Multinuclear solid-state NMR studies of ordered mesoporous bioactive glasses

The local structures of highly ordered mesoporous bioactive CaO-SiO2-P2O5 glasses were investigated for variable Ca contents. H-1 NMR revealed a diversity of hydrogen-bonded and isolated surface silanols as well as adsorbed water molecules. The structural roles of Si and P were explored using a combination of Si-29 and P-31 magic-angle spinning (MAS) nuclear magnetic resonance (NMR) techniques; the proximities of Si and P to protons were studied through cross-polarization-based experiments, including H-1-Si-29 and H-1-P-31 hetero-nuclear two-dimensional correlation spectroscopy. The results are consistent with SiO2 being the main pore-wall component, whereas P is present as a separate amorphous calcium orthophosphate phase, which is dispersed over the pore wall as nanometer-sized clusters. The excess Ca that is not consumed in the phosphate phase modifies the silica glass network where it associates at/near the mesoporous surface. This biphasic structural model of the pore wall leads to the high accessibility of both Ca and P to body fluids, and its relation to the experimentally demonstrated high in vitro bioactivities of these materials is discussed.