For some time now, scientists have known that certain nanostructures are very sensitive to the presence of various chemicals and gases, making them good candidates for use in explosives-detecting devices. Unfortunately, because they're so small, mounting a single nanostructure within such a device would be an extremely fiddly and costly process. They would also be quite fragile, plus it would be difficult to clean the detected gas from them, so they could be reused. Recently, however, scientists from New York's Rensselaer Polytechnic Institute have figured out a solution to those problems. They have created a postage stamp-sized piece of foam made from one continuous piece of graphene, that is easy to manipulate, flexible, rugged, simple to neutralize after each use ... and is ten times more sensitive than traditional polymer sensors.
To create the material, collaborating scientists at the Chinese Academy of Sciences grew graphene on a structure of nickel foam, then removed the nickel. What was left was a sort of graphene skeleton, with nanoscopic nooks and crannies, but that was macroscopic in its total size.
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The foam was tested by exposing it to ammonia and nitrogen dioxide gas, both of which are often produced by explosive substances. The gas particles stuck to the foam, changing the electrical resistance of the conductive graphene. By measuring those changes in resistance, the researchers were able to detect when the gases were present.
More specifically, in five to ten minutes the foam experienced a 30 percent change in resistance, when exposed to 1,000 parts-per-million of ammonia - that's at room temperature and atmospheric pressure. By contrast, in order to achieve the same change in resistance in the same amount of time, polymer sensors would require a concentration of at least 10,000 parts-per-million. That's ten times less sensitivity.
Additionally, the foam was found to still work at concentrations down to 20 parts-per-million, which is reportedly much lower than is possible using current commercial devices. It also didn't require high temperatures, unlike some commercial sensors.
Although the exact numbers weren't the same as those for ammonia, the graphene foam was also shown to be ten times more effective than polymer sensors at detecting nitrogen dioxide. According to the Rensselaer scientists, it should be relatively easy to engineer the foam to detect other substances, too.
Once the material has been used to detect a gas, it can be "cleaned" simply by applying an electrical current of about 100 milliamperes to the graphene. This causes the gas particles to release from the foam and fall off, leaving it ready to be used again.
A paper on the research was recently published in the journal Scientific Reports.