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

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 P-31 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 PO4 tetrahedral units embedded in the disordered polymeric silicate network is given.