Fluo-3
Fluo-3 is a fluorescence indicator of intracellular calcium (Ca), developed by Roger Y. Tsien and colleagues. It is used to measure Ca inside living cells in flow cytometry, and confocal laser scanning microscopy using visible light excitation (compatible with argon laser sources operating at 488 nm). Fluo-3 and derivatives (Fluo-4, Fluo-5 etc) have also been widely used with two-photon excitation microscopy. Fluo-3 is an essentially nonfluorescent compound, but upon binding of Ca its fluorescence increases sharply with an emission maximum at 525 nm suitable for conventionally used detectors designed for fluorescein isothiocyanate (FITC) measurements. This large change in  fluorescence coupled with a good yield of photons provides very high contrast which allowed the detection of microscopic Ca release events inside cells called "Calcium sparks". Whereas the salts of fluo-3 are unable to penetrate cells, loading can be achieved using its acetoxymethyl (AM) ester derivative. Once inside the cell, unspecific esterases cleave the ester effectively trapping fluo-3. As calcium is a key second messenger within cells, the specific properties of fluo-3 enable researchers to investigate the time-resolved dynamics of intracellular signal transduction in a diverse range of cells.