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A novel azathia-crown ether dye was synthesized and characterized and its binding ability with metal ions was investigated via UV–vis spectroscopy. When a series of alkali-metal, alkaline earth metal and transition metal ions were mixed with the dye in acetonitrile, only Hg 2+ induced a large blue shift from 478 nm to 369 nm corresponding to a distinct color change from red to yellow, which made it possible to distinguish Hg 2+ from other metal ions by the naked-eye. The excellent Hg 2+ -selectivity of the dye was attributed to the formation of a 1:2 dye:Hg 2+ coordination complex which was demonstrated by means of a Job's plot.