A turn-on bis-hydrazone fluorescent chemosensor for selective Cd2+ detection: synthesis, structural insights, and theoretical validation

Abstract

Heavy metal pollution, particularly from cadmium(II) ions (Cd²⁺), causes severe environmental and health risks due to its acute toxicity, carcinogenicity, and bioaccumulation, leading to kidney damage, neurological disorders, and other physiological issues. Herein, we report the one-pot synthesis of a bis-hydrazone-based fluorescent probe L·2H2O (1) for selective detection of Cd²⁺. Probe 1 was derived from isophthalaldehyde and 3-pyridylcarbonyl hydrazine and single-crystal X-ray diffraction discloses a well-defined binding pocket with pyridyl, imine, and carbonyl donor sites suitable for Cd²⁺ coordination. Probe 1 exhibits weak emission in CH₃CN/HEPES buffer (9 : 1, v/v, pH 7.4) due to photoinduced electron transfer (PET) and unrestricted intramolecular rotations. Upon selective binding to Cd²⁺, 1 displays a pronounced turn-on fluorescence response with intensity enhancements of at ∼324 nm and at 420 nm, accompanied by bathochromic shifts to 327 nm (Δλ = 3 nm) and 445 nm (Δλ = 25 nm) (λ_ex = 295 nm). The limit of detection (LOD) for probe 1 with metal Cd²⁺ is 3.39 µM, with a binding constant of 5 × 10³ M⁻¹. ¹H NMR titration, DFT-optimized geometries (B3LYP/6-31+G(d)/LANL2DZ), and simulated UV-Vis spectra further confirm binding of Cd²⁺, blocking PET and rigidifying the structure via chelation-enhanced fluorescence (CHEF). This work presents a modular hydrazone scaffold for developing selective Cd²⁺ sensors with potential application in environmental and biological monitoring.