Challenges and Opportunities of Tip-Enhanced Raman Spectroscopy in Liquids

Abstract

Tip-enhanced Raman spectroscopy (TERS) is an emerging technique with a large number of potential applications in different fields such as chemistry, physics, biology, life sciences, and materials science. TERS in liquids offers nondestructive and label-free topographical and chemical imaging of molecules and chemical reactions under native conditions at a nanometer scale. It combines Raman spectroscopy with scanning probe microscopy, providing a unique combination of chemical information and high spatial resolution. The versatility of TERS increases even further if it is modified for simultaneous electrochemical measurements. This paper reviews the recent progress of TERS in liquid, with a focus on molecular structures, interfaces, and chemical reactions studied to date. For example, TERS studies revealed nanoscale chemical images of various molecules containing a benzene ring such as thiophenol or carbon nanotubes. Additionally, heterogeneous catalytic reactions at the solid−liquid interface were investigated in a new way to gain a deeper insight into electrochemical processes. Furthermore, TERS measurements expanded our knowledge of several redox compounds and enabled new insights into various chemical transformations. Despite substantial progress, TERS in liquids is not yet a routine measurement method, and there are still several challenges to solve before TERS can unleash its full potential.

Progress of tip-enhanced Raman scattering for the last two decades and its challenges in very recent years

“…For TERS equipment, on the other hand, Raman microscopy has been combined with STM or AFM, which was initially used for observation at an air-solid interface. For observations in liquid, 48,63 an STM tip except for the apex is covered with an insulator to prevent electric current excluding a tunnelling effect, 98 and the AFM cantilever shows a lower resonance frequency. 62 The protection layer, which maintains both the chemical and mechanical stability of the tip, improves the TERS signals in liquid.…”

“…62 The protection layer, which maintains both the chemical and mechanical stability of the tip, improves the TERS signals in liquid. 62 In EC-TERS, 48,63 the noble metal tip and single crystal substrate, on which analyte molecules are adsorbed, play the role of working electrodes. The working electrodes, a Ag reference electrode, and a Pt counter electrode are connected to a bipotentiostat, by which the electrochemical potentials of the tip and sample are controlled.…”

“…42 Since that time, several research groups have developed various types of solution-based TERS. [41][42][43][44][45][46][47][48] Particularly, EC-TERS has shown rapid advances. 64,102,103 Liquid TERS and EC-TERS have already been reviewed in many review papers, and thus, in this section we focus on liquid TERS and TERS with chemically modified tips, which our group developed recently.…”

“…64 The configuration of side illumination and collection has been widely used for EC-TERS. 48 Here, the studies on EC-TERS, EC-AFM-TERS, applications of EC-TERS to electro-active catalysis, and energy-related electrochemical systems are introduced.…”

“…42 Since that time, several novel ideas for liquid TERS have emerged, and nowadays, liquid TERS has become a routine technique. [43][44][45][46][47][48] To improve the sensitivity, selectivity, and stability of TERS tips during liquid phase measurements, Pienpinijtham et al proposed the use of chemically modified tips with various probe molecules. 43,47 They used them for nanoscale pH measurement in solutions and chirality discrimination using TERS.…”