Researchers are using nanomaterials to help plants do everything from monitor drought to emit light. Lotuses grow in a fountain outside JJ Richardson’s apartment in Melbourne. One day, Richardson, a chemical engineer at the University of Melbourne, noticed the flowers while walking by and thought that these water-loving plants would be perfect for a new research project: teaching plants to detect dangerous chemicals.
Richardson isn’t a plant biologist. Most of the time, he researches metal-organic frameworks (or MOFs), a crystal-like nanomaterial that is useful for everything from gas storage to drug delivery to sensing compounds. With an undergraduate degree in philosophy and a master’s in systems engineering, Richardson has long been interested in “life and lifelike materials” and became intrigued by the possibilities of plants after coming across a 2015 Science Advances paper where researchers demonstrated a way to create an electronic circuit inside a plant. The trick was to put the plant in a particular solution and let the plant’s own internal architecture suck up the circuit material, similar to how a white rose placed in dye will suck up the color, turning it blue, pink, or rainbow-hued.
For their experiment, “we actually grew the MOFs inside the plants rather than using pre-formed materials,” says Richardson, who presented this research at the American Chemical Society meeting today in Orlando, Florida. The team put the MOF ingredients (like metal salts) in a water solution, and then placed the lotuses in that solution. The lotuses sucked up the solution and grew different types of fluorescent MOFs inside their vascular systems. These newly engineered plants became less fluorescent when placed in acetone, showing that they were able to detect the presence of the toxic chemical.
The vision, according to Richardson, is to one day have plants in airports helping sense for the presence of chemicals. That won’t be happening anytime soon, given that MOFs are still expensive and the lotus research is a proof-of-principle experiment. Still, it’s another entry in the growing field of plant nanobionics, or efforts to put nanostructures into plants to teach them to do everything from monitor droughts to replace street lights.
“It’s long overdue that we start to look at plants as the starting point of technology,” says Michael Strano, a chemical engineer at the Massachusetts Institute of Technology, and one of the pioneers of plant nanobionics. Plants collect their own energy from the sun and store that as fuel, which means they’re like a combination of solar cells and batteries. They repair themselves, can pump water hundreds of feet in the air with seemingly no energy input, and easily adapt to harsh environments. “As an engineering platform, they have a number of untapped advantages,” Strano adds.
Back in 2016, Strano’s team figured out how to engineer spinach plants to detect explosives in a multi-step process. First, the scientists planted sensors — which are able to detect the nitroaromatic compounds often found in land mines — into the leaves of plants. The spinach then absorbs the explosive compound through its roots; when the compound travels to the leaves, it activates the plant’s sensors. The sensors then emit a fluorescent signal that can be seen from an infrared camera nearby, which is hooked to a computer and can also send an email alert about the explosives.
“As an engineering platform, they have a number of untapped advantages”
Since then, the team has turned the sensors inward. Instead of measuring explosives in the groundwater, the new focus is seeing what is happening with the plants themselves. “Plants collect information in their environment and signal internally with chemical signals,” says Strano. Being able to measure these changes could tell us how thirsty the plant is and help with issues like water utilization, crop health, and monitoring drought. One of these sensors goes on the surface of a leaf and measures the opening of the plant pores and keeps track of the water stored inside. “We have some work we’re getting ready to submit now about how we’re able to interpret those signals and send them to a cell phone,” he adds. The next step will be connecting the sensors, which means “hooking plants up to the internet.”
Farmers and agriculture workers are interested in getting more fine-grained detail to improve the growing process, but there’s a lot of interest in genetically engineering the plants themselves. Imagine making photosynthesis more efficient, or engineering plants to be resistant to pests and disease, or creating better biofuels. All of that starts with getting the DNA into existing cells, says Markita Landry, a chemical engineer at the University of California at Berkeley.
By itself, DNA doesn’t have the rigidity necessary to go where it needs to. Landry describes it like trying to push a cooked needle through a piece of toast — it’s too floppy to really go anywhere. Current techniques involve putting DNA inside a bacteria inserted into the plant or shooting it into the cell. Both methods fail frequently, which is where Landry’s work comes in. Her idea was to deliver DNA using carbon nanotubes, which are essentially stiff cylinders that are incredibly small. It’s like threading the DNA to a needle that can actually get through and deliver the payload, she says, and it’s more efficient than the other methods. (The results were published in the journal Nature Nanotechnology earlier this year.)
Her team mixes DNA-coated carbon nanotubes into cell cultures and observes the transformations, like where the DNA goes and what it does. In other cases, they will take a needle out of a syringe and poke the waxy epidermal layer of the plant with the solution of nanoparticles. Plenty of questions remain over the ideal size and stiffness of the nanoparticles, and the studies so far have focused on getting the DNA into the plant at all. The next step is figuring out how to get it exactly where it might be most helpful.
Landry’s research works on the molecular level, but other work in plant nanobionics is easily visible to the naked eye. For 33 weeks this summer, the Cooper Hewitt Smithsonian Design Museum in New York will have an unusual display: a light-emitting plant. This is the next generation of a plant that Strano’s lab engineered in 2017 by embedding light-emitting nanoparticles into watercress. The team is now working on an switch that will help the plant shut off during the day and turn on at night.
For now, these plants can only emit light for about four hours, so they’d be best suited for replacing your desk lamp. Strano hopes that by improving the brightness and efficiency, we can think bigger and create a world where off-grid plants that power themselves can replace street lighting.
Before we get there, though, plenty of very basic questions remain. “We’re still learning how nanoparticles move and traffic inside living plants,” says Strano, whose team has also published a paper with a theory that tries to predict how the nanoparticles move. “The basic science is emerging, but we’re still testing the bounds of this theory and we’re still extending it to a broader range of nanoparticles.”
Richardson acknowledges that there are plenty of big questions left, but is eager to continue working with MOFs and plants. His team also experimented with dipping chrysanthemums in MOFs and found that these plants were less damaged by the sun than flowers without the coating, making him think that such a material could help protect the plants we might want to grow in space, where radiation is plentiful. Anywhere that humans go in the future, plants are likely to be right there with us. And as this recent research shows, there’s a good chance that this field of plant-based innovations will continue to grow for a long time to come.