"While our papers discuss sensor operation at elevated temperatures, which is amazing, the room (skin) temperature performance is what is exciting to us," researcher Craig Grimes of Penn State told nanotechweb.org. "It will enable a skin-temperature, bandage-like hydrogen sensor to be placed on the newborn infant for continuous, real-time monitoring. Our specific interest is the use of the transcutaneous sensor for monitoring for an unpleasant bacterial infection that is too common in premature infants and is generally fatal."

Grimes and colleagues prepared arrays of titania nanotubes by anodizing titanium foil in an electrolyte of 0.5% hydrofluoric acid. The electrical conductivity of the nanotubes increased dramatically on exposure to hydrogen. In fact, the tubes have a hydrogen sensitivity more than an order of magnitude larger than the next best material.

The electrical conductance of an array of titania nanotubes with an inner diameter of 22 nm went up by a factor of 104 on exposure of the tubes to 1000 ppm hydrogen at 290°C. But nanotubes with an inner diameter of 76 nm, which had half the surface area of the smaller tubes, were 200 times less sensitive to hydrogen.

"It's actually due to the nano-architecture of the tube arrays, not just a simple scaling of surface area," said Grimes. "Titania is a semiconductor and the hydrogen ions apparently diffuse through the 'necks' of the tube-to-tube contact points, dramatically changing the electrical resistance in response to hydrogen. Crucially, we are able to readily achieve sub 1 ppm hydrogen sensitivities with sensors at room temperature, hence you have minimal power requirements."

According to Grimes, the titania nanotubes also have outstanding photocatalytic properties. As a result, sensors using the tubes could be able to self-clean through exposure to ambient ultraviolet light. "Sensors get dirty with use, which typically ends their useful lifetime and, usually, the more sensitive the sensor the quicker it gets dirty, becomes of no use, and has to be replaced," he said. "A self-cleaning sensor would be a tremendous step forward in sensor technology."

Now the team is working on the effects of changes in humidity on the output from the titania nanotube hydrogen sensors. It looks like the answer could be cross-calibrating the output with that from a nanoporous alumina sensor that measures humidity well but doesn't react to hydrogen.

"Once all the loose ends associated with making a sensor suitable for real-world application are tied up - in about a year's time - we will go to clinical trials at the Penn State Hershey medical school and hospital to investigate the clinical utility of the sensor," said Grimes. "If it is a useful device we will seek to license the technology to a company that sells medical equipment."

The scientists reported their most recent work in Advanced Materials and Sensors and Actuators B.