Traditionally, gold evokes a sense of awe and extravagance because of its lustre and permanence. For centuries it was thought to be chemically unreactive but exploited for its physical properties of ductility, malleability and corrosion resistance, and in modern times, because of its excellent conductance, it is widely used in electronics. In the past 20 years, however, there has been a revival of interest in gold driven in the main by chemists. They have been seduced by a form that has been known since ancient times – colloidal gold, now generally referred to as nano-gold.
The alchemists discovered that gold would dissolve in aqua regia (HNO3/HCl) and that if a piece of tin was added to the solution, a purple pigment formed. This was used in the Middle Ages to colour glass ruby red. Although they thought that the colour was due to a tin–gold compound, when the early chemists, notably Michael Faraday, investigated it, they were amazed to find it was just another form of gold – the colour was determined by the size of the gold nanoparticles. Larger particles gave a yellow colour. Colloidal gold continued to be used as a pigment to colour glass and ceramics, and later as a contrast agent for biological electron microscopy.
Catalytic prowess
In 1985, Masatake Haruta, professor of chemistry at Tokyo Metropolitan University, Japan, discovered that nano-gold had another surprising property – it was a good catalyst. Nanoparticles of gold deposited on metal oxide substrate could catalyse the oxidation of CO to CO2 with molecular oxygen at temperatures as low as –70oC. As a result, gold is now incorporated in safety masks where exposure to CO is a threat. Haruta’s research started a revolution that is likely to see gold having many new roles this century.
Trevor Keel, head of technology at the World Gold Council, believes it has a glowing future: ‘Many new commercial applications for gold are on the horizon thanks to the enormous groundswell of research into the metal in recent years. In 2011, it was successfully introduced alongside platinum and palladium in diesel engine catalytic converters.’1
Haruta proved the catalytic activity of gold depended on cluster size and this depended on how the catalyst was prepared. Various particle sizes were tested, some as large as 30nm, but only the smaller ones could effect the necessary pre-dissociation of O2 to O• that is necessary for the reaction. His recipes for preparing nano-gold catalysts, which include a deposition–precipitation method among others, are now widely used and can produce clusters with as few as 20 atoms.
Other researchers have added merit to gold’s catalytic prowess. Richard Lambert and his group at the University of Cambridge, UK, for example, found that gold particles, which had diameters less than 3.5nm, were catalytically active and of these the most efficient appeared to be a 55-atom cluster, at least for the selective oxidation of styrene by O2. This stability and activity is thought to arise from the altered electronic structure intrinsic to these small gold nanoparticles.2
In the past 10 years, Graham J. Hutchings, director of the Catalysis Centre at Cardiff University, UK, and his group have greatly extended the range of reactions catalysed by supported nano-gold particles. Using a mixture of gold and palladium on a titanium dioxide substrate, he found he could catalyse the rapid synthesis of hydrogen peroxide, from hydrogen and oxygen, without the formation of water as a byproduct, and the oxidation of primary alcohols to aldehydes. Both of these reactions were carried out at 100oC in the absence of a solvent. More recently, he has shown that carbon is a better support for the catalyst, which has further increased the potential for his catalyst for reactions such as the epoxidation of propylene and the preparation of secondary amines.
In 2006, Avelino Corma of the Instituto de Tecnologia Quimica at Valencia, Spain, reported that nano-gold supported on a substrate of titanium oxide or iron(III) oxide can catalyse the reaction of nitroarenes with hydrogen gas.3 For example, 3-nitrosytrene undergoes 99% conversion and 96% selectivity to 3-vinylaniline and without the formation of the byproduct hydroxylamine styrene. According to Corma, this reaction suggests a greener route to the synthesis of industrially-important compounds such as cyclohexanone that currently produce this undesirable byproduct.
Gold catalysts are now being made and used by the chemical industry. There are several commercial gold catalysts on the market, including a gold–palladium catalyst on a silicate substrate produced by Evonik; nano-gold on a carbon substrate produced by 3M; and one made of 1% gold on titanium dioxide by Strem Chemicals. A gold-based catalyst developed by Johnson Matthey is being used by INEOS to produce 250,000t of vinyl acetate monomer (VAM), the precursor to the polyvinyls used to make adhesives, fibres, paints, and resins. Another process that might soon benefit from gold catalysts is the synthesis of vinyl chloride, the precursor to PVC.
However, gold need not be in the form of nanoparticles to be an excellent catalyst. Wayne Goodman and Mingshu Chen of Texas A&M University, US, have shown that a film of gold atoms is also excellent at oxidising CO. And Dean Toste of the University of California at Berkeley, US, has demonstrated that gold(I) can also be used as a catalyst, specifically for the oxidative rearrangement reactions of alkynes, with yields as high as 94% of the desired product. His group has since extended the use of gold catalysts to the rapid assembly of polycyclic ring systems.
Gold standard
Gold is also proving surprisingly useful in medicine, both in diagnostics and in therapeutics. Back in 1998, Chad Mirkin, professor of chemistry at Northwestern University, Evanston, Illinois, US, showed that it is possible using gold nanoprobes to identify the DNA of disease pathogens at extremely low concentrations, thereby aiding rapid diagnosis.4
His method is based on completing an electrical circuit between two electrodes 20μm apart, which can only be done by particular strands of DNA interacting with capture strands attached to a gold nanoparticles. The capture-strand is designed to have just the right sequence of bases to attract DNA from the disease microbe.
The Mirkin method offers a means of analysis that is 100 times faster than conventional DNA analysis techniques, which rely on the polymerase chain reaction (PCR) to replicate the DNA to a level at which it can be identified. Such is the interest in Mirkin’s work that some of his systems have already been commercialised and are being evaluated in major US hospitals.
Verigene, for example, is available from the Illinois-based company Nanosphere, and in theory it can diagnose a disease at point of contact with a patient, so may soon find its way to doctors’ surgeries.
Molly Stevens, professor of biomedical materials and regenerative medicine at Imperial College London, UK, has developed biological nanosensors consisting of gold nanoparticles, measuring ca 50nm across and red in colour. The nanoparticles can be coated with a range of biological molecules, such as antibodies, that can recognise and pick up antigens associated with certain disease states.
The nanosensors can be used, for example, to detect PSA (prostate specific antigen) at concentrations a billion times lower than currently used methods, such as ELISA. The gold nanoparticles are coated with an antibody that grabs PSA. A second antibody, which is bound to the enzyme glucose oxidase (GOx), latches on to the PSA. In the final step, the GOx acts to reduce silver ions in solution to silver atoms, which become attached to the gold nanoparticles, resulting in blue shift in the infrared, which is easily detected.5
Gold can also be used in other kinds of medical screening. Mostafa El-Sayed, director of the Laser Dynamics Laboratory, Georgia Institute of Technology, US, was one of the first to exploit gold nanorods as contrast agents in cancer imaging and in the treatment of cancer.6 The nanorods are easy to make, they can be grown from a dilute solution of auric acid (HAuCl4) seeded by adding gold nanoparticles (<5nm), and their absorption in the near infrared (NIR) means they can be used in vivo for clinical applications. The nanorods can be incorporated into monoclonal antibodies that selectively bind to the surface of malignant cells. On exposure to NIR, they generate intense local heat hot enough to kill the cancer cell.
Similar gold-based therapies that attack cancerous cells with gold-covered silica spheres have already been approved by the FDA in the US. Manufactured by Aurolase, these spheres are injected into the patient and accumulate in tumours because cancer cells have much weaker walls than healthy cells and are more easily penetrated. Again, when the tumour is exposed to NIR, the gold melts and the tumour’s goose is cooked.
Finally, nano-gold is also proving itself surprisingly useful in pregnancy kits. First Response, which contains nano-gold, has been on the market since 2010. This readily available device relies on the detection of the hormone human chorionic gonadotropin (hCG), which a woman’s body releases when she becomes pregnant and which is excreted in her urine. In the test, a sample of urine is put on the end of an immunoassay strip and moves along this by capillary action until it encounters monoclonal antibodies specifically designed to interact with hCG, and which have been attached to gold nanoparticles. When that happens, these particles cluster together and become much larger. They now move along the strip until they come up against a filter, which stops them and it turns the traditional red of colloidal gold. If there is no hCG present in the urine, then the gold nanoparticles don’t coagulate and so can pass through the first filter to be caught on a second barrier. This now shows red, indicating a successful test but a negative result. It will also show red even if the pregnancy indicator strip has turned red because some nano-gold particles don’t pick up hCG and so can pass through the first barrier.
Whatever the outcome of the test, don’t clap your hands in joy if that’s what you wish for. A typical 18-carat ring loses about 6mg of gold per year – more if you clap a lot. Assuming there are a billion such rings in the world, then this amounts to around 6t/year of the metal being lost for ever, which is equivalent to around 200,000 troy ounces in the archaic unit of weight still used by the gold industry.
Truly, gold never fails to surprise.
Gold’s hallmark
Gold’s quality is guaranteed by its hallmark and this is still measured in the traditional unit of carats, 24 being the 100% end of the scale. Ancient gold objects may need an extra guarantee to be validated as genuine, rather than the work of skilful forgers. This is done by the forensic analysis of the helium it contains, which increases with time and is generated by traces of uranium and thorium. Newly refined gold has levels of helium around 0.2ppm. whereas old gold objects have much higher levels, up to 60ppm.
John Emsley is a popular science writer based in Cambridge, UK.
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