In modern agriculture, controlled-release formulations (CRF) is greatly desirable for achieving valid utilization and loss reduction of pesticides. It can also alleviate the adverse effect on non-target organism. In this study, we used PDA as a depositing layer, constructed pyraclostrobin@SiO 2 @polydopamine microcapsule (Pyr@SiO 2 @PDA MC). The resulting microcapsule is a near-rod shape (about 1.152μm), which has a drug-loading efficiency of 55%. FT-IR and TG analysis revealed the successful entrapment of the pesticide. The microencapsulation had important function in the slow-release rate of the pyraclostrobin and superior UV-shielding properties. Moreover, bioactivity of the microcapsule against Fusarium oxysporum f.sp.vasinfectum was determined and it exhibited better inhibition activity than pyraclostrobin technical in the later stage. At last, acute toxicity tests demonstrated that Pyr@SiO 2 @PDA MC on zebrafish with lower toxicity on the first day. These results showed that Pyr@SiO 2 @PDA MC may process broader application potential in agriculture.
Research article
Fabrication a controlled-release pesticide for improving UV-shielding properties and reducing toxicity via coating polydopamine
https://doi.org/10.21203/rs.3.rs-28348/v1
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published 05 Apr, 2021
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Pyraclostrobin
Polydopamine
UV-shielding properties
Sustained release
Bioactivity
Acute toxicity evaluation
Pyraclostrobin (Fig. 1) is a methoxy acrylate fungicide developed by BASF in 1993 1. It acts on the cytochrome bcl complex in the fungal mitochondrial respiratory chain, preventing electron transfer and inhibiting mitochondrial respiration, making mitochondria unable to produce the energy (ATP) required for normal metabolism of cells, leading to cell death 2,3.Pyraclostrobin (pyr) has a very broad bactericidal spectrum with protective and therapeutic effects on crops 4. There is, however, a high ecological safety risk when used in paddy fields, which restricts the application of pyraclostrobin to a certain extent 5,6. A viable strategy to reducing the accumulation of pesticides in a water environment is to develop controlled release formulations (CRFs). Li fabricated pyr microcapsules (MCs) with Fe3+ and tannic acid, which showed excellent effcacy on rice blast and signifcantly lower toxicity to four model organisms 7. However, the drug loading of the microcapsules is not mentioned in this manuscript, and we cannot know the loading capacity of the carrier for pyraclostrobin. In addition, the preparation process needs to rely on the chelation of calcium lignosulfonate molecules with Fe3+ to prepare a pre-film and then perform a second film formation, which is cumbersome.
Controlled release technology (CRT) has been extensively employed in industries and fields such as pharmaceuticals, coatings, cosmetics, etc 8–12. In the field of pesticide formulation processing, CRT is also an ideal choice to advance the efficiency of pesticides and reduce environmental pollution. Therefore, it has become an important direction for the development of new pesticide formulation 13–15. To date, a host of inorganic materials like SiO2, and various polymers are used as a carrier to deliver the drug in order to achieve controlled-release 16–18. Study on silica has attracted a wide range of attentions due to its prominent properties, such as facile preparation, low cost, polymer coatings, etc 19–21. More importantly, it has a significant impact on enhancing plant tolerance against biotic and abiotic stresses 22. These advantages make it an ideal and suitable choice for drug delivery in agriculture.
Dopamine (Da), which can form an adherent polydopamine (PDA) coating in a weakly alkaline solution through oxidative self-polymerization 23. In addition, PDA coating can also be used for other reactions because of its abundant functional groups (amino, imino, and catecholyl) 24,25. Xin prepared a PDA coated avermectin microcapsules (Av@PDA MC) with prominent sustained-release performance and good adhesion properties 26. Gao fabricated a novel imidacloprid microcapsule with PDA followed by PU exhibited excellent dispersion stability 27. Undeniably, PDA is considered to be a promising pesticide encapsulant for obtaining novel controlled release formulation 28–30.
Microcapsules are one of the microparticulate systems which have shown tremendous potential and are used for controlled-release formulation and to improve the environment security and persistence of agrochemicals 31,32. Pesticide microcapsules made of reliable material as carrier could increase length of activity as well as reduce side effects on the environment. In the present work, a simple method has been attempted to encapsulate pyr for acquiring high drug-loading efficiency. The preparation of our microcapsules and the drug loading process are performed simultaneously to obtain Pyr@SiO2 MC. This can effectively improve the preparation efficiency. The deposition of PDA on the surface can give microcapsules excellent UV shielding properties. Prepared Pyr@SiO2@PDA MC was characterized on physicochemical properties including size, morphology, drug loading, release behavior and UV-shielding. The bactericidal activities againsted Fusarium oxysporum f.sp.vasinfectum as well as the acute toxicity to zebrafish were also investigated.
Materials
Pyraclostrobin technical (97% TC) was provided by Jiangsu Subin Agrochemical Co., Ltd (Jiangsu China). 250g/L Pyraclostrobin emulsifiable concentrate (EC) was purchased from BASF (Germany). Dopamine hydrochloride(DA) and Tris-HCl(1.5M, pH = 8.8) were supplied by Beijing solarbio science&technology Co., Ltd. Cetyl trimethyl ammonium bromide (CTAB) and tetraethyl orthosilicate (TEOS) were purchased from Aladdin reagent (Shanghai) co., Ltd. Ethyl acetate, tween–80 emulsifier, methanol and the other reagents were came from Sinopharm group chemical reagent co. Ltd and used as received without further purification. The chemicals what we used for the experiments were of analytical grade.
Preparation of Pyr@SiO2@PDA microcapsule
Microcapsules were prepared as reported by literature with some modification 33. Firstly, 0.5% w/v water phase was obtained by dissolving 0.5g cetyl trimethyl ammonium bromide in 100.0mL of deionized water. Next, 0.5g Pyr was dissolved in a mixture ethyl acetate and tetraethyl orthosilicate to prepare oil phase. Then, the oil phase was mixed with the aforementioned water solution, ammonia water (25%–28%, v/v) was added dropwise to adjust its pH to 8. The mixture was agitated in a magnetic stirrer (C-MAG HS 7, IKA Company, Germany) with the rotation speed 600rpm for 2h, placed at room temperature overnight. Subsequently, DA and 200μl Tris-HCl was added and stirred at room temperature for 24h (Fig. 2). After centrifugation, the solution was washed at least three times with deionized water and dried under vacuum at 45°C. The dried sample was named as Pyr@SiO2@PDA MC. In order to obtain the highest loading content of pyr, different amount of DA were used to prepare the microcapsules (Table 1).
Characterization of the Pyr@SiO2@PDA microcapsules
Particle size of Pyr@SiO2@PDA MC was measured by Mastersizer 2000 (Malvern instruments Ltd., Britain). The particles were suspended in deionized water for measurement, and each sample was measured three times. The appearance of the sample was observed by a JMS - 6360 scanning electron microscope (SEM; Tokyo, Japan) at 25 KV. Nicolet-IS 5 (United States) was used to record the characteristic absorption peak of the products in range of 4000–500 cm–1 at room temperature. For purpose of confirming the existence of the Pyr and determine loading ratio, thermogravimetric analysis (TG) was conduced over the temperature range from 25 to 600 ◦C at 10 ◦C/min rate by mean of Mettler Toledo thermal analyzer (TGA 2). Zeta potential was tested by micro-electrophoresis apparatus (JS94H2 Shanghai Zhongchen Digtal Technology Apparatus Co., Ltd., China). Equation 1 was utilized to calculate the effective components in the microcapsule 34. Samples containing the same quality of Pyr, including Pyr TC, Pyr@SiO2 MC, Pyr@SiO2@PDA MC were enclosed in quartz tube. Ultraviolet (UV) light was selected as light source, with the irradiance of 3.00 mW/cm2, rated power of 1.5 W × 2, at a distance of 15 cm, and set the dark control. Sampling was performed at 1, 2, 4, 8 and 10h, respectively. The released amounts of Pyr were monitored by HPLC (Agilent Technologies Inc., America) and the photodegradation rate of samples was calculated according to Equation (2).
In vitro release studies
The drug-loaded microcapsule was placed in a dialysis bag (MWCO 3500 Da) and immersed in 200.0 mL of release medium(phosphate buffer saline, ethanol and tween–80 emulsifier, 140:59:1 v/v/v), maintaining the temperature at 25±1◦C. Some of the solution was withdrawn at appropriate intervals for the HPLC assay, and the same amount of fresh release medium was added after each sampling at predetermined time intervals to keep a constant volume. Meanwhile, for the control study, 97% Pyr TC was used as a reference under the same condition.
Indoor virulence assay
Fungicidal activity of Pyr@SiO2@PDA MC was assessed by the growth rate method using Fusarium oxysporum f.sp.vasinfectum as the tested strain, and the 250g/L Pyr EC was used as a control dosage form 35. The tested strain was isolated and purified by the Laboratory of Pesticide Science, College of Plant Protection, Hunan Agricultural University. Firstly, the medicament was formulated into five concentrations of drug-containing medium at a concentration of 5, 10, 50, 100, and 200 mg/mL. Subsequently, the fungicidal evaluation under the first, third, fifth, seventh, and ninth days was then performed. Finally, the cross method was applied to determin the colony diameter, and the concentration for 50% of maximal effect (EC50) was calculated according to the formula (3). The control group was treated with sterile water of the same volume, each treatment was performed at least in triplicate.
Acute toxicity tests on Zebrafish
In order to evaluate the acute toxicity of the sample to aquatic organisms, zebrafish, which respond quickly and inexpensively, was used as a model organism. The zebrafish was the same as the zebrafish used in the previous experiments 36, purchased from a commercial supplier in Changsha China, weighing 0.2 to 0.4g and having an average length of 2.0 to 3.0cm. Acute toxicity tests were carried out according to guideline for “environmental safety evaluation tests of chemical pesticides standards" 37. The zebrafish were carried out in accordance with the basic principles of animal experiments at Hunan Agricultural University, and were approved by the Pesticide Chemistry Laboratory of the College of Plant Protection.
Acute toxicity studied were performed in a 96h semi-static test 38,39. Briefly, placed 10 tails of adult fish in 3.00 L aerated water with diverse concentrations of agents(Pyr@SiO2@PDA MC and Pyr EC) ranging from 0.02 to 0.12 mg/L. Control experiments with only aerated water were consistent with the test group. Three repetitions were set for each experiment. Keep room temperature at 25±1°C and light/dark cycle (16 hr light/8 hr dark) constant. Dead fish being removed in order to avoid affecting the test. With SPSS, software available in computer, LC50values and their 95% confidence limits could be obtained at 24, 48, 72, and 96 h, respectively.
Effect of dopamine addition on microcapsules
The particle size of microcapsules increased with the addition of DA, but decreased when DA increased to 80mg (Table 1).This may be due to the limited specific surface area that can be contacted by the microcapsule. As the covered PDA reaches an extreme value, increasing the dosage of DA, particle size will not continue to increase. Besides, the drug load will first increase and then decline with the increase of DA. When the addition amount of DA was 40mg, the drug load could reach 55%, which was higher than the single- and double-shelled cyhalothrin/SiO2 microcapsules (about 50.0% and 25.0%, respectively) 40. As depicted in Figure 3a, the color of the solution gradually deepens with the increase of DA. After tableting, samples S1, S2 and S3 were grayish white, grayish black and dark gray, respectively (Fig. 3b). The color of microcapsule samples became darker with the increase of DA content.
Characterization of samples
Samples S1, S2 and S3 were examined under different magnifications. The microstructure of the microcapsule is built on rod-like structure coated by PDA. Altered levels of DA showed different degrees of agglomeration. When the content of DA increased from 10mg to 20mg, the dispersion of the particles could be effectively improved and the coating effect of the PDA could be better seen (Figs. 4a-f). After adding more DA to the particles, the rod-like structure of the particles is destroyed and agglomeration occurs. In this case, the obtained particles visibly presented irregular shape (Figs. 4g-i). Finally, combining the results of drug loading and SEM, S2 (in Table 1) was selected for further study.
TG and DTG analysis of SiO2@PDA MC and Pyr@SiO2@PDA MC in nitrogen atmosphere have been carried out to further vertify the presence of the pry. As shown in Fig. 5a, TG analysis was also carried out for quantitative analysis. The curves of SiO2@PDA MC and Pyr@SiO2@PDA MC showed weight loss in the range of 300–600 ◦C due to the decomposition of PDA 41. A new weight loss started at about 200 ◦C, which was assigned to the decomposition of pyr. The weight loss of bare SiO2@PDA MC was about 15%, but that of Pyr@SiO2@PDA MC ascended to 73%. Therefore the weight of pyr loaded in the samples was about 58%. It also illustrated the successful encapsulation of pyr in the Pyr@SiO2@PDA MC. The calculation found that the drug loading rate was only 55%, which was significantly lower than that obtained by TG analysis (58%). This may be because despite grinding and ultrasound, the samples were not completely destroyed and some drugs were not completely released, resulting in a low calculated drug loading rate.
Zeta potential was used to monitor the change of particle potential before and after PDA coating. Potential beginning with the silica was −16.05 mV and turned to –20.11 mV after the deposit of PDA layer (shown in Fig.6). Since the Pyr@SiO2 MC have a large amount of Si-OH groups, the zeta potential values appeared to be negative at first, and when the PDA is coated, the PDA also has a large amount of -OH and -NH showing negative zeta potetntial so that the negative potential of the particle band increases 42. Zeta potential values shifted with the cladding of the PDA layers, which indicated the successfully deposition of the PDA.
IR Spectrum Analysis
The infrared spectroscopy of DA, PDA, SiO2, Pyr@SiO2 MC and Pyr@SiO2@PDA MC were performaned by FTIR spectra, which were utilized to verify successful encapsulation of pesticide. DA had many narrow peaks, which were characteristic of small molecule (Fig. 7a); while PDA showed only a few main peaks: the main peak of the aromatic ring was about 1623 cm–1, and the main peak of the catechol-OH group was about 3446 cm–1 43. In the FTIR spectra of the microcapsule, the main peaks of SiO2 and PDA overlapped with that of the Pyr (Fig. 7b). The infrared spectrum of the SiO2 exhibited a typical peak at 1100 cm–1(Si-O-Si) corresponding to the primary silica bonds 44. Compared with SiO2, the characteristic absorption peak of drug-loaded microcapsules appeared at 1717 cm–1, which was attributed to the stretching vibration of C = O in Pyr. Besides, a peak corresponding to the stretching vibration of Si-O-Si appeared at 1100 cm−1. The characteristic absorption peak from Pyr@SiO2@PDA MC (3446 cm–1) appeared as a wider peak of -OH groups compared to Pyr@SiO2 MC. The results stated that Pyr was encapsulated into the microcapsule. What’s more,no new IR peaks indicated no chemical reactions occurred 45.
Studies of the UV-shielding properties of the PDA layer for Pry
Statistically, plenty of applied pesticides are lost due to hydrolysis and photolysis 46. Effectively encapsulation can protect the active ingredients and prolonging the duration 47. The UV-shielding properties of the PDA carrier was studied in ethanol/water solution with different samples. Fig. 8 hows the photolysis curves of Pyr TC, Pyr@SiO2 MC and Pyr@SiO2@PDA MC. In the first two hours, the photolysis rates of the microcapsules were close to each other. After 4h of UV illumination, the photodegradation rate of TC was nearly 50%, while those of the Pyr@SiO2 MC and Pyr@SiO2@PDA MC were 22.10% and 18.66% respectively. The photodegradation rate of Pyr TC, Pyr@SiO2 MC were 2.2 times and 1.5 times than that of Pyr@SiO2@PDA MC after 12h of illumination. These results clearly showed that the PDA carrier had remarkable UV shielding properties, which can significantly reduce the photodegradation rate of Pyr and improve its stability. In addition, Tong 48 found that GO with PDA layer had obvious adhesion properties. When PDA coating was added, the persistence of pesticides on cucumber leaves was improved after simulated rain-wash test. Therefore, PDA carrier was an ideal carrier to extend the service life of pesticides.
Drug Release Property
The cumulative release curves of Pyr from the TC and Pyr@SiO2@PDA MC are shown in Figure 9 After 12 h immersed in phosphate buffer saline, the cumulative release rate of Pyr from TC was more than 35% w/w, compared to 10% w/w when loaded in microcapsule (Fig.9a). After 24h, the Pyr released from TC was over 80% w/w; however, the obtained microcapsule released less than 15% w/w of Pyr after 24h immersed, and 38% w/w after 192h. The cumulative release of the Pyr from TC is up to 98% w/w after 192h. It seems that Pyr@SiO2@PDA MC liberated much lesser Pyr molecules at the same period. Result revealed that the active ingredients from Pyr@SiO2@PDA MC was released slowly in vitro and the diffusion time was longer. Due to the presence of the PDA coating, the microcapsule was a good option for controlling the long-term release of the Pyr, which also helped to extend the duration.
Fungicidal activity of Pyr@SiO2@PDA microcapsule
With sufficiently demonstrated characterization of the Pyr@SiO2@PDA MC in size, morphology, IR and release property, we began to study its bactericidal activity. The results of the indoor virulence test are set out in the table 2. At the test concentration, Pyr@SiO2@PDA MC was significantly inferior to the EC in inhibiting the growth of mycelium. On the fifth day after the administration, the fungicidal activity of microcapsule was similar to that of the EC. Very interestingly, the EC50 values of MC and EC at ~9d post administration were 5.62 and 85.11 mg/mL, respectively. The former is about 15 times more toxic than the latter. With the extension of time, the active ingredients are continuously released in the microcapsule while are gradually decomposed in the EC. Thus, the virulence of the microcapsule gradually surpassed that of the EC, showing a very obvious sustained release effect.
Toxicity Evaluation
During the experiment, no death or abnormal behavior was detected in the blank control group of zebrafis. In contrast, the death has begun in the treatment group on the first day. At the beginning, after contact with the high-concentration EC, the zebrafish immediately turned upside down, swam rapidly, and jumped out of the water. Subsequently, poisoned zebrafish reacted dull and the swimming speed was slow. When fish died, their abdomen and cheeks turned reddish (Fig. 10)
The LC50 values for 24–96h and their 95% confidence limits were expressed in Table 3. LC50 values of Pyr EC and Pyr@SiO2@PDA MC in 24h were 0.065 and 0.118 mg/L. The value of LC50 was between 0.1 mg a.i./L and 1 mg a.i./L. Thus, according to guideline for “environmental safety evaluation tests of chemical pesticides standard” (GB/T 31270.12–2014), virulence of MC on zebrafish was high toxicity. While the Pyr EC was extreme toxicity(LC50≦0.1 mg a.i./L). The toxicity of MC to zebrafish was lower in the first 48h compared with Pyr EC (Table 3). This indicated that effective encapsulation can reduce the toxicity of pesticides to non-target organisms to some extent. This conclusion was similar to the result reported by Xu. He used poly(2-dimethylaminoethylmethacrylate) (PDMAEMA) and chitosan (CS) fabricated CS-g-PDMAEMA microcapsules. LC50 (24h) values of Pyr@CS-g-PDMAEMA were 0.1020 mg/L, which was less toxic than the control 49. While, in the next 48 h, the acute toxicity of MC against zebra fish became comparable with that of Pyr EC. LC50 (96 h) values of Pyr EC and Pyr@SiO2@PDA MC were 0.049 and 0.062 mg/L, respectively.
In this work, we have fabricated the Pyr@SiO2@PDA microcapsule in a straightforward way. The pyr-loaded microcapsules with maximum drug loading of 55% demonsrated sustained release for up to 200 hours. Contrasting with free pyr, PDA could effectively improve the photostability of pesticides under ultraviolet light. It comes up with an effective strategy for protecting such photounstable pesticide. The obtained microcapsule processes longer duration against Fusarium oxysporum f.sp.vasinfectum compared to traditional formulation (250g/L Pyr EC). On the other hand, a reduction in the amount of pesticide used helps to alleviate environmental pollution. Moreover, microcapsulation could provide a degree of protection against zebrafish on the first 24 h. Consequently, this formulation could enhance utilization efficiency while exert less unintended negative effects on environment. Therefore, successful development of Pyr@SiO2@PDA microcapsule has made it possible to improve its application properties.
Ethics declarations
Consent statement
The experimental animal programs involved in this experiment were carried out in accordance with the basic principles of animal experiments at Hunan Agricultural University, and were approved by the Pesticide Chemistry Laboratory of the College of Plant Protection.
Competing interests
The authors declare that they have no competing interests.
Funding
This work was funded by the National Key Research and Development Program, Special Research and Development of Fertilizers and Pesticides (Yfd0201200, 2016).
Acknowledgments
The author would like to thank all the authors for their guidance and help with the experiment. Thanks to the Hunan Agricultural University Testing Center for characterization of the materials.
Data availability
All data generated during this study are included in this published article and additional data will be shared upon request to the corresponding author.
Contributions
WY designed the study; WY and LCN performed the experiments and wrote the manuscript; ZX, CW and ZY analysed the data. All authors read and approved the final manuscript.
Pry: pyraclostrobin
PDA: polydopamine
MC : microcapsule
FT-IR: Fourier transform infrared spectroscopy
TG: Thermogravimetric Analysis
CRFs: controlled release formulations
CRT: controlled release technology
Da: dopamine
Av: avermectin
PU: polyuria
EC: emulsifiable concentrate
CTAB: cetyl trimethyl ammonium bromide
TEOS: tetraethyl orthosilicate
TC: technical
EC50: concentration for 50% of maximal effect
LC50: lethal concentration 50
HPLC: High Performance Liquid Chromatograph
SEM: scanning electron microscope
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Table 1 Composition of the microcapsule prepared for carrying purposes
|
Sample |
DA (mg) |
Particle size (μm) |
Loading efficiency (%) |
|
S1 |
20 |
1.126±0.13 |
50.21±0.23 |
|
S2 |
40 |
1.152±0.24 |
55.00±0.41 |
|
S3 |
80 |
0.980±0.14 |
53.38±0.66 |
Note: Results are reported as mean ± SD (n = 3)
Table 2 The result of indoor toxicity determination of pyraclostrobin EC and Pyr@SiO2@PDA MC against Fusarium oxysporum f.sp.vasinfectum
|
Tested agents |
Investigation time (d) |
Equation （y=）
|
EC50 value（mg/mL） r |
Related coefficient (R) |
|
pyraclostrobin emulsifiable concentrate (EC)
|
1 |
4.794+1.032x |
0.58±0.07 |
0.946 |
|
3 |
4.168+1.008x |
6.76±0.38 |
0.957 |
|
|
5 |
3.698+0.909x |
26.9±0.75 |
0.983 |
|
|
7 |
3.357+0.992x |
45.32±0.91 |
0.975 |
|
|
9 |
3.228+0.885x |
85.11±1.21 |
0.976 |
|
|
Pyr@SiO2@PDA microcapsule (MC) |
1 |
3.285+0.876x |
91.20±1.34 |
0.986 |
|
3 |
3.149+1.115x |
46.24±0.85 |
0.954 |
|
|
5 |
3.955+0.816x |
19.05±0.52 |
0.960 |
|
|
7 |
3.672+1.298x |
10.47±0.47 |
0.995 |
|
|
9 |
3.973+1.366x |
5.62±0.42 |
0.981 |
Note: Results are reported as mean ± SD (n = 3).
Table 3. Acute toxic effects of microsphere to zebrafish
|
Agents |
Time （h） |
Toxicity equation (y=) |
Correlation (R2) |
LC50 ( a.i. mg/L) |
95%confidence limits ( a.i. mg /L) |
|
pyraclostrobin emulsifiable concentrate (EC) |
24 |
4.050+3.800x |
0.961 |
0.065±0.017 |
0.053-0.089 |
|
48 |
4.392+3.655x |
0.919 |
0.063±0.015 |
0.047-0.099 |
|
|
72 |
3.915+3.169x |
0.902 |
0.058±0.012 |
0.037-0.125 |
|
|
96 |
2.368+1.802x |
0.960 |
0.049±0.010 |
0.042-0.057 |
|
|
Pyr@SiO2@PDA microcapsule (MC) |
24 |
2.213+2.382x |
0.986 |
0.118±0.028 |
0.097-0.157 |
|
48 |
2.992+2.888x |
0.978 |
0.092±0.026 |
0.081-0.124 |
|
|
72 |
2.987+2.539x |
0.993 |
0.067±0.013 |
0.059-0.077 |
|
|
96 |
3.238+2.685x |
0.977 |
0.062±0.011 |
0.056-0.070 |
Note: Results are reported as mean ± SD (n = 3). The a.i. in the table represented available ingredient.