Elsevier

Materials Letters

Volume 264, 1 April 2020, 127345
Materials Letters

Preparation of natural pyrethrum-polylactic acid microspheres with different particle sizes and surface morphology

https://doi.org/10.1016/j.matlet.2020.127345Get rights and content

Highlights

  • A series of pyrethrum-PLA microspheres with different morphology were prepared.

  • While the sizes change, the appearances of microspheres change regularly.

  • 100-800 mg of microspheres can be obtained per 100 mL of the reaction system.

Abstract

Controlled-release technology has been widely studied in the field of medicine, but it is rarely used in pesticides. The objective of this study was to obtain pyrethrin-polylactic acid microspheres with different particle sizes and morphologies. By adjusting the concentration of polylactic acid and the rotation speed, a series of microspheres can be prepared simply and efficiently by using the single emulsion-solvent evaporation method. The result of laser particle size analyzer showed that the average diameter of the microspheres ranged from 13.5 to 142.4 μm. The FESEM observation revealed that the obtained microspheres were spherical with pores on the surface. And as the particle size decreases, the area of the surface pores becomes larger and shallower. The spectrum of FTIR indicated the presence of pyrethrin in the prepared microspheres.

Introduction

The global annual expenditures on pesticides have reached $56 billion, and about 0.3 g of pesticides are used for the food of one meal. However, the effective utilization rate of traditional pesticide formulations is only about 10% due to the ease of degradation, volatilization and loss [1]. In order to maintain the control effect, people have to increase the amount or frequency of applications, which causes a large amount of organic solvents (especially benzene solvents) to be used on the field. This is not only a waste of economy, but also a serious environmental pollution. With the need for sustainable development, environment friendly pesticide formulations are gaining more and more attention. Controlled-release microspheres improve operational safety compared to conventional formulations. At the same time, they can reduce the impact of environmental factors, prolong the efficacy of the drug and reducing the amount and frequency of medication [2]. In addition, the sustained-release microspheres drift less and are more easily to store and transport.

Compared with the pharmaceutical industry, the wide application of the agricultural field requires lower costs and more convenient processes. The precipitation of microspheres obtained by emulsion cross-linking are irregular-shaped, and require lots of acetone to remove residue [3]. The premixed membrane emulsification can form regular microspheres. But the process needs to extrude the emulsion through SPG film under nitrogen pressure for multiple cycle, which is complicated [4]. Therefore, there is a need for a process which is easy to scale up and has a high yield. Solvent evaporation is a common method for preparing controlled-release microspheres. The double emulsion method (W/O/W) is more suitable for drugs with high water solubility to ensure a higher drug loading amount. With this method, about tens to 100 mg of microsphere can be obtained from 100 mL reaction system in many studies [5], [6].

The single emulsification-solvent evaporation technique disperses the drug and carrier in an organic solvent, emulsifies in the aqueous phase, and removes the organic phase to form microspheres. Some researchers have found that modify the emulsification process with ultrasound can significantly reduce the particle size to 20 µm or less [7]. Since most of the pesticides are hydrophobic, the use of the double emulsion method to prepare the pesticide microspheres will have a low drug loading, the single emulsion-solvent evaporation technique is more suitable for pesticide.

Pyrethrin is a natural insecticide extracted from Tanacetum cinerariifolium, which has low toxicity for plants and human. Exposure to light, oxygen, and high temperatures will cause rapid degradation of pyrethrin [8]. This is conducive to environmental protection, but correspondingly increases the total usage and frequency. It’s a good choice to develop some formulations which continuously release active substances without changing the degradation characteristics. Polylactic acid (PLA) is chosen as the carrier for pyrethrin, because it is an environment-friendly material which can be completely degraded into CO2 and H2O in the environment [9]. Considering the significant influence of particle size and microsphere characterization [10], we prepared natural pyrethrin-PLA microspheres with different particle size and porosity on the basis of emulsion solvent evaporation method.

Section snippets

Materials

Polylactic acid was purchased from eSUN Industrial Corporation (Shenzhen, China), with a molecular weight (Mw) of 100,000. Natural pyrethrin was purchased from pinobiotech Corporation (Xi'an, China). Dichloromethane (DCM, AR) and polyvinyl alcohol (PVA, the alcoholysis degree: 87.0–89.0%) was purchased from Aladdin Industrial Corporation.

Preparation of pyrethrum-loaded microspheres

The PLA was dissolved in 10 mL DCM to prepare a solution with a polymer concentration of 10–80 mg/mL, followed by the addition of natural pyrethrin as the

Result and discussion

The low effective utilization rate of pesticides leads to excessive application and has a serious impact on the environment. Meanwhile, the new pesticide variety has a long development cycle and a low screening success rate. Formulation of existing pesticides has a positive effect on sustainable development and environmental protection.

In our preliminary experiment, we found that the concentration of PLA and the stirring speed had an obvious influence on the particle size of the microspheres.

Conclusion

The application of pesticide ‘green’ preparation requires low cost and high yield. We prepared natural pyrethrin-PLA microspheres with different particle sizes and surface characteristics by adjusting the PLA concentration and stirring speed. The process is easy to scale up and efficient. Depending on the diameter, 100–800 mg of drug-loaded microspheres can be obtained per 100 mL of the reaction system. It is found that as the diameter decreases, the porosity of the pyrethrin-PLA microsphere

CRediT authorship contribution statement

Yan Li: Writing - original draft, Writing - review & editing, Methodology, Funding acquisition. Duo Cao: Visualization, Project administration. Feng-an Jia: Data curation. Fan Chang: Resources. Rui Lv: Validation. Jia-kun Dai: Conceptualization, Supervision, Funding acquisition.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This work was supported by the Key Research and Development Program of Shaanxi Province [No. 2017NY-158], the Science and Technology Plan Project of Shaanxi Academy of Science [No. 2017k-07], and the Science and Technology Plan Project of Xi'an [2019216614GXRC002CG003-GXYD3.0].

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    Therefore, controlling microsphere size and surface properties is necessary for their biomedical application. Various techniques have been used to produce polymer microspheres [8–11]. The emulsion solvent evaporation method is arguably the simplest to employ and can also be promptly optimized [12].

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