Modulating Squaraine Fluorescence via Hydrogen Bonding: A Selective Turn-Off Sensor for Thioglycolic Acid

Abstract Thioglycolic acid (TGA), a common ingredient in hair removal creams and other industrial formulations, owing to its hazardous nature, necessitates sensitive and selective analytical detection. Herein, we report a symmetrical squaraine-based fluorescent probe for the selective detection of TGA, investigated employing fluorescence spectroscopy, NMR, and density functional theory (DFT) calculations. The probe upon interaction with TGA, undergoes pronounced fluorescence quenching arising from hydrogen-bonding interaction, which perturbs the intramolecular charge-transfer (ICT) excited state. The probe exhibits a linear response over the 23.8–77.4 nM concentration range with a detection limit of approximately 4 nM and demonstrates excellent selectivity toward TGA. Practical applicability was validated by the quantification of TGA in commercial hair removal cream samples. In addition, the probe functions as a molecular logic gate based on its fluorescence response, highlighting its potential for intelligent molecular sensing.