Construction Of Rapid Pesticide Residue Detection Method Based On Carbon Quantum Dots And Its Environmental Toxicity

As a new type of fluorescent nanomaterial,carbon quantum dots（CQDs）could be prepared by a wide range of raw materials with various preparation methods.CQDs have unique optical properties and environmental friendliness,and could be widely used in broad aspects:such as in chemical analysis,biological imaging,photocatalysis and other fields.Fluorescence method is an important method for the detection of organophosphorus pesticides.The study of CQDs mainly concentrated on fluorescent probes for the detection of organophosphorus pesticides.The fluorescent probes have the following advantages:good biological compatibility,fast speed,and simple operation without using expensive apparatus.Choline bitartrate and choline dihydrogen citrate,two choline ionic liquids,were selected as the raw material to synthesize CQDs by high temperature pyrolysis,and the preparation conditions were optimized under different pyrolysis time and temperature.The prepared CQDs were charaterized with infrared spectrometer,ultraviolet spectrometer,fluorescence spectrophotometer and transmission electron microscopy and other instruments.The fluorescence quantum yield of the synthesized carbon quantum dots was calculated with quinine sulfate as the reference,and its stability and fluorescence lifetime were determined.The results are as follows:The optimal preparation conditions was determined:choline bitatrate as raw material and pyrolyzed at 300℃for 2h.The average particle size of the carbon quantum dots was 40 nm,with good dispersion,including C,H,O and N elements,and carboxyl and amino groups on the surface.The fluorescence quantum yield was 8.64%,showing good stability,bleaching resistance and fluorescence life.The rapid detection of organophosphorus pesticides based on fluorescence quenching mechanism of CQDs was developed.Firstly,the influence of copper ion on fluorescence quenching of carbon quantum dots was studied,and the detection conditions were optimized.The linear relationship of fluorescence quenching rate of CQDs and the concentrations of organophosphorus pesticides was determined,and the stability,reproducibility and the selectivity of the method was evaluated with chlorpyrifos,trichlorfon and dufulin as the target compounds.The developed method was applied to the real sample analysis.The fluorescence intensity of carbon quantum dots decreased with the increase of copper ion concentration,and the fluorescence quenching rate of copper ion on carbon quantum dots was also affected by p H value of the solutions.When the p H of the solutions ranged from 6.0to 9.0,copper ion had the best effect on the fluorescence quenching of carbon quantum dots,and salt concentration had no obvious effect on the reaction.The optimal detection conditions for chlorpyrifos:p H 8.0,temperature 35℃,concentration of thioacetylcholine iodide 100μmol/L,reaction for 9min.The linear ranges of chlorpyrifos,trichlorfon and dufulin with the developed method were 3.5×10^-3-5×10^-2mg/kg、1.2×10^-3-4×10^-2mg/kg、2×10^-3-6.4×10^-2mg/kg,respectively,with the limits of detection 3.5×10^-3mg/kg、1.2×10^-3mg/kg、2×10^-3mg/kg,respectively.The method had good selectivity,stability and reproducibility.The average recoveries of chlorpyrifos were in the range of 94.5%～110%at the spiked concentration of 0.03 mg/kg,0.08 mg/kg and 0.2 mg/kg,and the relative standard deviations were in the range of 1.6%～6.6%,which could be used for the determination of real samples.In order to study the toxicity of carbon quantum dots to aquatic organisms,Chlorella vulgaris and Daphnia magna were used as the target organisms.The effects of carbon quantum dots on the growth rate,chlorophyll content,respiration rate,soluble protein content,SOD activity,CAT activity and MDA content of Chlorella vulgaris was determined.Then the acute and chronic toxicity of carbon quantum dots to Daphnia magna were measured,and the LC₅₀of carbon quantum dots to Daphnia magna was determined.The results were as follows:The growth inhibition rate of Chlorella vulgaris increased with the higher concentrations of carbon quantum dots and the longer exposure time.The EC₅₀ of 96h was 271.021 mg/L,indicating the low toxicity of the prepared CQDs.The chlorophyll content and respiration rate of Chlorella vulgaris decreased with the increasing concentrations of CQDs and the longer exposure time,while SOD activity,CAT activity and MDA content increased after the exposure to CQDs solution,suggesting that the CQDs might cause oxidative damage of Chlorella vulgaris.In the test of acute toxicity and chronic toxicity of Daphnia magna,its mortality rate increased with the higher concentrations of CQDs and exposure time,and the LC₅₀ value of48 h was 179.622 mg/L,indicating its low toxicity of the prepared CQDs.In chronic toxicity test,the CQDs had significant effects on the production and molting of Daphnia magna at the concentration of 18mg/L,which indicated that the CQDs not only inhibited the growth and development of Daphnia magna,but also threatened it at the population level.In the study,CQDs were prepared with choline ionic liquids,and which was used to develop the rapid detection method of organophosphorus pesticide residues.The developed method has the advantages of simple operation,fast speed and environmental friendliness without using expensive apparatus,which could be used for rapid on-site screening of organophosphorus pesticide residues.