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Nitrogen-doped carbon quantum dots (N-CQDs) were synthesized by hydrothermal treatment of citric acid and triethylamine. The fluorescence resonance energy transfer (FRET)based fluorescence "switch-off−on" of N-CQD (energy donor) in the presence of MnO 2 nanowires (energy acceptor) has been successfully applied to fabricate a fluorometric probe for detection of cholesterol (ChO), glutathione (GSH), acetylcholinesterase (AChE), and chlorpyrifos. MnO 2 nanowires (MnO 2 NWs) significantly quenched the blue fluorescent emission of N-CQDs by the phenomenon of FRET. The redox reactions of MnO 2 with H 2 O 2 and thiolated compounds resulted in the decomposition of MnO 2 nanowires (brown) to give Mn 2+ ions (colorless), which induced the fluorescence recovery of N-CQDs (turn-on). The interruption of the FRET phenomenon of N-CQD−MnO 2 NW composites by the produced H 2 O 2 from the reaction of cholesterol oxidase in the presence of cholesterol, and thiocholine from the reaction of acetylthiocholine in the presence of acetylcholinesterase, causes FL recovery of N-CQDs. The inhibition of AChE by chlorpyrifos induces FL quenching (turn-off) of N-CQD−MnO 2 NW composites. The decomposition of MnO 2 NWs into Mn 2+ in the presence of glutathione resulted in the subsequent FL recovery of N-CQDs. The sensing system shows a sensitive response to cholesterol, glutathione, and chlorpyrifos pesticide, giving LODs and LOQs of 4.89 nM, 7.52 nM, 0.01 μM and 14.83 nM, 22.80 nM, 0.03 μM, respectively. The practical applicability of the proposed probe has been verified by detecting the ChO and GSH in human plasma with satisfactory results.

In this work, fluorescence (FL) quenching (turn-off) and recovery (turn-on) of carbon quantum dots (CQDs) in the presence of dispersed and aggregated gold nanoparticles (AuNPs) was used as a probe for monitoring the inhibition and reactivation of acetylcholinesterase (AChE).

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