Surface-enhanced Raman spectroscopy  Surface-enhanced Raman spectroscopy or surface-enhanced Raman scattering (SERS) is a surface-sensitive technique that enhances Raman scattering by molecules adsorbed on rough metal surfaces or by nanostructures such as plasmonic-magnetic silica nanotubes. The enhancement factor can be as much as 1010 to 1011, which means the technique may detect single molecules. SERS from pyridine adsorbed on electrochemically roughened silver was first observed by Martin Fleischmann, Patrick J. Hendra and A. James McQuillan at the Department of Chemistry at the University of Southampton, UK in 1973. This initial publication has been cited over 4000 times. The 40th Anniversary of the first observation of the SERS effect has been marked by the Royal Society of Chemistry by the award of a National Chemical Landmark plaque to the University of Southampton. In 1977, two groups independently noted that the concentration of scattering species could not account for the enhanced signal and each proposed a mechanism for the observed enhancement. Their theories are still accepted as explaining the SERS effect. Jeanmaire and Richard Van Duyne proposed an electromagnetic effect, while Albrecht and Creighton proposed a charge-transfer effect. Rufus Ritchie, of Oak Ridge National Laboratory's Health Sciences Research Division, predicted the existence of the surface plasmon. The exact mechanism of the enhancement effect of SERS is still a matter of debate in the literature. There are two primary theories and while their mechanisms differ substantially, distinguishing them experimentally has not been straightforward. The electromagnetic theory proposes the excitation of localized surface plasmons, while the chemical theory proposes the formation of charge-transfer complexes. The chemical theory is based on resonance Raman spectroscopy, in which the frequency coincidence (or resonance) of the incident photon energy and electron transition greatly enhances Raman scattering intensity. Research in 2015 on a more powerful extension of the SERS technique called SLIPSERS (Slippery Liquid-Infused Porous SERS) has further supported the EM theory. The increase in intensity of the Raman signal for adsorbates on particular surfaces occurs because of an enhancement in the electric field provided by the surface. When the incident light in the experiment strikes the surface, localized surface plasmons are excited. The field enhancement is greatest when the plasmon frequency, ωp, is in resonance with the radiation (formula_1 for spherical particles).