Full Spectroscopic Tip-Enhanced Raman Imaging of Single Nanotapes Formed from β-Amyloid(1–40) Peptide Fragments

Abstract

This study demonstrates that spectral fingerprint patterns for a weakly scattering biological sample can be obtained reproducibly and reliably with tip-enhanced Raman spectroscopy (TERS) that correspond well with the conventional confocal Raman spectra collected for the bulk substance. These provided the basis for obtaining TERS images of individual self-assembled peptide nanotapes using an automated, objective procedure that correlate with the simultaneously obtained scanning tunneling microscopy (STM) images. TERS and STM images (64 × 64 pixels, 3 × 3 μm²) of peptide nanotapes are presented that rely on marker bands in the Raman fingerprint region. Full spectroscopic information in every pixel was obtained, allowing post-processing of data and identification of species of interest. Experimentally, the "gap-mode" TERS configuration was used with a solid metal (Ag) tip in feedback with a metal substrate (Au). Confocal Raman data of bulk nanotapes, TERS point measurements with longer acquisition time, atomic force microscopy images, and an infrared absorption spectrum of bulk nanotapes were recorded for comparison. It is shown that the unique combination of topographic and spectroscopic data that TERS imaging provides reveals differences between the STM and TERS images, for example, nanotapes that are only weakly visible in the STM images, a coverage of the surface with an unknown substance, and the identification of a patch as a protein assembly that could not be unambiguously assigned based on the STM image alone.

Tip-enhanced Raman scattering—Targeting structure-specific surface characterization for biomedical samples

“…It is still not clearly understood what triggers the misfolding of proteins to fiber-like structures, and a correlation of morphology and chemical characterization via TERS could deliver complementary information in this field. TERS was used to investigate conformal changes (α-helix versus β-sheet) and amino acid distribution on fibril surfaces [34,79,108,109]. In first experiments, the direct molecular distinction of selected amino acids on insulin fibril surfaces with a lateral resolution of less than 2 nm in gap mode configuration was reported [79].…”

“…In an independent study, TERS images of self-assembled peptide nanotapes from β(16-22) (part of β-amyloid peptide) were correlated with STM topography images [108]. Using a gap mode configuration, TERS spectra were collected at every pixel point in a 64 × 64 pixel size image.…”

Characterization of semiconductor materials using synchrotron radiation-based near-field infrared microscopy and nano-FTIR spectroscopy

“…Under optimized conditions the illuminated tip acts as an antenna that confines the incident electric field around the tip-apex, thus providing a nanoscale light source for high-resolution imaging. Depending on the wavelength of the incident photons and by analyzing the scattered light with a spectrometer system, sample properties in the visible [9][10][11][12][13][14][15], IR [16][17][18][19][20], and THz [21][22][23][24][25][26] regime can be investigated with the additional benefit of a significantly improved spatial resolution. For wavelengths in the IR regime this technique has already been applied to many different sample systems, e.g.…”

Replacing a Century Old Technique – Modern Spectroscopy Can Supplant Gram Staining

“…In TERS, the local field enhancement is enabled when the incident laser light strikes the nanosize tip-apex 16 . TERS on bacteria 17–19 and for other biochemical applications was reported before 20, 21 .…”