A Naphthoquinoline-Dione-Based Cu²⁺ Sensing Probe with Visible Color Change and Fluorescence Quenching in an Aqueous Organic Solution

Copper metal ions (Cu²⁺) are widely used in various industries, and their salts are used as supplementary components in agriculture and medicine. As this metal ion is associated with various health issues, it is necessary to detect and monitor it in environmental and biological samples. In the present report, we synthesized a naphthoquinoline-dione-based probe 1 containing three ester groups to investigate its ability to detect metal ions in an aqueous solution. Among various metal ions, probe 1 showed a vivid color change from yellow to colorless in the presence of Cu²⁺, as observed by the naked eye. The ratiometric method using the absorbance ratio (A₄₁₃/A₄₇₆) resulted in a limit of detection (LOD) of 1 µM for Cu²⁺. In addition, the intense yellow-green fluorescence was quenched upon the addition of Cu²⁺, resulting in a calculated LOD of 5 nM. Thus, probe 1 has the potential for dual response toward Cu²⁺ detection through color change and fluorescence quenching. ¹H-NMR investigation and density functional theory (DFT) calculations indicate 1:1 binding of the metal ion to the small cavity of the probe comprising four functional groups: the carbonyl group of the amide (O), the amino group (N), and two t-butyl ester groups (O). When adsorbed onto various solid surfaces, such as cotton, silica, and filter paper, the probe showed effective detection of Cu²⁺ via fluorescence quenching. Probe 1 was also useful for Cu²⁺ sensing in environmental samples (sea and drain water) and biological samples (live HeLa cells).