Investigation of citric acid-assisted sol-gel synthesis coupled to the self-propagating combustion method for preparing bioactive glass with high structural homogeneity

In this research, the mechanism of an efficient strategy for the synthesis of 58S bioglass with high structural homogeneity by a citric acid assisted sol-gel route was investigated. This is an interesting approach to prepare bioactive glass via the sol-gel method with application potential in bone tissue engineering and also for the development of new biomedical devices. Herein, 58S bioglass was synthesized by two routes: conventional sol-gel method (CSG) and citric acid assisted sol-gel route coupled to the self-propagating combustion method (SPC). The effects of citric acid on the temperature required for 58S vitreous consolidation, long- and short-range ordering were investigated by several analysis techniques. Results suggested that citric acid molecules serve as an effective molecular template formed by molecular network raised from intermolecular forces, especially the hydrogen bonds, resulting from the chemical interactions between the COOH and hydroxyl groups (water, ethanol, P-OH, Si-OH). In this scenario, citric acid controls the phase segregation during the drying and combustion steps of the gel in the SPC method by establishing chemical interactions (hydrogen bonds) with the superficial silanol groups present on the small-sized silica nanoparticles present in the sol governing their growth. Besides these mentioned features, the self-propagating combustion behavior exhibited by the nitrate citrate in the SPC xerogel during the combustion step allowed the removal of the organic load and the consolidation of the vitreous structure at a temperature considerably lower than the sample obtained by the CSG method. Consequently, the SPC method leads to the formation of a glass structure with high homogeneity for the 58S, whereas the conventional sol-gel method produces a matrix enriched with calcium phosphate crystalline nuclei - glass-ceramic.