Plasmonic tool images biomolecules

Ashley N. Paddock, ashley.paddock@photonics.com

For the first time, optical nanoantennas can harness the power of plasmonics to study the dynamics of cell membrane biology. A unique artificial biological platform combines a fluid bilayer of lipid molecules with a fixed pattern of nanoantennas to observe biomolecules in motion. The method offers a tool for investigating the immune system’s cellular signaling network.

Researchers from the Department of Energy’s Lawrence Berkeley National Laboratory and the University of California, Berkeley, developed a technique for lacing artificial lipid membranes with billions of gold “bow tie” nanoantennas. Without touching the nanoantennas, a protein travels through plasmonic “hot spots,” which amplify its fluorescent or Raman optical signal thousands of times.

“Our technique is minimally invasive, since enhancement of optical signals is achieved without requiring the molecules to directly interact with the nanoantenna,” said team leader Jay Groves of Berkeley Lab’s Physical Biosciences division and UC Berkeley’s chemistry department.

Gold triangle nanoparticles paired tip-to-tip in a bow-tie formation serve as optical antennas. When a protein (green dot) bound to a fluorescently labeled SOS catalyst passes through the gaps between opposing tips of the triangles (plasmonic hot spots), fluorescence is amplified. Courtesy of Berkeley Lab.

“This is an important improvement over methods that rely on adsorption of molecules directly onto antennas, where their structure, orientation and behavior can all be altered,” he said.

The team produced billions of gold nanoantennas in a synthetic membrane using a combination of plasma processing and colloidal lithography methods. They created well-defined spacing between each pair of gold triangles in the final array, with a tip-to-tip space between neighboring nanotriangles in the 5- to 100-nm range.

Until now, it was not possible to decouple, or distinguish, the size of the gold nanotriangles – which determine surface plasmon resonance frequency – from the tip-to-tip distance between the individual nanoparticle features. With the team’s colloidal lithography approach, a self-assembling hexagonal monolayer of polymer spheres was used to shadow mask a substrate on which to deposit gold nanoparticles. The removal of the colloidal mask leaves behind large arrays of gold nanotriangles and nanoparticles, upon which the synthetic membrane can be fabricated.

“When we embed our artificial membranes with gold nanoantennas, we can trace the trajectories of freely diffusing individual proteins as they sequentially pass through and are enhanced by the multiple gaps between the triangles,” Groves said. “This allows us to study a realistic system, like a cell, which can involve billions of molecules, without the static entrapment of the molecules.

“The idea that optical nanoantennas can produce the kinds of enhanced signals we are observing has been known for years, but this is the first time that nanoantennas have been fabricated into a fluid membrane so that we can observe every molecule in the system as it passes through the antenna array. This is more than a proof of concept; we’ve shown that we now have a useful new tool to add to our repertoire.”

The work, which was supported by the DoE Office of Science, appeared in Nano Letters (doi: 10.1021/nl300294b).