Biomolecular Architecture for Nanotechnology - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 158

DNA metallization or device fabrication by sequence-specific molecular lithog-
raphy destroys the specific recognition properties of the DNA scaffold. This can be
avoided by embedding the metallization pattern into DNA via a sequence-specific
aldehyde derivatization, in which DNA molecules react with glutaraldehyde ( Keren
et al. 2004 ). In this way, the DNA scaffold is marked for metallization before
the metallization process and can be used for successive biological manipulation.
The silver clusters, which act as nucleation sites for the following electroless gold
deposition, form by reduction of silver ions by aldehyde groups bound to DNA in
the aldehyde-derivatized regions. Electroless deposition is suited for metallization
of templates in solution. If immobilized on a substrate, the metallized biological
templates are no longer curved or agglomerated.
Nanowire and nanowire arrays on silicon substrates can be fabricated using
microtubules as templates ( Zhou et al. 2008 ). Microtubules are rigid and hollow
protein tubes with a diameter of about 25 nm and a length of few micrometers,
which self-assembly from units called '“-tubulins. Because the microtubule end
exposing “-tubulins grows much faster than the end exposing '-tubulins, growth
of a microtubule on substrate is controlled and directed. Therefore, nanowires can
easily form by first growing a microtubule that connects two desired locations
on a substrate using a tau protein as mediator, followed by covalently binding
functionalized 1.4-nm colloidal gold nanoparticles on the microtubule, and, finally,
enhancing the diameter of the bound Au nanoparticles by a photochemical method.
Microtubule-templated nanowires with lengths between 1 and 20 m can be
obtained in this way, consisting of (possibly agglomerated) gold nanoparticles
with diameters of 12-15 nm that enhance the initial microtubule diameter up to
80-100 nm. The resistivity of a typical nanowire with a length of 2:5 mis
7:3 10 5 m. In a similar way, arrays of parallel microtubules can first self-
assemble and then suffer a metallization process, the average distance between
microtubules in the array being of 18 nm; the gold coverage in this case is not as
good as for single nanowires.
The fabrication of coaxial metal nanocables has been detailed in Carny
et al. ( 2006 ). The trilayered metal-insulator-metal structure used self-assembled
peptide nanotubes as scaffolds, on which small gold nanoparticles with a diameter
of 1.4 nm were attached at specific sites via a recognition molecule incorporated
in linker peptides. The linker peptides bind weakly to the surface of the peptide
nanotube and covalently to chemical inserts used to bind other molecules, in
particular gold nanoparticles. These bound Au nanoparticles act as nucleation sites
for selective reduction of gold ions on the exterior surface of peptide nanotubes, the
final cable having an external metal coating of 20 nm. An internal metallic layer is
obtained by reducing silver on the inner surface of the hollow peptide nanotube; the
coaxial nanocable has thus a silver-peptide-gold layered structure. The sequence
of processes for the fabrication of the nanocable is illustrated in Fig. 5.6 .
Peptide templating was also used to increase the efficiency of dye-sensitized solar
cells that incorporate a hollow TiO 2 nanoribbon network electrode ( Han et al. 2010 ).
The highly entangled framework of TiO 2 nanoribbons, which acts as photoanode,
is expected to reduce surface traps and the undesirable recombination rate of