Based on resonance energy transfer (FRET) from dansyl to rhodamine 101, a new fluorescent probe (compound 1 ) containing rhodamine 101 and a dansyl unit was synthesized for detecting Hg 2+ through ratiometric sensing in DMSO aqueous solutions. This probe shows a fast, reversible and selective response toward Hg 2+ in a wide pH range. Hg 2+ induced ring-opening reactions of the spirolactam rhodamine moiety of 1 , leading to the formation of fluorescent derivatives that can serve as the FRET acceptors. Very large stokes shift (220 nm) was observed in this case. About 97-fold increase in fluorescence intensity ratio was observed upon its binding with Hg 2+ .