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Tip-enhanced Raman spectroscopy (TERS) provides topographic and chemical information simultaneously with a spatial resolution well below the optical diffraction limit; however, the typically used noble-metal nanotips are prone to unwanted chemisorption, and when silver is used, the enhancement quickly fades due to chemical degradation during storage and use. In the present work, we demonstrate a method to produce strongly enhancing tips for TERS, which are protected against undesired chemisorption and (in the case of silver) have a significantly longer lifetime than unprotected tips. This was achieved by chemical coating of an ultrathin (few nanometers) and pinhole-free silica shell on top of the enhancing noble-metal nanostructures. The enhancement achieved with the protected silver tips was comparable with the typical tip enhancement for TERS; that is, no major constraints for TERS experiments resulted from the protective coatings. These shell-isolated tips could widen the scope of tip-enhanced Raman sp...

Stiftung Stipendien-Fonds des Verbandes der chemischen Industrie e.V.; German National Academic Foundation; Bruker; National Science Foundation (NSF CAREER) [CHE 0748226]; MOST of China [2011YQ03012406]; NSFC [21021120456]; EMRP

Raman microscopes are currently used in various fields of research because they allow for label-free sample investigation. Moreover, the inherently low scattering cross section of Raman spectroscopy, as well as its diffraction-limited lateral resolution, has been overcome by new Raman microscopy techniques. Nonlinear methods such as coherent anti-Stokes Raman spectroscopy and stimulated Raman spectroscopy reduce measurement times and improve z resolution, allowing for three-dimensional spectroscopic imaging of biological samples. Moreover, tip-enhanced Raman spectroscopy, a near-field optical technique that combines scanning-probe microscopy with the enhancement offered by surface-enhanced Raman scattering, enables Raman spectroscopic imaging far below the optical diffraction limit. We cover the theoretical and technical aspects of Raman microscopy and related new imaging techniques and review some very recent applications in graphene research and cell biology.

Chemische Analysemethoden auf der Nanometerskala sind von zentraler Bedeutung fur die Charakterisierung von Nanostrukturen moderner Materialien und biologischer Systeme. Spitzenverstarkte Raman-Spektroskopie (tip-enhanced Raman spectroscopy, TERS) kombiniert die chemische Information, welche durch Raman-Spektroskopie erhalten werden kann, mit der von der oberflachenverstarkten Raman-Streuung (SERS) bekannten Signalverstarkung und der hohen raumlichen Auflosung der Rasterkraftmikroskopie (AFM) bzw. der Rastertunnelmikroskopie (STM). Dabei wird eine metallische oder metallisierte Spitze in den Fokus eines Lasers gebracht, wodurch ein stark verstarktes elektromagnetisches Feld am Spitzenende resultiert, welches als sehr kleine Lichtquelle fur Raman-spektroskopische Messungen auf der Nanometerskala dient. Dieser Aufsatz beschaftigt sich hauptsachlich mit den Voraussetzungen fur eine effiziente Kopplung des Laserlichts an die Spitze sowie den verbliebenen Herausforderungen und Tucken der spitzenverstarkten Raman-Spektroskopie, welche fur die chemische Bildgebung mittels TERS berucksichtigt werden mussen. Beispiele aus aktuellen Publikationen demonstrieren das Potential von TERS fur die chemische Bildgebung mit einer raumlichen Auflosung von ca. 10 nm und einer Sensitivitat, welche den Nachweis weniger Molekule ermoglicht in Anwendungen, die von den Materialwissenschaften bis hin zu den Biowissenschaften reichen.

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.

Monolayer sheets have gained attention due to the unique properties derived from their two-dimensional structure. One of the key challenges in sheet modification/synthesis is to exchange integral parts while keeping them intact. We describe site-to-site transmetalation of Zn(2+) in the netpoints of cm(2)-sized, metal-organic sheets by Fe(2+), Co(2+), and Pb(2+). This novel transformation was done both randomly and at predetermined patterns defined by photolithography to create monolayer sheets composed of different netpoints. All transmetalated sheets are mechanically strong enough to be spanned over 20 × 20 μm(2) sized holes. Density functional theory calculations provide both a model for the molecular structure of an Fe(2+)-based sheet and first insights into how transmetalation proceeds. Such transmetalated sheets with random and patterned netpoints can be considered as two-dimensional analogues of linear copolymers. Their nanoscale synthesis presents an advance in monolayer/polymer chemistry with applications in fields such as surface coating, molecular electronics, device fabrication, imaging, and sensing.

Tip-enhanced Raman spectroscopy is a surface sensitive analytical method that combines the advantages of scanning probe microscopy and Raman spectroscopy. It holds great promises for imaging of biological samples with high spatial resolution (10–50 nm), well below the optical diffraction limit. It offers the opportunity to directly localize and identify proteins and their conformation in a complex (e.g., native) environment. Tip-enhanced Raman (TER) spectra in the so-called “gap-mode” configuration with a metal tip in scanning tunnelling feedback with a metal substrate coated with different proteins (bovine serum albumin, immunoglobulin G, trypsin, and β-lactoglobulin) as well as of model octapeptides (with and without an aromatic amino acid residue) are presented. The goal was to determine if it is possible to reliably assign marker bands for proteins and if different secondary structures of proteins can be distinguished in their gap-mode TER spectra as reliably as by IR and conventional Raman spectrosco...

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.

Full spectroscopic imaging by means of tip-enhanced Raman spectroscopy (TERS) was used to measure the distribution of two isomeric thiols (2-mercaptopyridine (2-PySH) and 4-mercaptopyridine (4-PySH)) in a self-assembled monolayer (SAM) on a gold surface. From a patterned sample created by microcontact printing, an image with full spectral information in every pixel was acquired. The spectroscopic data is in good agreement with the expected molecular distribution on the sample surface due to the microcontact printing process. Using specific marker bands at 1000 cm−1 for 2-PySH and 1100 cm−1 for 4-PySH, both isomers could be localized on the surface and semi-quantitative information was deduced from the band intensities. Even though nanometer size resolution information was not required, the large signal enhancement of TERS was employed here to detect a monolayer coverage of weakly scattering analytes that were not detectable with normal Raman spectroscopy, emphasizing the usefulness of TERS., Beilstein Journal of Nanotechnology, 2011 (2), ISSN:2190-4286

Synthetic covalent monolayer sheets and their subclass, two-dimensional polymers are of particular interest in materials science because of their special dimensionality which renders them very different from any bulk matter. However, structural analysis of such entities is rather challenging, and there is a clear need for additional analytical methods. The present study shows how tip-enhanced Raman spectroscopy (TERS) can be performed on monomer monolayers and the covalent sheets prepared from them by [4 + 4]-cycloaddition to explore rather complex structural and mechanistic issues. TERS is a surface analytical method that combines the high lateral resolution of scanning probe microscopy (SPM) with a greatly enhanced Raman scattering intensity. The high spatial resolution (60 nm) and the significantly improved sensitivity (contrast factor of4000) compared to confocal Raman microscopy provides new insights into the formation of this new and exciting material, namely significant consumption of the reactive units (anthracenes) and exclusion of the alternative [4 + 2]-cycloaddition. Moreover, due to the high lateral resolution, it was possible to find a first spectroscopic hint for step growth as the dominant mechanism in the formation of these novel monolayer sheets. In addition, TERS was used to get first insights into the phase behavior of a comonomer mixture.

Methods for chemical analysis at the nanometer scale are crucial for understanding and characterizing nanostructures of modern materials and biological systems. Tip-enhanced Raman spectroscopy (TERS) combines the chemical information provided by Raman spectroscopy with the signal enhancement known from surface-enhanced Raman scattering (SERS) and the high spatial resolution of atomic force microscopy (AFM) or scanning tunneling microscopy (STM). A metallic or metallized tip is illuminated by a focused laser beam and the resulting strongly enhanced electromagnetic field at the tip apex acts as a highly confined light source for Raman spectroscopic measurements. This Review focuses on the prerequisites for the efficient coupling of light to the tip as well as the shortcomings and pitfalls that have to be considered for TERS imaging, a fascinating but still challenging way to look at the nanoworld. Finally, examples from recent publications have been selected to demonstrate the potential of this technique for chemical imaging with a spatial resolution of approximately 10 nm and sensitivity down to the single-molecule level for applications ranging from materials sciences to life sciences.

Methods for chemical analysis at the nanometer scale are crucial for understanding and characterizing nanostructures of modern materials and biological systems. Tip-enhanced Raman spectroscopy (TERS) combines the chemical information provided by Raman spectroscopy with the signal enhancement known from surface-enhanced Raman scattering (SERS) and the high spatial resolution of atomic force microscopy (AFM) or scanning tunneling microscopy (STM). A metallic or metallized tip is illuminated by a focused laser beam and the resulting strongly enhanced electromagnetic field at the tip apex acts as a highly confined light source for Raman spectroscopic measurements. This Review focuses on the prerequisites for the efficient coupling of light to the tip as well as the shortcomings and pitfalls that have to be considered for TERS imaging, a fascinating but still challenging way to look at the nanoworld. Finally, examples from recent publications have been selected to demonstrate the potential of this technique for chemical imaging with a spatial resolution of approximately 10 nm and sensitivity down to the single-molecule level for applications ranging from materials sciences to life sciences.

The biological membrane plays an important role in numerous cellular processes and is linked to various diseases. Many of its biological functions have been shown to depend on local environments with specific compositions. 1 Self-assembled nanoscale compartments enriched in sphingolipids, cholesterol and proteins called “lipid rafts” are believed to be a locus for biochemical reactions taking place on the cell membrane. 2 Investigation of such domains with nanometer resolution is still a very challenging task, especially when it is done in their natural environment. Tip-enhanced Raman spectroscopy (TERS) is a non-destructive analytical technique capable of yielding vibrational spectra of samples with a lateral resolution below 30 nm. 3 It is essentially an apertureless near-field technique where conventional optics are used to illuminate a metal or metalized scanning probe microscopy (SPM) tip. This tip is brought in close proximity to a sample surface leading to a significant enhancement (several orders of magnitude) of the Raman scattering from the molecules located in the small region under the tip apex. Biological materials are in general very weak Raman scatterers due to their non-resonant character. Therefore, their investigation demands a highly optimized TERS setup, especially with respect to tip fabrication.

Deterioration of the outstanding optical properties of elemental silver due to atmospheric corrosion compromises its use in the field of plasmonics. Therefore, more chemically inert, but more lossy, metals (e.g., gold) are often used as a compromise. Silver tips for near-field optical microscopy are only utilized by specialized laboratories with in-house tip production facilities. This article presents a time-dependent study of the effect of atmospheric corrosion on the electromagnetic enhancement of solid silver tips. It was found that chemical degradation renders them unusable for tip-enhanced Raman spectroscopy (TERS) within the first two days after production. Furthermore, we present a simple electrochemical method for recovering the enhancing effect of corroded silver tips, as well as for storing freshly prepared probes, for example, for easy shipment. The present work greatly simplifies the experimental aspects of near-field optical microscopy, which should make near-field optical techniques, and, in particular, TERS, more accessible to the scientific community. (C) 2015 AIP Publishing LLC.

Lipid domains in supported lipid layers serve as a popular model to gain insight into the processes associated with the compartmentalization of biological membranes into so-called lipid rafts. In this paper, we present reproducible tip-enhanced Raman spectra originating from a very small number of molecules in a lipid monolayer on a gold surface, probed by the apex of a nanometer-sized silver tip. For the first time, we show large (128 × 128 pixels), high-resolution (< 50 nm) tip-enhanced Raman images of binary lipid mixtures with full spectral information at each pixel.