Selective detection of Cu2+ in aqueous medium using an acid hydrazide-based chemosensor: experimental and DFT/TDDFT studies

Abstract

An acid hydrazide-based Schiff base, N'-(4-hydroxybenzylidene)picolinohydrazide (HP), was designed and synthesized in a single-step process for the selective detection of Cu²⁺ ions. The structure of HP was thoroughly characterized by FTIR, ¹H NMR, ¹³C NMR, HRMS, SCXRD and elemental analysis. The cation-sensing performance of HP was investigated using UV-Vis spectroscopy, revealing a rapid response with high selectivity and sensitivity toward Cu²⁺ in MeOH/H₂O (6 : 4, v/v) with a 10 mM HEPES buffer at pH 7.4, with a detection limit of 8.94 µM. Job's plot analysis confirmed a 1 : 1 binding stoichiometry, with an association constant of 4.26 × 10⁴ M⁻¹. FESEM analysis showed distinct morphological changes upon Cu²⁺ coordination. The SCXRD result exhibits good agreement with DFT and TD-DFT of the probe HP. The hole–electron analysis clearly reveals locally excited (LE) characteristics for the free ligand and intramolecular charge transfer (ICT) behavior for the HP–Cu²⁺ complex. Furthermore, the HP probe demonstrated practical applicability for Cu²⁺ detection in real water samples.