I synthesized rGO/m-ZnO nanocomposites .One of applications of nanocomposites is photocatalytic degradation of an organic dye. Which is better , measure the z-potential of rGO/m-ZnO suspensions in presence of dye or in absence of it.
I agree with the answer from Jérémie- also, the change in zeta potential in the presence of dye can give you some indications of the extent of the interactions.
I suppose it is necessary to measure Zeta potential of your nanoparticles, both without dye, and in his presence. Because we can not exclude the possibility of exposure of your dye, on the structure of the electrical double layer of your particles.