Detection of melamine by a magnetic relaxation switch assay with functionalized Fe/Fe3O4 nanoparticles

The detection for melamine has attracted much attention due to its toxicity to the kidney of animals and humans. Herein, we reported a magnetic relaxation switch (MRSw) assay for melamine with Fe/Fe3O4 nanoparticles functionalized by the dopamine conjugated with methoxypoly (ethylene glycol) carboxyl acid (mPEG-COOH) and (2,4,6-trioxo-1,3,5-triazin-1-yl) acetic acid (TTAA) (TTAA-Fe/Fe3O4 NPs). The approach of the MRSw assay for melamine is based on the aggregation of TTAA-Fe/Fe3O4 NPs induced by the special triple hydrogen bonds between melamine and TTAA, which results in the increase of the spin-spin relaxation time (T2) of surrounding water protons of nanoparticles. Due to no limitation of light scattering, the detection of melamine with the MRSw assay can be performed in a turbid medium without pre-treatment and purification. Upon addition of the different concentration of melamine to an aqueous solution of TTAA-Fe/Fe3O4 NPs ([Fe] = 32 μM), the change of T2 value has a good linear relationship with the concentration of melamine below 16 μM. The limit of detection (LOD) for melamine is ∼2 μM. Furthermore, the MRSw assay exhibits a high selectivity for melamine over cytosine, uracil and thymine with the similar structure.