Sustainable mercury monitoring using graphitic carbon nitride as a 2D binding layer in diffusive gradient thin films

• GCN–agarose composite developed as a novel DGT binding layer. • Metal-free GCN provides selective Hg²⁺ adsorption through strong Hg–N coordination sites. • Composite exhibits >90% Hg²⁺ binding efficiency at near-neutral environmental pH. • Sustainable, non-toxic, and scalable material enables next-generation DGT mercury monitoring. Mercury (Hg) is a highly toxic and persistent environmental pollutant whose accurate monitoring remains challenging due to limitations in existing diffusive gradients in thin films (DGT) binding materials, including insufficient selectivity, complex synthesis, and sustainability concerns. In this study, we report the first application of graphitic carbon nitride (GCN) nanosheets as a metal-free and sustainable binding material for Hg²⁺ monitoring in DGT systems, addressing key limitations of current approaches. Few-layer GCN nanosheets were synthesized via thermal polymerization followed by protonation-assisted exfoliation and comprehensively characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), atomic force microscopy (AFM), Raman spectroscopy, and scanning electron microscopy (SEM). These analyses confirmed the formation of a structurally stable, nitrogen-rich framework with abundant coordination sites. The GCN–agarose (AG–GCN) composite binding layer demonstrated high mercury binding efficiency (>90%) at environmentally relevant concentrations (2.5–10 ng/mL) and near-neutral pH, attributed to strong coordination between Hg²⁺ ions and electron-donating nitrogen sites within the heptazine structure. Compared to conventional DGT binding phases, the proposed system offers enhanced sustainability, metal-free composition, and strong affinity toward mercury, highlighting its potential for next-generation passive environmental monitoring and advanced nanomaterial-based sensing platforms.