The Preparation And Application Of Manganese(Ⅱ) Enhanced Fluorescent Graphene Quantum Dots Containing Nitrogen

Graphene quantum dots（GQDs）have a wide application in biological imaging and chemical biology analysis,due to their excellent water solubility,low toxicity,high stability and unique optical properties.At present,amount of stable grapheme oxide quantum dots prepared by directly carbonization of citric acid,whose complicated preparation process and tedious postprocessing can lead to a limited application in biology.Thus,the facile and efficient synthesis for stable GQDs has become a hot research topic.In this dissertation,stable water-soluble Mn²⁺enhanced nitrogen-doped graphene quantum dots(Mn²⁺-NGQDs)and nitrogen-doped graphene（NG）were synthesized by hydrothermal method using manganese chloride as a dopant in aqueous solution,as well as glycine and sodium citrate as reaction precursors.The products of Mn²⁺-NGQDs were characterized by transmission electron microscopy（TEM）,high-resolution transmission electron microscopy（HRTEM）,X-ray photoelectron spectroscopy（XPS）,X-ray diffraction（XRD）,Raman（RAMAN）and Fourier Transform Infrared Spectroscopy（FT-IR）,respectively.The influences of different conditions including the molar ratio of dopants,the type of dopants,reaction time and reaction temperature to the fluorescence properties of Mn²⁺-NGQDs were systematically studied.The optimized synthesis conditions and the mechanism of the fluorescence enhancement of Mn²⁺-NGQDs were explored.The Mn²⁺-NGQDs prepared under optimized conditions were used as fluorescent probes,and their application in chemical analysis and cell imaging were also investigated.The main resultsare as follows:（1）Under hydrothermal conditions,the maximum fluorescence intensity is obtained when the dopant is manganese chloride,the molar ratios of glycine/sodium citrate are 1:2,the reaction temperature is 180℃,the reaction time is 10 h,the concentration of Mn²⁺is 2×10^-4mol/L.The Mn²⁺-NGQDs prepared under optimized conditions display a high quantum yield of 42.16%,increased by 15.1%compared with that of undoped NG.The results show that the doping of Mn²⁺can effectively change the morphology and enhance the fluorescence of NG.（2）As fluorescent probes,a method for detecting Hg²⁺quantitatively with Mn²⁺-NGQDs is established based on fluorescence quenching mechanism.By detecting other ions,it is proved that the prepared Mn²⁺-NGQDs show good sensitivity and selectivity for Hg²⁺detection.The best detection system is obtained in different buffer solution with a pH value of 7.38.In PBS buffer solution and a range of Hg²⁺concentration of 0.2×10^-62.0×10^-6mol/L,the linear relationship of fluorescence quenching（△F’）versus Hg²⁺concentration is very good,with a linear equation of△F’=F/F₀=0.8774-0.2637C,and a linear correlation coefficient R²=0.996.The detection limit for Hg²⁺is 4.2×10^-7mol/L,which is very similar to that in the HMTA buffer solution and ultrapure water.The fluorenscence quenching mechanism of Mn²⁺-NGQDs for Hg²⁺has been discussed and explained by static quenching and non-radiative energy transfer theory.（3）The biological evaluation of Mn²⁺-NGQDs as fluorescent probes were studied.The MTT test results of HepG2 cells prove that the Mn²⁺-NGQDs have good biocompatibility.The fluorescence imaging of different concentrations of quantum dots for HepG2 and RAW264.7 cells was performed.The results show that the Mn²⁺-NGQDs can rapidly enter the cell with lower concentrations（40μg/mL）.