Article pubs.acs.org/JAFC
Preparation and Characterization of Controlled-Release Avermectin/ Castor Oil-Based Polyurethane Nanoemulsions Hong Zhang, He Qin, Lingxiao Li, Xiaoteng Zhou, Wei Wang, and Chengyou Kan* Department of Chemical Engineering and Key Laboratory of Advanced Materials of Ministry of Education, Tsinghua University, Beijing 100084, People’s Republic of China ABSTRACT: Avermectin (AVM) is a low-toxic and high-active biopesticide, but it can be easily degraded by UV light. In this paper, biodegradable castor oil-based polyurethanes (CO-PU) are synthesized and used as carriers to fabricate a new kind of AVM/CO-PU nanoemulsion through an emulsion solvent evaporation method, and the chemical structure, colloidal property, AVM loading capacity, controlled-release behavior, foliar adhesion, and photostability of the AVM/CO-PU drug delivery systems are investigated. Results show that AVM is physically encapsulated in the CO-PU carrier nanospheres, the diameter of the AVM/ CO-PU nanoparticles is 85%. The release profiles indicate that the release rate is relatively high at the early stage and then slows, which can be adjusted by loaded AVM content, temperature, and pH of the release medium. The foliar pesticide retention of the AVM/CO-PU nanoemulsions is improved, and the photolysis rate of AVM in the AVM/CO-PU nanoparticles is significantly slower than that of the free AVM. A release mechanism of the AVM/CO-PU nanoemulsions is proposed, which is controlled by both diffusion and matrix erosion. KEYWORDS: avermectin, castor oil-based polyurethane, drug-loaded nanoemulsion, controlled release, photostability
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simple immersion loading method or supercritical fluid technology, which showed good controlled-release behavior and UV-shielding property.23,24 Jia et al. used polydopamine (PDA) microcapsules to encapsulate AVM, and they found that the AVM/PDA system could not only supply the controlledrelease and UV-shielding properties but also prolong the foliar pesticide retention by adjusting the adhesion property of the microcapsule surface.25,26 In addition, a novel double-shelled AVM microcapsule with the organic−inorganic composite of silica−glutaraldehyde−chitosan as the carrier was also prepared, and better controlled-release longevity was obtained in comparison with single-shelled microcapsules.27 However, the sizes of most AVM drug-loaded particles are not in the range of 1−100 nm, which does not conform to the rigid definition of nanotechnology.28 Taking advantage of the benefits of materials at nanoscale to prepare nanoscale pesticide delivery systems would improve their performances in agricultural application because the nanoscale pesticide delivery system has a smaller size and larger surface area, which is beneficial to spread uniformly over leaves and improve foliar pesticide deposition and retention.6,29 As is known, polyurethane (PU) is a polymer with a variety of building blocks and alternating soft and hard segments, which can be designed and adjusted to meet different requirements, and has been applied in biomedical devices and drug delivery systems.30−33 The polymers prepared by using
INTRODUCTION Pesticides are very important agrochemicals and largely used in agriculture to maintain high crop yields and sufficient food supplies, but most of the pesticides are lost or decomposed in application,1 and only about 0.1% can finally affect harmful organisms.2 Without doubt, the low bioavailability and overdosing will bring about problems of environmental pollution and human health.3,4 To overcome these drawbacks of conventional pesticides, pesticide delivery systems have been investigated with the development of nanotechnology over the past few years.5,6 Because an advanced pesticide delivery system can provide a sustained effect through maintaining a stable release rate and an appropriate effective concentration of active ingredient over a specified period of time, an appropriate controlled-release formulation would contribute to a decrease of the waste and harm of pesticides, as well as an improvement of bioavailability.7−9 Avermectin (AVM), an alternative of high-toxic pesticides, is recognized as a nuisanceless biological pesticide with a broad insecticidal spectrum and high activity. However, its disadvantages are also obvious, which lead to not only overdosing and high cost but also harm to the environment and humans. On the one hand, a large amount of organic solvent has to be used in the main AVM emulsifiable concentrate formulation for its water insolubility. On the other hand, AVM is easily photooxidized and degraded under irradiation of UV light, resulting in poor photostability and short half-life.10−13 Thus, controlledrelease AVM formulations have been a concern, and some AVM delivery systems with continuous release and UV degradation resistance have been investigated, in which some inorganic materials14−17 and polymers18−22 have been used to load AVM. Wen et al. used porous hollow silica nanoparticles (PHSN) as carriers to prepare AVM/PHSN systems by a © XXXX American Chemical Society
Special Issue: Nanotechnology Applications and Implications of Agrochemicals toward Sustainable Agriculture and Food Systems Received: Revised: Accepted: Published: A
March 27, 2017 May 26, 2017 May 31, 2017 May 31, 2017 DOI: 10.1021/acs.jafc.7b01401 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
Article
Journal of Agricultural and Food Chemistry renewable sources such as vegetable oil as starting material have attracted widespread attention for economic and environmental concerns in recent years.34,35 Specifically, as a biodegradable material, castor oil-based PU (CO-PU) has been synthesized due to its low cost, low toxicity, and renewability.36,37 However, there was little research on PU-containing pesticide delivery systems, especially for systems at nanoscale.38,39 Hence, COPU is considered to be a suitable material for the pesticide delivery system due to its biodegradability, optimized size at nanoscale, and other adjustable physical properties. In this paper, waterborne CO-PU was chosen as a novel carrier to load AVM, and a new kind of AVM/CO-PU drugloaded nanoemulsion was prepared through an emulsion solvent evaporation method. The colloidal property, the chemical structure, and the performances of the AVM/COPU drug-loaded systems were investigated.
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Table 1. Recipes for the Preparation of AVM/CO-PU Nanoemulsions CO-PU emulsion
sample
a
AVM/CO-PU (wt %)
amt of AVM (g)
amt of acetone (mL)
amt (g)
solid cont (wt %)
USMa
U1 U2 U3
20 30 50
0.80 1.20 2.00
20 20 30
13.74 13.74 13.74
29.1 29.1 29.1
HSDMb
H1 H2 H3 H4
20 30 40 50
1.00 1.50 2.00 2.50
35 35 42 49
18.90 18.90 18.90 18.90
26.5 26.5 26.5 26.5
Ultrasonic method. bHigh speed dispersing method.
MATERIALS AND METHODS Characterization. Hydrodynamic diameter (Dh), polydispersity index (PI), and zeta potential (ζ) of the nanoparticles were determined on a Zetasizer 3000HS (Malvern, UK) at 25 °C, and the samples were prepared by diluting the emulsions with water to a solid content of about 0.1 wt %. Fourier transform infrared (FTIR) spectra were recorded on a FTIR spectrometer (Thermo Fisher Scientific, Nicolet 560, USA) using the free AVM solid powder in KBr or the thin latex films of the nanoemulsions. UV−vis spectra were recorded on an UV−vis spectrophotometer (Pgeneral, T6, China), and the absorbance was determined under the maximum absorption wavelength (λmax) of 245 nm. The AVM concentration of the samples was 85% for all of AVM/ CO-PU nanoemulsions despite a slight decrease, indicating that the loss of the AVM in the preparation of AVM/CO-PU nanoemulsions was small and most of the AVM could be effectively encapsulated in CO-PU nanoparticles. It is worth
Mt = kt n M∞ G
(3) DOI: 10.1021/acs.jafc.7b01401 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
Article
Journal of Agricultural and Food Chemistry Mt is the mass of AVM released at time t, M∞ is the mass of AVM released as time approaches infinity, k is a constant, and n is the diffusional exponent characteristic of the release mechanism. The fitting results of the release profiles are given in Table 4. The correlation coefficients (r2) were >0.94, indicating that the release behavior of AVM from the AVM/CO-PU nanoemulsions was in good correlation with the Ritger−Peppas empirical equation. According to the literature,47 because the values of n in all of the AVM/CO-PU release profiles were between 0.45 and 0.55, the release mechanism of the AVM/ CO-PU nanoemulsions belonged to non-Fickian transport, and the release of AVM from the AVM/CO-PU nanoemulsions was controlled by both diffusion and matrix erosion.2,21 Adhesion Property of the AVM/CO-PU Nanoemulsions. To prove the AVM/CO-PU drug-loaded system could prolong foliar pesticide retention, measurements of the adhesion property were conducted according to the literature.25 The results indicated that for the AVM/CO-PU nanoemulsions, many small particles deposited on the corn leaves (Figure 7a,b), and most of these particles remained on the leaves after washing (Figure 7a′,b′). However, for the free AVM, although some larger particles deposited on the corn leaf at first, most of them were washed away from the leaf (Figure 7c,c′). It is clear that the AVM/CO-PU nanoemulsions had better adhesion property and could improve the foliar pesticide retention. Photostability of the AVM/CO-PU Drug-Loaded System. Because AVM is easily degraded by UV irradiation, the protection of AVM from photolysis is important for an AVM drug delivery system. Here, an acceleration test under the irradiation of a 1000 W UV lamp was used to investigate the photostability of AVM in the AVM/CO-PU nanoemulsion (sample H1), and the results are plotted in Figure 8. Apparently, the decomposition rate of AVM in the AVM/ CO-PU drug-loaded system was slower than that of the free AVM, which was attributed to the UV-shielding and protective effect of the CO-PU carrier on AVM. As shown in Figure 8, the irradiation time for the free AVM to decompose to 50% was 3.5 min, whereas for the AVM in the AVM/CO-PU drug-loaded system this was prolonged to 11.5 min, indicating that the AVM/CO-PU drug-loaded system had better photostability. It should be noted that because the decomposition rate of the AVM was accelerated greatly under the irradiation of a 1000 W UV lamp in this work, the actual decomposition rate will be much slower in the condition of normal sunlight. Thus, the AVM/CO-PU drug-loaded system had better photostability and would be helpful to reduce the loss of AVM caused by photolysis in agricultural application. In summary, a new kind of AVM/CO-PU nanoemulsion with biodegradable CO-based polyurethane as the carrier was successfully prepared by means of an emulsion solvent evaporation method, and the AVM loading capacity was up to 42.3 wt % with a high encapsulation efficiency >85%. The diameter of all the AVM/CO-PU nanoparticles was
