Michael Strano and co-workers, MIT, USA, report a nanotube system that detects the passage of individual ions. Previous systems have only allowed the detection of large molecules (DNA, polymers, etc.) that spend a significant time within the pore.

Strano’s team used a single-walled carbon nanotube to connect two water reservoirs each containing an electrode. Protons flowed freely across the nanotube (~108 protons/s), but larger cations required a voltage to allow them to pass through. The passage of individual Na⁺, K⁺, and Li⁺ ions could be observed by obstruction of the otherwise stable proton current.

The researchers hope to embed these nanotubes in membranes for water desalination or use them to detect and identify impurities in drinking water as the ion transport time is size dependent.

• Coherence Resonance in a Single-Walled Carbon Nanotube Ion Channel
  C. Young Lee, W. Choi, J.-H. Han, M. S. Strano,
  Science 2010, 329, 1320 – 1324.
  DOI: 10.1126/science.1193383