Structure–composition relationships of bioactive borophosphosilicate glasses probed by multinuclear 11B, 29Si, and 31P solid state NMR

Abstract

By combining ¹¹B, ²⁹Si, and ³¹P nuclear magnetic resonance (NMR) experimental results, we present a comprehensive structural investigation of 15 borophosphosilicate (BPS) glasses of the Na₂O–CaO–B₂O₃–SiO₂–P₂O₅ system: in two base compositions comprising 46 mol% (“S46”) and 49 mol% (“S49”) SiO₂, progressive replacements of SiO₂ by B₂O₃ were performed at a constant total Na₂O and CaO content. The S46 glass members constitute B-bearing analogs of “45S5 Bioglass” that is utilized extensively for bone grafting in periodontal and orthopedic surgery. Orthophosphate ions prevail throughout all structures, while the silicate network polymerization increases slightly with a growing amount of B₂O₃ in the glass. ¹¹B NMR revealed continuous BO₃ → BO₄ conversions for increasing B₂O₃ content, with asymptotic fractions of 34% and 43% of B^[4] coordinations out of the borate speciation observed for the series of S46 and S49 glasses, respectively. While all BPS glasses are homogeneous across a μm-scale, strong preferences for B^[3]–O–B^[3] and B^[4]–O–Si^[4] bond formation lead to structures comprising (sub)nm-sized domains of BO₃ groups in boroxol rings and borosilicate networks built by SiO₄ and BO₄ tetrahedra. These borate/borosilicate networks are merged mainly by B^[4](3Si) and B^[3](1Si) moieties in Si-rich BPS glasses (where each value in parentheses specifies the number of bonds to Si atoms), while B^[4](3Si) and B^[4](2Si) groups are the dominant network contact points in the B-rich glasses. We discuss the partitioning of non-bridging oxygen ions between the BO₃ and SiO₄ groups, the relative propensities for B^[4]–O–Si^[4] and B^[4]–O–B^[3] bond formation, as well as the expected bearings of our proposed BPS structural model for the glass degradation in aqueous media, where we identify the fractional population of B^[4] coordinations and the silicate network connectivity to constitute the dissolution-controlling parameters.