Biomedical Engineering Reference
In-Depth Information
electrical conductivity, physical rigidity, and chemical stability [1-3]. CNTs can be made
by various methods such as electric arc discharge, chemical vapor deposition, or laser
evaporation [2]. The structure of a CNT is quite different from conventional carbon fiber
and graphite. CNTs have tubular structure made of hexagonal honeycomb lattices built
from sp
2
carbon units and are seamless structure with typically several nanometers in
diameter and many microns in length. CNTs can be categorized into single-wall carbon
nanotube (SWCNT) and multiwall carbon nanotube (MWCNT). SWCNT has a single
cylinder formed by rolling up a single graphite sheet seamlessly into a tube as shown in
Figure 13.1.
MWCNT comprises several concentric graphite tubules and the distance between
graphite sheets is 3.4 Å, which is close to the interlayer spacing in graphite. Theoretical cal-
culation predicts that CNT will behave either as metal or semiconductor depending upon
its size and symmetry of the two-dimensional carbon lattice (helicity) [4].
The CNT represents a new kind of carbon-based material and is superior to other con-
ventional carbon-based materials. The unique structural and electronic properties of CNT
make them extremely attractive for the construction of chemical sensors and biosensors
based on electrochemical transduction. Over the last several years, extensive efforts have
been devoted into the preparation and characterization of CNT-based electrochemical sen-
sors and biosensors. Recently, the CNT-based electrochemical sensors and biosensors have
been reviewed [3, 5-6]. Recent studies demonstrate that the unique structural and elec-
tronic properties of CNT can enhance the electron-transfer reactions of hydrogen peroxide
and nocotinamide adenine diclucleotide (NADH), and thus highly sensitive electrochemi-
cal biosensors based on oxidase and dehydrogenase enzymes can be fabricated with the use
of CNT [7-8]. In addition to enhanced electrochemical reactivity, CNT-modified electrodes
can minimize surface-fouling effects commonly involved in the NADH-oxidation process.
These properties of CNT are extremely attractive for a wide range of electrochemical
biosensors.
Owing to the enhanced electroactivity of the product of the enzyme label as well as gua-
nine of target DNA, highly sensitive DNA hybridization sensors based on CNT can also
be prepared. Conductive properties of CNT permit direct electron-transfer reactions
between redox proteins and underlying electrodes; thus, simple reagentless biosensor can
be made.
In this chapter, recent advances in the use of CNT for the construction of electrochemi-
cal biosensors are summarized. Common preparation methods for CNT-modified elec-
trodes and their analytical characteristics are also discussed. In addition, CNT-modified
amperometric biosensors based on oxidase or dehydrogenase are illustrated with exam-
ples. DNA biosensors based on the enhanced detection of the product of the enzyme label
or of guanine base in the target DNA are also discussed. Finally, the field-effect transistors
functionalized with biomolecules are illustrated.
FIGURE 13.1
Structures of carbon nanotubes.
