ELECTROCHEMICAL RECOGNITION AND TRANSPORT OF IONS AT LIQUID/LIQUID INTERFACES AS A PRINCIPLE FOR ENVIRONMENTAL AND BIOMEDICAL ANALYSIS AND BEYOND

Recognition and transport of important species at the membrane of a biological cell are critical for regulation of intracellular communication, metabolic pathways, vital internal conditions, and pharmaceutical drug up-take. Both processes are mediated by membrane-bound proteins functioning as pores, channels, and transporters that recognize and facilitate the transport of ions, nucleic acids and sugars. This whole process can be driven actively by membrane potential against the concentration gradient of transported species. In my PhD work, I fundamentally characterized dynamics of active ion transport, both in the presence and absence of recognition events, at liquid/liquid interfaces to understand electrochemically-controlled interfacial ion recognition and transfer. A deeper understanding of the kinetic and thermodynamic properties is achieved to realize applications in biomedical and environmental science, sensor technology and nanotechnology. The interface between two immiscible solutions served as an artificial model of a cell membrane. By manipulation of the interfacial potential, the active transport of ionic species was mimicked, which was monitored by an ionic current. Micrometer and nanometer sized interfaces were formed experimentally at the orifice of micropipets and nanopipets to probe ion-transfer reactions. Micropipet/nanopipet voltammetry was advanced to accurately obtain quantitative kinetic and thermodynamic parameters through numerical simulations of ion transfer and diffusion. Ion transfer rates for reversible and nonreversible reactions were determined to demonstrate how the rate controls the current, which affects the sensitivity of ion transfer as a sensing principle. Molecular recognition and transport of biomedical ionic drugs by hydrophobic receptors was examined thermodynamically, demonstrating how the interfacial interactions influence the selectivity of the sensing principle. Kinetic and thermodynamic analysis of the transfer of perfluoroalkyl surfactants, an emerging class of environmental contaminants that accumulate in wildlife, yielded high lipophilic values to suggest a possible origin of their high toxicity. Although, the focus of my research was primarily fundamental in nature, I tested the ion transfer principle practically with an ion selective electrode, developed in our group. Hexafluoroarsenate, an arsenical biocide found recently in wastewater, was detected at sub-nanomolar levels to confirm a thermodynamic mechanism that controls the detection limit