Nanopartlcles of gold in solution are a deep red crimson colour and are used in modern medical diagnostics like pregnancy testing.
Whilst the term ‘nanotechnology’ is unfailingly modern, many of the scientific principles behind it have been unwittingly practised for millennia. The ancient mystery of the Lycurgus Cup illustrates this point perfectly. This beautiful Roman artefact, when held up to the light, mysteriously changes colour from an opaque green to a translucent red. Today, our understanding of nanotechnology explains the ‘mystery’ – the glass of the cup contains small quantities of colloidal gold and silver which interact with light to produce this unusual effect.
Gold on the nanoscale is a unique material. First and foremost, it is not gold coloured, but a deep crimson red or light blue (depending on the size and shape of the particles). This dramatic colour change is due to the minute particles of gold interacting with light differently to bulk gold. The particles themselves retain many of the characteristics of bulk gold (for example their biocompatibility and resistance to corrosion). They also develop other intriguing properties such as reactivity (making them useful as catalysts in chemical reactions).
These particles have already found practical use in a range of fields. Various diagnostic devices rely on their unusual optical properties; examples include the First Response pregnancy testing kit and the Merck Singlepath assay (which is used to detect salmonella). The US-based company Pointcare Technologies has developed and marketed a system based on gold nanoparticles that measures a key indicator in the progression of HIV.
Pioneering work in South Africa and Japan in the 1980s highlighted the practical uses of gold as a catalyst. Since then, interest in the field has exploded, leading to the commercial availability of a range of catalysts and the development of various industrial processes and products using these materials. Indeed, catalysts based on gold nanoparticles are now a hot topic in the world of chemistry.