Biomedical Engineering Reference
In-Depth Information
9.4.2
Modified CNTs
CNTs are intrinsically very gas-sensitive to the chemical environment
due to very-large surface area. However, various strategies to modify
their surface walls have been developed to enhance the chemical
detection up to sub-ppm level, at room temperature.
In this paragraph, CNT networks modified with
functional
groups
attached to walls and as nanofiller in composite materials
to engineer
with different matrix embedding CNTs
as one-dimensional structures are shortly surveyed for gas-sensing
applications.
The physically and chemically activated surface modifications
in the CNT networks involve noncovalent or covalent attachments
of foreign functionalizing materials and anchored functional
groups changing the electronic properties useful for chemical
sensing applications. In fact, the chemical modifications [169-171]
represent an important area in the sidewall chemistry, doping, and
solubilization of the CNTs for sensor nanomaterials applications.
nanocomposites
. [172] demonstrated the electrical resistance response
of the chemically functionalized MWCNTs (f-CNTs) to alcohol vapors
with ultralow power consumption. For the fabrication of the sensor,
purified MWCNTs were sonicated in 3:1 concentrated sulfuric acid
and nitric acid for different time intervals. By this method, the
MWCNTs can be oxidized and COOH groups will be grafted along the
sidewall and the tube ends of the MWCNTs, as shown in Fig. 9.15.
It is believed that with the polar COOH groups attached onto the
nanotube surface, the sensors will give higher responses toward the
alcohols vapors under test as their absorption efficiency with these
volatile organic compounds will be increased due to the dipole-dipole
interactions (mainly hydrogen bonding) between the COOH and the
polar organic molecules. Figure 9.15 shows a schematic diagram of
how the ethanol molecules interact with the COOH groups through
hydrogen bonds. The f-CNTs were batch manipulated across Au
microelectrodes by electrophoresis and their electrical resistance
change was measured for alcohol exposure. The sensor showed a
linear response to alcohol vapor concentrations from 1 to 21 ppm
and good selectivity, upon air flow, alcohol vapor of ethanol with
respect to water vapor. Compared to bare CNTs, the modified CNTs
increased their sensitivity from
Sin
et al
~
0.9% to 9.6%.
