SERS spectra silver electrodes

The first report of the SERS spectrum of a species adsorbed at the electrode/ electrolyte interface was by Fleischman et al (1974) and concerned pyridine on silver. The Raman spectrum of the adsorbed pyridine was only observed after repeated oxidation/reduction cycles of the silver electrode, which resulted in a roughened surface. Initially, it was thought that the 106-fold enhancement in emission intensity arose as a result of the substantially increased surface area of the Ag and thus depended simply on the amount of adsorbate. However, Jeanmarie and Van Duync (1977) and Albrecht and Creighton (1977), independently reported that only a single oxidation/reduction cycle was required to produce an intense Raman spectrum and calculations showed that the increase in surface area could not possibly be sufficient to give the observed enhancement. 

Spectrum of pyridine on a silver electrode from KCl solution was reported by Fleischmann et Later, the SERS spectra on other metals such as Au and Cu were also observed. The theory and practice of SERS spectra have been described in numerous monographs (e.g., Ref. 84). 

Benner et followed the development and changes in the SERS of cyanide chemisorbed on a silver electrode as a function of the electric potential. They see a band at 2165 cm appear at about -0.1 V, which is replaced by a band at 2140 cm cathodically to -0.3 V. This band is eventually replaced by a band at 2110 cm" These different bands are associated with AgCN, Ag(CN)2, and Ag(CN)3 or Ag(CN)4, respectively, on the basis of voltam-mograms and the position of the bands (see also Plieth et At slow sweep rates the cyanide anions have time to diffuse away from the electrode, so the higher complexes are not formed, as indicated by the absence of the relevant bands from the SERS spectrum. Dornhaus et followed in a similar method the... 

McMahon showed that M,2-bis-(4-pyridyl)ethylene, (t-BPE), adsorbed on a silver electrode exhibits an excitation profile with a definite maximum at about 600 nm, and then increases again toward the red. The intensities were normalized to perchlorate ion scattering. These maxima have no counterpart in the solution absorption spectrum. This is a case of an off-resonance SERS effect. 

Fig. 4.13. (a) Frequency-dependent SER spectrum of pyridine, INDO/S-SOS, with applied field being 2.81 eV. (b) Experimental SER spectrum of pyridine absorbed on a rough silver electrode in water at - 0.25 V vs a saturated Ag/AgCl/KCl reference electrode. 

Fig. 12. SERS-spectrum of native CT-DNA. DNA concentration 200 pgxmL, 0.15 M KCl, 10 M cacodylate pH 6.8. Laser excitation line 514 nm, laser power at electrode 100 mW. Prior activation of silver electrode by two triangular voltage sweeps between —0.1 and +0.2 V at a sweep rate of 50 mV s ...

Figure 14a shows an example of the SERS spectrum of 5 -AMP and its building stones adenine (Fig. 14c) and adenosine (Fig. 14b) adsorbed at a positively charged silver electrode in the spectral range of 100 to 1700 cm". The most characteristic internal band systems of the 5 -AMP spectrum are located at 730 and 1340 wave-numbers. They exhibit a significantly enhanced intensity. Moreover, one intense band is observed at 240 cm " This band has been assigned to the interfacial vibration of the phosphate group with the positive silver surface —POf /Ag (cf. Fig. 14d). The...

Figure 17 shows the SERS spectra of native and methylated DNA. In the SERS spectrum of the native DNA (cf. Fig. 17a) the Raman bands at 736cm and 1332 cm corresponding to adenine residues are more intense than the bands in the SERS spectrum shown in Fig. 12. As has already been mentioned, the Raman intensity of the adenine vibration can vary somewhat depending on the electrochemical pretreatment of the silver electrode and the available quality of the DNA samples. Before discussing the specific changes in DNA SERS spectra, due to the methylation, it is necessary to know the SERS data of the methylated guanine bases. The observed frequencies and relative intensities of the SERS bands of guanine and its derivates are given in Table 4. The methylation of guanine leads to a specific...

All Raman bands measured in DNA fibres or crystals appear in this chromosome Micro-Raman spectrum. In addition, typical vibrations of the protein component were observed (phenylalanine, tyrosine, S—S group and the amide I mode). Recently, Micro-SERS has been applied for the first time to investigate the chromosomes adsorbed at the silver electrode This Micro-SERS spectrum of Chinese hamster metaphase chromosomes shows a number of intense bands. The enhancement factor obtained was estimated to be about 100 for the 790 cm DNA backbone vibration. The most intense bands in this SERS spectrum are located at 730 cm " and 1330 cm and can be attributed to the adsorbed adenine base vibration of the DNA. The characteristic protein vibrations in the normal Raman spectrum are missing in the SERS spectrum. 

Another demonstration of the utility of SERS for studying somewhat larger proteins is shown in Fig. 25 where the SERS spectrum of bovine serum albumine (BSA) could be observed on silver electrodes at a concentration of 20 pg ml ... 

Fig. 5.80. Spectra of bis(4-pyridine)acetylene adsorbed on a silver electrode top SEHR spectrum, aqueous solution of 0.1 M KCl, Ag/AgCl = 0.5 V, Aq = 1064 nm, Pq = 2 bottom SER spectrum, aqueous solution of 0.1 M KCl, Ag/AgCl = 0.6 V, Aq = 532 nm, Pq = 10 mW data taken from [500]...

Over the past decades, smface enhanced Raman scattering (SERS) has became a valuable spectroscopic technique as a powerful smface diagnostic tool. In 1974 Fleischmann, Hendra, and McQuillan performed the first measurement of a surface Raman spectrum from pyridine adsorbed on an electrochemically roughened silver electrode. It has been explained that some vibrational bands of pyridine are selectively enhanced a million times. This increases the sensitivity of... 

Fig. 4b shows the SERS spectrum of cetyltrimethylam-monium bromide adsorbed on a silver electrode surface ( — 0.2 V vs. SCE). In this case, only two bands are clearly observed in the C-C stretching spectral range 500-1800 1/cm. The first one appears at 752 1/cm, and is due to C-N stretching mode at the headgroup. The second band is observed at 1450 1/cm and it has been assigned to the C-H scissoring mode of the alkyl chain. 

The electrode reaction of tetraheme cytochrome C3 adsorbed at the silver electrode surface was monitored by SERRS spectroscopy [104, 105] and compared with the results obtained by voltammetric techniques. The formal potential determined on the basis of the SERS measurement is more positive compared to the potential determined by the voltammetry, but it is in good agreement with the macroscopic formal potential of the heme-1. This is because cytochrome C3 is adsorbed on the silver electrode particularly via lysine residues surrounding heme-1, which is in fact responsible for the SERRS spectrum of this protein [105]. 

Ss nucleic acids are also adsorbed at carbon and silver electrodes in a broad range of potentials [107-109]. The details of this adsorption are not known, nevertheless it is evident that the base residues are located in close proximity of these solid electrodes. The base residues in ss nucleic acids adsorbed at the solid electrodes are electrooxidized (see Section 3.3) and yield specific bands in the SERS spectrum [109]. 

Figure 6. SER spectrum of pz adsorbed on silver electrode. E = -700 mV. The Ag electrode was activated in 1.0 M KBr solution in the presence of 0.01 M pz.

The SERS spectra of pz and pztf adsorbed on silver electrodes were measured. Orientations of these molecules on the electrode surface were obtained from the analysis of the relative intensities of the SERS bands, and subsequent application of the SERS selection rules. Pz adsorbs end-on, via the N lone pair. Several pz bands which are normally forbidden in the NRS appeared in the SER spectrum with fair intensity. The presence of these bands was explained, based on the field gradient mechanism nevertheless, this mechanism does not explain the origin of all forbidden pz bands. The pz SER spectrum is potential dependent however, there is no evidence of reorientation in the potential range studied. pzH is predominantly adsorbed flat on the halide-coated silver electrode at potentials more negative than -300 mV. An end-on adsorbed cation is observed at potentials more positive than -170 mV. The pz molecule was also electroreduced in halide medium at potentials more negative than -900 mV. Bands due to the reduction product were observed. The reduced pz was identified as the 1,4-dihydropyrazine cation. This cation may be trapped on the electrode surface, and thus its bands can be observed even at potentials more positive than -9(X) mV. 

Although SERS is inherently a very selective technique, Carrabba et al. [72] have used electrosorption of analytes onto silver electrodes to further increase the analytical selectivity of SERS for the identification of structurally similar compounds. By changing the potential of the electrode, the SERS spectrum was noted to change differently for all chemicals investigated. This is based on the fact that the chemically specific free energy of adsorption is changed as the electrode potential is varied, which gives rise to potential-dependent peak positions and intensities for individual compounds. 

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