A Microfluidic Platform for Streaming Potential Measurement: Design, Fabrication and Application

This thesis describes the construction and application of a platform for streaming potential measurement in a microfluidic channel. Streaming potential is an electrokinetic phenomenon that produces electric field from pressure-driven liquid motion on a charged surface. The assessment of streaming potential is a widely-used method to determine the surface zeta-potential in both fundamental researches and industrial testings.;The platform we constructed was comprised of three parts: a disposable microfluidic channel with integrated microelectrodes, wherein the liquid flowed and generated streaming potential; a syringe pump to control the movement of liquid in the microfluidic channel; and a high-performance multimeter to monitor the potential signals. The microchannels were fabricated by two different approaches from glass and polydimethylsiloxane (PDMS), namely PDMS-glass hybrid microchannel and capillary-based microchannel respectively. The measurement was automated as both the syringe pump and the voltmeter were controlled by computer programs. The platform could determine zeta-potentials of microchannel surfaces from streaming potentials following the Helmholtz-Smoluchowski equation. We investigated the influence of the volumetric flow rate, electrolyte concentration and pH of the electrolyte solution on streaming potentials. The platform was demonstrated to be a simple, sensitive and reliable tool to measure streaming potentials and examine surface polarities.;We employed the platform to construct a label-free DNA sensor based on the hybridization of peptide nucleic acid (PNA) and DNA. The uncharged PNA probes were immobilized on the surface of microfluidic channels through routine chemical reactions. Upon forming hybrids with DNAs, more negative charges appeared on the PNA-coated microchannel which were reflected in the shift of streaming potentials. We found that the DNA sensor using the capillary-based microchannel had a detection limit of 2 nM and could distinguish complementary DNAs from the DNA strands with a CC mismatch from original CG base pair. The sensor constructed by the hybrid microchannel could respond quantitatively to DNAs with a concentration of 10 -- 200 nM, whereas showed a lower specificity to the targeting DNAs. The streaming potential based DNA sensor enriches the tools for DNA diagnosis, and the sensor is inexpensive, straightforward, and requires no DNA labeling.;Finally, a pneumatic peristaltic micropump was fabricated to replace the bulky syringe pump to regulate the liquid motion in the platform. By combining together three consecutive active valves activated by positive pressure and one normally-closed passive valve, flow rate as high as 464 microL/min was generated and stable streaming potentials were obtained. The integration of the pneumatic micropump allowed recycling of liquids and fabrication of a more compact microchip for streaming potential measurement.