Three dimensional structures are able to be imaged by scanning the volume with a nanometer-size Brownian particle. The contribution of electrostatic interaction to the image formation in thermal noise imaging was studied. The problem was simplified to one dimension by replacing the complex three-dimensional structure with a planar coverslip. A simple fluorescence experiment was designed first to calibrate the distance from the trapping center to the planar surface. Strong electrostatic repulsion between like charged surfaces was observed in the experiments at low ionic strength as expected. Further fluorescence experiments shows that minimum separation decreases as salt concentration increases. It was found that 0.01mol/l is the optimal salt concentration for the given experimental condition. Higher concentrations lead to a permanent adhesion of particles to the surface making thermal noise imaging impossible.