Biomedical Engineering Reference
In-Depth Information
or even microwave resonators covered with CNTs and which experience a resonant
frequency shift when gas is detected. CNT-coated surface acoustic wave (SAW)
devices and other types of devices are used to sense gases when covered with CNT.
The functionalization of CNT, which was explained in Chap.
1
, confers to CNT
gas sensors the necessary selectivity to detect a certain gas from a mixture of gases.
A simple example is the noncovalent drop cast of Nafion or polyethylenimine (PEI)
on single-walled CNT chemFETs, which improves the gas sensing and selectivity
of these sensors to NO
2
and NH
3
(
Qi et al. 2003
). ChemFETs based on single-
walled CNT and functionalized with PEI were able to detect 1 ppb of NO
2
and
were insensitive to NH
3
. The same device functionalized with Nafion was very
sensitive to NH
3
and insensitive to NO
2
. PEI is used frequently to functionalize
CNT gas sensors because it changes CNT conduction from native p-type to n-type.
Nanoparticles deposited on the CNT surface confer to CNT the possibility to work
at higher temperatures and harsh environments. In principle, Pd nanoparticles are
used for H
2
detection and gold nanoparticles for NO
2
detection. The recovery of
such sensors is done either by UV exposure or by heating, using a resistor film
placed beneath the device.
The exhaled breath components contain tens of gases, which indicate certain
diseases. For example, nitric oxide (NO) fluctuation is a relaxant of the blood vessel,
and its concentration indicates heart problems before any clinical symptom. NO
is also a neurotransmitter and is the mediator of immune system to pathogens.
Also, changes in the concentration of NO signal neuron death and the Alzheimer
disease. The high level of NO is a fingerprint of asthma. So, NO is an indicator of
many diseases. A normal person exhales about 6-16 ppb of NO, while an asthmatic
person exhales 34-50 ppb of NO. NO detection was recently performed (
Kuzmych
et al. 2007
) using a functionalized CNT FET, with the impressive detection limit of
5 ppb at 30% relative humidity. The CNT FET used a network of CNTs covered by
PEI, as indicated in Fig.
2.42
. The detection limit of NO was studied under similar
conditions as in breath, i.e., in the presence of O
2
,H
2
O (humidity), and CO
2
.Itwas
found that O
2
does not influence the detection limit, the relative humidity should be
in the 15-30% range for optimal detection, and CO
2
deteriorates the detection limit
up to 10-15 ppb. A similar configuration can be used to detect the nonpolar volatile
NO
CNT
PEI
D
S
Fig. 2.42
The detection of
exhaled NO
Si/SiO
2
gate
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