Carry out a Stokes’ Law calculation. They can’t be nanoparticles if they’re settling. You probably have started with a powder of fused aggregates and loose agglomerates.

To prevent ZnO nanoparticles from settling in culture plates, use stabilizers like PVP or PEG and apply ultrasonication or vortex mixing. To test antibacterial activity, ensure standardized bacterial cultures and incubate with a stable NP solution, evaluating results through methods like zone of inhibition measurements.

You could try disk diffusion method. Use a sterile disc, first add the desired concentration of nanoparticles to the disc and dry them. Later place the disc on the plate for antibacterial activity.