Thiooxo-Rhodamine B hydrazone derivatives bearing bithiophene group as fluorescent chemosensors for detecting mercury(II) in aqueous media and living HeLa cells

Two sulfur-rich thiooxo-Rhodamine-bithiophene skeletons RB-TS and DRB-QS are established to function as Hg²⁺-specific chemosensors. Both of RB-TS and DRB-QS have excellent spectral responses towards Hg²⁺ in aqueous solution with low detection limit (ppb level) and high binding association constant. RB-TS exhibits FRET sensing mechanism in binding with Hg²⁺, while DRB-QS displays the traditional ‘Off-On’ fluorescence change with the 1:2 binding stoichiometry. With the addition of Hg²⁺, the fluorescence of Rhodamine B is switched on in both RB-TS and DRB-QS, meanwhile colour change of the solution can be clearly observed by naked-eye. Two chemosensors are further proven to be practical in sensitively monitoring trace Hg²⁺ in tap water and real lake water specimens, as well as in imaging Hg²⁺ internalized by living cells with low cytotoxicity.