Ratiometric sodium indicator, membrane impermeable Excitation: 340, 380 nm | Emission: 505 nm | Kd: 5 mM SBFI is a UV-excitable, ratiometric green indicator for intracellular sodium (Na⁺) measurements. Ex/Em: 340/505 nm can be used to measure Na⁺-bound SBFI, and Ex/Em: 380/505 nm can be used to detect Na⁺-free SBFI. It is ~18X more selective for Na⁺ over K⁺. Ratiometry is optimal for imaging applications where quantification of intracellular Na⁺ concentrations is desired, and reduces effects of photobleaching, heterogenous dye loading, and variable cell morphology. SBFI K⁺ salt is a membrane impermeable form of SBFI that can be used in lipid membrane-free systems, in liposomes, or can be introduced into cells by electroporation, microinjection or other methods.