UV Raman Spectroscopy

Cons of Conventional Raman Spectroscopy

Raman spectroscopy is an indispensable technique used in many laboratories, which is based upon molecular Raman scattering. Generally, Raman spectrum measurement possesses two limitations:

As the cross-sections of Raman scattering are lesser, there is a requirement of intense layers and sensitive detection systems for achieving adequate signal.

Fundamental and intrinsic noise sources such as sample autofluorescence mitigate the signal-to-noise ratio.

Rise of UV Raman Spectroscopy

Raman spectroscopy imaging is usually performed with red, green, or near-infrared (NIR) lasers due to the availability of established detectors and lasers at these wavelengths. If Raman shift is measured at the UV wavelength range, both of the above-mentioned limitations can be eventually removed.

UV Raman Spectroscopy to Resonance Enhanced Raman Scattering

Visible and NIR lasers possess photon energies below the electronic transitions of the bulk of the molecules, In UV lasers, the photon energy of the laser exists within the electronic spectrum of the molecule. This leads to enhancement in the intensity of Raman-active vibrations by many orders of magnitude. This phenomenon is known as “resonance-enhanced Raman scattering”.

UV Lasers 

Even though UV lasers can excite autofluorescence, it generally occurs at wavelengths after 300 nm only, which is also independent of the UV laser wavelength. When excited by a common 266 laser, greater Stokes shift (4000 cm^-1) leads to Raman emission below 300 nm. In this, autofluorescence does not interfere with the Raman signal, leading to the possibility of measurement of making a high signal-to-noise ratio. 

Availability of UV Lasers for Raman Spectrometer

Compact, affordable, and high-power UV lasers have become increasingly available. Examples are NeCu hollow-cathode metal-ion lasers at 248.6 nm and quadrupled, diode-pumped Nd: YAG lasers at 266 nm. This leads to UV Raman spectroscopy as a more common technique. However, optical fiber availability in the UV range has been lagging till now.

Pros of UV Raman Spectroscopy

Most often, there is a trade-off between fluorescence background and the strength of the signal. This situation dominates the choice of excitation wavelength for the technique of Raman spectroscopy. We can briefly see the advantages of UV Raman spectroscopy.

The efficiency of Raman scattering is inversely proportional to the fourth power of excitation wavelength. This case makes a good cause of UV excitation. Theoretically, excitation with UV light generates signals greater than twenty-fold than at 532 nm and 100 folds more than at 785 nm. 

Often, Raman active molecules exhibit a fluorescence, when excited with a laser, which results in the broad background, having several orders of magnitude more than the Raman signal. This broad emission can overlap the entire Raman spectrum when a visible light source is utilized. Further, it mitigates the signal-to-noise ratio completely and obstructs the spectrum. Additionally, fluorescence is greater when the molecular structure is complex, which is the case in biological samples and organic compounds. This also occurs because of fluorescent impurities in the sample. Fluorescence can be mitigated during the usage of excitation light sources at 785 nm, 830 nm, or 1064 nm. Still, some biological samples experience undesired heating at 1064 nm wavelength. 

Hence, another way came to completely mitigate the fluorescence window. Generally, the fluorescence occurs greater than 300 nm. So, when the sample is excited at 266 nm or below, full fingerprint and functional bands can be easily collected for the whole 4000 cm^-1spectrum, without any interfering background. 

Extension of Pros by Resonance Raman Spectroscopy

Usage of excitation source at UV range results in a large resonant increase of the Raman signal for some signals, which leads to the technique of resonant Raman spectroscopy. Also, UV Resonance Raman spectroscopy yields enhancement in signal and increased analytical specificity under the right circumstances.

When the energy of the laser coincides with the electronic transition within the sample molecule, the signal can be enhanced by 102-106 folds, which leads to the usage of resonant Raman spectroscopy. The excitation coming close to the electronic transition of the molecule can generate 5x to 10x amplified signals, yielding ‘pre-resonance’ conditions. 

Another gain of resonance Raman spectroscopy is enhanced specificity. This happens because signal enhancement takes place only when electronic transitions coincide with the wavelength of the laser. This permits the preferential excitation of a molecule within a complex sample. There is also selective enhancement of signal from a particular subgroup within a macromolecule. This technique takes advantage of resonance with UV light with aromatics and chromophores in complex biomolecular samples for studying the structure and dynamics, folding, interactions, and environment changes of nucleic acids and proteins. The choice of the wavelength is helpful in the determination of specific structure, for which resonance takes place, multiplying the signal selectively, yielding a relatively simple Raman spectrum for a complex molecule.

Applications of UV Raman Spectroscopy

Though UV Raman spectroscopy can be useful in many applications, only some of them have exploited UV Raman spectroscopy well.

UV Raman spectroscopy has been tested as a method for the trace detection of nitrogen.

•  It can also be used for pressure measurement.
• It is useful in combustion studies
• It is utilized in the analysis of the mixing of fuel and air.
• It is a valuable tool for solid-state materials analysis.
• It offers insights into optical, electronic, and structural properties of materials, which are fabricated for electronics.
• In the growth of diamond, UV Raman spectroscopy aids in the characterization and quantification of impurities, which would cause fluorescence in visible Raman spectroscopy
• It is used in the trace detection of analytes in complex samples. Examples are saliva, cocaine, carcinogenic pollutants like polyaromatic hydrocarbons in the environment.
• Colored food samples like edible oils and dark beverages can be easily characterized with background.reduction.
• It is a favored technique for stand-off detection of chemical warfare agents and explosives by security forces, although these systems need specialized optics and high power lasers for the achievement of longer working distance.
• It has a larger impact in probing molecular structure and dynamics in biological systems. 
• It permits selective excitation of a particular segment of interest within macromolecules like nucleic acid and amino acid. It enhances signal from that specific substructure, providing relatively clean Raman spectra from highly complex samples.
• It is an invaluable tool for protein folding and protein structure, the impact of environment, and molecular interactions.
• It has also been tested for monitoring the mode of action of antibiotics for specific bacteria and antibiotics pairs.

Conclusion

Initially, limitations of conventional Raman spectroscopy have been provided. Then, the rise of UV Raman spectroscopy and Resonance Raman spectroscopy have been briefed. UV lasers and their availability have been listed. Additionally, the pros of UV Raman spectroscopy have been elaborated and extended pros of Resonance Raman spectroscopy have been detailed. Subsequently, applications of UV Raman spectroscopy have been discussed.