Biomedical Engineering Reference
In-Depth Information
(domain-selective) DNA (or nanoparticle) attachments on an isotropic nano-object
by virtue of the spatial confinement of a solid substrate, which lowered the
spherical symmetry of a nanoparticle. These researchers modified the surface of
a commercially available paramagnetic bead with a monolayer of DNA strands
through biotin-streptavidin interaction. As well, gold nanoparticles were coated
with a mixed layer of two different DNA sequences via gold-thiol bonding. One
of the DNA sequences on the gold nanoparticles was complementary to a capture
DNA on the magnetic bead, while the other provided a freedom to hybridize with a
linker DNA in a following step. Upon hybridization with the DNA on the magnetic
bead, the gold nanoparticle was able to touch the magnetic bead by its one face.
This allowed the linker DNA to be attached on the unhindered part (opposite to
the magnetic bead touched area) of the gold nanoparticle. The asymmetrically
decorated linker DNA domain on the gold nanoparticle was then able to recognize
another gold nanoparticle bearing a homogeneous layer of complementary DNA
so that dimeric nanoparticle clusters were formed on the magnetic bead. The
dimer structure could be cleaved from the magnetic bead based on a DNA-strand-
displacement reaction. In another experiment, a larger gold nanoparticle (e.g., 50 nm
in diameter) was adopted, which was similarly decorated with multiple DNA linkers
on one patch of its surface. The 50 nm nanoparticle carrying the linker DNA strands
was then released from the bead, followed by the attachment of multiple smaller
gold nanoparticles (11 nm) through DNA hybridizations, resulting in two-faced
anisotropic Janus particles. Such a surface-assisted stepwise process remarkably
improved the assembly efficiencies of symmetric dimer clusters and asymmetric
Janus nanoparticles.
Besides the asymmetric DNA decorations, it is highly desirable to control the
placement and the relative orientation of DNA ligands on a nanoscale building
block. This will result in a rationally assembled structure with fine-controlled bond-
ing valence and geometry. Recently, Kim et al. presented a method that was able to
control the number, placement, and relative orientation of up to six DNA linkers on a
nanoparticle (Fig.
8.2
)[
32
]. The researchers employed a stepwise process for ligand
attachment and relatively stiff (shorter than the 50 nm persistence length of a DNA
duplex) DNA linkers during the nanoparticle assembly. The idea was somehow
analogous to the valence-shell electron-pair repulsion (VSEPR) model, where a
special geometrical configuration of chemical bonds must satisfy a minimized
repulsion between bonding and nonbonding electron pairs surrounding a central
atom. The method by Kim et al. relied on the assistance of silica particles that were
functionalized with carboxylic groups. The DNA ligand was modified with an amino
group at one end and a thiol group (persulfide) at the opposite end. The positively
charged amino terminus of the DNA strand could bind to a negatively charged
carboxyl group on the silica particle via electrostatic attraction. After washing away
excess DNA ligands, the thiol (persulfide) tag at the far end of the immobilized DNA
was activated by a chemical reductant (DTT) so that an AuNP was able to catch the
thiol group. The DNA linked with a gold nanoparticle via gold-thiol bonding was
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