A bright future with self-assembling nanocubes

Home pregnancy test

Home pregnancy test strip showing a positive result (Source: Martyn F. Chillmaid)

What does the home pregnancy test and stained glass have in common? Both contain nanometer sized particles of metal (nanoparticles) that play a key role in how they work. The home pregnancy test is basically a sensing strip used to detect the presence of a hormone produced in a pregnant woman. The strip contains a suspension of gold particles chemically linked to antibodies which bind to the hormone when they are present. When this happens, light is strongly scattered and produces a bright-red colour on the test strip. If no hormone is present, the gold-nanoparticles just stay the initial vaguely green colour.

The Lycurgus Cup (Source: British Museum)

An example of stained glass is the famous CE Lycurgus cup at the British Museum. Normally, the it looks like an opaque green cup, but it turns a glowing red colour when held up to the light. The cup is made from a special type of glass known as diachronic, which contains small amounts of gold and silver nanoparticles in suspension and gives the cup these unusual optical properties.

Despite these known uses of nanoparticles, researchers have had limited ability to tailor their optical properties.

Recently Dr. Andrea Tao, a professor at the University of California, San Diego Jacobs School of Engineering, and her colleagues have developed a technique to control the arrangement of metallic nanoparticles and their corresponding optical properties. Their findings are published online June 10 in Nature Nanotechnology

The team first made cube-shaped silver particles “nanocubes” that are about 80 nanometers (nm) wide (or about 100 times smaller than a strand of human hair). Then, they added polymer chains of various lengths to the surfaces of the nanocubes. Finally, they created a thin polystyrene film that’s about 150 nm thick with the nanocubes embedded within it.

The team controlled how the nanocubes organize within the polystyrene film by modifying how the cubes interact with each other. Normally, objects stack by packing face-to-face like Tetris blocks. In this case, the nanocubes tend to arrange edge-to-edge when the surfaces were covered with longer polymer chains. Conversely, shorter polymer chains on the nanocube surface promoted a face-to-face arrangement.

Showing the orientation of nanocubes within the polymer film (Source: Gao et al. 2012)

The team tested the optical response of the films by measuring how much light (of varying wavelengths) were transmitted through the film, and confirmed the arrangements of the nanocubes to be either edge-to-edge or face-to-face.

Although, it isn’t entirely clear how the optical response of the films differs depending on orientation of the nanocubes. From a photograph of the different films, it looks like the film with nanocubes arranged edge-to-edge reflects more light (looks brighter) than the film with nanocubes arranged face-to-face.

Unlike conventional metallic particles, metallic nanoparticles much smaller than the wavelength of light can help concentrate and channel light because light waves get trapped on the metal surface. This light-matter interaction produces unique optical properties.

“Our findings could have important implications in developing new optical chemical and biological sensors, where light interacts with molecules, and in optical circuitry, where light can be used to deliver information,” Tao said in the University’s press release.

The team’s nanocubes confine light into nanometer sized volumes, which could allow for optical sensors that are extremely sensitive. In the future, the new technique could be used to create complex materials for next-generation antennas and lenses according to the press release.

Barnes, W.L., Dereux, A., & Ebbesen, T.W. (2003). Surface plasmon
subwavelength optics Nature DOI: 10.1038/nature01937

Gao, B., Arya, G., & Tao, A.R. (2012). Self-orienting nanocubes for the assembly of
plasmonic nanojunctions Nature DOI: 10.1038/NNANO.2012.83

Hockmuth, C. (2012, June 13). Self-Assembling Nanocubes for Next Generation Antennas and Lenses [Press release]. Retrieved from http://ucsdnews.ucsd.edu/pressreleases/self_assembling_nanocubes_for_next_generation_antennas_and_lenses/

Stockman, M.I. (2011). Nanoplasmonics: The physics behind the applications Physics Today DOI: 10.1063/1.3554315


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