As a control, you should use the same solvent/buffer as that in which the samples to be tested are dissolved

I would like to agree with Pierre Béguin

Any solvents, buffers or even sterile water is sufficient to act as an effective negative control for disc diffusion method.

Hope this answer is helpful.

I don’t quite agree with the previous answer. The control has to be withe the SAME solvent/buffer, whatever it is, as the substances to be tested.

Thanks Pierre Béguin