Colorimetric sensing of cyanide anion in aqueous media with a fluorescein-spiropyran conjugate

A fluorescein-spiropyran conjugate (2) behaves as a receptor for colorimetric sensing of cyanide anion (CN^-) in aqueous media under irradiation of UV light. The compound 2, which exists as a spirocycle-closed (SP) form in the dark condition, is isomerized to the spirocycle-opened merocyanine (MC) form upon irradiation of UV light and shows absorption bands at 467 and 568 nm. Addition of CN^- to the solution leads to a decrease in these bands and an appearance of new absorption band at 512 nm, via a nucleophilic interaction between CN^- and the spirocarbon of MC form. This absorption change occurs selectively with CN^- and enables ratiometric quantification of CN^- by absorption analysis.