Biomedical Engineering Reference
In-Depth Information
polymerization and the template are then dissolved using appropriate acids or bases.
Organic solvents may be used to dissolve polymer templates. Figure 4.3 depicts a schematic
of the template-based synthesis process. It has been shown that template-directed CP nan-
otubes and nanowires have higher electronic conductivities than bulk samples of the same
material. Furthermore, the mechanism of electronic conduction in these template-directed
nanostructures can be varied at will by changing their diameter [29].
He and coworkers [30] synthesized PANI nanowires by stretching, aligning, and immo-
bilizing double-stranded
-DNA on a thermally oxidized Si chip by the molecular comb-
ing method [31,32]. Then the DNA templates were incubated in protonated aniline
monomer solution to emulsify and organize the aniline monomers along the DNA chains.
Finally, the aligned aniline monomers were polymerized enzymatically by adding horse-
radish peroxidase (HRP) and H
2
O
2
successively to form PANI/DNA nanowires.
Simmel and coworkers [33] also synthesized PANI nanowires templated by DNA using
three methods. They found that DNA templating worked best for PANI formed by oxida-
tive polymerization of aniline with ammonium persulfate, both in solution and on tem-
plates immobilized on a chip. DNA was also good template for PANI formed by
enzymatically catalyzed polymerization utilizing HRP. However, immobilization of these
structures between contact electrodes was compromised by extensive protein adsorption
to the surface. A photo-oxidation method using ruthenium
tris
(bipyridinium) complexes
as photo-oxidant resulted in less uniform PANI /DNA structures than the other methods.
4.2.5
Individually Addressable Single Nanowires
Although several examples of uses of these methods of synthesis and fabrication of 1-D
nanostructures have been reported, many properties of these nanostructures and their
fabrication methods had significantly limited their full-scale development, particularly for
high-density arrays [34-37]. For example,
(1)
the harsh conditions, highly concentrated sodium hydroxide or phosphoric acid,
required to dissolve most templates in template-synthesized CP NWs might not
be suitable for many biological applications.
(2)
Functionalization/modification for incorporation of specific sensing capabilities
can only be performed as postassembly processes.
(3)
Nanodevice fabrication requires complex postsynthesis assembly using sophis-
ticated manipulating tools.
1
2
3
4
FIGURE 4.3
Schematic of template-based synthesis of conducting polymer nanowires (CP NWs). 1) Deposition of inert con-
ducting thin film on one side of template 2) Polymerization of the monomers within the pores of the template 3)
Etching of inert conducting thin film 4) Dissolution of template.
