Efficient inner filter effect of gold nanoparticles on the fluorescence of CdS quantum dots for sensitive detection of melamine in raw milk

A simple, rapid and ultra-sensitive analytical method for the determination of melamine in raw milk based on inner filter effect (IFE) of Au nanoparticles (AuNPs) on the fluorescence of CdS quantum dots (QDs) is described in this study. With the presence of citrate-stabilized AuNPs, the fluorescence of l-cysteine (l-Cys)-capped CdS QDs was remarkably quenched by AuNPs via IFE. The fluorescence of the AuNPs-CdS QDs system was recovered upon addition of melamine, since melamine could induce the aggregation of AuNPs accompanying the reduction of their characteristic plasmon absorption at ∼522 nm, which resulted in “turn-on” of the IFE-decreased fluorescence of CdS QDs. Under the optimum conditions, a good linear correlation for detection of melamine in raw milk was exhibited from 0.05 mg·L−1 to 0.35 mg·L−1, and the detection limit was 0.017 mg·L−1, which was much lower than the detection limit of HPLC coupled with UV detector and the safety limit required by USA, UK and China. This method was successfully carried out for the assessment of melamine in raw milk samples, which revealed many advantages such as high sensitivity, low cost and non-time-consuming compared with traditional methods.