Microrobots for the Photocatalytic Degradation of Microplastics

Plastic waste is found almost everywhere, from urban environments to the wilderness. It can take hundreds of years for this waste to degrade completely. The smallest plastic particles—microplastics—are particularly hard to remove. In addition, they can adsorb harmful heavy metals and pollutants. There are some photocatalytic treatments that can generate reactive species such as radicals, which then degrade plastic waste. However, getting the photocatalysts into close enough contact with the microplastics to successfully degrade them is difficult.

Martin Pumera, University of Chemistry and Technology, Prague, Brno University of Technology, and Mendel University, Brno, all Czech Republic, and China Medical University Hospital, Taichung, Taiwan, and colleagues have developed visible-light-driven self-propelled microrobots that can attach to plastics and break them down. The team first prepared star-shaped particles of photocatalytic bismuth vanadate (BiVO₄) using a scalable hydrothermal procedure. Then they decorated the structures with magnetic iron oxide using a Fe₃O₄ nanoparticle solution.

The researchers found that resulting BiVO₄/Fe₃O₄ microrobots can move in a dilute H₂O₂ solution under visible light and adsorb onto common polymers, with high surface coverages on polylactic acid (PLA, 72 %) and polyethylene terephthalate (PET, 66 %) and lower coverages on polycaprolactone (PCL, 44 %) and polypropylene (PP, 27 %). The team then illuminated pieces of the four polymers covered with the microrobot catalyst for seven days in a dilute H₂O₂ solution. They observed that the plastic lost up to 3 % of its weight and that the surface texture for all four polymers changed from smooth to rough, indicating degradation.

The microrobots were easy to recover due to their magnetic properties. They could also move along a maze made from several small channels, either driven by light or steered using magnetic fields. This demonstrates the usability of the microrobots in complex environments and hard-to-reach spaces.