Facile Colorimetric Detection of Melamine in Milk Using a Magnetic Molecularly Imprinted Polymer

There is an increasing demand for rapid and easy-to-use detection methods for monitoring melamine in food products. However, conventional approaches rely heavily on high-end equipment such as mass spectrometry or fast test kits based on antigen–antibody interactions, which significantly hinders their widespread application for on-site detection. In this study, we developed a colorimetric sensor based on a molecularly imprinted polymer, prepared via in situ deposition of polydopamine onto a magnetite core. A nanoscale polymeric layer with a thickness ranging from 7 to 10 nm was obtained, featuring binding sites that conformationally mimic melamine molecules. When the imprinted polymer encounters melamine, the imprinted cavities selectively capture the target molecules and block H₂O₂ from reaching the magnetite surface. This suppression of •OH generation limits the oxidation of the colorimetric indicator, resulting in varying degrees of color change corresponding to the melamine concentration. The resulting imprinted polymer successfully detected melamine in spiked milk samples at concentrations ranging from 5 to 1000 μM, with good linearity (R² = 0.9828) using UV–vis spectroscopy. Notably, even low levels of melamine could be visually detected by a distinct color change. In addition, the sensor exhibited excellent selectivity when tested against various other molecules and ions in milk samples. Importantly, the limit of detection was determined to be 5 μM, corresponding to 0.32 mg in 500 mL of milk, which is well below the melamine toxicity threshold (14 mg for a 70 kg adult). These results suggest that the proposed sensor holds strong potential for real-world applications in the rapid, accurate, and on-site detection of melamine in food products.