N-Doped Carbon Quantum Dot-MnO₂ Nanowire FRET Pairs: Detection of Cholesterol, Glutathione, Acetylcholinesterase, and Chlorpyrifos

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 MnO2 nanowires (energy acceptor) has been successfully applied to fabricate a fluorometric probe for detection of cholesterol (ChO), glutathione (GSH), acetylcholinesterase (AChE), and chlorpyrifos. MnO2 nanowires (MnO2 NWs) significantly quenched the blue fluorescent emission of N-CQDs by the phenomenon of FRET. The redox reactions of MnO2 with H2O2 and thiolated compounds resulted in the decomposition of MnO2 nanowires (brown) to give Mn2+ ions (colorless), which induced the fluorescence recovery of N-CQDs (turn-on). The interruption of the FRET phenomenon of N-CQD–MnO2 NW composites by the produced H2O2 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–MnO2 NW composites. The decomposition of MnO2 NWs into Mn2+ 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.