Nano Goldfish

Counter-intuitively, the reddish-purple colour of this filmy fish is not the product of an organic or inorganic red dye, but the product of gold nanoparticles (GNPs) dispersed in a 4% PVA solution. 

The terms ‘nanoparticle’ and ‘nanotechnology’ became popularised in the 1980s, coinciding with the 1981 development of the scanning tunnelling microscope (STM) which allowed scientists to view and manipulate nanoparticles, and the 1985 discovery of fullerenes, a nanoscale form of carbon with unusual structural and bonding properties. The publication of engineer Eric Drexler’s 1986 Engines of Creation (a dystopian vision of the future where self-replicating nanobots escape from laboratories and destroy the world) also led to increasing public interest and concern about nanotechnology, inspired by his ‘grey goo’ theories. 

However, despite the relative novelty of the term and our ability to observe and control nanoparticles, technologies that use them have been around for hundreds of years. Recent archaeological analysis has shown that the spectacular red and yellow hues of 16th and 17th century Medieval stained glass windows have colloidal gold and silver nanoparticles to thank for their vibrancy. Even earlier than that, the British Museum’s famous Roman Lycurgus Cup owes its dichroic effect to the different sized nanocrystals of silver-gold alloy that are dispersed through a glassy matrix (it changes colour depending on the angle of the light passing through it), and red glasses dating back to the Bronze Age in Italy (1200-1000 BCE) are thought to owe their colour to the excitation of copper nanoparticles.

These early examples demonstrating the interesting optical effects of metal nanoparticles were not created by scientists,  but by alchemists and master glassworkers who understood that unusual colours could be achieved by the addition of noble metals to their materials, even if they could not see what was happening at the nanoscale or yet precisely control it. Gold nanoparticles are extremely effective at absorbing and scattering light, and the colour they produce can be ‘tuned’ by changing the size of the nanoparticle: smaller gold nanoparticles absorb light at the blue-green end of the spectrum, reflecting red light and creating a deep ruby colour, whereas larger particles absorb more red light, creating a purple or blue effect. 

In recent years, the unusual optical, electronic and photodynamic properties of gold nanoparticles have allowed us to find all sorts of uses for them. They have been used as catalysts in the development of new and more efficient fuel cells, as particularly effective electrical conductors in printable conductive inks and electronic chips, to detect biomarkers of heart disease and infection in the human body, and as a targeted medical treatment to kill cancerous cells with light therapy (light is applied to tumours containing gold nanoparticles, which rapidly heat up to kill the cancerous cells).