Preparation of large-sized hydroxyapatite single crystals using homogeneous releasing controls

Hydroxyapatite (HAP) single crystals with size of tens of micrometer have been synthesized by a controlled homogeneous precipitation method. A complex of calcium bis(2-ethylhexyl) sulfosuccinate (Ca(AOT)2) and Na2HPO4 used to supply the calcium and phosphate ions during the reactions. Hexamethylenetetramine (HMT) is applied as a pH regulator to control the homogeneous release of hydroxyl ions in the bulk solution. Since HAP formation is sensitive to pH, the controlled release of hydroxyl ions by the hydrolysis of HMT can trigger the crystallization of HAP, which is regulated by the solution temperature. The formed HAP crystals have large and smooth crystal facets and their lattice structure can be easily determined by using atomic force microscopy. The control experiments show that HMT plays an important role in the formation of the large HAP since it can provide the homogeneous supersaturation fields in the bulk solution. The application of Ca(AOT)2 is another helpful factor which reduces the supersaturation level during the crystallization process. This synthesis strategy provides a facile pathway to obtain HAP single crystals at least one order in magnitude larger than the conventional ones.