Every day, an estimated 3,000 to 6,000 people worldwide die from diseases caused by contaminated water. Filtration can reduce the risks, but traditional bacterial and viral filters trap pathogens inside granular carbon or porous ceramic or polymer materials, many of which are difficult to clean and must be changed frequently.

Now scientists are turning to carbon nanotubes. A team from Rensselaer Polytechnic Institute in Troy, NY, and the Banaras Hindu University in Varanasi, India, has devised a way to get millions of the large carbon molecules to collect on the inside surface of a quartz tube about a centimeter across. The resulting tube-inside-a-tube consists of radially oriented nanotubes, packed as tightly as a fistful of spaghetti and bonded together; this structure can be detached from the quartz and extracted whole. With one of its ends capped and water pumped in through the other, such a cylinder acts as a filter. Water molecules can squeeze out through nanometer-sized gaps in the walls, but bacteria like E. coli and viruses like the 25-nanometer-wide poliovirus get stuck.