In-Channel Salt Bridge for Amperometric Detection of Chip Based Capillary Electrophoresis and Structural Effect of Nanoporous Electrode for Conductometric Detection at High Ionic Strength

Abstract

Electrochemical techniques provide key solutions to the construction of miniaturized systems for bioanalysis, neuroscience, chemical, and environmental analysis. With the rapid developments in nano-electromechanical systems (NEMS) and micro-electromechanical systems (MEMS), electrochemical detection techniques along with electrochemical sample injection, mixing, and preparation have proven to be important components of miniaturized analytical devices. Although various electrochemical detection strategies for miniaturized systems have been proposed, there remain many challenges related to the microchannel-electrode design and electrode material and structure. This dissertation describes electrochemical strategies for use under strong electric fields for miniaturized analytical devices and a unique conductivity detection method based on a well-defined nanoporous electrode. Chapter 1 introduces the background and an overview of the challenges related to analytical miniaturized systems. This section particularly focuses on electrochemical detection techniques for analytical microsystems. In Chapter 2, we propose a novel method for in-channel electrochemical detection under a high electric field using a polyelectrolytic gel salt bridge (PGSB) that is integrated into the middle of the electrophoretic separation channel. The finely tuned placement of a gold working electrode and the PGSB on an equipotential surface in the microchannel provided highly sensitive electrochemical detection without any deterioration in the separation efficiency or interference of the applied electric field. To assess the working principle, the open circuit potentials between gold working electrodes and the reference electrode at varying distances were measured in the microchannel under electrophoretic fields using an electrically isolated potentiostat. In addition, in-channel cyclic voltammetry confirmed the feasibility of electrochemical detection under various strengths of electric fields (~400 V/cm). Effective separation on a microchip equipped with a PGSB under high electric fields was demonstrated for the electrochemical detection of biological compounds such as dopamine and catechol. The proposed in-channel electrochemical detection under a high electric field enables wider electrochemical detection applications in microchip electrophoresis. In Chapter 3, we examine electrochemical behavior in a nano-confined space and introduce well-defined nanoporous electrodes to improve conductivity detection for ion chromatography. Nanoporous electrified surface creates unique nonfaradaic electrochemical behavior that is sensitively influenced by the pore size, morphology, ionic strength, and electric field modulation. Here we report the contributions of ion concentration and applied ac frequency to the electrode impedance through electrical double layer overlap and ion transport along the nanopores. The impedance analysis based on the transmission line model revealed the elements of the equivalent circuit such as pore resistance (Rpore) and capacitance (Ce), which are characteristic parameters varying with surface morphology as well as ion concentration. Nanoporous Pt with uniform pore size and geometry (L2-ePt) was investigated in comparison to Pt black with a dendritic structure and broad distribution of pore size. In spite of similar real surface areas, L2-ePt responded more sensitively to conductivity changes in aqueous solutions than Pt black and enabled quantitative conductometry for high electrolyte concentrations, which is difficult in general. The nanopores of L2-ePt were more effective to reduce electrode impedance so as to exhibit superior linear responses to not only flat Pt but also Pt black, leading to successful conductometric detection in ion chromatography without ion suppressor at high ionic strength.