“Both of these nanostructures have long been sought by the [scientific] community for their unique optical properties but have not been achieved experimentally,” Younan Xia of the University of Washington told nanotechweb.org.

The scientists created the silver nanocubes by reducing silver nitrate with ethylene glycol at 160°C in the presence of poly(vinylpyrrolidone) (PVP). The PVP acts as a capping reagent, controlling the growth rates of some faces of the silver so that single-crystal nanocubes form rather than multiple twinned particles. The reaction temperature, the concentration of the silver nitrate and the ratio of PVP to silver nitrate were also crucial factors. The resulting cubes were slightly truncated along their edges and at their corners.

“The shape and structure - hollow versus solid - of nanostructures provides another two useful ways to tune the electronic and optical properties of nanostructured materials,” added Xia. “Particularly so for noble metals such as silver and gold because their plasmon resonance has been shown to be very sensitive to these two parameters.”

To create the gold nanoboxes, the team reacted the silver nanocubes with an aqueous solution of HAuCl4. This converted the silver into soluble silver chloride and gold formed in its place. The resulting hollow nanoboxes were bounded by two sets of facets - eight triangular facets and six square ones. The boxes, which were roughly 20% larger than the silver nanocube templates, self-assembled into a close-packed two-dimensional array.

“Silver nanocubes are expected to exhibit stronger Raman scattering enhancement than other morphologies,” said Xia. “And gold nanoboxes are expected to have a plasmon resonance band shifted to the near infrared - a spectral region particularly useful for many biorelated applications because our skins are transparent in this window.”

As a result, the silver nanocubes could supply Raman scattering enhancement for sensing and biodetection applications and the gold nanoboxes could be used as lightweight fillers for conductive composites and for phototriggered drug delivery. Both the gold and silver nanoparticles could serve as building blocks to construct more complex architectures, such as long-range ordered lattices.

Xia says the team will now seek to extend its approach to other materials, explore applications for the nanocubes and nanoboxes, and try to understand the growth and shape control mechanism. The researchers reported their work in Science.