Surface-enhanced resonance Raman spectroscopy SERRS

3 SURFACE-ENHANCED RESONANCE RAMAN SPECTROSCOPY (SERRS) 

We have seen that the RR and SER effects can each give rise to enhancement in the effective Raman scattering cross-section for a vibrating molecule of up to ca. 106. Clearly, if these two effects can be combined, even more spectacular enhancement of the resulting Raman spectra may result. 

A good illustration of the great sensitivity which can be achieved by means of SERRS is the set of spectra shown in Fig. 3 for the dye molecule R6G on colloidal silver [13]. As is clearly seen, the quality of these spectra is such that the solutions could be further diluted by at least another order of magnitude, yet already these are from nanomolar solutions. 

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The use of surface-enhanced resonance Raman spectroscopy (SERRS) as an identification tool in TLC and HPLC has been investigated in detail. The chemical structures and common names of anionic dyes employed as model compounds are depicted in Fig. 3.88. RP-HPLC separations were performed in an ODS column (100 X 3 mm i.d. particla size 5 pm). The flow rate was 0.7 ml/min and dyes were detected at 500 nm. A heated nitrogen flow (200°C, 3 bar) was employed for spraying the effluent and for evaporating the solvent. Silica and alumina TLC plates were applied as deposition substrates they were moved at a speed of 2 mm/min. Solvents A and B were ammonium acetate-acetic acid buffer (pH = 4.7) containing 25 mM tributylammonium nitrate (TBAN03) and methanol, respectively. The baseline separation of anionic dyes is illustrated in Fig. 3.89. It was established that the limits of identification of the deposited dyes were 10 - 20 ng corresponding to the injected concentrations of 5 - 10 /ig/ml. It was further stated that the combined HPLC-(TLC)-SERRS technique makes possible the safe identification of anionic dyes [150],... 

R.M. Seifar, M.A.F. Altelaar, RJ. Dijkstra, F. Ariese, U.A. Th. Brinkman and C. Gooijer, Surface-enhanced resonance Raman spectroscopy (SERRS) as an identification tool in column liquid chromatography. Anal. Chem., 72 (2000) 5718-5724. 

Certain roughened surfaces (Ag or Au colloids) exhibit another nice intensification of the Raman effect of 103 to 106 by exciting surface plasmons in the colloid particles this is surface-enhanced Raman, first seen by Fleischmann54, and explained by van Duyne.55 Combining resonance and surface-enhanced effects in surface-enhanced resonance Raman spectroscopy (SERRS), the Raman intensity can increase by factors as large as 1012, so that solutions of concentration down to 10 12 M can be detected. 

Since the pioneering work by Cotton et al. on heme proteins (Cotton et al., 1980), surface enhanced resonance Raman spectroscopy (SERRS), Sec. 6.1, has been used to study a large variety of biomolecules, such as retinal proteins (Nabiev et al., 1985), flavoproteins (Coperland et al., 1984 Holt and Cotton, 1987), chlorophylls (Cotton and Van Duyne, 1982 Hildebrandt and Spiro, 1988), and oxyhemoglobins (de Groot and Hesters, 1987). The advantages of this technique include low sample concentration and fluorescence quenching. The main question is whether or not the native structure and function of the molecule is preserved on the metal surface. 

Fundamental questions related to the electronic configuration of the open or colored forms and the number and structures of the photomerocyanine isomers are considered on the basis of the results of continuous-wave (stationary) and time-resolved (picosecond, nanosecond, and millisecond) Raman experiments. For spironaphthoxazine photochromic compounds, the Raman spectra may be attributed to the TTC (trans-trans-cis) isomer having a dominant quinoidal electronic configuration. Surface-enhanced resonance Raman spectroscopy (SERRS) is demonstrated as a new analytical method for the study of the photodegradation process in solution for nitro-BIPS derivatives. The development of this method could lead to the identification of the photoproducts in thin polymer films or sol-gel matrices and ultimately to control of degradation. 

Trace detection and analysis of species at pico- to femtomolar concentrations using surface-enhanced resonance Raman spectroscopy (SERRS). 

A further resonant contribution to the enhancement is possible when the molecule has an electronic transition in resonance, or dose to resonance, with the exciting laser so that one obtains surface-enhanced resonant Raman scattering or surface-enhanced resonant Raman spectroscopy (SERRS). In these circumstances the resonant enhancement typically contributes an additional factor of between 100 and 1000 to the intensity of the signal and this makes SERRS a particularly attractive approach for analytical apphcations because of its extremely high sensitivity. 

Developments in Raman spectroscopy, with applications for colorants, have included resonance Raman, surface enhanced Raman spectroscopy (SERS), surface enhanced resonance Raman spectroscopy (SERRS) and near-infrared Fourier transform Raman spectroscopy (NIR-FT-Raman), with the latter technique discussed in the next section. 

Because of such complications, the surface-enhanced resonance Raman spectroscopy (SERRS) was developed. As it exploits the best features of both the SERS and the RRE, the resulting enhancement of the Raman signal intensity can be as high as 10 ". Additionally, SERRS spectra resemble regular RRE spectra, which make the former much easier to interpret. 

These spectra were obtained with a smooth, polished surface. Thus no surface enhancement was effective. The molecules under investigation can alternatively be deposited onto roughened surfaces and the obtained spectra show a combination of surface and resonance enhancement. The method is called surface enhanced resonance Raman spectroscopy (SERRS). 

NOj(N02/N204) gas and the effect of gas adsorption was monitored using visible and infrared spectroscopy the high sensitivity surface-enhanced resonance-Raman spectroscopy (SERRS) allowed to observe the reversible chemical absorption of NO2 on a monomolecular LB layer of PrPc2 and PrPc. ... 

While the redox chemistry of metal- and flavin-based cofactors may be readily detected by voltammetry, that associated with thiol-disulfides and amino acids is not. It is also important to be aware that voltammetry by itself provides no direct insight into the chemical identity of the redox couple. This can be overcome by using electrodes that allow for simultaneous spectroscopic and voltammetric analysis of adsorbed proteins. Examples include Ag electrodes that allow for surface-enhanced resonance Raman spectroscopy (SERRS) and mesoporous nanocrystalline Sn02 electrodes that allow for electronic absorption or magnetic circular dichro-ism spectroscopies [3]. Another consideration is the need for a redox center to be positioned within ca. 14 A of the electrode, and so the surface of the protein, for facile interfacial electron exchange. As a consequence, proteins with only buried redox centers are not routinely addressed by direct electrochemical methods. 

The conditions for the applicability of surface enhanced Raman spectroscopy for studies of oligo and polynucleotides has been assessed. In particular, surface enhanced resonance Raman spectroscopy (SERRS) has been used to study the binding of chromophores to DNA strands at probe concentrations as low as 10 M. 

In the present project we have investigated whether surface enhanced resonance Raman spectroscopy (SERRS) could be used to study various types of DNA-chromophore interactions. In this technique, a silver colloid is added to the solution containing the molecule to be studied [34]. First, we had to test whether the complex, between dye and DNA, would remain unaltered upon adsorption to the silver colloid surface which is a necessary condition for a valid application of the method. A second system specific question concerns the useful information that can be extracted from frequency shifts and relative intensity alterations in the vibrational spectrum of the DNA-bound chromophore [35-37]. 

Bredirick et al adsorbed CCD on a citrate-reduced silver colloid, and found that adsorbed CCD retains 60-85% of its enzymatic activity in the reaction of catechol substrate with O2 to give cisxis-muconate [112]. The first surface-enhanced resonance Raman spectroscopy (SERRS) study has demonstrated that the native conformation of CCD is retained in the adsorbed state. 

The discovery and understanding of SERS was important not only because it made Raman a more viable analytical method but also because it introduced the concept of surface-enhanced spectroscopies in general. With the SERS precedent, surface-enhanced resonance Raman spectroscopy (SERRS) and surface-enhanced hyper-Raman spectroscopy (SEHRS) have both been discovered and put to use as analytical tools. In fact, enhancement factors as large as 10 have been measured in SEHRS experiments (see Section VII.B). This immense enhancement was only recently surpassed by the 10 " enhancement measured in single molecule SERS (see Section X). 

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