A Rare Natural Benzo[k,l]xanthene as a Turn-Off Fluorescent Sensor for Cu²⁺ Ion

Rapid and efficient analyses of copper ions are crucial to providing key information for Cu²⁺ in living cells because of their biological importance. In this study, we reported one new turn-off fluorescent sensor for Cu²⁺ with a benzo[k,l]xanthene core, which served as an efficient cation sensor for copper ion over a wide range of other cations (Na⁺, K⁺, Ag⁺, Hg²⁺, Cd²⁺, Co²⁺, Ni²⁺, Zn²⁺, Mg²⁺, and Fe³⁺) owing to the catechol group in the aromatic core. The sensor showed selectivity for Cu²⁺ over other ions; the logK_β for Cu²⁺ binding to compound 1 had a value of 13.265. In the presence of Cu²⁺, sensor 1 provided significant fluorescence decrement; Co²⁺, and Ni²⁺ caused a fluorescence decrement when employed at a higher concentration than Cu²⁺, while Na⁺, K⁺, Hg²⁺, Cd²⁺, Zn²⁺, and Mg²⁺ metal ions produced only minor changes in fluorescence intensity. Fluorescence experiments demonstrate that compound 1 may have an application as a fluorescent probe for detecting Cu²⁺ with a limit of detection of 0.574 µM.