Graphene-Based Sensors-on-Tape for Investigating Transfer of Water in Plants

Dr. Patrick Schnable, a Professor from Iowa State University, has reported on the way he analysed the time taken by two different types of corn plants, to move water from their roots to their lower leaves and subsequently to their upper leaves.

Iowa State University researchers have developed “plant tattoo sensors” to take real-time, direct measurements of water use in crops. (Photo credit: Liang Dong)

The researcher intended to work with innovative, less-expensive, easily developed, graphene-based sensors-on-tape that can be fixed to plants to enable retrieval of new types of data by farmers and researchers. The tool enabling the measurement of water transfer is a very small graphene sensor that can be affixed on plants. The researchers call it a “plant tattoo sensor.”

With a tool like this, we can begin to breed plants that are more efficient in using water,” stated Schnable. “That’s exciting. We couldn’t do this before. But, once we can measure something, we can begin to understand it.”

Graphene is a fascinating material. It appears as a honeycomb of carbon atoms (one atom thick), has excellent heat and electric conductivity and is stable and strong. The graphene-on-tape methodology adopted in this research has even been adopted to develop wearable pressure and strain sensors, such as sensors ingrained into a “smart glove” that measures movements of the hand.

Scientists report on the different sensors and the “simple and versatile method for patterning and transferring graphene-based nanomaterials” to develop the flexible sensors in a paper featuring on the cover of the Advanced Materials Technologies journal published in the December 2017 issue.

The Faculty Scholars Program of Iowa State’s Plant Sciences Institute has primarily supported the study.

The technology developer and lead author of the paper is Liang Dong, who is an Iowa State associate professor of electrical and computer engineering. Seval Oren, co-author of the paper, is a doctoral student in electrical and computer engineering and assisted in developing the sensor-fabrication technology. Other co-authors of the paper who assisted with test applications of the sensors are Schnable (director of Iowa State’s Plant Sciences Institute), 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 (a Professor of civil, construction, and environmental engineering).

We’re trying to make sensors that are cheaper and still high performing,” stated Dong.

In order to achieve this, the scientists have devised a technique for creating intricate graphene patterns on tape. Dong stated that the initial step is fabricating indented patterns on a polymer block’s surface by adopting 3-D printing or a molding process. The indented patterns are filled by the application of a liquid graphene solution to the block. A tape is used to remove the excess graphene. Subsequently, another strip of tape is used to strip the graphene patterns, thereby making a sensor on the tape.

The technique can be used to make accurate patterns with a width of just five millionths of 1 m, which is merely 1/20th of the diameter of a strand of human hair. According to Dong, if the patterns are made that small, the sensitivity of the sensors is increased. For instance, the technique was used to generate a detailed image of Iowa State’s Cyclone mascot with a width of less than 2 mm. “I think this is probably the smallest Cyclone,” stated Dong.

This fabrication process is very simple,” stated Dong. “You just use tape to manufacture these sensors. The cost is just cents.”

For the purpose of plant studies, the sensors are developed by using graphene oxide, a material that is highly sensitive to water vapor. The existence of water vapor alters the material’s conductivity, and this could be quantified to precisely evaluate transpiration, or the release of water vapor, from a leaf.

Dong stated that the plant sensors have been successfully investigated in pilot field experiments as well as in the lab.

A new $472,363, 3-year grant awarded by the U.S. Department of Agriculture’s Agriculture and Food Research Initiative will support further field investigation of water transport in corn plants. Michael Castellano, an Iowa State associate professor of agronomy and William T. Frankenberger Professor in Soil Science, will head the study. Dong and Schnable are the co-investigators.

The Iowa State University Research Foundation has filed for a patent for this sensor technology. The research foundation has also awarded an option to EnGeniousAg to commercialize the technique. EnGeniousAg is an Ames startup company co-founded by Dong, Schnable, Castellano, and James Schnable (Patrick Schnable’s son), an assistant professor of agronomy and horticulture at the University of Nebraska-Lincoln, a partner on another Iowa State sensor study that led to the founding of the company.

The most exciting application of the tape-based sensors we’ve tested so far is the plant sensor. 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.

Liang Dong, Lead Author

However, these are not the only functions of the sensors. According to the authors, the technique can “open a new route” for a broad array of applications, such as sensors for investigating the structural integrity of buildings, for biomedical diagnostics, for environmental monitoring, and, after suitable adjustments for investigating crops for the presence of pesticides or diseases.

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