Raman scattering appears when light is scattered by atoms or molecules in an inelastic process. In order for this process to happen, the light has to induce a dipole moment within the target molecule, which is proportional to the electromagnetic field strength E and the polarisability a. This classical Raman effect is described by the following equation:
In the case of surface enhanced Raman scattering, the surface plasmons of a suitable SERS substrate (picture on the right) are in resonance with the Raman frequencies of an adsorbed molecule thus enhancing the spectrum obtained from the molecule. The resonance yields enhancement factors of up to ~1015 allowing single molecule efficiency. In addition to this unique sensitivity, a further advantage of SERS in the context of interaction studies is that only Raman signals from molecules that are adsorbed on the substrate (in this case silver / gold NPs) surface will be enhanced. Raman signals from molecules that are further away from the surface will not be enhanced.
In case of small Raman shifts the SERS intensity is proportional to the forth power of the electromagnetic field strength E0 and a factor, which depends on the complex dielectric function of the metal (the e´s indicating for the dielectric constants of the metal and with index m for the media):
Schematic representation of the plasmon resonances of single nanoparticles