Potential dependence of SERS

By changing the surface morphology which then can influence any one of the four factors enumerated above. This last point is the reason why the electric potential dependence of SERS is included in this section, and the focus of the discussion will be on it. Note that such morphological changes may be associated with irreversible behavior as a function of potential, while one may expect a more reversible dependence for the first three effects. One should keep in mind that it is not always possible to determine which of these above points is operative in a given system. 

Billmann and Otto have shown that the potential dependence of SERS depends also on the presence of other constituents beside the Raman scattering molecule. Thus, pyridine produced a lower signal at -0.8 V than at -1.0 V in the presence of cyanide, while in its absence the inverse was seen. Similarly, the cyanide signal (at 2113 cm ) was higher at -0.8 V than at -1.0 V in the presence of pyridine, and vice versa in its absence. This behavior can perhaps be understood on the basis of competition of the two species over the available surface. The cyanide is more strongly adsorbed at the less cathodic potentials. 

The electric potential dependence of SERS of pyridine on a copper electrode was investigated by Temperini et They find (with 647.1-nm excitation) that the bands seen exhibit qualitatively similar dependence on the potential. The behavior depended on the sweep rate. At low sweep rates (1 mV s ) the signal was constant in the range -1.2 to -0.9 V and decreased monotonously as the voltage was changed to -0.3 V. At a 10-fold-higher scan rate the SERS showed a maximum at about -0.7 V. The ratio between the band intensities did not remain constant, indicating a different quantitative dependence on the potential. 

Although the foregoing account is greatly simplified in that it takes no account of the vitally important roughness features of the surface, nor of its atomic nature, the presence of defects, the pH of the electrolyte, etc., it nonetheless provides a useful model for understanding the observed potential dependence of SERS effects. 

An interesting difference, however, is seen in the potential dependence of the SERS va and features, in that the latter is only observed at the least negative potentials (E > -0.2 V), where the surface infrared vaa band also appears, whereas the former band survives until the most negative potentials (E < -0.8 V),... 

The application of surface-enhanced Raman spectroscopy (SERS) for monitoring redox and other processes at metal-solution interfaces is illustrated by means of some recent results obtained in our laboratory. The detection of adsorbed species present at outer- as well as inner-sphere reaction sites is noted. The influence of surface interaction effects on the SER spectra of adsorbed redox couples is discussed with a view towards utilizing the frequency-potential dependence of oxidation-state sensitive vibrational modes as a criterion of reactant-surface electronic coupling effects. Illustrative data are presented for Ru(NH3)63+/2+ adsorbed electrostatically to chloride-coated silver, and Fe(CN)63 /" bound to gold electrodes the latter couple appears to be valence delocalized under some conditions. The use of coupled SERS-rotating disk voltammetry measurements to examine the kinetics and mechanisms of irreversible and multistep electrochemical reactions is also discussed. Examples given are the outer- and inner-sphere one-electron reductions of Co(III) and Cr(III) complexes at silver, and the oxidation of carbon monoxide and iodide at gold electrodes. 

Ochsenkuhn MA, Borek J, Phelps R, Campbell CJ (2011) Redox potential dependence of peptide stmcture studied using SERS. Nano Lett 11 2684-2688... 

Xiao, Y.-). and Markwell, J.P. (1997) Potential dependence of the conformations of nicotinamide adenine dinuceletide on gold electrode determined by FT-near-IR-SERS. Langmuir, 13, 7068-7074. 

In a unique experiment Wetzel et determined the dependence of SERS of pyridine on a silver sol on the electric potential, which was maintained by a reversible redox system (europium +3, +2) added to the suspension. The relative concentration of the europium ions was maintained by means of an electrode whose potential was controlled with standard electrochemical equipment. They found that the ratio between the two main bands behaved similarly on an electrode and on a sol. Also frequency shifts were seen. They interpreted these results as indicating that the same SERS mechanisms are operative on the sol and in electrochemical systems. 

Pyridine certainly is the most extensively studied compound in the SERS literature. The potential dependence of its vibrational band intensities as a silver electrode is swept through its ORC at a rate of 5mVs-1 is shown [14] in Fig. 13. The intensity of the strongest band reaches a maximum at - 0.9 V (SCE), in keeping with the model described previously in this section and illustrated in Fig. 12. Also consistent with the model is the fact that the SERS-enhanced H20 band at ca. 3500 cm 1 becomes weaker as H20 is displaced from the electrode surface by Py. However, there is no increase in the H20 band intensity when the potential is swept beyond the PZC, as might be expected from the condition illustrated by Fig. 12(E). 

Martin Fleischmann and coworkers were the first to recognize that the high sensitivity and high energy resolution of EC-SERS, combined with an analysis of the potential dependence of frequency and intensity of the bands, could be used to characterize the details of layers involving the coadsorption of two species. Two types of electrochemical coadsorption... 

The polypeptide toxins from the scorpions Centruroides suffusus and Tityus ser-rulatus. These toxins act by shifting the voltage dependence of the activation of Na channels, thereby inducing a Na channel activity at negative potentials at which Na channels are normally closed 63,64). Site 4 toxins, because of their high affinity for the Na channel, have been efficient tools to elucidate the molecular structure of the Na channel 30,65,66). 

Figure 3 also contains an example of an ISER-flrel plot for a simple specifically adsorbed species, bromide on silver (solid curve). This plot was extracted from bromide coverage-potential data, obtained from differential capacitance measurements, along with the corresponding potential-dependent intensity of the SERS bromide-surface stretching mode at ca. 160 cm"1 (19.). In this case, the maximum (i.e. unity) value of 0r>1 corresponds to a close-packed bromide monolayer, ca. 1.4 x 10"9 mol cm 2. Again, the ISER-0t 1... 

Acetvlpyridine. 2. Relatively intense SERS spectra were obtained for 2 using 0.10 M KC1 (8) but there was little dependence of the relative intensities of the bands on potential. A band due to the unprotonated pyridine was seen near 1600 cm-1 and, at pH < 6, a band due to the protonated compound appeared near 1640 cm-1 and increased at the expense of the 1600 cm" band until only the protonated species could be detected at pH = 1.3. The pKa of 2 is 3.51 (9),... 

The adsorption mode of 2,6-dimethylphenyl isocyanide (DMPl) on Au electrodes [43] has been studied using potential-dependent SERS. The gold substrate... 

The mode of 2,6-dimethylphenyl isocyanide (DMPI) adsorption on Pt electrodes [43] has been studied using potential-dependent SERS. The Pt electrodes were obtained by depositing Pt nanoparticles ( 17nm) on a polycrystalline Pt film. Three bands appear, at 2166, 2124 and 1997cm" , when DMPI is adsorbed on the... 

Using SERS, Brolo et al. [249] have studied the potential-dependent orientation of 2,2 -bipyridine molecules adsorbed on a... 

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