"One of the major challenges was the synthesis of a highly crystalline tin nanowire and the conversion of the tin precursor nanowire to tin dioxide without changing the overall shape of the nanowire," Martin Moskovits told nanotechweb.org. "I think this is the first example of such a topotactic [structure-preserving] oxidation of a nanowire, but being first is not the main point. The ability to carry out desirable chemical processes at the nanoscale without losing one's shape is."

Moskovits and colleagues made the sensors by growing ß-Sn nanowires in a porous anodic alumina template. They then removed the wires from the template and converted them to a p-type metastable tin sub-oxide (SnO) phase, and then to n-type SnO2. The resulting nanowires were around 60 nm in diameter: the researchers say that this is small enough for the adsorption of oxidizing or reducing gases on the surface to alter the bulk electronic structure of the entire nanowire.

The conductance and current-voltage characteristics of the nanowires depended strongly on the temperature and ambient gas conditions. For example, in the absence of oxygen a nanowire was a fairly good conductor, but it became an insulator once a quantity of oxygen was added. By contrast, the presence of a combustible gas such as carbon monoxide increased the nanowire's conductivity.

"The synthesis we use is potentially a low-cost process," said Moskovits, "making it eventually possible to configure an entire building or complex, like an airport, with thousands of intelligent sensors networked together to provide an instantaneous, spatially-coherent picture of the chemical composition of the building's atmosphere - both the substances that should be there and those we wished were absent."

Now the team plans to functionalize nanowires, produce nanowires from different semiconductor materials and "multiplex the lot as a next step in creating highly sensitive intelligent sensors, approaching, in time, the sensing capability and cognitive pattern-recognition ability of a mammalian nose."

The scientists reported their work in Advanced Materials.