Green Synthesis of CuO Nanoparticles Using <i>Carissa</i><i> </i><i>edulis</i> Stem Bark Extract, and the Antibacterial Evaluation

In this work, copper oxide nanoparticles (CuO NPs) were efficiently synthesized using an environmentally friendly and simple process by applying the aqueous stem bark extracts of <i>Carissa edulis</i>. During synthesis of CuO the following parameters were varied so as to achieve the optimum conditions. The first parameter to be varied was precursor (Cu(NO<sub>3</sub>)<sub>2.</sub>2H<sub>2</sub>O) concentration. Secondly, was the ratio of extract to precursor salt. Thirdly, the pH was investigated between 6 and 11. Lastly, effects of synthesis temperature was investigated from 25°C to 70°C. To characterize the synthesized CuO NPs, Fourier transform infrared (FTIR) spectroscopy, powder X-ray diffraction (XRD), UV-visible spectrophotometer, and scanning electron microscopy (SEM) analysis were used. The average particle size was 20.84 ± 10.19 nm as determined using the XRD technique, which was mainly spherical in shape. The XRD also revealed a monoclinic crystal system of the synthesized CuO nanoparticles. Ultraviolet-Visible analysis showed characteristic peak at 630 nm indicating formation of the CuO NPs. The Tauc plot was used to calculate the optical band gap of CuO NPs from the absorption spectra, which was found to be approximately 2.7 eV. The FTIR peak at 420 cm<sup>−1 </sup>is associated with Cu-O-H stretching suggesting the formation of CuO NPs. Further, the CuO NPs antibacterial potentials were assessed using a standard disc diffusion method. Variety of microorganisms, including <i>Pseudomonas aeruginosa, </i><i>Escherichia coli,</i> <i>Bacillus subtilis, </i>and <i>Staphylococcus aureus </i>were used to test the antibacterial activity of the synthesized CuO NPs. The synthesized CuO NPs demonstrated highest antibacterial activity against <i>Pseudomonas aeruginosa, Escherichia coli,</i> and<i> Staphylococcus aureus </i>compared to both positive control (Erythromycin) and carissa edulis extract. The findings suggest that the CuO NPs synthesized using <i>Carissa Edulis</i> may be used as an alternative medication to fight bacterial infections.