Tiny metallic ‘nanosprings’ could soon be used to disrupt cell components or generate magnetically induced currents, according to a study by Hongyu Chen and coworkers from Nanyang Technological University and the National University of Singapore.1 The team has designed ultrasmall coils that can store and release mechanical energy by contracting and relaxing their polymer shells.

Fig. 1: Transmission electron microscopy image of ultrasmall polymer-coated gold springs in their coiled state. © 2010 X. Jun

Many coiled nanostructures have been prepared by gas-phase deposition of materials on substrates such as metal oxides or carbon nanotubes in solution. However, these approaches are unsuitable for the production of metallic coils. Chen and his co-workers have now developed a controllable and scalable solution-based method to prepare metal nanosprings by wrapping prefabricated ultrathin gold nanowires in polymer shells (Fig. 1).

“We observed that in polymer-coated gold nanorods, the shells were stretched non-uniformly. They were thinner on the ends and thicker on the sides,” says Chen. “Hence, we decided to put polymer shells on the thinnest nanowires we could find.”

The team mixed the nanowires with a block copolymer — composed of two distinct polystyrene (PS) and poly(acrylic acid) (PAA) subunits — in an organic solvent. The block copolymers form a swelled tubular shell around the metal, resulting in a cylindrical tube in which the PS subunits bind to the nanowire surface and the water-soluble PAA points outward.

The assembly was then diluted with a large amount of water, which increased the surface tension at the polymer–water interface as the organic solvent escaped from the swelled shell. This caused the polymer shell to contract and the nanowire to coil.

According to Chen, the team’s initial goal was to form random coils to demonstrate the force exerted by the polymer shells. Surprisingly, the obtained coils were highly uniform with multiple loops packed together in the same plane. “This is probably because the first loop formed provides a structural template for the subsequent loops, and the packing among the loops makes them more stable,” he says.

The researchers are currently investigating other systems that are even easier to manipulate and have recently produced coiled carbon nanotubes using a modified method. Their long-term goal is to use the nanosprings in biological and nanoelectronic applications.