[graph: see text] 1-(Beta-D-galactopyranosyl-1'-deoxy-1'-iminomethyl)-2-hydroxynaphthalene (L1), possessing an ONO binding core, was found to be selective for Cu2+ ions in N-[2-hydroxyethyl]piperazine-N'-[2-ethanesulfonic acid] buffer, at concentrations < or = 580 ppb, at physiological pH by eliciting switch-on behavior, whereas the other ions, viz., Mg2+, Ca2+, Mn2+, Fe2+, Co2+, Ni2+, Zn2+, and Cd2+, caused no significant change in the fluorescence. Whereas the binding characteristics were ascertained by absorption spectroscopy, the species formed were shown by Q-TOF ES MS.