Evidence of Nanometric-Sized Phosphate Clusters in Bioactive Glasses As Revealed by Solid-State ³¹P NMR

Abstract

Bioactive glasses are able to form strong bonds to bone. This property, crucial for medical applications, depends on the glass composition and structure. Dissolution of phosphates in melt-quenched silicate glasses raises the question of chemical homogeneity and possible formation of clusters. A detailed structural characterization of the bioactive glasses is thus highly desirable. In this work, the nature of the distribution of phosphate units in a melt-quenched bioactive glass is elucidated for the first time using 31 P spin-counting solid-state NMR experiments. The structure of a dense bioactive calcium silicate glass with 2.6 mol % of phosphorus oxide is shown to exhibit nanometric-sized chemical and structural heterogeneities. Clear experimental evidence of the presence of phosphate clusters of five and six PO 4 tetrahedral units embedded in the disordered polymeric silicate network is given.

The role of fluoride in the nanoheterogeneity of bioactive glasses

“…The extent of clustering also depends on the silicate content. MD simulations (45) have shown that for compositions based on Bioglass 45S5 with constant phosphate content the amount of phosphate clusters, typically containing four or five phosphate groups, a number consistent with NMR data from a comparable, sodium--free composition (38) , increases with increasing silicate content. By contrast, such clusters were negligible for compositions with silicate network connectivity lower than 2.9 (41,46) .…”

“…Data on P--P separation from combined MD simulation/solid--state NMR experiments later confirmed the finding that larger agglomerations of phosphate, containing three or more phosphate groups, only occurred in glasses with phosphate contents above 4 mol% (46) . By contrast, recent 31 P spin--counting solid--state NMR experiments showed nanometre--sized phosphate clusters consisting of five to six orthophosphate groups in a sodium--free version of Bioglass 45S5 (Figure 3 (38) ). These results suggest that the occurrence of clustering is dependent on the phosphate content of the glass, with phosphate clusters being more likely to be present in high phosphate content compositions.…”

“…Phosphate is present as orthophosphate, QP 0 (PO4 3--), mostly (37,38) . Some Si--O--P bonds have been reported, mostly based on MD simulation data (37,39) but also recently from solid--state NMR spectroscopy experiments (38) , while other studies claimed that no such bonds exist ( Figure 1; (37,40) ).…”

“…Some Si--O--P bonds have been reported, mostly based on MD simulation data (37,39) but also recently from solid--state NMR spectroscopy experiments (38) , while other studies claimed that no such bonds exist ( Figure 1; (37,40) ). Either way, one cannot really speak of a mixed silicate--phosphate network, as the majority of the phosphate is certainly present as isolated orthophosphate groups, charge--balanced by modifier ions.…”

“…units, indicating that the glass structure is heterogeneous on a nanometric length scale. (Image taken from (38) with permission, © American Chemical Society).…”