19F MAS-NMR and solution chemical characterization of the reactions of fluoride with hydroxyapatite and powdered enamel

Abstract

Solution chemical and ¹⁹F magic angle spinning-nuclear magnetic resonance (MAS-NMR) methods have been utilized to study the effects of fluoride dose, fluoridating pH, and mineral surface area on the dynamics of fluoride reactivity with hydroxyapatite and powdered human dental enamel in vitro. Both solution chemical fluoride uptake and NMR measurements demonstrated that the reaction products of ionic fluoride with apatite include mixtures of FAP, FHAP, and CaF₂, with increased amounts of CaF₂ promoted by increased F concentration or decreased pH. NMR analysis showed FAP or FHAP as a reaction product of fluoride uptake under all conditions, regardless of whether CaF₂ was formed, unambiguously demonstrating fluorite as an additive rather than substitute form of F reactivity. pH stat measurements demonstrated the release of OH⁻ during F reactivity with apatites corresponding to ion exchange formation of FAP/FHAP or dissolution/reprecipitation formation of CaF₂. Phosphate release into solution accompanied fluoride uptake under all conditions, including regions where ion exchange predominated. Whereas powdered dental enamel demonstrated fluoride uptake behavior similar to that of synthetic apatite, the resulting reaction products differed as analyzed by ¹⁹F MAS-NMR.