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In 1921, Sir Chandrasckhara Venkata Raman from India observed a new spectroscopic effect by utilizing simple optical tools, this observation leads him later to discover what we called nowadays, Raman effect. Later in 1930 the Nobel Prize granted to C. V. Raman for his discovery. At first Raman noticed that the new type of spectra, emitted from organic solutions, were differ from other well-known spectra like fluorescence. Moreover, many scientists today thought that what Raman saw with his eyes is anti-stock Raman scattering. Raman published his observations in Nature 1921 under "A New Type of Secondary Radiation". This is the story for discovering of Raman scattering effect which is later become one of the most advanced and powerful analytical technique.
Basic principles of Scattering processes
In order to fully understanding the essential concepts of surface enhanced Raman scattering effect, we should at first understand the principles of Raman spectroscopy. In general, spectroscopic techniques have been used widely for analysis samples from deferent materials and usually the properties of the emitted or absorbed or scattered spectrum depend on the nature of the analyte and the physical phenomena that happened during the interaction between the analyte and the incident spectrum (photons). There are two types of optical processes that involved when incident photons interact with substance. First one consists of two separated steps; absorption and emission. Second one involves optical scattering process which means simultaneous absorption and emission at the same time. The new emitted (scattered) photon will have different energy from the incident one and in most cases this photon reveals a specific structural property of the sample. Scattering process can be classified into the following category:
Elastic scattering: this type of optical scattering occurs when the scattered and incident photons have the same amount of energy but different polarization. If this happened in molecules, it usually called Rayleigh scattering which involves no energy transfer between molecules and photons as shown in figure (1)(a). Rayleigh scattering does not useful as analytical technique because it does not reveal any information about the internal structure of the molecules. If the elastic scattering happened on nanosacle materials, they usually referred to it as Mie scattering.
Inelastic scattering: this type of scattering can be considered when the scattered photons (ES) have different energy than that of incident photons (EL). Raman scattering is one of inelastic scattering phenomenon which involves electronic transitions in vibrational and rotational molecular energy levels as shown in figure (1) (b).
Raman scattering differs from fluorescence emission in many aspects. Fluorescence process involves two stages the first is excitation of electron by incident photon then followed by emission the absorbed energy as fluorescence photon. In Raman scattering there is now intermediate step, that means when incident photon interacts with molecule will replace simultaneously (directly) by another emitted photon. One of the major benefits from this is that Raman scattering can happen even the molecule does not have electronic transition at incident spectrum wavelength. Furthermore, fluorescence and absorption techniques usually cannot be used in transparent regions because there is no molecular absorption. However, Raman scattering can be used in transparent regions, it does not required absorption process. There is one disadvantage of scattering process when comparing with absorption and fluorescence is Raman scattering is a weak or very weak effect. This weakness in Raman scattering signal is the main reason that this techniques dose not useful for many years after the discovery, but later when a sophisticated developments have been occurred in both instrumentation and nano-science and the discovery of the surface enhanced Raman scattering, it became one of the most promise and advanced technology for both industrial and medical applications.
Stock and anti-Stock Raman scattering
Scattered photons in Raman scattering process may have higher or lower energy when compare with incident photons:
Stock process: this occurs when the scattered photon has lower energy than that for the incident photon. This process involves vibrational excitation of the molecule from level (v=0) to level (v=1), see figure (2).
Anti-Stock process: this occurs when the scattered photon has higher energy than that for incident one. The molecule will relax from higher vibrational level (v=1) to lower one (v=0), see figure (2).
Apparently, anti-stock Raman spectrum has much weaker peaks and intensities when compare with stock Raman spectrum, see figure (2).
Figure (2) From Ref. : Jablonski diagram for stock and anti-stock Raman spectrum
The difference in energy between incident and scattered photon is so called Raman shift and it usually measured by wavenumber units (cm-1). Raman shift can be calculated from the following equation Î”ER=EL-ES, its positive for Stock process and negative for anti-Stock process.
4. Raman Spectroscopy Instrumentation
Raman spectrometry as a powerful analytical technique dose not finds its way until the recent development in instrumentation. In C. V. Raman time, he used a highly concentrated solution so he saw the scattered light by his eyes. Unfortunately, most of the samples that required to be analyzed are much diluted, and thus Raman spectrum will be very weak and need special instrumentation. As a direct result of that, many applications did not find their way to Raman spectroscopy until the using of laser as light source in Raman spectrophotometer and the huge development in electronic detection devices such as photomultiplier tubes (PMT), multichannel charge coupled devices (CCD). Laser is used widely in Raman spectrophotometer because it has a very intense and monochromatic radiation which is the main requirement for Raman scattering to be occurred. Nowadays, it is possible to analyze many substances by Raman spectrometry that would never be analyzed by this technique at the time of Raman discovery. Furthermore, many commercial companies have made very simple and portable Raman spectrophotometers which allowed the using of this technique for a wide range of users.
5. Applications of Raman Spectroscopy
Because of the recent developments in Raman instrumentation, Raman spectroscopy has a wide range and rapid applications in different field. W. Kiefer , has published an intensive reviews of the recent application in Raman spectroscopy including linear and nonlinear applications. In the First review, Kiefer reviews about 299 papers published just in Journal of Raman Spectroscopy (JRS) during the time period 2005 to 2006. In the second one, he reviews 207 papers, published in 2007 and also only in JRS. The application fields that can be summarized from these two reviews stated as following:
Art and archaeology: pottery, ceramics, porcelain, shards, glasses, shells, textiles, books, journals, manuscripts, posters, and historical materials.
Biosciences: biomolecules, cells and bacteria, plants, fibers, and medicine.
Vibrational studies in chemistry
Solid state: minerals, crystals, glasses, ceramics, and organic and inorganic crystals.
Liquid and liquid interactions: binary mixture, and noncoincidence effect.
Nanomaterilas: carbon nanotubes, nanoparticles and nanocrystalline thin films.
Physics and chemistry of High pressured materials.
(10) Forensic science.
Kiefer said in his conclusion "Sometime I am asking myself what can not be done with Raman spectroscopy".
6. Surface Enhanced Raman Scattering (SERS)
Surface-enhanced Raman spectroscopy (SERS) was first observed by Fleischman et al. in 1974, and discovered by Jeanmarie and Van Duyne and Albrecht and Creightonin 1977. We should distinct between the observation and the discovery because at first Fleischman thought that this enhancement produced from the increasing in the surface area of the substrate. Later, Van Duyne and Creighton who explain that the amplification of the signal comes from the effective Raman cross section and that from the excess of the increased number of molecules required as a result of the surface's roughness factor. In 1978, it was pointed out that SERS was a result of the excitation of surface plasmons. At that time, many researchers predicted that SERS have to happen in metals colloids. Silver and alkali metals produced most intensive SERS followed by gold and copper, then aluminum, indium, and platinum, and finally transition elements. Besides that, other parameters like excitation wavelength, the polarization of the exciting and scattered light with respect to symmetry axes of the nanostructures elucidated, the precise structural features of the SERS-active system. Over the last 30 years, everything that has been learned about SERS was a result of the very close matching between SERS and the intensity of the Plasmon.
Raman scattering that coming from the molecule can be amplified by several magnitude of its original signal, this simply what called SERS. The original of this enhancement produces when electromagnetic radiation, usually laser beam, interacts with a substrate made from metals, this will produce high excitations called plasmon resonance. In order to profit from this phenomenon, the substances must be on or close enough to the surface of the substrate (~10 nm max).
7. SERS substrates
Suitable efficient substrate that supports good SERS is that substrate capable to support a strong plasmon resonance which in turn will generate a highly amplified SERS signals. However, many substrates can generate enhancement, but few of them capable to provide uniform and reproducible signals. In particular applications such as single molecule detection, uniform SERS signals are required. Moreover, SERS depends on the producing of resonant response of the substrate, so it depends on wavelength of the excitation radiation. Consequently, SERS substrate will exhibits different enhancements when exited by different wavelength, which means the substrate required a specific range of wavelength matching the perfect resonance to provide maximum magnitude of enhancement. Based on that explanation, any SERS substrate exited at wrong wavelength will be considered as a bad SERS substrate. Raman scattering spectrometry for molecules usually requires excitation wavelength in visible or near infrared regions (~400-1000 nm), so it is essential for any SERS substrate in order to work properly, it should be operated in this wavelength range.
As a general rule to achieve the enhancement the following points should be considered:
Silver (Ag) and Gold (Au) substrates are the two widely used metals to study and applied SERS and plasmonics. This extensively used of these metals is simply because they have a unique property which is supporting providing a highly sustain plasmon resonance in visible and near infrared regions.
Geometrical properties of the substrate: substrate in dimensions of less than 100 nm is required to generate sustain enhancement. In this particular point, we could understand the connection between SERS and the nanotechnology. Despite of this requirement, one should keep in mind that a large size metallic surface can be used also as SERS substrate because it already has nano-structural shapes on the surface.
Obviosluy, these two parameters or requirements are not compulsory, many substrates made from other metals such as copper (Cu) or platinum (Pt), or a large flat metallic can amplify Raman signals. However, these substrate will provide much lower enhancement when compare with silver and gold in nano-size substrates.
8. SERS on metallic nanorods
In this section, a brief survey for recent papers that used silver and gold as substrate has been conducted as following:
Seung Joon and etal. , reported that they conducted a simple strategy for placing analyte molecules in hot spots between closely spaced nanowires leading to intense SERS enhancement. The results are highly reproducible in all experiments, this is because of the regularity of the SERS substrate, which consists of highly ordered and regular silver nanowires fabricated in porous alumina.
In 2009, Oh Min Kyung and etal. , used a new method of collecting nanoparticles at a water-hexane mixture on a packed single layer of film. The packed nanoparticles were gold nanospheres with a 26 nm diameter or gold nanorods (NR) with a 31 nm diameter and 74 nm length. Moreover, they test variations in the surface enhanced Raman scattering (SERS) intensities from such nanoparticle films with respect of variation in the layer compositions.
Dhawan Anuj and his colleagues , have engineered surface enhanced Raman scattering substrate. They developed fiber optic sensor and planar substrates containing patterned nanomaterials such as nanoholes in gold films, gold nanoparticles, nanopillars, nanorods, and nanoislands. Several methods of producing gold nanofeatures on fiber tips and planar substrates were conducted such as annealing of thin gold films and focused ion beam (ion bombardment). Excitation of surface plasmons in gold nano-engineered materials leads to sustain enhancement in the Raman signal produced from molecules that attached to the nanostructure surface. It was noticed also that the SERS signal from these gold nanomaterials was much intense than that obtained from a gold film.
In 2007, Yoo Sang-Hoon and Park Sungho , have synthesized and studied a platinum coated gold nanorod arrays. They used electro method for deposition to deposit Au and Ag ions into porous alumina membranes. Platinum wire and Ag/AgO electrode were used as the counter and reference electrodes, respectively. Gold nanorods were electrodeposited in the interior of an anodic alumina template at a constant voltage,-1.0 V versus Ag/AgCl. However, the resulting cyclic voltammograms provided evidence that the gold nanorod surface was changed totally by platinum metal. Bare nanoporous gold nanorod and platinum-coated nanoporous gold nanorod arrays were imaged and scanned by SEM.
Yang Yong and Nogami Masayuki  reported that they have synthesized a self-assembled monolyer of silver nanorods for surface enhanced Raman scattering. They prepared monolayer silver nanorods on glass substrate and then used these nanorods in SERS.
9. Applications of SERS
Raman scattering spectrum provides very useful information of those particular molecules from which Raman scattered light. Raman spectrum for a particular sample (molecule) consists unique peaks (fingerprint) that can be detected with appropriate instrumentation. This feature in Raman spectrum makes from this technique a powerful analytical tool and attracts researchers from all over the world for various applications. As I mentioned before, the only negative aspect that is Raman signal is weak. However, SERS introduce huge changes of that negative concept, SERS has a very intense spectrum, and it is stronger than that for fluorescence and NMR.
In the following survey, samples of most recent interesting biological and medical applications have been briefly summarized;
Hyangah Chon and etal., Have described a surface enhanced Raman scattering (SERS) based gradient optofluidic sensor for a rapid and sensitive immunoassay. In this work, a new microfluidic sensor with functional internal structures has been designed and fabricated. Quantitative analysis of a specific target marker is performed by analyzing its characteristic SERS signals. The detection limits for immunoglobin (IgG) was 1-10 ng/mL. This SERS based sensor for immunoassay is expected to be a powerful analytical tool rapid and sensitive diagnostic perposes.
In 2010 a very interesting field which is cancer detection has been started by Shangyuan Feng . A surface enhanced Raman spectroscopy method was conducted for blood plasma chemicals analysis in the first time to develop a simple blood test for nasopharyngeal cancer detection. Silver nanoparticles as the SERS active substrate were directly mixed with blood plasma to enhance the Raman signals of blood constituents including; proteins, lipids, and nucleic acids. Within 10s a high quality SERS spectrum from blood plasma-Ag NP was recorded. The differences in specific biomolecules, including an increase in the relative amounts of nucleic acid, collagen, phospholipids and phenylalanine and a decrease in the percentage of amino acids and saccharide contents in the blood plasma, have been used to detect nasopharyngeal cancer patients as compared to that of healthy persons. The results from this exploratory study demonstrated great potentials for developing SERS system that can detect cancer in early stages.
Niranjan A. Malvadkar , and his colleagues have developed a new method based on SERS for detection of viral genes. SERS substrates were synthesized by electroless silver metallization and vapor phase gold deposition on nanofilm of poly(chloro-p-xylylene) templates. These substrates exhibit quasi periodic nanomorphology inherited from the underlying template, resulting in highly reproducible SERS signal. These substrates are used for detection of respiratory syncytial virus (RSV) gene which requires high sensitivity, stability, and reproducibility of the Raman signal.
Gobind Das and etal.,  from the University of Catanzaro, Italy have described a nano patterned SERS substrate for protein analysis. They have explain the fabrication method of nano-structures as a surface enhanced Raman scattering substrate by using electroplating and electronbeam lithography techniques to obtain an array of gold nanoparticles aggregation structures of diameter ranging between 80 and 100nm with interspace of 10-30 nm. Attomoles of myoglobin molecules can be detected by this SERS method. This SERS substrate was employed to investigate the structural changes of many proteins such as myoglobin, bovin serum albumin, lysozyme, and ribonuclease in the temperature range between âˆ’65 and 90 â-¦ C.
In 2006, Mahmoud M. A. and Badr Y., have used SERS on silver nanowires to study RNA bases structure. The surface enhanced Raman scattering (SERS) was used to analysis the structure of RNA bases that adsorbed on the surface of silver nanowires. The RNA bases are vertically oriented on the surface of Ag nanowires. Consequently, the in-plane bands were enhanced according to the SERS selection rule. Many new bands corresponding to the base surface bond were shown in the spectra.
Ghodke Harshad B. and etal., have used SERS to characterized a new hybrid nanowire prepared from i-tetraplex and non-Watson-Crick base-pairing DNA bases.
In 2008, Primera Pedrozo and his colleagues  have used SERS based on nanotechnology to detect some explosives and biological agents stimulants. Results have led to extend existing recorded limits of detection of 10-7 M to 10-8 M (10-15 g) in DNT and to 10-12 M (10 -19 g) for TNT.
In 2008, at the University of Georgia, Driskell Jeremy D. and his team  have utilized silver nanorods array with SERS to detect some infectious agents. They have prepared an aligned silver nanorod array substrates by oblique angle deposition method. This silver nanorod array exhibit fascinating enhancement factors (âˆ¼ 5 x 108). It was found that the enhancement factor depends directly on the length of silver nanorods, the substrate coating, the polarization of the excitation light, and the incident angle. The researchers have demonstrated that this method can be used as a sensor to detect many types of human pathogens such as the bacterium mycoplasma pneumonia, rotavirus, human immunodeficiency virus, and respiratory syncytial virus.