Intramolecular charge transfer (ICT) based fluorescent reagents containing a dansyl fluorophore have been synthesized and characterized. The reagent 1 and its complex, 1+Hg²⁺ in sodium acetate buffer (pH 6.7) revealed considerable fluorescence enhancement (switched-on) in the presence of bovine serum albumin (BSA) with 10 ppb detection sensitivity. ¹H NMR spectral analysis suggests complexation between 1 and Hg²⁺ ion involving the N,N-dimethylamino and carboxylic functions.