Ni2+-induced intramolecular excimer formation of a naphthalene-based novel fluorescent probe, 1-[(naphthalen-3-yl)methylthio]-2-[(naphthalen-6-yl)methylthio]ethane (L), has been investigated for the first time and nicely demonstrated by excitation spectra, a fluorescence lifetime experiment, and H-1 NMR titration. The addition of Ni2+ to a solution of L (DMSO:water = 1:1, v/v; lambda(cm) = 345 nm, lambda(ex) = 280 nm) quenched its monomer emission, with subsequent enhancement of the excimer intensity (at 430 nm) with an isoemissive point at 381 nm. The fluorescence lifetime of free L (0.3912 ns) is much lower than that of the nickel(2+) complex (1.1329 ns). L could detect Ni2+ as low as 1 x 10(-6) M with a fairly strong binding constant, 2.0 x 10(4) M-1. Ni2+-contaminated living cells of plant origin could be imaged using a fluorescence microscope.