Surface-enhanced Raman spectroscopy
[4][5] SERS from pyridine adsorbed on electrochemically roughened silver was first observed by Martin Fleischmann, Patrick J. Hendra and A. James McQuillan at the Department of Chemistry at the University of Southampton, UK in 1973.
The 40th Anniversary of the first observation of the SERS effect has been marked by the Royal Society of Chemistry by the award of a National Chemical Landmark plaque to the University of Southampton.
Rufus Ritchie, of Oak Ridge National Laboratory's Health Sciences Research Division, predicted the existence of the surface plasmon.
The Raman signal is then further magnified by the surface due to the same mechanism that excited the incident light, resulting in a greater increase in the total output.
When the frequency shift is large, the incident light and the Raman signal cannot both be on resonance with ωp, thus the enhancement at both stages cannot be maximal.
Silver and gold are typical metals for SERS experiments because their plasmon resonance frequencies fall within these wavelength ranges, providing maximal enhancement for visible and NIR light.
[29] In the current decade, it has been recognized that the cost of SERS substrates must be reduced in order to become a commonly used analytical chemistry measurement technique.
[37][38][39] The shape and size of the metal nanoparticles strongly affect the strength of the enhancement because these factors influence the ratio of absorption and scattering events.
Such substrates can be fabricated on a wafer scale and label-free superresolution microscopy has also been demonstrated using the fluctuations of surface enhanced Raman scattering signal on such highly uniform, high-performance plasmonic metasurfaces.
[43] Due to their unique physical and chemical properties, two-dimensional (2D) materials have gained significant attention as alternative substrates for surface-enhanced Raman spectroscopy (SERS).
[46][47] MXenes have a high surface area, good electrical conductivity, and chemical stability, making them attractive for SERS applications.
[47] As research and development continue, 2D materials-based SERS sensors will likely be more widely used in various industries, including environmental monitoring, healthcare, and food safety.
[52][53] The ability to analyze the composition of a mixture at a nanoscale makes the use of SERS substrates that are beneficial for environmental analysis, pharmaceuticals, material sciences, art and archaeological research, forensic science, drug and explosives detection, food quality analysis,[54] and single algal cell detection.
[55][56][57] SERS combined with plasmonic sensing can be used for high-sensitivity quantitative analysis of small molecules in human biofluids,[58] the quantitative detection of biomolecular interaction,[59] the detection of low-level cancer biomarkers via sandwich immunoassay platforms,[60][61] the label-free characterization of exosomes,[62] and the study of redox processes at a single-molecule level.
[66] One common way in which selection rules are modified arises from the fact that many molecules that have a center of symmetry lose that feature when adsorbed to a surface.
The loss of a center of symmetry eliminates the requirements of the mutual exclusion rule, which dictates that modes can only be either Raman or infrared active.
In some experiments, it is possible to determine the orientation of adsorption to the surface from the SERS spectrum, as different modes will be present depending on how the symmetry is modified.
[50][51] SERS can be used to target specific DNA and RNA sequences using a combination of gold and silver nanoparticles and Raman-active dyes, such as Cy3.
The gold nanoparticles facilitate the formation of a silver coating on the dye-labelled regions of DNA or RNA, allowing SERS to be performed.
This has several potential applications: For example, Cao et al. report that gene sequences for HIV, Ebola, Hepatitis, and Bacillus Anthracis can be uniquely identified using this technique.
