Electrochemical sensors based on carbon nanotubes - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 463

than that at the graphene hexagon in the basal graphite plane and would act as an elec-
trophilic reaction site. There have been numerous reports on the enhanced electron
transfer of analytes when CNTs are used as electrode materials. In carbon nanotubes,
it has been suggested that the presence of defects creating overall topological change
may inherently increase reactivity compared with their graphite counterparts [8]. The
moderate reactivity made it easy to introduce functional groups to the CNTs (both side
wall and end cap) which is essential for sensor design in many cases. For example,
during the purifi cation of CNTs with a strong acid, the carboxyl functional groups
are introduced to the surface of CNTs, especially at the tips. The carboxy functional
groups are involved in nanotube chemical modifi cation with amide-linked groups at
the tip ends of the CNTs [23, 24].

15.2.3 Preparation of CNTs
The preparation of CNTs is a prerequisite step for the further study and application of
CNTs. Considerable efforts have been made to synthesize high quality CNTs since their
discovery in 1991. Numerous methods have been developed for the preparation of CNTs
such as arc discharge, laser vaporization, pyrolysis, and plasma-enhanced or thermal
chemical vapor deposition (CVD). Among these methods, arc discharge, laser vaporiza-
tion, and chemical vapor deposition are the main techniques used to produce CNTs.
Arc discharge [25] is initially used for producing C 60 fullerenes. Nanotubes are pro-
duced by arc vaporization of two carbon rods placed in a chamber that is fi lled with
low pressure inert gas (helium, argon). The composition of the graphite anode deter-
mines the type of CNTs produced. A pure graphite anode produce preferably MWNT
while catalyst (Fe, Co, Ni, Y or Mo) doped graphite anode produces mainly SWNT.
This technique normally produces a complex mixture of components, and requires fur-
ther purifi cation to separate the CNTs from the soot and the residual catalytic metals
present in the crude product.
The laser ablation technique was developed in 1995 by Smalley's group [26] at Rice
University. Samples were prepared by laser vaporization of graphite rods with a cata-
lyst mixture of Co and Ni (particle size

m) at 1200ºC in fl owing argon followed
by heat treatment in a vacuum at 1000ºC to remove the C 60 and other fullerenes. The
material produced by this method appears as a mat of “ropes”, 10-20 nm in diameter
and up to 100

1 µ

m or more in length. The average nanotube diameter and size distribu-
tion can be tuned by varying the growth temperature, the catalyst composition, and
other process parameters.
Despite the frequent use of arc-discharge and laser ablation techniques, both of
these two methods suffer from some drawbacks. The fi rst is that both methods involve
evaporating the carbon source, which makes it diffi cult to scale up production to the
industrial level using these approaches. Second, vaporization methods grow CNTs
in highly tangled forms, mixed with unwanted forms of carbon and/or metal species.
The CNTs thus produced are diffi cult to purify, manipulate, and assemble for building
nanotube-device architectures in practical applications.

µ