Integration of colorimetric and SERS detection for rapid screening and validation of melamine in milk

Various analytical methods have been developed for detecting melamine in milk. Herein, we developed a novel method which integrated two gold nanoparticle (Au NP) based techniques, colorimetric and surface enhanced Raman spectroscopic (SERS) analyses, for rapid screening and validation of melamine in milk. The colorimetric method, which utilizes the color change of Au NPs from red to blue or purple upon interaction with melamine, was used for rapid screening. However, the colorimetric method presents false positive and inaccurate quantitative signals in the presence of interfering compounds. To overcome these limitations, the SERS method, which can directly utilize Au NPs from the colorimetric method, was employed as a rapid validation tool. In order to optimize the integration of these two methods, three sizes (15, 30, and 50 nm) of Au NPs were evaluated, and the 30 nm Au NPs were determined to be the best size for both colorimetric and SERS methods based on limits of detection (LODs) and quantification capability of melamine. By using the developed colorimetric-SERS method, we were able to rapidly screen and validate as low as 0.25 ppm melamine in milk within 20 min. Integrating colorimetric and SERS methods exploits the advantages of both methods, and provides a more rapid, accurate, and cost-effective way for monitoring melamine contamination in large amounts of food or feed products.