Ordered Hierarchical Ag Nanostructures as Surface-Enhanced Raman Scattering Platforms for (Bio)chemical Sensing and Pollutant Monitoring

Surface-enhanced Raman scattering (SERS), due to its extreme sensitivity down to the single molecule level, has been a superior analytical technique for (bio)chemical sensing and monitoring. However, the reliability of SERS has been a major issue since its discovery and has not yet been fully addressed, which is largely due to the lack of properly designed SERS substrates and facile, cost-effective fabrication strategies of metallic nanostructures. Herein, we employ hierarchical metallic nanopatterns as active SERS substrates, which can be facilely fabricated by superplastic nanomolding of bulk metals with hierarchical anodic aluminum oxide templates. The SERS signals of such hierarchical metal substrates show both high sensitivity (SERS intensity of up to 10(6) with an enhancement factor of similar to 7.0 X 10(5)) and reproducibility (relative standard deviation as low as 7%) with an optimized configuration, which significantly outperforms that without hierarchical nanostructures. Our results reveal that the microcavities of the hierarchical molds can significantly reduce the nonuniform plastic deformation of the bulk metals caused by surface roughness, while the nanoholes in the microcavities yield densely packed nanopillars with exceptional SERS signals. These hierarchical metallic nanostructures are further applied for the detection of biomolecules and organic dyes with concentrations as low as 10(-10) M, whose SERS intensity still presents a prominent signal-to-noise ratio. Such facile and scalable fabrication of hierarchical metallic nanostructures not only sheds light on the key factors that affect the uniformity and sensitivity of SERS but also provides guidelines for rational design of superior SERS substrates with outstanding performances, which is of great implication for (bio)chemical sensing and pollutant monitoring.