Modulating Affinities of Di-2-picolylamine (DPA)-Substituted Quinoline Sensors for Zinc Ions by Varying Pendant Ligands

Abstract

We have developed a series of di-2-picolylamine (DPA)-substituted quinoline sensors, HQ1–4, bearing a pendant ligand at the 8 position of quinoline. UV–vis spectra of HQ1–4 showed similar variations to that of HQ5 but with different varying extents upon the titration of zinc ions. Fluorescence intensities of HQ1, HQ3, and HQ4 were enhanced 4–6 times upon the addition of 1 equiv of zinc ions under an aqueous buffer. Somewhat unexpectedly, HQ2 is nonfluorescent in the presence of metal ions, including zinc ions. The affinities of HQ sensors are distributed in a broad range from nanomolarity to femtomolarity by varying the pendant ligands near the coordination unit. More importantly, these new sensors exhibited very high selectivity for Zn2+ over Na+, K+, Mg2+, and Ca2+ at the millimolar level and over other transition metal ions at the micromolar level, except for Cd2+. These findings indicated that the incorporations of the pendant groups exerted no effect on the spectroscopic properties and selectivity of the parent fluorescent sensor, with the exception of HQ2. Finally, X-ray crystal structures of ZnHQʼs revealed that the auxiliary pendant groups at the 8 position participated in zinc coordination and were able to tune the affinities of HQ sensors