Used in plastic dinnerware, colorants, and flame retardants, melamine is not approved for use in food. Yet, in 2008, the Chinese government discovered that manufacturers were illegally adding melamine to milk powder, a practice that led to six infant deaths that year and sickened 300,000, according to news reports. Melamine’s high nitrogen content fools simple tests for protein, indicating falsely high protein levels. Subsequently, both U.S. and Chinese governments imposed a limit of 1-ppm melamine in infant milk products.

A fast, portable test that could check for melamine adulteration anywhere along the chain of production would help manufacturers and consumers, says Yi Lu of the University of Illinois, Urbana-Champaign. Lu’s group has developed ways to use the PGM to determine concentrations of metal ions, antibodies, and more. Lu and collaborators at Tsinghua University, in Beijing, extended this to melamine in milk.

The researchers identified a DNA aptamer—a sequence that binds with high specificity to a molecule of interest—for melamine. To measure melamine concentrations, the researchers add the aptamer to milk along with two complementary DNA sequences: one linked to a magnetic bead and the other to invertase enzyme, which converts sucrose to glucose. In the absence of melamine, the aptamer binds to both the bead and to invertase, allowing the researchers to remove invertase from the solution with a magnet. But if the aptamer binds melamine, it can’t bind to the bead or to invertase. The magnet removes empty beads, leaving behind invertase in proportion to the melamine concentration. Then when the researchers mix the sample with sucrose, the invertase produces glucose for the PGM to measure. The aptamer can detect melamine at 67.5 ppb in the presence of melamine analogs and milk proteins.