Colloidal gold is a suspension (or colloid) of sub-micrometre-sized particles of gold in a fluid - usually water. The liquid is usually either an intense red colour (for particles less than 100 nm), or a dirty yellowish colour (for larger particles). Due to the unique optical, electronic, and molecular-recognition properties of gold nanoparticles, they are the subject of substantial research, with applications in a wide variety of areas, including electron microscopy, electronics, nanotechnology, and materials science. Synonyms: 60 nm Colloidal Gold.