UC Santa Barbara

Electrochemistry is the study of the interface between chemical species and electrified surfaces. These interfaces, which use electrical current to drive chemical transformations or derive electrical energy from chemical reactions, are critically important in the context of climate change. Using electrochemistry, renewably-generated electricity can be stored in the form of chemical potential in the chemical bonds of a fuel (such as hydrogen) or in the energy of electrons in a battery. These reactions are often catalyzed by nanoparticles of various materials, chosen due to their high surface areas and ostensibly tunable proparties. However, a significant gap in our understanding of these nanoparticles remains: as nanoparticles are not atomically precise, each individual particle has different properties (e.g., size, shape, or catalytic performance). However, conventional analysis methods study ensembles of many particles and cannot deconvolute each individual’s contribution. This means that the best-performing particles (compared to those which contributed little to the reaction) cannot be readily identified, and future syntheses cannot be tailored to target these particles.

The first three chapters of this dissertation describe my work to develop new methods and tools to study individual particles at the nanoscale, one particle at a time. First, I describe our efforts to understand the reactivity of platinum nanoparticles which catalyze the hydrogen evolution and oxygen reduction reactions. The broad distribution of catalytic activities we measured could not be explained simply by particle size, and our insights let us identify a possible mechanism degraded the particles' activity. In order to glean more information from each particle, I next developed a method to measure the electrochemical impedance of single nanoparticles. Electrochemical impedance spectroscopy has the ability to resolve different interfacial processes based on their timescales and was used to detect individual, insulating microparticles as they collided with an electrode; the time-resolved impedance spectra enabled accurate measurement of the particle-electrode contact areas. Finally, the single-particle impedance technique was applied to individual Prussian blue nanocubes, which reversibly intercalate sodium ions. In this case, spatially-resolved impedance spectra allowed us to measure the ionic and electronic conductivities of the particles as they stored sodium ions and electrons and revealed that these conductivities varied by up to an order of magnitude particle-to-particle.

The final chapter of this work extends the time-resolved impedance techniques developed for single particle analysis to the field of electrochemical biosensing. Biosensors aim to measure the presence or concentration of a particular species, and electrochemistry provides a natural way to translate between a chemical species and an electrical signal. Specifically, I describe the interrogation of a class of devices known as electrochemical aptamer-based sensors using electrochemical impedance spectroscopy. This technique enabled us to measure the state of the sensors more precisely and more rapidly than comparable methods, allowing real-time measurements of an antibiotic and an amino acid, even in the blood stream of living animals.