Control of surface characteristics for Raman spectroscopy


Precise control of the surface characteristics of substrate materials is essential to broaden the scope of Raman spectroscopy and make it a standard analytical tool. Scientists from South China University of Technology investigated this research area, publishing a review study on the development of Raman spectroscopy substrates and research milestones in the journal ACS Au Materials.

Study: Dimensional Design for Surface Enhanced Raman Spectroscopy. Image Credit: Forance/Shutterstock.com

Raman spectroscopy and surface enhanced Raman spectroscopy

Raman spectroscopy is a powerful non-destructive analysis technique that has been widely applied in fields such as life sciences, environmental sciences, and surface sciences. The technique is named after CV Raman, who discovered a previously unknown phenomenon of light scattering due to fluctuations of atoms and molecules from the normal state.

In Raman scattering, unlike the more common elastic Rayleigh scattering, small fractions of molecules or atoms lose or gain energy causing the photons to shift in frequency. The energy of the different rotational or vibrational states of a molecule determines the energy differences seen in Raman scattering. The measurement of Raman shift spectra reveals information about the structural and chemical composition of a target molecule. It is essentially a molecular fingerprint.

Raman spectroscopy is limited by low intensity Raman scattering, despite advances in laser technology. Recently, the discovery of surface-enhanced Raman spectroscopy has offered significant advantages to researchers. Raman intensity can be enhanced by adsorbing molecules onto the surface of a rough silver electrode composed of substantial amounts of nanostructured silver materials.

Research has indicated that surface-enhanced Raman spectroscopy can display an enhanced Raman intensity of approximately 106. Improvement factors as low as 107 can help realize single molecule signal observation. Electromagnetic mechanism hotspots are caused by the nanogap between adjacent silver nanoparticles, greatly enhancing the electromagnetic mechanism enhancement factor, which can reach orders of magnitude higher than the chemical mechanism.

Fabrication of Stable Surface-Enhanced Raman Spectroscopy Platforms

Studies have recently been conducted on improving the stability of platforms. Research has highlighted the importance of quantitative optimization of active sites. Advances in nanomaterials, manufacturing methods, and nanoscience have proven essential for this field, as better defined nanoscale substrates can be developed.

Researchers have developed tunable zero-dimensional nanoparticles, one-dimensional nanowires, and three-dimensional arrays. Materials developed include coinage metals, MXenes and graphene. The ability to tune the surface characteristics of materials at the nanoscale has provided scientists with powerful strategies to reduce the complexities of trace detection. Active sites, each with optimal enhancement factors, can be fabricated using powerful surface characteristic modification capabilities.

(a) Diagram of Raman and Rayleigh scattering of light by a molecule located between two metallic nanoparticles involving a hot spot.  (b) Jablonski diagram representing quantum energy transitions for Raman and Rayleigh scattering of a molecule.  Schematic diagrams illustrating (c) surface plasmon polaritons at the surface of a metallic thin film and (d) localized surface plasmon resonance at metallic nanoparticles.

(a) Diagram of Raman and Rayleigh scattering of light by a molecule located between two metallic nanoparticles involving a hot spot. (b) Jablonski diagram representing quantum energy transitions for Raman and Rayleigh scattering of a molecule. Schematic diagrams illustrating (c) surface plasmon polaritons at the surface of a metallic thin film and (d) localized surface plasmon resonance at metallic nanoparticles. Image Credit: Long, L et al., ACS Materials AU

The study

The authors presented a review of the dimensional design of substrates used for enhancement purposes in surface-enhanced Raman spectroscopy. Although the authors have not provided a truly comprehensive review of studies over the past half century, they have attempted to describe the development of these substrates.

The research investigated zero-dimensional, 1D, 2D, and 3D substrates classified according to their geometric dimension. For each of these classes of substrates, the composite configuration and geometric design can be modified to achieve the optimal enhancement factor at their active sites.

It is important to find an ideal substrate for this analytical technique. Substrates should possess several beneficial properties such as high sensitivity, selectivity, and stability, which will make them attractive for research and technological applications. Possessing these optimal characteristics enhances the versatility of the technique, improving the enhancement factor, fast response, and enhanced signal reproducibility.

Some milestones in the development of SERS substrates made over the nearly half-century history of SERS include 0D nanoparticles, 1D nanowires, 2D metallic and non-metallic thin films, and 3D nanostructure arrays.  Advances in SERS substrates have greatly expanded the scope of application of SERS.

Some milestones in the development of SERS substrates made over the nearly half-century history of SERS include 0D nanoparticles, 1D nanowires, 2D metallic and non-metallic thin films, and 3D nanostructure arrays. Advances in SERS substrates have greatly expanded the scope of SERS application. Image Credit: Long, L et al., ACS Materials AU

4D Surface Enhanced Raman Spectroscopy

In the review, the authors pointed out that femtosecond pulsed laser technology can be used to incorporate a time dimension into surface-enhanced Raman spectroscopic analysis. By incorporating a time dimension into this analysis technique, surface-enhanced Raman spectroscopy can provide insight into the ultrafast dynamics that occur when molecules undergo structural changes. This capability expands the technology’s reach beyond providing information about the chemical composition of molecules.

This technique is called 4D surface-enhanced Raman spectroscopy and provides unprecedented information about target molecules. The authors said this additional spatiotemporal resolution helps the technique answer fundamental questions and enhances its versatility as an ultra-fast analytical technique.

Providing ultrafast analysis of nanometer-resolution structures and single molecules is the ultimate goal of surface-enhanced Raman spectroscopy research. However, there is currently no suitable technique to achieve this goal. Observing the dynamics of molecular changes and visualizing the dynamics of specific important chemical reactions are currently beyond the reach of researchers.

Dimensional design can help advance spatiotemporal resolution for this analytical technique and enable quantitative analysis of a single molecule. A 4D design where the pump-probe process is carefully planned, in accordance with improving the geometric structures of a substrate, may be able to meet this demand. Additionally, the incorporation of strategies such as scanning probe methods can help achieve this goal of surface-enhanced Raman spectroscopy.

The future

Although there are still significant challenges in the field, this review article provides a wealth of information on current progress and advances in these techniques, and the authors stated that in the future it will be possible to visualize elementary chemical reactions and achieve four-dimensional analysis of molecules and their ultrafast dynamics.

Further reading

Long, L et al. (2022) Dimensional design for surface-enhanced Raman spectroscopy ACS Au Materials [online] pubs.acs.org. Available at: https://pubs.acs.org/doi/10.1021/acsmaterialsau.2c00005

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