Solid-state double nuclear magnetic resonance study of the local structure of calcium phosphate nanoparticles synthesized by a wet-mechanochemical reaction

Calcium phosphate synthesized by a wet-mechanochemical reaction between Ca(OH)(2) and H3PO4 is identified by X-ray diffractometry as an apparently single phase of hydroxyapatite (HAp). We examined more detail on the local structure of the product by high-resolution H-1 and P-31 solid-state double nuclear magnetic resonance (NMR) spectroscopy. Adsorbed and structural waters, as well as two kinds of OH groups in HAp, were observed in the inversion recovery 1H magic angle spinning (MAS) spectra. HAp and other calcium phosphate were observed in H-1-->P-31 cross polarization (CP)/MAS spectra. The P-31 differential cross polarization (DCP)/MAS and H-1 inversion recovery --> P-31 CP/MAS measurements revealed different chemical interactions between H-1 and P-31 and, hence, could distinguish between ordered and disordered HAp. This micro-inhomogenity results from interiors and near-surface of nanoparticles as well as Ca2+-deficiency.