Until now, scientists have only been able to assemble nanosized objects into relatively small structures, such as micron-sized crystals and nanoparticle coatings. "The problem with these approaches is that the small pieces do not want to stick together beyond a certain limit," explained team leader Bartosz Grzybowski of Northwestern University. "Even if one uses many nanoparticles and tries to cross-link them, there are some limitations due to nucleation and growth and one usually ends up with small clumps rather than macroscopic materials."

Grzybowski and colleagues have now developed a conceptually different approach that overcomes this challenge. Instead of going all the way from nano to macro, the researchers build intermediate-sized supraspheres, and then build these up into macromaterials. The supraspheres are no longer rigid objects but display properties such as plasticity and stickiness. "They are like little pieces of Playdoh that can easily be 'glued' into larger structures," says Grzybowski.

The Northwestern team begin with metal particles of either gold or platinum that are about 5 nm across and coat them in a surfactant. Next, the researchers expose the particles to ultraviolet radiation, which causes the particles to cross-link together into spherical, sticky aggregates that are between 50 and 300 nm in diameter. These supraspheres are malleable, show metallic properties and can be assembled into larger, millimetre-sized structures.

Another startling finding is that if the spherical building blocks are gently heated at 50 °C, they harden to form a solid metallic block. "From a practical perspective, this means we can go from the waxy to pure metallic state at almost room temperature," enthused Grzybowski. "To my knowledge, there are no other materials that have such properties."

The mouldable metals could be useful for metallurgy and because they are nanoporous, they could find use in catalysis applications too. "In this context, one of the bimetallic structures we made is really fascinating because it is both nanoporous and can have two catalytic materials 'mixed in' at the nanoscale," Grzybowski told nanotechweb.org. "As such, it can be used to catalyse more than one reaction at once – a very hot topic that our group is working on 24/7."

The team now plans to also make similar monoliths from non-metallic nanoparticles, such as semiconductors. "Finally, there are tonnes of fundamental questions to be answered, for example, why do these materials conduct in the waxy state and what is their band structure?" asked Grzybowski.

The researchers reported their work in Science.