Probing Redox Reactions at the Nanoscale with Electrochemical Tip-Enhanced Raman Spectroscopy

Abstract

A fundamental understanding of electrochemical processes at the nanoscale is crucial to solving problems in research areas as diverse as electrocatalysis, energy storage, biological electron transfer, and plasmon-driven chemistry. However, there is currently no technique capable of directly providing chemical information about molecules undergoing heterogeneous charge transfer at the nanoscale. Tip-enhanced Raman spectroscopy (TERS) uniquely offers subnanometer spatial resolution and single-molecule sensitivity, making it the ideal tool for studying nanoscale electrochemical processes with high chemical specificity. In this work, we demonstrate the first electrochemical TERS (EC-TERS) study of the nanoscale redox behavior of Nile Blue (NB), and compare these results with conventional cyclic voltammetry (CV). We successfully monitor the disappearance of the 591 cm(-1) band of NB upon reduction and its reversible reappearance upon oxidation during the CV. Interestingly, we observe a negative shift of more than 100 mV in the onset of the potential response of the TERS intensity of the 591 cm(-1) band, compared to the onset of faradaic current in the CV. We hypothesize that perturbation of the electrical double-layer by the TERS tip locally alters the effective potential experienced by NB molecules in the tip-sample junction. However, we demonstrate that the tip has no effect on the local charge transfer kinetics. Additionally, we observe step-like behavior in some TERS voltammograms corresponding to reduction and oxidation of single or few NB molecules. We also show that the coverage of NB is nonuniform across the ITO surface. We conclude with a discussion of methods to overcome the perturbation of the double-layer and general considerations for using TERS to study nanoscale electrochemical processes.

Advances in surface-enhanced vibrational spectroscopy at electrochemical interfaces

“…Further applications of liquid TERS have since been demonstrated, including the analysis of lipid bilayers [131], the adsorption of probe molecules on Au(111) [132], and TERS imaging in organic liquids [133]. It has taken several years since the first demonstration of liquid TERS to develop EC-TERS but 2015 saw major advances from two independent groups [134,135]. Ren et al employed an STM-TERS approach with horizontal illumination (see Figure 6(a)-(d)) to obtain EC-TERS spectra of molecular monolayers adsorbed at single crystal Au electrodes, and reported significant changes brought about by the potential-induced protonation/deprotonation of the surface-adsorbed species [135].…”

“…Ren et al employed an STM-TERS approach with horizontal illumination (see Figure 6(a)-(d)) to obtain EC-TERS spectra of molecular monolayers adsorbed at single crystal Au electrodes, and reported significant changes brought about by the potential-induced protonation/deprotonation of the surface-adsorbed species [135]. Using an AFM-TERS configuration, Van Duyne and co-workers studied the redox behaviour of NB at indium tin oxide (ITO) electrodes and postulated that, because of the scale of the experiment, the TERS probe perturbs the electrical double-layer and hence affects the local potential experienced by the molecules at the surface [134]. Moreover, the authors reported step-like features when plotting TERS intensity as a function of potential, characteristic of a single or few-molecule response, but this was highly dependent on the position on the surface, reflecting the non-uniform distribution of surface-adsorbed molecules (Figure 6(e)).…”

Unforeseen distance-dependent SERS spectroelectrochemistry from surface-tethered Nile Blue: the role of molecular orientation

“…[1][2][3] The redox dye Nile Blue (NB) has emerged as an excellent probe for probing fundamental electrochemistry with SERS. [4][5][6][7][8][9][10] A key feature of NB is that resonant excitation (e.g. 642 nm) leads to a strong SERS signal when the molecule is in its oxidized form and a weak SERS signal when it is in its reduced form, providing a spectral readout for reduction and oxidation events (e.g.…”

One‐Step Nanoextraction and Ultrafast Microanalysis Based on Nanodroplet Formation in an Evaporating Ternary Liquid Microfilm

“…[29,30] As summarized in Figures S2 and S3 in the Supporting Information, the limit of detection (LOD) of R6G is as low as 10 −9 m, which is comparable to surface-enhanced Raman scattering [31] and better than the LOD obtained using other approaches, such as well-defined Ag nanocrystals in solution or microfluidic nanopillars. [34][35][36] Remarkably, the LOD of Nile Red was found to be 10 −12 m, possibly due to its higher partition coefficient compared to Nile Blue and R6G.…”