A simple Schiff base molecular logic gate for detection of Zn²⁺ in water and its bio-imaging application in plant system

Selective and sensitive detection of Zn²⁺ is important both in chemistry and biology. A water soluble simple Schiff base ligand, 2-[(2-hydroxyethylimino)methyl]]phenol (H₂SalEt) was synthesized by one step condensation reaction between salicylaldehyde and 2-amino-1-ethanol in ethanol medium which is demonstrated to have utility in selectively determining Zn²⁺ ions over other common metal ions in (100 mM HEPES buffer, pH 7.4) solution displaying a significant colour change yellow to blue under UV light. The binding of HSalEt-Zn²⁺ was tracked by elemental analysis, mass spectra analysis, FT-IR, ¹H and ¹³C NMR spectroscopy in CH₃OH-d₄. Single crystal X-ray diffraction studies reaffirm the binding modes of HSalEt-Zn²⁺ aggregate. The ligand (H₂SalEt) exhibits a weak fluorescence intensity (quantum yield, Φ = 0.054) and the presence of zinc ions 14 fold enhancement of fluorescence intensity (quantum yield, Φ = 0.769). H₂SalEt allows the detection of Zn²⁺ at 10^-7 (M) level. Addition of chelating agent EDTA to a solution containing HSalEt-Zn²⁺ complex leads to immediate quenching of fluorescent intensity, making the chemosensor H₂SalEt a reversible one. In aqueous solution, H₂SalEt induces a (2:1) complex formation with Zn²⁺ ions indicated by Job's plot analysis. 'OR' and 'Keypad lock' logic gates were created that respond to Zn²⁺ and Al³⁺ inputs with absorbance (359 nm) and fluorescence output (454 nm) signals. In order to further explore the sensing potential of H₂SalEt, two different kinds of cells, Azotobacter chroococcum and Candida albicans were used for in vitro investigation of the intracellular Zn²⁺ by H₂SalEt. Fluorescence imaging of translocation of Zn²⁺ through the transparent stem of rice seedling, Oryza sativa, proved in vivo bio imaging capability of H₂SalEt.