Graphene Oxide Hybrid Nanosheets

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Zeolitic Imidazolate Framework/Graphene Oxide Hybrid Nanosheets Functionalized Thin Film Nanocomposite Membrane for Enhanced Antimicrobial Performance Jing Wang, Yuanming Wang, Yatao Zhang, Adam Andrew Uliana, Junyong Zhu, Jin-dun Liu, and Bart Van der Bruggen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06992 • Publication Date (Web): 02 Sep 2016 Downloaded from http://pubs.acs.org on September 6, 2016

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ACS Applied Materials & Interfaces

Zeolitic

Imidazolate

Nanosheets

Framework/Graphene

Functionalized

Thin

Film

Oxide

Hybrid

Nanocomposite

Membrane for Enhanced Antimicrobial Performance Jing Wang a, b, Yuanming Wang a, Yatao Zhang a*, Adam Uliana b, c, Junyong Zhu b, Jindun Liu a**, Bart Van der Bruggen b a

b

School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou 450001, China Department of Chemical Engineering, KU Leuven, Celestijnenlaan 200F, B-3001 Heverlee,

Belgium c

Department of Chemical Engineering, The Pennsylvania State University, University Park, PA

16802, U.S.A.

* Corresponding authors: Yatao Zhang, E-mail: [email protected] Jindun Liu, E-mail: liujindun@ zzu.edu.cn

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ACS Paragon Plus Environment


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Abstract: Inspired by the rational design concept, a novel antimicrobial agent zeolitic imidazolate framework-8 (ZIF-8)/graphene oxide (GO) was synthesized and utilized as a novel and efficient bactericidal agent to fabricate antimicrobial thin film nanocomposite (TFN) membranes via interfacial polymerization. The resultant hybrid nanosheets not only integrates the merits of both ZIF-8 and GO, but also yields a uniform dispersion of ZIF-8 onto GO nanosheets simultaneously, thus effectively eliminating the agglomeration of ZIF-8 in the active layer of membranes. ZIF-8/GO thin film nanocomposites (TFN-ZG) membrane with typical water permeability (40.63

L m-2 h-1 MPa-1) allows for efficient bivalent salts removal (rejections of Na2SO4 and MgSO4 were 100% and 77%, respectively). Furthermore, the synthesized ZIF-8/GO nanocomposites were verified to have an optimal antimicrobial activity (MIC,128 µg/mL) in comparison with ZIF-8 and GO separately, which sufficiently endowed the TFN-ZG membrane with excellent antimicrobial activity (84.3% for TFN-ZG3). Besides, the antimicrobial mechanisms of ZIF-8/GO hybrid nanosheets and TFN-ZG membranes were proposed. ZIF-8/GO functionalized membrane with high antimicrobial activity and salt retention denoted its great potential in water desalination, and we suggest that ZIF-8 based crystal may offer a new pathway for the synthesis of multifunctional bactericidal. Keywords: in-situ growth; graphene oxides; ZIF-8; antimicrobial; nanofiltration

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1. Introduction With limited water resources and increasing global water consumption, the world is forced to develop advanced water treatment technologies that can meet present and future water requirements in an environmentally sustainable and cost effective way.1 Membrane separation technology is likely to be the most effective solution to this water crisis, owing to its automatic and continuous operation, small space and energy requirements as well as eco-friendliness.2-3 Due to the straightforward pore forming mechanism, higher flexibilities, simpler preparation process and lower costs compared to inorganic membranes, polymeric membranes are currently the most widely used membrane type for water treatment.4-5 Nonetheless, polymeric membranes are still restricted by several challenges, especially their low resistance to fouling and the trade-off relationship between permeability and selectivity.6 Numerous researchers have attempted to fabricate high-performance membranes with enhanced organic antifouling capabilities by optimizing physicochemical properties of the membrane surface.7-8 Unfortunately, organic matter is not the only sources of membrane fouling. Biofouling, defined as microbial growth and biofilm formation, is also a bane in membrane separation processes for water and wastewater treatment.1,

9-10

Preparing membranes with antimicrobial

activities may significantly increase the separation efficiency and life span. Additionally, the use of antimicrobial membranes helps to provide clean and pathogen-free water.11-12 Thus, the development of antimicrobial membranes with high water permeability and selectivity is imperative for water purification under the context of energy efficiency and cost effectiveness. Nanomaterials are especially effective in conjunction with membrane technology due to their massive specific surface areas along with their distinctively tunable chemical, physical, and mechanical properties. Thus, the incorporation of nanomaterials into membranes had an explosive growth over the last two decades.3,

5, 13-15

Graphene oxide (GO) nanosheets, which contain

carboxyl, hydroxyl and epoxide functional groups, are one such nanomaterial that has emerged as an optimal starting material for making uniform, stable, and functional nanocomposite membranes with high chemical stability, strong hydrophilicity and excellent antifouling properties.16-19 Several studies have further demonstrated the strong antimicrobial properties of GO against a wide variety of

microorganisms,

including

gram-positive/-negative

bacteria,

3


phytopathogens,

and


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biofilm-forming microorganisms.20-21 Porous crystalline materials classified as metal-organic frameworks (MOFs) have also drawn widespread attention because of their outstanding characteristics: exceptionally high surface areas, large pore volumes, high degrees of crystallinity and alterable pore functionalities.22-24 Nowadays, rather than searching for available nanomaterials for enhancing membranes in a trial-and-error manner, researchers have shifted toward synthesizing hybrid nanocomposites in a rational design way since they realized that the chemistry and the ultimate functions of nanocomposites could be deliberately tailored for a desired purpose before embarking on nanocomposites synthesis. Hybrid nanocomposites based on GO and MOFs have elicited much interest;25 e. g., Hu et al. used two-dimensional ZIF-8/GO hybrid nanosheets as seeds to prepare defect-free ultrathin molecular sieving membranes for gas separation.26 Huang et al. developed a novel bicontinuous ZIF-8@GO membrane through layer-by-layer (LBL) deposition of a graphene oxide (GO) suspension on a semicontinuous ZIF-8 layer; the ZIF-8@GO membranes show a high hydrogen selectivity.27 Petit et al. synthesized MOF–GO nanocomposites with different ratios of GO and MOF for gas adsorption applications.28 These exciting developments show the significant advantages of using hybrid nanocomposites over single nanoparticle types. Nevertheless, most nanocomposites based on GO and MOFs were applied to gas separation or adsorption, while none of them have been exploited as water treatment membranes up to this point. Derived from the rational design concept, the present study attempts to design a novel hybrid nanosheets that integrates the virtues of GO and MOFs and apply them in the fabrication of antimicrobial thin film nanocomposite (TFN) nanofiltration membranes via interfacial polymerization. Interfacial polymerization is a remarkably simple process to prepare TFN membranes and endows them with a defect-free, continuous and ultra-thin film to maximize water permeability and minimize pumping energy.29 Zeolitic imidazolate framework-8 (ZIF-8), a commonly used zeolitic MOF material, was selected to prepare ZIF-8/GO hybrid nanosheets through facile in-situ growth because ZIF-8 is prone to be synthesized at room temperature and may carry antimicrobial properties.30 Generally, aggregates may be formed in the MOF dispersion and membranes, ultimately impacting the role of MOF in membranes even if the MOF dispersion is treated by sonication previously.24, 31 Nevertheless, the in-situ growth of ZIF-8 onto GO surface can largely obliterate the agglomeration of ZIF-8 in the membrane owing to the coordination 4


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between Zn2+ and carboxyl groups of GO which can disperse Zn2+ uniformly on the GO surface (Figure 1b). Conventional antimicrobial agents like metal (e.g., silver or copper) ions or nanoparticles, titanium dioxide, graphene oxides, chitosan and enzymes have been extensively utilized for the preparation of antibacterial membranes. All of these traditional antimicrobial agents, however, are restricted by some drawbacks such as low stability, activity and selectivity.9, 12, 32-36

Whereas, by superimposing the antimicrobial activity and the internal merits of both ZIF-8

and GO, ZIF-8/GO hybrid nanosheets may act as a more outstanding multifunctional antimicrobial agent over these traditional antimicrobial agents. Thereby, the improvement in overall membrane performance of the antimicrobial TFN nanofiltration membranes can be expected. Herein, in-situ growth of ZIF-8/GO hybrid nanosheets were synthesized and incorporated into membranes as antimicrobial agents. To study the effects of ZIF-8/GO hybrid nanosheets on membrane structural and separation properties, a series of antimicrobial TFN nanofiltration membranes were prepared by incorporating GO or varied amounts of ZIF-8/GO into aqueous solutions. The synthesis of ZIF-8/GO hybrid nanosheets was confirmed by TEM, XRD and

TGA. The physicochemical properties of membranes were systematically characterized by FT-IR, XPS, FESEM and TGA. Furthermore, the separation performance was investigated through water flux and salt rejection tests. Antibacterial activities of ZIF-8/GO hybrid nanosheets and the as-prepared membranes were assessed against gram-negative E. coli bacteria using typical counting methods.

2. Experimental 2.1. Materials

All reagents and solvents were commercially available and used as received. Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 99% purity) and 2-methylimidazole (99% purity, Hmim) used to synthesize ZIF-8 were obtained from Sigma-Aldrich. Natural graphite powder (spectral pure, ∼45 µm particle diameter) was procured from Sinopharm Chemical Reagent Co., Ltd. Polyethersulfone (PES) ultrafiltration membranes (MWCO: 10,000 Da, water permeation flux: 90 L m-2 h-1 bar-1) were purchased from Sepro Membranes Co., Ltd. Piperazine (PIP, 99% purity) and trimesoyl chloride (TMC, 99% purity) were acquired from Sigma-Aldrich. All other chemicals (analytical 5



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grade) used were obtained from Tianjin Kermel Chemical Reagent Co. Ltd., China. Deionized water was employed throughout the experiments. 2.2. Synthesis of ZIF-8/GO hybrid nanosheets

Graphene oxide (GO) powders were prepared from natural graphite through a modified Hummer’s method.37 ZIF-8/GO hybrid nanosheets were prepared by in-situ growth of ZIF-8 onto the GO nanosheets and synthesis procedure is illustrated in Figure 1b. To fabricate the ZIF-8/GO hybrid nanosheets, 50 mg of GO powder was first dissolved into a solution consisting of 0.1833 g of Zn(NO3)2·6H2O and 25 mL of deionized water; the resulting solution was then sonicated to obtain a GO suspension solution. Next, 2 g of trimethylamine (TEA) was added into a 25 mL aqueous solution containing 3.24 g of 2-methylimidazole while vigorously stirring. The GO suspension solution was subsequently poured quickly into the 2-methylimidazole solution. Stirring was stopped after 1 h while the prepared ZIF-8/GO hybrid nanosheets were washed with deionized water and recovered by centrifugation (6000 rpm, 10 min) for three times38. For comparison, pure ZIF-8 nanoparticles were synthesized via the same process without adding GO nanosheets into the zinc nitrate solution (Figure 1a). 2.3. Preparation of ZIF-8/GO thin film nanocomposite (TFN) membranes

PES ultrafiltration membranes were used as the support layer of the TFN membranes. Pretreatment of the membranes contained two steps. First, PES ultrafiltration membranes were soaked in aqueous solutions of sodium dodecyl sulfate (SDS, 0.5 g/L) for at least 24 h to remove impurities. Second, these membranes were rinsed by distilled water and dried by air blowing oven before being fixed in a cap device for interfacial polymerization. Aqueous solutions used for interfacial polymerization were prepared by adding X mg of GO or ZIF-8/GO hybrid nanosheets into the precursors including PIP (0.25%), SDS (0.1%) and Na2CO3 (0.1%) under sonication until the solutions were well-dispersed. The as-prepared TFN membranes were designated as TFC, TFN-GO, TFN-ZG1, TFN-ZG2, and TFN-ZG3 corresponding to 0 wt% GO and ZIF-8/GO, 0.2 wt% GO, 0.1 wt% ZIF-8/GO, 0.2 wt% ZIF-8/GO, and 0.3 wt% ZIF-8/GO, respectively. TFN membrane fabrication process began with the immersion of the PES substrate membranes into the interfacial polymerization aqueous solutions at room temperature for 30 min to execute the adsorption of PIP and the deposition of GO or ZIF-8/GO hybrid nanosheets onto the membrane 6


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surface. Immediately after, an equal volume of 0.1 wt% TMC/n-hexane solution was added to the top of the membrane surface to create the interface, and the polymerization was continued for 3 min. After the reaction, excess organic solution was drained, and the nanofilm surface was cleaned with a sufficient amount of n-hexane and dried for 30 min to evaporate the n-hexane. Afterwards, the membranes were dried at 70 ℃ for 15 min to promote the stability of the obtained TFN membranes. 2.4. Characterization of ZIF-8/GO hybrid nanosheets

Powder X-ray diffraction (XRD) analyses were performed on GO, ZIF-8, and ZIF-8/GO by PANalytical X’Pert Pro (PANalytical, Netherlands) in the scanning range of 2θ between 5o and 90o using copper Kα as the source of radiation. Transmission electron microscopy (TEM) was investigated via a FEI model TECNAI G2 transmission electron microscope (FEI, USA) under 200 kV acceleration voltages. Prior to imaging, the samples were well dispersed into the solvent with the aid of ultrasound, and the suspended particles were then transferred to a 400 mesh carbon-coated copper grid and dried. Thermogravimetric analysis (TGA) were carried out under nitrogen using a TG-DTA, DT-40 system (Shimadzu, Japan) in which each sample was heated from 25 to 800 °C with a heating rate of 10 °C/min. 2.5. Membranes Characterization

The functional groups on the membrane surfaces were verified by Fourier transform infrared spectroscopy (FTIR, Nicolet Magna-IR 560 Spectrometer). The transmittance spectra were conducted from 670 to 4000 cm-1 with a resolution of 4 cm-1 at room temperature. All detections were performed using air as the background. The surface chemical compositions of the membranes and the materials were analyzed by X-ray photoelectron spectroscopy (XPS, PHI-1600 X-ray photoelectron spectrometer, USA) using Mg Kα as the radiation source. The take-off angle of the photoelectron was set at 90o. Survey spectra of the membranes were collected over a range of 0 to 1300 eV. The cross-sectional and surface morphologies of the membranes were inspected by a field emission scanning electron microscope (JEOL Model JSM-6700F, 7



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Tokyo, Japan), which was also used to perform energy dispersive X-ray spectroscopy (EDS) compositional analysis. For cross-sectional studies, membrane samples were prepared by first soaking in ethanol, followed by freeze-fracturing in liquid nitrogen, and then drying in dry air. Finally, the membranes were sputtered with gold. FESEM imaging was carried out at 10 kV. To study the surface wetting nature of the membranes, dynamic water contact angle measurements were completed at room temperature using an EasyDrop contact angle instrument and software (Kruss, Germany). All membranes were dried at room temperature prior to measuring their contact angles. A drop of water (3.0 µL) was placed on the membrane surface using a micropipette. Contact angles were measured using a circle fitting method by the drop shape analysis software. 2.6. Antimicrobial activities of the membranes

In order to fully investigate the antimicrobial properties of the membranes, the antibacterial activity of the antimicrobial agents (GO, ZIF-8 and ZIF-8/GO) and membranes were characterized by determining the minimum inhibitory concentration (MIC) and bacteriostasis rate (BR) of each membrane. Escherichia coli (E. coli) were used as a model bacterium. The target E. coli cultures were grown overnight in a Luria–Bertani (LB) broth at 37 °C to a mid-log phase. The cultures were subsequently diluted in fresh medium and grown until log phase (2 h), which was confirmed by measuring the optical density at 600 nm. Afterward, the bacterial cells were suspended and diluted in sterile saline solution to 107 colony-forming units (CFU) mL-1. Bacteriostasis rates were used to quantitatively analyze the antibacterial activities of the membranes. In a typical procedure, a membrane sample (1 × 6 cm2) was put into a test tube containing 10 mL of E. coli suspension (105 CFU mL-1) before being incubated at 37 °C for 4 h. Then, 100 µL of diluted E. coli solution obtained by the standard serial dilution method was spread onto an agar plate and incubated at 37 °C for 12 h. Three measurements were carried out for each membrane, and the mean values were reported in the results. The numbers of colonies on the plates were counted, and the bacteriostasis rate, BR, was defined by the following equation:

BR =

A- B × 100% A

(1)

where A is the number of colonies on the control plates without membranes, and B is the number of colonies on the plates treated with membranes.

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For the MIC tests, the antibacterial agents were diluted to serial dilutions (512, 256, 128, 64, 32 g/mL) by sterile saline solution before being inoculated with E. coli suspension and tested for bacteriostasis rate (BR). The lowest concentration that can satisfied, BR> 90%, represented the MIC of the antibacterial materials. 2.7. Membrane separation experiments

The water filtration performances of PES, TFC, and TFN nanofiltration membranes were evaluated by measuring pure water flux and salt rejection. A cross-flow filtration system (i.e., system with the feed stream flowing tangentially to the membrane surface) was employed to test the performance of the prepared membranes. The detailed operation of the nanofiltration set-up was described in our previous study.39 The effective areas of the nanofiltration membranes were each 28.26 cm2. Each tested membrane was initially pre-pressurized with pure water at 0.6 MPa for 30 min to ensure that the membranes reached a steady state prior to performing the nanofiltration experiments. The membranes were then tested in the range of 0.2 to 0.8 MPa. The pure water flux of the prepared membranes was first measured and calculated using the following equation:

J = AV∆t

(2)

where V is the volume of permeate pure water (L), A is the effective area of the membrane (m2), and ∆t is the permeation time (h). After the pure water permeability tests, feed solutions containing 1 g/L concentrations of either NaCl, Na2SO4, MgCl2, or MgSO4 were applied to evaluate the water softening performance of these composite membranes. The salt rejection, R, of each tested membrane was calculated using the following equation:

R = (1 − Cp ) × 100% Cf

(3)

where Cp is the permeate concentration and Cf is the feed concentration. The concentrations of the inorganic salt solutions were measured with an electrical conductivity meter.

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3. Results and discussion 3.1. Characterization of ZIF-8/GO hybrid nanosheets

Powder XRD patterns of GO, ZIF-8, and ZIF-8/GO hybrid nanosheets are collected in Figure 2. Pristine GO shows its most intense peak at 2θ = 9.9o, corresponding to the (002) plane with the formation of a large amount of oxygenated functional groups on graphitic planes and representing an interlayer distance of 9.6 Å.40 In the case of ZIF-8, various sharp diffraction patterns of ZIF-8 are preserved; these patterns are in agreement with data published in the literature.41-42 As for ZIF-8/GO hybrid nanosheets, the characteristic diffraction peak of GO at 2θ = 9.9o is absent, which may be attributed to a distortion of the stacked GO. Nevertheless, the predominant diffraction patterns of ZIF-8 are preserved in ZIF-8/GO, although the intensity of the peaks is reduced. These patterns are consistent with the composition of ZIF-8/GO hybrid nanosheets, as ZIF-8 is the predominant component. The XRD results demonstrate that the presence of GO does not prevent the formation of linkages between zinc oxide and 2-methylimidazole and the formation of ZIF-8/GO hybrid nanosheets. ZIF-8/GO nanocomposites are expected to have a distinct structure, which provides fingerprints of their textural and chemical nature. The texture of the nanosheets can be observed on the TEM images presented in Figure 3. For comparison, TEM images of GO and ZIF-8 are also included. GO shows a distinctive transparent and distorted laminar structure (Figure 3a). The synthesized ZIF-8 exhibits a hexagonal morphology with a particle size in the range of 40 to 80 nm (Figure 3b). In the case of ZIF-8/GO nanosheets, hexagonal ZIF-8 crystals with a few irregular squares or rectangles are uniformly stabilized on the GO surfaces. Besides, ZIF-8 crystals preferably formed on the edge of the GO nanosheets instead of the middle area, and the ZIF-8 crystals existing on their edge have larger crystal sizes than those located in their middle range (Figure 3c, d). The GO sheets appear to provide a scaffold for the nucleation and growth of crystals; the ZIF-8 shapes and sizes are probably controlled by the epoxy groups occurring on the GO sheets or the carboxylic groups located at the edges through coordination modulation.28, 43 Therefore, using the in-situ growth technique, ZIF-8 can be homogeneously distributed on the surface of GO and that may be the best structure for ZIF-8/GO hybrid nanosheets to present their maximum activity. TGA diagrams of GO, ZIF-8, and ZIF-8/GO hybrid nanosheets are shown in Figure 4. The weight 10


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loss of ZIF-8/GO hybrid nanosheets consisted of three steps. The first stage was from room temperature to 120 °C with a total mass loss of 6.4%, which was mainly due to the loss of adsorbed water. The second stage was from 120 to 320 °C with a total mass loss of approximate 30%, attributed to the decomposition of GO. The decomposition of GO in ZIF-8/GO hybrid nanosheets contained two phases since the interaction between ZIF-8 and GO may delay the pyrolysis process of GO that coved by in-situ growth of ZIF-8. Therefore, in addition to the decomposition of exposed GO from 120 to 180 °C, the weight loss at 180-320 °C also relates to the decomposition of GO. The final step over the temperature range from 320 to 600 °C with an obvious mass loss of 63% was assigned to the removal of the organic linker molecules and the collapse of ZIF-8. The same trend has also been found in a related study.43 From the TGA analysis, it can be roughly calculated that the percentage of ZIF-8 in the composite is 63%. 3.2. Characterization of membranes

The FTIR spectra of PES support, TFC, TFN-GO, TFN-ZG1, TFN-ZG2 and TFN-ZG3 are shown in Figure 5. The characteristic bands of typical polyethersulfone at 1578 (aromatic bands), 1485 (benzene ring and C–C bond stretch) and 1240 (aromatic ether bands) cm-1 are observed in the FTIR spectra of the PES membranes.44 TFC and TFN membranes also show these characteristic bands of PES, but the bands become weaker. As shown in Figure 5, the TFC membranes exhibit new bands at 1650 and 1359 cm-1 compared with the PES membrane. These two peaks belong to C=O stretching and C-N stretching vibrations, respectively, corresponding to amide structure of the polyamide layer formed during interfacial polymerization.24, 31 The TFN membranes show the same peaks, which demonstrates that the polyamide layer is formed in the presence of GO or ZIF-8/GO nanocomposites. Furthermore, a new broad band peak around 3430 cm -1 appears in the spectra of TFN−GO membranes, which can be assigned to O–H stretching vibrations in water and hydroxyl groups in the GO.7 This broad band peak can also be found in TFN-ZG membranes and becomes much weaker than in the TFN−GO membranes, which is attributed to the O–H stretching vibrations in ZIF-8/GO nanosheets and confirms the presence of ZIF-8/GO nanocomposites in the polyamide thin layer. The characteristic bands of GO and ZIF-8 between 700 and 1700 cm− 1 are not well pronounced due to their minute amount in the membranes and may be masked by the peaks of PES and polyamides. Because of the limitation of our IR apparatus (for mid-IR

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measurements only), the most interesting Zn–N stretch mode which expected at 421 cm−1 was not observed.45 Therefore, XPS was used to further investigate the surface chemical structure of the membranes. The X-ray photoelectron spectroscopy of ZIF-8/GO membranes was shown in Figure 6, the chemical composition and the atomic percentage of the nanofilm were calculated from XPS and listed in Table 1. It was obvious from Figure 6a that the original PES substrate presented characteristic peaks of C 1s and O 1s, while peaks of C 1s, O 1s and N 1s were observed from the spectrum of TFC and TFN-ZG3. As an indication of the polyamide structure, the enhancement of the intensity of N 1s (from polyamide) was observed compared with the original PES substrate. Furthermore, the C 1s core-level spectra of TFN-ZG3 was shown in Figure 6b, which could be curve fitted with three peak components with binding energies of 284.6 eV for C-H, 285.5 eV for C-N/C-OH and 287.8 eV for C=O, certificating the formation of polyamide.

46-47

Besides, the

presence of Zn 2p in TFN-ZG3 indicates the existence of ZIF-8/GO on the membrane surface (Figure 6c, d).27 The results of XPS confirm the presence of ZIF-8/GO nanosheets and the formation of polyamide through interfacial polymerization in the TFN membranes. The top surface and cross-section morphology of the membranes were characterized by FESEM. The FESEM image of membranes surface (Figure 7) reveals that all the membranes have a finely dispersed nodular structure packed by the spherical globules on surface of the dense polyamide layer. These nodular structures have been reported for several TFC/TFN membranes prepared by interfacial polymerization, which is assumed to be a result from the growth of initial polyamide tufts caused by the defect in the interface at the beginning of crosslinking. Specifically, polyamide tufts act as a seeds which can attract more aqueous phase monomer (PIP) and boost the formation of nodules during the process of polymerization.48-50. Therefore, the size of the nodules is controlled by the regional concentration of monomer near the seeds along with the reaction rate between aqueous phase monomer and organic phase monomer.51 In the case of TFN-ZG membranes, the quantity and size of the nodular structure simultaneously decrease with an increase of ZIF-8/GO content. The polymerization process appears to be tailored by the content of ZIF-8/GO hybrid nanosheets. In detail, ZIF-8/GO nanocomposites may adsorb the PIP in aqueous solution, thereby reduce the amount of defects and the regional concentration of PIP in their vicinity, ultimately restricting the quantity and size of polyamide tufts while rendering the nodular 12


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structure loosely dispersed in TFN-ZG membrane surfaces. Dramatically, a volcano-like structure can be seen in TFN-ZG3 (Figure 7d). Owing to the increasing amount of ZIF-8/GO nanocomposites, more PIP would be adsorbed by ZIF-8/GO, thus the formation of the defects was limited and the growth of polyamide nodular structure can shift from the defects in the interface to the surface of ZIF-8/GO nanocomposites. Therefore, the formation of this volcano-like structure may originate from the integration of ZIF-8/GO nanocomposites and the polyamide nodular structure. EDS analysis was used to further study the elemental composition of the membrane surfaces modified by ZIF-8/GO nanocomposites (see Figure S2). The results demonstrated that no zinc element existed on the selected nodular of TFC membrane. Additionally, combing with the result of XPS, it can be concluded that the nodular structure was polyamide. As seen from Figure S2b, the measured 0.62 wt.% of zinc element verified the existence of ZIF-8/GO nanocomposites on the nodular structure of TFN-ZG3. The cross-section morphology of the membranes was shown in Figure 8. All the TFC/TFN membranes comprised a thin dense layer coated on top of the PES support. The thinnest active layer (92.6 nm) was observed for TFC membranes in Figure 8a. With increasing ZIF-8/GO content, the thickness of the selective layer increases from 131 nm for TFN-ZG1 to 145 nm for TFN-ZG2. In the case of TFN-ZG3, the thickness of the selective layer was significantly boosted, to 168 nm. This trend was opposite to the reduction in amount and size of nodular structure on the membrane surface, while it perfectly corroborated the constant amount of aqueous phase monomer and organic phase monomer used in these membranes as well as our hypothesis about the variation of the nodular structure. 3.3. Antimicrobial properties

To demonstrate our envision about the ZIF-8/GO hybrid nanosheets, we evaluated the antimicrobial activities of each membrane sample against E. Coli, (Figure 9a and Figure 10) The amount of viable colonies of E. coli shown in Figure 10 a-c was slightly changed, which indicated that the TFC and TFN-GO membranes can hardly remove bio-fouling caused by E. coli. TFN-GO membranes fails to exhibit its antimicrobial activity because GO belongs to a contact-based antimicrobial material. In the process of interfacial polymerization, GO as aqueous monomer is

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prone to submerged and covered by the polyamide (Figure 11a) due to its hydrophilicity, negligible thickness as well as the feature of IP (aqueous solution covered by organic solution while the polymerization happens between interfaces). Furthermore, it was found that the count for viable colonies of E. coli sharply decreased after incorporating ZIF-8/GO nanocomposites. Correspondingly, BR of the membranes rose from 50.7% for TFN-ZG1 to 66.3% for TFN-ZG2 and 84.4% for TFN-ZG3. In contrast to GO, the stereo structure of ZIF-8 will facilitate the exposure of hybrid ZIF-8/GO nanosheets on the surface of membranes (Fig. 1b, Fig. 7d and Fig. S1), thus endowing the TFN-ZG membranes with improved antimicrobial activity. Furthermore, the MIC of ZIF-8, GO and ZIF-8/GO showed that hybrid ZIF-8/GO nanosheets (128 µg/mL) had a more intensive antimicrobial activity than the other two individual materials (256 µg/mL for both of ZIF-8 and GO). Therefore, it was concluded that the ZIF-8/GO hybrid nanosheets with stronger antimicrobial activity are responsible for the sharp anti-biofouling property of TFN-ZG3. Theoretically, bacterial inactivation by GO comprises direct puncturing of the cell membrane, oxidative stress, extraction of phospholipids from the lipid bilayer, and adhesion of graphene sheets on the cell surface.21 There are no literatures reported on the application of ZIF-8 as an antimicrobial agent and its antimicrobial mechanism. In detail, the imidazole ring in ZIF-8 was accredited for antibacterial activity and imidazole derivatives were well-known for a class of compounds with diverse biological activities.52-53 The antimicrobial activity of imidazole originates in the disruption of liposomes which are composed of phospholipids containing unsaturated fatty acids.54 However, mammalian cells and gram-negative bacilli (like E. coli used in this study) which have limited free fatty acids are not very sensitive to imidazole, whereas fungi and many gram-positive bacteria rich in free fatty acids are sensitive to imidazole. Furthermore, the release of zinc ions will also give rise to the antimicrobial activity of ZIF-8 because of the natural antimicrobial property of metal ions.55-56 MOFs can act as a reservoir of metal ions, providing their gradual release and resulting in a sustained antibacterial action analogous to that proposed for metal/metal oxide nanoparticles (NPs).57 The release of zinc ions for TFN-ZG3 membrane was confirmed by ICP-MS (Table S1). Therefore, a synergistic effect involving graphene oxide and ZIF-8 is identified as a possible pathway that plays an important role in antimicrobial activity of TFN-ZG membrane, and the mechanism of TFN-ZG antimicrobial membranes is presented in Figure 11b. The oxidative stress was proposed to be the primary 14


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antimicrobial mechanism when GO was not in suspension condition.58 The oxidative stress of GO was triggered by the direct formation of reactive oxygen species (ROS) or by the depletion of cellular antioxidants, which can lead to the oxidation of lipids and can ultimately result in membrane damage and cell death.20 Cellular internalization,59 ROS generation, plasma membrane disorganization caused by the released Zn2+ was responsible for bactericidal activity of ZIF-8 in this study.57 3.4.

Filtration performance of membranes

Membranes hydrophilicity has an imperative influence on its antifouling property and filtration performance. In order to accurately measure the hydrophilicity of resultant membranes, we show the water contact angle (WCA) in Figure 9b. Owing to inherent hydrophilicity of polyamide and GO, the WCA rapidly (in a few seconds) declined to extreme low number for all the membranes, disclosing their excellent hydrophilicity. Furthermore, it is notable that the WCA of TFN-ZG membranes increase with ZIF-8/GO content (from 10.7 o for TFN-ZG1 to 16.8

o

for TFN-ZG3).

this is derived from the hydrophobic properties of ZIF-8. The effect of various ZIF-8/GO contents on the permeability at different pressures was examined and the results are shown in Figure 9c. It was found that the water flux linearly increased with the enhancement of operating pressure for all of the as-prepared membranes. In detail, the TFC membrane gave the lowest water permeability of 26.63 L m-2 h-1 MPa-1, whereas the TFN-ZG1 and TFN-ZG2 reached up to ca. 40.63 L m-2 h-1 MPa-1. This can be explained as follows: the occurrence of ZIF-8/GO hybrid nanosheets in the polyamide layer can reduce the interaction of the polyamide chains and undermine the polymer chain packing to a certain degree due to the relatively poor compatibility between inorganic and organic parts, thereby decreasing the compactness of the selective layer.18 The improved surface hydrophilicity ascribed to hydrophilic GO can further enhance the permeability of the modified membranes. In contrast, in the case of TFN-ZG3, the pure water flux significantly decreased to the same value as that of TFC membranes. A plausible explanation is that the thicker polyamide layer and partial agglomeration of ZIF-8/GO nanosheets may block the pathways for permeation and increase the resistance of water transport. Four types of salt solutions (MgCl2, MgSO4, NaCl and Na2SO4) as the single solute were applied

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to explore the NF performance under an operating pressure of 4 bar, and the results are presented in Figure 9d. For Na2SO4 and MgSO4, the rejection of all the presented membranes were around 100% and 77%, respectively. All membranes showed the lowest rejection when MgCl2 was as feed solution. TFC membranes have a rejection of 20.2%, while all the TFN membranes have a lower rejection for MgCl2 (10.5%). This may be triggered by the enhanced negative charge density of TFN membranes with the introduction of ZIF-8/GO nanocomposites. According to the electrostatic repulsive interaction mechanism, the negative charge of the membrane surface attracts a high-valent cation and repulse a high-valent anion, which results in a high rejection of Na2SO4 and a low rejection of MgCl2.60 Furthermore, it could be observed that the rejection data of TFC, TFN-ZG1, TFN-ZG2, and TFN-ZG3 for NaCl showed an obvious trade-off effect. The rejection order for all the membranes was Na2SO4>MgSO4>NaCl>MgCl2, which greatly corresponds to negatively charged NF membranes.61 Table S3 summarized the permselectivity perferance of other GO membrane that prepared by interfacial polymerization in the litureature. It is easy to see that our ZIF-8/GO membranes show better permselectivity perferance (higher water flux and good salts rejection).

4. Conclusion In this study, a facile approach for the development of ZIF-8/GO functionalized TNF membrane with an enhanced antimicrobial activity was firstly explored and studied. Firstly, uniformly dispersed ZIF-8/GO hybrid nanosheets that integrated the merits of the ZIF-8 and GO were achieved via an in-situ growth method; the character of ZIF-8/GO hybrid nanosheets was confirmed by TEM, XRD and TGA. Then, a series of ZIF-8/GO TFN membranes were fabricated by immobilizing ZIF-8/GO hybrid nanosheets onto a PES membrane via interfacial polymerization. The physicochemical properties of membranes were systematically characterized by FT-IR, XPS, FESEM and TGA. The permeability of the TFN membranes greatly improved from 26.63 L m-2 h-1 MPa-1 (TFC) to 40.63 L.m-2 h-1 MPa-1 (TFN-ZG1), without sacrificing their rejection. In addition, the antimicrobial activity of the ZIF-8/GO nanocomposites exhibited a higher antimicrobial activity (MIC, 128 µg/mL) contrasted to ZIF-8 and GO. Besides, surface modification with ZIF-8/GO hybrid nanosheets conferred effective antimicrobial activity (84.4% for TFN-ZG3) and high hydrophilicity to TFN nanofiltration membranes. The mechanism of 16


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antimicrobial activity of ZIF-8 and ZIF-8/GO membranes is postulated for the first time. Considering the superior overall performance derived from ZIF-8/GO hybrid nanosheets, we anticipate the promising future of ZIF-8 based nanocomposites as a multifunctional antimicrobial agent for versatile applications. Supporting information XPS survey spectra of ZIF-8/GO hybrid nanosheets, energy dispersive spectroscopy (EDS) analysis of the nodular structure on TFC and TFN-ZG3 membrane, separation performance comparison between GO membranes prepared by interfacial polymerization in the literature and this study. ACKNOWLEDGMENT This work has been funded by the National Natural Science Foundation of China (Nos. 21376225 and 21476215), Program for Science & Technology Innovation Talents in Universities of Henan Province (16HASTIT004) and Excellent Youth Development Foundation of Zhengzhou University (No. 1421324066). Jing Wang sincerely acknowledges the financial support provided by China Scholarship Council (CSC) of the Ministry of Education, P.R. China. References (1) Fane, A. G.; Wang, R.; Hu, M. X., Synthetic Membranes for Water Purification: Status and Future. Ange. Chem. Int. Edit. 2015, 54 (11), 3368-3386. (2) Pendergast, M. M.; Hoek, E. M. V., A Review of Water Treatment Membrane Nanotechnologies. Energ. Environ. Sci. 2011, 4 (6), 1946-1971. (3) Li, R.; Zhang, L.; Wang, P., Rational Design of Nanomaterials for Water Treatment. Nanoscale 2015, 7 (41), 17167-17194. (4) Mohammad, A. W.; Teow, Y. H.; Ang, W. L.; Chung, Y. T.; Oatley-Radcliffe, D. L.; Hilal, N., Nanofiltration Membranes Review: Recent Advances and Future Prospects. Desalination 2015, 356 (0), 226-254. (5) Yin, J.; Deng, B., Polymer-matrix Nanocomposite Membranes for Water Treatment. J. Membr. Sci. 2015, 479 (0), 256-275. (6) Han, Y.; Xu, Z.; Gao, C., Ultrathin Graphene Nanofiltration Membrane for Water Purification. Adv. Funct. Mater. 2013, 23 (29), 3693-3700. (7) Yu, L.; Zhang, Y.; Zhang, B.; Liu, J.; Zhang, H.; Song, C., Preparation and Characterization of HPEI-GO/PES Ultrafiltration Membrane with Antifouling and Antibacterial Properties. J. Membr. Sci. 2013, 447, 452-462. (8) Wang, J.; Lang, W.-Z.; Xu, H.-P.; Zhang, X.; Guo, Y.-J., Improved Poly(Vinyl Butyral) Hollow Fiber Membranes by Embedding Multi-Walled Carbon Nanotube for the Ultrafiltrations of Bovine Serum Albumin and Humic Acid. Chem. Eng. J. 2014. (9) Zhao, Q.; Hou, J.; Shen, J.; Liu, J.; Zhang, Y., Long-lasting Antibacterial Behavior of a Novel Mixed Matrix Water Purification Membrane. J. Mater. Chem. A 2015. (10) Wang, Y.; Liu, C.; Zhang, Y.; Zhang, B.; Liu, J., Facile Fabrication of Flowerlike Natural Nanotube/Layered Double Hydroxide Composites as Effective Carrier for Lysozyme Immobilization. ACS Sustain. Chem. Eng. 2015, 3 (6), 1183-1189. (11) Jiang, J.; Zhu, L.; Zhu, L.; Zhang, H.; Zhu, B.; Xu, Y., Antifouling and Antimicrobial Polymer Membranes Based on

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Figure 1 Illustration of the synthesis process of ZIF-8 (a), and the in-situ growth of ZIF-8/GO hybrid nanosheets (b).

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Figure 2 Power XRD patterns of GO, ZIF-8, and ZIF-8/GO hybrid nanosheets.

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Figure 3 TEM images of GO (a), ZIF-8 (b), and ZIF-8/GO hybrid nanosheets (c, d).

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Figure 4 Thermogravimetric analysis (TGA) curves of GO, ZIF-8, and ZIF-8/GO hybrid nanosheets

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Figure 5 FTIR spectra of PES, TFC, TFN-GO and TFN-ZG membranes.

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Figure 6 XPS survey spectra of pristine PES membranes, TFC membranes and TFN-ZG3 membranes (a); C 1s core-level spectra of TFN-ZG3 membranes (b); Zn 2p core-level spectra of TFN-ZG3(c) and TFC membranes (d).

Table 1 Atomic concentrations of different elements for PES subtract, TFC and TFN-ZG3 membranes Membrane

Atomic concentrations (%) of different elements

ID

S 2p

C 1s

N 1s

O 1s

Zn 2p

PES

4.07

71.73

4.38

19.82

/

TFC

0.59

70.78

12.76

15.87

/

TFN-ZG3

0.97

70.92

11.15

16.71

0.26

26


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Figure 7 Surface FESEM images of resultant membranes: TFC (a), TFN-ZG1 (b), TFN-ZG2 (c) and TFN-ZG3 (d).

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Figure 8 Cross-sectional FESEM images of resultant membranes: TFC (a), TFN-ZG1 (b), TFN-ZG2 (c) and TFN-ZG3 (d).

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Figure 9 Number of viable E. coli colonies growing on agar plates (a) and water contact angle (b) for resultant membranes. Pure water flux (c) and salt rejection ratios (d) of TFC membranes, TFN-ZG1 membranes, TFN-ZG2 membranes, and TFN-ZG3 membranes at 0.4 MPa.

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Figure 10 Antimicrobial activities against E. coli of blank sample (a), TFC membranes (b), TFN-GO membranes (c), TFN-ZG1 membranes (d), TFN-ZG2 membranes (e), and TFN-ZG3 membranes (f).

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Figure 11 Schematic diagrams for the antimicrobial mechanism of TFN-GO membranes (a) and TFN-ZG membranes (b).

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Graphical abstract


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Graphene Oxide Hybrid Nanosheets

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