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
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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<sup>–1</sup>, 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<sup>–8</sup>–5.0 ×
10<sup>–6</sup> M with a detection limit of 1.0 × 10<sup>–8</sup> 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