The interaction of light with free electrons in a gold or silver nanostructure can give rise to collective excitations commonly known as surface plasmons. Plasmons provide a powerful means of confining light to metal/dielectric interfaces, which in turn can generate intense local electromagnetic fields and significantly amplify the signal derived from analytical techniques that rely on light, such as Raman scattering. With plasmons, photonic signals can be manipulated on the nanoscale, enabling integration with electronics (which is now moving into the nano regime). However, to benefit from their interesting plasmonic properties, metal structures of controlled shape (and size) must be fabricated on the nanoscale. This issue of MRS Bulletin examines how gold and silver nanostructures can be prepared with controllable shapes to tailor their surface plasmon resonances and highlights some of the unique applications that result, including enhancement of electromagnetic fields, optical imaging, light transmission, colorimetric sensing, and nanoscale waveguiding.

Nanoscale materials are beginning to have an impact in the field of molecular diagnostics. In particular, gold nanoparticles surface-functionalized with DNA have garnered much recent interest. Due to the unusual optical and catalytic properties of gold nanoparticle labels, several distinct advantages for assay readout have been realized. This review focuses on the progress made in our group over the past seven years in the development of particle surface chemistry and ultrasensitive protein and nucleic acid assays based upon DNA-functionalized gold nanoparticles. For DNA targets, experiments demonstrate that assays based upon gold nanoparticle labels have enhanced target specificity and in certain cases the sensitivity of polymerase chain reaction (PCR), without the need for target amplification. For protein targets, similar experiments demonstrate that assays based upon gold nanoparticles are up to one million times more sensitive than conventional protein detection methods. Recent data using human samples demonstrate the utility of such assays.

One-dimensional metallic nanostructures such as nanorods and nanowires are of tremendous interest for electronic, sensing, and catalytic applications. Shape anisotropy introduces new optical properties in gold and silver nanoparticles, such as longitudinal plasmon resonance bands in the visible and near-IR portion of the spectrum. Different approaches employed for the shape-controlled synthesis of silver and gold nanocrystals include chemical, electrochemical, and physical methods. The chemical route for the synthesis of nanorods and nanowires, in which metal salts are reduced in an aqueous solution, is one of the most widely used methods. This route commonly employs a surfactant as the directing agent to introduce asymmetry in the nanocrystal shape. Variation in the concentration of precursor salt and the surfactant, the nature of the surfactant, the nature and concentration of reducing agents, the presence of external salts, and the pH of the reaction solution all affect nanocrystal shape, dimension, and yield. The size and shape of the nanocrystals affect the position of the plasmon bands, which in turn has been widely used in surface-enhanced spectroscopies that include both Raman and fluorescence. The aqueous, surfactant-directed route also promises the synthesis of more complex nanostructures with additional desirable properties.