SERS Sensing of Melamine

Melamine (2,4,6-triamino-1,3,5-triazine) is added to the food to elevate the measured protein content. A tolerance level for melamine is 1 ppm for infant formula and 2.5 ppm (2500 ppb, equivalent of 0.02 mmol F or 2.5 mg F ) for milk and other food products. Melamine was found as an adulterant in pet foods in 2007 in North America and in milk products in 2008 in China. The melamine is one of the most common food additives studied by SERS because of its distinct sharp peaks and emerging safety concern. 

Most of the previous SERS analysis required some pretreatment methods to extract melamine from real matrix. Betz and co-workers developed the fast SERS approach for detecting melamine in infant formula without the need for purification or additional equipment (Betz et al. 2012). Eor this purpose, a galvanic displacement of Cu by Ag was used to create Ag nanostructures on low-cost surfaces of Cu tape and coin and excellent SERS enhancement was obtained. A strong peak between 676 and 690 cm was observed and served to identify melamine contamination of the formula. Both the tape and coin enabled the detection of contamination down to the 5 ppb level which is up to 200x better than that using commercial Klarite substrates. 

Zhou and co-workers developed a new strategy for quantitative detection of melamine in milk based on Au Ag NPs with 4-ATP as the internal reference (Zhou et al. 2015). The internal reference is embedded between Au core and Ag shell while the target analyte is outside of the Ag shell. For melamine detection, the milk samples containing different amounts of melamine were mixed with Au 4-ATP Ag NPs and SERS spectra were taken. Two distinct peaks, 1075 and 700 cm associated with 4-ATP and melamine, respectively, were chosen for quantitative analysis. A strong correlation between their intensity ratio and the melamine concentration was observed. This allowed an amount of melamine in milk to be determined down to 5 pM concentration. 

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