SERS spectra electrode

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. 

Gao et al. (346) obtained SER spectra of toluene, isopropylbenzene, and ferf-butylbcnzene on roughened gold electrodes. The spectra are reasonably interpreted in terms of flat-lying molecules, 77-bonded to the surface via the aromatic ring. However, one imagines that the 77-bonding must be weaker in the presence of the bulky ferf-butyl substituent, and for this molecule all the bands in the spectrum of the liquid occur also in the SER spectrum. (The normal Raman spectrum shown in Fig. 2A of the Gao et al. paper (346) is that of isopropylbenzene rather than that of the indicated toluene.)... 

Figure 3.29 In situ SERS spectrum of hem in (Hm) adsorbed on a roughened Ag electrode (curve a, -0.50 V versus SCE) and its reduced counterpart (curve b, 0.1 V) recorded in an aqueous solution pH = 3, Xe%c = 532 nm.

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... 

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. 15. SER spectrum of ethene adsorbed at a gold electrode. (Reproduced with permission from ref. 24.)...

Fig. 3. SERS spectrum of cytosine (above). Conditions laser excitation line 514.5 nm laser power at the Ag electrode 10 mW E, —0.6 V, 0.1 M Ka and 10 M Na2HTO4 pH 8 cytosine concentration 1 x 10 M. (Lewinsky, Ref...

The cell design shown in Fig. 2c has been used for micro-SERS spectroscopy of chromosomes and related material This microcell is for use in a Raman microspectrometer using epi-illumination. By choice of the objective used in the microscope, the focus of the laser beam is about 6 pm in diameter for a typical chromosome micro-SERS-spectrum. The working electrode, with a diameter of 0.5 mm, is fitted into a perspex rod and can be screwed into a frame for positioning the electrode surface to the objective of the microscope. This small microcell needs only 0.08 ml of sample. 

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. 24. SERS spectrum at the Ag electrode 0.1 M KCl, 280 pg ml lysozyme recorded using 514,5 nm excitation and 6 mW power (Kisters, Ref.

Fig. 25. SERS spectrum of bovine serum albumin (BSA) adsorbed on the Ag electrode. Conditions 20 pgml BSA, 0.15 M KCl, 2mM Tris, exc., 514.5 nm, laser power 10 mW, E —0.9 V (Kisters, Ref. >)...

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]...

In Fig. 8.11, the observed SER bands of pyrazine adsorbed on Au electrodes are compared with the normal IR spectrum of an aqueous pyrazine solution (5 M) and the SEIRA spectrum of adsorbed pyrazine [71]. In the SER spectrum (c), the Raman-inactive modes for the free molecule are observed at 1479, 1155, 1123, and 1047 cm (shown by dashed lines) together with Raman-active modes (sohd lines). The counterparts of these Raman-inactive modes can be found in the solution IR spectrum (a), indicating that these bands are apparently ungerade modes. On the other hand, the SEIRA spectrum (b) is essentially identical to the solution... 

FT SER spectra have been obtained of many different types of compounds including some environmental contaminants. Initially, for environmental applications, very simple monosubstituted aromatic compounds are being studied. One of the simplest of these is 3-chloropyridine (CP). This compound gives very intense NIR-SER spectra on Cu colloids and on Cu electrodes (see Figure 7). Plot A of Figure 7 shows the SER spectrum of 1 mM CP on a Cu electrode and Plot B... 

FT SER spectra have also been measured for mixtures of CP and picoline with surprising results (Angel, S.M. Archibald, D.D. Appl. Spectrosc. in press). Plot A of Figure 8 shows the SER rectrum of 1 mM CP on a Cu decirode at a potential of -0.6 V versus the saturated calomel electrode (SCE). The most intense SER spectra were obtained for this compound at this potential. The SER spectrum of 1 mM picoline (Plot C of Hgure 8) also shows the greatest intendty at -0.6 V versus SCE. SER spectra obtained for a mixture containing O.SmM of each of these compounds is potential depeitdent (Plot B of Hgure 8). 

Even with the low output power of the diode laser, SER spectra were easily obtained of a variety of diffoent compounds on Ag and Cu electiodes. For 0.1 M pyridine on an Ag electrode, typical signal levels were about 10,000 cps. Hgure 9 shows the SER spectrum of 0.1 M pyridine on Cu (Plot A) and Ag (Plot B) electiodes. In each case, the SER enhanconent is about 10 to 10. However, it is somewhat Iowa on the C u electiodes than on the Ag electrodes. 

In some special cases, the two approaches can be combined with the normal Raman or resonance Raman spectroscopy to comprehensively study a film electrode. For example, a thin film of polymer, oxide, semiconductor, or Langmuir- Blodgett (LB) film is coated in different ways onto the SERS-active substrate, such as Fig. 18(a). The SERS spectrum recorded at this stage can provide the bottom layer structure of the film. The film can then be deposited as a thicker layer, which enables the Raman signal from the bulk phase to be sufficiently strong and detected by normal Raman... 

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. 

Therefore, pyridine and tetramethylammonium (TMA) were used as model molecules for the alkylpyridinium and alkyltrimethylammonium headgroups, while 1-Br octane was used as model for the tail. The strategy behind this procedure is to pre-adsorb the head model compounds on the SERS activated electrode, and after acquiring the corresponding SERS spectrum, to add the 1-Br octane solution to look whether bands due to CH2 vibrational modes appear. The measurements were carried out at s = — 0.8 V vs. SCE. The applied potential was kept negative to assure that no electrostatic attraction of the bromo octane towards the electrode was involved. 

Tetramethylammonium bromide before (bottom) and after the addition of 1-Br octane in a proportion of 1 5 (middle). For comparing purposes the SERS spectrum of Cj TAB (top). The used substrate was an ex situ electrochemically roughened Ag-electrode. The spectra were acquired at applied potential , = - 0.2 V vs. SCE. All the solutions were 10" M in 0.1 KCl. The excitation was 488 nm... 

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.

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