This dissertation presents a method and apparatus for rapid measurement of fluid pH value in a microscale volume. The fluorescence of indicators immobilized on polymer microbeads is a function of the concentration of an analyte. This was determined in a silicon microfabricated flow channel. The sizes of microbeads can be determined by the scattering signal, so this system could be used to analyze several analytes at the same time by using beads of different sizes which are sensitive to different analytes. The fluorescent indicator Carboxy-SNAFL I was immobilized by Molecular Probes on amino-functionalized polystyrene beads of 5 mum diameter. These pH-sensitive microbeads were used in this apparatus to measure the pH of sample solutions. The intensities of two fluorescent peaks (at 560 nm and at 620 run) were measured, their ratio was dependent on the pH value of the analyte. We discriminated bead diameter by monitoring the intensity of the scattered excitation signal.; In the measurement of the light scattering signal, the standard deviation/bead diameter (SD/D) for the 10.3 mum beads was ∼26%. The major error is caused by varying positions of the beads in the microchannel. In order to decrease the SD/D of scattering signal and thus increase the number of distinct bead types that can be identified, a low Reynolds number silicon/pyrex microfluidic device that forces particles flowing in a microchannel to a tightly-focused equilibrium position was designed and fabricated. When scattering signals from 10 mum polymer beads were measured, we observed that the tight equilibrium position attained by the particles resulted in a much smaller standard deviation of the scattering signal than was observed in other microflow channel devices that did not focus the particles to equilibrium positions.; Another applications of this device is also discussed. We show that this particle-focusing channel may be used in a micro-fluidic flow sensor.
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