Building with optical Legos

Krista D. Zanolli, krista.zanolli@photonics.com

Just as generations of children have enjoyed stacking and building with plastic Lego toys, scientists are now using light-activated nanoshells as Lego-like building blocks for both 2- and 3-D structures that could be used in chemical sensors, metamaterials and nanolasers.

In a collaborative effort, scientists from Rice University, Harvard University, the University of Texas at Austin and the University of Houston are using this self-assembly method to build complex structures that can trap, store and bend light.

“We used the method to make a seven-nanoshell structure that creates a particular type of interference pattern called a Fano resonance,” said Peter Nordlander, professor of physics and astronomy at Rice. “These resonances arise from peculiar light wave interference effects, and they occur only in man-made materials. Because these heptamers are self-assembled, they are relatively easy to make, so this could have significant commercial implications.”

The research team was led by Harvard University applied physicist Federico Capasso, whose efforts to fabricate the structure were spurred by Nordlander’s 2008 prediction that a heptamer of nanoshells would produce Fano resonances.

Artist’s rendition of two types of optical circuits: The three-particle trimer functions as a nanoscale magnet, while the seven-particle heptamer exhibits almost no scattering for a narrow range of wavelengths, due to interference. Courtesy of the laboratory of Federico Capasso, Harvard School of Engineering and Applied Sciences.

Coated with polyethylene glycol, the nanoshells are assembled into clusters when a droplet of particles is dried on a hydrophobic substrate. When the initial droplet evaporates, it breaks down on the surface of the substrate into smaller droplets, some of which contain three or seven nanoshells. As the droplets completely evaporate, capillary forces pack the nanoshells together into a cluster, and they remain held together by van der Waals forces.

Because of the Fano resonance pattern, the new materials can manipulate light in bizarre ways that no natural material can. According to Nordlander, the new materials are ideally suited for making ultrasensitive biological and chemical sensors. He added that they also might lend themselves to nanolasers and integrated photonic circuits that run off of light rather than electricity.

The spherical nanoshells, which are roughly one-twentieth the size of red blood cells, consist of a glass center and are coated with gold. By varying the size of the glass center and the thickness of the gold shell, researchers can create nano-shells that interact with specific wavelengths of light.

“Nanoshells were already among the most versatile of all plasmonic nanoparticles, and this new self-assembly method for complex 2-D and 3-D structures simply adds to that,” said nanoshell inventor Naomi Halas, Rice’s Stanley C. Moore professor of electrical and computer engineering. She has also helped develop a number of biological applications for nanoshells, including diagnostics and a minimally invasive procedure for treating cancer.

Halas’ extensive work in the field of photonics has led to human trials for cancer treatment involving near-infrared light heating gold nano-shells, which target and destroy cancerous tumors without affecting the surrounding healthy tissue.

Halas and collaborator Jennifer West, the Isabel C. Cameron professor of bioengineering, founded Nanospectra Biosciences Inc., a Houston company based on the patented light-activated gold nanoshell technology.

“It has been highly gratifying to see our discovery move from the research laboratory into the commercial sector, where it is now helping cancer patients,” West said.

In early June, the State Bar of Texas designated the two “Inventors of the Year” for their patented process. Halas and West are the first women to win the award since its inception in 1983. Theirs is also the first nanotechnology-based invention to win.