Qualitative Analysis on Crystal Growth of Synthetic Hydroxyapatite Influenced by Fluoride Concentration

Abstract

Fluorides are known to be a significant factor that promote remineralization and stabilize hydroxyapatite crystals within the enamel. In spite of many studies about the effect of fluoride on remineralization of tooth enamel, its mechanism has not yet been established. This study was designed to examine the effect of fluoride concentration on crystal growth of synthetic hydroxyapatite (HA) particles in pH 7.0 supersaturated solutions with different fluoride concentrations (0, 1, 2, 4, 8, 16, 32 and 64 ppm) by means of FE-SEM, FE-TEM, x-ray diffraction (XRD) and FT-IR. Eight groups of pH 7.0 calcium phosphate supersaturated solutions were prepared with different fluoride concentrations (0, 1, 2, 4, 8, 16, 32 and 64 ppm). Each solution was introduced into the reactive column containing the synthetic HA (Sigma Aldrich) by using a peristaltic pump for constant reaction rate (2.5 mL/h) for 48 hours. Calcium phosphates precipitated on the synthetic HA surface while solutions flew through the reactive column. The resulting products were prepared for FE-SEM and FE-TEM to evaluate the morphological changes on the crystals and examined by XRD and FT-IR to identify the crystals. The FE-SEM examination revealed various morphological changes of the crystals, where the size and the number of new crystals on the surface of the seed HA increased as the fluoride concentration was raised, with additional less-ordered crystallites at relatively high concentrations of fluorides. FE-TEM examination showed an additional amorphous layer on the surface of the crystals with the existence of fluoride, whereas definite lattice structures completely reached the surface of the crystals without fluorides. XRD data showed that all crystals had the same patterns as the unreacted synthetic HA, regardless of fluoride concentration. With FT-IR results, the intensity of the OH-libration mode decreased when adding fluoride, compared to that of pristine HA. This study found that fluorides had an effect on crystal growth of the synthetic hydroxyapatite, depending on the concentration of the fluorides. The introduction of fluoride promotes the growth of HA, resulting in crystal growth on the surface of seed HA at relatively low concentrations of fluoride and additional less-ordered HA crystallites at relatively high concentrations of fluoride. The resulting crystals were considered to be partially fluoridated HA under room temperature and pH 7.0 supersaturated solutions, regardless of the concentration of the fluoride. Under the experimental conditions in this study, fluorides mainly react with the surface of the seed HA and have impact on the growth of HA in a less effective manner as the concentration of fluoride increases.