Spectroscopy comparison with SERS

If an enhancement of five to six orders of magnitude can be achieved routinely, TERS for small molecules, which are not in resonance with the laser line, is within reach. These molecules have cross sections of the order of (dff/dfJ) 10 cm sr and are barely seen as adlayers on smooth interfaces by normal Raman spectroscopy. To test this, benzenethiol was chosen, which assumes an essentially vertical orientation to the surface due to its thiol group. The adlayer preparation is quick and easy the adsorption occurs from an etha-nolic solution, and a self-assembled monolayer is formed on previously flame-annealed gold or platinum surfaces. Fig. 10.26 shows TERS spectra for benzenethiol at Au(llO) and Pt(llO) surfaces. The comparison of the spectra reveals the characteristic benzenethiol bands but with sHghtly different band positions and relative intensities for the two samples and a nearly 20-fold lower intensity level for benzenethiol at Pt(llO). The comparison of these data with SER spectra for... 

Abstract Surface-enhanced Raman microprobe spectroscopy (micro-SERS) and near-infrared Fourier transform SERS spectroscopy (NIR-FT-SERS) are used to study, in situ, the adsorption process of alkylpyridinium bromide (C PyBr) and alkyltrimethylammonium bromide (C TAB) adsorbed on charged silver nanoparticle surfaces. Vibrational assignment was achieved by comparison of observed band position and intensity in the Raman spectra with wave numbers and intensities from ab initio LCAO-MO-SCF Hartree-Fock calculations at the 6-3IG level. 

A study that combined cyclic voltammetry with a time-resolved, surface-enhanced Raman spectroscopy (SERS) examination of the electroreduction of p-nitrobenzoic acid (PNBA) provided a more complete description of this complex mechanism. The reductive cyclic voltammogram to — 1.4 V is shown in Figure 3-21 (pH = 11), for smooth and roughened Ag electrodes. The use of roughened electrodes is necessary for the SERS experiment. While an adsorption prepeak was indicated on the roughened electrodes, the overall cyclic voltammetric responses for both electrodes were remarkably similar. In such cases, adsorption equilibrium is probably obtained, and qualitative comparisons between SERS studies on roughened electrodes and CV studies on smooth electrodes are possible. 

The mechanism of the PLP-dependent P-reaction involves a number of different chemical transformations (scheme 1B). The reaction requires the forma-tion/scission of C-C, C-O, C-N, C-H, N-H, and O-H bonds and the pathway for the synthesis of i-Trp from i-Ser and indole involves a minimum of at least eight distinct PLP-intermediates. RSSF spectroscopy allows direct detection and spectral characterization of the various catalytic intermediates, which accumulate during the course of the reaction (85,86). Information from RSSF spectroscopic investigations is greatly enhanced by the use of both isotopically labeled substrates (85) and substrate analogs (82), which alter the accumulation of intermediates during the presteady state phase of the reaction. Direct comparison of RSSF spectra for deuterium labeled substrates with the isotopically normal compounds is a powerful tool for the identification and assignment of chromophoric reaction intermediates (85). Finally, structure-function relationships within the bienzyme complex may be addressed by careful comparison of the time-re-solved RSSF spectra for reactions of native and mutant enzyme species (87-89). 

The reaction scheme of Bode [11] was derived by comparison of the X-ray diffraction patterns of the active materials with those for the model compounds. How the 8-Ni(OH)2 in battery electrodes differs from the model compound is discussed in Section 5.3.I.3. In recent years, the arsenal of in situ techniques for electrode characterization has greatly increased. Most of the results confirm Bode s reaction scheme and essentially all the features of the proposed a/y cycle. For instance, recent atomic force microscopy (AFM) of o -Ni(OH)2 shows results consistent with a contraction of the interlayer distance fiom 8.05 to 7.2 A on charge [61-63]. These are the respective interlayer dimensions for the model a-Ni(OH)2 and y-NiOOH compounds. Electrochemical quartz crystal microbalance (ECQM) measurements also confirm the ingress of alkali metal cations into the lattice upon the conversion of a-Ni(OH)2 to y-NiOOH [45,64,65]. However, in situ Raman and surface-enhanced Raman spectroscopy (SERS) results on electrostretching modes that are consistent with a weakening of the O-H bond when compared with results for the model a- and 8-Ni(OH)2 compounds [66]. This has been ascribed to the delocalization of protons by intercalated water and Na ions. Similar effects have been seen in passive films on nickel in borate buffer electrolytes [67]. 

A particularly salient example of the complementarity of normal Raman spectroscopy with other techniques is expressed in a recent publication of the combined data from infrared, normal Raman, SERS, SEHRS, and theoretical predictions for one molecule. First, ab initio theoretical predictions were made for the vibrational characteristics of fran5-l,2-bis(4-pyridyl)ethylene (BPE) at the Hartree-Fock 6-31G level. When the spectra were collected, comparisons were made between the theoretical and experimental results as well as among the different spectra. Based on the known selection rules for each spectroscopy and the matching of wavenumber shifts to theoretical predictions, all vibrational bands were assigned. 

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