The ancient art of ropemaking, which twists together thin threads into cables, can be applied to nanowires made of gold, a team at Nanjing Tech University has shown (J. Am. Chem. Soc. 2020, DOI: 10.1021/jacs.0c03445).
The researchers had previously found they could induce individual gold nanowires to twist around another to form double helices by depositing silver on the wires. This time, they first bundled multiple nanowires together and then triggered the twisting with a different metal. Yawen Wang, Hongyu Chen, and colleagues added polyvinylpyrrolidone and a commercially available surfactant to a gold nanowire solution, which caused the nanowires to aggregate into bundles of 20–500 strands apiece. The bundles ranged from 50 nm to almost 2 µm in diameter and were tens of micrometers long. The researchers then mixed the strands with a palladium salt, disodium tetrachloropalladate, and L-ascorbic acid. The acid caused the palladium to leave the salt and attach to the gold. The researchers hypothesize that the palladium partially alloys with the gold, introducing strain to the wires’ crystalline lattice, Chen says. The nanowires relax the strain by twisting, which induces all the wires in the bundle to twist in the same direction, forming the nanoropes.
Exactly what such nanoropes might be useful for isn’t clear, Chen says. The technique does demonstrate a way to mechanically transmit energy through a system, the way transmission gears do in a clock or a bicycle, and perhaps that would prove useful in some sort of nanodevice, he speculates. “Our work creates a fundamental capability and understanding, which hopefully someday would be utilized,” he says.