Surface Enhanced Raman Spectroscopy (SERS) is a powerful technique that is used to enhance sensitivity when using Raman spectroscopy. Raman scattering gives inherently weak signals, but the discovery of SERS in the 1970s by Martin Fleischmann and his research team at the University of Southampton helped to overcome this problem.
How it Works
SERS was first discovered in 1974 when Raman spectra of pyridine on roughened silver was observed, but the magnitude of this finding was not realized until 1977. Typical metals used are gold, silver, and copper, but examination of alkali metals and metal alloys as plasmonic substrates for SERS has now also been carried out. The surfaces are prepared through chemical roughening, metallic coating, or deposition of colloidal metallic nanoparticles onto a surface. A laser wavelength, compatible with the chosen SERS metal, is used for excitation.
Laser excitation of the nanostructures creates a plasmonic light field. When a molecule is absorbed or lies close to the enhanced field at the surface, an enhancement in the Raman signal is observed. The enhanced signals make it possible to detect low concentrations, so fluorescence labeling is not required.
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Image Credit: Iwane M, Fujii S, Kiguchi M. Surface-Enhanced Raman Scattering in Molecular Junctions. Sensors. 2017; 17(8):1901.
Type of Substrates
SERS substrates can be both roughened surfaces and nanoparticles in colloidal solutions. Both the surfaces and solutions consist of nanostructured metals of varying shapes and sizes. The shapes and size of the nanoparticles affects the degree of SERS enhancement, so this is an area of research that is still under exploration.
The most frequently used colloidal solutions are gold and silver nanoparticles. Common nanoparticle shapes are spheres, but stars, pyramids, and rods are being researched as possibilities due to the way they enhance signals. A major disadvantage of nanoparticle solutions is that the deposition and separation can create ‘hot-spots,’ meaning analysis and results are not evenly spread when mapping a surface.
Surface-confined nanostructures reduce the likelihood of ‘hot-spots,’ but they are less commonly used due to the difficulty of making them in a laboratory and the limited availability of surfaces available on the market. Unlike the simple chemical reaction of colloidal nanoparticle solutions, roughened surfaces often are produced through complicated techniques including various types of lithography.
What is it Used For?
SERS is used in a variety of areas, including forensics, medical and analytical testing, trace material analysis, drug discovery, point of care testing, and biological/chemical threat detection. The potential uses for SERS in biosensing are broad and include screening for diseases such as diabetes, cancer, Alzheimer’s and Parkinson’s.
Immunofluorescence staining is the gold standard for clinical diagnostics of tissues, but the toxicity of many fluorescent dyes means that analysis cannot be carried out in vivo. Raman spectroscopy is a chemically specific, label-free diagnostic technique, so is a great alternative. The fact that SERS is a fast, label-free technique has also proved to be a significant advantage for point-of-care tests for therapeutic drug monitoring.
SERS nanotags, gold spheres functionalized with reporter molecules encased in a silica shell, can be used as a way of labeling different items. The nanotags can be used to encode jewelry and banknotes for security as well as the identification of fraud in goods transportation.
With Raman and SERS now developed to be portable, the use of SERS in field testing for forensics and biological/ chemical threat detection has dramatically increased. Having mobile detection equipment is very important, with previous methods of analysis including liquid chromatography and mass spectrometry. However, these methods require lengthy sample preparation. SERS provides an excellent, sensitive and fast alternative.
New Research
SERS remains an area of analytical chemistry that is being developed further to produce even more concise and accurate results. Researchers are currently examining ways to use nanoparticles for SERS enhancement, as well as exploring different solid roughened surfaces and how they could enhance SERS signals.
As well as improving current methods of detection, SERS has the possibility in the future to be combined with other techniques. Combining ultraviolet spectroscopy with SERS, for example, would allow for protein and biomolecule detection. Ultrafast SERS could also be possible if the method can be connected with femtosecond stimulated Raman spectroscopy.
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