Highly efficient alginate sodium encapsulated chlorpyrifos/copper(II) Schiff base mesoporous silica sustained release system with pH and ion response for pesticide delivery

Abstract

Salicylaldehyde modified mesoporous silica (SA–MCM-41) was prepared through a co-condensation method. Through the bridge effect from copper ion, the chlorpyrifos (CH) was supported on copper(II)/Schiff base mesoporous silica (Cu–MCM-41). And then CH–Cu–MCM-41 was encapsulated by alginate sodium (ALG) to form a highly efficient sustained-release system (ALG–CH–Cu–MCM-41). The sustained-release performance of ALG–CH–Cu–MCM-41 at different pH values and different NaCl concentrations was investigated. The as-synthesized system showed significant pH responsiveness. Under the condition of pH ≤ 7, the release rate of chlorpyrifos decreased with pH increasing. Meanwhile, its release rate under weak base conditions was slightly larger than under weak acid conditions. The sustained release curves of ALG–CH–Cu–MCM-41 were consistent with the sustained release curves of CH–Cu–MCM-41 but with a smaller release rate because of the block effect from ALG. Meanwhile, the drug release rate of the sustained release system was also affected by ion concentration. Their sustained-release curves could be described by the Korsmeyer–Peppas equation.

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1. Introduction

In the past decade, mesoporous silica materials have attracted the attention of pharmaceutical researchers around the world. As we know, mesoporous silica materials have many unique properties, such as their non-toxic nature, large surface area and pore volume, and tunable pore size, as well as being chemically inert and having easily modified surface properties.^1–6 After over a decade of booming development, mesoporous silica nanoparticles have been regarded as one of the most promising biomedical platforms for therapeutic, diagnostic, prognostic, and combinatorial applications.^7,8 Benefiting from their stable mesoporous structures, large surface area, tunable pore size, easy surface functionalization, and good biocompatibility, mesoporous silica nanoparticles can not only be fine-tuned to achieve the desired physicochemical characteristics for accommodating multiple cargo molecules such as therapeutic drugs, proteins, genes, and imaging agents either alone or in combination, but also be engineered to facilitate on-demand drug release and multimodality imaging.^9–11 Meanwhile, various types of such materials have been developed into drug delivery systems.^12,13

Alginate (ALG) is a biodegradable, biocompatible, and natural marine based polysaccharides.^14–18 Under the specific pH conditions, the negatively charged carboxylate acid groups of alginate can form ionic bonds with the positively charged amino groups to create pH sensitive complexes.^19,20 As a result, the alginate complex has been applied to design pH responsive drug delivery carriers.

In the previous work, we prepared the pH responsive chlorpyrifos/copper Schiff base modified mesoporous silica (CH–Cu–MCM-41) which showed a high sustained-released performance.²¹ In this paper, we modified CH–Cu–MCM-41 with ALG to further improve its sustained released performance. ALG interacted with the copper ion from Cu–MCM-41 through ionic bond and encapsulated the CH–Cu–MCM-41 with the increase of the steric hindrance of the sustained released system. Besides, the system showed pH sensitivity and ion sensitivity which is useful for the pest control in agriculture.^22–24 The relationship between the NaCl concentrations and pH values with its sustained released performance was also investigated. Finally, the highly efficient sustained release system with pH-sensitivity and ion-sensitivity for pesticide delivery was developed which would thus be expected to bring positive effects in agricultural fields for pest control.

2. Experimental

2.1 Chemicals

Cetyl trimethyl ammonium bromide (CTAB), tetraethyl orthosilicate (TEOS), ethanol, dichloromethane, anhydrous magnesium sulfate, ammonia, copper nitrate, sodium hydroxide, hydrochloride were obtained from Tianjin Damao Chemical Reagents. 3-Aminopropyltriethyloxy silane (APTES), salicylaldehyde and alginate sodium (ALG) were obtained from Aladdin. And chlorpyrifos (Jiangsu Jinghong Chemical Engineering Co., Ltd.) were also used in this work. All chemicals were analytical grade and used as received without any further purification.

2.2 Preparation of salicylaldimine

According to the literature,²² 4.42 g of APTES, 2.44 g of salicylaldehyde and 100 mL of ethanol were added into a flask and reacted at 95 °C for 3 h. Ethanol was removed by rotary evaporated, and 20 mL of dichloromethane was added, then the products washed with deionized water 3 times. The organic layer was extracted and standing for 12 h. Then the product was filtered to remove dichloromethane to attain salicylaldimine.

2.3 Preparation of SA–MCM-41

According to previous research,²⁵ co-condensation method was adopted to prepare salicylaldehyde modified mesoporous silica (SA–MCM-41). 1.0 g of CTAB, 100 mL deionized water and 70 mL of ammonia were added to the flask to be dissolved at 60 °C with stirring. And 5 g of TEOS was added to the solution dropwise. 1 hour later, 1 g of as synthesized salicylaldimine was added and kept on reacting for 6 h before being crystallized at room temperature, filtered, washed and dried. Finally, the template was removed by ethanol to attain SA–MCM-41.

2.4 Preparation of Cu–MCM-41

20 mL of copper nitrate solution of 1.2 mol L⁻¹ was added to 200 mg of SA–MCM-41 at 35 °C under stirring for 24 h. Then Cu–MCM-41 was attained after being filtered, washed and dried.

2.5 The loading of chlorpyrifos

The supported chlorpyrifos was prepared via impregnation. Cu–MCM-41 was activated under vacuum at 80 °C for 6 h. And 100 mg of samples was immersed in 20 mL of chlorpyrifos ethanol solution (10 mg mL⁻¹) at 35 °C under stirring for 24 h, then filtered, washed, and dried. The obtained products were denoted CH–Cu–MCM-41. And the filter liquor was characterized by UV-vis to calculate the concentration after adsorption.

2.6 Preparation of ALG–CH–Cu–MCM-41

100 mg of samples was immersed in 20 mL deionized water under stirring, then injected with 10 mL alginate sodium solution (5.5 mg mL⁻¹) dropwisely. After under string for 4 h, the samples were filtered, washed, and dried to obtain the products denoted as ALG–CH–Cu–MCM-41.

2.7 Structural characterization of particles

The samples were analyzed using a Bruker AXS D8 X-ray diffractometer (Bruker AXS GmbH, Karlsruhe, Germany) with Cu radiation (λ = 1.5418 Å) and a graphite monochromator at 25 °C, 40 kV, and 30 mA. The measurements were scanned at 2° min⁻¹ (angular range 2θ = 0.5–10°) in 0.02° step size. The morphology of the particles was analyzed by a Spectrum100 Fourier infrared spectrometer (PerkinElmer Inc., USA) by using the KBr squash technique. The gold particles were sprayed on the surface of samples under protection of N₂ and the samples were characterized by an S4800 scanning electron microscope (Hitachi, Japan) to observe the surface topography. BET surface area of samples was determined by N₂ adsorption isotherms at 77 K, operated on Quadrasorb SI adsorption equipment. The samples were degassed at 200 °C for 12 h in vacuum before N₂ adsorption experiment. The zeta potential of the samples was investigated with a Zetasizer Nano ZS (Malvern Instruments) in water at pH = 7 through ultrasonic dispersion.

2.8 Loading amount calculation

A UV-2550 UV-vis spectrophotometer from Shimadzu Co., Japan, was applied to measure the amount of chlopyrifos adsorbed by mesoporous silica. Linear regression of the solution concentration (C) and absorbance (A) of chlorpyrifos standard solutions of different concentrations at λ = 290 nm was performed to obtain a standard curvilinear equation: C = 61.356A − 0.0613, R² = 0.9997. UV spectroscopy was performed to measure the absorbance of this solution before and after the adsorption in chlopyrifos ethanol solution (10 mg mL⁻¹). Loading content (LC) may be calculated by the following equation:where C₀ is the origin mass concentration (mg L⁻¹) of the chlorpyrifos in ethanol solution, C₁ is the mass concentration (mg L⁻¹) of the chlorpyrifos in ethanol solution after adsorption, and m is the mass (g) of mesoporous silica. V is the total volume of the solution.

2.9 Sustained-release performance test

The performance of sustained-release chlorpyrifos particles was tested according to the reference in room temperature and dispersed in 50 mL of 40% ethanol.²⁶ Linear regression of the solution concentration (C) and absorbance (A) of chlorpyrifos in 40% ethanol solutions of different concentrations at λ = 284 nm was performed to obtain a standard curvilinear equation: C = 48.672A + 0.0322, R² = 0.9994. The (M₁, mg) drug-loaded particles were weighed and placed in a conical flask filled with 50 mL of 40% ethanol. At intervals of (t), 1 mL of the sample solution was transferred and diluted to 25 mL. An equal volume of the original sustained-release solution was then added to the conical flask to replace the withdrawn sample. The absorbance of the 25 mL solution was obtained, and the cumulative release amount of chlorpyrifos was calculated as R_i. A t–R_i curve was drawn to study the release kinetics of chlorpyrifos. R_i and LC may be calculated by the following equation:where ρ_i is the mass concentration (mg L⁻¹) of chlorpyrifos for each sampling.

3. Results and discussion

3.1 Characterization of mesoporous materials

FTIR was carried out to compare the different composition of Cu–MCM-41, CH–Cu–MCM-41, and ALG–CH–Cu–MCM-41. As shown in Fig. 1. For Cu–MCM-41, the bands located at 3391 cm⁻¹, 1626 cm⁻¹ was the stretching vibration band from Si–OH and C=O respectively. Two bands appeared in 2855 cm⁻¹ and 2926 cm⁻¹ were ascribed to the characteristic peaks of methylene group. 1384 cm⁻¹ was the characteristic band of the coordination between copper ion and Schiff base. For CH–Cu–MCM-41, the characteristic peaks of chlopyrifos located at 1547, 1413 cm⁻¹ which proved that the chlopyrifos was successfully adsorbed by Cu–MCM-41. The characteristic peaks of ALG located at 3340 cm⁻¹ and 2926 cm⁻¹ for the spectra of ALG–CH–Cu–MCM-41. And the band of 1625 cm⁻¹ ascribed to COO⁻ group of ALG was significantly enhanced which confirmed the inclusion by ALG.

Fig. 1 FTIR spectra of Cu–MCM-41, CH–Cu–MCM-41, and ALG–CH–Cu–MCM-41.

Fig. 2 shows the XRD patterns of Cu–MCM-41, CH–Cu–MCM-41 and ALG–CH–Cu–MCM-41. There are three characteristic peaks which could be ascribed to (100), (110) and (200) crystal face respectively indicated that the particles had regular hexagonal pore structure.²⁷ As modified by salicylaldehyde and coordinated with copper ion, the strength of the XRD peaks of (110) and (200) crystal face decreased, which proved that APTES and copper ion were introduced to the system and decreased its degree of orderliness.²⁸ And loading of chlorpyrifos and encapsulated by ALG didn't change its structure indicating that the regular hexagonal pore structure remained in the samples.

Fig. 2 XRD patterns of Cu–MCM-41, CH–Cu–MCM-41 and ALG–CH–Cu–MCM-41.

As shown in Fig. 3a, the N₂ adsorption/desorption isotherms of Cu–MCM-41, CH–Cu–MCM-41 and ALG–CH–Cu–MCM-41 belong to Langmuir IV (the slope of it was decreasing) with H4 hysteresis loop (hysteresis loop was closed at p/p_o = 0.4) which confirmed their mesoporous structure according to the previous report.²⁹ What's more, loading of chlorpyrifos and inclusion by ALG would significantly decrease its BET surface and pore volume because of pores blocked by chlorpyrifos and ALG as shown in Table 1 and Fig. 3b, but draw slight effect on the pore size.

Fig. 3 N₂ adsorption/desorption isotherms (a) and pore size distribution (b) of Cu–MCM-41, CH–Cu–MCM-41 and ALG–CH–Cu–MCM-41.

Table 1 The pore structural parameter of Cu–MCM-41, CH–Cu–MCM-41 and ALG–CH–Cu-MCM-41

Sample	BET surface (m^2 g^−1)	Pore volume (cm^3 g^−1)	Pore size (nm)
Cu–MCM-41	545	0.173	3.60
CH–Cu–MCM-41	302	0.148	3.60
ALG–CH–Cu–MCM-41	243	0.118	3.50

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Fig. 4 depicts the SEM image of ALG–CH–Cu–MCM-41 and CH–Cu–MCM-41. As shown, the regular hexagonal pore structure was well-maintained without agglomeration for both samples was in consistent with the XRD results. In Fig. 4a the surface of CH–Cu–MCM-41 was rough for the coordination with copper ion. After encapsulated by ALG, a thin layer was detected on the surface of samples as shown in Fig. 4b.

Fig. 4 SEM image of CH–Cu–MCM-41 (a) and ALG–CH–Cu–MCM-41 (b).

Zeta potential studies of CH–Cu–MCM-41 and ALG–CH–Cu–MCM-41 were investigated at pH = 7. As listed in Table 2, the zeta potential of CH–Cu–MCM-41 shifted from 45.4 mV to −14.5 mV after encapsulated by sodium alginate caused by the negative ion from the carbonic of sodium alginate.³⁰ The results proved that the surface of CH–Cu–MCM-41 was modified with sodium alginate through ionic interaction.

Table 2 The zeta potential of CH–Cu–MCM-41 and ALG–CH–Cu–MCM-41

Samples	CH–Cu–MCM-41	ALG–CH–Cu–MCM-41
Zeta potential/mV	45.4	−14.5

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3.2 Sustained released test

Fig. 5b depicts the sustained release curves of ALG–CH–Cu–MCM-41 at various pH with a sequence of pH = 3 > pH = 9 > pH = 5 > pH = 7 in release rate. And the released rate was smaller comparing to Fig. 5a which has been done in our previous work²¹ in the same pH value. Under acid conditions, the lower the pH was, the faster the release rate was in consistence with the sustained release curves of CH–Cu–MCM-41. Because the Schiff base was unstable in acid, C=N tended to decompose. As a result, the coordination bond between copper ion and C=N broke down, and the nitrogen on Schiff base was protonated. So the interaction between chlorpyrifos and substrate was weakened and the release rate was accelerated.³¹ Under basic conditions, the coordination between hydroxide ion and copper ion was stronger than that between Schiff base and copper ion which apparently weakened the interaction between C=N and copper ion and between copper ion and ALG. So the release rate was slightly faster than in acid (pH = 5). While under more acid conditions at pH = 3, the release rate was even faster than in weak base. The results proved that ALG–CH–Cu–MCM-41 was pH sensitive with various sustained released performance according to the different pH value and had a smaller released rate comparing to CH–Cu–MCM-41 because of the block effect from ALG.

Fig. 5 Effect of pH values on sustained released performance of CH–Cu–MCM-41 (a)²¹ and ALG–CH–Cu–MCM-41 (b), and effect of NaCl concentrations on sustained released performance of ALG–CH–Cu–MCM-41 (c).

Fig. 5c shows the sustained released performance of ALG–CH–Cu–MCM-41 in different concentration of sodium chloride solution at pH 7. The larger the concentration of NaCl was, the faster its release rate was with a sequence of 1.0 mol L⁻¹ > 0.5 mol L⁻¹ > 0 mol L⁻¹. While the concentration of NaCl increased, ionic balance inside the control released system was broken. As a result, sodium alginate was peeled off from the particles leading to a faster release rate. And results confirmed the ion response of ALG modified chlorpyrifos/copper(II) Schiff base mesoporous silica sustained release system.

3.3 Kinetics study

To further understand the sustained release mechanism, the data of sustained release of chlorpyrifos from ALG–CH–Cu–MCM-41 in various pH values and various NaCl concentration were fitted to zero-order model, first order model, Higuchi model,³² and Korsmeyer–Peppas model³³ respectively. As shown in Table 3 which we had calculated in our previous work,²¹ the drug release behavior of sustained-release CH–Cu–MCM-41 particles was in consistent with the Korsmeyer–Peppas kinetic equation. When pH = 5, 7 and 9, the diffusion coefficient nth power for time (t) n are 0.3233, 0.3108 and 0.4087 calculated from the kinetic equation, while all of them were below 0.45. And the values obtained indicate that the sustained release of chlorpyrifos from the particles was controlled by a Fickian diffusion mechanism^34–36 which proved that the difference of the concentration is the main impact on the release process. When pH = 3, the diffusion coefficient n is 0.6485 which is larger than 0.45 and the mechanism become non-Fickian, namely synergic effect of diffusion and coordination bond breaking between the Schiff base and the copper ion as well as between the copper ion and chlorpyrifos. As a result, obvious “sudden release” activity was observed at pH = 3 as mentioned above.

Table 3 Fitting results for drug release curves of CH–Cu–MCM-41 in various pH values²¹

Kinetic model	Fitting equation	R^2
Zero-order		0.5072
		0.6311
		0.6840
		0.5631
First-order		0.6801
		0.7118
		0.7208
		0.6980
Higuchi		0.7603
		0.8623
		0.8981
		0.8023
Korsmeyer–Peppas		0.9887
		0.9485
		0.9439
		0.9249

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As shown in Tables 4 and 5 respectively. The drug release behavior of ALG–CH–Cu–MCM-41 sustained-release particles was in consistent with the Korsmeyer–Peppas kinetic equation. When under various pH value, pH = 3, 5 and 7, the diffusion coefficient n are all above 0.45 which is in contrary to CH–Cu–MCM-41. The mechanism become non-Fickian, namely synergic effect of diffusion and structure erosion. When pH = 9, the diffusion coefficient n is 0.4315 which is below 0.45, and the values obtained indicate that the sustained release of chlorpyrifos from the particles was controlled by a Fickian diffusion mechanism^32,33 which proved that the difference of the concentration is the main impact on the release process caused by the sodium alginate peeled off from the particles in basic conditions. While adding NaCl to the solution, the diffusion coefficient decreased below 0.45 again caused by the sodium alginate peeled off from the particles for the broken of ionic balance inside the sustained released system which is consistence with the sustain released performance mentioned above.

Table 4 Fitting results for release curves of ALG–CH–Cu–MCM-41 in various pH values

Kinetic model	Fitting equation	R^2
Zero-order		0.8794
		0.7780
		0.8124
		0.6115
First-order		0.9762
		0.8344
		0.8312
		0.7150
Higuchi		0.9835
		0.9439
		0.9638
		0.8319
Korsmeyer–Peppas		0.9914
		0.9659
		0.9750
		0.9028

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Table 5 Fitting results for release curves of ALG–CH–Cu–MCM-41 in different NaCl concentration

Kinetic model	Fitting equation	R^2
Zero-order		0.8124
		0.6784
		0.7006
First-order		0.8312
		0.7086
		0.7407
Higuchi		0.9638
		0.8848
		0.8993
Korsmeyer–Peppas		0.9750
		0.9424
		0.9652

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By now, the complete drug release of ALG–CH–Cu–MCM-41 was generally illustrated in consideration of kinetic mechanism and the data in this work, as shown in Fig. 6. The layer of ALG on the surface of the mesoporous silica prevented the chlorpyrifos from spreading outside the particles. Under acid or basic conditions, the C=N bond in Schiff base would decompose and the chlorpyrifos would be released faster. Under basic conditions, hydroxide ion would compete with ALG to interact with copper ion while further weaken the interaction between copper ion and ALG. As a result, the ALG would be peeled off and the released rate of CH–Cu–MCM-41 would be accelerated as well as adding NaCl.

Fig. 6 The schematic diagram of drug release of ALG–CH–Cu–MCM-41.

4. Conclusions

pH-Responsive and ion-responsive sustained release system of alginate sodium modified chlorpyrifos/copper(II) Schiff base mesoporous silica was prepared. The characterization confirmed the existence of ionic bond between ALG and CH–Cu–MCM-41 which formed a ropy network among the particles. The regular hexagonal pore structure and pore size were well-maintained without agglomeration. The as-synthesized system showed significant pH sensitivity with a smaller release rate after encapsulated by ALG because of the block effect. When pH ≤ 7, the release rate of chlorpyrifos decreases with pH increasing. And in weak base condition, its release rate is slightly larger than the weak acid condition. Their sustained-release curves could be described by Korsmeyer–Peppas equation in consistence with unFickian diffusion mechanism at 3 < pH < 7 and in consistence with Fickian diffusion mechanism at pH = 9 or adding NaCl caused by the sodium alginate peeled off from the particles for the broken of ionic balance inside.

Acknowledgements

This research was supported by National Natural Science Foundation of China (Grant No. 21576303, 21606262), Natural Science Foundation of Guangdong Province (Grant No. 2016A030313375), Science and Technology Planning Project of Guangdong Province (Grant No. 2014A020208126, 2015A020209197), Science and Technology Program of Guangzhou, China (Grant No. 201510010150).

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