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
 
Search WWH ::




Custom Search