Naked-eye detection of Pd2+ ion using a highly selective fluorescent heterocyclic probe by turn-off response and in-vitro live cell imaging

The heterocyclic probe 3-(1-isoquinolinyl)imidazo[5, 1-a] isoquinoline (IQ) in CH3OH/HEPES buffer system (5 mM, pH = 7.4, 6:4, v/v), exhibits blue fluorescence (lambda(em) = 454 nm) upon excitation with 359 nm light. Upon adding Pd2+ ion solution, IQ shows turn-off response when observed under long UV light. Using IQ bivalent palladium can be detected with a low limit of detection (0.210 mu M) within the pH range of 2-8. In the presence of large varieties of metal ion, colorimetric and fluorescence responses did not exhibit any perceptible change. A small change in fluorescence life-time in TRPL study indicates the static nature of quenching. Binding of probe IQ with Pd2+ ion can be reversed using Na(2)EDTA. Job's plot and ESI mass spectrometric analysis reveal a 1:1 ratio for binding of IQ with Pd(II) ion. DFT/TDDFT calculations performed on [Pd(IQ)Cl(CH3CN)](+) ion support the experimental findings. Cytotoxic studies showed that probe IQ suppressed the growth of L929 and Hela cells in a dose-dependent manner. Fluorescence imaging in the absence and presence of Pd2+ revealed that IQ can be successfully applied on living cells with outstanding sensitivity.