Study On Fate Of Graphene Quantum Dots With Different Surface Functional Groups In Water

With the increase yield of carbon-based nanomaterials and the widening application range,carbon-based nanomaterials will inevitably enter the aquantic environment and become potential pollutants,threatening the water supply ecosystem.However,the research of thefate of carbon-based nanomaterials in water is very limited.In order to further understand the fate of carbon-based nanomaterials in water,in this study we investigate the stability of graphene quantum dots with three different functional groups in water solution And we also study theinterficail mechanism between quantum dots and natural organic matter.In this experiment,transmission electron microscopy,dynamic light scattering,infrared spectroscopy and XPA diffraction were used to characterize the nanomaterials’ size,hydrodynamic radius,surface functional groups and structures of three commercial graphenes.It is found that oxygen-containing functional groups exist on the surface of three kinds of graphene quantum dots.The contents of oxygen-containing functional groups are following the order of carboxylated graphene quantum dots,aminated graphene quantum dots,hydroxylated graphene quantum dots.In this study,we also explored the dynamic light scattering and Zeta potential to investigate the influence of pH,natural organic matter and other environmental factors on the stability of graphene quantum dots in water.The results show that with the decrease of p H,the particle size of graphene quantum dots first decreases rapidly and then stabilizes,and the absolute value of the surface Zeta potential increases.The presence of natural organic matter can enhance the stability of graphene quantum dots in water.In this experiment,the time-resolved dynamic light scattering method was used to track the agglomeration process of graphene quantum dots,and also the effects of Ca²⁺,Mg²⁺and Na⁺on the agglomeration of graphene quantum dots were investigated.Studies have shown that there are two stags in the condensation process of graphene quantum dots in water:reaction limitation and diffusion limitation.The critical agglomeration concentrations of Ca²⁺,Mg²⁺and Na⁺of carboxylated graphene quantum dots were 1.81 m M,82 m M and 2819 mM,respectively.The critical agglomeration concentrations of hydroxylated graphene quantum dots Ca²⁺,Mg²⁺and Na⁺were 1.7mM,75 mM and 2819 m M,respectively.The critical agglutination concentrations of the graphene quantum dots Ca²⁺,Mg²⁺and Na⁺were 1.8 m M,80.2 m M,and 2912 mM,respectively.The condensation of graphene quantum dots in Mg²⁺and Na+basically accords with the Schultze-Hadi rule,but the aggregation in Ca²⁺does not comply with the Schultze-Hadi rule,presumably due to the aggregation of Ca²⁺in the graphene quantum dots.In addition to compressing the double layer,it also serves as a bridge.The stability of three graphene quantum dots in water is carboxylated graphene quantum dots,aminated graphene quantum dots,hydroxylated graphene quantum dots in order.The difference between the three is presumed to be caused by different contents of oxygen-containing functional groups on the surface..Both graphene quantum dots and humic acids are fluorescent.The fluorescence peak positions of the three graphene quantum dots and humic acid interacted blue shifted.Three fluorescent components were analyzed by parallel factor method（PARAFAC）.The excitation/emission wavelengths were 265（360）/480 nm and310/425 nm,and 290（more than 350）/540 nm.The characteristic peaks of the three graphene quantum dots were not isolated in the three-dimensional fluorescence spectra of the graphene quantum dots interacting with humic acid,demonstrating that the fluorescence of the former is quenched after the reaction of the graphene quantum dots with humic acid.The overall fluorescence intensity decreased most due to the interaction of carboxylated graphene quantum dots with humic acid,.Based on the infrared spectrum analysis of humic acid and graphene quantum dots before and after reaction,the carboxylic acid group in humic acid may be the key to the interaction between graphene quantum dots and humic acid.