Imaging electrochemical reactions at the nanoscale

As part of an interdisciplinary team, supported by an AFOSR MURI, we are devising new ways to study electrochemical reactions at the nanoscale based upon optical readouts.  We have constructed a scanning electrochemical microscope (SECM) on top of an optical microscope, allowing us to perform simultaneous SECM and optical experiments.  By integrating 3-dimensional super-resolution imaging techniques, we are able to precisely locate a scanning tip electrode above a substrate with 25 nm resolution.  To do this, we attach fluorescent nanodiamonds to the sheath of the SECM probe as well as the substrate, and then use a phase mask to imprint a distance dependent pattern on the emission (see figure at right).  This allows us to calculate the distance between the tip and the substrate.  The ability to use optical readouts to independently confirm and calibrate tip-substrate distances allows for more accurate determination of electrochemical parameters (such as rate constants) that depend strongly on this distance measurement.

3-dimensional super-resolved images of fluorescent nanodiamonds attached to the SECM tip (yellow circle) and the substrate (white circle).  The orientation of the double lobed pattern allows us to calculate the vertical position of both the tip and the substrate, resulting in a quantitative distance measurement between the two.

We also use molecules that change their emission properties based on their redox state.  For example, we were the first group to observe electrogenerated chemiluminescence at the surface of single nanoparticle electrodes, as shown below.

Representative publications:

V. Sundaresan, J.W. Monaghan, K.A. Willets.  "Visualizing the Effect of Partial Oxide Formation on Single Silver Nanoparticle Electrodissolution."   J. Phys. Chem. C.  DOI: 10.1021/acs.jpcc.7b11824 (2018).

Y. Yu, V. Sundaresan, S. Bandyopadhyay, Y. Zhang, M.A. Edwards, K. McKelvey, H.S. White, K.A. Willets.  “Three-Dimensional Super-Resolution Imaging of Single Nanoparticles Delivered by Pipettes.”  ACS Nano.  11, 10529 (2017).

V. Sundaresan, K. Marchuk, Y. Yu, E.J. Titus, A.J. Wilson, C.M. Armstrong, B. Zhang, K.A. Willets.  “Visualizing and Calculating Tip-Substrate Distance in Nanoscale Scanning Electrochemical Microscopy Using 3-Dimensional Super-Resolution Optical Imaging.”   Analytical Chemistry. 89, 922 (2017).

A.J. Wilson, K. Marchuk, K.A. Willets.  “Imaging electrogenerated chemiluminescence at single gold nanowire electrodes.”  Nano Lett. 15, 6100 (2015).

C. Renault, K. Marchuk, H.S. Ahn, E.J. Titus, J. Kim, K.A. Willets, A.J. Bard.  “Observation of nanometer-sized electro-active defects in insulating layers by fluorescence microscopy and electrochemistry.”  Anal Chem. 87, 5730 (2015).