|Title:||High performance organic electrochemical transistors for chemical and biological sensing|
|Advisors:||Yan, Feng (AP)|
|Subject:||Hong Kong Polytechnic University -- Dissertations|
Thin film transistors
|Department:||Department of Applied Physics|
|Pages:||xviii, 112 pages : color illustrations|
|Abstract:||Organic electrochemical transistors (OECTs) have gained great attention in various chemical and biological sensing applications due to its intrinsic signal amplification function combined with highly efficient interfacing with ionic fluxes in biological environments. Besides, the freedom of synthesis, facile solution processing, superior biocompatibility and mechanical matching of organic materials offer OECTs a whole range of imaginative possibilities for investigation from fundamental device physics to biosensing related healthcare and wearable applications. In this thesis, the microfabrication technique, electrical characterization, and sensing applications of rigid and flexible OECTs based on a series of semiconducting polymers were systematically investigated. These polymers included highly conductive poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) for device operated in depletion mode, thiophene, thiadiazole or diketopyrrolo-pyrrole based p-type conjugated polymers for device operated in accumulation mode. A simple and convenient photolithographic microfabrication process was established for miniaturization of OECTs into micrometre resolution, which was the fundamental technique for fabrication of the OECTs discussed over this whole thesis. Through miniaturization of channel area, the device response time could be dramatically reduced to 10⁻⁵ s, which opened up the possibility to introduce AC measurements into electrochemical sensing applications. Then ion strength sensing, dopamine sensing and monitoring of cell activity were successfully demonstrated for PEDOT:PSS based OECTs. The precisely extracted transconductance signal indicated that the AC measurements could be a high reliable and anti-noise sensing method for investigation into multifunctional organic bioelectronic systems.|
Then a series of high mobility p-type conjugated polymers were integrated into OECTs. Their electrical performance when operated in aqueous electrolyte was investigated. Process optimization, impedance analysis and ionic response behavior were carried out for better understanding of the working mechanism for OECTs employing these polymers. Through the analysis of the structure-property relationship, the mechanism of ionic penetration process and its interaction with polymer film would be promising to be clarified, and the results may shed light on further design and synthesis of novel conjugated polymers for the requirements of bioelectronic applications. At last, OECT based on a recently reported semiconducting polymer, p(g2T-TT), was successfully exploited as flexible, label-free RNA sensor. The device showed stable performance in accumulation mode operation and high sensitivity to RNA biomarkers in physiological environment, with the detection limit down to 10⁻¹² M. The capacitance modulated sensing mechanism was investigated through variation of channel thickness and impedance analysis. The interaction of RNA molecules and polymer backbone was further investigated by characterization of electrolyte size effect on the p(g2T-TT) based OECTs. The successful demonstration of this sensor platform for detecting IL-8 mRNA, one biomarker for early detection of oral squamous cell carcinoma, indicates the possibility to employ this sensor in noninvasive cancer diagnosis applications. In summary, the microfabrication technique by photolithography was established for design and fabrication of OECT in micrometer dimensions. The device physics and operation mechanism of OECT based on different types of organic materials were comprehensively investigated. Through carefully device optimization and functionalization strategies, the OECT could serve as a universal platform for various kinds of in vitro and in vivo sensing applications.
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