John Bahns, Subramanian Sankaranarayanan, Liaohai Chen, and Stephen Gray find new way to assemble nanoparticles. Credit: Argonne National Laboratory.
For many years, scientists
have searched for ways to assemble nanoparticles into larger structures of any
desired shape and form at will. This effect has been achieved in a new study by
using a laser as if it were a magic wand, creating an assembled, continuous
filament as the laser beam is moved around.
“It’s possible that we
could use this method to encapsulate pharmaceutical agents for new drug
delivery systems or build cathodes with very large surface areas for use in
batteries,” said Argonne biophysicist John Bahns, who led the invention of
the technology. “It could potentially help us find better materials that
could be used in everything from catalysts to semiconductors; the possibilities
Researchers at Argonne
National Laboratory shined a low-power laser—similar in intensity to ones used
in office laser pointers—into a solution of gold and carbon nanoparticles
suspended in water. Unexpectedly, they found that the carbon nanoparticles
decomposed or deformed to create a kind of glue that enabled the creation of
long gold and carbon chains that assembled continuously wherever the laser was
This new technique for
materials design, known as optically directed assembly or ODA, could provide
scientists and inventors with an uncharted route to new materials, technologies,
and even treatments for diseases. “It’s incredibly exciting to think about
the vast world of technology that could result from people using ODA,”
said Liaohai Chen, who helped to develop the technology. “This is just the
very beginning; we really don’t even know yet all the things that might be
The research leading to the
discovery of ODA sought to develop new methods for imaging dynamic biological
processes in living systems. “ODA provides a new way to encapsulate metal
particles with inert carbon, which can be harnessed to generate imaging probes
for studies of biological systems important to bioenergy research or medical
diagnostics,” Chen said. “ODA opens a new avenue to synthesize a new
generation of nanoparticle-based imaging probes (especially for the metal
isotopes) and therapeutic agents with a simple, inexpensive, safer and greener
(energy efficient) process” said Chen.
“It could potentially
help us find better materials that could be used in wide application spectrum
ranging from catalysts, drug delivery to semiconductors,” said Bahns.
The difference between ODA
and other light-based experiments in materials design lies in the fact that ODA
involves the creation of what Chen called a “structure within a
structure.” Rather than creating a completely continuous material like a
sheet of aluminum foil, ODA forms a larger coherent structure from the
individual nanoparticles. “You can think of it like going to the beach and
pointing a stick at the sand,” Chen said, “and then all of a sudden
having pebbles gather and join together wherever you decided to point the
“The laser basically
acts kind of like a pen, as opposed to a stylus, creating a thread of gold and
carbon as it’s moved along,” added Argonne
nanoscientist Subramanian Sankaranarayanan, who collaborated on the research.
“It surprised us that such a low-power laser could have such a big
Sankaranarayanan and Stephen
Gray of Argonne’s Center for Nanoscale
Materials have also developed a predictive computational model of the ODA
Optically directed assembly
works because the laser heats up the spot onto which it is focused, causing a
phenomenon known as convective flow in which the solution travels around the
hot spot. The action of the flow combined with laser heating brings the
particles together, creating the filaments.
The discovery of the ODA
technique happened by accident. Bahns and Chen were investigating carbon in
soil by using a technique called Raman spectroscopy.
The researchers added gold
nanoparticles to their sample because these particles are known to boost Raman
signals. Because Raman spectroscopy requires the use of a laser, the researchers
found that gold-carbon chains would form wherever they moved the laser.
“It looked almost like an Etch-a-Sketch,” Chen said.
Based on the researchers’
observations, ODA can also provide a new way to encapsulate metal particles
with inert carbon, which researchers believe will eventually allow for the
creation of new probes that would investigate the uptake of different molecules
by both human and plant tissues.
According to Gray, ODA
helps to bridge a gap that has existed in materials design between structures
of different sizes. “There’s a grand challenge to extend nanoscale
phenomena to the millimeter level,” Gray said. “ODA works best in the
region between the two called the ‘mesoscale,’ and so we believe that we have a
lot of different opportunities to explore with it.”
An article based on the
study appeared in Physical Review Letters.