Reversible evanescent-based opto-chemical sensor for in-situ detection of contaminants in static and dynamic fluid system

Ahmed Hasnain Jalal, Purdue University

Abstract

The main objectives of this thesis are to fabricate a reversible Clad-modified Opto-chemical Plastic Fiber Sensor (OCPFS) for the instantaneous detection of ammonia in both static and dynamic aqueous media and to evaluate its dynamic range, sensitivity, time response, and life cycle. In this work, Plastic Optical Fiber (POF) was modified and applied as an optical waveguide. A portion of the cladding of the POF was removed by chemical etching and was coated with a thin solid film of pH indicator named Oxazine 170 per Chlorate, followed by a layer of Poly Di-Mythyl Siloxane (PDMS) through the 'Layer by layer dip Coating Method'. The PDMS layer was needed because the Oxazine 170 per Chlorate pH indicator is water soluble. Experimental evidence revealed that Oxazine 170 per Chlorate required the presence of moisture to function effectively hence the relative humidity must be controlled. The detection of ammonia by the sensor developed in this study involves with interactions of several phenomena including transmission of electromagnetic wave through the fiber, generation of evanescent wave in the etched region of the fiber, volatization of ammonia in water solution, mass diffusion of ammonia vapor through the PDMS membrane layer driven by pressure and concentration gradients, deprotonation and protonation of ammonia/ Oxazine 170 per Chlorate, and optical intensity modulation. Variation in intensity due to ammonia was monitored in a spectrometer using Spectra Suite Software. The fiber sensor operates in the visible light range and wavelength shifting occurs from blue (455–490 nm) to yellow (575–595 nm) and red (620–780 nm) for long exposure and higher concentrations. The fabricated optical fiber ammonia sensor was employed in both static and dynamic fluid systems. In static fluid system, the intensity decreased with increasing ammonia concentrations at wavelength of 476nm. The experimental results showed that the absorbance increased in the blue light region as the intensity decreased, consistent with Beer Lambert law. In the yellow region (575–595 nm), however, absorbance decreased with the concentrations. For prolonged exposure of sensor to high ammonia concentrations, a change in color towards red region was observed. In dynamic water system, the absorbance curve also showed the trend as what had been found in stagnant water experiments at 478.31 nm wavelength (blue region). The minimum ammonia detection limit of the sensor fabricated in this study was 1.4 ppm and 3.9 ppm for the static and the flowing water system, respectively. This difference in detection limit was attributed to the external pressure propelling the ammonia solution across the sensor in the flowing water system. The sensor developed in this study showed excellent repeatability, response time (within 10 seconds of the ammonia concentration change), and reversibility (99% recovery within 50 seconds upon ammonia removal). Results showed that the sensor was very sensitive to ammonia at low ammonia concentration range. A change in ammonia concentration of 12.6 ppm (from 1.4 to 14ppm) resulted in a change in absorbance of 0.04. At higher concentration range, however, a change in concentration of 42ppm (from 28 to 70ppm) only resulted in approximately 0.03 change in absorbance. Further increase in concentration by 52 ppm only resulted in a marginal absorbance change of 0.02. These trends suggested that at higher ammonia concentrations, the sensor may become saturated and further increase in ammonia concentration would not result in appreciable increase in absorbance. The sensor developed in this study was therefore recommended for applications involved with low ammonia concentration detection. For the confirmation of practical feasibility of this sensor, it was employed 10 days in the flowing water system. For the same concentration of ammonia and same flow rate of water, the sensor shows almost similar intensity variation and same time response within these days. These outstanding and productive behaviors of this sensor established its practical implementation in the real life. (Abstract shortened by UMI.)

Degree

M.S.E.

Advisors

Nnanna, Purdue University.

Subject Area

Electrical engineering

COinS