Doctor of Philosophy

dissertationAir quality monitoring has become increasingly important because of the growing need for public health assessment under inevitable exposure to both indoor and outdoor air pollution. Monitoring air quality requires the detection of a wide range of gases, especially, at a part-per-billion (ppb) level. Gas chromatography (GC), despite its extensive volumes and power consumption, is the most widely utilized technique to separate and detect multiple compounds from a mixture. Among all miniaturized detectors for a gas chromatography system, only a few are capable of detecting gases down in ppb levels. To address such problems, a gas detector must avoid the use of any chemistry films, extensive peripheral equipment, and ion destructive mechanisms. This dissertation first presents the phenomenon of viscosity-induced pressure transients as a potential sensing element and thus as a solution to address the aforementioned challenges. Then, this dissertation secondly presents the use of gas bubbles, when gas is streamed into liquid flow through a micro nozzle, to enable quantification, which was named as microbubble chromatography. The proof-of-concept viscosity-based sensor was realized by constructing coupled microchannels with different diameters and placing a pressure detector in the second channel. This proof-of-concept viscosity-based sensor demonstrated (i) the detection of 12 volatile organic compounds, (ii) a minimum detection limit of 13.1 (15.3 ppb), 13.6 (13.7 ppb), and 17.5 picograms (22.6 ppb) for hexane, heptane, and benzene, respectively, (iii) an average sensitivity of 15×10-6 sccm/pg, and (iv) temperature stability of 94.3% in a temperature range of 40-110 0C for hexane compounds in a microgas chromatography system. Furthermore, this dissertation demonstrated successful quantification of the detected gases by utilizing the viscosity-to-pressure conversion mechanism in a microfluidic device. The device produced different sizes of microbubbles for different types of gases and enabled quantification via the counting of the number of bubbles. The fabricated bubble chromatography system (i) produced unique bubble diameters of 52.5, 93.0, 98.5, and 103.5μm from four types of gases (He, H2, N2, and CH4), (ii) demonstrated bubble chromatogram by injecting 0.1 μL pentane, and (iii) achieved a detection limit of 4.5 nanogram with sensitivity of 0.34 μm/ng and 6.94 bubbles/ng