Paper-Based Analytical Device Coated with a Fluorescent Polymer for the Visual Detection of Biothiols

The reducing capacity of biological and environmental aqueous samples is a key indicator of oxidative stress and redox balance. Thiol-containing compounds (biothiols) are essential in maintaining this balance within biological systems. In this study, we developed a paper-based analytical device (PAD) for the rapid and quantitative detection of biothiols in aqueous solutions and cell homogenates. The PAD uses a cysteine-specific fluorescent probe that generates a signal proportional to the biothiol concentration. Assay conditions and reaction kinetics were optimized to expand the dynamic range and lower the detection limit for cysteine (Cys). By incorporation of cystine, PAD enabled accurate quantification of other biothiols with high reproducibility and sensitivity. We validated the PAD using standard reducing agents and complex biological samples, demonstrating a strong correlation with the conventional spectroscopic method using 5,5'-dithiobis(2-nitrobenzoic acid). This cost-effective, portable, and user-friendly PAD offers a versatile solution for on-site and real-time redox monitoring, with potential applications in biomedical research, diagnostics, and environmental monitoring.