Simultaneous Nucleophilic-Substituted and Electrostatic Interactions for Thermal Switching of Spiropyran: A New Approach for Rapid and Selective Colorimetric Detection of Thiol-Containing Amino Acids

Abstract

Complementary electrostatic interaction between the zwitterionic merocyanine and dipolar molecules has emerged as a common strategy for reversibly structural conversion of spiropyrans. Herein, we report a concept-new approach for thermal switching of a spiropyran that is based on simultaneous nucleophilic-substitution reaction and electrostatic interaction. The nucleophilic-substitution at spiro-carbon atom of a spiropyran is promoted due to electron-deficient interaction induced by 6- and 8-nitro groups, which is responsible for the isomerization of the spiropyran by interacting with thiol-containing amino acids. Further, the electrostatic interaction between the zwitterionic merocyanine and the amino acids would accelerate the structural conversion. As proof-of-principle, we outline the route to glutathione (GSH)-induced ring-opening of 6,8-dinitro-1′,3′,3′-trimethylspiro [2H-1-benzopyran-2,2′-indoline] (<b>1</b>) and its application for rapid and sensitive colorimetric detection of GSH. In ethanol–water (1:99, v/v) solution at pH 8.0, the free <b>1</b> exhibited slight-yellow color, but the color changed clearly from slight-yellow to orange-yellow when GSH was introduced into the solution. Ring-opening rate of <b>1</b> upon accession of GSH in the dark is 0.45 s^–1, which is 4 orders of magnitude faster in comparison with the rate of the spontaneous thermal isomerization. The absorbance enhancement of <b>1</b> at 480 nm was in proportion to the GSH concentration of 2.5 × 10^–8–5.0 × 10^–6 M with a detection limit of 1.0 × 10^–8 M. Furthermore, due to the specific chemical reaction between the probe and target, color change of <b>1</b> is highly selective for thiol-containing amino acids; interferences from other biologically active amino acids or anions are minimal