Plasmon-Sampled Surface-Enhanced Raman Excitation Spectroscopy on Silver Immobilized Nanorod Assemblies and Optimization for Near Infrared (λ_ex = 1064 nm) Studies

Abstract

For many Surface-Enhanced Raman Spectroscopy (SERS) applications, the enhancing substrate must exhibit a number of critical properties that include low cost, robustness, and reproducibly high enhancement over large areas of the substrate. In this study we investigate the SERS fundamental enhancement factor of silver Immobilized Nanorod Assembly (AgINRA) substrates as a function of both the dielectric sphere diameter (310–780 nm) and the input laser wavelength (633–1064 nm) with a technique called plasmon-sampled surface-enhanced Raman excitation spectroscopy (PS-SERES). The nonresonant molecule benzenethiol was chosen as the probe molecule. Higher enhancement factors (EFs) were measured as the plasmon resonance and excitation wavelength’s relative separation were optimized and both moved toward the infrared region, ultimately eclipsing the 108 mark. This is the highest EF to date measured on this type of large-area substrate. The enhancement factors reported here are the result of efficient coupling between free space photons and the surface plasmon states in the metal INRA substrate. Coupled with their robustness and ease of fabrication, these results further underscore the value and versatility of metal INRA substrates in the field of surface-enhanced Raman spectroscopy.

Recent advances in plasmonic nanostructures for sensing: a review

“…Recently, many studies have focused on simple fabrication methods for the realization of high aspect ratio structures that maximize the electric edge effect. Such nanostructures have been fabricated on planar platforms (i.e., aligned nanorods), 85,66 on underlying high aspect ratio nanostructures (e.g., nanospheres, nanorods, and etched fiber bundles), 67,86 and as individual nanoparticles in solution (i.e., nanostars). 43,87,60 In the case of planar substrates, one of the simplest fabrication methods involves physical vapor deposition of aligned nanorods [see Fig.…”

“…67 One method recently developed for fabrication of such substrates results in the generation of immobilized silver nanorods assemblies (INRA) on polymeric spheres [see Fig. 5(c)].…”

“…5(c)]. 67 In this method, silver is deposited via physical vapor deposition on polymer nanospheres while being rapidly spun (∼550 rpm). Based on the particular deposition conditions employed (i.e., sphere size and rotation rate), an array of aligned silver nanorods is created.…”

“…By varying the size of the nanospheres used to make the underlying monolayer, the LSPR can be varied controllably to tune the SP absorption maximum. 67 Employing this method, silver substrates have been generated that can be tuned from 450 to 1100 nm (with nanospheres ranging in size from 320 and 790 nm) while having EF of 10 7 to 10 8 .…”

Optical Properties of Thin Gold Nanoshell Arrays and Their Application as NIR SERS Substrate

“…This huge Raman signal enhancement originates primarily from the largely enhanced local electric field near the surface of metallic nanostructures or nanoparticles due to the localized surface plasmon resonance (LSPR), which is defined as the collective oscillations of conduction electrons induced by an interacting light. It is reported that, the best Raman enhancement is reached when the LSPR wavelength is halfway between the incident laser wavelength and the molecular Stokes shifted wavelength for a fixed Raman shift [2]. Because of the special dielectric function characteristics of metallic materials, especially gold and silver, which are often used to fabricate or synthesize nanostructures, most SERS applications are limited in the visible region.…”