Indirect Approach for CN– Detection: Development of “Naked-Eye” Hg²⁺-Induced Turn-Off Fluorescence and Turn-On Cyanide Sensing by the Hg²⁺ Displacement Approach

The coumarin-thiol-based colorimetric and fluorimetric chemosensor ( E )-7-(diethylamino)-3-((2-mercaptophenylimino)methyl)-2 H -chromen-2-one 1a was designed, synthesized, and evaluated for anion detection through the metal displacement mechanism. Chemosensor 1a exhibited a high sensitivity and selectivity to Hg 2+ ion detection in HEPES-buffered solution [20 mM, 3:7 CH 3 CN/H 2 O (v/v), pH 7.0] with a 1:1 binding stoichiometry and a 5.549 × 10 5 M –1 binding constant. The fluorescence intensity of chemosensor 1a was selectively quenched by Hg 2+, and an obvious color change from yellowish green to orange was observed by the naked eye. Moreover, a solution of the in situ generated 1a + Hg 2+ complex displayed a high selectivity to CN – through the Hg 2+ displacement approach. The gradual addition of CN – to the resultant solution of the 1a + Hg 2+ complex induced its fluorescence recovery with a detection limit of 31 nM. This reversibility in fluorescence suggests the promising applicability of chemosensor 1a as an “on–off–on” naked-eye sensor.