Preparation Of N,S Co-doped Or Amphiphilic Graphene Quantum Dots And Their Application

As novel fluorescent carbon nanomaterials,graphene quantum dots（GQDs）usually possess small size（<less than 100 nm）and consists of single or few graphene layers.Similar with other graphene materials,GQDs exhibited high stability,large specific surface area andπ-πconjugated systems,as well as low bio-toxicity.Owing to the pronounced quantum confinement and edge effects,GQDs showed tunable photoluminescence properties and could be used in sensing,bioimaging and other fields.However,problems of traditional GQDs lie in low quantum yield,poor selectivity or active sites.Moreover,most GQDs only exhibit hydrophilic or lipophilic properties.Through the heteroatom doping or surface functional group modification,the physical and chemical properties of GQDs could be effectively adjusted.Additionally,the introduction of both hydrophilic and hydrophobic groups on the surface of GQDs endow GQDs with Amphiphilicamphiphilic properties.In this dissertation,N,S co-doped GQDs（GQDs）and Amphiphilic GQDs were easily produced through one-step solvothermal or hydrothermal methods and their applications in fluorescence sensing and active interfacial materials were studied.Detailed research was summarized as following.（1）One-step synthesis of N,S co-doped graphene quantum dots（N,S-GQDs）was proposed based on molecular fusion with 1,3,6-trinitropyrene,thiourea and DMF in a solvothermal process.The prepared N,S-GQDs exhibited excitation-independent photoluminescence behaviors,the maximum excitation wavelength was 375 nm and the maximum emission wavelength was 450 nm.N,S-GQDs showed blue fluorescence at 365 nm UV light and the absolute QY was 23.2%.As illustrated by transmission electron microscope（TEM）and atomic force microscopy（AFM）,the average diameter of N,S-GQDs was 2.0 nm and the thickness was between 0.8-0.9 nm,indicating single-layer graphene structure.Successful incorporation of N and S atoms into the carbon lattices of GQDs was proven by X-ray photoelectron spectroscopy（XPS）characterization.Fe³⁺,Cu²⁺and Ag⁺can significantly quenched the fluorescence of N,S-GQDs.To selectively detect only one ion species in a mixture of Fe³⁺,Cu²⁺and Ag⁺,the interaction between GQDs and the other two types of ions must be masked.The binding of N,S-GQDs with Fe³⁺or Ag⁺is suppressed in presence of ascorbic acid（AA）and fluorescence quenching only due to the interaction between N,S-GQDs and Cu²⁺will be observed.When cysteine（Cys）is added,the quenching of fluorescence is caused by Fe³⁺only.Lastly,the interaction of N,S-GQDs with Cu²⁺can be inhibited in order to isolate the effects of Fe³⁺and Ag⁺by（ethylenedinitrilo）tetraacetic acid disodium salt（EDTA）which is a chelator of Cu²⁺.N,S-GQDs are able to differentially detect Fe³⁺,Cu²⁺and Ag⁺ions in a mixture.The linear ranges of detection of Fe³⁺,Cu²⁺and Ag⁺were 0.01-25.0μM,0.4-180.0μM and 0.1-140.0μM,respectively.The detection limits of Fe³⁺,Cu²⁺and Ag⁺were 8 nM,250 nM and 50 nM,respectively.Moreover,detection of Fe³⁺,Cu²⁺and Ag⁺in complex biological（serum）and environmental samples（river water）is demonstrated with the same results as standard detection methods（ICP-MS and FAAS）.In addiyion,the fluorescence of N,S-GQDs quenched by Cu²⁺can be recovered by the addition of Cys.Thus,a fluorescence detection method（turn-off-on fluorescence detection mode）of Cys was established.The detection linear range of Cys was 0.5-140.0μM and the detection limit was 0.2μM.This method can be used for human urine Cys detection.（2）Amphiphilic GQDs was proposed by a facile one-pot hydrothermal process and applied as active interface materials.1,3,6-trinitropyrene,was chosen as the carbon source,NaOH was added to introduce hydrophilic groups and lauric acid was added to introduce hydrophobic groups.The prepared amphiphilic GQDs exhibited excitation-independent photoluminescence behaviors,the maximum excitation wavelength was 485 nm and the maximum emission wavelength was 520 nm.Amphipathic GQDs showed green fluorescence at 365 nm UV light and the absolute QY was 20.1%.As illustrated by TEM and AFM,the average diameter of amphipathic GQDs was 2.7 nm and the thickness was between 0.9-1.0 nm.The hydroxyl and C12chains onto GQDs was demonstrated by XPS and Fourier transform infrared spectroscopy（FTIR）.The amiphiphilic GQDs exhibited as interesting active-interfacial materials.A stable Pickering emulsion can be formed in two-phase systems containing water and immiscible chloroform solvent.The formed Pickering emulsions are water-in-oil reverse-phase emulsions in chloroform（oil）layer.As characterized by polarized light microscopy,all droplets in the Pickering emulsion are regular microspheres with similar size of about 35μm.Bright-field and confocal fluorescence images of water-in-chloroform Pickering emulsion stabilized by amphiphilic GQDs.Amphiphilic GQDs are all distributed on the two-phase interface beteween organic solvent and water.Owning to interesting surface properties,the amiphiphilic GQDs were able to act as active-interfacial materials in many fields as nanoparticle surfactant including performing Pickering interfacial polymerization,reducing high-speed liquid splashing on the superhydrophobic surface,dispersing insoluble carbon nanomaterials and form ultra-light aerogel.In addition,using amphiphilic GQDs as topography inducer,gold nanoparticle clusters with petal structure were prepared.（3）Using the prepared amphiphilic GQDs as raw materials,sulfonated GQDs（GQDs-SO₃H）were prepared by sulfonation reaction.GQDs-SO₃H remained amphiphilic characteristics and exhibited remarkable catalytic efficiency in interfacial catalysis transesterification of soybean oil for producing biodiesel.As illustrated by TEM and AFM,the morphology of GQDs-SO₃H had not been damaged after sulfonation.The average particle size of GQDs-SO₃H was 2.6 nm,the thickness was between 0.9-1.0 nm.The sulfonic acid groups onto GQDs was demonstrated by XPS and FTIR.The acid density of GQDs-SO₃H was 6.1 mmol H⁺g^-1 obtained by titration.GQDs-SO₃H were further used as interfacial catalyst for transesterification of soybean oil with methanol to prepare biodiesel.The best catalytic efficiency was achieved when the catalyst dosage was 3.0%,the reaction temperature was 120℃,the reaction time was 5 h and the ratio of methanol to oil was 6:1.Under these conditions,the conversion rate of biodiesel reached 85.3%.The conversion rate reached 85.3%.Interfacial catalysis of transesterification of soybean oil with amphiphilic GQDs-SO₃H was investigated.Results indicate that the emulsification effect exist in the period of the reaction,that will increase two-phase contact and improve mass transfer.The catalytic efficiency of GQDs-SO₃H was higher than that of commercially available Nafion@NR50 and Amberly15 solid acidsMoreove,it is 1.85times of non-amphiphilicsulfonated GQDs,indicating the effect ofemulsification caused by amphiphilicGQDs-SO₃H.Moreover,the catalytic performance of GQDs-SO₃H is close to that of concentrated sulphuric acid under the same reaction condition.GQDs-SO₃H can be recycled and reused.