Nowadays cups of yoghurt have a note telling you that the product contains L(+)-lactic acid, one of two forms that differ in the spatial arrangement of their atoms. The two"isomers", L(+) and D(-), have different "optical activities", turning the plane of polarized light to the right or left respectively. Of course the capacity to alter the polarization, intensity or color of light is not confined to solutions of yoghurt. Light interacts in one way or another with all matter. Now an international team led by LMU physicist Professor Tim Liedl has fabricated a new type of optically active, three-dimensional structures which allow one to tailor the properties of visible light in virtually any desired fashion. Their method is based on "DNA origami": DNA strands that fold spontaneously into precise shapes (determined by their nucleotide sequence) serve as scaffolds for the attachment of gold nanoparticles in predetermined patterns. In their experiments the researchers chose left- and right-handed helical arrangements for the gold particles. By the appropriate choice of parameters, the team was able to tune the interaction of light with the metal particles. This new approach opens up the route to the preparation of self-assembling metamaterials and eventually to the construction of novel types of lenses. The study was performed under the auspices of the Nanosystems Initiative Munich (NIM), a Cluster of Excellence. (Nature online, 14 March 2012)
With the molecular "origami"technique, the researchers can program the basic structure of their material at will by taking advantage of the structure of DNA. DNA molecules are built up of four types of subunits (A, C, G, T), which can pair with each other (A with T and G with C). By mixing DNA strands that pair with one another at specific sites, one can produce three-dimensional shapes of any required form. Liedl and colleagues formed an 85 nm long cylinder containing localized binding sites for 10-nanometer gold particles, on which the particles can be arrayed like beads on a string that wraps helically around the cylinder.
"The precision and yield of the expected structure is extremely high, and the degree of control achieved is greater than in any previous attempt to pattern metallic nanoparticles in a defined geometry," says coauthor Professor Friedrich Simmel, a physicist at the Technical University of Munich. By varying the size, disposition and composition of the nanoparticles or altering other features of the water-soluble structures, the investigators can customize the impact of the material on light rays passing through it.
For instance, by arranging the gold particles in right or left-handed helices, the properties of the transmitted light are altered accordingly. The magnitude of the optical response was found to be markedly dependent on the size of the particles, and their precise chemical nature also has a significant influence on how they interact with incident light. Thus, if the gold particles are coated with silver, the position of the optical resonance is shifted from the red to the blue range of the spectrum.
The phenomenon of circular dichroism can be used to characterize the optical activity of materials. It is measured using two light beams of defined wavelength which are circularly polarized in opposite senses and modulated in different ways by passage through the material. When the response of a given sample of the metamaterial was determined at various wavelengths, the results were found to agree with calculations based on a theoretical model. Hence, the model can be used to design materials that modulate light in specific ways. "We will now test whether we can also alter the refractive index of our materials," says Liedl. "Materials with a negative index of refraction could, for instance, serve as the basis for novel optical lens systems."(NIM/suwe)
|Contact: Dr. Kathrin Bilgeri|