Green synthesis of a manganese oxide-selenium nanocomposite using watermelon peel waste with antimicrobial and...

The present study demonstrates the successful green biosynthesis of a manganese oxide-selenium nanocomposite (MnO₂-Se NC) using watermelon peel waste extract, providing a sustainable approach for valorizing agro-industrial waste into a multifunctional nanomaterial. The biofabricated nanocomposite was characterized by UV-Vis, XRD, FTIR, DLS and TEM analysis. TEM analysis revealed predominantly spherical nanoparticles with an average size of approximately 55 nm, while UV-Vis spectra displayed two distinct absorption peaks at 266 and 300 nm, confirming successful nanocomposite formation. The MnO₂-Se NC exhibited potent antibacterial activity, with MIC values ranging from 3.9 to 31.25 µg ml^-1 and MBC values between 15.62 and 125 µg ml^-1. The highest efficacy was observed against Bacillus subtilis (MIC = 3.9 µg ml^-1), whereas comparatively lower activity was recorded against Pseudomonas aeruginosa (MIC = 31.25 µg ml^-1), with MBC/MIC ratios of 2-4 indicating a bactericidal mode of action. In addition, the nanocomposite demonstrated promising antifungal activity, with MIC/MFC values of 31.25/62.5 µg ml^-1 against Candida albicans and 125/500 µg ml^-1 against Aspergillus brasiliensis, suggesting greater effectiveness against yeast than filamentous fungi. Furthermore, the MnO₂-Se NC showed strong larvicidal and adulticidal activity against Culex pipiens, with significantly lower LC₅₀ and LC₉₀ values (5.6-7.0 ppm) compared to the crude watermelon peel extract (66-100 ppm). These findings indicate enhanced toxicity and efficacy of the bimetallic nanocomposite at substantially lower concentrations. In conclusion, the findings highlight the considerable multifunctional potential of watermelon peel-derived MnO₂-Se NC as a sustainable antimicrobial and vector-control agent. The combined biological activities, together with the use of an abundant agricultural waste resource for synthesis, support its future development as an environmentally friendly nanomaterial for biomedical and mosquito-management applications.