A novel fluorometric sensor bearing three dansyl moieties based on tris[2-(2-aminoethylthio)ethyl]amine was prepared by a simple approach using a conventional two-step synthesis. The sensor exhibits highly Hg 2+ -selective ON–OFF fluorescence quenching behavior in aqueous acetonitrile solutions and is shown to discriminate various competing metal ions, particularly Cu 2+ , Ag + , and Pb 2+ as well as Ca 2+ , Cd 2+ , Co 2+ , Fe 3+ , Mn 2+ , Na + , Ni 2+ , and Zn 2+ , with a detection limit of 1.15×10 −7 M or 23ppb.