Melamine is a nitrogen-rich compound used in resins, flame retardants and dry-erase boards. Shockingly, however, melamine has recently been incorporated into milk products to artificially boost the nitrogen concentration and thereby falsify the milk's protein content. Ingestion of these tampered products, especially infant formula, has led to cases of acute kidney failure, causing death in several newborns.

Fig. 1: Color change of cyanuric acid–gold nanoparticles upon addition of extracts from melamine-tainted infant formula.

Now, Lehui Lu at the Changchun Institute of Applied Chemistry of the Chinese Academy of Sciences and colleagues1 have designed gold nanoparticles that change color upon coming into contact with melamine, providing a method for the rapid and selective detection of the contaminant in milk products (Fig. 1).

“Our approach is simple and doesn't need any advanced instruments,” says Lu. In their strategy, the researchers coupled gold nanoparticles with cyanuric acid (CA), a compound that associates with melamine via hydrogen-bonding.

“The reaction of melamine and cyanuric acid or its derivatives can lead to the formation of special supermolecular structures,” says Lu. “Therefore, we reasoned that when combined with CA derivatives, gold nanoparticles may be used for detecting melamine.”

The researchers first synthesized a sulfur-bearing CA derivative. Next, they heated a gold salt with a reducing agent called sodium citrate to prepare 12 nm precursor gold nanoparticles. Finally, they mixed the CA derivative with the precursor nanoparticles to afford CA–gold nanoparticle conjugates.

Lu and his colleagues showed that the wine-red CA–gold nanoparticle suspensions progressively turned purple then violet blue when added to increasingly concentrated aqueous melamine solutions. Visible to the naked eye, the color changes occurred within 60 seconds at concentrations a thousand times lower than the US safety limits. Exposure to other hydrogen-bonding compounds produced weaker responses, suggesting that the nanoparticles are melamine-specific.

The researchers also applied their method to real milk and infant formula samples. They found that after extraction using acid followed by purification, tainted milk produced distinct and dramatic changes in color when added to the nanoparticle suspension. Infant formula produced a less prominent color change due to interference by the the larger and more complex constituents. The researchers are currently trying to improve the extraction procedure to facilitate detection in milk products.