Making Nanofluidic Devices out of Graphene Oxide Papers
Occam’s razor is a heuristic stating that among competing theories, the one that makes the fewest assumptions should be chosen. Researchers at Northwestern University would probably have to agree, having recently discovered a surprisingly easy way to make nanofluidic devices. Using paper and scissors, they can cut a nanofluid device into any shape and size they want, adding to the method’s versatility.
Nanofluidic devices are attracting researcher’s interest because their thin channels can transport ions, and with them a higher than normal electric current, making the devices promising for use in batteries and new systems for harvesting energy, water purification, and sorting DNA.
The two Northwestern researchers involved, Jiaxing Huang and Kalyan Raidongia, established that simply stacking up sheets of the inexpensive material graphene oxide creates a flexible paper-like form of the material that has tens of thousands of very useful channels.
A tiny gap forms naturally between neighboring sheets, and each gap is a channel through which ions can flow. This paper-and-scissors method one day could be used to manufacture large-scale nanofluidic devices without relying on expensive lithography techniques.
“In a way, we were surprised that these nanochannels actually worked, because creating the device was so easy,” said Jiaxing Huang. “No one had thought about the space between sheet-like materials before. Using the space as a flow channel was a wild idea. We ran our experiment at least 10 times to be sure we were right.”
With a pair of regular scissors, the researchers just cut the paper into the needed shape, which, in their experiment’s case, was a rectangle. The experiment is described in a paper published in the Journal of the American Chemical Society.
“Many people have studied graphene oxide papers but mainly for their mechanical properties or for making graphene,” Huang said. “Here we show that graphene oxide paper naturally generates numerous nanofluidic ion channels when layered.”
Graphene Oxide Sandwhich
Graphene oxide is essentially graphene sheets decorated with oxygen-containing groups. It is made from inexpensive graphite powders via chemical reactions that have been known for more than a century.
To fabricate a working nanofluidic device, researchers used a pair of scissors to cut a piece of their graphene oxide paper into a centimeter-long rectangle. They then encased the paper in a polymer, next drilled holes to expose the ends of the rectangular piece and then filled up the holes with an electrolyte solution, a liquid containing ions, to finish the device.
Finally they put electrodes at both ends and tested the electrical conductivity of the device. Huang and Raidongia saw higher than normal current, and not only that, but the device also worked whether flat or bent. According to Huang, the nanochannels have significantly different and more desirable properties from their bulk channel counterparts. The nanochannels have a concentrating effect, resulting in an electric current much higher than those in bulk solutions.
Scaling up the size of the device would be a simple matter. Tens of thousands of sheets or layers create tens of thousands of nanochannels, each channel approximately one nanometer high. There is no limit to the number of layers , and thus channels, one can have in a piece of paper.
In order to manufacture very large arrays of channels, one only needs to put more graphene oxide sheets in the paper or to stack up many pieces of paper. A larger device, obviously, can process larger quantities of electrolyte.