The DiO/DPA system detects cytoplasmic membrane potential changes using the principle of fluorescence resonance energy transfer (FRET), where DiO is a "stationary" FRET donor while DPA acts as a potentiometric FRET acceptor. The DiO/DPA system has been reported to produce a fluorescence signal change of >56% in HEK-293 cells and >25% in neuronal cultures and brain slices per 100 mV membrane potential change. The DiO/DPA membrane potential detection kit includes a vial of the green fluorescent membrane dye DiOC16 (3) in DMSO (1 mL at 2 mM) and a vial of the anionic membrane quencher dye DPA (dipicrylamine) in DMSO (1 mL at 20 mM) .