The zeta potential of the CuO NPs measured was -13.5 mV (Figure ). As previously described, visual observation of the plant extract after mixing with the metal precursor was the first signal for the biosynthesis of CuO NPs . The color change to dark greenish can be attributed to the vibration and resonance of charged ions on the surface of the NPs . A negative zeta potential indicates that the particles have a net negative charge on the surface, possibly due to the ionization of functional groups or the adsorption of negative ions. A zeta potential of -13.5 mV suggests moderate colloidal stability, leading to a tendency for agglomeration between the particles . In general, high zeta potential values (above ±30-40 mV) are indicative of more significant electrostatic repulsion between the particles, which helps prevent aggregation, keeps the particles more dispersed, and indicates good physical stability of the nanosuspension .