The eco-friendly synthesis of nanomaterials has emerged in modern science to resolve the increasing concerns of environmental pollution and sustainable development. Metal-oxide NPs, including TiO₂, ZnO, and CuO, are widely studied for their potential in facilitating a sustainable environment. Due to their ease of availability, low cost, chemical stability, and nontoxicity, they are considered the best candidates for environmental pollution control and bioremediation. Among these, CuO NPs have gained considerable focus for their antibacterial and catalytic properties. The green synthesis of nanomaterials using various plant parts promotes sustainable chemistry for the sustainability of the environmental and antibacterial applications. Thus, the plant-based biosynthesis of nanoparticles are highly necessary to achieve such environmental and health sustainability. In the present study, we prepared an eco-friendly, cheap, and straightforward sol–gel synthesis method of copper-oxide nanoparticles (CuO NPs) using the aqueous bark extract of Croton macrostachyus. At 200 mg/mL, the uncalcined CuO NPs demonstrated the highest inhibition diameter for Staphylococcus aureus (S. aureus), with 22±1.3 mm, and for Escherichia coli (E. coli), with 11±0.7 mm. Moreover, the calcined CuO NPs presented notable catalytic performance in reducing 4-nitrophenol to 4-aminophenol in 8 minutes with a removal of 98.79%. The kinetics of the process resulted in an apparent rate constant (K_app) of 0.507 min^-1 with a pseudo-first-order reaction. Therefore, this eco-friendly synthesis method not only eliminates the use of harmful reducing substances but also offers a hands-on solution to environmental pollution and disease-resistant bacteria problems.