Surface-concentrated chitosan-doped MIL-100(Fe) nanofiller-containing PVDF composites for enhanced antibacterial activity

Journal Pre-proofs Surface-concentrated chitosan-doped MIL-100(Fe) nanofiller-containing PVDF composites for enhanced antibacterial activity Kie Yong Cho, Cheol Hun Yoo, Young-June Won, Do Young Hong, Jong-San Chang, Jae Woo Choi, Jung-Hyun Lee, Jong Suk Lee PII: DOI: Reference:

S0014-3057(19)30971-1 https://doi.org/10.1016/j.eurpolymj.2019.109221 EPJ 109221

To appear in:

European Polymer Journal

Received Date: Revised Date: Accepted Date:

13 May 2019 29 August 2019 30 August 2019

Please cite this article as: Yong Cho, K., Hun Yoo, C., Won, Y-J., Young Hong, D., Chang, J-S., Woo Choi, J., Lee, J-H., Suk Lee, J., Surface-concentrated chitosan-doped MIL-100(Fe) nanofiller-containing PVDF composites for enhanced antibacterial activity, European Polymer Journal (2019), doi: https://doi.org/10.1016/j.eurpolymj. 2019.109221

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

© 2019 Published by Elsevier Ltd.

Surface-concentrated chitosan-doped MIL-100(Fe) nanofiller-containing PVDF composites for enhanced antibacterial activity Kie Yong Cho a,b,1, Cheol Hun Yooa,1, Young-June Wona, Do Young Hongc, Jong-San Changc,d, Jae Woo Choie, Jung-Hyun Leef,*, Jong Suk Leea,* a

Department of Chemical and Biomolecular Engineering, Sogang University, Seoul 121-742, Republic of Korea. b

Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, TX 77843-3122, United States. c Research

Center for Nanocatalysts, Korea Research Institute of Chemical Technology (KRICT), 141 Gajeong-ro, Yuesong-gu, Daejeon, 305-600, Republic of Korea d Department

of Chemistry, Sungkyunkwan University, Suwon 440-476, South Korea

e Water

Resource Cycle Research, Korea Institute of Science and Technology, Hwarangno 14-gil 5, Seongbuk-gu, Seoul, 02792, Republic of Korea f Department

of Chemical and Biological Engineering, Korea University, 5-1 Anam-dong, Seongbuk-gu, Seoul 136-713, Republic of Korea 1These

authors contributed equally to this work.

*Corresponding author email: [email protected] (J.-H. Lee) and [email protected] (J. S. Lee) KEYWORDS: PVDF composites; MIL-100(Fe); chitosan doping; antibacterial activity;

antifouling Abstract: Antibacterial properties are a major issue for the membrane-based wastewater treatment since the biofilm on membranes generated by bacterial growth can significantly reduce water flux. Here, surface-concentrated MIL-100(Fe)/chitosan (MIL100-CS)-embedded polyvinylidene fluoride (PVDF) composite membranes (i.e., PVDF-S/MIL100-CS) were successfully fabricated by the newly developed fabrication method, so-called solvent-assisted nanoparticle embedding (SANE). The SANE method was modified from a conventional nonsolvent-induced phase separation (NIPS) method by the addition of an intermediate step for the selective arrangement of hydrophilic nanofillers on top of the nascent PVDF film. The 1

additional step in the SANE method enabled the surface-selective filler distribution with the open surface of fillers, resulting in the hydrophilic surface. Also, it facilitated to form the sponge-like pore structures mostly due to the dilution effect of additional solvent. The PVDF/MIL100-CS microfiltration (MF) composite membranes acquired by SANE exhibited the higher antibacterial activity and the substantially enhanced biofouling resistance for E. coli cells than those of the pristine PVDF due to the surface-selective arrangement of MIL100-CS fillers which include both hydrophilic and biocidal properties. The live/dead test for antibacterial activities with E. coli cells further confirmed the enhanced suppression of the biofouling resistance in the asymmetric PVDF/MIL100-CS composite membranes relative to that of PVDF. Our current study offers a new platform for fabricating asymmetric MF composite membranes with enhanced antibacterial activity and biofouling resistance. 1. Introduction Membrane-based separation techniques have garnered extensive attention in water purification fields due to their inherent advantages, including small footprints, excellent energy efficiency, and high separation efficiency [1,2]. However, the membrane fouling issues caused by the undesired accumulation of foulants on the membrane surface or/and in membrane pores should be addressed since it can significantly reduce water flux, hence increasing operating costs [3,4]. In particular, biofouling has been considered most challenging to address since it involves the self-developing process [5]. Various passive antifouling strategies have been attempted to suppress the initial adhesion of bacterial foulants by tailoring the inherent physical and chemical properties of polymeric membrane surfaces [3,6]. Notably, the incorporation of hydrophilic fillers into the polymeric membranes is the most common strategy since hydrophilic membranes are less susceptible to fouling than hydrophobic analogues due to reduction of the hydrophobic interaction between microorganisms and membranes [7,8]. Passive antifouling strategies, however, fail to avoid bacterial colonization and biofilm formation on the membrane surface. With that in mind, many research groups have been developing new antifouling membranes with the active antifouling strategies to eliminate proliferative fouling by the destruction of the chemical structure and inactivation of the microorganisms [3,9-11]. For instance, silver nanoparticles deactivate microorganisms via directly contacting with the cell wall or releasing biocidal silver ions (Ag+) through oxidation in the aqueous media [10]. In addition, TiO2 particles are activated to generate photo-induced reactive oxygen species [11]. However, silver and TiO2 nanoparticles have technically low 2

compatibility with the polymeric matrix. Also, they can lead to the secondary pollutions by the released metal ions and lack stability by oxidation [10,11]. Among various fillers, metal-organic framework (MOF) nanofillers are attractive candidates for the composite membranes since the organic ligands can provide outstanding compatibility with polymer matrix, unlike the metal and metal oxide fillers [12,13]. MOFs, which have unique porous structures with inorganic metal clusters ligated by organic ligand linkers, have recently gained great interest due to a wide range of potential applications, including catalysts, sensors, gas adsorbents, antibacterial, and gas separation membranes [1417]. In particular, the metal sites and the functional groups in the organic ligands have shown an antibacterial interaction with bacterial cells, resulting in biocidal activity [14]. However, most MOFs are sensitive to water due to the lability of the metal-ligand bonds. Only a few MOFs have been reported to have good water stability, including UiO-66, MOF-808, DUT-67, MOF-545, and MIL-100(Fe) [15,16]. Among them, MIL-100(Fe) is attractive for water treatment applications because of their excellent water stability as well as hydrophilic properties [15]. The biocidal activity and hydrophilic properties make the MIL-100(Fe) as a promising filler in the composite membranes for antibacterial applications. With the selection of appropriate nanofillers, the surface-selective distribution of those nanofillers is critical for high-performance membranes, especially antibacterial activity. Recently, Koseoglu-Imer et al. demonstrated that the highly viscous polymer/silver nanoparticle composite solutions led to the fabrication of composite membranes with the unique asymmetric structure of Ag nanoparticle-mounted membranes on the surface, exhibiting the substantially enhanced antibacterial properties [17]. However, the immobilization of Ag nanoparticles on the membrane still needs to be addressed. The reported composite structure in the membrane fabrication has drawn our attention to developing the new fabrication method for asymmetric composite membranes with selectively distributed reinforcing fillers on the surface. In this study, we developed the new fabrication method, so-called solvent-assisted nanoparticle embedding (SANE), for microfiltration (MF) composite membranes by using the PVDF matrix and chitosan (CS)-doped MIL-100(Fe) (MIL100-CS) hydrophilic nanofillers. MIL100-CS was employed to imbue the PVDF composite membranes with the dual features of hydrophilic and biocidal properties as well as the enhanced interfacial interaction with PVDF support by CS. The SANE method led to the unique sponge-like morphology through the entire 3

membrane as well as an asymmetric structure of PVDF composite membranes with surfaceconcentrated MIL100-CS nanofillers (PVDF-S/MIL100-CS). The effects of our new fabrication method for MF composite membranes on the membrane morphology and performance were investigated and compared with those prepared by the conventional nonsolvent-induced phase separation (NIPS) method. More importantly, antifouling performance and mechanism of the newly developed MF composite membranes were elucidated. 2. Experimental 2.1. Materials Polyvinylidene fluoride (PVDF, MW: 275,000 g mol-1) was purchased from Sigma-Aldrich and used after dried at 110 oC for 12 h. Dimethylformamide (99.8% anhydrous), chitosan ( ≥ 75% deacetylated), ethyl alcohol (95%), ethanol (95% anhydrous), Iron(III) chloride hexahydrate (97%), 1,3,5-benzenetricarboxylic acid (BTC, 95%), hydrofluoric acid (HF, 48 wt% in H2O), acetic acid (95%), and nitric acid (70%) were purchased from Sigma-Aldrich and used as received. Phosphate buffered saline (PBS), Luria-Bertani (LB) broth miller (BD DifcoTM), tryptic soy broth (TSB) medium (BD DifcoTM), and agar (extra pure) were purchased from Daejung Chemical and used as received. De-ionized (DI) water (18.2 MΩ cm) was prepared by a Millipore Milli-Q purification system. Escherichia coli (E. coli, ATCC 47076) was purchased from American Type Culture Collection. 2.2. Synthesis of MIL-100(Fe) MIL-100(Fe) was synthesized through the modification of the reported procedure [19]. The hydrothermal reaction for MIL-100(Fe) was performed with the molar ratio of Fe/ BTC/ HF/ HNO3/ H2O = 1.0/ 0.67/ 2.0/ 0.6/ 277. The reactant mixture was loaded into a Teflon-sealed autoclave. The reaction was allowed by heating at 150 oC for 12 h. The light orange color precipitates were obtained by simple filtration and then washed with an excess of deionized water. The as-synthesized MIL-100(Fe) was further rinsed by two-step processes using 80 oC of water and 60 oC of ethanol, sequentially. The acquired resultants were dried at 80 oC under reduced pressure for 2 days. 2.3. Fabrication of chitosan-doped MIL-100(Fe) (MIL100-CS) CS-coated MIL-100(Fe) was fabricated by following the previous report, which showed the highest CS coating on MIL-100(Fe) [19]. The prepared MIL-100(Fe) (0.03 g) in ethanol (6 mL) and CS (0.032 g) in the acidic aqueous solution (pKa: 6.5, 7 mL) suspensions were mixed 4

and then stirred for half an hour. The resulting MIL100/CS was purified by centrifugation and washed with aqueous solutions containing acetic acid (1 vol%). 2.4. Fabrication of the PVDF-N/MIL100-CS membrane via a nonsolvent-induced phase separation (NIPS) method A typical NIPS method was applied to fabricate the PVDF/MIL100-CS membrane for comparison. The dope solution was prepared by mixing two separated solutions of (i) MIL100CS (0.04 g) in DMF (0.36 g) and (ii) PVDF (0.36 g) in DMF (3.24 g). For the MIL100-CS dispersion, horn-type sonication was performed for 10 min with 30 s bursts and 30 s breaks method [18]. The mixture solution was then stirred at 25 °C overnight. Before casting, the mixture solution was degassed by sonication for 10 min and repeated two more times. Once the PVDF/MIL100-CS solution was casted by a casting knife (130 m) on the non-woven polyester fabric, it was immediately immersed into a coagulation water bath and retained for 12 h. The nascent membranes were stored in distilled water until the subsequent use. The PVDF membranes fabricated by the NIPS method will be denoted as PVDF-N hereafter. The PVDFN and PVDF-N/MIL100 membranes were also fabricated by using the same procedure applied for PVDF-N/MIL100-CS. 2.5. Fabrication of the PVDF-S/MIL100-CS membrane via a solvent-assisted nanoparticle embedding (SANE) method The PVDF-S/MIL100-CS membranes were fabricated by the new SANE method, and the SANE method-induced PVDF membranes are denoted as PVDF-S hereafter. The dried PVDF (0.36 g) was dissolved in DMF (3.24 g). The MIL100-CS particle (0.04 g) was dispersed in DMF (0.36 g) by the horn-type sonication for 10 min with 30 s bursts and 30 s breaks method. Before casting, the separated solutions were degassed by sonication for 10 min and repeated two more times. The PVDF solution was casted by a casting knife on the non-woven polyester fabric. Thereafter, a rubber gasket frame was immediately placed on the casted membrane to guide the second solution. The separate MIL100-CS solution was poured onto the gasketguided PVDF solution. The casted membranes were shaken for 3 min using a shaking incubator (30 rpm), and then the acquired membranes were immersed in a water coagulation bath for 12 h. The all-nascent membranes were stored in distilled water until the subsequent use. Both PVDF-S and PVDF-S/MIL100 membranes were fabricated by using the same procedure for PVDF-S/MIL100-CS. However, it should be noted that the separate DMF solvent was gently poured onto the nascent PVDF film for the PVDF-S membrane, while the separate MIL5

100(Fe)/DMF solution was additionally poured for the PVDF-S/MIL-100 counterpart. 2.6. Evaluation of antibacterial activity E. coli cells were used as the model strain for the antibacterial test and the colony forming unit (CFU) numbers were counted to assess the antibacterial behavior of membranes. A single fresh colony from the agar plate was inoculated in 3 g L-1 of tropic soy broth and cultured for 18 h at 200 rpm and 37 °C to acquire a mid-exponential growth phase with a final optical density at 600 nm (OD600) of 0.16 (105 CFU mL-1). The cultured solution (1 mL) was centrifuged for 10 min at 9,000 rpm to obtain a pure bacterial cell pellet. After that, the cell pellet was re-dispersed in 20 ml of the LB solution and diluted in deionized water to an appropriate ratio (103 CFU mL-1 for the bacterial solution and 105 CFU mL-1 for the cross-flow filtration). The antibacterial effect of the prepared membranes was evaluated by following the previously reported protocol [20]. All membranes were disinfected by 60% of the aqueous ethanol solution and rinsed with DI water five times. After that, all membrane coupons were dipped into the prepared bacterial solution and incubated 37 °C for 24 h. All membranes were removed from the bacterial solution, the remaining supernatant solutions were diluted and spread on the nutrient agar plate, and then cultured at 37 °C for 24 h. The number of CFU on each agar plate was counted for the evaluation of the antibacterial activity of prepared membranes. The live and dead bacteria on the membrane surface were observed by fluorescence microscopy. The remaining supernatant solutions, which had contacted with prepared membranes, were stained with SYTO 9 and propidium iodide (PI) of the Live/Dead BacLight bacterial viability kit (Molecular Probes, Willow Creek, OR, USA) at RT for 30 min. Dyed samples were observed with the confocal laser scanning microscopy (CLSM, C1 plus, Nikon, Japan). The obtained images of the membrane surface were merged by an image analysis program (Imaris 6.1.5, Bitplane, Switzerland). 2.7. Evaluation of membrane water flux The pure water flux of all the membranes used in this work was evaluated by a dead-end type, laboratory-scale permeation system (Fig. S1). The membranes were cut into the specimen (membrane area, A = 12.56 cm2) and then placed in an Amicon cell (Amicon® Stirred Cells, Millipore). The pure feeding water was stirred at a rate of 400 rpm throughout the filtration experiment, and the permeate was collected and weighted by an electronic balance. Pure water flux (Jw, L m-2 h-1) was calculated from the collected volume of the permeate (DV) during a certain time interval (Dt) by using the Equation, Jw = DV/ ADt. Before the measurement of the 6

water flux, pre-operation was performed to induce membrane stabilization at 2 bar for 6 h. Then, the water flux of the membrane was monitored every 30 s at 1 bar for 2 h. 2.8. Evaluation of biofouling properties The biofouling resistance of the membranes was evaluated by monitoring the flux change during the lab-scale cross-flow membrane filtration. All of the cross-flow filtration devices such as tubes and membrane modules were dried at 121 °C for 5 h before use. The prepared bacterial solution, which is described in section 2.6., flowed through the membrane module at RT and a constant flow rate of 1 mL min-1 using a peristaltic pump. The transmembrane pressure was maintained at 32 ± 5 kPa. During the cross-flow operation for 24 h, the flux was monitored as a function of the retention time. 2.9. Characterizations Scanning electron microscopy (SEM, Inspect F50, FEI) was used for evaluation of particle and membrane morphologies. The elemental concentrations were analyzed by energydispersive x-ray spectroscopy (EDS, Inspect F50, METEK). The particle size of MIL-100(Fe) and MIL100-CS was examined at 25 °C by using an electrophoretic light scattering spectrometer (ELS-Z2, Otsuka Electronics, Japan). Zeta-potentials (ELS-Z2, Otsuka Electronics, Japan) were characterized by using the 10 mM NaCl solution with polystyrene latex monitor particles at room temperature and repeated five times. X-ray diffraction (PXRD) was performed using Dmax 2500/PC (Rigaku) with Cu Ka radiation (λ = 1.5406 Å). Thermogravimetric analysis (TGA) was performed using a TGA 1 STARe system (Mettler-Toledo International, Inc., Switzerland) under air at a heating rate of 10 oC min-1. Thermal analysis was performed by differential scanning calorimetry (DSC, TA instruments Q20). To quantify the actual amount of nanofillers in the MMMs and the chitosan of MIL100-CS, the temperature was increased to 800 C at a heating rate of 10 C min-1 within the air and then kept at 800 C for 1 h. The surface area was measured by Brunauer-Emmett-Teller (BET) using Micromeritics Tristar 3000. Fourier-Transform Infrared Spectroscopy (FT-IR) spectra were measured using a MAGNA-IR550 Spectrophotometer-Series II in the wavenumber range of 4000–400 cm−1 (at 2 cm−1 resolution). The typical sessile drop method was used to measure the contact angle by using a contact angle analyzer (KRÜSS, DSA100, Germany). An averaged value of the contact angle was obtained from at least 5 measurements for each sample. High-pressure liquid chromatography-mass spectroscopy (HPLC-MS, Agilent 1100 system equipped with degasser, quaternary pump, and UV detector) was used to quantify the amount of released chitosan. 7

3. Results and discussion 3.1. Fabrication of CS-doped MIL-100(Fe)

Fig. 1. (a) Schematic illustration for the fabrication of MIL100-CS. (b) XRD patterns of CS, MIL-100(Fe), and MIL100-CS. SEM images of (c1) MIL-100(Fe) and (c2) MIL100-CS. (d) Particle size distributions of MIL-100(Fe) and MIL100-CS. The hydrophilic CS-doped MIL-100(Fe) (MIL100-CS) nanofillers were fabricated by using a simple mixing approach in the pH-controlled aqueous solution by acetic acid (Fig. 1a) [19]. MIL-100(Fe) was first synthesized by a modified microwave-assisted hydrothermal method [19,22]. The crystallinity of the synthesized MIL-100(Fe) was characterized by XRD, and the acquired XRD patterns for our housemade MIL-100(Fe) were consistent with its simulated patterns, indicating that MIL-100(Fe) was well synthesized (Fig. 1b). The crystal shape of MIL-100(Fe) was examined by SEM, mostly exhibiting the octahedral shape (Fig. 1c1). The size distribution of as-synthesized MIL-100(Fe) was evaluated by DLS and SEM, and its average particle size was determined to be ca. 155 ± 45 nm (Fig. 1d). The surface of MIL100(Fe) was coated by chitosan to provide the biocidal properties as an active strategy for enhanced biofouling resistance [19,23,24]. Hidalgo et al. previously demonstrated that the 8

chitosan was dominantly located on the MIL-100(Fe) surface due to the interfacial interaction mainly driven by coordinative bonds between enriched hydroxyl (OH) groups in CS and Fe (III) in MIL-100(Fe) [19]. Furthermore, the CS coating on the MIL-100(Fe) can preserve the Fe ions from their reduction reactions, further enhancing the water stability of MIL-100(Fe) [19]. The acquired MIL100-CS was characterized by XRD and SEM (Fig. 1). The XRD pattern and the SEM image of MIL100-CS exhibited that the crystal structure and shape of MIL100CS were well maintained in comparison to those of MIL-100(Fe) albeit the acidic solutionbased modification processes (Figs. 1b and 1c). The MIL100-CS nanofillers showed an increase in the average particle size (175 ± 57 nm) compared to that for the as-synthesized MIL-100(Fe) counterpart based on SEM and DLS analysis. In addition, the CS-doping onto MIL-100(Fe) led to the significant decrease in a BET surface area from 1,511.9 m2 g-1 to 566 m2 g-1 as well as the formation of a strong characteristic peak at ca. 1,041 cm-1 which corresponds to C-O-C stretching derived from the glycosidic bond in chitosan (Fig. S2). All of those structural characterizations indicate that the fabrication of MIL100-CS was successfully performed. 3.2. Fabrication of PVDF composite membranes with MIL100-CS (PVDF-S/MIL100-CS)

Fig 2. Schematic illustration of the fabrication procedures for (a) conventional NIPS and (b) newly developed SANE methods. The PVDF composite membrane prepared by the NIPS method is made by casting a mixture of polymer and fillers on the fabric support (Fig. 2a). Consequently, the nanofillers are randomly distributed over the entire membrane, probably inducing reduced filler effects on the antibacterial activity and biofouling resistance. In contrast, a new fabrication method was 9

designed for the surface-selective arrangement of nanofillers by the modification of the conventional NIPS method (Fig. 2b), a so-called solvent-assisted nanoparticle embedding (SANE) method. The SANE method includes the additional unique step. Once the nascent PVDF film is casted on the fabric support, the separate MIL100-CS solution in DMF was poured onto the nascent PVDF film, which was quenched in water after a brief mixing. A surface-selective distribution of nanofillers was expected since diffusion of nanoparticles would be limited by viscous PVDF solution and the subsequent surface-concentrated nanofillers would be more effective for both enhanced antibacterial activity and biofouling resistance. The cross-sectional SEM images for the PVDF-S/MIL100-CS prepared by the SANE method and the PVDF-N/MIL100-CS prepared by the NIPS method were compared to each other in order to investigate the effect of different fabrication methods on both morphology and filler distribution (Fig. 3). A typical finger-like structure with an asymmetric dense surface layer was observed for the PVDF-N/MIL100-CS membrane, reflecting that the phase separation occurred under fast mixing conditions, while a sponge-like structure was prevailing for the PVDF-S/MIL100-CS counterparts (Figs. 3a1 and 3a2). An addition of hydrophilic nanofillers to the polymer matrix commonly turns the overall membrane morphology from the sponge-like structure to the finger-like structure because of the enhanced exchange rate between the non-solvent and solvent [26]. Initially, we expected to obtain the finger-like structure for the morphology of PVDF-S/MIL100-CS membranes because of the hydrophilic nature of MIL100-CS. However, the resulting SEM images showed that the sponge-like structures are still dominant for the PVDF-S/MIL100-CS membrane (Fig. 3a1). It should be noted that the PVDF solution was phase separated with the additional solution layer containing MIL100-CS nanofillers, which induced dilution of polymer concentration in the surface region. It can be speculated that the dilution effect of polymer concentration on the morphology was more dominant than the nanofiller-induced hydrophilic effect. To verify our hypothesis, both the pristine PVDF-S and the PVDF-S/MIL-100(Fe) membranes were fabricated by using the SANE method and confirmed that the sponge-like pore structures were prevailing for both of them (Fig. S4) [26]. Also, both of them exhibited similar microsized pore geometries, which is appropriate for microfiltration. Feijen et al. summarized the phase separation mechanism associated with the membrane morphology and explained the effect of polymer concentrations on the membrane morphology [26]. The interconnected pore structures can be formed by the 10

coalescence of poor polymer droplets via bimodal decomposition. In addition, the surface pores of PVDF-S/MIL100-CS membranes were much larger than those of PVDF-N/MIL100-CS counterparts due to the decreased polymer concentration in the membrane surface region (Figs. 3a1 and 3a2). (a1)

(a2) 100 nm

200 nm

3 m

3 m

Depth from membrane surface (m)

(b) (c1) Membrane Surface 0 Fig. 3. SEM images of membranes prepared by the two different casting methods, NIPS vs. (i) 20

SANE. Cross-sectional SEM images of (a) PVDF-N, (b) PVDF-S, (c) PVDF-N/MIL-100(Fe), 40

800 nm

(d) PVDF-S/MIL-100(Fe), (e) PVDF-N/MIL-100(Fe)/CS, and (f) PVDF-S/MIL-100(Fe)/CS 60 (c2) PVDF-S/MIL100-CS

membranes (scale bar: 3 μm) (inset: 80 magnified SEM images, scale bar: 150 nm). 100

(ii)

0

PVDF-N/MIL100-CS

2

6

4

Atomic% of Fe

8

800 nm

Fig 3. Cross-sectional SEM images of (a1) PVDF-S/MIL100-CS and (a2) PVDF-N/MIL100CS (inset: particle-resolved SEM images on the membrane surface). (b) The depth-dependent Fe atom distribution of the PVDF-S/MIL100-CS and PVDF-N/MIL100-CS membranes. Cross-sectional SEM images of the PVDF-S/MIL100-CS membrane at (c1) the top position (i) and (c2) the bottom position (ii). The site-selective arrangement of the nanofillers onto the membrane surface with the open surface of the fillers prevailing can enhance the reinforcing filler effects on the antibacterial activity since the interface of the fillers with water can be maximized despite the small loading content of fillers [21]. The SEM image for the surface of the PVDF-S/MIL100-CS membrane exhibited the MIL100-CS fillers were embedded on the PVDF membrane surface with the open surface of the fillers (Fig. 3a1 inset). In contrast, we hardly observed the MIL100-CS fillers on the surface of PVDF-N/MIL100-CS due to their random distribution throughout the membrane, and even some observed fillers were seemingly covered by PVDF (Fig. 3a2 inset). It is indicative that our new SANE method can lead to more enhanced filler effects for antibacterial activity than the NIPS method-based composite membranes. The actual amount of fillers in both PVDF-S/MIL100-CS and PVDF-N/MIL100-CS composite membranes was determined by using TGA. We confirmed that both composite membranes contained a similar amount of 11

MIL100-CS nanofillers (ca. 8.2 wt%) (Fig. S3) [25]. The number distribution of MIL100-CS nanofillers in vertical direction was investigated by quantifying the amount of Fe atoms along the longitudinal direction based on the SEM-EDS analysis. (Figs. 3b and 3c). The Fe atoms were mainly distributed within a region of 0–30 m from the top for the PVDF-S/MIL100-CS membrane (membrane thickness: 107 ± 9 m), indicating that most of MIL100-CS nanofillers were observed at the top regions (Figs. 3b and 3c). The Fe distribution in the PVDF-N/MIL100CS membrane, however, indicated that the MIL100-CS nanofillers were uniformly distributed along the longitudinal direction of the membrane (Fig. 3b). The SEM analysis verified that the SANE method indeed enabled the composite membranes with the surface-concentrated filler distribution, unlike the NIPS method. Table 1. The profiles of PVDF-based membranes fabricated by SANE and NIPS methods. Method SANE NIPS

Thickness Contact angle Zeta-potential (µm) (˚) (mV) PVDF-S 102 ±12 76.2 ± 4.4 -9.5 ± 1.1 PVDF-S/MIL-100(Fe) 106 ±10 46.1 ± 5.5 -18.3 ± 2.7 PVDF-S/MIL100-CS 107 ±9 41.1 ± 3.1 +19.2 ± 3.2 PVDF-N/MIL100-CS 112 ± 5 64.2 ± 4.8 +1.3 ± 4.6 Membrane

The unique asymmetric distribution of MIL100-CS nanofillers in the PVDF composite membranes is expected to result in more hydrophilic surface than that of PVDF composite membrane prepared by NIPS. It should be noted that the effect of different fabrication methods on the surface roughness is negligible since the difference in water contact angle for both PVDF-S and PVDF-N is within the error range (i.e., 76.2 ± 4.4o vs. 77.4 ± 3.5o). The extent of hydrophilicity of each membrane was examined by the water contact angle (Table 1). The PVDF-S membrane exhibited the water contact angle of ca. 76.2o, which is almost the same as that of the conventional PVDF films (79.9o) [27]. The embedding of MIL-100(Fe) nanofillers to the PVDF membrane by the SANE method led to a substantial decrease in the contact angle value (ca. 46.1o), reflecting the hydrophilic nature of MIL-100(Fe). Also, a marginal reduction in the average contact angle for PVDF-S/MIL100-CS was observed compared to that of PVDFS/MIL-100(Fe) although the difference in the water contact angle for the PVDF-S/MIL100-CS and the PVDF-S/MIL-100(Fe) was within the error range (Table 1). Meanwhile, the MIL100CS-containing PVDF composite membrane prepared by the NIPS method showed the much higher water contact angle value (ca. 64.2o) than that of PVDF-S/MIL100-CS (Table 1). Such characterizations of the examined membranes demonstrated that the SANE method is more 12

efficient for the enhancement of surface hydrophilicity than the NIPS method due to the presence of surface-concentrated hydrophilic nanofillers (Fig. 3). 3.3. Stability of PVDF-S/MIL100-CS composite membranes (a)

Chitosan Doping (b) HO HO

MIL100-CS Filler

O O

N H

H

O

H

NH2

O

O

n

O

OH

exo up

O

PVDF

OH

OH

OH

NH2 H

H

H-bonding

PVDF/CS

F F F F F F F F F F

Tm

H HH H H HH H H H

120

PVDF Matrix

(c)

PVDF/CS

PVDF/MIL100-CS Membrane

80

900

880

C-F

Weight (%)

Absorbance

860

180

Temperature ( C)

C-F

PVDF

160 o

(d)100

PVDF/CS

840

140

before after 24 h after 100 h

60 40 20

PVDF 1120 1140

1160

1180

1200 -1

1220

Wavenumber (cm )

0

200

400

600

Temperature (oC)

800

Fig. 4. (a) Illustration for the interfacial interaction between chitosan and PVDF. (b) DSC curves of PVDF and PVDF/CS. (c) FT-IR spectra of PVDF and PVDF/CS. (d) TGA curves of PVDF-S/MIL100-CS before and after water flux tests (a dead-end flow system at 2 bars). The stability of MIL100-CS nanofillers in PVDF-S/MIL100-CS composite membranes should be substantiated, especially due to the surface-concentrated distribution of MIL100-CS nanofillers. The enriched functional groups including hydroxyl (OH) groups in CS molecules on the surface of MIL-100(Fe) presumably form intermolecular hydrogen bonds with fluorine groups (CF2) in the PVDF matrix (Fig. 4a). Such hydrogen bonds may facilitate the stable immobilization of MIL100-CS fillers onto the PVDF membrane surface. To confirm their interfacial interactions, DSC was performed for the PVDF composite containing 10 wt% of CS (PVDF/CS) instead of PVDF-S/MIL100-CS because the interaction between PVDF and CS in PVDF-S/MIL100-CS was challenging to detect due to insufficient interaction sites caused by its unique morphology (ca. 6 wt% of CS in PVDF-S/MIL100-CS was determined by TGA) (Figs. S3 and S6). The PVDF/CS composite was prepared by a solution process using the 13

NMP/acetic acid (99/1 v/v) mixture, and the resulting product was obtained by using hexane based on a coagulation method. The PVDF showed a melting temperature (Tm) of 162.2 oC, while the PVDF/CS composite showed a slightly lower Tm of 161.1 oC (Fig. 4b). In addition, the degree of crystallinity (Xc) of PVDF/CS decreased compared to that for PVDF (28.6 vs. 40.0%). [28]. It is indicative that the interaction of CS with PVDF molecules affects the crystalline properties of PVDF. Our previous report demonstrated that the reduction of Tm and crystallinity of PVDF may originate from the decrease in the lamella crystalline structure of PVDF by the compatible polymer blends [28]. Arunbabu et al. also reported that the compatible blend by the hydrogen bonds between secondary amine groups (NH) in polybenzimidazole and fluorine groups (CF2) in PVDF exhibited the decrease in both Tm and crystallinity of PVDF [24]. To further clarify the interfacial interaction between PVDF and CS, FT-IR was employed, and PVDF showed the broad peaks at ca. 1170 and 877 cm-1, which correspond to the vibrational frequencies of C-F, C-C-F, and F-C-F bonds (Fig. 4c) [24]. After blending with CS, the FT-IR spectra for PVDF/CS exhibited the red-shift in C-F bonds to ca. 1167 and 873 cm-1, respectively since the C-F bond was becoming weaker and elongated by a form of hydrogen bonds (Fig. 4c). Arunbabu et al. reported a similar observation that the free N-H band of polybenzimidazole (PBI) in FT-IR was red-shifted upon blending with PVDF due to hydrogen bonding between the imidazole ring of PBI and C-F groups of PVDF [24]. As such, the hydrogen bonding between PVDF and CS is the significantly important parameter for the stable immobilization of MIL100-CS fillers in the PVDF-S/MIL100-CS composite membranes [24]. The TGA analysis for the PVDF-S/MIL100-CS composite membranes before and after the 24 and 100 h water filtration tests was used to further confirm that the composite membranes maintain the stability associated with the filler immobilization (Fig. 4d). The TGA curves for the PVDF-S/MIL100-CS composite membranes before and after the water filtration testing were almost identical, and the deviation in the actual loading content of the nanofillers was negligible (ca. 0.2 wt% difference even after 100 h) (Fig. 4d), confirming that the surfaceconcentrated MIL100-CS nanofillers were well immobilized on the PVDF matrix. In addition, the SEM images for the top surface of PVDF-S/MIL100-CS before and after the 24 h water filtration testing indeed suggested the robust membrane stability because they exhibited almost identical morphology (Fig. S7). 3.4. Evaluation of antibacterial activities 14

The antibacterial activity of the PVDF-based MF composite membranes was investigated by evaluating the normalized number of live E. coli bacteria on the agar plate. The bacterial broth contacted with PVDF-S membranes showed a large number of live E. coli colonies, while the PVDF-S/MIL-100(Fe) analogues slightly reduced the number of viable E. coli colonies (Figs. 5b2 and 5b3). Some metal-organic frameworks (MOFs) exhibited excellent biocidal behaviors since both the metal sites and the organic ligands may have an antibacterial interaction with bacterial cells [29,30]. More specifically, the metal ions or organic functional groups released from MOF can react with the thiol groups of proteins and cations in bacterial cells, respectively, resulting in disruption of the integrity of bacterial film or the modification/ fragmentation of DNA [30-32]. However, such high biocidal activity occurs mostly at limited defect sites. With that in mind, the poor antibacterial activity of PVDF-S/MIL-100(Fe) is probably associated with the minuscular releasing of metal ions and organic ligands due to the water-stable characteristic of MIL-100(Fe) (Fig. 5a).

Fig. 5. (a) The normalized number of live bacteria on the agar plate for E. coli bacteria. 15

Photographs of E. coli grown agar plates using the bacterial solution after (b1) no treatment for a control sample or a treatment with (b2) PVDF-S, (b3) PVDF-S/MIL-100(Fe), and (b4) PVDF-S/MIL100-CS membranes at 37 oC for 24 h, respectively. (c) Schematic illustration of the antibacterial activity of the PVDF-S/MIL100-CS composite membrane. (d) Chitosan releasing percentages from the PVDF-S/MIL100-CS composite membrane. The fluorescence images of stained (live/dead stain) membrane surfaces: (e1) PVDF-S, (e2) PVDF-S/MIL100(Fe), and (e3) PVDF-S/MIL100-CS (green fluorescence indicates the live bacteria are colonizing the surface, while red fluorescence represents the dead ones). To enhance the antibacterial activity, we designed the chitosan-doped MIL-100(Fe) as the intensive filler because the deacetylated chitosan has enriched functional groups including hydroxyl (OH) and amino (NH2) groups, facilitating a form of bonding with bacterial cells and providing extremely high hydrophilicity [23]. The bacterial broth contacted with the PVDFS/MIL100-CS composite membrane exhibited no E. coli colonies (Fig. 5b4), demonstrating the significantly enhanced antibacterial activity. Meanwhile, PVDF-N/MIL100-CS, which was prepared by NIPS, showed some of E. coli colonies albeit it contained the similar MIL100-CS content (ca. 8.2 wt%) to that of PVDF-S/MIL100-CS (Fig. S8a). Such a distinct difference in antibacterial activity between PVDF-S/MIL100-CS and PVDF-N/MIL100-CS justifies the significance of the surface-selective arrangement of MIL100-CS biocidal fillers for high efficiency. The accepted antibacterial mechanism for CS is based on the interaction between positively charged amino groups and negatively charged bacterial cells, inducing the leakage of protein and other intracellular constituents [23,33]. Based on the antibacterial mechanism for CS, we can propose the plausible antibacterial mechanism on PVDF-S/MIL100-CS that the interaction between positively charged CS and negatively charged E. coli can kill the E. coli cells, disturbing the growth of biofilm. Besides, it may repel E. coli cells due to the hydrophilic surface (Fig. 5c) [34]. To support our suggestion regarding the plausible antibacterial mechanism by MIL100-CS, the surface zeta potential for PVDF-S, PVDF-S/MIL-100(Fe) and PVDF-S/MIL100-CS was evaluated (Table 1). It was confirmed that the PVDF-S/MIL100-CS composite membrane exhibited the positive zeta potential of +19.2 mV while the other two PVDF-S and PVDF-S/MIL-100(Fe) exhibited negative zeta potential of -9.5 mV and -18.3 mV, respectively. Our zeta potential measurements were almost consistent with Mady’s study, exhibiting an increase in positive zeta potential with increasing protonated CS concentrations in the drug encapsulation work [35]. It was further presumed that the CS molecules released from the MIL100-CS nanofillers 16

may enhance the antibacterial activity of the PVDF-S/MIL100-CS composite membranes. In order to evaluate whether the CS is released, the PVDF-S/MIL100-CS composite membrane was immersed into an acidic aqueous solution (pKa: 6.5, 200 ml), and then the solution was stirred at 37 °C for six days. The collected samples at the predetermined time were characterized by HPLC-MS based on the protocol reported by Yan et al. [36]. The total mass of CS (ca. 6 wt%) in examined PVDF-S/MIL100-CS, which were calculated using TGA results, was regarded as 100%, and the percent of released chitosan was calculated based on the HPLCMS results (Figs. S3, S6, and 5d). The amount of released CS was small and then showed dramatic downhill after three days stirring, indicating that the doped CS on the MIL-100(Fe) and the released CS can induce the antibacterial activity. However, we anticipated that the strong interaction of CS with MIL-100(Fe) and PVDF might prohibit CS from being released. It was consistent with our TGA analysis for the PVDF-S/MIL100-CS composite membranes, showing that change in the amount of MIL100-CS nanofillers is negligible before and after the water flux testing (Fig. 4d). As such, the physically entangled CS molecules by themselves can affect the outcome by the harsh external stimulations. The low content of CS released (ca. < 0.3%) and the reduction of the CS releasing content (ca. 0.1%) after three days water flux testing well supported our hypothesis (Fig. 5d). The antimicrobial activity of each MF membrane was evaluated by monitoring the live and dead cells on the membranes using the fluorescence with live and dead staining (Fig. 5e). Most E. coli cells were alive in PVDF-S membranes (Fig. 5e1), while only a few dead cells were observed for the PVDF-S/ MIL100-CS counterparts (Fig. 5e3), supporting the strong biocidal activity of MIL100-CS. Meanwhile, the PVDF-S/ MIL-100(Fe) membrane exhibited both live and dead cells albeit the number of those cells were much less than that for the PVDF-S analogue (Fig. 5e2), reflecting that MIL-100(Fe) has hydrophilic properties in addition to a limited biocidal activity probably due to the limited number of reactive sites induced by the water-stable nature of MIL-100(Fe) [15,22,29]. We also performed a comparative evaluation for the PVDF-N/MIL100-CS composite membrane, and the fluorescence image also showed live E. coli cells, which can be explained by the lower filler effects by a PVDF capping than that of PVDF-S/MIL100-CS (Fig. S8). We also confirmed the smaller filler effects derived from the NIPS method using zeta potential, exhibiting that the relatively lower positive zeta potential value (ca. +1.3 mV) for PVDF-N/MIL100-CS than that of PVDF-S/MIL100-CS (ca. +19.2 mV) (Table 1). As such, the PVDF-S/MIL100-CS composite membrane has the 17

substantially enhanced antibacterial activity derived from the unique asymmetric MIL100-CS filler distribution and the open surface of MIL100-Cs fillers. 3.5. Evaluation of water flux and biofouling resistance The pure water flux of three different MF membranes prepared by the SANE method was evaluated by a dead-end type permeation system (Fig. 6a) [37]. The PVDF-S membrane exhibited the lowest water flux of ca. 270 L m-2 h-1 and both PVDF-S/MIL-100(Fe) and PVDFS/MIL100-CS membranes enhanced water flux (ca. 281 L m-2 h-1 and 295 L m-2 h-1) due to the enhanced hydrophilic properties although changes in the cross-sectional morphology and the surface pore size (1-2 µm) are negligible (Figs. 3 and 6a) [8,21]. It is well-known that the addition of hydrophilic fillers contributes to improving water flux [38-40]. Previously, Zhao et al. also reported that the hydrophilic additives of o-aminobenzoic acid–triethylamine (o-ABA– TEA salt) containing reverse osmosis membrane showed the enhanced water permeability [40]. On the other hands, all three membranes prepared by the NIPS method showed almost similar water flux (Fig. S9), reflecting the significance of the surface-selective distribution of nanofillers.

0

PVDF-S/MIL100-CS

100

PVDF-S/MIL-100(Fe)

200

PVDF-S

Water Flux (L m-2 h-1)

300

Normalized Water Flux (Jw/Jw0)

(b)

(a)

1 0.8 0.6 0.4 PVDF-S PVDF-S/MIL-100(Fe) PVDF-S/MIL100-CS

0.2 0.0 0

5

10

15

20

25

Retention Time (h)

Fig. 6. (a) Pure water flux tests for PVDF-S, PVDF-S/MIL-100(Fe), and PVDF-S/MIL100-CS by using a dead-end type permeation system. (b) Normalized water flux of PVDF-S, PVDFS/MIL-100(Fe), and PVDF-S/MIL100-CS as a function of retention time in biofouling experiments using the E. coli feeding solution. The biofouling resistance was further examined by monitoring the water flux change as a function of time in a cross-flow system with the E. coli broth feed solution (Fig. 6b) [30,41]. It should be noted that the biofouling resistance in Fig. 6(b) was characterized by using the E. coli broth solution in the cross-flow system in order to avoid concentration polarization. All 18

examined membranes showed a gradual decline in the water flux, which is attributed to biofouling induced by the adhesion of the E. coli cells on the membrane surface (Fig. 6b). The PVDF-S membrane showed a significant reduction of ca. 50% in the water flux, reflecting severe biofouling. In contrast, the PVDF-S/MIL100-CS composite membrane exhibited the significantly high preservation in the normalized water flux (Jw/Jw0 = ca. 0.85) and followed by PVDF-S/MIL-100(Fe) (Jw/Jw0 = ca. 0.65), indicating the improved biofouling resistance than the PVDF-S (Jw/Jw0 = ca. 0.51) (Fig. 6b). The biofouling resistance results by the examined membranes were in good agreement with those of the antibacterial tests (Figs. 5 and 6b). For comparison, the biofouling resistance of the membranes prepared by NIPS was characterized; the biofouling resistance was increasing in the order of PVDF-N < PVDFN/MIL-100(Fe) < PVDF-N/MIL100-CS, which is identical to the biofouling results of the membranes prepared by SANE (Fig. S9b). However, the Jw/Jw0 value of PVDF-N/MIL100-CS (ca. 0.70) is lower than that of PVDF-S/MIL100-CS. It reflects that the newly invented SANE method can be more efficient for biofouling resistance than the conventional NIPS counterpart because of dual features of antibacterial activity and hydrophilic properties by surfaceselectively distributed CS-MIL100 fillers. 4. Conclusions The newly suggested fabrication method for MF composite membranes, so-called solventassisted nanoparticle embedding (SANE) enabled the unique composite membrane structures of asymmetrically surface-concentrated nanofillers and sponge-like substructures. The SANE method was modified from the conventional nonsolvent-induced phase separation (NIPS) method by the addition of an intermediate step where the chitosan (CS)-doped MIL-100(Fe) (i.e., MIL100-CS) solution was gently introduced onto the nascent polymeric membranes. The acquired PVDF/MIL100-CS composite membrane slightly enhanced water flux compared to the pristine PVDF counterpart prepared by the SANE method (295 vs. 270 L m-2 h-1) due to the presence of surface-concentrated hydrophilic nanofillers with an uncovered open surface. More importantly, the SANE-based PVDF/MIL100-CS composite membranes exhibited the excellent antibacterial properties for E. coli cells owing to synergistic features of antibacterial activity and hydrophilic properties by surface-selectively distributed CS-MIL100 fillers. The excellent antibacterial properties of the PVDF-S/MIL100-CS composite membranes enabled the much higher biofouling resistance (Jw/Jw0 = ca. 0.85) compared to that for PVDF-S (Jw/Jw0 = ca. 0.51) after 24 h operation in the cross-flow system with the E. coli broth feed solution. 19

Hence, the newly approached SANE method is promising for the fabrication of MF composite membranes with substantially enhanced antibacterial activity and biofouling resistance. Acknowledgments This research was supported by the Samsung Research Funding Center of Samsung Electronics under Project Number SRFC-MA1602-06 and “Human Resources Program in Energy Technology” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea. (No.20174010201150). DYH and JSC are grateful to the Global Frontier Center for Hybrid Interface Materials (GFHIM, Grant No. NRF-2013M3A6B1078879) for financial support. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at References [1]

M.A. Shannon, P.W. Bohn, M. Elimelech, J.G. Georgiadis, B.J. Marinas, A.M. Mayes, Science and technology for water purification in the coming decades, Nature. 452 (2008) 301–310.

[2]

S. Alzahrani, A.W. Mohammad, Challenges and trends in membrane technology implementation for produced water treatment: a review, J. Water Process Eng. 4 (2014) 107–133.

[3]

R. Zhang, Y. Liu, M. He, Y. Su, X. Zhao, M. Elimelech, Z. Jiang, Antifouling membranes for sustainable water purification: strategies and mechanisms, Chem. Soc. Rev. 45 (2016) 5888–5924.

[4]

N. Shahkaramipour, T. Tran, S. Ramanan, H. Lin, Membranes with surface-enhanced antifouling properties for water purification, Membranes 7 (2017) 13.

[5]

C.S. Ong, P.S. Goh, W.J. Lau, N. Misdan, A.F. Ismail, Nanomaterials for biofouling and scaling mitigation of thin film composite membrane: A review, Desalination. 393 (2016) 2–15.

[6]

Q. Wei, T. Becherer, S. Angioletti-Uberti, J. Dzubiella, C. Wischke, A.T. Neffe, A. Lendlein, M. Ballauff, R. Haag, Protein interactions with polymer coatings and biomaterials, Angew. Chem. Int. Ed. 53 (2014) 8004–8031.

20

[7]

K.L. Prime, G.M. Whitesides, Self-assembled organic monolayers: model systems for studying adsorption of proteins at surfaces, Science 252 (1991) 1164–1167.

[8]

S. Liang, Y. Kang, A. Tiraferri, E.P. Giannelis, X. Huang, M. Elimelech, Highly hydrophilic

polyvinylidene

fluoride

(PVDF)

ultrafiltration

membranes

via

postfabrication grafting of surface-tailored silica nanoparticles, ACS Appl. Mater. Interfaces 5 (2013) 6694-6703. [9]

M. Ben-Sasson, K.R. Zodrow, Q. Genggeng, Y. Kang, E.P. Giannelis, M. Elimelech, Surface functionalization of thin-film composite membranes with copper nanoparticles for antimicrobial surface properties, Environ. Sci. Technol. 48 (2014) 384–393.

[10] S.-H Park, Y.-S. Ko, S.-J. Park, J. S. Lee, J. Cho, K.-Y. Baek, I. T. Kim, K. Woo, J.-H. Lee, Immobilization of silver nanoparticle-decorated silica particles on polyamide thin film composite membranes for antibacterial properties, J. Membr. Sci. 499 (2016) 80-91. [11] J.-H. Li, B.-F. Yan, X.-S. Shao, S.-S. Wang, H.-Y. Tian, Q.-Q. Zhang, Influence of Ag/TiO2 nanoparticle on the surface hydrophilicity and visible-light response activity of polyvinylidene fluoride membrane, Appl. Surf. Sci. 324 (2015) 82-89. [12] G. Dong, H. Li, V. Chen, Challenges and opportunities mixed-matrix membranes for gas separation, J. Mater. Chem. A 1 (2013) 4610-4630. [13] S.H. Lee, H.Y. Seo, Y.S. Yeom, J.E. Kim, H. An, J.S. Lee, H.K. Jeong, K.Y. Baek, K.Y. Cho, H.G. Yoon, Rational design of epoxy/ ZIF-8 nanocomposites for enhanced suppression of copper ion migration, Polymer 150 (2018) 159–168. [14] W. Zhuang, D. Yuan, J.-R. Li, Z. Luo, H.-C. Zhou, S. Bashir, J. Liu, Highly potent bactericidal activity of porous metal-organic frameworks, Adv. Healthcare Mater. 1 (2012) 225-238. [15] C. Wang, X. Liu, N.K. Demir, J.P. Chen, K. Li, Applications of water stable metalorganic frameworks, Chem. Soc. Rev. 45 (2016) 5107–5134. [16] K.Y. Cho, J.Y. Seo, H.-J. Kim, S.J. Pai, X.H. Do, H.G. Yoon, S.S. Hwang, S.S. Han, K.Y. Baek, Facile control of defect site density and particle size of UiO-66 for enhanced hydrolysis rates: insights into feasibility of Zr(IV)-based metal-organic framework (MOF) catalysts, Appl. Catal., B 245 (2019) 635-647. [17] M. Sile-Yuksel, B. Tas, D.Y. Koseoglu-Imer, I. Koyuncu, Effect of silver nanoparticle (AgNP) location in nanocomposite membrane matrix fabricated with different polymer type on antibacterial mechanism, Desalination 347 (2014) 120–130. 21

[18] H. An, S. Park, H.T. Kwon, H.-K Jeong, J.S. Lee, A new superior competitor for exceptional propylene/propane separations: ZIF-67 containing mixed matrix membranes, J. Membr. Sci. 526 (2017) 367-376. [19] T. Hidalgo, M. Gimenez-Marques, E. Bellido, J. Avila, M.C. Asensio, F. Salles, M.V. Lozano, M. Guillevic, R. Simon-Vazquez, A. Gonzalez-Fernandez, C. Serre, M.J. Alonso, P. Horcajada, Chitosan-coated mesoporous MIL-100(Fe) nanoparticles as improved bio-compatible oral nanocarriers, Sci. Rep. 7 (2017) 43099. [20] S. Zivanovic, J. Li, P.M. Davidson, K. Kit, Physical, mechanical, and antibacterial properties of chitosan/PEO blend films, Biomacromolecules 8 (2007) 1505–1510. [21] D. Rana, T. Matsuura, Surface modifications for antifouling membranes, Chem. Rev. 110 (2010) 2448–2471. [22] P. Horcajada, S. Surble, C. Serre, D.Y. Hong, Y.K. Seo, J.S. Chang, J.M. Greneche, I. Margiolaki, G. Ferey, Synthesis and catalytic properties of MIL-100(Fe), an iron(III) carboxylate with large pores, Chem. Commun. (2007) 2820–2822. [23] A. Jain, L.S. Duvvuri, S. Farah, N. Beyth, A.J. Domb, W. Khan, Antimicrobial polymers, Adv. Healthcare Mater. 3 (2014) 1969–1985. https://doi.org/10.1002/adhm.201400418. [24] D. Arunbabu, A. Sannigrahi, T. Jana, Blends of polybenzimidazole and poly(vinylidene fluoride) for use in a fuel cell, J. Phys. Chem. B 112 (2008) 5305–5310. [25] K.Y. Cho, H. An, X.H. Do, K. Choi, H.G. Yoon, H.K. Jeong, J.S. Lee, K.-Y. Baek, Synthesis of amine-functionalized ZIF-8 with 3-amino-1,2,4-triazole by postsynthetic modification for efficient CO2-selective adsorbents and beyond, J. Mater. Chem. A 6 (2018) 18912-18919. [26] P. van de Witte, P.J. Dijkstra, J.W.A. van den Berg, J. Feijen, Phase separation processes in polymer solutions in relation to membrane formation, J. Membr. Sci. 117 (1996) 1– 31. [27] K.Y. Cho, A.S. Lee, H. Jeon, S.H. Park, M. Jang, H.G. Yoon, S.M. Hong, K.-Y. Baek, S.S. Hwang, Tuning the interface between poly(vinylidene fluoride)/UV-curable polysilsesquioxane hybrid composites: Compatibility, thermal, mechanical, electrical, and surface properties, Polymer 77 (2015) 167–176. [28] K.Y. Cho, Y.J. Lee, H.J. Kim, H.G. Yoon, S.S. Hwang, Y.K. Han, K.-Y. Baek, Interfacial control of PVDF-TrFE/SWCNT nanocomposites using P3HT-PMMA block copolymer for ultra-low percolation threshold, Polymer 77 (2015) 55–63. 22

[29] G. Wyszogrodzka, B. Marszalek, B. Gil, P. Dorozynski, Metal-organic frameworks: Mechanisms of antibacterial action and potential applications, Drug Discovery Today 21 (2016) 1009–1018. [30] A. Zirehpour, A. Rahimpour, A.A. Shamsabadi, M.S. Gh., M. Soroush, Mitigation of thin-film composite membrane biofouling via immobilizing nano-sized biocidal reservoirs in the membrane active layer, Environ. Sci. Technol. 51 (2017) 5511–5522. [31] M.D. Pulido, A.R. Parrish, Metal-induced apoptosis: mechanisms, Mutat. Res. 533 (2003) 227–241. [32] Q.L. Feng, J. Wu, G.Q. Chen, F.Z. Cui, T.N. Kim, J.O. Kim, A mechanistic study of the antibacterial effect of silver ions on Escherichia coli and Staphylococcus aureus, J. Biomed. Mater. Res. 52 (2000) 662-668. [33] Y. Jiao, L. Niu, S. Ma, J. Li, F.R. Tay, J. Chen, Quaternary ammonium-based biomedical materials: State-of-the-art, toxicological aspects and antimicrobial resistance, Prog. Polym. Sci. 71 (2017) 53–90. [34] X. Yan, H.M. Evenocheck, Chitosan analysis using acid hydrolysis and HPLC/UV, Carbohydr. Polym. 87 (2012) 1774–1778. [35] M.M. Mady, M.M. Darwish, Effect of chitosan coating on the characteristics of DPPC liposomes, J. Adv. Res. 1 (2010) 187-191. [36] M.S. Ganewatta, K.P. Miller, S.P. Singleton, P. Mehrpouya-Bahrami, Y.P. Chen, Y. Yan, M. Nagarkatti, P. Nagarkatti, A.W. Decho, C. Tang, Antibacterial and biofilm-disrupting coatings from resin acid-derived materials, Biomacromolecules. 16 (2015) 3336–3344. [37] X. Zheng, M. Ernst, M. Jekel, Identification and quantification of major organic foulants in treated domestic wastewater affecting filterability in dead-end ultrafiltration, Water Res. 43 (2009) 238–244. [38] J.-A Park, K.Y. Cho, C.H. Han, A. Nam, J.-H. Kim, S.-H. Lee, J.W. Choi, Quaternized amphiphilic block copolymers/graphene oxide and a poly(vinyl alcohol) coating layer on graphene oxide/poly(vinylidene fluoride) electrospun nanofibers for superhydrophilic and antibacterial properties, Sci. Rep. 9 (2019) 383. [39] S.-H. Park, Y. Ahn, M. Jang, H.-J. Kim, K.Y. Cho, S.S. Hwang, K.-Y. Baek, Effects of methacrylate based amphiphilic block copolymer additives on ultra filtration PVDF membrane formation, Sep. Purif. Technol. 202 (2018) 34-44.

23

[40] L. Zhao, W.S.W. Ho, Novel reverse osmosis membranes incorporated with a hydrophilic additive for seawater desalination, J. Membr. Sci. 455 (2014) 44–54. [41] S.E. Kwan, E. Bar-Zeev, M. Elimelech, Biofouling in forward osmosis and reverse osmosis: Measurements and mechanisms, J. Membr. Sci. 493 (2015) 703–708.

24

Supplementary Material for:

Surface-concentrated chitosan-doped MIL-100(Fe) nanofiller-containing PVDF composites for enhanced antibacterial activity Kie Yong Cho a,b,1, Cheol Hun Yooa,1, Young-June Wona, Do Young Hongc, Jong-San Changc,d, Jae Woo Choie, Jung-Hyun Leef,*, Jong Suk Leea,* a

Department of Chemical and Biomolecular Engineering, Sogang University, Seoul 121-742, Republic of Korea. b Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, TX 77843-3122, United States. c Research Center for Nanocatalysts, Korea Research Institute of Chemical Technology (KRICT), 141 Gajeong-ro, Yuesong-gu, Daejeon, 305-600, Republic of Korea d Department of Chemistry, Sungkyunkwan University, Suwon 440-476, South Korea

Water Resource Cycle Research, Korea Institute of Science and Technology, Hwarangno 14-gil 5, Seongbuk-gu, Seoul, 02792, Republic of Korea e

f Department

of Chemical and Biological Engineering, Korea University, 5-1 Anam-dong, Seongbuk-gu, Seoul 136-713, Republic of Korea 1These

authors contributed equally to this work.

*Corresponding author email: [email protected] (J.-H. Lee) and [email protected] (J. S. Lee)

25

Fig. S1. Schematic diagram of the lab-scale microfiltration system.

(b)

600 MIL-100(Fe) 400

MIL100-CS

200

0 0.0

0.2

0.4

0.6

MIL-100(Fe) Chitosan MIL100-CS

Absorbance

Quantity Adsorbed (cm3 g-1)

(a)

0.8

1.0

1200

1100

1000

900

Wavenumber (cm-1)

Relative Pressure (P/Po)

Fig. S2. BET N2 adsorption-desorption isotherm curves of MIL-100(Fe) and MIL100-CS. (B) FT-IR spectra of MIL-100(Fe), chitosan, and MIL100-CS. 26

Fig. S3. TGA curves of PVDF, MIL-100(Fe), PVDF-N/MIL100-CS, and PVDF-S/MIL100CS at 10 oC min-1 under air.

The actual loading content of fillers was determined by TGA, resulting in ca. 8.5 wt% of MIL-100(Fe) and MIL-100(Fe)/CS in the PVDF-S/MIL-100(Fe) and PVDF-S/MIL100-CS composite membranes, respectively. The final weight at 700 C corresponds to the mass of FeO from the MIL-100(Fe) because the organic ligands were completely decomposed at 700 oC.

Thus, we obtained the ratio between the mass of MIL-100(Fe) and FeO by dividing the

average weight of the residual sample at 700 C by that of the corresponding initial sample weight. The final weight of PVDF-S/MIL100-CS at 700 C indicated the loaded mass of FeO in the membrane, and we calculated the actual loading mass of MIL-100(Fe) by multiplying the ratio to average mass of FeO in the membrane (Fig. S3).

27

2 m Fig S4. A cross-sectional SEM image of PVDF-S.

Fig. S5. Cross-sectional SEM images of PVDF-N/MIL100-CS for (a) overview, (b) a top region, and (c) a bottom region.

28

Fig. S6. TGA curves of MIL-100(Fe), chitosan, and MIL100-CS at 10 oC min-1 under air.

(a)

(b)

5 m

5 m

Fig. S7. SEM images for the top surface of PVDF-S/MIL100-CS membrane (a) before and (b) after water flux tests (a dead-end flow system at 2 bars for 24 h).

29

(a)

(b)

10 m

Fig. S8. (a) A photograph of E. coli colonies growing on the agar plate after-treatment of the bacterial solution with the PVDF-N/MIL100-CS composite membrane at 37 oC for 24 h. (b) The fluorescence microscopy image of stained (live/dead stain) surfaces of the PVDFN/MIL100-CS composite membrane after incubating at 37 oC for 24h.

PVDF-S/MIL-100(Fe)

N/MIL-100(Fe)

(b)

50 0

PVDF-N/MIL100-CS

100

PVDF-N/MIL-100(Fe)

150

Normalized Water Flux (Jw/Jw0)

200

PVDF-N

Water Flux (L m-2 h-1)

(a)

1 0.8 0.6 0.4 PVDF-N PVDF-N/MIL-100(Fe) PVDF-N/MIL100-CS

0.2 0.0 0

5

10

15

20

25

Retention Time (h)

Fig. S9. (a) Pure water flux tests for PVDF-N, PVDF-N/MIL-100(Fe), and PVDF-N/MIL100CS by using a dead-end type permeation system. (b) Normalized water flux of PVDF-N, PVDF-N/MIL-100(Fe), and PVDF-N/MIL100-CS as a function of retention time in biofouling experiments using the E. coli feeding solution.

30

Graphical abstract

31

Highlights ● A solvent-assisted nanoparticle embedding (SANE) method was developed for MF composite membranes. ● Chitosan-doped MIL-100(Fe) nanoparticles exhibited both hydrophilic and biocidal properties. ● Poly(vinylidene fluoride) exhibited good interfacial interaction with chitosan via hydrogen bonding. ● The SANE method enabled surface selective distribution of nanoparticles for enhanced antibacterial activity.

32