Biology Reference
In-Depth Information
ordered 1D anisotropic (meaning directionally dependent) structures find
applications, especially in nanoelectronic devices. Conducting wires, for
example, could be used to interconnect the different electronic components
that make up a cell phone or iPod. The rod-shaped VNPs with their high
aspect ratio are excellent building blocks for the fabrication of such
structures. The aspect ratio of VNPs is defined as length:diameter. M13 and
Tobacco mosaic virus
(TMV) have been extensively studied, and have aspect
ratios of approximately 150 and 17, respectively.
1
..1 Bundles, Wires, and Fibers
Rods can be self-assembled end to end (also referred to as head to tail) and
side to side. Structures with high aspect ratios can be built. A collection of
rods aligned into a 1D structure is referred to as
bundle
. When these
bundles
are metalized
or mineralized (recall Chapter 6), they are referred to as
wires
or
.
Self-assembly can be spontaneously induced. For example, when exposed
to acidic conditions, TMV particles spontaneously self-assemble head to tail.
As described in Chapter 6, during the sol-gel silica mineralization process of
TMV, which is performed at acidic pH, TMV particles tend to align head to tail
and long wires can be formed (see Fig. 6.5) (Shenton
fibers
., 1999). The head-
to-tail alignment is most likely a product of complementary hydrophobic
interactions between the dipolar ends of the helical structure. In an acidic
environment, the head-to-tail alignment is favored as in this conformation
repulsion between the carboxylic acid residues at the assembly interface
is minimal (Shenton
et al
., 1999). When such structures are mineralized,
a continuous metal coat that is bridging two or several TMV particles is
formed, for example, see the silica-coated TMV wires (Fig. 6.5) (Shenton
et al
et
al
., 1999). Continuous wires can also be synthesized in the interior channel
of the aligned rods. Figure 7.2 shows a linear TMV aggregate. Straight
nickel wires were grown within the central channel that reached lengths
of more than 500 nm without any gaps; these results show that linear TMV
aggregates have a single uninterrupted central channel and wires can bridge
two and potentially multiple virions (Knez
., 2003).
The head-to-tail assembly of TMV can also be assisted and stabilized by
aniline (Niu
et al
., 2007). Aniline or phenylamine is an
aromatic amine (Fig. 7.3a). Aniline binds to TMV via electrostatic interactions
and hydrogen bonding. Polymerization of aniline on the surface of TMV
leads to the formation of a thin layer of the polymer on the surface of TMV.
et al
., 2007a,b; Wang
et al
1
M13 has a length of about 1 µm and a diameter of 6.5 nm. The aspect ratio is thus length:
diameter = 1000 nm : 6.5 nm = 154. TMV is about 300 nm in length and has a diameter of 18 nm;
thus, the aspect is 17.
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