Facile and innovative method for bioglass surface modification: Optimization studies

- Modification of 45S5 bioglass surface was carried out by ion exchange technique.
- Modified surface of bioglass may act as a “catalyst” for physicochemical reactions.
- The modification of surface favors the formation of Si-NBO at the expense of Si-BO-Si bonds.
- Ion exchange time higher than 30 min induces the devitrification of bioglass.

In this work it is presented a facile and novel method for modification of bioglass surface based on (Camolten salt bath2 +| Naglass+) 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 46.1 mol% SiO2, 24.4 mol% Na2O, 26.9 mol% CaO, and 2.6 mol% 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 60 min. The optimization studies by using XRF and XRD showed that ion exchange time of 30 min 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 O1s XPS results showed that the modification of glass surface favors the formation of Si-ONBO groups at the expense of SiOBOSi 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.

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