Picomolar melamine enhanced the fluorescence of gold nanoparticles: spectrofluorimetric determination of melamine in milk and infant formulas using functionalized triazole capped gold nanoparticles.

We wish to report a simple and sensitive method to determine the melamine in milk and infant formulas using 3-amino-5-mercapto-1,2,4-triazole capped gold nanoparticles (AMTr-AuNPs) as fluorophore. The AMTr-AuNPs were synthesized by a wet chemical method and were characterized by high-resolution transmission electron microscopy (HR-TEM), and X-ray diffraction, UV-visible and fluorescence spectroscopic techniques. The AMTr-AuNPs show the absorption maximum at 520 nm and emission maximum at 759 nm (λ(ex)=520 nm). While adding 10 μM melamine, the wine red color of AMTr-AuNPs was changed into purple and the absorption band at 520 nm was decreased. The observed changes were ascribed to the hydrogen bonding interaction between melamine and AMTr-AuNPs, which led to the aggregation of the nanoparticles. This was confirmed by dynamic light scattering and HR-TEM measurements. No appreciable absorption change was observed for AMTr-AuNPs in the presence of less than micromolar concentrations of melamine. But, the emission intensity of AMTr-AuNPs was enhanced even in the presence of picomolar concentration of melamine. Based on the enhancement of emission intensity, the concentration of melamine was determined. The present fluorophore showed an extreme selectivity towards the determination of 100 nM melamine in the presence of 500-fold common interferents. The good linearly was observed from 1×10⁻⁹ to 100×10⁻¹² M melamine and a detection limit was found to be 10 fM/L (S/N=3). The proposed method was successfully applied to determine melamine in cow milk and infant formulas. The obtained results were validated with HPLC.