Abstract: Purpose: A novel fluorescent quenching immune chromatographic strip (FQICS) using upconversion nanoparticles (UCNPs) was developed to detect sulfaquinoxaline (SQX) in milk samples based on the principle of fluorescence resonance energy transfer (FRET). Methods: UCNPs was prepared using a pyrolytic process, and colloidal gold was prepared using sodium citrate reduction method. Then OVA-UCNPs were distributed to the upper side to form C line. OVA-UCNPs and SQX-OVA coating antigen were added to the lower side of the NC membrane to form the T line. When a negative sample was added to the sample pad, the liquid sample containing colloidal gold-labeled antibody migrated via capillary action toward the end of the strip. When the mixture reached the T line, which was coated with SQX-OVA coating antigen and OVA-UCNPs, the gold-labeled antibody bound to SQX-OVA coating antigen. Since the UCNPs were simultaneously coated at the same location as the SQX-OVA coating antigen, the binding of gold-labeled antibody to SQX-OVA coating antigen could provide a suitable distance between the UCNPs and colloidal gold to cause FRET. Using a 980 nm laser, the test results could be evaluated visually according to fluorescence intensity. Results: The limit of detection (LOD) of this assay for SQX in PBS buffer was 1 μg/L, and the LOD for SQX in milk samples was 8 μg/L. The whole detection process could be completed within 15 min. Conclusion: The FQICS based on UCNPs allowed for simple, sensitive, time-saving, and rapid on-site detection of SQX residues in milk.

Key words: sulfaquinoxaline, upconversion nanoparticles, fluorescence quenching immune chromatographic strips, milk