Solid-state 1H and 31P NMR and FTIR spectroscopy study of static and dynamic structures in sol-gel derived calcium hydroxyapatites

Abstract

Calcium hydroxyapatite containing amorphous phosphate phase (ACP-CaHA) and nano-structured hydroxyapatite (CaHA) have been prepared by two sol-gel synthesis routes. The structural organization of hydroxyl groups in both materials has been determined by means of ¹H MAS NMR and FTIR spectroscopy. It has been shown that the amount of structural –OH groups in nano-structured CaHA is significantly higher than that from adsorbed water and vice versa in ACP-CaHA. A precise signal shape analysis has been carried out for both studied samples. The ³¹P NMR signals have been found being Voigt-shaped in the wide-line as well as in MAS spectra. The ¹H and ³¹P spin-lattice and spin-spin relaxation time measurements have revealed that the fast spin motion takes place in ACP-CaHA. The corresponding correlation time τ ~ 7 · 10^–7 s at ~300 K has been determined. The effect of MAS rate on the 31P signal shape also confirms that this motion runs in the time scale of microseconds or even nanoseconds. The magnitude of the anisotropic broadening 1220 ± 20 Hz determined for nano-structured CaHA is very close to the maximum of the dipolar ¹H–³¹P coupling distribution profile estimated using CP MAS kinetics. The dynamics of ¹H–³¹P spin interactions in nano-structured CaHA (τ ~ 3.3 · 10^–5 s) is much slower than in ACP-CaHA.