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NIM nanosystems initiative munich

Wednesday, 14 March, 2012

Sculpting Light with Gold Nanoparticles

NIM scientists develop a nanomaterial that precisely modulates light

Cups of dairy products nowadays often contain a label that informs about the content of D- and L-lactic acid, the two “isomers” – i.e. the possible configuration of atoms – of lactic acid.  The two isomers, L(+) and D(-), have different “optical activities”, turning the plane of polarized light to the right or left respectively. Using a technique similar to this naturally occurring optical phenomenon, an international team led by scientists in Munich has developed a synthetic, three-dimensional material that allows visible light to be modified in specific and desired ways.

To produce this “metamaterial,” physicists utilize synthetic DNA segments that fold into a designed shape, here a cylinder that is enveloped by a helical arrangement of gold nanoparticles. Billions of parallely assembled copies of these nanostructures together then constitute the “wonder material” that manipulates light in specific ways. NIM scientists Prof. Tim Liedel of LMU Munich and Prof. Friedrich Simmel of TU Munich discuss the results of their study in the current issue of Nature.

Using the so-called “DNA origami” method, NIM scientists are able to predetermine the underlying structure of the material. To this end, they utilize the characteristic structure of DNA, which is made up of four different bases. Two complementary base pairs can naturally form a bond with one another. These pairs find each other in a self-assembly process, forming a synthetic, three-dimensional object of the desired shape. In their experiments, the physicists chose a 85 nm long cylindrical structure to which they attached many gold nanoparticles close together, like a string of pearls that wraps the cylinder in a helical way. The gold particles have a diameter of about 10 nanometers and are connected to the DNA object at specific binding sites.

“The precision and yield of the technique is extremely high. It has also been better controlled than previous attempts to arrange metallic nanoparticles in a defined geometry using DNA self-assembly,” explains Friedrich Simmel.

When the physicists vary the parameters of the structures that are dissolved in water, the optical effects of the material change, in turn changing the properties of the light beam that emerges. Variable parameters include the arrangement, size, and quality of the gold particles. Scientists can make a distinction, for example, about whether the orientation of the gold particle helix around the DNA construct is left-handed or right-handed. Experimentation has shown that the intensity of the optical response strongly increases with the size of the particles. The chemical composition of the particles is also very influential. If the physicists coat the gold particles with an additional layer of silver, the optical resonance shifts from the red end of the spectrum to the shorter wavelengths of the blue end.

In the study, the property of circular dichroism was used by the physicists to characterize the materials. To measure circular dichroism, the scientists sent two circularly polarized light beams with a defined wavelength through the examined material. One beam had a positive direction of rotation and the other a correspondingly negative direction of rotation. As the beams passed through the material, they were modified in different ways. The combination of a series of measurements across various wavelengths yielded values specific to that material. In the experiment, these values agreed completely with the theoretical values calculated beforehand. Using these theoretical calculations, the scientists are now able to adjust the material parameters in such a way that in the end the desired light emerges. Prof. Tim Liedl explains where their research could lead in the future: “We are now going to study whether we can alter the refractive index of the materials we create using this method. Materials with a negative refractive index could for example be used to develop a range of applications, such as novel optical lens systems.”

DNA-based Self-Assembly of Chiral Plasmonic Nanostructures with Tailored Optical Re-sponse. Anton Kuzyk, Robert Schreiber, Zhiyuan Fan, Günther Pardatscher, Eva-Maria Roller, Alexander Högele, Friedrich C. Simmel, Alexander O. Govorov und Tim Liedl. Nature Volume 482 Number 7389 pp245-368

Prof. Tim Liedl
Department für Physik - Lehrstuhl Rädler
Geschwister-Scholl-Platz 1
D-80539 München, Germany

Tel: +49 89 2180 3725
Fax: +49 89 2180 3182
email: tim.liedl@physik.lmu.de

To Liedl group
To Simmel group
To Högele group



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