Surface sensitivity, SERS

The label-free detection of biomolecules is another promising field of application for SERS spectroscopy. Tiniest amounts of these molecules can be adsorbed by specific interactions with receptors immobilized on SERS-active surfaces. They can then be identified by their spectra, or specific interactions can be distinguished from unspecific interactions by monitoring characteristic changes in the conformation sensitive SERS spectra of the receptors. 

Numerous SERS studies of adsorbed molecules have appeared in the literature. Obviously, it is a useful method for the identification of species at the interface, and its inherent surface sensitivity is an attractive feature. In this context it should be noted that the adsorption of a molecule can change the selection rules for Raman scattering, and modes that are Raman inactive in the isolated molecule may show up in SERS. 

IETS is well suited for investigating the adsorption of organofunctional-alkoxy silanes on alumina. Surface sensitivity at the sub-monolayer level, and orientational selectivity makes this technique particularly attractive, especially when used in conjunction with other techniques such as FT-IR, and SER spectroscopies. 

It has now been demonstrated that many molecules adsorbed on appropriately prepared metal surfaces display Raman cross-sections several orders of magnitude greater than the corresponding quantity for an isolated molecule or from a solution. Together with other surface-sensitive techniques, SERS has catalyzed the study of condensed phases on surfaces. It has demonstrated promise as a vibrational probe of in situ gas-solid, liquid-solid, and solid-solid environments, as well as a high-resolution probe of vacuum-solid interfaces. 

R. A. Dluhy, S.M. Stephens, S. Widayatl and A.D. Williams, Vibrational Spectroscopy of Biophysical Monolayers. Applications of IR and Raman Spectroscopy to Biomembrane Model Systems at Interfaces, Spectrochim. Acta Part A51 (1995) 1413. (Review on biomembrane model systems studied by surface-sensitive vibrational spectroscopic methods. In particular the following methods are surveyed external reflectance IR spectroscopy, wave-guide Raman spectroscopy cmd SERS.)... 

Raman spectroscopy is not particularly surface sensitive but a surface enhanced Raman scattering (SERS) effect is observed on some metals (copper, gold, silver, nickel etc). On an appropriately prepared (roughened) surface or on metal colloids the surface coverage of molecules can be measured by Raman spectroscopy with high sensitivity. 

The tremendous advances that have occurred in the spectroscopic analysis of the electrode/electrolyte interface have begun to provide a fundamental understanding of the elementary processes and the influence of process conditions. Surface-sensitive spectroscopic and microscopic analyses such as surface-enhanced Raman scattering (SERS) [1], potential-difference infrared spectroscopy (PDIRS) [2], surface-enhanced infrared spectroscopy (SEIRS) [3], sum frequency generation (SFG) [4], and scanning tunneling microscopy (STM) [5,6] have enabled the direct observation of potential-dependent changes in molecular structure [2,7] chemisorption [8,9], reactivity [10], and surface reconstruction [11]. 

Ionic liquids at the gas-liquid and solid-liquid interface have been extensively studied by a variety of surface analytical techniques. The most prominent technique for surface orientational analysis proves to be SFG. Other vibrational spectroscopic and surface-sensitive techniques such as surface-enhanced Raman spectroscopy (SERS) and total internal reflection Raman spectroscopy (TIR Raman) have been employed for studying surface processes these techniques, however, have not been applied yet specifically for the study of ionic hquids. 

SERS does, however, have some limitations. For the SERS effect to occur, the analyte needs to be adsorbed onto or in close proximity to the roughened metal surface, and only a few metals have so far been shown to be efficient at providing surface enhancement. SERS intensities are also dependent on the roughness of the metal surface, and there are significant problems associated with the preparation of reproducible substrates with uniform roughness features. The spectra obtained are also dependent on the orientation of the molecule on the metal surface, and vibrations with little to no intensity in normal Raman scattering can become relatively intense. This can in some cases make it difficult to identify the analyte positively. Eurther, contamination can also be a problem. Since SERS is very sensitive, it is possible that very small amounts of a contaminant can be enhanced, and if the analyte to be considered does... 

Quite recently the complex of 2-(4-hydroxyphenylazo) benzoic acid (HABA) and avidin was presented as a model system of chromophore-containing proteins [79]. HABA was chosen because its spectrum is known to be very sensitive to the molecule environment, which makes it an ideal probe for eventual structural changes accompanying adsorption of the complex on the Ag electrode surface [79]. The fact that normal solution resonance Raman (RR) and the SERRS spectra of avidin-HABA complex are almost identical and those of free (uncomplexed) HABA are different clearly demonstrates that the native form of the complex is preserved on the surface. HABA-SERS active groups of its complex with avidin are buried within the protein matrix. Hence, it is evident that the long-range electromagnetic effect operates, so that it should be possible to obtain SERS spectra of chromophores buried in more or less native proteins [79]. 

I Surface enhanced Raman spectroscopy (SERS) involves adsorbing molecules in a metal surface or depositing metal nanoparticles on a surface. The amount of Raman signal obtained can be enhanced by 1 Oj, making what is normally a bulk technique extremely surface sensitive. 

The surface enhanced Raman spectroscopy is a surface sensitive technique with a greatly enhanced signal by molecules adsorbed on rough metal surfaces the enhancement factor can be as much as 10 " -10, which permits this technique to be sensitive enough to detect single molecules. In most experiments the SERS... 

The advent of surface-enhanced Raman spectroscopy [435] allows studying extremely low concentrations of molecules on surfaces. Yet SERS is still a rarely applied vibrational technique. Because SERS provides both rich spectroscopic information and high sensitivity as a result of the large enhancement effect, it is an ideal tool for trace analysis as well as for in situ investigations of interfacial phenomena. A number of investigations has explored the possibility of using SERS for direct analysis of species separated by TEC, HPLC and GC. Tran [436] reported sub-ng detection of dyes on filter paper by SERS. 

The ability to probe surfaces using in situ SERS can be exploited in polymer chemistry to characterise the surface of polymers for comparison with the bulk properties and to study polymer-metal composites such as adhesives and coatings. Interactions between adhesives and metal polymeric surfaces have been investigated [437]. The applicability of SERS to polymer surfaces has also been reported in a study on poly-/ -nitrostyrene [438], and in another study on the effects of chromic/sulfuric acid etching on PE films [439]. The desired surface sensitivity... 

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