Naturally Occurring\nFlavonol, Quercetagetin 5,6,7,3′,4′-Pentamethyl\nEther (Marionol), as a Nontoxic Plant-Based Fluorescent Probe for\nRapid, Sensitive, and Selective Detection of Cu<sup>2+</sup> in Water

Herein, we report a naturally occurring flavonol, quercetagetin\n5,6,7,3′,4′-pentamethyl ether (known as marionol), as\na nontoxic fluorescent probe for rapid, sensitive, and selective detection\nof Cu<sup>2+</sup> in water. The interaction between marionol and\nCu<sup>2+</sup> ions is studied by various state-of-the-art spectroscopic\ntechniques such as <sup>1</sup>H NMR, UV–vis, fluorescence,\nand high-resolution mass spectrometry (HRMS). Marionol shows strong\nfluorescence quenching in the presence of Cu<sup>2+</sup>, whereas\nother interfering cations such as Fe<sup>3+</sup>, Co<sup>2+</sup>, Al<sup>3+</sup>, Pb<sup>2+</sup>, Hg<sup>2+</sup>, Zn<sup>2+</sup>, Mg<sup>2+</sup>, Mn<sup>2+</sup>, Ca<sup>2+</sup>, Cu<sup>+</sup>, Ag<sup>+</sup>, Na<sup>+</sup>, and K<sup>+</sup> produce negligible\nchanges in fluorescence. The binding stoichiometry between marionol\nand Cu<sup>2+</sup> is found to be 2:1, according to Job’s\nplot and HRMS analyses. The binding constant (<i>K</i><sub>a</sub>) of Cu<sup>2+</sup> with marionol is calculated to be 2.89\n× 10<sup>9</sup> M<sup>–2</sup>, and the limit of detection\nis found to be 0.15 μM. In addition, marionol is further used\nfor the quantification of Cu<sup>2+</sup> in natural spring water\nsamples. The good recoveries (92–110% with RSD <5%) and\nits low toxicity to MCF-7 human breast cancer cells make marionol\na unique plant-based fluorescent probe for the detection of Cu<sup>2+</sup> ions.