Deciphering Interfaces at the Micro- and Nano- Scale

Abstract

Interface emerges where distinct material phases meet each other. Although macroscopic models often represent interfaces as idealized geometric boundaries, these microscopic regions are structurally and chemically heterogeneous and frequently govern the behavior of the overall system. By focusing on three specific boundary types, the electrochemical interface, the excitonic interface, and the nano-bio interface, this thesis explores how spatial and temporal variations at the micro- and nano- scale determine system performance. Chapter 1 establishes the conceptual framework for interfacial characterization. Interfaces are classified according to the phases in contact and this thesis centers on three types of interface. The electrochemical interface is considered as the electrical double layer and diffusion layer that regulate charge transfer and mass transport. The excitonic interface is the boundary for energy and charge transfer processes between quantum dots and molecular acceptors. The nano-bio interface is the boundary between synthetic nanomaterials and living systems. This Chapter also introduces the principal characterization methods used throughout the thesis, including confocal fluorescence microscopy, focused ion beam milling, scanning transmission electron microscopy, and transient absorption spectroscopy. Chapter 2 examines the electrochemical interface by spatially resolving local pH during the carbon dioxide reduction reaction. Although highly structured electrodes are known to alter product selectivity, the origin of this effect remain under debate. Photolithographically fabricated gold microwire arrays were used as model systems to determine how surface microstructure modifies the local catalytic environment. Confocal microscopy coupled with a soluble ratiometric fluorescent probe enabled spatial mapping of pH within the diffusion layer under operating conditions. Microstructured electrodes were found to generate a higher local pH than planar electrodes. When faradaic efficiency for carbon monoxide was normalized to the measured local pH, both microstructured and planar electrodes exhibited the same dependence. This result shows that the apparent selectivity enhancement associated with electrode microstructuring arises primarily from localized alkalization and suppresses the competing hydrogen evolution reaction. Chapter 3 further studies the electrochemical interface by assessing the structural stability of the topological semimetal PtGa during the hydrogen evolution reaction. Topological materials have been proposed as promising electrocatalysts because their nontrivial surface states are expected to promote rapid intrinsic reaction kinetics. However, establishing a direct relationship between these electronic features and catalytic activity requires careful characterization of the solid-liquid interface under operating conditions. Using focused ion beam milling, scanning transmission electron microscopy, this Chapter demonstrates that PtGa is not structurally stable during electrocatalysis. Under practical reaction conditions, the surface undergoes substantial reconstruction through gallium loss, producing a highly porous, platinum-rich layer, indicating that the high activity of PtGa originates from an increased density of platinum active sites rather than from topology-driven enhancement of the intrinsic reaction rate. Chapter 4 addresses the excitonic interface by resolving the mechanistic competition between Förster resonance energy transfer and charge transfer. Quantum dots are widely used as donors in energy transfer systems with organic dyes, yet experimentally observed efficiencies often fall below theoretical expectations. To investigate this discrepancy, commercially available core-shell quantum dots were coupled to sulforhodamine B acceptors. Transient absorption spectroscopy revealed the spectroscopic signature of the sulforhodamine B cation, providing direct evidence for interfacial charge transfer. This competing process substantially reduces the efficiency of the intended energy transfer, even in donor-acceptor pairs with strong spectral overlap. Additional experiments using tunable core-shell structured quantum dots showed that energy transfer efficiency depends on shell thickness. These results demonstrate that donor emission characteristics alone are insufficient for designing efficient quantum dot energy transfer systems, as shell thickness also determines the balance between competing interfacial pathways. Chapter 5 investigates the nano-bio interface through confocal fluorescence microscopy, enabling the observation of living systems with subcellular spatial resolution. The interaction between mammalian cells and synthetic silicon nanowire arrays was first evaluated. Resonance energy transfer imaging and propidium iodide assays confirmed that the nanowires penetrate the cell membrane following electroporation. Intracellular pH was then monitored during electrochemical operation using a ratiometric fluorescent indicator, revealing that reducing potentials increase intracellular pH. Imaging across the boundary between planar and nanowire regions suggests that this electrochemical activity is localized primarily to the extracellular space adjacent to the membrane rather than to the cytosol itself. This Chapter then extends the analysis of the nano-bio interface to the free-living diazotroph Xanthobacter autotrophicus as a potential microbial biofertilizer. Confocal imaging of labeled bacteria demonstrated successful epiphytic colonization across the roots of multiple plant species, supporting the potential of this organism to interact with agricultural crops and contribute to nutrient delivery.