Green synthesis of copper nanoparticles from onion peel for wastewater bacteria detection and methyl orange degradation

Abstract

Wastewater from industrial and domestic sources is frequently contaminated with pathogenic bacteria, such as coliforms, posing a significant threat to human and environmental health. Conventional methods for synthesising metal nanoparticles are often hazardous and energy-intensive, highlighting the need for sustainable alternatives. Green synthesis using plant-derived biomolecules offers an eco-friendly and cost-effective route, yet systematic studies exploring Allium cepa L. (onion) peel extract (OPE) for multifunctional copper nanoparticle (CuNP) synthesis remain limited. In this study, OPE was employed as a natural reducing and stabilising agent to synthesise CuNPs via a sand-bath-mediated method. The reaction parameters (pH, temperature and time) were optimised to achieve stable nanoparticles, with characterisation by UV–Vis spectroscopy, FT-IR, HRTEM, DLS and XRD confirming spherical, crystalline OPE-CuNPs with an average size of ∼3–21 nm. The biosynthesised OPE-CuNPs exhibited effective antibacterial activity against Klebsiella pneumoniae, obtaining a minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of 12.5 μg/mL, values lower than several previously reported plant-mediated CuNPs. At concentrations of 50 and 100 μg/mL, inhibition zones of 12.5 ± 0.3 mm and 25.5 ± 0.3 mm were recorded, demonstrating concentration-dependent activity. Ciprofloxacin was used as a positive control and OPE as a negative control, confirming that the antibacterial activity originated from the OPE-CuNPs. In addition, the OPE-CuNPs showed excellent photocatalytic activity, with 96% degradation efficiency for methyl orange under sunlight within 25 min, which surpasses several green-synthesised CuNP studies reported in the literature. These findings suggest that onion peel, a readily available biowaste, can be effectively valorised for the sustainable synthesis of OPE-CuNPs with dual antibacterial and photocatalytic applications, offering a promising strategy for wastewater remediation and environmental protection.