Gold nanoparticles have tunable electronic & optical properties. Gold nanoparticles show different colors with monodisperse various particles size. Metallic gold nanoparticles display optical properties, or interactions with light in the visible to near-infrared regions which make these particles very attractive for imaging and sensing applications. In case of 10-30nm monodisperse gold nanoparticles, light absorbs in the blue-green portion of the spectrum (~450 nm) while red light (~700 nm) is reflected, results in red color. As particle size increases, the wavelength shifts to red shift. Red light is then absorbed, and blue light is reflected, results in solutions with purple color. As particle size continues to increase wavelengths move into the IR portion of the spectrum and most visible wavelengths are reflected, giving the nanoparticles clear or translucent color.

Metallic nanoparticles have applications far and wide for optical imaging, photo-acoustic (light combined with ultrasound) imaging, nuclear medicine imaging, assays based on color changes, i.e. colorimetric assays, and many other medical and sensing techniques.