ABSTRACT OF THE DISSERTATION
Synthesis, characterization and utility of carbon nanotube based hybrid sensors in bioanalytical applications
Doctor of Philosophy, Department of Electrical Engineering
University of California, Riverside, USA
Prof. Ashok Mulchandani, Chairperson
The detection of gaseous analytes and biological molecules is of prime importance in the fields of environmental pollution control, food and water - safety and analysis, and medical diagnostics. This necessitates the development of advanced and improved technology that is reliable, inexpensive and suitable for high volume production. The conventional sensors available for catering to these needs are often traditional thin film based sensors which lack sensitivity due to the phenomena of current shunting across the charge depleted region when an analyte binds with them. Thus one dimensional nanostructures are heralded for wide applications which apart from eliminating the current shunting due to their one dimensional geometries also facilitate device miniaturization and low power operations.
Carbon nanotubes are a class of popularly known one dimensional nanostructures that exhibit excellent electronic and mechanical properties. Their properties of small size, high strength, high electrical and thermal conductivity and high specific area have resulted in their wide spread applications in sensor technology. However, to overcome the issue of low sensitivity of pristine carbon nanotubes and to widen their scope, efforts have been made to configure hybrid devices that combine the properties of carbon nanotubes along with other materials like metals, metal oxides, conducting polymers etc. Conducting polymers exhibit electronic, magnetic and optical properties of metals and semiconductors while retaining the attractive mechanical properties and processing advantages of polymers. Their high chemical sensitivity, room temperature operation and tunable charge transport properties has made them ideal for use as tranducing materials in chemical sensors. In this dissertation, various applications of carbon nanotube based hybrid devices such as CNT-conducting polymer and graphene-CNT based sensors have been demonstrated. Electrochemical polymerization enabled the synthesis of CPs and metal nanoparticles in a simple and efficient way on the surface of carbon nanotube based platforms thus resulting in the fabrication of hybrid devices exhibiting superior properties.
In the first study towards demonstrating the synergistic behavior of the hybrid devices, PEDOT: PSS was electropolymerized on the surface of SWNTs and their response towards volatile organic compounds was evaluated. In terms of performance, when compared with bare SWNTs, these hybrid sensors exhibited better sensitivity over a wide dynamic range of concentrations of saturated vapors of VOCs. The second application involved electrodeposition of Polyaniline boronic acid on SWNTs by charge controlled electrochemical polymerization and their subsequent evaluation as chemiresistive sensors towards detection of D-fructose and D-glucose. The regeneration of the sensors based on the basis of the reversible nature of the binding between PABA and 1, 2 or 1, 3-diols at lower values of pH was also studied. These enzyme free sensors exhibited a limit of detection of 2.92 mM for D-fructose and 3.46 mM for D-glucose. Molecular imprinting of conducting polymers (MICPs) is a very versatile technique that is used towards detecting biological entities such as receptors, enzymes, and antibodies with high specificity. A self-doped, molecularly imprinted conducting polymer (MICP) on aligned SWNTs surface has also been discussed which displayed better sensitivity towards D-fructose. Imprinting imparts higher selectivity towards D-fructose over D-glucose by template directed formation of imprinted cavities that are of same chemical nature in terms of shape, size and functionality as D-fructose. In the next study, nonenzymatic glucose sensor based on Platinum nanoflowers on multi-walled carbon nanotube-graphene hybrid has been developed. The hybrid sensor was synthesized of Carbon nanotube (MWNT)/ graphene hybrid by one step chemical vapor deposition process and electrodeposition of high surface area platinum nanoflowers. Direct oxidation of glucose due to the electrocatalytic property of the Pt-nanoflowers resulted in the non-enzymatic detection of glucose in the physiological range at neutral pH.