Surface enhanced Raman scattering is phenomenon whereby the normally faint light signals that are 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 in every 108 incoming photons subject to this kind of scattering. Surface enhanced Raman scattering results in this effect being greatly amplified, typically by a factor of 103 to 106, and in some circumstances up to 1015. The enhancement is achieved when the molecules under investigation 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 employed in the form of nanoparticles.
It is thought that the effect is produced when plasmons are created at the metal surface by the laser light used to achieve surface enhanced Raman scattering. Plasmons are electromagnetic waves which travel a short distance across the surface of the metal when the metal’s electron cloud is stimulated by light. Tiny irregularities on the nanoparticles’ surfaces seem to concentrate the effect, which is increased still further when the nanoparticles are arranged in clusters. The electromagnetic field generated then appears to cause molecules in the immediate vicinity to demonstrate much more intense Raman scattering than would normally be the case. It is also thought that chemistry might play a role in some cases, but research toward a full explanation is ongoing.
This effect has led to the development of surface enhanced Raman spectroscopy (SERS), a technique that has greatly extended the scope of Raman spectroscopy, allowing detection of extremely small amounts of various substances without the need for expensive instruments. To maximize the surface enhanced Raman scattering effect, the material under investigation is deposited onto suitable metal nanoparticles, often in a colloid. As with traditional Raman spectroscopy, a monochromatic laser is used to produce the required scattering. Before the scattered light is analyzed, the more intense 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 numerous chemical compounds in trace amounts. It therefore has applications in forensic science, environmental monitoring and medicine. Metal nanoparticles can be introduced into living cells, making it possible to use SERS to investigate cellular biochemical activity.
