Walter Scott, Jr. College of Engineering

Graduate Exam Abstract

Manasi Katragadda
M.S. Final
Feb 23, 2009, 2:15 pm
Eng B103
A multi-channel fiber-optic phosphorescent sensor for monitoring dissolved oxygen
The scope of this research study involves the design, fabrication and testing of a multi-channel instrument for use with fiber-optic phosphorescence based biosensors. This instrument finds application in in-situ ground water quality monitoring of contaminants like toluene and chlorinated ethenes with genetically engineered enzymes via indirect monitoring of dissolved oxygen level in water. While the majority of existing sensors available are capable of monitoring only a single analyte, there is a need for the development of suitable multiple sensor systems capable of measuring different multiple analytes simultaneously. Accordingly, a multi-channel oxygen-sensitive optode instrument based on fiber optic chemical sensing has been developed. Tris-(4, 7-diphenyl-1, 10-phenanthroline) ruthenium (II) [Ru (dpp) 3]2+ forms the oxygen sensitive element on each optode. Each channel in this system measures red phosphorescence of a dye when illuminated with blue LED light. Dissolved oxygen concentration range was monitored in each channel based on phosphorescence quenching. An innovative electronic time division multiplexing approach via LabVIEW was used for switching between channels, which has proved to be highly economical, costing one-half as much as a mechanically switched system. Other multi-channel system configurations such as a fiber optic switch approach and replicated hardware approach have also been evaluated.
The average power of the LED source for each channel was monitored on a long-term i.e. overnight and short-term basis, i.e. for nearly two hours. Results indicate an initial warm-up period of 6-7 minutes for each channel LED before actual measurements could be taken. The LED power appears to be stable during the overnight monitoring with fluctuations of only 0.3%. In a separate measurement, noise in the LED power was also monitored. In this process, noise was monitored while using different power supplies to power the LEDs to ensure that the power supplies were not the dominant source of noise. After proper shielding, it was found that the noise of 1 mV peak-to-peak in the LED power was due to the quantization noise of an A/D converter that has been incorporated into the sensor system.
Stability of the Transimpedance Impedance Amplifier (TIA) circuitry in the detector system of the sensor has also been monitored. At the end of about two hours, a percentage drop of 0.2% was observed suggesting that the TIA was very stable. The dark voltage output of the sensor system was monitored for about 15 minutes by the PMT in the detector system with the source system turned off. Precaution was taken to cover the fiber connection ports on the front panel of the instrument to avoid interference from any background light. The dark voltage was very stable at approximately 0.07 V.
The source and detector insertion loss were also measured in triplicate for all channels. The source insertion loss was found to lie between 14.5 dB to 18.8 dB and the detector insertion loss was found to lie between 11 dB to 14.9 dB. The source cross talk was measured for some channels to be -21dB.
The limit of detection at a signal-noise ratio (SNR) of three with an optode was determined to be approximately 0.2 ppm of oxygen for each channel. The sensitivity of each individual channel to oxygen was calculated to be nearly 0.2 V/ppm. Uniformity was calculated to be 89.4%. Crosstalk for all channels was found to vary between -12 dB to -22 dB.

Adviser: Kevin Lear
Co-Adviser: NA
Non-ECE Member: Kenneth Reardon
Member 3: Diego Krapf
Addional Members: NA
Program of Study: