Generating electricity, desalinating water, and sensing trace chemicals are some of the uses of graphene just added to the growing list of this material's potential applications. They all feature graphene in a liquid environment, where one wouldn't expect it due to its hydrophobic nature.

Research at the University of Pennsylvania, published last month in the journal Nano Letters, pushes mass fabrication of graphene devices to new limits. The devices, fabricated by a scalable shadow-mask process, serve as fine biosensors for traces of opioids – drugs. The detection limit ranged down to 10 picograms per millileter of liquid.

Picture: Graphene biosensor for drugs, American Chemical Society.

The researchers employed the molecule naltrexone, which has a high affinity for opioids. Opioid molecules tend to bind to the naltrexone receptor in a liquid environment. The receptor was chemically attached to graphene transistors. In the experiment, the presence of opioids near graphene changed the conductivity of the transistors, showing up as a change in an electronic signal. The fabrication yield of more than 98%, the high sensitivity and specificity of the sensor make it a serious candidate for mass production of biosensors for any application in which the detection of opioids, such as alcohol or morphine is desired. The sensors featured arrays of hundreds of graphene transistors, all working in parallel, fabricated on standard graphene on silicon.

Around the same time, a new method for generating electricity, by simply dipping a piece of graphene sheet into a common ionic solution, was reported in Nature Communications ("Waving potential in graphene"). This electric energy harvesting approach could be integrated into cost-effective, self-powered sensor designs, reported Nanowerk News.

Researchers showed that moving a sheet of graphene across the surface of an ionic liquid, such as regular sea water, produces a small amount of electricity. They show that with a piece of graphene about the size of a small ruler (2 x 10 square centimeters), the electric potential generated is approximately 100 millivolts and, while this is still a relatively small output, the authors demonstrate that it is enough to stimulate a sciatic nerve of a frog. Importantly, they also show that the electricity generated is proportional to the size of the graphene sheet and the dipping speed, suggesting that this device may be scalable, opening up its potential use as, for example, a tsunami monitor.

It seems that technology is getting closer to a real-life graphene water desalination filter, with latest research from Oak Ridge National Labs (ORNL) pointing to a durable graphene-based desalination membrane with 100 times better performance compared to current filters.

A year ago, we wrote about the idea to use graphene as a water desalination filter, based on nanopores in a graphene sheet. In the meantime, technology has moved faster than one would expect, with nanometer-sized pores becoming a reality in lab results. While this fast development could lead to a demonstration of such water filters in the near future, the current research from ORNL offers an alternative approach, based on graphene oxide, or more precisely graphene oxide frameworks (GOFs).

“This is basically sheets of oxidized graphene connected by specific chemical linkers from some of the oxidation sites,” said ORNL’s Bobby Sumpter. “Because it’s composed mainly of strongly bonded carbon, it doesn’t decompose in water and has good mechanical properties. It’s an exciting material with potential for numerous applications.”

Although the results up to date are only computer simulations, graphene oxide is readily available in large quantities and experiment should soon follow. There is good reason to go in this direction, as water desalination is one of the big obstacles of mankind, and the simulated GOF filters would desalinate water 100 times faster than current technology. Furthermore, the technique would be completely scalable.

To summarize, although hydrophobic, graphene is making itself comfortable in liquid environments. We think that graphene's use in generating electricity, detecting drugs in blood and filtering water is only the beginning of a broad field of research and application.