Facile synthesis of bright yellow fluorescent nitrogen-doped carbon quantum dots and their applications to an off–on probe for highly sensitive detection of methimazole

In this work, bright yellow fluorescent nitrogen-doped carbon quantum dots (YN-CQDs) are produced by a facile, inexpensive and eco-friendly hydrothermal technique using extract of chlorophyll from lettuce leaf as precursor. The quantum yield (QY) of as-synthesized YN-CQDs was up to 38%, which was higher than the previously reported quantum yield of most CQDs prepared using other biomass. Silver ion (Ag+) can covalently bond with carboxyl, hydroxyl and amino groups on the surface of YN-CQDs and quench the fluorescence of YN-CQDs through static quenching. Upon addition of methimazole (MZ) to the YN-CQDs/Ag+ dispersion system, Ag+ releases from the YN-CQDs/Ag+ complex due to the high affinity of MZ for Ag+ and the quenched fluorescence of the YN-CQDs was recovered. Hence, we developed an off–on sensing method for the detection of MZ using YN-CQDs/Ag+ as a fluorescent probe. Under optimal conditions, with addition of MZ, the fluorescence recovery of YN-CQDs/Ag+ system happens with a good linearity in the range of 0.003 ~ 60 μmol/L and a detection limit of 0.8 nmol/L. The YN-CQDs/Ag+ system exhibited good MZ determination performance. The established off–on fluorescence sensing platform was successfully employed for detecting MZ in mouse plasma.