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

Solid state H-1 and P-31 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 (1) 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 associated with defective hydrogenophosphate groups present in the bulk. Despite the well-ordered surface terminations of the nanoparticles, specific surface signals associated with nonprotonated and protonated surface terminating phosphate groups could be identified. In addition, from the three resolved H-1 signals associated with columnar OH channels, two from the bulk and one from the surface, a structural model describing the relative organization of hydroxyl groups running along the c axis inside the columnar OH channel in the well-crystallized particles is proposed: the two types of bulk hydroxyls are associated with the presence of both up and down orientations of their related protons in a same tunnel. Corresponding H-1 signatures of the surface-terminating hydroxyls or structured water molecules emerging from the OH channels were also identified. Moreover, in addition to the broad 5.1 ppm line associated with water adsorbed on calcium cations and hydrogenophosphate groups, the 1.1 ppm line is ascribed to structured external water molecules stacking in continuity to the OH channels.