A unique naphthaldehyde-dipyrromethane based ‘on-off’ chemosensor DPN was synthesized and utilized as a reversible AIE based fluorescent probe for the discerning sensing of Cu2+ ions, in the presence of other competing metal ions with the limit of detection of about 3.3 nM under semi-aqueous medium. Cu2+ ion interacts with the receptor DPN to form a 2:1 stoichiometry complex, ensuing a notable blue shift of absorption maximum from 445 nm to 407 nm, as well as nearly complete quenching of fluorescence intensity of the receptor at the wavelength 515 nm. These extreme variations can be clearly perceived by our visible eye under 365 nm UV light. The optimized geometries of DPN and DPN + Cu2+ adduct were augmented at the B3LYP/LANL2MB energy level through theoretical studies. Fascinatingly, the receptor DPN was marvelously employed to make test strips for the quick sensing of Cu2+ ions. In addition, the determination of Cu2+ ions in real water samples were performed to demonstrate its practical applications in the environment. By using fluorescent imaging in live A549 and AGS gastric cancer cells, we examined the biological effects of DPN on detecting copper ions.