A Selective Pyridinedicarbohydrazide-based "naked-eye" Fluorescent Probe for Detecting Cu²⁺ Ions in Aqueous Solutions

A novel fluorescent and colorimetric probe (L3), based on 2,6-pyridinedicarbohydrazide, was rationally designed and synthesized for the highly selective and reversible detection of Cu²⁺ ions. The structural and optical properties of the probe were comprehensively characterized by FT-IR, ¹H NMR, UV-Vis spectroscopy, fluorescence analysis, ESI⁺-MS, and elemental analysis. The probe L3 displayed a high selectivity, high sensitivity and quick response to Cu²⁺ ions in the presence of other competing cations including Ba²⁺, K⁺, Pb²⁺, Hg²⁺, Al³⁺, Zn²⁺, Fe²⁺, Tl⁺, Mg²⁺, Ni²⁺, Nd³⁺, Fe³⁺, Ag⁺, Ca²⁺, Cs⁺, Co²⁺, Cd²⁺, Sr²⁺, and Mn²⁺ ions). Upon interaction with Cu²⁺, probe L3 exhibited distinct optical responses, including significant fluorescence quenching, a notable redshift in the UV-Vis spectrum, and a rapid, visible color change from colorless to greenish-yellow under aqueous conditions and across a broad pH range. The detection limit was calculated to be as low as 2.28 × 10⁻⁹ M, with an association constant (Ka) of 2.2 × 10¹¹ M⁻², indicating strong binding affinity. Job's plot and ¹H NMR titration confirmed a 1:2 stoichiometric complex formation between L3 and Cu²⁺ ions. The probe also exhibited excellent reversibility, as confirmed by fluorescence recovery upon adding Na₂EDTA, suggesting its suitability for real-time and reusable sensing applications. Furthermore, the practical utility of L3 was demonstrated using test strips for Cu²⁺ detection in aqueous environments, and its effectiveness was further validated in real water samples, including tap water, drinking water, and seawater, achieving recovery rates up to 99.2%. These findings suggest that L3 is promising for the sensitive, rapid, and cost-effective monitoring of copper ions in environmental and drinking water systems.