A Dual Turn-Off Colorimetric and Fluorescent Sensor for Trace Water in Alcohols Based on a Rhodamine B-Al³⁺ Complex

Rapid-response sensors are promising tools to detect trace water in organic solvents; however, accurate quantification in alcoholic solvents remains challenging due to competitive coordination from hydroxyl groups. Herein, a colorimetric/fluorescent dual-mode sensor based on rhodamine B-aluminum complex (RhB-Al³⁺) has been developed for the detection of trace water in methanol, ethanol, isopropanol, and 1-butanol. The RhB-Al³⁺ complex demonstrates rapid response (< 10 s), high sensitivity (detection limits [v/v]): 0.0134% (ethanol), 0.0074% (methanol), 0.0224% (isopropanol), and 0.0498% (1-butanol), and excellent specificity. In addition, the color of the RhB-Al³⁺ complex in alcohols solution changed from pink to colorless under visible light and from yellow to colorless under a 365-nm UV light, which enabled naked-eye detection of trace water in alcohol solvents. The sensing mechanism was investigated by ¹H-NMR, HR-MS, pH, and spectroscopy analysis. It was demonstrated to involve water molecules coordination to the Al³⁺ of the RhB-Al³⁺ complex, which leads to the dissociating of the complex and in turn promotes the reformation of the rhodamine spirolactam ring. Moreover, the RhB-Al³⁺ complex was successfully used for sensing trace water in commercial alcohol solvents, demonstrating its potential for industrial quality control applications.