Nanocrystals adenine

In this section, we will describe some experiments which we have performed using the above-mentioned nano-Raman microscope. Figure 2.6a shows the Raman spectmm of an adenine nanocrystal of height 7 nm and width 30 nm [19]. Several Raman bands are observed as the probe tip is near enough to the sample (AFM operation is made in contact mode). These bands, except the one appearing at 924 cm, are assigned as the vibrational modes, inherent to the adenine molecule, according to the molecular orbital calculation. For examples, two major bands, one at... 

Figure 2.6 Raman spectrum of an adenine nanocrystal obtained (a) with and (b) without the metallic tip. Spectrum (a) corresponds to the tip-enhanced near-field Raman spectrum while spectrum (b) shows the conventional micro-Raman spectrum.

Figure 2.7 (a) Tip-enhanced near-field Raman spectral mapping ofthe adenine nanocrystal at 30 nm intervals, (b) Raman intensity distribution of two major bands at 739 cm and 1328cm. ... 

Figure 2.9 Spectra of a single adenine nanocrystal, (a) TERS spectrum, (b) ordinal SERS spectrum, and (c) ordinary near-infrared (NIR) Raman spectra. For the SERS measurement, a silver island film was used. For the NIR Raman measurement, i thick sample of adenine was used with a 1 h exposure.

Moreover, we have found temporal fluctuation in TERS spectra of an adenine nanocrystal when we left the silver-coated cantilever (operated in contact mode) on the surface of the nanocrystal for 600 s [27]. Figure 2.11a shows a waterfall plot of a... 

Fig. 16.16 (a) Raman spectra of an adenine nanocrystal at several tip-sample distances indicated in the schematic of (b). The peaks marked by coo and coi represent the unperturbed ring breathing mode ( 720 cm ) [107] and the frequency shifted one, respectively. Deconvoluted Lorentzian peaks for each spectrum are also shown by the dotted curves... 

TERS on DNA bases was demonstrated for the hrst time in 2004 by Watanabe et al. In that work, experiments and density-functional theory (DFT) calculations on adenine molecules in a nanocrystal were presented. From the acquired spectra, which differed from standard Raman spectra, it was concluded that those crystals were mechanically deformed in contact-mode TERS [74]. Consequently, band shifts were observed and attributed to interactions of molecule and metal tip. Comparing SER and TER spectra, a band shift could be observed, too, mainly caused by more specihc interactions of adenine and metal. In SERS of adenine on silver island hims, molecules were evenly attached to the rough surface by the amino group and its adjacent nitrogen atoms (N1 and N7). In contrast, in contact-mode TERS experiments (i.e., tip always touches the sample), the silver tip was constantly moved over the molecules with a force of - 5 pN per molecule. Based on theoretical calculations, the authors concluded that the TERS probe was selectively pressed to one nitrogen atom (namely N3). Later it was shown that in contact-mode TERS, an adsorption-desorption process of the molecule at the tip could be responsible for the band shift [62]. 

Fig. 8.6 (a) SEM image of aluminum-coated probe, showing a nanoparticle attached at the probe apex. The deposition thickness was 25 nm. The inset shows a low-magnification image, (b) DUV-TERS spectra of adenine nanocrystals excited with 266-nm laser wavelength measured with and without tip. The inset shows the absorption spectrum of adenine nanocrystals... 

The measured Raman spectra of adenine nanocrystals with and without the tip are shown in Fig. 8.6b. Strong enhancement of the Raman peaks was observed when the tip was in contact with the sample. The inset of Fig. 8.6b shows the absorption spectrum of the adenine nanocrystals. The Raman excitation wavelength (266 nm) used in the experiment was closely matched to the maximum absorption peak of adenine nanocrystals. The observed spectra of the adenine nanocrystals were consistent with the reported resonance Raman spectrum of the deoxyribonucleotides of adenine (dAMP) for 266-nm excitation [72-74]. This experiment is the first demonstration of plasmonically enhanced DUV resonance Raman scattering from biologically relevant molecules. The quartz peaks observed around 500 cm remained unchanged with and without the tip, indicating the localized character of the tip-enhancement effect. The estimated enhancement factor was 1.3 x 10. ... 

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