Chem. Mater., 20 (7), 2583–2591, 2008.

Dispersion of Silicate in Tricalcium Phosphate Elucidated by Solid-State NMR
A. Rawal,† X. Wei,‡ M. Akinc,‡ and K. Schmidt-Rohr*†

Abstract:
The dispersion of silicate in tricalcium phosphate, a resorbable bioceramics for bone replacement, has been investigated by various solid-state nuclear magnetic resonance (NMR) methods. In samples prepared with 5 and 10 mol % of both 29SiO2 and ZnO, three types of silicate have been detected: (i) SiO44− (Q0 sites) with long longitudinal (T1,Si) relaxation times (~10 000 s), which substitute for ~1% of PO43−; (ii) silicate nanoinclusions containing Q2, Q1, and Q0 sites with T1,Si ~ 100 s, which account for most of the silicon; and (iii) crystalline Q4 (SiO2) with long T1,Si. Sensitivity was enhanced >100-fold by 29Si enrichment and refocused detection. The inclusions in both samples have a diameter of ~8 nm, as proved by 29Si{31P} REDOR dephasing on a 30-ms time scale, which was simulated using a multispin approach specifically suited for nanoparticles. 29Si CODEX NMR with 30-s 29Si spin diffusion confirms that an inclusion contains >10 Si (consistent with the REDOR result of >100 Si per inclusion). Overlapping signals of silicate Q2, Q1, and Q0 sites were spectrally edited based on their J-couplings, using double-quantum filtering. The large inhomogeneous broadening of the Q2, Q1, and Q0 29Si subspectra indicates that the nanoinclusions are amorphous.