Polyethylene glycol (PEG) functionalized double-walled carbon nanotubes (DWNTs) have been synthesized by a [2 + 1] cycloaddition reaction and characterized by transmission electron microscopy, atomic force microscopy, Raman spectroscopy, thermal gravimetric analysis, and UV–visible spectroscopy. Functionalization affords a large increase in the aqueous solubility of DWNTs. The saturated concentrations of DWNTs functionalized with diazido-terminated PEG₈₀₀ (with a molecular weight of 800) and azido-terminated PEG₇₅₀ monomethylether (with a molecular weight of 750) are very similar—0.36 and 0.37 mg/mL (DWNTs equivalent concentration), respectively.

Titania nanotubes (TiO₂-NTs) are a potential drug vehicle for use in nanomedicine. To this end, a preliminary study of the interaction of a model cell with TiO₂-NTs has been carried out. TiO₂-NTs were first conjugated with a fluorescent label, fluorescein isothiocyanate (FITC). FITC-conjugated titania nanotubes (FITC-TiO₂-NTs) internalized in mouse neural stem cells (NSCs, line C17.2) can be directly imaged by confocal microscopy. The confocal imaging showed that FITC-TiO₂-NTs readily entered into the cells. After co-incubation with cells for 24 h, FITC-TiO₂-NTs localized around the cell nucleus without crossing the karyotheca. More interestingly, the nanotubes passed through the karyotheca entering the cell nucleus after co-incubation for 48 h. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) were also employed in tracking the nanotubes in the cell. These results will be of benefit in future studies of TiO₂-NTs for use as a drug vehicle, particularly for DNA-targeting drugs.