Surface-enhanced Raman scattering greatly magnifies the weak light signals associated with Raman scattering, allowing for the detection of trace chemical compounds. This effect is achieved when molecules are in contact with metal nanoparticles, causing plasmons to be created on the surface of the metal. This led to the development of surface-enhanced Raman spectroscopy (SERS), which has applications in various fields, including forensic science, environmental monitoring, and medicine.
Surface enhanced Raman scattering is a phenomenon whereby the normally weak light signals associated with Raman scattering become much more powerful and more easily detectable. While Raman spectroscopy is a useful means of identifying molecules present in a material or solution, it is limited by the fact that the effect is very weak, with normally only one of every 108 incoming photons subject to this type of scattering. Surface-enhanced Raman scattering causes this effect to be greatly magnified, typically by a factor of 103 to 106, and up to 1015 in some circumstances. The enhancement is achieved when the molecules under study are in contact with or in close proximity to a metal surface that has roughness on the scale of 10-100 nanometers (nm). Silver, gold and copper give the best results and are usually used in the form of nanoparticles.
The effect is thought to be produced when plasmons are created on the surface of the metal by laser light used to achieve enhanced Raman scattering at the surface. Plasmons are electromagnetic waves that travel a short distance across a metal surface when the metal’s electron cloud is stimulated by light. Small irregularities on the surface of the nanoparticles appear to concentrate the effect, which is further enhanced when the nanoparticles are arranged in clusters. The electromagnetic field generated then appears to cause molecules in close proximity to demonstrate much more intense Raman scattering than would normally be the case. It is also thought that chemistry may play a role in some cases, but the search towards a full explanation is ongoing.
This effect led to the development of surface-enhanced Raman spectroscopy (SERS), a technique that greatly expanded the scope of Raman spectroscopy, allowing the detection of extremely small amounts of various substances without the need for expensive instruments. To maximize the surface-enhanced Raman scattering effect, the test material is deposited on suitable metal nanoparticles, often in a colloid. As with traditional Raman spectroscopy, a monochromatic laser is used to produce the required dispersion. Before the scattered light is analyzed, the stronger signal due to Rayleigh scattering is filtered out to prevent it from overwhelming the Raman signals.
The greatly improved sensitivity of surface-enhanced Raman scattering allows the technique to be used to detect many trace chemical compounds. It therefore has applications in forensic science, environmental monitoring and medicine. Metallic nanoparticles can be introduced into living cells, making it possible to use SERS to study cellular biochemical activity.
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