Since 1928, both linear and non-linear Raman Spectroscopy have continued to grow and develop. The use of shape analysis of Raman spectra to investigate chemical exchange reactions now has applications in many research fields including industrial, pharmaceutical, medical, and agricultural science and technology.
Linear Raman spectroscopy is defined as a non-intrusive single cell method for the biochemical analysis of cells. The label free technology can be used to analyze mammalian cells, organelles, bacteria, viruses, and nanoparticles. The spectrum derived from this technique is able to give detailed and vital chemical information of the single cells, such as nucleic acids, protein, carbohydrates, and lipids. This is also done without needing any external labels.
In addition to this, the Raman spectrum can determine the so-called ‘fingerprint’ of single cells. This is particularly useful when needing to differentiate between cell types in terms of physiological states, nutrient condition, and variable phenotypes.
Linear Raman spectroscopy is able to undertake analysis that cannot be achieved with other spectroscopies. An example of this is the fact that Raman spectroscopy can accurately measure a sample under extreme conditions. Recently some of the more fascinating areas of study using linear analysis is “Raman spectroscopic signature of life” and, specifically, how it can be applied in order to monitor the life cycle of a single yeast cell.
However, it should be noted that this type of linear spectroscopy also has limitations. An emerging field of research into Raman spectroscopy has revealed numerous techniques that can adapt to and give solutions to these problems. Non-linear techniques are able to provide signal strength much greater than the more traditional linear Raman Spectroscopy.
Furthermore, non-linear Raman effects produce resolutions which are many orders of magnitude higher than could be previously obtained. These non-linear Raman spectroscopic techniques are thought to be substantial and can be used in chemistry, physics and biology and include hyper Rayleigh and hyper Raman spectroscopy, coherent anti-Stokes Raman Spectroscopy (CARS), Raman Gain and Inverse Raman Spectroscopy, Photoacoustic Raman Spectroscopy (PARS) and the Raman Induced Kerr Effect (RIKE).
The coherent anti-Stokes Raman scattering (CARS) method is able to produce signal spatial distribution measurements and has applications for probing local structures and evaluating their temporal evolution in a mix of ethanol and water.
Additionally, Hyper Raman spectroscopy is especially valuable as a non-linear Raman spectroscopic technique due to its ability to investigate transitions which are not active during the infrared effect and cannot be achieved through linear Raman spectroscopy. Conversely, this technique is experimentally challenging and therefore the benefits of using this method must outweigh the difficulty of achieving the desired results.
Both linear and non-linear Raman Spectroscopic techniques have their benefits and emerging applications. Like any other method, the use of either must depend upon the desired outcome.
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