A fluorescent "turn-off" naphthalimide-derived Schiff base for sensing Cu²⁺ ions: experimental and computational...

A naphthalimide-based Schiff base (probe 1) was synthesized for the selective detection of Cu²⁺ ions in a MeOH: H₂O (9:1, v/v) solvent system. The optical properties of the probe were investigated using UV-visible and fluorescence spectroscopy. In the UV-visible spectrum, probe 1 displayed absorption bands at 260 nm and 330 nm. Fluorescence measurements revealed an emission peak at 445 nm, accompanied by a shoulder at 470 nm. Upon the addition of Cu²⁺ ions, a significant quenching of the fluorescence intensity was observed, whereas other metal ions showed negligible interference. Quantitative analysis revealed a limit of detection (LOD) of 1.22 × 10^-5 M and a limit of quantification (LOQ) of 4.08 × 10^-5 M for Cu²⁺. The association constant was calculated from the B-H plot which was determined to be 1.43 × 10² M^-1 with the quenching constant of 1.99 × 10² M calculated from the Stern-Volmer plot. Also, the binding stoichiometry of 2:1 was determined by Job's plot between the probe 1 and Cu²⁺ ions, respectively. Further, the results were validated by HR-MS, DLS, and computational studies. The DFT studies revealed a charge transfer from the thiophene ring towards the naphthalimide moiety which was significantly inhibited in the presence of Cu²⁺ ions.