Salicylidene-based dual-responsive ‘turn on’ fluorometric chemosensors for the selective detection of Zn2+, Al3+ and F− ions: theoretical investigation and applications in the live cell imaging of zebrafish larvae and molecular logic gate operation

Abstract

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 Pbcn and monoclinic P2₁/n 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.