A Selective, Nontoxic, OFF–ON Fluorescent Molecular Sensor Based on 8-Hydroxyquinoline for Probing Cd2+ in Living Cells

In spite of the fact that cadmium( II) has been recognized as a highly toxic element and that excessive exposure to this metal ion has been reported to have many adverse effects on human health, very few selective and specific fluorescent probes are available for imaging Cd2+ in living cells. Herein, we report the spectroscopic and photochemical characterization of 5-(5-chloro-8-hydroxyquinolinylmethyl)- 2,8-dithia-5-aza-2,6-pyridinophane (L) as a fluorescent sensor for the selective imaging of Cd2+ in living cells. In particular, the response of L to Cd2+ was first assessed in aqueous solutions, sodium dodecyl sulfate micelles, and liposomes, and subsequently in living cells by fluorescence microscopy techniques. Cytofluorimetric analyses of leukemic HL-60 cells loaded with L also allowed evaluation of the toxicity of the probe and the selective analysis of its intracellular fluorescence in the presence of Cd2+. Furthermore, the 1:1 complex species [Cd(L)H2O]2+ responsible for the OFF–ON chelation enhancement of fluorescence (CHEF) effect on L was structurally characterized; time-dependent DFT calculations allowed the prediction of theoretical excitations, which were comparable with the experimental ones.