Properties of Nano Calcium Phosphates of Various Ca/P Ratios

Previous studies on nano calcium phosphates (NCP) have focused nearly exclusively on nano hydroxyapatite (HA) because nano particles of other more soluble calcium phosphate phases of biomedical interests could not be readily prepared by the existing methods. OBJECTIVES: To prepare and study the properties of NCP with a range of Ca/P ratios. METHODS: The NCP were prepared using a spray-drying technique by simultaneously feeding to a two-liquid nozzle calcium hydroxide and phosphoric acid solutions at rates corresponding to a Ca/P ratio of 1.67, 1.5 or 1.33. X-ray diffraction (XRD), Infrared spectroscopy (FTIR) and BET measurements were used for product characterizations. Solubility properties were assessed by equilibrating the NCP sample in a physiological-like solution (PLS) at pH7.4 and 37°C. RESULTS: As the Ca/P ratio of prepared NCP decreased, XRD pattern changed from that of amorphous calcium phosphate (ACP) to mixed phases of ACP and crystalline acid phosphates. FTIR displayed spectra of ACP with the presence of acid phosphate, carbonate and water. NCP of lower Ca/P ratio contained lower amounts of ACP and carbonate, possibly due to the lower pH of the reactants. BET showed a surface area (mean±s.d.; n=3) of (36±6) m²/g for the NCP, corresponding to a calculated particle size of (59±9) nm. ANOVA showed that Ca/P ratio had a significant (p<0.05) effect on NCP's solubility. The measured ion activity product for HA, pIAP(HA), were (mean±s.d.; n=2) 93.8±0.08, 94.2±0.4, 94.9±0.02 for NCP with Ca/P ratios of 1.67, 1.5 and 1.33 respectively. These values were much lower (p<0.05) than the established value (115) for HA. CONCLUSIONS: NCP of various Ca/P ratios can be prepared using a spray drying method. All NCP were significantly more soluble than HA and have potential for applications such as tooth remineralization/desensitization and controlled drug release. Supported by NIDCR R01DE16416, NIST, and ADAF.