Color control

Hi all,

I belive that you all know me from my previous post, but if new on the blog, I am Karol Kolataj from Tim Liedl’s group in Munich.

I hope that up till now you like our blog. As we already moved to more science-focused part of the blog, in this entry I will show you colourful side of nanotechnology, and explain how scientists can control colour and shape of noble metal nanoparticles. Everyone knows noble metals such as gold or silver. These metals are widely used in jewellery industry, hence one could assume that their optical properties are well known for the general public. It is obvious that gold is yellowish, and silver greyish, isn’t it? However, as this statement is true in macroscale, it turns out that in nanoscale gold and silver exhibit completely different optical properties.

When a piece of metal becomes very small, in the range of one billionth of a meter (nanometer), it is called nanoparticle. Optical properties of such structures differs from ones that we know from our everyday life. As bulk silver is grey and bulk gold is yellow their nanoparticles can have all colours of the rainbow from red through green to violet, depending on their size and shape. Thanks to their outstanding optical properties gold and silver nanoparticles were used for centuries as a dye. One of the best known example of such use is the Lycurgus cup, a 4th-century Roman cup in which the exceptional optical appearance was achieved by the use of gold and silver nanoparticles dispersed in the glass material.
The Lycurgus cup, which is shown in the top of this post, looks different in reflected (lit from the front, left side in the image above) and transmitted light (lit from behind, right side of the image above). The cup is exhibited at the British Museum (Image source: https://www.britishmuseum.org)

Even though gold and silver nanoparticles has been used for centuries, people could barely control the shape and the colour of obtained nanostructures. The reason for that is the abundance of factors that play a role in the synthesis of noble metal nanoparticles, and have to be precisely controlled throughout the process. Moreover, the synthesis of noble metal nanoparticles is extremely sensitive. Therefore, even slight change in reaction conditions can completely alter the final outcome. Explicit example of this sensitivity is the synthesis of gold nanorods and concave cubes. Both syntheses are carried out by a slow growth of small gold nanocrystals in a solutions containing chloroauric acid, silver nitrate, ascorbic acid, and CTAB (cetrimonium bromide) or CTAC (cetrimonium chloride) respectively. Hence, changing one ion (bromide to chloride) one can obtain blue solution of concave cubes instead of brown solution of nanorods.

Currently, scientists can precisely control many crucial factors during the synthesis of noble metal nanoparticles, which allows them to synthesise specific nanoparticles in high yield. Moreover, due to high sensitivity, which might have looked like an inconvenience, scientists can easily obtain new shapes of nanoparticles by playing around with the synthesis conditions. Up to now, by adjusting basic reaction conditions such as temperature, pH, type of solvent, and ionic composition, scientists were able to synthesize various nanoparticles such as: prisms, cubes, tetrahedrons, rods or bipyramids, having the whole spectrum of colours.

This is all, I wanted to show you this time. I hope that from now on you will be eager to spare your golden ring to obtain beautiful solution of gold nanoparticles. Stay tuned!