An organoselenium-based NSe3 type of tripodal system 2 as a Hg2+-selective fluorescence "turn-on" probe is described. The "turn-on" fluorescence behavior of this selenotripod 2 is significant because it depends on Hg-Se bond formation and acts as a reporting unit for this system. The system exhibits immediate response (15 s) with a subnanomolar detection limit (0.1 nM) for the Hg2+ ion. It efficiently detects both aqueous and nonaqueous He2+ at 2 nM concentration.