As Bunes explains, the composite itself combines single-walled carbon nanotubes with a rare organic compound polymer known as carbazole. Inside the scanner, the composite helps transfer electrons from one electrode to the other. The goal, he says, is to make it difficult for charges to move.
“In order for a charge to move from one of our electrodes to the other, it has to jump through these nanotube junctions, and the nanotubes are separated by two layers of polymer,” Bunes says. “When an alkane is present, the alkane molecule likes to sit in between the polymers, and that increases the spacing between the nanotubes.” When that happens, Bunes adds, fewer charges are able to make the trip between electrodes, signaling a change in the electrical current. That change lets the detector know that alkane is present.
Bunes says the team picked carbon nanotubes because of their high surface area to volume ratio, as well as their high conductivity. “The other thing that the carbon nanotubes gives us is that their structure is conjugated so it’s easy to get stuff to stick to it,” he adds. “That enables us to functionalize [the nanotubes] with the polymer.”
While carbon nanotubes have great structural properties, it was difficult to figure out how they would behave. So difficult, in fact, that the results of their experiment ended up being the opposite of the team’s hypothesis. Carbazole is a very strong electron donor, Bunes says, so the team expected the electrons to go from the polymers to the carbon nanotubes. “But instead, the electrons went from the nanotubes to the carbazole, which is frankly bizarre,” Bunes says. “Figuring that out was huge.”
What makes the Utah researchers’ scanner stand out is the very small amount of polymer on the surface of the carbon nanotubes, whereas others “look like a big chunk of polymer” according to Bunes. “Rather than looking like a complete film, our material looks more like spaghetti noodles on a plate,” Bunes explains. “It’s very, very porous [and has] a very, very high surface area. That’s the major advantage of our sensor over others – the increased porosity and the increased surface area improves the sensitivity.”