Synthesis Of N-doped Graphene Quantum Dots And Its Application In Fluorescence Sensor

Graphene quantum dots(GQDs)have attracted much attention due to their excellent optical stability,multifunctional surface modification,high photobleaching resistance and excellent electronic properties,but the quantum yields(QY)of GQDs are very low which limits its development.Studies have shown that the atom doping is an effective method to adjust the inherent properties of carbon nanomaterials.Doping GQDs with nitrogen atoms could adjust the band gap of GQDs,electronic,chemical and optical properties,so it has the advantages of high quantum yield,good solubility,low toxicity and good biocompatibility.Therefore,N-GQDs have broad applications in biosensing,environmental monitoring and biological imaging.In this study,we use several different carbon and nitrogen sources as raw materials,and use a simple hydrothermal synthesis method to synthesize N-GQDs with different nitrogen contents.We finally selected a synthesis method with high fluorescence and quantum yields for subsequent research in this paper by calculating the QY and analyzing the fluorescence spectrum.And then three different fluorescence sensing systems were constructed to monitor the content of chromium ions(Cr⁶⁺),adenosine triphosphate(ATP)and thiourea(TU)in the actual samples.The main researches are as follows:1.Synthesis of N-GQDs by three different materials,and the Fourier infrared spectroscopy(FTIR),X-ray photoelectron spectroscopy(XPS),fluorescence spectroscopy(FL)and ultraviolet-visible absorption spectroscopy(UV-vis)were used to characterize and analyze the performance of N-GQDs.Finally,N-GQDs with better performance were selected for following research.2.N-GQDs were used to detect the content of Cr⁶⁺in water samples.During the experiment,citric acid was used as the carbon source and urea was used as the nitrogen source.N-GQDs were synthesized by a simple hydrothermal synthesis method.Then the fluorescence was quenched when Cr⁶⁺was added to the N-GQDs solution.Because of the absorption band of Cr⁶⁺completely covers the excitation band of N-GQDs,causing most of the excitation energy of N-GQDs to be absorbed by Cr⁶⁺,and N-GQDs only absorbed a small amount of excitation energy.Based on this principle,we use N-GQDs as a fluorescence sensor to sensitively detect Cr⁶⁺in water due to the internal filtering effect(IFE)that caused the fluorescence of N-GQDs to be quenched.The sensor has high selectivity and sensitivity to Cr⁶⁺during the experiment.The experimental results show that the linear range is 0.06-105?mol L^-1,and the detection limit is 36.7 nmol L^-1.Finally,the Cr⁶⁺in the water was detected by spiked,and the recovery was 100%-102%.3.Detection of ATP in human serum by N-GQDs combined with Fe³⁺as a fluorescence probe.Non-radiative electron transfer occurs between Fe³⁺and N-GQDs,which could the fluorescence of N-GQDs to be quenched.After adding ATP,the fluorescence of N-GQDs gradually restored.This is because ATP has a high affinity with Fe³⁺through the Fe-O-P bound,and Fe³⁺wound be detanched from the surface of N-GQDs,and the fluorescence wound be recovered.The fluorescence sensor constructed in this experiment has good selectivity,high sensitivity and wide linear range to ATP,so it can be applied to the detection of ATP in serum successfully.The linear range was 4-140?mol L^-1,and the detection limit was 0.4?mol L^-1.The recoveries were 98%-103%.4.Construction of"off-on"fluorescence sensing system based on N-GQDs to detect TU in fruit juice.The surface of N-GQDs contains a number of oxygen-containing groups such as hydroxyl groups and carboxyl groups,which could combine with mercury ions(Hg²⁺)causing non-radiative electron transfer,and the fluorescence of N-GQDs was quenched(off).The fluorescence of N-GQDs was immediately restored(on)when thiourea(TU)was present.This is because the TU surface has thiol functional groups which has a particularly strong binding force with Hg²⁺to form Hg²⁺-TU complexes,resulting the recovery of fluorescence of the N-GQDs.The linear range of the sensor was 0.5-14?mol L^-1,and the detection limit was41.7 nmol L^-1.Finally,the recovery of the actual sample was 99%-106%.So the sensing system cound be successfully applied to the detection of TU content in juice and water sample.