Carbon nanotube based electronic detection of biomolecules and chemicals
Salila Vijayalal Mohan Hari Krishna
Date of Issue2015
School of Mechanical and Aerospace Engineering
Most current nanostructure-based sensors for detecting biomolecules and chemicals rely on time-consuming optical detection principles, require large sample volume, or involve complex data analysis. This has compelled the need to explore an electronic alternative to bio/chemical sensing. The highly sensitive surface and size compatibility of carbon nanotubes (CNTs) with bio/chemical systems, and the charged nature of bio/chemical species have been the cardinal reasons for adopting CNTs as the transducing surface in electronic platforms such as field-effect transistors (FETs) and chemiresistors. Along the development of this new type of devices, two major issues are identified to be related to sensitivity and selectivity. For most interested molecules, bare-CNT devices show very poor response. This requires scientists to modify CNT surfaces to enhance the device performance. Even for surface modified devices which exhibit strong response, how to translate the device outputs into molecular recognition (receptor-target interaction) is another challenge, because different targets bond differently with different receptors, and display different electronic sensing mechanisms depending both on the molecular nature and device parameters. In this thesis, we design two types of devices (CNT FET/chemiresistor) to study the sensing mechanisms, the effects of surface modification and device structure on the transduction outputs in detecting three types of typical biomolecular and chemical entities. Particularly, individual and network CNTs were successfully grown using chemical vapor deposition (CVD) process under optimized conditions and fabricated into FETs/chemiresistors. The surface modification was extensively studied in terms of the role of functionalization entities, the effect of different receptors in identifying a single target, and the effect of one receptor in identifying different targets. By using various non-covalent surface modification entities such as pyrene butanoic acid succinimidyl ester (PASE), single-stranded DNA (ssDNA) and 3, 4, 9, 10-perylene tetracarboxylic acid (PTCA), we detected a wide range of biomolecular and chemical targets including DNA and RNA, DA, 9-ACA, DCFNa and curcumin. For studying the effect of device parameter, we analyzed the impact of FET channel length on nucleic acid hybridization detection. From the studies pertaining to electronic detection of molecular recognition, the results are divided into the following categories. Firstly, with respect to the electronic sensing mechanism, the FET-based detection exhibited a combination of charge transfer, carrier scattering and charge trapping mechanisms, which were apparent from the change in transistor electrical parameters including conductance, transconductance, threshold voltage and hysteresis gap; while chemiresistor-based detection exhibited an increase/decrease in conductance. Secondly, surface modification not only improve the sensitivity and selectivity of the detection, but also enable CNT sensors to detect wide range of receptor-target interactions such as hydrogen bonding, electrostatic/polar interactions and host-guest complex formation. Pyrene-derivative functionalization improved the selectivity in detecting DNA-DNA hybridization and DNA-RNA hybridization. SsDNA decoration improved the FET response to DA, and enhanced its selectivity in the presence of UA. PTCA linker improved the device response, sensitivity, limit of detection (LOD), and selectivity of β-cyclodextrin (CD)-decorated CNT chemiresistors in identifying different host-guest interactions. Finally, with respect to device parameter, FETs with channel length in the range of a few hundred micrometers displayed better response for hybridization detection compared to FETs with few micrometer or millimeter long channels. The 300 µm long channel FETs make use of their excellent coverage area, freedom from contacts, thereby showing better response compared to micrometer-channel FETs. In addition, they do not suffer from the very high channel resistance hindering the detection sensitivity compared to millimeter-channel FETs.