Surface-Enhanced Raman Scattering (SERS), Applications

Surface enhanced Raman scattering (SERS) is a sensitive and selective spectroscopic technique for the detection and characterization of analytes, which are adsorbed on suitable metal surfaces. The effect was first discovered experimentally in 1974 by Fleischmann et al., who reported very intense Raman bands from pyridine adsorbed onto an electrochemically roughened silver surface. They attributed this strong signal to the presence of a large number of pyridine molecules present at the roughened electrode surface due to its large surface area. However, in 1977 Jeanmaire and Van Duyne, and Albrecht and Creighton, independently recognized that the increase in intensity could not be accounted for simply by the number of scatterers present. They observed that compared with Raman scattering from the equivalent concentration of pyridine in solution, SERS gives an enhancement of ∼ 106 and concluded that an intrinsic surface enhancement effect played a fundamental role in producing the enhanced Raman scattering. In the basic process of SERS, the analyte is adsorbed onto a roughened metal surface of a suitable metal, usually silver or gold. On excitation of this surface with a laser beam, a change in polarizability of the analyte occurs in a direction perpendicular to the surface, leading to the enhanced scattering. The rough surface required for scattering can be provided in many ways. Common methods use aggregated colloidal particles, roughened electrodes, or thin cold-deposited metal films. However there are now many specially designed surfaces some of which are available commercially. With a technique which was discovered experimentally and which involves both the physics and chemistry of the interaction of light with a roughened surface and of the adsorption an analyte on a metal surface in water, air or vacuum at the molecular level it was perhaps inevitable that in the early stages of development the theory would be hotly disputed. However the process is now much better understood and it is possible to use the advantages of SERS to develop very sensitive and informative detection methods.