This ‘Etch-a-Sketch’ is laser guided

U. CHICAGO (US) — A low-power laser—similar to the common laser pointer—can cause gold and carbon nanoparticles to assemble into long chains that follow the laser beam as it moves.

“You can think of it like going to the beach and pointing a stick at the sand,” says Liaohai Chen, an Argonne National Laboratory researcher who helped develop the technology, “and then all of a sudden having pebbles gather and join together wherever you decided to point the stick.”

“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,” adds Subramanian Sankaranarayanan, who collaborated on the research. “It surprised us that such a low-power laser could have such a big effect.”

The researchers shined laser light into a solution of gold and carbon nanoparticles suspended in water, and unexpectedly found the carbon nanoparticles decomposed or deformed to create a kind of “glue.” This enabled the creation of long gold and carbon chains that assembled continuously wherever the laser was pointed. The work is detailed in the journal Physical Review Letters.

For years, scientists have searched for ways to assemble nanoparticles—tiny bits of matter less than a millionth of an inch across—into larger structures of any desired shape. This new assembly technique could allow scientists to do just that.

“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,” says 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 are endless.”

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,” says Chen. “This is just the very beginning; we really don’t even know yet all the things that might be possible.”

Argonne biophysicist John Bahns holds samples of gold and carbon nanoparticles, which when combined show an unexpected and fascinating property under low-intensity laser light. The gold nanoparticles appear red due to a phenomenon known as plasmon resonance. (Credit: Argonne National Laboratory)

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.

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 completely 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 surprisingly found that gold-carbon chains would form wherever they moved the laser.

“It looked almost like an Etch-a-Sketch,” Chen says.

Argonne is managed by the University of Chicago for the U.S. Department of Energy, which funded the study.

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