Cobalt oxide nanoparticles (Co 3 O 4 -Nps) have many applications and now a days the green methods of synthesis of these NPs are preferred over other methods because of associated benefits. In this study, Co 3 O 4 -Nps were synthesized by using leaves extract of Populus ciliata (safaida) and cobalt nitrate hexa hydrate as a source of cobalt. The synthesized NPs were analyzed by different techniques such as fourier transform spectroscopy (FTIR), x-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Antibacterial activities of the synthesized Co 3 O 4 -Nps were evaluated against gram negative and gram positive bacteria and found active against Escherichia coli (E. coli), Klebseilla pneumonia (K. pneumonia), Bacillus subtillus (B.subtillus) and Bacillus lichenifermia (B. lichenifermia). The activity results were analyzed statistically by one-way ANOVA, with 'Dunnett's Multiple Comparison Test'. The maximum mean activity (21.8±0.7) was found for B. subtilis and minimum mean activity (14.0±0.6) was observed for E. coli.

Green synthesis of nanoparticles has received a lot of attention from scientists globally because it is eco-friendly, relatively rapid, and a cost-effective method. This work presents a method for the green synthesis of nickel oxide nanoparticles (NiO-NPs) using leaf extracts of Populus ciliata as a reducing and stabilising agent. The synthesised NiO-NPs were characterised by ultraviolet-visible spectroscopy, Fourier transform infrared spectroscopy, powder X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. It was found that the synthesised NiO-NPs shapes varied, were highly crystalline, and had a face-centred cubic geometry. The calculated crystallite size of the synthesised nanoparticles was 44 nm. Moreover, the antibacterial activity of the synthesised NiO-NPs was also conducted against Gram-positive bacteria (Bacillus subtilis, Bacillus licheniformis) and Gram-negative bacteria (Escherichia coli and Klebsiella pneumoniae). Bacillus subtillus showed a maximum inhibition zone of 28.1±1.6 mm, whereas Klebsiella pneumonia showed a minimum inhibition zone of 9.2±0.5 mm. It was also found that the antibacterial activity increased with an increase in the concentration of NiO-NPs. Keywords: antibacterial activity, green synthesis, nanoparticles, NiO, Populus ciliata.