Amperimetric Biosensor Based on Carbon Nanotube and Plasma Polymer

Abstract

Carbon nanotubes (CNTs) have been the focus of considerable studies since their discovery by Iijima (Iijima, 1991). CNTs are considered to be formed by the folding of graphene layers into carbon cylinders. They are of two distinct structural types: single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). A SWCNT is a single shell extending from end to end with 1.3-2 nm tube diameter. A MWCNT is composed of several coaxial shells, each formed with rolled graphite sheets, with diameters varying from 2 to 50 nm and the distance between sheets being about 0.34 nm. CNTs have many new applications to electronic devices, such as field-effect transistors (Maki et al., 2006; Ohnaka et al., 2006) and gas sensors (Ueda et al., 2006; Wongwiriyapan et al., 2005), because of their unique electronic properties, unique geometric structure, high mechanical strength, and high chemical stability. Recently, amperometric biosensors using CNTs have been increasingly reported. The typical structure of a CNT-based amperometric biosensor is the combination of biomacromolecules (e.g., enzymes) and CNTs in the vicinity of the electrode. Because of the well-defined nanostructure of CNTs, a good connection between CNTs and enzymes can be obtained. CNTs also enhance the electron transfer from reaction center of enzyme to electrode. Therefore, high performance of biosensor characteristics can be realized. Many strategies has been reported: the treatment of CNTs with nitric or sulfonic acid in order to introduce the chemically active functional groups (Liu et al., 2005a; Gooding et al., 2003; Azamian et al., 2002; Kim et al., 2006; Li et al., 2005; Liu et al., 2006), which enables subsequent modification. Another strategy is the dispersion of CNTs with a binder such as Nafion (Tang et al., 2004; Tsai et al., 2005; Hrapovic et al., 2004; Lee et al., 2007), Teflon (Wang & Musameh, 2003), sol-gel (Gavalas et al., 2004; Salimi et al., 2004; Yang et al., 2006; Kandimalla et al., 2006), poly(methyl methacrylate) (PMMA) (Rege et al., 2003), redox hydrogel (Joshi et al., 2005; Wang et al., 2006), poly(dimethyldiallylammonium chloride) (Zhao & Ju, 2006; Yan et al., 2007), chitosan (Liu et al., 2005b; Rivas et al., 2007), and electropolymerized film (Tsai et al., 2006; Pan et al., 2005). However, these methods require that enzymes and other biomolecules receive careful treatment in order to retain their tertiary structure. Moreover, when using such wet processes, it is difficult to control the nanoscale fabrication. A method where the CNTs are directly treated by plasma has recently been reported (Khare et al., 2005; Khare et al., 2004; Khare et al., 2002; Plank et al., 2005). The