Site-Directed Functionalization of MXene/GO with Cysteine for Dual-Metal Ion Detection: A Plug-and-Play Paper Electrodes

Mercury and cadmium contamination pose serious environmental and public-health risks, requiring rapid and selective detection at trace levels. In this work, a site-directed functionalization strategy is developed using a cysteine-modified MXene-graphene oxide composite (MX-GO-C) for dual metal-ion sensing. The intrinsic chemical asymmetry of cysteine enables controlled orientation of active sites: coordination of the amine (−NH) group with MXene leaves the thiol (−SH) group available for selective Hg2+ binding, while anchoring through the thiol group on graphene oxide exposes the amine site for Cd2+ recognition. This deliberate molecular arrangement allows metal-specific interactions within a single sensing platform. The successful functionalization and selective metal binding were confirmed by FT-IR, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and field-emission scanning electron microscopy (FESEM). Electrochemical studies using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) reveal excellent sensitivity and selectivity toward Hg2+ and Cd2+, with well-resolved oxidation signals. For practical applicability, a plug-and-play paper-based microfluidic analytical device (μPAD) was fabricated using the MX-GO-C composite, enabling rapid, low-cost, and portable on-site detection. Both glassy carbon electrode and μPAD platforms achieve nanomolar-level detection limits, highlighting the robustness and versatility of the sensing system for environmental monitoring applications.