— Besides looking good under a scanning electron microscope, the nanoflowers detect ethanol through changes in their electrical resistance (Image: IOP/Harbin Engineering University)
(Image: IOP/Harbin Engineering University)
Spectacular flower-like nanostructures grown in a laboratory in China can detect alcohol and might also be useful as catalysts.
The "nanoflowers" were made from zinc oxide by Yujin Chen and colleagues at Harbin Engineering University. Conventional ethanol sensors are made from the same material and work by detecting the change in electrical resistance when a wad of zinc oxide powder or a layer of the material is exposed to ethanol vapour.
But these conventional sensors look set to be replaced by a new generation of detectors. "The sensor materials will probably be replaced with tailored nanostructures since they appear to give a higher sensitivity," explains Edman Tsang, who researches new nanomaterials at the University of Reading in the UK.
Zinc oxide sensors need to be heated to temperatures of up to300°C before they become sensitive to ethanol. Chen's nanoflowers become sensitive at just 140°C.
"The observation of ultra-high sensitivity for ethanol at such an unusually low temperature is striking," says Tsang, who was not involved in the research. The new operating temperature is low enough to be practical to be built in solid state sensors such as lab-on-a chip devices.
The advantage of nanostructures is that they are more compact and use less power than conventional ones. Nanosensors consume just nanowatts of power, while those on a larger scale can use milliwatts.
Zinc oxide changes resistance when molecules of ethanol vapour stick onto it in a process called adsorption. The flower-like structures work at lower temperatures because their tiny size enhances adsorption. Each flower is made up of bundles of nanorods 15nm wide. They were made by blasting a zinc-containing solution with ultrasound.
To make a sensor the researchers wired up two patches of the flowers into a circuit. One was exposed to the sample and the other kept sealed. By comparing the resistance of the exposed patch with the control flowers, it is possible to tell if ethanol is present.
The sensor can detect ethanol at levels as low as about 50 parts per million, far more sensitive than is required for applications such as breathalysers. But for practical use, they need to be "doped" with platinum or gold to increase sensitivity even more a technique common in zinc oxide sensors.
Tsang says the flowers are the most sensitive un-doped zinc oxide structure he has seen, and that doping them with the right metal should make them even better. "I can envisage a wide range of potential applications," he says, "including monitoring combustion gases from factories or vehicles, food and odour analysis, or breath analysis."
"I suspect the new structure may form a good catalyst material," Tsang adds. Zinc oxide with added copper is already used in the manufacture of methanol and used to remove sulphur from natural gas. "It would be interesting to study this new material for these applications," he says.
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