Colorimetric detection of melamine in pretreated milk using silver nanoparticles functionalized with sulfanilic acid

Multiple methods have been published to detect melamine, only a few offer sensitivities below 50 nM with complicated procedures and sophisticated equipments. We demonstrate here a simple, rapid and lower-cost assay with high sensitivity for the melamine detection in milk samples using sulfanilic acid-modified silver nanoparticles (SAA-AgNPs). Due to the special chemical structures, SAA shows similar response to its analogues, which reveals that the selectivity and sensitivity of SAA itself is poor. However, the formation of SAA-AgNPs dramatically improves the selectivity of SAA and only melamine reacts with SAA-AgNPs. The possible mechanism is discussed. The interaction between exocyclic amine of melamine and SAA induces rapid aggregation of SAA-AgNPs accompanied by a naked-eye visible color change, resulting in precise quantification of melamine that can be monitored by a simple UV-visible spectrometer. Metal ions, amino acids and sugars that are common in milk have negligible interference influences. The extinction ratio (A620nm/A390nm) is correlated with the melamine concentration over the range of 0.1-3.1 μM. The detection limit is 10.6 nM, which is much lower than the safety limits (8 μM for infant formula in China, 20 μM in both the USA and EU and 1.2 μM in the CAC review for melamine in liquid infant formula). The method is applied successfully to determine melamine in pretreated milk products, indicating the potential practical use for the products suspected of melamine exposure.