Carbon nanotubes get a lot of attention when researchers discuss creating molecular-scale devices such as chemical sensors and drug-delivery systems. However, making nanotubes out of other organic molecules could also offer advantages, for example easy chemical modification of the tube structure. Mitsuru Akashi and co-workers from Osaka University in Japan1 have now discovered a simple route to produce poly(lactic acid) (PLA) nanotubes through the spontaneous fusion of individual ‘nanocapsules’.

Fig. 1: Schematic illustration of the formation of PLA nanotubes from individual hollow capsules.From Ref. 1. Reproduced with permission. © 2010 ACS

Lactic acid is a familiar biochemical molecule that can be transformed into a polyester, making it useful for applications ranging from clothing to automobiles and packaging. The versatility of PLA arises from its ‘stereocomplex’ structure; because lactic acid has an asymmetric chiral site within its framework, it forms left- and right-handed isomers that pack together side-by-side after polymerization. Controlling the ratio of left- to right-handed PLA isomers allows precise control over the polymer’s thermal and mechanical properties.

Akashi and his colleagues previously exploited the side-by-side packing tendencies of PLA helixes to make ultrathin polymer films one layer at a time — an ideal way to fabricate biocompatible pharmaceutical delivery devices. In their latest effort, the researchers turned to a templating method to create nanostructured polymers for drug encapsulation. The team first coated 300 nm-wide spherical silica particles with a film composed of ten double-layers of left- and right-handed PLA isomers. They then dissolved the silica away using hydrofluoric acid, leaving behind hollow polymer shells.

Unexpectedly, drying the hollow PLA capsules from a watery dispersion led to the formation of cylindrical structures. “When we observed the hollow capsules with scanning electron microscopy, we found that tubular nanostructures as well as hollow nanocapsules were formed,” says Toshiyuki Kida, a member of the research team. The images revealed that some nanocapsules had spontaneously merged into peapod-like one-dimensional cylinders, thousands of nanometers long. Increasing the surface deposition density of nanocapsules enabled the team to produce well-defined polymer nanotubes with high efficiency.

In addition to uncovering the mechanisms behind this new nanotube synthesis, the researchers are looking to expand the range of organic components. “We are now examining nanotube formation from other types of hollow polymer capsules, including polyelectrolyte and nonionic multilayers,” says Kida.