Squaraines as Fluoro−Chromogenic Probes for Thiol-Containing Compounds and Their Application to the Detection of Biorelevant Thiols

A highly selective colorimetric chemodosimeter for thiol-containing compounds in aqueous solutions is reported. The design protocol makes use of a highly specific reaction between thiols and the electrophilic four-membered ring of highly colored, fluorescent squaraine backbones. At neutral pH selective decoloration and total emission quenching was found due to the rupture of the highly delocalized squaraine framework upon selective nucleophilic addition of thiol-containing derivatives. The squaraine derivatives have been successfully applied to the determination of low-molecular mass aminothiols in human plasma. The method utters the high potential applicability of the chemodosimeter approach in the search for new or improved chromogenic selective or specific probes for target guests.