Preparation and characterization of single- and double-shelled cyhalothrin microcapsules based on the copolymer matrix of silica–N-isopropyl acrylamide–bis-acrylamide

Abstract

Silica, as a controlled release material, has been applied widely in many fields owing to its high thermal and mechanical stability, corrosion resistance, biocompatibility and low cost. We herein synthesized pesticide cyhalothrin/SiO₂ microcapsules and cyhalothrin/silica–N-isopropyl acrylamide (NIPAM)–bis-acrylamide (MBA) microcapsules. A silica shell was formed through the hydrolysis and polycondensation of tetraethyl orthosilicate under alkaline conditions, and then the single-shelled silica microcapsules were modified by triethoxyvinylsilane and cross-linked with NIPAM and MBA through free radical polymerization. The microcapsules were characterized by scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy and thermogravimetric analysis, and their release kinetics and biological activity were also detected. The single- and double-shelled microcapsules had excellent loading abilities for cyhalothrin (about 50.0% and 25.0%, respectively). The release curves showed that the double-shelled microcapsules allowed better controlled release, probably through a combination of diffusion and erosion. The microcapsules showed longer persistence than that of cyhalothrin emulsifiable concentrate. This study provides a novel material for fabricating pesticides with controlled release.

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1. Introduction

Widely used all over the world, pesticides have seriously polluted the soil, groundwater and atmosphere and have thus threatened human health, which require reasonable and effective methods to reduce the environmental burden. For instance, controlled release techniques can improve the utilization of pesticides as well as reduce the frequency of agrochemical application.

This technique has been used in many fields such as food,¹ medicine,² pesticides,³ environmental engineering,⁴ cosmetics⁵ and coatings.⁶ Microcapsule, which is one of the controlled release formulations of pesticides, is prepared using natural or synthetic polymeric materials to encapsulate pesticides via physical methods, chemical methods or a combination of them. Microencapsulation is commonly performed by in situ polymerization,⁷ emulsion polymerization,⁸ interfacial polycondensation,⁹ solvent evaporation,¹⁰ colloidal templating,¹¹ and coacervation/phase separation,¹² with the common shell materials being polyurea,¹³ polyamide,¹⁴ polyurethane,¹⁵ and polystyrene.¹⁶ All of them are chemically synthesized polymers, leaving undesirable organic solvent residues during monomer polymerization. Besides, hydrophobic polymers may adsorb active ingredients and prevent their release. Furthermore, organic polymers, with difficult to control degradation rates, easily cause environmental pollution.

As a functional material, silica has been widely applied in medicine,¹⁷ cosmetics,⁵ phase change materials¹⁸ and pesticides¹⁹ owing to its high thermal and mechanical stability, corrosion resistance, biocompatibility and low cost. However, there are many active hydroxyl groups on the surface of silica microcapsules that are prone to agglomeration; thus, the microcapsule suspension is unstable, which is usually circumvented by passivating these groups with alkane-substituted silanes such as 3-(methacryloyloxy)propyl trimethoxysilane²⁰ and (3-aminopropyl)triethoxysilane.¹⁹

In this study, we used tetraethyl orthosilicate as a precursor to prepare silica-coated cyhalothrin microcapsules using an emulsion polymerization method. To prevent microcapsule agglomeration and prolong the controlled release time, we modified the surface of the microcapsules using triethoxy-2-propenyl-silane and then reacted them with N-isopropyl acrylamide (NIPAM) through free radical polymerization to obtain hybrid inorganic and organic microcapsules. Cyhalothrin belongs to pyrethroid insecticides, targeting a wide range of insects, including sucking pests, soil pests and health pests. The solubility of cyhalothrin in water is 0.004 mg L⁻¹ (pH 5.0, 20 °C), and it is irritating to the eyes.²¹ When loaded into the microcapsules, cyhalothrin showed a high loading content and sustained release behavior. The preparation conditions of the cyhalothrin microcapsules, the effects of pH on sustained release performance and the biological activity were studied herein.

2. Experimental

2.1 Materials

Tetraethyl orthosilicate (TEOS), hexadecyltrimethylammonium bromide (CTAB), triethoxyvinylsilane, sodium hydroxide and potassium persulfate (KPS) were analytical-grade chemicals purchased from Sinopharm Chemical Regent Co., Ltd. (Beijing, China). NIPAM and bis-acrylamide (MBA) were provided by Alfa Aesar. Cyhalothrin at a purity of 97.5% was purchased from Jiangsu Yangnong Chemical Group Co., Ltd. (China). Acetonitrile and methanol were of HPLC grade and purchased from Thermo Fisher Scientific Inc. (USA). Musca domestica containing both males and females was provided after the third day of emergence by China Agricultural University.

2.2 Encapsulation of cyhalothrin into SiO₂ microspheres

Cyhalothrin-loaded SiO₂ microspheres were prepared by in situ polymerization as previously described by Magdassi⁵ with a slight modification. Briefly, a certain quantity of CTAB was dissolved in 100 mL of deionized water, giving a solution of the surfactant (m/v, g mL⁻¹). The oil phase was prepared by dissolving cyhalothrin in TEOS. The cyhalothrin to TEOS ratios were 1/1, 1/2 and 1/3, and those of the oil phase to water phase were 22%, 24%, 26%, 28% and 30%. The oil phase was poured into the water phase and sheared at 18 000 rpm; the resulting emulsion was poured into a three-necked flask equipped with a mechanical stirrer containing 100 g aqueous NaOH solution at pH 11.3, and stirred at 400 rpm. Subsequently, the stirring rate was lowered to 200 rpm. The emulsion was stirred at room temperature for 24 h, followed by stirring at 60–65 °C for 3 h. The precipitate obtained by centrifugation (10 000 rpm) was dried at 40 °C.

2.3 Synthesis of cyhalothrin-loaded NIPAM–MBA–SiO₂ microcapsules

Briefly, 10 mL of the SiO₂ microspheres suspension loaded with cyhalothrin was added to a three-necked flask equipped with a mechanical stirrer containing 20 mL of water and stirred at 300 rpm. Subsequently, 1 mL of triethoxyvinylsilane was slowly added to the mixture, followed by stirring at 40 °C for 5 h. The microcapsules were collected by centrifugation and then freeze-dried overnight.

The microcapsules obtained by the abovementioned step were suspended in 25 mL of NIPAM (625 mg) and MBA (625 mg) aqueous solutions, and the mixture was stirred at 400 rpm at room temperature. 5 mL of 2% KPS was then added to the suspension, and stirred at 400 rpm at 65 °C for 3 h. The double-shelled microcapsules were obtained by centrifugation and dried under vacuum.

2.4 Characterization

2.4.1 Scanning electron microscopy (SEM). A scanning electron microscope (HITACHI S4800, Japan) was used to characterize the morphology and structure of cyhalothrin microcapsules.

2.4.2 Transmission electron microscopy (TEM). A Transmission electron microscope (JEM-2010, JEOL Ltd., Japan) was used to characterize the internal structure of the prepared microcapsules.

2.4.3 Fourier transform infrared (FTIR) spectroscopy. FTIR spectroscopy was performed on an FTIR spectrometer (BRUKER-VECTOR22, Germany) over potassium bromide pellets and the wavelength was set from 4000 to 450 cm⁻¹.

2.4.4 Particle size analysis. Mastersizer particle size analyzer (Mastersizer 2000, Malvern Instruments Co., UK) was used to determine the size and polydispersity of cyhalothrin microcapsules.

2.4.5 Thermodynamic properties of cyhalothrin microcapsules. The microspheres were evaluated by DSC (Shimadzu DSC-50, Germany) thermal analyzer under a nitrogen atmosphere.

2.4.6 Measurements of the encapsulation efficiency (EE). The microspheres suspension was dispersed in a certain amount of acetonitrile. The mixture was then centrifuged and the supernatant was analyzed by high-performance liquid chromatography (HPLC, Agilent 1100, USA) with a diode array detector. The HPLC separation of cyhalothrin was carried out on a Spuril-C18 column (4.6 mm × 250 mm, 5 μm, Dikma Technologies Inc., China) with isocratic elution of methanol–water (90/10, v/v) as the mobile phase. The analyte (10 μL) was injected into the HPLC system and separated at 25 °C, using a constant flow rate of 1.0 mL min⁻¹ at the detection wavelength of 230 nm. EE was calculated according to eqn (1) as follows:

2.4.7 Measurement of cyhalothrin release. The release profiles of loaded cyhalothrin from the prepared microcapsules were investigated. A certain quantity of the microcapsules suspension was added to a dialysis bag (size: 5 M, MW: 8000–14 000), which was then placed into a 20 mL release medium; the medium was acetonitrile–water solution (50 : 50, v/v) and adjusted the pH of 3, 5, 7, and 9 by HCl and NaOH. The release was investigated in vitro at 25 °C using a controlled environmental/orbital shaker incubator (Huamei Co., Jiangsu Province, China) at 200 rpm. At different time intervals, 1 mL of solution was sampled and 1 mL of a fresh acetonitrile–water solution was added to the reagent bottle to maintain constant volume and unsaturated condition. The sampled solution was filtered through a cellulose membrane filter (diameter, 13 mm; pore size, 0.22 μm; Dikma Technologies Inc.) and analyzed by HPLC under the same conditions mentioned above.

2.4.8 Controlled release kinetics studies. The cumulative release of cyhalothrin from the microcapsules was determined using an empirical equation to estimate the kinetic parameter n, as described previously by the Korsmeyer–Peppas model²² (2), where M_t is the amount of drug released in time t, M_∞ is the initial amount of drug in the solution, and k is the first order release constant.

M_t/M_∞ = ktⁿ (2)

2.4.9 Evaluation of biological activity. The biological activity was evaluated according to GB/T 13917.1-2009 (China). The cyhalothrin microcapsule suspension was diluted with water and uniformly coated on three different surfaces (specification: 20 × 20 cm²), i.e., glass plate, paint wood and cement board. Water and emulsifiable concentrate (EC) of cyhalothrin were used as controls. The materials coated with the insecticide were stored under natural conditions for 1, 30, 60 and 90 days. The test insects were then forced to come into contact with the coated surface for 30 min on the first, thirtieth, sixtieth, and ninetieth days. Subsequently, they were transferred to clean utensils with feeding, and the mortality was observed and calculated 24 h later, according to eqn (3).

3. Results and discussion

3.1 Preparation of cyhalothrin microcapsules

The preparation procedure of single- and double-shelled cyhalothrin microcapsules is illustrated in Fig. 1 and 2. In the first step, TEOS-dissolved cyhalothrin was added to the continuous phase, which became small droplets under high shear conditions. At the oil–water interface, TEOS was hydrolyzed to orthosilicic acid and consequently polycondensed to yield silica shells for the poor solubility of the polymer under alkaline conditions. In the second step, the cyhalothrin/SiO₂ microcapsules were modified with triethoxyvinylsilane, and then the modified silica particles were polymerized with NIPAM through free radical polymerization, giving double-shelled, inorganic/organic hybrid microcapsules.

Fig. 1 Schematic diagram of the possible formation mechanism of single- and double-shelled microcapsules.

Fig. 2 The reaction mechanism for the formation of silica/cyhalothrin and silica–NIPAM–MBA/cyhalothrin microcapsules.

The formation of cyhalothrin/SiO₂ microcapsules is mainly affected by four parameters, i.e. shear rate, surfactant content, oil–water ratio and drug–wall material ratio. These parameters were optimized to increase EE and to improve the particle size distribution (Tables 1–3). The D₁₀, D₅₀, D₉₀ means the largest particle equivalent diameter, when the particle size distribution cumulative distribution reached 10%, 50%, and 90%, which was used to measure the range of the particle size distribution. As summarized in Table 1, with decreasing shear rate, agglomeration occurs and the particle size distribution becomes non-uniform, while the encapsulation efficiency increases due to agglomeration. Therefore, with the comprehensive consideration of encapsulation efficiency and particle size distribution, the optimum shear rate was 18 000 rpm (the particle size distribution is 0.15–1.15 μm). The particle size distribution was most uniform when 1.0% CTAB was used (Table 2), and the microcapsules were agglomerated and EE increased when the content exceeded 1.5%. In addition, when the drug–wall material ratio was 1 : 1 and the oil–water ratio was 30%, EE reached a maximum (Table 3). As suggested by Tables 1–3, the optimum conditions for preparation are a shear rate of 18 000 rpm, CTAB content of 1%, oil–water ratio of 30% and cyhalothrin/TEOS ratio of 1 : 1 and EE of 93.92%. The particle size distribution under these conditions conformed to a normal distribution (Fig. 3) with a mean particle size of 0.47 μm.

Table 1 The effect of shear rate on the encapsulation efficiency and particle size distribution

Sample	Cyhalothrin (g)	TEOS (g)	CTAB content (%)	Shear rate (rpm)	EE%	Size distribution (μm)
						D10	D50	D90
1	5.5	16.5	1	18 000	77.18	0.4	0.61	1.15
2	5.5	16.5	1	13 000	77.88	0.43	4.79	11.79
3	5.5	16.5	1	7000	79.04	0.81	34.07	107.12

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Table 2 The effect of surfactant content on the encapsulation efficiency and particle size distribution

Sample	Cyhalothrin (g)	TEOS (g)	CTAB content (%)	Shear rate (rpm)	EE%	Size distribution (μm)
						D10	D50	D90
4	5.5	16.5	0.5	18 000	79.94	0.39	5.60	13.97
5	5.5	16.5	1.0	18 000	77.18	0.40	0.61	1.15
6	5.5	16.5	1.5	18 000	77.20	0.47	16.88	203.65
7	5.5	16.5	2.0	18 000	80.27	13.90	24.89	35.56
8	5.5	16.5	3.0	18 000	79.25	48.80	69.46	100.04

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Table 3 The effect of oil–water ratio and drug–wall material ratio on the encapsulation efficiency

Sample	Oil–water ratio (%)	Cyhalothrin/TEOS	Cyhalothrin	TEOS	EE%
9	22	1 : 3	5.5	16.5	77.18
10	22	1 : 2	7.3	14.7	75.93
11	22	1 : 1	11	11	85.41
12	24	1 : 3	6	18	83.29
13	24	1 : 2	8	16	88.26
14	24	1 : 1	12	12	92.35
15	26	1 : 3	6.5	19.5	84.43
16	26	1 : 2	8.7	17.3	88.61
17	26	1 : 1	13	13	93.69
18	28	1 : 3	7	21	84.69
19	28	1 : 2	9.4	18.6	89.14
20	28	1 : 1	14	14	93.81
21	30	1 : 3	7.5	22.5	70.07
22	30	1 : 2	10	20	80.54
23	30	1 : 1	15	15	93.92

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Fig. 3 Size distribution of cyhalothrin/SiO₂ microcapsules.

3.2 Characterization of cyhalothrin microcapsules

The FTIR spectra of silica, vinyl-functionalized silica, silica–NIPAM–MBA, silica/cyhalothrin microcapsules, vinyl-functionalized silica/cyhalothrin microcapsules, cyhalothrin/silica–NIPAM–MBA microcapsules, and cyhalothrin are shown in Fig. 4. In Fig. 4(a), the peaks at 1086 cm⁻¹ and 470 cm⁻¹ are attributed to the stretching vibration and bending vibration of the silicon–oxygen bond, respectively, and the new absorption peak at 1685 cm⁻¹ in the spectrum of vinyl-functionalized silica (Fig. 4(b)) corresponds to the vinyl bending vibration. In Fig. 4(c), the peaks at 3434 cm⁻¹ and 3077 cm⁻¹ are the stretching vibrations of free and associated N–H from MBA, respectively, and that at 1659 cm⁻¹ is the stretching vibration of carbonyl group from NIPAM and MBA. As indicated by curves a–c, the polymer of silica–NIPAM–MBA was obtained. The peaks at 1724 cm⁻¹ and 1586 cm⁻¹ represent the stretching vibrations of carbonyl group and aromatic group from cyhalothrin, respectively (Fig. 4(d)). In Fig. 4(g), the characteristic peaks of cyhalothrin are located at 1720 cm⁻¹ and 1587 cm⁻¹, and that the peak at 1653 cm⁻¹ is the stretching vibration of the carbonyl group, which is very similar to that in Fig. 4(c). In short, cyhalothrin/silica–NIPAM–MBA microcapsules were successfully prepared.

Fig. 4 FTIR spectra of silica (a), vinyl-functionalized silica (b), silica–NIPAM–MBA (c), cyhalothrin (d), cyhalothrin/silica microcapsules (e), cyhalothrin/vinyl-functionalized silica microcapsules (f), and cyhalothrin/silica–NIPAM–MBA microcapsules (g).

The morphologies of cyhalothrin/silica and cyhalothrin/silica–NIPAM–MBA microcapsules are presented in Fig. 5. It can be observed that the microcapsules were dispersed without aggregation, and the particle size was uniform (Fig. 5(a and c)). The surface of cyhalothrin/silica microcapsules was rather rough (Fig. 5(b)), while that of cyhalothrin/silica–NIPAM–MBA microcapsules was relatively smooth and compact (Fig. 5(d)), which was responsible for the lower release rate of the double-shelled microcapsules. Moreover, the particle size of the double-shelled microcapsules was higher than that of single-shelled ones due to the polymerization of NIPAM and MBA.

Fig. 5 Morphology of cyhalothrin/silica (a and b) and cyhalothrin/silica–NIPAM–MBA microcapsules (c and d).

The TEM images of single- and double-shelled microcapsules are shown in Fig. 6. In Fig. 6(a), the TEM figure shows that the single SiO₂ microcapsule is uniform; therefore, the single SiO₂ microcapsule proved to be solid. The clear core–shell structure is shown in Fig. 6(b), the dark colour of the microcapsule in the core-region is SiO₂, and the light colour around the SiO₂ is the polymer layer of NIPAM–MBA. It can be assumed that the formation of the double-shelled structure can slow down the pesticide release rate to some extent compared with the single-shelled microcapsule.

Fig. 6 The TEM images of cyhalothrin/silica (a) and cyhalothrin/silica–NIPAM–MBA microcapsules (b).

The amount of loaded cyhalothrin and the thermal stability of prepared microcapsules were detected by thermogravimetric analysis (TGA). The TGA curves of cyhalothrin, silica, silica–NIPAM–MBA, cyhalothrin/silica microcapsules, cyhalothrin/NIPAM–MBA, cyhalothrin/silica microcapsules and cyhalothrin/silica–NIPAM–MBA microcapsules are shown in Fig. 7. The weight loss between 200 °C and 310 °C may be due to the volatilization and decomposition of cyhalothrin (Fig. 7(a)), and that in the range of 310–450 °C can be attributed to the decomposition of NIPAM–MBA shells (Fig. 7(d and e)). As shown in curves c and e, cyhalothrin microcapsules lose approximately 50.0% and 25.0% of their original weights, respectively, from 200 °C to 310 °C, corresponding to the ratios of loaded cyhalothrin. Moreover, the shell materials remained stable before 200 °C because the microcapsules barely lost weight (Fig. 7(b and d)).

Fig. 7 TGA curves of cyhalothrin (a), silica (b), cyhalothrin/silica microcapsules (c), silica–NIPAM–MBA (d), and cyhalothrin/silica–NIPAM–MBA microcapsules (e).

3.3 Controlled release kinetics

The dialysis method was used to study the release behavior of cyhalothrin-loaded microcapsules. The cyhalothrin release behaviors from single- and double-shelled microcapsules were studied by changing the pH, while keeping the temperature (25 °C) constant at a vibrating speed of 200 rpm. Fig. 8 shows the effects of different pH values (3.0, 5.0, 7.0, and 9.0) on the cyhalothrin release behaviors from the cyhalothrin microcapsules. For the single-shelled microcapsules, the release rate was highest at pH 7, and the cumulative release of cyhalothrin reached 70% on the 12th day, which was followed by that at pH 5 (cumulative release reached 70% on the 19th day). At pH 3 and 9, the cumulative release of cyhalothrin reached 70% on the 22nd day and 26th day, respectively. Probably, there were many active hydroxyl groups on the silica surface; thus, the SiO₂ microcapsules were prone to aggregation under acidic or basic conditions, which decelerated cyhalothrin release. The release rate of the double-shelled microcapsules was lower than that of single-shelled ones because the polymer of NIPAM–MBA shelled the SiO₂, reduced the hydrolysis of the silica shell and delayed the release of cyhalothrin. The release data were also analyzed by applying the Korsmeyer–Peppas model “M_t/M_∞ = ktⁿ”, from which k, n and T₅₀ (time of 50% cyhalothrin release from the microcapsules) were calculated (Table 4). There was a good correlation between the release profiles of cyhalothrin from the microcapsules with correlation coefficients r exceeding 0.9433. The release mechanism depended on the value of n; when n < 0.45, diffusion mechanism; 0.45 < n < 0.89, combined diffusion and erosion mechanism; n > 0.89, erosion mechanism. In Table 4, the n values ranged from 0.45 to 0.89; thus, the microcapsules may be released according to a combined diffusion and erosion mechanism.

Fig. 8 Effects of pH on the release behavior of cyhalothrin from single- and double-shelled cyhalothrin microcapsules.

Table 4 The constants of fitting the Korsmeyer–Peppas model to the release rate of cyhalothrin from the microcapsules under different conditions

Microcapsules	pH values	n	k	R	T50 (d)
Single-shelled	3	0.6818	8.0421	0.9837	14.58
	5	0.6875	8.8516	0.9697	12.41
	7	0.5687	13.8710	0.9433	9.53
	9	0.7502	5.9015	0.9935	17.26
Double-shelled	3	0.7777	5.9073	0.9846	15.58
	5	0.6910	8.5079	0.9519	12.97
	7	0.6622	9.0847	0.9572	13.14
	9	0.7682	5.1706	0.9960	19.17

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3.4 Biological evaluation

The control effects of the prepared microcapsules on Musca domestica are shown in Tables 5–7. The cyhalothrin microcapsules persisted longer than cyhalothrin EC. The former still had good control effect on the 90th day, but the latter lost this effect on the 60th day. Overall, the control effect was enhanced with increasing dose. On the first day, the control effect of cyhalothrin EC (100%) surpassed that of the microcapsules. The permeability of the spraying surface influenced the control effect; thus, the impermeable glass plate had the best outcomes, followed by those of the semipermeable paint wood and the completely penetrable cement board. No significant differences were observed in the control effect between the single- and double-shelled microcapsules on the glass plate or paint wood. However, on cement board, the control effect of double-shelled microcapsules (67.2%, (Table 6, 20 mg m⁻²)) was better than that of single-shelled ones (15.3%, (Table 5, 20 mg m⁻²)) because it was more difficult for the larger double-shelled microcapsules to penetrate the cement board surface.

Table 5 Effects of different doses of cyhalothrin/SiO₂ microcapsules on killing Musca domestica on different surfaces

	Spraying surface	Dose (mg m^−2)	Corrected mortality (%)
			1st day	30th day	60th day	90th day
SiO2 microcapsules	Glass plate	1	15.8	56.6	76.6	39.9
		10	80.4	80.0	93.7	84.2
		20	87.5	94.4	100.0	92.8
	Paint wood	1	26.6	63.6	75.0	56.2
		10	85.7	90.0	94.1	93.7
		20	85.4	92.8	95.2	95.7
	Cement board	1	0	0	0	10.0
		10	0	0.4	8.3	2.8
		20	0	5.4	30.7	15.3

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Table 6 Effects of different doses of cyhalothrin/SiO₂–NIPAM–MBA microcapsules on killing Musca domestica on different surfaces

	Spraying surface	Dose (mg m^−2)	Corrected mortality (%)
			1st day	30th day	60th day	90th day
SiO2–NIPAM–MBA microcapsules	Glass plate	1	64.4	76.9	70.9	70.3
		10	70.9	91.6	92.3	86.2
		20	90.9	91.6	92.3	100
	Paint wood	1	50.0	32.6	75.0	70.3
		10	83.3	90.0	100.0	96.8
		20	100.0	91.0	100.0	100.0
	Cement board	1	0	9.09	27.3	28.3
		10	8.3	14.2	36.2	29.6
		20	0	28.5	50.0	67.2

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Table 7 Effects of different doses of 4.5% cyhalothrin EC on killing Musca domestica on different surfaces

	Spraying surface	Dose (mg m^−2)	Corrected mortality (%)
			1st day	30th day	60th day	90th day
Cyhalothrin EC	Glass plate	1	100.0	57.3	36.3	10.0
		10	100.0	78.6	58.3	18.7
		20	100.0	100.0	85.0	36.2
	Paint wood	1	100.0	45.6	17.3	10.0
		10	100.0	76.3	35.2	12.5
		20	100.0	90.0	75.0	30.7
	Cement board	1	87.5	25.0	12.5	0
		10	100.0	37.6	26.6	10.0
		20	100.0	56.2	36.2	20.0

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4. Conclusion

In summary, we developed an environmentally friendly and cost-effective wall material, as well as prepared single- and double-shelled cyhalothrin microcapsules by an emulsion polymerization method using TEOS as the precursor and NIPAM as the outer shell. The resulting microcapsules, which were characterized by SEM, FTIR and TGA, had a high drug loading (single 50% and double 25%, w/w) and excellent sustained release performance. Furthermore, the double-shelled microcapsules had longer controlled release than that of the single-shelled ones, and they exhibited a better control effect and persisted for longer than cyhalothrin EC. Therefore, SiO₂–NIPAM–MBA is a potential material for fabricating controlled release pesticides.

Acknowledgements

This project supported by ‘‘key projects in the national science and technology pillar program during the Twelfth Five-year Plan Period’’ (nos: 2011BAE06B06).

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