Polarized Raman microspectroscopy and atomic force microscopy were used to obtain quantitative information regarding the molecular structure of individual diphenylalanine (FF) nano- and microtubes. The frequencies of the Raman spectral bands corresponding to the amide I (1690 cm(-1)) and amide III (1249 cm(-1)) indicated that the FF-molecules interact by hydrogen bonding at the N-H and not at the C═O sites. The calculated mean orientation angles of the principal axes of the Raman tensors (PARTs) obtained from the polarized Raman spectral measurements were 41 ± 4° for the amide I and 59 ± 5° for amide III. On the basis of the orientation of the PART for the amide I mode, it was found that the C═O bond is oriented at an angle of 8 ± 4° to the tube axis. These values did not vary significantly with the diameter of the tubes (range 400-1700 nm) and were in agreement with the molecular structure proposed previously for larger crystalline specimens.
Polarized Raman microspectroscopy and atomic force microscopy were used to measure molecular orientation in individual diphenylalanine nanotubes (diameters ranging from 100 nm to 1000 nm). Analysis of the amide I Raman bands (1686 cm(-1)) indicated that the C=O side chains have a parallel alignment with the nanotube axis. The amide III Raman band (1249 cm(-1)) associated with the peptide backbone C-N vibrations showed that these bonds are preferentially aligned perpendicular to the nanotube axis. However, the Raman band corresponding to the symmetric breathing mode of the aromatic rings (1002 cm(-1)) indicated a rather random orientation. These results support the theoretical molecular structure models proposed recently.
We report a new approach in tip-enhanced Raman spectroscopy (TERS) in which TERS-active tips with enhancement factors of ∼10(-5)× can be rapidly (1-3 min) produced in situ by laser-induced synthesis of silver nanoparticles at the tip apex. The technique minimizes the risks of tip contamination and damage during handling and provides in situ feedback control, which allows the prediction of the tip performance. We show that TERS tips produced by this technique enable the measurement of spatially resolved TERS spectra of self-assembled peptide nanotubes with a spatial resolution of ∼20 nm.
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