DNA-Directed Assembly of Nanophase Materials: An Updated Review - DNA Nanotechnology: From Structure to Function - page 155

template requires a high-ionic-strength buffer to maintain its structural integrity.
Well-defined 2D tetragonal arrays of gold nanoparticles with easily tuneable
interparticle spacings were successfully assembled. Seeman and coworkers realized
the importance of structural rigidity of a DNA nanoscaffold in order to achieve a
high-fidelity replication of a periodical DNA lattice into well-defined nanoparticle
arrays. They designed a triangular DNA motif with rigid DX (DX D double
crossover) edges and used them to direct nanoparticle assembly [ 24 ]. Such a rigid
motif offered significantly improved control on the interparticle spacings in the
as-obtained two-dimensional nanoparticle arrays, as evidenced by TEM imaging,
highlighting the crucial role of the structural rigidity of a DNA supramolecular
template.
Apart from a periodical gold nanoparticle array, Alivisatos group took a step
forward by placing four gold nanoparticles with different sizes on the vertices of
a DNA tetrahedron so that a chiral nanoparticle superstructure could be obtained
[ 25 ]. Helical assembly of gold nanoparticles provides another way to form chiral
nanoassemblies. For example, Sharma and Yan et al. achieved a helical assembly
of gold nanoparticles on a DNA nanotubule and found that the gold nanoparticles
could in turn regulate the conformations of the DNA tubules via a steric hindrance
interaction related to the sizes of the nanoparticles [ 26 ]. Ding et al. reported a
rolling-up strategy for the preparation of a gold nanoparticle helix with the use of a
DNA origami template [ 27 ]. These experiments clearly demonstrated the possibility
of using DNA-guided self-assembly to build chiral metamaterials that may result in
important optical applications.

8.3 Recent Methodological Developments

in DNA-Programmed Nanoparticle Assembly

Despite the aforementioned achievements in DNA nanotechnology, which have
clearly verified the irreplaceable role of DNA in organizing inorganic nanophase
materials, some challenges still exist. For example, it is still hard to achieve a site-
specific attachment of DNA ligands on the surface of a spherical nanoparticle. This
is especially important when a DNA multifunctionalized nanoparticle is employed
as an assembly motif, which requires not only a control on the numbers of nearest
neighbors of the nanoparticle but also a desired DNA bonding direction. On the
other hand, orientation-controlled alignments of one-dimensional nanomaterials
including nanotubes, nanowires, and nanorods on a 2D DNA lattice need to be
realized toward electrical measurements and inter-device connections. One other
challenge is the preparation of asymmetrically functionalized nanoparticles (Janus
particles) to obtain another dimension of control in DNA-guided self-assembly.
The past several years have witnessed some important methodological break-
throughs that provide great promises to solve the problems we are currently being
challenged.