Biomolecular Architecture for Nanotechnology - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 155

an almost neutral buffer solution that contains divalent cations such as Mg 2C ,
are heated in order to denaturate, and then are gradually cooled such that ssDNA
self-assembly by adopting the most energetically favorable conformation reached
when the oligonucleotides find the (partially) complementary strands and hybridize.
Hybridization takes place under mild environmental conditions and can be reversed.
It is important that all hybridized components of a tile have the same hybridization
efficiencies. The final motif depends on both tile geometry, which determines the
unit cell of the pattern, and sticky ends, which impose the connectivity between
tiles and, in particular, the dimensionality of the final structure ( Lin et al. 2009 ).
Self-assembly is not, however, a perfect process: it occurs always with a less-than-
unity efficiency.
It should be mentioned that, apart from symmetric and repetitive architectures
based on interconnected tiles, two-dimensional structures with areas as large as
8;500 nm 2 consisting entirely of distinguishable nucleotides can be obtained by the
DNA origami process ( Kuzuya and Komiyama 2010 ). In this case, the scaffold runs
back and forth through the whole area and multiple parts in the scaffold are bound
by staple strands. Alternatively, DNA origami can be considered as a folded long
ssDNA with a resulting shape determined by multiple helper strands.
To use DNA structures as scaffolds for inorganic molecules in nanoelectronic
applications, it is necessary to immobilize these molecules on DNA strands,
which is achieved either by hybridization between target molecules labeled with
DNA strands complementary to that of extensions in the DNA scaffold or by
chemically/covalently linking functionalized target molecules with corresponding
functional groups on the scaffold ( Lin et al. 2009 ).

5.2 Self-Assembled DNA Nanowires

In general, bare DNA has semiconducting properties, its intrinsic conductivity being
too low to render it useful as molecular wire. Therefore, metallization of DNA
strands is required to increase DNA conductivity. DNA metallization is one of
the most common examples of nanotechnologies involving biomolecular templates
and among the first to be implemented. In most cases, the metallization process
consists of producing first metallic clusters on DNA, which act as nucleation sites
for the following selective deposition of a metal layer. The deposition process,
which implies the growth of preformed nucleation sites, continues until the gaps
between metallic clusters are bridged and a continuous metallic wire is obtained.
The metallic nucleation sites are either small metallic particles bound to DNA
or bounded metal complexes or ions, which are transformed to metallic clusters
by a reduction process. Uniform metal-coated DNA molecules can then bind
to macroscopic electrodes by specific molecular recognition. In general, DNA
metallization is accompanied by a loss of DNA structural details, and the nanowires
are typically several times thicker than the DNA template. Various methods used
to metallize and manipulate (stretch and position) DNA nanowires are reviewed in
Gu et al. ( 2006 ).