Colloidal gold, also known as "nanogold", 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). The nanoparticles themselves can come in a variety of shapes.

Spheres, rods, cubes, and caps are some of the more frequently observed ones. Known since ancient times, the synthesis of colloidal gold was originally used as a method of staining glass. Modern scientific evaluation of colloidal gold did not begin until Michael Faraday's work of the 1850s.

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 electronics, nanotechnology, and the synthesis of novel[peacock term] materials with unique properties.[peacock term] Generally, gold nanoparticles are produced in a liquid ("liquid chemical methods") by reduction of chloroauric acid (H[AuCl4]), although more advanced and precise methods do exist. After dissolving H[AuCl4], the solution is rapidly stirred while a reducing agent is added. This causes Au3+ ions to be reduced to neutral gold atoms.

As more and more of these gold atoms form, the solution becomes supersaturated, and gold gradually starts to precipitate in the form of sub-nanometer particles. The rest of the gold atoms that form stick to the existing particles, and, if the solution is stirred vigorously enough, the particles will be fairly uniform in size.

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