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Analytica Chimica Acta journal homepage: www.elsevier.com/locate/aca

In situ preparation of powder and the sorption behaviors of molecularly imprinted polymers through the complexation between polymer ion of methyl methacrylate/acrylic acid and Ca++ ion Sung Hyo Chough a , Kwang Ho Park a , Seung Jin Cho b , Hye Ryoung Park a, * a b

Department of Chemical Engineering and Research Institute for Catalysis, Chonnam National University, Gwangju 500-757, South Korea Medical and Healthcare Business Center, Sharp Corporation1-9-2 Nakase, Mihamaku, Chibashi, Chiba 261–0852, Japan

H I G H L I G H T S

G R A P H I C A L A B S T R A C T

 In situ MIP powder was prepared by the complexation between polymer anion and metal cation.  The powders were obtained through solvent/non-solvent process, and maintained original binding sites without cracking.  The obtained MIP powder were very porous to give higher uptakes.  Possible binding types formed in this procedure were considered.

A R T I C L E I N F O

A B S T R A C T

Article history: Received 7 January 2014 Received in revised form 14 May 2014 Accepted 25 May 2014 Available online xxx

Molecularly imprinted polymer (MIP) powders were prepared using a simple complexation strategy between the polymer carboxylate groups and template molecule followed by metal cation cross-linking of residual polymer carboxylates. Polymer powders were formed in situ by templating carboxylic acid containing polymers with 4-ethylaniline (4-EA), followed by addition of an aqueous CaCl2 solution. The solution remained homogeneous. The powders were prepared by precipitation by slowly adding a nonsolvent, H2O, to the mixture. The resulting particles were very porous with uptake capacity that approached the theoretical value. We suggest two types of complexes are formed between the template, 4-EA, and polymer. The isolated entry type forms well defined cavities for the template with high specific selectivity, while the adjacent entry type forms wider binding sites without specific sorption for isomeric molecules. To evaluate conditions for forming materials with high affinity and selectivity, three MIPs were prepared containing 0.5, 1.0, and 1.5 equivalents of template to the base polymer. The MIP containing 0.5 eq showed higher specific selectivity to 4-EA, but the MIP containing 1.5 eq had noticeably lower selectivity. The lower selectivity is attributed to poorly formed binding sites with little selective sorption to any isomer when the higher ratio of template was used. However at the lower ratio of template the isolated entry is preferably formed to produce well defined binding cavities with higher selectivity to template. ã 2014 Elsevier B.V. All rights reserved.

Keywords: Moleculaly imprinted polymer (MIP) In situ MIP powder Complexation of MIP, Polymer metal ion complex Ethylaniline Selective sorption

1. Introduction * Corresponding author. Tel.: +82 62530 1885; fax: +82 62530 1889. E-mail address: [email protected] (H.R. Park).

Molecular imprinting is an attractive technique to make specific recognition sites for a target molecule in a polymer. This technique

http://dx.doi.org/10.1016/j.aca.2014.05.044 0003-2670/ ã 2014 Elsevier B.V. All rights reserved.

Please cite this article in press as: S.H. Chough, et al., In situ preparation of powder and the sorption behaviors of molecularly imprinted polymers through the complexation between polymer ion of methyl methacrylate/acrylic acid and Ca++ ion, Anal. Chim. Acta (2014), http://dx. doi.org/10.1016/j.aca.2014.05.044

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is widely used to prepare materials for separation processes due to the high selectivity to a target molecule, and in research of the artificial enzymes and antibodies, synthetic receptors, and molecular catalyst and sensors [1–11]. Molecularly imprinted polymers (MIPs) are generally prepared through co-polymerization of a functional monomer and divinyl monomer as a crosslinker in the presence of the template. Functional monomer interacts with complementary functional groups of the template molecule to form a complex. The cross-linking monomer functions to solidify a three-dimensional cavity for the template in the MIPs. In general, most researches have used a very large portion of the cross-linker to obtain the specific rigid binding cavities. The rigidity makes it very difficult to remove the template from the MIPs. Thus, MIPs are used as in the form of fine particles by grinding and sieving for the effective extraction of the template. The number of molecular recognition cavities in the particles becomes much smaller than the number of cavities calculated from the amount of the template used because some of the binding sites are buried in the MIP even in the powdered state. Practically, the binding sites existing only on the surface of the particles are used for the rebinding of the template because of the difficulty of diffusion into the rigid polymer. These processes have a number of drawbacks such as the irregularity of the distribution of the binding sites, low binding capacity and selectivity due to poorly formed binding sites produced on the surface of the particles. Their preparation is also tedious and time consuming. In order to overcome these drawbacks, there are many modifications proposed to increase the number of binding sites on the surface of the particles through synthesis of monodisperse microspheres for MIP preparation [12–16], by methods to produce binding sites on the surface of particles [17–20], and by using an appropriate porogen to make highly porous particles by polymerization [21–23]. However, these processes have their own complications including separation of the particles from solvent, removal of all stabilizing agents after polymerization, or grinding and sieving to obtain fine particles, etc. There are several researches to prepare MIP thin films to separate enantiomers of amino acid by the phase inversion process [24,25]. In this work, we prepared MIP powders in situ by a very simple method using complex formation between polymer carboxylate anions and a metal ion (Ca2++). This was conducted by mixing two solutions of polymer including the template and a metal ion in a co-solvent system, and precipitating the polymer by the addition of a non-solvent to obtain porous particles. This method does not need labored grinding and sieving processes to obtain high porous powders, and can have highly binding capacity with relatively high selectivity. 2. Experimental 2.1. Materials Acrylic acid (AA), methyl methacrylate (MMA), 2,20 -azobis isobutyronitrile (AIBN), ethyl aniline (EA) isomers (2-. 3-, 4-), HCl, CaCl2.2H2O (purity 98 +%), ethyl alcohol, and 1,4-dioxane are purchased from Aldrich. Solvents were HPLC grades. The materials were used without further purification. 2.2. Determination of the amount of template bound by the polymer The base polymer was prepared by bulk polymerization using a mixture of 5 mol of MMA and 3 mol of AA with 0.2 wt% of AIBN to the total weight of the monomers in water bath at 70  C. The base polymer (5MMA-3AA, FW 716) will contain on average 3 carboxyl groups in one repeat unit. One equivalent was regarded as an amount to make the imprint complex

Table 1 Compositions of MIP preparation.

MIP 0.5 eq (FW: 763.5) MIP 1 eq (FW: 754.5) MIP 1.5 eq (FW: 725.5) NIP 1 eq (FW > 754.5)

5MMA-3AA(FW:716)(g)

4-EAa (g)

CaCl2b (g)

5

0.422

1.395

5

0.845

0.929

5

1.267

0.465

5



0.929

a

The values are amount bound to the base polymer (5MMA-3AA). The values are used amounts containing 20% excess to the theoretical value to form Ca-carboxylate complex with the remnant carboxyl groups after complex formation between the template and base polymer. b

corresponding to one third of the carboxylates in the base polymer (5MMA-3AA). One equivalent of template to 5 g of the base polymer corresponds to 0.845 g of ethylaniline (FW 121). Prior to cross-linking with CaCl2, the template amount bound to the base polymer was investigated at various template concentrations from 1 eq to 5 eq. The procedures were as follows: (1) dissolve 0.5 g of the base polymer in 4 mL dioxane in each vial, (2) add 1 mL of dioxane solution containing 1 eq mL1 to 5 eq mL1 concentration of 4ethylaniline, respectively, and agitate for 5 h, (3) then add 6 mL of H2O slowly to the mixture to precipitate with agitation, (4) measure the concentration of the filtrate by UV spectrometer at the wavelength of 286 nm for 4-EA. 2.3. Preparation of MIP The base polymer(5MMA-3AA) of 5 g was dissolved in 50 mL of dioxane, and then mixed with the appropriate amount of 4ethylaniline dissolved in dioxane solution to adsorb the needed equivalent of the base polymer as in Table 1. After agitating for 5 h to form the complex between template and base polymer, 2 mL of an aqueous solution of calcium chloride was added containing 20% excess to the theoretical amount to form a complex with the remaining carboxyl groups. After stirring overnight, a non-solvent, H2O, was added drop by drop to the solution under stirring until the precipitation was finished. The precipitation began from the volume ratio of 3/10 of water/dioxane, and was obtained in a powdered state. The powdered MIP(5MMA-3AA-1Ca; FW754) was obtained after removing the template by washing with 0.1 N HCl and 10% ethanol alternately several times until ethylaniline was not detected by UV, then rinsed with water and dried at 50  C under vacuum. This was designated as MIP 1 eq. By the same procedure, MIPs of MIP 0.5 eq containing 1.25 atoms of Ca and MIP 1.5 eq containing 0.25 atoms of Ca were prepared. The control non imprinted polymer(NIP) was prepared under the same conditions as the MIP without template. The whole processes is illustrated in Scheme 1. 2.4. Uptakes and selectivity The uptake of EA isomers was measured by UV spectrophotometer at 280 nm for 2-EA and 282 nm for 3-EA, respectively, after sufficient time to achieve equilibrium for a day. MIP(50 mg) was added into a conical flask with 20 mL of aqueous template solution containing 0.5, 1, 1.5, 2, 2.5, 3 and 4 eq, respectively, estimated from the amount of MIP. The uptake (Q) was calculated with the following equation: Q¼

VðC O  C i Þ m

(1)

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MMA + AA

3

Polymerization

COOH COOH COOH

1. Dissolve in Dioxane 2. Add 4-ethylaniline

C O OH NH2

O

HO NH2

Add CaCl2 solution below Dioxane/H2O = 10/3 COOH HOOC CO O

Ca

OC O

1. Add H2O to precipitate 2. Remove template

C O OH

CO O

Ca

OC O

Scheme 1. Procedure for MIP preparation through the complexation between polymer anion and metal ion.

where C0 (g mL1) is the initial concentration, Ci (g mL1) is the concentration at sorption equilibrium, V (mL) is the volume used at the sorption measurement, m (g) is the weight of the MIP. Selectivity was defined as the ratio of uptakes of competitor, 2or 3- isomer, with respect to 4-ethylaniline used as the template at the same concentration. 3. Results and discussion 3.1. Determination of the amount of template bound to the polymer Since MIPs were prepared using complexation between template and base polymer, the bound amount of template should be determined to evaluate the theoretical binding sites in MIPs. The results are shown in Fig. 1. The base polymer adsorbed nearly half of the template of the concentration of mother solution. Using Table 1 and the result of Fig. 1, we prepared MIPs of MIP 0.5 eq, MIP 1 eq,and MIP 1.5 eq, respectively.

3.2. Sorption behavior of MIP 1 eq All MIPs in this work were obtained in the powdered state precipitated by the addition of non-solvent, H2O, slowly under agitation to the dioxane solution containing base polymer, template, and CaCl2. This method was very simple to obtain MIP powders compared to the general method of MIP preparation with a cross-linker. The MIP 1 eq can theoretically have 1 equivalent of binding sites since it was prepared to bind one equivalent of template molecule. The NIP was prepared without template but under the same conditions as MIP 1eq. The sorption characteristics of the MIP 1 eq and NIP to ethylaniline isomers are shown in Fig. 2. The uptake of 4-ethylaniline, the template molecule, was up to 0.9 EQ which was 90% of the theoretical value. The EQ was defined as the dimensionless ratio of the experimental uptake to the theoretical value calculated from amount of the template used in the complex formation with base polymer. This is very high sorption compared to MIPs prepared by conventional cross-linking

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3.0

Uptake (EQ)

2.5

2.0

1.5

1.0

0.5

0.0 0

1

2

3

4

5

6

Template concentration(eq/5mL dioxane) Fig. 1. Determination of uptake of 4-EA to the base polymer, 5MMA-3AA, in dioxane solution. 1 eq for 50 mg of base polymer is corresponding to 8.45 mg EA.

agents which generally have lower uptake to the theoretical value [13]. In the case of MIPs with cross-linker using appropriate porogen, the uptake can be higher due to the development of

1.0 2-EA 3-EA 4-EA

Uptake(EQ)

0.8

0.6

0.4

0.2

0.0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Concentration(eq/20mL H2O/50mg MIP)

OhPtpwG 0.8 0.7

2-EA 3-EA 4-EA

Uptake(EQ)

0.6 0.5 0.4 0.3 0.2 0.1 0.0 0

1

2

3

4

5

Concentration(eq/20mL H2O/50mg MIP)

OiPupwG Fig. 2. Uptakes of EA isomers to the MIP (MIP 1 eq) and NIP. The theoretical maximum uptake for 50 mg of the above MIP will be 8.02 mg EA which can be calculated by (0.845)(716)/(754.5). The concentration of 8.02 mg/20 mL H2O/50 mg MIP was considered as 1 eq. EQ was the ratio of experimental uptake to the theoretical maximum uptake.

micro-pores, but some binding sites can be still buried in the particle interiors However, in the present method, more binding sites in the MIP particles may accessible due to the slow precipitation of the homogeneous complex solution by the solvent/non-solvent procedure. In addition, the MIP particles have a highly developed sponge-like micro-porosity permitting target molecules to penetrate into the particles shown in the SEM micrographs Fig. 3. Thus the uptake was high and close to the theoretical value. However, uptake of 3-EA saturated at 0.57 eq, and 2-EA saturated at 0.3 eq, based on the onset points of the plateau of the sorption curve at the relatively lower concentration of 3 eq and 2 eq, respectively. This corresponds to 2-EA occupying approximately 30% of the binding sites of 4-EA imprinted polymer and 3-EA occupying approximately about 60% of the sites. Saturation was observed in both cases since the sorption curves did not increased at higher concentration. The uptake of EA will be affected mainly by the geometrical shapes of binding sites in which a molecule can be accommodated, the interaction between amine of EA and carboxyl groups of the polymer, and hydrophobic interaction between the polymer and EA. Since the interaction between amine and carboxyl groups will be very similar regardless of EA isomers, the uptakes can be mainly attributed to the shape factor of the binding sites. As the MIP was prepared by forming a complex with Ca++ ion after mixing polymer and template in a solvent, the shapes of binding sites could be formed by intra- and inter- chain complexation. However, we propose four types as shown in Scheme 2. Complex formation between the base polymer and template can adapt two types, the isolated entry type (I) and the adjacent entry type (II). We proposed the isolated entry can form type A and type B binding domains, and the adjacent entry can form type C and type D domains by addition of CaCl2. By viewing the shape and molecular structure of EA, type A may geometrically have a specific adsorption to 4-EA, and type B may be selective to both 3- and 4-EA, but may not bind 2-EA which has the bulkier ethyl group adjacent to the amine. 2-EA will be usually adsorbed at the sites being very shallow in which carboxyl groups were nearly exposed. The type C and D have wide binding sites that have no specific adsorption behavior due to sufficient space to accommodate any isomer. In type C and D, the first bound molecule can interfere with the second molecule in the rebinding sorption process. Thus the ratio of type C and D can contribute to adsorption less than the theoretical amount in the rebinding process. The bound amount and selectivity may be dependent on the distribution of type A–D in the MIP. 4-EA can be adsorbed by all types of the binding shapes, 3-EA can be bound to type B–D, and 2EA can be bound to type C and D. Thus, the uptake of 4-EA can be higher, but 2-EA could have a lower uptake as in Fig. 2(A). In addition, since some of the Ca-carboxylate can be dissociated, the dissociated carboxyl groups can also nonspecifically adsorb any isomeric EA but the amount may be negligible. For the NIP shown in Fig. 2(B), the sorption of each isomer was very similar, and the uptake was very high, up to 75%. The NIP is also be very porous as a result of stacking fine particles during the precipitation. The free carboxyl groups will be randomly oriented and not form any specific binding sites because the amount of Ca ion is much lower than the amount of carboxyl groups for the exact complexation as shown in Table 1. The uptake curves of 3- and 4-EA overlapped. This implies that most of the binding sites could accommodate either isomers without any preference, but not for 2-EA. It may also be possible that the solvent, dioxane, can be imprinted more or less by the hydrogen bond with carboxyl groups. The binding site formed by dioxane could be shallow like type B which would be more suitable for 3- and 4-EA rather than 2EA, in view of their molecular size. Since 2-EA will be adsorbed mainly at the free carboxyl groups and not at the sites formed by dioxane, the uptake can be lower than other isomers.

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Fig. 3. FE-SEM micrographs of MIP(MIP 1eq) powder.

The selectivity is an important factor for the separation of each isomer. When the selectivity of an isomer is higher, the separation can be better. The selectivity of the each isomer was compared in Fig. 4 with respect to the template, 4-EA. The selectivity was 45– 40% for 2-EA, and 80–70% for 3-EA, respectively. To increase the selectivity of the template, the MIP should have higher portion of the specific binding sites for template. As discussed already, type A sites should have more specific binding to template (4-EA). A higher portion of type A sites should improve selectivity. However, because it is difficult to control directly the formation of type A binding sites, we instead attempted to decrease the formation of the adjacent entry type. When the ratio of the adjacent entry type is reduced, the formation of type C and D that gives rise to non specific adsorption is reduced. When a smaller amount of template is used, the probability of adjacent entry type sites is expected to decrease statistically. As the ratio of adjacent entry sites is

decreased, the ratio of type A formation is relatively increased. Thus we prepared two MIPs containing 0.5 and 1.5 equivalents of template to the base polymer, designated as MIP 0.5 eq and MIP 1.5 eq, respectively. 3.3. Sorption behavior of MIP 0.5 eq and MIP 1.5 eq Uptakes were shown for MIP 0.5 eq and MIP 1.5 eq in Figs. 5 and 6. The uptake of template, 4-EA, was much higher than other isomers in Fig. 5, while the uptake for 2- and 3-EA increased compared to the template (Figs. 2 and 6). The results imply that the type A and B from the isolated entry were formed preferentially at a lower ratio of template to the base polymer. When the probability of adjacent entry formation was decreased, the formation of less selective binding sites such as type C and D were decreased. In the case, uptakes of 2- and 3-EA were decreased to give higher

Scheme 2. Expected main types of binding sites formed with Ca++ ion after complex formation between 4-EA and base polymer.

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1.2 1.2

2-EA(MIP0.5Eq) 3-EA(MIP0.5Eq) 4-EA(MIP0.5Eq)

2-EA(MIP1.5Eq) 3-EA(MIP1.5Eq)

1.0 1.0

Selectivity

Selectivity

0.8

0.6

0.8

0.6

0.4 0.4 2-EA 3-EA 4-EA

0.2

0.2

0.0 0

1

2

3

4

0.0 0.5

5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Concentration(eq/20mL H2O/50mg MIP)

Concentration(eq/20mL H2O/50mg MIP) Fig. 4. Selectivity of EA isomers for MIP 1 eq.

Fig. 7. Selectivity comparison of EA isomers for MIP 1.5 eq and MIP 0.5 eq.

1.0

2-EA 3-EA 4-EA

0.8

Uptake(EQ)

form type C and D, by the increased amount of template in the process of complex formation with the base polymer. The selectivity was compared for MIP 0.5 eq and MIP 1.5 eq in Fig. 7. The selectivity was 25% for 2-EA and 47% for 3-EA at MIP 0.5 eq, and 55–60% for 2-EA and 87–90% for 3-EA. When a smaller portion of template was used, the selectivity of the template was markedly increased. It can be reasoned that the formation of the less selective sorption sites is decreased.

0.6

0.4

4. Conclusions 0.2

0.0 0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Concentration (eq/20mL H2O/50mg MIP) Fig. 5. Uptakes of EA isomers to the MIP (MIP 0.5 eq). The theoretical maximum uptake for 50 mg of the above MIP will be 3.96 mg EA which can be calculated by (0.422)(716)/(763.5). The concentration of 3.96 mg/20 mL H2O/50 mg MIP was considered as 1 eq. EQ was the ratio of experimental uptake to the theoretical maximum uptake.

selectivity for 4-EA due to the higher formation of type A as in MIP 0.5 eq. At the MIP 1.5 eq, uptakes of 2- and 3-EA increased. This could be explained by an increase of the adjacent entry sites to

MIP powders prepared in situ by a simple complexation procedure, between carboxylate anions contained in the polymer, an amine containing template and metal cations. Porous polymer powders were prepared by precipitation using a solvent/nonsolvent treatment. The target or imprint molecules could easily penetrate into the MIP and gave an uptake close to the theoretical value. Two types of complexes between the base polymer and template are proposed, isolated entry and adjacent entry. To investigate the ratio of the adjacent entry formation, three MIPs were prepared containing 0.5, 1.0, and 1.5 equivalents of template to the base polymer. The MIP of 0.5 eq showed higher specific selectivity to 4-EA, but the MIP of 1.5 eq had noticeably lower selectivity. The results suggest that the isolated entry can form suitable binding sites for the template, while the adjacent entry sites can have low specific sorption of an isomer.

0.9

Acknowledgements

2-EA 3-EA 4-EA

0.8

We are grateful to Professor Kenneth J. Shea of Chemistry Department, University of California, Irvine for his careful review, illuminating comments and fruitful correspondence, and to unkown reviewers to be refined paper with the excellent suggestions. This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF2012R1A1A3002725).

Uptake(EQ)

0.7 0.6 0.5 0.4 0.3 0.2

References

0.1 0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Concentration(eq/20mL H2O/50mg MIP)

Fig. 6. Uptakes of EA isomers to the MIP (MIP 1.5 eq). The theoretical maximum uptake for 50 mg of the above MIP will be 12.50 mg EA which can be calculated by (1.267)(716)/(725.5). The concentration of 12.50 mg/20 mL H2O/50 mg MIP was considered as 1 eq. EQ was the ratio of experimental uptake to the theoretical maximum uptake.

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acrylic acid and Ca++ ion.

Molecularly imprinted polymer (MIP) powders were prepared using a simple complexation strategy between the polymer carboxylate groups and template mol...
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