From kitchen to lab: Unveiling the potency of Allium cepa peels in green synthesis of silver nanoparticles and assessing their antioxidant, catalytic, antibacterial, cytotoxic, and melamine detecting properties

Abstract

Onion peels are a significant portion of global kitchen waste generated from the annual production of over 107 million tonnes of onions. Despite being enriched with bioactive compounds with potential antioxidant and antimicrobial properties, this bio-waste is often discarded, presenting an environmental challenge. Allium cepa peels derived from local and imported onion varieties, including small onion, Bombay red, Pakistan, India, and China, were utilized in this study to synthesize silver nanoparticles (AgNPs). The AgNPs were assessed for antioxidant, catalytic, antibacterial, cytotoxic, and melamine detecting properties. AgNPs formation was confirmed by yellow to dark-brown color change and UV-Vis peak at 420-480 nm. Scanning electron microscope analysis revealed spherical AgNPs ranging from 25-85 nm. Antioxidant activity of the AgNPs was assessed through total antioxidant capacity (TAC), total flavonoid content (TFC), total phenolic content (TPC), and DPPH radical scavenging assays. The AgNPs showed higher TFC and TAC compared to the water extracts (WEs), as the one-way ANOVA revealed a significant difference between the TFC and TAC values of WEs and AgNPs (P < 0.05; TFC: P = 0.001; TAC: P = 0.0004). China-AgNPs exhibited the most potent antioxidant activity with an IC50 value of 1.49 mg/mL. Catalytic activity was assessed using paranitrophenol degradation with 4000 ppm AgNPs, achieving complete degradation within 240 minutes. Cytotoxicity of AgNPs was evaluated using the viability of Artemia salina (brine shrimps) and proved to be biocompatible and non-toxic at room temperature. AgNPs exhibited higher antibacterial activity against Staphylococcus aureus than Escherichia coli relative to WEs. Melamine was detected using three melamine concentrations and in raw milk, using China-AgNP-based colorimetric method. The China-AgNPs showed sensitivity for detecting melamine at 1-8 ppm concentrations and identified melamine adulteration in raw milk. These findings suggest an eco-friendly and convenient method for synthesizing AgNPs using onion peels with potential applications in various industries.