Label-free colorimetric sensing of melamine based on chitosan-stabilized gold nanoparticles probes

A sensitive and simple label-free colorimetric method for determination of melamine was development based on the aggregation of chitosan-stabilized gold nanoparticles (AuNPs). AuNPs were prepared by reducing gold salt with a polysaccharide, chitosan, in the presence of tripolyphosphate (TPP). Here, chitosan acted as a reducing/stabilizing agent. The obtained gold nanoparticles were characterized with UV-vis spectroscopy and transmission electron microscopy. The presence of melamine led to a shift in the surface plasmon bond and a color change of chitosan-stabilized AuNPs from red to dark blue. Results showed that the absorbance ratio (A650/A520) was linear with the logarithm of melamine concentration in the range of 10−5-10−2 g/L with a correlation coefficient of 0.996. The limit of detection (LOD) obtained by UV-vis spectrum was 6 × 10−6 g/L. The proposed method was applied successfully to the determination of melamine in pretreated liquid milk products, and the recoveries were from 85% to 107%. The method is rather simple, and the whole process including sample pretreatment takes only 20 min at room temperature. The merits (such as simplicity, rapidity, low cost and visual colorimetry) make the proposed method useful for on-site screening melamine levels well below the current safety limit in raw milk.

► AuNPs were prepared by reducing gold salt with chitosan in the presence of TPP. ► The solution showed a color change from red-to-blue when melamine was added. ► The absorbance ratio was linear with the logarithm of melamine concentration. ► The proposed method was applied successfully to the determination of melamine.