UCLA

Microorganisms often exist in complex assemblages, where chemical gradients play a crucial role in influencing their metabolic processes. These gradients, resulting from endogenous microbial activities, involve a dynamic transfer of nutrients, signaling molecules, and metabolic waste. Understanding the effects of such gradients on microbial physiology is important for advancing our knowledge of microbial systems. Traditional methods for creating artificial microenvironments to study these interactions, such as chamber systems, pipette injection, hydrogels, and microfluidic devices, have provided significant insights in the physiological heterogeneity in the microbial systems. However, existing platforms are limited by their spatial resolution and temporal response times. My research aims to improve the spatiotemporal resolution of artificial microenvironments by developing a novel biocompatible electrochemical platform for the generation and regulation of chemical gradients. Leveraging the fast and controllable kinetics of electrochemistry, this platform will allow for higher resolution and quicker response times compared to conventional methods. Through meticulous electrochemical design and advanced simulation guidance, I have established and optimized the electrochemical platforms for controllable gradients of oxygen (O2), hydrogen peroxide (H2O2) and pH.My first research project focused on the electrochemical control of O2 and H2O2 gradients (Chapter 2). O2 and H2O2 as one of reactive oxygen species (ROS) are two critical biologically relevant species in extracellular microenvironment. To artificially creating O2 and H2O2 gradients, I performed electrochemical oxygen reduction catalyzed by Au and Pt deposition layer on Si microwire arrays. Microwire electrodes allowed for customized morphology, facilitating the exploration of the relationship between the electrode morphology and the gradient profiles. However, the inverse design of the desired gradients is still challenging considering the large variations of extracellular O2 and H2O2 gradients in different microbial systems. To address this challenge, a machine learned based inverse design strategy was developed to quickly program a desired concentration profile at the microscopic level. Finite element method (FEM) models and machine learning algorithms were integrated to corelated the electrochemical parameters with the gradient profiles, and the targeted microenvironments of O2 and H2O2 generated through inverse design was experimentally validated. The concept of the inverse design assisted by artificial intelligence was demonstrated on both O2 and H2O2 gradients, enhancing our ability to understand and control extracellular spaces with precise spatiotemporal resolution. With the success in O2 and H2O2 gradients, my research was extended to the investigation of pH microenvironment (Chapter 3). pH, which reflects the local proton availability, is a critical factor in biological metabolism and physiology. To achieve a controllable pH gradient through electrochemical strategy, a proton-coupled electron transfer reaction (PCET) was meticulously designed and evaluated. Instead of using microwire electrodes, interdigitated electrodes consisting of a pair of microelectrode array strips was used to provide more flexibility in tuning the range and slope of the pH gradients. Key parameters, including oxidation and reduction potential, the gap between electrodes, and the electrolyte concentration, were systematically optimized to achieve the desired pH differential, guided by a comprehensive simulation model. The pH gradients generated by electrochemical reactions were mapped using confocal microscopy, revealing a high spatial resolution of ~100 μm and a rapid response time of ~101 s. Additionally, the biocompatibility of this electrochemical platform was evaluated, confirming its suitability for further applications in microbial systems. During my graduate career, I have demonstrated that electrochemistry represents a highly promising approach for constructing controllable microenvironments within microbial systems, advancing the spatiotemporal resolution of the artificial gradients. This strategy can be extended to the regulation of various small molecules that play crucial roles in cellular physiology across diverse assemblages. The development of this platform provides a novel toolkit for investigating microbial behavior in heterogenous microenvironments, thereby contributing to the broader field of microbial physiology and ecology.