**4. Surface-enhanced Raman spectroscopy**

The Raman signal can also be amplified by the adsorption of molecules in certain metallic surfaces. This behavior was discovered in 1974 by Fleishmann, Hendra and McQuillan [22]. Initially, the phenomena was associated to the increase of metallic surface area during oxidation–reduction cycles, but soon, it was evident that the strong increase of the Raman signal cannot be explained only considering the changes in metallic surface [23–25]. Finally, this new intensification process is known as surface enhanced Raman spectroscopy (SERS), evidently this process can be combined to RR effect and the acronym for this behavior is SERRS.

There are two main mechanisms to explain the SERS effect: (1) the electromagnetic model and (2) the charge transfer or chemical model. In the electromagnetic model, the interactions among the incident electromagnetic radiations (laser lines) with the surfaces are the main concerns. In this model, the form, size, alignment and type of metallic substrate are the main characteristics to shape the absorption of the surface plasmon resonance. The enhancement of the local electromagnetic field promoted by surface plasmon resonances is more efficient in metallic regions named hot-spots (nanoscale gaps or nanoholes), and as a consequence, the SERS signal from molecules adsorbed in hot-spots dominates the overall detected intensity [26, 27].

In the charge-transfer or chemical model, the interactions between the adsorbed molecules (mainly molecular orientation) with the metallic atoms involved in adsorption (atoms organization) are considered. The adsorption increases the polarizability of the molecule, which can be further increased by charge transfer between the adsorbed molecule and the metallic surface induced by the incident radiation [28, 29]. The applied electrical potential can also be considered in cases where the molecules are adsorbed in an electrode. The SERS technique permits the study of surface interactions, adsorption process, electrode reactions, biological systems, single molecule detection, and so on [30–33].
