Collections > Electronic Theses and Dissertations > Conductivity based photothermal absorbance detection for microfluidic devices

The research presented describes the development of microfluidic devices for conductivity detection and its application for high-sensitivity photothermal detection. The process of photothermal absorbance detection combines the universality of standard absorption detection with the path length-independence of conductivity detection. Due to limitations in fabricating capillary-based contactless conductivity detectors, the laser excitation region is extremely small in comparison to total sensing region, which reduces the detected signal. This problem can be circumvented by applying the technique to a microfabricated system. A microfabricated contact conductivity detector using metal film electrodes was incorporated into a device with a simple cross channel. Several inorganic salts were detected with varying excitation voltages, frequencies, and electrode geometries. Using the data from these conductivity studies, the device was modified to include a three-electrode design for use in photothermal detection. Two types of electrodes were investigated, metal thin film electrodes and polyelectrolyte salt bridge electrodes (PSBE). Studies were performed in order to characterize both types of electrodes. Conditions investigated include alteration of fabrication and bonding techniques, electrode and channel dimensions, laser settings, and conductivity excitation parameters. Initial studies were carried out using DABSYL-tagged analytes excited by 488 nm laser light. Successful separation and detection of several DABSYL-tagged amino acids was achieved. One advantage of conventional optical absorbance detection techniques is the ability to detect native analytes, but the visible wavelength used in the previous studies restricted analytes to those that could be tagged with DABSYL chloride. In order to study the photothermal response of several native biological analytes, further characterization of the device's behavior was performed using 266 nm laser excitation. A number of analytes including fluorescent dyes, native amino acids and peptides, and nucleotide bases and oligonucleotides were detected with the system. Measurements indicated comparable behavior to that observed in the visible spectrum with respect to both signal and noise responses. A similar detection device was fabricated for use with high performance liquid chromatography (HPLC). Conductivity based photothermal detection was tested with both an end-column detector and an integrated packed bed/conductivity detector device.