Modeling and Characterization of Au/Liquid and Liquid/Liquid Interfaces for Application in Capacitive-Based Sensing Devices

Capacitive-based MEMS sensing has found use in a wide array of fields; however, the continuing trend of miniaturization necessitates a change in the overall sensing approach of these systems due to the inherent challenges associated with measuring increasingly smaller changes in capacitances. This work seeks to solve this issue through the study of the ultra-high capacitance per unit area of the interface between two immiscible electrolyte solutions (ITIES).

This thesis focuses on the modeling and characterization of the impedance behaviors of the water/1,2-dichloroethane (DCE), gold/water, and gold/DCE interfaces for potential use in capacitive-based microelectromechanical systems (MEMS) sensing devices. Each interface is probed through the use of electrochemical impedance spectroscopy (EIS). Equivalent circuit models are developed for each of these interfaces, and it was found that a modified Randles equivalent circuit best fits the impedance data for both the Au/liquid and water/DCE interfaces. The benefits of using nanoporous gold (NPAu) electrodes as opposed to conventional planar electrodes are discussed and the impacts of initial gold concentration, electrode thickness, and etch time on the NPAu pore structure and impedance behavior of the NPAu/water and NPAu/DCE interfaces are studied in detail. It was found that a pure constant phase element (CPE) model was able to accurately describe the impedance behavior for these interfaces.

Characterization of the impedance data is conducted using a multivariate optimization script developed in MATLAB to simultaneously solve for each variable in the equivalent circuit models through minimization of the difference between the impedance data and the fit given by the equivalent circuit models. The capacitance of the water/DCE interface was found to have a linear dependence on the interfacial area, with C/A=0.4168±0.042 F/m2, which is significantly larger than what is observed in a conventional capacitive-based MEMS device. The clear dependence of water/DCE interfacial capacitance on interfacial area for the water/DCE ITIES makes it a promising candidate to couple with input mechanical signals for an ITIES-based sensing device.