From Bottle Rockets to Bionic Spinach
For a chemical engineering lab, Michael Strano’s workspace has an unusual array of greenery: spinach, watercress, and arugula. Strano has introduced nanoparticles into the plants, by way of their leaves, to give them new capabilities. “You wouldn’t want to eat them,” he says, pointing to the lush salad greens. “This one is designed to glow like a desk lamp, and that one is trying to talk to your cell phone.” He has also created spinach plants that are able to sense explosives in soil and transmit that information to a remote observer.
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Strano has long worked with carbon nanotubes and pioneered their use as sensors in plants as well as in humans and animals. He has also used nanoparticles to enhance plants’ capabilities, creating devices that seem to verge on science fiction, like the plant-as-desk-lamp. Along the way, he has discovered how nanotubes can serve as an entirely new source of energy. His work spans thermodynamics, materials science, nanotechnology, and now plant biology—a confluence of interests that remains rare in the upper reaches of academia.
“We have several ecosystems going on in this lab,” says Mike Lee, a Strano grad student who works on sensors in fish. “He’s such a passionate guy and there’s so much to learn.”
Strano’s history of creative risk-taking began early on. Growing up in Pennsylvania, he tinkered with chemistry and electronics. His father, who started out as an electrician for Bell Telephone in Philadelphia, later moved the family out of the city to start an audio electronics shop. At age 10 or 11, Strano figured out how to electrolyze salt water, running a current through it to release hydrogen gas, which he stored in glass bottles in the freezer. When his older brother, John, questioned whether he had, in fact, captured hydrogen, Strano showed that by inserting a heated wire through the cap to ignite the gas, he could shoot a Snapple bottle 100 feet into the air. “After that, everyone was terrified,” he says. “But to me it was almost like magic. I could take this clear gas and astound people.”
When Strano was 12, his father died suddenly of a heart attack, leaving his mother with five kids (Strano was the second-oldest). “We did struggle,” he says. “And science was a productive outlet from all that.” In 1993, he became the first member of his family to attend college—Brooklyn Polytechnic, where he focused on chemical engineering. He credits the school with providing a strong mathematical and analytical foundation. It was the kind of place that emphasized teaching and held students to rigorous standards: “The entire class might get nothing higher than a B,” he says.
In 1997, Strano began a PhD at the University of Delaware, working to make porous carbon membranes that could facilitate chemical reactions. He also worked on a project, sponsored by DuPont, for which he designed a membrane that could separate nitrogen and oxygen from pressurized air. While virtually all of his PhD colleagues went on to jobs in industry, Strano decided to pursue a career in academia, moving on to a postdoctoral fellowship at Rice University, in Houston.
At Rice, Strano joined the lab of Richard Smalley, a chemist who’d won the Nobel Prize for his work on a form of carbon called buckyballs. “I was the only postdoc in the lab,” Strano recalls, explaining that Smalley, notoriously demanding, “had sort of chased all the other postdocs away.” Still, he managed to thrive under Smalley, shining lasers at nanoparticles to learn about their light-emitting properties. In particular, he focused on particles that emit light in the near-infrared portion of the spectrum. This property struck him as significant because the human body is transparent to near-infrared light.
When he became an assistant professor at the University of Illinois at Urbana-Champaign in 2006, Strano decided to investigate whether he could introduce these nanoparticles into the body, where they might act as sensors, binding to molecules and transmitting information through the skin by way of fluorescence. As he set up his new lab, he devoted himself to that vision. “Back then it was an off-the-wall idea,” he says, noting that little work had been done on whether nanoparticles could serve effectively as sensors—or even whether they would be safe. (One skeptical early reviewer likened the idea of developing such nanosensors to putting a can opener in the human body.) Strano’s approach was to take carbon nanoparticles and coat them with a polymer that could bind to glucose and respond to different sugar concentrations. When an external device worn by the patient shined light through the skin, it caused the nanoparticles to fluoresce, with signatures that varied according to these glucose concentrations. (He also conducted animal experiments to show that the nanoparticles appeared safe.) Early on, Strano focused on glucose because he believed that implantable sensors would help people with diabetes, like his wife’s nephew. His short-term goal was to eliminate the need for them to stick themselves frequently to measure blood sugar. But the ultimate goal is to develop an artificial pancreas that could measure blood sugar and deliver insulin in real time.
In 2007, Strano was recruited to MIT, where he earned tenure in just two years. He continued the work on nanosensors, and in 2009 he showed that carbon nanotubes injected under the skin as a kind of “tattoo” could be used to measure blood glucose levels. Over time, he developed a host of other sensors as well. The technology detected such molecules as fibrinogen, which is important to blood clotting, and nitric oxide, which serves as a key signaling molecule in the cardiovascular system and elsewhere. He also worked to increase his sensors’ sensitivity. In a paper this year in Nature Nanotechnology, he unveiled sensors that could identify single protein molecules.
Early in his time at MIT, Strano also discovered a new mechanism for energy generation—entirely by accident. In 2008, he had started working on a project, funded by the United States Air Force, to develop novel actuators that would respond quickly to a chemical trigger. As part of the work, he placed the explosive TNT around the surface of carbon nanotubes in order to observe the rate at which it ignited in that environment. As he made the measurements, however, he noticed that the reaction was also producing a large and unexpected electrical pulse. In other words, the reaction of TNT on the surface of a carbon nanotube appeared to be converting heat from the reaction to electricity, generating bursts of power much larger than scientific theory at the time predicted.
Strano was the first to observe this phenomenon, which he dubbed the “thermopower wave.” It was a “new and exciting idea for transforming a volatile chemical reaction into a very high-power pulse of electricity,” says Kourosh Kalantar-Zadeh, director of the Centre for Advanced Electronics and Sensors at RMIT University in Australia. In addition to the theoretical interest it held for chemists, the discovery has opened the door to new power sources. In some contexts, researchers or consumers may want sources capable of providing large amounts of power over short periods of time. Thermopower waves would make this possible, and they could in theory remain unused for indefinite periods without losing energy. Because they rely on chemical bonds for storage, power sources that use this mechanism would be more like fuel in a gas tank than, say, a lithium-ion cell-phone battery that slowly discharges and goes dead, says Strano. Recently, he has also shown that thermopower waves can be generated without the use of explosives or high temperatures. Specifically, he has placed acetonitrile on nanotubes and allowed it to evaporate; that chemical change, it turns out, is enough to generate the electrical current. Strano and others around the world have begun to experiment with simple devices that might make use of thermopower waves, but he says the applications remain in early stages.
Strano first turned his prodigious imagination to plant biology in 2009. Initially, he says, he thought that plants’ remarkable ability to regenerate key proteins could inspire solutions to a problem in solar power: exposure to sunlight gradually degrades many materials used to capture the sun’s energy. In 2010, Strano’s team developed a set of synthetic molecules that, held in a solution, can spontaneously assemble themselves into a photovoltaic structure; the cell breaks down when a surfactant is added to the solution but quickly reassembles itself once the surfactant is filtered out. Shortly thereafter, he began to focus on plants’ other physiological capabilities: for instance, they produce their own power, pump their own water, and consume more carbon dioxide than they produce. “In my field, engineers haven’t been trained to look at plants as the starting point for technology,” says Strano. But he began to think of them as microfluidic networks that have a mechanism for transporting fluids internally, while their chloroplasts, the structures where photosynthesis takes place, can be likened to chemical batteries. “You can put together a simplified perspective on plants that has great appeal to engineers like me,” he says.
In 2011, Strano hired biologist Juan Pablo Giraldo as a postdoctoral fellow. Giraldo, who is now an assistant professor at the University of California at Riverside, says he felt energized to help “merge the worlds of nanomaterials and the living organic.” In a 2014 paper in Nature Materials, the team showed that plants would take up carbon nanoparticles through pores on the undersides of their leaves, called stomata—and that these particles would then enter the plants’ chloroplasts. Following this discovery, the researchers introduced nanoparticles that allowed leaves to absorb wavelengths of light that plants normally do not make use of, effectively expanding their photosynthetic range. The researchers also supplied particles that serve as antioxidants and thus protect the chloroplasts from damage caused by intensive exposure to sunlight. As a result, they were able to boost the photosynthetic potential of the plants by 30 percent. This finding could theoretically be used to help plants grow better in high-density environments, where they receive limited light in the visible range and might benefit from the ability to harness other wavelengths.
In the same paper, Strano, Giraldo, and their colleagues also showed that they could turn plants into sensors for nitric oxide, a pollutant that contributes to acid rain. The researchers did this by wrapping nanoparticles in polymers that could interact selectively with nitric oxide and then introducing those particles into plants. When nitric oxide was present, it changed the way the underlying nanoparticles emitted light. In 2016, Strano’s group turned spinach leaves into sensors for nitroaromatic compounds, a type of explosive, by incorporating carbon nanotubes coated in selective polymers into the leaves. In theory, if nitroaromatics were present in groundwater, the plants could detect them—and send a notification to a nearby device such as an infrared camera connected to a small computer, or even a cell phone without an infrared filter. (The practical relevance of this system is not entirely clear, but it suggests an approach to detecting land mines or pollutants in groundwater.)
Strano says that this plant work is especially gratifying to discuss with his daughters, who are 11, nine, seven, and four years old. “Bionic plants—now that is actually a conversation you can have with a seven-year-old,” he says. Strano and his family live in Lexington, Massachusetts, which has excellent public schools; but since his eldest daughter was old enough for preschool, he and his wife, Sally, a former mathematician, have chosen to homeschool the kids instead. They belong to a homeschooling co-op, which meets once a week and provides some structure, but much of their approach is free-wheeling (with the exception of math, which Strano says requires daily drills). The girls spend long hours at the library, reading independently. Two of them recently accompanied their father on a trip to Japan, where they learned about Japanese culture and a few words of the language.
Of course, Strano is especially excited to teach the kids science. “I take my mandate seriously,” he says. “I have four girls, and I can’t guarantee that they will all go into STEM, but it is heading that way.” He and his wife encourage the children to design experiments and pursue whatever areas of inquiry interest them. The 11-year-old, who likes whittling and crafts, did a project on how cornstarch-based slime slides down an inclined plane. The nine-year-old, who loves birds, used a time-lapse camera to study which species fed together at a bird feeder. The girls also make regular visits to Strano’s lab, where they have a firsthand view of their father’s projects.
Among the more playful pursuits under way in the lab are electronic devices made from plants. Strano and his students are focusing in particular on remote controls, which normally communicate using infrared energy. By modifying plants with nanoparticles so that they release infrared energy on cue—in response, say, to a consumer’s desire to turn on the TV—the researchers might enable plants to serve the same function as these gadgets. “We think we can replace some of the devices that are currently stamped out of plastic,” Strano says, adding that the goal is to create a greener class of electronics. A challenge in mimicking remote controls, however, is that their signaling occurs very rapidly, on the order of milliseconds, and “plants don’t like to move that fast.” (One also wonders about the fate of plant-based electronics, which require water, when their human owners go on vacation. The devices would have to water themselves, Strano says.)
Mike Lee, who is a second-year graduate student, is responsible for keeping a large aquarium of goldfish in the basement of the lab and developing nanoparticle-based sensors that can be injected into them to sense their concentrations of stress hormone, or cortisol. The project is a collaboration with scientists at King Abdullah University of Science and Technology in Saudi Arabia, who plan to release fish into the Red Sea, analyzing their cortisol levels in response to environmental conditions.
Most recently, Strano made a surprising discovery about the behavior of water itself within the confines of carbon nanotubes. In a paper published in Nature Nanotechnology in November, he showed that inside a nanotube of a very specific diameter, at temperatures of up to at least 105 °C, water formed a solid, much like ice. “When fluid is in a confined environment its phase behavior becomes distorted, but this is an extreme case,” he says. Researchers might be able to create tiny wires made of solid water, which would be stable at room temperature and would conduct protons with high efficiency, as water is known to do. This could, for example, prove helpful for developing better hydrogen fuel cells. Strano says he’s excited to explore the properties of “ice nanowires,” but he adds, “the jury’s out if they’ll be of any use.”
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