Colloidal gold

Colloidal gold, or more precisely gold nanoparticles are sub-micrometre sized particles of gold, usually found in the form of a suspension in water (hence "colloid"), in which case the liquid usually appears to be either an intense red colour (for particles less than 100 nm), or a dirty yellowish colour (for larger particles). Known since ancient times, it was originally used as a method of staining glass; however, serious research into colloidal gold did not begin until Michael Faraday's work of the 1850s.

Currently, it is a subject of intense research, with applications in a wide variety of areas, including medicine, electronics, and the synthesis of novel materials with unique properties. The nanoparticles themselves can come in a variety of shapes: spheres, rods, cubes and caps are some of the more frequently observed ones.

Synthesis

Generally, gold nanoparticles are produced in a liquid ("liquid chemical methods") by reduction of hydrogen tetrachloroaurate (H Au Cl₄), although more advanced and precise methods do exist. After dissolving HAuCl₄, the solution is rapidly stirred while a reducing agent is added. This causes Au³⁺ ions to turn into plain gold atoms. As more and more of these gold atoms appear, the solution becomes supersaturated, and gradually gold starts to precipitate in the form of small sub-nanometer particles. The rest of the gold atoms that appear stick to the existing particles, and if the solution is stirred vigorously enough, the particles will be fairly monodisperse.

To prevent the particles from aggregating, some sort of stabilizing agent that sticks to the nanoparticle surface is usually added.

Turkevitch et al. Method

Pioneered by J. Turkevitch et al. in 1951 and refined by G. Frens in 1970s, this recipe is the simplest one available. Generally, it is used to produce modestly monodisperse spherical gold nanoparticles suspended in water of around 10–20 nm in diameter. Larger ones can be produced, but this comes at the cost of monodispersity and shape.

Take 5.0×10⁻⁶ mol of HAuCl₄, dissolve it in 19 ml of deionized water (should be a faint yellowish solution)
Heat it until it boils
While stirring vigorously, add 1 ml of 0.5% sodium citrate solution; keep stirring for the next 30 minutes
The colour of the solution will gradually change from faint yellowish to clear to grey to purple to deep purple, until settling on wine-red.
Add water to the solution to bring the volume back up to 20 ml (to account for evaporation).

The sodium citrate first acts as a reducing agent, and later the negative citrate ions are adsorbed onto the gold nanoparticles and introduce the surface charge that repels the particles and prevents them from aggregating.

To produce bigger particles, less sodium citrate should be added (possibly down to 0.05%, after which there simply would not be enough to reduce all the gold). The reduction in the amount of sodium citrate will reduce the amount of the citrate ions available for stabilizing the particles, and this will cause the small particles to aggregate into bigger ones (until the total surface area of all particles becomes small enough to be covered by the existing citrate ions).

Brust et al. method

This method was discovered by Brust and Schfrinn in early 1990s, and can be used to produce gold nanoparticles in organic liquids that are normally not miscible with water (like toluene).

Dissolve 9.0×10⁻⁴ mol of HAuCl₄ (about 0.3051 g) in 30 ml of water
Dissolve 4.0×10⁻³ mol of tetraoctylammonium bromide (TOAB) (about 2.187 g) in 80 ml of toluene
Add the HAuCl₄ solution to the TOAB and stir vigorously for about 10–20 minutes. The colour of the aqueous phase should become clear, and the colour of the organic phase (the toluene) should become orange.
While stirring vigorously, add (preferably dropwise, but really doesn't matter) sodium borohydride (Na B H₄); the colour should change from orange to white to purple to eventually reddish, although the latter colours will be poorly discernible, since the solution will be quite concentrated and thus will look very dark.
Keep stirring the solution for up to 24 hours to ensure monodispersity (especially if NaBH₄ was not added dropwise; otherwise just an hour or two is enough).
Separate the organic phase, wash it once with dilute H₂S O₄ (sulfuric acid) to neutralize it, and several times with distilled water.

Here, the gold nanoparticles will be around 5–6 nm. NaBH₄ is the reducing agent, and TOAB is both the phase transfer catalyst and the stabilizing agent.

It is important to note that TOAB does not bind to the gold nanoparticles particularly strongly, so the solution will aggregate gradually over the course of two weeks or so, which can be very annoying! To prevent this, one can add a stronger binding agent, like a thiol (in particular, alkanethiols seem to be popular), which will bind to gold covalently, and hence pretty much permanently. The neat thing with alkanethiol protected gold nanoparticles is that one can precipitate them and then later resuspend them, which is in fact a superior purification mechanism than the last step in the process above.

History

A so-called Elixir of Life, a potion made from gold, was discussed, if not actually manufactured, in ancient times. Colloidal gold has been used since Ancient Roman times to colour glass an intense red. In the 16th century, the alchemist Paracelsus claimed to have created a potion called Aurum Potabile (Latin: potable gold). In the 17th century the glass-colouring process was refined by Andreus Cassius and Johann Kunchel. In 1842, John Herschel invented a photographic process called chrysotype (from the Greek word for gold) that used colloidal gold to record images on paper. Paracelsus' work is known to have inspired Michael Faraday to prepare the first pure sample of colloidal gold, which he called 'activated gold', in 1857. He used phosphorus to reduce a solution of gold chloride. Faraday was the first to recognise that the colour was due to the minute size of the gold particles.

References

Bernhard Wessling, Conductive Polymer / Solvent Systems: Solutions or Dispersions?, 1996 (on-line here)

Michael Faraday, Philosophical Transactions of the Royal Society, London, 1857

University of Edinburgh School of Physics: Colloids (mentions Elixir of Life)

Paul Mulvaney, University of Melbourne, The beauty and elegance of Nanocrystals, Use since Roman times

C. N. Ramachandra Rao, Giridhar U. Kulkarni, P. John Thomasa, Peter P. Edwards, Metal nanoparticles and their assemblies, Chem. Soc. Rev., 2000, 29, 27-35. (on-line here; mentions Cassius and Kunchel)

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