Melamine is a cyclic molecule based on carbon, nitrogen, and oxygen atoms. Because it possesses many nitrogen atoms, it can be used to fool certain analytical assays for the protein content of food.
However, it is illegal to add melamine to food because it is toxic; the safety limit in the United States and the European Union is 2.5 parts per million. Melamine poisoning has recently sickened and killed many pets and infants.
In light of these incidents, a sensor that can be used to readily detect melamine in food with no sample preparation, that works rapidly with no instrumentation, is needed. Lehui Lu (Chinese Academy of Sciences) and coworkers have worked towards developing such a method.
The sensor.
Similarly to other news posts on this website, the assay is based on a visually observable color change of a gold nanoparticle solution. The color of gold nanoparticles in a water solution depends on the diameter of the nanoparticles.
For example, gold nanoparticles that are 200 nanometers in diameter will be a different color than those that are 15 nanometers in diameter. Aggregating gold nanoparticles in solution, in response to an appropriate stimulus, gives a readily observable color change.
The trick is to design the nanoparticles to aggregate in response to an analyte, such as melamine. In this case, the scientists coated the nanoparticles with a derivative of cyanuric acid.
Such molecules are known to form multiple stable chemical bonds with melamine molecules. Thus, when melamine molecules are present in a solution of these specially coated gold nanoparticles, the nanoparticles will aggregate, giving a color change, and the amount of melamine in the solution can be determined.
Evaluation of the sensor.
The scientists found that melamine caused the nanoparticles to aggregate (color change) within 1 minute. Even adding as little as 2.5 parts per billion melamine to the solution lead to a readily observable color change by eye.
This melamine concentration is 1000 times below the safety limit. Furthermore, it is approximately the concentration that is detectable with very expensive instrumentation, that also happens to be much slower.
The sensor is specific for melamine, in that molecules of a similar structure do not interfere in the assay. Most importantly, the sensor also works in purified milk.
However, for use in infant formula, the amino acids, vitamins, and numerous other molecules and minerals need to be removed prior to melamine analysis. Such extraction procedures are available.
Overall evaluation.
With all due respect, the scientists seem a bit sketchy as to whether the assay is effective in processed baby formula. In any event, this is progress towards developing a more practically useful assay for melamine, which may lead towards an assay that works with no sample processing.
for more information:
Ai, K., Liu, Y., & Lu, L. (2009). Hydrogen-Bonding Recognition-Induced Color Change of Gold Nanoparticles for Visual Detection of Melamine in Raw Milk and Infant Formula Journal of the American Chemical Society, 131 (27), 9496-9497 DOI: 10.1021/ja9037017