Preparation and performance of a BTDA-modified polyurea microcapsule for encapsulating avermectin

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Highlights

  • Chitosan oligomer (CO) was applied in the preparation of polyurea microcapsules.

  • The UV-resistance of the microcapsule was enhanced by grafting BTDA to CO.

  • Photodegradation of avermectin was greatly reduced when embedded in the microcapsule.

  • The microcapsule itself was subject to photodegradation in water.

Abstract

A pesticide microcapsule was prepared by encapsulating avermectin (AVM) in a polyurea microcapsule via interfacial polymerization in acetic ether/water emulsion. The polyurea microcapsule was consisted of chitosan oligomer (CO) as the membrane material and diphenyl methane-4,4′-diisocyanate (MDI) as the crosslinker. A chemical modification was carried out by grafting a UV-absorbent, 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BTDA), to CO before interfacial polymerization to enhance the UV-resistance of the microcapsule. The BTDA grafted CO (CO-BTDA) and the AVM microcapsules were characterized by a variety of instrumental techniques, including NMR, FTIR, UV–vis, GPC-LS, DLS, SEM and TEM. The in vitro release test showed that the polyurea microcapsule maintained the sustained release of AVM for a longer period (up to 120 h) in comparison with the commercial AVM formulations (within 24 h). The photodegradation test revealed that the polyurea microcapsule significantly reduced the AVM degradation and extended the half-life of AVM from 4.16 h to 9.43 h. The AVM degradation was further reduced by using the BTDA-modified polyurea microcapsule. The corresponding half-life was extended up to 17.33 h and can be mediated by changing the mass ratio of BTDA: CO during the synthesis of CO-BTDA. The use of polyurea microcapsule did not raise a concern about pesticide residue as no AVM was detected after the photodegradation test. In addition, the polyurea microcapsule itself was subject to degradation under sunlight exposure, which reduced its residue in the environment.

Introduction

Pesticides are important agricultural inputs that protect agro-products from pests and diseases to achieve a high productivity. However, the intensive use of traditional chemical pesticides has caused environmental pollution and food safety issues due to their relatively poor degradability and high toxicity [[1], [2], [3]]. There is an urgent need to reduce the use of such pesticides. An alternative approach is to develop new pesticides which are eco-friendly, highly efficient and safe. Biopesticides are chemicals obtained from naturally occurring substances (biochemical pesticides) or microorganisms (microbial pesticides) that control pests [4,5]. Due to their high efficiency, superior degradability and relatively low toxicity, biopesticides are increasingly used as substitutes for the traditional chemical pesticides.

Avermectin (AVM) is one of the most widely used biopesticides exhibiting broad-spectrum anthelmintic and insecticidal activities [6]. It is very sensitive to sunlight especially at the UV range, resulting in rapid degradation and therefore low efficacy toward pest control [7,8]. One solution is to directly add antioxidant or UV-absorbent to the traditional AVM formulations (e.g., AVM emulsion), while some studies have reported the low efficiency of this approach, as AVM lost protection from those additives after dilution and spraying during pesticide application [9,10]. In addition, the use of harmful organic solvents (e.g., toluene and xylene) in the AVM emulsion may bring additional risks to the environment and eco-systems. A better solution could be microencapsulation which refers to a technique to encapsulate the pesticide in a microcapsule [11]. A microcapsule can be made of an inorganic particle (e.g., mesoporous silica) or a polymer, while the latter one is more frequently used. The main function of the microcapsule is to provide a layer to the pesticide and protect it from the outer environment [12,13]. Other advantageous features of the microcapsule include: a) improved pesticide (hydrophobic) dispersion in water [14], b) sustained and controlled pesticide release [15,16], and c) reduced pesticide toxicity during handling and transportation [17]. Because of these advantages, pesticide microcapsules have become a subject of intensive research and development in recent years.

There are several methods to prepare a polymer-based microcapsule. One common method is interfacial polymerization occurring at the interface between an aqueous solution containing one reactant and an organic solution containing a second reactant. For instance, a polyurea microcapsule is prepared by the polymerization of a di- or polyamine reactant in the aqueous solution and a diisocyanate reactant in the organic solution [18,19]. A polyurethane microcapsule is formed by changing the reactant to di- or polyhydric alcohol in the aqueous solution [20,21]. Although there are many selections of reactant in the aqueous solution, the use of chitosan oligomer (CO) as a reactant is scarce. CO is the pyrolytic or enzymolytic product of chitosan, a well-known polysaccharide-based biopolymer derived from chitin with good degradability and biocompatibility [22]. The monomer of CO contains both amine and hydroxyl groups that can react with diisocyanate, while the reaction between amine group and diisocyanate is in priority, leading to the formation of polyurea. Besides, the amine and hydroxyl groups are able to react with a number of active ingredients, bringing novel functionalities to the microcapsule [[23], [24], [25]]. For instance, a UV-resistance property could be obtained when using a UV-absorbent as the active ingredient, providing extra protection to the encapsulated pesticide. Hence, it is interesting to carry out research on the potential application of CO in the preparation of microcapsules with good degradability and improved performance.

This study aimed to prepare a CO-containing polyurea microcapsule and investigate its performance on encapsulating AVM as a model pesticide. The AVM microcapsule formulation was prepared via interfacial polymerization in an acetic ether/water emulsion system. Before interfacial polymerization, CO was modified by grafting a UV-absorbent to enhance the UV-resistance of the microcapsule. The prepared microcapsules were characterized by various instrumental techniques and investigated on sustained-release and anti-photolysis performances. The outcome of this study could provide an approach for the functionalization of microcapsules with wide applications in the agricultural field.

Section snippets

Chemicals

Avermectin was purchased from Shandong Qilu King-Phar Pharmaceutical Co., Ltd. (Jinan, China) and used as a model pesticide representing a list of photosensitive pesticides. The chemical reagents used in the synthesis of polyurea were chitosan oligomer (Shanghai Yuanye Bio-Technology Co., Ltd., China), diphenyl methane-4,4′-diisocyanate (MDI, crosslinker, J&K Scientific Ltd., China), and 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BTDA, UV-absorbent, J&K Scientific Ltd., China). Two

Characteristics of CO-BTDA

The grafting of BTDA to CO was an important step for the preparation of AVM microcapsules with enhanced resistance to photodegradation. NMR analysis was carried out to confirm the reaction between CO and BTDA, and the results are shown in Fig. 2a. The three peaks around 8 ppm in the BTDA spectrum represent the 1H chemical shifts of benzene ring. These peaks are also shown in the CO-BTDA spectrum around 8.5 ppm. The difference in chemical shift values can be explained by the successful reaction

Conclusions

In summary, a BTDA-modified polyurea microcapsule was successfully prepared by first grafting BTDA to CO followed by emulsion interfacial polymerization between CO and MDI. The prepared microcapsules showed a sustained release of AVM for a longer period and protected the encapsulated AVM from sunlight. The grafting of BTDA to CO was approved an effective way to reduce the photodegradation of AVM and the degradation rate was influenced by the BTDA: CO ratio. A reverse relation was obtained

Acknowledgements

We appreciate the financial support from the National Natural Science Foundation of China (No.21806182) and the Key Laboratory of Agrifood Safety and Quality, Ministry of Agriculture and Rural Affairs of China.

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    These authors contributed equally to this work.

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