Electrochemical sensors based on carbon nanotubes - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 460

15.1 INTRODUCTION
Since the discovery of carbon nanotubes (CNTs) in 1991 [1], there has been growing
interest in using CNTs in chemical and biochemical sensing [2-5] and nanoscale elec-
tronic devices due to their remarkable electronic and mechanical properties. CNTs behave
as a metal or semiconductor depending on their structures. CNT-modifi ed electrodes have
better conductivity than graphite [6, 7] and show a superior performance compared with
such electrodes as Au, Pt, and other carbon electrodes. CNTs have a hollow core, which is
suitable for storing guest molecules. Proteins and enzymes can be immobilized in the hol-
low core as well as to the surface of CNT without losing biological activity. The electri-
cal conductivity of nanotubes can be improved by modifying the original CNT structure.
CNTs possess fascinating mechanical strength and are the strongest and stiffest material
known [8]. CNTs, especially the side walls, are relatively inert. However, the ends of nan-
otubes are more reactive than the cylindrical parts [8, 9]. For the application of CNTs
to electrochemical sensing, CNTs show the enhanced electrochemical response to some
important biomolecules [5, 10] and promote the electron transfer reactions of proteins
[4]. These characteristics demonstrate clearly that nanotubes have signifi cant potential for
the design of electrochemical sensors. Indeed, in the past several years, there have been
extensive studies on the applications of CNTs in electrochemical sensors [11]. Other appli-
cations include hydrogen storage [12], catalysis, micro-chemical and micro-biological
detectors [13-15], biological cell electrodes [16], nanoscale electronic and mechanical
systems [17], scanning probe microscope, and electron fi eld emission tips [18, 19].
In this chapter, we will discuss electrochemical sensors based on CNTs. First, the
properties and structures of CNTs, the preparation and purifi cation of CNTs, and the
advantages of electrochemical sensors based on CNTs are described, then, the fabri-
cation of electrochemical sensors based on CNTs, applications of electrochemical
sensors based on CNTs, and the spectroscopic characterization of CNT sensors are
described. In conclusion, we will look into some aspects of the future direction for
CNT sensors in clinical and biomedical research.
15.2 THE STRUCTURE AND PROPERTIES OF CNTS
15.2.1 The structure of CNTs
CNTs are unique tubular structures of nanometer diameter and have large length/
diameter ratio. CNTs are divided into two main groups: single-walled carbon nano-
tube (SWNT) and multi-walled carbon nanotube (MWNT). SWNT can be considered
as a long wrapped graphene sheet by rolling it in certain directions. The properties
of the nanotubes are mainly dictated by the rolling direction as well as the diameter.
SWNT consists of two separate regions (the side wall and the end cap) with very dif-
ferent physical and chemical properties. The end-cap structure is similar to or derived
from a small fullerene in which the carbon atoms are in both pentagon and hexagon
rings. The side wall only consists of hexagon rings. MWNT can be considered as a