Synthesis of plate-shaped hydroxyapatite via an enzyme reaction of urea with urease and its characterization

Plate-shaped hydroxyapatite (HAp) particles with preferred orientation to the c-plane have been synthesized by a homogeneous precipitation method via an enzyme reaction of urea with urease. The starting solutions with a Ca/P ratio of 1.67 were prepared by mixing calcium carbonate, phosphoric acid, and urea to which, then, an aqueous nitric acid solution was added for adjusting the pH at 3.0. While the products obtained from the starting solutions with high concentrations of calcium ion and phosphate ion were a mixture of calcium hydrogen phosphate (DCPD) and octacalcium phosphate (OCP), those from the starting solution with low concentrations of calcium ion and phosphate ion were a mixture of plate-shaped HAp and OCP. The particle size depended on the increasing rate of pH; a slower increasing rate of pH gave larger plate-shaped particles. The mixture product of plate-shaped HAp with preferred orientation to the c-plane and OCP changed to HAp single phase one by hydrothermal treatment at 120 °C for 24 h in pure water. The resulting HAp powders were kept plate-shaped still after the hydrothermal treatment. The end product was heated from 700 °C to 1200 °C for 1 h, and the phase changes during the heating process were examined. The product thermally decomposed into β-tricalcium phosphate (β-TCP) at 900 °C and then the β-TCP changed into α-TCP at 1200 °C.