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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 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. [ABSTRACT FROM AUTHOR]

The determination of thiol based biological molecules and drugs, such as cysteine (Cys), glutathione (GSH), a-lipoic acid, and sodium 2-sulfanylethane sulphonate in human plasma are becoming progressively more important, due to exploration of their essential role in numerous biological pathways. Herein we demonstrate the development of a highly sensitive colorimetric method for the determination of medicinally important thiol drugs (I-IV), based on aggregation of the citrate capped silver nanoparticles (Ag NPs). This was exploited from high affinity of thiols towards the Ag NPs surface, which could tempt replacement of the citrate shell by the thiolate shell of target molecules, resulting in aggregation of the NPs through intermolecular electrostatic interaction or hydrogen-bonding. Because of aggregation, the plasmon band at around 396 nm decreases gradually, along with formation of a new red shifted band. The calibration curves are derived from the intensity ratios of absorbance at 520 to 540 nm for compound I-IV respectively, to the original wavelength of 396 nm. The calibration curves display a linear relation in the range of lμM-100 μM (for I), 5pM-200 pM (for II) 10 pM-100pM (for HI) and 10 μM -150 μM (for IV), respectively. The obtained detection limits (3a) were found to be 1.5 μM, 6.6 μM, 10.5 μM and 11.6 pM for compound I-IV, respectively. The proposed method has been successfully applied for the detection of thiol compounds in different kind of real samples. [ABSTRACT FROM AUTHOR]