Electrochemical sensors based on carbon nanotubes - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 493

1600/1500 cm 1 intensity ratio indicating that PANI in the composite is richer in quinoid
units than the pure ES PANI possibly because the nanotube-PANI interactions pro-
mote and (or) stabilize the quinoid ring structure; (2) the N ß H stretching region near
3400 cm 1 is broad and strong in the composite but weak in the pure ES PANI since
the sp 2 carbon of CNTs competes with the chloride ion, and thus perturbs the H-bonding
environment and increases the N ß H stretching intensity.
15.5 CONCLUSIONS
This chapter described the general aspects of carbon nanotubes (CNTs) and electro-
chemical sensors based on carbon nanotubes. There are several methods for forming
CNT-modifi ed electrodes that can be used for detecting analytes. Both advantages
and examples of application of each method are described. This chapter also included
examples of a fairly large number of analytes which have been determined by CNT-
modifi ed electrodes as effective and sensitive transducers. Since there have been a
large number of publications in this exciting fi eld of analytical chemistry in the last ten
years, some of the important papers, particularly those published recently, might not
be included.
Among those several different types of transducers based on CNTs, the CNT-
composite electrode, which was the fi rst CNT electrode tested in 1996, is still widely
used with different composite materials such as conducting polymers, nanoparticles,
sol-gel, etc. The usefulness of these electrodes is based on their high sensitivity, quick
response, good reproducibility, and particularly long-term stability. We expect to see
continued research activities using CNT-composite electrodes.
In recent years, there are more applications based on the layer-by-layer fabrication
techniques for CNT-modifi ed electrodes. This technique clearly provides thinner and
more isolated CNTs compared with other methods such as CNT-composite and CNT
coated electrodes in which CNTs are in the form of big bundles. This method should
help biomolecules such as enzymes and DNA to interact more effectively with CNTs
than other methods, and sensors based on this technique are expected to be more sen-
sitive. Important biosensors such as glucose sensors have been developed using this
technique, and further development of other sensors based on the layer-by-layer tech-
nique is expected.
Vertically aligned CNT-modifi ed electrodes are based on a more elaborated tech-
nique than other methods, and microscopic images are used to characterize the inte-
grity of this type of electrode. The technique has been applied for the immobilization
of enzymes and DNA, and the sensors based on this technique have shown a lower
detection limit than those based on other methods. More research activities using this
technique, particularly with low density CNT arrays, are expected in the near future
because of its sensitivity and versatility.
Even though the number of reports on the electrochemistry based on an individual
nanotube is very limited, mainly due to technical diffi culty, the development of sensors