An 8-aminoquinoline derivative as a molecular platform for fluorescent sensors for Zn(II) and Cd(II) ions

The amide derivatives of 8-aminoquinoline with two types of amino acid pendants, i.e. glycine and β-alanine, are evaluated for fluorescence sensing of various metal ions. In Tris-HCl aqueous buffer solution, the derivative containing glycine exhibits selective fluorescence enhancement with Zn2+. The fluorescence turn-on signal at a longer wavelength is a result of the binding between N-8-aminoquinolinylglycinamide and Zn2+ which is promoted by the deprotonation of the amide proton. However, both Zn2+ and Cd2+ induce the turn-on signal in ethanol solution. X-ray crystallography of single crystals of both Zn2+ and Cd2+ complexes formed in ethanol reveals 1-D coordination polymer structures in which the metal ions are tethered with N and O of the amide groups. Simultaneous detection of Zn2+ and Cd2+ are possible by using paper chromatographic separation and the ligand as a visualizing agent. The fluorescence imaging of either Zn2+ or Cd2+ in plant tissue is demonstrated. The conjugation of the ligand with dimethylaminophenylacetylene effectively tunes the emission color of the ligand while retaining the sensing selectivity. Therefore, N-8-aminoquinolinylglycinamide is a simple yet effective binding probe for Zn2+ and Cd2+ with excellent fluorescence signal transduction.