Facile electrostatic self-assembly of Ag NWs/GO/Ag NCs sandwich structure on paper for ultrasensitive detection of trace agricultural pollutants

The engineering of multifunctional plasmonic interfaces on flexible substrates is paramount for advanced molecular sensing. In this study, a high-performance, hierarchical bimetallic SERS substrate, featuring an Ag NWs/PDDA/GO/Ag NCs sandwich architecture, was fabricated on cellulose filter paper via a facile electrostatic self-assembly strategy. By employing poly(diallyl dimethylammonium chloride) (PDDA) as a molecular linker to regulate interfacial charges, we achieved the sequential, high-density loading of 1D silver nanowires (Ag NWs), 2D graphene oxide (GO), and 0D silver nano cubes (Ag NCs).Physicochemical characterizations revealed that the ultrathin GO layer functions as a dielectric nano spacer, which effectively facilitates vertical plasmonic coupling between the bimetallic components to generate high-density 3D “hotspots” while simultaneously serving as a passivation barrier against metallic oxidation. The synergy of electromagnetic enhancement (EM) from the hierarchical sandwich structure and chemical enhancement (CM) via interfacial charge transfer resulted in an exceptional enhancement factor of 2.82 × 10 6 .The engineered substrate enabled the ultra-sensitive detection of paraquat with a limit of detection (LOD) of 10 -10 M. Furthermore, it demonstrated remarkable matrix-robustness for the in situ analysis of pesticide residues in untreated tea broth, maintaining high signal reproducibility (RSD < 7%). This work provides a versatile interfacial engineering strategy for the controllable assembly of heterogeneous nanostructures, paving the way for high-performance, field-deployable surface-sensitive applications in food safety and environmental monitoring.