Magnetic-Sensitive Nanoparticle Self-Assembled Superhydrophobic

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Magnetic-Sensitive Nanoparticle Self-Assembled Superhydrophobic Biopolymer-Coated Slow-Release Fertilizer: Fabrication, Enhanced Performance and Mechanism Jiazhuo Xie, Yuechao Yang, Bin Gao, Yongshan Wan, Yuncong C. Li, Dongdong Cheng, Tiqiao Xiao, Ke Li, Yanan Fu, Jing Xu, Qinghua Zhao, Yanfei Zhang, Yafu Tang, Yuanyuan Yao, Zhonghua Wang, and Lu Liu ACS Nano, Just Accepted Manuscript • Publication Date (Web): 28 Feb 2019 Downloaded from http://pubs.acs.org on February 28, 2019

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ACS Nano

Magnetic-Sensitive

Nanoparticle

Self-Assembled

Superhydrophobic

Biopolymer-Coated

Slow-Release Fertilizer: Fabrication, Enhanced Performance and Mechanism

Jiazhuo Xie,† Yuechao Yang,*,†,§ Bin Gao,‡ Yongshan Wan,† Yuncong C. Li,§ Dongdong Cheng,† Tiqiao Xiao,┴,#,∆ Ke Li,┴,∆ Yanan Fu,┴,# Jing Xu,║ Qinghua Zhao,║ Yanfei Zhang,║ Yafu Tang,† Yuanyuan Yao,† Zhonghua Wang,† and Lu Liu†

†National

Engineering Laboratory for Efficient Utilization of Soil and Fertilizer Resources;

National Engineering & Technology Research Center for Slow and Controlled-release Fertilizers, College of Resources and Environment, Shandong Agricultural University, Taian, Shandong 271018, China ‡Agricultural

and Biological Engineering, Institute of Food and Agricultural Sciences,

University of Florida, Gainesville, FL 32611-0570, USA §Department

of Soil and Water Science, Tropical Research and Education Center, IFAS,

University of Florida, Homestead, FL 33031, USA ║College

of Chemistry and Materials Science, Shandong Agricultural University, Taian,

Shandong 271018, China ┴Shanghai

Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China

#Shanghai

Synchrotron Radiation Facility/Zhangjiang Lab, Shanghai Advanced Research

Institute, Chinese Academy of Sciences, Shanghai 201210, China ∆University

of Chinese Academy of Sciences, Beijing 100049, China

*Corresponding author Yuechao Yang Tel: +86-538-824 2900 E-mail: [email protected] 1 ACS Paragon Plus Environment

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ABSTRACT Despite commercialized slow-release fertilizers coated with petrochemical polymers have revolutionarily promoted agricultural production, more research should be devoted to developing superhydrophobic biopolymer coatings with superb slow-release ability from sustainable and ecofriendly biomaterials. To inform the development of the superhydrophobic biopolymer-coated slow-release fertilizers (SBSF), the slow-release mechanism of SBSF needs to be clarified. Here, the SBSF with superior slow-release performance, water tolerance and good feasibility for large-scale production was self-assembly fabricated using a simple, solvent-free process. The superhydrophobic surfaces of SBSF with uniformly dispersed Fe3O4 superhydrophobic magnetic-sensitive nanoparticles (SMNs) were self-assembly constructed with the spontaneously migration of Fe3O4 SMNs towards the outermost surface of the liquid coating materials (i.e., pig fat-based polyol and polymethylene polyphenylene isocyanate in a mass ratio 1.2 : 1) in a magnetic field during the reaction-curing process. Result revealed the SBSF showed longer slow-release longevity (more than 100 days) than those of unmodified biopolymer-coated slow-release fertilizers and excellent durable properties under various external environment conditions. The governing slow-release mechanism of SBSF was clarified by directly observing the atmosphere cushion on the superhydrophobic

biopolymer

coating

using

the

synchrotron

radiation-based

X-ray

phase-contrast imaging technique. Liquid water only contacts the top of the bulges of solid surface (89.1%) and air pockets are trapped underneath the liquid (10.9%). The atmosphere cushion allows the slow diffusion of water vapor into the internal urea core of SBSF, which can decrease the nutrient release and enhance the slow-release ability. This self-assembly synthesis of SBSF through the magnetic interaction provides a strategy to fabricate not only ecofriendly biobased slow-release fertilizers but also other superhydrophobic materials for various applications.

KEYWORDS: self-assembly · superhydrophobic · biopolymer · Fe3O4 magnetic-sensitive nanoparticles · atmosphere cushion · slow-release mechanism · durable properties 2 ACS Paragon Plus Environment

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In nature, many species show extreme water repellency (superhydrophobic characteristic)1,2 to avoid the invasion of water droplets (lotus leaf),3,4 acquire water source (desert beetle),5 survive upon the water (legs of water striders),6 etc. Based on the observations of the microstructures of these species, researchers have found that their superhydrophobic characteristics can be attributed to the micro-nanoscale heaves within the microstructures, which can lower surface energy.7,8 Biomimetic superhydrophobic technologies, inspired by the micro-nanoscale structures of natural superhydrophobic surfaces, have been developed rapidly to fabricate diverse

biomimetic

superhydrophobic

materials

with

many

performances,

such

as

self-cleaning,9,10 self-healing,11 antifreezing,12-14 antisnow,15 antimicrobial,16 anticorrosion,17,18 antifouling,19-21 anti-bioadhesion22 and water/oil separation.23-25 Artificial superhydrophobic materials are commonly prepared by enlarging surface roughness treatment, such as spray coating,26 template method,27 lithography,28 plasma treatment,29 laser ablation,30 and deposition,31 and subsequent reducing surface energy process by incorporating the low surface energy substance such as fluoroalkyl silanes,32 alkyl silanes, thiols, fatty acids with long alkyl chains,33 perfluorinated polymers,34 polysiloxane35, silicones,36,37 and carbon tetrafluoride.38 The development and application of biomimetic superhydrophobic materials have been promoted recently. Cassie and Baxter firstly developed relevant equation to describe the superhydrophobic state.39 Barthlott et al. developed biomimetic superhydrophobic self-cleaning materials.40,41 Jiang

and

coauthors

binary collaboration

firstly

theory

raised

and

superhydrophobicity

contributed

significantly

and to

superhydrophilicity

bioinspired

intelligent

superhydrophobic interfacial materials.3,42-44 Fujishima’s group is well-known for the work on the light-induced super-amphiphobic interface materials.45-47 Despite numerous functions of the bio-inspired superhydrophobic materials have been recognized, additional attention is needed to explore and expand their applications, such as slow-release fertilizers, slow-release drugs, etc. Slow-release fertilizers have been worldwide developed and applied to improve nutrient use efficiency and reduce environmental pollution.48 Commercialization of slow-release fertilizers coated with water-repellent petroleum-based polymer (polyethylene and polyurethane etc.) have 3 ACS Paragon Plus Environment

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revolutionarily

promoted

agricultural

production;

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however,

the

nonrenewable

and

nonbiodegradable disadvantages and the potential environmental risks of the coating materials have limited the large-scale and long-term application of commercialized slow-release fertilizers.49 In recent years, increasing interest has been devoted to developing biopolymer coatings from sustainable ecofriendly biobased materials, such as starch,48,50 lignin and cellulose,51-53 chitosan,54 L-aspartic acid, castor oil,55 and waste frying oil56 to substitute petroleum-based coating materials. However, the poor hydrophobic and mechanical behaviors of biobased coating materials result in undesirable nutrient release characteristics, which cannot synchronize the nutrient release longevity of crops’ nutrient requirement.53,57 To improve the water repellency of the biobased polymer materials, superhydrophobic treatment using hydrophobic substances has been proposed.53 However, the slow-release performance of biobased polymer coatings still needs to be further enhanced. Considering the extreme non-wetting characteristic of superhydrophobic surfaces,58-61 incorporating the biomimetic technologies to fabricate superhydrophobic biobased slow-release fertilizers can be an effective mean to the advancement of the slow-release technology. In a previous work, we enhanced the slow-release performance of a biopolymer-coated slow-release fertilizer by firstly introducing the biomimetic superhydrophobic technique.62,63 However, we did not clarify the governing mechanisms of the significant slow-release improvement because we could not obtain any direct evidences to confirm the presence of atmosphere “outwear” on the outermost surfaces of the superhydrophobic slow-release fertilizer. Furthermore, the complicated and solvent-employed process, the non-environmentally friendly modifier and the low productions have limited the large-scale development and application of superhydrophobic slow-release fertilizers. With synthetically considering ecofriendly feedstock (e.g., long-chain fatty acid and edible materials etc.),33,64-67 simple and solvent-free fabrication process,43,68-70 and the self-assembly methods,30,71-74 superhydrophobic slow-release fertilizers can be manufactured by simple, cost-effective and ecofriendly processes. 4 ACS Paragon Plus Environment

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Here, a superhydrophobic biopolymer-coated slow-release fertilizer with the enhanced slow-release performance and good feasibility for large-scale production was one-step fabricated with a simple, solvent-free and self-assembly process. The key aspects of this work are as follows: (1) Superhydrophobic-modified magnetic-sensitive nanoparticles Fe3O4 was employed to construct the superhydrophobic surfaces of SBSF through self-assembly. With the spontaneous migration of SMNs towards the outermost surface of SBSF from the interior of liquid coating materials through the magnetic interaction between SMNs and ferruginous wall of the coating machine in a magnetic field, SMNs were dispersed uniformly on the outermost surface of the biopolymer coatings to construct the superhydrophobic microstructure of SBSF. To satisfy the high demand of SMNs in future large-scale production of SBSF, high-energy ball milling was firstly employed in the superhydrophobic-modification of SMNs in one pot with long-chain fatty acid. (2) Green, sustainable and ecofriendly pig fat was applied as feedstock to prepare the biopolymer coatings of SBSF without any by-products. (3) The mechanism of superhydrophobic surfaces of SBSF to enhance the slow-release performance was revealed by directly detecting the atmosphere cushions on the surface of outermost biopolymer coatings of SBSF using the synchrotron radiation-based X-ray phase-contrast imaging technique.75 Furthermore, the durable properties (durability, wear resistance, water resistance and durablilty to soil environment) of SBSF to external environment were also systematically studied. The self-assembled fabrication of SBSF by magnetic interaction provides a direction for developing ecofriendly biopolymer-coated slow-release fertilizers. The method to self-assembly fabricate superhydrophobic surface can also be extended to develop superhydrophobic materials for other applications.

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Figure 1. Schematic diagram of the fabrication process of the SBSF. RESULTS AND DISCUSSION Here, we report the self-assembly fabrication of superhydrophobic biopolymer-coated slow-release fertilizers using a simple, solvent-free process. Low-cost, eco-friendly, and renewable pig fat was selected as the raw material, which is one of the eight commonly used oils and fats. Pig fat has excess saturated fatty acid (57.95%), the highest trans-fatty acids contents (4.07%), and relatively low melting point (≈ 32 °C) (Figure S1, Table S1-S3). When used in the production process of slow-release fertilizer, it may introduce multiple benefits including reducing human health risks and saving energy. Specially, glycerin, polyol, and fatty acid in the fabrication of SMNs and biobased polyurethane coating material were all derived from pig fat with sufficient chemical reactions without any by-products (Figure S2-S5). FTIR, 1H-NMR,

13C-NMR,

SEM and ATR-FTIR (Figure S4, S5) were employed to confirm the successful

synthesis and soil degradation of pig fat-based biopolymer coating materials. The relevant results and discussion can be found in Supporting Information. Superhydrophobic Modification of Magnetic-Sensitive Nanoparticles. The Fe3O4 6 ACS Paragon Plus Environment

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magnetic-sensitive nanoparticles of two different sizes (20 nm, abbreviated as MN-20; 500 nm, abbreviated as MN-500) were employed to construct the superhydrophobic surfaces. The Fe3O4 magnetic-sensitive nanoparticles were modified with long-chain fatty acid derived from pig fat to make them superhydrophobic and thus facilitate their uniform dispersion in the liquid coating materials (i.e., pig fat-based polyol and isocyanate). To overcome the disadvantages of traditional

preparation

magnetic-sensitive

methods

nanoparticles,

(e.g. such

hydrothermal as

synthesis

high-temperature,

method)

of

high-pressure,

Fe3O4 and

solvent-employed preparation process with low yield, this work firstly used the high-energy ball milling to prepared SMNs from natural Fe3O4 mineral in one pot, utilizing the violent grinding and the accompanying generated heat to satisfy the large energy demand. Two Fe3O4 superhydrophobic magnetic-sensitive nanoparticles with different size (20 nm, abbreviated as SMN-20; 500 nm, abbreviated as SMN-500) were obtained to construct superhydrophobic surfaces of slow-release fertilizers by the spontaneous movement of SMNs in the liquid coating materials due to the magnetic interaction between SMNs and ferruginous wall of the coating machine.


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Figure 2. Microstructure, elemental composition and corresponding valence, and elemental distribution of two SMNs with different sizes (SMN-20 and SMN-500): TEM images of SMN-20 (A1) and SMN-500 (B1); XPS spectra of MN-20/SMN-20 (A2-A5) and MN-500/SMN-500 (B2-B5) before (a) and after (b) superhydrophobic modification; EDX maps corresponding to the TEM images of SMN-20 (A6) and SMN-500 (B6); Photograph of SMNs (mixture of SMN-20 and SMN-500) in water showing white atmosphere membrane (C). The TEM was employed to determine the size and microstructure of SMNs. The SMN-20 was ~20 nm size (Figure 2A1) and SMN-500 was ~500 nm size (Figure 2B1), which were verified by the dynamic laser scattering particle size analysis (Figure S6). Uniform dispersion of the nanoparticles was also observed. XRD results showed that the crystalline state of the nanoparticles did not change after the superhydrophobic modification (Figure S7). TEM and XRD results revealed that long-chain fatty acid derived form pig fat reacted only on the surface of Fe3O4 magnetic-sensitive nanoparticles without change the microstructure and crystalline state, obtaining good dispersion in the liquid coating materials. To understand the surface changes of the nanoparticles after superhydrophobic modification, FTIR was employed to detect the functional groups of Fe3O4 magnetic-sensitive nanoparticles and SMNs (Figure S8). The differences of functional groups confirmed the surface changes of SMN-20 and SMN-500 after superhydrophobic modification. The emerging absorption peaks -CH2- (ν, 2923 cm-1), -CH3 (ν, 2853 cm-1), C=O (ν, 1741 cm-1), C-H (δ, 1456 cm-1), C-O (ν, 8 ACS Paragon Plus Environment

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1163 cm-1) were from long-chain fatty acid76 and the disappeared absorption peaks O-H (ν, 3437 cm-1), O-H (β, 1634 cm-1) belonged to the -OH on the surface of Fe3O4 nanoparticles (Figure S8A,B), indicating the successful reaction between fatty acid and hydroxy functional group on the surfaces of Fe3O4 magnetic-sensitive nanoparticles. For comparison, the long-chain fatty acids were also grated to the non-magnetic sensitive Fe2O3 nanoparticles. The XPS was also employed to determine the elemental compositions and corresponding elemental valence states on the surface of Fe3O4 nanoparticles before and after superhydrophobic modification. In comparison of those of MN-20 (Figure 2A2-A5), the emerging peaks of SMN-20 belong to COOR (288.85 eV), C1s (284.78 eV) (Figure 2A3) and RO-Fe (533.22 eV), O1s (531.83 eV) (Figure 2A4), and the decreasing bonds can be assigned to HO-Fe (529.88 eV) (Figure 2A4), Fe2p1 (724.08 eV) and Fe-OH (710.68 eV) (Figure 2A5), suggesting the presence of long-chain fatty acid molecules on the surface of SMN-20. The XPS peaks of MN-20 (Figure 2A3) before superhydrophobic modification were mainly due to the inevitable adsorption of CO2 by the powder samples during the measurement. Compared to the binding energy of Fe2p1 (722.95 eV) and Fe-OH (700.99 eV) (Figure 2A5) on the surfaces of MN-20, the significantly increased binding energy of Fe2p1 (724.08 eV) and Fe-OH (710.68 eV) of SMN-20 (Figure 2A5), indicating the successful graft of long-chain fatty acid molecules with the OH groups on the surface of the nanoparticles (as illustrated in Figure S2,S3). The XPS spectra of SMN-500 (Figure B2-B5) were similar to those of SMN-20. When the atoms bond to other atoms with lesser electronegativity, the depressed electron binding energy can be obtained. In this work, the electronegativity of O atoms bonded to Fe atoms in long-chain fatty acid molecules was bigger than that of O atoms bonded to H atoms on the surfaces of Fe3O4 before reaction process. Therefore, the enhanced electron binding energy was observed. These results showed that the long-chain fatty acid molecules were grafted to the surface of the Fe3O4 magnetic-sensitive nanoparticles by the nucleophilic substitution reaction with -OH groups instead of a simple adsorption interaction. 9 ACS Paragon Plus Environment

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To observe the distributions of long-chain fatty acid on the surface of SMNs (SMN-20 and SMN-500), the TEM-EDX maps were employed to detect the surface elemental distributions (C, O, and Fe) (Figure 2A6,B6). Results revealed not only the already-existing O and Fe elements but also the emerging C element derived from long-chain fatty acid. These elements were equally distributed on the surfaces of the nanoparticles. The C element did not exist on the Fe3O4 magnetic-sensitive nanoparticles before surface-modification, which was verified by the FTIR analysis (Figure S8A,B). The TEM-EDX results confirmed the loading of long-chain fatty acid on the nanoparticle surfaces. From the above TEM, FTIR, XPS and TEM-EDX results, we can conclude that SMNs were successfully prepared by grafting the long-chain fatty acid onto the surfaces of Fe3O4 magnetic-sensitive nanoparticles. Furthermore, the superhydrophobicity of SMNs did not change after immersion in water for 4 months, suggesting the graft of long-chain fatty acid was through strong chemical reactions rather than weak physical adsorption. The accompanying excellent superhydrophobicity of SMNs was demonstrated by the water contact angle test (Figure S9). The excellent superhydrophobicity can endow the SMNs superior consistency with the hydrophobic coating materials. Interestingly, the SMNs can move freely with the magnetic interaction between SMNs and ferruginous object under magnetic field (Video S1), indicating the feasibility of the self-assembly fabrication process of SBSF.


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Figure 3. AFM images and SEM images (2000×) with corresponding WCAs of SF (A1-A3), NBSF (B1-B3), and SBSF (C1-C3) surfaces; SEM images of SBSF surface at 50000× (C4) and 200000×(C5) magnification; and schematic diagram of the superhydrophobic structure of SBSF (C6). Fabrication of Superhydrophobic Slow-Release Fertilizer through Self-Assembly. Recently, increasing interests have been devoted to developing slow-release fertilizers coated with biopolymers from sustainable and ecofriendly biomaterials. To overcome the hydrophilicity of biopolymers, biomimetic superhydrophobic technology has been employed to enhance the water repellency of biopolymer coating materials and thus improve their slow-release performance. Different from traditional slow-release fertilizers that often require complicated spraying, heat-curing and immersing process and the non-environmentally friendly hydrophobic

reagent

(fluoroalkyl

silanes),

the

advantages

of

the

self-assembly

superhydrophobic slow-release fertilizer (SBSF) are obvious, such as the simple, solvent-free 11 ACS Paragon Plus Environment

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fabrication process, the ecofriendly hydrophobic reagent (fatty acids with long alkyl chains) and the high yield. To better understand self-assembly process, the fabrication process of SBSF is summarized. The liquid coating materials were sprayed equably onto the surfaces of urea granules at 60 °C. After the heat-curing polymerization reaction of coating materials, the formed polyurethane coatings (0.5% relative to the urea) were adhered to the surfaces of urea granules. The slow-release fertilizers with various coating rates (3%, 5%, and 7%) were fabricated by repeating the spray-curing process for 6, 10, and 14 times, respectively. To form the superhydrophobic surfaces of SBSF, the liquid coating materials mixed with superhydrophobic magnetic-sensitive nanoparticles (SMNs) were employed in the last step of the coating. The Fe3O4 SMNs in the liquid coating materials (i.e., pig fat-based polyol and polymethylene polyphenylene isocyanate in a mass ratio 1.2 : 1) spontaneous migrated towards the outermost surface of liquid coating materials to construct the micro-nanoscale structures due to the magnetic interaction between the SMNs and ferruginous wall of the coating machine in a magnetic field. With the heat-curing process of biopolymer coating, the microstructures of the superhydrophobic surfaces of SBSF were constructed through self-assembly. For comparisons, slow-release fertilizers coated with pig fat-based polyurethane without any nanoparticles (abbreviated as SF), non-superhydrophobic slow-release fertilizers coated with pig fat-based polyurethane and non-magnetic sensitive superhydrophobic α-Fe2O3 nanoparticles (abbreviated as NBSF) were also prepared. To confirm the formation of superhydrophobic surfaces of on the fertilizers, SEM was employed to examine the surface morphology of SF, NBSF, and SBSF. The surface of SF was quite smooth (Figure 3A3), while NBSF surface was slightly rougher due to the addition of α-Fe2O3 nanoparticles (Figure 3B3). After the incorporation of Fe3O4 SMNs into liquid coating materials, the rough superhydrophobic structure of SBSF was observed (Figure 3C3). The significant enhancement of surface roughness can be attributed to the spontaneous movement of 12 ACS Paragon Plus Environment

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SMNs towards the outermost layer of the coating materials due to the magnetic interaction. The surfaces of uniformly dispersed SMN-500 and SMN-20 thus showed the typical superhydrophobic structure (Figure 3C3-C6). After the addition of SMNs with low surface energy, the surfaces of SBSF also had very low surface energy and demonstrated the superior superhydrophobic characteristic (water contact angle of 154.6°, water contact angle hysteresis of 1.5°). The SEM-EDX maps (Figure S10) were also used to study the surface elemental distributions of SF, NBSF and SBSF. Only C and O elements were observed on SF surfaces, while Fe element emerged on the surfaces of NBSF and SBSF besides C and O. SBSF showed denser Fe than NBSF, because Fe3O4 SMNs can move towards surfaces due the magnetic interaction. This demonstrates the important role of magnetic-sensitive Fe3O4 nanoparticles in the self-assembly fabrication process of SBSF. To further understand the surfaces characteristic of SF, NBSF, and SBSF, AFM was employed to observe the nanoscale surfaces roughness. The surface of the SF was again very smooth (Figure 3A1-A2). Only few non-magnetic sensitive α-Fe2O3 nanoparticles emerged on the surface of NBSF (Figure 3B1-B2). For SBSF (Figure 3C1-C2), the denser nanoscale bulges on the surfaces reflect the spontaneous migration and distribution of Fe3O4 SMNs onto out layer surface. The surface-roughness degrees of SF, NBSF and SBSF were 3.36 nm, 4.41 nm, and 20.52 nm. The surface of SBSF had the biggest surfaces roughness degree and the corresponding lowest surface energy. Therefore, the results confirm the superior superhydrophobic characteristic of SBSF, which can endow the coated fertilizer excellent slow-release performance.


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Figure 4. SEM images (left), corresponding EDX maps (middle), and EDX spectra of Fe elemental distribution (right) of the fracture surfaces of SF (A1-A3), NBSF (B1-B3) and SBSF (C1-C3). To further understand the self-assembly fabrication process of SBSF, SEM was employed to observe the microstructure of the fracture of the coatings (Figure 4A-C). The fractures of SF and NBSF showed no (Figure 4A) and uniformly distributed nanoparticles (Figure 4B). For the fracture of SBSF (Figure 4C), the distinct phenomena were observed on SEM-EDX results, which clearly demonstrate that the magnetic interaction can move the SMNs towards the outerlayer of the SBSF coating. This direct evidence strongly points out the self-assembly process of SBSF due to magnetic interaction of SMNs and ferruginous wall of the coating machine in magnetic field (illustrated in Figure 1,S2). The C and O distributions in the fractures of the coating materials were also measured and did not show obvious differences (Figure S11). In this work, the rough superhydrophobic surface was prepared through spontaneous self-assembly of SMNs during the coating rather than subsequent spraying treatment. The surface energy of SBSF was reduced by grafting superhydrophobic nanoparticles to replace the subsequent superhydrophobic treatment that requires toxic perfluoroalkyl triethoxysilane.62 More important, high-energy ball milling was firstly used to modify the SMNs with the long-chain fatty acid derived from pig fat. Therefore, the method to prepare the SBSF developed 14 ACS Paragon Plus Environment

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in this study is simple and ecofriendly. It can be applied to large-scale fabrications using existing industrial coating apparatus to produce SBSF for field application.


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Figure 5. Nutrient release rates of SF, NBSF, and SBSF in water at 25 °C with 3% (A), 5% (B), and 7% (C) coating percentages. 3D reconstruction image from SRXPCI technique and polarization micrograph of SF (D1-D3), NBSF (E1-E3), and SBSF (F1-F3) with water phase (blue), air phase (purple), and sample (yellow). The air (purple) and water (blue) phases are extracted and highlighted (F4-F5). The air (purple) phase is extracted and highlighted (F6-F7). Schematics of the solid-air-liquid interfaces on the superhydrophobic surface of SBSF when immersed in water (F8-F9). Slow-Release Performance and Mechanism of the Self-Assembled SBSF. To study the slow-release ability of SF, NBSF, and SBSF, we measured the nutrient releasing rates of the three fertilizers with 3%, 5%, and 7% coating percentages in water at 25 °C. The superhydrophobic surface had significant effect on N release characteristics of the slow-release fertilizers (Figure 5A-C). For the fertilizers with 3% coating materials, the 24 h N release rate and N release longevity of SBSF were 5.35% and 37.87 days, respectively; while those of SF and NBSF were 23.21% and 12.26 days, 22.62% and 12.00 days, respectively (Figure 5A). With the increasing coating percentage of other samples, the 24 h N release rates and N release longevity showed the same trends. With 5% coating, the 24 h N release rate and N release longevity of SBSF were 0.55% and 69.51 days, respectively; while those were 5.66% and 36.49 days, 7.46% and 37.59 days for SF and NBSF, respectively (Figure 5B). Similarly, with 7% coating, the 24 h N release rate and N release longevity of SBSF were 0.71% and 104.34 days, respectively; while those were 3.17% and 47.84 days, 4.55% and 47.71 days for SF and NBSF, respectively (Figure 5C). These results revealed SBSF had the best slow-release ability with 16 ACS Paragon Plus Environment

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longevity doubled that of SF, indicating the superhydrophobic surface significantly improved the nutrient release performance. On the other hand, the nutrient release characteristics of NBSF were similar to those of SF, suggesting the importance of the magnetic self-assembly of SMNs to the surface hydrophobicity of the SBSF. The slow-release abilities of SF, NBSF, and SBSF with 7% coating in the soil at 25 °C were also measured (listed in Supporting Information, Figure S13). In a previous work, we speculated the presence of atmosphere membrane on the surfaces of superhydrophobic slow-release fertilizers when immersed into water, which can enhance the slow-release performance. However, we could not find direct evidence to show the presence of atmosphere membrane in the SBSF surfaces. Therefore, an in-depth study has been made in this work to confirm the presence of the atmosphere “outerwear” on the surface of SBSF, and thus to clarify the governing mechanism of the significant improvement of slow-release performance by the superhydrophobic surfaces. To makes atmosphere membrane “visible” at the solid-air-liquid interface of SBSF, the polarizing microscope was used to indirectly visualize the atmosphere membrane through the different refractive index of water and atmosphere (Figure 5D3-F3). The clear contrast of brightness and darkness indicates the presence of air cushion between the microscopic structure of SBSF and water (Figure 5F3). The dark region can be attributed to the light reflection from the interface between the superhydrophobic surface of SBSF and water drops, while the bright region is due to the light reflection from the interface between the air and the water drops.77 For a comparison, polarizing microscope images of water drops on the surfaces of SF and NBSF were also taken (Figure 5D3 and E3), which did not show the solid-air-liquid interface. To further determine the morphology and distribution of atmosphere membrane, the synchrotron radiation-based X-ray phase-contrast imaging (SRXPCI) technique was employed to obtain the 3-D images of the interfaces of SF, NBSF, and SBSF (Figure 5D1-F1, D2-F2 and D4-D9).75 The images were obtained and reconstructed by calculating the phase-shift of X-ray 17 ACS Paragon Plus Environment

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after penetrating the water, atmosphere and coating materials with different density.78,79 The air membrane on the surface of SBSF showed abundant network-like air cushions buried in the valley of the superhydrophobic surface underneath the water. Liquid water only contacts the top of the bulges of solid surface and air pockets are trapped underneath the liquid.75,80,81 Furthermore, the air portions occupied the most space of the surface (89.1%), only a limited amount of water contacted the solid phase (10.9%). The air cushion served as a covering layer to reduce the contact between water and the superhydrophobic surface (Figure 5F1-F9). This provides direct evidence of air cushion on the surface of SBSF. The air cushion can reduce the contact area between liquid water and SBSF, and thus decrease the nutrient releasing rate and enhance the slow-release ability. For a comparison, the surfaces of SF and NBSF were also measured (Figure 5D1-D2 and E1-E2), showing no sign of air membrane. Water directly contacted the surfaces of SF and NBSF, causing the fast entrance of liquid water into the coating materials of slow-release fertilizers and consequently accelerating the nutrient release. From the above analysis, it can be concluded that we have successfully prepared the SBSF with enhanced slow-release ability. In addition, we obtained direct evidences of the presence of atmosphere membrane on the surface of SBSF with the polarizing microscopy and SRXPCI technique. The significant improvement of slow-release ability of SBSF can be attributed to the atmosphere “outerwear”. As we know, water is the main factor affecting the slow-release performance. It can fast permeate into the internal urea core in liquid water through the micropores on the polymer coatings of slow-release fertilizer. However, water cannot completely wet the superhydrophobic surface of SBSF because of the specific atmosphere cushions, which only allow the slow diffusion of water vapor into the internal urea core of SBSF. The water accumulating speed in vapor is significantly slower than that in liquid. Therefore, the atmosphere cushions can significantly reduce the nutrient release and enhance the slow-release ability of SBSF by decreasing the water accumulation rate into the interior of SBSF. The atmosphere membranes are derived from the surface tension on the interface of water and 18 ACS Paragon Plus Environment

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superhydrophobic surface. On the rough superhydrophobic surfaces with low surface energy, the water molecules can only form hydrogen bonds with each other to maintain the lowest energy state (equilibrium state). The equilibrium states are the minima of Gibbs free energy, which makes water suspending upon the superhydrophobic surface. Cassie and Baxter developed the Cassie-Baxter theory to describe this state,39,82 at which, liquid only contacts the top of the bulges of solid surface (89.1%) and air pockets are trapped underneath the liquid (10.9%). Therefore, the surfaces under the air cushion are perfectly nonwetting. As indicated above, the enhanced slow-release performance of the fertilizer is owing to the air cushion between water and SBSF. To confirm the change of the fraction of the air cushion with increasing release time, in this work, we detected the accurate fraction of the air cushion directly by SRXPCI technique. Furthermore, only the water drops on the superhydrophobic surfaces were in the Cassie-Baxter state, the air cushion occurred between water and SBSF. Contact angle hysteresis is an effective mean to detect the Cassie-Baxter states of superhydrophobic surfaces. Therefore, we also detected the contact angle hysteresis to confirm the change of the air cushion at the liquid-solid interface. The fraction of the air cushion was 89.1%, 86.4%, 83.9% and 80.3%, respectively; while the water contact angle hysteresis was 1.5°, 1.6°, 1.7° and 1.7° with increasing release time (0, 42, 84, 126 days) (Figure S15). The superhydrophobicity and the fraction of the air cushion only decreased slightly during the nutrient release longevity of SBSF. That means the Cassie-Baxter state and the accurate fraction of the air cushion of SBSF only changed little with increasing release time. The superior superhydrophobicity was due to the firm combination of SMNs with the biopolymer during the self-assembly process of SBSF.


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Figure 6. Durablilty of SBSF to external environment (left), SEM images (middle) and the corresponding elemental distribution detected by SEM-EDX (right) of durability (A1-A3), wear resistance (B1-B3), water resistance (C1-C3). Optical photograph and the 3-D scanning photograph of SBSF before (D1-D2) and after (E1-E2) release in water, microstructure (80000×) of the fracture of SBSF after release in water (E3), the composition detected by GC-MS (E4), and molecular weight (detected by GPC) of water soluble substance from coating materials (E5). Durable Properties of SBSF and Future Directions of Slow-Release Fertilizer. Durability, wear resistance, and water resistance of SBSF were determined (Figure 6A-C). The superhydrophobic characteristic (detected by water contact angle) and slow-release ability 20 ACS Paragon Plus Environment

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(measured by nutrient releasing longevity) were employed as indicators to evaluate the change of durable properties of the SBSF. For the durability, the SBSF sample with increasing storage period (14, 28, 42, 56, 70, 84, 98, 112, 126, and 140 days) under specific environmental conditions (Temperature: 25 °C; humidity: 60%RH) were measured (Figure 6A). The wear resistance of SBSF was also studied by the continuous rotation of coating machine on constant speed (50 rpm) with growing sampling time (1, 3, 5, 7, 9, 13, 17, 25, 33, and 40 min) (Figure 6B). To study the water resistance, the SBSF was soaked in 25 °C water with different time (7, 14, 21, 28, 35, 42, 49, 56, 63, and 70 days) (Figure 6C). These results revealed that the superhydrophobic characteristic and slow-release ability only changed little to the changes of external environment. The superior durability can be attributed to the firm self-assembly incorporation of SMNs onto the surfaces of SBSF drove by the magnetic interaction. The durability property of SBSF to soil environment was also studied with a slight influence in the nutrient release longevity (listed in Supporting Information, Figure S14). Considering the nutrient releasing curves and the superhydrophobic characteristics, the release process of SBSF can be divided to two stages. On the starting release stage, the slow nutrient releasing rate and corresponding improvement of nutrient release longevity can be attributed to the atmosphere “outerwear” between water and superhydrophobic surface of SBSF, which acts as a barrier that only allows penetration of water vapor into the fertilizer core. In this stage, the superhydrophobic characteristic of SBSF is the main factor to dominate nutrient release. The superhydrophobic modification of slow-release fertilizer can significantly prolong the nutrient release longevity for 1-2 months. On the subsequent release stage, with the accumulation of water inside of SBSF, the enlarging volume of slow-release fertilizer (e.g., superficial area: before, 66.45 mm2; after, 98.44 mm2. Figure 6D1-D2, E1-E2) can be observed. At this stage, the main influence factor to dominate the accelerated nutrient releasing rate is the change of biopolymer coating itself instead of the superhydrophobic surface of SBSF. To clarify the change of coating materials during the nutrient releasing process, the 21 ACS Paragon Plus Environment

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microstructure of the fracture surface and the substances dissolved from the coating materials were detected to confirm the structure and composition changes of coating materials (Figure 6E3-E5). SEM images showed obvious micropores on the of fracture surface of coating materials of SBSF after 60 days water immersion at 25 °C (Figure 6E3). These micropores were generated from the dissolution of soluble substances in coating materials as well as the swelling and enlarging effect of the slow-release fertilizer. The compositions of soluble substances in coating materials were mainly long-chain alkyl and aromatic compounds (detected by GC-MS, Figure 6E4,S12 and Table S5) with small molecular weight (300-1000 g/mol, determined by GPC, Figure 6E5). Subsequent improvement should be devoted to improving the performance of biopolymer coating to further enhance the slow-release performance. Therefore, one of the research directions to further enhance the nutrient release longevity of slow-release fertilizer is to improve the elasticity and water tightness of biopolymer coating, such as by reducing the soluble substances and grafting other superhydrophobic matters in the coatings. We may also prepare functional slow-release fertilizers with self-healing biopolymer coating. For example, we can introduce microspheres containing of consolidant into the coating materials to seal the micropores during the release. In addition, we can also incorporate water swelling substances to plug up the micropores to reduce the nutrient releasing rate. CONCLUSION In conclusion, we self-assembly fabricated a superhydrophobic biopolymer-coated slow-release fertilizer (SBSF) with superior slow-release performance, water tolerance and good feasibility for large-scale production. Due to the autonomous migration of SMNs towards the outside surface of SBSF from the interior of liquid coating materials, SMNs were dispersed uniformly on the outermost surface of the biopolymer coatings. With the heat-curing process of liquid biopolymer coatings, the microstructure of superhydrophobic surface of SBSF was constructed through self-assembly. As a result, SBSF showed much better slow-release performance than that of unmodified biopolymer-coated slow-release fertilizers. The slow-release longevity of 22 ACS Paragon Plus Environment

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SBSF was more than 100 days. In addition, SBSF in the soil environment also exhibited superior durable properties. This study also clarified the governing mechanism of superhydrophobic surfaces of SBSF to enhance the slow-release performance by directly observing the atmosphere cushions on the surface of outermost biopolymer coatings of SBSF using the synchrotron radiation-based X-ray phase-contrast imaging technique. The self-assembly fabrication of SBSF by magnetic interaction provides a direction for developing ecofriendly biobased slow-release fertilizers. The method to fabricate superhydrophobic surface through self-assembly can also be extended to other applications. MATERIALS AND METHODS Materials. Urea granules (2.00-4.75 mm in diameter and 46.4% of N, Ruixing Group Ltd., China), polymethylene polyphenylene isocyanate (PM-200, Wanhua Chemical Group Ltd., China), Pseudomonas lipase (lipase PS, 30000 U/g, optimum pH 7.0 and temperature 50 °C, Hangzhou Chuangke Biotechnology Co., Ltd., China) were used as received. Magnetic-sensitive nanoparticles (Fe3O4, 20nm and 500nm, saturated magnetization was 98 emu/g) and nonmagnetic-sensitive nanoparticles (α-Fe2O3, 20 nm and 500nm, Beijing DK nano technology CO. LTD, China) were used after magnetization in magnetic field. Pig fat was obtained after milling, filtration, and dehydration process. Preparation of Superhydrophobic-Modified Magnetic-Sensitive Nanoparticles. Fe3O4 superhydrophobic-modified magnetic-sensitive nanoparticles (SMNs) were prepared by a two-step reaction. The first step was the saponification of pig fat into fatty acid and glycerin.83 The mixture of pig fat (150 g), sodium hydroxide (80 g) and deionized water (120 mL) was firstly heated to 80 °C with continuous stir until the upper grease disappeared. After 30 min precipitation by adding NaCl salt, the upper liquid (crude glycerol) and the under precipitate (fatty acid sodium) were separated. Then, the sodium fatty acid was neutralized into fatty acid with the addition of hydrochloric acid. The organic layer was separated and purified by washing with deionized water and heated (60 °C) in vacuum to obtain the purified clear, light yellow pig 23 ACS Paragon Plus Environment

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fat-based fatty acids. The purified pig fat-based glycerol was also obtained with same treatment and employed in the synthesis of pig fat-based polyol. In the second step, the SMNs were firstly processed by high-energy ball milling. The mixture of Fe3O4 magnetic-sensitive nanoparticles (18.5 g), pig fat-based fatty acid (22.6 g), and absolute ethyl alcohol (20 mL) was milled for 8 h with an auto rotation rate of 400 rpm and a revolution speed of 200 rpm. The obtained slurry product was washed 3 times with ethyl alcohol and dried at 80 °C for 6 h. Finally, the obtained SMNs product was ground and filtrated with the 800 mesh (0.015mm) sieve. For comparison, the α-Fe2O3 superhydrophobic-modified non-magnetic sensitive nanoparticles were also obtained with the same treatment. Synthesis of Pig Fat-Based Polyol. Pig fat-based polyol was synthesized by an enzymic catalytic reaction on a acceptable temperature (42 °C) with a yield of approximately 90 wt%.84 The mixture of pig fat (81.38 g, 0.1 mol), pig fat-based glycerin (18.40 g, 0.2 mol, obtained from pig fat), deionized water (0.8464 g, 4.6% water/g pig fat-based glycerin), and Pseudomonas lipase (0.1356 g, catalyst, 500 U/g pig fat) were firstly stirred in a 500 mL three-neck round-bottom flask using a magnetic rotor at 800 rpm for 8 h in a 42 °C water bath and then placed in a 5 °C cooling environment for another 4 days. The obtained product was washed 3 times with 60 °C deionized water, filtrated, and heated in vacuum to obtain the purified pig fat-based polyol. The synthesis of pig fat-based polyol is described in Figure S2,S3. Fabrication of Self-Assembled Superhydrophobic Slow-Release Fertilizer. The self-assembled superhydrophobic slow-release fertilizer (SBSF) was fabricated by the spontaneous movement of SMNs due to the magnetic interaction between SMNs and ferruginous coating machine. Firstly, the mixture of liquid coating materials (pig fat-based polyol and polymethylene polyphenylene isocyanate in a mass ratio 1.2 : 1) was sprayed equably onto the surfaces of constantly-rolling urea granules (0.8 kg) in a rotating coating machine (WKY-300, Jingcheng Medical Equipment Manufacture Ltd., China) at 60 ± 5 °C. After the heat-curing polymerization reaction at 60 ± 5 °C for 10 min, the formed polyurethane 24 ACS Paragon Plus Environment

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coating (0.5% relative to the urea) would be adhered to the surfaces of urea granules. The SBSF with expected coating rates (3%, 5%, and 7%) were fabricated by repeating the spray-curing process for 6, 10, and 14 times, respectively. Before the last coating process, the mixture of liquid coating materials (pig fat-based polyol and polymethylene polyphenylene isocyanate in a mass ratio 1.2:1) and SMNs (liquid coating materials and SMNs in a mass ratio 10:1) was treated with subsequent ultrasonic concussion for 30 min. Then the superhydrophobic structure was constructed in the last coating process by the spontaneous migration and self-assembly of SMNs onto the surfaces of the biopolymer coatings under the magnetic field. The applied magnetic filed was formed by equipping Neodymium magnets in the outside surface of ferruginous wall of the coating machine. NBSF was also fabricated with the addition of nonmagnetic-sensitive nanoparticles and SF was prepared without the addition of nanoparticles. The coating conditions were as follows: flow temperature of hot air, 60 °C; flow rate of hot air, 380 m3/h; spray temperature, 65 °C; spray rate, 4 g/min; and rotational speed of the pan, 40 rpm. The synthesis of SBSF was illustrated in Figure 1,S2. Characterization. The surface structures of the SF, NBSF, and SBSF were observed using a Hitachi SU8200 scanning electron microscope (SEM) and a Bruker Dimension Icon atomic force microscope (AFM). The surface elemental distribution was measured by an energy dispersive X-ray spectroscopy (EDX) detector attached to the SEM. The micro-morphologies of the SMNs were observed using a FEI Tecnai G2 F20 transmission electron microscopy (TEM) and a Bruker D8 Advance X-ray diffractometer (XRD). The elemental distribution was measured by a Thermo Escalab 250Xi X-ray photoelectron spectrometer (XPS) with advance sculpture and an energy dispersive X-ray spectroscopy (EDX) detector attached to the TEM using a particular copper wire mesh without C element. The size analysis, structure, and hydrophobicity were obtained by a Malvern Zetasizer Nano ZS laser particle analyzer (LPA), a Thermo Nicolet 380 Fourier transform infrared (FTIR) spectrometer, and a Shanghai Zhongchen JC2000C2 contact angle meter, respectively. The atmosphere membrane on the surface of SBSF 25 ACS Paragon Plus Environment

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was observed with a Leika DM4P polarizing microscope and a synchrotron radiation-based X-ray phase-contrast imaging at the BL13W1 beamline in Shanghai Synchrotron Radiation Facility (SSRF). The structures of coating materials were measured by a Thermo Nicolet 380 Fourier transform infrared (FTIR) spectrometer, a Bruker Avance III 600M

13C

nuclear

magnetic resonance (NMR) spectrometer in CDCl3 using tetramethylsilane as the internal standard, and a Thermo Nicolet IS10 attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectrometer. The composition of fatty acids of eight fats and oils were measured by a Shimadzu GC-2010 gas chromatograph (GC) using the external standard method. The morphologies of SBSF before and after release in water were observed by a Zeiss Spectrum II 3-D scanning photograph. The composition and molecular weight of soluble substances in coating materials were measured by an Agilent 7000C (GC-MS) and an Agilent PL-GPC 50 gel permeation chromatography (GPC), respectively. The nutrient release characteristics of SF, NBSF, and SBSF were tested according to the ISO 18644. Durability Measurement. The durable properties of SBSF to external environment (water, soil, atmosphere and grind) were studied. For the durability, the SBSF sample was treated in a specific environment (Temperature: 25 °C and humidity: 60%RH) with increasing storage period (14, 28, 42, 56, 70, 84, 98, 112, 126, and 140 days). The wear resistance test proceeded in coating machine with continuous rotation (50 rpm) and sampling at 1, 3, 5, 7, 9, 13, 17, 25, 33, and 40 min. For the water resistance, the SBSF was soaked in 25 °C water with different time (7, 14, 21, 28, 35, 42, 49, 56, 63, and 70 days). The durability property of the SBSF in the soil environment was measured with increasing sampling time (14, 28, 42, 56, 70, 84, 98, 112, 126, and 140 days). The soil conditions were as follows: soil microbial biomass carbon, 0.15 g/kg dry soil; soil pH, 7.2; soil conductivity, 180.2 μS/cm; soil temperature, 25 °C; soil humidity, 60-100% (irrigation once a week). The superhydrophobic characteristic and slow-release longevity were employed to evaluate the changes of durable properties of the SBSF sample. ASSOCIATED CONTENT 26 ACS Paragon Plus Environment

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Supporting Information Descriptions of the selection of raw materials and synthesis, characterization, degradation properties of coating materials; gas chromatogram of four animal fats and four vegetable oils; Illustration

of

the

synthesis,

fabrication,

release,

application

and

degradation

of

superhydrophobic biopolymer-coated slow-release fertilizer; FTIR spectra and the physical properties of the raw materials derived from pig fat; structural characterization (13C-NMR, 1H-CMR

and ATR-FTIR spectra) of pig fat-based polyurethane materials; degradation

characterization of pig fat-based polyurethane materials during soil buried process (ATR-FTIR spectra and SEM images); size distributions, XRD patterns, FTIR spectra, and water contact angles of Fe3O4 magnetic-sensitive nanoparticles before and after superhydrophobic modification; SEM-EDX maps of surface and fracture surface element distributions, SEM-EDX spectra of fracture surface element distributions of three slow-release fertilizer; GC-MS spectra of soluble substances in coating materials; Nutrient release rates of slow-release fertilizer in 25 °C soil environment; Durablilty of SBSF to soil environment; Change of the fraction of the air cushion and the water contact angle hysteresis with increasing release time. Video S1: Magnetic interaction between SMNs and ferruginous object in a magnetic field (AVI) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Jiazhuo Xie: 0000-0001-8720-1953 Yuechao Yang: 0000-0003-4045-0252 Bin Gao: 0000-0003-3769-0191 ACKNOWLEDGMENTS This research was funded by the National Key R&D Program of China (2017YFD0200702), National Natural Science Foundation of China (Grant No. 31572201), Shandong Agricultural 27 ACS Paragon Plus Environment

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Innovation Team (SDAIT-17-04), the Projects of Commercialization of Rresearch Findings of Shandong Province (Grant No. [2014] 183), and the Great Innovation Projects in Agriculture of Shandong Province (Grant No. [2013] 136). REFERENCES (1) Peng, C.; Chen, Z.; Tiwari, M. K. All-Organic Superhydrophobic Coatings with Mechanochemical Robustness and Liquid Impalement Resistance. Nat. Mater. 2018, 17, 355-360. (2)

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Table of Contents (TOC)


Magnetic-Sensitive Nanoparticle Self-Assembled Superhydrophobic

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