A coumarin-appended pyridine dicarbohydrazide-based chemosensor for the highly sensitive and selective colorimetric...

A coumarin-appended pyridine dicarbohydrazide (H1) is developed for the ultra-fast, naked-eye recognition of Ni²⁺ ions. H1 exhibits an immediate visual response (≤1 s), supported by a distinct spectral change in the UV-visible region. The sensor displays remarkable sensitivity (LOD = 19.4 nM), surpassing EPA drinking water standards. It shows an evident 1 : 1 binding stoichiometry (k _a = 0.97 × 10⁴ M^-1), validated through Job's plot and Benesi-Hildebrand analysis. Interestingly, the titration studies exhibit a sharp isosbestic point, confirming a clean and well-controlled interconversion between the free and complexed forms. UV-vis and ¹H NMR spectroscopies confirm the complexation through the carbohydrazide unit, which is supported by DFT and TD-DFT calculations. H1 retains its selectivity in the presence of competing metal ions, remains effective over a broad pH range, and shows reversible chelation with EDTA, facilitating recyclability. Furthermore, paper-strip and silica-gel-based solid-state formats provide rapid on-site sensing. The development of an INHIBIT logic gate and a restorable "write-read-erase-read" memory element demonstrates its multifunctional capability. Importantly, real-sample studies (tap water, river water, fruit juice and tea) validate its performance in real-world samples. With its ultrafast response, high sensitivity, multifunctional capability, reliability and real-world applicability, H1 emerges as a highly effective, economical, and field-ready platform for practical Ni²⁺ monitoring in environmental and analytical settings.