Conductive and Chiral Polymer Modified Metal-Organic Framework for

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Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 26365−26371

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Conductive and Chiral Polymer-Modified Metal−Organic Framework for Enantioselective Adsorption and Sensing Xudong Hou, Tingting Xu, Yang Wang, Shengjun Liu, Runrun Chu, Junxiang Zhang, and Bo Liu* Hefei National Laboratory for Physical Sciences at the Microscale, Department of Chemistry, Hefei Science Center of CAS, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China

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S Supporting Information *

ABSTRACT: We reported integration of conductivity, chirality, and porosity into MIL-101@chiral-PANI composite for synchronous chiral recognition, adsorption, and sensing toward enantiomers. The core−shell structure of MIL-101@chiral-PANI was characterized in detail by Fourier transform infrared and circular dichroism spectroscopy as well as scanning electron microscopy and transmission electron microscopy. Adsorption behaviors of carvone enantiomers over chiral PANI and MIL-101@chiral-PANI are satisfied with pseudo-first-order fitting. In comparison with chiral PANI, MIL-101@ c-PANI exhibits a better enantioselectivity and much higher (>5-fold) adsorption amount over L-carvone than D-carvone. And MIL-101@cPANI is able to recognize the chirality of carvone via electrochemical sensing, taking advantage of the electric conductivity of chiral PANI. Our result demonstrated the feasibility of applying achiral MOF for enantioselective sensing and adsorption via installing chiral and conductive gates. And this chiral polymer modification strategy represents a universal way to entitle achiral MOFs with chiral functions. KEYWORDS: metal−organic framework, chiral polymer, enantioselective adsorption, enantioselective sensing, hybrid composite

1. INTRODUCTION Metal−organic frameworks (MOFs), a class of the most porous materials, have exhibited great potentials in adsorption/ storage, separation and catalysis, and so on.1,2 Homochiral MOFs, a subclass of MOF family, are particularly interesting owing to their chiral properties and consequently promising applications in enantioselective separation, asymmetric catalysis, and so on.3−7 Enantiopure organic ligands are often costly, which greatly hinders the applications of the most homochiral MOFs in practice. Postsynthetic strategy by chiral ligand modification in MOFs has been applied to tune the chirality of achiral MOFs for chiral applications.8−11 For example, chiral modification of the preassembled achiral MIL100 has shown remarkable catalytic activities in asymmetric aldol reactions.10 This inside modification seriously decreased efficient space in nanosized pores of parent MOFs and hence reduced the adsorption capability for further applications. Meanwhile, postsynthetic modification requires active sites (unsaturated coordination sites/functional groups) in parent MOFs, which also limited the spread of postsynthetic modification strategy. It has been proven that surface modification is an efficient approach to tune the properties of MOFs.12−15 Surface modification, instead of modification inside parent MOFs, will not alter the pore sizes and intrinsic properties of the parent MOFs. Depending on the functions of the materials used for surface modification, multifunctions can be integrated © 2018 American Chemical Society

into the composites. For example, composites of UiO-66NH2@PANI have been used for cadmium-ion detection16 and the MOF−poly(vinylpyrrolidone) composite membranes exhibited enhanced low-humidity proton conductivity.17 Polydimethylsiloxane modification on the surface of MOF-5 dramatically enhanced its stability to moisture.18 However, chiral surface modification on achiral MOFs has been rarely reported so far. Previously, we reported homochiral MOF thin films for enantioselective adsorption over enantiomers.19 Very recently, we have demonstrated for the first time a superficial chiral etching process (SCEP) to install chiral porous gate on achiral MOF surface for enantioselective sorption toward limonene enantiomers.20 In the process, chiral ligand etches the surface of presynthesized achiral MOF and reacted with the released metal ions for the production of chiral MOF on the surface of achiral MOF, thus giving rise to core−shell-structured hybrid composite. SCEP introduces chiral function onto achiral MOFs, but does not change the porosity and pore structure, enabling the core−shell composition enantioselective sorption. Although SCEP does not ask for lattice match between chiral and achiral MOFs, it requires carefully selecting the candidates of chiral ligand and parent MOF to enable the etching and Received: April 25, 2018 Accepted: July 13, 2018 Published: July 13, 2018 26365

DOI: 10.1021/acsami.8b06540 ACS Appl. Mater. Interfaces 2018, 10, 26365−26371

Research Article

ACS Applied Materials & Interfaces

((R)-camphorsulfonic acid-doped polyaniline) on preformed MIL-101(Cr) led to a core−shell-structured MIL-101@cPANI composite. The resultant composite displays enantioselective adsorption for L-carvone against D-carvone and distinctive electrochemical response over L- and D-carvone as a sensor (Figure S1). In comparison to chiral PANI, MIL101@c-PANI exhibits a better enantioselectivity and much higher (>5-fold) adsorption amount over L-carvone than Dcarvone (the structures of (R)-camphorsulfonic acid and L- and D-carvone are provided in Figures S2 and S3).

reacting process. In 2016, Gu and co-workers reported the growth of a homochiral MOF thin film on poly(L-DOPA)functionalized substrate to improve enantiomer separation.21 Li group fabricated MOF on β-cyclodextrin-modified gold for amperometric sensing of phenylalanine.22 MOF-based materials exhibit great potential for enantioselective application. However, it is still a great challenge to introduce more functions into one composite for enantioselective recognition, adsorption, and storage. In this work, we prepared an MOF−chiral polymer composite, which integrates chirality, porosity, and conductivity into the composite for enantioselective adsorption and sensing. Its chirality is responsible for chiral recognition, conductivity is used for sensing, and porosity is adopted for adsorption, as schemed in Figure 1. In situ coating c-PANI

2. EXPERIMENTAL SECTION 2.1. Materials. Chromium(III) nitrate nonahydrate (99.0% metals basis powder, Macklin); p-phthalic acid (99%, Macklin); 1R(−)-camphorsulfonic acid (99%, Macklin); aniline (AR, Sinopharm); ammonium persulfate (>99%, Macklin); L-(−)-carvone (99%, Macklin); D-(+)-carvone (97%, Macklin); and cyclohexane (AR, Sinopharm). 2.2. Preparation of MIL-101, Chiral PANI, and MIL-101@cPANI. 2.2.1. Syntheses of MIL-101. MIL-101 was synthesized according to a modified method from the literature.23 The synthesis of MIL-101 consists in the hydrothermal reaction of H2BDC (166 mg at 1 mmol) with Cr(NO3)3·9H2O (400 mg at 1 mmol) and H2O (4.8 mL at 265 mmol) at 210 °C for 18 h. After cooling to room temperature, the blue powder was collected by centrifugation, washed with N,N-dimethylformamide and water, and dried under vacuum. 2.2.2. Preparation of Chiral PANI. Chiral PANI was synthesized according to a modified method from the literature.24 Aniline (100 μL) monomer and 0.3 mol camphor sulfonic acid were well dispersed in 15 mL of 1 mol L−1 hydrochloric acid and then ultrasonicated for 20 min. Afterward, 114 mg of ammonium persulphate (APS) in 2 mL of 1 mol L−1 hydrochloric acid was added dropwise into the above solution at 3 °C and then the mixture was stirred overnight to ensure complete polymerization. The resulting precipitates were centrifuged and rinsed three times with doubly distilled water and alcohol in turns. Finally, the obtained products were dried in a vacuum oven at 80 °C for 12 h.

Figure 1. Schematic illustration of integrating chirality, porosity, and conductivity into MIL-101@c-PANI composite for enantioselective adsorption and sensing.

Figure 2. Characterization of the synthesized MIL-101 (black), MIL-101@c-PANI (blue), and chiral PANI (red): (a) PXRD patterns; (b) N2 sorption isomers at 77 K (solid symbols: adsorption; empty symbols: desorption); (c) FT-IR spectra; and (d) CD spectra. 26366

DOI: 10.1021/acsami.8b06540 ACS Appl. Mater. Interfaces 2018, 10, 26365−26371

Research Article

ACS Applied Materials & Interfaces

Figure 3. SEM and TEM images of MIL-101 (a, c) and MIL-101@c-PANI (b, d). 2.2.3. Preparation of MIL-101 (100 mg)@c-PANI (30 μL). MIL101 (100 mg) was well dispersed in 15 mL of 1 mol L−1 hydrochloric acid. Aniline (30 μL) monomer and 0.1 mol camphor sulfonic acid were added into the above mixture and then ultrasonicated for 20 min. Afterward, 114 mg of APS in 2 mL of 1 mol L−1 hydrochloric acid was added dropwise into the above solution at 3 °C and then the mixture was stirred overnight to ensure complete polymerization. The resulting precipitates were centrifuged and rinsed three times with doubly distilled water and alcohol in turns. Finally, the obtained products were dried in a vacuum oven at 80 °C for 12 h. 2.3. Carvone Sorption Kinetic Rate Measurements. Adsorption kinetics were recorded using a UV−vis spectrometer in 1 cm quartz cuvettes. Cyclohexane solutions of enantiopure L-carvone and −1 D-carvone (7 mg mL ) were prepared. The evacuated samples of 10 mg were immersed in 2.5 mL of the carvone cyclohexane solution and optical absorption spectra were recorded versus time. Upon uptake of carvone by the porous materials, the intensity of the peak measured in solution was monitored until it reached a plateau. 2.4. Characterization of Chiral Sensors. A source/measure unit (Keithley 6487) was used to monitor the real-time current changes under an applied voltage of 10 V direct current. The sensor is protected by Ar at 25 °C. After the baseline becomes stable, we inject 0.1 mL of carvone into the chamber and record the change of current over time.

preparation (see Experimental Section). The powder X-ray diffraction (PXRD) in Figure 2a shows that the structure of MIL-101 keeps intact, but the color changes from green to blackish green after c-PANI coating. As shown in the Fourier transform infrared (FTIR) spectra in Figures 2c and S6, characterized vibrations from c-PANI appeared in MIL-101@ c-PANI composite, in which peaks at 1302 and 1245 cm−1 correspond to stretching of C−N and CN, respectively.28,29 The peaks at 1734 cm−1 (CO stretching vibration) and 1043 cm−1 (SO3− vibration) from SO3− imply existence of (R)camphorsulfonate in the composite.30 The circular dichroism (CD) spectrum of MIL-101@c-PANI composite in the solid state gives rise to the same CD signal as c-PANI, which also supports the success of c-PANI coating on MIL-101 (Figure 2d). These characterizations reveal that the composite contains both c-PANI and MIL-101 components. 3.1. Gas Adsorption. Brunauer−Emmett−Teller (BET) specific surface area of resultant MIL-101@c-PANI composite was determined to be 728 m2 g−1 from the nitrogen sorption isotherm. Coating of c-PANI led to much decreased BET specific surface area of pristine MIL-101 (1908.2 m2 g−1). For comparison, BET specific surface area of c-PANI is measured to be 40 m2 g−1. Analyses of pore size distribution of the three samples suggested that c-PANI coating does not alter the pore structure of MIL-101 (Figure S7), which excludes the possibility of c-PANI existence in the pores of MIL-101. 3.2. SEM and TEM Images. Figure 3 provides direct and powerful evidence of the core−shell structure of the composite.16,31,32 By comparing SEM images, it is found that the smooth surface of MIL-101 becomes wrinkled after cPANI coating (see also Figures S8 and S9). The TEM images reveal the core−shell structure of the MIL-101@c-PANI

3. RESULTS AND DISCUSSION On the basis of acid/base chemistry, polyaniline can be doped with strong acid via protonating the imine nitrogen in the backbone.25 In case of chiral anions as counterbalanced ions, the polyaniline backbone could adopt a helical conformation with optical activity.26,27 (R)-Camphorsulfonic acid-modified polyaniline (c-PANI) and MIL-101 were prepared according to the reported process (see the structures of c-PANI and MIL-101 in Figures S4 and S5). Chiral PANI was coated onto preformed MIL-101 under the same condition as c-PANI 26367

DOI: 10.1021/acsami.8b06540 ACS Appl. Mater. Interfaces 2018, 10, 26365−26371

Research Article

ACS Applied Materials & Interfaces Table 1. Carvone Adsorption Data for MIL-101, c-PANI, and MIL-101@c-PANI amount (mg g−1)

D-carvone L-carvone

adsorption ratea (min−1)

D-carvone L-carvone

enantioselectivityb

MIL-101

c-PANI

MIL-101@c-PANI

332.7 338.3 0.0174 0.0179 1.028

12.9 24.0 0.0278 0.0416 1.496

26.8 157.1 0.0351 0.0575 1.638

Adsorption rates (min−1) are simulated from linear driving force (LDF).33−35 bEnantioselectivity was evaluated as the ratio of adsorption rate of Lcarvone to D-carvone.

a

Figure 4. D-carvone (black) and L-carvone (red) adsorption profiles over (a) chiral PANI, (b) MIL-101@c-PANI, and (c) MIL-101 at room temperature. (d) Relationship between the saturated adsorption amount of carvone and specific surface area of chiral PANI (black), MIL-101 (blue), and MIL-101@c-PANI (red) (solid symbols: L-carvone; slash symbols: D-carvone).

tion amount and kinetics) are summarized in Table 1. After adsorption, no obvious crystallinity loss was detected from PXRD, suggesting that the MIL-101@c-PANI composite is stable (Figure S14). It can be seen from Figure 4 and Table 1 that MIL-101 exhibits almost the same saturated adsorption amount for Dcarvone (332.7 mg g−1) and L-carvone (338.3 mg g−1). As there are no chiral centers in MIL-101, it shows no discrimination effect toward carvone enantiomers and hence displays the similar adsorption kinetics (Figure 4c). Chiral PANI shows a much smaller but distinct saturated adsorption capacity over D-carvone (12.9 mg g−1) and L-carvone (24.4 mg g−1) (Figure 4a). In contrast, the MIL-101@c-PANI composite displays a huge saturated adsorption difference over D-carvone (rate, 26.8 mg g−1) and L-carvone (157.1 mg g−1) (Figure 4b). As shown in Figure 4d, the saturated adsorption amount of Lcarvone is proportional to the specific surface area of absorbents, in the sequence MIL-101 > MIL-101@c-PANI > c-PANI. D-carvone are blocked by c-PANI on the shell layer for freely accessing the pores in MIL-101 owing to the obvious steric effect, which results in much decreased saturated adsorption amount in comparison to L-carvone. In terms of enantioselective adsorption, the saturated adsorption amount is the critical parameter. For separation, the adsorption rate and enantioselectivity are more important.

composite with the outer layer of c-PANI and the inner core of MIL-101, in which smooth edges and sharp corners of MIL101 microcrystals turn irregular (Figure 3b). Electronic microscopy observations show us successful coating of cPANI on the surface of MIL-101, rather than a simple mixture of both components. Judging from the PXRD, FTIR, CD, electron microscopy, and N2 adsorption isotherm data, we conclude that we prepared core−shell-structured MIL-101@cPANI, rather than a simple mixture or c-PANI inside of MIL101. 3.3. Enantioselectivity Adsorption of L- and DCarvone. We modified porous MIL-101 with c-PANI, aiming at enantioselective adsorption over enantiomers. Chiral PANI as shell layer is responsible for recognition of molecules bearing different chirality. The chiral molecules discriminated by c-PANI enter the pores of MIL-101 for adsorption and storage. Carvone enantiomers are selected as probe molecules owing to their good solubility, weak interaction with adsorbents, and suitable molecular dimensions. Adsorption experiments were carried out via immersing activated absorbents (MIL-101, MIL-101@c-PANI, and chiral PANI) in a cyclohexane solution containing pure L- or D-carvone. The adsorption amounts and kinetic behaviors were monitored by UV−vis absorbance spectra (see Supporting Information (SI) for details, Figures S10−S13). The adsorption data (adsorp26368

DOI: 10.1021/acsami.8b06540 ACS Appl. Mater. Interfaces 2018, 10, 26365−26371

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ACS Applied Materials & Interfaces

Figure 5. Time-dependent electrochemical response of D-carvone (black) and L-carvone (red) over (a) chiral PANI and (b) MIL-101@chiral PANI.

carvone and NH in protonated PANI chain, besides van der Waals interaction. Configuration of D-carvone is somehow linear, whereas L-carvone exists a fold configuration, as shown in Figure S2. More compacted configuration of L-carvone with higher adsorption rate could be the reason for higher electrochemical response. It has been reported that alcohol doping can decrease the resistance of PANI and alcohol with larger molecular sizes and gave rise to longer response time.39−42

The adsorption rate is evaluated from adsorption curve using LDF fitting (see details in the SI, Figures S15−S17). Enantioselectivity is calculated as the ratio of adsorption rate of L-carvone to D-carvone. As summarized in Table 1, in spite of high adsorption amount, MIL-101 exhibited no enantioselectivity, owing to the absence of chiral sites in the structure. MIL-101@c-PANI exhibits slightly higher enantioselectivity than c-PANI (Table 1). Enantioselective adsorption of c-PANI over carvone enantiomers originates from the steric effect (size and shape selectivity). It is worthy of note that the adsorption rate of carvone enantiomers in c-PANI is higher than that in MIL-101. This might be explained as surface adsorption on chiral PANI is faster than carvone diffusion in pores in MIL101.36 However, chiral PANI-modified MIL-101 results in a much higher adsorption rate than c-PANI and MIL-101 owing to their synergetic effect. The synergetic effect of chirality from c-PANI for recognition and porosity in MIL-101 for storage leads to higher enantioselectivity and adsorption capability toward L-carvone in core−shell-structured MIL-101@c-PANI. 3.4. Enantioselectivity Sensing of L- and D-Carvone. Excellent conductivity enables MIL-101@c-PANI as electrochemical sensor for detection of enantiopure carvone (see the SI for experimental methods and experimental setup). The device is exposed to D-carvone and L-carvone vapor at the identical conditions, and Keithley 6487 instrument is used to monitor the real-time current change. The control experiment using PANI gave rise to the same response over D-carvone and L-carvone (Figure S18). The time-dependent enantioselective discrimination is denoted as response = (Ig − Ia)/(Ia), where Ia and Ig are the electric currents of the sensor in Ar and target gas (vapor of L- and D-carvone), respectively. As displayed in Figure 5a, the c-PANI-based device shows obvious electrochemical sensing behavior toward L/D-carvone. The normalized response increases dramatically in the first 30 min and then reaches the plateau. It clearly shows that Lcarvone renders much higher response than D-carvone. MIL101@c-PANI displayed a similar response trend toward carvone enantiomers, but a higher response value (Figures 5b and S19, S20). Because surface adsorption on c-PANI is faster than the diffusion in pores in MIL-101, MIL-101@cPANI takes a longer time to reach equilibrium (ca. 400 min) due to the coexistence of surface adsorption and diffusion in pore. The conductivity of c-PANI could be altered by adsorption of carvone via tuning the polymer structure.37,38 The interaction of carvone and c-PANI is mainly ascribed to hydrogen bond (O···H−N) between ketonic oxygen in

4. CONCLUSIONS In summary, we have designed an MOF@chiral polymer composite with core−shell structure to integrate chirality, porosity, and conductivity into one material and achieve the integration of enantioselective adsorption, sensing, and storage. Via installing a chiral gate on achiral MOF, MIL-101@chiral PANI composite exhibits a better enantioselectivity and much higher (>5-fold) adsorption amount than chiral PANI over Lcarvone than D-carvone. The good conductivity of chiral PANI allowed us to study the sensing behavior of the composite toward carvone enantiomers. Using a little and cheap chiral species ((R)-camphorsulfonic acid in this work), we reached superior enantioselective adsorption and discrimination to the pure chiral counterpart. Considering the abundance of MOF materials with varied pore sizes, our result demonstrated a novel and universal strategy to apply achiral MOF for enantioselective adsorption and sensing with high efficiency and low cost.

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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/acsami.8b06540. Experimental details, test of carvone calibration curve, characterization of chiral sensors, physical models for diffusion mechanism; three-dimensional molecular structures of L/D-carvone; PXRD patterns; SEM images; FT-IR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Bo Liu: 0000-0002-1150-3709 Notes

The authors declare no competing financial interest. 26369

DOI: 10.1021/acsami.8b06540 ACS Appl. Mater. Interfaces 2018, 10, 26365−26371

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ACS Applied Materials & Interfaces

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (NSFC, 21571167, 51502282), Chinese Academy of Sciences, the Fundamental Research Funds for the Central Universities (WK2060190053), and Anhui Province Natural Science Foundation (1608085MB28).

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DOI: 10.1021/acsami.8b06540 ACS Appl. Mater. Interfaces 2018, 10, 26365−26371

Research Article

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DOI: 10.1021/acsami.8b06540 ACS Appl. Mater. Interfaces 2018, 10, 26365−26371