Date of Award

5-2007

Level of Access

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Electrical and Computer Engineering

Advisor

John F. Vetelino

Second Committee Member

David J. Frankel

Third Committee Member

Carl P. Tripp

Abstract

The increasing worldwide use of organophosphate pesticides on commercial fruit and vegetable crops to control pests has led to the demand for a low-cost, portable pesticide sensor. Although these pesticides help farmers maximize crop yield, excessive pesticide can create health risks for the consumer as well as have adverse effects on the environment. While the United States has created government agencies to regulate the amount of pesticides that may be used, other countries export many agricultural products for which pesticide use may not be regulated. Current methods of pesticide detection, such as GC/MS, are costly and time consuming. However, recently a lateral field excited (LFE) acoustic wave device on AT-cut quartz coated with a polymer film, polyepichlorohydrin (PECH), has been developed with the capability of detecting phosmet (C11H12NO4PS2), a commonly used organophosphate pesticide. The LFE sensor is excited by parallel electrodes carefully positioned on the reference surface. Application of an AC voltage to these electrodes excites the transverse shear mode (TSM), creating a sensor with an unmetallized sensing surface, coated only with the target analyte sorbing film. This sensor has a simpler structure than the standard AT-cut quartz crystal microbalance (QCM) in which the same TSM is excited with electrodes that wrap around its edge and cover the sensing surface. Unlike the QCM, in which the electric field is shorted by the sensing electrode, the LFE sensor is capable of detecting both mechanical and electrical property changes at the sensor surface. In this thesis a PECH film is evenly spun onto the sensing surface of QCM and LFE sensors to detect phosmet in an aqueous environment. The sensors were exposed to the pesticide using injection and flow through system techniques to demonstrate important sensing properties such as sensitivity, reproducibility, and response time. Comparing the LFE sensor to the standard QCM as a pesticide sensor, some results showed that the LFE sensor has greater sensitivity, with a lower limit in the parts per billion (ppb) range, shorter response times and more consistent reproducibility. The acoustic energy distribution of the LFE sensor was mapped and found to exhibit a circular pattern with maximum sensitivity at the center of the sensor. A method was developed to reduce the response time of the sensor by using the derivative of the frequency response. In a real world sensing application, in which the sensors were exposed to apple wash, the LFE proved to be too sensitive. The additional sensitivity to electrical properties, though possibly very useful in other applications, caused the LFE sensor to react to non-pesticide exposures. However, the simplicity in structure and improved sensing properties of the LFE sensor results in a sensor platform that may replace the standard QCM in many sensing applications.

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