Off-the-shelf devices that detect contaminants are bulky and expensive and require frequent recalibration. At the University of California, Los Angeles, engineers Tom Harmon and Jack Judy have built a tiny sensor that could improve soil monitoring of nitrates, which can leak into ground water and cause health problems. The sensor consists of a carbon rod coated with a mix of polymer and nitrate. Placing it in water or moist soil generates a voltage that corresponds to the difference between the nitrate concentrations of the soil and the coating.The hair-thin, 1.5-centimeter-long device could be mass-produced on silicon chips, says Harmon, lowering the cost to one-tenth that of conventional nitrate detectors. Eventually, the sensor will self-calibrate based on models of how environmental factors, such as temperature, affect the signal. In five years, Harmon says, the effort could yield arrays of cheap, reliable sensors that need little human intervention once deployed.