Revisiting silicate substituted hydroxyapatite by solid-state NMR

Silicon-substituted hydroxyapatite (Si-HAp) has shown promising properties such as high-bone remodeling around implants. So far, the techniques used for the structural characterization of the Si-HAp have given indirect evidence of the presence of silicon inside the structure (by X-ray and neutron diffraction). In this paper, we focus on Si-HAp derivatives obtained by a precipitation method (widely described in the literature). We demonstrate here by solid-state NMR spectroscopy that only a fraction of the silicon atoms are incorporated into the HAp lattice in the form of Q⁰ (SiO₄^4-) species, for 4.6 wt% Si-HAp. A large amount of silicate units are located outside the HAp structure and correspond to silica-gel units. All results were established through ²⁹Si MAS, ¹H → ²⁹Si CP MAS and T₁ρ(¹H) edited ¹H → ²⁹Si CP MAS experiments. This last pulse scheme acted as a powerful editing sequence, leading to unambiguous spectroscopic conclusions, concerning the location of the SiO₄^4- moieties.