Sr2+ and Eu3+ Co-Doped Whitlockite Phosphates Ca8−xSrxZnEu(PO4)7: Bioactivity, Antibacterial Potential, and Luminescence Properties for Biomedical Applications

Calcium phosphates are one of the main materials used in biomedicine for bone regeneration purposes. To improve the properties of biocompatible β-Ca3(PO4)2, doping by bioactive, antibacterial is actively used, as well as luminescent ions. Co-doped phosphates Ca8−xSrxZnEu(PO4)7 with a β-Ca3(PO4)2 (β-TCP)-type structure were synthesized through solid-state synthesis. The β-TCP-type structure was confirmed using X-ray powder diffraction and FTIR spectroscopy. Photoluminescence data, including excitation and emission spectra, decay curves, lifetime values and quantum yields, were collected for all samples. Ca8−xSrxZnEu(PO4)7 phosphates exhibit strong red-emission due to 4f-4f transitions of Eu3+ ions in disordered oxygen surrounding, with quantum yields reaching 54%. The phosphates demonstrated biocompatibility through MTT assay, with successful differentiation of aMSCs into the osteogenic lineage. Antibacterial activity was tested against four bacteria (E. coli, S. aureus, P. aeruginosa, and E. faecalis) and a fungus (C. albicans). It was found that the samples demonstrated antibacterial properties. The growth of E. coli and E. faecalis is significant inhibited by Ca8−xSrxZnEu(PO4)7 samples with 0 ≤ x ≤ 6.0. Analysis of mixed salt solubility using Eu3+ ions as a fluorescent probe showed that increasing Sr2+ concentration in Ca8−xSrxZnEu(PO4)7 delays both β-TCP phase resorption and HAP phase precipitation. These results demonstrate the potential of Ca8−xSrxZnEu(PO4)7 phosphates for bioimaging and bone healing control.