Biomolecular Architecture for Nanotechnology - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 162

between stacked two-dimensional lipid layers. Then, arrays of CdS nanorods with
tunable crystallographic orientation and widths, for example, can be obtained by
placing Cd 2C ions into the interhelical pores situated between strands followed by
a reaction with H 2 S( Liang et al. 2003 ). The CdS (002) polar planes are aligned
with the negatively charged DNA backbone due to strong electrostatic interactions,
the 60 ı tilt angle between (002) planes and the rod axis having no counterpart
in nanorods fabricated from II to VI semiconductors. As the charge density of
lipid membranes changes, the inter-DNA spacing can be tuned between 2.5 and
5.7 nm, the type of DNA having no significant influence on the semiconducting
nanorod array. However, the inter-DNA spacing decreases drastically for a large
Cd 2C concentration, for which the DNA strands condense.
Three-dimensional crystals with applications in X-ray optics have also been
fabricated using the Cowpea mosaic virus extracted from the leaves of California
blackeye as template ( Prasad et al. 2006 ). This virus can crystallize in two structures:
a body-centered-cubic structure with a lattice constant a D 31:7 nm and a hexagonal
structure with lattice constants a D 45:1 nm and c D 103:8 nm. By introducing
palladium and platinum inside the body-centered-cubic viral crystal using the
electroless deposition method, the interconnected void spaces are filled with metal.
A 1 m-thick film of these metal-infiltrated three-dimensional viral crystals with
unit cell dimension in the X-ray optic range has a maximum reflectivity of 7% for
X-rays with a wavelength of 35 nm incident on the f 111 g crystal face.
Quantum dot arrays with controlled periodicity have been fabricated on func-
tionalized three-dimensional DNA origami nanotubes, constructed by folding a
single-stranded M13 mp18 scaffold into a six-helix nanotube bundle by 170
different staple strands; the resulting nanotubes have a diameter of 6 nm and a
length of 412 nm and were dispersed on mica substrates for AFM monitoring of
the resulting structures ( Bui et al. 2010 ). A programmable quantum dot pattern can
be achieved by incorporating biotin-labeled staple strands at specific evenly spaced
binding sites along the nanotube axis, on which streptavidin-coated CdSe/ZnS core-
shell quantum dots can then bind. Successful fabrication of DNA nanotubes with
5, 9, 15, and 29 binding sites has been reported, corresponding to periodicities of
71, 43, 29, and 14 nm, respectively, but quantum dot attachment on the last two
structures was not entirely successful due to quantum dot bridging of multiple sites
or steric hindrance. Figure 5.11 illustrates an array of quantum dots bound on a DNA
nanotube.
Because DNA origami is synthesized in solution, random arrangements can
result from uncontrolled deposition on surfaces. A technique to create selective
binding sites for DNA origami on SiO 2 and diamond-like carbon surfaces, which
are of interest in nanotechnology, is described in Kershner et al. ( 2009 ). It involves
quantum dots
Fig. 5.11 Array of quantum
dots bound to a DNA
nanotube