Biomedical Engineering Reference
In-Depth Information
CNTs can offer various advantages with respect to other
counterpart sensor materials such as metal oxides, conducting
polymers and hybrid organic/inorganic materials in terms of fast
response due to short transport distance, high sensitivity due to
the large surface area, as well as the potential for miniaturization
via integration with IC-based technology leading to low power
consumption, low operating sensor temperature even at room
temperature with subsequent low power consumption, and mass
production for scale-up devices.
Advanced transducers such as chemiresistors, field-effect
transistors (FETs), surface acoustic wave (SAW) devices, optical
fibers, electrochemical cells, and MEMS have been developed and
used as transducing platforms for ultrasensitive micro/nanosensors
based on gas-sensitive CNTs to detect chemical species at high
sensitivity up to very low ppb-level of detection.
In the next paragraphs, the gas sensing properties of the CNTs
are reviewed in the different variants — pristine material, modified
material with functional groups, purified material, and functionalized
material with foreign structures, by using the several transducers
properly adapted.
9.4.1
Pristine Carbon Nanotubes
Kong
. [33] fabricated the first transistor based on individual
SWCNTs to study the gas sensing properties of raw carbon nanotube.
Upon exposure to gaseous molecules such as oxidizing NO
et al
or
2
reducing NH
, the electrical resistance of a semiconducting SWCNTs
was found to dramatically increase or decrease, respectively, at room
temperature. This proved the
3
-type semiconducting character
of the CNTs due to shift of the Fermi level upon chemical gating.
The nanotube sensors exhibited a fast response (<1-2 min), slow
recovery at room temperature and faster recovery by heating, and
sensitivity substantially higher than that of existing solid state
sensors at room temperature. In fact, the electrical resistance of
individual semiconducting SWCNTs changed by up to three orders of
magnitude within several seconds of exposure to NO
p
gas.
Figure 9.9 shows the I-V characteristics of the individual SWCNT
upon exposure of target molecules. After a 10 min exposure to NH
and NH
2
3
, a
3
conductance depletion was observed; while NO
exposure increased
the conductance of SWCNT by about three orders of magnitude. The
room temperature electrical resistance of the SWCNT under zero
2
