Surface-enhanced Raman spectroscopy (SERS) uses both roughened metal surfaces and nanoparticles in colloidal solutions to enhance the Raman signal. Recent research has also had a focus on zinc oxide nanostructures.
Nanoparticles are more commonly used due to the fact they can be easily made in the lab whereas roughened surfaces require complicated lithography techniques, with not many surfaces available to buy commercially.
What are Nanoparticles?
Metallic nanoparticles come in a variety of different metals, shapes, sizes, and compositions. The nanoscale size causes electron confinement in metal nanoparticles, which results in surface plasmon resonance. Plasmonic nanoparticles such as silver and gold are the most commonly used due to their unique physical properties.
As well as direct detection of samples mixed with nanoparticles, nanoparticles can be deposited or spin-coated onto various surfaces for a more accurate spread for detection, which is similar to using solid roughened surfaces.
The particle shape and size can be altered to give a better enhancement of SERS, but other factors can also be changed. A variety of metals can be used, as well as changing the reducing agent, stabilizer, concentration, temperature, and addition rate.
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Different Types
Gold nanoparticles were first to be used over twenty years ago, but other metals such as silver and copper have since been explored. Copper is less commonly used due to the fact gold and silver are more air stable and less reactive.
Gold is biocompatible and has advantageous optical properties for biomedical applications. It has a strong excitation close to the IR region of light, which is of great interest due to its possible use in biotechnological systems.
Silver nanoparticles became of interest to research groups because they give a larger enhancement of Raman scattering than gold nanoparticles. The most common ways to synthesize colloidal silver include a simple chemical reduction of a silver salt with trisodium citrate, or by Glucose-assisted reduction of silver sulfate with sodium citrate as a capping agent
Different Shapes
Gold and silver nanospheres are the most common types of nanoparticle used in SERS, but other shapes such as nanostars, nanorods, nanocubes, and nanowires can be developed through a polymer-mediated polyol process. Additionally, Nanoparticles can be capped or hollowed using various chemical methods. Both the shape and size of nanoparticles have been shown to affect SERS enhancement.
Nanospheres can suffer from low enhancement levels that vary from particle-to-particle due to deposition and separation creating ‘hot-spots,’ meaning analysis and results are not evenly spread when mapping a surface. Shapes with edges extending from the surface such as nanostars and nanopyramids give rise to a stronger scattering of light from the surface that flattened surfaces.
How They Are Used
SERS nanoparticles can be used across numerous fields. SERS is used for a wide range of testing, including forensics, medical, analytical, trace material analysis and point of care testing. It can be used in research for drug discovery and has the potential to detect diseases such as diabetes, cancer, Alzheimer’s and Parkinson’s. A big area of interest is in the detection of the latest biological and chemical threats, as there are handheld instruments for field testing available.
Nanoparticles can be used as nanotags to label and authenticate different objects. Gold spheres can be functionalized with reporter molecules encased in a silica shell. The nanotags can be used to encode banknotes and jewelry for security, as well as being used for identification of fraud during transportation of goods.
SERS is a fast, label-free technique that can be used for point-of-care testing for therapeutic drug monitoring. SERS is also a chemically specific, label-free technique that provides a non-toxic alternative to immunofluorescence for looking at tissues in vivo.
Handheld instruments allow a fast way of analysis for in field for forensics and biological and chemical threat detection. Other methods previously used such as liquid chromatography and mass spectrometry have lengthy sample preparation, so SERS gives a fast and sensitive alternative.
SERS has been around since the 1970s, but there are still a lot of areas left to explore. The different shapes and structures of nanoparticles are constantly being researched for the best optimization of SERS, as well as looking at the possibility of tip-enhanced Raman spectroscopy, and combining SERS with UV spectroscopy for the detection of proteins and biomolecules.
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