Drug delivery: keeping tabs on nanocarriers.

T he unique geometry of carbon nanotubes allows them to be used as drug-delivery vehicles or ‘nanocarriers’ in cancer therapy and other areas of medicine1. Nanocarriers work by bringing drugs directly to diseased areas of the body, thereby minimizing the exposure of healthy tissues while increasing the accumulation of the drug in the tumour area. This reduces both the dose necessary for treatment and the damage that can be caused to healthy tissue by powerful (and expensive) pharmaceuticals. To convert a nanotube into a nanocarrier, it must be able to target tumours, and this ability could be introduced by attaching a peptide or an antibody to its outer surface — an approach that is already widely used in nanomedicine2. On administration of the nanocarrier to the body, the peptide or the antibody would bind to its target, and the drug — which could be inside the nanotube, or attached to it — would then be released. The release of drug molecules from a nanotube can be triggered by either a change in pH or by enzymes produced by the tumour that ‘cut’ the drug molecules from the nanotube. These techniques would ensure that the nanotube reaches its target, but it is also important to know where the nanocarriers are in the body. Writing in Advanced Materials, Donglu Shi and co-workers at the University of Cincinnati, Shanghai Jiao Tong University, the University of Michigan and the Argonne National Laboratory report a significant step forward in this direction3. Shi and co-workers used a process called plasma surface polymerization to make novel nanostructures by combining quantum dots and multiwalled carbon nanotubes (Fig. 1). As carbon nanotubes are not chemically reactive, this technique treats them with a variety of chemical compounds so that the quantum dots can attach. The new nanostructures combine the drugdelivery potential of the nanotubes with the fluorescence properties of quantum dots, which allow the position of the nanocarrier in the body to be monitored. To demonstrate the utility of their approach, Shi and co-workers injected mice with varying amounts of their new nanostructures and found that the fluorescence signal was qualitatively correlated to the amount injected. Moreover, they showed that the new nanoparticles were bright enough to be seen even when injected into the liver, kidney and leg muscle of a mouse. In other words, the fluorescence signal was strong enough to travel through several millimetres of tissue to reach the detector. The team focused on two types of quantum dots: CdSe/ZnS, which emit in the visible range, and InGaP/ZnS, which emit in the near-infrared and hence are better suited for deep-tissue imaging. Combining the optical properties of quantum dots with the ability of carbon nanotubes to carry pharmaceutical cargos could prove highly beneficial in the field of drug delivery. DruG DelIvery