"Our 60 ppb detection limit is the one of the best reported so far under ambient conditions," team leader Goutam Koley of the University of South Carolina told nanotechweb.org. "To our knowledge, this is the best result using a large carbon-based sensing layer."

Koley and co-worker Muhammad Qazi's nanostructured graphite layer is easy and inexpensive to make through mechanical exfoliation of graphite. The layer can be put on any type of substrate – metal, insulator or semiconductor.

The device detects in "non-contact" mode. This makes it unique because it is entirely surface-based, unlike commonly employed sensors that rely on amperometry, or changes in the bulk conductivity of the sensing layer.

Instead, detection is based on the work function changes of the surface caused by the adsorbed gas molecules. These potentiometric changes are measured using a highly sensitive resonant microcantilever, which is located a few microns away from the sensing surface. "Our device also avoids one of the major obstacles in carbon-based sensing, which is predominantly dependent on carbon nanotubes that are difficult to control and expensive to produce," said Koley.

"NO2 is a toxic gas that can affect the respiratory system upon prolonged exposure," added USC Nanocenter collaborator Thomas Vogt. "It is thus one of the primary health hazards that needs to be sensed down to very low concentrations."

The new detector could find use in monitoring vehicle exhausts, industrial environments and when assessing indoor air quality.

The work was reported in Appl. Phys. Lett.