Nanostructures for Surface Enhanced Raman Spectroscopy (SERS) and Their Applications

Dear Colleagues,

More than thirty years ago, researchers discovered that the Raman signal of a molecule adsorbed on a nanostructured metal surface is significantly amplified. This enhancement effect is due to the coupling of incident radiation with the collective oscillations of surface metal electrons, which produce an electromagnetic field that then enhances the incoming electron. The areas in which this effect is particularly intense are in between adjacent metal nanostructures (if the distance is only a few nanometers), in interstitial crevices, and on sharp tips. However, the electromagnetic model cannot explain why the enhancement factor also depends on the chemical nature of the adsorbed molecule. Therefore, it has been hypothesized that further enhancement is provided by an increase in molecule polarizability due to the deformation of electron cloud distribution, as in the case of aromatic molecules; this may also be due to the formation of a resonant charge transfer complex between metals and adsorbed molecules.

Within the method of surface-enhanced Raman spectroscopy (SERS), the high specificity of the Raman signal is combined with a degree of sensitivity in suitable conditions, which can achieve single-molecule detection.

Signal enhancement depends on the dielectric constant of the metal, meaning coin metals have emerged as the most appropriate materials for this purpose, particularly silver and gold. Usually, substrates have nanostructured surface designs produced by different techniques, and they are gold-coated because gold is more resistant to oxidation in air (which can act to degrade the EF of the surface). The SERS substrate should enhance the RS enough to enable consistent and uniform detection sensitivity across the surface, maintaining its properties for as long as possible and providing a high number of sites for molecular detection. Over the last 30 years, many studies have been devoted to designing new substrate structures and configurations to maximize the aforementioned enhancement factors.

More recently, the coupling of Raman spectroscopy with optical microscopy in confocal geometry has presented a method of obtaining Raman maps with high spatial resolution. Spectral imaging has become an invaluable tool for researchers because it allows them to view the chemical distribution of the sample to be analyzed, taking us well beyond what the eyes can see.

By coupling the Raman spectrometer with an AFM, under the suitable conditions of optical excitation and collection optics, it is possible to perform tip-enhanced Raman spectroscopy (TERS) with sub-diffraction-limited imaging capabilities. In TERS, electromagnetic field enhancement occurs at the sharp metallic tip of the AFM. When the tip is brought close to the sample of interest, it provides a localized region of SERS enhancement, meaning structural and compositional imaging of the sample under study, with a spatial resolution of a few nanometers, can be obtained.

The aim of this Special Issue is to showcase the most recent advancement in SERS and TERS, including the development of new substrates with improved efficiency and the exploration of new fields of application such as biology, medicine, cultural heritage, material science, functionalized materials, and aereospace.

This cutting-edge research proceeds from three areas:

• novel low-cost, easy-to-produce substrates for the detection of parameters suitable for health (biology and health applications);
• nanoscale chemical imaging obtained by SERS and TERS, and the problems associated with them;
• new fields of application for SERS/TERS.

We solicit two kinds of papers: articles with novel results and reviews that can elucidate the state of the art in this ever-evolving field.

Dr. Sabina Botti
Guest Editor

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• SERS
• TERS
• nanoscale chemical imaging
• nanostructures for SERS
• plasmonic