Optical properties of gold nanoparticles

A solution of gold nanoparticles has a ruby-red color when the particle size is of the order of ten nanometers, a color quite different from the yellow we know when the gold is in ingot form. This change in color is the immediately visible sign of the confinement of the electromagnetic wave that describes light within NPs. This interaction between an electromagnetic wave and a nano-object is of great interest to physicists who want to understand and use these phenomena. In fact, this interest dates back to Michael Faraday, who in 1857 was the first to understand the relationship between the color of gold and the size of the NPs he had just synthesized (see §