Reaction characteristics of a tooth-bleaching agent containing H2O2 and NaF: in vitro study of crystal structure change in treated hydroxyapatite and chemical states of incorporated fluorine

This in vitro study was performed to elucidate the reaction mechanism of sodium fluoride (NaF), which is added to tooth-bleaching agents to lessen the adverse effect of hydrogen peroxide (H₂O₂) on teeth. Both hydroxyapatite (HAP) and dihydrated dicalcium phosphate (DCPD), model substances for dental hard tissues, dissolved easily in a simple H₂O₂ solution. In the H₂O₂/NaF solutions, however, fluorine compounds that could not be identified by X-ray diffraction (XRD) due to the smallness of the products were formed on the surface of the HAP. X-ray photoelectron spectroscopy (XPS) studies demonstrated that fluoridated hydroxyapatite (FHAP) was formed on HAP, and that calcium fluoride (CaF₂) formation was accelerated by increasing the concentrations of fluorine and H₂O₂ along with the partial dissolution of HAP. In H₂O₂/NaF solution, DCPD also transformed easily to FHAP and CaF₂, which are favorable to the remineralization process on the tooth surface. Thus, the mechanism of NaF was elucidated, and its use together with H₂O₂ for tooth bleaching was proved to be effective. Methodologically, the XPS two-dimensional plot made it possible for the first time to directly estimate the ratio of FHAP and CaF₂ in the reaction products, in contrast to the conventional wet-analytical method, which is simply based on the difference in solubility of the two components.