Novel magnetic multi-templates molecularly imprinted polymer for selective and rapid removal and detection of alkylphenols in water

Accepted Manuscript Novel magnetic multi-templates molecularly imprinted polymer for selective and rapid removal and detection of alkylphenols in water Xiaowen Xie, Xiaoguo Ma, Lihui Guo, Yinming Fan, Guolong Zeng, Mengyuan Zhang, Jing Li PII: DOI: Reference:

S1385-8947(18)31791-1 https://doi.org/10.1016/j.cej.2018.09.080 CEJ 19919

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

9 July 2018 27 August 2018 9 September 2018

Please cite this article as: X. Xie, X. Ma, L. Guo, Y. Fan, G. Zeng, M. Zhang, J. Li, Novel magnetic multi-templates molecularly imprinted polymer for selective and rapid removal and detection of alkylphenols in water, Chemical Engineering Journal (2018), doi: https://doi.org/10.1016/j.cej.2018.09.080

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Novel magnetic multi-templates molecularly imprinted polymer for selective and rapid removal and detection of alkylphenols in water

Xiaowen Xie, Xiaoguo Ma*, Lihui Guo, Yinming Fan, Guolong Zeng, Mengyuan Zhang, Jing Li School of Environmental Science and Engineering Guangdong University of Technology, Guangzhou 510006, China

Corresponding author: Xiaoguo Ma Postal address: School of Environmental Science and Engineering, Guangdong University of Technology, Guangzhou 510006, PR China Email: [email protected] Tel./ Fax.: +86 20 3932 2290

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ABSTRACT A novel multi-templates molecularly imprinted polymer (MIP) was prepared on the surface of mesoporous silica coated magnetic graphene oxide (MGO@mSiO2), and applied for the rapid and selective detection of alkylphenol compounds including bisphenol A (BPA), 4-tert-octylphenol (4-tert-OP) and 4-nonylphenol (4-NP) in water. The synthesized composite was characterized by Fourier transform infrared spectrometer, vibrating sample magnetometer, transmission electron microscopy and Brunauer-Emett-Teller surface area analyzer. The MIP showed high adsorption capacity, good selectivity and fast kinetic for these alkylphenols. The maximum adsorption capacities of the MIP for BPA, 4-tert-OP and 4-NP were 16.81, 35.97 and 61.73 mg g-1, respectively, which were significantly higher than those of the non-molecularly imprinted polymer (NIP) due to the imprinting effect. The adsorption of alkylphenols reached equilibrium within 30 min, and followed pseudo-second-order kinetic model and Langmuir model well. The MIP exhibited good sorption selectivity towards BPA, 4-tert-OP and 4-NP, 2

and could be reused 5 times. Furthermore, a new method for ultrasensitive determination of BPA, 4-tert-OP and 4-NP in water by high performance liquid chromatography was developed, using MIP-based magnetic solid-phase extraction coupled with dispersive liquid-liquid microextraction. Under the optimum conditions, the limits of detection (LODs) for BPA, 4-tert-OP and 4-NP were 0.013, 0.010 and 0.010 μg L-1, respectively. The proposed method was used to the detection of BPA, 4-tert-OP and 4-NP in real water samples, with spiked recoveries of 81.5-104.1% and relative standard deviations (RSDs) of 1.0-7.6%, indicating the multi-templates MIP could be a promising adsorbent for separation and analysis of alkylphenols. Keywords: Molecularly imprinted polymer; Alkylphenols; Multi-templates; Removal; Detection

1. Introduction Endocrine-disrupting compounds (EDCs), a class of man-made and natural chemicals, have been proved to cause an outsized influence on the ecosystem, human reproduction systems and immune function owing to their potential estrogenic activity in disturbing the normal function of endocrine systems [1,2]. Among them, alkylphenol compounds, like bisphenol A (BPA), 4-tert-octylphenol (4-tert-OP) and 4-nonylphenol 3

(4-NP), belong to one of this category [3]. BPA，which is pointed out that animals exposure to even low-dose BPA prenatally may lead to cognitive deficits and spatial learning, is an essential ingredient in the production of epoxy resins, polycarbonate plastics and other products [4-7]. 4-tert-OP and 4-NP were usually used as non-ionic surfactants in industries to produce detergents, emulsifiers and other compounds. Generally speaking, the detected concentrations of BPA, 4-tert-OP and 4-NP in world river waters were respectively 0.0005~4.0, <0.001~1.44 and 0.006~32.8 µg L-1 [8]. Though at trace concentrations, both 4-tert-OP and 4-NP have been listed as the priority hazardous substances in water by the European Union (EU) while BPA was not included in water legislations but restricted to be used in plastics infant feeding to reveal its toxicity [9,10]. Therefore, to research a quick and simple way to separate and remove BPA, 4-tert-OP and 4-NP in complexed matrices is imperative. Up to now, solid phase extraction (SPE), dispersive liquid-liquid microextraction (DLLME) and chromatographic techniques like liquid chromatography (LC) and gas chromatography (GC) coupled to different detectors were most frequently employed in the extraction and analysis of phenols [11-15], but with weaknesses of large consumption of solvents, low enrichment efficiency and selectivity. Consequently, it is of great necessity to develop specific materials with high selectivity as sorbent in SPE. Molecularly imprinted polymer (MIP), which possesses artificially specific sites complementary in size and shape to template molecules compared with other structure similar compounds, has drawn increasingly public attention due to its “memory” effect [16-18]. As regards its easy preparation, stability, high binding ability and selectivity, MIP often serves as an adsorbent to extract and remove target contaminants from 4

environmental water [19-22], sediment samples [23,24] and foods [25,26]. Classical methods to prepare MIP cover bulky organic co-polymerization, in-situ polymerization, precipitation polymerization and others, but all of them fail to realize straightforward process, strong bonding capability and high binding rate [27]. However, surface imprinting technology, which forms specific recognition sites onto the surface of the support, is able to effectively avoid the defects mentioned above and weaken the influence of “embedding phenomenon” in the traditional synthetic process of MIP [28]. An excellent carrier plays a vital role in the synthetic procedure of surface molecular imprinting polymers. Graphene oxide (GO), a new type of high-performance carbon materials with a high specific surface area and surface-rich functional groups [29,30], is considered as a superior supporter for preparing MIP to heighten the adsorption capacity, binging kinetic rate. In practical situations, one deficiency of MIP that not can be ignored is the nanoparticle sizes make it hard to be separated from aqueous samples, which limits its application in environmental monitoring. However, in recent years, the marriage of magnetic materials with GO have faultlessly overcome the above imperfection [31-34]. Besides, what is worth mentioning is that mesoporous silica (mSiO 2) with large surface areas and tunable pore sizes is also deemed as an ideal sorbent. Coating mesoporous silica onto magnetic graphene oxide (MGO) would further enlarge the surface area and then strengthen adsorption capacity of MIP [35-37].

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Much work so far has focused on using single template molecule to synthesize MIP, which prevent it from extracting and determining a group of organic pollutants. Though there are some reports synthesizing MIP using more than one template in acidic pharmaceut icals [38-40], acutely hazardous (p-type) chemicals [41] and phenolic compounds [42,43], the literature applying alkylphenols (BPA, 4-tert-OP and 4-NP) as the target molecules jointly, to our best knowledge, has not appeared until now. In this article, a novel multi-templates molecularly imprinted polymer (MIP) with BPA, 4-tert-OP and 4-NP as the common templates, 4-vinylbenzoic acid, ethylene dimethacrylate (EGDMA) and 2,2’-azobisisobutyronitrile (AIBN) as the functional monomer, cross-linking agent and initiator, respectively, has been successfully synthesized. Furthermore, by conducting orthogonal optimization experiment, the optimal value of relevant factors has been acquired. The multi-templates MIP was characterized by several technologies and its qualities such as adsorbability, binding rate, selectivity and reusability were also studied. Finally, the multi-templates MIP was applied to enrich and detect BPA, 4-tert-OP and 4-NP in environmental water samples based on magnetic solid-phase extraction assisted dispersive liquid-liquid microextraction.

2. Materials and methods 2.1. Materials

6

Bisphenol A (BPA), 4-tert-octylphenol (4-tert-OP), 4-nonylphenol (4-NP), bisphenol AF (BPAF), 4-tert-Butylphenol (4-PTBP), vinyltrimethoxysilance (VTTS), tetraethyl orthosilicate (TEOS), cetyl trimethylammonium bromide (CTAB), 4-vinylbenzoic acid, ethylene glycol

dimethacrylate

(EGDMA),

2,2’-azobisisobutyronitrile

tetrachloroethylene were purchased from Aladdin Reagent

(AIBN),

tetrachloroethane,

chlorobenzene,

carbon

tetrachloride,

(Shanghai, China). Natural flake graphite (325 meshes) was obtained from China

National Medicine Corporation Ltd. Other analytical grade reagents and HPLC grade methanol, acetone were purchased from Da Mao Chemical Reagent Plant (Tianjin, China). Standard stock solution of alkylphenols was prepared in HPLC grade methanol, and adsorbate solutions was diluted in methanol/water (2:3, V:V) by their each standard stock solution.

2.2. Apparatus and analytical methods The surface functional groups and chemical component were explored by a Fourier transform infrared spectrometer (FT-IR, Thermo Fisher Scientific, USA) and thermal gravimetric analyzer (TGA, Q600 SDT, TA, USA). The surface morphology and microstructure were examined by scanning electron microscopy (SEM, S4800, Hitachi, Japan) and transmission electron microscopy (TEM, JEM-2100F, JEOL, Japan). Vibrating sample magnetometer (VSM, Squid-VSM) was used to analyze the magnetic torque of the prepared materials. The materials’ 7

specific area was obtained from nitrogen adsorption-desorption tests using a Brunauer-Emett-Teller (BET) V-Sorb 2800 Series Analyzer (ASAP2020HD88, MIKE, America). X-ray diffraction (XRD) pattern analysis was performed with a D8; X’ Pert PRO MPD (Bruker AXS, Germany). High performance liquid chromatography (HPLC) with a Photodiode Array Detector (PDA) was performed with a LC-20A solution system (Shimadzu Corporation, Japan). Chromatography analysis was achieved on an Inert Sustain C 18 column (250 mm×4.6 mm, 5 µm). The analytical methods was following this: mobile phase, methanol/water (90/10, V:V); flow rate, 1mL min -1; column temperature, 40℃; PDA detection, at 277 nm; injection volume, 20 µL.

2.3. Preparation of multi-templates MIP 2.3.1. Preparation of magnetic graphene oxide (MGO) The modified Hummers method to synthesize GO nowadays has been completely full-fledged. On the basis of the works reported by Chen [44], some improvements including amount of chemicals, reaction time and operation process have been made. First concentrated sulfuric acid (250 mL, 98 wt%) and sodium nitrate (2.5 g) was stirred under ice bath condition, then natural flake graphite (5 g) was spoonful put into the above mixtures, reacting for 40 mins. Next potassium permanganate (35 g) was slowly added and allowing the reaction for 2 h under 10℃ to 8

prevent the oxidation reaction. Subsequently followed by the medium temperature for 1 h. After that, deionized water (240 mL) was dropwise added and maintained the temperature at 371 K for 40 min. Deionized water (500 mL) was used to dilute the obtained bright-yellow suspension and then following by H2O2 (30 mL, 30%). The next step was to wash the product with 3.4 wt% HCl and acetone successively, finally vacuum-dried at 80℃ for 24 h. MGO was prepared according to the inverse chemical co-precipitation method [44]. GO dispersed in deionized water after sonication for 30 min was mixed with FeCl3·6H2O (8 g) and FeSO4·7H2O (3.5 g) under mechanical stirring at 25℃. Then ammonia solution (30 wt%) was added to adjust the pH at 11 and then continued to react for 60 min at 80℃. The above reaction was performed under nitrogen atmosphere. The last step was to wash the black precipitate with deionized water and ethanol each three times, and vacuum-dried at 60℃.

2.3.2. Preparation of MGO@mSiO2 modified with vinyl groups (VTTS-MGO@mSiO2) MGO@mSiO2 was synthesized on the basis of reported methods with some improvements [45]. To begin with, MGO (0.5 g) as well as CTAB (1.5 g) were pet into a conical flask, then adding absolute ethanol (250 mL), deionized water (100 mL) and aqueous ammonia (5 mL, 30 wt%) one after another. The above solution was sonicated for 30 min to disperse the mixtures evenly. Following this, TEOS (4 mL) was added dropwise under on-going stirring, and allowing for 24 h at room temperature. Under the action of external magnetic field, the product was 9

collected and washed with ethanol for three times. To purify the mesoporous silica materials, a mixed solution of ethanol (150 mL) and NH4·NO3 (1.5 g) was used to completely remove CTAB before vacuum-drying at 60℃. In order to facilitating the subsequent polymerization, a vinyl group was grafted onto the surface of MGO@mSiO 2 [45]. Simply saying MGO@mSiO2 (0.5 g) and VTTS (10 mL) was dispersed in toluene (50 mL) by ultrasonic uniformity for 30 min. Then purging with nitrogen and reacting for 24 h at 50℃ under stirring. The obtained product was separated by a magnet and washed with toluene, finally dried in vacuum at 60℃.

2.3.3. Preparation of VTTS-MGO@mSiO2@MIP To enhance the adsorbability, extraction efficiency and selectivity for BPA, 4-tert-OP and 4-NP in environmental medium, a multi-templates MIP was prepared via surface molecular imprinting technique. Firstly, BPA, 4-tert-OP and 4-NP (0.175 mmol, 1:1:1, molar ratio) blended with 4-vinylbenzoic (1.05 mmol) were dispersed in toluene (50 mL) by sonicating for 15 min, then reacting for 4 h at 15℃ under air bath shaking table. After that, VTTS-MGO@mSiO2 (300 mg), EGDMA (2.63 mmol) and AIBN (50 mg) were in succession placed into the above solution and then the nitrogen was deoxygenated under ice-cooling. The reaction was proceed for 24 h at 60℃. To completely

10

remove templates, the obtained MIP was eluted with the mixtures of methanol and acetic-acid (9:1, V:V) until no templates were detected by HPLC. The NIP, in a similar way, was prepared using the same means in the absence of the target templates.

2.4. Batch adsorption experiments The effects of pH, sorption kinetics and isotherm of alkylphenols onto the multi-templates MIP were explored in this study. In the pH effect experiment, the multi-templates MIP (20 mg) was dispersed in a methanol/water solution of BPA, 4-tert-OP and 4-NP (50 mg L-1, 20 mL) with various pH values (from 3 to 10), then shaken in a thermostatic bath (200 rpm) at 298 K for 60 min. The kinetic tests at different temperature (298 K-318 K) were carried out under defined time intervals from 1 to 240 min with 50 mg L -1 alkylphenols initial concentration at pH 6. The sorption isotherm experiments were performed with different concentrations of adsorption solutions ranging from 5 to 120 mg L-1 at pH 6 for 60 min. After reaching adsorption equilibrium, the saturated solution was separated under external magnetic field and then filtered through a 0.45 µm membrane. The residual concentration of BPA, 4-tert-OP and 4-NP was detected by HPLC system. The binding capacity was calculated as follows: Qe = (Co-Ce) V/M 11

(1)

where Qe (mg g-1) is the adsorption capacity of the multi-templates MIP/NIP, Co and Ce are the initial and equilibrium concentrations (mg L -1) respectively. V (L) and M (mg) are the solution volume and adsorbent mass.

2.5. Adsorption selectivity To prove the specific selectivity of the multi-templates MIP toward BPA, 4-tert-OP and 4-NP, BPAF and 4-PTBP were chosen as the competitive agents due to their similar chemical structures with the template molecules. The molecule structures of these five alkylphenols were listed in Fig. S1. MIP and NIP (20 mg) was first respectively dispersed in a mixture solution contained BPA, 4-tert-OP, 4-NP, BPAF and 4-PTBP (50 mg L-1, 20 mL), then shaken at 298 K for 60 min to achieved a balanced adsorption. To get a better understanding of selectivity of the multi-templates MIP, the static distribution coefficient (Kd), selectivity coefficients (k) and relative selectivity coefficients (k o) were calculated as follows: Kd = Qe/Ce

(2)

k = Kd(template molecule)/Kd(competitive molecule) ko = kMIP/kNIP

12

(3) (4)

2.6. Regeneration experiments The multi-templates MIP (20 mg) were added into 20 mL of 50 mg L-1 of alkylphenols mixtures. After thoroughly adsorption, the collected filter liquor was detected by HPLC. And the recycle material was washed with the mixtures of methanol and acetic-acid (9:1, V:V) to completely remove the objective templates, then dried in vacuum. Repeat the above steps several times to investigate the reusability of the multi-templates MIP and NIP.

2.7. SPE-DLLME procedures SPE and DLLME were applied as pretreatment methods to enrich the trace alkylphenols in environmental water samples. The SPE-DLLME procedures were presented as follows: firstly 20 mg of MIP was placed in a round-bottom flask, meanwhile 50 mL of water sample was added to adsorbent for 10 min under 298 K at 200 rpm. After that, the MIP was collected under the external magnetic field and washed with deionized water. The templates BPA, 4-tert-OP and 4-NP were eluted with 1.0 mL of acetone for 5 min. The above acetone eluent using as the dispersant in DLLME was transferred to a glass centrifuge tube, and 30 μL of tetrachloroethane was added as the extractant. Subsequently 5.0 mL of 1% NaCl solution at pH 6 was blended with the mixtures and shaken to form a homogeneous emulsion. The was centrifuged at 4000 rpm for 5 min. Finally, the sedimentary solvent was collected and analyzed by HPLC. 13

3. Results and discussion 3.1. Preparation of multi-templates MIP The prepared VTTS-MGO@mSiO2 serving as the supporter in the multi-templates MIP synthetic process was able to provide high surface area, great chemical stability and biocompatibility [33]. 4-vinylbenzoic acid, not being used in other published literatures yet, was selected as an extremely good functional monomer to prepare the multi-templates MIP on account of its carboxyl and benzene ring, which is able to bind with BPA, 4-tert-OP and 4-NP via hydrogen bonds and π-π interactions in the process of polymerization. Orthogonal optimization experiment was conducted to obtained the optimal value of relevant factors (Test results in Table S1). The synthetic route of the multi-templates MIP is illustrated in Fig. 1, involving the following steps: ⅰ. To prepare the supporter VVTS-MGO@mSiO2. The preparation of the supporter has been described detailedly in previous work, so we needn’t repeat here; ⅱ. The three template molecules and 4-vinylbenzoic acid were allowed to react for 4 h at 298 K to form the prepolymerized complexity. ⅲ. Next the cross-linking agent EGDMA, initiator AIBN as well as the VVTS-MGO@mSiO2 were added to proceed a high-temperature polymerization for 24 h. At last, using the mixtures of methanol and acetic-acid (9:1, V:V) to entirely wipe off the templates and leaving the specific cavities in accord with BPA, 4-tert-OP and 4-NP in shapes, sizes, and chemical properties. 14

3.2. Characterization of multi-templates MIP The typical SEM and TEM images of MGO (A), MGO@mSiO2 (B) and multi-templates MIP (C) were presented in Fig. S2 and Fig. 2. As seen in Fig. S2A, MGO appeared a rough surface morphology comprising of small packed particles by reason of the agglomeration phenomenon in preparation process [34]. After coating with a mesoporous silica layer, the prepared MGO turned into a spherical structure with a smooth surface [33], which played a great role in preventing oxidation of magnetic core. This result was further confirmed by MIP microspheres with diameters of 65-70 nm in Fig. S2C. As presented in Fig. 2A, there were some lathy sticks like cudgels, which we speculated the crystal lattic of Fe3O4. In Fig. 2B, MGO@mSiO2 showed a distinct honeycomb-like mesoporous structure with diameter of 60 nm [52], which was beneficial to offer accessible channels for the adsorption and desorption of objective molecules. TEM micrograph of MIP (Fig. 2C) indicated the diameters of MIP microspheres were estimated about 66 nm, which matched well with SEM analysis results. In summary, the thin MIP with imprinted cavities was in favor of the mass transport between solution and MIP surface, and enhancing the adsorbability and shortening the equilibrium time. BET N2 adsorption/desorption measurement was also performed to certify the porous nature of the prepared materials. In Fig. 3A, as we can see, MGO@mSiO2, with an H4-type hysteresis loop at relative pressures (p/p0) in the range of 0.2 to 1.0, acted as IV isotherms, indicating the typical characteristics of mesoporous materials. Nevertheless, the phenomenon was not obvious in MIP due to lower N 2 adsorption amounts. 15

This also makes the surface area and total pore volume declined from 931 m2 g-1 and 0.38 cm3 g-1 of MGO@mSiO2 to 122 m2 g-1 and 0.09 cm3 g-1 of MIP, conversely the pore size calculated by Barrett-Joyner-Halenda (BJH) model increased from 2.31 to 7.09 nm, which revealed the MIP has been introduced into the mesopores onto the surface of MGO@mSiO2. All these tested results illustrated the synthetic compounds were equipped with a preeminent mesoporous structure and huge surface area, which contributed to the MIP adsorption amounts. To certify that the expected functional groups were grafted on the corresponding materials, FT-IR was employed as an analytical tool. The FT-IR spectra of GO (a), MGO (b), MGO@mSiO 2 (c), VTTS-MGO@mSiO2 (d), VTTS-MGO@mSiO2@MIP (e) and NIP (f), unwashed MIP (g) were shown in Fig. 4A. In curve a, the bands at 3450, 1727, 1631 cm-1 demonstrate the existence of –OH, C=O and C=C, which indicates GO has been admirably synthesized [36, 51]. However, in curve b, the C=O stretching vibration at 1727 cm -1 vanishes, and a new characteristic absorption peak of Fe-O at 590 cm-1 comes out [31]. Curve c shows us that the sharp peak at 1085 cm-1 is the asymmetric vibration of Si-O-Si [35,46,50], which suggests the mesoporous silica was succeeded to clad onto the surface of MGO. In curve d, the band at 1631 cm-1 strengthens compared with GO, suggesting MGO@mSiO2 surface is bound to form the vinyl groups [31]. From the spectra in curve e and f, it can be clearly noticed that MIP and NIP possess similar functional groups, shape and position, which is an indication that the templates have been completely removed from the MIP [38]. Nevertheless, in curve g, the stretching frequency of the C-H and C=O weakens to a certain extent, which can account for the hydrogen bonding hydroxyl groups contained in templates and MIP [21]. 16

The magnetization curves obtained from VSM in Fig. 4B convey a message that at room temperature, no matter MGO, MGO@mSiO 2 or multi-templates MIP, there were no hysteresis in their magnetization curves and no remanence or coercivity observed, indicating the prepared materials were equipped with superparamagnetic. What should be noteworthy was that, the saturation magnetization of MGO (22 emu g-1) was much stronger than MGO@mSiO2 (4 emu g-1) and MIP (3 emu g-1), which may be described to the mesoporous SiO2 and MIP layer in succession loaded on the surface of MGO, thus shielding magnetite to a large extent. Though the saturation magnetization value of VTTS-MGO@mSiO2@MIP was far fainter than other reported magnetic MIP [36,37], it had no influence on an effective magnetic separation from the adsorption solution with the help of an external magnetic field. The XRD images of MGO (a), MGO@mSiO2 (b) and multi-templates MIP (c) are presented in Fig. 4C. As we can see, the diffraction peak at 30.29o, 35.49o, 43.39o, 53.93o, 57.75o, 62.80o in curve a was in accord with (220), (311), (400), (422), (511) and (440) of Fe 3O4 (JCPDS Card: 019-0629) [31], indicating the prepared MGO satisfied the spinel structure. Additionally, the diffraction peaks of mSiO2 at 20o were also appeared in curve b and curve c. Compared with curve a, the corresponding peaks in curve b and c were getting weaker and even disappeared, which further corroborated the successful coating of mesoporous silica and MIP layers. The peaks of the prepared materials were not obviously presented, which corresponded to the relatively weak magnetism in Fig. 3, and also accounted for the outstanding adsorption quality of the multi-templates MIP. 17

Fig. 4D shows us the TGA weight loss curves of MGO, MGO@mSiO 2 and multi-templates MIP. Comparing curve a and b, both of them decline gently and at about 600℃, the weight loss of MGO reaches 29% while MGO@mSiO2 just 17%. This may be ascribed to the luxuriant oxygenated functional groups on MGO surface and the high temperature resistance [47]. After mesoporous silica being coated onto MGO, the oxygen-containing functional groups were muffled and hence MGO@mSiO2 becomes thermally stable, and not easy to be decomposed even in high temperature [49]. The TGA curve of the multi-templates MIP have a mild downtrend below 300℃ and a sharp deceleration between 300℃ and 550℃，which is respectively attributed to the degradation of laible oxygen groups and carbon skeleton for the MIP [49]. When at 600℃, the curve achieves equilibration with approximately 57% weight loss.

3.3. Adsorption study 3.3.1. Effects of pH on adsorption The solution pH, which may produce an effect on the surface change of the adsorbent and speciation of the contaminants, plays a key role in the chemisorption process [48]. As the findings in Fig. 5, it can be concluded that the maximum adsorptions of the multi-templates MIP for BPA, 4-tert-OP and 4-NP were realized at pH 6. A possible explanation for this is the hydrogen bond and π—π interaction between alkylphenols and the multi-templates MIP. When under acidic conditions, with the increasing solution pH, the interference caused by hydrogen 18

ions cuts down and the hydrogen bond affinity is getting much stronger, which accounts for the increasingly adsorption ability of MIP to the three objectives. In contrast, when pH is above 6, the sorption amount of MIP decreases. This is supported by the electrostatic repulsive interactions between template molecules and MIP, which surmount the binding affinity and become increasingly momentous during the sorption process [38]. 3.3.2. Adsorption kinetic The adsorption equilibrium time of the multi-templates MIP and NIP under different temperatures was explored by performing the dynamic study. As can be seen in Fig. 6, the adsorption of MIP and NIP increases at an express rate during the first 10 min and subsequently grows gently with time. On the basis of these results we concluded that whether the MIP or NIP, both of them can reach equilibration within 30 min. Beyond this, another anticipatory discovery is that the adsorption capacity of MIP is nearly 3 times than that of NIP, which can be attribute to the specific recognition sites formed on the multi-templates MIP. For the purpose of simulating the adsorption mechanism of MIP for BPA, 4-tert-OP and 4-NP, the dynamic models like pseudo-first order, pseudo-second order and intra-particle diffusion were applied to analyze the experimental datas, which are described as follow: Ln (Qe-Qt) = LnQe - k1t

(5)

t/Qt = 1/ k2Qe2 + t/Qe

(6) 19

Qt = kpt1/2 + C

(7)

where Qe (mg g-1) and Qt (mg g-1) are the adsorbed total amounts of BPA, 4-tert-OP and 4-NP at equilibrium and at various times t (min), respectively. k1 (min-1) and k2 (mg g-1 min-1) are the pseudo-first order and pseudo-second order rate constants of adsorption, respectively. k p and C are the intra-particle diffusion rate constant and boundary layer thickness, respectively. The three dynamic models’ linear fitting graphs and corresponding kinetic parameters at various temperatures are generalized in Fig. S3, 4, 5 and Table 1. As can be seen, among the three models, the pseudo-second order kinetics model could match the MIP adsorption process best with a highest regression values R2 (>0.9998), which demonstrates the chemisorption may be the rate-limiting step.

3.3.3. Adsorption isotherm Fig. 7 shows us the adsorption isotherms of the alkylphenols on the multi-templates MIP and NIP under three different temperatures, with initial concentrations ranging from 1 to 120 mg L-1. As shown, the binding capacity first increases rapidly, whereafter remains flat with the enhancement of the three templates initial concentration, which can be ascribed to the recognition sites for BPA, 4-tert-OP and 4-NP reaching a saturation. Additionally, NIP also exhibies the same behavior but with lower adsorption capacity. Langmuir, Freundlich and Tempkin models were studied to comprehend the adsorption process: 20

Ce/Qe = Ce/Qm + 1/KLQm

(8)

LnQe = LnKF + LnCe/n

(9)

Qe = a + bLnCe

(10)

where Ce (mg L-1) is the equilibrium concentration of template molecules; Q e (mg g-1) and Qm (mg g-1) is the equilibrium and maximum adsorption capacity, respectively; KL (L mg-1) and KF (mg g-1) is Langmuir and Freundlich binding coefficient, respectively; a, b, n is the constant. The fitting graphs and parameters of the corresponding models are shown in Fig. S6, 7, 8 and Table 2. The R2 values lead us to make a conclusion that the Langmuir model matches the experimental datas best compared with the other two models.

3.3.4. Adsorption thermodynamics The influence of temperature on the sorption amount of the multi-templates MIP and NIP for BPA, 4-tert-OP and 4-NP was executed. In Fig. S9, as we have seen, a greater adsorption amount was obtained at a lower temperature, which evidences the conclusions obtained in dynamic and isotherm experiments are consistent with each other. The thermodynamic behavior of the multi-templates MIP was validated according to the following equations: 21

LnQe/Ce = -ΔH/RT + ΔS/R ΔG = ΔH - TΔS

(11) (12)

where R and T is the universal gas constant (8.314 J mol-1 K-1) and Kelvin temperature (K). ΔH (KJ mol-1), ΔS (KJ mol-1) and ΔG are the standard enthalpy changes, entropy changes and gibbs free energy changes, respectively. Dependent on the thermodynamic graphs and relative parameters shown in Fig. S9 and Table 3, we can arrive at a conclusion that the adsorption process belongs to a spontaneously endothermic reaction through physisorption.

3.3.5. Adsorption selectivity The specific selectivity of the multi-templates MIP for BPA, 4-tert-OP and 4-NP has been confirmed by comparing with the competitive molecules (BPAF and 4-PTBP). The images and data in Fig. 8 and Table S2, 3, 4 convey us the expected information that the adsorption amount of the multi-templates MIP toward BPA, 4-tert-OP and 4-NP is obviously higher than BPAF, 4-PTBP. Conversely, the NIP displays similar adsorption capacity to those of other two interferential molecules. This means that the specific recognition sites to the three templates have successfully formed on the MIP surface. The carrier VTTS-MGO@mSiO2 was also chosen as the adsorbent material to prove the imprinting effects of the multi-templates MIP. The obtained results tell us that the absorption efficiency of the VTTS-MGO@mSiO2 is much 22

weaker than the NIP, needless to compare with the MIP. A feasible reason for such phenomenon is, through the cross-linking polymerization, the NIP surface possesses carboxyl groups which can combine with the templates, even without the specific cavities.

3.3.6. Regeneration of multi-templates MIP The adsorption-desorption experiments were repeatedly performed in the same procedures to examine the regeneration ability of the multi-templates MIP. As shown in Fig. 9, the adsorption capacity of the multi-templates MIP still remains at a relatively high level even after five times cycles, indicating the outstanding regeneration ability of the multi-templates MIP.

3.4. SPE-DLLME-HPLC determination of the three alkylphenols in environmental water samples 3.4.1. Optimization of SPE-DLLME conditions To obtain the desired experimental effect, related condition parameters of SPE and DLLME have been optimized in our work. As seen in Fig. S10, in SPE, 50 mL water was experimented under pH 6 and reached equilibrium within 8 min, then 1 mL acetone was applied to elute for 5 min. In DLLME (Fig. S11, 12), when at pH 6, 30 μL tetrachloroethane was served as extractant in 1 mL acetone dispersant with 1% NaCl

23

concentration. In the above optimal conditions, excellent recovery and enrichment coefficient of BPA, 4-tert-OP and 4-NP were acquired, leaving a strong foundation for the subsequent detection of alkylphenols in real water.

3.4.2 Environmental water analysis In the optimized experimental conditions, the VTTS-MGO@mmSiO2 MIP uniting with SPE-DLLME-HPLC technique was applied to realize the enrichment and detection of trace BPA, 4-tert-OP and 4-NP in real environmental water samples. The water samples were collected from a well, a lake, a pond and the Pearl River (Guangzhou, China). Recovery tests at different spiking amounts of the three alkylphenols were also carried out to examine the reliability of this method. The results are listed in Table 4. As we see, BPA was only detectable in well water and lake water with a value of 0.13、0.15 μg L-1, while 4-tert-OP and 4-NP were found at concentrations of 1.05, 1.15, 1.17, 1.19 μg L-1 and 0.41, 0.60, 2.04, 0.80 μg L-1 in well water, lake water, pond water and river water, respectively. The recovery rates and RSDs were in a range of 81.5-104.1%, 1.0-7.6%, which illustrated the excellent applicability and feasibility of this method for the analysis of BPA, 4-tert-OP and 4-NP in real water samples.

3.4.3. Comparison with other methods 24

To highlight the distinct merits of the imprinted material and present method, a comparison with other reported researches have been presented in Table 5. As we can see, solid-phase extraction (SPE) and dispersive liquid-liquid microextraction (DLLME), as the simple and rapid pretreatment techniques, were widely used in the trace detection of alkylphenols. In our study, under the linear concentration range from 0.05~10 μg/L with a correlation coefficient (R2) above 0.9963, the LODs (S/N=3) of BPA, 4-tert-OP and 4-NP were 0.013, 0.010 and 0.010 μg L-1, respectively. The intraday and interday RSD of BPA, 4-tert-OP and 4-NP were 5.01%、6.53%、5.62% and 5.73%、7.15%、5.98%, respectively. Compared with other studies, the multi-templates MIP prepared in this work possessed a better adsorbability and imprinting factor [25,27], additionally a lower LOD towards BPA, 4-tert-OP and 4-NP has been achieved [19,25,27], indicating the feasibility and sensitivity of the proposed method.

4. Conclusions A multi-templates molecularly imprinted polymer in this work has been successfully synthesized via surface imprinting technology. A series of characterization methods and binding adsorption tests were conducted to inspect the properties of the multi-templates MIP. Experimental results indicate that the prepared multi-templates MIP makes a good performance in term of adsorption capacity, equilibration time, selectivity and reusability. As the striking features mentioned above, the multi-templates MIP, uniting solid phase extraction with 25

liquid-liquid microextraction (SPE-DLLME), makes the simultaneous removal of bisphenol A, 4-tert-octylphenol (4-tert-OP) and 4-nonylphenol in environmental samples a reality. The unique characteristics of the multi-templates MIP make the analytical method a high concentration factor, low limit of detection, excellent recovery, indicating its superiorities.

Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 41272262), the Science and Technology Planning Project of Guangdong Province, China (No. 2016A040403112), and the Major Projects (Natural Science) of Education Department of Guangdong Province, China (261555101).

26

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Figure Captions Fig. 1. Synthesis schematic diagram of VTTS-MGO@mSiO2 @MIP. Fig. 2. TEM images of MGO (A), MGO@mSiO2 (B), VTTS-MGO@mSiO2 @MIP (C). Fig. 3. N2 adsorption-desorption isotherms (A) and BJH pore size distribution (B) of MGO@mSiO2 and VTTS-MGO@mSiO2@MIP. Fig. 4. FTIR spectras (A) of GO (a), MGO (b), MGO@mSiO 2 (c), VTTS-MGO@mSiO2 (d), washed VTTS-MGO@mSiO2@MIP (e) and NIP 33

(f), unwashed VTTS-MGO@mSiO2@MIP (g); Hysteresis loops (B) of MGO , MGO@mSiO2 and VTTS-MGO@mSiO2 @MIP; XRD patterns (C)

of

MGO

(a),

MGO@mSiO2

(b),

VTTS-MGO@mSiO2 @MIP

(c);

TGA curves

(D)

of

MGO,

MGO@mSiO2

and

VTTS-MGO@mSiO2@MIP. Fig. 5. Effect of sample pH on the adsorption of BPA, 4-tert-OP and 4-NP by VTTS-MGO@mSiO2 @MIP. Fig. 6. Adsorption kinetic curves of BPA , 4-tert-OP and 4-NP on VTTS-MGO@mSiO2@MIP and NIP with the fitting to the pseudo-second-order model (----) under 298 K. Fig. 7. Adsorption isothermic curves of BPA (A), 4-tert-OP (B) and 4-NP (C) on VTTS-MGO@mSiO2@MIP and NIP with the fitting to the Langmuir model (----) at various temperatures. Fig. 8. Competitive adsorption for four analogue of different adsorbent by VTTS-MGO@mSiO2, VTTS-MGO@mSiO2 @MIP and NIP. Fig. 9. The reusability analysis of VTTS-MGO@mSiO2@MIP for BPA, 4-tert-OP and 4-NP.

34

Fig. 1. Synthesis schematic diagram of VTTS-MGO@mSiO2@MIP.

35

Fig. 2. TEM images of MGO (A), MGO@mSiO2 (B), VTTS-MGO@mSiO 2@MIP (C).

36

Fig. 3. N2 adsorption-desorption isotherms (A) and BJH pore size distribution (B) of MGO@mSiO2 and VTTS-MGO@mSiO 2@MIP.

37

Fig. 4. FTIR spectras (A) of GO (a), MGO (b), MGO@mSiO2 (c), VTTS-MGO@mSiO2 (d), washed VTTS-MGO@mSiO2@MIP (e) and NIP (f), unwashed VTTS-MGO@mSiO2@MIP (g); Hysteresis loops (B) of MGO , MGO@mSiO2 and VTTS-MGO@mSiO2@MIP; XRD patterns (C) of MGO (a), MGO@mSiO2 (b), VTTS-MGO@mSiO2@MIP (c); TGA curves (D) of MGO, MGO@mSiO2 and VTTS-MGO@mSiO2@MIP. 38

Fig. 5. Effect of sample pH on the adsorption of BPA, 4-tert-OP and 4-NP by VTTS-MGO@mSiO2@MIP.

39

Fig. 6. Adsorption kinetic curves of BPA , 4-tert-OP and 4-NP on VTTS-MGO@mSiO2@MIP and NIP with the fitting to the pseudo-second-order model (----) under 298 K.

40

Fig. 7. Adsorption isothermic curves of BPA (A), 4-tert-OP (B) and 4-NP (C) on VTTS-MGO@mSiO2@MIP and NIP with the fitting to the Langmuir model (----) at various temperatures.

41

Fig. 8. Competitive adsorption for four analogue of different adsorbent by VTTS-MGO@mSiO2, VTTS-MGO@mSiO2@MIP and NIP.

42

Fig. 9. The reusability analysis of VTTS-MGO@mSiO2@MIP for BPA, 4-tert-OP and 4-NP.

43

Table 1 Kinetic parameters of BPA, 4-tert-OP and 4-NP adsorption onto the VTTS-MGO@mSiO2@MIP and NIP under 298K. Pseudo-first order model -1

BPA

4-tertOP

4-NP

-1

Pseudo-second order model 2

-1

Particle internal diffusion model 2

C

R2

Qe,exp (mg g )

K1

Qe,cal (mg g )

R

K2

Qe,cal (mg g )

R

Kid

MIP

11.42

0.0161

0.26

0.9557

0.019

11.48

0.9988

0.3672

6.7572

0.6545

NIP

4.19

0.0182

0.76

0.9357

0.068

4.20

0.9996

0.139

2.4918

0.6305

MIP

17.88

0.0230

0.18

0.9503

0.019

18.01

0.9999

0.5860

10.928

0.6308

NIP

5.90

0.0180

0.54

0.9117

0.049

5.89

0.9994

0.1725

3.7208

0.7033

MIP

40.54

0.0182

0.13

0.9135

0.0123

40.49

0.9997

0.8026

30.586

0.6000

MIP

19.62

0.0157

0.44

0.8622

0.0464

19.65

1

0.2924

16.166

0.6432

44

Table 2 Adsorption isotherm parameters of BPA, 4-tert-OP and 4-NP onto the VTTS-MGO@mSiO2@MIP under different temperatures. Langmuir model T (K)

Qmax

308

318

Tempkin model

R2

KF

n

R2

a

b

R2

0.0646

0.9985

0.575

1.951

0.9269

0.4567

0.2671

0.9915

35.97

0.0814

0.9955

0.235

2.056

0.9329

-0.0935

0.1432

0.9946

61.73

0.1379

0.9749

0.1070

2.028

0.742

-0.6751

0.0797

0.9363

BPA

13.04

0.0305

0.9923

1.218

1.800

0.9899

1.0441

0.3691

0.975

4-tert-OP

32.47

0.0259

0.9911

0.7886

1.488

0.9681

1.0657

0.1542

0.9766

4-NP

55.56

0.1111

0.9868

0.1420

1.9500

0.8563

-0.4398

0.0917

0.9778

-1

298

b

Freundlich model

(mg g )

(L mol-1)

BPA

18.81

4-tert-OP 4-NP

BPA

12.58

0.0196

0.9986

2.410

1.485

0.9754

1.3499

0.4017

0.9821

4-tert-OP

25.32

0.0166

0.9746

0.9186

1.5516

0.9714

1.1415

0.1901

0.9566

4-NP

49.26

0.0889

0.9933

0.1319

2.1711

0.9559

-0.682

0.107

0.9531

45

Table 3 Thermodynamic parameters of BPA, 4-tert-OP and 4-NP adsorption onto the VTTS-MGO@mSiO2@MIP at various temperatures.

B (L mol-1)

KL

Ln KL

△G0 (KJ·mol-1)

△H0 (KJ·mol-1)

△S0 (KJ·mol-1·K-1)

0.0646

18.217

2.902

-7.191

-47.099

-0.1343

0.0814

16.794

2.821

-6.989

-76.872

-0.2349

4-NP

0.1379

30.386

3.414

-8.458

-17.286

-0.0296

BPA

0.0305

8.601

2.152

-5.510

-47.099

-0.1343

0.0259

5.344

1.676

-4.292

-76.872

-0.2349

4-NP

0.1111

24.481

3.198

-8.189

-17.286

-0.0296

BPA

0.0196

5.527

1.710

-4.520

-47.099

-0.1343

0.0116

2.393

0.873

-2.307

-76.872

-0.2349

0.0889

19.589

2.975

-7.865

-17.286

-0.0296

1/T (K-1)

T (K) BPA 298

308

318

4-tert-OP

4-tert-OP

4-tert-OP 4-NP

0.003356

0.003247

0.003145

46

Table 4 Determination of BPA, 4-tert-OP and 4-NP in environmental water samples by SPE-DLLME-HPLC method.

Sample

Compound

BPA

Well water

4-tert-OP

4-NP

BPA

Lake water

4-tert-OP

4-NP

BPA

Pond water

4-tert-OP

4-NP

BPA Pearl river

4-tert-OP

water 4-NP

Add

Found

Recovery

RSD

(μg L )

(μg L )

(%)

(n=3，%)

0

0.13

--

10.34

1

1.02

89.19

4.35

10

10.54

104.09

7.39

0

1.05

--

2.75

-1

-1

1

2.04

98.36

1.03

10

11.02

99.66

2.91

0

0.41

--

9.72

1

1.38

97.85

2.67

10

10.48

100.71

4.67

0

0.15

--

6.33

1

0.97

81.54

5.76

10

10.48

103.26

1.72

0

1.15

--

8.89

1

2.12

96.81

3.86

10

10.56

94.06

4.86

0

0.60

--

5.42

1

1.57

97.06

3.12

10

9.33

87.28

6.63

0

0

--

--

1

1.07

106.70

5.52

10

10.21

102.13

2.03

0

1.17

--

4.48

1

2.22

104.29

7.57

10

9.87

86.95

2.90

0

2.04

--

6.71

1

2.99

95.19

2.10

10

11.61

95.75

7.54

0

0

--

--

1

0.97

97.42

7.05

10

9.84

98.36

7.94

0

1.19

--

7.42

1

2.13

93.95

6.20

10

10.62

94.33

4.90

0

0.80

--

7.37

1

1.81

100.93

4.83

10

9.31

85.16

4.45

47

Table 5 Comparison of the MSPE-DLLME-HPLC method based on VTTS-MGO@mSiO2@MIP with other analytical methods.

Qm Material type

Target

(mg g-1)

Imprint ing facror

Analytical method

NP

4 mg

2.88

SPE-HPLC

g-1 BPA Ag/GO-dual-MIP NP

0.3 91 mg g-1

2.6 MISPE-HPLC-FLD 2.9

1.3 MIPMS

BPA

2 mg

6.4

SPE-HPLC-DAD

500 MIPs

OP

ol

4.19

SPE-UPLC

g-1 18. rGO-Fe3O4

BPA

00 mg

--

MSPE-DLLME-HPL C

g-1 14. MMWCNTS

BPA

01 mg

x

range

Water

0.1-20

0.15

sampl

0μg

ng

es

mL-1

mL-1

Fish

0.1-10

sampl

00 μg

--

MMSPD-DLLME-HP LC-FLU

g-1

-1

BPA

mg

1.8

an

0 mg

ng

-1

g

2@MIP

OP

mg

Referre

(%)

nce

2.3-5.1

[19]

0.5-35

0.15

sampl

μmol

μmol

-1

L

L

Water

0.05-1

0.010

sampl

0 μg

μg

es

Water sampl es

L

0.01-1 0μg L-1

3.71

C

Water

0.05-1

sampl

0 μg

es

L-1

0.003 μg L-1

0.010 μg -1

L

μg L-1

mg 48

2.5-6.4

[27]

5.3

[32]

1.9-3.0

[52]

L

0.010 2.62

3.8

-1

L-1

MSPE-DLLME-HPL

[25]

-1

es

-1

4.1

mL

μg

61. 73

RSD

-1

Water

4.18

g-1

4-NP

ng L-1

0.013

35. 97

4.7

20-200

g

4-tert-

ng L-1

Hum

-1

VTTS-MGO@mSiO

2.4

L

16. 81

LOD

es

urine

g-1 μm

Linear

(IF)

52. Mag-DMIPs

Matri

4.21-6. 85

[53]

5.01-5. 73

6.53-7. 15

5.62-5. 98

Present study

g-1

49