TER Spectra Research Articles

Understanding tip‐enhanced Raman spectra of biological molecules: a combined Raman, SERS and TERS study

Although conventional Raman, surface‐enhanced Raman (SERS) and tip‐enhanced Raman spectroscopy (TERS) have been known for a long time, a direct, thorough comparison of these three methods has never been carried out. In this paper, spectra that were obtained by conventional Raman, SERS (on gold and silver substrates) and TERS (in ‘gap mode’ with silver tips and gold substrates) are compared to learn from their differences and similarities. Because the investigation of biological samples by TERS has recently become a hot topic, this work focuses on biologically relevant substances. Starting from the TER spectra of bovine serum albumin as an example for a protein, the dipeptides Phe–Phe and Tyr–Tyr and the tripeptide Tyr–Tyr–Tyr were investigated. The major findings were as follows. (1) We show that the widely used assumption that spectral bands do not shift when comparing SER, TER and conventional Raman spectra (except due to binding to the metal surface in SERS or TERS) is valid. However, band intensity ratios can differ significantly between these three methods. (2) Marker bands can be assigned, which should allow one to identify and localize proteins in complex biological environments in future investigations. From our results, general guidelines for the interpretation of TER spectra are proposed. Copyright © 2012 John Wiley & Sons, Ltd.

Observation of Multiple Vibrational Modes in Ultrahigh Vacuum Tip-Enhanced Raman Spectroscopy Combined with Molecular-Resolution Scanning Tunneling Microscopy

Multiple vibrational modes have been observed for copper phthalocyanine (CuPc) adlayers on Ag(111) using ultrahigh vacuum (UHV) tip-enhanced Raman spectroscopy (TERS). Several important new experimental features are introduced in this work that significantly advance the state-of-the-art in UHV-TERS. These include (1) concurrent sub-nm molecular resolution STM imaging using Ag tips with laser illumination of the tip-sample junction, (2) laser focusing and Raman collection optics that are external to the UHV-STM that has two cryoshrouds for future low temperature experiments, and (3) all sample preparation steps are carried out in UHV to minimize contamination and maximize spatial resolution. Using this apparatus we have been able to demonstrate a TERS enhancement factor of 7.1 × 10(5). Further, density-functional theory calculations have been carried out that allow quantitative identification of eight different vibrational modes in the TER spectra. The combination of molecular-resolution UHV-STM imaging with the detailed chemical information content of UHV-TERS allows the interactions between large polyatomic molecular adsorbates and specific binding sites on solid surfaces to be probed with unprecedented spatial and spectroscopic resolution.

Single-Molecule Tip-Enhanced Raman Spectroscopy

An existence proof for single-molecule tip-enhanced Raman spectroscopy (SMTERS) is given using the frequency domain approach involving the two isotopologues of Rhodamine 6G (R6G) that were previously employed for single-molecule surface-enhanced Raman spectroscopy (SMSERS). A combination of experimental and theoretical studies provides a detailed view of the isotopic response of R6G–d0 and R6G–d4 in the 600 – 800 cm–1 region. The single-molecule nature of the TERS experiment is confirmed through two lines of evidence. First, the vibrational signature of only one isotopologue at a time was observed from multiple TER spectra. Second, the spectral wandering of the 610 cm–1 mode of R6G–d0 was less than ±4 cm–1, which in turn is less than the 10 cm–1 isotopic shift so that no confusion in assignment resulted. As a consequence, the total TERS enhancement factor can now be accurately established as EFTERS = 1.0 × 1013 because only one molecule at a time is measured. Furthermore, EFTERS can be partitioned into an...

Characterization of dye molecules and carbon nanostructures by tip‐enhanced Raman spectroscopy

AbstractTip‐enhanced Raman spectroscopy (TERS) is a high sensitivity and high spatial resolution analytical technique based on the strong field enhancement provided by a sharp metallic tip. Using a commercial atomic force microscope (AFM) combined with a confocal optical setup in reflection geometry, we are targeting samples on non‐transparent electrodes. The resulting design challenges with respect to quantum efficiency, optical accessibility of the tip and the optics themselves, were balanced and taken into consideration for the presented ultra‐high vacuum–TERS setup. Second, we describe the development of a TERS setup at normal pressure in air, and we present the confocal Raman and TERS spectra of dye molecules and other nanostructures.

Nanoscale Probing of a Polymer‐Blend Thin Film with Tip‐Enhanced Raman Spectroscopy

Fundamental advances have been made in the spatially resolved chemical analysis of polymer thin films. Tip-enhanced Raman spectroscopy (TERS) is used to investigate the surface composition of a mixed polyisoprene (PI) and polystyrene (PS) thin film. High-quality TER spectra are collected from these nonresonant Raman-active polymers. A wealth of structural information is obtained, some of which cannot be acquired with conventional analytical techniques. PI and PS are identified at the surface and subsurface, respectively. Differences in the band intensities suggest strongly that the polymer layers are not uniformly thick, and that nanopores are present under the film surface. The continuous PS subsurface layer and subsurface nanopores have hitherto not been identified. These data are obtained with nanometer spatial resolution. Confocal far-field Raman spectroscopy and X-ray photoelectron spectroscopy are employed to corroborate some of the results. With routine production of highly enhancing TERS tips expected in the near future, it is predicted that TERS will be of great use for the rigorous chemical analysis of polymer and other composite systems with nanometer spatial resolution.

Tip-Enhanced Raman Spectroscopy Can See More: The Case of Cytochrome c

The heme protein, cytochrome c (Cc) has been studied using tip-enhanced Raman spectroscopy (TERS). By virtue of its sensitivity and superior spatial resolution, TERS detected both the heme and amino acid vibrational bands of Cc using resonance excitation at 532 nm. This is in contrast to conventional surface-enhanced Raman spectroscopy (SERS) where ensemble information is obtained, leading to the strongest Raman bands obscuring the weaker ones; i.e., only the resonantly enhanced heme bands are observed. Matrix-assisted laser desorption/ionization mass spectrometry supported the interpretation of the Raman data by showing that Cc remains intact on Ag surfaces. This work demonstrates the sensitivity of TERS for analyzing proteins (complete with band assignments) and that a collection of TER spectra gives a more complete description of large biomolecules. The latter is an important advantage usually not found in SERS measurements.

DFT Vibrational Calculations of Rhodamine 6G Adsorbed on Silver: Analysis of Tip-Enhanced Raman Spectroscopy

The tip-enhanced near-field Raman (TERS) bands of Rhodamine 6G (R6G), that we reported earlier [Chem. Phys. Lett. 2001, 335, 369.], are assigned on the basis of density-functional theory (DFT) calculations at the 6-311++G(d,p) level. The Raman and infrared intensities as well as frequencies of the vibrational modes are used for band assignments. These vibrational modes, in combination with characterization of resonant electronic transitions using time-dependent DFT calculations, predict spectral changes in resonant Raman and surface-enhanced resonant Raman scatterings of R6G. Moreover, the TERS spectra of R6G are analyzed in detail, where interactions between the tip and R6G molecules and their enhancement mechanisms are discussed. Finally, we propose a novel Raman spectroscopy technique capable of detecting molecular vibrations at sub-nanometer scale.