The prospect of new electronics, materials and biomedical applications has boosted research on the synthesis of carbon and metal-based nanotubes. However, no metal–organic supramolecular solids have yet been shaped into nanotube backbones.

Now, Hua Chun Zeng and Yu Xin Zhang at the National University of Singapore1 have for the first time synthesized nanotubes that incorporate hybrid metal–organic complexes into their wall structure.

The researchers used a self-assembly approach to produce bilayer structures of thin platelets of gold ions bound to alkanethiolate ligands — hydrocarbon chains terminated by anionic sulfur. The resulting sheets then curled into nanotubes over time.

“We were surprised to see the gold–alkanethiolate sheets curling into tubular structures with time,” says Zeng. “The forces holding the different layers in the nanotube structures are van der Waals interactions between the alkanethiolates.”

Nanotubes were produced by adding a gold ion precursor–ligand mixture to an alkaline solution containing a non-ionic surfactant. The surfactant was found to improve nanotube quality and uniformity, and the hybrid sheets curled into tubes at specific hydrocarbon chain lengths. Noting that the inner layers of the nanotubes contained less gold ions than the outer layers, Zeng believes that this optimal chain length balances attractive inter-chain van der Waals forces and repulsive interactions between gold ions in neighboring layers, stabilizing the tubular structure.

The nanotubes remained intact when subjected to various strong acids, bases and reducing reagents, as well as common solvents. This exceptional chemical stability and robustness is thought to originate from the protection provided by the hydrophobic alkanethiolate chains.

“Gold ions are difficult to oxidize and alkanethiolates have straight hydrocarbon chains, which are also very stable,” says Zeng.

Fig. 1: Schematic illustration showing the reduction of gold ion–alkanethiolate nanotubes into metallic gold–alkanethiolate supernanotubes.

The researchers also investigated several potential applications for their nanotubes. They demonstrated that an electron beam can reduce the gold ions into metallic gold (Fig. 1). The gold–alkanethiolate nanotubes can therefore be used as precursors for hybrid supernanotubes combining the properties of gold nanoparticles and nanotubes, with many new and promising applications. The team is currently optimizing gold reduction and ordering of the resulting gold nanoparticles.