Comparison of Hydrothermal and Sol-Gel Synthesis of Nano-Particulate Hydroxyapatite by Characterisation at the Bulk and Particle Level

Hydrothermal and sol-gel synthesis methods have been used to prepare nano-particulate hydroxyapatite (HA) powders for detailed characterisation. Bulk elemental analysis data are compared from X-ray diffraction, X-ray fluorescence and Fourier transform infrared spectroscopy. These show the presence of secondary phases in the sol-gel powders which can be attributed to evaporative loss of precursor phosphite phases during specimen preparation and breakdown of the primary HA phase during calcination. Only the primary HA phase is detected in the hydrothermally prepared powder. In addition, Ca/P ratios of each powder are determined at the particle level using transmission electron microscopy with energy dispersive X-ray spectroscopy (TEM-EDX), having first established a threshold electron fluence below which significant electron-beam-induced alteration of the composition of HA does not occur. The TEM-EDX results show a greater compositional variability of particles from the sol-gel preparation route compared to the hydrothermal route. Overall it is the combination of the analysis techniques that shows the hydrothermal synthesis route produces nearstoichiometric, single phase, hydroxyapatite.