Colloidal gold
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2008-06-24T19:57:50Z
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{{Nanomat}}
[[Image:ColloidalGold aq.png|100px|thumb|Aqueous 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 [[nanometre|nm]]), or a dirty yellowish colour (for larger particles)<ref name="ref1">Bernhard Wessling, ''Conductive Polymer / Solvent Systems: Solutions or Dispersions?'', '''1996''' ([http://www2.ormecon.de/Research/soludisp/ on-line here)]</ref><ref name="ref3">University of Edinburgh School of Physics: [http://www.ph.ed.ac.uk/~hs/Colloids/colloids.html Colloids (mentions Elixir of Life)]</ref>. 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 [[stained glass|staining glass]]. Modern scientific evaluation of colloidal gold did not begin until [[Michael Faraday|Michael Faraday's]] work of the 1850s<ref name="ref2"> V. R. Reddy, "Gold Nanoparticles: Synthesis and Applications" '''2006''', 1791, and references therein</ref> <ref>Michael Faraday, ''Philosophical Transactions of the Royal Society'', London, 1857</ref>. 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]]<ref name="ref4">Paul Mulvaney, University of Melbourne, ''The beauty and elegance of Nanocrystals'', [http://uninews.unimelb.edu.au/articleid_791.html Use since Roman times]</ref><ref name="ref5">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. ([http://pubs.rsc.org/ej/CS/2000/a904518j.pdf on-line here; mentions Cassius and Kunchel])</ref>, and the synthesis of novel materials with unique properties.
==Synthesis==
Generally, gold nanoparticles are produced in a liquid ("liquid chemical methods") by [[reduction (chemistry)|reduction]] of [[chloroauric acid]] (H[AuCl<sub>4</sub>]), although more advanced and precise methods do exist. After dissolving H[AuCl<sub>4</sub>], the solution is rapidly stirred while a [[reducing agent]] is added. This causes Au<sup>3+</sup> [[ion]]s to be reduced to neutral gold [[atom]]s. 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. <ref name="ref2" />
=== 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 10–20 nm in diameter. Larger particles can be produced, but this comes at the cost of monodispersity and shape.
*Take 5.0×10<sup>−6</sup> [[Mole (unit)|mol]] of HAuCl<sub>4</sub>, dissolve it in 19 [[milliliter|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 [[minute]]s.
*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 [[milliliter|ml]] (to account for [[evaporation]]).
The sodium citrate first acts as a [[reducing agent]]. Later the negatively-charged citrate ions are [[adsorbtion|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. <ref> BK Pong et al. J. Phys. Chem. C, 111 (17), 6281 -6287, 2007. New Insights on the Nanoparticle Growth Mechanism in the Citrate Reduction of Gold(III) Salt: Formation of the Au Nanowire Intermediate and Its Nonlinear Optical Properties </ref>
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 liquid]]s that are normally not [[miscible]] with water (like [[toluene]]).
*Dissolve 9.0×10<sup>−4</sup> mol of HAuCl<sub>4</sub> (about 0.3 [[gram|g]]) in 30 ml of water.
*Dissolve 4.0×10<sup>−3</sup> mol of [[tetraoctylammonium bromide]] (TOAB) (about 2.187 g) in 80 ml of toluene.
*Add the HAuCl<sub>4</sub> solution to the TOAB and stir vigorously for about 10– minutes. The colour of the [[aqueous]] [[phase (matter)|phase]] should become clear, and the colour of the [[organic compound|organic]] phase (the toluene) should become orange.
*While stirring vigorously, add (preferably dropwise, but really doesn't matter) [[sodium borohydride]].([[Sodium|Na]][[boron|B]][[H]]<sub>4</sub>); 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 [[hour]]s to ensure monodispersity (especially if NaBH<sub>4</sub> was not added dropwise; otherwise just an hour or two is enough).
*Separate the organic phase, wash it once with dilute [[hydrogen|H]]<sub>2</sub>[[sulfur|S]][[oxygen|O]]<sub>4</sub> ([[sulfuric acid]]) to neutralize it, and several times with [[distilled water]].
Here, the gold nanoparticles will be around 5–6 nm. NaBH<sub>4</sub> 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, [[alkanethiol]]s seem to be popular), which will bind to gold [[covalent]]ly, 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 HAuCl<sub>4</sub> with [[glucose]] {{Fact|date=June 2007}}, the [[reducing agent]]s are hydroxyl radicals and sugar [[pyrolysis]] [[radical (chemistry)|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 Rome|Ancient Roman]] times to colour glass intense shades of yellow, red, or mauve, depending on the concentration of gold. In the 16th century, the [[alchemy|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 <ref name="ref6">''Sonochemical Formation of Single-Crystalline Gold Nanobelts '' Jianling Zhang, Jimin Du, Buxing Han, Zhimin Liu, Tao Jiang, Zhaofu Zhang [[Angewandte Chemie International Edition]] Volume 45, Issue 7 , Pages 1116 - 1119 '''2006''' [http://dx.doi.org/10.1002/anie.200503762 Abstract]</ref>. ''[[Bacillus cereus]]'' was deposited on a [[silicon]] / [[silicon dioxide]] wafer lined with gold [[electrode]]s. This device was covered with [[Lysine|poly(L-lysine)]]. The bacterium's surface has a negative [[electrical charge|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 [[Redox|reduction]] of [[hydrogen tetrachloroaurate]] by [[sodium borohydride]] in the presence of one of the [[enantiomer]]s of [[penicillamine]] results in [[optical activity|optical active]] colloidal gold particles <ref name="ref7">''Self-Assembly of Nanoparticles on Live Bacterium: An Avenue to Fabricate Electronic Devices '' Vikas Berry, Ravi F. Saraf [[Angewandte Chemie International Edition]] Volume 44, Issue 41 , Pages 6668 - 6673 '''2005''' [http://www3.interscience.wiley.com/cgi-bin/abstract/112100782/ABSTRACT Abstract]</ref> <ref>''Large Optical Activity of Gold Nanocluster Enantiomers Induced by a Pair of Optically Active Penicillamines'' Hiroshi Yao, Kanae Miki, Naoki Nishida, Akito Sasaki, and Keisaku Kimura [[J. Am. Chem. Soc.]]; '''2005'''</ref>; 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 [[Fraction (chemistry)|fraction]]ated by [[electrophoresis]] into three fractions, Au<sub>6</sub>, Au<sub>50</sub> and Au<sub>150</sub> 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]] <ref name="RA Therapy 1">[http://www.ncbi.nlm.nih.gov/sites/entrez?Cmd=ShowDetailView&TermToSearch=17265489 Amelioration of collagen-induced arthritis in rats by nanogold.]</ref><ref name="RA Therapy 2">[http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1030729 Repeated Colloidal Gold Tests in Rheumatoid Arthritis]</ref>. In a related study<ref name="For the Dogs">[http://www.ncbi.nlm.nih.gov/sites/entrez?Cmd=ShowDetailView&TermToSearch=17381835 Two years follow-up study of the pain-relieving effect of gold bead implantation in dogs with hip-joint arthritis.]</ref>, 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]] <ref name="Alzheimer's">[http://findarticles.com/p/articles/mi_m0857/is_3_12/ai_n17212704 Gold is newest weapon in battle against Alzheimer's]</ref>. The possibilities for numerous similar radiative applications are also currently under exploration<ref name="Other Radiative">[http://www.ncbi.nlm.nih.gov/sites/entrez?Cmd=ShowDetailView&TermToSearch=16380179 Therapeutic possibilities of plasmonically heated gold nanoparticles.]</ref>.
Gold nanoparticles are being investigated as carriers for drugs such as [[Paclitaxel]] <ref>''Paclitaxel-Functionalized Gold Nanoparticles'' Jacob D. Gibson, Bishnu P. Khanal, and Eugene R. Zubarev [[J. Am. Chem. Soc.]] '''2007''', 129, 11653-11661 {{DOI|10.1021/ja075181k}}</ref>. 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.<ref>Qian, Ximei. "In vivo tumor targeting and spectroscopic detection with surface-enhanced Raman nanoparticle tags." Nature Biotechnology. 2008. Vol 26 No 1. </ref>
==See also==
* [[Colloidal silver]]
* [[Nanovid microscopy]]
==References==
{{reflist}}
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