Determination of Melamine by Label-Free Ratiometric Fluorescence Using the Inner-Filter Effect (IFE) between Cadmium Tellurium/Cadmium Sulfide Quantum Dots (QDs) and Gold Nanoparticles (Au NPs)

A simple and sensitive ratiometric fluorescent assay has been developed for the determination of melamine based on the inner-filter effect (IFE) between gold nanoparticles (AuNPs) and fluorescent quantum dots (QDs). In the IFE-based fluorescent assay, a complementary overlap was established between the absorption spectrum of AuNPs and emission spectrum of cadmium tellurium/cadmium sulfide quantum dots (CdTe/CdS QDs). Consequently, AuNPs in the dispersion state quench the emission of green CdTe/CdS QDs at 520 nm (QDs520), while the aggregated AuNPs quench emission of red CdTe/CdS QDs at 680 nm (QDs680). Along with the addition of melamine, the fluorescence of the QDs at 680 nm was quenched while the fluorescence of at 520 nm was recovered by the aggregation of AuNPs so the ratiometric fluorescent detection of melamine was complete. The system determined melamine from 30 to 1030 nM and was employed to determine melamine in milk products with satisfactory recoveries. The simple proposed sensing approach may improve the selectivity and sensitivity of IFE-based ratiometric fluorescent assays.