Nanotube noses

Sniffing out gaseous substances with a sensitivity comparable to that of a sniffer dog has remained a major engineering challenge.

Research groups have approached the challenge in fundamentally different ways, using either mechanical or electrochemical changes induced in the sensor by the transient interaction with gas molecules.

Researchers working for the US homeland security program now argue that a capacitance-based sensor consisting of carbon nanotubes and a specificity-defining thin coating outperforms commercial vapour sensors in terms of speed, sensitivity and reversibility.

To construct their nanoscale nose, Eric Snow and colleagues at the US Naval Research Laboratory in Washington DC used chemical vapour deposition to grow a network of single wall carbon nanotubes (SWNTs) on a doped and oxidized silicon chip. To make the sensor selective for specific groups of chemicals, the researchers coated the tubes with ultrathin layers of chemoselective materials, in some cases as little as a single self-assembling monolayer. On top of the mesh of coated nanotubes, they deposited palladium electrodes allowing them to measure simultaneously the capacitance (across the oxidised surface of the silicon substrate) and the resistance of the nanotubes.

Testing the device with a range of small molecules, the researchers found that the capacitance change triggered by the chemicals, which were all present at a pressure corresponding to one per cent of the equilibrium vapour pressure, showed a weak correlation with the dipole moment of the molecules.

Testing their device against a commercial sensor to detect dimethylmethylphosponate (DMMP, a substance used to simulate the presence of the nerve agent sarin), the authors measured a fourfold higher sensitivity and eightfold shorter recovery time for their system.

Not content with this successful outcome, they are seeking further improvements. ’We estimate that we can increase the series capacitance by about a factor of 10, which should produce a comparable increase in response,’ they claim.

Michael Gross