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One of the most interesting spectroelectrochemical techniques combines the fields of electrochemistry and Raman spectroscopy. Although the Raman effect was theoretically predicted by Smekal in 1923 [1] as well as by Kramers and Heisenberg in 1925 [2], the first physical evidence was found in 1928 by the Indian scientist Chandrasekhara Venkata Raman [3] and almost simultaneously by Soviet scientists Landsberg and Mandelstam [4]. The «New Type of Secondary Radiation» referred to by C. V. Raman [3] had great importance and he was subsequently awarded the Nobel Prize in physics in 1930 for this discovery.
The beginnings of Raman spectroscopy
C. V. Raman discovered the eponymous Raman effect while sailing from London to Bombay. During this trip, he became fascinated with the deep blue color of the Mediterranean Sea. Although a previous explanation from Lord Rayleigh considered this deep blue color to be just a reflection of the color of the sky, Raman was unable to accept this theory [5]. While still on board, he summarized all of his thoughts about this phenomenon and sent a letter to the editors of the scientific journal Nature when the ship docked in Bombay.
After that, Raman focused his research on the study of the scattering of light by liquids as well as by some solids. A short time later, he was able to demonstrate that the blue color of the sea resulted from the scattering of sunlight by water molecules and not from the reflection of the sky as was previously suggested.
An unexpected phenomenon: Surface-enhanced Raman scattering (SERS) effect
The Raman effect is very weak – only one in a million of the scattered light particles (or photons) exhibits a change in its wavelength. Despite these odds, a breakthrough took place in 1974 when Fleischmann observed an unexpected enhancement in the Raman signal of pyridine adsorbed on an electrochemically roughened silver electrode [6]. This phenomenon was dubbed the «Surface-Enhanced Raman Scattering (SERS)» effect, and its discovery opened up several new horizons for Raman spectroscopy.
The main difference between the SERS effect and conventional Raman spectroscopy is the enhancement of Raman intensity due to the presence of metal nanostructures as a fundamental factor. The SERS effect was controversially discussed for many years but is currently explained by the contribution of two mechanisms: electromagnetic and chemical (also called the «charge transfer» mechanism) [7].
The SERS effect depends on several factors that can be classified into three main categories:
1. SERS substrates. Ideal substrates must show high SERS activity, uniformity or ordered structure, offer stability and reproducibility. Au, Ag, and Cu are the most used metals for SERS applications, but other metals (e.g., Pt, Pd, Co, Fe, Ni, and Rh) as well as the combination of two to three different metals are currently used. It is important to note that careful control of the physical properties (size, shape, composition, distribution, etc.) of SERS substrates is required for success.
2. Laser wavelength. The interaction between the excitation wavelength and the metallic nanostructures on the substrate is crucial for SERS applications. Depending on the sample, the practicality of different lasers can be demonstrated—the most common being those centered in the visible range (i.e., 785 nm, 638 nm, and 532 nm).
3. Sample composition. Not all analytes are able to be detected by SERS scattering, and only certain properties induce the system’s SERS response (e.g., orientation, interaction with the metal substrate, concentration, etc.).
Unfortunately, there is no universal substrate that can be used for SERS enhancement for any kind of molecule because this powerful effect is very system-dependent. Taking into account the excellent enhancement of the optical signal, the development of new substrates is currently one of the most important research areas in Raman spectroscopy.
An interesting alternative to overcome the lack of sensitivity of conventional Raman spectroscopy is the so-called electrochemical surface-enhanced Raman scattering (EC-SERS) effect, where the enhancement of Raman intensity is produced or initiated via electrochemical routes. Electrochemical activation of metallic screen-printed electrodes (SPEs) leads to the reproducible generation of nanostructures with excellent SERS properties. In this way, gold, silver, and copper SPEs circumvent the traditional reproducibility limitation and produce the Raman intensity enhancement after the activation of their SERS features [8].
Learn more about your research projects with double the data
Raman spectroelectrochemistry provides users with two different signals from the same experiment, acting as a powerful tool to collect even more knowledge about the analyzed system.
Nowadays, Raman spectroelectrochemistry experiments are easily performed using SPELEC RAMAN, the only instrument on the market dedicated to performing such experiments. This compact device (25 × 24 × 11 cm) integrates a (bi)potentiostat/galvanostat, laser (785 nm, 638 nm, and 532 nm wavelengths available), and spectrometer. All of the integrated elements are synchronized and controlled with DropView SPELEC software, a dedicated software for spectroelectrochemistry that allows acquisition of electrochemical and optical data, and also includes specific tools for data treatment purposes.
Additionally, the instrument offers three working configurations: one dedicated to performing electrochemical experiments, one for Raman optical measurements and finally, one specifically for Raman spectroelectrochemistry.
SPELEC RAMAN can be used with SPEs as well as with conventional electrodes thanks to the development of a new cell that facilitates the performance of Raman spectroelectrochemical measurements with standard electrodes [9]. This cell overcomes limitations displayed by other setups like tedious and complex assembly protocols or needing to use higher solution volumes.
Fingerprint features for multiple applications
The excellent properties shown by this technique have facilitated the development of new applications in a variety of scientific fields. For instance, Raman spectroelectrochemistry is not only used for understanding fundamental processes better, but also for the development of new sensing platforms and protocols which in turn yield even more new analytical applications based on the SERS effect. The enhancement of Raman intensity allows for the detection of very low concentrations of different analytes that would simply not be possible with conventional Raman techniques.
At present, this combination of Raman spectroscopy and electrochemistry is one of the most interesting techniques regarding the characterization of materials due to the vibrational information that can be gathered. In addition, fingerprint properties are crucial for monitoring electrocatalytic reactions, energy storage devices, and corrosion processes. Furthermore, the position and intensity of Raman bands (as well as their changes with potential) are key points in the characterization of organic and inorganic compounds.
Find out more about the benefits of using SPELEC RAMAN for your research needs by reading our free Application Notes.
Application Note: In situ, fast and sensitive: Electrochemical SERS with screen-printed electrodes
References
[1] Smekal, A. Zur Quantentheorie der Dispersion. Naturwissenschaften 1923, 11 (43), 873–875. DOI:10.1007/BF01576902
[2] Kramers, H. A.; Heisenberg, W. Über die Streuung von Strahlung durch Atome. Z. Physik 1925, 31 (1), 681–708. DOI:10.1007/BF02980624
[3] Raman, C. V.; Krishnan, K. S. A New Type of Secondary Radiation. Nature 1928, 121 (3048), 501–502. DOI:10.1038/121501c0
[4] Landsberg, G.; Mandelstam, L. Eine neue Erscheinung bei der Lichtzerstreuung in Krystallen. Naturwissenschaften 1928, 16 (28), 557–558. DOI:10.1007/BF01506807
[5] Raman, C. V.; Walker, G. T. On the Molecular Scattering of Light in Water and the Colour of the Sea. Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character 1922, 101 (708), 64–80. DOI:10.1098/rspa.1922.0025
[6] Fleischmann, M.; Hendra, P. J.; McQuillan, A. J. Raman Spectra of Pyridine Adsorbed at a Silver Electrode. Chemical Physics Letters 1974, 26 (2), 163–166. DOI:10.1016/0009-2614(74)85388-1
[7] Schlücker, S. Surface-Enhanced Raman Spectroscopy: Concepts and Chemical Applications. Angewandte Chemie International Edition 2014, 53 (19), 4756–4795. DOI:10.1002/anie.201205748
[8] Hernandez, S.; Garcia, L.; Perez-Estebanez, M.; et al. Multiamperometric-SERS Detection of Melamine on Gold Screen-Printed Electrodes. Journal of Electroanalytical Chemistry 2022, 918, 116478. DOI:10.1016/j.jelechem.2022.116478
[9] Ibáñez, D.; Begoña González-García, M.; Busto, J.; et al. Development of a Novel Raman Cell for the Easy Handling of Spectroelectrochemical Measurements. Microchemical Journal 2022, 180, 107614. DOI:10.1016/j.microc.2022.107614
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