The Regulation Of Carbon Quantum Dot Phase Transfer And Microemulsion Phase Behacviour By Carbon Dioxide

Stimulus-responsive system refers to the system sensitive to external stimuli,and the structure of some components in the system will change upon receiving stimulus?such as pH,light,temperature,magnetism or gas?,resulting in a great change in the relevant properties of the system.Due to the advantages of nanoparticles and their response to the environment,stimulus-responsive nano-system has been a hot research topic.Due to the low-cost,biocompatibility,photostability and facile functionalization,carbon quantum dots?CDs?have emerged as a new class of carbon-based nanomaterials and showed potential applications in sensors,catalysis,bio-imaging and so on.Microemulsion is a highly dispersed nano-system with small particle size and large specific surface area.The special microenvironment of microemulsion has been widely used in chemical reactions and preparation of functional materials.However,in practical applications,we often need to use different hydrophilic/hydrophobic CDs to meet the needs of practical work.At the same time,in the process of utilization,we hope that the stability of microemulsion is better.Subsequently,we hope that they can be easily demulsified,which is more conducive to the separation and purification of the product and the recycling of microemulsion components.In view of this point,functionalized CDs with adjustable hydrophilicity/hydrophobicity and ionic liquid microemulsions with adjustable phase behavior were designed and developed in this paper.In addition,the regulation of CO₂ on dispersion and phase behavior of these systems was studied,which provided scientific basis for their application in chemical sensing and catalytic reactions.The main research contents are as follows:?1?The polyethyleneimine?PEI?was grafted on the surface of the original CDs by a condensation reaction to produce different molecular weight polyethyleneimine?PEI?functionalized CDs?PEI-CDs?.By alternately bubbling and removing CO₂,the reversible dispersion and precipitation of PEI-CDs in dimethylsulfoxide-water mixture were achieved,which was accompanied by the reversible enhancement and attenuation of fluorescence intensity.¹³C NMR spectra showed that the possible mechanism of this regulation involves the reaction of amino groups on PEI-CDs surface and CO₂,which produced the polyamino bicarbonate,which had a poor solubility in the dimethylsulfoxide-water mixture and precipitated from the solution,thus reducing the concentration of PEI-CDs in the mixture,so the fluorescence intensity of the solution decreased.When CO₂ was removed,the precipitate was dispersed again and the fluorescence intensity of the solution returned to its original value.A new method for the quantitative determination of CO₂ and SO₂ acid gases in water and air was established based on the novel CO₂-switchable property of PEI-CDs.?2?According to the relationship between structure and performance,a novel class of amine-functionalized CDs has been designed and developed.These CD can be well dispersed in organic solvents such as n-octanol,benzene,toluene,dichloromethane,and trichloromethane at atmospheric pressure.They can spontaneously transfer from the organic phase to the aqueous phase after CO₂ was bubbled in;After removing CO₂,they can be returned from the aqueous phase to the organic phase,and so that CO₂ can be used to modulate the reversible phase transfer of the functionalized CDs from the organic phase to the aqueous phase.The mechanism of the CO₂-switchable reversible phase transfer was investigated by the measurements of interfacial tension,water contact angle and ¹³C NMR spectroscopy.The results showed that the hydrophilic ammonium salt was produced by acid-base reaction of CO₂ with amines on the surface of CDs,and the CDs were changed from hydrophobic to hydrophilic,so they transferred from organic phase to aqueous phase.When CO₂ was removed,these functionalized CDs were in turn changed from hydrophilic to hydrophobic,thus transferring from the aqueous phase to the organic phase.Based on the unique phase-transfer behavior of the functionalized CDs,a highly efficient Knoevenagel reaction,product separation and catalyst recycling were achieved where the CDs were used as catalysts.?3?Based on the tunableness of the ionic liquids structure,a novel class of CO₂-responsive microemulsions comprising ILs,n-pentanol and water was developed,where the ILs with N-alkyl-N,N-dimethylethylamine cation[C_nDMEA]⁺?n=8,10,12,14,16?and azole-based anions,including imidazolium?[Im]^-?,trizolium?[Triz]^-?,and pyrazolium?[Pyr]^-?,were employed as the surfactants in the formulation of our microemulsion systems.The microstructure of these microemulsion systems and the phase separation of the microemulsion systems regulated by CO₂ were studied by phase diagrams,electrical conductivity,dynamic light scattering,cryogenic freeze transmission electron microscopy and optical microscopy:these microemulsion systems underwent a reversible switching from an apparent monophase to complete phase separation by alternatively bubbling and removal of CO₂ at atmospheric pressure.The mechanism of CO₂-driven reversible phase separation of microemulsion was studied by ¹³C NMR spectroscopy.The results show that the reversible phase separation of microemulsion mainly involves the reversible formation of hydrophilic bicarbonate and carbamate by the reaction of ionic liquid anion and CO₂,which increases the ionic strength in the mixture?and vice versa?,and the salting-out effect causes the phase separation of microemulsion.On the basis of CO₂-responsive phase separation,microemulsions have been used as microreactors for knoevenagel reaction,and homogeneous reaction of reactants,heterogeneous separation of products and recycling of microemulsions were achieved.?4?A class of CO₂-responsive microemulsions comprising ILs,ethyl acetate and water has been developed,where the ILs with N-alkyl-N,N-dimethylethylamine cation[C_nDMEA]⁺?n=12,14,16?and[Im]^-anions,were employed as the surfactants in the formulation of our microemulsion systems.The microstructure of these microemulsion systems was studied by phase diagram,electrical conductivity,dynamic light scattering,cryogenic transmission electron microscopy and optical microscopy.The results show that the system can be reversibly transformed from microemulsion to emulsion by alternately bubbling or removing CO₂ into the system.¹³C NMR spectra provided that the mechanism of microemulsion-to-emulsion transition may be due to the reversible reaction of ionic liquid anion and CO₂,which produced hydrophilic bicarbonate,the emulsifying ability of bicarbonate was poor,so the microemulsion system was destroyed and the system was emulsified.After CO₂ removal,the ionic liquid returns to its original state and the emulsion becomes microemulsion again.The ionic liquid microemulsion system with controllable structure is a bifunctional emulsion system with potential application value.