Researchers at the University of Massachusetts, Amherst, United States, have developed a nanoparticle-based sensor that changes color from yellow to red as the concentration of bacteria in water increases. Drinking water in developing countries is often contaminated with bacteria, which kills millions of people each year. Methods to test such water require expensive lab equipment and expert training, two resources many such communities lack. Vincent Rotello, a researcher, said, “There’s basically no easy way to tell if water has bacteria in it.” He and his colleagues wanted to offer a cheap and simple alternative to current testing methods. The team embedded a biosensor in a strip of filter paper, which changes from yellow to red within 10 minutes after exposure to bacteria. The sensor, says Rotello, does not require large amounts of the nanoparticles, or enzyme, so the strip’s cost should be low. The next step is to improve the sensitivity of the strips, he says, as the most virulent microbes can still cause diseases even at very low concentrations.