Tip-enhanced Raman scattering TERS

STM-Raman spectroscopy utilizes the effect that Raman scattering is enhanced for a molecule in the vicinity of a metal nanostructure. This enhancement effect is generally called surface-enhanced Raman scattering (SERS). When a sharp scanning probe, such as a tunneling tip for STM, is used as a metal nanostructure to enhance Raman intensity, it is called tip-enhanced Raman scattering (TERS). The concept of STM combined with Raman spectroscopy is presented in Figure 1.1. 

Deckert-Gaudig, T., Bailo, E., and Deckert, V. (2009) Tip-enhanced Raman scattering (TERS) of oxidised glutathione on an ultraflat gold nanoplate. Physical Chemistry Chemical Physics, 11, 7360-7362. 

Tip-Enhanced Raman Scattering (TERS) 481 Table 11.1 Selected nonbiological samples used in TERS experiments. 

Wood, B.R. et al (2011) Tip-enhanced Raman scattering (TERS) from hemo-zoin crystals within a sectioned erythrocyte. Nano Lett, 11, 1868-1873. doi 10.1021/nll03004n... 

As described so far, the origin of SERS enhancement is the localized surface plasmons generated on a metal surface. It is possible to obtain plasmon resonance at the tip of a sharpened metallic probe. SERS using a metallic nanotip is called tip-enhanced Raman scattering (TERS) [47-51]. Figure 8.5 shows the principle... 

Plasmon-assisted RS can operate in different modalities depicted in Eig. 3.17. The first modality is represented by bare Au NPs deposited as a film on a solid support and the molecules to be probed are in direct contact with the metal surface (Pig. 3.17a). This contact mode also applies to transition metal-coated NPs (Pig. 3.17b), which extend the SERS application to other wavelength regions and transition-metal catalyzed reactions. In contrast, tip-enhanced Raman scattering (TERS) operates in a noncontact mode the Au tip, acting as the Raman signal amplifier and the probed molecules on the surface are separated from each other (Pig. 3.17c). With this powerful approach of chemical imaging at the nanometre scale, any substrate can be probed without additional constraints on material composition and surface topography. 

Finally, it is evident that SERS should be considered from a wider perspective and comparing the performance of SERS with other techniques can always be useful. For example, fluorescence microscopy and spectroscopy are extremely sensitive and widely used techniques in many areas of the life sciences. Further development and expanding of other surface-enhanced spectroscopic techniques such as surface-enhanced infrared absorption (SEIRA), surface-enhanced fluorescence (SEF) and tip-enhanced Raman scattering (TERS) are also highly desirable. It is highly appreciable so that these techniques will help to refine information obtained by SERS in many bioanalytical, biomolecular and medical studies. 

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