| Graphene has the advantages of good electrical conductivity,large specific surface area,and strong catalytic ability,these properties make graphene an excellent electrochemical sensing material.Currently,graphene oxide and reduced graphene oxide have been widely used in electrochemical sensing applications.As the research of preparating methods of graphene progresses,it has been found that ultrasonic exfoliation and electrochemical exfoliation in liquid phase exhibit many superiorities,such as simple operation,high exfoliating efficiency,cost-effective,economy and environmental protection,strong controllability,et al.However,the electrochemical sensing applications of sonicating/electrochemical exfoliated graphene are still in active exploration.Besides,how the structure and morphology of graphene prepared by these two methods affect their electrochemical sensing ability is still lack of discussion.Therefore,the aim of this thesis is to study the preparation of graphene exfoliated by sonication and electrochemistry,and methods and approaches of tuning the structure and morphology of graphene.On these basis,we focus on the studies on the electrochemical properties of graphene with different structures and morphologies,stating of the relationship between them,as well the mechanism of the significantly improved electrochemical sensing ativity,finally constructing highly sensitive electrochemical sensing platforms for practical sample analyses.The main works of this thesis are as the following three parts.(1)Tuning the structure,morphology,and electrochemical behavior of sonicating exfoliated graphene nanosheets(GS)by centrifugal speed-based grading.GS was prepared by sonicating exfoliation,using graphite powder as raw material,N-methyl-2-pyrrolidone(NMP)as the solvent,sodium citrate as the assistant reagent,and a series of GS were prepared under different centrifugal speed.Results of particle size analysis,scanning electron microscopy,transmission electron microscopy and Raman spectroscopy indicated that the centrifugal speed has a significant effect on the size,thickness,and defect content of graphene.The electrochemical behavior of a probe of potassium ferricyanidem showed that the active area and interfacial electron transfer ability of GS prepared under different centrifugal speed showed obvious differences,and GS prepared under 2000 rpm has a bigger active electrode area,with a faster electron transfer rate.Further studies found that GS prepared under different centrifugal speed showed various signal amplification effects for bisphenol A,sunset yellow,tartrazine,and carbendazim,with GS prepared under 2000 rpm exbihiting the best effect.These results indicated that the structure and morphology of GS can be easily tuned by changing the centrifugal speed,thereby realizing the regulation of its electrochemical properties.Based on the signal amplification effect of sonicating exfoliated GS tuned by centrifugal speed,electrochemical detecting platforms with high sensitivities towards bisphenol A,sunset yellow and tartrazine,and carbendazim were respectivily developed,with LODs of 31 nM,1.4 nM,12 nM,and 0.78 nM,respectively.This method was used for the analyses for shopping receipts,drinks,and ground water/soil samples,with results well consistent with high-performance liquid chromatography.(2)Study on the structure and electrochemical properties of sonicating exfoliated graphene by sodium salt.Sodium citrate is a widely-used assistant reagent to improve the ultrasonic exfoliating efficiency of graphene in NMP.In order to screen out more assistant reagents with better performance,to further improve the electrochemical perfermance of exfoliated graphene,study on different sodium salts-assisted sonicating exfoliation of graphene was carried out.GS was prepared by sonicating exfoliation in NMP for 2 h,with sodium citrate,sodium phosphate,and sodium pyrophosphate as the assistant reagent separately,graphite powder as raw material.It was found that these sodium salts can not only efficiently improve the graphene yield in various degrees,but also reduce the number of graphene layers,with defect content increased.Compared with that of sodium citrate,the yield of GS exfoliated with sodium pyrophosphate is 30% higher,almost five times of that in pure solvent.Cyclic voltammetric behaviors and studies on a rotating disk electrode of aprobe potassium ferricyanide showed that GS prepared with the assistance of sodium pyrophosphate has a bigger efficient active area and a faster electron-transfer rate.Besides,the response behaviors of biomolecules(dopamine,uric acid,xanthine,and hypoxanthine),fungicide(malachite green),phenolic pollutants(4-chlorophenol and 4-nitrophenol),and azo colorants(ponceau 4R and rhodamine B)were studied on the surface of GS prepared with different sodium salts,results showed that the electrochemical signal enhancement effect of GS prepared with sodium pyrophosphate is significantly higher than that of sodium citrate.Thus it can be deduced that the structure,morphology,defect level,and electrochemical performance of GS can be tuned by the assistance of sodium citrate,and the performance of sodium pyrophosphate is obviously better than sodium citrate which is currently reported as the best.Based on GS prepared with sodium pyrophosphate,a series of electrochemical sensing platforms with high sensitivities towards a wide range of targets were successfully constructed.(3)Study on the structure and morphology,and electrochemical sensing properties of electrochemically exfoliated graphene tuned by pH control.Using a graphite foil working electrode,a saturated calomel reference electrode,and a platinum foil counter electrode,utilizing the intercalation of sulfate ion,electrochemical exfoliated graphene(EEG)was prepared under a constant potential of 5 V in a mixed electrolyte of sulfuric acid/ammonium sulfate/ammonia.The influence of the pH value of electrolyte on the exfoliating efficiency,morphology,oxygen content and defect level of graphene were studied.It was found that as the pH value increases from 1 to 7,the exfoliation of graphene slows down,and graphene has a more uniform morphology and fewer layers,with oxygen content and defect level both decreased.On that basis,the electrochemical behaviors of three biological molecules(dopamine,xanthine and hypoxanthine)on different EEGs prepared with different pH were studied.It was found that the electrochemical signal enhancement performances of EEGs towards them are closely related to the pH value.At last,an electrochemical sensing system for simultaneous detection of dopamine,xanthine and hypoxanthine was constructed based on EEG exfoliated at a pH of 5 as the sensitive material,with detection limits of 1.2 nM,7.5 nM,and 18 nM,respectively.This newly developed method was employed for the analysis of human urine samples,with spiked recoveries between 96.5%-104.7%. |
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