Overlapping plasmonic fields, created by positioning two gold nanoparticles on DNA origami, form a nanolens that acts as a nanoantenna capable of enhancing the fluorescent emission of a single molecule, researchers in Germany report. Drs. Guillermo Acuna and Philip Tinnefeld of the Institute for Physical and Theoretical Chemistry at the Technical University Braunschweig led the interdisciplinary group that developed the nanolenses by self-assembly, using DNA as a construction material. Developed in 2006, DNA origami structures are made up of a single strand of viral DNA, known as the scaffold, and several long "staple" strands that bind to specific regions of the scaffold, prompting the DNA to fold into a predetermined shape. The DNA origami nanopillar (in gray) immobilized on a coverslip. Two 80- to 100-nm-diameter gold nanoparticles serve as nanoantenna and focus the light in the hotspot between the nanoparticles. A fluorescent dye attached in the hotspots acts as an active optical source and reports on the fluorescence enhancement. Courtesy of TU Braunschweig. The team used DNA origami to assemble two gold nanoparticles next to a fluorescent dye trapped within the structure. When the two gold nanoparticles came close together, their overlapping plasmonic fields created the nanolens region that could help scientists visualize single molecules. Although other techniques are currently used for this purpose, they are complex and expensive. The team says that its self-assembly approach can produce many nanolenses quickly and cheaply. In nanophotonics, it is known that a pair of gold nanoparticles can focus light to a spot one thousandfold smaller than conventional lenses, the focusing ability of which is limited by diffraction. Such tight focusing has the potential to advance nanoscale signal processing in optical computers as well as boost the sensitivity of biotech applications such as DNA sequencing. The challenge has been in placing the gold particles, with their dimensions of 80 to 100 nm, at a defined distance and to bring molecules to study into the "hotspot" between the particles. Analogy between a conventional lens (left) focusing a light beam and the nanolens (right) made with two spherical gold nanoparticles on a DNA origami pillar structure. The nanolens can focus the light beam between the particles in an extremely reduced volume. Courtesy of Agustin Acuna. The scientists are confident that their technique might have an impact on a broad range of research disciplines. "Concentrating the light into very reduced volume in the zeptoliter range allows us to perform studies on individual objects with better signals and at higher concentrations where biologically relevant processes like DNA replication occur," Tinnefeld said. "Additionally, we can now investigate how light interacts with nanoparticles, a key component for the field of nanophotonics." The research appears in Science (doi: 10.1126/science.1228638). For more information, visit: www.tu-braunschweig.de For more on nanoplasmonic self-assembly techniques, see: Self-Assembling Optical Legos