Dual pH responsive azoxystrobin controlled release microspheres constructed from porous calcium carbonate and its biosecurity

Pesticides play a crucial role in modern agricultural production. However, traditional pesticide formulations suffer from low target utilization and serious environmental pollution during use, which not only leads to huge economic losses but also poses serious hazards to human health. Therefore, the development of stimuli-responsive pesticide controlled release systems provides a powerful strategy to improve pesticide utilization efficiency and reduce environmental pollution. In this paper, a drug-carrying calcium carbonate microsphere encapsulated with tannic acid and Cu complexes was constructed by co-precipitation-adsorption method to explore its physicochemical properties, release properties and safety studies.

In this study, porous calcium carbonate microspheres (CaCO) were prepared by co-precipitation method using soluble starch, calcium acetate and ammonium carbonate as raw materials, and then porous calcium carbonate microspheres loaded with pyrimethanil (Az) were obtained by impregnation and adsorption method (Az/CaCO), and tannic acid-Cu complex was further coated on the surface of the composite microspheres, to construct a pyrimethanil with a bi-pH responsive controlled release system (Az/CaCO@TA-Cu).

We analyzed the morphology and structure of Az/CaCO@TA-Cu in detail using scanning electron microscopy (SEM), energy spectrometry (EDS), Fourier transform infrared (FTIR), thermogravimetric analyzer (TGA), and Malvern's Zeta potentiostat. Next, we investigated the release behavior of pyrimethanil in soil and examined the responsive release properties at different pH values. In addition, we evaluated the bacteriostatic and biosafety properties of Az/CaCO@TA-Cu by mycelial growth rate experiments and zebrafish safety tests. (1) Electron microscopy (SEM) and energy spectrometry (EDS) studies demonstrated the successful encapsulation of Az/CaCO by tannic acid-Cu complexes. Fourier transform infrared (FTIR), thermogravimetric analyzer (TGA), and Malvern zeta potentiostat further demonstrated the successful encapsulation of Az/CaCO by tannic acid-Cu complexes and measured the drug loading of 16.42%. (2) Simulated release studies on the release behavior of pyrimethanil in soil and the responsive release properties at different pH values were investigated, and the results showed that Az/CaCO@TA-Cu had good slow and pH-controlled release properties, and the cumulative release rate in phosphate buffer solution at pH = 7 for 96 h was 36.99%, whereas the cumulative release rates at pH = 5 and pH = 9 conditions were respectively 74.32% and 58.79%, respectively. (3) The inhibitory effect of different samples on Fusarium graminearum was determined by mycelial growth rate. The mycelial growth rate experiments showed that Az/CaCO@TA-Cu had a better inhibitory effect on the growth of Fusarium graminearum, with median inhibitory concentrations of 6.58 and 3.28 times those of pure Az and Az/CaCO, respectively. (4) Pyrimethanil, a methoxyacrylate fungicide, is susceptible to wheat and significantly inhibits seed germination under high dose conditions. For this reason, we investigated the effect of different formulations on the germination rate of wheat seeds through wheat seed germination experiments. Wheat germination rate statistics showed that Az/CaCO@TA-Cu exhibited superior biosafety relative to Az/CaCO and pure Az. (5) The biosafety of the different agents was further compared using zebrafish acute toxicity test. The toxicity of Az/CaCO@TA-Cu to zebrafish was 1.2~4.6 times lower than that of Az and Az/CaCO. The results showed that Az/CaCO@TA-Cu showed better biosafety for zebrafish than Az and Az/CaCO.Conclusion: The pesticide carrier system with dual pH-responsive release function (Az/CaCO@TA-Cu) was prepared by surface coating of tannic acid-Cu complexes based on Az/CaCO prepared by co-precipitation-adsorption method. SEM and EDS elemental mapping verified the structure and composition of Az/CaCO@TA-Cu. ζ-potential, infrared absorption spectroscopy and thermogravimetric analyses further verified the successful loading of pyrimethanil as well as the successful coating of tannic acid-Cu complex. In the simulated release studies, Az/CaCO@TA-Cu exhibited a pH-responsive release function, and both acidic and alkaline environments promoted the release rate of Az in the loading system. The bacterial inhibition experiments showed that Az/CaCO@TA-Cu had good bacterial inhibition performance against Fusarium graminearum. In addition, Az/CaCO@TA-Cu showed good biosafety against wheat germination and zebrafish.