Carbon nanotubes are hollow one-nanometre-thick cylinders of rolled up carbon sheet and could be ideal as biosensors because they naturally fluoresce in the near-infrared region of the electromagnetic spectrum when exposed to near-infrared light. Biological tissue and fluids are particularly transparent in this part of the spectrum.

In previous work, the team, led by Michael Strano and Nicole Iverson, found that carbon nanotubes can be used to detect nitric oxide (NO) if the tubes are wrapped in DNA with a particular sequence. Once a NO molecule binds to the DNA-carbon nanotube complex, the fluorescence signal is quenched and this quenching is used to monitor NO binding. NO is an important signalling molecule in living cells since it is a neurotransmitter and coordinates immune system functions. NO levels are also often disrupted in cancer cells.

Implantable or injectable

Now, the researchers have further modified the nanotubes to make two new types of in vivo biosensor. The first can be injected into the bloodstream thanks to it being wrapped in PEG – a biocompatible polymer that prevents the tubes from clumping together in liquid environments, like blood. The second sensor was made by embedding the nanotubes in an alginate gel – a polymer extracted from algae.

To test their devices, the researchers injected the first type of sensor into live mice and implanted the second under the skin of a different batch of animals. By again monitoring nanotube fluorescence quenching, they found that the injectable sensor enters the bloodstream and once here can be used to monitor NO levels in organs like the liver before it is naturally eliminated by the kidneys. The implantable device can last much longer – as long as 400 days – while continuing to function perfectly during this time. This kind of sensor could be used to monitor cancers and other inflammatory diseases thanks to long-term detection of NO, says Iverson.

“These devices are minimally invasive and can be used over and over again,” she explains. “The nanotubes fluoresce when we shine infrared light on them but quickly quench when in the presence of NO. However, the fluorescence returns once NO is removed from the system.”

“Our work opens a new field for CNT sensors, pushing into the in vivo realm,” she told nanotechweb.org. “They could be used for long or short term detection of a large array of chemical analytes in the body and the NO sensor, in particular, could be a new way to detect inflammation, whether from cancerous cells or other disease states.”

The MIT team is now busy working on adapting its technology to detect glucose, by wrapping different kinds of molecule around the CNTs. “We are also looking at altering the hydrogels that encapsulate the sensors to allow the devices to travel further into tissue,” added Iverson.

More details about the research can be found in Nature Nanotechnology doi:10.1038/nnano.2013.222

Further reading

Single protein detection goes label free (Jul 2011)
Anomalous thickness-dependence of photocurrent explained for planar nano-heterojunction organic solar cells (Feb 2012)
CNT transistors detect cancer biomarkers (Feb 2013)
Nanotube transistors make good glucose sensor (May 2013)