The unusual optical properties of noble metal nanoparticles were employed long before scientists could even conceive of nanoscale objects. The Lycurgus Cup, which is part of a collection at the British Museum and was likely made in the 4th century in Rome, is a prime example. The glass portion of this vessel contains both colloidal Au and Ag and, thus, has the unusual property of appearing green when illuminated externally and red when illuminated internally. Similarly, many stained glass windows dating from the Medieval period contain red panels colored by Au colloids and yellow panels colored by Ag colloids, and Cu and Ag colloids were included in ceramic glazes used during the Renaissance period to give art objects an iridescent or metallic sheen. In each of these cases, the artists were capitalizing on the size-dependent optical properties of the noble metal nanoparticles without knowing it. It was Michael Faraday who first recognized that the intensely colored solutions were attributable to "highly divided", or colloidal, Au. The phenomenon responsible for the unusual scattering and absorption (extinction) properties of noble metal nanoparticles is the localized surface plasmon resonance (LSPR); excitation of the LSPR is achieved when the appropriate wavelength of light excites a collective oscillation of the conduction band electrons within a nanostructure. In 1908, Gustav Mie presented an analytical solution to Maxwell's Equations for the extinction of electromagnetic radiation by a metallic sphere. The energy that initiates the LSPR is highly sensitive to the nanostructure's composition, size, shape, dielectric environment, and spacing as well as the electron-withdrawing or electron-donating character of any chemisorbed species.
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