A novel fluorescent turn on probe derived from Schiff base for highly selective and sensitive detection of Cu²⁺ ion

A fluorescent probe (L) of bis Schiff base secondary amine ligand was designed and synthesized from 2,6-pyridinedicarboxylic acid, ethylenediamine and salicylaldehyde by condensation and reduction reaction. Its structure was characterized by IR, UV-Vis, LIF, ¹HNMR, ¹³C NMR, MS and elementary analysis. Investigation of binding test indicated that probe L could sensitively and selectively detect Cu²⁺ ion with striking fluorescent signaling responses in anhydrous ethanol solvent. Results from Job's plot, fluorescent titration, and MS experiments indicated a 1:1 binding ratio between probe L and Cu²⁺, with a complexation constant of 1.28 × 10⁸ M^-1. According to MS, IR and molar conductivity analysis, the mechanism of the probes' detection of Cu²⁺ may due to CHEF mechanism by the lone electron pairs in the N atom, and the enhancement of the molecular rigidity caused by disruption of intermolecular hydrogen bonding force, which leading to the occurrence of chelation fluorescence enhancement. The detection limit of complexation constant was 2.69 × 10^-8 M in the linear range of 0-40 μM, which provided an effective and convenient testing for trace copper in surface water and drinking water.