Making gold and silver nanoparticles
I was truly amazed when we were shown the range of colours that silver nanoparticles can produce depending on their size.
We were shown pictures of the different colours. I found the above image from the above web link. Each colour is caused by different size silver nanoparticles suspended in a collide (Ag NPs). The peak light absorptions range from 393 to 738 nm. The scientists responsible for making the colloids used many techniques including high-resolution transmission electron microscopy to determine the size and shape of the different nanoparticles causing the different colours.
The above image shows silver nanoplates
The above image shows 3rd prize in NanoArt 2013. It is called “Window on silver nanoparticles” by Inam Mirza, CRANIN with Prof James Lunney. Silver nanoparticles.
Gold nanoparticles were a bit more boring!
Gold Citrate synthesis of nanoparticles
Dispersions of discrete gold nanoparticles in transparent media provide a fascinating range of colours, only recently exploited in the manufacture of paints and coatings. The shape of the particles and the viewing conditions determine the colour we see. The gold particles in the test tubes on the left are shown in transmitted light, while the image on the right shows the same gold nanoparticles viewed in reflected light.
The diameter of gold nanoparticles determines the wavelengths of light absorbed. The colours in this diagram illustrate this effect.
The above right image shows gold nanoparticles.
The above image shows how both gold and silver nanoparticles affect the colour of the colloidal solution.
The image below left shows gold and silver nanoparticles of varying sizes and shapes. From left to right: 80nm silver nanospheres, 20 nm silver nanospheres, 40nm gold nanospheres, 12nm gold nanospeheres, 200nm silver nanoplates, 120 nm silver nanoplates and 60nm nanoplates. The image below right shows plasmonic nanoparticles can have simultaneously large absorption and large scattering cross sections. Here the particles absorb green and red light causing transmitted light to appear blue, and scatter red light, causing scattered light to appear red.
Our ancestors did not know about nanoparticles but they knew how to use the different colours from gold and silver in stained glass windows.
Theodosius Arrives at Ephesus from a Scene from the Legend of the Seven Sleepers of Ephesus, ca. 1200–05. Made in Rouen, France. The Metropolitan Museum of Art, New York, The Cloisters Collection, 1980 (1980.263.4)
The above pictures show pre-prepared silver nanoparticles (left) and gold nanoparticles (right).
The image below left shows the effect of shining a laser light through one of the solutions. The image below right shows white light being shone through one of the solutions.
Making our own samples of gold and silver nanoparticles
Nanoparticles have many applications and are important in nanotechnology. Gold nanoparticles show promise as a cancer treatment, whereas silver nanoparticles are useful for killing microbes. In this activity we had a go at making gold and silver nanoparticles and filtered them with ceramic filters having 20-nm pores.
1 mM Au3+ solution (hydrogen gold tetrachloride)
1 mM Ag+ solution (silver nitrate)
1% Sodium citrate solution
Disposable 3 mL pipettes
250 mL beakers
Anotop® 10 filters
3 mL Luer-Lok™ syringes
Test tube holders
Test tube rack
Goggles, gloves and old clothes (instead of a lab coat as I didn’t own one) should be worn as in all chemistry laboratory activities.
Silver nitrate solutions darken with exposure to light and, upon contact, will leave dark stains on skin or clothing.
Skin coming into contact with chemicals should be washed thoroughly with soap and water. If solutions come into contact with the eyes, flush the eyes for 10 minutes at an eyewash station and seek medical help.
1) We add 3ml of 1.0 mM of silver nitrate to one small test tube and added 3ml of 1.0 mM hydrogen gold tetrachloride to another small test tube.
2) We placed the test tubes in a 250-ml beaker of boiling water.
3) We left the test tubes in the boiling water bath for about 10 minutes.
4) As we put into two groups one group added 10 drops of 1% sodium citrate to both of their test tubes and the other group added 15 drops to their test tubes.
5) We continued to heat the test tubes until there was a colour change in all the test tubes. The gold changed after about 5 minutes but the silver took 15 minutes.
In the above right image the silver is just starting to change colour.
6) The test tubes were removed and put into a test tube rack to cool.
7) We checked for the Tyndall effect using a laser pointer. You can also check for gold and silver in clean water using this method.
The Tyndall effect, also known as Tyndall scattering, is light scattering by particles in a colloid or particles in a fine suspension. It is named after the 19th-century physicist John Tyndall. It is similar to Rayleigh scattering, in that the intensity of the scattered light depends on the fourth power of the frequency, so blue light is scattered much more strongly than red light.
Under the Tyndall effect, the longer-wavelength light is more transmitted while the shorter-wavelength light is more reflected via scattering. An analogy to this wavelength dependency is that longwave electromagnetic waves such as radio waves are able to pass through the walls of buildings, while shortwave electromagnetic waves such as light waves are stopped and reflected by the walls. The Tyndall effect is seen when light-scattering particulate-matter is dispersed in an otherwise-light-transmitting medium, when the cross-section of an individual particulate is the range of roughly between 40 and 900 nanometres, i.e., somewhat below or near the wavelength of visible light (400–750 nanometres).
It is particularly applicable to colloidal mixtures and suspensions; for example, the Tyndall effect is used commercially to determine the size and density of particles in aerosols and other colloidal matter.
8) We carefully poured some of the contents of each test tube into clean petri dishes.
9) We first drew up 1ml of the gold nanoparticles into a syringe followed by 1ml of air (we kept some of the mixture for later).
10) We attached the filter to the end of the syringe.
11) We pushed the nanoparticles through the filter.
12) We cut open the filter across the seam using a Stanley knife.
13) We repeated stages 9 to 12 for the silver nanoparticles.
The above image shows the gold filter (left) and silver filter (right).
Change the spacing between Gold Nanoparticles using NaCl
14) We added some sodium chloride grains to the unfiltered gold nanoparticles and gently mixed until there was a colour change.
The above images show SEM images of colloidal silver (left) and colloidal gold (right) on the surface of a 20 nm pore size ceramic filter.