CdSe/ZnS Quantum Dots with Multi-Shell Protection: Synthesis and Application in the Detection of Ractopamine Residue in Swine Urine

In this paper, we aimed to prepare ultra-stable aqueous CdSe/ZnS quantum dots (QDs) as fluorescent labels to detect ractopamine (Rac) residue in swine urine based on a competitive lateral flow immunoassay (LFIA) biosensor system. Such CdSe/ZnS QDs with multi-shell protection were prepared by various shellencapsulating strategies, which included preparing aqueous amphiphilic oligomer-CdSe/ZnS QDs, encapsulating aqueous CdSe/ZnS QDs within a silica shell, grafting the silica surface with long-chain hydrocarbons, and re-transferring the hydrophobic QDs-SiO₂ nanoparticles into water with amphiphilic oligomer. The size of multishell protected CdSe/ZnS QDs tested by TEM could be tuned from 11.7 to 27.3 nm. The QDs can be stabilized in a broad range of chemical conditions, including strong acidic solutions (pH = 1) and high concentrations of salts, and can withstand thermal treatment at 80 °C. Even in swine urine, the multi-shell protected CdSe/ZnS QDs can still retain their high fluorescence. Eventually, the reliable detection limit for Rac residue by using CdSe/ZnS QDs with multi-shell protection in swine urine is successfully pushed down to 0.1 ng/mL, which is over 30 times higher than commercial Rac LFIA strips.