Article updated on 5 November 2020.
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In 1928, the famous Indian physicist CV Raman discovered the inelastic scattering of photons from an atom or a molecule. This effect was thus called Raman scattering or the Raman Effect.
Most of the photons are scattered elastically from an atom or a molecule. This means that the scattered photons have the same energy and wavelength as the photons that are incident on the molecules. This phenomenon of elastic scattering is called Rayleigh scattering and accounts for most optical scattering processes such as the blue color of the sky and color of the ocean. But a significantly small fraction (i.e. less than 1x10-7) of these scattered photons have a frequency different from the incident photons.
Raman received a Nobel Prize in 1930 for observing this inelastic scattering effect in liquids. Later, Russian scientists Grigory Landberg and Leonid Mandelstam observed this effect in crystals as well. But, why is this peculiar anomaly in the scattering process so significant? Well, to know the answer to that we must first understand why it occurs.
Raman shifted photons have either higher or lower energy which is dependent on the vibrational state of the molecule by which it’s scattered. If the scattered photons have energy lower (longer wavelength) than the incident photons, it’s called the Stokes shift. If the scattered photons have energy higher (shorter wavelength) than the incident photons, it’s called anti-Stokes shift.
Structural and Chemical Information of Molecules
This means that inelastically scattered radiation can give us the structural and chemical information of the molecule. Each material has a specific signature of these inelastic scatterings and thus a detailed study of these signals can be utilized for efficient spectroscopic techniques. Thus, in condensed matter physics, Raman spectroscopy allows the study of vibrational (phonons), rotational and other low-frequency modes.
A light source mainly in the near-ultraviolet to the near-infrared range is used for this purpose. A fluorescent molecule has a relaxation of the vibration occurring in a rigid crystal lattice and Raman scattering is very sensitive to these relaxations, thus acting as fingerprints for the specific chemical bonds in the molecules.
Now that we have had a brief overview of what Raman scattering is, we need to know what the limitation of this technique is. Spontaneous Raman scattering is extremely weak when compared to Rayleigh scattering. Therefore, the detection signal is extremely low. This offers a serious limitation for practical use of this technique, especially for a material with an extremely low concentration.
Surface-Enhanced Raman Scattering (SERS)
In 1974, ‘Surface-Enhanced Raman Scattering (SERS)’ was accidentally discovered by Fleischmann, Hendra, and Mcquillan at The University of Southampton. They were trying to do Raman of pyridine which has a high Raman cross-section on the roughened silver electrode. They attributed the multifold enhanced signal to the increased surface area due to roughening.
Later in 1977, Jeanmaire and Van Duyne at Northwestern University and Albrecht and Creighton at the University of Kent reported that the enhanced signal is not primarily due to the increase in the surface area. They demonstrated that the enhancement in the signal strength is generated by a real enhancement of Raman scattering efficiency.
It is worth noting that although the first SERS studies were done on an electrochemical system based on pyridine and roughened silver electrodes, it can be used to study pretty much all the surface reactions. The working mechanism for this phenomenon underwent decades of debate and finally, a consensus was built around a local electromagnetic enhancement process which we will discuss in brief.
The enhancement in the output signal is attributed to the amplification of light at localized points known as localized surface plasmon resonances (LSPRs). LSPs are tightly bound enhancements in localized electromagnetic fields. These are generated in the gaps, crevices or sharp features on the surface of a plasmonic responsive material which are generally metals such as silver, gold, aluminum, and platinum.
A well-designed substrate with enhanced LSP response can be used to generate high signal intensities and is capable of detecting even a single molecule under the right conditions. Theoretically, electromagnetic enhancement for SERS can reach a factor of 1010 to 1011.
In most cases, the magnitude can reach an enhancement actor of the fourth power of the electromagnetic field. Dye molecules with a high Raman cross-section are often used for SERS applications. Maximizing the enhancement factors for SERS is still an active area of research.
Salient Features of Surface-Enhanced Raman Scattering
Some salient features of SERS are as following:
- It’s a non-destructive, in situ, vibrational technique.
- Surfaces with features that can enhance the LSPR signal such as edges, crevices, and sharp edges are required for improved signals.
- Since LSP is a surface-bound enhancement of electromagnetic fields, a molecule has to be guided to the vicinity of the surface for detection.
- SERS has a very high spatial resolution with an enhancement range of several nanometers that is capable of sensing even a single layer of an analyte.
- Different plasmonic materials, especially metals, have a different plasmonic response at different wavelengths. For example, silver and copper are good for enhancements in a visible range while gold is a good candidate for near IR wavelengths.
Applications of Surface-Enhanced Raman Scattering
Surface-enhanced Raman spectroscopy has numerous applications, especially in material characterization and molecular sensing. It can provide molecular-level information about the formation or the rapture of bonds in any chemical reaction. It can be used to observe the intermediates formed during the reaction and thus distinguish the reaction products as well.
For electrochemical reactions involving interfacial water molecules and electrodes, and electrolytes, the entire dynamics can be studied using SERS. SERS belong to the group of extremely sensitive surface techniques used to study molecular composition. This can be coupled with different characterization tools to make a holistic characterization device for highly efficient molecular sensors.
Recently, scanning probe microscopy (SPM) techniques such as the atomic force microscope (AFM) tip has been coupled with the Raman spectrophotometer to enhance the localized electromagnetic field response in the vicinity of the tip and the surface. The tunneling mode operated tip when illuminated with a laser beam generates localized surface plasmons in the gap between the tip and the surface, which results in very high intensities of light confined in very tiny spaces. These local hotspots act as the perfect locations for low concentration molecular sensing.