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Jiang Liu1 ∗ Le Li1 ∗ Hui Tang1 Feilang Zhao2 Bang-Ce Ye3 Yingchun Li1,3 Jun Yao4 ∗∗ 1 Key
Laboratory of Xinjiang Endemic Phytomedicine Resources, Ministry of Education; School of Pharmacy, Shihezi University, Shihezi 832002, China 2 Jiangsu Devote Instrumental Science & Technology Co., Ltd., Huai’an, China 3 Key Laboratory for Green Processing of Chemical Engineering of Xinjiang Bingtuan, School of Chemistry and Chemical Engineering, Shihezi University, Shihezi, China 4 School of Pharmacy, Xinjiang Medical University, Urumqi, Xinjiang, China Received April 24, 2015 Revised June 13, 2015 Accepted June 13, 2015
Research Article
Preparation and characterization of erythromycin molecularly imprinted polymers based on distillation–precipitation polymerization Erythromycin-imprinted polymers with excellent recognition properties were prepared by an innovative strategy called distillation–precipitation polymerization. The interaction between erythromycin and methacrylic acid was studied by ultraviolet absorption spectroscopy, and the as-prepared materials were characterized by Fourier-transform infrared spectroscopy and scanning electron microscopy. Moreover, their binding performances were evaluated in detail by static, kinetic and selective sorption tests. It was found that the molecularly imprinted polymers afforded good morphology, monodispersity, and high adsorption capacity when the fraction of the monomers was 7 vol% in the whole reaction system, and the adsorption data for imprinted polymers correlated well with the Langmuir model. The maximum capacity of the imprinted and the non-imprinted polymers for adsorbing erythromycin is 44.03 and 19.95 mg/g, respectively. The kinetic studies revealed that the adsorption process fitted a pseudo-second-order kinetic model. Furthermore, the imprinted polymers display higher affinity toward erythromycin, compared with its analogue roxithromycin. Keywords: Distillation-precipitation polymerization / Equilibrium / Erythromycin / Kinetics / Molecularly imprinted polymers DOI 10.1002/jssc.201500448
Additional supporting information may be found in the online version of this article at the publisher’s web-site
1 Introduction A molecularly imprinted polymer (MIP) is a kind of synthetic material formed in the presence of target molecules or structural analogue molecules as template. Removal of the templates from the polymeric network is able to produce cavities with molecular recognition ability, allowing for selective adsorption to the original templates [1]. As an artificial material, an MIP can be prepared easily and rapidly and is very stable in harsh conditions (organic solvents, extremes of pH, and temperature, etc.). Therefore, MIPs have been gaining significance in a wide range of applications including SPE [2–4], sensors [5–7], chromatography [8–10], mimetic catalysis [11–13] and so on. Traditionally, an MIP is produced as a bulk polymeric monolith, followed by me-
Correspondence: Dr. Yingchun Li, School of Pharmacy, Shihezi University, Shihezi 832000, China E-mail: [email protected]
Abbreviations: AIBN, azobisisobutyronitrile; DPP, distillation–precipitation polymerization; DVB, divinylbenzene; EM, erythromycin; MAA, methacrylic acid; MIP, molecularly imprinted polymer; MIPMS, molecularly imprinted polymeric microsphere; NIP, non-imprinted polymer; ROX, roxithromycin C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
chanical grinding to obtain micrometer-sized particles. Although the obtained materials are somewhat inelegant, this straightforward method is still useful and convenient [14–16]. However, in practical application, performance of MIP can be largely enhanced by controlling the morphology of the products, among which spherical polymer with uniform size and shape is especially favorable, for example, as chromatographic [4,10,17] and SPE packing materials [18–20]. Another aspect in producing monodispersed molecularly imprinted polymeric microspheres (MIPMSs) is the synthesis of binding sites located on or close to the polymer surface, to shorten response time and improve the steric accessibility. The methods of fabricating MIPMSs mainly include precipitation polymerization [21, 22], emulsion polymerization [23, 24], swelling suspension polymerization [25, 26], etc. However, addition of surfactant or stabilizer in these strategies leads to product with complicated constituent and poor controllability of polymerization process results in polymers with broad-dispersed size. Distillation precipitation polymerization (DPP) is a newly developed technique by Yang and Huang [27, 28], through which microspheres with various
∗ These
authors contributed equally to this work. correspondence: Dr. Jun Yao, E-mail: [email protected]
∗∗ Additional
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monomer composition can be obtained in the absence of any surfactant or stabilizer. The DPP reaction system starts with homogeneous mixtures of monomer and initiator dissolved in suitable solvent. During distillation of the solvent out of the system, the growing polymer chains phase separate from the continuous medium by enthalpic or entropic precipitation, where cross-linking prevents the produced polymer from free mixing with solvent, and finally affords the narrow/mono-dispersed polymeric microspheres [29, 30]. We recently extended this strategy into molecular imprinting, and MIPMSs with uniform size, simple component and high binding capacity have been successfully prepared [4]. However, the whole process contains two-step DPP, which is somehow time-consuming and lowers efficiency [31]. Herein, we designed direct synthesis of monodispersed MIPs by onestep DPP by using erythromycin (EM) as the model template. The polymeric spheres generated were characterized using FTIR spectroscopy and SEM, and their binding behavior, adsorption isotherm and kinetics were studied in detail.
2 Materials and methods 2.1 Materials and chemicals Erythromycin (EM) was supplied by Guangdong Jiqi Drug (Guangdong, China). Roxithromycin (ROX) was obtained from Adamas Reagent (Shanghai, China), and their structures are illustrated in Supporting Information Fig. S1. Methacrylic acid (MAA) and azobisisobutyronitrile (AIBN) were obtained from Tianjin Guangfu Chemical (Tianjin, China). Before use, MAA was distilled twice under vacuum and AIBN was recrystallized in ethanol. Divinylbenzene (DVB) was purchased from Sigma–Aldrich (Shanghai, China) and purified by extraction with 10% NaOH aqueous solution, and then dried over anhydrous magnesium sulfate. Water was doubly distilled. Solvents for HPLC were of chromatographic grade. All the other chemicals were of analytical grade and used without further treatment.
2.2 Instruments HPLC experiments were implemented using an Essentia LC–15C system equipped with two LC–15C Solvent Delivery Units, an LC Solution 15C workstation and an SPD–15C UV–Vis Detector (Shimadzu, Japan). LC condition was as follows: chromatographic separation was performed on a Shimadzu WondaSil C18 column (150 mm × 4.6 mm i.d., 5 m), the mobile phase was acetonitrile/phosphate buffer (2:8, v/v, pH 8.2) with a flow rate of 1.0 mL/min, injection volume was 20 L, and the detection wavelength was set at 215 nm for EM and 210 nm for ROX, respectively. All the measurements were carried out in triplicate. To identify the interaction between EM and MAA, a UV-2401 UV spectrophotometer (Shimadzu, China) was
C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
employed. IR spectra were recorded by a Nicolet EXUS–470 FTIR apparatus (Shimadzu, Japan). SEM images were taken by a JSM–6490LV scanning electron microscope (Tokyo, Japan) with an accelerating voltage of 10 kV.
2.3 Preparation of polymers EM-MIPs were prepared based on the method reported by Liu et al. [31] and a modified DPP tactic was adopted in this work. Briefly, the template (EM, 1.30 g), functional monomer (MAA, 0.6 mL) and cross-linker (DVB, 5.0 mL) were well dissolved in 80 mL acetonitrile by ultrasonication for 2 min. The solution was stored over night for the formation of a complex of EM and MAA. After addition of AIBN (2 wt% relative to the monomers) as initiator, the solution was poured into a three-necked flask that was attached with a dasher, a fractionating column and a receiver. The reaction apparatus is displayed in Supporting Information Fig. S2. The reaction mixture was purged with nitrogen for 10 min, and then heated from ambient temperature to boiling state within 30 min with continuously stirring at 120 rpm. The reaction was ended when 40 mL acetonitrile was distilled from the reaction system in 1.5–2 h. After polymerization, the resulting polymers were treated through ultrasonication and vacuum filtration, followed by washing with methanol for three times. The template molecules were removed by using mixture of methanol and acetic acid (90:10, v/v) in Soxhlet extractor until the complete absence of EM in the rinse solution. The obtained EM-MIPs were further washed with methanol and dried at 50⬚C. The non-imprinted polymers (NIPs) particles were prepared and washed using the same recipe but without the addition of the template EM.
2.4 Binding experiments In static adsorption test, 50.0 mg polymers were placed in a conical flask with stopper containing 5.0 mL EM standard methanol solution at the concentrations of 0.3–20.0 mg/mL. After incubation for 12 h at room temperature, the samples were centrifuged and filtered, and the concentration of free EM after adsorption was determined by HPLC–UV. Kinetic binding test was carried out in a similar way except that the initial concentration was constant at 2.0 mg/mL and the determination was made at different time points (5, 10, 20, 30, 60, 90, 120, 180, 240, and 300 min). The adsorption quantity (Q) was calculated through Eq. (1), where C0 and Cfree are the initial and free concentration of EM, V stands for the volume of the solution and W refers to the mass of the polymer powder. Q=
(C0 − Cfree ) × V W
(1)
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Figure 1. UV spectra of mixtures of EM and MAA under different molar ratio (A), and IR spectra of EM (B), EM-MIPs before (C) and after elution (D) of EM, and NIPs (E).
2.5 Selectivity of polymers The selectivity of EM-MIPs was evaluated with EM and ROX. 50.0 mg of EM-MIPs and NIPs were separately added to a conical flask containing 5 mL standard solution of each compound at 3 mg/mL. All the mixtures were incubated for 3 h at ambient temperature and then filtered through a 0.22 mm filter membrane. The filtrates of EM and ROX were determined by HPLC–UV.
3 Results and discussion 3.1 Confirmation of interaction between EM and MAA The key factor of preparing highly selective materials together with good adsorption capacity is that a complex must be formed between functional monomers and template molecules through strong interactions. EM molecule has Hbond acceptors while MAA owns H-bond donors. Therefore, these two compounds are very likely to form stable complex due to formation of hydrogen bonds in prepolymerization solution. This presumption was further examined by UV/Vis spectrophotometric analysis of a series of solutions containing different molar ratios of EM to MAA in acetonitrile. As shown in Fig. 1A, the maximum absorption wavelength of EM shifted from 217 to 225 nm with adding MAA and further to longer wavelength with increasing the concentration of MAA. This could arise from the formation of hydrogen bonds between the carbonyl group of MAA and the hydroxyl group of EM, which is also in accord with early reports [31,32].
3.2 Structure information of the polymers The IR spectra of EM, MIPs before and after elution of EM and NIPs were recorded to verify the synthesis of the desired products. The broad absorption band at 1175 cm−1 is assigned to the O–H stretching vibration of EM molecules, and the peak observed at 1365 cm−1 corresponds to the stretching vibration of C–N bonds of the tertiary amine group in EM molecules (Fig. 1B, C). These two characteristic absorp C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
tion peaks appearing in EM-MIPs before elution of templates (Fig. 1C) demonstrates that EM molecules are successfully imprinted into the polymers. After EM extraction (Fig. 1D), the polymer exhibits similar absorption bands as NIPs (Fig. 1E) do, implying that the template EM has been removed from the MIP matrix. The schematic illustration of the EMMIP preparation and the interaction between MIP and EM is shown in Supporting Information Fig. S3.
3.3 Effect of monomer ratio on morphology and binding behavior of polymers DPP begins with a homogeneous system containing monomer and initiator. As the solvent distills, polymeric microspheres form in the solution. Optimization of initial concentration of monomer is very important to get the products with narrow size distribution. The SEM images of EM-MIP microspheres prepared under different monomer concentrations are shown in Fig. 2. With the increase of the concentration from 3 to 7% (Fig. 2A, B, and C), the size of the polymeric spheres gets more uniform, the reason of which has been elaborated in our previous reports [4, 31]. However, when the monomer concentration went up to 9%, the reaction mixture got so viscous and uneven that it is difficult to distill off the expected volume of acetonitrile. Besides, increase of the amount of monomers resulted in wider size distribution and the formation of small-sized particles could be the consequence of shortened period of particle nucleation (Fig. 2D). Figure 2E illustrates the adsorption quantity (Q) and the imprinting factor (IF, IF = QMIPs /QNIPs ) of the polymers prepared at the four different monomer concentrations. The highest Q is 8.9 mg/g with the largest IF of 1.6, obtained with MIPs prepared at monomer concentration of 7%. QMIPs decreases when the concentration is lower than 7%, which can be explained by the fact that insufficient monomers yield less binding sites for EM adsorption. On the other hand, QMIPs also goes down with the monomer concentration surpassing 7%, which might be related to the fact that high quantity of monomers generates polymers with larger particle size, which makes lots of imprinted sites locate inside polymeric network, and consequently reduces effective binding sites and binding capacity, and prolongs equilibrium time. Hence, www.jss-journal.com
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Figure 2. SEM photos of EMMIPs prepared at different concentrations of monomer (A, 3%; B, 5%; C, 7%; D, 9%), and adsorption quantity and imprinting factor of the polymers prepared at different concentrations of monomer in standard solution of EM at 1.0 mg/mL (E).
monomer concentration of 7% was applied in the subsequent experiments for preparing EM-MIPs.
3.4 Adsorption capacity The polymers prepared under the optimized condition were used to adsorb EM at a series of concentrations and the binding isotherm corresponding to QMIPs or QNIPs versus EM concentration is displayed in Fig. 3A. It can be seen that both QMIPs and QNIPs increase significantly with increasing the concentration of EM from 0.3 to 10 mg/mL. After that, the raise in concentration from 10 to 20 mg/mL brings about slight Q enhancement, which is probably due to saturation of the binding sites in the sorbents. The maximum static adsorption capacities (Qmax ) of the imprinted and non-imprinted polymers for EM are 44.03 and 19.95 mg/g, respectively, indicating that imprinting effect plays an important role in adsorption performance. During the preparation of MIPs, template molecules and functional monomers bring out imprinting behavior and the subsequent removal of template leaves imprinted cavities and specific binding sites with functional groups existing in a predetermined orientation in the polymeric matrix. By contrast, such procedure does not occur in the case of NIPs so that they do not exhibit such specific binding ability. Langmuir and Freundlich models are applied to study the adsorption behavior of MIPs and NIPs theoretically by using Langmuir isotherm Eq. (2) and Freundlich isotherm Eq. (3), where Ce (mg/mL) and Qe (mg/g) are the concentration and the amount of adsorbed EM at adsorption equilibrium, Qm is the maximum adsorption capacity (mg/g), Kl is the Langmuir constant (mL/g), and Kf is the Freundlich constant (mg/g). Qe =
Qm · K l · Ce 1 + Ce · K l
Qe = K f · Ce n
(2)
(3)
The calculated Langmuir and Freundlich parameters are summarized in Table 1. By comparing the experimental adsorption isotherm (Fig. 3A) with the Langmuir (Fig. 3B) and Freundlich (Fig. 3C) models, one can find that the experimental data of MIPs is in better conformity with Langmuir isotherm model (R2 = 0.992), indicating that the binding sites C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
of MIPs for EM are mainly homogeneously distributed and the type of adsorption is monolayer adsorption [33–35]. Furthermore, the experimental Qm of MIPs is 44.03 mg/g, which is very close to the calculated value (45.69 mg/g) by Langmuir model. As for NIPs, the adsorption behavior agrees better with Freundlich model (R2 = 0.985), implying that EM adsorption on NIPs can be considered as a multilayer adsorption process [36, 37].
3.5 Adsorption kinetics The kinetics of EM adsorption on MIPs and NIPs were investigated, and the results are shown in Fig. 3D. It is apparent that MIPs display much higher adsorption capacity than NIPs do, which is the same as that in the static adsorption experiment. In addition, both polymers exhibit fast binding process in the initial period (about 60 min), and then the adsorption rate slows down and levels off to sorption equilibrium. The adsorption capacity of MIPs and NIPs at 60 min is 84.7 and 55.3% of their respective maximum values. To probe the kinetic mechanism for EM adsorption, the kinetic data of batch experiments were fitted to the pseudofirst-order and pseudo-second-order model. The pseudo-firstorder equation is given in Eq. (4), where K1 (g/min) is the adsorption rate constant of pseudo-first-order model, and Qe and Qt (mg/g) are the amount of adsorbed EM at equilibrium and at time t, respectively. ln ( Qe − Qt ) = ln ( Qe ) − K 1 × t
(4)
The pseudo-second order equation is given in Eqs. (5) and (6), where K2 (g/mg/min) is the adsorption rate constant of pseudo-second-order model, h0 is the initial adsorption rate (mg/g/min), and all the other variables are the same as described in the pseudo-first-order equation. 1 1 t = + ×t Qt K 2 × Q2e Qe
(5)
h 0 = K 2 × Q2e
(6)
The adsorption kinetic parameters and regression coefficients are listed in Table 1. The calculated equilibrium www.jss-journal.com
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Figure 3. Binding isotherms of MIPs and NIPs for EM (A), Langmuir (B) and Freundlich plots (C) of EM binding with MIPs and NIPs; Binding kinetics curves of MIPs and NIPs for EM (D), pseudo-first-order kinetic model (E) and pseudo-second-order kinetic model (F) for the adsorption of EM onto MIPs and NIPs. Table 1. Adsorption isotherm parameters of Langmuir and Freundlich, and kinetic parameters of the pseudo-first-order and the pseudosecond-order rate equation for EM adsorption on MIPs and NIPs
Sorbent
MIPs NIPs
Langmuir model
Freundlich model
Pseudo-first-order kinetics
Pseudo-second-order kinetics
Qe,cal (mg/g)
K1 (mL/g)
R2
N
Kf (mg /g)
R2
K1 (g/min)
Qe,cal (mg/g)
R1st 2
K2 (g/mg/min)
Qe,cal (mg/g)
h0 (mg/g/min)
R2nd 2
45.69 19.95
0.037 0.18
0.992 0.895
0.71 0.78
6.05 2.07
0.933 0.985
0.01 0.002
4.36 6.61
0.869 0.925
0.005 0.002
7.35 4.48
0.27 0.04
0.994 0.996
adsorption capacity (Qe,cal ) obtained from pseudo firstorder is 4.36 mg/g with a poor correlation coefficient of R1st 2 = 0.869 (Fig. 3E). By contrast, Qe,cal estimated from pseudo-second-order kinetic model is 7.35 mg/g with a fine correlation coefficient of R2nd 2 = 0.994 (Fig. 3F). Therefore, it is safe to conclude that adsorption of MIPs toward EM fits well with pseudo-second order model, and the process was controlled by chemical adsorption [33, 38, 39].
Table 2. Binding behavior of MIPs and NIPs for EM and ROX
3.6 Selectivity
lated from Eqs. (7)–(9), where Kd1 and Kd2 are the distribution coefficient of EM and its analogue ROX, respectively.
Table 2 summarizes the data obtained in the competitive binding experiments, including uptake capacity (Q), distribution coefficient (Kd ), selective coefficient of the sorbent (␣) and relative selective coefficient (). Kd , ␣, and  are calcu C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Polymers
Analyte
Q (mg/g)
Kd
␣

MIPs
EM ROX EM ROX
19.95 6.91 6.43 6.49
0.068 0.024 0.021 0.022
2.89
3.04
NIPs
Kd =
C0 − Cfree Cfree
0.95
(7)
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␣=
K d1 K d2
(8)
=
␣MIPs ␣NIPs
(9)
In the case of MIPs, Kd1 is much higher than Kd2 , and ␣ is 2.89, which is three times larger than that of NIPs (0.95), indicating the significant selectivity of MIPs toward the templateEM over the structurally similar compound ROX. Just as mentioned above, during MIP preparation, EM was incorporated into polymeric networks by non-covalent interaction and the following elution of EM created imprinted cavities and binding sites, imparting selectivity to the polymer due to the steric and structural matching between MIP and template molecules. As for NIPs, the absence of EM failed to generate imprinted cavities but there are still non-specific binding sites attributed by functional monomers.
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4 Concluding remarks
[14] Liu, J., Song, H., Liu, J., Liu, Y., Li, L., Tang, H., Li, Y., Talanta 2015, 134, 761–767.
Erythromycin molecularly imprinted materials were prepared by a novel approach called one-step distillation–precipitation polymerization. The synthesis conditions have been optimized, and the resultant MIPMSs showed good morphology, monodispersity and high adsorption capacity. The MIP adsorbent exhibits excellent characteristics such as fast adsorption kinetics, and relatively high selectivity toward EM. The experimental data fit well into the pseudo-second-order kinetic model, and match well with Langmuir isotherm. Based on the performance demonstrated in the present study, distillation– precipitation polymerization is a promising technique for preparation of MIPs due to the outstanding merits of the obtained products, the relatively simple reaction system and the more easily-controlled feature in the preparation process.
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The project financially supported by the Pairing Program of Shihezi University with Eminent Scholar in Elite University (SDJDZ201502), the Major Program for Science and Technology Development of Shihezi University (gxjs2014-zdgg04), National Natural Science Foundation of China (81260487, 81460543), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars from Ministry of Human Resources and Social Security of China (RSLX201301). The authors have declared no conflict of interest.
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Jiang Liu1 ∗ Le Li1 ∗ Hui Tang1 Feilang Zhao2 Bang-Ce Ye3 Yingchun Li1,3 Jun Yao4 ∗∗ 1 Key
Laboratory of Xinjiang Endemic Phytomedicine Resources, Ministry of Education; School of Pharmacy, Shihezi University, Shihezi 832002, China 2 Jiangsu Devote Instrumental Science & Technology Co., Ltd., Huai’an, China 3 Key Laboratory for Green Processing of Chemical Engineering of Xinjiang Bingtuan, School of Chemistry and Chemical Engineering, Shihezi University, Shihezi, China 4 School of Pharmacy, Xinjiang Medical University, Urumqi, Xinjiang, China Received April 24, 2015 Revised June 13, 2015 Accepted June 13, 2015
Research Article
Preparation and characterization of erythromycin molecularly imprinted polymers based on distillation–precipitation polymerization Erythromycin-imprinted polymers with excellent recognition properties were prepared by an innovative strategy called distillation–precipitation polymerization. The interaction between erythromycin and methacrylic acid was studied by ultraviolet absorption spectroscopy, and the as-prepared materials were characterized by Fourier-transform infrared spectroscopy and scanning electron microscopy. Moreover, their binding performances were evaluated in detail by static, kinetic and selective sorption tests. It was found that the molecularly imprinted polymers afforded good morphology, monodispersity, and high adsorption capacity when the fraction of the monomers was 7 vol% in the whole reaction system, and the adsorption data for imprinted polymers correlated well with the Langmuir model. The maximum capacity of the imprinted and the non-imprinted polymers for adsorbing erythromycin is 44.03 and 19.95 mg/g, respectively. The kinetic studies revealed that the adsorption process fitted a pseudo-second-order kinetic model. Furthermore, the imprinted polymers display higher affinity toward erythromycin, compared with its analogue roxithromycin. Keywords: Distillation-precipitation polymerization / Equilibrium / Erythromycin / Kinetics / Molecularly imprinted polymers DOI 10.1002/jssc.201500448
Additional supporting information may be found in the online version of this article at the publisher’s web-site
1 Introduction A molecularly imprinted polymer (MIP) is a kind of synthetic material formed in the presence of target molecules or structural analogue molecules as template. Removal of the templates from the polymeric network is able to produce cavities with molecular recognition ability, allowing for selective adsorption to the original templates [1]. As an artificial material, an MIP can be prepared easily and rapidly and is very stable in harsh conditions (organic solvents, extremes of pH, and temperature, etc.). Therefore, MIPs have been gaining significance in a wide range of applications including SPE [2–4], sensors [5–7], chromatography [8–10], mimetic catalysis [11–13] and so on. Traditionally, an MIP is produced as a bulk polymeric monolith, followed by me-
Correspondence: Dr. Yingchun Li, School of Pharmacy, Shihezi University, Shihezi 832000, China E-mail: [email protected]
Abbreviations: AIBN, azobisisobutyronitrile; DPP, distillation–precipitation polymerization; DVB, divinylbenzene; EM, erythromycin; MAA, methacrylic acid; MIP, molecularly imprinted polymer; MIPMS, molecularly imprinted polymeric microsphere; NIP, non-imprinted polymer; ROX, roxithromycin C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
chanical grinding to obtain micrometer-sized particles. Although the obtained materials are somewhat inelegant, this straightforward method is still useful and convenient [14–16]. However, in practical application, performance of MIP can be largely enhanced by controlling the morphology of the products, among which spherical polymer with uniform size and shape is especially favorable, for example, as chromatographic [4,10,17] and SPE packing materials [18–20]. Another aspect in producing monodispersed molecularly imprinted polymeric microspheres (MIPMSs) is the synthesis of binding sites located on or close to the polymer surface, to shorten response time and improve the steric accessibility. The methods of fabricating MIPMSs mainly include precipitation polymerization [21, 22], emulsion polymerization [23, 24], swelling suspension polymerization [25, 26], etc. However, addition of surfactant or stabilizer in these strategies leads to product with complicated constituent and poor controllability of polymerization process results in polymers with broad-dispersed size. Distillation precipitation polymerization (DPP) is a newly developed technique by Yang and Huang [27, 28], through which microspheres with various
∗ These
authors contributed equally to this work. correspondence: Dr. Jun Yao, E-mail: [email protected]
∗∗ Additional
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monomer composition can be obtained in the absence of any surfactant or stabilizer. The DPP reaction system starts with homogeneous mixtures of monomer and initiator dissolved in suitable solvent. During distillation of the solvent out of the system, the growing polymer chains phase separate from the continuous medium by enthalpic or entropic precipitation, where cross-linking prevents the produced polymer from free mixing with solvent, and finally affords the narrow/mono-dispersed polymeric microspheres [29, 30]. We recently extended this strategy into molecular imprinting, and MIPMSs with uniform size, simple component and high binding capacity have been successfully prepared [4]. However, the whole process contains two-step DPP, which is somehow time-consuming and lowers efficiency [31]. Herein, we designed direct synthesis of monodispersed MIPs by onestep DPP by using erythromycin (EM) as the model template. The polymeric spheres generated were characterized using FTIR spectroscopy and SEM, and their binding behavior, adsorption isotherm and kinetics were studied in detail.
2 Materials and methods 2.1 Materials and chemicals Erythromycin (EM) was supplied by Guangdong Jiqi Drug (Guangdong, China). Roxithromycin (ROX) was obtained from Adamas Reagent (Shanghai, China), and their structures are illustrated in Supporting Information Fig. S1. Methacrylic acid (MAA) and azobisisobutyronitrile (AIBN) were obtained from Tianjin Guangfu Chemical (Tianjin, China). Before use, MAA was distilled twice under vacuum and AIBN was recrystallized in ethanol. Divinylbenzene (DVB) was purchased from Sigma–Aldrich (Shanghai, China) and purified by extraction with 10% NaOH aqueous solution, and then dried over anhydrous magnesium sulfate. Water was doubly distilled. Solvents for HPLC were of chromatographic grade. All the other chemicals were of analytical grade and used without further treatment.
2.2 Instruments HPLC experiments were implemented using an Essentia LC–15C system equipped with two LC–15C Solvent Delivery Units, an LC Solution 15C workstation and an SPD–15C UV–Vis Detector (Shimadzu, Japan). LC condition was as follows: chromatographic separation was performed on a Shimadzu WondaSil C18 column (150 mm × 4.6 mm i.d., 5 m), the mobile phase was acetonitrile/phosphate buffer (2:8, v/v, pH 8.2) with a flow rate of 1.0 mL/min, injection volume was 20 L, and the detection wavelength was set at 215 nm for EM and 210 nm for ROX, respectively. All the measurements were carried out in triplicate. To identify the interaction between EM and MAA, a UV-2401 UV spectrophotometer (Shimadzu, China) was
C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
employed. IR spectra were recorded by a Nicolet EXUS–470 FTIR apparatus (Shimadzu, Japan). SEM images were taken by a JSM–6490LV scanning electron microscope (Tokyo, Japan) with an accelerating voltage of 10 kV.
2.3 Preparation of polymers EM-MIPs were prepared based on the method reported by Liu et al. [31] and a modified DPP tactic was adopted in this work. Briefly, the template (EM, 1.30 g), functional monomer (MAA, 0.6 mL) and cross-linker (DVB, 5.0 mL) were well dissolved in 80 mL acetonitrile by ultrasonication for 2 min. The solution was stored over night for the formation of a complex of EM and MAA. After addition of AIBN (2 wt% relative to the monomers) as initiator, the solution was poured into a three-necked flask that was attached with a dasher, a fractionating column and a receiver. The reaction apparatus is displayed in Supporting Information Fig. S2. The reaction mixture was purged with nitrogen for 10 min, and then heated from ambient temperature to boiling state within 30 min with continuously stirring at 120 rpm. The reaction was ended when 40 mL acetonitrile was distilled from the reaction system in 1.5–2 h. After polymerization, the resulting polymers were treated through ultrasonication and vacuum filtration, followed by washing with methanol for three times. The template molecules were removed by using mixture of methanol and acetic acid (90:10, v/v) in Soxhlet extractor until the complete absence of EM in the rinse solution. The obtained EM-MIPs were further washed with methanol and dried at 50⬚C. The non-imprinted polymers (NIPs) particles were prepared and washed using the same recipe but without the addition of the template EM.
2.4 Binding experiments In static adsorption test, 50.0 mg polymers were placed in a conical flask with stopper containing 5.0 mL EM standard methanol solution at the concentrations of 0.3–20.0 mg/mL. After incubation for 12 h at room temperature, the samples were centrifuged and filtered, and the concentration of free EM after adsorption was determined by HPLC–UV. Kinetic binding test was carried out in a similar way except that the initial concentration was constant at 2.0 mg/mL and the determination was made at different time points (5, 10, 20, 30, 60, 90, 120, 180, 240, and 300 min). The adsorption quantity (Q) was calculated through Eq. (1), where C0 and Cfree are the initial and free concentration of EM, V stands for the volume of the solution and W refers to the mass of the polymer powder. Q=
(C0 − Cfree ) × V W
(1)
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Figure 1. UV spectra of mixtures of EM and MAA under different molar ratio (A), and IR spectra of EM (B), EM-MIPs before (C) and after elution (D) of EM, and NIPs (E).
2.5 Selectivity of polymers The selectivity of EM-MIPs was evaluated with EM and ROX. 50.0 mg of EM-MIPs and NIPs were separately added to a conical flask containing 5 mL standard solution of each compound at 3 mg/mL. All the mixtures were incubated for 3 h at ambient temperature and then filtered through a 0.22 mm filter membrane. The filtrates of EM and ROX were determined by HPLC–UV.
3 Results and discussion 3.1 Confirmation of interaction between EM and MAA The key factor of preparing highly selective materials together with good adsorption capacity is that a complex must be formed between functional monomers and template molecules through strong interactions. EM molecule has Hbond acceptors while MAA owns H-bond donors. Therefore, these two compounds are very likely to form stable complex due to formation of hydrogen bonds in prepolymerization solution. This presumption was further examined by UV/Vis spectrophotometric analysis of a series of solutions containing different molar ratios of EM to MAA in acetonitrile. As shown in Fig. 1A, the maximum absorption wavelength of EM shifted from 217 to 225 nm with adding MAA and further to longer wavelength with increasing the concentration of MAA. This could arise from the formation of hydrogen bonds between the carbonyl group of MAA and the hydroxyl group of EM, which is also in accord with early reports [31,32].
3.2 Structure information of the polymers The IR spectra of EM, MIPs before and after elution of EM and NIPs were recorded to verify the synthesis of the desired products. The broad absorption band at 1175 cm−1 is assigned to the O–H stretching vibration of EM molecules, and the peak observed at 1365 cm−1 corresponds to the stretching vibration of C–N bonds of the tertiary amine group in EM molecules (Fig. 1B, C). These two characteristic absorp C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
tion peaks appearing in EM-MIPs before elution of templates (Fig. 1C) demonstrates that EM molecules are successfully imprinted into the polymers. After EM extraction (Fig. 1D), the polymer exhibits similar absorption bands as NIPs (Fig. 1E) do, implying that the template EM has been removed from the MIP matrix. The schematic illustration of the EMMIP preparation and the interaction between MIP and EM is shown in Supporting Information Fig. S3.
3.3 Effect of monomer ratio on morphology and binding behavior of polymers DPP begins with a homogeneous system containing monomer and initiator. As the solvent distills, polymeric microspheres form in the solution. Optimization of initial concentration of monomer is very important to get the products with narrow size distribution. The SEM images of EM-MIP microspheres prepared under different monomer concentrations are shown in Fig. 2. With the increase of the concentration from 3 to 7% (Fig. 2A, B, and C), the size of the polymeric spheres gets more uniform, the reason of which has been elaborated in our previous reports [4, 31]. However, when the monomer concentration went up to 9%, the reaction mixture got so viscous and uneven that it is difficult to distill off the expected volume of acetonitrile. Besides, increase of the amount of monomers resulted in wider size distribution and the formation of small-sized particles could be the consequence of shortened period of particle nucleation (Fig. 2D). Figure 2E illustrates the adsorption quantity (Q) and the imprinting factor (IF, IF = QMIPs /QNIPs ) of the polymers prepared at the four different monomer concentrations. The highest Q is 8.9 mg/g with the largest IF of 1.6, obtained with MIPs prepared at monomer concentration of 7%. QMIPs decreases when the concentration is lower than 7%, which can be explained by the fact that insufficient monomers yield less binding sites for EM adsorption. On the other hand, QMIPs also goes down with the monomer concentration surpassing 7%, which might be related to the fact that high quantity of monomers generates polymers with larger particle size, which makes lots of imprinted sites locate inside polymeric network, and consequently reduces effective binding sites and binding capacity, and prolongs equilibrium time. Hence, www.jss-journal.com
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Figure 2. SEM photos of EMMIPs prepared at different concentrations of monomer (A, 3%; B, 5%; C, 7%; D, 9%), and adsorption quantity and imprinting factor of the polymers prepared at different concentrations of monomer in standard solution of EM at 1.0 mg/mL (E).
monomer concentration of 7% was applied in the subsequent experiments for preparing EM-MIPs.
3.4 Adsorption capacity The polymers prepared under the optimized condition were used to adsorb EM at a series of concentrations and the binding isotherm corresponding to QMIPs or QNIPs versus EM concentration is displayed in Fig. 3A. It can be seen that both QMIPs and QNIPs increase significantly with increasing the concentration of EM from 0.3 to 10 mg/mL. After that, the raise in concentration from 10 to 20 mg/mL brings about slight Q enhancement, which is probably due to saturation of the binding sites in the sorbents. The maximum static adsorption capacities (Qmax ) of the imprinted and non-imprinted polymers for EM are 44.03 and 19.95 mg/g, respectively, indicating that imprinting effect plays an important role in adsorption performance. During the preparation of MIPs, template molecules and functional monomers bring out imprinting behavior and the subsequent removal of template leaves imprinted cavities and specific binding sites with functional groups existing in a predetermined orientation in the polymeric matrix. By contrast, such procedure does not occur in the case of NIPs so that they do not exhibit such specific binding ability. Langmuir and Freundlich models are applied to study the adsorption behavior of MIPs and NIPs theoretically by using Langmuir isotherm Eq. (2) and Freundlich isotherm Eq. (3), where Ce (mg/mL) and Qe (mg/g) are the concentration and the amount of adsorbed EM at adsorption equilibrium, Qm is the maximum adsorption capacity (mg/g), Kl is the Langmuir constant (mL/g), and Kf is the Freundlich constant (mg/g). Qe =
Qm · K l · Ce 1 + Ce · K l
Qe = K f · Ce n
(2)
(3)
The calculated Langmuir and Freundlich parameters are summarized in Table 1. By comparing the experimental adsorption isotherm (Fig. 3A) with the Langmuir (Fig. 3B) and Freundlich (Fig. 3C) models, one can find that the experimental data of MIPs is in better conformity with Langmuir isotherm model (R2 = 0.992), indicating that the binding sites C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
of MIPs for EM are mainly homogeneously distributed and the type of adsorption is monolayer adsorption [33–35]. Furthermore, the experimental Qm of MIPs is 44.03 mg/g, which is very close to the calculated value (45.69 mg/g) by Langmuir model. As for NIPs, the adsorption behavior agrees better with Freundlich model (R2 = 0.985), implying that EM adsorption on NIPs can be considered as a multilayer adsorption process [36, 37].
3.5 Adsorption kinetics The kinetics of EM adsorption on MIPs and NIPs were investigated, and the results are shown in Fig. 3D. It is apparent that MIPs display much higher adsorption capacity than NIPs do, which is the same as that in the static adsorption experiment. In addition, both polymers exhibit fast binding process in the initial period (about 60 min), and then the adsorption rate slows down and levels off to sorption equilibrium. The adsorption capacity of MIPs and NIPs at 60 min is 84.7 and 55.3% of their respective maximum values. To probe the kinetic mechanism for EM adsorption, the kinetic data of batch experiments were fitted to the pseudofirst-order and pseudo-second-order model. The pseudo-firstorder equation is given in Eq. (4), where K1 (g/min) is the adsorption rate constant of pseudo-first-order model, and Qe and Qt (mg/g) are the amount of adsorbed EM at equilibrium and at time t, respectively. ln ( Qe − Qt ) = ln ( Qe ) − K 1 × t
(4)
The pseudo-second order equation is given in Eqs. (5) and (6), where K2 (g/mg/min) is the adsorption rate constant of pseudo-second-order model, h0 is the initial adsorption rate (mg/g/min), and all the other variables are the same as described in the pseudo-first-order equation. 1 1 t = + ×t Qt K 2 × Q2e Qe
(5)
h 0 = K 2 × Q2e
(6)
The adsorption kinetic parameters and regression coefficients are listed in Table 1. The calculated equilibrium www.jss-journal.com
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Figure 3. Binding isotherms of MIPs and NIPs for EM (A), Langmuir (B) and Freundlich plots (C) of EM binding with MIPs and NIPs; Binding kinetics curves of MIPs and NIPs for EM (D), pseudo-first-order kinetic model (E) and pseudo-second-order kinetic model (F) for the adsorption of EM onto MIPs and NIPs. Table 1. Adsorption isotherm parameters of Langmuir and Freundlich, and kinetic parameters of the pseudo-first-order and the pseudosecond-order rate equation for EM adsorption on MIPs and NIPs
Sorbent
MIPs NIPs
Langmuir model
Freundlich model
Pseudo-first-order kinetics
Pseudo-second-order kinetics
Qe,cal (mg/g)
K1 (mL/g)
R2
N
Kf (mg /g)
R2
K1 (g/min)
Qe,cal (mg/g)
R1st 2
K2 (g/mg/min)
Qe,cal (mg/g)
h0 (mg/g/min)
R2nd 2
45.69 19.95
0.037 0.18
0.992 0.895
0.71 0.78
6.05 2.07
0.933 0.985
0.01 0.002
4.36 6.61
0.869 0.925
0.005 0.002
7.35 4.48
0.27 0.04
0.994 0.996
adsorption capacity (Qe,cal ) obtained from pseudo firstorder is 4.36 mg/g with a poor correlation coefficient of R1st 2 = 0.869 (Fig. 3E). By contrast, Qe,cal estimated from pseudo-second-order kinetic model is 7.35 mg/g with a fine correlation coefficient of R2nd 2 = 0.994 (Fig. 3F). Therefore, it is safe to conclude that adsorption of MIPs toward EM fits well with pseudo-second order model, and the process was controlled by chemical adsorption [33, 38, 39].
Table 2. Binding behavior of MIPs and NIPs for EM and ROX
3.6 Selectivity
lated from Eqs. (7)–(9), where Kd1 and Kd2 are the distribution coefficient of EM and its analogue ROX, respectively.
Table 2 summarizes the data obtained in the competitive binding experiments, including uptake capacity (Q), distribution coefficient (Kd ), selective coefficient of the sorbent (␣) and relative selective coefficient (). Kd , ␣, and  are calcu C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Polymers
Analyte
Q (mg/g)
Kd
␣

MIPs
EM ROX EM ROX
19.95 6.91 6.43 6.49
0.068 0.024 0.021 0.022
2.89
3.04
NIPs
Kd =
C0 − Cfree Cfree
0.95
(7)
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␣=
K d1 K d2
(8)
=
␣MIPs ␣NIPs
(9)
In the case of MIPs, Kd1 is much higher than Kd2 , and ␣ is 2.89, which is three times larger than that of NIPs (0.95), indicating the significant selectivity of MIPs toward the templateEM over the structurally similar compound ROX. Just as mentioned above, during MIP preparation, EM was incorporated into polymeric networks by non-covalent interaction and the following elution of EM created imprinted cavities and binding sites, imparting selectivity to the polymer due to the steric and structural matching between MIP and template molecules. As for NIPs, the absence of EM failed to generate imprinted cavities but there are still non-specific binding sites attributed by functional monomers.
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4 Concluding remarks
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Erythromycin molecularly imprinted materials were prepared by a novel approach called one-step distillation–precipitation polymerization. The synthesis conditions have been optimized, and the resultant MIPMSs showed good morphology, monodispersity and high adsorption capacity. The MIP adsorbent exhibits excellent characteristics such as fast adsorption kinetics, and relatively high selectivity toward EM. The experimental data fit well into the pseudo-second-order kinetic model, and match well with Langmuir isotherm. Based on the performance demonstrated in the present study, distillation– precipitation polymerization is a promising technique for preparation of MIPs due to the outstanding merits of the obtained products, the relatively simple reaction system and the more easily-controlled feature in the preparation process.
[15] He, Q., Fang, B., Su, Y., Zeng, Z., Yang, J., He, L., Zeng, D., J. Sep. Sci. 2013, 36, 301–310.
The project financially supported by the Pairing Program of Shihezi University with Eminent Scholar in Elite University (SDJDZ201502), the Major Program for Science and Technology Development of Shihezi University (gxjs2014-zdgg04), National Natural Science Foundation of China (81260487, 81460543), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars from Ministry of Human Resources and Social Security of China (RSLX201301). The authors have declared no conflict of interest.
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