SERS in Plain Sight: A Polarization Modulation Method for Signal Extraction

Abstract

Surface-enhanced Raman spectroscopy (SERS) is a powerful analytical spectroscopy offering advantages ranging from "vibrational fingerprints" to multiplexed detection. However, the use of this technique in real-world applications has been limited due to difficulties in detecting inherently weak Raman signals often embedded in strong interfering background signals. A variety of plasmonics-active platforms have been developed to increase Raman signals but are not sufficient to extract weak SERS signals from intense interfering background signals. Herein, we describe a practical method, referred to as polarization modulation-SERS (PM-SERS), which utilizes the polarization dependence of anisotropic SERS-active nanostructures to modulate the plasmonic effect to extract SERS signals and remove background. The modulation is obtained by switching the polarization of the excitation source at a specific frequency involving addition of only few optical components such as liquid crystal polarizers to a typical Raman setup. In this work, we characterized the polarization-dependent response of the SERS substrates fabricated using the oblique angle evaporation (OAV) technique and their response under laser excitation using a polarization modulated source. We demonstrated that the PM-SERS method can extract the analyte weak SERS signals from the strong interfering background signal in different situations, involving a fluorescent sample and a strong background light, and we show the possibility of using PM-SERS at a quasi-real time rate (0.5 Hz). We believe that the PM-SERS method will help expand the translation of applications that utilize SERS-substrates to realworld settings.

Dynamic Imaging of Multiple SERS Hotspots on Single Nanoparticles

“…In some events, the SERS emission is intense for both polarizations, as shown in the simulations. This suggests that SIFs from different polarization could be separated as previous reported. , Along with the dual wavelength experiments shown in Figure , these polarization experiments also demonstrate the presence of three types or classes of hotspots, namely, those excited with x -polarization, y -polarization, or both. These various polarization-sensitive hotspots can also be imaged over the surface of the particle.…”

“…Surface-enhanced Raman scattering (SERS) is an increase in Raman efficiency due to extremely localized electromagnetic fields at the surface of, for instance, Au and Ag nanoparticles . These fields can be excited using visible radiation and are tightly confined to only certain areas, known as “plasmonic hotspots”. − Molecules in these hotspots experience enhanced excitation and scattering when probed with the proper laser wavelength and/or polarization. , In recent years, there have been several spectroscopic studies of SERS hotspots consisting of either random structures − or nanofabricated plasmonic antennas. − Plasmonic hotspots are widely explored not only for chemical analysis via SERS, but also for particle/molecule trapping, , enhanced photochemistry, − and even nanolithography. , Images of plasmonic hotspots have been obtained with ∼10 nm resolution by near-field scanning optical microscopy − and with higher resolution by scanning (transmission) electron microscopes using electron energy-loss spectroscopy. − However, while many experiments concentrate on the average optical properties (e.g., the extinction spectrum from a nanoparticle suspension or the transmission spectrum from a nanofabricated substrate) to predict SERS performance, fewer experiments directly probe Raman scattering from single nanostructures (e.g., imaging of subnanoparticle interactions ,− ). Moreover, most reports do not completely explore the time evolution or local origin of fluctuations in the SERS signal.…”

“…2−6 Molecules in these hotspots experience enhanced excitation and scattering when probed with the proper laser wavelength 7 and/or polarization. 8,9 In recent years, there have been several spectroscopic studies of SERS hotspots consisting of either random structures 10−12 or nanofabricated plasmonic antennas. 13−15 Plasmonic hotspots are widely explored not only for chemical analysis via SERS, but also for particle/molecule trapping, 16,17 enhanced photochemistry, 18−20 and even nanolithography.…”

Unveiling practical considerations for reliable and standardized SERS measurements: lessons from a comprehensive review of oblique angle deposition-fabricated silver nanorod array substrates

“…For AgNR-based SERS detection, as discussed in Section 6.6.1, a polarization modulation method has been suggested for hardware-based baseline removal. 187 The other widely used strategy is mathematical baseline removal. Currently, the most popular methods for baseline removal are polynomial methods, where the featureless baseline is approximated using a polynomial function.…”

Polarized SERS substrates with directionality, repeatability and orderability: an anisotropic Ag nanocavity array

“…However, this problem can be solved by using the anisotropy of the sample to get polarization-modulated Raman signals. 30 For the polarization-modulated infrared spectrum, the baseline of the spectrum can be corrected, and only the anisotropic signal is retained after the data processing, so the background signals from a random orientation will be eliminated. Similarly, in the study of polarization-modulated Raman signals, the background signals, which have nothing to do with the polarization of nanostructures, can be eliminated by subtraction of Raman intensity in different polarization directions.…”