The dissertation presented here describes two research projects that may, at first glance, seem unrelated. Their unifying principle is the measurement of electrical impedance for the detection and analysis of biological materials. Impedance measurements have long been employed and studied in scientific fields, and the dissertation begins with a summary of historical methodology, applications, and terminology. Utilizing impedance measurements for microscopic imaging is the driving motivation for Scanning Impedance Imaging (SII). This technique manifests the distribution of electrical impedance inside biological tissues and is described after the dissertation's introduction. SII can provide micron-scale imaging resolution by scanning a shielded microprobe over a sample placed on a conducting plane. A known voltage is applied to the microprobe and the sample immersed in a conductive aqueous medium. Both the probe height and the shield spacing were evaluated and optimized and two hardware configurations were developed to implement image scanning. A combined value ρh (resistivity × height) was used to represent impedance for heterogeneous samples. Experiments on photosensitive polymers, a butterfly wing, an oxide coated silicon wafer and cancer cells were performed and impedance images obtained. Impedance measurements for sensitive detection are demonstrated using a microchip capable of capillary electrophoresis separations. The chip was built using exclusively thin film deposition techniques, fully compatible with microelectronics batch processing. Standard photolithography provided control of the spacing between impedance measurement electrodes used in and overall channel geometry. The chip's performance was tested using concentrations of sodium chloride and calcium chloride ranging from 1 µM to 1 mM in a 5 mM MES/His buffer. Separations were performed by applying different voltages to reservoirs positioned at the four fluid channel openings. Impedance detection was performed by applying a small AC voltage (1 Vrms, 250 kHz) to the insulated electrodes positioned inside the fluid channels. Electropherograms were obtained and signal-to-noise ratio (S/N) was calculated. Measurements indicated that the trend of S/N as a function of molar concentrations was consistent with bulk conductivity tests and the chip's detection limit was below 1 µM for both sodium and calcium cations.
College and Department
Ira A. Fulton College of Engineering and Technology; Electrical and Computer Engineering
BYU ScholarsArchive Citation
Shang, Tao, "Impedance Imaging and Measurements by Micro Probes in Aqueous Environments" (2010). Theses and Dissertations. 2236.
impedance, conductivity, imaging, contactless, thin film, microchip, electrophoresis