4. Modular optical properties for imaging and therapy

4.1 The origins of gold's scalable optical properties

The modular nature of the optical properties of gold nanoparticles is reflected in the wide range of colors offered by gold nanoparticle suspensions. Metallic gold is yellow at the macroscopic scale, while a suspension of gold nanoparticles around ten nanometers in diameter is red, and a suspension of gold nanobatons 30 nm long and 5 nm in diameter is blue. This change in color with size and shape is attributed to the phenomenon of localized surface plasmon resonance observed in metal particles whose size is smaller than the wavelength of the incident radiation. The collective oscillation of free electrons in the conduction band induced by the incident electromagnetic field is then strongly confined within the particle. The resulting...