Synthesis and characterisation of magnesium substituted calcium phosphate bioceramic nanoparticles made via continuous hydrothermal flow synthesis

Continuous hydrothermal flow synthesis (CHFS) technology has been used as an efficient and direct route to produce a range of largely crystalline magnesium substituted calcium phosphate bioceramics. Initially, magnesium substituted hydroxyapatite, Mg-HA, according to the formula [Ca10−xMgx(PO4)6(OH)2] was prepared in the CHFS system for x = 0.2 [where x:(10 − x) is the Mg:Ca ratio used in the reagents]. Biphasic mixtures of Mg-HA and Mg-whitlockite were obtained corresponding to x values in the range x = 0.4–1.6. The direct synthesis of phase pure crystalline Mg-whitlockite [based on the formula (Ca3−yMgy(HPO4)z(PO4)2−2z/3] was also achieved using the CHFS system for the range y = 0.7–1.6 (this corresponds to the range x = 1.6–5.3). With increasing substitution of magnesium for calcium, the material became ever more amorphous and the BET surface area generally increased. All the as-precipitated powders (without any additional heat treatments) were analyzed using techniques including X-ray powder diffraction, Raman spectroscopy and Fourier transform infra-red spectroscopy. Transmission electron microscopy (TEM) images revealed that in the case of y = 1.2, the Mg-whitlockite material comprised of ca. 28 nm sized spheres. The use of the CHFS system in this context facilitated rapid production of combinations of particle properties (crystallinity, size, shape) that were hitherto unobtainable in a single step process.