Scientists have created a stick-on, tape-based sensor for plant leaves. It measures how much water a plant drinks up from the soil. With this information, plant breeders will know which individuals to choose as parent stock for crops that can survive droughts by making the most of the available water. That’s important because as Earth’s climate warms, periods of severe drought are expected to become more common.
Liang Dong led the team that developed the new sensor. He works at Iowa State University in Ames. As an electrical engineer, he designs, builds and analyzes electrical systems to meet particular needs.
Dong calls his sensor a “plant tattoo.” It is a piece of clear tape embedded with a tattoo-like design made from multiple layers of graphene flakes. Graphene is a sheet of carbon that is only a single atom thick. It not only is great at conducting electricity, but also mechanically strong, which means it won’t easily break apart.
When carbon bonds with itself to form graphene, each layer is so thin that you can’t even see it. The tattoo’s pattern is visible because it has been made from several layers.
For the tattoos, it was important that the graphene be laid down in a pattern instead of as a uniform sheet. Certain patterns conduct electricity better than others. Dong used mathematical formulas to figure out which patterns should work best.
When taped to a plant’s leaf, the tattoo can be attached to small wires. These connect to a battery and a device that can measure an electrical current. The battery sends a current through the graphene. That current will change depending on how fast water moves up the plant. This will reveal how quickly the plant drinks in moisture from the soil.
Dong and his team described their sensors in the December 2017 issue of Advanced Materials Technologies.
How they did it
The first step in creating the new tattoo was to make a mold from a plastic block. “We made indents and channels in the block and then poured a graphene solution on it,” explains Dong.
Once the graphene dried into flakes, Dong used tape to remove the excess from the block’s surface. He then applied a second piece of tape and pressed down on it firmly. When he pulled it up again, the graphene was now stuck to the tape.
Dong and his team experimented with different compounds created using graphene They settled on one called graphene-oxide. It’s a compound of carbon and oxygen that is very sensitive to moisture. When it comes in contact with water vapor, the speed at which a charge, or electrical current, moves through the graphene-oxide slows.
This is a useful trait. As plants take up water from the soil, water vapor escapes from tiny holes in their leaves called stomata. By measuring how much the electrical current slows after the sensor is stuck to a plant’s leaf, researchers can figure out how fast the plant is slurping up water.
The researchers tried out the plant sensor with corn plants. They measured how long it took for two varieties of corn plants to move water from their roots to their lower leaves, and then to their upper leaves. For one plant, it took 80 minutes, on average, to move water from its fourth to its ninth leaf. The other plant did this in only 28 minutes.
Elizabeth Lee is a plant scientist in Canada at the University of Guelph, in Ontario. This corn expert says some varieties of corn close their leaves’ stomata faster than others when the soil is dry. This helps them conserve water and better survive drought.
The new sensor is “a nice way to easily identify which varieties are better are conserving water,” observes Lee. Breeders might want to focus on these when developing drought-tolerant cultivars.
Dong says the next step in his research is to use the sensor to see how a crop’s use of water affects how many ears of corn are produced and how big they are.
“The ultimate goal of our work is to develop drought-tolerant crops that can deal with climate change,” he says.
atom The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and uncharged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.
average (in science) A term for the arithmetic mean, which is the sum of a group of numbers that is then divided by the size of the group.
battery A device that can convert chemical energy into electrical energy.
bond (in chemistry) A semi-permanent attachment between atoms — or groups of atoms — in a molecule. It’s formed by an attractive force between the participating atoms. Once bonded, the atoms will work as a unit. To separate the component atoms, energy must be supplied to the molecule as heat or some other type of radiation.
carbon The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.
climate The weather conditions that typically exist in one area, in general, or over a long period.
climate change Long-term, significant change in the climate of Earth. It can happen naturally or in response to human activities, including the burning of fossil fuels and clearing of forests.
compound (often used as a synonym for chemical) A compound is a substance formed when two or more chemical elements unite (bond) in fixed proportions. For example, water is a compound made of two hydrogen atoms bonded to one oxygen atom. Its chemical symbol is H2O.
conductor (in physics and engineering) A material through which an electrical current can flow.
conserve To protect, as from loss or degradation.
cultivar Short for cultivated variety, such plants have been bred to exhibit particular prized traits.
current (in electricity) The flow of electricity or the amount of charge moving through some material over a particular period of time.
drought An extended period of abnormally low rainfall; a shortage of water resulting from this.
electrical engineer An engineer who designs, builds or analyzes electrical equipment.
electric current A flow of electric charge — electricity — usually from the movement of negatively charged particles, called electrons.
electronics Devices that are powered by electricity but whose properties are controlled by the semiconductors or other circuitry that channel or gate the movement of electric charges.
engineer A person who uses science to solve problems. As a verb, to engineer means to design a device, material or process that will solve some problem or unmet need.
graphene A superthin, superstrong material made from a single layer of carbon atoms connected together.
information (as opposed to data) Facts provided or trends learned about something or someone, often as a result of studying data.
moisture Small amounts of water present in the air, as vapor. It can also be present as a liquid, such as water droplets condensed on the inside of a window, or dampness present in clothing or soil.
oxide A compound made by combining one or more elements with oxygen. Rust is an oxide; so is water.
oxygen A gas that makes up about 21 percent of Earth's atmosphere. All animals and many microorganisms need oxygen to fuel their growth (and metabolism).
plastic Any of a series of materials that are easily deformable; or synthetic materials that have been made from polymers (long strings of some building-block molecule) that tend to be lightweight, inexpensive and resistant to degradation.
sensor A device that picks up information on physical or chemical conditions — such as temperature, barometric pressure, salinity, humidity, pH, light intensity or radiation — and stores or broadcasts that information. Scientists and engineers often rely on sensors to inform them of conditions that may change over time or that exist far from where a researcher can measure them directly.
trait A characteristic feature of something. (in genetics) A quality or characteristic that can be inherited.
water vapor Water in its gaseous state, capable of being suspended in the air.
Journal: S. Oren et al. High-resolution patterning and transferring of graphene-based nanomaterials onto tape toward roll-to-roll production of tape-based wearable sensor. Advanced Materials Technologies. Vol. 2, December 2017. doi: 10.1002/admt.20170022.