Interfacial Engineering of Dual-Layer Au-Shelled Ag Nanowires: Decoupling Chemical Stability From Ultrasensitive Surface-Enhanced Raman Spectroscopy Activity.

One-dimensional (1D) silver nanowires are promising surface-enhanced Raman spectroscopy (SERS) substrates, yet their plasmonic hotspots are spatially confined to sharp tips or junctions, leaving the extensive body optically inactive. Additionally, realizing high-performance Ag-based substrates faces a fundamental trade-off between chemical stability and surface activity. To simultaneously resolve these geometric and chemical limitations, we present a stepwise interfacial engineering strategy to synthesize 3D hierarchical dual-layer porous@smooth Au-shelled Ag nanowires. This architecture decouples functional requirements: A hermetic smooth Au inner shell passivates the Ag core for long-term stability, while the nanowire body serves as a structural scaffold for a porous Au outer shell. This design activates the entire 1D surface, creating a volumetric distribution of high-density hotspots localized at the interparticle gaps. Consequently, the hierarchical nanowires exhibit exceptional sensitivity for model analytes, including malachite green, crystal violet, and methylene blue, with detection limits ranging from 10-11 down to 10-14 M. Notably, for malachite green the sensitivity approaches the single-molecule regime. Finally, leveraging their high aspect ratio and mechanical flexibility, the nanowires demonstrate versatile utility in two distinct applications: enabling both stretchable SERS substrates for conformal sensing and colloidal dispersions for the rapid "mix-and-measure" fingerprinting of microplastics (PE, PP, PS, and PET).