Assessing the Phosphate Distribution in Bioactive Phosphosilicate Glasses by ³¹P Solid-State NMR and Molecular Dynamics Simulations

Abstract

Melt-derived bioactive phosphosilicate glasses are widely utilized as bone-grafting materials for various surgical applications. However, the insight into their structural features over a medium-range scale up to ∼ 1 nm remains limited. We present a comprehensive assessment of the spatial distribution of phosphate groups across the structures of 11 Na2O-CaO-SiO2-P2O5 glasses that encompass both bioactive and nonbioactive compositions, with the P contents and silicate network connectivities varied independently. Both parameters are known to strongly influence the bioactivity of the glass in vitro. The phosphate distribution was investigated by double-quantum (31)P nuclear magnetic resonance (NMR) experiments under magic-angle spinning (MAS) conditions and by molecular dynamics (MD) simulations. The details of the phosphate-ion dispersion were probed by evaluating the MD-derived glass models against various scenarios of randomly distributed, as well as clustered, phosphate groups. From comparisons of the P-P interatomic-distance spreads and the statistics of small phosphate clusters assessed for variable cutoff radii, we conclude that the spatial arrangement of the P atoms in phosphosilicate glasses is well-approximated by a statistical distribution, particularly across a short-range scale of ≤ 450 pm. The primary distinction is reflected in slightly closer P-P interatomic contacts in the MD-derived structures over the distance span of 450-600 pm relative to that of randomly distributed phosphate groups. The nature of the phosphate-ion dispersion remains independent of the silicate network polymerization and nearly independent of the P content of the glass throughout our explored parameter space of 1-6 mol % P2O5 and silicate network connectivities up to 2.9.

The role of fluoride in the nanoheterogeneity of bioactive glasses

“…MD simulations suggested that in low--phosphate--content bioactive glass compositions, phosphate groups are distributed randomly, while phosphate clustering was observed for high phosphate contents (12 mol% P2O5) with the glass separating into distinct silicate--rich and phosphate--rich regions (Figure 2 (39) ). 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) ).…”

“…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) .…”

Composition-dependent in vitro apatite formation at mesoporous bioactive glass-surfaces quantified by solid-state NMR and powder XRD

“…As discussed in our previous work,9,22,25,28,33,34 P is present as nmsized amorphous calcium phosphate (CaP) clusters dispersed across the main silicate pore-wall component. The results from the S90-τ SBF series are shown in (c).…”

Proton Environments in Biomimetic Calcium Phosphates Formed from Mesoporous Bioactive CaO–SiO2–P2O5 Glasses in Vitro: Insights from Solid-State NMR

“…35,44 The latter portion only exists in Ca-rich (e.g., S58) or P-free (e.g., S90) MBGs, whereas essentially all Ca 2+ species of the S85 structure are concentrated in the surface-associated and 1–2 nm-sized CaP clusters 75 that readily dissolve on their contact with aqueous solutions. 42,43,75 In contrast, the CaP clusters of the S58 MBG are much smaller and partly embedded within the silicate network building the pore wall, 75,76 thereby leading to a slower release of their Ca 2+ and PO 4 3– constituents. The distinct CaP cluster scenarios are illustrated in Figure 2 of Turdean-Ionescu et al 43 …”