Biomolecular Architecture for Nanotechnology - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 164

nanoshell plasmon spectrum. A recent review on the design principles of building
plasmonic nanostructures on DNA scaffolds can be found in Tan et al. ( 2011 ).
In another virus-assisted deposition process ( Szuchmacher Blum et al. 2004 ),
gold nanoparticles with diameters of 2 and 5 nm bind via gold-sulfur links at
specific sites, separated by precise distances, on a cowpea mosaic virus with an
average diameter of 28.4 nm. Because the protein capsid of this virus has an
icosahedral shape that results from the assembly of 60 copies of an asymmetric
unit, only multiple-of-60 numbers of gold nanoparticles can be attached on the
capsid. Gold attachment is only possible if cysteine groups are introduced at
designed locations on the virus capsid, on which thiol-reactive nanoparticles are
linked by sulfur-gold covalent bonds. The cysteine groups can be arranged in
various symmetric patterns, which correspond to different distances between gold
nanoparticles that can vary from 5.3 to 8.4 nm.
Viruses such as M13 are real toolkits for synthesizing semiconducting nanowires
from materials like ZnS and CdS, as well as magnetic nanowires from CoPt and FePt
ferromagnetic alloys, by incorporating specific nucleating peptides into its highly
ordered capsid ( Mao et al. 2004 ). The filamentous viral template can afterwards
be removed by an annealing process, which also promoted transformation of
polycrystalline into single-crystal nanowires. Modified M13 scaffolds have been
used to genetically engineer high-power lithium-ion batteries ( Lee et al. 2009 ). In
such batteries, nanostructured composite electrodes are essential to enhance the
transfer of electrons and Li C ions through the structure. Amorphous anhydrous
FePO 4 nanowires that act as cathodes for batteries have been templated on modified
M13 viruses loaded with uniformly distributed silver nanoparticles to enhance
conductivity. A further increase in performance of lithium-based batteries implies
good electrical contact between the active materials, which could be achieved by
a percolating network throughout electrodes. An additional genetic modification of
M13 allows conducting CNT to bind on the virus and thus to produce nanoscale
wiring to the active material.
A similar hybrid structure forms when a semiconducting polymer is encapsulated
in a self-assembled protein vault ( Ng et al. 2008 ). Such filled nanocapsules exist in
large numbers in human cells but can be engineered to contain fluorescent nanopar-
ticles useful in nanotechnology, for example, or nanoscale deliverable drugs. Natural
vaults are cylindrical structures that resemble a capped barrel, with typical diameters
and lengths of 41 and 72 nm, respectively, composed of two identical cuplike halves.
When deposited on functionalized mica surface, the vaults can adopt the closed
and open conformations, the latter referring to separated halves that assume a
flower-like shape with eight petals. In solution, the vaults dissociate for pH<4.
After encapsulation in a protein vault in an acid buffer solution with pH D 6:5,the
environment of an MPS-PPV [poly(2-methoxy-5-propyloxy sulfonate phenylene
vinylene)] fluorescent polymer changes and its emission spectrum becomes more
intense and red-shifted.
It should be mentioned that in some cases, the biological scaffolds should be
alive during the growth process. For example, when a percolating gold monolayer
with a thickness of 30 nm is selectively deposited on rod-shaped Bacillus cereus