No Nose, but a Heck of a Sniffer

IRA FLATOW: This is Science Friday. I am Ira Flatow. You know that spinach you have sitting in your refrigerator at home, waiting to be turned into a tasty salad, a side dish, or getting ready to be recycled, you know what I mean, the compost heap? Well, it’s actually good for more than just eating. My next guest gave this super food super powers. And, that is, the ability to detect buried explosives– spinach that can detect buried explosives.

And not only can it detect those bombs, it can send the signal to people about what it’s found. It’s a plant to human communication, the first of its kind. Joining me to discuss it is Michael Strano, Professor of Chemical Engineering at MIT. Welcome to Science Friday.

MICHAEL STRANO: Great, Ira. It’s my pleasure to be here.

IRA FLATOW: So what you’ve actually, essentially created is a bomb-sniffing plant?

MICHAEL STRANO: That’s right. It’s a plant that can detect explosive molecules in the groundwater or in the air and send that information to your phone.

IRA FLATOW: So how does it do this? What did you do to the spinach?

MICHAEL STRANO: Well, we used two types of very small particles. We call them nanoparticles. They’re small enough to get into pores within the plant. So the plant leaf has pores called stomata. And the plant uses them to evaporate water from the leaf. But you can use those pores to get two types of nanoparticles. And these are single-walled carbon nanotubes, or rolled cylinders of graphene. And they emit light in the near infrared. This is light that’s so red that your eye can’t see it. Your television remote control uses this kind of light.

IRA FLATOW: So the plant is shining in infrared?

MICHAEL STRANO: Right. It fluoresces So if you shine light on these particles, they will send light back at you in this infrared region, so red that your eye can’t see it, like your television remote control.

IRA FLATOW: So how long does it take for the plant to do this?

MICHAEL STRANO: It takes about 10 minutes. It depends on the size of the plant, but also the evaporation rate, so how much sun is shining on the plant.

IRA FLATOW: And so what’s the idea? How could you make practical use of this now?

MICHAEL STRANO: So this is part of an effort that we have at MIT to try to modify living plans to replace some of the devices that we make today and stamp out of plastic and circuit boards. So I call this area plant nanobionics. And the technology we developed could have applications to monitoring groundwater contamination around chemical plants, monitoring public spaces for terrorism. There may be defense applications.

But we’re also taking these same sensors and we’re turning them inward. And we’re connecting them to the plant to help us study and understand plants. Plants have an extensive chemical signaling inside. They know when they’re under attack from pests. They know when there’s a drought coming. So this information is useful to humans. And so we’re also interested in getting this information out of the plant and into a form that humans can use.

IRA FLATOW: So if you can do this in spinach, I imagine you could do it in other kinds of plants, too.

MICHAEL STRANO: That’s right. In the lab we’ve used spinach, watercress, arugu arugula. We also use plants that are commonly used for plant research, like there’s a plant called Arabidopsis, but we made a point in our paper to show that we could use these techniques on any plant, any mature plant that you find in the environment or in a nursery. We call them wild-type plants, so you don’t have to genetically engineer or use a very special kind of plant or plant species. So that was an important part of what we showed, that you could go up to any mature plant and modify it in this way.

IRA FLATOW: And how hard is it to modify? Do you just sprinkle on the nanoparticles and it just goes right into those little holes in the leaves?

MICHAEL STRANO: You essentially can just sprinkle on. We use a needless syringe to apply a solution of the nanoparticles. We pressurize the solution on the surface of a leaf. And then this backfills spaces within the leaf where water is normally stored, waiting to be evaporated in the plant. However, we’re working on ways that you could treat the water so that just the nanoparticles will infuse into the plant without that pressure. So there you could, in fact, just spray the leaves and you would have the particles be incorporated.

IRA FLATOW: Could you find plants that could detect, let’s say, traces of explosives in the air, like sniff it out instead of taking it in through the roots?

MICHAEL STRANO: So, yes. We showed in our paper that if you apply the explosives on the leaf surface, actually the response is faster. So you can detect molecules both from the air or the water. And the method isn’t just useful for explosives. We showed that you could detect dopamine. And, in fact, we’re working on a wide variety of sensors that you could put into the plant, and, also, combining them together to get lots of information in one set of infrared signals that come from the plant.

IRA FLATOW: Now, I’m going to parse this a little bit. You said dopamine, which, to me, is a neurotransmitter, which I move that to the next category of nerve gas.

MICHAEL STRANO: That’s right. So dopamine is interesting. It’s present in plants. But, also, we were interested in showing that we could detect molecules that are foreign to the environment, so we chose explosives. They shouldn’t be there in the groundwater or the air. But we’re also interested in detecting biological molecules that you can detect that are part of the plant and part of the plant’s signaling network. So, really, we were showing examples– we can detect almost any molecule in this way. So we’re trying to demonstrate that it’s a very generic technology.

IRA FLATOW: Wow, that certainly would be a cheap way to create a detector, just using any plant that you can incorporate.

MICHAEL STRANO: So plants have some advantages for being used in this way. And I don’t think engineers like myself have really seriously considered them as a platform. They provide their own power. They can move water from the ground to, in some cases, hundreds of feet in the air with no additional energy added. They use just water evaporation. And they self-repair. And they’re naturally adapted to the outdoor environment. If you think about what it takes to have your cell phone sit outside for year after year in extreme cold and in the sun, it’s an engineering challenge, but nature has essentially adapted to this task.

IRA FLATOW: So if you wanted to create a protective perimeter around something, you can just grow a lot of rhododendrons or something.

MICHAEL STRANO: Or just approach existing plants and modify them and set up a sensor network. That’s right.

IRA FLATOW: And I imagine the pentagon must be interested in your research.

MICHAEL STRANO: Well, I don’t know anything about that, but we’re certainly interested in–

IRA FLATOW: You haven’t got a call from DARPA yet?

MICHAEL STRANO: Not yet, but we’re interested in both– so plants are naturally adapted to detect things in their environment. For as large and extensive as a tree is on the surface, its branches, if you imagine it upside down, it’s root network is just as extensive. So plants and trees are really ideal for monitoring really large areas of groundwater.

IRA FLATOW: Can you triangulate the plants to actually find, let’s say, there’s a bomb in the soil, you get two or three, well, three plants, and can you actually find out the exact location?

MICHAEL STRANO: Ira, you are brilliant. Yeah, no this is actually–

IRA FLATOW: You caught that again? I’m sorry.

MICHAEL STRANO: [LAUGHS] No, this is actually what your cell phone does. In fact, if you have three plants a distance apart, right, you can actually pinpoint the location, approximately, by using the arrival time of the chemical signal to each of the plants.

IRA FLATOW: So where do you go next with this? I’ll give you my blank check question I give to a lot of scientists– if you could spend an infinite amount of money, what would you like to know next and to create next?

MICHAEL STRANO: So the thing that we’re really excited about now is actually turning the sensors inward towards the plant and being able to tap chemical signaling in the plant that can tell whether the plant is stressed. And this has many applications to understanding and improving agriculture, but also applications to what’s called urban farming and improving our ability to cultivate plants.

But plants are also used to produce specialty chemicals. And it turns out that in order to increase their production, plants are used for making anti-cancer drugs and for producing flavorence, and so forth. If you want the plant to produce more of those molecules, you have to activate a pathway in the plant that turns on when the plant is under stress. So we’re interested in tapping into the signaling within a plant and seeing what kinds of information that it gathers in the environment that could be sent to a user.

IRA FLATOW: Now, that’s really brilliant. To heck with me. That’s great, interesting stuff. Thank you for taking time to be with us today. And good luck to you, Dr. Strano.

MICHAEL STRANO: Thanks a lot, Ira. Thank you.

IRA FLATOW: You’re welcome. Michael Strano is Professor of Chemical Engineering at MIT.

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