Single particle detection based colorimetric melamine assay with MnO2-modified gold nanoparticles

In this work, we demonstrate a simple and label-free single particle detection (SPD) method for the quantification of melamine (Mel) based MnO 2 -modified gold nanoparticles (Au@MnO 2 NPs). Without Mel, MnO 2 shell of Au@MnO 2 NPs are able to be directly decomposed by gallic acid. In the existence of Mel, the decomposition process is restrained due to the strong hydrogen bonding between Mel and gallic acid. The selective etching of MnO 2 shell on AuNPs surface results in evident color change from single Au@MnO 2 NPs. Through quantitatively enumerating the number of color coded particles, trace amounts of Mel can be accurately detected. A linear range from 0.02 to 1 μM (R 2  = 0.98) and a limit of detection of 16.7 nM are acquired. In milk sample, satisfactory recovery was acquired. Therefore, the SPD method is expected to be used for the detection of various chemical contaminants in food.