The coumarin-thiol-based colorimetric and fluorimetric chemosensor (E)-7-(diethylamino)-3((2-mercaptophenylimino) methyl)-2H-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 Hg2+ ion detection in HEPES-buffered solution [20 mM, 3:7 CH3CN/H2O (v/v), pH 7.0] with a 1:1 binding stoichiometry and a 5.549 X 10(5) M-1 binding constant. The fluorescence intensity of chemosensor la was selectively quenched by Hg2+, 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 + Hg2+ complex displayed a high selectivity to CN- through the Hg2+ displacement approach. The gradual addition of CN- to the resultant solution of the 1a + Hg2+ complex induced its fluorescence recovery with a detection limit of 31 nM. This reversibility in fluorescence suggests the promising applicability of chemosensor la as an "on-off-on" naked-eye sensor.