| As a polysaccharide abundant in nature,chitosan,with its good biocompatibility and biodegradability,is widely used as a drug carrier in the pharmaceutical industry.However,chitosan has poor solubility and is insoluble in water,alkali and common organic solvents,which greatly limits its application in all aspects.Chemical modification of chitosan to obtain chitosan derivatives with required functions has become a research hotspot in recent years.In this study,with chitosan as the raw material,hydrophilic carboxymethyl chitosan(CMCS)was prepared by carboxymethylation.Using carboxymethyl chitosan as the wall material,avermectin microcapsules were prepared and characterized.Then,the lipophilic methyl methacrylate(MMA)was grafted onto the carboxymethyl chitosan to obtain the amphiphilic carboxymethyl chitosan-grafted-methyl methacrylate copolymer(CMCS-co-MMA).Using CMC S-co-MMA as the wall material,avermectin microcapsules were prepared and characterized.Finally,avermectin microcapsules prepared using these two kinds of wall materials were studied for photostability,release characteristics and insecticidal persistence.The specific contents of this study are as follows:Chitosan was modified with chloroacetic acid by carboxymethylation to obtain the hydrophilic chitosan CMCS.With the degree of substitution as the evaluation criterion,the effect of preparation parameters on the degree of substitution of CMCS was studied and the optimum preparation process was finalized,as follows:the amount of sodium hydroxide solution was 24 g,the amount of chloroacetic acid was 10 g,the amount of isopropanol was 20 g,the reaction temperature was 60℃,and the reaction time was 5 h.The chemical structure of CMCS was characterized by Fourier-transform infrared spectroscopy(FTIR),Carbon-13 nuclear magnetic resonance(13C NMR)and differential scanning calorimetry(TGA).The results showed that the target product of carboxymethylation was consistent with the expected structure and the highest degree of substitution was 0.81.Avermectin microcapsules were prepared by double emulsion-emulsion cross-linking method with CMCS as the carrier.The impact of CMCS concentration,emulsifier concentration,amount of oil phase and amount of cross-linking agent on the encapsulation efficiency and drug loading of avermectin microcapsules was investigated with encapsulation efficiency and drug loading as the evaluation criteria.The optimum preparation process was finalized,as follows:the CMCS concentration was 2.5%,the emulsifier concentration was 5%,the amount of oil phase was 16 g,and the amount of cross-linking agent was 0.3 g.The physicochemical properties of CMCS microcapsules were characterized by appearance observation,FTIR and particle size analysis.The results showed that the particle size of AV@CMCS microcapsules was 18.2 μm,the distribution was uneven,there was breakage of microcapsules,the encapsulation efficiency was 71.5%,and the drug loading was 17.7%.Carboxymethyl chitosan was modified with MMA by grafting modification to obtain the amphiphilic copolymer CMCS-co-MMA.With the grafting rate as the evaluation criterion,the effect of preparation parameters on the grafting rate of CMCS-co-MMA was studied and the optimum preparation process was finalized,as follows:the amount of MMA was 4 g,the amount of initiator was 2 g,the reaction temperature was 60℃,and the reaction time was 5 h.The chemical structure of CMCS-co-MMA was characterized by Fourier-transform infrared spectroscopy(FTIR),Carbon-13 nuclear magnetic resonance(13C NMR)and differential scanning calorimetry(TGA).The results showed that the target product of grafting modification was consistent with the expected structure and the highest grafting rate was 0.77.Avermectin microcapsules were prepared by double emulsion-emulsion cross-linking method with CMCS-co-MMA as the carrier.The impact of CMCS-co-MMA concentration,emulsifier concentration,amount of oil phase and amount of cross-linking agent on the encapsulation efficiency of avermectin microcapsules was investigated with encapsulation efficiency as the evaluation criteria.The optimum preparation process was finalized,as follows:the CMCS-co-MMA concentration was 2.5%,the emulsifier concentration was 5%,the amount of oil phase was 16 g,and the amount of cross-linking agent was 0.3 g.The physicochemical properties of CMCS-co-MMA microcapsules were characterized by appearance observation,particle size analysis and FTIR.The results showed that the particle size of AV@CMCS-co-MMA microcapsules was 11.0 μm,the distribution was even,there was not breakage of microcapsules,the encapsulation efficiency was 85.0%,and the drug loading was 37.2%.With technical avermectin as the control,the photostability,release characteristics in 20%ethanol-water solution and the persistence against Plutella xylostella larvae of avermectin microcapsules prepared using the above two kinds of wall materials were studied.The results showed that after the two kinds of avermectin microcapsules were exposed to outdoor natural conditions for 10 days,the photodegradation rate of avermectin in AV@CMCS microcapsules was 30.2%,and the degradation time was about 1 times longer than that of technical avermectin;and the photodegradation rate of avermectin in AV@CMCS-co-MMA microcapsules was 16.9%,and the degradation time was about 3 times longer than that of technical avermectin.The release characteristics of avermectin in 20%ethanol-water solution were also studied.The fitting results of Ritger-Peppas release model showed that AV@CMCS microcapsules exhibited irregular diffusion,and the t50 was 28.35 h,while AV@CMCS-co-MMA microcapsules had non-Fickian diffusion,and the t50 was 74.23 h.In the persistence test,although the initial control effect was not as good as that of emulsion oil in water,the microcapsules prepared using these two wall materials showed a longer persistence and more stable efficacy,and the mortality of Plutella xylostella was 50.2%and 79.7%respectively 21 days after application. |
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