A probe with hydrazinecarbothioamide and 1,8-naphthalimide groups for “turn-on” fluorescence detection of Hg2+ and Ag+ ions and live-cell imaging studies

•Hydrazinecarbothioamide appended 1,8-naphthalimide based probe (L) was synthesized.•Detection limits of L were in nanomolar range for both ions through a “turn-on” fluorescence response.•Easily observable color changes were observed after addition of Hg2+ and Ag+ ion into a solution of L.•“Turn-on” fluorescence response was due to restriction of PET and operation of CHEF processes.•Intracellular detection of Hg2+ and Ag+ ions in MDA-MB-231 and HDF cells was assessed.