In this work, we use optical methods to study electrochemical reactions and processes occurring on the nanometer length scale. Optical methods are advantageous over traditional electrochemical methods because of their high spatial resolution and sensitivity at both the single nanoparticle and single molecule level. This dissertation describes a series of studies in which super-localization and dark-field optical imaging is used to provide insight into spatial and temporal heterogeneity in nanoscale electrochemical systems with <25 nm spatial resolution. In the first set of experiments, three-dimensional (3-D) super-resolution imaging is used to determine the tip-substrate distance in nanoscale scanning electrochemical microscopy (SECM) with precision better than 25 nm. Correlating the tip-substrate distance using both optical and electrochemical techniques showed excellent agreement. Second, single nanoparticles (NP) were delivered through a nanopipette, and their resistive-pulse signals were correlated with a fluorescence optical signal. The diffusion trajectories of individual NP delivered to the external solution and to an electrified interface were obtained by 3-D super-resolution imaging, and showed signatures of both sub-diffusive and super-diffusive behavior, depending on the balance of forces between the flow from the pipette and the applied potential at the electrified substrate. Next, we studied the influence of surface oxide layers on single silver NP electrodissolution by tracking the intensity and spatial variation of scattering from single nanoparticles over time. We discovered that silver NPs can undergo electrodissolution in either a spatially symmetric or asymmetric manner, based on the nature of the surface oxide layer. Moreover, we also reported the simultaneous electrodeposition of silver oxide at the electrode surface during the electrodissolution of silver NPs, which enabled us to study the effect of multiple simultaneous redox reactions and their effects on one another. Overall, these experiments reveal local heterogeneity in nanoscale electrochemical processes and allow for many single nanoparticles to be measured in parallel, revealing relationships that are hidden using traditional electrochemical measurements. / Chemistry

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. / Chemistry