EIS using adjustable nanometer-gap electrodes

Abstract

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2007.Includes bibliographical references (p. 151-154).Electrochemical Impedance Spectroscopy (EIS) is a simple yet powerful chemical analysis technique for measuring the electrical permittivity and conductivity of liquids and gases. Presently, the limiting factor for using EIS as a portable chemical detection technology is the lack of absolute accuracy stemming from uncertainties in the geometrical factor used to convert measurable quantities of capacitance and conductance into the intrinsic parameters of permittivity and conductivity. The value of this geometrical conversion factor can be difficult to predict since it is easily affected by fringing electric fields, manufacturing variations, and surface chemistry. Existing impedance test cells typically address this problem using a calibration liquid with known permittivity and conductivity, however, this correction is not feasible in many applications since the calibration liquid may irreversibly contaminate the test electrodes. This thesis presents a technique for accurately measuring the permittivity and conductivity of liquids and gases without requiring the use of calibration liquids. This technique is made possible by precisely controlling the separation between two spherical electrodes to measure capacitance and conductance of the sample medium as a function of electrode separation. By leveraging the geometrical accuracy of the spherical electrodes and precise control of the electrode separation, the permittivity and conductivity of the sample can be determined without wet calibration. The electrode separation is adjusted using a flexure stage and a servomechanical actuator, which enables control the electrode separation with 0.25 nm resolution over a range of 50 gm. The nanometer smooth surfaces of the spherical electrodes also enable electrode gaps of less than 20 nm to be created.(cont.) The technique for measuring permittivity and conductivity presented in this thesis could eventually be adapted to make miniaturized disposable impedance test cells for chemical analysis. Such systems could take advantage of conductivity assays to determine the presence and concentration of specific substances. The adjustable nanometer electrode gap can also be used to study the properties of chemical and biological systems in highly confined states. These studies are fundamentally important for understanding biochemical processes in natural systems where reactions often take place inside confined structures such as cells, organelles, and the intercellular matrix.by Hongshen Ma.Ph.D