X-ray microanalysis in STEM of short-term physicochemical reactions at bioactive glass particle/biological fluid interface. Determination of O/Si atomic ratios

Short-term physicochemical reactions at the interface between bioactive glass particles and biological fluids are studied and we focus our attention on the measurements of O/Si atomic ratio. The studied bioactive glass is in the SiO2-Na2O-CaO-P2O5-K2O-Al2O3-MgO system. The elemental analysis is performed at the submicrometre scale by scanning transmission electron microscopy associated with energy-dispersive x-ray spectroscopy (EDXS) and electron energy-loss spectroscopy (EELS). We previously developed an EDXS quantification method based on the ratio method and taking into account local absorption corrections. In this way, we use EELS data to determine, by an iterative process, the local mass thickness, which is an essential parameter for correcting absorption in EDXS spectra. After different immersion times of bioactive glass particles in a simulated biological solution, results show the formation of different surface layers at the bioactive glass periphery. Before 1 day of immersion, we observe the presence of an already shown (Si,O,Al)-rich layer at the periphery. In this paper, we demonstrate that a thin 'electron dense' (Si,O)-layer is formed on top of the (Si,O,Al)-layer. In this (Si,O)-layer, depleted in aluminium, we point out an increase of oxygen weight concentration that can be interpreted by the presence of Si(OH)4 groups, which permit the formation of a (Ca,P)-layer. Aluminium plays a role in the glass solubility and may inhibit apatite nucleation. After the beginning of the (Ca,P)-layer formation, the size of the 'electron dense' (Si,O)-layer decreases and tends to disappear. After 2 days of immersion, the (Ca,P)-layer grows in thickness and leads to apatite precipitation. Copyright (C) 2004 John Wiley Sons, Ltd.