A coumarin based visual and fluorometric probe for selective detection of Al(III), Cr(III) and Fe(III) ions through “turn-on” response and its biological application

A Schiff base (L) containing coumarin and pyrene moieties was synthesized and characterized. It showed excellent selective recognition of M(III) ions (M = Al, Cr and Fe) in presence of monovalent, divalent and other trivalent metal ions. Sensor L itself is non-fluorescent upon excitation with 353 nm light (λex = 353) but a very large enhancement in fluorescence intensity was observed during the addition of these three trivalent metal ions into a CH3OH/aqueous HEPES buffer (5 mM, pH = 7.4, 6:4, v/v) solution of L. This prominent “OFF-ON” fluorescence switching can be observed with naked eye, in which “OFF” is due to operation of PET process in free L but PET is inhibited upon complexation thereby allowing CHEF (chelation enhanced fluorescence) process to operate. Detection limits of Al3+, Cr3+ and Fe3+ using L have been found to be 0.79 nM, 1.15 μM, and 1.28 μM respectively in the pH range of 6‒10. Additionally, “OFF-ON” fluorescence can be reversed by sequestering M(III) ions using [EDTA]4– solutions. Analysis of mass spectra and Job’s plot revealed a binding ratio of 1:1 for all three metal ions with L. The imine nitrogen and carbonyl oxygen atoms of L are involved in binding with Al3+, Cr3+ and Fe3+ ions, as revealed by NMR titration and IR spectroscopic analysis. Results of quantum yield (Φ), time resolved photoluminescence (TRPL) and computational studies (DFT/TDDFT) studies were also reported. Fluorescence intensity of these M(III) bound L complexes is quenched by fluoride ions. From cytotoxicity analysis, 7.5 μM of probe L was selected to analyze its fluorescence attributes in cellular imaging. Using human breast cancer cell line (MDA-MB-231) and primary human dermal fibroblasts (HDF), the potentiality of the probe L was confirmed through fluorescence cell imaging experiments.