Preparation Of Graphene Quantum Dots And Detection Of Paraquat

The widespread use of paraquat pesticide has brought about the increase of crop production,and the environmental pollution caused by its residues,which is harmful to human health.Therefore,it is urgent to find a fast and convenient method to detect paraquat residues.Compared with the traditional detection method,fluorescence analysis method has the advantages of high sensitivity,wide linear range,simple instrument structure and low cost,which has developed rapidly into a popular detection and analysis method.Graphene quantum dots are nano materials that can emit light by themselves,with low toxicity and stable luminescence.Due to their unique functions and structures,graphene quantum dots have attracted the attention of scientists.At present,graphene quantum dots are used in the fields of biological imaging,medical diagnosis,analysis and detection.In this paper,pyrolysis and hydrothermal methods were firstly introduced to change the reaction temperature,reaction time and amount of carbon source through single-factor variable method to determine the optimal parameters of preparation conditions.The maximum emission wavelength of the quantum dots obtained by both methods were 470nm.The results showed that the fluorescence intensity of the quantum dots prepared by pyrolysis method was the highest at 180℃,45 min and0.5g citric acid,the fluorescence quantum yield was 10.53%,the particle size distribution was between 4-8nm,and the zeta potential was-29.1mv.The quantum dots prepared by hydrothermal method at 180℃,10h,1ml of 1mg·ml-1 graphene oxide have the highest fluorescence intensity,the fluorescence quantum yield is8.79%,the particle size distribution is between 5-13nm,and the zeta potential value is-17.1mv.Secondly,the single factor variable method was used to determine that the optimal conditions for the preparation of GQDs were 1mg?m L^-1 of graphene oxide and 1m L of ammonia at 180℃,5 h and 10 ml,under which the quantum yield of GQDs was 6.97%.The potential distribution on the surface of GQDs was changed by adding ammonia and amino groups,which improved the fluorescence properties of GQDs.The maximum emission wavelength of GQDs is about 520nm,the particle size is mainly distributed between 1-3nm,and the zeta potential is-35.2mv.Furthermore,the microwave-assisted method was used to determine the microwave oven power as high and the digestion time as 5min,which was the best preparation condition.The fluorescence yield was 8.87%and the maximum emission wavelength was 630nm.The synthesized quantum dot shows an absorption peak at280nm,corresponding to the n-π*electron transition of the carbon structure.The peak at 320nm corresponds to the n-π*transition of the conjugate olefin.Infrared spectrum of R-GQDs indicated the existence of-OH,C-C and C=C bonds,particle size was mainly distributed between 2-8nm,and Zeta potential value was-3mv.Finally,the detection of paraquat by quantum dots of different colors was studied,and the results showed that the emission spectra of graphene quantum dots with red emission spectral band could overlap the absorption spectra of paraquat,and the red quantum dots could achieve resonance transfer of fluorescence energy with paraquat,quenching the constant 9758L?mol^-1.It is much higher than the quenching constants of blue and green quantum dots.In order to explore the quenching effect of paraquat on quantum dots in different detection environments,the p H,temperature and ion concentration of the detection environment were respectively changed.It was found that when the ambient solution was acidic,the fluorescence intensity of paraquat solution with different concentration gradient was increased and the phenomenon of red shift occurred.Paraquat has the best quenching effect at normal temperature,and different ion concentration will affect paraquat quenching quantum dots.Therefore,paraquat was tested under the condition of neutral（p H=7）,temperature of 25℃and ion concentration of 0mol?L^-1.In order to evaluate the efficacy of R-GQDs as a fluorescent sensor for selective detection of paraquat,R-GQDs was used to detect dimethicone,cypermethrin,chlorpyrifos and glyphosate isopropylamine as a comparative experiment.The study found that,except paraquat,the uv-visible absorption spectra of the four pesticides did not overlap with the fluorescence emission spectra of the red quantum dots,and no obvious quenching was found in the solution with concentration gradient.The quenching constants were 446,710,535 and1007L?mol^-1,respectively,which were all smaller than the quenching constants of paraquat on R-GQDs.