A colloidal crystal is an ordered array of colloid particles and fine grained materials analogous to a standard crystal whose repeating subunits are atoms or molecules.[1] A natural example of this phenomenon can be found in the gem opal, where spheres of silica assume a close-packed locally periodic structure under moderate compression.[2][3] Bulk properties of a colloidal crystal depend on composition, particle size, packing arrangement, and degree of regularity. Applications include photonics, materials processing, and the study of self-assembly and phase transitions.

Colloidal gold is a sol or colloidal suspension of nanoparticles of gold in a fluid, usually water.[1] The colloid is usually either an intense red colour (for spherical particles less than 100 nm) or blue/purple (for larger spherical particles or nanorods).[2] Due to their optical, electronic, and molecular-recognition properties, gold nanoparticles are the subject of substantial research, with many potential or promised applications in a wide variety of areas, including electron microscopy, electronics,[3] nanotechnology, materials science, and biomedicine.