A disposable dual-mode electrochemical/colorimetric paper-based analytical device for simultaneous detection of...

In this work, we developed a paper-based analytical device (PAD) for the simultaneous detection of hydroquinone (HQ) and mercury (Hg²⁺) in skin whitening products. The device integrates electrochemical analysis and colorimetric detection (E/C-PAD). HQ was determined electrochemically using a bismuth oxide-graphene ink electrode screen-printed on the paper (Bi₂O₃-GI/PAD). Hg²⁺ was detected by colorimetry using a gel of Thio Michler's Ketone and carboxymethylcellulose coated on paper (TMK-CMC gel/PAD). The morphology and structure of the E/C-PAD were characterized by scanning electron microscopy, energy dispersive X-ray analysis, and Fourier transform infra-red spectroscopy. The electrochemical properties of Bi₂O₃-GI/PAD were investigated, and HQ was quantified by differential pulse voltammetry. The colorimetric determination of Hg²⁺ was based on the value of the red channel obtained from processing digital images of reaction products. Once optimized, the Bi₂O₃-GI/PAD determined HQ concentrations from 1.0 to 20 microg mL^-1 and 20 to 100 microg mL^-1, with an LOD of 0.22 microg mL^-1 and an LOQ of 0.74 microg mL^-1. The TMK-CMC gel/PAD detected Hg²⁺ from 0.05 to 0.50 microg mL^-1 and 0.50 to 40 microg mL^-1, with an LOD of 0.01 microg mL^-1 and an LOQ of 0.04 microg mL^-1. The developed E/C-PAD demonstrated good reproducibility and stability, outstanding anti-interference capability, and high precision for the simultaneous detection of HQ and Hg²⁺ in real whitening products, showing results consistent with spectrophotometry. Moreover, this paper-based device is the first to combine electrochemical analysis with colorimetry for the simultaneous detection of HQ and Hg²⁺.