Optical switch developed from single molecules

Green light for nanoelectronics

A DNA breadboard is used to accurately position individual dye molecules to nanometers. Using a green "Springer" dye, light is directed either to the red or infrared starting dye, as visualized by four-color single-molecule spectroscopy. © JACS / LMU / NIM
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In nanophotonics, the behavior of light in the nanometer range is investigated and manipulated. In the future, for example, light could take on the role of electrical currents in optical circuits. In a small space, these optical circuits have the potential to far exceed the performance and operating speed of electronic circuits. Physicists have now shown in a novel approach how the propagation direction of light or light energy can be manipulated at the level of individual molecules.

The biophysicists placed a cascade of four different fluorescent dye molecules on a nanoscale DNA platform. Using a so-called "Springer" dye, they managed to control the direction of the light path or energy transfer. The success of this strategy could be visualized with a new four-color single-molecule technique, the researchers report in the journal "Journal of the American Chemical Society" (JACS).

Control light on the nanoscale

To control light on the nanoscale requires new optical components that act as wires and switches. As a kind of wire, the energy transfer between individual dyes could act. In nature, there is already a prominent example of this transfer: In photosynthesis, light energy is transported in light-harvesting complexes between molecules.

The principle of this so-called fluorescence resonance energy transfer (FRET) was used by the team of the Ludwig Maximilian University of Munich (LMU) headed by Professor Philip Tinnefeld, who has since moved to the Technische Universität Braunschweig, to transfer light from a fluorescent dye molecule to a fluorescent dye molecule conduct. For this purpose, the scientists use dyes that have their absorption maximum in the blue, green, red and infrared wavelengths.

Tiny breadboard in use

For the molecules to interact with each other, for example in artificial light circuits, they must be only about five nanometers apart. The scientists succeed in doing so with the help of a tiny plug-in board, for which they use the biomolecule DNA as building material. First, they bind each dye molecule to a short artificial DNA strand. These loaded sections and about 200 additional short DNA strands then serve as a kind of staples: They help a single, very long DNA thread to fold itself into a two- or even three-dimensional structure. display

This is predefined in such a way that the dye molecules optimally look out of the "DNA carpet", which is typically less than 100 nanometers by 100 nanometers in size. The targeted use of this molecular self-assembly and folding is called "DNA Origami", based on the Japanese Papierfalt technique.

DNA as a carrier material combined with four-color single-molecule spectroscopy

In the experiment, biophysicists now first stimulate the blue input dye with the appropriate wavelength of light. This is then transferred a portion of the excitation energy by means of FRET as fluorescence radiation to a nearby other dye . And here sits in the truest sense of the word the clou of the proposed breadboard design, the green "Springer" dye. Depending on where it is positioned, it directs the light energy either in the direction of the red or in the direction of the infrared "exit" dye. Which way was taken shows the color of the output signal.

In this novel approach, the scientists around Tinnefeld for the first time combined the use of DNA as carrier material with four-color single-molecule spectroscopy to visualize the switching of energy transfer pathways. The DNA origami objects generally have numerous binding sites for anchoring other molecules and can thus be considered as a molecular breadboard or "nanoplate". According to the scientists, the presented four-color spectroscopy with alternating laser excitation can also provide comprehensive information about objects on the nanoscale, both about their structure and their interactions.

Highly sensitive analytics

The new method, according to the scientists, is also suitable for highly sensitive analysis. For this they can construct the system so that they can already detect the binding of individual molecules of a sought-after substance via light signals. (Journal of the American Chemical Society, 2011; http://pubs.acs.org/doi/abs/10.1021/ja1105464

(University of Munich, 01.04.2011 - DLO)