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Grains of ‘nanorice’ may improve spectroscopic techniques

Apr 2006
Composite metallic particles shaped like rice grains combine the plasmonic properties of nanorods and nanoshells, suggesting potential applications in surface plasmon resonance and surface-enhanced Raman spectroscopies for biomedical diagnostics and pharmacology. Developed at the Laboratory for Nanophotonics at Rice University in Houston, the “nanorice” is described in the March 11 online edition of Nano Letters.

Such nanoscale metallic particles enhance local electromagnetic fields through the action of plasmons, optical excitations coupled with oscillations of their conduction electrons, enabling novel subdiffraction-limited amplification and focusing phenomena.

Courtesy of Hui Wang, Rice University.

Heretofore, explained laboratory director Naomi J. Halas, the highest field enhancements have been obtained at the junction of two nanoshells — spherical structures comprising a dielectric core coated with a metallic shell whose optical resonance can be tuned by changing their relative size.

It would be more convenient, however, to employ particles shaped like nanorods, which display strong field enhancements at their easily accessible tips. The investigators thus produced nanorodlike spindles of hematite roughly 350 nm long and coated them with gold nanoshells of various thicknesses by electroless plating.

Experimental analysis revealed that the local field enhancements of nanorice grains are several times larger than that from nanoscale bow-tie junctions and are similar in magnitude to that from nanoshell junctions. The researchers also believe that the grains display the highest surface plasmon resonance sensitivity of any metal nanostructure that has been produced to date.

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