Raman scattering is a very weak effect, with typical Raman cross sections being 1030-1026 cm2 per molecule. As described in a large body of literature, in surface-enhanced Raman scattering (SERS), we observe an increase in Raman cross section from molecules that are in close proximity to a noble metal nanostructure.
The potential of SERS in bioanalytics lies in the combination of sensitivity that can be achieved with the structural information that is generated in Raman spectroscopy as a vibrational method. In addition to the increased sensitivity, SERS offers the opportunity of probing very small volumes, owing to the fact that the SERS signal comes from the immediate vicinity of a metal nanostructure whose size and shape dictate the range of the electro-magnetic field enhancement. Individual metal nanostructures or their aggregates can act as tiny probes that can be brought to the analyte of interest or be arranged in space. The concept of a SERS label, where ‘reporter’ molecules with a specific SERS signature are attached to metal nanostructures, is generally accepted and already being commercialized.
SERS can provide new vibrational spectroscopic perspectives on biological and physiological processes. The spectra of the biological samples themselves, rather than those of label molecules, are of great interest in many bioanalytical applications of SERS.
Enhancement in SERS
The so-called electromagnetic field enhancement, the major contributor to the enhancement observed in SERS, is brought about by resonances of the optical fields (excitation and Raman scattering field) with surface plasmons of gold or silver nanostructures. Since the plasmon resonance covers a relatively wide frequency range compared to the frequency shift between excitation and Raman scattered light, both excitation and scattering fields can be in resonance with the surface plasmons of the metal nanostructures. In addition to the electromagnetic field enhancement, so-called chemical or electronic enhancement takes place, which yields an increase in Raman cross section (σRads) due to the electronic interaction of the analyte molecule with the metal surface. The following expression summarizes electromagnetic and chemical enhancement contributions:
with N being the number of molecules involved in the Raman process, I the laser intensity, A(νL) and A(νS) enhancement of the excitation and the scattering field, and σRads the Raman cross section of the adsorbed molecule.