Facile and innovative method for bioglass surface modification: Optimization studies

In this work it is presented a facile and novel method for modification of bioglass surface based on (Ca_{molten salt bath}²⁺|Na_glass⁺) ion exchange by immersion in molten salt bath. This method allows changing selectively the chemical composition of a surface layer of glass, creating a new and more reactive bioglass in a shell that surrounds the unchanged bulk of the original BG45S5 bioglass (core-shell type system). The modified bioglass conserves the non-crystalline structure of BG45S5 bioglass and presents a significant increase of surface reactivity in comparison with BG45S5. Melt-derived bioactive glasses BG45S5 with the nominal composition of 46.1mol% SiO₂, 24.4mol% Na₂O, 26.9mol% CaO, and 2.6mol% P₂O₅ have been subjected to ion exchange at 480°C in molten mixture of Ca(NO₃)₂ and NaNO₃ with molar ratio of 70:30 for different time periods ranging from 0 to 60min. The optimization studies by using XRF and XRD showed that ion exchange time of 30min is enough to achieve higher changes on the glass surface without alters its non-crystalline structure. The chemical composition, morphology and structure of BG45S5 and bioglass with modified surface were studied by using several analytical techniques. FTIR and O_1s XPS results showed that the modification of glass surface favors the formation of Si-O_NBO groups at the expense of SiO_BOSi bonds. ²⁹Si MAS-NMR studies showed that the connectivity of _SiQⁿ species decreases from cross-linked _SiQ³ units to chain-like _SiQ² units and finally to depolymerized _SiQ¹ and _SiQ⁰ units after ion exchange. This result is consistent with the chemical model based on the enrichment with calcium ions of the bioglass surface such that the excess of positive charges is balanced by depolymerization of silicate network. The pH changes in the early steps of reaction of bioactive glasses BG45S5 and BG45Ca30, in deionized water or solutions buffered with HEPES were investigated. BG45Ca30 bioactive glass exhibited a significant increase in the pH during the early steps of the reaction compared to BG45S5.