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Research Article Cite This: ACS Appl. Mater. Interfaces 2017, 9, 41512-41520

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Zirconium-Based Nanoscale Metal−Organic Framework/Poly(εcaprolactone) Mixed-Matrix Membranes as Effective Antimicrobials Ming Liu,†,‡ Lei Wang,*,† Xiaohua Zheng,†,‡ and Zhigang Xie*,† †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, P. R. China ‡ The University of Chinese Academy of Sciences, Beijing 100049, P. R. China

ACS Appl. Mater. Interfaces 2017.9:41512-41520. Downloaded from pubs.acs.org by TULANE UNIV on 01/20/19. For personal use only.

S Supporting Information *

ABSTRACT: Metal−organic framework (MOF)−polymer mixed-matrix membranes (MMMs) have shown their superior performance in gas separation. However, their biological application has not been well-explored yet. Herein, a series of zirconium-based MOF MMMs with high MOF loading and homogeneous composition have been prepared through a facile drawdown coating process. Poly(ε-caprolactone) (PCL) has been selected as a binder for its good biocompatibility and biodegradability. Zr-MOF nanoparticles, UiO-66, and MOF-525, have been utilized as “filler” because of their superior chemical stability, good biological safety, and versatile functions. Both UiO-66/PCL MMMs and MOF-525/PCL MMMs have a uniform appearance even at the highest loading of 50 wt % for UiO-66 and 30 wt % for MOF-525, respectively. The integrity of pore structures of UiO-66 within MMMs maintains well, which is evidenced by dye separation. All obtained MMMs possess good biocompatibility and mechanical property. Upon irradiation, MOF-525/PCL MMMs generate reactive oxygen species and serve as effective antibacterial photodynamic agents against Escherichia coli. This study offers an alternative system for forming homogeneous MOF/polymer MMMs and represents the first example of exploiting hybrid MMMs for biological applications. KEYWORDS: metal−organic frameworks, zirconium, mixed-matrix membranes, poly(ε-caprolactone), antimicrobial

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INTRODUCTION Metal−organic frameworks (MOFs) are a new type of nanoporous crystalline materials periodically constructed by inorganic nodes and organic linkers.1−3 Their high surface area, tunable pore size, and rich physicochemical properties have also enabled them as promising materials for gas separation and storage, sensing, catalysis, and biomedicine.4−11 Current studies have been focused on how to improve the manipulation and processability of these MOF materials due to the crystalline and robust nature. The incorporation of MOF microparticles or nanoparticles into polymers to fabricate MOF-based mixedmatrix membranes (MMMs) is a feasible strategy.12,13 The MOFs within these MMMs are usually utilized as “filler”, whereas the polymeric matrix primarily serves as a binder. These as-synthesized MOF-based MMMs not only combine the molecular sieving effect of MOFs but also largely overcome the brittleness and difficult syntheses of polycrystalline MOF membranes and limited mechanical strength of pure polymer membranes.14,15 Since the first report of MOF-based MMMs in 2004, several structural types of MOFs have been successfully introduced into MMMs. The driving forces for the development of MOFbased MMMs are their superior gas separation performance.16−22 The provision of open gas transport channels and some additional free volume at the interface between polymers © 2017 American Chemical Society

and MOF particles may enhance the gas permeabilities of MOF-based MMMs.23,24 Recent work has shown that the encapsulated MOF particles within MMMs are accessible for further chemical modification without destroying the structure of membranes.25 For fabricating “defect-free” MOF-based MMMs with a high MOF loading, the key factor is the compatibility between MOF particles and the selected polymer matrix. Many strategies, including reducing the size of MOF particles,26 chemical modification of MOF surfaces,22,27 utilization of MOF defects,28 and use of the polymers with limited inherent porosity (fluoropolymer poly(vinylidene fluoride))29 or in situ polymerization,30,31 have been proposed. The highest loading example for MOF-based MMMs is reported using styrene/butadiene copolymers as a binder, and the content of doped UiO-66 reaches 90 wt %.32 For antimicrobial applications of MOFs, metal ions (Ag+,33,34 Co2+,35−37 and Cu2+38), organic ligands,39 or a combination of them40,41 is mainly utilized as an effective biocide to provide antibacterial activity during the stepwise and slower decomposition of MOF skeletons, but these cannot be used repeatedly. Moreover, antimicrobial agents could also be Received: October 18, 2017 Accepted: November 8, 2017 Published: November 8, 2017 41512

DOI: 10.1021/acsami.7b15826 ACS Appl. Mater. Interfaces 2017, 9, 41512−41520

Research Article

ACS Applied Materials & Interfaces

Scheme 1. Schematic Presentation of Zirconium-Based Nanoscale Metal−Organic Framework (NMOF)/PCL Mixed-Matrix Membranes (MMMs) Synthesis and Antimicrobial Property

ultrasonically dispersed in dichloromethane (DCM) to obtain a homogeneous nanoparticle dispersion. After fully mixing with PCL solution (20 wt % in DCM), a uniform milklike UiO-66/ PCL suspension with increasing viscosity has been formed (Figure S4). The MMMs are formed by casting the suspensions onto slides (cover glass) placed on a hot stage maintained at 37 °C and following slow vaporization of the DCM solvent. Figure 1a shows the images of UiO-66/PCL MMMs with a wide range of UiO-66 loadings from 10 to 50 wt %. All MMMs are transparent and have a homogeneous appearance (Figure S5). With an increase in the amount of UiO-66 loadings, the colors of UiO-66/PCL MMMs become much whiter. When the loading amounts reach 55 and 60 wt %, the corresponding UiO-66/PCL MMMs are brittle and their membrane structures could not be maintained (Figure S6). The micromorphology of the UiO-66/PCL MMMs under the top view and crosssectional view is observed by SEM in Figure 1b,c. Without UiO-66 doping, the pure PCL membrane gives a neat appearance. With an increase in the loading of UiO-66, the surface of MMMs becomes folding and UiO-66 nanoparticles are dispersed evenly in the PCL matrix. Some of the macroporous structures were seen at higher loadings of 40 and 50 wt %. In the case of 40 wt % loading, regular macroporous structures with the diameter about 10 μm can be seen on the surface of MMMs and lead to the formation of hierarchical porous materials. The wall of these macroporous structures is formed by homogeneous UiO-66 nanoparticles. When the UiO-66 content is about 50 wt %, the macroporous structure maintains well but the size distribution becomes irregular (Figure S7). The pure PCL film still has an even cross section. When the loading of UiO-66 nanoparticles is below 30 wt %, the MMMs show a uniform composition and the UiO-66 nanoparticles finely disperse in the films. No apparent particle agglomerations have been observed even at the highest UiO-66 loading of 50 wt %, indicating good compatibility between UiO-66 nanoparticles and the PCL matrix. These results validated that the structural integrity is well-maintained during

encapsulated into the pores of MOFs to achieve a controllable release and a more lasting antibacterial efficacy.42,43 However, few studies have been performed on MOFs or MOF-based MMMs for antibacterial photodynamic therapy (aPDT). Herein, a series of zirconium-based MOF (Zr-MOF) MMMs have been prepared and their photodynamic antimicrobial property has been studied for the first time. As shown in Scheme 1, poly(ε-caprolactone) (PCL) has been selected as a binder for its good biocompatibility and biodegradability.44,45 Zirconium-based MOFs (Zr-MOFs), UiO-66, and MOF-525, have been utilized as filler to fabricate MOF-based MMMs due to their superior chemical stability and base platform for building a functional system.46−50 The particle size of Zr-MOF can be reduced to nanoscale using a “modulator approach”.51 Previous studies have revealed that “modulators”, such as benzoic acid, may partly substitute carboxylate linkers within frameworks to form “defects” and have a direct influence on the interface properties.52−57 Hence, we speculate that these defects from open Zr sites or modulators may improve the adhesion on the interface of the PCL matrix and Zr-MOF by coordination interaction between Zr sites of MOFs and carbonyl groups of PCL.

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RESULTS AND DISCUSSION Preparation and Characterization of UiO-66/PCL MMMs. Octahedral UiO-66 nanocrystals with an average size of 170 nm were first chosen as an archetypal Zr-MOF for fabricating MMMs. Their morphology and bulk crystallinity were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and powder X-ray diffraction (PXRD) (Figures S1 and S2). The porosity of UiO-66 nanocrystals has been confirmed by gas sorption measurements after being degassed at 393 K for 12 h, giving the Brunauer−Emmett−Teller surface area value of 1591 m2 g−1 (Figure S3). UiO-66/PCL MMMs with different amounts of UiO-66 loading from 10 to 60 wt % were prepared by the drawdown coating process. Typically, UiO-66 powders are 41513


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Figure 1. (a) Photographs of UiO-66/PCL MMMs prepared with a variety of UiO-66 loadings. SEM images of as-synthesized UiO-66/PCL MMMs under the top view (b) and cross-sectional view (c). Scale bars: 5 μm (the insert images are at a higher magnification with a scale bar of 1 μm).

istic peaks at around 2950 cm−1 for C−C bonds and 1720 cm−1 for CO bonds from the PCL matrix appeared in the infrared (IR) spectrum of UiO-66/PCL MMMs (Figure 3b). The thermal stabilities of UiO-66, PCL, and UiO-66/PCL MMMs have been determined through thermogravimetric analysis (TGA) by heating from room temperature to 800 °C at a rate of 10 °C min−1 under an air atmosphere (Figure 3c). Typically, the weight loss of UiO-66/PCL MMMs mainly consists of three steps. The first stage is from room temperature to 250 °C with an insignificant mass loss, which is mainly because of the loss of adsorbed water. The second stage from 250 to 380 °C is attributed to the first-stage decomposition of PCL, which contributes more than 90 wt % to the weight loss by the thermal degradation of PCL.58,59 The final step over the

the process of membrane preparation. The thickness of resultant MMMs (Figure 2) shows a loading-amount-dependent behavior from 32.8 μm thickness for 10 wt % loading to 132 μm thicknesses for 50 wt % loading, which may be ascribed to the increased NMOF particle loading and enhanced viscosity of UiO-66/PCL suspensions. The crystallographic structures of UiO-66/PCL MMMs, UiO-66 nanoparticles, and PCL matrix have been investigated by PXRD (Figure 3a). All sharp diffraction peaks match very well with the pattern of corresponding UiO-66 nanoparticles, confirming that the membrane preparation procedure does not affect the crystallinity of the UiO-66 particles. The crystalline peaks at 22.01 and 24.17 are attributed to the [110] and [200] crystallographic planes of the PCL matrix. Two new character41514


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Figure 2. Cross-sectional SEM images of UiO-66/PCL MMMs with various UiO-66 contents and the thickness of related films. (Scale bars: 50 μm.)

Figure 3. Characterization of as-synthesized UiO-66/PCL MMMs with different UiO-66 loadings. (a) PXRD patterns of UiO-66 nanoparticles, pristine PCL membrane, and UiO-66/PCL MMMs. (b) Fourier transform infrared (FT-IR) spectra of UiO-66 nanoparticles, pristine PCL membrane, and UiO-66/PCL MMMs. (c) TGA curves of UiO-66 nanoparticles, pristine PCL membrane, and UiO-66/PCL MMMs. (d) Static water contact angles of UiO-66 nanoparticles, pristine PCL membrane, and UiO-66/PCL MMMs.

temperature range of 380−550 °C is assigned to the removal of the organic linker and the collapse of the UiO-66 framework and the partial unzipping depolymerization process of PCL in its second-stage decomposition. The percentages of UiO-66 in

each MMM are calculated and listed in Table S1, which are in good agreement with the theory value. Static water contact angle is recorded to evaluate the surface wettability of membranes. In contrast to the UiO-66 and PCL matrix (Figure 41515


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Figure 4. Characterization of as-synthesized MOF-525/PCL MMMs. (a) TEM image of MOF-525 nanoparticles. Scale bar: 100 nm. (b) Photographs of MOF-525/PCL MMMs with 10 and 30 wt % MOF-525 loading. (c) CLSM image of MOF-525/PCL MMMs with MOF-525 loading of 30 wt %. SEM images of MOF-525/PCL MMMs with MOF-525 loading of 30 wt % under top view (d) and cross-sectional view (e). Scale bars: 2 μm (the insert images are at a higher magnification with a scale bar of 1 μm). (f) Reactive oxygen species (ROS) generation by MOF525/PCL MMMs with 10 and 30 wt % MOF-525 loading after irradiation with a LED lamp (λ = 510 nm, with a power density of 12 mW cm−2).

current synthesized MMMs have enough flexibility for further use in the biological field, such as antimicrobial materials. Preparation and Characterization of MOF-525/PCL MMMs. Antibacterial photodynamic therapy (aPDT) has drawn great attention for its markedly different treatment modality to treating bacterial infection compared with conventional antibiotics.60,61 Porphyrin and its derivatives are widely used because they act as an efficient photosensitizer by producing reactive oxygen species (ROS).62−66 Some of the previous studies have also demonstrated that Zr-porphyrin MOFs exhibit excellent chemical stability and effective ROS generation for photodynamic therapy (PDT).67 However, there have been no reports about nanoscale porphyrinic MOFs and their hybrid materials for antimicrobial application. Nanoscale MOF-525 with the particle size of 100 nm has been synthesized by the solvothermal approach, and its morphology and bulk crystallinity are well-characterized (Figures 4a, S10, and S11). The UV−vis absorption spectrum of MOF-525 exhibits a typical Soret band at 423 nm and four less intense Q bands in the range of 500−700 nm (Figure S12). MOF-525/PCL MMMs with loadings of 10 and 30 wt % are prepared. The prepared MOF-525/PCL MMMs are violet, which is the characteristic color of porphyrin and its derivatives (Figure 4b). The successful formation of MMMs is validated by PXRD, IR, and SEM (Figures S13−S15). The amount of MOF-525 in each MMM is determined by TGA (Figure S16), and the calculated results are listed in Table S3. The top and crosssectional morphologies of MOF-525/PCL MMMs with 30 wt % loading are given in Figure 4d,e. Similar to UiO-66/PCL MMMs, MOF-525 nanoparticles also have good compatibility with the PCL matrix and can be well-dispersed, as confirmed by the homogeneous red color in confocal laser scanning

3d), the contact angles of each UiO-66/PCL MMMs exhibit a downward tendency and the least value of 77° has been observed at the 50 wt % of UiO-66 loading. To confirm the integrity of pore structures, the adsorption performance of assynthesized UiO-66/PCL MMMs toward rhodamine B at the concentration of 0.01 mmol L−1 has been investigated. As shown in Figure S8, after dipping UiO-66/PCL MMMs into rhodamine B aqueous solution overnight, the color of membranes is changed from white to pale red, whereas the dye solution is almost colorless after the UiO-66/PCL MMM treatment. Little color change has been observed in the pure PCL membrane. All above results demonstrate that the smaller particle size and the modulator-induced defects of assynthesized UiO-66 nanoparticles may enhance the adhesion between MOF particles and the polymer matrix, ensuring successful formation of homogeneous MMMs with a higher loading. To further evaluate the mechanical properties of assynthesized MMMs, the stress−strain curves of the PCL control and UiO-66/PCL MMMs with various UiO-66 loadings (Figure S9) have been examined and their corresponding tensile data, such as elastic modulus, ultimate tensile strength, and elongation at break, have been listed in Table S2. With the increasing content of UiO-66 nanoparticles, the elastic modulus of as-synthesized MMMs enhances rapidly from 231 MPa for PCL to the max value of 917 MPa for 50 wt % UiO-66/PCL MMMs, whereas their values of elongation at break largely decrease from 14.5% of PCL to 0.7% of 50 wt % UiO-66/PCL MMMs. Also, the ultimate tensile strength of MMMs increases first and then decreases, giving a value close to that for the PCL control even at a very high UiO-66 loading concentration of 50 wt %. These above results demonstrate that 41516


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ACS Applied Materials & Interfaces microscopy (CLSM) imaging (Figure 4c). Subsequently, the ROS generation ability of MOF-525/PCL MMMs upon irradiation with a light-emitting diode (LED) lamp (λ = 510 nm, with a power density of 12 mW cm−2) has been monitored according to the UV absorbance of ROS scavenger 1,3diphenyl-isobenzofuran (DPBF) at 413 nm (Figures 4f and S17). Its relative ratios with initial value (I0) and selected time value (Ii) at 413 nm are plotted according to various irradiation times. The UV absorbance of DPBF decreases remarkably in the presence of MOF-525/PCL MMMs under irradiation, whereas negligible spectral changes have been observed in DPBF itself, indicating the effective and well-retained ROS generating ability of as-synthesized MMMs and thus they can be potentially used as antimicrobials. Biocompatibility and Antimicrobial Properties. Mouse fibroblastic cell, L929, has been first selected to study the biocompatibility of MOF-525/PCL MMMs by the established 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. As shown in Figure S18, more than 90% of cells remain viable in all cases even at a higher MOF-525 loading or the extended culture time with the MMMs. This result demonstrates the good safety of MOF-525/PCL MMMs for potential biological applications. To ascertain the antimicrobial photodynamic ability of MOF-525/PCL MMMs, a classical colony counting method against Escherichia coli (a Gram-negative bacteria) was used to measure bacterial viability. Typically, MOF-525/PCL MMMs were incubated with bacterial cell suspensions in the dark at 37 °C for 16 h, and the same suspensions without any treatment were used as a blank control. The number of viable colonies of E. coli (Figure 5a,b) is slightly changed, which indicated that the MOF-525/ PCL MMMs have good biocompatibility and do not show antimicrobial activity without irradiation. The antibacterial ability of MOF-525/PCL MMMs by generating ROS was investigated with E. coli under irradiation by a LED lamp (power density of 12 mW cm−2). A negligible effect on the growth of E. coli has been observed in control experiments. The time-dependent antibacterial testing results show that the growth of E. coli can be more effectively inhibited within the first 30 min and is slightly changed even when the irradiation time reached 90 min. Moreover, MMMs with 30 wt % MOF525 loading possessed the stronger inhibitory effect than that with 10 wt % loading in all tested concentrations. As shown in Figure S19, the amount of viable E. coli bacterial cells after the treatment with the PCL membrane or UiO-66/PCL MMMs under irradiation is slightly higher than that in the control group, further indicating the good biocompatibility of the PCL matrix and Zr-MOFs. The high coordination affinity between outer phospholipid bilayers of E. coli and the zirconium oxide nodes on the external surface of the MOF-525 nanoparticle may play a pivotal role in the antibacterial response of MOF525/PCL MMMs.37 As we know, silver, copper, zinc, etc. metal ions and their oxides are widely used as an effective biocide exhibiting antibacterial properties by releasing active metal ions, but these cannot be used repeatedly.68 Encouraged by the excellent antimicrobial properties of MOF-525/PCL MMMs, their reusability has been investigated in detail. All tested MMMs well-maintain their crystallinity and topological structures even when completely immersed into the lysogeny broth (LB) medium after 72 h, indicated by their PXRD patterns (Figure S20). Their superior stability is significantly helpful for their repeated use. MOF-525/PCL MMMs with a 30 wt % loading

Figure 5. (a) Plate photographs for E. coli on a lysogeny broth (LB) agar plate and (b) average number of viable colonies corresponding to treatment. Dark toxicity experiments: the bacterial suspension was incubated in the dark with MOF-525/PCL MMMs overnight. PDT experiments: the bacterial suspension was irradiated with a LED lamp (λ = 510 nm, with a power density of 12 mW cm−2). The bacterial suspension was incubated with or without MOF-525/PCL MMMs for 30 min and was then irradiated for different times, respectively.

have been selected to test the reusability. The MMMs exhibit similar antibacterial properties during three trials (Figure S21). All of these results prove the advantages of MOF-525/PCL MMMs for antibacterial photodynamic therapy.

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CONCLUSIONS

In this study, we have successfully prepared a series of zirconium-based MOF (Zr-MOF)/PCL MMMs with high MOF loading by the drawdown coating method. UiO-66 and MOF-525 nanoparticles have good compatibility with the PCL matrix by the coordination interaction between Zr cluster nodes and the carbonyl groups of PCL. No obvious nanoparticle aggregation has been observed even at the highest loading of MOFs. Both as-synthesized UiO-66/PCL MMMs and MOF-525/PCL MMMs also have good biocompatibility. After irradiation, these MOF-525/PCL MMMs exhibit effective ability to generating ROS and can be repeatedly applied as antibacterial photodynamic agents against E. coli. This facile MMM design approach could open new opportunities for combining polymers and MOFs and expanding their potential applications in various fields, such as “Smart” bioresponsive materials.69 41517


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For control experiments, DPBF absorption without MOF-525/PCL MMMs was also recorded under the same conditions. Cell Culture and Cytotoxicity Test. L929 cells (mouse fibroblastic cells) were grown at 37 °C in a humidified atmosphere containing 5% (v/v) CO2 in Dulbecco’s modified Eagle medium (DMEM, GIBCO) supplemented with 10% fetal bovine serum (GIBCO) and 100 U mL−1 penicillin and 100 mg mL−1 streptomycin. For the cytotoxicity test, cells were implanted in a 24-well plate with 500 μL of DMEM at a density of 2 × 104 cells per well for 24 h. Then, a membrane with a size of 0.5 cm × 0.5 cm was added into each well of the plate in three groups and 500 μL of fresh DMEM was also added. After different time intervals, 50 μL of the MTT solution (5 mg mL−1) in PBS was added and the plate was incubated for another 4 h at 37 °C. After that, the supernatant and membranes were removed and 500 μL of dimethyl sulfoxide was added to each well to dissolve MTT formazan crystals. After they were fully dissolved, the solution was transferred into a 96-well plate, 150 μL per well. The absorbance of the formazan product was measured at 570 nm by a microplate reader (BioTek, EXL808). Bacterial Cell Culture. Escherichia coli was cultured in lysogeny broth (LB) medium under shaking (180 rpm) at 37 °C overnight to a mid-log phase. E. coli was collected by centrifuging (at 10 000 rpm for 5 min) and washing by 0.9% NaCl solution three times. The harvested E. coli cells were suspended in fresh medium, and the optical density (OD600) of the bacterial suspensions was adjusted to 1.0 via UV−vis spectroscopy to prepare the stock solution. Antibacterial Activity Measurement. The antibacterial activity of as-synthesized Zr-MOF-based MMMs was evaluated against E. coli (Gram-negative bacterial strains) by measuring the colony forming units (CFUs). Before experiment, the films were sterilized by soaking in 70% ethanol for 30 min and then washed three times with 0.9% NaCl solution. PDT Experiment. The phototoxicity of as-synthesized Zr-MOFbased MMMs was determined by incubation with E. coli suspensions (100 μL) in 2 mL of fresh LB medium for 30 min in the dark at 37 °C and then by exposure to an LED lamp (λ = 510 nm, with a power density of 12 mW cm−2) for different times. Then, the bacterial suspensions were serially diluted in a 0.9% NaCl solution. A 100 μL portion of the diluted bacterial E. coli suspension was plated on a solid LB agar plate, and the colonies formed after 12−16 h incubation at 37 °C were counted. At the same time, bacterial suspensions with any treatment were used to determine the initial CFU. (Three experiments were performed for each membrane, and the mean values are reported in the results.) A control experiment with E. coli suspensions exposed at different times without MMMs was also performed the same way as above. The PDT antibacterial experiments by the same MOF-525/ PCL MMMs with a 30 wt % loading were carried out three times to verify their reusability. Dark Toxicity Experiment. The dark toxicity of as-synthesized Zr-MOF-based MMMs was determined by incubation with E. coli cell suspensions (100 μL) in 2 mL of fresh LB medium for overnight in the dark at 37 °C. Then, the bacterial suspensions were serially diluted and plated on a solid LB agar plate and the colonies formed after 12− 16 h incubation at 37 °C were counted. At the same time, bacterial suspensions with any treatment were used to determine the initial CFU. (Three experiments were performed for each membrane, and the mean values are reported in the results.)

MATERIALS AND EXPERIMENTS

Materials. PCL (Mn = 80 000) was purchased from Sigma-Aldrich and was used as received without any further treatment. Solvents were purchased from Sinopharm Chemical Reagent Co., Ltd. All were used directly without additional purification. Slide cover glasses (24 mm × 24 mm) were ultrasonically cleaned in acetone, ethanol, and ultrapure water sequentially for 15 min each to remove inorganic and organic surface contaminants. Characterization. The UV−vis spectrum was recorded on a Shimadzu UV-2450 spectrometer. The FT-IR spectrum was recorded by a Nicolet Impact 410 Fourier transform infrared spectrometer. TEM images were recorded by a JEOL JEM-1011 electron microscope operated at an acceleration voltage of 100 kV. SEM images were obtained using an FEI/Philips XL-30 scanning electron microscope at an accelerating voltage of 5 kV. For the clear cross section of films, the samples were made by bending the films under fragile conditions, for which the films were immersed in liquid nitrogen for a short time. The samples were sputtered with gold (10 nm thick) using a Bal-Tec SCD050 sputter Coater (Germany) prior to SEM examination. Powder X-ray diffraction (PXRD) patterns were recorded on a modified Xeuss SAXS/WAXS system of Xenocs France equipped with a semiconductor detector (Pilatus 100 K, DECTRIS, Swiss) attached to a multilayer focused Cu Kα X-ray source (GeniX3D Cu ULD, Xenocs SA, France), generated at 50 kV and 0.6 mA. The wavelength of the X-ray radiation was 0.154 nm, and the sample-to-detector distance was 39.8 mm. The beam was focused onto the sample position with a size of 40 μm × 60 μm. Each pattern was collected in 60 s, and the background was subtracted. Thermogravimetric analysis (TGA) was performed using a TGA/DSC 1 (Mettler-Toledo, Switzerland) at the rate of 10 °C min−1 from room temperature to 800 °C under the flow of air. Fluorescence and confocal microscopy (CLSM) images were taken using Zeiss LSM 700 (Zurich, Switzerland). Water contact angles were measured with a Ramé-Hart 200-F1 standard goniometer at room temperature with ultrapure water (3 μL) chosen as the probe liquid. The mechanical testing was performed using Instron 5869 conducted at room temperature at a tensile speed of 10 mm min−1. The viscosity was obtained with rotatory viscometer NDJ-1. Synthesis of Nanoparticles. UiO-66 nanoparticles were synthesized via the modified solvothermal method according to our previous work.46 MOF-525 nanoparticles were synthesized by the modified solvothermal method.50 Preparation of MMMs. In a typical process of preparing MMMs, 20 mg of UiO-66 was dispersed into DCM (400 μL) to give suspension A and 40 mg of PCL was dissolved in DCM (200 μL) to give solution B. Suspension A was sonicated for 30 min and then mixed with solution B. After sonicating for another 30 min, it gives the membrane casting solution, which was carefully casted onto a flat glass substrate placed on a hot stage maintained at 37 °C, followed by slow vaporization of solvent. About 3 h later, MMMs were peeled off the glass substrate and dried in vacuum at room temperature. Pure PCL membranes and MOF-525/PCL MMMs were fabricated by the same procedure, as described above. After drying, the size of the films is almost the same as that of the cover glass. Dye Absorption of MMMs. The prepared UiO-66/PCL MMMs were immersed in 2 mL of 0.01 mM rhodamine B aqueous solution for 24 h at room temperature. The pure PCL film was also tested for dye absorption under the same conditions for comparison. Detection of Singlet Oxygen Generated from MOF-525/PCL MMMs. To evaluate the singlet oxygen generation ability of the prepared MOF-525/PCL MMMs, 1,3-diphenyl-isobenzofuran (DPBF) was employed as a scavenger and monitored by timedependent electronic absorption spectroscopy. Briefly, 2 mL of ethanol solution containing MOF-525/PCL MMMs and DPBF (1.85 × 10 −5 M) was introduced into a 6-well plate in the dark. An LED lamp (λ = 510 nm, with a power density of 12 mW cm−2) was used to irradiate the solution at room temperature. The absorbance intensity of DPBF at the maximum wavelength of 413 nm was detected at different times.

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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.7b15826. TEM and SEM images, PXRD patterns, nitrogen adsorption−desorption isotherms, viscosity and transmittance, adsorption toward rhodamine B, stress−strain curves, UV−vis adsorption spectra, FT-IR spectrum, TGA curves, cell viability, plate photographs for E. coli, and reusability for antibacterial properties (PDF) 41518


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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (L.W.). *E-mail: [email protected] (Z.X.). ORCID

Lei Wang: 0000-0003-4395-5002 Zhigang Xie: 0000-0003-2974-1825 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support was kindly provided by the National Nature Science Foundation of China (Project Nos. 21401188 and 51522307).

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