Antibacterial and Photoactive Niobium Pentoxide-Loaded Chitosan–Gelatin Hydrogels with Favorable Cytocompatibility

The increasing prevalence of multidrug-resistant bacterial infections has created an urgent need for antibacterial biomaterials that combine effective antimicrobial activity with good biocompatibility. In this study, niobium pentoxide (Nb2O5)-loaded chitosan–gelatin (CG-Nb) hybrid hydrogels containing 1.1% (G1), 1.6% (G2), and 2.0% (G3) (w/w) Nb2O5 were evaluated for their antibacterial activity, photo-induced singlet oxygen generation, and cytocompatibility. Antibacterial performance against Staphylococcus aureus and Escherichia coli was assessed using agar diffusion, direct-contact colony reduction, and scanning electron microscopy, while singlet oxygen generation was investigated using the 9,10-diphenylanthracene (DPA) bleaching assay. Cytocompatibility was evaluated using L929 fibroblasts according to ISO 10993-5. The CG-Nb hydrogels exhibited concentration-dependent antibacterial activity, with G3 showing the highest antibacterial activity among the investigated formulations. G3 produced estimated bacterial reductions of 95.7% against S. aureus and 93.3% against E. coli in the direct-contact assay, while maintaining 87.3 ± 4.8% L929 fibroblast viability after 24 h. SEM analysis revealed progressive morphological changes in bacterial cells with increasing Nb2O5 content. The DPA bleaching assay demonstrated that G3 generated photo-induced singlet oxygen under blue-light irradiation. Because the antibacterial assays were performed without blue-light irradiation, the present results do not allow the photo-induced singlet oxygen generation observed in the DPA assay to be directly associated with the antibacterial activity of the hydrogels. Overall, the Nb2O5-containing chitosan–gelatin hydrogels exhibited enhanced antibacterial activity across the evaluated concentration range while maintaining favorable cytocompatibility under the tested conditions. These findings provide a biological evaluation of the previously developed hydrogel platform and support further investigation of CG-Nb hydrogels for antibacterial biomaterial applications.