Direct and indirect fluorescent detection of tetracyclines using dually emitting carbon dots

The authors describe dual-emission carbon nanodots containing blue emitters (BE; peak emission at 385nm under 315 nm excitation) and yellow emitters (YE; peak emission at 530 nm under 365nm excitation), and how they can be applied to direct and indirect determination of tetracyclines (TCs). The direct detection scheme is based on the finding that tetracycline (TET), oxytetracycline, chlortetracycline and doxycycline quench the two emissions of the carbon dots. While direct determination is rapid and convenient, it cannot differentiate between TCs. The indirect detection scheme, in contrast, is based on the finding that Al (III) ions enhance the fluorescence of the YE in the carbon dots, and that they cause a blue shift in emission. It is, however, known that TET forms a strong complex with Al (III), and this can inhibit the interaction between Al (III) and the YE, so that the fluorescence of YE is not enhanced and blue-shifted by Al (III) in the presence of TET. This finding is exploited in a fluorescence turn-on/off assay for TET that can distinguish TET from other TCs. The linear range of indirect determination for TET extends from 1 nM to 30 μM, and the limit of detection is 0.52 nM. The indirect method was successfully applied to the determination of TET in spiked milk, fish and pork, and recoveries ranged from 91.7 to 102 %. High concentrations of tetracyclines quench the dual (blue and green) emission of carbon dots. Additionally, based on the fluorescence enhancement of yellow emitters by Al 3+ , an indirect turn-on/off detection of tetracycline is established with high selectivity and sensitivity.