Biomedical Engineering Reference
In-Depth Information
to changes in the chemical environment. They are mechanically
and chemically robust and stable, and thus, compared to polymer-
functionalized sensors, metal-modified sensors are able to operate
not only at room temperature but also at higher temperatures in
harsh ambients. Typically, Pd and Pt are widely used in commercial
sensors to catalyze combustible gases such as H
, CO, LPG, and
so on for rapid detection at room temperature in order to avoid a
possible trigger of explosion in the chemical detection at elevated
temperatures.
, CH
2
4
sensors
based on palladium (Pd) nanoparticles modified SWCNTs prepared
by electron-beam evaporation of
Kong
et al
. [232] demonstrated room temperature H
2
~
Å
of palladium over the entire
substrate containing the SWCNTs device. Pd-functionalized SWCNTs
were shown to be highly sensitive to H
5
, with 50% greater response
of up to 50% relative resistance change toward 400 ppm H
2
in air
compared to bare SWCNTs bundles. Figure 9.27 shows the scheme
of device and its response to hydrogen. Response time was recorded
as 5-10 s, and the recovery time was measured as
2
~
400 s. It was
established that at room temperature, the adsorbed H
molecules
on the surface of Pd nanoparticles are dissociated by spillover
effect into hydrogen atoms that dissolve into Pd at high solubility,
leading to a decrease in the Pd work function. This causes electron
transfer from Pd to SWCNTs and reduces the hole carriers in the
2
-
type carbon nanotube, and hence causes a decrease in the electrical
conductance. This process is reversible as dissolved atomic hydrogen
in Pd can combine with adsorbed O
p
in air to form OH groups which
further combine with atomic hydrogen to form H
2
O and leave the
Pd-SWCNT system, thus recovering the sensor initial conductance
giving reversibility to the device.
Penza
2
. [31, 248] demonstrated the impact of the tailored
load of Au nanoclusters functionalizing the sidewalls of the MWCNT
networks on gas sensing properties of a chemiresistor, working at a
temperature ranging from 20 to 250°C. Au clusters with increasing
size of 5-15, 5-30, and 5-60 nm enhanced gas response compared
to unmodified CNT sensors for various reducing gases (NH
et al
, H
S)
3
2
and an oxidizing gas (NO
), down up to a sub-ppm level of detection
limit. Neglibile responses were recorded to CO, N
2
. Figure 9.28
shows the Au-nanoclusters onto CNTs sidewalls and the gas sensing
performance. The effect of sensor temperature on gas sensitivity
is also reported. An optimal operating temperature for each Au-
modified CNT sensor exposed to NO
O, SO
2
2
gas has been recorded, e.g.,
2
