Page 402 - Organic Electronics in Sensors and Biotechnology

Electrophoretically Deposited Polymers for Organic Electronics 379

of infiltrating the CNT structure, or producing a layered structure.
Alternatively, composite CNT/nanoparticulate coatings can be
obtained by co-EPD from stable suspensions containing two or more
components. The various components may be separately dispersed,
coming together only during EPD, or may be preassembled to form a
more complex building block. These opportunities have yet to be
investigated systematically, but some indicative promising results
have been obtained. Singh et al. have reported homogeneous and
thick deposits of CNTs that can be coated and infiltrated with TiO
2
nanoparticles obtained by co-EPD. Co-EPD is carried out at a con-
83
stant electric field of 20 V/cm. Due to the complementary surface
charge of CNT (negatively charged) and TiO nanoparticles (posi-
2
tively charged), the two components attract each other in aqueous
suspensions at the selected pH. These forces result in the deposition
of TiO nanoparticles on the surface of individual CNTs. Similarly,
2
CNT/SiO nanoparticle composite films have been obtained by EPD
2
84
from aqueous suspensions, as discussed elsewhere. The deposit is a
three-dimensional network of interwoven CNTs coated and infiltrated
by the SiO nanoparticles. This type of porous CNT/titania and CNT/
2
85
silica nanostructures may be useful for nanoelectronic devices. More
straightforwardly, the coating and infiltration of porous CNT assemblies
with nanoparticles can be seen as a useful step toward homogeneous
incorporation of CNTs in hard, structural, and functional matrices. 85
Kaya has recently investigated EPD for coating of MWCNT rein-
41
forced ultrafine (20 nm) hydroxyapatite powders with Ti-6Al-4V for
biomedical applications, such as total hip replacement.
EPD of Polymers EPD is an important commercial method of apply-
ing films to irregularly shaped metal articles, and adaptation of this
method to polymers could increase their utility significantly. Li et al. 86
have presented a convenient approach for the formation of polyani-
line (PANI) colloids with a size of ∼100 nm. It has been demonstrated
that the polyaniline colloidal suspensions have excellent processabil-
ity when applied electrophoretically. More significantly, the method
provides the means for delivering controlled amounts of materials to
desired locations by manipulating the electric field. This makes pos-
sible the patterning of polyaniline, a technique that could be attrac-
tive, particularly for practical device applications. Li et al. have used
this technique for the incorporation of polymeric coating on the
MEMS platform, the NIST micro-hotplate, a conductometric gas sen-
sor with an embedded microheater. The signal magnitude of this
micro-hotplate device with an electrophoretically integrated polyani-
line film correlates well with the gas concentration, with relatively
short response and recovery times. The EPD of polyaniline colloids
can be controlled with great flexibility by adjusting various process
parameters such as the duration of the deposition, the colloid con-
centration, or the applied voltage. In addition, the electrophoretic