Development of in-line fiber-optic interferometric sensors by using biopolymer : chitosan.

• Main Author: Chen, Li Han.
• Other Authors: Chan Chi Chiu
• Format: Theses and Dissertations
• Language: English
• Published: 2013
• Subjects: DRNTU::Engineering::Bioengineering
• Online Access: http://hdl.handle.net/10356/54548
• Institution: Nanyang Technological University

In recent years, polymers have increasingly been utilized and studied as possible waveguide materials. The reason for this trend is attributed to the advantages that polymers possess, such as the ease of structure modification, high structural integrity, excellent biocompatibility, good optical and mechanical properties. Among all the polymeric materials, chitosan which is a natural polymer, is one of the most promising candidates to be used as a sensitive medium for optical sensors in various sensing applications, and it is the subject of this thesis. This study explores possibilities of designing chitosan to possess versatile sensing response and integrating it into in-line fiber-optic interferometric sensors for acoustic, humidity and biosensing applications. The thesis begins with a comprehensive survey on the properties of chitosan, various types of polymer based sensors and in-line interferometric fiber-optic sensors. Following that, the chitosan based Fabry-Perot fiber-optic acoustic sensor is proposed and experimentally demonstrated. The transduction mechanism of the sensor is based on the deformation of the chitosan sensing diaphragm when the acoustic pressure is imposed. Due to its excellent elastic property, the sensor is able to define dynamic air pressure and has shown a high sensitivity of 0.002 V/mPa at 1 kHz with frequency response ranging from 20 Hz to 20 kHz. The success of integrating chitosan diaphragm into the Fabry-Perot fiber sensor had motivated us to further employ the sensor for in-vivo ultrasound measurement. The theoretical analysis results show that the chitosan membrane can provide maximum acoustic impedance matching for in-vivo ultrasound measurement and optimize matched-loading condition due to its permeable property. Similar to the acoustic application, the transduction mechanism is based on acoustically induced mechanical deformation of the chitosan sensing interferometer which exhibits a voltage sensitivity of 0.5 mV/MPa or -306 dB re 1V/µPa without using any filtering and external preamplifiers. Hence with such wideband response, biocompatibility and easy functionalization of chitosan membrane, it could suggest a potential for an accurate and reliable in-vivo ultrasound measurement. Fiber-optic relative humidity sensor can also be realized by using chitosan. Since chitosan has abundant of amino and hydroxyl groups present in its porous structure, these functional groups will enhance reversible adsorption of water vapor molecules from the gas phase through hydrogen bond formation. As a result, chitosan swells in the presence of water vapour. This swelling effect forms a basis for the chitosan-coated Polarization Maintaining (PM) fiber based Sagnac relative humidity sensor as the degree of swelling varies as a function of relative humidity. Such a response will induce Birefringence modulation through the swelling effect of chitosan when relative humidity is changed. Another possible sensing application offered by chitosan is the immunosensing. As chitosan will be a cationic polyelectrolyte for surface modification at a lower pH due to protonation of the primary amine group presented in each chitosan unit. Hence, Layer-by-Layer (LBL) modified, fiber-optic interferometry immunosensors for real-time affinity-based protein sensing applications had been proposed and experimentally demonstrated. Different fiber-optic interferometric configurations had been adopted for the development of chitosan based immunosensors, such as in-line Fabry-Perot interferometer (FPI), Mach-Zehnder interferometer (MZI) and Michelson interferometer (MI). In each sensor, the chitosan substrate was functionalized with self-assembled polyelectrolyte layers (Chitosan (CS)/ Polysodium Styrene Sulfonate (PSS)). For immunosensing evaluation, immunoglobulin G (IgG) was immobilized on the polyelectrolyte layer and anti-immunoglobulin G (anti-IgG) molecular binding event was monitored through the measurement of wavelength shift. The anti-IgG detection sensitivity and concentration detection limit are evaluated by Langmuir isotherm parameters and anti-IgG surface density. In all, these sensors can directly detect the binding of analytes to the immobilized antigen/antibodies without using any labelling reagents. In addition, these sensors are able to trace the presence of an analyte in a tested medium in real time and estimate the concentration of analyte by kinetics analysis.