= Gold nanoparticle
A <gold nanoparticle> is a small particle of <gold> whose <optics>[optical] and <surface chemistry> depend on its geometry and surroundings. A dispersion of such particles can appear red or purple rather than resembling a piece of bulk <gold>.<What Is So Special about Nano? by the National Nanotechnology Coordination Office>[Nanoscale gold and size-dependent colour]{ref}
In 1857, <Michael Faraday> described ruby-coloured fluids containing finely divided <gold>. He compared the transmitted colour of these fluids with coloured glass and observed changes toward blue or violet under conditions that altered the dispersed particles. These observations established a striking connection between the physical state of <gold> and its interaction with <light>.<Experimental Relations of Gold and Other Metals to Light by Michael Faraday>[Philosophical Transactions 147, pages 159–168, especially the ruby fluids and comparison with ruby glass]{ref}
A modern explanation involves <localized surface plasmon resonance>: the collective response of conduction <electrons> to an incident <electromagnetic field> changes <absorption> and <scattering>. The resulting spectrum depends on particle shape and the surrounding <dielectric>, as well as size. Thus colour alone is not a universal measure of <nanoparticle> diameter.
Experiments on <gold nanorings> show how geometry changes the response: varying the ratio of ring thickness to radius shifts the <localized surface plasmon resonance>. This is evidence for control through particle shape rather than a change of chemical element.<Optical Properties of Gold Nanorings by Javier Aizpurua and others>[Physical Review Letters 90 (2003), abstract: measured shifts and coupling of inner and outer surface modes]{ref}
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