Nqunqa, SphamandlaFeleni, UsisiphoMulaudzi, TakalaniMini, SixolileHitzeroth, Arina CNgece-Ajayi, Rachel F2026-05-052026-05-052026Nqunqa, S., Feleni, U., Mulaudzi, T., Mini, S., Hitzeroth, A.C. and Ngece-Ajayi, R.F., 2026. Green Synthesis of Copper Nanoparticles From Onion Peel for Wastewater Bacteria Detection and Methyl Orange Degradation. Journal of Nanotechnology, 2026(1), p.2680810.https://doi.org/10.1155/jnt/2680810https://hdl.handle.net/10566/22331Wastewater 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.enAllium cepaAntibacterial activityCopper nanoparticlesKlebsiella pneumoniaeMethyl orange degradationArticle