SERS spectrum

The label-free detection of biomolecules is another promising field of application for SERS spectroscopy. Tiniest amounts of these molecules can be adsorbed by specific interactions with receptors immobilized on SERS-active surfaces. They can then be identified by their spectra, or specific interactions can be distinguished from unspecific interactions by monitoring characteristic changes in the conformation sensitive SERS spectra of the receptors. 

The sensitivity limitations of TLC-FT-Raman spectroscopy may be overcome by applying the SERS effect [782]. Unlike infrared, a major gain in Raman signal can be achieved by utilising surface activation and/or resonance effects. Surface-enhanced Raman (SER) spectra can be observed for compounds adsorbed on (rough) metahic surfaces, usually silver or gold colloids [783,784], while resonance Raman (RR) spectra... 

The requirement for atomic-scale sites in the CT mechanism is thought to be due to the formation of adatom-adsorbate complexes. Such complexes require a coordination site on the adsorbate through which the strong interaction can occur, as was demonstrated by the controlled adsorption of isonicotinic acid and benzoic acid on thin island films of silver (see the work of Chen et ai, 1980). This clearly showed that SERS spectra could only be obtained from the molecules when coordinating sites were exposed to the Ag film. 

Figure 3.92 shows SERS spectra of adsorbed SSBipy and PySH at 0 V in the absence of the solution species, together with the Raman spectra of PySH in solution and crystalline SSBipy. The activities of the modified electrodes were first confirmed in solution containing cytochrome c. 

Figure 3.92 SERS spectra of (a) SSBipy-Au and (b) PySH-Au electrodes at 0 V vs. SCE in pH 7.0 phosphate buffer/0.1 M NaC104, together with Raman spectra of(c) the buffer employed in (a) and (b) saturated with PySH (c. 50mM). (d) SSBipy in the solid state as a powder. A He/Ne laser (632.8 nm, 30 mW) was used. The signal la belied with an asterisk is due to aqueous CI04. From Taniguchi et at. (1982).

The 550 cm"1 absorption present in the spectrum of the solid SSBipy, due to the S-S stretch, is absent in both the SERS spectra. 

The SERS spectra of the SSBipy and PySH are almost identical. 

The band at 1120cm"1 in the PySH solution spectrum is the substituent-sensitive band which shifts to 1100cm "1 in the SERS spectra of the two adsorbed species and increases in intensity. 

Narayanan V., Begun G., Stokes D., Sutherland W., Vo-Dinh T., Normal Raman and surface-enhanced Raman scattering (SERS) spectra of some fungicides and related chemical compounds,/. Raman Spectrosc., 1992 23 281-286. 

The SERS spectra can be obtained on a conventional Raman spectrometer (see Figure 2). In this particular system, 632.8 nm radiation from a Helium-Neon laser is used with an excitation power of 5 mW. Signal collection is performed at 0° with respect to the incident laser beam. This coaxial excitation/collection geometry is achieved with a small prism, which is used to direct the excitation beam to the sample while allowing... 

Surface-enhanced Raman (SER) spectra for adsornea lectrode... 

A set of SER spectra for adsorbed azide on silver, obtained for the same surface and solution conditions and for a similar sequence of electrode potentials as for the PDIR spectra in Figure 1, is shown in Figure 2. (See the figure caption and reference 7 for experimental details.) Inspection of these SER spectra in comparison with the PDIR results illustrate some characteristic differences in the information provided by the two techniques. Most prominently, in addition to the Nj" j/as band around 2060 cm"1, the former spectra exhibit three other features at lower frequencies attributable to adsorbed azide vibrations. By analogy with bulk-phase spectra for free and coordinated azide (15), the 1330 cm"1 SERS band is attributed to the N-N-N symmetric stretch, vt (2). The observation of both i/a and j/aa features in the SER spectra differs from the surface infrared results in that only the v band is obtained in the latter (2). The appearance of the vn band in SERS is of interest since this feature is symmetry forbidden in the solution azide Raman spectrum. 

Examination of the azide bending-mode region (600-700 cm 1) in the SER spectra (Figure 2) is also instructive with regard to adsorbate orientation. Thus, the pair of bands (at ca. 610 and 670 cm"1) seen at the least negative potentials are characteristic of end-on coordinated azide (15) the loss of the lower-frequency partner for E < -0.15 V is therefore also indicative of the removal of azide bound in this adsorbate geometry, again in harmony with the interpretation of the infrared spectra (7). 

The surface-enhanced Raman spectra (SERS) provide information about the extent of protonation of the species adsorbed at the silver/aqueous solution interface. The compounds investigated were 4-pyridyl-carbinol (1), 4-acetylpyridine (2), 3-pyridine-carboxaldehyde (3), isonicotinic acid (4), isonicotinamide (5), 4-benzoylpyridine (6), 4-(aminomethyl)pyridine (7) and 4-aminopyridine (8). For 1, the fraction of the adsorbed species which was protonated at -0.20 V vs. SCE varied with pH in a manner indicating stronger adsorption of the neutral than the cationic form. The fraction protonated increased at more negative potentials. Similar results were obtained with 3. For all compounds but 4, bands due to the unprotonated species near 1600 cm-1 and for the ring-protonated species near 1640 cm-1 were seen in the SERS spectra. 

In cases where the substituent on the pyridine ring contains a carbonyl group, a weak band for the C-0 stretch can be detected in the SERS spectra, generally near 1700 cm. ... 

The SERS spectra of pyridines contain many other bands but those mentioned above have proven to be particularly useful in characterizing the surface species. 

Pyridylcarbinol. 1. This compound has been investigated previously (8) and it was noted that its SERS spectra were almost identical to those of 4-pyridinecarboxaldehyde. SERS spectra in the 1500-1700 cm-1 region are shown in Figure 1. Buffers with pH bracketing the pKa of 4-pyridylcarbif>ol (5.76 (9)) were employed and it can be seen that the band for the unprotonated species near 1600 cm-1 is predominant at pH 6.88 but decreases as the band due to the protonated species (near 1640 cm-1) grows in when the pH is lowered. 

Figure 2. Surface mole fraction of protonated 4-pyridylcarbinol vs. pH. Points are from SERS spectra obtained at -0.20 V vs.

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),... 

Figure 3. SERS spectra of 0.050 M 4-pyridylcarbinol in pH 6.88 buffer at various electrode potentials (vs. SCE).

In 0.10 M KC1, the SERS spectra of 2 show a small band at about 1690 cm-1, attributable to the carbonyl stretch. Allen and Van Duyne (12) have considered orientational effects on band intensities and have concluded that the relative intensities should be given by Equation 2, where the intensity of the carbonyl band... 

In comparison to 1 and 2, the SERS spectra of 3-pyridine-carboxaldehyde (3) are relatively featureless (8). The spectra are dominated by the symmetrical ring-breathing mode at 1030 cm 1 but the features associated with the unprotonated species (about 1600 cm ) and the protonated species (about 1640 cm 1) are definitely present along with a weak carbonyl band at about 1710 cm . The variation in the relative population of protonated species is as expected (Figure 5) though a detailed analysis reveals some surprises. As can be seen in Figure 5, about equal intensities of the 1600 and 1640 cm bands are obtained at pH = 3.86, near the pKa (3.73 (9)). However, the band associated with the unprotonated pyridine persists at pH = 1.3, where less than 1J of the solution species remains unprotonated. 

Isonicotinic Acid. 4. It is difficult to obtain a spectrum of 4 because the neutral form is not very soluble. At low pH, however, the ring nitrogen is protonated (13) and the cationic isonicotinic acid is sufficiently soluble to obtain SERS spectra. A relatively intense spectrum was obtained at -0.20 V with 0.050 M isonicotinic acid, 0.10 M KC1 and 0.10 M HC1. Many of the spectral features seen with other pyridines are present but the inability to vary solution pH made it impossible to investigate the relative surface populations of protonated and unprotonated forms. 

Isonicotinamide, 5. This compound was sufficiently soluble to allow SERS spectra to be obtained at the 50 mM level in 0.10 M KC1 and 0.10 M KC1 + 0.10 M HC1 at -0.20 V. The spectra resembled those seen with other pyridines. In particular, an intense band at 1600 cm-1 was seen with the neutral electrolyte and it was replaced by a band at 1640 cm-1 in the acidic electrolyte. Of the two basic sites, only the ring nitrogen will be protonated in 0.10 M HC1 (22) so, with this compound also, the 1640 cm-1 band appears to be due to the protonated pyridine. No carbonyl band was seen in either spectrum. 

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