Naturally Occurring Flavonol, Quercetagetin 5,6,7,3′,4′-Pentamethyl Ether (Marionol), as a Nontoxic Plant-Based Fluorescent Probe for Rapid, Sensitive, and Selective Detection of Cu²⁺ in Water

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 ¹H 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 (<i>K</i> _a) of Cu²⁺ with marionol is calculated to be 2.89 × 10⁹ M^-2, and the limit of detection is found to be 0.15 μM. 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.