Fluorescein is nearly inactive as a photodynamic sensitizer. Fluorescein consequently is almost useless as a photodynamic sensitizer, but it is of great value in fluorescence immunoassays.
The fluorescein dye is a common fluorescent probe. Its very high molar absorptivity at the wavelength of the argon laser (488 nm), large fluorescence quantum yield and high photostability makes it a very useful and sensitive fluorescent label. Fluorescein is commercially available in many derivatives, such as fluorescein isothiocyanate and fluorescein succinimidyl ester, that can be covalently attached to macromolecules and to amino acids. The labeled molecules can be detected with very high sensitivity which is utilized in, for example, capillary electrophoresis. The emission spectrum of fluorescein overlaps extensively the absorption spectrum of tetramethyl rhodamine, which is a related strongly fluorescent dye, making this pair very suitable for energy transfer experiments to determine distances within and between labeled macromolecules. Fluorescein in aqueous solution occurs in cationic, neutral, anionic and dianionic forms making its absorption and fluoresence properties strongly pH dependent.