Effect of Aluminum Ion Incorporation on the Bioactivity and Structure in Mesoporous Bioactive Glasses

Abstract

This work is dedicated to biokinetic and structural investigations of Al2O3 doping (0.5 to 15 mol %) in ordered mesoporous bioactive glasses (MBGs), based on the composition 80% SiO2–15% CaO–5% P2O5 (in mol %). The synthesis was performed with the sol–gel method in combination with a structure directing agent (P-123) for the formation of mesopores. Structural investigations reveal that the incorporation of additional Al2O3 in a range of 1 to 10 mol % reduces the order of the mesostructure, whereas doping with 15 mol % Al2O3 creates well ordered mesopores again. Nitrogen adsorption–desorption isotherms show that specific surface area, pore volume, and pore diameter decrease only slightly upon incorporation of Al2O3. In vitro bioactivity tests exhibit a small decrease in bioactivity upon incorporation of small amounts and a sudden drop beyond 3 mol %. This can be related to the strong interaction of Al3+ and PO43–, which could be proven by multinuclear single and double resonance solid state nuclear magnetic resonance (NMR) spectroscopy. The number of P–O–Al linkages has been estimated by 27Al(31P) rotational echo double resonance (REDOR) experiments. The interaction of Al3+ and PO43– and the trapping of Ca2+ required for charge compensation hamper the release of P, Ca, and Si ions and thus provide less PO43– and Ca2+ ions for the crucial formation of hydroxycarbonate apatite (HCA).