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.
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.
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).
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. 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.
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) . 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.
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 sometimes overexpressed in cells of certain cancer types. Using SERS, these pegylated gold nanoparticles can then detect the location of the tumor.
Gold nanorods are being investigated as photothermal agents for in-vivo applications. Gold nanorods are rod shaped gold nanoparticles whose aspect ratios tune the surface plasmon resonance (SPR) band from the visible to near infrared wavelength. The total extinction of light at the SPR is made up of both absorption and scattering. For the smaller axial diameter nanorods (~10nm), absorption dominates, whereas for the the larger axial diameter nanorods (>35nm), scattering can dominate. Consequently, for in-vivo applications, small diameter gold nanorods are being used as photothermal converters of near infrared light due to their high absorption cross sections. Since near infrared light transmits readily through human skin and tissue, these nanrods, can be used as ablation components for cancer, and other targets. When coated with polyethylene glycol (PEG), gold nanorods have been known to circulate in-vivo for greater than 15 hours half life.