Biosynthesis and Characterization of Manganese Oxide Nanoparticles (MnO-NPs) from Non-Edible Onion Peels for Antimicrobial and Antioxidant Applications

In this study, aqueous extract of non-edible onion (Allium cepa L.) peel was employed as a chelating, capping and stabilizing agent in the green synthesis of MnO-NPs by alkaline precipitation of Mn(II) followed by low-temperature calcination. The particles were characterized and evaluated for their antimicrobial, antioxidant and in vitro biocompatibility activities. Qualitative phytochemical screening revealed tannins, flavonoids, alkaloids, phenolic compounds, anthraquinones, glycosides, saponins, and reducing sugars, which coordinate and cap Mn(II) during precipitation and remain associated with the particle surface. The MnO-NPs were characterized by UV–visible spectroscopy, FTIR, TEM, SEM-EDX, and PXRD. A characteristic UV–visible absorption peak at 240 nm confirmed nanoparticle formation, while FTIR revealed phytochemical capping groups and a distinct Mn–O stretching vibration at 530 cm⁻¹. TEM showed predominantly spherical to quasi-spherical particles with a mean size of 14.69 nm, and SEM-EDX showed aggregated nanoscale features in which manganese and oxygen were the only detectable elements. PXRD gave a pattern indexable to cubic MnO with no secondary crystalline phase resolved. The nanoparticles exhibited broad-spectrum antimicrobial activity against MRSA, Escherichia coli, Streptococcus pyogenes, Bacillus subtilis, Klebsiella pneumoniae, and Candida krusei (inhibition zones 24–27 mm at a single screening concentration of 100 µg/mL) and concentration-dependent antioxidant activity in DPPH and ABTS assays, to which the residual phytochemical capping layer is expected to contribute. Importantly, the MnO-NPs were highly biocompatible with L929 mouse fibroblasts, retaining >85% viability at antimicrobially effective concentrations (≤50 µg/mL), with a calculated IC₅₀ exceeding 150 µg/mL. Overall, onion-peel-derived MnO-NPs combine favorable physicochemical properties, selective antimicrobial activity, modest antioxidant capacity and good in vitro biocompatibility, identifying them as candidates for localized antimicrobial use and for the mechanistic, quantitative and in vivo studies that would support it.