Construction And Application Of Common Pesticide Electrochemical Sensors Based On Surface Imprinting

Pesticides are chemically stable and have a long degradation cycle,resulting in long-term residues in fruits and vegetables,the atmosphere,soil and water.This not only will bring about serious pollution to the environment,but also will eventually enter the human body through the enrichment of the food chain,causing various potential threats.Since there are too many types of pesticides with complex composition and hard to residue,the sensitivity and selectivity of the methods for detecting pesticide residue are highly required.Molecularly imprinted polymers（MIPs）are also called“artificial antibodies”,which are polymerized by a number of monomers to form a polymer with high selectivity for specific target substances.Through sorting the recent studies on the rapid detection of pesticide residues,the author found that electrochemical sensing methods have the advantages of speediness,simplicity and low cost.The research of high-performance electrochemical sensors for pesticide residues is attracting the attention of many scholars at home and abroad,and MIP,as an identification material,is often involved in the construction of electrochemical sensors for pesticide residues.However,the MIP polymers obtained by traditional preparation methods such as bulk polymerization and emulsion suspension polymerization are too deeply embedded in the imprinted sites,resulting in incomplete elution and slow mass transfer rate and poor electrical conductivity,which to a certain extent will limit the response signal transmission at the recognition interface of their modified sensors and reduce the sensitivity of the sensors.Therefore,this thesis focuses on the preparation of novel surface molecularly imprinted polymers and their research in the construction of electrochemical sensors for pesticide residues.It is proposed to explore novel surface molecular imprinting techniques to enhance the selectivity of sensing methods and to combine the synergistic use of novel nanomaterials and imprinted polymers to improve the sensitivity of electrochemical sensors for pesticide residues.Objects:Using three classes of pesticides typically representing methyl parathion（MP;organophosphates）,carbendazim（CBD;benzimidazoles）and cypermethrin（CYP;pyrethroids）as the detection target substances,the author employ the surface molecular imprinting technology and nanomaterial modification technology,and construct a new surface molecular imprinting electrochemical sensor.This not only attains the goal for detecting the sensitivity and selectivity of the above three substances,but also explores the effects of surface imprinting and nanomaterial modification on the sensor to provide theoretical and technical foundation for the practical application of the sensor for the detection of pesticide residues.Methods:Through the two-step distillation precipitation method and an electropolymerization method to prepare various pesticide molecularly imprinted polymers,to investigate the morphology,structure,adsorption behavior,selectivity and other performance indexes of the polymers,to screen the synthesis formulations/optimize the preparation conditions,and to obtain the best performance of surface imprinted polymers;meanwhile,a two-step modification technique was used to construct surface molecularly imprinted electrochemical sensors for each representative pesticide in combination with nanomaterials.Some classical electrochemical methods such as cyclic voltammetry,electrochemical impedance spectroscopy,and square wave voltammetry（SWV）were also used for the electrochemical characterization of the sensor preparation;scanning electron microscopy,transmission electron microscopy,X-ray diffraction,and X-ray photoelectron spectroscopy were used to characterize the morphology and elemental composition of the prepared imprinted polymers,the used nanomaterials and the final sensors;a complete electrochemical pesticide residue detection method was established and a comprehensive method validation study was conducted.Results:（1）Firstly,one-step and two-step distillation precipitation polymerization methods were used to prepare non-surface imprinted polymer microspheres（MIPMs）and surface molecularly imprinted polymer microspheres（SMIPMs）to investigate the effect of surface molecular imprinting on the electrochemical sensing performance.MP as a detection model,imprinted polymer microspheres were used as functional materials in carbon paste electrodes（CPE）to obtain non-surface-imprinted/surface-imprinted electrochemical sensors,and their performances were compared.The results showed that the current response of SMIPMs/CPE to MP was 2.5times higher than that of MIPMs/CPE.In addition,the electrochemically active area of SMIPMs/CPE is much larger than the latter.Surface-imprinted electrochemical sensors can adsorb more MPs,making their response signals higher than those of non-surface-imprinted sensors.A comparative examination of the morphological structure and adsorption behavior of the two polymers revealed that SMIPMs have larger particle size and higher specific surface area,which are more favorable for MP adsorption,and their adsorption amount is 2.5 times higher than that of MIPMs,and the time to reach adsorption equilibrium is shorter.Theoretical calculations of the separation factor（α）and relative separation factor（β）as well as thermodynamic parameters（ΔG°）of the two polymers confirmed that SMIPMs were better imprinted than MIPMs,and the binding energy of SMIPMs to MP was smaller than that of MIPMs,indicating that surface imprinting was more favorable to enhance the analytical performance of electrochemical sensors.Meanwhile,the prepared SMIPMs/CPE showed a good linear response to MPs with concentrations ranging from1.0×10^-12 to 8.0×10^-9 M,and the limit of detection（LOD）was 3.4×10^-13 M.This study provides an experimental basis and theoretical guidance for the use of surface imprinting technology to enhance the sensing performance.（2）Under the guidance of the above theory,a CBD surface imprinted electrochemical sensor was constructed by double modification of two nanomaterials（N,S-Mo₂C and C-ZIF67@Ni）with MIP membranes prepared by electropolymerization on the sensing electrode using CBD as the detection model.The effects of nanomaterials on the performance of surface molecularly imprinted electrochemical sensors were investigated through a series of single-factor examination and control experiments.The results showed that the nanomaterial-based modified MIP/N,S-Mo₂C/GCE and MIP/C-ZIF67@Ni/GCE had wider linear detection range and lower detection limit than MIP/GCE,and the surface imprinted sensor obtained by C-ZIF67@Ni modification had better analytical performance for CBD than N,S-Mo₂C.modification of surface nanomaterials can significantly improve the responsiveness of the sensor.A comparison of the physicochemical properties of the two nanomaterials revealed that C-ZIF67@Ni(A=647.35 m² g^-1)has a larger specific surface area compared to N,S-Mo₂C(588.74 m² g^-1),and the pore size of C-ZIF67@Ni（27.47 nm）is larger than that of N,S-Mo₂C（13 nm）.The differences in their structures lead to different current responses of their modified sensors and also to differences in the amount of molecularly imprinted polymers attached to the electrode surface,which in turn affects the sensor’s performance and sensitivity in the selection of detection substances.The comparison of these two studies provides theoretical guidance and experimental basis for the selection of nanomaterials in the construction of surface molecularly imprinted electrochemical sensors.（3）In order to improve the detection capability of the sensor for weakly electroactive pesticides（CBD）,and to solve the problem that its weak oxidation signal is susceptible to environmental interference and small changes in buffer composition under electrocatalytic conditions,an electrochemical sensing method was developed for the highly stable detection of trace CBD in complex samples using surface molecular imprinting technology and ratiometric sensing strategy.The surface imprinted electrochemical sensor showed good linearity for CBD in the concentration range of 1×10^-8～1×10^-3 M with an LOD of 3.4×10^-9 M.The ratiometric approach resulted in a better correlation than the non-ratiometric sensor.This new ratiometric electrochemical sensor directly adds the internal standard to the electrolyte solution to be measured,reducing other detection steps,which can effectively reduce the human error in the experimental process and improve the accuracy of the results.This ratiometric sensing strategy is expected to provide an important reference in the design and construction of future electrochemical sensors,and provide a strong reference for achieving highly stable and sensitive detection of other electroactive molecules.（4）In the field of electrochemical sensing,the detection of compounds with large molecular weight and complex structure often requires a more scientific and rational design,which puts higher requirements on the recognition element-MIP.Compared with MP and CBD,the chemical structure formula of CYP is more complex,which puts higher requirements on the surface molecular blotting preparation technology.Therefore,in the construction of CYP sensors,a novel bifunctional monomer surface-imprinted electrochemical sensor was prepared in situ on the electrode surface using Ag-N@ZnO/CHAC as the sensitizing material and bifunctional monomers（dopamine and resorcinol）as raw materials to investigate its sensing performance for CYP.The response surface methodology was used to investigate the effect of bifunctional monomers on the imprinting performance of the imprinted polymers and the performance of the constructed sensors.The effect of bifunctional monomers on the formation of molecular imprinting was also investigated based on response surface quadratic model ANOVA.Compared with the monofunctional monomer MIP,the introduction of bifunctional monomer increased the diversity of imprinting site types and imprinting affinity in the MIP structure,which in turn improved the selectivity and sensitivity of the bifunctional imprinted electrochemical sensor for CYP in the range of 2×10^-13 to 8×10^-9 M and LOD of 6.7×10^-14 M.Conclusion:Surface molecularly imprinted polymers can significantly improve the selectivity of sensors in complex matrices.Nanomaterials with large specific surface area and good electrical conductivity exhibit amplification of the sensor response signal and increase the molecularly imprinted polymer attachment,which in turn enhances the detection sensitivity.The effective combination of the two significantly enhances the accuracy,reliability,reproducibility and stability of the prepared sensors to some extent.On this basis,the constructed surface molecularly imprinted electrochemical sensor achieved highly sensitive and selective detection of MP,CBD and CYP.In addition,the effect of the relationship between monomer and template on the formation of bi-template molecular imprinting was explored by response surface methodology,which provided a theoretical basis for the synthesis and preparation of bifunctional monomer surface molecular imprinting and its excellent performance.