Biomedical Engineering Reference
In-Depth Information
15.2.5 Advantages of electrochemical sensors based on CNTs
CNTs have been one of the most actively studied electrode materials in the past few
years due to their unique electronic and mechanical properties. From a chemistry point
of view, CNTs are expected to exhibit inherent electrochemical properties similar to
other carbon electrodes widely used in various electrochemical applications. Unlike
other carbon-based nanomaterials such as C 60 and C 70 [31], CNTs show very different
electrochemical properties. The subtle electronic properties suggest that carbon nano-
tubes will have the ability to mediate electron transfer reactions with electroactive spe-
cies in solution when used as the electrode material. Up to now, carbon nanotube-based
electrodes have been widely used in electrochemical sensing [32-35]. CNT-modifi ed
electrodes show many advantages which are described in the following paragraphs.
First, the CNT-modifi ed electrodes catalyze the redox reactions of analytes. It has
been reported that the oxidation of such analytes as dopamine, H 2 O 2 , and NADH are
catalyzed at the various types of CNT-modifi ed electrodes. Second, the biomacro-
molecules such as enzymes and DNA can be immobilized on the CNT-modifi ed elec-
trodes and maintain their biological activities. Third, the CNTs are a good material for
constructing the electrodes. CNTs are small, straight, and strong, and they also have
chemical stability. These characteristics of CNTs are advantageous for constructing
CNT-modifi ed electrodes. A couple of examples are vertically aligned CNT modifi ed
electrodes and the electrodes based on an individual nanotube. Fourth, CNTs can be
functionalized mostly through the carboxyl group on the tips which helps to immobi-
lize the enzymes, etc. for the development of various types of sensors. The fi fth advan-
tage of CNTs is their good electronic conductivity. The conductivity of CNTs is also
affected by the structural changes such as twisting and bending of CNTs which may be
applied for the sensing purpose.
These advantages combined with others such as the porous structure may contribute
to having good wetting property for the solvents, a better electrode-electrolyte inter-
face, and a large surface area. The central hollow cores and outside walls are a supe-
rior material to adsorb and store gases such as oxygen, hydrogen, and nitrogen oxide
[4, 36, 37]. The investigation of gas sensors using the adsorptive properties of carbon
nanotubes to detect oxygen and carbon oxide has been reported [4, 38]. The CNTs
in many cases can serve as molecular wires that connect the electrode surface to the
active site of enzymes. The direct or enhanced electrochemistry of several proteins and
enzymes has been observed without the needs of mediators. Many enzyme-based elec-
trochemical biosensors have been reported using CNT-modifi ed electrodes.
15.3 FABRICATION AND APPLICATION OF
ELECTROCHEMICAL SENSORS BASED ON CARBON
NANOTUBES
A large number of papers on CNT-based sensors have been published in the last several
years mainly because CNTs have the following advantages for the electrochemical
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