Preparation Of N-doped Grapheme Quantum Dots And Their Applications In Biological Fluid Detection

Graphene quantum dots?GQDs?are carbon nanomaterials with zero-dimensional structure with particle sizes less than 100 nm.GQDs present unique optical and electronic properties because of the edge effect and quantum confinement effect.The studies demonstrated that nitrogen atoms can be doped in the structure of grapheme,and then obtain nitrogen-doped graphene quantum dots?N-GQDs?.Doping GQDs with nitrogen atoms can effectively adjusted the optical properties and chemical properties of GQDs.Compared with GQDs,N-GQDs have some fascinating advantages,such as stable photoluminescence,low cytotoxicity,excellent biocompatibility and solubility,adjustable band gap and high specific surface area.Therefore,N-GQDs have broad application prospects in the fields of biosensing,bio-laser and imaging as well as optoelectronic devices.In this paper,N-GQDs have been established using citric acid and urea by one-pot hydrothermal method.The structure and properties of N-GQDs were characterized.Then,we constructed an N-GQDs-based fluorescence sensor that can detect dopamine?DA?,histidine?His?and pyrophosphate?PPi?in biological fluids.The main contents of the study include:1.N-GQDs as fluorescent probe for the detection of DA.We synthesized N-GQDs by hydrothermal method with citric acid as carbon source and urea as nitrogen source,respectively.The prepared N-GQDs were characterized by transmission electron microscopy?TEM?,X-ray diffraction?XRD?,Raman spectroscopy,Fourier transform infrared spectroscopy?FTIR?and UV-Vis absorption spectra.We also investigated the stability of N-GQDs over a range of pH,ionic strength?NaCl?and time.In addition,under alkaline conditions,DA could self-polymerize into pDA and adsorbed on the surface of N-GQDs to form a thin film,leading to quenching of N-GQDs fluorescence by resonance energy transfer?FRET?.Based on this quenching mechanism,the fluorescent probe can detect DA sensitively.With the optimized conditions,including pH,reaction time and reaction temperature,the linear range of DA was 0-90?mol L^-1,and the limit of detection?3?/R?was80.5 nmol L^-1.The fluorescent probe was successfully applied for DA.We synthesized N-GQDs by detecting in human urine samples,with a recovery range of 96%-102%.2.Cu²⁺modulated N-GQDs as fluorescent probe for the detection of histidine.On the surface of N-GQDs exist abundant oxygen-containing functional groups?such as-COOH,-OH?,Cu²⁺could form covalent bonds with them and adsorbed on the surface of N-GQDs.The fluorescence of N-GQDs was significantly quenched by Cu²⁺because of non-radiative electron transfer.When His was added,His has an imidazole group and the high affinity of His to Cu²⁺enables Cu²⁺to be dissociated from the surface of N-GQDs and formation of a more stable Cu²⁺-His complex eventually leads to fluorescence recovery of N-GQDs.A highly sensitive fluorescence sensor for detecting His was constructed,the linear range was0-35?mol L^-1,and the limit of detection?3?/R?was 72.2 nmol L^-1.The fluorescent probe was successfully applied for His detecting in human urine samples,with a recovery range of96%-102%.3.N-GQDs-Pb???system as fluorescent probe for the detection of PPi.The fluorescence of N-GQDs could be significantly quenched in the presence of Pb???through the coordination occurring between Pb???and oxygen-containing groups on the surface of N-GQDs and forming N-GQDs-Pb???complexes by an inner filter effect.In the presence of PPi,the fluorescence of N-GQDs-Pb???system was recovered gradually because of the separation of Pb???from the N-GQDs-Pb???complexes due to strong bond force between Pb???and PPi.In addition,the fluorescence intensity of N-GQDs was not affected by PPi.Therefore,the fluorescence probe could be used for the detection of PPi,the linear range was0.2-40?mol L^-1,and the limit of detection?3?/R?was 69.2 nmol L^-1.The fluorescent probe was successfully applied for His detecting in human urine samples,with a recovery range of96%-103%.