Probing Nanoscale Electrochemical Processes on Single Gold Nanoparticles using Optical Microscopy

Abstract

In this work, we use optical techniques to provide insight into how various components within electrochemical cells can impart apparent heterogeneity to single gold nanoparticle electrodes. Optical methods are advantageous in comparison to traditional electrochemical techniques due to their high sensitivity and spatial resolution, allowing us to study the impact of heterogeneity with single nanoparticle and single molecule sensitivity. Throughout the course of this dissertation, two optical techniques are discussed in detail, dark-field microscopy, and single molecule fluorescence imaging. We first began by studying the impact of the substrate using dark-field microscopy to monitor the electrodissolution kinetics of gold nanoparticles on thin films of tin-doped indium oxide (ITO), which is a commonly used supporting electrode for correlated optical and electrochemical studies. We found that ITO from two different suppliers showed marked differences in the gold electrodissolution kinetics, with ITO from one of the suppliers even showing poor sample-to-sample reproducibility across substrates within the same lot number. These results showed that the supporting electrode cannot be ignored when performing single nanoparticle structure-function studies. In the second work, we analyzed the electrodissolution of gold nanoparticles on well-behaved ITO substrates to investigate heterogeneity in their electrodissolution kinetics. The rate constants associated with the electrodissolution of Au NPs were extracted by fitting the intensity-time traces to a first-order kinetic model. We found that a non-negligible population of Au NPs didn’t fit the predictive kinetics model leading us to further probe whether surface effects play a role in the electrodissolution process. Super-localization imaging was used to track the center position of the Au NPs as they electrodissolved revealing three distinct electrodissolution behaviors, and a mechanism for the electrodissolution of Au NPs was proposed. Furthermore, calcite-assisted localization and kinetics (CLocK) microscopy was used to visualize changes in anisotropy and provide information as to how the shape of the Au NP changes as it electrodissolves. Lastly, in our third work, we provide insight as to how heterogeneity from all the different components of a single nanoparticle electrochemical sample impacts the apparent electrode performance. We proposed dark-field microscopy and single molecule fluorescence imaging as tools capable of detangling these effects. Moreover, we established Cresyl Violet as a reporter of single molecule electrochemistry and developed a two-working electrode optical system capable of visualizing single molecule activity. Lastly, we explored the relationships between Au NP size, Cresyl Violet activity and Au NP electrodissolution and found no clear trend between them suggesting the need for more studies to deconvolute these effects and provide meaningful insight into the structure-property relationships. Overall, this dissertation highlights the complexity of single nanoparticle studies and how heterogeneity can be induced from all the components of an electrochemical cell.