Zinc(II) Ion Sensing in Aqueous Micellar Solution Using Modified Bipyridine‐Based “Turn‐On” Fluorescent Probes and its Application in Bioimaging

Modified bipyridine-based constructs 4-(pyridine-2-yl)-3H-pyrrolo [2,3-c] quinoline (PPQ), (6-(3H-pyrrolo [2,3-c]quinolin-4-yl)pyridin-2- yl)methanol (PPQ-OH) and their assemblies with surfactants to function as turn-on fluorescent sensors for Zn2+ in aqueous solution are evaluated. Earlier, we have shown fluorescence-based selective ratiometric detection of Zn2+ by PPQ in DMF [RSC Adv. 2014, 4, 25605]. This work attempts to overcome the water solubility problem of the hydrophobic PPQ and PPQ-OH, by using micelles. While, the ligands show selective sensing behavior for Zn2+ over important biological cations including Na+, K+, Ca2+, Mg2+ in anionic sodium dodecyl sulphate (SDS) and non-ionic Tween 80 micelles, no Zn2+ sensing is observed in cationic cetyltrimethylammonium bromide (CTAB) micelles. Unlike in DMF case, Cd2+ interference is observed in aqueous condition, which can be avoided either by performing the study at pH  9 or by carrying out time resolved fluorescence study. By analysing the jobs plot data, fluorescence lifetimes, experiments with varying micellar shape and pH, we confirm that coordination volume of the octahedral metal complex and five-membered chelate ring formation are critical factors for Cd2+ interference. The described sensing systems, are capable of detecting Zn2+ at micromolar level. Additionally, it is shown that PPQ and PPQ-OH can be used to detect Zn2+ in HeLa cells at physiological conditions, by bioimaging studies.