Interface-engineered hydrogel enables hotspots densification and stable thiram capture for synergistic SERS...

Surface-enhanced Raman scattering (SERS) has emerged as a powerful technique for the rapid and sensitive detection of pesticide residues, particularly targeted pesticides of concern in food safety. However, hotspot intensification strategies in hydrogel-based SERS substrates that rely on solvent-exchange-induced shrinkage often suffer from concomitant analyte loss, thereby limiting detection reliability. Herein, an interface-engineered Ag-PAM-GO hydrogel was developed by integrating graphene oxide (GO) into a polyacrylamide (PAM) network to simultaneously achieve structural regulation and interfacial interactions. The incorporation of GO tailors the pore architecture and promotes coordination-driven Ag⁺ enrichment together with confined in situ nanoparticle growth, enabling the uniform generation of plasmonic hotspots. Meanwhile, solvent-exchange-induced volumetric shrinkage effectively increases hotspot density, leading to an enhanced electromagnetic (EM) effect. Focusing on thiram, a widely used dithiocarbamate fungicide, the proposed substrate achieves ultrasensitive detection with a limit of detection of 7.12 × 10^-4 mg/L, while maintaining excellent signal uniformity and reproducibility. Interfacial Lewis acid-base interactions and hydrogen bonding cooperatively drive the preferential retention and local enrichment of thiram molecules during solvent exchange. Density functional theory calculations further revealed pronounced charge-transfer (CT) characteristics, confirming the contribution of the CT mechanism, while the increased adsorption energy supported strengthened molecule-substrate interactions. Benefiting from the synergistic interplay between the EM and CT mechanisms, the developed platform enables the reliable detection of thiram residues in fruit juices as well as on curved fruit surfaces, providing an effective strategy for the rational design of high-performance flexible SERS substrates for selective thiram residue detection.