An up-conversion test paper based on "switch-off" of fluorescence is constructed to sensitively and selectively detect cancer-causing dye Sudan III in lipstick.

Cancer-causing dye Sudan III is banned adding to cosmetics, so a method for detecting trace Sudan III in cosmetics is established. A single dispersed up-conversion molecularly imprinted fluorescent nanoprobe is constructed and coated on the filter paper. The mechanism for detecting Sudan III by this composite fluorescent nanoprobes-paper is systematically analyzed. The fluorescent response (max emission peak is at 541 nm) is linearly related to 10-1000 nM Sudan III, and Sudan III can be selectively recognized (imprinting factor increased to 4.1). The limit of detection and quantitation are further reduced to 2.89 nM and 9.63 nM, respectively. The recoveries of Sudan III in lipstick samples are between 93.18 and 108.3%, and relative standard deviation is less than or equal to 4.6%. Trace Sudan III in cosmetics are detected accurately and sensitively by this method due to up-conversion nanoparticles with little interference of background fluorescence and molecularly imprinted polymers with selective enrichment.