Eu3+-Doped Gadolinium Niobate as a Potential Tool for Bioimaging and Phototherapy in Breast Cancer Cell Lines

The development of stable luminescent host matrices for Eu³⁺ incorporation remains an important research topic. To evaluate the potential of Eu³⁺-doped Gd_³NbO_⁷ for bioimaging and photoactivated biomedical applications, Gd_³NbO_⁷:0.5% Eu³⁺,2.0% Na⁺ particles were synthesized by a polyol-mediated sol–gel route followed by thermal treatment. XRD confirmed the formation of the cubic Gd_³NbO_⁷ phase, while SEM and EDS analyses revealed homogeneous particle morphology and elemental distribution. Photoluminescence measurements showed the characteristic Eu³⁺ emission bands, with intense red emission centered at 612 nm, indicating successful incorporation of Eu³⁺ into low-symmetry lattice sites. Biological evaluation in MCF-7 human breast cancer cells demonstrated low dark cytotoxicity, with high cell viability maintained at all tested concentrations. Upon irradiation at 405 nm, however, cell viability decreased significantly, evidencing a marked light-dependent cytotoxic effect. Two-way ANOVA identified irradiation as the main factor affecting cell viability (p < 0.0001). Confocal microscopy confirmed particle internalization by cells.