Citation
Girei, Saad Hayatu
(2021)
Optical microfiber sensor coated with nanomaterials for ammonia sensing applications.
Doctoral thesis, Universiti Putra Malaysia.
Abstract
Ammonia (NH3) is a colorless compound with a distinctive odor composed of nitrogen
and hydrogen atoms that can be found in water as ammonia nitrogen (NH3-N) and in the
air as NH3 gas. It is commonly used in several industrial processes, agricultural activity
and several biological systems. However, at high concentrations, NH3 can be toxic to
plants, animals as well as human beings. Its detection is important for environmental and
industrial safety. The previous development of NH3 sensors were mostly concentrated
on the thick films and electrical based sensors rather than optical.
In recent years, tapered optical microfibers sensors have been attracting greater attention
due to their simplicity and immunity to various sources of interferences. This research
work presents the development of a tapered optical microfiber sensor coated with
nanomaterials for the detection of NH3 in liquid and gaseous forms. The working
principle of the sensor is based on the interaction between the evanescent fields of the
tapered microfiber and different NH3 concentrations. The interactions alter the properties
of light propagating through the optical fiber and consequently producing a measurable
response that allows quantifying the concentration of NH3. To enhance the sensing
performance of the developed sensor, zinc oxide (ZnO) and graphene oxide (GO)
nanomaterials were deposited on the fiber surface providing a higher surface area and
suitable chemical reaction between NH3 and the sensing layer.
The GO nanostructures and ZnO nanorods were deposited using the optical and
hydrothermal deposition techniques, respectively. The optical deposition technique was
successfully implemented to produce uniform GO coating with corrugated and wrinkled
structures on the cylindrical optical microfiber surface. The GO deposition is due to the
occurrence of the thermophoresis effect resulting from the interaction of optical radiation
with the GO solution at the tapered area of the microfiber interferometer (MFI). A unique
hydrothermal method is designed for uniform coating around the cylindrical optical
microfibers. This method is a simple and environmentally-friendly deposition technique that produced ordered arrays of ZnO nanorods directing outwards from the surface of the
tapered optical microfiber.
The NH3-N sensing response investigated in a wide wavelength range of 1500 – 1800
nm by monitoring the wavelength shift shows sensitivities of 0.0894 nm/ppm and 0.1748
nm/ppm at 1785 nm for bare and GO-coated MFI sensor, respectively. The developed
NH3-N sensor showed excellent properties of high sensitivity, stability, and fast response
at room temperature as compared to the conventional sensors. The ZnO nanorods and
GO-coated optical microfiber sensor for NH3 gas exhibit maximum absorbance response
with the optimum sensing layer thickness of 750 nm and 692 nm, respectively. Sensing
performance results reveal the sensitivities of 59.18 AU/% and 61.78 AU/% for sensors
coated with ZnO nanorods and GO, respectively. The results of this investigation further
reveal a promising method to improve NH3 gas sensitivity by prolonging the
hydrothermal growth duration of ZnO nanorod arrays on the optical microfiber. It was
also discovered that the GO-coated sensor produced negative and positive absorbance
responses at the visible and near-infrared wavelength regions, respectively. These
interesting sensing characteristics provide a new understanding of the behavior of
absorbance response of the GO-coated sensor to the operational bandwidth.
Above all, the combination of appropriate sensitive materials and optical fiber devices
provides a convenient platform to detect the chemical concentrations of liquid and gas.
As a result, these sensors may find applications in the manufacture of fertilizer, medical
diagnostics, and in rivers and drinking water monitoring.
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