Development of a Coumarin-Based Schiff Base Fluorescent Probe and its Application in Detection of Cu²⁺

A highly practical Schiff base fluorescent probe, (E)-3-amino-N'-((7-(diethylamino)-2-oxo-2 H-chromen-3-yl)methylene)thiophene-2-carbohydrazide (M), with a facile synthetic route has been successfully developed. M has been utilized for the specific detection of Cu²⁺ in THF/H₂O Tris buffer solution (v/v = 9:1, 0.01 M, pH = 7.4) via the fluorescence quenching mechanism. The detection of Cu²⁺ by M has been largely unaffected by interfering ions and has demonstrated a distinct dual-channel response in both colorimetry and fluorescence. The response time of M towards Cu²⁺ is remarkably fast, taking only 30 s. Additionally, M exhibits exceptional sensitivity with a limit of detection (LOD) as low as 1.76 × 10^- 7 M. The stoichiometric ratio between M and Cu²⁺ has been determined to be 1:1 through Job's Plot, while the binding constant has been calculated as 1.19 × 10⁴ M^- 1 using the Benesi-Hildebrand equation. The structure of M has been elucidated by ¹H NMR and ESI-MS analyses, thereby confirming the binding mode between M and Cu²⁺. Further validation has been achieved through DFT calculations. The test paper based on M has finally been prepared for the rapid and convenient detection of Cu²⁺. The M has also been utilized for the detection of Cu²⁺ in real samples, including lake water, onions, and coffee, demonstrating favorable recovery rates. Moreover, successful visual detection has been achieved in food samples such as bean sprouts and rice. The aforementioned examples have collectively illustrated the practical applicability of M in authentic samples.