Discrimination of Surface and Bulk Structure of Crystalline Hydroxyapatite Nanoparticles by NMR

Solid state 1H and 31P NMR spectroscopy was used to characterize well-crystallized hydroxyapatite samples of different stoichiometry prepared by a precipitation route. The aim of the paper was to investigate the bulk structural features of samples with different stoichiometry and to discriminate signals related to the surface from those related to the bulk. Thanks to the implementation of (i) in situ thermal pretreatment at 623 K, (ii) filling of the NMR rotor in a controlled atmosphere, (iii) relative proton enrichment of the surface performed under controlled isotopic H-D exchanges, and (iv) specific NMR sequences including inversion recovery measurements, two-dimensional HETCOR and DQSQ spectra, new resolved NMR signals originating from the surface and from the bulk were identified alongside already reported signals associated with adsorbed water, structural phosphates, and OH groups. In particular, considering the influence of the stoichiometry, it was possible to identify a specific signature associa...