Raman spectroelectrochemistry (Raman-SEC) is a technique that studies the inelastic scattering or Raman scattering of monochromatic light related to chemical compounds involved in an electrode process. This technique provides information about vibrational energy transitions of molecules, using a monochromatic light source, usually from a laser that belongs to the UV, Vis or NIR region. Raman spectroelectrochemistry provides specific information about structural changes, composition and orientation of the molecules on the electrode surface involved in an electrochemical reaction, being the Raman spectra registered a real fingerprint of the compounds. 
When a monochromatic light beam samples the electrode/solution interface, most of the photons are scattered elastically, with the same energy than the incident light. However, a small fraction is scattered inelastically, being the energy of the laser photons shifted up or down. When the scattering is elastic, the phenomenon is denoted as Rayleigh scattering, while when it is inelastic it is called Raman scattering. Raman spectroscopy combined with electrochemical techniques, makes Raman spectroelectrochemistry a powerful technique in the identification, characterization and quantification of molecules.
The main advantage of Raman spectroelectrochemistry is that it is not limited to the selected solvent, and aqueous and organic solutions can be used. However, the main disadvantage is the intrinsic low Raman signal intensity. Different methods as well as new substrates were developed to improve the sensitivity and selectivity of this multirresponse technique. 
For researchers, a few experimental considerations related to Raman spectroelectrochemistry include electrode preparation, cell design, laser parameters, electrochemical sequence and data process.
Methods.
The Raman resonance effect produces an increase in Raman intensity up to 106 times. In this phenomenon, the monochromatic light interaction with the sample produces the transition of the molecules from the fundamental state to an excited electronic state, instead of a virtual state as in normal Raman spectroscopy. This phenomenon of increased intensity could be observed in materials such as carbon nanotubes.
Surface-Enhanced Raman Scattering (SERS) is a technique capable of increasing Raman signal intensity up to 1011 times. This phenomenon is based on the interaction of monochromatic light with materials that exhibit plasmonic properties. The most common metals used in SERS are nanostructured metals with plasmonic band (gold, silver or copper). Nanostructured electrode surfaces can be generated by depositing metallic nanostructures of these materials. A disadvantage of this phenomenon is, sometimes, the lack of reproducibility of the spectra due to the difficulty of obtaining identical nanostructured surfaces in each experiment. 
Surface-oxidation enhanced Raman scattering (SOERS) is a process similar to SERS, which allows the Raman signal to be enhanced when a silver electrode is oxidized in a particular electrolyte composition. This process is carried out at sufficiently positive potentials to ensure the oxidation of the electrode surface. There are significant differences with the SERS effect, but it is a phenomenon that also enhances the Raman signal. 
In SHINERS, metallic nanoparticles with plasmonic properties are coated with ultra-thin homogeneous silica or alumina layers, forming isolated nanoparticles. The metallic nucleus (Au or Ag) is responsible of the enhancement of the Raman signals of the nearby molecules, while the coating layers eliminate the influence of the metallic nucleus on the Raman and electrochemical signals by preventing the molecules from being directly adsorbed onto them. Silica and alumina coating can improve the chemical and thermal stability of nanoparticles. This fact has great importance in the "in-situ" study of catalytic reactions. The high sensitivity of the SHINERS surfaces makes these nanostructures a promising tool for the study of liquid-solid interfaces, especially in spectroelectrochemistry. 
Tip-enhanced Raman scattering (TERS) is a technique that provides molecular information at nanoscale. In these experiments, metal nanostructures are replaced by a sharp metal tip of nanometric size, concentrating the roughness directly on a small region that improves the spatial resolution of scanning techniques in Raman spectroscopy. 
Configuration.
Different configurations can be used to perform Raman-SEC experiments. Raman scattering provides spectra with very weak Raman bands, therefore, a very well aligned optical configuration is required. Laser has to be focused on the electrode surface and an efficient collection of the scattered photons is mandatory. Many of the instruments used for Raman-SEC are based on the combination of a spectrometer, a potentiostat and a confocal microscope, since it is possible to focus and collect the scattered photons in a highly efficient way. Low resolution Raman spectrometers can be also used, providing suitable results. Using this setup, the sampling area is larger and average information about the electrode surface is obtained.
Typical configurations in Raman-SEC:
Instrumentation.
The experimental setup to perform Raman spectroelectrochemistry consists of a light source, a spectrometer, a potentiostat, a spectroelectrochemical cell, a three-electrode system, radiation beam conducting devices, data collection and analysis devices. Nowadays, there are commercial instruments that integrate all these elements in a single instrument, significantly simplifying the performance of spectroelectrochemical experiments. 
Applications.
In recent years Raman-SEC has become an important tool in the study of electrochemical processes and in the characterization of many molecules, providing specific "in situ" information about them. Some applications are: 
References.
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