phosphate ions in the fluoride treatment solution reduced the amount of CaF2 deposited. Acid resistance evaluation and phase composition analysis indicated that the initial dissolution rate of the fluoride-treated HAp in Ac-AcNa buffer solutions decreased on surfaces densely covered with CaF2 particles.