Development of Novel Urease-Responsive ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Development of Novel Urease-Responsive Pendimethalin Microcapsules Using Silica-IPTS-PEI As Controlled Release Carrier Materials You Liang, Mingcheng Guo, Chen Fan, Hongqiang Dong, Guanglong Ding, Wenbing Zhang, Gang Tang, Jiale Yang, Dandan Kong, and Yongsong Cao* College of Plant Protection, China Agricultural University, Beijing, China ABSTRACT: Microcapsules are highly desirable for attaining the most effective utilization of the pesticide as well as reducing environmental pollution. In this work, a novel urease-responsive system was prepared using isocyanate-functionalized silica cross-linked with polyethylenimine (silica−IPTS−PEI) via urea bonds. The results demonstrated that the silica−IPTS−PEI microcapsules had a high pendimethalin loading efficiency (approximately 30% w/w) and could effectively enhance the thermal and light stability of pendimethalin. The release curves agreed with the Ritger and Peppas equation, and the release of pendimethalin was diffusion-controlled. The release rates of synthesized silica-IPTS-PEI microcapsules showed positive correlation with the temperature. In weak acid and base conditions, the pendimethalin release rates were higher than under neutral conditions, and the silica−IPTS−PEI microcapsules displayed excellent urease-responsive property with controlled release performance. Compared with pendimethalin emulsifiable concentration, the silica-IPTS-PEI microcapsules had a longer duration and higher herbicidal activity against weeds in a greenhouse experiment. Allium cepa chromosome aberration assays showed that the microcapsules had lower genotoxicity than pendimethalin technical. Thus, the urease-responsive silica−IPTS−PEI microcapsules have a great potential application as an environmentally friendly herbicide formulation. KEYWORDS: Pendimethalin, Urease-responsive, Microcapsules, Silica, Controlled release, Genotoxicity

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INTRODUCTION The widespread use of pesticides in agriculture increases the yield and quality of crop production. However, approximately 90% of the pesticides are lost during application due to their volatilization, degradation, photolysis, leaching, and runoff, and less than 0.1% of the pesticides finally reach the harmfully biological targets.1,2 The long-term excessive application of pesticides has induced adverse effects on the environment and human health.3,4 Controlled release technology in pesticides is an effective way to solve these problems. Previous research has reported that encapsulation of pesticides into polymeric carriers could achieve continuous and stable release of active ingredients, which would prolong the effective duration and reduce the application amount of pesticides.5,6 The microcapsules have many advantages compared to traditional pesticide formulations, which include reduced plant, fish, and mammalian toxicity; improved storage stability; and protected pesticides from environmental hazards.7−9 Various synthetic polymers have been used as shell materials to prepare pesticide microcapsules, such as polyurethane, polyamide, polystyrene, and polyvinyl chloride.10−12 Unfortunately, the biodegradation rate of some polymers is difficult to control because of their hydrophobicity and semicrystalline morphology, which has resulted in secondary pollution.13,14 © 2017 American Chemical Society

The mesoporous silica sphere is a good candidate for encapsulation of drug molecules attributing to its low production cost, good biocompatibility, high drug loading capability, and being easily functionalized.15−20 However, the release rate, pattern, and duration of single silica-shelled microcapsules are not well controlled, which limits the application of the microcapsules in the field of agriculture.21 Currently, stimuli-responsive double-shelled microspheres have emerged as promising candidates to improve the controlled release properties and realize intelligent release of pesticides. Several studies have reported that stimuli-responsive systems could respond to plenty of stimuli including pH, enzymes, redox, temperature, and ultrasound.22−26 Among different types of stimuli, enzymes have attracted much attention due to their high specificity, accuracy, and efficiency for certain substances.27,28 Pendimethalin (N-(l-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine) is a dinitroaniline herbicide used for the selective control of weeds in a variety of crops, such as cotton (Gossypiumhirsutum L.), maize (Zea mays L.), tomato Received: January 19, 2017 Revised: April 13, 2017 Published: April 19, 2017 4802

DOI: 10.1021/acssuschemeng.7b00208 ACS Sustainable Chem. Eng. 2017, 5, 4802−4810

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ACS Sustainable Chemistry & Engineering

RPW) seeds were obtained from the China Agriculture University experiment station. Preparation of Pendimethalin Microcapsules. Synthesis of Silica Microcapsules. The silica microcapsules with pendimethalin were synthesized by using the modified reference methods.18 First, 1.5 g of CTAC was dissolved in 89 mL of deionized water (15 g/L of the whole system) to obtain the aqueous phase. Then, 0.5 g of pendimethalin and 4 mL of TEOS were dissolved in 5.0 mL of ethyl acetate to obtain the oil phase. A stabilized oil/water emulsion was prepared by adding the aqueous phase to the oil phase using a homogenizer at 6000 rpm. Subsequently, 0.1 mL of ammonia solution (0.9 mol/L) was added dropwise into the system and stirred at 300 rpm for 4 h. After allowing it to age 8 h at room temperature and washing it with distilled water three times, the single-shelled silica microcapsules were obtained. The preparation process of silica microcapsules without pendimethalin was the same as that of producing single-shelled silica microcapsules except that the oil phase was obtained by mixing 4 mL of TEOS and 5.0 mL of ethyl acetate. Synthesis of Isocyanate-Functionalized Silica Microcapsules. Approximately 1.0 g of the single-shelled silica microcapsules in 100 mL of toluene was sonicated for 1 h, and 2.0 mL of IPTS was added into the previous suspension under a nitrogen atmosphere. The resultant precipitate was centrifuged at 6000 rpm for 10 min and washed thrice with toluene. The dry isocyanate-functionalized silica microcapsules were obtained after drying at 60 °C. Synthesis of Silica−IPTS−PEI Microcapsules. Approximately 1.0 g of isocyanate-functionalized silica microcapsules and 0.2 g of PEI were dissolved in tetrahydrofuran. Then PEI solution was added dropwise into the microcapsules suspension. The mixture was stirred for 2 h at room temperature and washed with tetrahydrofuran several times. Finally, the dry silica−IPTS−PEI microcapsules were obtained after drying for 24 h at 60 °C. Characterization. The morphology and structure analysis of the samples were measured by scanning electron microscopy (Hitachi S4800, Japan) at an acceleration voltage of 20 kV. The functional groups present in the microcapsules were analyzed using a Fourier transform spectrophotometer (Jasco 5300, Japan) by using the KBr disc method, and spectra were obtained in transmission mode in the region of 4000−450 cm−1 at a resolution of 4 cm−1.42 The size distribution of the samples was determined by a Mastersizer 3000 laser diffraction particle size analyzer (Malvern, UK). The heat stability of the microcapsules at a heating rate of 10 °C/min from 25 to 700 °C was measured by a SDT-Q600 thermogravimetric analyzer (TA Instruments-Waters LLC, USA). The concentration of pendimethalin was monitored by high performance liquid chromatography (HPLC) with an ultraviolet detector (Shimadzu, Japan). The chromatographic separation was conducted by a Kromasil ODS C18 column (250 mm × 4.6 mm, 5 mm; Dikma, USA) equipped with a guard column (4 mm × 3 mm). A flow rate of 1 mL/min was carried out with a mobile phase composition of acetonitrile and 1 g/L acetic acid (80:20, v/v). The injection volume was 20 μL, and the column temperature was at room temperature. All the solvents used for HPLC measurements were filtered with a 0.45 μm membrane filter. The Stability of Silica−IPTS−PEI Microcapsules. To examine the stability of silica−IPTS−PEI microcapsules under different conditions, approximately 2 mg of silica−IPTS−PEI microcapsules was added into 20 mL of methanol−water solvents (30:70, v/v) and packed in glass tubes. The samples were stored at different temperatures (40, 50, and 60 °C) for 60 days; then the effect of temperature on the release of the microcapsules was analyzed. The UV-shielding properties of the microcapsules were investigated by exposing the samples to UV light (illuminated wavelength: 254 nm light intensity at 20 cm = 700 μW/cm2) for 72 h, and the pendimethalin concentration changes were analyzed by HPLC. The technical grade pendimethalin was used as control at the same time. Controlled Release Kinetics. The release properties of the encapsulated pendimethalin from the silica−IPTS−PEI microcapsules at different pH values, temperatures, and enzymes were investigated by

(Lycopersicon esculentum L.), and wheat (Triticum aestivum L.).29 The most widely used formulation type of pendimethalin is emulsifiable concentrate due to its inferior water solubility (0.3 mg/L at 20 °C).30,31 However, pendimethalin is easily lost through volatilization and photodegradation, and can be absorbed by a variety of soils, which leads to the utilization efficiency decrease of pendimethalin.32,33 In addition, traditional pendimethalin microcapsules have usually been observed to have a large burst release during the first 1−24 h.34 Therefore, there is a compelling need to design a novel pendimethalin formulation, which can reduce the loss of volatilization and photodegradation, improve the utilization of pendimethalin, and achieve sustained release. Soil is a living dynamic system containing many free enzymes, immobilized extra-cellular enzymes, and enzymes within microbial cells.35 These enzymes are generally of bacterial or fungal origin, and their properties are similar to enzymes in other systems. The enzymes mostly found in soil are urease, alkaline phosphatase, dehydrogenase, and catalase.36 The reaction rates of these enzymes have a positive correlation with soil temperature and moisture.36,37 Previous research has also reported that weed germination and growth are distinctly dependent on soil temperature and moisture.38,39 So soil enzymes were selected for the stimuli-responsive system in this study. Among the various soil enzyme-responsive pesticide delivery systems, the urea bond was of great interest because it can be designed intelligently and cleaved easily by urease.40 Several studies have reported that urea bonds were formed by the reaction of amines with isocyanates or carbamates.27,41 Hence, by introducing urea bonds between functionalized mesoporous silica microcapsules and polymers with amino groups, a urease-responsive delivery system of pendimethalin was established. Initially, pendimethalin was encapsulated into the silica shell through an interfacial polymerization method using tetraethyl orthosilicate as a precursor. And then the resulting silica microcapsules were modified with 3-isocyanatopropyltriethoxysilane through a covalent cross-linking reaction. Subsequently, the novel urease-responsive pendimethalin microcapsules were prepared using isocyanate-functionalized silica cross-linked with polyethylenimine (silica−IPTS−PEI), which can release pendimethalin “on site.” The preparation conditions of silica−IPTS−PEI microcapsules; the effects of pH, temperature, and enzymes on the release performance; the stability of microcapsules; the herbicidal activity; and genotoxicity were also investigated.

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EXPERIMENTAL SECTION

Materials. Pendimethalin technical (98% purity) was acquired from Shandong Huayang Technology Co., Ltd. Pendimethalin 330 g/ L emulsifiable concentration (EC) was supplied by the BASF Chemical Company. Cetyltrimethylammonium chloride (CTAC), polyethylenimine (PEI), ammonia, tetraethyl orthosilicate (TEOS), 3-isocyanatopropyltriethoxysilane (IPTS), ethyl acetate, toluene, tetrahydrofuran, hydrochloric acid, acetic acid, methanol, and ethanol were analytical chemicals provided by Sinopharm Chemical Reagent Beijing Co., Ltd. Carbol fuchsin, cellulose (3−10 units/mg; from Trichoderma viride), and pectinase (≥5 units/mg protein (Lowry), from Aspergillus niger) and urease (20 units/mg; from Canavalia ensiformis) were purchased from Sigma-Aldrich. Acetonitrile and methanol (HPLC grade) were supplied from J. T. Baker. Deionized water used to prepare all solutions was collected from a Millipore water purification system (Milli-Q water). Barnyard grass (Echinochloa cruss-galli L., BYG) and redroot pigweed (Amaranthus retrof lexus L., 4803


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Figure 1. Possible mechanism for the formation of the silica−IPTS−PEI microcapsule.

Figure 2. SEM images of the silica microcapsules prepared with different amounts of CTAC, TEOS, and PEI. (A1) CTAC 10 g/L, (A2) CATC 15 g/L, (A3) CTAC 20 g/L, (B1) TEOS 4.0 mL, (B2) TEOS 5.0 mL, (B3) TEOS 6.0 mL, (C1) PEI 0.1 g, (C2) PEI 0.2 g, (C3) PEI 0.4 g. characteristics of pendimethalin and the microcapsule system, and n is the diffusion parameter that is indicative of the transport mechanism.43 Allium cepa Chromosome Aberration Assay. Several researchers have reported that higher plants can be used as an indicator organism to study the impact of pesticides on the environment.44,45 Plant systems have a variety of well-defined genetic end points including alterations in ploidy, chromosomal aberrations, and sister chromatid exchanges.46 Allium cepa chromosome aberration assay is an established plant bioassay validated by the United Nations Environment Programme, which has been used as an efficient and standard test to monitor the genotoxicity of pesticides.47 The onion bulbs were grown in distilled water at room temperature until the root length had reached 2−3 cm. The bulbs were treated with different concentrations of pendimethalin technical and silica−IPTS−PEI microcapsules (100, 200, and 500 mg/L); a control test was carried out with distilled water. After 24 h, the roots were harvested from each onion bulb and fixed in the ethanol−acetic acid mixture (3:1, v/v) for 12 h. Then, the roots were washed with distilled water several times, incubated in 1 mol/L

HPLC. In different pH value and temperature experiments, approximately 50 mg of silica−IPTS−PEI microcapsules was suspended in 200 mL of methanol−water solvents (30:70, v/v) and placed onto a rotary shaker at 100 rpm. A total of 2.0 mL of the mixture was collected at predetermined intervals from the suspension. Then, the sample was centrifuged to obtain supernatant without the microcapsules for HPLC analysis. In the urease experiment, 20 mg of silica−IPTS−PEI microcapsules was suspended in 37.5 mL of methanol−water solvents (5:95, v/v), and then 12.5 mL of the enzyme solution (0.4 g of enzyme was dissolved in 100 mL of water at pH 7.0) was added into the system. This mixture was continuously stirred at 100 rpm and collected at different intervals. After centrifugation at 1000 rpm for 10 min, the pendimethalin content in the supernatant was measured by HPLC. The release data were analyzed by the Ritger and Peppas equation Mt/Mz = ktn, where Mt/Mz represents the percentage of pendimethalin released at time t, k is the constant which depends on incorporate 4804


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ACS Sustainable Chemistry & Engineering HCl at 60 °C for 9 min, and washed again. Subsequently, approximately 2 mm root tips were cut and placed on the slide with 20 g/L aceto-orcein. After staining for 3 min, the root tips were squashed on the slide and covered with a coverslip for cytological analysis. Approximately 1500 cells (500 cells for each of the three slides) were observed at ×1000 magnification. All of the treatments were carried out in triplicate. The mitotic index (MI) and chromosome aberration index (CAI) were calculated as follows:

MI =

CAI =

TCM × 100 TC TAC × 100 TCM

(1)

(2)

where TCM, TAC, and TC represent the number of cells in mitosis, the number of aberrant cells, and total number of cells observed, respectively. Greenhouse Herbicidal Activity. The greenhouse herbicidal activities of pendimethalin EC and silica−IPTS−PEI microcapsules against Echinochloa cruss-galli L. (BYG) and Amaranthus retrof lexus L. (RPW) were evaluated at different concentrations using water as a control. Weed seeds were cultivated in plastic boxes with size 7 × 7 cm, which were completely filled with a 1:4 (v/v) mixture of vermiculite and nutrient soil (pH = 7.3). Twenty seeds were planted in each plastic box and kept under greenhouse conditions of 25−30 °C and 50−70% humidity. In the weed control experiments, different concentrations of pendimethalin EC and microcapsules (500, 1000, and 1500 mg/L) were applied to the surface of mixed soil by spraying soon after planting the seeds, and the same amount of water was sprayed as a blank control. After 21 days, the stem control efficacy and fresh weight reduction were investigated. In the effective duration experiments, the weed seeds were planted at the 30th day after different concentrations of pendimethalin EC and microcapsule (500, 1000, and 1500 mg/L) treatment; the same amount of water was sprayed as a blank control. The stem control efficacy and fresh weight reduction were studied at the 51th day after treatment. All of the treatments were replicated five times in a completely randomized design. The whole experiment was repeated three times. The calculating formula for stem control efficacy was SE = (M2 − M1)/M2 × 100%, where the number of plants in the treatment and blank control were recorded as M1 and M2, respectively. The calculating formula for fresh weight reduction was FR = (N2 − N1)/ N2 × 100%, where the average fresh weight of plants in the treatment and blank control were counted as N1 and N2, respectively. Data Analysis. The data of herbicidal activity and genotoxicity were analyzed by one-way analysis of variance technique (ANOVA) using SPSS 20.0 statistical analysis software (SPSS, Chicago, IL, USA). Duncan’s multiple range test was performed to compare the treatment difference. Statistical significance was determined by p < 0.05.

Figure 3. Particle size distribution of the microcapsules.

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RESULTS AND DISCUSSION Preparation of Pendimethalin Microcapsules. Figure 1 shows the formation mechanism of the silica−IPTS−PEI microcapsule. During the formation of the pendimethalin silica microcapsule, pendimethalin and TEOS were dissolved in ethyl acetate to obtain the oil phase, and the resulting hydrophobic liquid was emulsified in the aqueous phase containing the CTAC to form an oil/water emulsion. The mesoporous silica shell was formed by utilizing ammonia solution as a catalyst via hydrolysis and polycondensation of TEOS at the water/oil interface. And then the pendimethalin silica microcapsule was dispersed in toluene for the surface functionalization with a silane coupling agent of IPTS, which generated a −NCO on the surface of the pendimethalin silica microcapsule. Subsequently, the isocyanate-functionalized silica microcapsule could react with the amino group of PEI to obtain a PEI cross-

Figure 4. FT-IR spectra of silica (Aa), isocyanate-functionalized silica (Ab), silica-IPTS-PEI (Ac), and PEI (Ad). TGA and DTG curves of the microcapsules (Ba) blank silica microcapsules, (Bb) silica microcapsules with pendimethalin, (Bc) silica-IPTS-PEI microcapsules without pendimethalin, and (Bd) silica-IPTS-PEI microcapsules with pendimethalin.

linking isocyanate-functionalized silica microcapsule (silica− IPTS−PEI). Effects of CTAC. CTAC was selected as a surfactant to disperse oil droplets in water. The preliminary studies showed that the concentration of CTAC had an effect on the formation and the diameter of silica microcapsules.18 As shown in Figure 2, the diameters of silica−IPTS−PEI microcapsules were decreased with an increase in the concentration of CTAC. 4805


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Figure 5. Stability of the resulting pendimethalin microcapsules affected by temperature (A) and UV radiation (B).

Figure 7. Effect of pH value (A), temperature (B), and urease (C) on the release behaviors of pendimethalin from silica-IPTS-PEI microcapsules.

Figure 6. Synthetic process of enzyme-responsive microcapsules and the mechanism of triggered pendimethalin release.

Table 1. Constants from Fitting the Generalized Model, Mt/ Mz = ktn, to the Release Data of Pendimethalin from SilicaIPTS-PEI Microcapsules under Different Conditions

When the concentration of CTAC was 10 g/L, it was difficult to obtain the stable microemulsion due to the oil phase undispersible in the water phase, resulting in the rupture of microcapsule shells without a pendimethalin coating (Figure 2A1). When the concentrations of CTAC were 15 g/L and 20 g/L, the resultant silica microcapsules appeared as spherical particles with good monodispersity (Figure 2A2 and A3). The particle size distribution of the microcapsules variation with CTAC is shown in Figure 3. The mean particle sizes of the microcapsules were 5.0 and 3.0 μm as the concentrations of CTAC were 15 g/L and 20 g/L, respectively, which suggested that the mean particle sizes of the resulting microcapsules depended on the concentrations of CTAC. Hence, the

conditions pH values

temperature (°C)

urease

4806

5 7 9 25 35 45

n

K (× 10−2)

r

T50 (d)

0.39 0.49 0.35 0.49 0.38 0.36 0.36

12.71 7.77 17.07 7.77 13.20 15.30 27.37

0.9973 0.9937 0.9973 0.9937 0.9883 0.9879 0.9841

33.51 44.68 21.55 44.68 33.27 26.83 0.22


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2C3), which reduced the dispersion of the microcapsules and seriously restricted the application of the microcapsules in the fields of agriculture. Based on this, the desired pendimethalin microcapsules with a stable outer shell and excellent dispersion were prepared by using 0.2 g of PEI (Figure 2C2), and this structure could reduce the initial burst release and realize sustained-release of pendimethalin. Characterization. Fourier Transform Infrared Spectroscopy (FT-IR). The infrared spectra of silica, isocyanatefunctionalized silica, silica-IPTS-PEI microcapsules, and PEI were investigated by FTIR spectra. Compared with silica (Figure 4Aa), two characteristic absorption peaks appeared at 2323 and 2970 cm−1 for isocyanate-functionalized silica attributed to the −NCO and −CH 2− stretching vibrations, respectively (Figure 4Ab), indicating that the silica was successfully modified with IPTS.48,49 The PEI showed a broad band that appeared at 3286 cm−1, ascribed to the stretching vibration of amino groups (−NH2), and two weak bands appeared at 2936 and 2818 cm−1, belonging to the C−H stretching vibration bands (Figure 4Ad). After isocyanatefunctionalized silica interacting with PEI, the spectrum of the silica-IPTS-PEI exhibited two characteristic peaks of urea linkages (−NH−CO−NH−) at 1634 and 1560 cm−1. Meanwhile, the absorption band of the isocyanate groups (−N CO) at 2323 cm−1 peaks disappeared, which proved that PEI was conjugated with the isocyanate-functionalized silica (Figure 4Ac). TGA and DTG Curves of Microcapsules. Figure 4B shows the TGA and differential thermal gravimetric (DTG) curves of the blank silica microcapsules, silica microcapsules with pendimethalin, silica-IPTS-PEI microcapsules without pendimethalin, and silica-IPTS-PEI microcapsules with pendimethalin. The weight loss below 180 °C was probably due to the evaporation of water in the microcapsules, and that in range of 180−330 °C could be attributed to the decomposition and evaporation of pendimethalin. Similar results were found by Yu et al.50 At the stage between 235 and 470 °C, the endothermic peak of the DTG curve was ascribed to decomposition of the PEI outer shells. Therefore, the total weight loss of silica-IPTSPEI microcapsules without pendimethalin and silica-IPTS-PEI microcapsules with pendimethalin from 25 to 700 °C was approximately 29.0% and 59.0%, respectively, and the ratio of pendimethalin in the silica-IPTS-PEI microcapsules was approximately 30%. The Stability of Silica-IPTS-PEI Microcapsules. Thermal Stability. Figure 5A shows the decomposition rates of silicaIPTS-PEI microcapsules (approximately 5 μm) and pendimethalin technical at different temperatures (40, 50, and 60 °C) for a period of 60 days. For pendimethalin technical, at 40 and 60 °C, the decomposition rates were increased from 13.66% to

Figure 8. Genotoxicity of silica-IPTS-PEI microcapsules and pendimethalin technical against Allium cepa. Each data point represents the mean value from at least three independent experiments.

diameter of pendimethalin microcapsules could be easily tuned through changing the amount of CTAC during synthesis. Effects of TEOS. The mesoporous silica shells were formed by TEOS diffusing from the interior of the micromulsion droplets to the interface, and their sizes increased with the TEOS amount (Figure 2B1 and B2). However, the results did not mean that more TEOS will be able to obtain larger microcapsules (Figure 2B). When the amount of TEOS reached 6 mL, grapelike clusters of microcapsules were formed (Figure 2B3). These results were consistent with our previous research.18 Effects of PEI. PEI was used as the outer shell material, and the double-shelled microcapsules were formed through urea bonds between isocyanate groups of isocyanate-functionalized silica and the amino groups of PEI. The morphology of doubleshelled microcapsules influenced by PEI concentration can be observed in Figure 2C. When the amount of PEI was 0.1 g, the outer shell of the microcapsules was too thin to cover completely the surface of the silica microcapsules (Figure 2C1), and these microcapsules made it hard to control the release of pendimethalin effectively and led to high initial burst release. However, when excessive PEI (0.3 g) was used, a huge number of microcapsules were coalesced in a chunk (Figure

Table 2. Efficacy of Silica-IPTS-PEI Microcapsules against BYG and RPGa stem control efficacy (%) treatment pendimethalin EC

pendimethalin microcapsule

a

concentration (mg/L) 500 1000 1500 500 1000 1500

BYG 37.93 58.62 68.97 42.29 60.71 72.43

fresh weight reduction (%)

RPW b a a b a a

35.71 57.14 67.86 38.04 59.26 70.37

c b ab c ab a

BYG 78.48 89.77 98.36 81.68 91.54 99.14

RPW c b a c b a

76.31 88.92 97.87 80.13 89.82 98.15

c b a c b a

Herbicidal activities within each column followed by different letters are significantly different at p < 0.05. 4807


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ACS Sustainable Chemistry & Engineering Table 3. Efficacy of Silica-IPTS-PEI Microcapsules against BYG and RPG (Sowing after Spraying 30 d)a stem control efficacy (%) treatment pendimethalin EC

pendimethalin microcapsule

a

concentration (mg/L) 500 1000 1500 500 1000 1500

BYG 14.81 25.93 48.15 34.48 55.17 68.97

d cd b c b a

fresh weight reduction (%)

RPW 11.11 22.22 40.74 32.14 53.57 64.29

e de bc cd ab a

BYG 35.44 47.19 55.81 65.45 79.86 89.08

RPW f e d c b a

31.36 44.21 54.16 63.42 75.19 84.55

f e d c b a

Herbicidal activities within each column followed by different letters are significantly different at p < 0.05.

from the microcapsules increased with the rise of temperature. In the temperature range of 25 to 45 °C, the cumulative release rates were from 54.25% to 63.49% after 60 days. The release data were also analyzed by applying the generalized model Mt/ Mz = ktn. There was good correlation of the release curves of pendimethalin from the microcapsules with the empirical equation (25, 35, and 45 °C). The correlation coefficients (r) were higher than 0.9879. The n values were 0.49, 0.38, and 0.36 at 25, 35, and 45 °C (Table 1). All of the n values below 0.5 indicated that the release of pendimethalin was diffusioncontrolled.53 The T50 values were also calculated and shown in Table 1. Effects of Enzyme. As shown in Figure 7C, a relatively high release rate of the pendimethalin was found in the presence of urease; approximately 25.16% of the pendimethalin was released from silica-IPTS-PEI microcapsules within 1 h, and the cumulative release rate was up to 81.94% after 30 h. However, the pendimethalin released from the microcapsules was not detected in the presence of nonrelated enzymes (such as cellulase and pectinase). The release data were analyzed by applying the generalized model. As shown in Table 1, the release of pendimethalin was consistent with the empirical equation (in the presence of urease). The correlation coefficient (r) was 0.9841. The n value was 0.36 (Table 1). All of the n values below 0.5 indicated that the release of pendimethalin from the microcapsules was diffusion-controlled. The T50 value was also calculated and presented in Table 1. Allium cepa Chromosome Aberration Assay. Figure 8 shows the genotoxicity of silica-IPTS-PEI microcapsules and pendimethalin technical against Allium cepa at the concentrations of 100, 200, and 500 mg/L. The MI was decreased when the concentration of silica-IPTS-PEI microcapsules and pendimethalin technical increased. And the MI of the microcapsules treatment was significantly higher than that of the technical treatment at the same concentration. In addition, the CAI increased gradually along with the increase of microcapsules and technical concentration, and the CAI of the microcapsules treatment was lower than that of the technical treatment at the same concentration. This experiment indicated that the shell of the silica-IPTS-PEI microcapsule could significantly reduce the genotoxicity of pendimethalin technical. Greenhouse Herbicidal Activity. Table 2 shows the herbicidal activity of silica-IPTS-PEI microcapsules against BYG and RPG in concentrations ranging from 500−1500 mg/L. The results indicated that there were no significant differences in control efficacy between pendimethalin EC and silica-IPTS-PEI microcapsules at the same concentration, indicating the microcapsules could achieve the same effects as pendimethalin EC.

36.73% at 60 days. While, for silica-IPTS-PEI microcapsules, the accumulated decomposition rates at different temperatures were less than 5% after 60 days. These results indicated that silica-IPTS-PEI microcapsules were more stable than the technical under high temperatures. Light Stability. Figure 5B shows the effects of UV radiation on stability of silica-IPTS-PEI microcapsules (approximately 5 μm) at room temperature under stirring. Without UV radiation, the decomposition of pendimethalin from silica-IPTS-PEI microcapsules was not detected, and the pendimethalin technical exhibited 8.7% decomposition after 60 h. Pendimethalin technical was completely decomposed after 48 h of UV radiation. Nevertheless, the pendimethalin wrapped in microcapsules exhibited less than 25% after 72 h of UV radiation. These results demonstrated that the pendimethalin could be protected by the microcapsules shells. Controlled Release Kinetics. Figure 6 shows the synthetic process of enzyme-responsive microcapsules and the mechanism of triggered release of pendimethalin. PEI was conjugated onto the surface of isocyanate-functionalized silica microcapsules via urea bonds as enzyme cleavage sites. The urea bonds can easily be broken by urease, which triggers the release of pendimethalin from the microcapsules. Effects of pH. Figure 7A shows the effects of different pH values (5.0, 7.0, and 9.0) on the pendimethalin release behaviors of silica-IPTS-PEI microcapsules (approximately 5 μm) at room temperature. The cumulative release rates of pendimethalin were in the order pH 9.0 > pH 5.0 > pH 7.0. For the microcapsules at pH 5.0 and pH 9.0, the cumulative release of pendimethalin reached 62.79% and 69.58% after 60 days, respectively, whereas at pH 7.0, the cumulative release of pendimethalin reached 54.25% after 60 days. This was probably because the shell of the microcapsule could be hydrolyzed under basic conditions, and the PEI shell could be decomposed under acid conditions, and the cumulative release rates of pendimethalin were faster than that under neutral conditions.51,52 The data of pendimethalin released from the microcapsules in various pH values were fitted to the generalized model Mt/Mz = ktn (Table 1). There was good correlation of the release curves of pendimethalin released from the microcapsules with the empirical equation. The correlation coefficients (r) were higher than 0.9927. The n values were 0.39, 0.49, and 0.35 at pH 5.0, 7.0, and 9.0. All of the n values below 0.5 indicated that the release of pendimethalin was diffusion-controlled. The time taken for 50% of the active ingredient to be released (T50) was also calculated, and it is shown in Table 1. Effects of Temperature. Figure 7B shows the release behaviors from silica-IPTS-PEI microcapsules (approximately 5 μm) at different temperatures (25, 35, and 45 °C) at pH 7. It was found that the cumulative release rates of pendimethalin 4808


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coronary heart disease in the chinese population. Environ. Sci. Technol. 2017, 51 (1), 664−670. (5) Dailey, O. D.; Dowler, C. C.; Mullinix, B. G. Polymeric microcapsules of the herbicides atrazine and metribuzin: preparation and evaluation of controlled-release properties. J. Agric. Food Chem. 1993, 41 (9), 1517−1522. (6) Mogul, M. G.; Akin, H.; Hasirci, N.; Trantolo, D. J.; Gresser, J. D.; Wise, D. L. Controlled release of biologically active agents for purposes of agricultural crop management. Resour. Conserv. Recy. 1996, 16 (1), 289−320. (7) Frederiksen, H. K.; Kristensen, H. G.; Pedersen, M. Solid lipid microparticle formulations of the pyrethroid gamma-cyhalothrin incompatibility of the lipid and the pyrethroid and biological properties of the formulations. J. Controlled Release 2003, 86 (2−3), 243−252. (8) Grillo, R.; Pereira, A. E. S.; Nishisaka, C. S.; de Lima, R.; Oehlke, K.; Greiner, R.; Fraceto, L. F. Chitosan/tripolyphosphate nanoparticles loaded with paraquat herbicide: An environmentally safer alternative for weed control. J. Hazard. Mater. 2014, 278, 163−171. (9) Liu, B.; Wang, Y.; Yang, F.; Wang, X.; Shen, H.; Cui, H.; Wu, D. Construction of a controlled-release delivery system for pesticides using biodegradable PLA-based microcapsules. Colloids Surf., B 2016, 144, 38−45. (10) Abraham, S.; Park, Y. H.; Lee, J. K.; Ha, C.-S.; Kim, I. Microfluidic synthesis of reversibly swelling porous polymeric microcapsules with controlled morphology. Adv. Mater. 2008, 20 (11), 2177−2182. (11) Wang, Y.; Gao, Z.; Shen, F.; Li, Y.; Zhang, S.; Ren, X.; Hu, S. Physicochemical characteristics and slow release performances of chlorpyrifos encapsulated by poly(butyl acrylate-co-styrene) with the cross-linker ethylene glycol dimethacrylate. J. Agric. Food Chem. 2015, 63 (21), 5196−5204. (12) Zhang, D.-x.; Li, B.-x.; Zhang, X.-p.; Zhang, Z.-q.; Wang, W.-c.; Liu, F. Phoxim microcapsules prepared with polyurea and urea− formaldehyde resins differ in photostability and insecticidal activity. J. Agric. Food Chem. 2016, 64 (14), 2841−2846. (13) Wen, J.; Kim, G. J. A.; Leong, K. W. Poly(d,llactide−co-ethyl ethylene phosphate)s as new drug carriers. J. Controlled Release 2003, 92 (1−2), 39−48. (14) Huang, Z.-M.; Zhang, Y. Z.; Kotaki, M.; Ramakrishna, S. A review on polymer nanofibers by electrospinning and their applications in nanocomposites. Compos. Sci. Technol. 2003, 63 (15), 2223−2253. (15) Li, G. L.; Wan, D.; Neoh, K. G.; Kang, E. T. Binary polymer brushes on silica@polymer hybrid nanospheres and hollow polymer nanospheres by combined alkyne−azide and thiol−ene surface click reactions. Macromolecules 2010, 43 (24), 10275−10282. (16) Li, Z.; Barnes, J. C.; Bosoy, A.; Stoddart, J. F.; Zink, J. I. Mesoporous silica nanoparticles in biomedical applications. Chem. Soc. Rev. 2012, 41 (7), 2590−2605. (17) Yang, P.; Gai, S.; Lin, J. Functionalized mesoporous silica materials for controlled drug delivery. Chem. Soc. Rev. 2012, 41 (9), 3679−3698. (18) He, S.; Zhang, W. B.; Li, D. G.; Li, P. L.; Zhu, Y. C.; Ao, M. M.; Li, J. Q.; Cao, Y. S. Preparation and characterization of double-shelled avermectin microcapsules based on copolymer matrix of silicaglutaraldehyde-chitosan. J. Mater. Chem. B 2013, 1 (9), 1270−1278. (19) Martina, K.; Baricco, F.; Berlier, G.; Caporaso, M.; Cravotto, G. Efficient green protocols for preparation of highly functionalized βcyclodextrin-grafted silica. ACS Sustainable Chem. Eng. 2014, 2 (11), 2595−2603. (20) Yao, M.-Y.; Huang, Y.-B.; Niu, X.; Pan, H. Highly efficient silicasupported peroxycarboxylic acid for the epoxidation of unsaturated fatty acid methyl esters and vegetable oils. ACS Sustainable Chem. Eng. 2016, 4 (7), 3840−3849. (21) Barbé, C. J.; Kong, L.; Finnie, K. S.; Calleja, S.; Hanna, J. V.; Drabarek, E.; Cassidy, D. T.; Blackford, M. G. Sol−gel matrices for controlled release: from macro to nano using emulsion polymerisation. J. Sol-Gel Sci. Technol. 2008, 46 (3), 393−409.

Table 3 shows the effective duration of silica-IPTS-PEI microcapsules against BYG and RPG in concentrations ranging from 500 to 1500 mg/L. Compared with pendimethalin EC, the silica-IPTS-PEI microcapsules had a longer duration and higher herbicidal activity in the same concentration at the 51th day. Hence, the urease-responsive silica-IPTS-PEI microcapsules could prolong the effective duration of pendimethalin in controlling weeds.

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CONCLUSIONS In this work, novel urease-responsive silica-IPTS-PEI microcapsules were developed through the introduction of urea bonds using isocyanate-functionalized silica conjugated with PEI. The prepared microcapsules showed a high pendimethalin loading efficiency (approximately 30% w/w) and could effectively protect pendimethalin against thermal degradation and photodegradation. The release data can be described by the generalized model Mt/Mz = ktn. The n values below 0.5 indicated that the release of pendimethalin from silica-IPTSPEI microcapsules was diffusion-controlled. In addition, the release of silica-IPTS-PEI microcapsules showed a positive relation with temperature. Under weak acid and weak base conditions, the pendimethalin release rates of the microcapsules were higher than those under neutral conditions. Meanwhile, the microcapsule displayed an excellent urease-responsive property and controlled release performance. Furthermore, the microcapsules had longer duration in controlling weeds than pendimethalin EC and lower genotoxicity than pendimethalin technical. Thus, urease-responsive silica-IPTSPEI microcapsules demonstrated a great potential as a preemergence herbicide in agricultural application.

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AUTHOR INFORMATION

Corresponding Author

*Address: No. 2 Yuanmingyuan West Road, China Agricultural University, Beijing, China, 100193. Telephone: 86-1062734302. Fax: 86-10-62734302. E-mail: [email protected], [email protected]. ORCID

You Liang: 0000-0001-8972-6788 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support of this work by the National Natural Science Foundation of China (31471799, 31260441) and the National Department Public Benefit Research Foundation of China (201303031).

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REFERENCES

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