Fluoride Analysis of Apatite Crystals with a Central Planar OCP Inclusion: Concerning the Role of F⁻ Ions on Apatite/OCP/Apatite Structure Formation

Abstract.

To study the roles of F⁻ ions in the formation of apatite crystals embedding octacalcium phosphate (OCP) lamella in the center of apatite (Ap), a range of the Ap/OCP/Ap lamellar-mixed crystals were synthesized under various concentrations of fluoride ion (F⁻) from 0.1–1.0 ppm at pH 6.5 and 37°C. The products were analyzed for the F⁻ incorporation, F⁻ distribution, and the amount of OCP and Ap by chemical analysis, X-ray diffraction (XRD), electron probe microanalysis (EPMA), and nuclear magnetic resonance (NMR) techniques. The F⁻ content and the amount of apatite in the crystalline product increased with an increase in the F⁻ concentration in solution, whereas the amount of OCP and the yield of total product decreased. EPMA indicated that F⁻ ions are distributed in the crystals almost homogeneously. The combined analysis suggested that a low-substituted fluoridated hydroxyapatite (FHAp) grew on a small amount of F⁻-containing OCP or on a surface-reaction layer of OCP, which has accumulated a small amount of F⁻. The roles of F⁻ ions were hypothesized as the reduction of the growth rate and/or the critical thickness in the a*-axis direction of OCP, the enhancement of hydrolysis of OCP, and the activation of the growth of FHAp, resulting in thinner OCP lamella and thicker apatite lamella in the a*-axis direction with an increase in F⁻ concentration.