A methionine-based turn-on chemical sensor for selectively monitoring Hg2+ ions in 100% aqueous solution

Dansyl-labeled methionine is synthesized by solid-phase synthesis, and found to be a highly sensitive and selective sensor for Hg2+. The sensor sensitively detects Hg2+ ions in aqueous solution by a turn-on response; however, the sensor detects Hg2+ ions by a turn-off response in organic and mixed aqueous-organic solutions. We investigated the binding stoichiometry, binding constant, and binding mode of the sensor under various solvent conditions. In 100% aqueous solution, 2 : 1 complexation of the sensor with Hg2+ ions is more favorable than 1 : 1 complexation, whereas the sensor preferentially forms a 1 : 1 complex in 100% CH3CN or in 50% CH3CN-aqueous solutions. Results reveal that the stoichiometry of the sensor-Hg2+ complex plays an important role in the type of response to Hg2+ ions, and that 2 : 1 complexation is required for a turn-on response to Hg2+ ions in aqueous solution.