Green synthesis of copper oxide nanoparticles using Eucalyptus globulus extract with effective bactericidal activity for river water treatment

•We obtained copper oxide nanoparticles from green synthesis of Eucalyptus globulus.•A low concentration of the salt helps the formation of nanoparticles.•Copper oxide nanoparticles show bactericidal activity against Gram-positive and negative bacteria.•Nanoparticles are a successful alternative for the treatment of river water. Bacterial resistance is a major global issue traditionally addressed from a health-focused perspective. However, recent research has revealed that contaminated river water significantly contributes to the spread of this resistance. In this context, treating bacterially contaminated river water using bactericidal compounds obtained through green synthesis is essential. Currently, nanotechnology emerges as an innovative approach for obtaining nanomaterials, such as copper oxide nanoparticles (CuONPs), which exhibit bactericidal properties useful for water remediation. In this study, copper oxide nanoparticles (CuONPs) were synthesized using an aqueous extract of Eucalyptus globulus and characterized by UV–Vis spectroscopy, infrared spectroscopy, Energy Dispersive X-ray Spectroscopy (EDS), and Scanning Electron Microscopy (SEM). Bactericidal activity was assessed via bacterial inhibition tests on water samples collected from three strategic points along the Chibunga River in Riobamba, Chimborazo Province. The optimal synthesis of CuONPs was achieved with a 0.05 M concentration of precursor salt and a temperature of 80 °C. Under these conditions, UV–Vis analysis showed an absorption wavelength of 327 nm for CuONPs, while FTIR analysis identified a characteristic Cu-O bond band at 574 cm⁻¹. SEM analysis revealed nanoparticles sized between 30 and 300 nm, predominantly oval in shape, with irregular edges and a rough surface. The EDS spectrum indicated an atomic ratio of 58:29:4 for C:O:Cu, with a strong signal around 0.35 eV corresponding to carbon. Finally, Pearson analysis, demonstrated an inverse correlation between the number of bacterial colonies and exposure time to CuONPs, indicating a reduction in the bacterial load from the first hour of contact with the nanoparticles. Based on these results, it is concluded that CuONPs exhibit bactericidal activity against Gram-positive and Gram-negative bacteria resistant to antibiotics present in river water samples and aid in their purification. [Display omitted]