Fluorescein (FRC)

Fluorescein is a synthetic organic compound available as a dark orange/red powder slightly soluble in water and alcohol. It is widely used as a fluorescent tracer for many applications. Fluorescein is a fluorophore commonly used in microscopy, in a type of dye laser as the gain medium, in forensics and serology to detect latent blood stains, and in dye tracing.

Fluorescein has an absorption maximum at 494 nm and emission maximum of 521 nm (in water). The major derivatives are fluorescein isothiocyanate (FITC) and, in oligonucleotide synthesis, 6-FAM phosphoramidite. The fluorescence of this molecule is very intense; peak excitation occurs at 494 nm and peak emission at 521 nm. Fluorescein has a pKa of 6.4, and its ionization equilibrium leads to pH-dependent absorption and emission over the range of 5 to 9.