The device: Flexible self-assembled nanotubes have arrived. Researchers at Seoul National University used crimped macromolecules that are hydrophobic on one side and hydrophilic on the other to create rings that, in aqueous solution, stack of their own accord, creating nanotubules that expand and contract in response to changing temperature. They published their technique today (September 20) in Science.
Although the molecules fit together easily, they’re not connected by covalent bonds, “which endows the rings with flexible diameter through sliding motion between the molecules,” lead author Myongsoo Lee explained in an email. Lee and his team could prompt this behavior by adjusting the temperature, causing the nanotubule interiors to fluctuate in size by 3 to 4 nanometers. The nanotubes expanded as temperatures dropped and contracted as their environment heated, creating a temperature-dependent response “similar to a pulsating blood vessel,” explained Lee.
The hollow nature of the nanotubules also has potential benefits. Medical researchers hope that by packing nanotubes with drugs they can help target therapeutics to specific areas of the body while limiting side effects elsewhere.
In order to test how their nanotubes interacted with internal particles, Lee and his team seeded the system with light-sensitive fullerene molecules, which have been used in applications ranging from anti-cancer therapy (sensitizing cancer cells to light-induced damage) to capturing the sun’s energy in solar cells. The fullerenes are taken up during nanotubule self-assembly due to their hydrophobic nature. Lee and his colleagues demonstrated that the fullerenes packed more densely in contracted tubules than those that were expanded, Contracting the nanotubes further caused them to expel some of the fullerenes.
What’s new: Previous tubules are assembled via covalent or hydrogen bonds, Lee explained, which give the tubules very little flexibility. “To overcome this limitation,” Lee said, “we make hexameric rings by using 6 bent-shaped molecules as cyclic building blocks,” allowing flexibility through “simple sliding motions between adjacent molecules.”
Additionally, “it is not an easy task to assemble molecules into really well-defined nanostructure,” Ling Zang, a professor of nanotechnology at the University of Utah who was not involved in the work, explained in an email. But Lee and his team were able to use the attractions between aromatic rings in their macromolecules to create the desired structures. “The group has now come up with a smart molecular design and highly controlled dynamic assembling processing,” Zang said.
Importance: Such flexible nanotubes have potential to act as templates for other nanostructures, said Lee. Many nanotechnology applications are designed to take advantage of the electrical or magnetic properties of nanoparticles, such as in solar cells that rely on an electron cascade to transfer solar energy to battery electrodes. But to be arranged properly, such particles need templates, Lee explained.
Having flexible templates could also allow researchers to alter the nanostructure’s properties. For example, contracted nanotubes hold nanoparticles closely packed, forming a highly conductive material, but expanding the nanotubule makes the particles separate, reducing their conductivity. “In this regard, our tubules provide a switch between conductor or magnetic wire and insulator triggered by external stimuli,” Lee said.
Needs improvement: So far Lee’s group has only investigated how fullerenes interact with their nanotubules, but the researchers are beginning to examine a range of different nanoparticles and even proteins to understand how they switch between packed and loose states. Nanotubes used to deliver drugs must be able to release them into target cells, and the authors of an accompanying perspective article in Science speculate that tubule expansion and contraction could act in a pump-like fashion to move materials through the tube.
Furthermore, the nanotubes themselves have potential for modification to enable their use in future applications, said Zang. Though the conducting properties Lee’s current nanotubes haven’t been tested, Zang expects that if properly modified, they could potentially act as conducting polymers, engaging with their internal fullerenes to enable electron transport “for a new generation of organic solar cells.”
Z. Huang et al., “Pulsating tubules form noncovalent macrocycles,” Science, 337:1521-1526, 2012.