Nanoscale photonics: Nanoshells: gifts in a gold wrapper.

I n the past few years,metallic nanoparticles have emerged as powerful building blocks for photonic devices in which electromagnetic waves can be controlled at the nanometre length scale.Structures such as metal nanoparticle waveguides exploit the unique properties of collective electron oscillations, known as plasmons.At the March 2003 symposium* on Photonics and the Nanoscale at the American Physical Society Meeting in Austin,Texas,Naomi Halas and Jennifer West from Rice University reported their efforts to take plasmonic nanoparticles into the bio-arena.In a series of clever experiments,they demonstrated how silica–gold nanoshells are uniquely suitable for use in ‘instantaneous’whole-blood immunoassays,optically triggered drug delivery,and targeted photothermal destruction of cancer cells. The use of metallic nanoparticles dates back to the Renaissance. In that era,artists handcrafted vibrantly coloured church windows and glass vases by dissolving minute amounts of noble metal impurities in a glass melt to induce precipitation of nanometre-size metallic clusters. In 1908,Gustav Mie provided the theory explaining this phenomenon.This theory predicts that metallic nanoclusters strongly absorb visible light at a well-defined plasmon-resonance frequency.At this frequency,electromagnetic energy is efficiently converted into a collective motion of the free electrons in the metal.The resonance frequency of the particles depends on the particle size and shape, the presence of other particles,and the dielectric environment. At the APS meeting,George Schatz from Northwestern University demonstrated how the resonance frequency of two-dimensional metallic nanoparticle arrays can be tuned over a wide range of frequencies from blue to near-infrared by controlling the electromagnetic interaction between the particles. Experimental and theoretical work by Naomi Halas and Peter Nordlander at Rice University showed that silica–gold nanoshells offer enormous flexibility to tune the resonance frequency by varying the relative dimensions of the silica core and gold shell. In contrast to solid-core metallic nanoparticles, the resonance of a silica–gold nanoshell particle can easily be positioned in the ‘water window’in the near-infrared (800–1,300nm), where absorption by biomatter is low.Together with the high degree of biocompatibility of gold nanoshells, this result opens the door to a wide variety of biological applications. Halas and West showed how near-infrared resonant nanoshells could be used to enable fast whole-blood immunoassays.For conventional blood immunoassays,optical tests are performed at visible wavelengths.Because a purification step needs to be Analyte