Lithography-free fabrication of scalable 3D nanopillars as ultrasensitive SERS substrates

Surface-enhanced Raman spectroscopy (SERS) detection of analyte molecules at ultra-low concentrations requires highly-efficient plasmonic nanostructures enabling a high hot-spot density. However, a facile and cost-effective strategy toward large-area fabrication of efficient nanostructures with significant electromagnetic field enhancement remains a great challenge. Further, SERS faces reliability issues with the molecular fingerprint at ultra-low concentrations. This work shows a one-step rapid fabrication technique utilizing glancing angle deposition for growing 3D nanopillars of Ag or Au, which is facile, scalable and cost-effective. The 3D nanopillar substrates can reliably detect analyte molecules with concentrations as low as 10^-18 M with a high signal-to-noise ratio molecular fingerprint proven for Cresyl violet, p-aminothiophenol and Rhodamine 6G. The ultra-high enhancement is realized in conjunction with the formation of a high hot-spot density due to localized surface plasmons and surface plasmons at metal/air interface. A portable handheld Raman spectrometer is used to evaluate the potential application of the nanopillars for on-site diagnostics. It avoids the need for sophisticated tabletop instruments yet provides high-precision molecular specificity outside specialized laboratories. The 3D nanopillar substrates show excellent molecular detection limits at 10⁻¹⁵ M concentrations when tested with a handheld Raman spectrometer. The uniqueness of the 3D nanopillar features with the formation of a high density of hot-spots and one-step nanofabrication methods provide a platform to unravel on-site diagnostics with cost-effective approaches.