MIT researchers have built the first sensor array that can detect single molecules produced by living cells, using a carbon nanotube sensor array that can detect hydrogen peroxide molecules and could help scientists learn more about that molecule’s role in cancer.

The sensor consists of a film of carbon nanotubes embedded in collagen. Cells can grow on the collagen surface, and the collagen also attracts and traps hydrogen peroxide released by the cell. When the nanotubes come in contact with the trapped hydrogen peroxide, their fluorescence flickers. By counting the flickers, one can obtain an accurate count of the incident single molecules.
More info: MIT news