Model for Dental Enamel Apatite Depth Profiling

Structural and chemical changes that arise from fluoride substitution of synthetic hydroxyapatite (Ca sub(5)(PO4) sub(3)OH or "HAp") as a model of tooth enamel, were investigated by X-ray photoelectron spectroscopy (XPS). Elemental depth profiles with a depth resolution on a nanometer scale were determined to reveal the effect of "fluoridation" in neutral (pH = 6.2) and acidic media (pH = 4.2). With respect to the chemical composition and the crystal structure, XPS depth profiling revealed different effects of the two treatments. In both cases, however, fluoridation affected the surface only on a nanometer scale, which is in contrast to recent literature reports concerning XPS analysis performed on teeth, where depth profiles of F extending to several micrometers were reported. In addition to the elemental depth profiles, as published in various other studies, we also present quantitative depth profiles of the compounds CaF sub(2), Ca(OH) sub(2), and fluorapatite (FAp) that were recently proposed as a three-layer model for fluoridation of HAp under acidic conditions. The analysis of our experimental data exactly reproduces the structural order of this model, but on a scale that differs by nearly two orders of magnitude from previous predictions. The results also reveal that the amount of Ca(OH) sub(2) and FAp is small compared to that of CaF sub(2). Therefore, it is questionable whether such thin Ca(OH) sub(2) and FAp layers really can act as protective layers for dental enamel.