Copper doped zinc sulfide quantum dots as ratiometric fluorescent probes for rapid and specific detection of tetracycline residues in milk

In this work, copper-doped zinc sulfide quantum dots (ZnS:Cu QDs) were developed as ratiometric fluorescent probes to detect tetracycline (TC) in milk. With the increasing of TC concentration, the fluorescence at 578 nm of ZnS:Cu QDs was quenched, while a new fluorescence at 520 nm was significantly enhanced due to the formation of a green-emitting complex of Zn2+ and TC on the surface of probes. The fluorescence intensity ratio (I520/I578) have a good linear relationship (R2 = 0.98) with the concentration of TC in the range of 0.2-2.0 μM. The detection limit was calculated to 149 nM, which is lower than the allowable level of TC (about 180 μM) in milk set by the US Food and Drug Administration (FDA). Finally, the fluorescent test papers have been also prepared using ZnS:Cu QDs as ink by painting on a piece of filter paper. With the addition of TC, the orange test paper produced green spots that became more and more obvious with a dose-discerning ability as low as 2 μM. This work demonstrated a great potential of ratiometric fluorescent probe for the easy and visual detection of TC in complicated real samples.