Biomedical Engineering Reference
In-Depth Information
The electrical properties of CNTs are extremely sensitive to
charge transfer and chemical doping effects by various molecules.
When electron-withdrawing molecules (e.g., NO
, O
, O
) or electron-
2
3
2
donating molecules (e.g., NH
, CO, H
S, SO
, H
, CO
, C
H
OH, etc.)
3
2
2
2
2
2
5
interact with the
-type semiconducting CNTs, they will change the
density of the main charge carriers (i.e., holes) in the nanotube,
changing the electrical conductance. This behavior is the basis for
applications of CNTs as electrical chemical gas sensors [29-32].
Generally, gas sensors based on pristine CNTs exhibit certain
limitations, such as some low chemical affinity, lack of selectivity, or
irreversibility, or long recovery time. To overcome these limitations,
various strategies are currently used such as purification,
manipulation, functionalization, modification, and activation of the
CNTs with foreign materials to alter their chemical reactivity and
enhance their gas sensing performance.
Gas sensors based on CNTs have been largely studied in the
form of individual nanotube, bundle of nanotubes, networked films,
vertically aligned nanostructures, or nanocomposites with CNT
fillers for highly sensitive gas detection applications [33-40]. Due
to very high surface-to-volume ratio, hollow nanostructure, high
electron mobility, great surface reactivities, and high capability of
gas adsorption, CNTs have been investigated as building blocks for
fabricating novel devices at nanoscale such as high-performance gas
sensors and nanoplatforms for biosensing.
This contribution reviews the integration of CNTs as advanced
nanostructured sensitive coatings with the different transducers
for the development of high performance chemical nanosensors
exploitable for various applications such as environmental air
monitoring, industrial processes control, safety, security, healthcare,
medical diagnosis, and automation. As it will be seen in the following
sections, ppm and sub-ppm chemical detection limits have been
obtained in most of the investigated cases. Particularly, the physical
properties of the CNTs will be reviewed, including their methods of
fabrication. Special session will be devoted to gas sensors based on
pristine, modified, purified, and functionalized CNTs by considering
the assessment of the gas sensing performance. Moreover, the
different transducers employing CNTs will be considered by outlining
the optimal performances for the specific applications. Comparative
analysis and future challenges in the CNT-based chemical sensors
will be depicted as well.
p
