Credit: © 2007 AAAS

Metals can generally be shaped into arbitrary structures by heating them to very high temperatures, at which point they can either be poured into molds or pounded with the blacksmith's hammer. Now, researchers from Northwestern University in the USA have used metal nanoparticles as building blocks to create plastic-like materials that can be molded into macroscopic objects.

Bartosz Grzybowski and co-workers1 use long molecular chains with thiol groups at each end to link individual gold nanoparticles into larger assemblies called 'supraspheres'. The size of these spheres ranges from 50–300 nm and can be controlled by changing the concentration of the nanoparticles or linkers. Although the supraspheres predominently consist of metal, they are easily deformed and behave as a plastic solid. Further assembly of these 'sticky' building blocks onto master templates results in freestanding materials that can be hardened by heating at relatively low temperatures (˜50 °C). Moreover, the porosity of the heat-treated materials can be easily tuned as it is determined by the initial size of the supraspheres.

This technique is a general one and has been applied to other metals, such as silver, platinum and palladium. By using supraspheres made from two different kinds of nanoparticles, bimetallic structures can be formed, and it is suggested that these porous metallic materials of controlled composition could find applications in separation science and chemical catalysis.