Two 4-(aminoalkyl)aminonaphthalimide compounds which exhibit fluorescence enhancement and associated spectral changes in the presence of Cu 2 + , Mn 2 + and Ni 2 + are reported. The compounds are believed to undergo intramolecular quenching by the distal nitrogen on the 4-substituent. When metal ions are present, this quenching mechanism is partially or wholly removed due to complexation of the metal ions with the 4-substituent. The two compounds are particularly sensitive to Cu 2 + such that concentrations in the 0-1 ppm range are sufficient to effect complete restoration of the naphthalimide fluorescence.