Colloidal gold
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 materials with unique properties.

# Synthesis
Generally, gold nanoparticles are produced in a liquid ("liquid chemical methods") by reduction of chloroauric acid (H), although more advanced and precise methods do exist.  After dissolving H, 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.
To prevent the particles from aggregating, some sort of stabilizing agent that sticks to the nanoparticle surface is usually added. They can be functionalized with various organic ligands to create organic-inorganic hybrids with advanced functionality. 

## Turkevich et al. method
Pioneered by J. Turkevich 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 1020 nm in diameter.  Larger particles can be produced, but this comes at the cost of monodispersity and shape.
- Take 5.0106 mol of HAuCl4, dissolve it in 19 ml of deionized water (the result should be a faintly yellowish solution) .
- Heat it until it boils.
- Continue the heating and, 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 as necessary to bring the volume back up to 20 ml (to account for evaporation).
The sodium citrate first acts as a reducing agent. Later the negatively-charged citrate ions are adsorbed onto the gold nanoparticles, introducing the surface charge that repels the particles and prevents them from aggregating.
Recently, the evolution of the spherical gold nanoparticles in the Turkevich reaction has been elucidated. Interestingly, extensive networks of gold nanowires are formed as a transient intermediate. These gold nanowires are responsible for the dark appearance of the reaction solution before it turns ruby-red. 
To produce larger 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 Schiffrin in early 1990s, and can be used to produce gold nanoparticles in organic liquids that are normally not miscible with water (like toluene).
- Dissolve 9.0104 mol of HAuCl4 (about 0.3 g) in 30 ml of water.
- Dissolve 4.0103 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 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.(NaBH4); 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 H2SO4 (sulfuric acid) to neutralize it, and several times with distilled water.
Here, the gold nanoparticles will be around 56 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.  Alkanethiol protected gold nanoparticles can be precipitated and then redissolved. Some of the phase transfer agent may remain bound to the purified nanoparticles, this may affect physical properties such as solubility. In order to remove as much of this agent as possible the nanoparticles must be further purified by soxhlet extraction.

## Sonolysis
Another method for the experimental generation of gold particles is by sonolysis. In one such process based on ultrasound, the reaction of an aqueous solution of HAuCl4 with glucose, the reducing agents are hydroxyl radicals and sugar pyrolysis radicals (forming at the interfacial region between the collapsing cavities and the bulk water) and the morphology obtained is that of nanoribbons with width 30 -50 nm and length of several micrometers. These ribbons are very flexible and can bend with angles larger than 90. When glucose is replaced by cyclodextrin (a glucose oligomer) only spherical gold particles are obtained suggesting that glucose is essential in directing the morphology towards a ribbon.

# 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 intense shades of yellow, red, or mauve, depending on the concentration of gold.  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.

# Current research
Research in 2005 demonstrated that nanogold-coated bacteria can be used for electronic wiring . Bacillus cereus was deposited on a silicon / silicon dioxide wafer lined with gold electrodes. This device was covered with poly(L-lysine). The bacterium's surface has a negative charge, even more so due to the presence of flexible teichoic acid brushes. Poly(L-lysine)-coated nanogold particles carry a positive charge when washed with nitric acid and therefore the particles will only stick to the bacteria and nothing else.  The bacteria survive this treatment. When the humidity increases in a sample, the bacterium absorbs water and the resulting membrane expansion can be monitored by measuring the electrical current flowing through the bacteria. The Fowler-Nordheim equation is obeyed when the interbacterial distance is very small.
The reduction of hydrogen tetrachloroaurate by sodium borohydride in the presence of one of the enantiomers of penicillamine results in optical active colloidal gold particles  ; 127(44) pp 15536 - 15543; (Article) doi:10.1021/ja053504b. Penicillamine anchors to the gold surface by virtue of the thiol group. In this study the particles are fractionated by electrophoresis into three fractions, Au6, Au50 and Au150 as evidenced by Small angle X-ray scattering (SAXS). The D and L isomers have a mirror image relationship in circular dichroism.

# Health/Medical Applications
Colloidal gold has been successfully used as a therapy for rheumatoid arthritis .  In a related study, the implantation of gold beads near arthritic hip joints in dogs has been found to relieve pain.
An in vitro experiment has shown that the combination of microwave radiation and colloidal gold can destroy the beta-amyloid fibrils and plaque which are associated with Alzheimer's disease .  The possibilities for numerous similar radiative applications are also currently under exploration.
Gold nanoparticles are being investigated as carriers for drugs such as Paclitaxel .  The administration of hydrophobic drugs require encapsulation and it is found that nanosized particles are particularly efficient in evading the reticuloendothelial system.
In cancer research, colloidal gold can be used to target tumors and provide detection using SERS,Surface Enhanced Raman Spectroscopy in vivo.  These gold nanoparticles are surrounded with Raman reporters which provide light emission that is over 200 times brighter than quantum dots.  It was found that the Raman reporters were stabilized when the nanoparticles were encapsulated with a thiol-modified polyethylene glycol coat.  This allows for compatibility and circulation in vivo.  To specifically target tumor cells, the pegylated gold particles are conjugated with an antibody (or an antibody fragment such as scFv), against e.g. Epidermal growth factor receptor, which is overexpressed in cancerous cells.  Using SERS, these pegylated gold nanoparticles can then detect the location of the tumor.