Paper Versus Pathogen

For the one billion people worldwide who don't have access to clean drinking water, purifying water usually involves costly filtering devices or toxic chemicals. But a new study shows that killing pathogenic bacteria may be as easy as filtering water through paper containing silver nanoparticles (Environ. Sci. Technol., DOI: 10.1021/es103302t). The concept could provide a cheap, portable, and nontoxic way to purify water on a small scale.

"Silver has had millennia of use as a bactericide," says Derek Gray, a chemist at McGill University. He and chemist Theresa Dankovich dipped 0.5 mm thick sheets of filter paper in a silver nitrate solution. To form the nanoparticles, they reduced the silver by bathing the paper in sodium borohydride. They then used the paper to filter a brew of pathogenic bacteria, at a concentration commonly found in ditches.

Paper containing 5.9 mg silver per gram of paper killed nearly all the bacteria and met Environmental Protection Agency standards for water purification devices, which require reducing bacterial numbers by 5 orders of magnitude. The amount of silver leaching from the paper—just 0.0475 ppm—also met the EPA safe drinking water standard.

"However, we need to use real ditch water to verify the technology under field conditions," Dankovich says. Ditch water contains a complex mix of inorganic and organic contaminants that could interfere with the bactericidal action of the paper, she says. She and Gray hope that once commercialized, the purifying sheets would cost little more than a piece of paper.