Electrochemical atomic force microscopy tip-enhanced Raman spectroscopy (EC-AFM-TERS) was employed for the first time to observe nanoscale spatial variations in the formal potential, E0′, of a surface-bound redox couple. TERS cyclic voltammograms (TERS CVs) of single Nile Blue (NB) molecules were acquired at different locations spaced 5-10 nm apart on an indium tin oxide (ITO) electrode. Analysis of TERS CVs at different coverages was used to verify the observation of single-molecule electrochemistry. The resulting TERS CVs were fit to the Laviron model for surface-bound electroactive species to quantitatively extract the formal potential E0′ at each spatial location. Histograms of single-molecule E0′ at each coverage indicate that the electrochemical behavior of the cationic oxidized species is less sensitive to local environment than the neutral reduced species. This information is not accessible using purely electrochemical methods or ensemble spectroelectrochemical measurements. We anticipate that quantitative modeling and measurement of site-specific electrochemistry with EC-AFM-TERS will have a profound impact on our understanding of the role of nanoscale electrode heterogeneity in applications such as electrocatalysis, biological electron transfer, and energy production and storage.
Bibliographical noteFunding Information:
M.M., G.K., G.G., G.C.S., and R.P.V.D. acknowledge support from the Air Force Office of Scientific Research MURI (FA9550-14-1-0003). L.J. and D.V.C. acknowledge support from the NSF award CHE-1362825.The authors thank Dr. Allen J. Bard, Dr. Henry S. White, Dr. Katherine A. Willets, and Dr. Stephanie Zaleski for helpful discussions.
- Laviron model
- Tip-enhanced Raman spectroscopy (TERS)
- cyclic voltammetry
- single-molecule electrochemistry