Construction of Molecularly Imprinted Polymer Microspheres by Using

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Construction of Molecularly Imprinted Polymer Microspheres by Using Helical Substituted Polyacetylene and Application in EnantioDifferentiating Release and Adsorption Junya Liang,‡ Yi Wu,‡ and Jianping Deng* State Key Laboratory of Chemical Resource Engineering, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China S Supporting Information *

ABSTRACT: Chiral molecularly imprinted polymer microspheres (MIPMs) reported so far are majorly limited to being constructed by using achiral polymer together with chiral template. The present contribution reports on a unique type of chiral MIPMs consisting of chirally helical substituted polyacetylene, which are prepared through suspension polymerization by using (a)chiral acetylenics as monomer and chiral Boc-D/L-proline as template. The resulting MIPMs after removing the template show optical activity that is derived from the chirally helical structures of substituted polyacetylene. The microspheres demonstrate enantio-differentiating ability in releasing the enantiopure templates. A complete release of the template provides the chiral MIPMs. Worthy to mention is that the two chiral sources (chirally helical conformation and chiral template configuration) work in a synergistic way, obviously increasing the MIPMs’ enantiodiscrimination ability. The present study develops a strategy for preparing chiral MIPMs, which are expected to find significant applications in chiral separation, enantioselective release of chiral drugs, etc. KEYWORDS: chiral, enantioselective adsorption, helical polymers, molecular imprinting, templates

1. INTRODUCTION

Chirally helical polymers possess optical activity, chiral recognition,11 and the well-known “chiral amplification” effect,12,13 and have been broadly exploited in the last decades.14−16 Chirally helical polymers can be prepared by introducing chiral sources in the polymerization system, such as chiral monomer,17 chiral additive,18 and chiral solvent.19 Reportedly, chirally helical polymers with single helix,17,18 duplex helix,20 and supramolecular helix21−23 have been established. Moreover, chirally helical polymers have been applied to achieve chiral separation by enantioselective crystallization,24 chiral adsorption,25 and chiral release.26 Our group has focused on the synthesis of chirally helical substituted polyacetylenes and based on them a series of optically active materials have been fabricated including hybrid hollow chiral particles,17 chiral magnetic microparticles,27 and chiral nanoparticles/graphene oxide hybrid materials.28 The studies further promote us to apply chirally helical polymers toward MIP technique. In particular, the chiral source of MIPs used so far is generally limited to a single one (chiral configuration4 or chiral helical conformation29,30). Studies on MIPs with multichiral sources are hardly found in literature. We hypothesize that judiciously combining different chiral sources in one entity is expected to provide novel MIP materials. Accordingly, the

Molecular imprinting aims to develop tailored binding materials that have a “lock and key” relationship with a certain molecule (template).1 Molecularly imprinted polymers (MIPs), as a unique class of synthetic tailor-made polymers with the capability of selective recognition, have found various applications especially in molecule detection,2 (bio)sensors,3 drug delivery,4 and separation processes.5 Nowadays, molecular imprinting technique has been successfully combined with a variety of materials such as carbon-nanotubes,2 gold nanoparticles,3 and silver particles.6 Meanwhile, the template molecules also have been expanded from traditional small molecules7 to metal-ion complexes8 and polymers.9 When chiral molecules are used as template, the as-prepared MIPs possess potentials in chiral separation.10 To improve the efficiency and to widen the application range of MIPs in chiral separation, functional polymers were employed to achieve MIPs. Functional polymers, together with matching chiral templates, would realize the desired target in chiral separation. In the present contribution, we will report a type of molecularly imprinted polymer microspheres (MIPMs) that are constructed by the functional polymerschirally helical polymers, together with chiral template. Synergistic effect occurred between the helical polymer chains and the chiral cavities after removing the template, when the chiral MIPMs were used for chiral adsorption purpose. © XXXX American Chemical Society

Received: April 5, 2016 Accepted: April 27, 2016

A

DOI: 10.1021/acsami.6b04057 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces present work is aimed at synthesizing MIPMs consisting of chiral helical substituted polyacetylene with chiral template and studying the effect of the two chiral elements in enantioselective discrimination. Worthy to be highlighted is that when the handedness of the helical conformation matched the chiral template’s configuration, the two chiral elements worked synergistically, largely improving the chiral MIPMs’ enantioselectivity; however, when the polymer chains’ helicity did not match the template’s configuration, the two chiral elements would cancel out. This phenomenon helpfully enhances our understanding of the interaction between multichiral sources. Meanwhile, the present study also provides new materials for enantio-differentiating release and adsorption purposes.

Table 1. Microspheres Synthesized under Different Conditionsa sample P-M1D

P-M2D P-M2L P-M2LD

M1 (achiral, 0.100 g) M1 (achiral, 0.100 g) M1 (achiral, 0.100 g) M1 (achiral, 0.100 g) M1 (achiral, 0.100 g) M1 (achiral, 0.100 g) M1 (achiral, 0.100 g) M2 (chiral, 0.118 g) M2 (chiral, 0.118 g) M2 (chiral, 0.118 g)

MIPM-2D MIPM-2L MIPM-2LD

M2 (chiral, 0.118 g) M2 (chiral, 0.118 g) M2 (chiral, 0.118 g)

P-M2

M2 (chiral, 0.118 g)

P-M1L P-M1LD MIPM-1D MIPM-1L MIPM-1LD

2. EXPERIMENTAL SECTION

P-M1

2.1. Materials. Rhodium catalyst, (nbd)Rh+B−(C6H5)4 (nbd = 2,5norbornadiene) was prepared in a reported method.31 Achiral monomer (M1) was prepared according to a method reported early,32 and chiral monomer (M2, S) was synthesized as reported.33 A bifunctional butynyl ester (dibutynyl adipate) was prepared by referring to the method in literature34 and was used as cross-linker to prepare cross-linked polymer microspheres. Boc-D/L-proline and Boc-D/L-alanine were purchased from Aldrich and used as received. Polyvinylpyrrolidone (PVP K30) was purchased from Beijing Chemical Reagent Co. and used without further purification. Solvents were purified by distillation. Deionized water was used for the experiments. 2.2. Measurements. The morphology of the microspheres was observed with a Hitachi S−4800 scanning electron microscope (SEM). Circular dichroism (CD) and UV−vis absorption spectra were recorded on a Jasco-810 spectropolarimeter. Optical rotations were conducted on a JASCO P-1020 digital polarimeter with a sodium lamp (λ = 589 nm) as the light source at room temperature. FTIR spectra were measured on a Nicolet NEXUS 870 infrared spectrometer. Raman spectra were recorded on a Renishaw inVia-Refl exconfocal Raman microscope with an excitation wavelength of 785 nm. Thermogravimetric analysis (TGA) was carried out with a Q50 TGA at a scanning rate of 10 °C/min under N2. 2.3. Preparation of (a)Chiral Microspheres with Chiral Template Molecules. The achiral and chiral microspheres were synthesized by suspension polymerization of M1 (for achiral microspheres) and M2 (for chiral microspheres) with Boc-D-proline and Boc-L-proline as the chiral templates. Detailed information on the samples is shown in Table 1. Taking M1 with Boc-D-proline as an example, the major process for preparing the microspheres was described in Scheme 1 as follows. M1 (0.1 g, 0.5 mmol), dibutynyl adipate (0.0125 g, 0.05 mmol), and Boc-D-proline (0.02 g, 0.1 mmol) were dissolved in CHCl3 (1 mL) under ultrasonification for 1 min. The solution was added in a PVP aqueous solution (1 wt %, 50 mL) under vigorous stirring at a rate of 350 rpm with nitrogen atmosphere in ice bath. After 30 min, a solution of Rh catalyst (0.0026 g, 0.005 mmol, 0.5 mL CHCl3) was added in the above reaction system. The solution was maintained at 0 °C for 4 h. Subsequently the reaction system was heated at a rate of 8 °C/h until the temperature reached 30 °C, and kept for 4 h at the temperature to complete the polymerization and then to evaporate CHCl3 thoroughly by bubbling N2. The obtained yellow microspheres were collected after filtration, repeatedly washing by deionized water, and drying. The other microspheres were synthesized in the same way. We also prepared microspheres consisting of M1 or M2 without template molecules (denoted as P-M1 and P-M2, respectively). The microspheres were also subjected to enantioselective adsorption process, serving as control test. 2.4. Release of Chiral Template to Form Chiral MIPMs. The preprepared microspheres (P-M1D, P-M1L, P-M1LD, P-M2D, PM2L, P-M2LD, Table 1) were transformed to the corresponding MIPMs (MIPM-1D, MIPM-1L, MIPM-2D, MIPM-2L, MIPM-1LD, MIPM-2LD) after immersing into ethanol to remove template

monomer

template

removal of template

Boc-D-proline

no

Boc-L-proline

no

racemic Bocproline Boc-D-proline

no yes

Boc-L-proline

yes

racemic Bocproline none

yes

Boc-D-proline Boc-L-proline racemic Bocproline Boc-D-proline Boc-L-proline racemic Bocproline none

no no no yes yes yes

a

All the samples were synthesized with monomer (0.5 mmol), catalyst (0.0026 g, 0.005 mmol), cross-linker (0.0125 g, 0.05 mmol), template (0.02 g, 0.1 mmol), water−oil ratio (50/1.5 mL/mL) and stabilizer concentration (1 wt % PVP aqueous solution) under N2 at the stirring speed of 350 rpm. molecules. Herein we monitored the release process of the chiral template molecules as follows. The above microspheres with a certain amount (m) were immersed in ethanol (V) at room temperature. After a certain time of release, the ethanol solution was measured by UV−vis spectroscopy to determine the concentration of the solution (c). The release amount of the templates can be calculated by the following equation: release amount = cV/m. As P-M1LD and P-M2LD contained racemic Boc-proline as template, competitive release of template enantiomers was also measured by optical rotation to calculate the value of enantiomeric excess (ee), ee = α/αmax, where α is the measured optical rotation and αmax is the optical rotation of the pure enantiomer. After reaching the release equilibrium, the microspheres were furtherly subjected to dialysis to ensure the complete removal of templates. 2.5. Enantioselective Adsorption by Chiral MIPMs. The enantioselective adsorption by the MIPMs was tested in similar method as release process. Herein the adsorbate solutions are Boc-Dproline and Boc-L-proline in chloroform. With Boc-D-proline as an example, a chloroform solution of Boc-D-proline (c0 = 1 mg mL−1, V) was prepared. Subsequently, a predetermined amount of MIPMs (m) was encased in a filter paper and then immersed in the preprepared adsorbate solution. The solution was subjected to UV−vis spectroscopy to determine the concentration (ct) at certain intervals. Then, the chiral compound adsorbed by the microspheres can be determined according to the equation: enantioselective adsorption = V(ct − c0)/m. Herein, the ee value of adsorption toward racemic Boc-proline solution was measured as mentioned in Section 2.4.

3. RESULTS AND DISCUSSION 3.1. Characterization of (a)Chiral Microspheres with Chiral Template Molecules. In the present study, we designed and successfully prepared molecularly imprinted microspheres consisting of helical substituted polyacetylene using chiral template through catalytic suspension polymerization. The aims are to study the effects of different chiral B


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Scheme 1. Illustrative Strategy for Preparing the MIPMs and the Process for Enantioselective Release and Adsorption

elements (chiral template and chiral helical polymer) on enantioselective discrimination. The strategy is depicted in Scheme 1. The obtained microspheres (P-M1D, P-M1L, P-M2D, PM2L, P-M1LD, and P-M2LD) were first characterized by SEM to observe the morphology, as presented in Figure 1. Herein, SEM images of the microspheres containing Boc-D-proline are presented as representative because of the same morphology of the microspheres prepared with Boc-D-proline and Boc-Lproline separately as template. This is also true for the SEM images of MIPMs (as discussed below). SEM images of the microspheres in Figure 1 show the smooth surface of the microspheres and a mean diameter of approximately 300 μm. The polymers derived from M1 (achiral) and M2 (chiral) were proved to adopt helical conformations in our previous studies.32,33 Moreover, circular dichroism (CD) and UV−vis absorption spectroscopies have been proved as effective and straightforward methods to identify helical conformations in synthetic polymers. With the microspheres (P-M1D, P-M1L, PM2D, P-M2L, P-M1LD, and P-M2LD) in hand, we next characterized them by CD and UV−vis absorption spectroscopies, as illustrated in Figure 2. The microspheres without template (P-M1 and P-M2) were taken as control samples and also measured, as presented in Figure 2. Because the microspheres were cross-linked and did not dissolve in any solvent, they were qualitatively measured by pressing into a transparent film.25 Because of the different thicknesses of the sample films, the CD signals and UV−vis adsorption peaks show a slight variation. In Figure 2B, a broad UV−vis adsorption peak appeared in the spectra of P-M1D, P-M1L, P-M1LD, and P-M1 at wavelength about 375 nm due to the helical conformations formed in the substituted polyacetylene; however, no CD signal appeared at the corresponding wavelength (Figure 2A) because of the equal amount of left- and right-handed helical conformations.32 In addition, CD signals and corresponding UV−vis adsorption peaks appeared at about 200 nm in the spectra of P-M1D and P-M1L; and the CD signal was positive

Figure 1. SEM images of (A, C, E, G) microspheres with templates and (B, D, F, H) the corresponding surfaces. The microspheres with Boc-L-proline as template have the same morphology as the microspheres with Boc-D-proline, so the SEM images are omitted (the microspheres were directly stuck to the conductive adhesive on the sample stage and were dealt with by high vacuum gold sputtering before SEM observation).

in P-M1D but negative in P-M1L. Nevertheless, neither CD signal nor UV−vis adsorption peak appeared around 200 nm in P-M1’s spectra. These observations are attributed to the existence of Boc-D-proline in P-M1D and Boc-L-proline in PM1L. Moreover, P-M1LD had only UV−vis adsorption peak but no CD signal around 200 nm. This is because the template C


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Figure 2. (A) CD and (B) UV−vis spectra of microspheres derived from M1 (P-M1D, P-M1L, P-M1LD, MIPM-1D, MIPM-1L, MIPM-1LD, and PM1); (C) CD and (D) UV−vis spectra of microspheres derived from M2 (P-M2D, P-M2L, P-M2LD, MIPM-2D, MIPM-2L, MIPM-2LD, and PM2). The spectra were qualitatively measured by pressing samples into a transparent film at room temperature.

Figure 3. (A) Time-release profiles and (B) maximum release amount of template molecules from P-M1D, P-M1L, P-M2D, and P-M2L.

measurements. The FTIR spectra in Figures S1 show the successful synthesis of the microspheres (detailed analysis is presented in the Supporting Information). The Raman spectra in Figures S2 indicate the high cis contents and high stereoregularity of the polymer chains constructing the microspheres. The TGA results in Figures S3 illustrate that the thermal decomposition temperatures of achiral and chiral microspheres are similar, ranging from 330 to 380 °C. This demonstrates the high thermostability of the microspheres. 3.2. Release of Chiral Template to Form Chiral MIPMs. To obtain the final MIPMs, the above microspheres (P-M1D, P-M1L, P-M2D, P-M2L, P-M1LD, and P-M2LD) underwent releasing the template molecules. It should be noted that the release of monomers and oligomers can be neglected because of the quantitative yield of the microspheres and their good

in P-M1LD is racemic. In Figure 2C, D, a CD signal and a corresponding broad UV−vis adsorption peak appeared at about 400 nm, due to the polymer chains forming helical conformations with a predominant screw sense.33 Meanwhile, the CD signals and UV−vis adsorption peaks at 200 nm in the spectra of P-M2D, P-M2L, P-M2LD, and P-M2 are ascribed to the same reason as in the case of P-M1D, P-M1L, P-M1LD, and P-M1. Herein it is worth mentioning that Boc-D/L-proline did not perform as chiral additives to induce helix-sense-selective polymerization35 of the achiral monomer (M1), for no changes appeared in the CD spectra of P-M1D and P-M1L at 300−500 nm when compared to that of P-M1 (Figure 2A). The microspheres were then characterized by FTIR, Raman spectra, and TGA techniques, as shown in Figure S1−S3. For clarity, P-M1 and P-M2 were taken as examples to perform the D


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Figure 4. (A) Time-release profiles, (B) time-enantiomeric excess profiles, and (C) maximum release amount of racemic templates from P-M1LD and P-M2LD (solvent, EtOH; at r.t.).

cross-linking. Meanwhile, the cross-linking density of the microspheres is suitable for the later release and adsorption processes. The microspheres derived from M2 are expected to show enantioselective interaction toward a pair of enantiomers because of the helical polymer chains with predominantly handed screw sense, as proved above. This hypothesis was experimentally justified, as shown in Figure 3. The release rate of Boc-D-proline from P-M2D was higher than that of Boc-Lproline from P-M2L (Figure 3A), and the maximum release amount was also achieved in the former case (Figure 3B). This is most likely because the one-handed polymer helices in the microspheres derived from M2 formed stronger interaction toward Boc-L-proline than toward the other isomer.26 So Boc-Lproline rather than the D-isomer can combine tightly with the helical polymers chains. As a consequence, Boc-D-proline is easier to be released from P-M2D than Boc-L-proline from PM2L. On the contrary, the microspheres derived from achiral M1 have the racemic helical conformations as discussed earlier, so the chiral recognition of the left- and right-handed helical segments in the microspheres derived from M1 is equal. Consequently, the release ability toward the two enantiomeric templates in P-M1D and P-M1L is nearly the same, as demonstrated in Figure 3. Although the chiral recognition offset mutually, the macromolecular helices with opposite helicity in the microspheres still had enantioselectively binding force toward the corresponding Boc-proline enantiomer. So the maximum amounts of the templates released from P-M1D and P-M1L were lower than that in P-M2D, but higher than that in P-M2L. For the microspheres with racemic Boc-proline as template (P-M1LD and P-M2LD), the release results are shown in Figure 4. At initial stage, the release speed of P-M1LD was obviously faster than P-M2LD; and P-M1LD earlier reached the release equilibration in 2 h, whereas P-M2LD reached the equilibration in 3 h, as shown in Figure 4A. To further illustrate the enantioselective discrimination, we also measured the enantiomeric excess (ee) of the release process in Figure 4B. For P-M1LD, the ee value was kept constant across the whole release process (approximately 0). This is because the equal amount of left- and right-handed helical conformations makes P-M1LD fail to show enantioselectivity, so the racemic template (L and D) simultaneously released without competition. For M2LD, the result was quite different. The ee value increased rapidly in the first 2 h, and reached the maximum (49%) at 2.5 h, subsequently decreased slightly in 2.5−4 h, and then reached the final equilibration (34%). The ee value

remained positive during release, illustrating that the released substance was predominantly Boc-D-proline with positive optical rotation. As mentioned above, polymers derived from M2 have stronger binding force toward Boc-L-proline, so BocD-proline released faster at the initial stage until reaching the maximum ee value. Subsequently, the ee value decreased (2.5− 4 h), implying that the binding force between Boc-L-proline and the chirally handed macromolecular helicces was weaken by solvation and so Boc-L-proline predominantly released at this stage. In addition, Figure 4A, B shows that the equilibration time of the ee value lagged behind that of the release amount. It is speculated that there existed an exchange process of enantiomers after release equilibration. According to the results, P-M1LD has higher saturated release amount than PM2LD, but P-M2LD has more remarkable enantioselectivity (Figure 4C). When the release process reached equilibrium, the ratio of the release amount to the originally loaded value was calculated. The release ratios of P-M1D, P-M1L, P-M2D, P-M2L, PM1LD, and P-M2LD were 74, 74, 85, 14, 76, and 58%, respectively. So it is obvious that the microspheres still held template residues. To completely remove the template molecules, the microspheres after reaching release equilibrium were further repeatedly subjected to dialysis until the optical rotation reached zero in the eluates. Thereby the corresponding MIPMs were obtained: MIPM-1D, MIPM-1L, MIPM-2D, MIPM-2L, MIPM-1LD, and MIPM-2LD (see Table 1). The MIPMs were also characterized by SEM, CD, and UV− vis spectroscopies. The SEM images of the MIPMs are shown in Figure 5. The MIPMs show no change in diameter after removal of templates, but slight pores appeared on the surface of MIPMs (Figure 5B, D, F, H). This is because the template molecules aggregated and then, together with the solvent, released out of the microspheres in the release process. This process undoubtedly resulted in the formation of the pores. We also attempted to measure the BET surface area, pore volume, and pore size distribution by nitrogen adsorption isotherm. However, the specific surface area of the MIPMs was found to be too small, at the same level as test error. Therefore, BET approach is not applicable in this study. The CD and UV−vis spectra of the MIPMs are shown in Figure 2. After releasing the template molecules, the CD signal and UV−vis adsorption peak at the wavelength of 200 nm disappeared in the spectra of the MIPMs, further demonstrating the total removal of the template molecules from the microspheres after dialysis. Therefore, the obtained MIPMs were considered no longer containing template molecules. E


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3.3. Enantioselective Adsorption by Chiral MIPMs. The obtained MIPMs have two favorable factors potentially contributing to enantioselective adsorption. One is the stereoselective cavities left after removing the template.36 The other is the one-handed helical conformations of the polymer chains in the microspheres.25 Herein, the enantioselective adsorptions of the MIPMs (MIPM-1D, MIPM-1L, MIPM-2D, MIPM-2L, MIPM-1LD, and MIPM-2LD) toward Boc-Dproline and Boc-L-proline were systemically measured, as shown in Figures 6, 7, and Figure S4. As a comparison, the microspheres without template molecules (P-M1 and P-M2) were also tested, as shown therein. For enantioselective adsorption of MIPM-1D, MIPM-1L, and P-M1 (Figure 6), MIPM-1D showed a preferential adsorption toward Boc-D-proline while MIPM-1L preferred Boc-L-proline, however P-M1 did not show any selectivity. The two MIPMs showed enantioselectivity toward their respective templates, because of the cavities that the template molecules left. The stereospace in chiral helical polymer chains would bind a certain enantiomer. In this case, the equal amount of leftand right-handed helices in microspheres derived from M1 had equal binding force toward L and D enantiomers, which makes the enantioselective recognition ability canceled out; so the helical structures hardly contributed to the enantioselective discrimination observed in the MIPMs. This is in accordance with the adsorption results of P-M1 containing no template molecules in the preparation process (Figure 6C, D). Therefore, the enantio-excess in adsorption is attributed to the successful imprinting. In addition, the adsorption result of MIPM-1LD (Figure S4) is similar to P-M1’s, illustrating that

Figure 5. SEM images of (A, C, E, G) MIPMs and (B, D, F, H) the corresponding surfaces. The MIPMs with Boc-L-proline as template have the same morphology as the microspheres with Boc-D-proline, so the SEM images are omitted (the microspheres were directly stuck to the conductive adhesive on the sample stage and were dealt with by high vacuum gold sputtering before SEM observation).

Figure 6. Enantioselective adsorption of (A) MIPM-1D, (B) MIPM-1L, (C) P-M1 toward Boc-D-proline and Boc-L-proline, respectively; (D) saturated adsorption amount of MIPM-1D, MIPM-1L and P-M1 toward the enantiomers. (c = 1 mg mL−1, CHCl3, r.t.). F


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Figure 7. Enantioselective adsorption of (A) MIPM-2D, (B) MIPM-2L, and (C) P-M2 toward Boc-D-proline and Boc-L-proline, respectively; (D) saturated adsorption amount of MIPM-2D, MIPM-2L and P-M2 toward the enantiomers (c = 1 mg mL−1, CHCl3, r.t.).

enantioselective adsorption ratio of MIPM-2LD (2.2) is also lower than that of MIPM-2L (5.8). A combination of the above results illustrates that the polymer helical structures and template cavities can simultaneously improve the enantioselective capacity. To further get a deep insight into the roles of helical structure and template cavity, we chose the MIPMs with single template (MIPM-1D, MIPM-1L, MIPM-2D, MIPM-2L) to carry out the enantioselective adsorption toward Boc-D/Lalanine, as demonstrated in Figure 8. Some adsorption results toward Boc-D/L-proline in Figures 6 and 7 are also included in Figure 8 for a vivid comparison. For the microspheres derived from achiral monomer, MIPM-1D and MIPM-1L, the different adsorption amount toward Boc-D-alanine and Boc-L-alanine was not significant. However, for the microspheres derived from

the racemic template is invalid for enantioselectivity. This also supports the above explanation. Enantioselective adsorption of MIPM-2D, MIPM-2L, and PM2 is illustrated in Figure 7. MIPM-2L and P-M2 showed preferential adsorption toward Boc-L-proline, whereas MIPM2D did not show any selectivity. As discussed above, the onehanded helical structures in microspheres consisting of M2based polymer had stronger binding force with Boc-L-proline. So just as expected, P-M2 preferably adsorbed Boc-L-proline, as shown in Figure 7C, D. In the case of the adsorption of MIPM2D in Figure 7A, D, the template cavities had a trend to absorb Boc-D-proline, which is opposite to the effect of one-handed helical structures. As a consequence, the adsorption of MIPM2D toward Boc-D-proline and Boc-L-proline was almost the same and MIPM-2D did not show obvious enantioselective discrimination. However, in the case of MIPM-2L in Figure 7B, D, the helical structures and the template cavities both had a trend to absorb Boc-L-proline. As a consequence, they performed a synergistic adsorption in MIPM-2L and thus a faster adsorption rate toward Boc-L-proline than toward Boc-D-proline was observed in this case. As a control test, the adsorption of PM2 is shown in Figure 7(C, D). As no template molecules were added in P-M2, only the one-handed helical polymer chains worked in the enantioselective adsorption process. As a consequence, the enantioselective adsorption ratio of P-M2 toward Boc-L-proline to Boc-D-proline (1.9) is obviously lower than the ratio of MIPM-2L (5.8). In addition, the enantioselective adsorption ability of MIPM-2LD was also measured, as shown in Figure S4. As racemic Boc-proline worked as the template in MIPM-2LD, the cavities are not enantioselective. This case is similar to P-M2. The

Figure 8. Saturated adsorption amount of MIPM-1D, MIPM-1L, MIPM-2D, and MIPM-2L toward Boc-D/L-proline and Boc-D/Lproline solution, respectively. (c = 1 mg mL−1, CHCl3, r.t.). G


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Figure 9. (A-a) Time-adsorption profile and (A-b) time-ee profile of MIPM-2L toward racemic Boc-proline solution; (B-a) the saturated adsorption amount and (B-b) ee value of 5 times’ cycle in MIPM-2L (c = 1 mg mL−1, CHCl3, r.t.).

4. CONCLUSIONS In summary, molecularly imprinted polymer microspheres (MIPMs) were successfully prepared by suspension polymerization of achirally and chirally substituted acetylenic monomers in the presence of template molecules, Boc-D/Lproline. The microspheres were obtained in regular morphology and the polymer chains in the microspheres were found to adopt helical conformations. The chiral microspheres enantioselectively released the chiral templates. The MIPMs derived from achiral monomer possessed only limited enantioselective adsorption toward Boc-D/L-proline. However, in one of the MIPMs derived from chiral monomer (MIPM-2L), we found that the enantioselective binding sites formed by the templates and the chirally helical macromolecular conformations worked synergistically in the chiral adsorption process. The synergistic effect largely increased the chiral MIPMs’ enantioselective adsorption ability. Furtherly, the MIPMs demonstrated the desirable reusability. The present study establishes a strategy for preparing chiral MIPMs with multichiral sources. In addition, after optimizing the polymerization method, chiral MIPMs with the size of 1−10 μm are highly anticipated, which potentially can be used as stationary phase. Our research is currently ongoing along this direction.

chiral monomer, MIPM-2D and MIPM-2L, the adsorption toward Boc-L-alanine was obviously larger than that toward Boc-D-alanine. These results were clearly different from the adsorption toward Boc-D/L-proline, as summarized in Figure 8. As discussed above, both helical structure and template cavity worked in the enantioselective adsorption toward Boc-D/Lproline, whereas for Boc-D/L-alanine, only helical structure played the dominant role, because the molecular size of Boc-D/ L-alanine is not suitable for the template cavities, namely, they are not matching in stereoconfiguration. Therefore, the enantioselectivity of template cavities is invalid toward Boc-D/ L-alanine. However, both MIPM-2D and MIPM-2L can enantioselectively adsorb Boc-L-alanine. This is because the one-handed helical structures dominate the adsorption results. The helical structures are still effective in this case because of both the chiral stereospaces in the helical grooves and the formation of hydrogen bonding between the helical main chains and the adsorbate. Given the above, the enantioselectivity from the template cavity has a requirement for the molecular size of adsorbates, while the enantioselectivity from helical structures shows a wider applicability. In addition, Figure 8 also shows that the adsorption amount of the four MIPMs toward Boc-proline is much larger than toward Boc-alanine, and especially the enantioselectivity of MIPM-2L toward Boc-D/L-proline is obviously better than toward Boc-D/L-alanine. This further demonstrates that when helical structures and template cavities are judiciously utilized together in the adsorption process, a positive synergistic effect can come into play. To test the MIPMs’ potential applications, we chose MIPM2L with the best enantioselectivity as discussed above to carry out the adsorption test in racemic Boc-proline solution. The relevant results are displayed in Figure 9. The adsorption amount and enantiomeric excess were measured simultaneously. In the racemic solution, the adsorption amount reached equilibration in 2 h, and the saturated adsorption amount was ∼530 mg/g, without desorption. However, the equilibration of ee lagged behind the adsorption amount’s. The final ee value was ∼57% (Figure 9A). In addition, to examine the reusability of the MIMPs, MIPM-2L underwent an adsorption−desorption process, in which the desorption was accomplished by dialysis approach. This process was repeated for 5 times. Figure 9B depicts the related recycling results. After 5 times’ cycle, the adsorption amount and ee value slightly decreased by 4.2 and 5.8%, respectively. Accordingly, the MIPMs are reusable, possessing significant potentials in enantioselective adsorption.

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ASSOCIATED CONTENT

* Supporting Information S

These materials are available free of charge via the Internet at The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b04057. FTIR spectra, Raman spectra, TGA curves of the microspheres, and enantioselective adsorption of MIPM-1LD and MIPM-2LD (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86-10-6443-5128. Fax: +86-10-6443-5128. Author Contributions ‡

J.L. and Y.W. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21474007, 21274008), the Funds for H


Research Article


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Creative Research Groups of China (51221002), and the “Specialized Research Fund for the Doctoral Program of Higher Education” (SRFDP 20120010130002).

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Construction of Molecularly Imprinted Polymer Microspheres by Using

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