Iowa State University plant scientist Patrick Schnable has described how he measured the time it takes for two kinds of corn plants to move water from their roots to low leaves and then the upper leaves.
He was interested in working with a new, low-cost, easily produced, graphene-based, sensors-on-tape that can be attached to plants and can provide new kinds of data to researchers and farmers.
"With a tool like this, we can begin to breed plants that are more efficient in using water," he said. "That's exciting. We couldn't do this before. But, once we can measure something, we can begin to understand it."
The tools that make these water measurements possible is a tiny, graphene sensor that can be taped to plants. Researchers have called this a “plant tattoo sensor.” Graphene is a key material in this sensor. The carbon honeycomb is an atom thick and it is great for conducting electricity and heat, as well as staying strong and stable. The graphene-on-tape technology in the study has also been used to produce wearable strain and pressure sensors, including sensors that are built into a “smart glove” that measures hand movements.
Liang Dong, an Iowa State associate professor of electrical and computer engineering is the lead author of the paper and developed the technology. Seval Oren, doctoral student in electrical and computer engineering, is a co-author who helped develop the sensor-fabrication technology. Co-authors who helped test applications of the sensors are Schnable, director of Iowa State’s Plant Sciences Institute, a Charles F. Curtiss Distinguished Professor in Agriculture and Life Sciences, the Iowa Corn Promotion Board Endowed Chair in Genetics and the Baker Scholar of Agricultural Entrepreneurship; and Halil Ceylan, professor of civil, construction and environmental engineering.
"We're trying to make sensors that are cheaper and still high performing," Dong said.
To do that, the researchers have developed a process for fabricating intricate graphene patterns on tape. The first step was to create indented patterns on the surface of a polymer block, with either a molding process or 3D printing. Engineers applied a liquid graphene solution to the block, which filled the indented patterns. They use tape to remove the excess graphene. Then they take another strip of tape to pull away from the graphene patterns, creating a sensor on tape.
The process can produce precise patterns as small as 5 millionths of a meter wide—just a twentieth of the diameter of the average human hair. Dong said making the patterns so small increases the sensitivity of the sensors.
"This fabrication process is very simple," Dong said. "You just use tape to manufacture these sensors. The cost is just cents."
In the case of plant studies, the sensors are made with graphene oxide, a material very sensitive to water vapor. The presence of water vapor changes the conductivity of the material and that can be quantified to accurately measure transpiration (the release of water vapor) from a leaf.
The plant sensors have been successfully tested in a lab and pilot field experiments.
"The most exciting application of the tape-based sensors we've tested so far is the plant sensor," Dong said. "The concept of wearable electronic sensors for plants is brand new. And the plant sensors are so tiny they can detect transpiration from plants, but they won't affect plant growth or crop production."
This technology could open a new route for a variety of applications, including sensors for biomedical diagnostics, for checking the structural integrity of buildings, monitoring the environment and testing crops for diseases or pesticides.
The paper on this research was published in Advanced Materials Technologies.