Ratiometric potassium indicator, membrane impermeable Excitation: 340, 380 nm | Emission: 505 nm | Kd: 5 mM PBFI is a UV-excitable, ratiometric green indicator for intracellular potassium (K⁺) measurements. Ex/Em: 340/505 nm can be used to measure K⁺-bound PBFI, and Ex/Em: 380/505 nm can be used to detect K⁺-free PBFI. It is 1.5X more selective for K⁺ over Na⁺, and works well in cytosolic conditions where [K⁺]/[Na⁺] is ~10. Ratiometry is optimal for imaging applications where quantification of intracellular K⁺ concentrations is desired, and reduces effects of photobleaching, heterogenous dye loading, and variable cell morphology. PBFI K⁺ salt is a membrane impermeable form of PBFI that can be used in lipid membrane-free systems, in liposomes, or can be introduced into cells by electroporation, microinjection or other methods.