Date of Award

Summer 2008

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Micro and Nanoscale Systems

First Advisor

Kody Varahramyan

Abstract

Organic electronic devices, based on Poly (3,4-ethylenedioxythiophene)-Poly (styrene sulfonic acid) (PEDOT-PSS) as the active layer for sensor applications, have been studied. Two sets of sensors have been developed. In one case, sensors consisting of PEDOT-PSS resistors have been realized and demonstrated for soil moisture monitoring. The resistor model for the soil moisture sensor enables the sensor device to be fabricated at low cost and easily tested with a simple structure. Unlike the large dimension device used in Time Domain Reflectometry (TDR), the sensors are small and are capable of capturing microscale behavior of moisture in soil which is useful for geological and geotechnical engineering applications.

The Field Effect Transistors (FETs) based on PEDOT-PSS and GOx have been developed for a glucose sensing application. The sensitivity of the developed FET-based sensors is enhanced by selecting the channel as the active sensing region as compared with the previously reported devices which use the gate as the active sensing region. This also allows the devices to be designed by a simple and cost-effective means, unlike other complex platform designs for polymer-based sensor devices.

PEDOT-PSS based sensors showed higher sensitivity and reversible electrical properties when compared to early versions of sensors fabricated using polymer electrolytes which showed irreversible change in the electrical properties when exposed to high moisture content. The output characteristics, which is the change in electrical sheet resistance of the PEDOT-PSS film versus the percentage change in relative humidity (%RH), show that the conductivity of the film decreases when it is exposed to increasing levels of moisture content. The change in the output resistance of the developed PEDOT-PSS based sensor device was observed to be from 2.5 MΩ to 4.0 MΩ when exposed to soil samples (e.g. Buckshot Clay, CH) with 15–35 % change in gravimetric water content.

The FET-based glucose sensor using PEDOT-PSS and GOx as the channel materials, is designed and developed with the capability of precise, fast, and wide sensing range of measurement compared to that of traditional glucose sensors, which are costly and operate on a complex electrochemical based principle. The fabrication and characteristics testing steps of the present glucose sensor are also simpler in comparison to other glucose sensors, which use electrochemical cells for measurements. In the present device, GOx was immobilized on PEDOT-PSS conducting polymer film using a simple cost effective spin-coating technique. A linear increase in the FET drain current was observed, which was resulted from the increase in glucose concentration. The sensitivity of the glucose sensor was determined to be 0.3 Ampere per 1 mg/ml of glucose concentration. A linear range of response was found from 0.2 to 3 mg/ml of glucose, with a response time of 10–20 s. The results indicated that the reported FET-based glucose sensor retains the enzyme bioactivity and can be applied as a glucose biosensor. Moreover, the glucose sensor presented in this dissertation has displayed a reasonable level of sensitivity, repeatability, and stability. The evaluated range of glucose detection shows that the developed biosensor can be used to detect glucose concentration for normal and diabetic patients. This finding also opens a potential pathway for further development of novel biosensor devices.

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