A method of assembling metal nanoparticles into a clay-like structure that can be moulded and fired has been developed by researchers at Northwestern University, Chicago, USA.

The pliable ‘supraspheres’ were created by covering different metal nanocrystals (such as gold, silver, platinum and palladium) of five-nanometres in diameter with photoresponsive azobenzene groups. These were then irradiated with UV light, which isomerised the trans-azobenzene into cis-azobenzene and caused the nanocrystal coating to become hydrophilic. The presence of the hydrophobic solvent, toluene, induced self-assembly of the nanoparticles to form spheres.

After evaporating the solvent, the group was left with pure supraspheres (see image above) that are over 100nm in diameter and behave like ‘mouldable metal’.

‘The great advantage of this approach is that our mouldable metals have a consistency of plasticine or wax, which allows them to be moulded at room temperature,’ explains Rafal Klajn, a PhD student at Northwestern University who helped develop the material.

‘Once a desired shape has been moulded, heating at just 50ºC hardens it like pottery, yielding pure metal of an arbitrary shape,’ he adds.

Klajn says the technique will work with any metal, as ‘the properties of the nanocrystals depend on the coating, and not on the core’. Potential applications include flexible electronics, as well as the development of separation science.