Chemical and biological sensing applications based on graphene field-effect transistors

Abstract

Chemical and biological sensors based on graphene field-effect transistors (G-FETs) were investigated. A single-layer of graphene was prepared by mechanical cleavage of natural graphite. The G-FETs were driven by a reference-gate operating in buffer solution, and exhibited very good transport characteristics. The G-FETs detected the pH value of the solution with high precision. The Dirac point shifted in the positive direction with increasing pH of the solution. The detection limit (signal/noise = 3) for measuring changes in the pH of the solution was estimated to be 0.025, indicating the high sensitivity of the G-FETs. Moreover, the devices electrically detected proteins with different charge types. The drain current decreased (increased) when positively (negatively) charged proteins were added to the solution. These results indicate that the G-FETs are among the most suitable candidates for FET-based chemical and biological sensors.

Introduction

Systems for detecting simple biomolecules have become increasingly important in the field of life sciences. Generally, optical detection methods are used in conventional biosensors. Although optical detection is extremely sensitive, this approach needs expensive instruments, professional knowledge of labeling processes, fluorescent dyes and complex experimental techniques. Electrical detection of biomolecules by using nanomaterials such as silicon nanowiress (Cui et al., 2001, Elfström et al., 2008) and carbon nanotubes (Sánchez-Acevedo et al., 2009, Kim et al., 2009, Maehashi et al., 2007, Maehashi et al., 2009, Okuno et al., 2007) have been intensively investigated in past decade with respect to application to diagnostic biochips. Among these biosensing techniques using carbon nanotubes are among the most promising candidates for label-free biosensing, owing to their high aspect ratio and outstanding electrical characteristics. Several label-free biosensors based on carbon nanotube technology have been reported, including electrical detection of bisphenol A (Sánchez-Acevedo et al., 2009), a prostate cancer marker (Kim et al., 2009, Okuno et al., 2007) and immunoglobulins (Maehashi et al., 2007, Maehashi et al., 2009). However, controlling the diameter of carbon nanotubes remains a critical issue, because the electrical characteristics of carbon nanotubes strongly depend on their diameter and metal work function (Chen et al., 2005). At present, diameter-controlled carbon nanotube growth has not been successfully achieved.

Graphene, which consists of single to several layers of crystalline carbon, can potentially be used to address these problems. Graphene is a zero-gap semiconductor whose conduction band and valence band are connected at the K-point. Therefore, the transport characteristics of graphene exhibit distinctive ambipolar behavior with high minimum conductivity. Single-layer graphene has extremely high carrier mobility (>20,000 cm²/Vs at room temperature) with large carrier concentrations (∼10¹² cm⁻²); furthermore, the material consists of highly stable, perfectly two-dimensional carbon crystal (Geim and Novoselov, 2007, Novoselov et al., 2004). Graphene is expected to have potential applications in next-generation high-speed logic devices (Lin et al., 2009). In particular, chemical and biological sensors based on graphene technologies has advanced rapidly in recent years, owing to the high stability of the material. Sensors using graphene or graphene oxide have been developed for the electrochemical detection of glucose (Kang et al., 2009, Shan et al., 2010), and solution pH sensors using few-layer epitaxial graphene on a 6H–SiC substrate have been demonstrated (Ang et al., 2008). Binding reactions occur close to the surface of graphene field-effect transistors (G-FETs); accordingly, detection of gas molecules (Schedin et al., 2007) and biomolecules (Mohanty and Berry, 2008, Ohno et al., 2009) has been reported. In graphene, high carrier mobility is only achieved in single-layer graphene due to its linear energy dispersion at K-point (Nagashio et al., 2008). Since the sensitivity using FET-based chemical and biological sensors depend on their transconductance (=∂I_D/∂V_G = C_GV_Dμ; where C_G and μ are the gate capacitance and field-effect mobility, respectively), G-FETs with single-layer graphene are thought to be suitable for the sensing devices.

In the present study, we carried out highly sensitive solution pH sensing and monitoring of the charge-type dependence of protein adsorptions onto the surface of graphene using single-layer G-FETs. As results, the lowest detection limit (signal/noise = 3) of the change in solution pH value was estimated to be 0.025 and the G-FETs could be electrically distinguished between positive and negative charged proteins in a buffer solution, indicating the high potential of G-FETs for applications in chemical and biological sensors.