Full Paper Aptamer-Based Label-Free Immunosensors Using Carbon Nanotube Field-Effect Transistors Kenzo Maehashi, a * Kazuhiko Matsumoto, a Yuzuru Takamura, b Eiichi Tamiya c a Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan *e-mail: maehashi@sanken.osaka-u.ac.jp b School of Materials Science, Japan Advanced Institute of Science and Technology (JAIST), 1-1 Asahidai, Nomi City, Ishikawa 923-1292, Japan c Department of Applied Physics, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan Received: October 31, 2008 Accepted: January 23, 2009 Abstract Aptamer-based label-free immunosensors were fabricated for immunoglobulin E (IgE) detection using carbon nanotube field-effect transistors (CNTFETs). Aptamers are artificial oligonucleotides and thus are smaller than the Debye length. After aptamers were covalently immobilized on CNT channels, the electrical properties of the CNTFETs were monitored in real time. Introduction of IgE at various concentrations caused a sharp decrease in the source-drain current and gradual saturation at lower currents. From the measurement of the dependence of CNTFET electrical properties on IgE concentration, the dissociation constant between aptamer and IgE reactions was estimated to be 1.9  10 9 M using the Langmuir adsorption isotherm. Keywords: Aptamers, Label-free immunosensors, Carbon nanotubes, Field-effect transistors, Debye length, Immunoglobulin E, Dissociation constant DOI: 10.1002/elan.200804552 1. Introduction Biomolecule detection has attracted much attention in the many areas of practical pharmacy, genomics, clinical diag- nosis for health care, and life sciences [1 – 11]. In particular, the label-free electrical monitoring of biorecognition events provides a promising platform that is simpler and less expensive and requires less energy than conventional methods. Rapid testing of different proteins is also required in various applications, including clinical diagnostics, envi- ronmental testing, food analysis, and bioterrorism detection technologies. One promising approach to the label-free electrochem- ical detection of biomolecules uses carbon nanotubes (CNTs) [12 – 14]. CNTs are one of the most attractive materials in terms of fundamental science and technology owing to their unique characteristics [15]. Since CNTs are formed by rolling graphite sheets, they have quasi-one- dimensional structures. Single-walled CNTs (SWNTs) exhibit conducting or semiconducting behavior with high carrier mobility [16, 17]. Therefore, they are promising materials for nanoscale electronic devices [18 – 22]. In particular, CNT field-effect transistors (CNTFETs) [23, 24] are notable candidates for highly sensitive label-free biosensors owing to their unique geometries with a high surface-to-volume ratio. Biomolecules such as DNA and proteins has been detected using CNTFETs [25 – 30]. Figure 1 shows a simple detection mechanism for biomol- ecules using CNTFETs. Figures 1a and 1b show a typical CNTFET structure and a band structure of a p-type CNTFET, respectively. The source-drain current in the CNTFET is monitored using the measurement system shown in Figure 1c. When a charged biomolecule adsorbs on the channel of the CNTFET (Fig. 1d), the CNTFET band structure is modulated (Fig. 1e). As a result, a reduction in source-drain current is observed (Fig. 1f). Because the proteins are much larger than the diameter of the CNT channels (1 – 2 nm), CNTFETs are expected to have a high sensitivity for protein detection. In this study, we detected immunoglobulin E (IgE) using CNTFETs. IgE is a subclass of antibodies found only in mammals and exists in human serum at low concentration (ca. 1 nM), which is only 0.05% of immunoglobulin G concentration. IgE plays a key role in the allergic response, for example, hay fever, atopic dermatitis and allergic asthma, and is especially associated with type 1 hypersensitivity [31, 32]. Therefore, it is important for rapid detection of IgE with high sensitivity. The measurement of antigen-antibody reactions is very common in protein detection. However, the typical size of antibodies is around 15 nm [33, 34], making them much larger than the Debye length in a buffer solution of typical concentration [35]. As a result, antigen – antibody reactions occur outside the electrical double layer. Thus, it may be difficult to detect proteins with high sensitivity using antibody-modified CNTFETs [30]. In this study, aptamers 1285  2009 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim Electroanalysis 2009, 21, No. 11, 1285 – 1290