Ratiometric calcium indicator, membrane impermeable. Excitation: 340, 380 nm | Emission: 505 nm | Kd: 145 nM Fura-2 is the most popular UV-excitable, ratiometric green indicator for intracellular calcium (Ca²⁺) measurements. Ex/Em: 340/505 nm can be used to measure Ca²⁺-bound Fura-2, and Ex/Em: 380/505 nm can be used to detect Ca²⁺-free Fura-2. Ratiometry is optimal for imaging applications where quantification of intracellular Ca²⁺ concentrations is desired, and reduces effects of photobleaching, heterogenous dye loading, and variable cell morphology. Fura-2 K⁺ salt is a membrane impermeable form of Fura-2 that can be used in lipid membrane-free systems, in liposomes, or can be introduced into cells by electroporation, microinjection or other methods.