Surface-Enhanced Raman Instrumentation

In surface-enhanced Raman spectroscopy (SERS), increased Raman signals are observed from molecules attached to metallic clusters ranging in size of the order of tens of nanometres (a. 16). Enhancements as high as lO have been observed. These enhancement factors can lead to single-molecule Raman spectroscopy (a. 17). Using a Raman microscope the probe volume can be as small as 10 picolitres. Spectra can be measured with a one-second collection time. 

The enhancement of Raman scattering at noble metal (gold, platinum, etc.) surfaces can be attributed to two factors. One is an electromagnetic effect (discussed above) observed for molecules near roughened noble metal surfaces, and the second is a chemical interaction 

When the incident radiation interacts with the surface, it causes the free electrons to oscillate with the incident electric field and polarises the noble metal particles. This creates a strong local electric field at the particle surface known as a surface plasmon. When a molecule is in close proximity to a noble metal particle the molecule is polarised by the electric field of the noble metal particle. This leads to an enhancement of the Raman signal because the Raman scattering is proportional to the square of the local electric field. 

A dual-fibre optical probe has been developed for Raman spectroscopic monitoring of a number of polymer processes. In the dual-fibre configuration the internal bundles transmit the light to the sample and the outer fibres carry the signals back to the spectrometer for processing. 

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Vo-Dinh T., Stokes D.L., Griffin G.D., Volkan M., Kim U.J., Simon M.I., Surface-enhanced Raman scattering (SERS) method and instrumentation for genomics and biomedical analysis, J. Raman Spectrosc. 1999 30 785-793. 

Fortunately, in favorable cases enhancement mechanisms operate which increase the signal from the interface by a factor of 105 — 106, so that spectra of good quality can be observed - hence the name surface-enhanced Raman spectroscopy (SERS). However, these mechanisms seem to operate only on metals with broad free-electron-like bands, in particular on the sp metals copper, silver and gold. Furthermore, the electrodes must be roughened on a microscopic scale. These conditions severely limit the applicability of Raman spectroscopy to electrochemical interfaces. Nevertheless, SERS is a fascinating phenomenon, and though not universally applicable, it can yield valuable information on many interesting systems, and its usefulness is expected to increase as instrumentation and preparation techniques improve. 

Ever since Albrecht and Creighton [85], and Jenmarie and van Duyne [86], observed that the Raman cross-section for pyridine absorbed on a roughened silver electrode was larger than that in solution by six orders of magnitude, surface enhanced Raman spectroscopy (SERS) has steadily gained ground in analytical instrumentation. The sensitivity of this technique... 

Surface-Enhanced Raman Spectroscopy (SERS) SERS is a technique for increasing the sensitivity of a Raman instrument. This module explores its application to the detection of chemicals indicative of brain injury. 

Near-infrared surface-enhanced Raman spectroscopy Some of the major irritants in Raman measurements are sample fluorescence and photochemistry. However, with the help of Fourier transform (FT) Raman instruments, near-infrared (near-IR) Raman spectroscopy has become an excellent technique for eliminating sample fluorescence and photochemistry in Raman measurements. As demonstrated recently, the range of near-IR Raman techniques can be extended to include near-IR SERS. Near-IR SERS reduces the magnitude of the fluorescence problem because near-IR excitation eliminates most sources of luminescence. Potential applications of near-IR SERS are in environmental monitoring and ultrasensitive detection of highly luminescent molecules [11]. 

A prerequisite for the development indicated above to occur, is a parallel development in instrumentation to facilitate both physical and chemical characterization. TEM and SPM based methods will continue to play a central role in this development, since they possess the required nanometer (and subnanometer) spatial resolution. Optical spectroscopy using reflection adsorption infrared spectroscopy (RAIRS), polarization modulation infrared adsorption reflection spectroscopy (PM-IRRAS), second harmonic generation (SFIG), sum frequency generation (SFG), various in situ X-ray absorption (XAFS) and X-ray diffraction spectroscopies (XRD), and maybe also surface enhanced Raman scattering (SERS), etc., will play an important role when characterizing adsorbates on catalyst surfaces under reaction conditions. Few other methods fulfill the requirements of being able to operate over a wide pressure gap (to several atmospheres) and to be nondestructive. 

This chapter has been organized by considering several aspects. An introduction concerning the relevance of the electronic properties and applications of the azamacrocycles related to surface phenomena as well as the general aspects and characteristics of the vibrational techniques, instruments and surfaces normally used in the study of the adsorbate-surface interaction. The vibrational enhanced Raman and infrared surface spectroscopies, along with the reflection-absorption infrared spectroscopy to the study of the interaction of several azamacrocycles with different metal surfaces are discussed. The analysis of the most recent publications concerning data on bands assignment, normal coordinate analysis, surface-enhanced Raman and infrared spectroscopies, reflection-absorption infrared spectra and theoretical calculations on models of the adsorbate-substrate interaction is performed. Finally, new trends about modified metal surfaces for surface-enhanced vibrational studies of new macrocycles and different molecular systems are commented. 

Sampling, sample handling, and storage and sample preparation methods are extensively covered, and modern methods such as accelerated solvent extraction, solid-phase microextraction (SPME), QuEChERS, and microwave techniques are included. Instrumentation, the analysis of liquids and solids, and applications of NMR are discussed in detail. A section on hyphenated NMR techniques is included, along with an expanded section on MRI and advanced imaging. The IR instrumentation section is focused on FTIR instrumentation. Absorption, emission, and reflectance spectroscopy are discussed, as is ETIR microscopy. ATR has been expanded. Near-IR instrumentation and applications are presented, and the topic of chemometrics is introduced. Coverage of Raman spectroscopy includes resonance Raman, surface-enhanced Raman, and Raman microscopy. 

Despite the extensive studies of the anodic layers on Pt with various ultraviolet-visible optical methods, they have not provided a clear indication of the electronic or structural properties of the layers. Rather these optical methods have been more than just another form of readout to complement the electrochemical measurements of charge and current response of the layer to potential and time. Vibrational spectroscopic data from infrared and Raman measurements would be more helpful in establishing the nature of the layers but it is difficult to use these techniques to study metal-electrolyte and similar interfaces because of solvent interference and sensitivity problems. A noteworthy exception is the quite successful in situ use of Raman spectroscopy to study the electrochemically formed oxide layers on silver by Kotz and Yeager. In the instance of silver electrodes, there is a large surface enhanced Raman effect and the signal-to-noise ratio is not a problem. Unfortunately this is not the situation with other metal surfaces such as Pt. Even so, with improved instrumentation there is hope that in situ Raman studies of the anodic layers on Pt will become practical. 

S. Farquharson, P. Maksymiuk, K. Ong, and S. D. Christesen, Chemical agent identification hy surface-enhanced Raman spectroscopy, in Vibrational Spectroscopy-Based Sensor Systems, S. D. Christesen and A. J. Sedlacek El, Eds., Society of Photo-Optical Instrumentation Engineers, Bellingham, WA, 2002, Vol. 4577, p. 166. 

See also Matrix Isolation Studies By IR and Raman Spectroscopies Nonlinear Optical Properties Nonlinear Raman Spectroscopy, Instruments Nonlinear Raman Spectroscopy, Theory Photoacoustic Spectroscopy, Theory Raman Optical Activity, Applications Raman Optical Activity, Theory Rayleigh Scattering and Raman Spectroscopy, Theory Surface-Enhanced Raman Scattering (SERS), Applications. 

This article describes the elements of Raman spectrometers for routine analyses which are available commercially. Instruments designed only for special research are not covered. Only spectrometers for classical (linear) Raman scattering are mentioned, not those for observing resonance Raman scattering (RRS), surface-enhanced Raman scattering (SERS) and all nonlinear Raman techniques they are described elsewhere in this Encyclopedia. 

See also Biochemical Applications of Fluorescence Spectroscopy Biomacromolecular Applications of UV-Visible Absorption Spectroscopy Chiroptical Spectroscopy, General Theory Chiroptical Spectroscopy, Orientated Molecules and Anisotropic Systems Ellipsometry Surface Plasmon Resonance, Instrumentation Surface Plasmon Resonance, Theory Surface-enhanced Raman Scattering (SERS), Applications. 

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