Synthesis And Fluorescence Detection Performance Of Element-doped Graphene Quantum Dots

Graphene quantum dots(GQDs)have become one of the research hotspots in various fields such as physics,chemistry,materials science and medicine because of their special fluorescence properties.GQDs exhibit good solvent solubility due to a mass number of functional groups on their edges,which provide a good sensing performance in biology,chemistry,medicine.At present,most of the detection mechanisms of GQDs as fluorescent probes are ambiguous in the field of fluorescence detection,and the quantum yield(QY)of GQDs used in the detection field is generally low.This paper carries out the following research works:(1)First,the effects of the doping elements of N and S on the chemical structure of GQDs and optical properties were investigated.We used citric acid(CA)as the carbon source,urea(Ur)and ethylenediamine(EDA)as the nitrogen source,sodium sulfide(Na2S)as the sulfur source,and thiourea(Us)as the sulfur-nitrogen co-doping material.One-step hydrothermal method was performed to prepare GQDs with different doped elements.All synthesized GQDs exhibit blue fluorescence,and the doping of elements S and N significantly enhances the QY of GQDs,where QY is: undoped GQDs(14%),Sdoped GQDs(28%),sulfur Nitrogen-doped GQDs(32%),N-doped GQDs,78%)with urea as nitrogen source(N-GQDs),and N-doped GQDs(93%)with EDA as N-nitrogen source.We investigated the effects of S and N doping on the fluorescence detection performance of dopamine(DA)and further studied the fluorescence detection mechanism.It is found that the nitrogen-doped GQDs prepared by urea as a nitrogen source have the best DA detection performance.The the nitrogen-doped GQD probes have high sensitivity and selectivity for the detection of DA,showing a bi-linear relationship,and linear range is as wide as 10-7000 nM.The DA limit of detection(LOD)was 3.3 nM in a 0.1 M Phosphate buffer saline(PBS)solution.Meanwhile,the nitrogen-doped GQDs has good selectivity for the detection of DA.When detecting DA in human serum,the recovery is 98.4-105.8% with relative standard deviation(RSD)less than 3%.(2)For the S-and N-doped GQDs system,firstly,we investigated the effect of elemental doping on the QY of GQDs.It is believed that the structure of pyridinic nitrogen and graphene nitrogen have a significant effect on the QY.Secondly,we studied the fluorescence detection mechanism of DA in detail.Results show that the detection mechanism of DA is derived from the pyridine N structure on the surface of nitrogendoped GQDs.More importantly,the catechol group in the DA molecule can form a specific chemical bond with N-GQDs to form a ground state non-fluorescent.(3)We also explored the influence of the doping element of B on the chemical structure of GQDs(B-GQDs)and optical properties.The study used CA as the carbon source and sodium tetraphenylboron sodium(NaTPB)as the boron source to synthesize B-GQDs by hydrothermal method.The optimal preparation conditions were as follows: molar ratio of CA and NaTPB was 14: 1,hydrothermal reaction was carried out at 180 °C for 8 hours.The highest QY of B-GQDs was 41%.Compared with undoped GQD,BGQDs have more alkali resistance,temperature resistance,light resistance and storage stability.The fluorescence detection performances of B-GQDs to catechol(CC)and glutathione(GSH)were studied.B-GQDs have higher sensitivity,good linearity and better selectivity in detecting CC and GSH.The linear range of CC detected in 0.1 M PBS was 1-50 nM,the LOD was 0.25 nM.The linear range of sensing GSH was 2-100 nM,and the LOD was 0.5 nM.In actual samples,the recovery of B-GQDs to CC in river water was 103.3%-106.0%(RSD <5.3%).When B-GQDs were used to detect GSH in human serum,the recovery was 99.5%-105.0%(RSD<4.7%).(4)The fluorescence detection mechanism of CC and GSH were disscussed.Boric acid groups and CCs on the surface of B-GQDs are chemically bonded to form BGQDs@CC complex,which achieves the purpose of fluorescence quenching to detect CC;In addition,GSH and CC can form a stable compound GSH@CC,which promotes the desorption of CC molecules from the B-GQDs@CC complex,thus realize the reversible fluorescence detection of GSH.