Herein, we report a naturally occurring flavonol, quercetagetin 5,6,7,3 ',4 '-pentamethyl ether (known as marionol), as a nontoxic fluorescent probe for rapid, sensitive, and selective detection of Cu2+ in water. The interaction between marionol and Cu2+ ions is studied by various state-of-the-art spectroscopic techniques such as H-1 NMR, UV-vis, fluorescence, and high-resolution mass spectrometry (HRMS). Marionol shows strong fluorescence quenching in the presence of Cu2+, whereas other interfering cations such as Fe3+, Co2+, Al3+, Pb2+, Hg2+, Zn2+, Mg2+, Mn2+, Ca2+, Cu+, Ag+, Na+, and K+ produce negligible changes in fluorescence. The binding stoichiometry between marionol and Cu2+ is found to be 2:1, according to Job's plot and HRMS analyses. The binding constant (K-a) of Cu2+ with marionol is calculated to be 2.89 x 10(9) M-2, and the limit of detection is found to be 0.15 mu M. In addition, marionol is further used for the quantification of Cu2+ in natural spring water samples. The good recoveries (92-110% with RSD <5%) and its low toxicity to MCF-7 human breast cancer cells make marionol a unique plant-based fluorescent probe for the detection of Cu2+ ions.
