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NANOMATERIALS
DNA brings quantum dots to order
Semiconductor quantum dots coated with strands of DNA can self-assemble into a variety of structures.
Yan Liu
A
Size / colour
FEI ZHANG / ARIZONA STATE UNIV.
Valency
n important goal in nanotechnology
is to construct elaborate superassemblies of nanoscale components
while maintaining strict control over the
organization of these building blocks.
Among the various types of potential
building blocks are quantum dots, which
are the focus of much research because
they have unique optical and electronic
properties that could prove useful in
applications such as photovoltaics, lightemitting devices and medical imaging 1.
The primary challenge when constructing
super-assemblies from quantum dots is to
control the number of binding sites on the
surface of the dots while simultaneously
maintaining their physical stability and
useful properties. Writing in Nature
Nanotechnology, Shana Kelley, Ted Sargent
and co-workers2 at the University of
Toronto report that quantum dots coated
with DNA ligands can be reliably organized
into complex super-assemblies.
DNA is a polymer in which various
bases are attached to a backbone made
of sugars and phosphate groups held
together by phosphodiester bonds. There
are four different bases (adenine, cytosine,
guanine and thymine), and DNA can be
programmed to form different two- and
three-dimensional structures because
adenine always pairs with thymine, and
cytosine always pairs with guanine.
Similarly, any material that can be
attached to DNA can take advantage of
this complementary base pairing to form
precisely designed structures.
Kelley and coworkers2 devised a simple
but clever one-pot method to synthesize
cadmium telluride quantum dots that
had a discrete number of DNA ligands
attached to their surface, with molecules
of mercaptopropionic acid (MPA) acting
as a co-ligand and occupying the sites left
open by the DNA. The dots were stable
and had good optical properties, and
their size (and hence the wavelengths
at which they absorb and emit light)
could be controlled by adjusting the
experimental conditions.
The DNA ligands consisted of three
domains: a domain containing between
5 and 20 guanine bases that was attached
Figure 1 | DNA-programmed self-assembly of quantum dots. Kelley and co-workers2 synthesized
quantum dots of different sizes and with different numbers of DNA ligands attached them (left). Both
the size of the dots and the number of ligands (the valency) can be controlled by varying the reaction
time and the length of the dot-binding domain in the DNA ligands. The structure on the right is a ternary
complex containing a large symmetric four-valent quantum dot at its centre (red); this is surrounded by
four medium-sized asymmetric bivalent dots (orange) and four small monovalent dots (green). In this
example, one of the two DNA-binding domains attached to the orange dot is complementary to the DNAbinding domains attached to the green dots, and the other is complementary to those attached to the
red dots (which is why the orange dot is said to be asymmetric bivalent). The red and green dots cannot
bind to each other because their DNA-binding domains are not complementary. Such assemblies of
nanoparticles can be used to study the transfer of energy between quantum dots (curved black arrows).
to the quantum dot (shown in purple
in Fig. 1); a spacer region (blue); and a
domain in which the sequence of bases was
chosen by the researchers (yellow or pale
pink). The backbone of the quantum-dotbinding domain contained 5–20 sulphur
atoms because it was held together by
phosphorothioate bonds, rather than the
normal phosphodiester bonds (which do
not contain sulphur). These sulphur atoms
reacted with the cadmium atoms at the
surface of the quantum dot to form a CdS
shell around the CdTe core; this monolayer
of CdS ensured that the dot was stable and
that the fluorescence-emission efficiency
was high. The presence of the guanine
bases also helped to secure the quantumdot-binding domain to the surface of
the dot. The backbones of the other two
domains were held together by normal
phosphodiester bonds, leaving the DNA
binding domain (yellow) free to hybridize
with complementary DNA sequences.
NATURE NANOTECHNOLOGY | VOL 6 | AUGUST 2011 | www.nature.com/naturenanotechnology
© 2011 Macmillan Publishers Limited. All rights reserved
The key to achieving strict control
over the organization of the quantum
dots in any super-assembly is to control
the ‘valency’ of the dots (that is, the
number of DNA ligands attached to their
surface). Kelley and co-workers found
that the valency was 1 when there were 20
phosphorothioate linkages in the quantumdot-binding domain, and that it increased
to 4 or 5 as the number of linkages was
reduced to 5 (depending on the size of the
dot). The right panel of Fig. 1 illustrates
the complexity of quantum dot assemblies
that can be constructed with high yield;
quantum dots with asymmetric bivalency
can also be seen.
Kelley and co-workers then studied
the transfer of energy between the
quantum dots within these assemblies. In
particular, they found that the colour of the
fluorescent emission from a linear ternary
complex could be reversibly switched by
changing the pH of the solution. This can
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be explained by a reversible protonation
and deprotonation of the MPA co-ligand
in neutral and alkaline pH. These results
suggest that the programmed quantum
dot complexes could be used to monitor
enzymatic reactions that involve a pH
change in cellular compartments. An
obvious future direction is to explore
quantum dot complexes that can respond
to other environmental triggers, such
as changes in ion concentrations or the
presence of biomarkers. The strategy
developed by Kelley and co-workers
could also be readily used to create hybrid
structures containing both semiconductor
quantum dots and metallic nanoparticles,
which would allow plasmonic interactions
between the two to be studied in detail3.
Challenges for the future include
developing a better understanding of how
the experimental conditions and other
factors (such as the length of the various
domains in the DNA ligands) influence the
size and valency of the quantum dots, and
extending the approach to other materials
systems to make quantum dots that are
less toxic and emit over a wider range of
wavelengths. Combining this approach
with DNA scaffolds4 would increase control
over long-range order, which would allow
for the creation of bigger assemblies.
Overcoming these challenges could lead to
numerous applications in nanophotonics
and nanoelectronics.
❐
Yan Liu is at the Biodesign Institute and in the
Department of Chemistry and Biochemistry, Arizona
State University, Tempe, Arizona 85287, USA.
e-mail: [email protected]
References
1. Choi, C. L. & Alivisatos, A. P. Annu. Rev. Phys. Chem.
61, 369–389 (2010).
2. Tikhomirov, G. et al. Nature Nanotech. 6, 485–490 (2011).
3. Jin, Y. & Gao, X. Nature Nanotech. 4, 571–576 (2009).
4. Seeman, N. C. Annu. Rev. Biochem. 79, 65–87 (2010).
GRAPHENE ELECTRONICS
Thinking outside the silicon box
The best applications of graphene will be those that exploit its basic characteristics, rather than try to change them.
Tomás Palacios
G
raphene, a single layer of
carbon atoms arranged in a
honeycomb lattice, is on its way
to revolutionizing electronics. The
combination of perfect charge-carrier
confinement with very high carrier
mobility in a two-dimensional structure
makes it an ideal material for highly
scaled electronics. These properties are
attracting more and more electrical
engineers to the ‘flatland’ of graphene,
and the effort is paying off 1. Recently,
transistors with frequency performance in
excess of 300 GHz have been reported2 and
companies and research groups around the
world are engaged in a race to demonstrate
circuits of increasing complexity 3–5.
However, in spite of this progress, the
most promising applications of graphene
in electronics for the immediate future are
unlikely to involve competing directly with
conventional transistors.
Although graphene transistors are
very fast, at present they lack traditional
current saturation, have poor on–off
current ratios, and are much slower
(typically 100 times) in real circuit
applications. These serious limitations
are the result of graphene not having a
bandgap, and they are often overlooked
in the fast-moving world of research.
They mean that graphene transistors,
although excellent tools to probe the
transport properties of this material, still
have a long way to go to compete with
state-of-the-art silicon and compound
semiconductor electronics.
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Infrared photons
Doped silicon
Metal
Silicon
Dielectric Si MOSFET
Figure 1 | Schematic diagram of an infrared detector in which graphene nanoribbons (top) are integrated
onto a prefabricated silicon chip. Graphene-based ambipolar electronic devices and various types of
sensor could also be readily integrated onto silicon chips. Figure used and modified with permission from
Kyeong-Jae Lee, MIT.
As I argued at a recent conference in
Boston organized by Nature Publishing
Group, ‘Graphene: The Road to Applications’,
amplifiers and digital logic may not be the
best applications for this amazing material,
at least in the short term. In the past few
months, there have been several proposals
for graphene devices and circuits that attempt
to exploit the fundamental properties of the
material, including its zero bandgap, rather
than trying to overcome them. Examples
include ambipolar electronics5–10, infrared
detectors11,12 and heterogeneous graphene–
silicon systems13.
Ambipolar electronics is based on a new
family of nonlinear devices that rely on the
fact that the conduction and valence bands
in graphene meet at a point called the Dirac
point. This band structure makes it possible
to continuously sweep the Fermi level from
the conduction band to the valence band by
just changing the gate voltage of a fieldeffect transistor. This in turn changes the
channel conduction from electron-based to
hole-based and gives graphene transistors
their characteristic V-shape transfer
curve. This ambipolar conduction, which
is responsible for the poor performance
NATURE NANOTECHNOLOGY | VOL 6 | AUGUST 2011 | www.nature.com/naturenanotechnology
© 2011 Macmillan Publishers Limited. All rights reserved