<b>Confined Multiphase Electrochemistry</b>
Creators: name: Vannoy, Kathryn J
Publisher: name: Purdue University Graduate School
Year: 2024
Subjects: classificationCode: 340103
subject: Electroanalytical chemistry

description: Scientists across many disciplines have observed a striking phenomenon: chemical reactions that do not appreciably occur in large volumes often proceed readily in microdroplets. At the core of suggested mechanisms is the influence of interfacial chemistry on the overall reaction; when the interfacial area dominates the reactor volume, the measured reaction rate is often accelerated. For instance, microdroplets with a high surface area-to-volume ratio (generally with radii smaller than 10 µm) provide a unique reaction environment and have been observed to accelerate a wide variety of chemical reactions. This is likely surprising to most readers, as much of our chemical intuition comes from experiments performed on benchtops in beakers (large, single-phase systems). However, microdroplets are regularly exploited by nature, from multiphase atmospheric aerosols to biomolecular condensates in cells. Thus, it is vital to have measurement tools capable of studying multiphase, nanoscale reactors. An electrochemical perspective on measuring multiphase chemistry under nanoconfinement is given in Chapters 2-4. To my knowledge, there were no reports of accelerated reactivity in microdroplets from electrochemical studies until the 2021 observation that enzyme turnover rates are inversely-related to the size of the containing nanodroplet (given in Chapter 6). In this dissertation work, we developed new electroanalytical tools to probe chemical transformations/reactions at micro- and nano-interfaces and made use of new reaction schemes that capitalize on multiphase microenvironments.Much of the method development was built on the foundation of stochastic nanoelectrochemistry, a technique that is reviewed thoroughly in Chapters 2, 4, and 5. Briefly, stochastic nanoelectrochemistry is the measurement of single nano-entities, one-at-a-time, as the collide with a micron-sized electrode. The nano-entities studied in this dissertation were aqueous droplets, either suspended in an immiscible oil continuous phase or propelled through air. We dove deeply into these studies, from using correlated microscopy to watch how these micro- and nanodroplets collide with other interfaces to building simulations to quantify changes to the chemistry inside. We showed how the surface environment directs water nanodroplet collisions (Chapter 10) and measured the sub-diffraction-limited nanometer contact area that forms between a microdroplet and a metal surface (Chapter 11). Using the nanodroplets as tiny reactors, we measured accelerated rate constants and promoted unfavorable nucleation events in attoliter-femtoliter aqueous droplets (see Chapter 6-7 and Chapter 12, respectively) and in microliter aqueous droplets (see Chapter 8 and Chapter 9, respectively).As mentioned above, microdroplets are ubiquitous in air (<i>e.g.,</i> aerosols). However, electrochemistry is not an obvious choice for the measurement of intact aerosols because electrochemistry is traditionally performed in a conductive solution, and electrochemistry in air is difficult. In this dissertation we laid the groundwork for a path forward that allows electrochemical access the air|microdroplet interface. We designed and characterized a novel electrochemical cell, where the working electrode is a microwire traversing a suspended liquid film (Chapters 13-15). The early results were born from pure curiosity: Can we do electrochemistry in a soap bubble wall? Chapter 13 shows that the answer is “Yes!”, and that electrochemistry can report on aerosol contents that are collected from the air into this thin film. However, the soap bubble wall was severely limited by the lifetime of the bubble wall (bubbles pop), so in Chapters 14 and 15, we introduce a suspended ionic liquid film that does not pop from evaporation. With the more robust system, we realized the ability to probe intact single microdroplets, one-at-a-time (Chapter 14), giving electrochemical access to the air|water interface.As detection of illicit substances from aerosols has the potential for immediate impact on first responder, user, and bystander safety, we employed the new technology to electroanalyze aerosolized methamphetamine (Chapter 13) and fentanyl (Chapter 15). Electrochemistry is small, simple, and affordable, making it a realistic candidate for an in-field sensor. We overcame selectivity challenges by using our understanding of interfacial microenvironments to leverage local pH changes, as demonstrated by the reliable detection of low purity cocaine in mixed powders (Chapter 16). This patented method provides a highly selective technique for cocaine identification in the presence of adulterants without the need to bring any chemicals to the scene (water is our only reagent!).In sum, this body of work contributes to the electrochemical studies in nano- and microdroplets, extending the reach to account for droplet size on measured rates and to include microdroplets with a water|air boundary. Applications of the work were focused on in-field detection of illicit substances.