Aqueous Preparation of Highly Dispersed Hydroxyapatite Nanorods for Colloidal Liquid Crystals

Hydroxyapatite (HA) nanorods were synthesized using a citrate-assisted hydrothermal method. NaH2PO4, Na2HPO4, and Na3PO4 were used as the phosphate sources and the influences of pH value were investigated. The XRD results show that pure hexagonal HA can be synthesized using Na3PO4·12H2O as the phosphate source with the citrate solution pH ranging from 5.0 to 7.6. The zeta potential evaluation demonstrates that as-synthesized HA nanorods are colloidally stable and the aqueous dispersion can be maintained homogenous without any sediment or creaming for more than at least a month. The Ca/P molar ratio of the HA nanorods is about 1.60, indicating that the HA nanorods are calcium-deficient hydroxyapatite. Besides, owing to the excellent colloidal stability and rod-like morphology with a high aspect ratio (>6), the HA aqueous dispersion undergoes a phase transition from an isotropic state to a liquid crystalline state upon increasing the particle concentration to 17wt%. The completely liquid crystalline phase forms when the particle concentration reaches above 30wt%.