Salicylidene-based dual-responsive ‘turn on’ fluorometric chemosensors for the selective detection of Zn ²⁺ , Al ³⁺ and F ⁻ ions: theoretical investigation and applications in the live cell imaging of zebrafish larvae

Four salicylidene-based dual-responsive chemosensors 1,5-bis(5-bromosalicylaldehyde)carbohydrazone (R1), 1,5-bis(5-bromosalicylaldehyde)thiocarbohydrazone (R2), 1,5-bis(3-ethoxysalicylaldehyde)carbohydrazone (R3) and 1,5-bis(3-ethoxysalicylaldehyde)thiocarbohydrazone (R4) were synthesized and characterized. The molecular structures of R1 and R3 were confirmed by single crystal X-ray diffraction technique, which crystallized in the orthorhombic <i>Pbcn</i> and monoclinic <i>P</i>2₁/<i>n</i> space groups, respectively. The chemosensor molecules were investigated for their recognition properties against the selected cations (K⁺, Ca²⁺, Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Fe³⁺ and Al³⁺) and anions (F^-, Cl^-, Br^-, I^-, HSO₄^-, H₂PO₄^-, ClO₄^-, N₃^- and NO₃^-) by colorimetry, absorption spectroscopy, fluorescence spectroscopy, ¹H NMR spectroscopy and theoretical studies. The sensor molecules showed colorimetric responses for the Co²⁺, Ni²⁺, Cu²⁺ and Fe³⁺ cations and the F^- anion. Interestingly, the Zn²⁺ and Al³⁺ cations showed only the 'turn on' fluorometric response, whereas the F^- anion showed both colorimetric and fluorometric responses. The binding constants were determined using the Benesi-Hildebrand (B-H) equation from the fluorescence titrations and found to be higher for R3 towards the Al³⁺ cation (2.03 × 10⁶ M^-1) with a low limit of detection (1.79 μM) and for R4 towards the F^- anion (5.13 × 10⁵ M^-1) with a low limit of detection (5.23 μM). The chemosensors established 1 : 2 and 1 : 1 binding stoichiometries with the sensed cations and anion, respectively, as confirmed by Job's plots. The computational studies show a lower band gap of HOMO-LUMO when the chemosensors bind with the sensed inorganic ions compared to the free chemosensors. Furthermore, the observed fluorescent behaviour of the Zn²⁺ and Al³⁺ cations have motivated us to investigate the practical applications in the live cell-imaging of zebrafish larvae as well as in the development of a molecular logic gate.