Design And Study Of Acid-responsive CaCO₃-Based Pesticide Controlled Release System

Cruciferous root disease,a highly damaging soil-borne disease,was triggered by Plasmodiophora brassicae Woronin,and clubroot infection and development were found to occurmore commonly between soil p H 4.0 and 6.5.In this thesis,an acid-stimulated responsive slow-release pesticide system was designed based on the pathogenic characteristics of rhizoctonia and nano slow-release technology.Effective utilization of drugs can be increased and drugs’pollution to the environment can be decreased while conducting chemical treatment.In this work,CaCO₃@SiO₂@PEI composite system was prepared with micro/nano CaCO₃ as a carrier loaded with drug.The system could release drugs slowly under weakly acidic conditions,while releasing CaCO₃ to regulate environmental p H and provide Ca²⁺for plant growth.It could provide an effective solution for the integrated management of diseases such as cruciferous root disease.1.Controllable synthesis of micro/nano CaCO₃:nano CaCO₃ was prepared by gas diffusion method.Using the thermal decomposition of NH₄HCO₃ solid in a sealed desiccator,CO₂ being produced to diffuse into the Ca Cl₂ solution to react and form nano CaCO₃ particles.Both the concentration of Ca Cl₂ solution（c,g/L）and the rate of CO₂diffusion into the solution affected the particle size of CaCO₃（d,nm）.The parafilm was used to seal the mouth of the beaker containing Ca Cl₂ solution to reserve small holes as the diffusion channels of CO₂.Decreasing the concentration of Ca Cl₂ solution and increasing the gas diffusion rate were beneficial to control the nano CaCO₃ particle size.micro CaCO₃was prepared by biomineralization method.In Ca Cl₂ solution,Ca²⁺was complexed with soluble starch used as a template to react with Na₂CO₃ solution.Increasing the reaction temperature or the substrate concentration will augment the particle size of CaCO₃.The CaCO₃ was a spherical structure of spherulite crystal type with porous interior,which was characterized by SEM,TEM,FT-IR and XRD.2.Preparation of CaCO₃@SiO₂@PEI core-shell structures and properties of controlled drug release.The surface layer of nano CaCO₃ was coated with mesoporous SiO₂ to form a core-shell structure（noted as CaCO₃@SiO₂）.SEM and TEM showed that the particles are core-shell structure and have a shell thickness of about 5 nm.The SiO₂ shell layer could maintain the nano CaCO₃ vaterite crystal shape to prevent nanoparticle agglomeration better.Using impregnation loading method,thiamethoxam,uniconazole and thiophanate-methyl were loaded by CaCO₃@SiO₂ and CaCO₃ respectively.The results showed that the loading of CaCO₃@SiO₂ for water soluble pesticides-thiamethoxam,reachs 124±4 mg/g,which was much higher than that of CaCO₃ drug loading（49±2 mg/g）.The optimal loading conditions were ultrapure water as solvent,thiamethoxam concentration of 800 mg/L and shaking at 150 r/min for 12 h at room temperature.Another important role of the SiO₂ shell layer was to significantly enhance the drug-carrying capacity of the system.Polyethyleneimine（PEI）was used further to coat CaCO₃@SiO₂-THI,being noted as CaCO₃@SiO₂@PEI.The slow-controlled release effect of this system was investigated in PBS buffers of different p H.The consequents claimed that the slow-release equilibrium was reached after 120 h at p H=4.5,with a cumulative drug release of 40%and sudden release completed at 24 h.3.Effect of CaCO₃@SiO₂@PEI system on the growth activity of oilseed rape seedlings.Oilseed rape seeds grow at p H=4.5 under hydroponics in the presence and abscence of CaCO₃@SiO₂@PEI.The methods made the p H of the culture solution increasing to about 7.5 after 7 days and the growth of oilseed rape seedlings was not significantly affected while the p H only increased to about 5 without added materials.The germination of rape seeds received a certain degree of inhibition when the materials application rate was increased to 1500 mg/L.CaCO₃@SiO₂@PEI possessed good biocompatibility for rapeseed growth,not inhibiting the germination rate and seedling survival of rapeseed seeds.This material is suitable for acidic environments and can significantly improve environmental p H,with promising applications for the development and integrated management of diseases such as cruciferous root disease.