Grains of ‘nanorice’ may improve spectroscopic techniques
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
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
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