Article updated on 21 May 2020
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Studying nanostructures and its applications, using modern Raman spectroscopy, is a very active and burgeoning area in physics, material research, chemical sciences, and biological and biomedical applications.
‘A new type of secondary radiation’ – is the title of the Nature paper published in 1928, by Indian physicists, C V Raman and K S Krishnan. The significance of the study was so evident that within two years, the discoverer was awarded the Nobel Prize for Physics in 1930.
When Raman was fascinated with the deep blue color of the Mediterranean Sea aboard the S. S. Narkunda from England to India, he pondered: “why is the sea blue?”
As opposed to Lord Rayleigh’s explanation that the blue color of the sea is merely a reflection of the blue sky, Raman is convinced that the blue ‘color’ of the sea is due to molecular scattering, just as the blue of the sky is. This was the beginning for the discovery of the Raman Effect. He became interested in the mechanism of light scattering. Upon incidence of monochromatic light on different highly purified liquids, Raman and his team observed change in frequency of the scattered light (unlike Rayleigh scattering, where there is no change in frequency, called elastic scattering) and change in polarization. ‘Feeble florescence’ – was how they termed the observation for want of a better word or a possible explanation of the observed phenomenon.
Raman, later, called it ‘modified scattering’, or a ‘new type of secondary radiation’ when he realized what he was observing as the visible light analog of Compton Effect in X-ray photons. This discovery was the experimental proof of the theoretical Kramers-Heisenberg effect (Quantum theory of Dispersion); now this effect in the visible light corresponding to the vibrational modes is called the Raman Effect.
The Raman Effect was hailed by an American physicist R. W. Wood as “…one of the best convincing proofs of the quantum theory”.
“…someone drew his attention to the discovery of the Compton Effect some few years earlier, and Raman responded with, ‘Ah, but my effect will play a very great role for chemistry and molecular structure!’ That statement was indeed prophetic”
Quantum mechanically, the energy transition, involved in the vibrational state of the scattering molecule, is observed as the light due to Raman effect/scattering. It comprises ~1 in 107 of the incident photons. Raman spectroscopy is the study of the spectrum of the monochromatic light (due to Raman effect) from substances giving information about the molecular structure. If a molecule has a centre of symmetry then Raman active vibrations are infra-red inactive, and vice versa. If there is no centre of symmetry then some (but not necessarily all) vibrations may be both Raman and infra-red active. Study of Raman spectroscopy reveals interplay between atomic positions, electron distributions and inter-molecular forces.
Each type of bond has characteristic modes of vibration; therefore each type of molecule has its own spectral “fingerprint”. A plot of Raman intensity vs. Raman shift is a Raman spectrum. Thus the Raman spectrum is a chemical fingerprint that brings in a wide range of application tools and study. Few advantages of Raman spectroscopy can be listed as follows: water does not interfere strongly, simultaneous direct sample analysis and estimation of multiple analytes is possible, small sample volume and minimum sample preparation is required, it is non-destructive, quantitative information can be obtained, any sample can be analyzed, spatially and temporally resolved spectra can also be obtained. It is useful for high-sensitive label-free identification of molecular species. This is an advantage as an analytical tool in biological sciences, chemical sciences, material sciences, and geosciences. Several biological molecules have distinct Raman features that yield structural and environmental information. It is also a giant tool for nano-scientists. How does this tool that identifies chemical and structural fingerprint aid in studying nanostructures?
Nanostructures are defined as structures which has at least one or all of the dimensions in the range of 1-100 nm. These nanostructures fall in between macroscopic matter and molecular/atomic species; their structure-size rendering them their unique properties. The nanostructures can be engineered to different properties, and have a wide range of applications in any field today. Nanotubes, nanospheres, nanopores, nanosurfaces, nanocubes, nanowires are a few nanostructures to name. The advent of current technology is in making use of these nanostructures. Optical study of nanostructures is an interesting field and Raman spectroscopy adds an altogether different perspective. It is often important to study the chemical or structural nature of these nanostructures at the surface or interior (using confocal techniques); this can be studied with Raman spectroscopy. A few applications of Raman spectroscopy in studying nanostructures are cited here.
Raman spectroscopy can be used for biophysical structure analysis of molecules present on the nanostructures. Polymeric nanostructures, silica nanoparticles, Single Walled Carbon Nanotubes (SWCNTs) and metal nanoparticles such as gold or silver nanoparticles can be detected by normal or resonant Raman scattering. These nanoparticles are used as drug vehicles or in phototherapy in biomedical applications.
SWCNTs show remarkable chirality-dependent properties; SWCNTs often produce random chiralities while some SWCNT chiralities are metallic and the others are semiconductors. It can be easily observed due to their strong resonant Raman spectrum. In one study, Raman spectroscopy is integrated with optical tweezers, to analyze chemical and physical properties of single optically trapped particles via molecular vibrational fingerprints. These Raman tweezers are used to monitor molecules of single living cells, to identify microorganisms and surface molecules on different nanostructures. Raman tweezers were reported for analysis and manipulation of single-walled carbon nanotubes (SWCNTs), graphene flakes, and SERS (surface-enhanced Raman spectroscopy)-active metal nanoparticles.
A size-dependent study of Quantum dots using Raman shift reveals that the electron-phonon coupling decreases with decreasing quantum dot size.
Fiber-laser-based stimulated Raman scattering microscopy is employed to detect “essential diagnostic features”, such as information on specific protein nanostructures/nano-pores, structural changes leading to aggregation in cells help in intra-operative histology. Raman spectroscopy is also used in the study of toxicity of different nanostructures, in living systems. The Raman characterization helps in looking at the cells response to these nanostructures. The cellular reaction can also be mapped, classified and investigated. This helps in developing an understanding of the nano-bio interaction, covering both sides of the story: the nanostructure chemistry and the cellular reactions.
Though poor/weak signal is a problem often encountered in Raman Spectroscopy there are other great ways to amplify the intensity: plasmon-enhanced Raman spectroscopy (PERS), including surface-enhanced Raman spectroscopy (SERS), tip-enhanced Raman spectroscopy (TERS) and shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS).
A single molecule was accurately positioned at the 5 nm gap of a metallic bowtie-shaped nanostructure. The nanostructure-design produced repeatable high local electric field enhancement by several orders of magnitude. Single‐molecule surface‐enhanced Raman scattering (SM‐SERS) of individual nanostructures (such as metallic bowtie nanostructures) can be utilized in the design of highly‐sensitive photonic devices.
Tip-enhanced Raman spectroscopy (TERS) has emerged as a powerful analytical technique providing high chemical sensitivity for surface molecular mapping with nanoscale spatial resolution under ambient conditions. Nanoscale mapping of structural defects and functional groups with a -10nm spatial resolution is reported using TERS. Such studies optimize optoelectronic devices based on 2D materials such as graphene, graphene oxide, single-layer MoS2 and others.
The highly specific and sensitive spectrum, that is a Raman fingerprint of each structure/molecule makes Raman spectroscopy a burgeoning technique in all facets of science.
References
- G. Venkataraman, Raman and his effect (Hyderabad, Universities Press (India), 2005)
- C.V. Raman and K.S. Krishnan, 'A new type of secondary radiation', Nature121, 501 (1928)
- Singh, Rajinder, and Falk Riess. "The 1930 Nobel Prize for Physics: A Close Decision?" Notes and Records of the Royal Society of London 55, no. 2 (2001): 267-83. http://www.jstor.org/stable/532100
- Shu‐Lin Zhang, Raman Spectroscopy and its Application in Nanostructures (2012) Online ISBN:9781119961659 |DOI:10.1002/9781119961659
- DNA Origami Directed Assembly of Gold Bowtie Nanoantennas for Single‐Molecule Surface‐Enhanced Raman Scattering
- Stable optical trapping and sensitive characterization of nanostructures using standing-wave Raman tweezers
- Nanostructure-based plasmon-enhanced Raman spectroscopy for surface analysis of materials
- Nanomaterials in complex biological systems: insights from Raman spectroscopy
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