Biofabrication of Co₃O₄ and Ag-doped Co₃O₄ nanoparticles using aqueous extracts of Celtis occidentalis leaves and their antimicrobial activity

The synthesis of nanoparticles (NPs) using plant extracts is a green chemistry approach that has attracted considerable interest due to its sustainability, safety, and cost-effectiveness. This research describes the preparation of cobalt oxide (Co₃O₄) and silver-doped Co₃O₄ (Ag-Co₃O₄) NPs via a sustainable green synthesis technique employing aqueous leaf extracts of Celtis occidentalis. The synthesized nano-materials were analyzed using various techniques including Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and UV–vis spectroscopy. Their antimicrobial properties were tested against three bacterial strains: Listeria monocytogenes (ATCC 7644), Enterococcus faecalis (ATCC 29212), and Escherichia coli (ATCC 25922); and three fungal strains: Aspergillus niger, Penicillium sp, and Mucor sp. The assessment involved determining the zone of inhibition and minimum inhibitory concentration (MIC). The XRD spectra confirmed the formation of face-centered cubic spinel Co₃O₄, with the crystal structure undergoing slight modification upon doping with Ag. The results with both bacterial and fungal strains showed enhanced antimicrobial activity following the doping of Co₃O₄ with Ag. In the antibacterial assay, Co₃O₄ exhibited the highest (P < 0.05) zone of inhibition against the three bacterial strains at a concentration of 3.2 mg/mL compared to 1.8 mg/mL observed for Ag-doped Co₃O₄ NPs. Furthermore, the Co₃O₄ NPs showed MICs of0.1 ± 0.02 mg/mL, 0.3 ± 0.02 mg/mL, and 0.4 ± 0.0mg/mL for A.niger, Penicillum sp and Mucor sp, respectively as opposed to 0.027 ± 0.01 mg/mL, 0.05 ± 0.00 mg/mL and 0.04 ± 0.01 mg/mL recorded for Ag-doped Co₃O₄ NPs with respect to A. niger, Penicillium sp, and Mucor sp, respectively. In conclusion, Ag-doped Co₃O₄ NPs in an aqueous medium with C. occidentalis leaf extract allowed bio-fabrication of Ag-doped Co₃O₄ NPs with enhanced antibacterial and antifungal activities.