Fluoridated apatite synthesized using a multi-step fluoride supply system

Fluoridated apatite was synthesized at 80±1°C, pH 7.4±0.2, using a 5-step fluoride supply system. During the synthesis experiment, 0, 5, 10, 15 and 20 mmol/l of fluoride were each supplied for one-fifth of the experimental period with calcium and phosphate. X-ray diffraction analysis showed a typically apatitic pattern, although the (3 0 0) reflection was broader than that of homogeneous fluorapatite. Scanning electron micrographic observation indicated that the apatite was composed of needle-like crystals similar to hydroxyapatite and fluorapatite. High-resolution transmission electron microscopy showed a slender hexagonal shape similar to homogeneous hydroxyapatite in cross-sections perpendicular to the c-axis and the structural damage in the core of the crystal, although no boundary of step-like layers was observed. The apparent solubility in 0.5 mol/l acetate buffer solution (37°C and pH 4.0) was 12.1±0.2 mmol/l, much less than that of homogeneous hydroxyapatite 32.3±1.9 mmol/l, and similar to that of heterogeneous two-layer fluoridated apatite with an outer fluoride-rich layer 12.5±0.6 mmol/l, which was synthesized previously by supplying fluoride during the latter half of the experimental period.