The Expanding Frontiers of Tip-Enhanced Raman Spectroscopy

Fundamental understanding of chemistry and physical properties at the nanoscale enables the rational design of interface-based systems. Surface interactions underlie numerous technologies ranging from catalysis to organic thin films to biological systems. Since surface environments are especially prone to heterogeneity, it becomes crucial to characterize these systems with spatial resolution sufficient to localize individual active sites or defects. Spectroscopy presents as a powerful means to understand these interactions, but typical light-based techniques lack sufficient spatial resolution. This review describes the growing number of applications for the nanoscale spectroscopic technique, tip-enhanced Raman spectroscopy (TERS), with a focus on developments in areas that involve measurements in new environmental conditions, such as liquid, electrochemical, and ultrahigh vacuum. The expansion into unique environments enables the ability to spectroscopically define chemistry at the spatial limit. Through the confinement and enhancement of light at the apex of a plasmonic scanning probe microscopy tip, TERS is able to yield vibrational fingerprint information of molecules and materials with nanoscale resolution, providing insight into highly localized chemical effects.

Atomic Force Microscopy Based Tip-Enhanced Raman Spectroscopy in Biology

Most biological phenomena occur at the nanometer scale, which is not accessible by the conventional optical techniques because of the optical diffraction limitation. Tip-enhanced Raman spectroscopy (TERS), one of the burgeoning probing techniques, not only can provide the topography characterization with high resolution, but also can deliver the chemical or molecular information of a sample beyond the optical diffraction limitation. Therefore, it has been widely used in various structural analyses pertaining to materials science, tissue engineering, biological processes and so on. Based on the different feedback mechanisms, TERS can be classified into three types: atomic force microscopy based TERS system (AFM-TERS), scanning tunneling microscopy based TERS system (STM-TERS) and shear force microscopy based TERS system (SFM-TERS). Among them, AFM-TERS is the most widely adopted feedback system by live biosamples because it can work in liquid and this allows the investigation of biological molecules under native conditions. In this review, we mainly focus on the applications of AFM-TERS in three biological systems: nucleic acids, proteins and pathogens. From the TERS characterization to the data analysis, this review demonstrates that AFM-TERS has great potential applications to visually characterizing the biomolecular structure and crucially detecting more nano-chemical information of biological systems.

Tip-enhanced Raman spectroscopy - from early developments to recent advances

An analytical technique operating at the nanoscale must be flexible regarding variable experimental conditions while ideally also being highly specific, extremely sensitive, and spatially confined. In this respect, tip-enhanced Raman scattering (TERS) has been demonstrated to be ideally suited to, e.g., elucidating chemical reaction mechanisms, determining the distribution of components and identifying and localizing specific molecular structures at the nanometre scale. TERS combines the specificity of Raman spectroscopy with the high spatial resolution of scanning probe microscopies by utilizing plasmonic nanostructures to confine the incident electromagnetic field and increase it by many orders of magnitude. Consequently, molecular structure information in the optical near field that is inaccessible to other optical microscopy methods can be obtained. In this general review, the development of this still-young technique, from early experiments to recent achievements concerning inorganic, organic, and biological materials, is addressed. Accordingly, the technical developments necessary for stable and reliable AFM- and STM-based TERS experiments, together with the specific properties of the instruments under different conditions, are reviewed. The review also highlights selected experiments illustrating the capabilities of this emerging technique, the number of users of which has steadily increased since its inception in 2000. Finally, an assessment of the frontiers and new concepts of TERS, which aim towards rendering it a general and widely applicable technique that combines the highest possible lateral resolution and extreme sensitivity, is provided.

Rational fabrication of a gold-coated AFM TERS tip by pulsed electrodeposition

Reproducible fabrication of sharp gold- or silver-coated tips has become the bottleneck issue in tip-enhanced Raman spectroscopy, especially for atomic force microscopy (AFM)-based TERS. Herein, we developed a novel method based on pulsed electrodeposition to coat a thin gold layer over atomic force microscopy (AFM) tips to produce plasmonic TERS tips with high reproducibility. We systematically investigated the influence of the deposition potential and step time on the surface roughness and sharpness. This method allows the rational control of the radii of gold-coated TERS tips from a few to hundreds of nanometers, which allows us to systematically study the dependence of the TERS enhancement on the radius of the gold-coated AFM tip. The maximum TERS enhancement was achieved for the tip radius in the range of 60-75 nm in the gap mode. The coated gold layer has a strong adhesion with the silicon tip surface, which is highly stable in water, showing the great potential for application in the aqueous environment.

Tip-enhanced Raman spectroscopy: tip-related issues

After over 15 years of development, tip-enhanced Raman spectroscopy (TERS) is now facing a very important stage in its history. TERS offers high detection sensitivity down to single molecules and a high spatial resolution down to sub-nanometers, which make it an unprecedented nanoscale analytical technique offering molecular fingerprint information. The tip is the core element in TERS, as it is the only source through which to support the enhancement effect and provide the high spatial resolution. However, TERS suffers and will continue to suffer from the limited availability of TERS tips with a high enhancement, good stability, and high reproducibility. This review focuses on the tip-related issues in TERS. We first discuss the parameters that influence the enhancement and spatial resolution of TERS and the possibility to optimize the performance of a TERS system via an in-depth understanding of the enhancement mechanism. We then analyze the methods that have been developed for producing TERS tips, including vacuum-based deposition, electrochemical etching, electrodeposition, electroless deposition, and microfabrication, with discussion on the advantages and weaknesses of some important methods. We also tackle the issue of lifetime and protection protocols of TERS tips which are very important for the stability of a tip. Last, some fundamental problems and challenges are proposed, which should be addressed before this promising nanoscale characterization tool can exert its full potential. Graphical Abstract ᅟ.

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

Tip-enhanced Raman scattering (TERS) has become a powerful tool for nanoscale structural analysis for several branches of organic, inorganic, and biological chemistry. This highly sensitive technique enables molecular characterization with a lateral resolution far beyond Abbe's diffraction limit and correlates structural and topographic information on a nanometer scale. In this review, the current experimental concepts with respect to their strengths and obstacles are introduced and discussed. A further focus was set to biochemistry comprising applications like nucleic acids, proteins, and microorganisms, thus demonstrating the potential use towards the pharmaceutically relevant challenges where nanometer resolution is required.