Disclosed are nanoparticles, such as carbon nanotubes or other materials having extended aromatic surfaces (e.g., graphene sheet or nanotube), 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 to render them stable in suspension. This molecule may be covalently attached to the nanoparticle, or may be adsorbed thereto as an amphiphilic molecule. The nanoparticles are coupled to the drug through supramolecular bonding i.e., binding to the exterior of the nanoparticle through π-stacking. The drug may also be covalently bonded to the hydrophilic polymer, which is coupled to the nanoparticle through supramolecular bonding. 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-coupled, functionalized nanoparticles may also be targeted to specific cells through modification of the hydrophilic polymer, e.g., by adding an RGD peptide, or an antibody, which is targeted to cells expressing integrins, or an antibody directed to a cell surface marker. 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.