A thin-film transistor based acetylcholine sensor using self-assembled carbon nanotubes and SiO2 nanoparticles
Introduction
Acetylcholine (ACh) is one of the most important neurotransmitters in human nervous systems. It is involved in many nervous activities including learning, attention, memory, and muscle contraction. The dysfunctional ACh regulation in the brain causes a number of neuropsychiatric disorders such as Parkinson disease, Alzheimer disease, and myasthenia gravis. Therefore, there has been growing interest in the development of accurate sensing methods to measure ACh concentration [1], [2], [3]. High-performance liquid chromatography (HPLC) method on microdialysis samples is a widely used technique for ACh measurement [4]. It is a high-resolution approach but the system setup is very expensive. A promising alternative approach is to use microscale devices because of their low cost and miniaturized size. Most microsensors use immobilized acetylcholinesterase (AChE) enzyme as the sensing material. The measurement of the ACh concentration is based on the hydrolysis process of ACh catalyzed by the AChE. The AChE-based microsensors have also been used to detect AChE inhibitors such as pesticides [5]. Common microsensors that have been developed for ACh sensing include amperometric devices [6], potentiometric electrodes [7], luminescent detectors [8], and ion-sensitive field-effect transistors (ISFETs) [9]. Among these devices, ISFET biosensor is an effective technique and provides a number of advantages: low workload on pre-sampling and post-analysis; high compatibility with the current microfabrication techniques; and easy integration with control circuits. Furthermore, the ISFET biosensors are standalone devices; they are robust and can operate in harsh conditions with wide temperature and pH ranges [10], [11]. It is expected that the ISFET-based devices will continue to provide high-performance sensors and systems in the future.
However, most reported ISFET sensors are fabricated on bulk silicon (Si) substrates because the basic ISFET design evolves from the traditional metal-oxide-semiconductor field-effect transistor (MOSFET) structure. Even though these ISFETs demonstrate higher performance and wider applicability than most electrochemical sensors, their sensitivities are restricted by the planar structures: the accumulation or depletion of charge carriers only occurs in the surface region of the device. In contrast, the design and development of novel field-effect sensors using nanomaterials, especially carbon nanotubes and nanowires, provide an effective approach to overcome the limitations of the Si planar structures because of the scale and morphology of the nanomaterials [12]. Nanomaterial-based electronic devices have proven to be a powerful class of high-performance sensors.
Recently, we reported a single-walled carbon nanotube (SWNT) thin-film biosensor [13]. It demonstrates a high resolution, defined as the lowest measurable chemical concentration, of 100 pM for ACh sensing. The two-terminal resistor-like structure simplifies the fabrication and measurement procedures. However, the current shift range is small; the characterized sensitivity of the sensor is 7.2 μA/decade. Our group has also developed ISFET-based ACh sensors using novel materials. Y. Liu et al investigated the ISFET technology by using low-cost materials including polyaniline (PANI) and indium oxide (In2O3) nanoparticles [14], [15]. The ISFET sensors demonstrate promising results for ACh sensing. However, these materials have low conductivities, which restrict the sensitivities of the fabricated sensors. The measured sensitivities for the PANI and In2O3 nanoparticles ISFETs are 1.4 and 12.4 μA/decade, respectively.
In order to further investigate the potential of nanomaterial-based microsensors, we modify the device design and develop a high-sensitivity ISFET ACh sensor using SWNTs and silicon dioxide (SiO2) nanoparticles. The sensor is based on an SWNT thin-film transistor (TFT) structure. The SWNTs are layer-by-layer (LbL) self-assembled on the Si substrate as the semiconducting film; SiO2 nanoparticles are coated as the dielectric film; and AChE enzyme molecules are immobilized on the surface as the sensing film. Compared with the PANI and In2O3 nanoparticles, the SWNTs have a much higher conductivity, which provides the ISFET with an enhanced working current. The current variation caused by the target analytes with difference concentrations is increased as well. The SWNT ISFET demonstrates a high sensitivity of 378.2 μA/decade. In addition, the sensor shows promising performance for ACh sensing in terms of resolution and response time. The structure, fabrication, and characterization of the SWNT ISFET sensor are described and discussed in this paper.
Section snippets
Experiments
All the chemicals used in the experiments were commercially available. They were diluted with deionized water to obtain optimum concentrations. The pristine SWNTs (powder, 1.1 nm in diameter, 50 μm in length, density of 2.1 g/cm3, purity >90%, purchased from Chengdu Organic Chemical Co. Ltd.) were treated with a mixture of 1:3 HNO3:H2SO4 acids at 110 °C for 45 min to increase the solubility in water. The final concentration of the SWNT dispersion was 1 mg/ml. The SiO2 nanoparticle dispersion
Results and discussion
The SWNT is first measured for the functionality, i.e., the field-effect, in a 10 mM ACh solution. The output characteristics of the SWNT ISFET are shown in Fig. 3(a). The device shows an explicit field effect and typical p-type transistor characteristics. The gate and drain voltages VG and VD are negative; VG is swept from −2 to 0 V with a 0.4 V step and VD is swept from 0 to −1 V with a −20 mV step. A higher |VG| results in a higher drain current |ID|. The gate transfer characteristics of the same
Conclusion
We have successfully developed a high-sensitivity and low-cost ACh sensor using the SWNT thin-film transistor as a platform. The SWNTs, SiO2 nanoparticles, and AChE enzyme molecules are deposited on the device as the semiconducting, dielectric, and sensing layers, respectively. The transistor is highly sensitive to the concentration change of hydrogen ions due to the ACh hydrolysis reaction. The gate voltage dramatically enhances the drain current and the sensitivity of the sensor. The
Acknowledgements
We thank Prof. Stephen A. Campbell at the Department of Electrical and Computer Engineering at the University of Minnesota for using his laboratory for the electrical characterization. We thank the staff members in the Nanofabrication Center and the Characterization Facility at the University of Minnesota for their help with the experimental work. This work is partially supported by the Defense Advanced Research Projects Agency (DARPA) MEMS/NEMS Fundamental Research Program through the
Wei Xue received the BS and the MS degrees in electrical engineering from Shandong University, Jinan, China, in 1997 and 2000, respectively, and the PhD degree in mechanical engineering from the University of Minnesota, Minneapolis, MN, in 2007. He is currently an assistant professor of mechanical engineering at Washington State University, Vancouver. Before he joined WSU, he was a postdoctoral research associate at the Department of Mechanical Engineering, University of Minnesota. His main
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Wei Xue received the BS and the MS degrees in electrical engineering from Shandong University, Jinan, China, in 1997 and 2000, respectively, and the PhD degree in mechanical engineering from the University of Minnesota, Minneapolis, MN, in 2007. He is currently an assistant professor of mechanical engineering at Washington State University, Vancouver. Before he joined WSU, he was a postdoctoral research associate at the Department of Mechanical Engineering, University of Minnesota. His main research interest includes microfabrication techniques, nanotechnology, polymer/silicon microelectromechanical systems (MEMS), micro/nano electronics, and chemical/biological sensors.
Tianhong Cui received the BS degree from Nanjing University of Aeronautics and Astronautics in 1991, and the PhD degree from the Chinese Academy of Sciences in 1995. He is currently a Nelson associate professor of mechanical engineering at the University of Minnesota. From 1999 to 2003, he was an assistant professor of electrical engineering at Louisiana Technical University. Prior to that, he was a STA fellow at National Laboratory of Metrology, and served as a postdoctoral research associate at the University of Minnesota and Tsinghua University. He received research awards including the Nelson Endowed Chair Professorship from the University of Minnesota, the Research Foundation Award from Louisiana Tech University, the Alexander von Humboldt Award in Germany, and the STA & NEDO fellowships in Japan. He is a senior member of IEEE and a member of ASME. His current research interests include MEMS/NEMS, nanotechnology, and polymer electronics.