Electrochemical sensors based on carbon nanotubes - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 466

sensor applications: (1) small size with large surface area, (2) high sensitivity, (3)
fast response time, (4) enhanced electron transfer, (5) easy protein immobilization
with retention of activity, and (6) alleviating surface fouling effects. For the fabrica-
tion of CNT-modifi ed electrodes, several methods have been developed. Most of these
methods involve the immobilization of enzyme molecules on the electrode surface.
In the following sections, the preparation methods and some representative results
using such electrodes as (1) CNT-composite electrodes, (2) vertically aligned nano-
tube electrode arrays, (3) layer-by-layer electrodes, and (4) CNT coated electrodes will
be described. In addition, advantages as well as the applications of each type of these
CNT-modifi ed electrodes are discussed. Since the effi cient charge transfer between the
electrode surface and macrobiomolecules such as proteins and DNA is important for
the development of biosensors, the direct electrochemistry of proteins and DNA is dis-
cussed in this section. The discussion in this section also includes CNT enzyme-based
biosensors.
15.3.1 Preparation of carbon nanotube electrodes and their
electrochemical characteristics
15.3.1.1 CNT-composite electrodes
The fi rst CNT-modifi ed electrode was reported by Britto et al. in 1996 to study the
oxidation of dopamine [16]. The CNT-composite electrode was constructed with bro-
moform as the binder. The cyclic voltammetry showed a high degree of reversibility in
the redox reaction of dopamine (see Fig. 15.3). Valentini and Rubianes have reported
another type of CNT paste electrode by mixing CNTs with mineral oil. This kind
of electrode shows excellent electrocatalytic activity toward many materials such as
dopamine, ascorbic acid, uric acid, 3,4-dihydroxyphenylacetic acid [39], hydrogen per-
oxide, and NADH [7]. Wang and Musameh have fabricated the CNT/Tefl on compos-
ite electrodes with attractive electrochemical performance, based on the dispersion of
CNTs within a Tefl on binder. It has been demonstrated that the electrocatalytic proper-
ties of CNTs are not impaired by their association with the Tefl on binder [15].
Other CNT-composite electrodes are prepared by using conducting polymers such
as polypyrrole [40] and polyaniline [41]. The PPy/CNT composite can be prepared
electrochemically by repeated scanning the potential in the oxidation range in an aque-
ous solution containing CNTs, surfactant SDS, and pyrrole. A CNT-composite elec-
trode has also been prepared using the sol-gel matrix as a glucose sensor [42]. The
preparation of binderless biocomposite with CNTs has been reported for the prepara-
tion of glucose sensor [15]. After mixing CNTs with glucose oxidase (GOx), the com-
posite was packed inside of a needle and was used for the detection of glucose.
15.3.1.2 Vertically aligned CNT-modifi ed electrode
The electrodes modifi ed with vertically aligned CNTs have recently received much
interest for the purpose of designing CNT-based electrodes and electrochemical sensors.