The lack of clean water in many areas around the world is a persistent, major public health problem. One day, tiny robots could help address this issue by zooming around contaminated water and cleaning up disease-causing bacteria. Scientists report a new development toward this goal in the journal ACS Applied Materials & Interfaces.

Drinking water contaminated with pathogenic bacteria can cause serious illnesses that, in areas with spotty medical services, are potentially life-threatening without proper treatment. Water can be disinfected with chlorine or other disinfectants, but there are some hardy bacteria and other microorganisms that are hard to remove. Treating water with a combination of disinfectants or increasing their concentrations can help. But they remain in the water, and their byproducts can be harmful to human health. In recent years, researchers have been exploring the use of self-propelled micromotors to degrade and capture pollutants in water. Building on this work, Diana Vilela, Samuel Sánchez Ordóñez and colleagues wanted to see if they could engineer tiny robots to remove waterborne bacteria.

The team designed "two-faced" spherical particles to perform the task. One face is made with magnesium, which reacts with water to produce hydrogen bubbles to propel the microbots. The other face is made out of alternating iron and gold layers topped by silver nanoparticles. Bacteria stick to the gold and are killed by the silver nanoparticles. Lab testing showed that the particles can motor around in water for 15 to 20 minutes before the magnesium is spent. And they trapped more than 80 percent of E. coli in water spiked with a high concentration of the bacteria. Then, because of the iron's magnetic properties, the microbots are removed easily with a magnet, without leaving behind any harmful waste in the water.