Tuning of metal ion selectivity by varying number of coordinating atoms in quinoline-derived fluorescent sensors

Abstract

Both Cd2+ and Zn2+ are metal ions in group 12 elements with very different toxicities but similar binding properties to most organic and biological ligands. In this work, a series of quinoline derivatives were synthesized and studied with an aim for Cd2+ selective turn-on fluorescent sensors. The structures of the compounds were designed to systematically evaluate the effects of amino protons, number of N-coordinating atoms, quinoline/picoline moieties and substituents on the sensing properties. The association constants of all synthesized ligands were studied by UV-vis absorption titration in aqueous solution. The results revealed that the ligands having 6- and 4- donor atoms and absent of amino proton gave higher association constants for Cd2+ and Zn2+, respectively. L6, a ligand with 2 quinolines/2 picolines and 6 N atoms without amino proton, exhibited the highest Cd2+ sensitivity providing 118-fold of fluorescence enhancement (I/I0) at 480 nm and highest Cd2+/Zn2+ selectivity giving the Cd2+/Zn2+ association constant ratio of 32. L5, a tetradentate ligand containing 1 quinoline/2 picolines without amino protons showed highest association constant for Zn2+. The greater number of the quinoline units led to lower binding affinity and lower I/I0. Fluorescence quenching was even observed for ligands having solely 3 and 4 quinoline units (L12 and L13). Electron withdrawing substituents gave higher I/I0 but lower association constants. L6 was thus selected for further optimization as Cd2+ fluorescent sensor. The selectivity of Cd2+ detection by L6 was significantly improved by an addition of tripicolylamine (TPA), a non-fluorescent tetradentate ligand, that provided the limit of detection Cd2+ of 69 nM. In the fluorescence cell imaging, L6 could monitor Cd2+ in the concentration range of 0.1–10 µM.