Colorimetric and fluorescent "turn-off" sensors for Hg²⁺ based on mercury-induced hydrolysis of dithioacetals

Three fluorescent probes for Hg²⁺, Hg-5, Hg-6, and Hg-7, were designed and synthesized by incorporating a cyclic dithioacetal recognition unit to an N, N-diethyl-7-amino-coumarin fluorophore. Under physiological pH conditions, all the probes demonstrated high selectivity and strong anti-interference capability toward Hg²⁺ over other competing ions. These probes enable both colorimetric and fluorescent quantification of Hg²⁺ in aqueous solution and show promising potential for confocal fluorescence imaging of Hg²⁺ in living cells. Notably, the ring size of the dithioacetal moiety significantly influences the response kinetics due to steric effect and heterocyclic stability. The response time toward Hg²⁺ decreases in the order of Hg-5 > Hg-6 > Hg-7. In addition of fast responses to Hg²⁺ (response time less than 1 min), excellent linear relationships between the absorbance ratio A₄₄₆/A₄₀₀ as well as the fluorescence intensity at 485 nm of Hg-7 and Hg²⁺ concentration were observed over the range of 0.5-20 μM (R² = 0.99), and the detection limit (LOD) for Hg²⁺ was calculated to be 0.04 μM.