**6. Summary**

has been used to study surface reactions on single crystal and smooth surfaces, as surface

Surface-enhanced Raman scattering spectroscopy (SERS) can be used for the identification short-live reaction intermediates such as radical and radical ions on the electrode surface and elucidation of the reaction mechanism in general. Tian and his research group at Xiamen University, China, carried out the first *in situ* electrochemical SERS (EC-SERS) investigation

The benzyl radical anion as a reaction intermediate and 3-phenylpropanenitrile as the major reaction product were detected from the SERS study for the above surface reaction. The com-

and all other possible interactions including the solvent molecule have been determined from the systematic SERS study. The SERS results were further validated by quantum mechanical density functional theory (DFT) calculations, which confirm the detection of the reaction inter-

It was established by Mulvihill et al. that LB assemblies made of various polyhedral Ag nanocrystals can be utilized as high quality SERS active substrates for the high-sensitivity detection of arsenate and arsenite ions in aqueous solutions with a detection limit of 1 ppb [31]. The detection limit resulted from the analysis carried out by the SERS-based technique is an order of magnitude lower than the existing yardstick set by the World Health Organization (WHO). The SERS substrate can be used as a chemical sensor, which is simultaneously highly reproducible and portable, and therefore, this could be easily executed in field detection. The SERS technique can be further employed in environmental analysis. Pesticides, herbicides, pharmaceutical chemicals in water, banned food dyes, aromatic chemicals in regular aqueous solutions and in sea water, chlorophenol derivatives and amino acids, chemical warfare species, explosives, and various organic pollutants [32, 33] can qualitatively and quantitatively analyzed by the SERS-based detection technique. The partition property of SERS substrates as well as surface chemistry facilitates the complete separation of pollutants and analysis of complex environmental samples in real environ-

Immobilized metal nanoparticles in the form of SERS substrates can be used for biomedical diagnostics. For instance, the SERS substrate can be used as a glucose sensor to detect glucose in human blood. Although glucose is most commonly monitored by electrochemicalbased sensors, a substitute protocol using SERS substrates fabricated by the NSL technique has been employed to detect glucose in blood [34]. In this new protocol, the SERS-based glucose sensor was developed by growing silver film over nanospheres (AgFON) surfaces prepared by the NSL technique. Nevertheless, glucose sensing on a bare AgFON surface was not successful and glucose was brought within the range of electromagnetic enhancement of the AgFON surface by formation of a self-assembled monolayer (SAM) on its surface to partition the analyte of interest, in a manner similar to the technique used to generate the stationary phase in high-performance liquid chromatography. Numerous SAMs were studied to partition glucose effectively to the AgFON surface and it was observed that both

Cl in acetonitrile (CH3

CN) on the Ag electrode [30].

Cl on the Ag surface

roughness of the substrate does not play any role in this enhancement.

plete reaction mechanism enlightening the adsorption process of PhCH<sup>2</sup>

on the electrochemical reduction of PhCH<sup>2</sup>

mediate and products.

308 Raman Spectroscopy and Applications

mental analysis and monitoring.

Even though, the appropriate theory and principles to elucidate the correct mechanism of the SERS phenomenon is yet to be developed, the 40 years of this versatile technique has reached a new height due to the increased efficiency of the modern Raman instrumentation and recent advancement in nanoscience and nanotechnology. Controlled and reproducible fabrication of SERS-active substrates [36] and understanding the in depth connection between the nanoparticle structure and SERS activity remains noteworthy challenges in this field of research. We believe that the fast and gradual development of nanoscience and nanotechnology will eventually allow an absolute understanding of the SERS effect and a broad range application of SERS in both analytical sciences and biomedical sciences.
