Fluorescein

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Image:Fluorescin in dropper.jpg Image:Fluoresceine.png Image:Fluorescein Isothiocyanate.gif Fluorescein is a fluorophore commonly used in microscopy, in a type of dye laser as the gain medium, and in forensics and serology to detect latent blood stains. Fluorescein has an absorption maximum at 490 nm and emission maximum of 514 nm (in water). Also, fluorescein has an isoabsorptive point (equal absorption for all pH values) at 460 nm.

Molecular Formula: C₂₀H₁₂O₅

Molecular Weight: 332.32 grams per mole

CAS Number: 2321-07-5

In biology, the isothiocyanate derivative of fluorescein is often used to label and track cells in fluorescence microscopy applications. The fluorescence of this molecule is very high, and excitation occurs at 494 nm and emission at 525 nm. The isothiocyanate group (-N=C=S) replaces a hydrogen atom on the bottom ring of the structure, and is reactive with amine groups on proteins inside cells. Fluorescein isothiocyanate is often abbreviated as FITC. Additional biologically active molecules (such as antibodies) may also be attached to fluorescein, allowing biologists to target the fluorophore to specific proteins or structures within cells. This application is common in yeast display.

Fluorescein has a pK_a at 6.4 and multiple ionization equilibria. This leads to pH dependent absorption and emission over the range of 5 to 9. Also, the fluorescence lifetimes of the protonated and deprotonated forms of fluorescein are approximately 3 and 4 ns, which allows for pH determination from non-intensity based measurements. The lifetimes can be recovered using time-correlated single photon counting or phase-modulation fluorimetry.

There are many other fluorescein derivatives, such as Oregon Green, Tokyo Green, SNAFL, and carboxynapthofluorescein. These have been tailored for various chemical and biological applications where higher photostability, different spectral characteristics, or different attachment groups are needed.

One of its more recognizable uses is in the Chicago River, where fluorescein is used to dye the river green on St. Patrick's Day.

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