Fluo-4 is used to measure calcium (Ca2+) concentrations inside living cells, and is often used for high-throughput screening of receptor ligands and calcium permeable ion channels.
The green-fluorescent calcium indicator, Fluo-4, is an improved version of the calcium indicator, Fluo-3. It is commonly used as the non-fluorescent acetoxymethyl ester (Fluo-4 AM) which is cleaved inside the cell to give the free, fluorescent Fluo-4. It loads faster, is brighter at equivalent concentrations and is well-excited by the 488 nm line of the argon-ion laser which is often used in biological research laboratories. The sole distributor and owner of the patents is Invitrogen which acquired the original inventor Molecular Probes.
^Gee KR, Brown KA, Chen WN, Bishop-Stewart J, Gray D, Johnson I (February 2000). "Chemical and physiological characterization of fluo-4 Ca(2+)-indicator dyes". Cell Calcium27 (2): 97–106. doi:10.1054/ceca.1999.0095. PMID10756976.