Rational design of aromatic hydrazones for fluorescent Zn2⁺ sensing and colorimetric detection of Zn2⁺ and Cu2⁺ ions

Zinc (Zn2⁺) and copper (Cu2⁺) are the second and third most abundant transition metals in the human body, and their dysregulation is associated with various pathological conditions, necessitating sensitive and selective detection strategies. Fluorescence-based sensing is particularly attractive due to its high sensitivity, simplicity, and biocompatibility. Herein, we report a series of hydrazone-linked Schiff base chemosensors incorporating pyrene (L1), anthracene (L2), and triphenylamine (L3) moieties. These probes exhibit distinct colorimetric responses toward Zn2⁺/Cu2⁺ and a selective fluorescence “turn-on” response toward Zn2⁺ in DMSO–HEPES buffer (1:1, v/v, pH 7.4), with negligible interference from competing metal ions. The fluorescence enhancement originates from Zn2⁺-induced chelation-enhanced fluorescence (CHEF) accompanied by suppression of PET and C = N isomerization. Job’s plot and spectroscopic analyses indicate a 2:1 ligand-to-metal binding stoichiometry, corroborated by HRMS, 1HNMR titrations and DFT calculations. The sensors display moderate to strong binding affinities (Ka = 3.5–25.8 × 105 M⁻1) and micromolar (µM) detection limits for Zn2⁺ and Cu2+ ions. Combined spectroscopic and computational studies establish clear structure–property relationships, while reversibility experiments confirm the robustness and reusability of these probes.