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126 Smart Nanomaterials for Sensor Application, 2012, 126-148 Songjun Li, Yi Ge and He Li (Eds) All rights reserved - © 2012 <strong>Bentham</strong> <strong>Science</strong> Publishers CHAPTER 8 Advanced Carbon Nanotubes and Carbon Nanotube Fibers for Biosensing Applications Zhigang Zhu 1* , Andrew J. Flewitt 1 , William I. Milne 1 and Francis Moussy 2 1 Electrical Engineering Division, Department of Engineering, University of Cambridge, 9 JJ Thomson Avenue, Cambridge, CB3 0FA, UK and 2 Brunel Institute for Bioengineering, Brunel University, Uxbridge, Middlesex, UB8 3PH, UK Abstract: Since the discovery of Carbon Nanotubes (CNTs) by Iijima in 1991[1, 2], there has been an explosion of research into the physical and chemical properties of this novel material. CNT based biosensors can play an important role in amperometric, immunosensor and nucleic-acid sensing devices, e.g. for detection of life threatening biological agents in time of war or in terrorist attacks, saving life and money for the NHS. CNTs offer unique advantages in several areas, like high surfacevolume ratio, high electrical conductivity, chemical stability and strong mechanical strength, and CNT based sensors generally have higher sensitivities and lower detection limit than conventional ones. In this review, recent advances in biosensors utilising carbon nanotubes and carbon nanotube fibres will be discussed. The synthesis methods, nanostructure approaches and current developments in biosensors using CNTs will be introduced in the first part. In the second part, the synthesis methods and up-to-date progress in CNT fibre biosensors will be reviewed. Finally, we briefly outline some exciting applications for CNT and CNT fibres which are being targeted. By harnessing the continual advancements in micro and nano- technology, the functionality and capability of CNT-based biosensors will be enhanced, thus expanding and enriching the possible applications that can be delivered by these devices. Keywords: Advanced carbon nanotubes; fibres; biosensors. 1. INTRODUCTION OF CARBON NANOTUBES Biosensors are a type of bio-molecular probe that measure the concentration of biological molecules by transducing a biochemical interaction into a quantifiable electrical signal. Biosensors hold the promise of early diagnosis of diseases and genetic disorders through detection of associated molecules such as DNA, enzymes, proteins and peptide aptamers. There is intensive interest in the use of nanomaterials for such applications, and CNTs are one of the most promising materials due to the following properties: i) large length-to-diameter aspect ratios produce high surface-to-volume ratios, and are thus responsible for the efficient capture and promotion of electron transfer reactions from analytes [3]; ii) carbon nanotubes can be functionalized by most biomolecules both at the tube ends and on the walls, which allows the design of novel biosensors or enhances the solubility of the tubes in a polymer matrix through adjusting the hydrophobicity or hydrophilicity of the surface; iii) the conductivity of the tubes is critical in controlling electron transfer kinetic for use in biosensors as an electrode. For example, Multi-Wall Carbon Nanotubes (MWNTs) are metallic conductors and can be used directly as electrodes in electrochemical biosensors; however, the semi-conductor Single-Wall Carbon Nanotubes (SWNTs) are ideal For Nano-Scale Field- Effect transistors (FETs) to detect single molecules [4, 5]. But the separation and purification of CNTs are real problems and haven’t been completely solved. CNTs are well-ordered, hollow graphitic nanomaterials made of cylinders of sp 2 - hybridized carbon atoms, and they have two main structural types. SWNTs consist of a single cylindrical tube of graphite sheet, while MWNTs comprise an array of such nanotubes that concentrically nest like rings of a tree trunk to share a common longitudinal axis [6-8]. As shown in Fig. 1, SWNTs can be either metallic or semiconducting, *Address correspondence to Zhigang Zhu: Electrical Engineering Division, Department of Engineering, University of Cambridge, 9 JJ Thomson Avenue, Cambridge, CB3 0FA, UK; Tel: +44 01223 748304; Fax, +44 1223 748348; Email: zz259@cam.ac.uk
Advanced Carbon Nanotubes and Carbon Nanotube Fibers Smart Nanomaterials for Sensor Application 127 depending on the direction in which the sheets roll to form the tube cylinders. The direction and the nanotube diameter are obtainable from a pair of integers (n, m) that denote the nanotube type. Depending on the appearance of a belt of carbon bonds around the nanotube diameter, the nanotube is either of the achiral armchair (n = m), achiral zigzag (n = 0 or m = 0), or chiral (any other n and m) type. All achiral armchair SWNTs are metals; while chiral and achiral zigzag tubes are semi-conducting unless the vector indices give a whole number when the calculation (n-m)/3 is performed [9]. Thus, with small diameter SWNTs, approximately two-thirds are semi-conducting and one-third are metallic. The electrical properties between MWNTs and SWNTs are similar, since the coupling between the cylinders is weak in MWNTs. In the meantime, electrical transport in metallic SWNTs and MWNTs occurs ballistically (i.e., without scattering) along the nearly one-dimensional electronic structure, enabling them to carry high currents with essentially no heating [3]. Apart from the conductivity, another important structural aspect of CNTs is their local anisotropy. The sidewalls, comprising the vast majority of the tubes, are a relatively inert layer of sp 2 hybridized carbon atoms and are highly hydrophobic. The open ends of CNTs have carbon atoms bonded to oxygen to give far more reactive species, which are quite hydrophilic and critical for the good electrochemical properties of nanotubes. This hydrophobicity presents a major challenge when it comes to dispersing and manipulating carbon nanotubes to give controlled modification of electrode surfaces [10]. There is a tendency to rapidly coagulate both in aqueous solution and polar solvents, although Sano et al. have reported that acid shortening can extend the storage time for homogenous CNT solution [11]. Meanwhile, dispersing CNTs is usually performed in nonpolar organic solvents such as in Dimethylformamide (DMF) [12, 13] or with the aid of surfactants or polymers [14]. The difficulty in dispersing nanotubes in aqueous solutions has sometimes been used as an advantage in preparing nanotube modified electrodes, where nanotubes dispersed in an organic solvent are dropped onto an electrode surface following solvent evaporation [10, 11]. Figure 1. Idealized representation of defect-free (n,m) SWNTs with open ends. A) A metallic conducting (10,10) tube ( armchair ), B) a chiral, semiconducting (12,7) tube, and C) a conducting (15,0) tube ( zigzag ). D) Schematic representation of a 2D graphite layer with the lattice vectors a1 and a 2, and the roll-up vector C h= na 1 + ma 2. Achiral tubes exhibit roll-up vectors derived from (n,0) (zigzag) or (n,n) (armchair). The translation vector T is parallel to the tube axis and defines the 1D unit cell. The rectangle represents an unrolled unit cell, defined by T and Ch. In this example, (n,m) =(4,2), Reprinted with permission from [9]. 1.1 CNTs Synthesis Methods Three main methods are used to synthesize SWNTs and MWNTs: arc-discharge, laser-ablation and Chemical Vapour Deposition (CVD), and each method will be considered here. 1.1.1. Arc-Discharge Arc-discharge is the easiest and most common way to produce CNTs [15]. Indeed, CNTs were firstly discovered by Japanese scientist, Iijima, who was planning to utilize an arc-discharge method to synthesize a 1 a 2 T Cn (n,n) (n,0)
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