Insights into the composition-structure-property relationship in P₂O₅–CaO–Na₂O–CuO bio-relevant glasses

Copper-containing calcium phosphate glasses possessing antimicrobial properties have attracted attention for biomedical applications in tissue engineering. Yet, composition-structure-property evaluations linking structural, thermal and optical properties with the oxidation states of copper are lacking. In this work, bio-relevant P₂O₅–CaO–Na₂O–CuO glasses were prepared by melting with increasing [CuO]/[Na₂O] molar ratios (0.1–1.5), and studied by X-ray diffraction (XRD), Fourier transform-infrared (FT-IR) spectroscopy, thermal/dilatometric analysis, optical transmission, X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL) spectroscopy. The XRD analysis confirmed the non-crystalline structure of the glasses. The FT-IR evaluation indicated that increasing the [CuO]/[Na₂O] ratio significantly supports the connectivity of the glass network. The thermal characterization revealed a trend of increase in the glass transition and softening temperatures, while the coefficient of thermal expansion decreased with increasing CuO thus harmonizing with a glass strengthening. The optical evaluation showed substantially decreased transmission from the red toward the near infrared region due to d-d transitions in Cu²⁺ ions. A significant red shift in the glass transmission edge was also exhibited, wherein a decrease in the optical band gap energies with increasing CuO content was indicated by Tauc plot assessment. Further evaluation of the Urbach energies was performed which suggested increased structural disorder for high [CuO]/[Na₂O] ratios. While XPS did not support a trend of increasing contents of non-bridging oxygens in the glasses, it indicated copper occurred most significantly as Cu⁺ besides Cu²⁺. It is supported that the decrease in optical band gaps and increase in Urbach energies follows the incorporation of copper ions rather than an increase in non-bridging oxygens. PL spectra showed features ascribed to Cu⁺ merely for the glass with lowest [CuO]/[Na₂O] ratio, whereas an emission quenching was indicated for higher CuO content coherent with a Cu⁺ → Cu²⁺ energy transfer.