Construction and evaluation of controlled-release delivery system of Abamectin using porous silica nanoparticles as carriers
© Wang et al.; licensee Springer. 2014
Received: 29 October 2014
Accepted: 25 November 2014
Published: 4 December 2014
Photolysis and poor solubility in water of Abamectin are key issues to be addressed, which causes low bioavailability and residual pollution. In this study, a novel hydrophilic delivery system through loading Abamectin with porous silica nanoparticles (Abam-PSNs) was developed in order to improve the chemical stability, dispersity, and the controlled release of Abamectin. These results suggest that Abam-PSNs can significantly improve the performance of controllable release, photostability, and water solubility of Abamectin by changing the porous structure of silica nanoparticles, which is favorable to improve the bioavailability and reduce the residues of pesticides.
KeywordsAbamectin Silica nanoparticles Controlled release Delivery system
Nowadays, chemical control is still an important way in protecting crops from biological disasters and reducing the yield loss. Compared with chemical pesticides, biopesticides have attracted increasing attention because of its high bio-efficiency, safety, and environmental friendliness. However, as a typical biopesticide, although Avermectin possesses such advantages over chemical pesticides, its conventional formulations still suffer from some shortages, such as environmental sensitivity, poor water solubility, and short duration and low bioavailability, which limited their large-scale applications in crop production.
The rapid development of nanoscience and nanotechnology provides new ways to improve the performances of conventional pesticide formulations by constructing nanodelivery system using nanomaterials as carriers. In nanomedicine field, many studies suggested that nanoscale carriers can improve the timed release of drug molecules, enable precisive drug targeting, and enhance the drug bioavailability[1–6]. But limited work has been performed on nanoparticle-based pesticide delivery systems. However, the pesticide nanodelivery system has great potential to precisely control the release of pesticide and remarkably reduce effective dosage by maintaining an effective concentration in the target for longer periods of time[7–9], which is favorable to improve the utilization, reduce the residues of pesticides, and avoid the pollution of environment and agricultural products. Moreover, pesticide nanodelivery systems have been proposed to produce a better spatial distribution on leaf surfaces of crops due to the nanoscale size and thereby improve effectiveness of pesticide applications. Besides, pesticide nanodelivery systems also have better penetration through the cuticle and allow slow and controlled release of active ingredients on the target[5, 6].
Silica nanoparticles have attracted many attentions due to their low cost, non-toxicity, high surface area, and high reactivity as drug carriers[10–15]. Although limited works have been done on the pesticide delivery system using silica nanoparticles as carriers, some functional defects still need to be improved. Currently, the silica nanoparticles as pesticide carriers did not show controllable porous surface properties, and release of pesticides was generally controlled by adjusting pesticides concentration and the thickness of coating layer, resulting in the relatively limited tuning range of release rate. Moreover, the size uniformity of silica nanocarriers need to be further improved because monodisperse carriers are favorable to promote adhesion and permeability of the pesticide on target crops.
In this study, porous silica nanoparticles (PSNs) with controlled surface properties were employed as novel carriers for loading Abamectin to form Abamectin with porous silica nanoparticles (Abam-PSNs) delivery system. Morphology, pesticide-loading capacity, release rate, and the photolysis of Abamectin were studied for Abam-PSNs delivery system with different porous structures.
Tetraethylorthosilicate (TEOS), poly(vinylpyrrolidone) (PVP, Mw approximately 10,000), sodium hydroxide (NaOH), and ammonium hydroxide (NH3 · H2O, 28% by weight in water) were purchased from Sigma-Aldrich (Beijing, China). Ethanol and isopropanol were of analytical grade and obtained from Thermo Fisher Scientific (Beijing, China). All chemicals were directly used as received without further purification.
Preparation of PSNs and Abam-PSNs
Fifty milliters of a mixture deionized H2O (49.5%), NH3 · H2O (18%), and ethanol (32.5%) was injected into 50 mL of a mixture of TEOS (9%) and ethanol (91%) at room temperature under magnetic stirring. After reacting for 2 h, the silica (SiO2) nanoparticles were collected by centrifugation. The prepared SiO2 nanoparticles were re-dispersed in deionized water and added with PVP solution (0.1 g/mL) at 100°C to synthesize SiO2@PVP nanoparticles. Finally, NaOH solution was added to the SiO2@PVP nanoparticles solution as an etching agent and reacted respectively for 45, 75, 105, and 120 min at 30°C to obtain four kinds of PSNs with controllable porous structure. The four kinds of PSNs with different porous structures were used as pesticide carriers to load Abamectin and form Abam-PSNs. In detail, PSNs with different porous structures were mixed with Abamectin in ethyl alcohol solution respectively, and then the mixtures were ultrasonicated for 5 min and vortexed for 24 h in a shaker. Finally, the Abam-PSNs powder was obtained by centrifugation at 13,000 × g for 15 min.
The morphologies of the PSNs and Abam-PSNs were investigated by the transmission electron microscopy (TEM, Hitachi-7650, Hitachi Ltd., Chiyoda-ku, Japan) and scanning electron microscope (SEM, JSM-6700 F, JEOL Ltd., Akishima-shi, Japan). The samples of PSNs and Abam-PSNs dispersed in water were cast onto a carbon-coated copper grid, followed by evaporation under vacuum at room temperature for TEM measurement. The prepared PSNs samples were added dropwise onto the silicon slice surface and dried at room temperature for SEM measurement. The specific surface area of the PSNs was measured by Brunauer-Emmett-Teller (BET) method. The pore structure analysis and nitrogen adsorption-desorption isotherms were conducted using a TristarII3020 surface area analyzer (Micromeritics Instrument Corporation, Norcross, GA, USA). The pore size distribution was calculated by Barrett-Joyner-Halenda (BJH) method.
Investigation on loading performance of Abam-PSNs
(V is the solution volume and M is the mass of porous silica carrier).
Investigation on controlled release behaviors of Abam-PSNs
The release profiles of Abamectin from the Abam-PSNs were investigated as follows: 1 mg Abam-PSNs samples were suspended in 20 mL ethanol solution. The suspension was transferred to the dialysis bag, which were sealed into the 100 mL ethanol solution as the release medium. The release rate of Abamectin from the Abam-PSNs sample was calculated by measuring the concentrations of Abamectin dissolved in the release medium at different times to evaluate the sustained release property. The concentrations of Abamectin were measured by UV-2250 spectrophotometer by collecting 5.0 mL of outside fluid at different intervals of 24, 48, 72, 100, 155, and 200 h. The pure Abamectin (TC) was used as a control.
Photolysis experiments of Abamectin in Abam-PSNs
The photolytic behavior of Abamectin in the as-prepared Abam-PSNs was evaluated with the pure Abamectin (TC) as a control. The samples were dissolved in ethanol and were irradiated under the light source of an UV fluorescent tube with an emitting light of 365 nm wavelength. The distance of the light source from the upper level of the samples solution was 15 cm. The Abamectin concentration of samples was analyzed at a specified time interval (0, 12, 24, 36, 48, 60, and 72 h).
Results and discussion
Preparation and characterization of PSNs
The results indicated that the PSNs can be successfully synthesized using NaOH as an appropriate etching agent, while carbonyl groups of PVP as the surface protecting agent can form the strong hydrogen bonds with the hydroxyls on the surface of the PSNs. The protection effect of PVP is favorable to increase the stability of silica spheres against collapsion in the etching process and leads to the formation of porous silica nanoparticles. In addition, PVP is favorable to improve pesticide-loading capacity and the water dispersity of the Abam-PSNs.
Effects of etching time on porous structure of PSNs
Effects of porous structure on Abamectin-loading capacity of Abam-PSNs
Controlled release behaviors of Abam-PSNs
The release rate of Abam-PSNs samples was relatively fast at initial stage and then gradually became slow with increased time, as the Abamectin adsorbed on the surface of Abam-PSNs was easier to release than that loaded within the carriers. For the sample with less internally porous structure (Abam-PSNs-1), Abamectin concentrated on the surface nanopores of silica carrier, which make it easier to release pesticide with the fastest release rate than other Abam-PSNs samples. Whereas, for the sample with richer porous structure (Abam-PSNs-3, Abam-PSNs-4), the Abamectin were mainly loaded on the internal porous structure of silica carriers, resulting in longer release time due to relatively slow and smooth release. For the sample of Abam-PSNs-2, the irregular release rate of Abamectin may be due to the mutual interference between fast release on the surface and slow release resulting from decreased surface area which leads to the decreased diffusion gradients and driving force of Abamectin. The results indicated that Abamectin loaded by Abam-PSNs can slowly and controllably be released by adjusting the porous structure of PSNs carriers, resulting in the validity of Abamectin in the target lasting for a longer time. The properties could not only increase the efficiency of pesticide applications and decrease the spraying dosage but also reduce residues and improve the environmental protection.
Effects of Abam-PSNs on photolysis of Abamectin
In order to improve controlled release, chemical stability, and bioactivity of Abamectin, we developed a nanodelivery system of Abam-PSNs by loading Abamectin with PSNs as carriers. PSNs are synthesized by the process of surface-protected etching, which have developed porous structure controlled by adjusting the etching time. As a novel pesticide carrier, PSNs show excellent pesticide-loading capacity and delivery performance in controlled release, anti-photolysis, and water dispersity. This delivery system is favorable to overcome the shortages of biopesticides, such as environmental sensitivity, poor water solubility, and hence improve efficacy in crops protection, decreased the spraying dosage, and reduce residues and pollution in food and environment.
YW is an assistant professor, HC is a professor, CS and BC are assistant professors, and XZ is a graduate student in the Institute of Environment and Sustainable Development in Agriculture, Chinese Academy of Agricultural Sciences.
This research was supported by the Major National Scientific Research Program of China (2014CB932200), National High Technology Research and Development Program of China (863 Program) (2011AA10A201), Agricultural Science and Technology Innovation Program and Basic Scientific Research Fund of National Nonprofit Institutes.
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