Raman and SERS

Raman and SERS

Introduction

RAMAN SPECTROSCOPY

Raman spectroscopy is a light scattering strategy, and can be considered in its simple complex structure as a process where a photon of light interacts with sample to create scattered radiation of various wavelengths. Raman spectroscopy is essential analytical and research tool. It can be utilized for applications as wide rage as pharmaceuticals, polymers, semiconductors and even for the investigation of fullerene structures and carbon nano-materials. Raman spectroscopy is extremely information rich , helpful for compound distinguishing proof, portrayal of sub-atomic structures, impacts of holding, environment and weight on a specimen.

The strategy of Raman spectroscopy was not that generally taught inside college courses, despite the fact that the dissipating procedure itself was built up as far back as 1928 by Professor C.V Raman. FTIR, UV-VIS, and NMR and so forth .Were by generally more ordinary. In the mid 1990’s,   the next generation of smaller   more conservative instruments began to develop. They used more up to date lasers, optics and locators and started the smaller scale Raman unrest

THE THEORY OF RAMAN SPECTROSCOPY

When monochromatic radiation is incident upon a sample then this light will interact with the sample in some fashion. It may be reflected, absorbed or scattered in some manner. It is the scattering of the radiation that occurs which can tell the Raman spectroscopist something of the samples molecular structure. If the frequency (wavelength) of this scattered radiation is analyzed, we see both the incident radiation wavelength seen (Rayleigh scattering) and a small amount of radiation that is scattered at some different wavelength (Stokes and Anti-Stokes Raman scattering). (approx. only 1 x 10-7 of the scattered light is Raman). Structural and chemical information is provided by the change in wavelength of the scattered photon.

This fig 1.Light scattered from a molecule

Light scattered from a molecule contain components such as the Rayleigh scatter and the Stokes   Anti-Stokes Raman scatter and the Rayleigh scatter and

In a molecular system, the above frequencies are found in the ranges based on Vibrational, rotational, and electronic level transitions. The scattered radiation occurs in all directions and has observable change in its polarization along with its wavelength. Rayleigh scattering is a change without change in frequency   Lord Rayleigh describes it as responsible for the blue color of the sky. A change in the wavelength of the light is known as Raman scattering(Jarvis et al 2004). Raman shifted photons of light are of higher or even lower energy it determined by    the vibration state of the molecule involved.

Fig 2. Energy Diagram for Raman Scattering

So far the importance  of these two processes is the Stokes scattering, here lower energy is used to scatter the  photon   (shifted wavelength towards the red end of the spectrum) the reason being that  at room temperature the population state of a molecule is principally in its ground vibrational state this is the larger Raman scattering effect as shown in the diagram above. A small number of molecules are in higher vibrational level, and hence the scattered photon can actually be scattered at a higher energy, a gain in energy and a shift to higher energy and a blue shifted wavelength. This is the weaker Anti-Stokes Raman scattering.

The incident photons will therefore interact with the present molecule, and the amount of energy change it is lost or gained by a photon is characteristic of the nature of each bond present. It iss true that all vibrations will not  be observable with Raman spectroscopy it depends with  the symmetry of the molecule. Enough information is gained and one can make very precise characterization of the molecular structure.

Hence, the amount of energy shift for a C-H bond is different to that seen with a C-O bond, and different again to that seen with a Metal-O bond. By looking at all these various wavelengths of scattered light, one can detect a range of wavelengths related with the various bonds and vibrations, for a table of Raman band positions

APPLICATION OF RAMAN

Raman spectroscopy is used in different fields to be perfect , any application where non-damaging, minuscule, substance examination and imaging is required. Whether the objective is subjective or quantitative information, Raman investigation gives essential data effectively and rapidly. It can be utilized to quickly portray substance arrangement and structure of an example, whether strong, fluid, gas, gel, slurry or powder. Another essential territory where it is connected is in redox science research. Reverberation Raman (RR) spectroscopy is the perfect instrument for redox biology research. It indicates high sensitivity to haem proteins; it can likewise illustrate their oxidation and oxygenation states in situ. RR imaging demonstrates compound and spatial data that empowers connections between’s haem protein circulation, oxidation state, and protein cell function to be made.

Raman spectroscopy is effectively applied in analysis of extensive variety of materials and systems. It is used to extract substance data without the need to control qualities, or use stains or antibodies used to separate a full range of chemical information without the requirement for focusing on bio-molecules, markers, stains or dyes.Micro-organisms.Raman spectroscopy empowers the fast discovery and distinguishing proof of micro organisms   an assortment of settings.

EFFECTS OF RAMAN

Molecular vibrations can be energized either through the absorption of light or the inelastic dispersing. Infrared spectroscopy depends on the immediate retention of light and the procedure is like light assimilation in the UV-Vis range. The distinction is that in the UV-Vis extend light causes the purported vibronic transitions that all the while include changes in the vibration and electronic energy levels.

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Figure 1 A simplified schematic diagram showing light absorption (UV-Vis and IR), Rayleigh (elastic) scattering – R, Stokes and anti-Stokes Raman scattering processes, S and A respectively.

The Raman diffusing phenomenon is not the same as IR spectroscopy. In a disentangled way it can be clarified as far as versatile (Rayleigh disseminating) and inelastic (Raman dispersing) crashes between occurrence light and target particles. In the Rayleigh dissipating, the incident of light is transmitted through an objective media with no change. While in the Raman dispersing a little divide of light is scattered and the energy exchange between the two frameworks happens. The Raman impact is a feeble procedure that produces

Scattering  intensities 3-5orders of magnitudes smaller in contrast with the Rayleigh disseminating. In the energy exchange prepare, the occurrence photon may lose energy by energizing higher sub-atomic vibrational states. The photon then rises with a lower recurrence (longer wavelength) and the procedure is alluded to as Stokes Raman scrambling. The occurrence photon may likewise procure the energy from the atom and impel rot into lower sub-atomic vibrational state(Corrado et al 2016). For this situation the photon develops with a higher recurrence (shorter wavelength) and the procedure is known as hostile to Stokes Raman diffusing, see Figure 1.

WHAT IS SURFACE ENHANCED RAMAN SCATTERING, OR SERS?

Surface enhanced Raman scattering (SERS) is a technique which offers orders of magnitude increases in Raman intensity, overcoming the traditional drawback of Raman scattering – its inherent weakness.  Enhancement factors can be as high as 1014-15, which are sufficient to allow even single molecule detection using Raman.  SERS is of interest for trace material analysis, flow cytometry and other applications where the current sensitivity/speed of a Raman measurement is insufficient.

The enhancement takes place at a metal surface which has nanoscale roughness, and it is molecules adsorbed onto that surface which can undergo enhancement.  Typical metals used are gold or silver – preparation of the surface can be through electrochemical roughening, metallic coating of a nano-structured substrate, or deposition of metallic nanoparticles (often in a colloidal form).  Many researchers create their own SERS substrates, but commercially available kits offer a more routine approach (Chen et al 2015).

For all intents and purposes, the upsides of SERS can be investigated on any Raman framework, and the genuine estimation is made in the standard way. Commonly it is important to utilize a laser wavelength which is perfect with the picked SERS metal, yet past this there are no real troubles. SERS spectra do at times vary from an “ordinary” Raman range of the same material, so elucidation of information must be considered.

Raman Scattering is a week impact. We find  that analyte atoms are adsorbed on a plasmonic metal molecule surface, the Raman disseminating sign can be massively upgraded. The impact is then alluded to as surface-improved Raman disseminating (SERS). In amazing cases the upgrade is strong to the point that even single atom discovery gets to be conceivable. The SERS upgrade system is quite entangled. From a hypothetical perspective, the aggregate SERS upgrade includes

(i)            Chemical improvement

(ii)           Resonance Raman improvement

(iii)          Charge-exchange resonance improvement

(iv)          Plasmon reverberation improvement forms

By a variable of around 106. Raman spectra emerge from the vibrational frequencies of particles and give ‘sub-atomic unique finger impression’ data that is especially profitable in science. The characteristically low affectability of traditional Raman disseminating limits its pertinence, yet affectability upgrade by SERS has brought about more boundless applications, particularly in surface science where the ecological affectability of vibrational spectra uncovers how particles associate with surfaces

DISCOVERY OF SERS

Van Duyne coined the SERS acronym the fundamental donor to the intensity improvement is an electromagnetic impact emerging from the laser excitation of limited surface Plasmon (aggregate electron motions) at unpleasant metal surfaces, which makes an upgraded electric field (E). Both the occurrence and scattered light are impacted by this field improvement, bringing about an aggregate Raman signal upgrade relative to E4. The trial circumstance is indicated schematically in figure 1 .A small commitment to SERS improvements originates from a charge exchange instrument for adsorbed particles with suitable acceptor or giver orbitals that collaborate with the metal substrate. The metals showing the biggest SERS improvements are silver, gold and copper.

Figure 3

A schematic representation of a SERS experiment with pyridine adsorbed on silver, showing the incident laser and Raman scattered light, the intensities of which are both influenced by the enhanced field at the silver surface resulting from surface Plasmon excitations.

At that stage, from the spectra and the reliance on potential, it appeared that the band at 1008 and 1037 cm−Being mindful of unusually substantial watched signs, and this was puzzling and unexplained. Was an insoluble compound being framed at the surface that was more dissolvable on decrease? Observing over the more extensive range they found that there were additionally adsorbed pyridine groups in the areas around 3060, 1210 and 650 cm−1 and a wide band topping at 235 cm−1. I noticed, ‘The main band of interest is the Ag-Py 235 cm−1 metal-ligand vibration. This seems to associate with the 1025 cm−1 band.’ The specialists on had a Discussion on Intermediates in Electrochemical Reactions at Oxford on 19 September. Having set up sensible reproducibility with the Ag/pyridine framework, they looked to build up the simplification of the marvel in leading trials on silver with collidine, 3-and 4-methyl pyridine, and different amines and also Pt/benzene. There were some reassuring signs for the fairly insoluble collidine however nothing from the Pt/benzene framework.

The scientists Martin, Pat and others met intermittently to talk about the outcomes and technique. These were enthusiastic gatherings, rich with thoughts, and a decent arrangement of filtering was required toward the end to deal with the most ideal path forward. They when Martin thought of the possibility that the movements in the physisorbed pyridine groups with potential may be because of terminal surface water reorientation as the surface charge changes sign( Granger et al 2016). They had not found any major new exploratory results, regardless of various thoughts having been sought after, and started assembling a paper for Chemical Physics Letters. This was at last sent to David Buckingham on 21 February 1974 after impressive examination over uncertain viewpoints, for example, the abnormally high watched Raman intensities. At last no remark was incorporated on the intensities. The reaction from David Buckingham not exactly a week later was to say that the paper had been acknowledged and they had found a fascinating phenomena and this is how SERS was discovered.

THE THEORY OF SURFACE ENHANCED RAMAN SCATTERING

By considering the molecule and metal to form a conjoined system, we derive an expression for the observed Raman spectrum in surface-enhanced Raman scattering. The metal levels are considered to consist of a continuum with levels filled up to the Fermi level, and empty above, while the molecule has discrete levels filled up to the highest occupied orbital, and empty above that( Carvalho et al 2015). It is presumed that the Fermi level of the metal lies between the highest filled and the lowest unfilled level of the molecule.

The molecule levels are then coupled to the metal continuum both in the filled and unfilled levels, and using the solutions to this problem provided by Fano, we derive an expression for the transition amplitude between the ground stationary state and some excited stationary state of the molecule-metal system. It is shown that three resonances contribute to the overall enhancement; namely, the surface plasmon resonance, the molecular resonances, as well as charge-transfer resonances between the molecule and metal. Furthermore, these resonances are linked by terms in the numerator, which result in SERS selection rules. These linked resonances cannot be separated, accounting for many of the observed SERS phenomena ( Bonal et al 2016). The molecule-metal coupling is interpreted in terms of a deformation potential which is compared to the Herzberg-Teller vibronic coupling constant. We show that one term in the sum involves coupling between the surface plasmon transition dipole and the molecular transition dipole. They are coupled through the deformation potential connecting to charge-transfer states. Another term is shown to involve coupling between the charge-transfer transition and the molecular transition dipoles. These are coupled by the deformation potential connecting to Plasmon resonance states. By applying the selection rules to the cases of dimmer and trimmer nanoparticles we show that the SERS spectrum can vary considerably with excitation wavelength, depending on which Plasmon and or charge-transfer resonance is excited.

DIFFERENCENCES BETWEEN SERS AND RAMAN

SERS has the enhancement of the signal, that sometimes cannot be measured by classical Raman spectroscopy because of the low concentration of the analyte.SERS spectra and Raman spectra are basically similar, but they are not always exactly the same. The SERS effect is highly localized (the electrical field normal to the surface), therefore, only a signal from part of the molecule, close to the substrate, will be enhanced. When the molecule is adsorbed on the surface, its symmetry might slightly change, and so do the selection rules. Also, because Plasmon resonance at the origin of the electromagnetic field enhancement is wavelength dependent, the different spectral regions of the spectrum may be enhanced differently(Grass et al 2015).

SERS is a Raman Spectroscopy technique that provides a greatly enhanced Raman signal from Raman active molecules adsorbed onto certain metal surfaces. This is because the surface Plasmon resonance   is the resonant oscillation of conduction electrons at the interface between a negative and positive permittivity material stimulated by incident light. in synthesis, the difference with Raman Spectroscopy is the use of a material with Plasmonic resonances and the increases in the intensity of Raman signal have been regularly observed on the order of 10⁴-10⁶,andcanbeashighas10⁸and 10⁴ for some systems ( Fan et al 2015).

SERS greatly enhances the normal Raman signal in the order of 6-8 and even it can go up to 14 orders of magnitude. Forbidden signals of Raman can be appear in SERS.

PLASMONIC SURFACE

It has been known since Roman times, that by blending silver and gold colloids with glass, the last obtains dynamic hues. This property was generally used during the time to brighten windows in houses of worship and churches (recolored glass). In the nineteenth century, Michael Faraday was most likely the first to think about the shades of gold particles deductively (Bergman et al 2013). However  the mystery of stained glass was not divulged until a German physicist Gustav Mie introduced a hypothesis that clarified the optical properties of circles in 1908 The purported Mie hypothesis demonstrated that the circle measure, its dielectric capacity and the refractive file of the encompassing medium decides the shade of metal nanoparticles, for instance gold or silver. The electronic excitations in charge of these lively nanoparticle hues are presently known as confined surface plasmons (LSP’s) or molecule plasmons (Bryche et al 2016).

In the previous two decades the plasmonics range encounters an amazing and about exponential development. It would be a troublesome assignment to specify all applications identified with the nano plasmonics field. In any case, likely the biggest application territories would be bio detecting, restorative diagnostics and biomedical examination, nanoantennas, Plasmon waveguides, met materials, surface-upgraded Raman dissipating spectroscopy and optical gadgets. It is critical we see surface Plasmon (SP’s (Kurouski et al 2015). A surface Plasmon polariton (SPP) is a coupled charge-thickness electromagnetic wave that is bound to the interface between a material with a negative dielectric consistent (for occasion, silver or gold) and a material with a positive dielectric steady (for occurrence, air or glass).

Figure 4

Theoretical example of surface Plasmon generation utilizing a 60 nm slit in 20 nm thick Au film, A and B (zoom out). Both maps show y-component of the electric field. The incident wave (720 nm) is TM polarized and is perpendicular to the metal surface.

The SPP’s attract important  interest because it opens some new of manipulating  light on nanometer scales. efforts have been there aimed to  integrate optical devices  circuits and futurimg all-optical photonic chips.  A SPP propagation along a  metallic surfaces   reaches a few tens of micrometers       mirrors   waveguides and interferometers   have been totally fabricated. The SPP’s is as applied in  observing  molecular binding occasions with a detection limitation of approximately 0.003 nm in adsorb ate thickness    . Techniques  are there   used to launch SP’s. In Figure 1, a theoretical example using a 60 nm slit in 20 nm thick Au film is shown.

THE USED OF RAMAN&SERS IN BIOLOGICAL SAMPLES ( BACTERIA ) TAKE E. COLI AS AN EXAMPLE

The  recovery of live bacteria is useful  for consistent whole cell SERS characterization. The SERS biosensor can  detect , identify, and classify   bacteria  from human serum. Bacteria can be identified at  species level, and the potential for detecting poly-microbial cultures by the unique spectra   they generate and  can be demonstrated.

Raman signal for recovered bacteria is mostly generated from   combination of inactive cells, active viable cells, and changed cell membranes, let us understand that SERS measurement is sensitive to     smallest changes in biomolecular composition. Lysis filtration  purified hydrophilic bacteria  without affecting Raman spectra. The is shifts in relative peak intensity  of SERS spectra for hydrophobic bacteria because of   lysis filtration  are there  and should not be seen.

The  inspection  of both spectra uncovers numerous differences . Be that as it may, distinction is seen in the locale from 500 to 700 cm−1. While the five tops at 526, 564, 582, 610, and 664 cm−1 show up in this area on the spectra gained from the example arranged with basic blending, just three crests at 520, 571, and 621 cm−1 are seen in microwells. The tops around 500 cm−1 on the SERS of proteins and peptides were credited to S–S stretch and the crest around 520 and 526 cm−1 on both spectra can be allocated to S–S stretch ]. Because of the way that thiol moieties communicate firmly with AgNPs, the band movements might be seen on the SERS endless supply of such gatherings on the bacterial cell divider with the AgNPs while considering the free AgNPs present after blending with bacterial cells. The tops on basic blending spectra at 564, 582, 610, and 664 cm−1 and 660 and 630 cm−1 could be ascribed to starches and COO-separately. Note that we prior confirmed that the wellspring of phantom elements was beginning from the microscopic organisms However, as seen above, even with the utilization of same laser and kind of the NPs, minor departure from the ghastly components are observed

THE DIFFICULTIES OF USING RAMAN &SERS IN BIOLOGICAL SAMPLE

The process is too tiresome and an error is likely to occur during the experiment which might consequently give wrong results and conclusion. Another difficult is that there is variation on the spectral features observed which might give wrong information. There might also it is not easy to make the comparison since the SERS and Raman looks similar though they are quite different.

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