Disclosed are nanoparticles, such as carbon nanotubes or other graphitic sheet materials having extended aromatic surfaces, which are used to deliver active agents such as drugs, labels or dyes (termed for convenience a “drug”) to the interior of cells. The nanoparticles are functionalized by a hydrophilic polymer or adsorption of an amphiphilic molecule to render them stable in suspension. The drug is therefore capable of release in the cell exterior. The drug is more rapidly released at lower pH, as found e.g., in tumor cells. The drug may also be linked to a branched chain hydrophilic polymer, so that each polymer molecule carries more than one drug bound by a cleavable linker.
