John Watt demonstrates one way to make gold nanoparticles. They form by chemical reaction inside tiny spheres called micelles, created by surfactants (soaps).
Making gold nanoparticles in micelles.
We start with gold chloride – a salt that will be the source of our gold. To this, we add a surfactant – a soap that will help control nanoparticle growth. Octane – our main organic solvent – is added next, followed by butanol – another solvent. Water is added next.
The surfactant has molecules with a hydrophobic tail and hydrophilic head. Because of this, it forms tiny spheres called micelles. The water is trapped inside these micelles, separating it from the organic solvent.
Because the gold chloride is soluble in water, it too ends up inside the micelles.
We now add sodium borohydride, a few drops at a time. Because this is water soluble, it enters the micelles.
Here, it reduces the gold chloride and forms metallic gold, which crystallises into gold nanoparticles.
You can imagine the micelles as nanoreactors containing a chemical reaction in water. Because of the small size of the micelles, once the gold chloride has been reduced, the nanocrystals of gold stop growing.