Metal ion fluorescent sensor from amide derivatives of 8-aminoquinoline

Abstract

The design and synthesis of fluorescent sensors for metal ions are interesting due to their high sensitivity, high selectivity and imaging capability desirable for analysis of biological and environmental systems. Herein, amide derivatives of 8-aminoquinoline with three types of amino acid pendants, i.e. glycine, β-alanine and glycylglycine, are evaluated as turn-on fluorecent sensors for metal ions to explore their effects on metal ion binding selectivity. In Tris-HCl aqueous buffer solution, only the derivative of 8-aminoquinoline containing glycine (1) exhibits selective fluorescence enhancement, a remarkable 24-fold increase of the fluorescence quantum yield, with Zn2+. The fluorescence enhancement is a result of the strong binding with Zn2+ (K = 8.03 × 105) which is promoted by the depronation of the amide proton as confirmed by 1H NMR, MS, absorption and emission spectroscopy. In ethanol and on filter paper, 1 exhibited dual fluorescence turn on signals and stronger binding to Zn2+ (K = 1.20 × 106) and Cd2+ (K = 4.61 × 105) that allows simultaneous detection of both metal ions using a simple paper chromatographic technique. Sensor 1 also allows fluorescence imaging to locate either Zn2+ or Cd2+ in plant tissue depending on either water or ethanol treatments. The results demonstrate that the simple structure of 1 is very effective for turn-on fluorescent sensors for Zn2+ and Cd2+ and it may be used as a core structure for further development of other effective metal sensing ligands.