A Selective Pyridinedicarbohydrazide-based "naked-eye" Fluorescent Probe for Detecting Cu 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 Cu ions in the presence of other competing cations including Ba, K, Pb, Hg, Al, Zn, Fe, Tl, Mg, Ni, Nd, Fe, Ag, Ca, Cs, Co, Cd, Sr, and Mn 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.