Formation of calcium phosphates by vapour diffusion in highly concentrated ionic micro-droplets

In this work we have used the sitting drop vapour diffusion technique, employing the "crystallization mushroom" to analyze the evolution of calcium phosphate crystallization in micro-droplets containing high initial concentrations of Ca(2+) and HPO(4)(2-). The decomposition of NH(4)HCO(3) solution produces vapours of NH(3) and CO(2) which diffuse through the droplets containing an aqueous solution of Ca(CH(3)COO)(2) and (NH(4))(2)HPO(4). The result is the increase of pH by means of the diffusion of NH(3) gas and the doping of the calcium phosphate with CO(3)(2-) ions by means of the diffusion of CO(2) gas. The pH of the crystallization process is monitored and the precipitates at different times are characterized by XRD, FTIR, TGA, SEM and TEM techniques. The slow increase of pH and the high concentration of Ca(2+) and HPO(4)(2-) in the droplets induce the crystallization of three calcium phosphate phases: dicalcium phosphate dihydrate (DCPD, brushite), octacalcium phosphate (OCP) and carbonate-hydroxyapatite (HA). The amount of HA nanocrystals with needle-like morphology and dimensions of about 100 nm, closely resembling the inorganic phase of bones, gradually increases, with the precipitation time up to 7 days, whereas the amount of DCPD, growing along the b axis, increases up to 3 days. Then, DCDP crystals start to hydrolyze yielding OCP nanoribbons and HA nanocrystals.