Solid State NMR Characterization of Nano-Crystalline Hydroxy-Carbonate Apatite using 1H-31P-13C Triple Resonance Experim
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0984-MM06-05
Solid State NMR Characterization of Nano-Crystalline Hydroxy-Carbonate Apatite using 1H-31P-13C Triple Resonance Experiments Florence Babonneau1, Christian Bonhomme1, Satoshi Hayakawa2, and Akiyoshi Osaka2 1 Universite P. et M. Curie, Paris, 75252, France 2 Faculty of Engineering, Okayama, 700 8530, Japan
ABSTRACT The local structure around hydrogen atoms, phosphate ions and carbonate ions in nano-crystalline nonstoichiometric hydroxy-carbonate apatite was investigated in this study. The use of 13C enriched precursors allowed to perform 1D and 2D CP MAS HETCOR (HETeronuclear CORrelation) experiments with 13C as a target spin. 2D triple resonance experiments involving the 1H/13C/31P spins were also performed. All these experiments led to the partial editing of the corresponding projection spectra and revealed fine structural details for the corresponding material. At least four 13C peaks, corresponding to carbonate ions in A-sites (OH-) and B-sites (PO43-) were evidenced.
INTRODUCTION Mineral crystals of human and animal hard tissues (bones and teeth) are analogous to the synthetic calcium hydroxyapatite (HAP), Ca10(PO4)6(OH)2 but correspond to structurally disordered nano-crystalline entities with nonstoichiometric compositions. Impurities such as carbonate ions are especially known to enhance the demineralization of hard tissues (e.g., caries). The carbonate content in the bone mineral is about 5–8 wt % [1,2]. The anionic PO43- tetrahedral sites in the apatite lattice may be occupied by both the CO32- and HPO42- ions, while the substitution of OH- is possible by CO32- ion. Generally, one can distinguish type B carbonated apatite, where the PO43- groups are partially replaced by CO32anions and type A carbonated apatite, where the OH- groups located at the channel sites along the c-axis of the hexagonal structure are partially replaced by CO32- anions. Calcium sites can also be occupied by other cations and can accept vacancies up to a maximum of two out of the ten existing in stoichiometric apatites. Therefore, much controversy still exists about the introduction of carbonate ions or HPO42- ions into the apatite lattice. Moreover, the elucidation of the crystal structure of nonstoichiometric carbonated apatites is fundamental for the understanding of issues related to the structure and chemistry of bone mineral and bone resorption in vivo. However, the numerous possibilities of compensation of charge unbalance occurring when CO32- ions enter the apatite lattice structure have not been fully clarified because the local structure of the carbonate ions and phosphate ions depends on the synthesis methods. It follows that the most interesting nonstoichiometric apatites are the CO32- and HPO42- containing nano-crystalline apatites. In this work, 13C enriched carbonate species were incorporated into HAP structures (4.8 wt %). 1H, 13C and 31P single pulse experiment (SPE) and cross polarization (CP) MAS
spectra were systematically recorded. The CP dynamics were studied by variable contact time 1 H/31P and
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