A Solid-State NMR Study of Selenium Substitution into Nanocrystalline Hydroxyapatite

The substitution of selenium oxyanions in the hydroxyapatite structure was examined using multinuclear solid-state resonance spectroscopy (ssNMR). The study was supported by powder X-ray diffractometry (PXRD) and wavelength dispersion X-ray fluorescence (WD-XRF). Samples of pure hydroxyapatite (HA(300)) and selenate (HA(300)-1.2SeO(4)) or selenite (HA(300)-1.2SeO(3)) substituted hydroxyapatites were synthesized using the standard wet method and heated at 300 °C to remove loosely bonded water. PXRD data showed that all samples are single-phase, nanocrystalline hydroxyapatite. The incorporation of selenite and selenate ions affected the lattice constants. In selenium-containing samples the concentration of Se was very similar and amounted to 9.55% and 9.64%, for HA(300)-1.2SeO(4) and HA(300)-1.2SeO(3), respectively. PXRD and ssNMR data showed that the selenite doping significantly decreases the crystallite size and crystallinity degree. (31)P and (1)H NMR experiments demonstrated the developed surface hydrated layer in all samples, especially in HA(300)-1.2SeO(3). (1)H NMR studies showed the dehydroxylation of HA during the selenium oxyanions substitution and the existence of hydrogen bonding in structural hydroxyl group channels. (1)H→(77)Se cross polarization NMR experiments indicated that selenites and selenates are located in the crystal lattice and on the crystal surface.