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 US 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. Gold triangle nanoparticles paired tip-to-tip in a bow-tie formation serve as optical antennas. When a protein (green) bound to a fluorescently labeled SOS catalyst passes through the gaps between opposing tips of the triangles (plasmonic hot spots), fluorescence is amplified. (Berkeley Lab) “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. “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.” 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 range of 5-100 nm. An array of gold nanoparticles in the shape of triangles that are paired in a tip-to-tip formation, like a bow tie, can serve as optical antennas, capturing and concentrating lightwaves into well-defined hot spots, where the plasmonic effect is greatly amplified. (Image: Groves et al, Berkeley Lab) 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. An array of gold nanoantennas laced into an artificial membrane enhances the fluorescence intensity of three different molecules when they pass through plasmonic hot spots in the array. Watch for the blue, green and red flashes. The photobleaching at the end of each fluorescence event (white flashes) is indicative of single-molecule observations. “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,” Groves said. “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. For more information, visit: www.lbl.gov