A simple, reliable and sensitive colorimetric visualization of melamine in milk by unmodified gold nanoparticles

Abstract

In this paper, we report a simple, reliable and sensitive colourimetric visualization of melamine in milk products using citrate-stabilized gold nanoparticles (Au NPs). Upon exposure to ppb-level melamine, gold nanoparticle solution exhibits a highly sensitive colour change from red to blue and rapid aggregation kinetics within the initial 5 min, which can directly be seen with the naked eye and monitored by UV-vis absorbance spectra. As confirmed by the comparison with six other typical amino compounds, the melamine molecule itself contains multiple strong-binding sites to the surface of Au NPs and thus plays a role of molecular linker to efficiently crosslink Au NPs. Further evidence is that the sensitivity is significantly improved when NaHSO₄ is added to promote the ligand exchange between citrate and melamine at the surface of Au NPs. The NaHSO₄-optimized Au NPs system provides a rapid colourimetric assay for the rapid detection of melamine down to ∼25 ppb in real milk products.