Scientists at the U.S. Department
of Energy's Brookhaven National Laboratory have found a new way to use a
synthetic form of DNA to control the assembly of nanoparticles - this
time resulting in switchable, three-dimensional and small-cluster structures
that might be useful, for example, as biosensors, in solar cells, and as new
materials for data storage. The work is described in Nature Nanotechnology,
published online December 20, 2009.
These illustrations show how a 3-D crystal made from nanoparticles changes between two distinct states via an intermediate structure (top row, middle) when looped (left) versus unlooped (right) double-stranded DNA chains are used to link the particles. The scientists were able to measure the distance between the particles in each structure by recording x-ray scattering patterns (bottom row). Switching from looped to unlooped DNA increased the interparticle distance by about 6 nanometers.
The Brookhaven team, led by physicist Oleg Gang, has been refining techniques
to use strands of artificial DNA as a highly specific kind of Velcro or glue
to link up nanoparticles. Such DNA-based self-assembly holds promise for the
rational design of a range of new materials for applications in molecular separation,
electronics, energy conversion, and other fields. But none of these structures
has had the ability to change in a programmable manner in response to molecular
stimuli - until now.
"Now we're using a special type of DNA-linking device - a kind
of 'smart glue' - that affects how the particles connect to make
structures that are switchable between different configurations," says
Gang. This reliable, reversible switching could be used to regulate functional
properties - for example, a material's fluorescence and energy transfer
properties - to make new materials that are responsive to changing conditions,
or to alter their functions on demand.
Such responsiveness to changes in environmental conditions and the ability
to adopt new forms are hallmarks of living systems. In that way, these new nanomaterials
more closely mimic biological systems than any previous nanostructures. Though
far from any form of truly "artificial life," these materials could
lead to the design of nanoscale machines that, at a very simple level, mimic
cellular processes such as converting sunlight into useful energy, or sensing
the presence of other molecules. Responsive materials would also have benefits
in the field of optics or to produce regulated porous materials for molecular
separations, Gang says.
The scientists achieved the goal of responsiveness by creating structures where
the distance between nanoparticles could be carefully controlled with nanometer
accuracy.
"Many physical characteristics of nanomaterials, such as optical and
magnetic properties, are strongly dependent on the distance between nanoparticles,"
Gang explains.
In their previous studies, the scientists used single strands of DNA attached
to individual nanoparticles as linker molecules. When the free ends of these
DNA strands had complementary genetic code, they would bind to attach the particles.
Constraining the interactions by anchoring some of the particles on a surface
allowed the scientists to reliably form a variety of structures from two-particle
clusters (called dimers) to more complex 3-D nanoparticle crystals.
In the new work, the scientists have added more complicated double-stranded
DNA structures. Unlike the single strands, which coil in uncontrollable ways,
these double-stranded structures are more rigid and therefore constrain the
interparticle distances.
Additionally, some of the strands making up the double-stranded DNA molecules
have complicated structures such as loops, which pull the bound particles closer
together than when both strands are exactly parallel. By varying the type of
DNA device, between looped and unlooped strands, and measuring the interparticle
distances using precision techniques at Brookhaven's National Synchrotron
Light Source (NSLS) and at the Center for Functional Nanomaterials (CFN), the
scientists demonstrated that they could effectively control the distance between
the particles and switch the system from one state to another at will.
The approach resulted in two-configuration, switchable systems both in dimers
and nanocrystals, with a distance change of about 6 nanometers - about
25 percent of the interparticle distance. By comparing kinetics in the two systems,
they found that the switching between states is faster in the simpler, two-particle
system. The dimers also retain their ability to return to their initial state
more precisely than the 3-D crystals, suggesting that molecular crowding may
be an issue to further investigate in the 3-D materials.
"Our hope is that the ability to induce post-assembly reorganization
of these structures by adding DNA or other molecules as external stimuli, and
our ability to observe these changes with nanometer resolution, will help us
understand these processes and find ways to apply them in new kinds of nanomachinery
in which the system's functionality is determined by the nanoparticles
and their relative organization," says Gang.
Future studies will make use of precise imaging capabilities, such as advanced
electron microscopy tools at the CFN and higher-resolution x-ray techniques
that will become available at Brookhaven's new light source, NSLS-II,
now under construction.