A new 1,8-naphthalimide-based fluorescent turn-off sensor for detecting Cu2+ and sensing mechanisms

In this article, a new turn-off fluorescent sensor N-n-butyl-4-{2-[(ethylimino)methyl]phenol}-1,8-naphthalimide (HL) for Cu-II ions is synthesized, which contains a 1,8-naphthalimide moiety as the fluorophore and a Schiff base as the recognition group. As expected, it exhibits high selectivity and sensitivity for detecting Cu-II ions over other common metal ions in acetonitrile-2-(4-(2-hydroxyethyl)-1-piperazinyl)-ethanesulfonic acid (HEPES) (1:1 v/v, pH = 7.4) solution. In addition, the fluorescence intensity for HL showed a good linearity with the concentration of Cu-II ions in the range of 0.5-5.0 mu M. The 2:1 binding stoichiometry between HL and Cu-II ions was established on the basis of combined fluorescence titrations, a Job's plot, single-crystal X-ray analysis and mass spectrometry. The quenching response of HL toward Cu-II ions is attributed to the reverse photoinduced electron transfer mechanism. The proposed sensor HL is preliminarily applied to quantify Cu-II ions in water samples from the Yellow River and tap water.