Biomedical Engineering Reference
In-Depth Information
exchange between an adsorbate and the CNTs changes the electron
density in the nanomaterial, modifying the carrier density in the
nanostructure. The surface modifications in the nanostructures
can enhance the charge transfer by applying external perturbations
(e.g., electric field, electromagnetic radiation, thermal heating,
ultrasonication), attachment of functional groups or catalytically
functionalizing sidewalls of carbon-based nanomaterials. Various
methods of functionalizations [22, 187, 188] have been explored
to fabricate modified CNTs for chemical sensing applications
and increased gas adsorption properties. Functionalizations of
the CNTs have been accomplished by three different approaches
or technological methods: (i) thermally activated surface
modifications; (ii) photochemically activated surface modifications;
and (iii) physically and chemically activated surface modifications.
Particularly, foreign materials functionalizing the carbon nanotube
sidewalls involve the mentioned third method, where functional
materials provide to the CNTs additional and new functionalities for
advanced gas sensors.
Generally, the covalent functionalization of the CNTs could
damage their nanostructure with an undesired change from sp
2
3
to sp
structure and a consequent loss of conjugation [189]. Alternatively,
it is possible to functionalize CNTs noncovalently to preserve sp
2
structures and their electronic properties that are very attractive for
chemical sensing performance as well.
Recently, gas sensors based on functionalized CNTs have been
proposed capable of detecting small gas concentrations with high
specificity. In fact, successful attempts of functionalized CNT sensors
have been covered by a wide variety of functionalizing materials such
as polymers [38, 143, 176, 178, 181, 190-194, 230], enzymes [195-
197], proteins [16, 198-202], antibodies [203-208], DNA [209-217],
semiconducting metal oxides [218-229, 252], metalloporphyrins
[231], and metallic nanoclusters [26, 31, 36, 232-249] using different
processing methods for functional materials integrated in several
transducers. Furthermore, nanoclusters of noble metals (Au, Pt, Pd,
Ag, Ru, etc.) have been used as metal decorations onto sidewalls to
enhance the gas sensitivity [36, 232, 234, 241, 242, 246-249] of the
CNTs networks, operating at a sensor temperature ranging from 20
to 250°C, to detect various gas molecules of NO
, NH
, H
, CH
, H
S,
2
3
2
4
2
C
, and volatile organic solvents. A summary of functionalized
CNTs gas sensors is shown in Table 9.5.
H
6
6
