The structural role and coordination environment of cobalt in 45P2O5-CaO-Na2O phosphate glasses: thermal properties and Raman, UV-vis-NIR, and EPR spectroscopy.

Cobalt-containing materials are of interest for a wide range of applications, from biomaterials to solid-state lasers in optics. For instance, Co2+ is known to trigger the formation of new blood vessels, i.e. angiogenesis. Here, the use of phosphate glasses as a vehicle for local release of Co2+ ions is an attractive strategy to overcome the vascularisation limitation in tissue engineering. This study aimed to establish structure-property correlations as a function of the coordination environment of cobalt in 45P2O5-(30 - x)CaO-25Na2O-xCoO (x: 0.01 to 10 mol%) glasses. Constant polymerization and O/P ratio, resulting ultimately in constant basicity, were shown by ICP-OES and Raman spectroscopy. The latter, combined with EPR analysis, indicated that Co2+ was the predominant oxidation state and the presence of Co3+ can be excluded. UV-vis-NIR absorption spectra showed that the ratio between Co2+ in four- and six-fold coordination remained constant throughout the glass series. Their thermal properties measured by DSC and heating microscopy did not change much in the substitution range studied here. The steady trend in Tg values suggests a compensation between two opposite effects caused by the presence of four and six-fold coordinated Co2+, both being present at a constant ratio throughout the glasses. Accordingly, the higher field strength of Co2+ compared to that of Ca2+ is expected to strengthen the glass network. In contrast, four-fold coordinated cobalt is expected to weaken the network by connecting fewer fragments of the phosphate glass network than six-fold coordinated cobalt. These results indicate that the structural properties of the glasses with constant basicity are influenced by the coordination number of Co2+.

Unravelling the dissolution mechanism of polyphosphate glasses by 31P NMR spectroscopy: ligand competition and reactivity of intermediate complexes.

Phosphate glass dissolution can be tailored via compositional and subsequent structural changes, which is of interest for biomedical applications such as therapeutic ion delivery. Here, solid-state 31P nuclear magnetic resonance characterisation of 45P2O5-xCaO - (55 -x)Na2O glasses was correlated with dissolution studies using time-dependent liquid 31P NMR spectroscopy and quantitative chemical analysis. Glasses dissolved congruently in aqueous media, and the first dissolution stage was the hydration of phosphate chains. In deionised water and Tris buffer (pH0 7.4 or 7.9), trimetaphosphate rings and orthophosphates were the predominant species in solution, indicating relatively fast degradation. By contrast, long phosphate chains were identified in EDTA (pH0 10.0). Besides pH differences, coordination of phosphate species by metal cations appears to play a catalytic role in the hydrolysis mechanism via turning phosphorus atoms into suitable electrophiles for the subsequent nucleophilic attack by water. Hydrolysis rates were proportional to phosphate complex stability, with stronger complexes for chains than for rings. A competition between solvent and phosphate species for the metal ion occurred in the order EDTA > Tris > deionised water.