Novel imprinted nanocapsule with highly enhanced hydrolytic activity for organophosphorus pesticide degradation and elimination

Accepted Manuscript Novel Imprinted Nanocapsule with Highly Enhanced Hydrolytic Activity for Organophosphorus Pesticide Degradation and Elimination Heguang Shi, Ruiyu Wang, Jixiang Yang, Hongqi Ren, Shuai Liu, Tianying Guo PII: DOI: Reference:

S0014-3057(15)00482-6 http://dx.doi.org/10.1016/j.eurpolymj.2015.09.016 EPJ 7073

To appear in:

European Polymer Journal

Received Date: Revised Date: Accepted Date:

15 July 2015 14 September 2015 16 September 2015

Please cite this article as: Shi, H., Wang, R., Yang, J., Ren, H., Liu, S., Guo, T., Novel Imprinted Nanocapsule with Highly Enhanced Hydrolytic Activity for Organophosphorus Pesticide Degradation and Elimination, European Polymer Journal (2015), doi: http://dx.doi.org/10.1016/j.eurpolymj.2015.09.016

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Novel Imprinted Nanocapsule with Highly Enhanced Hydrolytic Activity for Organophosphorus Pesticide Degradation and Elimination Heguang Shi, Ruiyu Wang, Jixiang Yang, Hongqi Ren, Shuai Liu, Tianying Guo* Key laboratory of Functioal Polymer Materials (Nankai University), Ministry of Eduction, Institute of Polymer Chemistry, College of Chemistry, Nankai University, Weijin Road, No.94, Tianjin 300071, China. *Corresponding Author. E-mail: [email protected]

The removal of organophosphorus pesticide residues is of great importance to environment treatment nowadays. In this work, we designed a novel imprinted hollow nanocapsule which could effectively eliminate organophosphorus pesticide residues remained in water environment. The shell of the methyl parathion imprinted nanocapsule was prepared based on a multi-pyridine-ligand functional monomer, N-(4-vinylbenzyl)-di(2-picolyl)amine. The obtained nanocapsule showed a high catalytic efficiency towards methyl parathion, with an initial hydrolysis rate of 3.1×10-2 mM/h, 355-fold relative to the self-hydrolysis of methyl parathion. And a low Km value, 0.6 mM, was observed, indicating a good affinity of functional ligand on the nanocapsule towards methyl parathion. Additionally, the obtained hollow nanocapsule could further remove p-nitrophenol generated in the catalytic degradation system, another hazardous substance, by enriching part of p-nitrophenol in its central cavity. Keywords: Pesticide Degradation, Molecular Imprinting, Nanocapsule, Methyl Parathion, p-Nitrophenol

1. Introduction Organophosphate compounds (OCs), such as paraoxon, parathion, malathion and so on, have been widely used as pesticides in agricultural productivity [1, 2]. However, they have been proved to be extremely hazardous for human health because they may inhibit main metabolic pathways by inhibiting acetylcholinesterase, the key enzyme in the transmission of nerve impulses [3-5]. Due to their high resistance to spontaneous hydrolysis, OCs may persist in earth for a long time, adverse to the ecology. So pesticide residues in vegetables, fruits, water and earth are drawing more and more attention. To remove OCs effectively, a number of catalysts to date have been discovered to accelerate their degradation, including inorganic metal ions or metal-oxides [6, 7], metal ion complexes [8, 9], bio-enzymes [10, 11] and so on. Among these catalysts, bio-enzymes have been considered to be the ideal decontaminating agent towards OCs, with the advantages of high catalytic activity, no environment pollution. The typical one is organophosphorus hydrolases, which can detoxify a broad range of toxic organophosphate pesticides [12]. Although owing impressive catalytic capabilities, natural bio-enzymes may possess some disadvantages such as instability of enzyme structure, sensitivity to harsh process conditions, non-recovery of enzyme from the reaction mixture, and high cost of separation, thereby restricting their practical applications for large-scale enzymatic degradation [13]. To avoid these shortcomings mentioned above and simultaneously obtain high catalytic capabilities, chemists began to devote to designing molecular structures similar to bio-enzyme several years ago, so mimicking of bio-enzyme by synthetic chemistry gradually became the top subject of organophosphate degradation. So far, outstanding progress has been made to mimic bio-enzyme with several model systems, including

synthetic macrocyclic compounds [14, 15], molecular assemblies, catalytic antibodies [16], and molecularly imprinted polymers (MIPs) [17-20]. Among these methods, molecular imprinting has been demonstrated to be a promising and facial method, which can effectively mimic bio-enzyme with high catalytic capability towards OCs [20-26]. In comparison with natural bio-enzyme, molecularly imprinted polymers(MIPs) possess many advantages such as low cost, superior stability, easy preparation and storage [27-29]. Molecular imprinting is a technique to generate active sites in highly cross-linked polymers with specific shape having functional groups in a defined orientation. The formation of imprinting efficiency is mainly based on the interactions between template and functional monomer such as covalent interactions, non-covalent interactions, electrostatic interactions, and metal ion coordination interactions. It has been demonstrated that constructing active sites by metal ion coordination can remarkably increase the catalytic activity. Recently, much research has been made to deeply investigate the structure of phosphoesterase [30-34] and a series of biomimetic compounds based on multi-pyridine ligands have been synthesized to mimic active functional domain of phosphoesterase, such as 2,6-bis((bis(pyridin-2-ylmethyl)amino) methyl)-4-methylphenol, where the pyridine residues are mimics for histidine, part of posphoesterase[32]. The obtained Zn(II) complex showed a high k cat, 5.08×10-3 s-1, towards bis(2,4-dinitrophenyl)phosphate hydrolysis. Triel and co-workers [35] synthesised 3-(bis(pyridin-2-ylmethyl)amino)propan-1-ol derived from di(2-picolyl)amine and its Zn(II) complex, which also exhibited high catalytic activity towards bis(p-nitrophenyl) phosphate hydrolysis. These results imply that the introduction of pyridine group can help to increase the catalytic activity of mimics. Thus in this work, to improve the coordination interaction between template and functional monomer, a multi-pyridine-ligand functional monomer containning di(2-picolyl)amine was synthesized to prepare MIPs. To the best of our knowledge, this is the first time to investigate multi-pyridine ligand as functional monomer applied in molecular imprinting for organophosphorus pesticide degradation. Besides, p-nitrophenol, hydrolysisi product of methyl parathion, is another hazardous substance to the environment, which has always been ignored by other groups. Herein, to remove simultaneously p-nitrophenol, a methyl parathion imprinted nanocapsule with hollow structure was also designed as similar to our previous work [36, 37]. Methyl parathion could be hydrolyzed in the imprinted sites, and the generated p-nitrophenol could be enriched in the void. Therefore the obtained hollow nanocapsule was expected to be able to hydrolyze methyl parathion rapidly and absorb the generated p-nitrophenol simultaneously.

2. Experimental 2.1. Chemicals Methyl parathion was purchased from Best Reagent Ltd. (Chengdu, China). Di(2-picolyl)amine (DPA), 4-Vinylbenzyl chloride (VBC), azodiisobutyronitrile (AIBN) and γ-methacryloxypropyl- trimethoxysilane (KH-570) were supplied by Heowns Biochem Technologies (Tianjin, China). Tetraethylorthosilicate (TEOS) was purchased from Beijing InnoChem Science & Technology Co. Divinylbenzene (DVB) containing 80% DVB was obtained from Aldrich (Stein-heim, Germany) and purified by treating with 5wt% aqueous NaOH to remove the inhibitor. 2.2. Synthesis of N-(4-vinylbenzyl)-di(2-picolyl)amine (VBDA)

Under an inert gas atmosphere, DPA (2 mmol), VBC (2 mmol) and anhydrous Na 2CO3 (10 mmol ) was dissolved in 50 mL dichloromethane (CH 2Cl2) and stirred for 24 h at room temperature (as displayed in Scheme 1). Then 50 mL deionized water was added to the solution. The organic phase was washed once with 50 mL water, dried over anhydrous Na 2SO4 and concentrated under vacuum. Silica gel column purification with CH2Cl2:methanol (16:1, v/v) gave 0.5 g of yellowish oil (81%). 1H NMR (400 MHz, CDCl3) δ 8.52 (s, 2H), 7.74–7.06 (m, 10H), 6.69 (dd, J = 17.6, 10.9 Hz, 1H), 5.73 (t, J = 14.1 Hz, 1H), 5.22 (t, J = 11.7 Hz, 1H), 3.81 (s, 4H), 3.68 (s, 2H).

Scheme 1. Synthesis of N-(4-vinylbenzyl)-di(2-picolyl)amine

2.3. Preparation of SiO2 precursor Firstly, uniform silica nanoparticles were prepared using Stöber method through hydrolysis of TEOS with aqueous ammonia [38]. Specifically, 11 mL 25% aqueous ammonia and 2.5 mL deionized water were dissolved in 100 mL ethanol, then the mixture of 5 mL TEOS in 100 mL ethanol was added rapidly and stirred at a rate of 250 rpm under room temperature for 20 h. The silica nanoparticles were obtained by centrifugation and washed with water and ethanol, dried under vacuum. Then, 1 g of the obtained silica nanoparticles and 10 mL KH-570 were dispersed in 100 mL ethanol and stirred under nitrogen at 50 oC for 24 h. The product (denoted as vinyl-SiO2) was centrifuged and washed with ethanol for several times, then dried under vacuum before use. 2.4. Preparation of methyl parathion imprinted hollow nanocapsule 0.05 g VBDA was dissolved in 2 mL DMSO, then 0.047 g Zn(NO 3)2•6H2O was added. The mixture was stirred overnight to get VBDA-Zn(II) complex, followed by adding 25 mL ethanol. Then 0.02 g vinyl-SiO2 as cores, a certain amount of DVB, AIBN and methyl parathion was added to the above solution followed by ultrasonication for 2 min. Subsequently, the resulted solution was purged with dry N2. Polymerization was performed at 70 oC under N2 for 21 h. The product was centrifuged and washed with ethanol. Then the silica cores were removed by 330 μL HF in the mixture of ethanol and deionized water (4:1, v/v) for 12 h. After that, the template was removed in the mixture of 0.1 M NaOH aqueous solution and ethanol to give imprinted hollow nanocapsule, which was denoted as VBDA-MIP nanocapsule. Non-imprinted nanocapsules were prepared in the absence of methyl parathion, following the same procedure described above. Correspondingly, another kind of imprinted nanocapsule was prepared with Zinc dimethacrylate (MAA-Zn(II)) as functional monomer in equal proportion, which was denoted as MAA-MIP nanocapsule. 2.5. Characterization Infrared spectra of the samples in KBr were recorded on a FTS6000 FT-IR spectrometer manufactured by Bio-rad of USA.

1

H NMR spectra were measured by AVANCE III 400MHz made by Bruker in Switzerland.

Transmission electron microscope (TEM) observations were conducted using a JEM100C II microscope with 100 kV. The dried samples were dispersed in ethanol and loaded on a 300 mesh carbon-coated copper grid. A UV-2550 spectrometer (Shimadzu, Suzhou, China) was used to detect the absorption strength of the solution at certain wave-length to obtain the concentrations of p-nitrophenol and methyl parathion according to the standard curves of concentration and absorption strength. 2.6. Hydrolytic experiments Typically, 5 mg VBDA-MIP nanocapsule was added to a solution of 4 mL PBS (0.1 M, pH=9.0) and 0.9 mL ethanol by ultrasonication for 10 s, then stirred for a while to obtain stable mixture. Kinetics measurement was started by injecting 100 μL methyl parathion/ethanol solution (10 mg/mL) at 30 oC. 100 μL mixture solution was taken at regular intervals and centrifuged rapidly, and 50 μL supernatant was diluted with 350 μL PBS. The concentration of hydrolyzate p-nitrophenol was determined by UV-vis spectrophotometer at 400 nm. And the kinetics data of methyl parathion hydrolysis catalyzed by VBDA-NIP nanocapsule and MAA-MIP nanocapsule were conducted under the same conditions. Based on the kinetics data, linear fit was made with the first three points of each kinetic curve, and the slope was defined as initial hydrolysis rate (kini).

3. Results and discussion 3.1. Preparation of methyl parathion imprinted nanocapsule

Scheme 2. Schematic procedure for the preparation of the VBDA-MIP nanocapsule

The overall procedure for producing methyl parathion imprinted nanocapsule involved several steps, as illustrated in Scheme 2. Firstly, uniform silica nanoparticles were prepared using Stöber method through hydrolysis of TEOS in aqueous ammonia. Silica nanoparticle was chosen as core due to its stability in organic solvents and the fact that a variety of functional groups could be easily modified on its surface. In the next step, the silica nanoparticles were chemically modified with KH-570 to introduce vinyl group onto the nanoparticle surface, which was denoted as vinyl-SiO2. Afterwards, precipitation polymerization was carried out in ethanol to produce MIP@SiO2 with VBDA-Zn(II) complex as functional monomer, DVB as crosslinking agent, methyl parathion as template and AIBN as initiator. Then the imprinted

nanocapsule was obtained by removing the SiO 2 core with HF. After wiping off template molecules, recognition sites complementary to the template molecule in shape, size, and chemical functionality were formed in the highly cross-linked polymer matrix. Fig. 1 displays the TEM images of MIP@SiO 2 and the VBDA-MIP nanocapsules. And Fig. 1(A) reveals that SiO 2 nanoparticles are spherical in shape and almost uniform in size, with a diameter of approximately 150 nm. As illustrated in Fig. 1(B), the MIP shell with lower contrast than that of encapsulated silica nanoparticles can be distinguished clearly, indicating that methyl parathion imprinted polymer shells with an average thickness of about 20 nm have been coated onto the silica cores uniformly. The formation of MIP shell could be attributed to the copolymerization of cross-linking agent and the pre-assembly complexes formed by VBDA-Zn(II) and template on the surface of vinyl-SiO2. From Fig. 1(C) we can see that uniform MIP nanocapsules with obvious hollow interiors were obtained, demonstrating silica cores have been removed by HF. Without methyl parathion added, VBDA-NIP nanocapsules show the same morphology with VBDA-MIP nanocapsule, as presented in Fig. 1(D).

Fig. 1 TEM images of (A) SiO2 (B) MIP@SiO2 (C) VBDA-MIP nanocapsule (D) VBDA-NIP nanocapsule

To further confirm the successful preparation of imprinted hollow nanocapsule, the samples SiO 2 nanoparticle, vinyl-SiO2, MIP@SiO2, VBDA-MIP and VBDA-NIP nanocapsule were characterized by using FT-IR spectroscopy, as shown in Fig. 2. Compared with the infrared data of pure silica nanoparticles(a), vinyl-SiO2(b) exhibits clearly two characteristic peaks at 2982 cm -1 and 1720 cm-1, which can be attributed to C-H asymmetry stretching vibration of methylene and C=O stretching vibration of the carbonyl group, respectively [39, 40], indicating the covalent coupling of KH-570 onto SiO2 surface successfully. The strong absorbance peak at 2924 cm -1 and the bands at the range of 1450-1600 cm-1 on curve (c), corresponded to the C-H stretching vibration in the methyl units and C=C stretching vibration in benzene ring of VBDA respectively, implying the synthesis of MIP@SiO 2 through the copolymerization of VBDA-Zn(II) and cross-linking agent on the surface of vinyl-SiO2. Moreover, the disappearance of the

strong absorbance peak at 1103 cm -1 on curve (d) and (e), ascribed to the stretching vibration of Si-O bond, demonstrating the successful removal of SiO 2 cores.

Fig. 2 The FT-IR spectra of (a)SiO2 (b)vinyl-SiO2 (c)MIP@SiO2 (d)VBDA-MIP nanocapsule (e) VBDA-NIP nanocapsule

3.2. Optimization of the imprinted nanocapsule composition The catalytic activity of the VBDA-MIP nanocapsule was determined by the quantity of imprinted cavity on VBDA-MIP nanocapsule, which is closely related to the ratio of functional monomer to template and the degree of crosslinking (weight percentage of crosslinking agent accounts for the total weight of functional monomer and crosslinking agent). To optimize the degree of crosslinking, we firstly fixed the molar ratio of VBDA to methyl parathion at 5:1. A batch of VBDA-MIP nanocapsules with different degree of crosslinking (75%, 78%, 82%, 86%) was prepared. And the initial hydrolysis rates of methyl parathion catalyzed by these nanocapsules were displayed in Fig. 3. The results point out that decreasing the degree of cross-linking leads to the increase of catalytic activity of the VBDA-MIP nanocapsules, because of the increase of the quantity of imprinted site. But when the degree of crosslinking was reduced to 75%, regular spherical nanocapsules with rigid structure were difficult to form, which could be confirmed by the TEM images of corresponding MIP@SiO2 and VBDA-MIP nanocapsule, as shown in Fig. 4. The irregular nanoparticles displayed in Fig. 4(a) might be attributed to serious self-seeding nucleation occurred during the polymerization, which will cause the decrease of the proportion of effective imprinted sites on the MIP layer. And Fig. 4(b) exhibits irregular nanocapsule with loose skeleton was obtained owing to the low degree of crosslinking. This nanocapsule is easy to be broken during use, resulting in a poor catalytic activity. Thus 78% was selected as the optimal degree of crosslinking.

Fig. 3. Effect of degree of cross-linking of the VBDA-MIP nanocapsule on the initial hydrolysis rate

Fig. 4 TEM images of (A) MIP@SiO2 (B) VBDA-MIP nanocapsule with the degree of crosslinking 75%

After the degree of crosslinking was fixed at 78%, a series of nanocapsules with different molar r atio of VBDA to methyl parathion (10:1, 8:1, 5:1, 2:1) were prepared. And the initial hydrolysis rates of methyl parathion catalyzed by these nanocapsules were shown in Fig. 5, from which we can see that the highest kini, 3.1×10-2 mM/h, was obtained by the VBDA-MIP nanocapsule prepared with 5:1 (molar ratio) of VBDA to methyl parathion. These results imply that the catalytic activity of VBDA-MIP nanocapsule increases with the decrease of molar ratio of VBDA to methyl parathion, resulting from the increase of the quantity of imprinted site. But when the molar ratio was reduced to 5:1, the yield of imprinted sites reaches a maximum, and when the molar ratio was further reduced to 2:1, the catalytic activity could not be improved, instead, a lower initial hydrolysis rate was observed, demonstrating the yield of imprinted sites reached a maximum when the molar ratio of VBDA to methyl parathion was designed as 5:1. Thus the VBDA-MIP nanocapsule prepared with the optimum molar ratio of VBDA to methyl parathion as 5:1 and a crosslinking degree of 78% was used in the following investigation.

Fig. 5. Effect of the molar ratio of VBDA to methyl parathion added in the polymerization on the initial hydrolysis rate of VBDA-MIP nanocapsule

3.3. Hydrolytic activity of the VBDA-MIP nanocapsule Fig. 6 exhibits the self-hydrolysis kinetics curve of methyl parathion and kinetics curves catalyzed by VBDA-MIP and VBDA-NIP nanocapsules and Table 1 displays the corresponding initial hydrolysis rates (kini values). The self-hydrolysis rate was found to be 8.73×10 -5 mM/h. High catalytic enhancements, 355-fold and 61-fold higher than the self-hydrolysis rate, were obtained with VBDA-MIP nanocapsule and VBDA-NIP nanocapsule, respectively, indicating that both MIP and NIP nanocapsules showed rate enhancement for methyl parathion hydrolysis relative to non-catalyzed reaction. This result confirms that the nanocapsules prepared using VBDA as functional monomer can catalyze methyl parathion hydrolysis effectively. The catalytic action of VBDA-NIP nanocapsule could be attributed to the partial functional groups on the surface of the nanocapsule, which can coordinate with methyl parathion. Furthermore, the VBDA-MIP nanocapsule exhibited considerably higher catalytic activity than VBDA-NIP nanocapsule, owing to the rapid rebinding of methyl parathion onto the imprinted sites of the VBDA-MIP nanocapsule, where methyl parathion coordinated with VBDA-Zn(II) complex and was catalytically hydrolyzed by the strong interaction with the zinc catalytic center. Table 1. Initial hydrolysis rates of methyl parathion catalyzed by VBDA-NIP and VBDA-MIP nanocapsules

kini(mM/h)

self-hydrolysis

VBDA-NIP nanocapsule

VBDA-MIP nanocapsule

8.73×10-5

5.32×10-3

3.1×10-2

Fig. 6. The self-hydrolysis kinetics curve of methyl parathion (a) and kinetics curves catalyzed by (b)VBDA-NIP nanocapsule and (c) VBDA-MIP nanocapsule

For comparison, ethyl parathion, structural analogue of methyl parathion, was selected as control substrate to investigate further the imprinting effect of the VBDA-MIP nanocapsule. Fig. 7 illustrates the kini values of methyl parathion and ethyl parathion catalyzed by VBDA-MIP nanocapsule and VBDA-NIP nanocapsule respectively. Apparently, the VBDA-MIP nanocapsule exhibited higher catalytic capacities towards methyl parathion than ethyl parathion, demonstrating that VBDA-MIP nanocapsule has higher selectivity towards methyl parathion than its structural analogue. This should be attributed to that the imprinted sites on imprinted nanocapsules are highly specific and selective towards methyl parathion based on its shape, size and functionality, while its structural analog was unable to bind at the sites exactly, resulting in much slower hydrolysis of ethyl parathion than methyl parathion. However, the VBDA-NIP nanocapsule did not show big difference on the catalytic capacities toward methyl parathion and ethyl parathion, just because there are no specific recognition sites on the VBDA-NIP nanocapsule. The targeted molecules could only coordinate to the functional groups on the surface of VBDA-NIP nanocapsule, leading to significantly low catalytic activities.

Fig. 7. The comparative kini values of methyl parathion and ethyl parathion catalyzed by the VBDA-MIP nanocapsule and VBDA-NIP nanocapsule

As an important parameter of enzymatic reaction, the Michaelis-Menten constant (Km), which reflects the affinity of an enzyme for a particular substrate, is usually used to estimate the catalytic activity of an enzyme. The lower the value of Km, the higher the affinity. In this work, to determine the Km value of the VBDA-MIP nanocapsule, a batch of hydrolytic experiments with different concentrations of methyl parathion (0.5 mM, 1 mM, 2 mM, 4 mM) were conducted and the initial hydrolysis rates (k) were measured. According to the Lineweaver-Burke equation, 1/k=1/Vm+Km/(Vm•[S]), 1/k versus reciprocal of initial substrate concentration (1/[S]) was plotted as shown in Fig. 8. A low Km value was calculated from the plots to be 0.6 mM, indicating that the obtained VBDA-MIP nanocapsule possesses a high affinity towards methyl parathion, leading to a remarkable catalytic activity [41].

Fig. 8. Lineweaver-Burke plot of kinetics data of hydrolysis for methyl parathion catalyzed by VBDA-MIP nanocapsule

To investigate the reutilization of the VBDA-MIP nanocapsule, the VBDA-MIP nanocapsule after used was regenerated by removing the template again in NaOH aqueous, followed by uploading the Zn 2+. The initial hydrolysis rate of methyl parathion catalyzed by the regenerated VBDA-MIP nanocapsule was measured under the same condition. As displayed in Fig. 9, after three circles, up to

82.4%

of

the

catalytic activity was still retained, compared with the initial utilization, implying the sufficient mechanical properties of the VBDA-MIP nanocapsule to suffer from the damage during catalysis hydrolysis and ultrasonic dispersion process. A spot of decline of catalytic capacity may be attributed to the part loss of nanocapsule amount due to the repeated wash and centrifugation.

Fig. 9. Reutilization cycles of VBDA-MIP nanocapsule

3.4. Effect of introducing tridentate pyridine ligand on the catalytic activity To investigate the effect of introducing tridentate ligand, DPA, to the functional monomer on the catalytic activity of VBDA-MIP nanocapsule, we compare the catalytic ability of the VBDA-MIP nanocapsule with that of nanocapsule prepared with equimolar amounts of MAA-Zn(II) as monomer, which has been used by our group before. Fig. 10 presents the kinetics curves of methyl parathion catalyzed by the VBDA-MIP nanocapsule and the MIP-MAAZn(II) nanocapsule with kini values of 3.1×10-2 mM/h and 1.18×10-2 mM/h. An enhancement of 135-fold, relative to self-hydrolysis rate, was observed, significantly lower than the 355-fold of VBDA-MIP nanocapsule, implying that VBDA is more effective than MAA-Zn(II). This could be mainly attributed to the difference of structures of their transition state complexes, as shown in Scheme 3. Firstly, nitrogen atom possesses lower electronegativity, giving it stronger coordination capacity than oxygen atom. So Zn(II) can exist more stably in the structure of DPA. Secondly, in the structure of MAA-Zn(II), Zn(II) exists in the center of a dimer formed by two methacrylic acid molecules,

producing a tetra-coordinated configuration. During the polymerization, methyl parathion coordinated to Zn(II) to generate a penta-coordinated intermediate. And the synthesized functional monomer VBDA, a tridentate ligand, may form a tetra-coordinated intermediate after interacting with methyl parathion. As widely known, it is easy for Zn(II) to form tetra- or hexa-coordinated complexes, and the penta-coordinated form is not stable. So it is easier to produce a stable tetra-coordinated transition state for VBDA combination with methyl parathion during the polymerization. This result implies the introducing of tridentate ligand to functional monomer can obviously help to increase the catalytic activity towards methyl parathion.

Fig. 10. The kinetic curves of MP catalyzed by (a)MIP-MAAZn(II) nanocapsule (b)VBDA-MIP nanocapsule

Scheme 3. Comparison of the transition state complexes derived from methyl parathon with MAA-Zn(II) and VBDA-Zn(II)

3.5. Ability of the VBDA-MIP nanocapsule for p-nitrophenol elimination As the hydrolyzate of methyl parathion, p-nitrophenol acts also as a kind of poisonous compound and is even harder to be eliminated [42, 43]. So it is significant to evaluate the ability of the VBDA-MIP nanocapsule for p-nitrophenol elimination. For this purpose, the identical hydrolytic experiment was conducted for 50 h and the practical concentrations of methyl parathion and p-nitrophenol were measured synchronously. The theoretical concentrations of generated p-nitrophenol in different intervals were equal to the consumed methyl parathion concentrations, which could be calculated by deducting the practical concentrations with initial concentration of methyl parathion. In this way, theoretical and practical kinetic curves of p-nitrophenol can be depicted, as exhibited in Fig. 11. During the first 10 h, the practical concentration was in good agreement with the theoretical concentration. As the reaction continued, the theoretical concentration increased while the practical concentration tended to be constant. The

discrepancy between theoretical concentration and practical concentration of p-nitrophenol can be attributed to the enrichment of p-nitrophenol in the nanocapsule. Concretely, when the concentration of p-nitrophenol in the catalytic system reached a high level, part of p-nitrophenol was absorbed by the VBDA-MIP nanocapsule, then got through a channel formed by the imprinted cavities with larger volume than p-nitrophenol, and enriched in the inner cavity of the VBDA-MIP nanocapsule finally. Thus, the duel functions of methyl parathion degradation and p-nitrophenol elimination were achieved simultaneously, which was consistent with the result of our previous work [37].

Fig. 11. (a) theoretical and (b) practical kinetic curves of p-nitrophenol generated in the catalytic system

Scheme 4. Proposed mechanism for methyl parathion hydrolysis and p-nitrophenol enrichment by VBDA-MIP nanocapsule

To further understand this working mechanism, a possible pathway for the VBDA-MIP nanocapsule towards methyl parathion degradation and p-nitrophenol elimination is proposed, as shown in Scheme 4. Firstly, methyl parathion is selectively absorbed in the imprinted sites rapidly, where hydrolysis of methyl parathion happens based on a catalytic reaction [44, 45]. The functional monomer-Zn(II) complex coordinates with water molecule to form A, and the water molecule can be deprotonated in alkaline

environment to produce B with a Zn(II)-OH active specie. Then the central phosphorus atom of methyl parathion is attacked nucleophilically by the active species, while the Zn(II) simultaneously withdraws electron density away from the phosphorus atom by interacting with the phosphoryl sulfur, forming penta-coordinated intermediate C. Subsequently, with the P-OAr bond weakened, the good leaving group p-nitrophenol is released to form D. In the final step, O,O-dimethyl phosphorothioate in D is substituted by H2O to regenerate the starting zinc complex A, ready for the next catalytic cycle. As the catalytic reaction happens, the produced p-nitrophenol accumulates continuously in the catalytic system. Finally, part of p-nitrophenol diffuses into the inside of the VBDA-MIP nanocapsule through the imprinted cavities. The elimination of p-nitrophenol will accelerate the catalytic reaction in return. After 50 h, about 0.1467 mM p-nitrophenol was enriched in the cavity of the VBDA-MIP nanocapsule, indicating an adsorption amount as high as 20.4 mg/g towards p-nitrophenol.

4. Conclusions In this work, we have successfully prepared a novel methyl parathion imprinted nanocapsule with N-(4-vinylbenzyl)-di(2-picolyl)amine as functional monomer based on the monodisperse SiO 2 nanoparticle precursor. A high catalysis efficiency, 355-fold relative to self-hydrolysis of methyl parathion, was observed. And the MIP nanocapsule showed higher catalytic activity than the nanocapsule prepared with MAA-Zn(II) as functional monomer, indicating the positive effect of inducing di(2-picolyl)amine to functional monomer on the catalytic activity. Moreover, the MIP nanocapsule was proved to be capable of eliminating the generated p-nitrophenol effectively, by enriching part of p-nitrophenol in its central cavity. This wok demonstrates that multi-pyridine ligand is expected to be used to prepare more effective biomimetic catalyst towards organophosphorus pesticide degradation.

Acknowledgements The authors were grateful to Doctoral Fund of Ministry of Education of China (RFDP, Proj. No. 20130031110012), the National Natural Science Foundation of China (50978138), PCSIRT (IRT1257) and NFFTBS (No. J1103306) for financial support.

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Graphical abstract

Highlights 

We have probed the affinity of a multi-pyridine-ligand based functional monomer, N-(4-vinylbenzyl)-di(2-picolyl) amine, in the catalytic degradation of organophosphorus pesticide.



A novel nanocapsule was prepared with N-(4-vinylbenzyl)-di(2-picolyl) amine as functional monomer, divinylbenzene as crosslinking-agent and methyl parathion as template.



The obtained hollow nanocapsule not only showed high catalytic activity towards methyl parathion but also could further remove the generated p-nitrophenol in the catalytic degradation system, another hazardous substance, by enriching part of p-nitrophenol in its central cavity.