Aqueous Preparation of Highly Dispersed Hydroxyapatite Nanorods for Colloidal Liquid Crystals

Hydroxyapatite (HA) nanorods were synthesized using a citrate-assisted hydrothermal method. NaH 2 PO 4 , Na 2 HPO 4 , and Na 3 PO 4 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 Na 3 PO 4 ·12H 2 O 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%.