Bacteria-sized robots attack microplastics and win by breaking them down

PICTURE: Metallic microrobots (dark blue dots) colonize a piece of shredded microplastic under visible light, breaking down the plastic into smaller molecules. view After

Credit: Adapted from Applied materials and interfaces ACS 2021, DOI: 10.1021 / acsami.1c04559

Small pieces of plastic are everywhere, ranging from urban environments to pristine wilderness. Left on their own, they can take hundreds of years to completely degrade. Catalysts activated by sunlight could speed up the process, but it is difficult to make these compounds interact with microplastics. In a proof of concept study, researchers Applied materials and interfaces ACS developed self-propelled microrobots capable of swimming, attaching to and breaking down plastics.

While plastic products are ubiquitous indoors, plastic waste and broken pieces are now littering the exterior as well. The smallest of them – microplastics less than 5mm – are difficult to pick up and remove. In addition, they can adsorb heavy metals and pollutants, potentially harmful to humans or animals if accidentally consumed. So, previous researchers have proposed a low-energy way to get rid of plastics in the environment by using catalysts that use sunlight to produce highly reactive compounds that break down these types of polymers. However, contacting the catalysts and tiny pieces of plastic is difficult and usually requires large pretreatments or mechanical stirrers, which are not easy to scale. Martin Pumera and his colleagues wanted to create a catalyst powered by sunlight that travels to and locks onto microparticles and dismantles them.

To transform a catalytic material into light-actuated microrobots, the researchers made star-shaped particles of bismuth vanadate, then evenly coated the 4-8 µm wide structures with magnetic iron oxide. Microrobots could swim through a maze of channels and interact with pieces of microplastic along their length. The researchers found that under visible light, microrobots focused heavily on four common types of plastic. The team then illuminated pieces of the four plastics coated with the microrobot’s catalyst for seven days in a dilute solution of hydrogen peroxide. They observed that the plastic lost 3% of its weight and that the surface texture for all types changed from smooth to pitted, and that small molecules and components of the plastic were found in the remaining solution. The researchers say self-propelled microrobot catalysts pave the way for systems that can capture and degrade microplastics in hard-to-reach places.

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The authors acknowledge funding from the European Regional Development Fund.

The summary that accompanies this article can be viewed here.

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