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

•Two thiooxo-Rhodamine-bithiophene derivatives are established to function as Hg2+-specific chemosensors.•RB-TS exhibits FRET sensing mechanism in binding with Hg2+ with ppb level detection limit.•DRB-QS displays the traditional ‘Off-On’ fluorescence change with the 1:2 binding stoichiometry.•RB-TS and DRB-QS can sensitively monitor trace Hg2+ in tap water and real lake water specimens.•Both of RB-TS and DRB-QS can be applied in imaging Hg2+ in living HeLa cells with low cytotoxicity.