(93b) A Novel Biosensor Using Laterally Aligned Single-Walled Carbon Nanotube Resistor for Detection of Pathogen DNA in the Environment

Pathogen detection and identification using genetic markers such as DNA and RNA are receiving great interests due to the rapidly increasing availability of genetic information for most major pathogens. The real-time quantitative polymerase chain reaction (qPCR) and DNA microarray assay have recently emerged as promising tools for environmental pathogen monitoring and they allow for faster, more sensitive and high throughput detection of multiple genetic targets of pathogens. However, the complicated procedure and expensive equipments required prohibit their practical application for water quality monitoring. In addition, the inherent bias associated with DNA extraction and PCR protocols often influence the accuracy and reliability of the results. Pathogenic sensing is another alternative that relies on the interaction between the target cells or nucleic acid with the probes immobilized on surfaces, which can be transduced by various sensing methods such as electrical signal, fluorescence and surface plasmon resonance. Recent advances in nanotechnology provide unique advantages to pathogenic sensing, because the nanomaterials of size comparable to those of target pathogens improve the sensitivity and detection limit of sensor enormously. Nanowire, nanotubes and nanoparticles have been applied for nano-biosensor either for constructing miniaturaized sensor probes, as biomarker, or as a signal amplifier. They enhance the sensitivity, selectivity, response time and stability of these devices, and are suitable for simultaneous detection of multiple genes of multiple pathogens. Direct measurement of conductance/resistance between two electrodes with a micro-sized gap bridged with a sensing element can also be a highly sensitive technique for biosensing purpose. The gap between the two electrodes reduces the conducting path of electron transport, enabling noise analysis of the transport and enhancing sensitivity. Among the various nano-materials, single walled carbon nanotube (SWNT) has shown promising with its nano-dimension, sensitivity to molecular adsorption or reaction at its surface, easy-to-functionalize surface chemistry, high surface-to-volume ratio, high mechanical strength and unique electronic properties. The one dimensional structure of nanotubes offers the smallest confinement for an electron transport along the longitudinal direction. With single-stranded DNA (ssDNA) as probes immobilized on SWNT, hybridization event of target-probe DNAs could be detected as electronic signal. In the last decade, studies have been reported on the methods for ssDNA-probe immobilization, target-DNA detection, DNA-SWNT interaction and sensing mechanism. Current study with SWNT as support for probe DNA has claimed to be able to detect DNA as low as picomolar level. However, most of these studies are exploration in nature for demonstration of SWCNT-based devices. Systematic investigation of the factors that may affect the sensor performance has been scarce. For potential application of SWCNT-based nanobiosensors for environmental monitoring, comprehensive evaluation of the sensitivity, specificity, reliability and reusability of the sensors is needed. In this study, we construct and investigate the application of a novel electrical resistance-transducing, SWNT-based DNA biosensor. SWNTs are aligned between two Au electrodes using dielectrophoresis (DEP), and the electrical property of the structure formed is determined to be metallic as a resistor. The single-stranded DNA molecules as probes are immobilized onto the sidewall of carboxyl group functionalized SWNTs via amide linkage, a covalent bonding strong enough to persist in the presence of physical disturbance. The remaining open sites on the sidewall are blocked through polymer (polyethylene glycol) wrapping, whose effect was evaluated later. The electronic resistance changes, as a result of hybridization between probe DNA on the SWNTs and target DNA to be detected in the solution, are used as a signal for detection. Electrical signal response as a function of target DNA concentrations demonstrates that our device could be used to detect DNA in a wide range of concentration.