Electrochemistry At Nanoscopic Liquid/Liquid Interface Array And Its Analytical Application

Electrochemistry at the liquid/liquid interface, or so-called the interface between two immiscible electrolyte solutions （ITIES）, provides a facile biomimetic approach to study and understand mass transport and exchange at the biological interfaces and membranes. In comparison with the conventional electrochemistry method, namely, that at the solid/liquid interface, not only the electron transfer reaction but also ion transfer reaction as well as the coupled reactions （e.g., the oxygen reduction and hydrogen evolution reactions） can be addressed. It has been widely applied in the nonredox electrochemical detection of various ions, ranging from inorganic and organic ions to biochemical substances, in particular when their oxidation/reduction is difficult.For decades, miniaturization of the ITIES （micro-/nano-ITIES） have attracted increasing research interests. Miniaturization of the ITIES can effectively lower large iR drop bring by the organic liquids, improve detection sensitivity and limit of detection due to enhanced mass transfer and rapid interfacial processes and stabilize the liquid/liquid interface by miniaturizing its area. In addition, the electrochemical measurements can be performed in a two-electrode format using the conventional potentiostat. Till now, many approaches have been developed to create nano-ITIES, under the support of glass nanopipet, nanoporous materials with extremely high pore density （such as nanoporous silica films, track etched polymer, the solid-state silicon nitride nanopore membranes and silicate/zeolite membranes） and metal particles. With the development of nanotechnology and bioscience, the studies of ITIES focus on developing novel strategies to fabricate miniaturized ITIES and extending their applications in various fields. This thesis has been divided into four parts:The first chapter briefly introduced the development of ITIES and relevant basic theory （ion transfer reaction and steady-state current）; then emphatically introduced the strategies to fabricate nano-ITIES and its characterization. Electrochemical detection methods and applications of micro-/nano-ITIES were also included.The second chapter demonstrated a novel and facile method to build nano-ITIES array with the support of ultrathin freestanding silica nanochannel membrane （SNM）.The array consists of a high density (4.0 × 10⁴ pores μm^-2) of individual nano-ITIES of 2-3 nm and behaves as a microinterface characteristic of diffusion geometry with two back-to-back inlaid microdisc interfaces. Thus, ion transfer across the nano-ITIES array yielded symmetric steady-state current waves. Furthermore, because of the ultrasmall size of each independent nano-ITIES, an apparent size-selective ion transfer was observed. In addition, since the SNM surface is negatively charged, the transfer of anions encountered strong electrostatic repulsion from the nanochannel walls, with the magnitude of ion transfer current apparently dependent on the ionic strength of aqueous solutions.In the third chapter, SNM/nano-ITIES has been developed for investigation of the ion-transfer voltammetric behavior of metoprolol （MTP）, an antihypertensive drug, and for its detection in complex samples. Important thermodynamic parameters, such as the partition coefficient and apparent diffusion coefficient of MTP, were obtained. Seven different interfering substances, namely glucose, urea, ascorbic acid, glycine, MgCl₂, Na₂SO₄ and BSA, were chosen as models of interferents in biological samples to study their influence on the ion-transfer current signal of MTP. The results confirmed that the steady-state current waves barely changed in the presence of these interferents except BSA. The presence of BSA shortened the potential window and decreased the ion-transfer current magnitude. Moreover, the determination of MTP in complex samples （i.e. urine, serum and blood） and pharmaceutical forms were conducted without sample pretreatment. A wide linear dynamic range and a low limit of detection were achieved. SNM/nano-ITIES was proved to be stable and repeatable.The last chapter summarized the work presented in this thesis. Attempts were made to propose the future research trend of SNM/nano-ITIES.