Tiny magnetic robots capture pollutants and release them on demand

By covering the surface of magnetic nanoparticles with brush-like polymer strands, researchers have made tiny machines that they say could be an economical way to clean up contaminated water (Nat. Commun. 2022, DOI: 10.1038/s41467-022-28406-5). The polymer brushes act like “miniature hands” to pick up pollutants, says their developer Martin Pumera, a chemist at the University of Chemistry and Technology, Prague.

The nanorobots, which are controlled by heat and magnetic fields, could be reused many times to clean up large volumes of water. They should also be cheap and easy to make on a large scale, he says.

Researchers have in the past 2 decades made different molecular machines to remove contaminants. These microrobots typically rely on chemical reactions to move around and then break down the pollutants, but being fueled by such reactions restricts the robots’ lifespan.

Pumera and his colleagues wanted to make long-lasting, reusable micromachines. They covered iron oxide nanoparticles with negatively-charged, heat-responsive polymers that stick out from the surface. The polymers attract contaminants that have a slight positive charge, such as arsenic ions and the toxic pesticide atrazine. The polymers are hydrophilic at 5 °C, but become hydrophobic at 25 °C.

When the researchers add the particles to water containing arsenic and atrazine and slowly raise the temperature, the particles adsorb the toxins at their surfaces and clump together, trapping the toxins in the mass. The researchers apply a magnetic field to drive the clusters to move around and pick up more pollutants. Then they remove the nanorobots using a magnet and cool the particles down to their hydrophilic temperature, causing the particles to separate and release their load, after which they are ready for reuse.

In tests of water containing 5 mg/mL of arsenic and atrazine, the particles could remove about 72% of the contaminants. The researchers could reuse them 10 times, although their pollutant-trapping efficiency decreased over time. By tailoring the chemical groups in the polymer, Pumera says they can make nanorobots that pick up other pollutants.

The use of heat-responsive polymers for trapping and releasing the contaminants is a new approach, “as most of the micro-nanobots used for environmental applications could only trap or degrade the contaminants until now,” says Samuel Sánchez Ordóñez, a chemist at the Institute for Bioengineering of Catalonia.

Pumera’s team is now collaborating with a local waterworks company to test their microrobots and evaluate how the system’s economics work out on a large scale. “The magnetic setup is easy to scale up and the whole setup is envisioned to be flow through,” he says. “The parts are cheap, but they need to become dirt cheap.”