Flexible Plastic Electrodes Based on Thiosulfate-Modified gC₃N₄ Nanosheets for Simultaneous Handheld Monitoring of Hazardous Cationic Dyes in Drinking Water

The critical concerns regarding hazardous organic dyes, which cause significant harm to human health and environmental sustainability, require immediate attention. This study demonstrates the development of an innovative disposable electrochemical sensor that employs thiosulfate-modified graphitic carbon nitride (gC3N4-S2O3) nanosheets as class-selective and cost-efficient flexible plastic electrodes for the remote, handheld monitoring of cationic dyes─a class of perilous water-soluble organic pollutants. This sensor is capable of detecting several cationic dyes, such as methylene blue (MB), rhodamine B (RhB), and thionine (Th), simultaneously in drinking water. Modified gC3N4-S2O3 nanosheets were produced using a straightforward sonochemical-assisted method, followed by thermal condensation treatment. The produced material is characterized by Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and differential pulse voltammetry (DPV). The DPV measurements span a wide range of dye concentrations, ranging from 1 nM to 1 mM. Under optimized conditions, the electrochemical gC3N4-S2O3 sensor exhibits excellent performance, with picomolar limits of detection (LOD), i.e., 118 pM for Th, 161 pM for RhB, and 209 pM for MB, respectively. The gC3N4-S2O3 sensor utilizes electrostatic interactions to distinguish between cationic and anionic dyes and is capable of simultaneously detecting several cationic dyes. The gC3N4-S2O3 sensor also demonstrates enhanced capability to detect several cationic dyes in solutions, complex mixtures, and drinking water samples, indicating minimal interference from ionic species inherently present in drinking water. This work highlights the potential of gC3N4-S2O3 sensors for application in monitoring and remediation of the environment.