This stability of NPs from plant extracts can be attributed to the protective agents in the plant extract . According to Suárez-Cerda et al. , electrostatic mechanisms promote the stabilization of NPs in colloidal solution. The presence of carbonyl groups, coming from the plant extract, on the surface of the CuO NPs suggests the formation of an electrostatic sphere, like an electrical charge, consequently causing repulsion between charged NPs during their growth phase.