Fluorogenic mercury ion sensor based on pyrene-amino mercapto thiadiazole unit

A highly selective and sensitive determination of Hg²⁺ in water samples with bioimaging applications in living cells using a fluorogenic pyrene-amino mercapto thiadiazole (PYAMT) probe is described. The probe PYAMT exhibited three absorption peaks at 333, 348 and 394nm and emission maxima at 378, 388 and 397nm (λ_ex=348nm). It showed significant fluorescent quenching (96%) with I/I₀=0.051 upon the addition of 2.5μM Hg²⁺ ion in CH₃CN(ACN):H₂O (1:1, v/v; pH 7.2), whereas its fluorescence remained unaltered in the presence of other metal ions. The quenching phenomenon is attributed to the heavy atom effect of Hg²⁺ ion followed by electron transfer. The fluorescence intensity decreased linearly against a wide range from 100nM to 2.5μM Hg²⁺ (R²=0.9937) with a limit of detection as low as 0.35nM (S/N=3). The binding stoichiometry ratio of PYAMT-Hg²⁺ is proved to be 1:1 by fluorescence and DFT measurements. The sensor possesses high association constant with Hg²⁺ ion in the order of 9.08×10⁵M^-1 and it is also capable of reversibly detecting cysteine with OFF-ON mechanism. Finally, the proposed method is successfully applied to selectively detect Hg²⁺ ion in real water samples and bioimaging studies in live cells.