Multifunctional ZnO Nanoparticles Synthesized Using <i>Spirodela polyrhiza</i> Extract: Characterization, Photocatalytic Activity, Antimicrobial Assessment, and In Silico Modeling

This study investigates the green synthesis of zinc oxide nanoparticles (ZnO NPs) using the aqueous extract of the aquatic plant <i>Spirodela polyrhiza</i> (greater duckweed) and evaluates their multifunctional properties. The ZnO NPs were synthesized via a sustainable method and characterized using UV-visible spectroscopy, TEM, FESEM, EDX, FTIR, and XRD analyses. UV-visible spectroscopy confirmed the formation of ZnO NPs with a characteristic absorption peak at ∼349 nm. TEM and FESEM analyses revealed spherical and nonspherical particles ranging from 20 to 70 nm. The antimicrobial activity of ZnO NPs was assessed against three bacterial strains (<i>Escherichia coli</i>, <i>Staphylococcus aureus</i>, and <i>Bacillus subtilis</i>) and three fungal strains (<i>Aspergillus niger</i>, <i>Penicillium chrysogenum</i>, and <i>Candida albicans</i>). Notably, <i>B. subtilis</i> showed a maximum inhibition zone of 18 mm at 100 mg/mL, while <i>A. niger</i> exhibited the highest antifungal response with a zone of 22 mm and an activity index (AI) of 1.15, indicating comparable or superior activity to ketoconazole at higher concentrations. Molecular docking simulations using the crystal structure of <i>B. subtilis</i> YmaH (Hfq) protein (PDB ID: 3HSB) revealed strong noncovalent interactions with Zn atoms of the NPs, particularly involving HIS57 and LEU26 residues. Additionally, ZnO NPs demonstrated a noteworthy photocatalytic degradation (90.4%) of methylene blue dye under sunlight exposure. These results highlight the potential of <i>S. polyrhiza</i>-mediated ZnO NPs for use in antimicrobial therapies and environmental remediation applications.