Many people associate with gold prosperity in our time, expensive jewelry and a solid investment. But now the use of small particles of this precious metal, down to the nanoscale, makes it even possible that gold nanoparticles could contribute to solving some critical global problems of today and the near future, because these tiny gold particles have a variety of unique properties that lead to materials covering a wide range of options. Moreover they are still very cost effective, very different from the usual thinking of this expensive precious metal that reaches continuously new record nominal highs.
These gold nanoparticles have been used for centuries, albeit unconsciously. Roman artisans took advantage of the effect, which was created when they added gold chloride into molten glass: fine gold particles were created, the glass coloring purple-red. Today we know that these particles are nanoscaled. In many cathedrals throughout Europe such colored glass windows can be found.
 

Properties and Applications
 

The earliest medical applications of gold went up to the year 2500 BC. The Chinese used this precious metal to achieve a healing effect.
The full potential in biomedical applications, however, revealed nanotechnology. Tumor-targeting techniques that exploit the existing biocompatibility of gold, are being developed as well as other simple to implement, cost effective and reliable diagnostic tests for rapid detection of prostate cancer or other tumors. Currently, tumor-targeting drug delivery systems with gold nanoparticles are in Phase 1 and Phase 2 clinical testing. They showed promising results so far. In the field of diagnostics, the precious metal convinces by its testimony reliability and sensitive power with minimal material usage. Thus rapid tests, such as for pregnancy or for salmonella, E. coli and Campylobacter bacteria, are unimaginable without gold nanoparticles. Likewise, it is already possible to measure by means of functionalised colloidal gold nanoparticles the occurrence of HIV disease and its progress in the body by appropriate rapid diagnostic tests. Similarly, the commercialisation of early prostate cancer diagnostics seems to be possible in the near future by the use of a system, based on gold nanoparticles rapid test methods.
However, the applications of nano gold are not only limited to the medical sector. The precious metal in its nanoscaled form may also contribute to the more efficient and “greener” production of some of today's chemical intermediates used in the production of our everyday commodities and food products. Also the tiny gold particles can be used in air and water purification, for example via the efficient binding of mercury emissions into the atmosphere.
Attempts were made to produce cheap and efficient fuel cells with (nano-)gold, a major clean-energy technology of the near future.
One of the largest and still growing industries, the information and communication technology (ICT), can also hardly refrain from the use of gold. Here it is used for the development of conductive nanoparticle inks, for example, to print on plastics for electronic applications, because it has high durability and excellent resistance. Further investigations on touch-sensitive displays with gold nanoparticles show enhanced values, as well as these particles can cause a further improvement of flash memory devices (eg USB flash drives or SSD hard drives).
In particular, the current standard conductive material for touch screens, ITO, could be replaced by the combination of nanoscale gold and CNTs in a transparent layer. Other possible applications are in the area of ​​solar cells.
The numerous applications of gold nanoparticles are mainly based on the unique properties of these tiny particles. They show very high biocompatibility and are thus non-toxic, which makes gold interesting for biomedical applications. Due to the so-called surface plasmon resonance effect the color of the particles changes, when hormones, antigens, etc. are attached. Hits a suitably modified gold nanoparticle an only during pregnancy released and in a urine or blood sample existing hormone, so it changes color and pregnancy can be detected.
 

Natural resources and manufacturing
 

Gold occurs on earth in the primary raw materials as gold-bearing rock (gold ore) and in secondary deposits as fine metal. The world's annual production is currently about 2,600 tons, about one hundred times more than in the 19^th Century. Significant amounts of gold are produced during the refining of other metals such as copper, nickel or other metals. So sometimes only these "impurities" lead to an economical exploitation.
Large parts of gold nanoparticles are now obtained via the reduction of gold chloride solution (so-called tetra-chloro-aureate). This process uses different materials as a reducing agent, such as citric acid, oxalic acid, boron hydrides or others. Alternatively, it is also possible to generate such nanoparticles via laser radiation or UV light irradiation. In addition to the reducing agent a stabiliser is added during the process. This and the choice of the process parameters significantly affect the size, shape and morphology of nanoparticles.

Literature:

1. Wikipedia: Gold
2. Gold for good. Gold and nanotechnology in the age of innovation , T. Keel, R. Holliday, T. Harper, World Gold Council, January 2010, London, UK.