Direct Probing of the Phosphate-Ion Distribution in Bioactive Silicate Glasses by Solid-State NMR: Evidence for Transitions between Random/Clustered Scenarios

Abstract

By employing 31P multiple-quantum coherence-based solid-state nuclear magnetic resonance spectroscopy, we present the first comprehensive experimental assessment of the nature of the orthophosphate-ion distributions in silicate-based bioactive glasses (BGs). Results are provided both from melt-prepared BG and evaporation-induced self-assembly-derived mesoporous bioactive glass (MBG) structures of distinct compositions. The phosphate species are randomly dispersed in melt-derived BGs (comprising 44–55 mol % SiO2) of the Na2O–CaO–SiO2–P2O5 system, whereas a Si-rich (86 mol % SiO2) and Ca-poor ordered MBG structure exhibits nanometer-sized amorphous calcium phosphate clusters, conservatively estimated to comprise at least nine orthophosphate groups. A Ca-richer MBG (58 mol % SiO2) reveals a less pronounced phosphate clustering. We rationalize the variable structural role of P in these amorphous biomaterials.