A device for optical mapping of protein binding sites, in particular, transcription factor binding sites, on single DNA molecules, includes an insulating substrate having two parallel channels and at least one slit connecting the two channels, a coverslip on the substrate, at least two reservoirs on the substrate connecting the channels of the insulating substrate, and at least two electrodes in the reservoirs. When the reservoirs are filled with a buffer solution, the electrodes are in electrical contact in the buffer solution.