Silver Nanostructures with Unique Optical Properties Created

Michael A. Greenwood

Silver nanobars and nanorice with the ability to scatter and absorb light in the near-infrared have been produced by controlling the assembly of silver atoms in solution. These nanostructures could compete with nanorods made of gold as contrasting agents for the optical imaging of cancer.

Figure 1. An SEM image of silver nanobars (left) is tilted 45° (right) to provide a better view of the shape of the nanobars.

A team of researchers led by Younan Xia from the University of Washington in Seattle grew the nanobars, which had an average aspect ratio of 2.7, in a solution of ethylene glycol (the main component of antifreeze) and polyvinylpyrrolidone (a polymer in toothpaste). Rice-shaped particles were fashioned subsequently by smoothing the corners and edges of the nanobars.

Silver particles smaller than 60 nm in diameter scatter light twice as efficiently as gold and are two orders of magnitude better at enhancing the Raman signals of adsorbed molecules, but nanorods of silver have not been as widely studied as those of gold, the researchers said.

By correlating the spectra of single nanoparticles with their electron microscope images, the researchers determined how sharp edges — along with different aspect ratios — affected the frequency of plasmon resonance. Single nanoparticle spectroscopy was performed with a Nikon inverted microscope and an Ocean Optics CCD spectrometer.

Figure 2. These plots show the relative field amplitude around the front (A, B) and the center (C, D) of a nanobar measuring 150 × 55 × 50 nm at its two resonance frequencies.

Because the particles grew anisotropically, they exhibited two plasmon resonance peaks. This resulted in both the nanobars and nanorice being able to scatter and absorb light in the visible and near-infrared regions. The researchers found that the dimensions of the silver nanostructures affected not only the wavelength of the light they scattered but also the intensity with which they enhanced near-fields for molecular spectroscopy.

More research is needed to better control the size and aspect ratio of nanobars before they can be effectively employed as a labeling and contrast agent.

Nano Letters, published online March 8, 2007, doi: 10.1021/nl070214f.