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 ¹H and ³¹P 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 ¹H→³¹P cross polarization (CP)/MAS spectra. The ³¹P differential cross polarization (DCP)/MAS and ¹H inversion recovery →³¹P CP/MAS measurements revealed different chemical interactions between ¹H and ³¹P 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.