Massachusetts Institute of Technology, Cambridge, USA

A chemist predicts a bright future for sensors based on carbon nanotubes.

I am struck by the parallels between the development of polymer-based chemical sensors and those made from carbon nanotubes.

About ten years ago, I started to develop sensors from conjugated organic polymers, which took advantage of the materials’ optical properties, rather than the electrical properties that had been exploited in devices until that time. This work led to fluorescent sensors, which are now being used in Iraq to detect explosives.

As with polymers, early work on nanotube sensors focused on detecting changes in a tube’s electrical conduction when it binds to a molecule of interest. But electrical responses are sensitive to stray electric fields, which create interference in the signal.

Now, researchers working with nanotubes are also moving towards optical methods. A demonstration of a biosensor for glucose (P. W. Barone and M. S. Strano Angew. Chem. Int. Edn 45, 8138–8141; 2006) sets the stage.

To make the sensor, the team first attached glucose groups to nanotubes. They then mixed these nanotubes with a large molecule, known as concanavalin A, which can bind to four glucose molecules at once. The glucose-decked nanotubes end up caught in clumps around the concanavalin A, which attenuates their emission. This system is sensitive to glucose because any glucose in solution loosens the nanotube clusters, and so boosts fluorescence.

A significant advantage of nanotubes is that they emit near infrared light, a longer wavelength than that accessible with polymers. And it just happens that human tissue is almost transparent in this spectral region. As a result, sensors based on these materials might be used for in vivo clinical diagnostics.