Electrochemistry and Electrokinetics in Microchannels

Abstract

The main body of this work considers the design and development of a microfluidic, continuous electrochemical sensor capable of measuring accurate potential differences. The key challenge in creating such a device is the implementation of a miniaturized reference electrode and salt bridge. The purpose of a salt bridge is to allow ionic conduction between the reference and working electrodes while maintaining a physical separation between the two systems. Macro reference electrode and salt bridge techniques are difficult to implement on a micro scale. Instead of attempting to conform one of these techniques to function in a micro system, new methods were developed that take advantage of the conditions in a continuous microfluidic device. In particular, laminar flow and slow relative diffusion times allow for a reference electrode that does not require a physical salt bridge. Ionic conduction is maintained between neighboring reference and analyte streams while slow mixing effectively separates the two systems. Several different device designs were investigated focusing on the prevention of reference electrode contamination. If the reference electrode is chemically contaminated it will no longer behave as expected and can not be used as a reference point. Contamination at the reference electrode was evaluated while varying flow rates and the geometry of the microfluidic device. Mathematical models were simulated in order to understand the mass transport in each device design. Based on these simulations, dimensionless groups were found that defined the dominant physics in each system. These dimensionless numbers were then validated experimentally and numerically over a range of device parameters. Subsequently, operation criteria were developed to ensure that the reference electrode remains stable and uncontaminated. By creating a stable reference electrode on chip, any homogeneous electrochemical system that was previously studied on the macro scale can now be studied continuously in a microfluidic device. A secondary portion of this work investigates the role of surface charge with respect to electrodynamics in a microchannel. As the surface area to volume ratio increases, the concentration of charge at a channel wall may begin to approach the electrolyte concentration in the bulk solution. This phenomenon is studied numerically, with and without convection, in particular as it relates to a possible mechanism for overlimiting current. Additionally, a potential de-ionization device is theorized based on this mechanism along with scaling arguments that can be used to aid device design.