The negative control should be a mixture of NaDes and PBS in the same proportion as they are in the protein sample, unless this has been shown to be unnecessary. The protein standards should also contain the same mixture in the same concentration as the protein sample. If you can show that the residual concentration of NaDes has no effect on the slope of the standard curve, then you can leave it out of the negative control. PBS should have no effect on the Bradford assay, but I suggest you convince yourself of this experimentally.

Use PBS alone (without protein or NaDes) as the negative control (blank) in your Bradford assay.