Functionalized C₆₀ Peas in a Pod

Using supercritical carbon dioxide, scientists in England have inserted fullerene molecules with exterior organic functional groups into single-walled carbon nanotubes (SWNTs). The team also showed that encapsulation of the functionalized fullerenes can be enhanced or inhibited by altering the functional group.

The work was carried out by Andrei N. Khlobystov and David A. Britz of Oxford University, Martyn Poliakoff and Jiawei Wang of the University of Nottingham, and their coworkers [Chem. Commun., 2004, 176].

SWNTs incorporating fullerenes such as C₆₀ or C₇₀ have so-called pea pod structures. They are designated C_n@SWNT and are generally prepared in the gas phase at 300 to 500 °C. Organic functional groups decompose in this temperature range.

"We have demonstrated that filling SWNTs in supercritical CO₂ allows insertion of the C₆₀ molecules bearing chemical functionalities at low temperatures without affecting the functional groups," Khlobystov tells C&EN. "Our low-temperature technique opens up the possibility of generating new forms of nanotube-based structures and, in turn, the potential to explore applications such as catalysis, molecular separations, and nanoscale drug delivery."

Khlobystov and colleagues showed that fullerenes with exterior ester groups can enter SWNTs in supercritical CO₂ at temperatures as low as 30 to 50 °C, forming C₆₁(COOC₂H₅)₂- @SWNT pea pod structures.

High-resolution transmission electron microscopy (HRTEM) revealed that the pea pod structures formed in 60% of the nanotubes observed.

"This is the first example of a nano pea pod containing functionalized fullerenes," Khlobystov says.

The group also used HRTEM to show that fullerenes bearing carboxylic acid groups, C₆₁(COOH)₂, aggregate by hydrogen bonding to form a dimeric supramolecular complex. The dimer sterically hinders encapsulation and preferentially coordinates to the outside walls of the SWNTs.

The low-temperature filling technique takes advantage of the hybrid gas-liquid properties of supercritical CO₂ such as low viscosity and zero surface tension.

"There is also no solvent residue," points out Poliakoff, an expert on the use of supercritical fluids. "The method is an interesting application of CO₂ that opens up exciting new possibilities for nanotechnology."

The Oxford group is particularly interested in using the technique to generate nanotubes containing geometrically regular arrays of magnetically active molecules, such as organic radicals or metal-containing fullerenes.

"These structures are candidate materials for solid-state quantum computing," Britz says. "The supercritical CO₂ method enables us to assemble such arrays using thermally unstable molecules that exhibit unique magnetic properties. We can effectively create one-dimensional molecular arrays with periodic and controlled spacing."

The group also notes that SWNTs containing electronically active functionalities could potentially be useful for creating nanotube-based electronic devices.