If there is a spectrum that can be detected by sensors, from very small to very big, then the sensor array built by engineers at the Massachusetts Institute of Technology (MIT, Cambridge, USA) can stake a claim for the very smallest – a single molecule. The array uses carbon nanotubes, which are rapidly becoming the Swiss Army Knife of nanotechnology, to detect the molecule of hydrogen peroxide on as small a surface as a single cell. You may ask, why hydrogen peroxide?

It’s more than a disinfectant and hair bleach, that’s for sure. It’s an organic compound (H2O2) found in all organisms that use oxygen for metabolism. It has been known for some time that hydrogen peroxide can damage cells and DNA, but recently it’s also become known that it is used as a signaling molecule for the stimulation of cell growth. Signaling molecules are like messengers; they get sent when something needs to happen – in this case, the chemistry of cell metabolism comes to the point where cell growth or division is ‘necessary’ and it generates hydrogen peroxide molecules. These molecules, in turn, work with a common growth factor called EGF (Epidermal Growth Factor), which binds to its receptor on the outer membrane of cells (EGFR). This induces cells to grow or divide. In some way (still unknown) hydrogen peroxide amplifies the EGFR signal.

The hydrogen peroxide molecule is very small and typically does not leave the vicinity of a single cell – hence the need for detection at the level of one molecule and one cell.

The sensor constructed at MIT is a film of carbon nanotubes embedded in collagen (a gel-like protein that makes up about 20-35% of animal tissue). Cells are grown on the surface of this nanotube ‘carpet’ and the collagen attracts the hydrogen peroxide produced by the cells. To get to the collagen, however, the hydrogen peroxide must enter a nanotube. The nanotubes have been treated with a fluorescent material that reacts in the presence of hydrogen peroxide. The nanotubes ‘flicker’ when reacting, which can be recorded and counted – thus providing an accurate count of the production of hydrogen peroxide, one molecule at a time.

This has a variety of uses…

The team also found that in skin cancer cells, believed to have overactive EGFR activity, the hydrogen peroxide flux was 10 times greater than in normal cells. Because of that dramatic difference, Strano believes this technology could be useful in building diagnostic devices for some types of cancer.”You could envision a small handheld device, for example, which your doctor could point at some tissue in a minimally invasive manner and tell if this pathway is corrupted.”

Strano points out that this is the first time an array of sensors with single-molecule specificity has ever been demonstrated. He and his colleagues derived mathematically that such an array can distinguish “near field” molecular generation from that which takes place far from the sensor surface. “Arrays of this type have the ability to distinguish, for example, if single molecules are coming from an enzyme located on the cell surface, or from deep within the cell,” says Strano.

Strano’s team is also working on carbon nanotube sensors for other molecules. The team has already successfully tested sensors for nitric oxide and ATP (the molecule that carries energy within a cell). “The list of biomolecules that we can now detect very specifically and selectively is growing rapidly,” says Strano, who also points out that the ability to detect and count single molecules sets carbon nanotubes apart from many other nanosensor platforms.

[Source: EurekAlert]

At the moment, the single molecule sensor array is more of a laboratory tool. It will be useful in pursuing the role of hydrogen peroxide in cell growth, of course; but if the approach is easily manufactured, and adaptable to other molecules – then this will become part of the ever increasing range of sensor technology.