Metal–organic nanotubes (MONTs) offer an attractive alternative to carbon nanotubes because the backbones of MONTs incorporate metal cations. This hybrid structure makes them potential useful materials for gas storage, catalysis and drug delivery. MONTs could also be used as cylindrical vehicles, micro-reactors or hollow nanofibers. However, few researchers have focused on the development of these materials.

Fig. 1: (a) Molecular representation of a self-assembled dragon column-like nanotube. (b) Photograph of a Chinese dragon column.

Now, Kuang-Lieh Lu at the Academia Sinica in Taipei, Taiwan and colleagues1 have designed cadmium metal–organic frameworks that ’roll up“ to self-assemble into arrays of single-walled nanotubes. Their unique nanotubes resemble dragon columns, a type of ancient Chinese totem (Fig. 1).

“The fabrication of MONTs is simple, safe and cheap,” says Lu. “Moreover, the target structures are easy to design and tune.”

The nanostructures result from careful selection and combination of metal centers as nodes and organic scaffolds as linkers. In their nanotube design, the researchers selected cadmium ions, which can coordinate to four ligands, and banana-shaped organic linkers displaying a 120° bending angle between metal-connecting extremities.

The research team simply reacted the cadmium(II) ions and ligand precursors at 4 °C in the presence of alkali metal cations in an organic solvent–water mixture to generate their nanotubes in a single step.

After isolating the nanotubes as crystalline structures, the researchers analyzed their nanotubes by single-crystal X-ray diffraction. They discovered that the alkali metal cations were positioned in close proximity to the cadmium(II) ions along the nanotubes, acting as a cement holding the cadmium ions together in a three-dimensional array.

The X-ray measurements also revealed that the bending angle in the organic linker was crucial in determining the nanotube cross-section. The 120° bending angle gave cadmium-based MONTs with a hexagonal cross-section. “If the bending angle between the connecting functional groups was 60 or 90°, the metal–organic framework would tend to form nanotubes with triangular or rectangular cross-sections, respectively,” says Lu.

The researchers are currently preparing different MONTs with tunable diameters by modifying the topology of a variety of two-dimensional sheets, which roll up to form the nanotubes.