Soluble Molecularly Imprinted Polymer-Based Potentiometric Sensor

Letter Cite This: Anal. Chem. XXXX, XXX, XXX−XXX

pubs.acs.org/ac

Soluble Molecularly Imprinted Polymer-Based Potentiometric Sensor for Determination of Bisphenol AF Huan Zhang,†,‡ Ruiqing Yao,† Ning Wang,‡ Rongning Liang,*,‡ and Wei Qin*,‡ †

School of Chemical Engineering, Northwest University, Xi’an 710069, P. R. China Key Laboratory of Coastal Environmental Processes and Ecological Remediation, Yantai Institute of Coastal Zone Research (YIC), Chinese Academy of Sciences (CAS), Shandong Provincial Key Laboratory of Coastal Environmental Processes, YICCAS, Yantai, Shandong 264003, P. R. China

‡

S Supporting Information *

ABSTRACT: Molecularly imprinted polymer (MIP)-based polymeric membrane potentiometric sensors have been successfully developed for determination of organic compounds in their ionic and neutral forms. However, most of the MIP receptors in potentiometric sensors developed so far are insoluble and cannot be well dissolved in the polymeric membranes. The heterogeneous molecular recognitions between the analytes and MIPs in the membranes are inefficient due to the less available binding sites of the MIPs. Herein we describe a novel polymeric membrane potentiometric sensor using a soluble MIP (s-MIP) as a receptor. The s-MIP is synthesized by the swelling of the traditional MIP at a high temperature. The obtained MIP can be dissolved in the plasticized polymeric membrane for homogeneous binding of the imprinted polymer to the target molecules. By using neutral bisphenol AF as a model, the proposed method exhibits an improved sensitivity compared to the conventional MIP-based sensor with a lower detection limit of 60 nM. Moreover, the present sensor exhibits an excellent selectivity over other phenols. We believe that s-MIPs can provide an appealing substitute for the traditional insoluble MIP receptors in the development of polymeric membrane-based electrochemical and optical sensors.

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compounds in their ionic and neutral forms.11−14 These MIP receptors include MIP microbeads,15,16 nanobeads,17 and amorphous particles.18 Unlike the traditional ionophores which can be well dissolved in the plasticized polymeric membranes, MIPs are rigid and highly cross-linked polymers so that they cannot be easily dissolved but rather be dispersed in the membranes.19 The formed sensing membranes could be heterogeneous, thus inducing less available binding sites in the membrane and higher membrane impedance as compared to the homogeneous membranes used in the conventional ISEs. Clearly, such insoluble MIP receptors are rather undesirable for development of membrane sensors.20 Nowadays, soluble MIPs (s-MIPs) have exhibited great potential to significantly change such a situation.21 Inspired by the breakthrough from this area, we report here for the first time an s-MIP-based potentiometric sensor. The new s-MIP receptor is synthesized by the swelling of the traditional MIP at a high temperature in the presence of a high boiling point solvent. The obtained MIP can be well dissolved not only in the organic solvents but also in the plasticizers which are usually used as the membrane solvents for the polymeric membrane

n recent years, carrier-based solvent polymeric membrane ion-selective electrodes (ISEs) have evolved to be a very promising tool for potentiometric analysis of a broad variety of ions in environmental and biological applications.1−3 In order to achieve selective recognition, an efficient molecular receptor (i.e., ionophore) which acts as a complexing agent for a target ion is usually required as the sensing element.4 Nowadays, many compounds which can form stable and selective complexes with the target ions have been synthesized and even commercially available. However, most of these recognition receptors are used for inorganic ions such as electrolyte ions and heavy metal ions. The synthetic receptors for selective recognition of organic species are still rather rare.5,6 As highly suitable receptors for organic species, molecularly imprinted polymers (MIPs) have gained wide acceptance as new artificial recognition receptors in chemical sensors since they have affinities and selectivities comparable to natural receptors such as antibodies and enzymes.7−9 Especially, MIPs are more stable, less costly, and easier to produce. These materials are generally synthesized in the presence of functional monomers, template molecules and cross-linking agents. After template removal, binding sites with molecular recognition properties are formed in the polymerized material.10 Currently, potentiometric sensors based on various MIP receptors have been successfully developed for determination of organic © XXXX American Chemical Society

Received: August 23, 2017 Accepted: December 11, 2017 Published: December 11, 2017 A

DOI: 10.1021/acs.analchem.7b03432 Anal. Chem. XXXX, XXX, XXX−XXX

Letter

Analytical Chemistry Scheme 1. Schematic Illustration for the Synthesis of the s-MIP

for the subsequent experiments. The traditional insoluble MIP was prepared under identical conditions with the s-MIP except for omission of the swelling step. The soluble and insoluble nonimprinted polymers (s-NIP, NIP) were prepared by the similar procedures in the absence of the template. The obtained imprinted and nonimprinted beads were characterized by using scanning electron microscopy (SEM, Hitachi, S-4800). Membranes and Electrodes. The s-MIP (s-NIP)-based membranes were prepared by adding 432 μL of the s-MIP (sNIP) solution (50 mg/mL) into the membrane matrix containing 3.6 mg of tridodecylmethylammonium chloride (TDMAC), 7.2 mg of tetradodecylammonium tetrakis(4chlorophenyl)borate (ETH 500), 131.1 mg of poly(vinyl chloride) (PVC), and 196.6 mg of dioctylphtalate (DOP). The obtained mixtures were then degassed by sonication for 2 min. The membrane cocktails were prepared by dissolving these components in 4.5 mL of tetrahydrofuran (THF). For comparison, the traditional insoluble MIP (NIP)-based membranes contained 21.6 mg of MIP (NIP), 3.6 mg of TDMAC, 7.2 mg of ETH 500, 131.1 mg of PVC, and 196.6 mg of DOP. Glassy carbon electrodes (GCEs) were polished and washed with ethanol and deionized water. The solution of poly(3octylthiophene) (POT, 25 mM with respect to the monomer in CHCl3, 5 μL) was drop-cast onto the polished GCE three times and left to dry. Then 60 μL of membrane cocktail was drop-cast onto the obtained glassy carbon electrodes and allowed to dry for 6 h. All the membrane electrodes were conditioned overnight in a 30 mM phosphate buffer solution (PBS) of pH 6.0 before measurements.

potentiometric sensors. The new concept has been evaluated and illustrated for potentiometric detection of neutral phenols by using a solid-contact polymeric membrane electrode with the proposed s-MIP as the receptor. In this work, bisphenol AF (BPAF), a new bisphenol analogue which is considered to be more harmful than bisphenol A,22 is selected as a model of neutral phenols. It will be shown that the s-MIP-based polymeric membrane electrode could offer remarkably improved sensitivity for potentiometric detection of neutral phenols.

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EXPERIMENTAL SECTION Synthesis of the Soluble Molecularly Imprinted Polymer. The s-MIP beads were synthesized by a noncovalent method. Briefly, the template BPAF (0.5 mmol), methacrylic acid (MAA, 2 mmol), divinylbenzene 80 (DVB 80, 10 mmol), and free-radical initiator 2,2′-azobis(isobutyronitrile) (AIBN, 25 mg) were dissolved in acetonitrile (12.5 mL) in a 50 mL flask. The mixture was sonicated for 5 min to maintain homogeneity. Then the solution was purged with a gentle flow of N2 for 15 min and sealed under N2 atmosphere. Polymerization was carried out by submerging the flask in an oil bath at 80 °C for 24 h. After polymerization, the template was removed by batch-mode solvent extraction with methanol/ acetic acid (8/2, v/v) and methanol. The complete removal of the template was confirmed by the Fourier-transform-infrared spectroscopy (FT-IR) (Figure S1 in the Supporting Information). The resulting polymer was then dried in vacuum overnight at 40 °C. Finally, 100 mg of the obtained polymer was added in 2 mL of 1,2,4-trichlorobenzene (TCB), and then the mixture was heated with stirring at 150 °C for 5 h for MIP swelling. The obtained mixture with 50 mg/mL s-MIP was used B

DOI: 10.1021/acs.analchem.7b03432 Anal. Chem. XXXX, XXX, XXX−XXX

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Analytical Chemistry

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RESULTS AND DISCUSSION

As artificial mimetics of natural receptors such as antibodies and enzymes, MIPs have proven to be excellent receptors for the selective recognition of a variety of organic targets in chemical sensors.23 Notably, almost all of the recognition processes of natural receptors take place in homogeneous systems. However, it is unfortunate that the conventional MIPs are highly crosslinked polymers and thus cannot be easily dissolved in the plasticized sensing membrane phases of chemical sensors.12 It is highly desired to prepare s-MIPs for efficient molecular recognition. In this work, we explore for the first time the feasibility for fabricating a potentiometric sensor by using an sMIP in the polymeric membrane as a receptor. In contrast to the conventional methods for the synthesis of insoluble MIPs, the synthesis approach for the s-MIP involves two steps including precipitation polymerization and hightemperature swelling. The schematic representation of the synthesis of the s-MIP is shown in Scheme 1. In the polymerization step, MAA and DVB are used as the functional monomer and the cross-linker, respectively. The carboxyl group of MAA can interact via strong hydrogen bonding with the hydroxyl and fluorine groups of BPAF. In addition, DVB provides additional π−π interactions with the aromatic moiety of the template, thereby improving the binding affinity of the MIP.24 In the swelling step, a highly polarizable solvent TCB, which is usually utilized as the solvent to dissolve polymers in high-temperature gel permeation chromatography,25 is employed to induce swelling of the MIP. At a high temperature (i.e., 150 °C), TCB can be incorporated into the imprinted polymer matrix through π−π interactions between its aromatic moiety and the phenyl groups of the imprinted polymer matrix. Such incorporation can lead to swelling of the MIP and thus effectively enlarge the distances in between the polymer chains.26−28 This will significantly improve the solubility of MIP in the organic phase.29,30 Notably, TCB could be incorporated into the imprinted polymer matrix but not inside the cavities. In this case, the distances between the imprinted cavities can be enlarged by the TCB swelling, while the cavities may not be affected by such swelling. The obtained imprinted cavities are filled with hydrophilic −COOH groups, while the imprinted polymer matrix mainly consists of hydrophobic polystyrene. Since TCB is a hydrophobic solvent, it can be readily incorporated into the hydrophobic imprinted polymer matrix according to the selective swelling mechanism.31,32 The obtained s-MIP and the traditional insoluble MIP beads were characterized by using SEM and the results are shown in Figure 1. As illustrated, both MIP beads are spherical and the diameter of the s-MIP beads is larger than that of the insoluble MIP beads (Figure 1a,b). In order to clearly illustrate the diameters of the s-MIP and MIP beads, SEM images for more beads are shown in Figure S2 in the Supporting Information. The particle size distributions of the s-MIP and MIP beads were tested and calculated by the Nano Measurer software (http://nano-measurer.software.informer.com/). The results are shown in Figure S3 in the Supporting Information. As can be seen, the soluble MIP beads have a size distribution of 2.0−4.8 μm (Figure S3a), while the insoluble MIP beads have a size distribution of 1.1−2.7 μm (Figure S3b). This difference in particle size is probably due to the swelling of the imprinted polymer in the presence of TCB. Additionally, the SEM images also illustrate that the s-NIP and insoluble NIP beads have the

Figure 1. SEM images of the obtained (a) s-MIP, (b) MIP, (c) s-NIP, and (d) NIP beads.

morphological structures and particle sizes similar to those of the s-MIP and MIP beads, respectively (Figure 1c,d). In order to evaluate the solubilities of the s-MIP in organic solvents, the solubility characteristics of the conventional MIP and the s-MIP were compared. Experiments show that the proposed soluble imprinted beads can be well dissolved in common organic solvents (22 mg/mL) such as dimethylformamide (DMF) and acetonitrile, and the solutions are stable, while the traditional ones cannot be dissolved but only dispersed in these solvents after ultrasonic treatment (Figure S4 in the Supporting Information). Indeed, obvious precipitation of insoluble beads was observed after the dispersion of the conventional MIP standed for 1 h. These results suggest that the proposed s-MIP can be well dissolved in organic solvents. To confirm that the proposed imprinted polymer can be effectively applied in plasticized polymeric membranes, the solubilities of the s-MIPs in different plasticizers were tested. As shown in Figure S5 of the Supporting Information, the s-MIP exhibits superior solubilities compared to the traditional MIP. Interestingly, the s-MIP beads produced cannot only be dissolved in apolar DOP (dielectric constant, εr = 5.0) but also in polar o-nitrophenyl octyl ether (o-NPOE, εr = 24.0). Such excellent solubilities in these solvents can be reasonably attributed to the incorporation of TCB into the polymer matrix and the resultant swelling of the imprinted beads. The feasibility of using the s-MIP as the receptor for sensitive and selective detection of neutral BPAF was examined with the solid-contact polymeric membrane electrodes. Previous studies show that undissociated neutral phenols and their derivatives can induce strong anionic potential responses on quaternary ammonium salt-doped polymeric sensing membranes.24,33,34 Such unexpected anionic responses are attributed to the net movement of hydrogen ions from the sensing membrane phase to the aqueous phase stimulated by neutral phenols.33 As shown is Figure S6, both s-MIP- and MIP-based membrane electrodes (curves a and b) show much larger anionic responses than the s-NIP- and NIP-based and blank membrane electrodes (curves c−e), which suggests the specific interactions between BPAF and its MIPs in the polymeric membranes. Moreover, it can also be seen that the s-MIP-based membrane electrode (curve a) exhibits a much better performance than the traditional MIP-based membrane electrode (curve b). For measurement of 1 μM BPAF in 30 mM PBS, the EMF change obtained by the insoluble MIP-based electrode is only less than C

DOI: 10.1021/acs.analchem.7b03432 Anal. Chem. XXXX, XXX, XXX−XXX

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Analytical Chemistry

Figure 2. (a) Typical potential responses to BPAF in 30 mM pH 6.0 PBS of the solid-contact electrodes based on s-MIP and insoluble MIP (inset). (b) Calibration curves for the corresponding s-MIP- and MIP-based potentiometric sensors. s-MIP membrane: 21.6 mg of s-MIP, 3.6 mg of TDMAC, 7.2 mg of ETH 500, 131.1 mg of PVC, and 196.6 mg of DOP. MIP membrane: 21.6 mg of MIP, 3.6 mg of TDMAC, 7.2 mg of ETH 500, 131.1 mg of PVC, and 196.6 mg of DOP. The potential difference between the baseline and the potential measured at 250 s after BPAF addition was used for quantification. Each error bar represents 1 standard deviation for three measurements.

Figure 3. Responses of the s-MIP (a) and traditional MIP (b)-based potentiometric sensors to neutral phenols: BPAF (black filled square), 4,4′dihydroxybiphenyl (red filled down-triangle), 2-nitrophenol (blue open circle), and phenol (filled circle). Other conditions are as given in Figure 2. Error bars represent 1 standard deviation for three measurements.

2a). Indeed, much smaller potential responses were observed for the sensor based on the traditional MIP beads (Figure 2b). Especially, the detection sensitivity of the insoluble MIP electrode (1.32 mV/μM) is much lower than that of the electrode based on s-MIP (4.93 mV/μM). These results are due to the decrease in the number of the binding sites for molecular recognition with the insoluble MIP membrane. The comparison of the present s-MIP-based electrode with other detection methods for BPAF is summarized in Table S1 in the Supporting Information. As illustrated, the response characteristics of the proposed electrode are comparable to or better than those methods. A series of three repetitive measurements of 1 μM BPAF was utilized for evaluating the detection precision. This series yielded reproducible potential signals with a relative standard deviation (RSD) of 5.5% (Figure S7). Additionally, the RSD was found to be 4.2% for five electrodes prepared from different batches (1 μM). The selectivities of the proposed s-MIP-based potentiometric sensor for BPAF over other neutral phenols were investigated. To guarantee that all the phenols tested were in their neutral forms, the sample pHs were adjusted to certain values according to their acid/dissociation constants by using suitable buffer solutions (see the Supporting Information). The responses of the proposed sensor based on the s-MIP to other phenols such as 4,4′-dihydroxybiphenyl, 2-nitrophenol, and phenol are shown in Figure 3a. As can be seen, the sensor exhibits an excellent selectivity over other phenols. Compared to the responses to neutral BPAF with respect to increasing the

a quarter of that by the electrode with the s-MIP beads as the sensing element. This remarkable improvement in sensitivity could be attributed to the fact that the s-MIP beads can be well dissolved in the polymeric sensing membrane, thus leading to more available binding sites in the membrane. These results indicate that the proposed s-MIP may be an appealing substitute for the traditional insoluble MIP receptors in the development of polymeric membrane-based potentiometric sensors. Figure 2a displays the typical dynamic potential responses of the potentiometric sensor based on s-MIP for measuring neutral BPAF at concentrations ranging from 0.1 to 10 μM in 30 mM PBS of pH 6.0. As illustrated, the membrane electrode shows the nonclassical potential responses to neutral BPAF, which is consistent with the results of previous studies.24,34,35 In this work, the potential difference between the baseline and the potential measured at a fixed time (i.e., 250 s) after BPAF addition was used for quantification. Detailed analysis of the experimental results indicates that the potential response is proportional to the concentration of neutral BPAF in the range of 0.1−1 μM (Figure 2b). In this case, a lower detection limit of 60 nM (3σ) could be obtained. Such detection limit could allow the applications of the proposed sensor to determination of BPAF in wastewater samples from the BPAF manufacturing plants22 or in some biological samples.36 For comparison, the potential responses of the traditional insoluble MIP-based electrode to neutral BPAF were also tested over the concentration range of 0.1−10 μM (see the inset of Figure D

DOI: 10.1021/acs.analchem.7b03432 Anal. Chem. XXXX, XXX, XXX−XXX

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Analytical Chemistry concentration from 0.1 to 1 μM, negligible changes in potential difference can be observed for other three phenols. In contrast, the potentiometric sensor using the traditional insoluble MIP as the receptor exhibits relatively poor selectivities in terms of much smaller potential differences between BPAF and other phenols as compared to those of the s-MIP-based sensor (Figure 3b). These results suggest that the improvement in selectivity is mainly caused by the excellent solubility of the sMIP receptors in the sensing membrane. Thus, it can be demonstrated that the proposed s-MIP is a promising and reliable receptor for development of polymeric membrane potentiometric sensors, especially for organic species. In order to validate its feasibility for real applications, the proposed sensor was used to analyze BPAF in mixtures containing three interferences and in spiked river water samples. The results are shown in Tables S2 and S3 in the Supporting Information, respectively. As can be seen, the present sensor based on the s-MIP exhibits a better accuracy and a higher precision compared to the sensor based on the traditional insoluble MIP (Table S2). Additionally, the recoveries for the river sample analysis vary from 93 to 103% (Table S3). These results indicate that the s-MIP-based sensor has promising potential for determination of BPAF in complex environmental samples.

Wei Qin: 0000-0002-9606-7730 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grants 21475148, 41576106), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant XDA11020702), the Youth Innovation Promotion Association of CAS (Grant 2014190), and the Taishan Scholar Program of Shandong Province (Grant TS20081159).

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(1) Afshar, M. G.; Crespo, G. A.; Bakker, E. Anal. Chem. 2016, 88, 3945−3952. (2) Bakker, E.; Bühlmann, P.; Pretsch, E. Chem. Rev. 1997, 97, 3083− 3132. (3) Bobacka, J.; Ivaska, A.; Lewenstam, A. Chem. Rev. 2008, 108, 329−351. (4) Chen, L. D.; Mandal, D.; Pozzi, G.; Gladysz, J. A.; Bühlmann, P. J. Am. Chem. Soc. 2011, 133, 20869−20877. (5) Meyerhoff, M. E.; Fu, B.; Bakker, E.; Yun, J. H.; Yang, V. C. Anal. Chem. 1996, 68, 168A−175A. (6) Bühlmann, P.; Badertscher, M.; Simon, W. Tetrahedron 1993, 49, 595−598. (7) Erdő ssy, J.; Horváth, V.; Gyurcsányi, R. E. TrAC, Trends Anal. Chem. 2016, 79, 179−190. (8) Banerjee, S.; König, B. J. Am. Chem. Soc. 2013, 135, 2967−2970. (9) Fuchs, Y.; Soppera, O.; Mayes, A. G.; Haupt, K. Adv. Mater. 2013, 25, 566−570. (10) Wulff, G. Chem. Rev. 2002, 102, 1−28. (11) Rao, T. P.; Kala, R. Talanta 2008, 76, 485−496. (12) Liang, R. N.; Song, D. A.; Zhang, R. M.; Qin, W. Angew. Chem., Int. Ed. 2010, 49, 2556−2559. (13) Liang, R. N.; Ding, J. W.; Gao, S. S.; Qin, W. Angew. Chem., Int. Ed. 2017, 56, 6833−6837. (14) Moreira, F. T. C.; Kamel, A. H.; Guerreiro, J. R. L.; Sales, M. G. F. Biosens. Bioelectron. 2010, 26, 566−574. (15) D'Agostino, G.; Alberti, G.; Biesuz, R.; Pasavento, M. Biosens. Bioelectron. 2006, 22, 145−152. (16) Liang, R. N.; Gao, Q.; Qin, W. Chin. J. Anal. Chem. 2012, 40, 354−358. (17) Liang, R. N.; Kou, L. J.; Chen, Z. P.; Qin, W. Sens. Actuators, B 2013, 188, 972−977. (18) Mazzotta, E.; Picca, R. A.; Malitesta, C.; Piletsky, S. A.; Piletska, E. V. Biosens. Bioelectron. 2008, 23, 1152−1156. (19) Rao, T. P.; Daniel, S.; Gladis, J. M. TrAC, Trends Anal. Chem. 2004, 23, 28−35. (20) Zhu, J. W.; Qin, Y.; Zhang, Y. H. Anal. Chem. 2010, 82, 436− 440. (21) Wulff, G.; Chong, B. O.; Kolb, U. Angew. Chem., Int. Ed. 2006, 45, 2955−2958. (22) Song, S. J.; Ruan, T.; Wang, T.; Liu, R. Z.; Jiang, G. B. Environ. Sci. Technol. 2012, 46, 13136−13143. (23) Ye, L.; Haupt, K. Anal. Bioanal. Chem. 2004, 378, 1887−1897. (24) Kou, L. J.; Liang, R. N.; Wang, X. W.; Chen, Y.; Qin, W. Anal. Bioanal. Chem. 2013, 405, 4931−4936. (25) Liu, H. P.; Tanaka, T.; Urabe, Y.; Kataura, H. Nano Lett. 2013, 13, 1996−2003. (26) Huang, H. H.; Niu, H.; Dong, J. Y. Macromolecules 2010, 43, 8331−8335. (27) Huang, Y. J.; Qin, Y. W.; Wang, N.; Zhou, Y.; Niu, H.; Dong, J. Y.; Hu, J. P.; Wang, Y. X. Macromol. Chem. Phys. 2012, 213, 720−728. (28) Dong, C.; Wang, N.; Qin, Y. W.; Niu, H.; Dong, J. Y. Macromol. Chem. Phys. 2014, 215, 1776−1784. (29) Wulff, G. Angew. Chem. 1995, 107, 1958−1979.

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CONCLUSIONS We have developed a novel polymeric membrane potentiometric sensor for detection of neutral BPAF using an s-MIP as the receptor. The proposed sensor shows an improved sensitivity for BPAF compared to the classical MIP-based sensor, which offers a detection limit of 60 nM for neutral BPAF. The sensor also exhibits an excellent selectivity over other phenols. It can be demonstrated that the proposed s-MIP would be an appealing substitute for the traditional rigid MIP in the development of potentiometric sensors. Since the proposed detection system has the flexibility of incorporating different MIPs for other analytical targets, we envision that the novel concept will be widely applicable for sensitive and selective detection of a wide range of targets not only for electrochemical but also for optical membrane sensors. In this early work, the sensing system was not yet optimized for optimal synthesis of MIPs, and there is significant scope to further develop such system to reach a lower detection limit. These efforts are currently in progress in our laboratory.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.7b03432. Experimental details including materials, EMF measurements, electrode renewal, synthesis conditions, as well as the characterizations of polymers (FT-IR and SEM) and sensors (sensitivity, regeneration, and application) (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *Fax: +86-535-2109000. E-mail: [email protected]. ORCID

Rongning Liang: 0000-0002-9693-1242 E

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Analytical Chemistry (30) Sarhan, A.; Wulff, G. Makromol. Chem. 1982, 183, 1603−1614. (31) Yin, J.; Yao, X. P.; Liou, J. Y.; Sun, W.; Sun, Y. S.; Wang, Y. ACS Nano 2013, 7, 9961−9974. (32) Fan, H. L.; Jin, Z. X. Macromolecules 2014, 47, 2674−2681. (33) Ito, T.; Radecka, H.; Tohda, K.; Odashima, K.; Umezawa, Y. J. Am. Chem. Soc. 1998, 120, 3049−3059. (34) Wang, X. W.; Ding, Z. F.; Ren, Q. W.; Qin, W. Anal. Chem. 2013, 85, 1945−1950. (35) Wang, T. T.; Liang, R. N.; Yin, T. J.; Yao, R. Q.; Qin, W. RSC Adv. 2016, 6, 73308−73312. (36) Yang, Y. J.; Yin, J.; Yang, Y.; Zhou, N. Y.; Zhang, J.; Shao, B.; Wu, Y. N. J. Chromatogr. B: Anal. Technol. Biomed. Life Sci. 2012, 901, 93−97.

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DOI: 10.1021/acs.analchem.7b03432 Anal. Chem. XXXX, XXX, XXX−XXX