2.3.2. Surface-Enhanced Raman Scattering (SERS)

To enlarge the applicability of Raman techniques on biological tissues and fluids, it is possible to amplify the signal coming from the samples thanks to the Surface Enhanced Raman Spectroscopy (SERS, also named as Surface-Enhanced Raman Scattering). SERS is a vibrational spectroscopy technique that exploits the intensity amplification of Raman signal by metallic nanostructures or metal nanoparticles. At the beginning SERS was adopted to the study of dilute aqueous solutions of a rather simple system such as small molecules or molecular ions, recent nanotechnology, and photonics developments stimulated the application of SERS to more complex bio-systems such as macromolecules, cells, tissues, and bio-fluids. Qualitative and quantitative detection of one or more analytes with SERS can be achieved in a direct or indirect way [31]: in the direct approach (label-free SERS) the spectroscopic signal is due to all those analytes which adsorb on the SERS substrate, whereas in the indirect approach the signal results form a specific SERS label that binds to a target analyte of the sample (see Figure 4). SERS enhancement factor is a very significant parameter in SERS, and it is used for quantifying the overall signal enhancement. To experimentally evaluate SERS enhancement factor measurements of the SERS intensity for the adsorbed molecule on the metal surface, relative to the normal Raman intensity of the “free” molecule in solution are required. The enhancement factor can assume values from 10¹⁰ to 10¹¹, which allows detecting single molecules.

Graphic representation of the two approaches for SERS: the indirect approach (SERS immunoassay) on the left and the direct approach (label-free SERS) on the right.