In vivo inspired conditions to synthesize biomimetic hydroxyapatite

We present a simple one-pot crystallization method, inspired by biological conditions, for the synthesis of hydroxyapatite (Ca₅(PO ₄)₃OH) nanocrystals. The reaction proceeds via NH ₃ vapor diffusion into a CaCl₂-NaH₂PO ₄ mixed solution that is free of any organic additives. The advantage of relying on acidic calcium-phosphate precursors here is, first, that the reaction can be performed at room temperature within a short time and without direct pH control and, second, that it does not produce any secondary phases or byproduct. Furthermore, the addition of NaHCO₃ to the salt solution or the introduction of (NH₄)₂CO₃ instead of NH₃ lead, respectively, to the precipitation of B- or A-type carbonate-apatite phases according to the FT-IR data. Multinuclear solid state NMR studies and especially ¹³C CP experiments allow an in-depth characterization showing the presence of A/B substitutions in carbonated samples as well and indicate a close similarity to deproteinated bovine compact bone. A precipitation mechanism accounting for the precipitation of mainly A- or B-type carbonated apatite under the respective experimental conditions is proposed.