The F content in sol-gel derived FHA coatings: an XPS study

Fluoridated hydroxyapatite (FHA) coatings have good bioactivity and small solubility in body fluid and thus have good potential as alternative for pure hydroxyapatite (HA) in bioactive applications. Bioactivity and solubility are sensitive to fluorine content in the coatings. In this work, Ca(NO3)2, P2O5 and HPF6 are used to prepare FHA coatings. The fluorine content of the coatings is characterized by means of X-ray photoelectron spectroscopy (XPS) analysis and a pF-meter with a fluorine-sensitive electrode. The XPS results of the F/Ca ratio are in good agreement with the HPF6/Ca ratio designed in the dipping sol when F/Ca is below 0.2. The F/Ca ratio saturates at 0.2 as fluorapatite forms. Upon saturation of F in FHA, the extra fluorine is believed to vaporize and evade the coating as HF formed from the reaction between CaF2 and the acidic P-containing groups. For the same reason, the fluorine content in the coating is also affected by the Ca/P ratio in the dipping sol.