Biomedical Engineering Reference
In-Depth Information
devices; the gravimetric effects in the piezoelectric mass-sensitive
devices; the optical properties changes (refractive index and
dielectric constant) in the optic fiber devices; the mass loading in
the micro-nano-electromechanical systems (MEMS-NEMS).
Here, a brief account of the various transducers coated by CNTs
for gas sensing applications will be reviewed.
9.5.1
Chemiresistors
Chemiresistors are the most commonly utilized classes of gas sensing
devices. They are largely produced in commercial gas sensors
mainly based on metal oxides, conducting polymers, and composite
materials. These devices work on the principle that a change in some
property of the gas-sensitive material resulting from interaction with
a gas/odor leads to a change in the electrical resistance of the sensor.
The sensing mechanisms that lead to these resistance changes are
different for each material type; however, the fundamental principle
consists in the electron charge transfer between targeted molecules
and sensor material. The structure of device is quite simple and easy
to fabricate and is essentially the same for each sensing material
used. The material is deposited over interdigitated or two parallel
electrodes, which form the electrical connections through which
the relative electrical resistance change (equivalently conductance
change) is measured due to chemical interactions caused by the gas
adsorption into sensor material. The heater is required when metal
oxides are used as sensing material because high temperatures are
required for effective operation of these sensors. In the contrast,
polymer sensors operate currently at room temperature. The
CNT-chemiresistors can work both at room temperature and high
temperatures (less than 250°C) depending on performance required
and specific application. Many CNT-chemiresistors with SWCNTs or
MWCNTs, CNT networked layers, raw, and functionalized CNT films,
vertically aligned or DEP-aligned CNTs, individual and bundled
ropes have been extensively studied in the last years [26-33, 36, 38-
40, 146, 147, 149, 150, 167, 173-175, 220, 225, 227, 228, 230-232,
234-238, 241-249, 253-258, 260-265, 267].
Meyyappan and coworkers [28] fabricated a gas sensor by casting
of a SWCNT-networked layer on interdigitated electrodes (IDE) for
gas and organic vapor detection, at room temperature. The sensor
response is linear for gas concentrations from sub-ppm to hundreds
