Within resonance Raman spectroscopy, energy of the incident photon is tuned to a specific resonance in the species of interest. One of the key advantages of resonance Raman spectroscopy versus non-resonant Raman variants is the enhancement in the Raman signal intensity on-resonance.1
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Challenges for Raman Spectroscopy
One of the biggest challenges for Raman spectroscopy applications is the inherently weak signal levels.
In a Raman experiment, an incident photon is inelastically scattered from the target species. The resulting inelastically scattered photons have an energy difference equal to a specific energy level in the sample. For Raman with visible incident photons, the transitions probed are typically vibrational transitions.
The reason the signal levels are so weak in Raman spectroscopy is that only about 1 in every 108 photons undergoes the Raman scattering process spontaneously.2
Another factor that affects the signal-to-noise levels in a Raman experiment can also be poor collection efficiency on the detection side as it is challenging to design spectrometers with high collection efficiency for the Raman scattering.
Another issue that leads to weak Raman scattering signals is the intensity-frequency scaling factors.
The Raman scattering strength is proportional to the fourth power of the excitation frequency, so using shorter wavelength light produces more Raman signal. However, using shorter wavelengths for excitation also leads to a greater fluorescence background and poorer signal-to-background contrast so a careful trade-off between the fluorescence effects and scattering intensity needs to be considered.
Raman Enhancement
One approach to enhancing signal levels in a Raman experiment is to use a very intense light source to increase the overall number of scattering events. Another is to make use of resonances in the sample to enhance the probability of a given scattering event.
While molecules and materials will inherently have resonances due to their electronic and vibrational structure, there are many relatively recent developments in resonance Raman spectroscopy where attempts are made to use strong local electromagnetic fields near the sample of interest to enhance the Raman scattering signal through the use of species such as nanoparticles.3
The use of nanoparticles and other species to enhance Raman scattering probabilities has given rise to surface-enhanced Raman scattering (SERS) and the closely related resonance-enhanced version of the technique.
SERS is becoming increasingly used by the medical community for biomarker detection for rapid screening of various diseases.4 Many portable devices for trace chemical detection, including for explosives, also use SERS and its resonance-enhanced equivalent.
Resonance Raman
The advantage of using a method such as resonance Raman for biomarker and biomolecule screening is that, unlike many fluorescence-based methods, it is a label-free technique.
The lack of need to add labels simplifies the experimental preparation and makes resonance Raman a very widely applicable technique that can be used with a large number of sample types.
Resonance Raman methods are compatible with solids and liquids. Where sufficient signal levels can be achieved, gases are now widely used in the pharmaceutical industry for studying proteins and other biological species.
The structural information that can be recovered on the composition of molecular species and their characteristic vibrations can also be used to learn about the interactions between different molecular species or solute-solvent interactions.5
Developments in ultrafast laser systems that can produce sub-picosecond pulses have also opened new opportunities for using resonance Raman to explore the fundamentals of protein folding dynamics and interactions.6
The use of temporally short laser pulses to perform time-resolved resonance Raman experiments means that these dynamics can be probed in real-time and the timescales of even relatively fast processes can be measured directly.
Using Resonance Raman Techniques in Materials Science
Resonance Raman methods are also widely used in materials science. Many versions of resonance Raman, such as tip-enhanced resonance Raman spectroscopy, have been used to investigate the properties of 2D monolayer materials.7
The chemical sensitivity of Raman spectroscopy is useful for identifying the local chemical bonding environment in the material, but the resonance enhancement helps improve the sensitivity. These properties can then be combined with spatial resolution in an imaging experiment to build up a complete profile of the local chemical environments in a material and how they interact with other regions.
Imaging structures on the nanoscale with conventional optical microscopy can be very challenging due to the diffraction limit of visible light being several hundred nanometers.
The information recovered from resonance Raman is inherently selective due to the selection rules that govern the technique and that not all molecular vibrations are Raman active.
For molecules adhered to surfaces, this selectivity can help identify the nature of the bonding and interaction between the molecule and substrate rather than relying on direct imaging of the structure.
The ability of Raman spectroscopy to recover information on the ‘chemical fingerprint’ of molecules makes it a highly useful technique in many areas of analytical science.
Resonance enhancement helps overcome one of the main limitations of the technique, the poor signal levels, and new variations to enhance Raman signal levels mean that resonance Raman spectroscopy is likely to remain a popular choice in many industries for years to come.
References and Further Reading
- Robert, B. (2009). Resonance Raman spectroscopy. Photosynthesis research, 101(2), 147-155.
- Jones, R. R., Hooper, D. C., Zhang, L., Wolverson, D., & Valev, V. K. (2019). Raman Techniques : Fundamentals and Frontiers. Nanoscale Research Letters, 14, 231. https://doi.org/10.1186/s11671-019-3039-2
- Faulds, K., & Graham, D. (2011). Surface-Enhanced Raman Scattering ( SERS ) and Resonance Raman Scattering ( SERRS ): A Review of Applications. Applied Spectroscopy, 825–837. https://doi.org/10.1366/11-06365
- Sharma, B., Frontiera, R. R., Henry, A., Ringe, E., & Duyne, R. P. Van. (2012). SERS : Materials, applications, and the future. Materials Today, 15(1–2), 16–25. https://doi.org/10.1016/S1369-7021(12)70017-2
- Gómez, S., Egidi, F., Puglisi, A., Giovannini, T., Rossi, B., & Cappelli, C. (2022). Unlocking the power of resonance Raman spectroscopy: The case of amides in aqueous solution. Journal of Molecular Liquids, 346, 117841. https://doi.org/10.1016/j.molliq.2021.117841
- Mizutani, Y. (2017). Time-Resolved Resonance Raman Spectroscopy and Applications to Studies on Ultrafast Protein Dynamics. Bulletin of the Chemical Society of Japan, 90, 1344–1371. https://doi.org/10.1246/bcsj.20170218
- Shao, F., & Zenobi, R. (2019). Tip-enhanced Raman spectroscopy : principles , practice , and applications to nanospectroscopic imaging of 2D materials. Analytical and Bioanalytical Chemistry, 411, 37–61. https://doi.org/10.1007/s00216-018-1392-0
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