Study On The Determination Of Imidacloprid Pesticides In Fruits And Vegetables By Ratio-based Voltammetry

Imidacloprid[IMI,1-((6-Chloro-3-pyridiny)methyl)-N-nitro-imidazolidinimine],as one of the neonicotinoid pesticides,is generally used in crops,fruits and vegetables to control piercing-sucking mouthpart pests.By competitively combining it with the acetylcholine receptor,IMI can interfere with stimuli transmission in the central nervous system of pests.Even though IMI has certain advantages,such as a considerable insecticide spectrum,relatively low toxicity and water stability,it still creates hazards to human beings and the biosphere.Some countries have banned the use of IMI,but the problems caused by IMI residues are still emerging.Therefore,there is an urgent need for a feasible and reliable IMI detection method.This study presents an electrochemical detection platform to combine supramolecular recognition materials and molecularly imprinted materials,and applies the ratio strategy to the electrochemical detection system to achieve specific recognition and sensitive detection of IMI.The main research contents and results are as follows:(1)This work introduces a?-cyclodextrin/multi-walled carbon nanotube(?-CD/MWCNT)modified microelectrode array(MEA)for rapid determination of imidacloprid in vegetables.The MEA,fabricated on a silicon wafer,contains 20parallel-connected working electrodes,a counter electrode and a reference electrode.MWCNT is drop-casted onto the working electrode area,and?-CD is decorated onto MWCNT layer with electropolymerization.Electrochemical behaviors of the as-fabricated sensor are investigated with cyclic voltammetry(CV),electrochemical impedance spectroscopy(EIS)and differential pulse voltammetry(DPV).Obtained results validate that the proposed sensor has a large electroactive area,great electrical conductivity and high sensitivity.During preparation and application of the sensor,0.5mg m L^-1 MWCNT suspension and phosphate buffer solution(0.1 mol L^-1,p H 7)are found to be optimized experimental conditions.Under optimized conditions,a wide linear range as 5 to 100?mol L^-1is obtained for target imidacloprid.and the limit of detection(LOD,S/N=3)is well defined as 0.629?mol L^-1.The sensor is used for sensing imidacloprid in cabbage,cucumber and tomato.The results from this method are in accordance with high performance liquid chromatography-mass spectrometry(HPLC-MS)data.(2)We introduce the ratiometric IMI analysis on a flexibly-fabricated vibratory electrode module.In this system,?-cyclodextrin is chosen as a supramolecular recognizer to capture IMI,and Prussian blue(PB)acts as an internal reference.The reference current(I_PB)presents remarkable stability with different IMI concentrations,while shows an identical fluctuant tendency to IMI current(I_IMI)against interferences.The ratiometric readout(I_IMI/I_PB)is proven to be a more reliable indicator to quantize IMI.The as-fabricated sensor integrates a tri-electrode system and a vibration motor to improve IMI adsorption.The flexible module is prepared following well-proven industrial techniques of flexible print circuit.Fabrications and modifications of the sensor are morphologically examined.Synergistic characteristics in the ratiometric analysis are validated and optimized with electrochemical methods.Under optimum conditions,vibration-enhanced determinations are applied to laboratory and natural samples.Detection limit and linear range are well found as 6.1×10^-8 mol L^-1 and2×10^-7-1.2×10^-4 mol L^-1,respectively.The applicability for the natural samples is further validated with recovery tests and certified HPLC.Given this,this method can be treated as a promising tool for IMI determination in broad sources.(3)This work presents a signal on-off ratiometric electrochemical sensor coupled with a molecular imprinted polymer(MIP)for imidacloprid(IMI)determination.The ratiometric strategy corrects the detection results by setting 6-(Ferrocenyl)hexanethiol(FcHT)as an internal reference.The MIP membrane,as a molecular recognition receptor,has a three-dimensional structure and is complementary in the shape and chemical functionality to the template.The combination of a ratiometric strategy and MIP improves the sensitivity and selectivity of the sensors.The experimental parameters,such as monomer/template ratio,electropolymerization cycle and adsorption time,are optimized to improve the IMI sensing performance of the as-fabricated electrode.Under optimal electrochemical conditions,the proposed sensor is used to detect IMI in laboratory and real samples.In a range as wide as 5×10^-7-1×10^-4mol L^-1,a linear relationship is well established,and the value of the correlation coefficient is 0.9984.The limit of detection(LOD,S/N=3)is well defined as 4.7×10^-8mol L^-1.The recovery rates range from 97.4%to 103.5%while testing real samples.The fabricated electrode shows superior anti-interference ability for IMI determination in the presence of analogs at a higher level.The electrochemical sensor with high selectivity and sensitivity prepared in this research has realized the sensitive and stable detection of IMI in fruits and vegetables,and provided theoretical and experimental basis for the development of rapid detection equipment for pesticide residues in fruits and vegetables.