Establishment of Pesticide-loaded Microcapsule System and Its Activity Regulation

Author:Li Bei Xing

Supervisor:liu feng

Database:Doctor

Degree Year:2019

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Pages:96

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In recent years,microencapsulation technology has developed rapidly in the domain of pesticides,mainly due to its advantages in improving the utilization efficiency and safety of pesticides.Commonly used microencapsulation methods include in-situ polymerization,interfacial polymerization,spray drying and solvent evaporation.However,these methods have more or less some limitations in cost,time consumption,simplicity and environmental compatibility.In recent years,easily degradable polymer such as polylactic acid,polyhydroxyalkanoate,polypropylene carbonate,and natural gelatin,alginate,modified cellulose and chitosan were extensively used in the fabrication of chlorpyrifos,imidacloprid and other insecticide microcapsules(MCs).Due to the limitation of manufacturing technique and financial cost,the application of these methods in pesticide industry is still insufficient.Seeking for natural products and easily degradable materials is still the main concern of pesticide microencapsulation studies as well as developing microencapsulation technology that has the peculiarities of simple manufacturing process,low cost and easy industrialization.After the microencapsulation of pesticides,the surface of the particles(core material)is covered by oligomer or eupolymer.Thus,their appearance,size,rigidity,polarity and other characteristics will change significantly,which would cause changes in the affinity of active ingredients to the target organisms,and thereby influence the distribution and release of pesticides.Therefore,the dosage of pesticides exposed to harmful organisms would change and ultimately affect the efficacy.However,there is still a lack of systematic theoretical support on how to construct efficient and convenient pathways for the bioactivity regulation of pesticide microencapsulation which requires further study and clarification.The main results of this study are summarized as follows:1.The establishment and characterization of MCs by using a coordination assemblyIn this study,coordination assembly on liquid-liquid interface was achieved by coordination reaction between metal ions and tannic acid.The deposition was confirmed by zeta potential,energy dispersive spectroscopy and X-ray photoelectron spectroscopy.Scanning electron microscopy and transmission electron microscopy were integrated to characterize the MCs.According to atomic force microscopy height analysis,membrane thickness of the MCs increased linearly with sequential deposition.For MCs prepared using the Fe3+-TA system,the average membrane thicknesses of MCs prepared with 2,4,6 and 8deposition cycles were determined as 31.3±4.6,92.4±15.0,175.4±22.1 and 254.8±24.0nm,respectively.Dissolution test showed that the release profiles of all the four tested MCs followed Higuchi kinetics.Membrane thicknesses of MCs prepared using the Ca2+-TA system were much smaller.We can easily tune the membrane thickness of the MCs by adjusting metal ions or deposition cycles according to the application requirements.In this study,a reliable,convenient and general method was proposed to evaluate the release kinetics of MCs under artificial simulation conditions.The dissolution test was carried out using an elaborate apparatus comprising a syringe and a needle filter.The optimal test conditions were determined as follows:3 ml of newly configured 80%methanol aqueous solution(v/v)was used as the release medium,and Nylon 6 filters(13 mm diameter,0.22μm pore size)were used as diffusion barriers,and a sampling regime was determined based on predetermined intervals.Validation tests demonstrated that this strategy possessed favorable universality for three wall materials(polyurea,polyurethane and urea-formaldehyde)and three core materials(phoxim,pyraclostrobin and clomazone),as all sampling points had standard errors below 3.4%.Our proposed method also showed apparent advantages in experimental precision and the demand for a release medium compared with dynamic dialysis.2.Optimization of physicochemical properties of pesticide MC system by using a coordination assemblyA model chlorpyrifos MC was prepared using coordination assembly between Fe3+and tannic acid.The influence of independent variables such as the dropping rate of tannic acid(X1)and Fe3+(X3),the concentration of tannic acid(X2)and Fe3+(X4),and the reaction temperature(X5)on the encapsulation efficiency(R1)and release characteristics(R2)of the MC had been investigated,based on a central composite design with five factors and five levels.The results showed that the main factors influencing R1 and R2 were X4 and X2,then the interaction between X2 and X4,followed by X5 and X3.The optimal formula mainly based on higher R1 and lower R2 were determined and then tested.The optimized conditions led to an encapsulation efficiency and cumulative release proportion of 97.12±60.72%and40.07±60.53%,along with the average relative errors of predicted values being 1.78%and21.60%,respectively.3.Bioactivity regulation approaches of pesticide-loaded MC system by using coordination assemblyWhen Mg2+,Al3+,Cu2+or Mn2+were use as central ions,the pyraclostrobin-loaded MCs had a relatively high inhibitory activity against the mycelial growth of Colletotrichum acutatum,whereas the inhibitory activity was the lowest for that of Fe3+.The inhibitory activity of pyraclostrobin on the spore germination of C.acutatum was higher.In terms of MCs prepared with Mg2+,Ca2+or Al3+,the inhibitory activity on the spore germination of C.acutatum were high,with the EC50s all around 0.02mg/l.When Cu2+,Zn2+,Mn2+or Fe3+were used as central ions,the inhibitory activity of the MCs on the spore germination of C.acutatum was slightly lower(EC50 of 0.0572-0.0800 mg/l);the inhibitory activity of the MCs gradually increased with the increaseing assembly layers.Pyraclostrobin MCs assembled with different central ions also showed high inhibitory activity on the spore germination of Magnaporthe grisea.When the central ions were Mg2+,Ca2+,Al3+,Cu2+or Mn2+,the EC50s0s were all around 0.03mg/l,and the bioactivity of the MCs changed insignificantly with the assembly layers.As for MCs prepared with Zn2+or Fe3+,the EC50s of against the spore germination of M.grisea were 0.0047 and 0.0154 mg/l,respectively;their inhibitory activity decreased first and then increased with the assembly layers.Pyraclostrobin MCs assembled with different organic ligands also showed high inhibitory activity against the mycelial growth of C.acutatum.As for the MCs of one assembly layer,the EC50s ranged between 0.0184 and 0.0729 mg/l.The bioactivity of the MCs hardly changed with the assembly layers.The inhibitory activities of pyraclostrobin MCs assembled by different organic ligands on the spore germination of C.acutatum were much different.In terms of MCs assembled one cycle,the EC50s of MCs assembled by dopamine,gallic acid,catechol,caffeic acid and phytic acid were 0.0516-0.0827 mg/l,while that of tannic acid-assembled MCs was significantly lower(EC50 of 0.4302 mg/l).Except for the pyraclostrobin-loaded MCs with an initial average particle size higher than6μm,there were no significant differences in the inhibitory activity of the MCs with particle size of 2.03-5.61μm(prepared by coordination assembly)against the mycelial growth of C.acutatum.These pyraclostrobin-loaded MCs also exhibited high inhibitory activity on the spore germination of C.acutatum;however,the inhibitory activity of these MCs changed insignificantly with the decrease of initial average particle size.In addition,these MCs had extremely high activity against the spore germination of M.grisea;the EC50s of the MCs assembled one cycle were all lower than 0.001 mg/l and there were no significant differences between MCs with different particle size.4.Bioactivity regulation approaches of pesticide-loaded MC system prepared with polyureaThe inhibitory activities of pyraclostrobin-loaded MCs with different particle size against the mycelial growth of C.acutatum were significantly different.When the average particle size of the pyraclostrobin-loaded MCs was 15.02μm,the EC50 value was 1.1638 mg/l.Their inhibitory activity against the mycelial growth of C.acutatum gradually increased with the decreasing average size of the MCs;when the average particle size of the MCs was 2.26μm,the EC50 was as low as 0.0277 mg/l.The activities of pyraclostrobin-loaded polyurea MCs with different particle size also differed greatly in inhibiting the spore germination of M.grisea.When the average particle size was 15.02μm,the EC50 was 0.2421 mg/l.Their inhibitory activity against the spore germination of M.grisea gradually increased with the decreasing average size of the MCs;when the average particle size of the MCs was lower than 3.67μm,the EC50 hardly changed.The amount of wall materials can also affect the inhibitory activity of pyraclostrobin-loaded MCs on the mycelial growth of C.acutatum.When the content of wall materials was 0.01%(mass fraction),the EC50 of the MCs was 0.0855 mg/l;the inhibitory activity of the MCs on the mycelial growth of C.acutatum decreased rapidly with the increasing content of wall materials;when the content of wall materials was 1.00%,the EC50of the MCs was 16.5050 mg/l,which was only 1/193 of the inhibitory activity compared with that of 0.01%.The inhibitory activities of pyraclostrobin-loaded MCs with different wall material content against the spore germination of M.grisea were also dramatically different.When content of wall material was 0.01%,the EC50 of pyraclostrobin-loaded MCs was0.1580 mg/l;the inhibitory activity of the MCs on the spore germination of M.grisea decreased gradually with the increase of wall material content;when the content of wall materials was 1.00%,the EC50 of the MCs was 4.4671 mg/l,which was only 1/28 of the inhibitory activity compared with that of 0.01%.The efficacy of pyraclostrobin-loaded MCs with different content of wall materials in controlling tomato root rot were varied greatly.After treated with pyraclostrobin-loaded MCs,the incidences of tomato root rot significantly decreased in all treatments.When the wall material content was 0.01%,the incidence of tomato root rot was the lowest(25.0%),which was significantly lower than that of control(75.0%).In conclusion,selecting appropriate wall materials and microencapsulation parameters can effectively regulate the activity of pesticide MCs by mediating the membrane thickness,membrane composition,and the particle size of the MCs.