“Smart” Fertilizer with Temperature- and pH-Responsive Behavior via

Research Article pubs.acs.org/journal/ascecg

“Smart” Fertilizer with Temperature- and pH-Responsive Behavior via Surface-Initiated Polymerization for Controlled Release of Nutrients Chen Feng,† Shaoyu Lü,*,† Chunmei Gao,† Xinggang Wang,‡ Xiubin Xu,† Xiao Bai,† Nannan Gao,† Mingzhu Liu,*,† and Lan Wu§ †

State Key Laboratory of Applied Organic Chemistry, Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province and Department of Chemistry, Lanzhou University, Lanzhou 730000, People’s Republic of China ‡ Research Institute of Lanzhou Petrochemical Corporation, Petrochina Lanzhou Petrochemical Company, Lanzhou 730060, People’s Republic of China § College of Chemical Engineering, Northwest University for Minorities, Lanzhou 730030, People’s Republic of China ABSTRACT: With the fast development of sustainable modern agriculture, there has been a shift toward the development of controlled/slow release fertilizers (CSRFs) that can overcome current limitations of conventional fertilizers. Ideally, CSRFs would release nutrient matching plants demands, even when environmental conditions fluctuate. However, no current CSRFs meet this ideal, especially as nutrient release is affected by temperature and pH. In this study, a “smart” fertilizer with polymer brushes of poly(N,N-dimethylaminoethyl methacrylate) grafting from polydopamine-coated ammonium zinc phosphate via surfaceinitiated atom-transfer radical polymerization is reported. The structure and morphology of the fertilizer were measured by transmission electron microscopy (TEM); the composition of the product was determined with Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), and inductively coupled plasma (ICP) emission spectrometer. Dualresponsive controlled-release behaviors were characterized in water by spectrophotometry and ICP, and results demonstrate the “smart” fertilizer shows excellent temperature- and pH-responsive behavior to release nutrients according to the ambient environment. The nutrients release rate can be obviously accelerated in an acidic pH (below pKa) medium at a definite temperature. In addition, low temperature (below LCST) can accelerate the nutrients release rate in a basic medium (above pKa), which is contrasting to the reduction of nutrients release rate at high temperature (above LCST) in the same medium. The pHand temperature-responsive “smart” fertilizer will improve nutrients availability and avoid excessive release of nutrients causing damage to plant roots at high temperature, which indicating that the stimuli-responsive system has potential application in sustainable modern agriculture. KEYWORDS: Polydopamine, Poly(N,N-dimethylaminoethyl methacrylate), Surface-initiated atom-transfer radical polymerization, Temperature- and pH-response, Controlled-release fertilizer

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INTRODUCTION Dramatic growth of the global population is considered as the largest impact on the natural environment. To satisfy demands of a growing population, such as food, fuel, and clothe, fertilizers are increasingly needed to supply nutrients to crops.1,2 It is thought that 70% of nutrients will come from fertilizers to support plant growth by the year 2020.3 However, many research results demonstrate that only a small part of fertilizers can be taken up by plants. Most of nutrients deriving from fertilizers that are applied to the fields are lost into groundwater by leaching or atmosphere by evaporating.4,5 To enhance fertilizer use efficiency and the effective nutrients uptake, in recent years, CSRFs have been widely researched, and become more and more important.6,7 © 2015 American Chemical Society

CSRFs have many remarkable advantages over traditional chemical fertilizers, for instance, decreasing greenhouse gas emissions, sustaining agriculture and environment, and increasing food production. Typically, inorganic or organic materials are used to encapsulate chemical fertilizer cores to reduce nutrients release rate.8,9 For example, Gao et al. reported that coated fertilizer with graphene oxide films using a simple approach tends to slow nutrient release rate.10 Lubkowski et al. developed a new biodegradable material consisting of a copolymer of poly(butylene succinate) and a butylene ester of dilinoleic acid to encapsulate the multiReceived: July 21, 2015 Published: November 17, 2015 3157

DOI: 10.1021/acssuschemeng.5b01384 ACS Sustainable Chem. Eng. 2015, 3, 3157−3166

Research Article

ACS Sustainable Chemistry & Engineering component fertilizer that can prolong nutrient release.11 Recently, micro/nanonetworks that were made of a highenergy electron beam dispersed attapulgite and hydrophilic polymer as the controlled-release platform were widely researched.12,13 Meanwhile, coating materials based on natural polysaccharides are drawing great attention because of their biodegradable, renewable, and low-cost.14−16 However, the amount of nutrients taken up by crops varies according to the different ambient condition. It is obvious that temperature, pH, and moisture can easily influence the nutrient uptake, so the development of environmental-responsive fertilizers is necessary and urgent. As far as we know, there are few environmental-responsive controlled-release fertilizers reported. Since Lee et al. have reported that catecholic amine (dopamine) can spontaneously deposit on various materials, this arises large interest in the attachment strategy on the basis of self-polymerization of dopamine.17 Mussel-inspired surface chemistry has provided a simple and efficient approach to modify substrates by forming polydopamine (Pdop) layers. The process of self-polymerization is only dissolving dopamine hydrochloride in a dilute aqueous solution that buffered to pH 8.5 using tris(hydroxymethyl)amino-methane and HCl. Furthermore, the Pdop layers’ thickness could be simply regulated by immersion time, the number of deposition cycles, and the concentration of dopamine hydrochloride.18,19 Additionally, many reports demonstrate that Pdop coating possesses latent reactivity, which could be used as a platform for further modification. Lee et al. found that biomolecules can easily immobilize onto the surfaces of Pdop coating on the basis of the thin polymer films’ high reactivity toward nucleophile (amine or thiol groups) at alkaline solution.20 However, one of the most interesting applications is a monomeric dopaminebased initiator or polymerdopamine-based macroinitiator for surface-initiated atom transfer radical polymerization (SIATRP). Polymer brushes can be easily immobilized onto the Pdop coating by atom transfer radical polymerization (ATRP) taking place according to previous research.21−23 Therefore, a “smart” fertilizer with stimuli-responsive brushes to control nutrients release using “graft-from” method via SI-ATRP technique is achieved in this study. Applications of poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) have attracted enormous attention in biotechnology and biomedicine due to its temperature- and pHresponsiveness and less cytotoxicity.24,25 Interestingly, the lower critical solution temperature (LCST) and the corresponding pH values in solution strongly determine the environmental response of PDMAEMA. Consequently, PDMAEMA is thought to be a polymer with double-stimuli response.26−28 By making use of the responsiveness to the environmental stimuli of PDMAEMA, Müller et al. prepared the double-stimuli-responsive porous membrane, of which the pore size can be monitored by temperature and the environmental pH values in water. On the basis of those, the selectivity of particle size can be achieved.29 Majewski et al. reported the synthesis of dual-responsive magnetic core−shell nanoparticle for nonviral gene delivery and cell separation.30 As the previous studies reported, the lower critical solution temperature (LCST) of PDMAEMA ranges from 30 to 53 °C in accordance with the molecular weight of the polymer chains and the corresponding pH values in solution.31,32 PDMAEMA is hydrophilic below LCST because the hydrogen bond could be formed between polymer chains and water; nevertheless, the hydrogen bond will be broken at high

temperature (above LCST) causing PDMAEMA to exhibit hydrophobicity. Moreover, PDMAEMA is a weak electrolyte (pKa ∼ 7.4). At low pH, such as pH 4.0, the tertiary amine groups of PDMAEMA could be completely protonated, and this contributes to its hydrophilicity owing to the electrostatic repulsions between polymer chains; whereas PDMAEMA chains exhibit hydrophobicity at high pH because the tertiary amine groups are unprotonated, and the polymer chains are shrunk. To the best of our knowledge, there have been few research efforts on a “smart” fertilizer with temperature- and pH-responsiveness to achieve stimuli-responsive release according to the ambient environmental conditions. Herein, we put forward a simple strategy for modification of fertilizer cores with PDMAEMA via SI-ATRP. The Pdop layers were deposited on the surface of ammonium zinc phosphate that was used as multielement compound fertilizer cores to supply nutrients. Subsequently, PDMAEMA chains were immobilized onto the Pdop layers using the method of SIATRP. The stimuli-responsive release behavior of nutrients affected by temperature and pH was systematically examined.

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EXPERIMENTAL SECTION

Materials. N,N-dimethylaminoethyl methacrylate (DMAEMA, 99%, Aladdin) was distilled under reduced pressure to remove the polymerization inhibitor prior to use. Dopamine hydrochloride (Acros), 2-bromoisobutyryl bromide (BIBB, 98%, Aladdin), and N,N-dimethylaminopyridine (DMAP, 99%, Aladdin) were used as received, without further purification. 2,2-Bipyridine (Bpy) and Tris(hydroxymethyl)amino-methane (Tris) were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). Cuprous bromide (Cu(I)Br) was stirred with acetic acid overnight, vacuum filtrated, washed with diethyl ether and anhydrous ethanol, and then dried under vacuum at room temperature. Triethylamine (TEA) was dried over CaH2 overnight and distilled under reduced pressure and stored with activated molecular sieves. Dichloromethane (DCM) was distilled over CaH2 under reduced pressure prior to use. All other reagents, methanol (MeOH), diammonium phosphate, zinc nitrate, and ammonia were analytical reagent grade and used directly without further purification. Water used throughout the experiment was deionized. Synthesis of Ammonium Zinc Phosphate Multielement Compound Fertilizer (MCF). The process of preparing the multielement compound fertilizer (MCF) refers to the previous literature.7 Briefly, diammonium phosphate solution (0.3 M) was added dropwise into a solution of zinc nitrate (0.3 M). After complete addition, ammonia (25−28 wt %) was utilized to adjust the pH of the solution to approximately 9.0. Subsequently, the reaction was carried out at room temperature under vigorous stirring for 12 h. After the reaction, the suspension was left to age for 24 h, then the suspension was filtered and the solid was dried under vacuum. Synthesis of Polydopamine Coated Multielement Compound Fertilizer (MCF@Pdop). The MCF@Pdop was prepared by spontaneous oxidative polymerization of dopamine hydrochloride.33 First, ammonium zinc phosphate (2.0 g) and 200 mL of Tris− HCl buffer solution (pH 8.5) were added to a 500 mL three-necked flask. The whole system was sonicated for 10 min to make sure the good dispersion of MCF in the buffer solution. Afterward, dopamine hydrochloride (0.2 g) was dissolved in the solution and the system was stirred for 24 h at room temperature. The color of the solution gradually turned dark because of the self-polymerization of dopamine hydrochloride. Finally, the mixture was filtered and washed with deionized water for three times. The final product was dried under vacuum at room temperature to a constant weight. According to the previous report, the thickness of the Pdop layer can be regulated by the deposition numbers of dopamine.34 With the purpose to obtain a certain thickness layer of Pdop coated MCF, we designed the deposition numbers to 2 times in the same way. 3158

DOI: 10.1021/acssuschemeng.5b01384 ACS Sustainable Chem. Eng. 2015, 3, 3157−3166

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ACS Sustainable Chemistry & Engineering Scheme 1. Schematic Illustration for the Fabrication of PDMAEMA-Functionalized Fertilizer

Synthesis of MCF@Pdop Supported ATRP Macroinitiator (MCF@Pdop-Br). Typical procedures for the preparation of coated fertilizer supported initiator (MCF@Pdop-Br) are as follows.35 MCF@Pdop (2.0 g) and dry DCM (20 mL) were mixed in a 100 mL three-necked round-bottomed flask and afterward sonicted for 10 min to make sure the good dispersion of MCF@Pdop in DCM. DMAP (0.5 g) was dissolved in the mixture, then TEA (1.0 mL) without any traces of water was added. The whole system was immersed in an ice−water bath with vigorous stirring. Subsequently, BIBB (1.0 mL) was dissolved in dry DCM (10 mL) and the solution was added dropwise at 0 °C over 1 h. After the addition of BIBB, the reaction was slowly returned to the room temperature and stirred under a nitrogen atmosphere for 24 h. The product was obtained by filtration and washed with ethanol/water (1:1, v/v) mixture several times to make sure that the extra reactants were completely removed. The MCF@Pdop-Br was dried under vacuum at 40 °C for 2 days. Synthesis of PDMAEMA-Functionalized Multielement Compound Fertilizer (MCF@Pdop-g-PDMAEMA). The ATRP technique was employed to synthesize MCF@Pdop-g-PDMAEMA. MCF@ Pdop-Br (1.0 g), DMAEMA (6.4 mL), and a methanol/deionized water mixture (8.0 mL, 1:1, v/v) were transferred to a 100 mL twonecked round-bottomed flask. The whole system was sonicated for 10 min to make sure the good dispersion of MCF@Pdop-Br in the mixture. The system was degassed using freeze−pump−thaw nitrogen three cycles, followed immediately by the addition of Bpy (48 mg) and Cu(I)Br (18 mg). After another freeze−pump−thaw three times, the reaction was maintained at 35 °C for 24 h in an oil bath with stirring in a nitrogen atmosphere. To terminate the polymerization, the flask was exposed to air and submerged in cold water, then methanol (40 mL) was added to the flask. The product was filtered and washed by methanol and deionized water for several times to remove the residual catalyst. Finally, MCF@Pdop-g-PDMAEMA was dried under vacuum overnight.

Techniques of Characterization. Fourier transforms infrared (FTIR) spectra of MCF, MCF@Pdop, MCF@Pdop-Br, and MCF@ Pdop-g-PDMAEMA were determined at room temperature by Nicolet NEXUS 670 FTIR spectrometer (USA) with a KBr pellet at a scanning range from 4000 to 500 cm−1. Transmission electron microscopy (TEM) images were taken using a JEM-1200EX/S (Hitachi, Japan) with an accelerating voltage of 200 keV. Thermogravimetric analysis (TGA) was carried out using a STA PT1600 (Linseis, Germany) from room temperature to 800 °C with a heating rate of 10 °C/min under a dry nitrogen purge. Differential scanning calorimetry (DSC) was employed to determine the LCST of MCF@Pdop-g-PDMAEMA using a Sapphire DSC (PerkinElmer, USA). At the beginning of the measurement, each sample was immersed in an aqueous solution of pH 10 which was adjusted by dilute solution of NaOH for at least 24 h to ensure equilibrium at room temperature. Then, each sample (about 20 mg) was sealed in an aluminum pan. Thermal analysis was carried out under nitrogen atmosphere at a heating rate of 1.0 °C/min. Meanwhile, standard procedure was followed to calibrate the temperature scale to ensure reliability of the data obtained. The LCST was obtained as the maximum of the DSC endothermic peak. The content of nitrogen of samples was measured by an elemental analysis instrument (Germany Elemental Vario EL Corp., Model 1106), and the content of ammonium of samples was measured according to Nessler’s reagent colorimetric method.36 The content of phosphorus and zinc was determined by an IRIS Advantage ER/S inductively couple plasma emission spectrometer (TJA, USA). Controlled-Release Behavior of MCF@Pdop and MCF@ Pdop-g-PDMAEMA in Water. To investigate the stimuli-responsive release of zinc (Zn), phosphorus (P2O5), and nitrogen (NH4+), the experiments were carried out as follows: 0.3 g of MCF@Pdop or MCF@Pdop-g-PDMAEMA was placed into a dialysis membrane tube (molecular weight cutoff = 7000−14000). Then, the dialysis tube was 3159

DOI: 10.1021/acssuschemeng.5b01384 ACS Sustainable Chem. Eng. 2015, 3, 3157−3166

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MCF (Figure 1a), a peak around 3265 cm−1 is ascribed to the NH stretching vibration, whereas the absorptions at round 1430 and 697 cm−1 are attributed to NH scissoring vibration. Meanwhile, the absorption peak at 1035 cm−1 is attributed to the PO bond asymmertric stretching vibration of phosphorus−oxygen tetrahedron, and the absorption peak at 614 cm−1 attributed to the ZnO band vibration of zinc−oxygen tetrahedron is also found. These results can confirm that MCF was successfully prepared as we expect. For MCF@Pdop (Figure 1b), the new absorption peaks are observed at 1605 and 1442 cm−1 because of the existence of the indole or indoline structures and the vibrations of the NH bond, as reported previously.37 Macroinitiator is confirmed by the weak bands at 1650 and 1710 cm−1 indicating the amides and esters formed by the reaction of BIBB with primary amine and hydroxyl groups on the surface of Pdop layer, respectively, as shown in Figure 1c. Finally, for the FTIR spectrum of MCF@ Pdop-g-PDMAEMA (Figure 1d), the CO band, the ester of PDMAEMA absorption peak at 1730 cm−1 and the characteristic absorptions of −CH3 and −CH2 vibration at 2822 and 2771 cm−1 indicate that PDMAEMA is successfully immobilized on the surface of macroinitiator. TEM Observation. To confirm further the successfully functionalized fertilizer, the samples were determined by TEM observations. TEM images (Figure 2) demonstrate the changes of the surface morphology of MCF, MCF@Pdop, and MCF@ Pdop-g-PDMAEMA. Before functionalization, the surface morphology of MCF (Figure 2a) shows that the boundary of the salty core is dark; however, after self-polymerization of dopamine hydrochloride in Tris−HCl buffer solution, a thin light polymer film with an average thickness of 10 nm is found on the boundary of MCF@Pdop (Figure 2b), indicating the existence of Pdop coating, which agrees with the previous work reported by Ma et al.38 After the SI-ATRP of DMAEMA taking place on the surface of macroinitiator, the outer layer coated MCF@Pdop is approximate 100 nm and is very rough, as identified by TEM images (Figure 2c). Hence, the successful functionalization of MCF by Pdop and DMAEMA is obviously evidenced on the basis of the images examined by TEM observation. Thermal Gravimetric Analysis (TGA). TGA was employed to investigate the weight percentage of Pdop layers and polymer brushes from room temperature to 800 °C under nitrogen, and the TGA profiles are shown in Figure 3. According to the TGA profiles, the weight loss of the MCF is 14.40% when the temperature reaches 800 °C, which can be attributed to the decomposition of MCF and the loss of a small amount of absorbed water. After modification with Pdop, the weight loss of 24.49% is observed that indicates that MCF is

immersed in 100 mL aqueous solutions of different pH (4.0, 7.0, and 10.0) and incubated at different temperature (25 and 40 °C) in a water bath. At selected time points, 5.0 mL of the solution was collected and the equal volume of fresh medium was added to keep the total volume to 100 mL. After microwave-assisted digestion conducted with the addition of a catalyst, an inductively coupled plasma atomic-emission spectrometry was used to determine the content of zinc (Zn) and phosphorus (P2O5) released from fertilizers. Meanwhile, the content of ammonium (NH4+) released from fertilizers was calculated by making use of Nessler’s reagent colorimetric method.

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RESULTS AND DISCUSSION Preparation of MCF, MCF@Pdop, MCF@Pdop-Br, and MCF@Pdop-g-PDMAEMA. MCF, MCF@Pdop, MCF@ Pdop-Br, and MCF@Pdop-g-PDMAEMA were successfully prepared as described in Scheme 1. First, MCF@Pdop was obtained by the deposition of dopamine hydrochloride on MCF in Tris−HCl buffer solution at pH = 8.5. Afterward, MCF@Pdop-Br was prepared by the reaction between BIBB and amine groups or hydroxyl groups on the surface of MCF@ Pdop. Finally, double-stimuli-responsive polymer, PDMAEMA, was grafted from MCF by SI-ATRP technique using MCF@ Pdop-Br as a macroinitiator to obtain MCF@Pdop-gPDMADMA. The postfunctionalization of fertilizer core with PDMAEMA chains is adopted in order to convert the conventional fertilizer to the “smart” fertilizer. FTIR Analysis. The structures of MCF, MCF@Pdop, MCF@Pdop-Br, and MCF@Pdop-g-PDMAEMA were confirmed by FTIR, as shown in Figure 1. In the spectrum of the

Figure 1. FITR spectra of MCF (a), MCF@Pdop (b), MCF@PdopBr (c), MCF@Pdop-g-PDMAMEA (d).

Figure 2. TEM images of MCF (a), MCF@Pdop (b), and MCF@Pdop-g-PDMAEMA (c). 3160

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carbon reaches 45.9% in MCF@Pdop-g-PDMAEMA, which is consistent with the large mass loss appears in TGA curves. Morphologies and Characteristics of MCF, MCF@ Pdop, and MCF@Pdop-g-PDMAMEA. Photographic images show the morphologies of the core fertilizer (MCF) (Figure 4a), MCF@Pdop (Figure 4b), and MCF@Pdop-g-PDMAEMA (Figure 4c). The characteristics of MCF@Pdop, and MCF@ Pdop-g-PDMAMEA are presented in Table 1.

Figure 3. TGA curves of MCF (a), MCF@Pdop (b), MCF@Pdop-Br (c), and MCF@Pdop-g-PDMAEMA (d).

obviously coated by the Pdop layer and the weight percentage of Pdop layer is calculated to be 10.09%. In the case of MCF@ Pdop-Br, the weight loss is 27.54% owing to the decomposition of small functionalized organic matter. From the difference between MCF@Pdop and MCF@ Pdop-Br, we can calculate that there are 0.204 mmol of initiator groups per gram of macroinitiator. Because the polymer brushes graft from the surface of MCF@Pdop-Br via the SI-ATRP technique, the TGA profiles of MCF@Pdop-g-PDMAEMA have a remarkably higher weight loss up to 71.92%. From this phenomenon, we can imply that DMAEMA monomers have been successfully carried out the polymerization reaction. As shown in the TGA profiles, the MCF@Pdop-g-PDMAEMA shows a two-stage weight loss processes. The decomposition of fertilizer core coated by Pdop and a small part of PDMAEMA may account for the prior weight loss until 300 °C, whereas the major weight loss from 300 to 800 °C owes to the complete decomposition of the outer layer of PDMAEMA chains. There results further confirm that MCF@Pdop-g-PDMAEMA was successfully prepared. As we can see, the weight loss of 44.38% is observed between MCF@Pdop-Br and MCF@Pdop-g-PDMAEMA. However, it is noteworthy that the weight loss of MCF@Pdop-gPDMAEMA appears to be not consist with the thickness of the polymer layer showed in the TEM images. As the previous study reported, TEM images just show a two-dimensional (2D) structure of samples, but the fact is that MCF is a threedimensional (3D) crystal.35 Because PDMAEMA was coated onto the surface of MCF, it is reasonable to realize the difference between TGA and TEM results. To confirm further this phenomenon, elemental analysis was employed to verify the high mass loss in TGA. As shown in Table 1, the rapid growth of the carbon content was observed, and the content of

Figure 4. Photographs of MCF (a), MCF@Pdop (b), and MCF@ Pdop-g-PDMAEMA (c).

LCST Behavior. As we know, PDMAEMA has temperatureresponsive behavior that has been thoroughly investigated in recent years.39−41 On the basis of this property of PDMAEMA, the “smart” fertilizer can realize thermoresponsive release in accordance with the environmental temperature. To determine the temperature-responsive behavior of the “smart” fertilizer (MCF@Pdop-g-PDMAEMA), DSC analysis was carried out under nitrogen. In general, the LCST of the polymers is deemed to be the temperature at the maxima of the DSC endothermic peak. As shown in Figure 5a, there is no endothermic peak in the DSC thermogram of MCF@Pdop in the examined temperature range. However, Figure 5b shows an endothermic peak in the DSC thermogram after the modification of MCF@Pdop with PDMAEMA. Hence, the LCST of MCF@Pdop-g-PDMAEMA is around 34 °C referring to the endothermic peak in Figure 5b. Numerous experiments have been carried out to investigate the mechanism of the temperature-response of polymer.42−45 One of the most widely acknowledged and acceptable mechanisms is the volume phase transition resulting from the breakdown of a hydrophilic/hydrophobic balance between hydrogen bonds and hydrophobic segments of the polymer chains. Generally, intermolecular hydrogen bonds between PDMAEMA and water molecule favor the hydrophilicity of PDMAEMA brushes when the temperature is below LCST. As the environmental temperature increases, the hydrophilic/

Table 1. Characteristics of the Controlled-Release Fertilizer wt % sample

P2O5

Zn

NH4+

C

H

MCF MCF@Pdop MCF@Pdop-g-PDMAEMA

17.3 13.7 3.76

36.1 28.8 7.78

7.80 6.28 1.71

0.12 8.02 45.9

2.28 2.45 6.85

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DOI: 10.1021/acssuschemeng.5b01384 ACS Sustainable Chem. Eng. 2015, 3, 3157−3166

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DSC analysis, the temperature-responsive release is selected at the temperature of 25 °C (below LCST) and 40 °C (above LCST). During the controlled-release stage, we observed that the nutrient release rate and the cumulative release of the fertilizers are remarkably poor because of the low water solubility property of MCF as the fertilizer core. Therefore, release curves were obtained by plotting the concentration of the released nutrient normalized with the highest value versus time. First of all, we examined the controlled-release behavior of phosphorus from MCF@Pdop, MCF@Pdop-g-PDMAEMA at different conditions that are mentioned above within 28 days. Figure 6 shows the release behavior of phosphorus (PO43−) in pH 4.0, pH 7.0, and pH 10.0 aqueous solutions at 25 and 40 °C, and nutrient release plots obviously exhibit the stimuliresponsive release behavior of the PDMAEMA-functionalized fertilizer. For MCF@Pdop, the cumulative release amount of phosphorus is 76.84%, 71.72%, and 82.33% in pH 4.0, pH 7.0, and pH 10.0 aqueous solutions at 25 °C, respectively. When the temperature rises to 40 °C, the release rate significantly increases, and the cumulative release amount of phosphorus reaches 92.83%, 87.26, and 100% in acidic solution, neutral solution, and alkaline solution, respectively. It is clear that higher temperatures can accelerate the nutrient release rate because of an increase in the average molecular diffusion rate. Furthermore, the hydrolysis rate of salty core in acidic/basic solution is faster than that in neutral solution that can also contribute to an increase in the nutrient release rate. For MCF@Pdop-g-PDMAEMA, the release rate is slower than that of MCF@Pdop due to the higher diffusion resistance since PDMAEMA grafting from the Pdop layer. In addition, there is a large difference in the nutrient release rate comparing to MCF@Pdop because of the stimuli-responsive behavior of PDMAEMA brushes according to pH values and temperature. At low temperature (25 °C), the cumulative release amount of phosphorus is 59.22%, 47.36%, and 40.89% in pH 4.0, pH 7.0, and pH 10 aqueous solutions, and the cumulative release amount is calculated to be about 68.98%, 56.26%, and 34.21% in the same condition at 40 °C. Furthermore, it is noted that the cumulative release amount appears higher in the basic solution at 25 °C (below LCST) than that at 40 °C (above LCST). Interestingly, it is contrary in acidic and neutral

Figure 5. DSC thermograms of MCF@Pdop (a), MCF@Pdop-gPDMAEMA (b) at pH 10.0.

hydrophobic balances shifts to a more hydrophobic nature resulting in a compact aggregation process occurring in the polymer brushes. This assists in the volume phase transition of PDMAEMA chains at the temperature of LCST. Controlled-Release Behavior of MCF@Pdop and MCF@Pdop-g-PDMAEMA. The controlled-release behavior is one of the most significant characteristics of the controlledrelease fertilizer. To investigate the nutrient release rate and the cumulative amount of released nutrient in this study, MCF@ Pdop and MCF@Pdop-g-PDMAEMA are chosen as the samples to study the double-stimuli-responsive release properties according to the ambient environmental conditions. It is well-known that the typical LCST behavior of PDMAEMA is strongly dependent upon the environmental temperature and the corresponding pH values in aqueous solution; for instance, the LCST of PDMAEMA at pH 3.0 is above 70 °C, at pH 7.0 is around 55 °C, and is below 35 °C if the pH raises to 10.0.31,32 Because the pKa of the PDMAEMA is approximately to 7.4, the stimuli-responsive behaviors of surface-grafted polymer brushes are examined with the pH values below, near and above the pKa (pH 4.0, pH 7.0, and pH 10.0). Meanwhile, according to the

Figure 6. Controlled-release behavior of PO43− from PDMEMA-functionalized fertilizer according to temperature and pH changes. (a) 25 °C, and (b) 40 °C. 3162

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Figure 7. Controlled-release behavior of Zn2+ from PDMEMA-functionalized fertilizer according to temperature and pH changes. (a) 25 °C, and (b) 40 °C.

Figure 8. Controlled-release behavior of NH4+ from PDMEMA-functionalized fertilizer according to temperature and pH changes. (a) 25 °C, and (b) 40 °C.

shrunk to form the compact polymer shells that can act as barriers that make the nutrient release rate lower than that at 25 °C (below LCST). While, below LCST, extended PDMAEMA brushes form channels throughout the PDMAMEA outer layer leading to high permeability of the nutrient, which makes nutrients pass through the swollen PDMAEMA brushes layers more easily. Besides, as the previous study reported, LCST can shift to a higher temperature that is above 50 °C with the pH of the solution varies from 7.0 to 4.0.32 Accordingly, polymer chains cannot be completely shrunk to be used as an effective hindrance to slow down the release rate in an acidic or neutral medium. Meanwhile, the release behavior of zinc and ammonium has a similar trend toward phosphate owing to the temperature- and pH-responsive of PDMAEMA brushes which agrees with the above analysis. Figure 7 displays the controlled-release behavior of zinc in different conditions. The cumulative release amount of zinc from MCF@Pdop is 76.84%, 72.61%, and 84.06% in pH 4.0, pH 7.0, and pH 10.0 aqueous solutions at 25 °C. When the temperature increases to 40 °C, the cumulative release amount reaches 91.64%, 86.61%, and 100% in pH 4.0, pH 7.0, and pH 10.0 aqueous solutions, respectively. For MCF@Pdop-gPDMAEMA, the release of zinc is 57.07%, 46.84%, and

solution. These results suggest that the cumulative amount of released nutrient from MCF@Pdop-g-PDMAEMA strongly depends on the pH values and temperature. On the basis of the property of PDMAEMA, the most likely reason for the significant differences in the release process is ascribed to the configurational changes of the grafted PDMAEMA brushes at different environmental conditions. For the pH-induced release plots, in an acidic media, PDMAEMA brushes are completely stretched because of the entire protonization causing the electrostatic repulsion of polymer chains and the fully extended polymer chains form open channels that contribute to moisture penetrating into the fertilizer core and nutrients dissolving out. With the increase of pH value, PDMAEMA brushes are gradually shrunk to the incompact polymer shells that can slightly block the nutrient release in neutral or alkaline solution. For the temperatureinduced release plots, it is noteworthy that the cumulative release amount at 25 °C is higher than that at 40 °C in alkaline solution, which is contrasting to the fact that high temperature favors the nutrient release. Presumably factor accounting for this phenomenon is the temperature-responsive switch behavior of the PDMAEMA brushes. At 40 °C (above LCST) in alkaline solution, polymer brushes are completely 3163

DOI: 10.1021/acssuschemeng.5b01384 ACS Sustainable Chem. Eng. 2015, 3, 3157−3166

Research Article

ACS Sustainable Chemistry & Engineering 38.54% in pH 4.0, pH 7.0, and pH 10.0 solutions at 25 °C. At a higher temperature (40 °C), the release of zinc is 70.36%, 55.13%, and 31.67% in pH 4.0, pH 7.0, and pH 10.0 solutions. It is important to note that the cumulative release amount of zinc is minimal in an alkaline solution at 40 °C. Similarly, as shown in Figure 8, the release of ammonium from MCF@Pdop is 82.59%, 76.62%, and 85.72% in pH 4.0, pH 7.0, and pH 10 aqueous solution at 25 °C. At 40 °C, the release of ammonium from MCF@Pdop raises to 96.23%, 88.64%, and 100% in pH 4.0, pH 7.0, and pH 10.0 aqueous solution, respectively. For MCF@Pdop-g-PDMAEMA, at 25 °C, the release amount is 63.85%, 54.13%, and 48.79% in acidic solution, neutral solution, and alkaline solution. The release of ammonium reaches 75.71%, 64.34% and 42.36% in pH 4.0, pH 7.0, and pH 10.0 aqueous solution at 40 °C. It is worth noting that some changes in the nutrients release behaviors appeared when the fertilizer was applied to different environmental conditions. More experiments on this point would be carried out in further studies for the practical application. Compared to immediately available forms of fertilizer and conventional CSRFs, the “smart” fertilizer can avoid excessive release of fertilizer nutrients into soil, which may cause damage to plant roots, especially at high temperature such as at noon or in summer. It is reported that temperature affects nutrient release rates more than any other extrinsic factor.46 Data showed that double nutrient release rate for a 10 °C rise in temperature (10% per °C).47 Du et al. also reported that nutrient release increased 16% per °C as temperature increased from 20 to 40 °C, and in 20 to 30 °C, 18% per °C increase achieved.48 The strong effect of temperature may lead to an earlier and higher peak in nutrient release and nutrient deficiencies. Our “smart” fertilizer coated by Pdop inner layer and PDMAEMA brushes can effectively regulate nutrients release according to the environmental condition. The release rate can be reduced at high temperature (above the LCST of PDMAEMA), which will overcome the above limitation. Soil pH directly affects the availability of nutrients. Some nutrients become insoluble when the soil’s pH is too low or too high, limiting the availability of these nutrients to the plant root system. Most nutrients are more soluble in acid soils with pH range of approximately 6.0 to 7.0 than in neutral or slightly alkaline soils. The pKa of PDMAEMA is approximately to 7.4. Therefore, in an acidic medium with pH below its pKa, PDMAEMA brushes are completely stretched to release nutrients. When the pH is above its pKa, PDMAEMA brushes are gradually shrunk to polymer shells which hinder nutrients release. The pH responsive property of MCF@Pdop-gPDMAEMA may improve the availability of nutrients. With the technological developments and the commercial availability of CSRFs, a wide range of applications of CSRFs have been achieved than the past, yet their use in modern agriculture is still limited. High cost is the main reason why the limited use of CSRFs. In this work, the cost of the materials for the preparation of MCF@Pdop-g-PDMAEMA is much higher than urea fertilizer owing to SI-ATRP taken place on the surface of macroinitiator with 6.4 mL of DMAMEA. However, “smart” fertilizer could realize the stimuli-responsive controlled release of nutrients according to the environmental conditions. Meanwhile, the use of “smart” fertilizer can effectively decrease the risk of environmental pollution causing by possible losses of nutrients or excessive fertilization, which meets the requirements of the

modern sustainable agriculture. Considering these advantages, this fertilizer would be promising in the continuing research of CSRFs and may be largely used for high-value crops in the future.

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CONCLUSIONS In this work, a “smart” fertilizer functionalized with a Pdop inner layer and double-stimuli-responsive PDMAEMA brushes was successfully prepared by SI-ATRP. Because of the configurational changes of PDMAEMA brushes in response to different environmental conditions, different release profiles can be found by altering the environment. The nutrients relese rate can be obviously accelerated in an acidic pH (below pKa) medium at a certain temperature. In addition, low temperature (below LCST) can accelerate the nutrients release rate in a basic medium (above pKa), which is contrasting to the reduction of nutrients release rate at high temperature (above LCST) in the same medium. These results demonstrate excellent controlled-release properties of the fertilizer based on the PDMAEMA brushes grafting from the Pdop inner layer. Therefore, the as-prepared fertilizer shows promising applications in sustainable modern agriculture.

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AUTHOR INFORMATION

Corresponding Authors

*S. Lü. Tel.: +86-931-8912387. Fax: +86-931-8912582. E-mail: [email protected]. *M. Liu. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (grant no. 51273086, 51503091, 31260500), Special Doctorial Program Fund from the Ministry of Education of China (grant no. 20130211110011), and the Fundamental Research Funds for the Central Universities (grant no. lzujbky-2015-26).

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