Reproducible Spectroscopy Research Articles

Optimizing silver‐capped silicon nanopillars to simultaneously realize macroscopic, practical‐level SERS signal reproducibility and high enhancement at low costs

AbstractThe ideal surface‐enhanced Raman spectroscopy (SERS) substrate should fulfil the following: (a) predictable SERS enhancement, (b) macroscale SERS signal uniformity, and (c) suitability for mass production at low costs. Macroscale SERS uniformity and reproducibility at practical levels are big obstacles, which have been preventing most SERS substrates from reliable sensing applications. We have previously shown that SERS‐active nanopillar structures, fabricated by lithography‐free processes, exhibit high average SERS enhancements and are mass producible. Here, we report an optimized process and show that the improved structures exhibit unrivalled macroscale SERS uniformities (RSD: ∼2.5% in millimeter scale, ∼7% in wafer scale) and reproducibility (RSD: ∼1.5% across 3 wafers), while at the same time exhibiting a very large average SERS enhancement factor of >108. The obtained SERS uniformity (~2.5% RSD in millimeter scale) is the best to date measured on large‐area solid SERS substrates. Fast and reproducible SERS analysis of trans‐1,2‐bis (4‐pyridyl) ethylene down to 4 × 10−13 mol is demonstrated using the optimized structures. We emphasize that achieving simultaneously macroscopic, practical‐level SERS signal reproducibility and high enhancement via a lithography‐free process is a notable advance towards industrialization of substrate‐based SERS sensors.

Silver Nanodesert Rose as a Substrate for Surface-Enhanced Raman Spectroscopy

Silver galvanic displacement on silicon has been employed to produce large-area reproducible substrates, with morphology similar to that of the natural desert rose but on the micrometer scale. The process is based on an extremely simple wet chemistry approach using only AgF and KF, as silver and fluoride sources. A key element is the absence of HF in the deposition solution, which has been commonly used in previous silver galvanic displacement processes. The new process affords a higher degree of control in the redox reaction than those reported previously. The structures formed in this manner possess a large area-to-volume ratio with a high density of rough silver flakes uniformly distributed across the substrate. The silver morphology on the nanometer scale is shown to provide an excellent platform for surface-enhanced Raman spectroscopy (SERS), yielding detection levels for trans-1,2-bis(4-pyridyl)ethylene, 4-mercaptopyridine, and Rhodamine 6G in solution down to ppb, ppt, and ppq limits, respectively. The SERS reproducibility on the substrate was verified by monitoring the signal intensity variations across the sample. The simplicity of the substrate fabrication process, as well as the excellent uniformity, opens up opportunities for the quantitative and in-field chemical trace analysis using these substrates.