Date on Master's Thesis/Doctoral Dissertation

8-2011

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Chemistry

Committee Chair

Baldwin, Richard P.

Author's Keywords

Electrochemistry; Sensors; Microfabrication

Subject

Electrochemical sensors; Microfabrication

Abstract

In this work, micro fabrication techniques are explored not only to simplify the production of complex lab on-a-chip devices (LOC), but also micro fabrication will be utilized to create intelligent design features that will enhance an electrochemical sensor's capabilities. First, a low temperature adhesive bonding procedure for LOC glass devices was evaluated for capillary electrophoresis (CE) applications. This low temperature method utilizes UV adhesive to bond the glass microchips under the assistance of a mask aligner. The bonding process was carried out at room temperature in < 30 minutes, and provided a near 100% success rate. Microchips exhibited similar electroosmotic flow, separation characteristics, stable long-term performance, excellent chip-to-chip reproducibility, as their thermally bonded counter parts. This bonding approach required new but easily implemented structural features. In addition to cost effective and reliable fabrication techniques, microchips designed for long-term unattended electrochemical sensing have been evaluated. Specific advantages of the micro fabrication approach include the capability to create an intelligent design containing features such as redundant sensing electrodes, on-chip reference and auxiliary electrodes, and in situ electrode regeneration/calibration. One system targeted involves continuous pH monitoring in drinking water at solid-state iridium oxide electrodes. Microchips utilized consist of a flow-through silicon platform containing patterned gold electrodes onto which iridium oxide was deposited electrochemically. To simulate drinking water detection scenarios, sensors were integrated into a flow system. Elven equivalent pH electrodes where evaluated for electrode-to-electrode reproducibility, long-term drift, and response to expected interfering agents. With on-chip voltage treatment, absolute potentials measured for an electrode array are within ± 4 mY, with identical (±1 mY/pH unit) calibration slopes. This performance level is sustainable over weeks. Sensors for exhaustive coulometry were designed, fabricated and evaluated. Microchips contained thin-film gold working and Ag/AgCI pseudo-reference electrodes. A custom flow cell containing a counter electrode chamber was constructed to integrate the sensor and to create an electrolysis chamber with a fixed volume. Different chip designs were evaluated for reproducibility and longevity using Fe(CN)63-/4- as model analytes. The relative standard deviation (RSD) for a chip (over 42 days) was 5.5% whereas the sensor-to-sensor reproducibility was within 6.3%. A more practical application for utilizing exhaustive coulometry by the determination of free chlorine in drinking water is briefly evaluated. Initially studies will outline the challenges involved by analyzing hypochlorite.

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