Transport and encapsulation of gold nanoparticles in carbon nanotubesElectronic supplementary information (ESI) available: Detailed description of nanotube processing and characterisation procedures. See DOI: 10.1039/c0nr00035c

Nanoparticles confined in small volumes exhibit functional properties different from that of the bulk material. Furthermore, the smaller the volume available then the greater the effects of confinement are observed to be. Metallic nanoparticles encapsulated within carbon nanotubes have been proposed for many applications ranging from catalysis to quantum storage devices. In this study we examine encapsulation of discrete gold nanoparticles (AuNP) within multi-wall carbon nanotubes (MWNT), with internal diameter less than 10 nm. During the encapsulation process AuNP undergo Ostwald ripening allowing them to reach a diameter that precisely matches the internal diameter of MWNT (snug fit). The use of supercritical CO 2 as a processing medium enables efficient transport and irreversible encapsulation of AuNP into narrow nanotubes. Once inside MWNT, the nanoparticles are unable to grow further and retain their spheroidal shape. This dynamic behaviour observed for AuNP differs significantly from the behaviour of molecular guest-species under similar conditions. Carbon nanotubes with internal diameters less than 10 nm have been filled with discrete gold nanoparticles using supercritical CO 2 .