Fluorescein isothiocyanate stability in different solvents

Well-established label in biomolecules analysis, fluorescein isothiocyanate is commercially available in its two different isomers with equal fluorescence sensitivity, differing in meta- vs. para-isothiocyanate position. It was found in this study that the stability of the isomers depends on the polarity of different solvents and the pyridine (stabiliser) presence. The lowest stability has fluorescein isothiocyanate in water solvent. In acidic solution, their quinoid (acid) form switches to the lactone form. The p-quinoid form has higher tendency to create dimers and trimers and is less soluble in acetone than the lactone form.