5-Carboxyfluorescein is a highly soluble dye which is utilized as a pH indicator. 5-Carboxyfluorescein displays strong fluorescence and is concentration dependent. This compound labels proteins, nucleotides and peptides from carboxylic acid interacting with primary amines.5-Carboxyfluorescein is a single isomer derivative of 5(6)-carboxyfluorescein that can be used to fluorescently label biomolecules through the interaction of carboxylic acid with primary amines.1 It demonstrates excitation/emission maxima of 492 and 518 nm, respectively