In current drug development, the visualization of therapeutic-agent translocation to the cell interior represents one of the essential problems. The transport across cellular membranes is frequently executed by cell-penetrating peptides (CPPs), and the process is routinely studied with fluorescence microscopy when the CPP is attached to a fluorescent marker. Nevertheless, the use of existing labelled CPPs is connected with several drawbacks. Especially, bonding of CPPs to the external face of the cell membrane make interpretation of the translocation difficult and can result in false positive results. Therefore, the development of novel electrochemically switchable fluorescent probe for the detection of CPP translocation is suggested. In this project, a fluorescent organic dye will be coupled with an electro-controlled fluorescent quencher, which could be activated/deactivated in response to applied electric potential. As the electron transfer cannot occur through the cell membrane, labelled external CPPs would be switched off through photoinduced electron transfer, and fluorescence only of internalized CPPs could be studied. Reversibility of the system represents a remarkable advantage as it would allow repetitive on/off switching of the fluorescent probe.