TY - JOUR A1 - Jäger, Christian A1 - Welzel, T. A1 - Meyer-Zaika, W. A1 - Epple, M. T1 - A solid-state NMR investigation of the structure of nanocrystalline hydroxyapatite JF - Magnetic resonance in chemistry N2 - Nanocrystalline hydroxyapatite (HAp) prepared by a precipitation route was investigated. The X-ray diffraction (XRD) powder patterns of the elongated nanocrystals with a typical diameter of about 10 nm and length of 30-50 nm (by transmission electron microscopy (TEM)) revealed the presence of HAp with significantly broadened XRD reflections. However, Ca deficiency was found, as the Ca/P ratio was 1.5 only (so-called calcium-deficient hydroxyapatite (CDHA)), and not 1.67. This Ca deficiency of nanocrystalline HAp is explained using NMR. It is shown unambiguously that (i) the nanocrystals consist of a crystalline core and a (disordered) surface region with a relative phosphate content of about 1:1, (ii) the crystalline core is HAp, and (iii) the surface region is dominated by hydrogen phosphate anions (with no hydroxyapatite-like structural motif) and structural water (hydrate). From the relative phosphate content and taking into account the crystal shape, the thickness of the surface layer along the main crystal axis could be estimated to be about 1 nm, and the average chemical composition of the surface layer has been determined. Finally, a Ca/P ratio of 1.52 was estimated from the NMR data that compares well with the value of 1.51 from chemical analysis. The important consequences are that the surface of nanocrystalline HAp has nothing in common with the bulk composition and that the chemistry of such materials (e.g. the binding of protein molecules to phosphate surfaces) must be reconsidered. KW - NMR KW - H-1 KW - P-31 KW - Biomineralization KW - Hydroxyapatite KW - Calcium phosphate KW - Nanoparticles PY - 2006 DO - https://doi.org/10.1002/mrc.1774 SN - 0749-1581 SN - 1097-458X VL - 44 IS - 6 SP - 573 EP - 580 PB - Wiley CY - Chichester AN - OPUS4-12366 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER -