Frequency dependence SERS spectra

Fig. 4.12 presents the static SER spectra of pyridine, a CU4 cluster, and the CU4-pyridine complex. The structure of the Cu4-pyridine complex is also presented, while Fig. 4.13 gives the experimental and the calculated frequency-dependent SER spectra of pyridine only. Note that we have used a Y-shaped CU4 cluster structure, rather than the global minimum rhombus shape, as this leads to better behaved DFT calculations. As we can see from Fig. 4.12, the static INDO/S and DFT spectra are in reasonable agreement for all except the mode at 1253 cm for pyridine, and 1253 and 1500 cm for the CU4-pyridine complex. The intensities of these modes are suppressed in the INDO/S results relative to DFT, but as shown in Fig. 4.13, the frequency-dependent INDO/S-SOS results match experiment reasonably well, with the four major SERS-active vibrational modes at around 630, 993, 1050, and 1600 cm having about the right relative intensities. Moreover, both the DFT and INDO/S results in Fig. 4.12 suggest that for the CU4-pyridine complex orientation considered here, the spectrum of Cu4-pyridine complex is approximately a combination of the spectmm of pyridine and that of CU4. 

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. 

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...

SERS spectrum is affected by the LSPR of metallic substrates which is wavelength dependent. Thus, the peaks in the SERS spectrum can be enhanced by a different factor depending on LSPR of metallic substrate and a particular vibration frequency. 

First, there are many variables that determine SERS enhancement, not all of which are controlled. Even if particle size and shape can be reproduced, surface chemistry is difficult to control and is generally unstable. Chance exposure to contaminants or reconstruction of the metal surface can significantly vary the observed enhancement over time. Second, chemical enhancement depends on the adsorbate-surface interaction and varies both with surface chemistry and with adsorbate structure. Analytes differ greatly in the strength of this interaction, and surface contamination can prevent it altogether. SERS is not a general phenomenon as far as the wide range of species encountered in chemical analysis. Third, relative intensities and even peak frequencies can be quite different for an adsorbed molecule compared to the spectrum observed in bulk. 

The splitting between the levels /-) and /+) may nor may nor be close to the splitting between the original levels /i) and I/2). In the former case, the frequency of the bands would not change very much from the field-free case, i.e., the NR spectrum of the molecule in bulk solution. However, in either situation, the SERS intensities of the bands would be dependent on electric field strength, which would be manifested in the matrix elements in the numerator of the polarizability components. This effect can be referred to as a SERS Stark effect. 

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