Photografting Graphene Oxide to Inert Membrane Materials to Impart

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Environmental Aspects of Nanotechnology

Photo-Grafting Graphene Oxide to Inert Membrane Materials to Enhance Antibacterial Activity Masashi Kaneda, Xinglin Lu, Wei Cheng, Xuechen Zhou, Roy Bernstein, Wei Zhang, Katsuki Kimura, and Menachem Elimelech Environ. Sci. Technol. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.estlett.9b00012 • Publication Date (Web): 04 Feb 2019 Downloaded from http://pubs.acs.org on February 5, 2019

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Environmental Science & Technology Letters

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Photo-Grafting Graphene Oxide to Inert Membrane

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Materials to Enhance Antibacterial Activity

5 6 7 8

Masashi Kaneda1, 2, Xinglin Lu*, 1, Wei Cheng1, 3, Xuechen Zhou1, Roy Bernstein4,

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Wei Zhang4, Katsuki Kimura2, and Menachem Elimelech*, 1

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

1Department

of Chemical and Environmental Engineering, Yale University, New Haven, CT 06520-8286, United States

2Division

of Environmental Engineering, Hokkaido University, N13W8, Kita-ku, Sapporo 060-8628, Japan

3State

Key Laboratory of Urban Water Resource and Environment, School of Environment, Harbin Institute of Technology, Harbin 150090, China

4Department

of Desalination and Water Treatment, Zuckerberg Institute for Water Research, The Jacob Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Sede-Boqer Campus 84990, Israel

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* Corresponding Authors

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* E-mail: [email protected] (X.L.); [email protected] (M.E.)

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Abstract

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Surface modification with bactericides is a promising approach to imparting membrane materials

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with biofouling resistance. However, chemical modification of membranes made from inert

31

materials, such as polyvinylidene fluoride (PVDF) and polysulfone, is challenging due to the

32

absence of reactive functional groups on these materials. In this study, we develop a facile

33

procedure using benzophenone as an anchor to graft biocidal graphene oxide (GO) to chemically

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inactive membrane materials. GO nanosheets are first functionalized with benzophenone through

35

an amide coupling reaction. Then, benzophenone-functionalized GO nanosheets are irreversibly

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grafted to the inert membrane surfaces via benzophenone-initiated crosslinking under UV

37

irradiation. The binding of GO to the membrane surface is confirmed by scanning electron

38

microscopy and Raman spectroscopy. When exposed to a model bacterium (Escherichia coli),

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GO-functionalized PVDF and polysulfone membranes exhibit strong antibacterial activity,

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reducing the number of viable cells by 90% and 75%, respectively, compared to the pristine

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membranes. Notably, this bactericidal effect is imparted to the membranes without compromising

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membrane permeability and solute retention properties. Our results highlight the potential

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application of benzophenone chemistry in membrane surface modification as well as its promise

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in developing antimicrobial surfaces for a variety of environmental applications.


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TOC Art



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INTRODUCTION

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Biofouling is a major technical obstacle in membrane-based separation processes because it leads

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to a decrease in membrane performance and an increase in energy consumption and operational

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cost.1,2 Although numerous techniques have been implemented to mitigate membrane biofouling,

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such as pretreatment of the feed water and optimization of operating conditions,3 it is still very

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challenging to fully eliminate the occurrence of biofouling. The inevitable adhesion of bacteria to

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the membrane surface results in the colonization and multiplication of microbial cells, thereby

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leading to the formation of a biofilm,4,5 which is the major cause of membrane biofouling.5

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Therefore, surface modification of membranes with bactericidal materials against proliferative

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bacteria has been of great interest in fabricating anti-biofouling membranes for sustainable water

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purification.6

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Graphene oxide (GO), an emerging two-dimensional carbon-based nanomaterial, has been

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extensively investigated as a bactericidal modifier to impart antimicrobial activity to various

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engineered surfaces, including cotton fabric,7 polymer films,8 and water treatment membranes.9,10

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GO nanosheets induce physical or chemical damage to the cell membrane upon direct contact,6,11

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which allows for nondepleting and environmentally friendly antimicrobial surface coatings.

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Owing to the high specific surface area, GO is also employed as a structural scaffolding for

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biocidal metal nanoparticles (e.g., Ag, ZnO, or TiO2)12–14 to achieve synergistic antibacterial

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properties. Previous studies have demonstrated membrane surface functionalization with GO or

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GO-silver nanocomposites to reduce biofilm formation on the membrane surface, thereby

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mitigating the deleterious effects of biofouling.15,16

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Although surface grafting of GO nanosheets onto membrane materials (e.g., polyamide

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desalination membranes17 or polyacrylonitrile membrane18) with active groups is quite

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straightforward, chemical modification of membranes made from inert materials such as

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polyvinylidene fluoride (PVDF) and polysulfone (PSf), which are commonly employed for water

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purification processes,19 is challenging due to the absence of reactive functional groups on these

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materials. While GO can be physically blended into the casting solution during the phase-inversion

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process for fabricating GO-composite membranes (e.g., PES,20 PSf,21 or PVDF22), the majority of

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nanosheets are inevitably embedded in the bulk polymer and remain unavailable for contact-

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mediated bacterial inactivation. To expand the realm of biocidal GO material in water purification -4ACS Paragon Plus Environment

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membranes as well as other environmental surfaces where biofouling resistance is needed, novel

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approaches are crucially needed to functionalize inert materials with GO nanosheets.

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Benzophenone is a widely used photo-initiator and crosslinker in biochemistry and material

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science,23 due to its unique photochemical properties and low risk to the environment.24 Upon UV

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irradiation at 365 nm, benzophenone forms a triplet ketyl biradical that can create a covalent bond

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through abstraction of hydrogen atom from accessible C-H bonds and subsequent recombination

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processes.23,25,26 Recently, this versatile photochemical technique has been applied to modify the

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surface of electrode,27 photocatalyst,28 and biosensor29 materials. Notably, this benzophenone

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chemistry is also effective in grafting polymer brushes to nonpolar surfaces such as

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polydimethylsiloxane (PDMS) and even polytetrafluoroethylene (PTFE),30 thereby holding

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potential in surface functionalization of inert membrane materials while serving as a versatile

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anchor and crosslinker.

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In this study, we demonstrate, for the first time, a facile technique using 4-benzoylbenzoic

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acid as an anchor to graft GO nanosheets to inert membrane materials, including PVDF and

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polysulfone. The functionalized membranes are extensively characterized to confirm the

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successful grafting of GO and to assess its effect on the membrane’s intrinsic transport properties.

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The grafted GO nanosheets impart strong antibacterial activity to the inert membranes, as

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evidenced by the decreased viability of bacterial cells in contact with the membrane surface. Our

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results highlight the potential application of benzophenone chemistry in surface modification of

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inert materials for a variety of environmental applications.

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MATERIALS AND METHODS

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Synthesis of Benzophenone-Functionalized Graphene Oxide (GO). GO nanosheets

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were functionalized with 4-benzoylbenzoic acid (benzophenone) through ethylenediamine-

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mediated amide coupling reaction (Figure 1A). Native carboxyl groups of GO were first converted

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to amine-reactive esters in the presence of 1.5 mM 1-ethyl-3-[3-(dimethylamino) propyl]

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carbodiimide hydrochloride (EDC) and 2.5 mM N-hydroxysuccinimide (NHS). Ethylenediamine

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was then reacted with the amine-reactive esters of GO for 2 h, yielding amine-terminated GO

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nanosheets.17 Activated carboxyl groups of 4-benzoylbenzoic acid, formed by reaction with 2.25

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mM EDC and 3.75 mM NHS,31 were used to link the benzophenone molecule to the amine-5ACS Paragon Plus Environment


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functionalized GO nanosheets via amide coupling. X-ray photoelectron spectroscopy (XPS, PHI

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VersaProbe II, USA) was performed with a 0.47 eV system resolution to identify the atomic

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composition and relevant chemical structure of control GO and benzophenone-functionalized GO.

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More details about the synthesis of benzophenone-functionalized GO nanosheets are given in

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Supporting Information as well as the schematic in Figure S1.

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FIGURE 1

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Surface Modification of Inert Membranes. Benzophenone-functionalized GO

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nanosheets were photo-grafted to polysulfone membrane (Mw = 22 kDa, SEPRO, USA) and two

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types of commercial PVDF membranes (hydrophobic and hydrophilic, models VVHP and GVWP,

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Millipore) (Figure 1B). A membrane coupon was placed on a glass slide and sealed with a frame,

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leaving the top surface exposed. The membrane surface was first immersed in benzophenone-

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functionalized GO dispersion at room temperature for 2 h. Thereafter, the surface was thoroughly

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rinsed twice with DI water. The surface with freshly adsorbed GO was then irradiated under UV

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light (365 nm, 4 W, F4T5/BLB) for one hour in air. Lastly, the GO-functionalized membrane was

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bath sonicated (26 W·L−1, FS60 Ultrasonic Cleaner, Fisher Scientific) for 10 minutes to remove

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unbound GO nanosheets and stored in dry condition until use. Additional experimental procedures

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of surface modification are given in the Supporting Information.

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Characterization of Modified Membranes. Scanning electron microscopy (SEM, XL-

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Philips, USA) was used at an acceleration voltage of 10 kV to verify the successful grafting of GO

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to the membrane surface. Samples were dried in a desiccator and sputter-coated with 8 nm of

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iridium prior to SEM imaging. The presence of GO nanosheets on the membrane surface was

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confirmed using Raman spectroscopy (Horiba Jobin Yvon HR-800, USA) at 532 nm laser

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excitation. Atomic force microscopy (AFM, Bruker, USA) was carried out to characterize the

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surface roughness and morphology of GO-functionalized membranes in a peak force tapping mode

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with silicon probes (Scanasyst-Air, Bruker). XPS spectra were collected to identify the surface

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elemental composition of membranes. Membrane surface hydrophilicity was determined by

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measuring the contact angle of DI water using the sessile drop method (Video Contact Angle

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System, AST Products, USA) as described in previous publications.15,32

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The water permeability of GO-functionalized membranes was evaluated in a dead-end

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filtration unit (Model 8010, Amicon Stirred Cell, Millipore) at an operating pressure of 3.4 bar for -6ACS Paragon Plus Environment

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polysulfone membrane and 0.1 bar for PVDF membrane. Prior to water permeability tests,

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membranes were compacted for 7 h at 3.4 bar to achieve stable water flux (Figure S2). DI water

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was used as the feed solution at room temperature (23 ºC), and permeate flux was monitored

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throughout the experiment by a computer at one-minute intervals. Molecular weight cut-off

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(MWCO) of membranes before and after surface functionalization was determined using a dead-

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end filtration cell at 2.5 bar and room temperature (25  2 ºC). In these experiments, the rejection

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of polyethylene glycol (PEG) with different molecular weights (10, 20, 35, and 100 kDa) was

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determined by a total organic carbon (TOC) analyzer (Analytik Jena, Germany) and the MWCO

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of the membrane corresponded to the MW of PEG exhibiting 90% rejection.

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Antibacterial Activity of Modified Membranes. A colony-forming unit (CFU)

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enumeration assay and a live/dead fluorescent staining assay were employed to evaluate the

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antibacterial activity of GO-functionalized membranes, following the protocol provided in

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previous publications.11,17,33,34 For both assays, Escherichia coli (E. coli, American Type Culture

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Collection BW26437) suspension (108 CFU·mL−1, 0.9% NaCl) was in contact with the membrane

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surface for 3 h at room temperature. Cells were removed from the top surface after 10-minute bath

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sonication in saline solution (0.9%, NaCl) and immediately cultured on Luria-Bertani agar media,

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followed by incubation overnight at 37 °C for CFU enumeration. To determine the live/dead ratio,

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the detached bacteria from membranes were stained with 1.17 μM SYTO 9 and 10 μM propidium

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iodide (Live/Dead Baclight Bacterial Viability Kit, Thermo Fisher, USA) and then visualized by

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an epifluorescence microscope (Axiovert 200 M, Zeiss, USA). Morphology of bacteria deposited

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on membranes was imaged by SEM following the procedure described in previous studies.34,35

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Briefly, after a three-hour exposure to membranes, cells were fixed using Karnovsky's solution at

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pH 7.4 for 3 h, and then sequentially dehydrated by immersing samples every 10 minutes in

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water/ethanol and ethanol/freon mixtures. Samples were dried in a desiccator overnight at room

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temperature, sputter-coated with 16-nm iridium, and visualized by SEM using 10-kV acceleration

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voltage.

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RESULTS AND DISCUSSION

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GO Nanosheets Are Irreversibly Grafted to Inert Membrane Materials. Benzophenone-

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functionalized GO nanosheets were synthesized via EDC- and NHS-mediated amide coupling -7ACS Paragon Plus Environment


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reaction using ethylenediamine as a cross-linker, as schematically illustrated in Figure 1A. The

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elemental composition of control and benzophenone-functionalized GO (GO-BPh) was surveyed

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by XPS analysis (Table S1). In particular, the nitrogen content was 1.8% in GO-BPh composite,

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whereas no nitrogen content was detected in the control GO. Additionally, the XPS N1s spectrum

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was deconvoluted into two peaks, at 399.6 and 401.6 eV, which are ascribed to the amide linkage

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(N-C=O) and cationic ammonium nitrogen (C-NH3+), respectively (Figure 2A).36,37 In comparison

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to that of control GO, the nitrogen peak for N-C=O significantly emerged after GO nanosheets

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were functionalized with benzophenone groups, verifying the successful synthesis of GO-BPh

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composite through the formation of an amide bond. The deconvoluted XPS C1s spectrum revealed

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the presence of hydroxyl, epoxide, and carboxyl functional groups of GO and GO-BPh composite

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(Figure 2B).

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FIGURE 2

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The membrane top surface was exposed to benzophenone-functionalized GO dispersion, and

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GO nanosheets were irreversibly bound to the surface through benzophenone-initiated

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crosslinking under UV irradiation (Figure 1B). When in contact with the membrane surface, the

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hydrophobic nature of benzophenone facilitates adsorption of GO-BPh composite, thereby

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forming a GO adsorbed layer on the membrane surface.30 Benzophenone groups are activated into

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a biradicaloid triplet state via an n−π* transition under UV irradiation.23,38 The triplet

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benzophenone abstracts a hydrogen atom directly from adjacent aliphatic C-H bonds of the

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membrane material and subsequently recombines with the substrate,25,26 eventually resulting in

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anchoring GO nanosheets to the membrane surface. Membranes were modified with two different

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GO concentrations, 100 and 500 mg/L, and the corresponding functionalized membranes were

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denoted as GO100 and GO500, respectively. Because all GO-functionalized membranes

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underwent bath sonication for 10 minutes to remove unbound GO nanosheets, any GO nanosheets

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observed on the surface were ascribed to grafting through covalent bonding.

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After the functionalization, both membrane surfaces showed a slight color change from white

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to light brown, providing preliminary evidence of the successful modification (Figure S3). SEM

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images enable us to take a closer look at the deposited nanosheets on the membrane surface (Figure

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3). Pristine membranes (Figures 3A and 3C) displayed a smooth surface whereas GO nanosheet

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wrinkles were observed on both the functionalized membrane surfaces (Figures 3B and 3D).39 -8ACS Paragon Plus Environment

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Surface functionalization with GO was further confirmed by Raman spectroscopy (Figures 3E and

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3F). Both GO-functionalized polysulfone and PVDF membranes exhibited two dominant peaks at

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1350 and 1590 cm-1, which correspond respectively to characteristic D and G bands of GO,40

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whereas no significant peak was observed on the pristine membrane except for the polysulfone

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substrate. For the Raman spectra of GO-functionalized polysulfone membrane, the presence of

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GO (G band) and phenyl ring vibration of polysulfone backbone both contributed to the peak

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positioned at 1590 cm-1.41,42 Notably, the hydrophobic PVDF membrane, which is a pristine

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material without any additives received from the manufacturer,43 was also successfully

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functionalized with GO via the benzophenone chemistry, as verified by SEM imaging and Raman

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spectroscopy (Figures S4 and S5), suggesting the versatility of benzophenone chemistry in

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modifying hydrophobic inert materials.

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FIGURE 3

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Minimal Impact of Surface Functionalization on both Membrane Transport and

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Surface Properties. Water permeability and molecular weight cut-off (MWCO) of polysulfone

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and PVDF membranes were evaluated to determine the effect of GO functionalization on the

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intrinsic membrane transport properties. Taking polysulfone membrane as an example (Figure

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S6A), GO-functionalized membranes i.e., GO100 (276  92 L m-2 h-1 bar-1) and GO500 (264  28

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L m-2 h-1 bar-1), displayed water permeability comparable to that of the pristine polysulfone

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membrane (308  68 L m-2 h-1 bar-1). However, UV exposure of the pristine polysulfone membrane

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surface decreased its water permeability to 188  63 L m-2 h-1 bar-1, even though UV light at a

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wavelength larger than 350 nm is unlikely to cause polymer degradation of polysulfone.44 This

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result implies that benzophenone groups or GO nanosheets absorbed a large portion of the UV

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light while photo-grafting GO to polysulfone membranes, thereby protecting the membrane

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surface from UV degradation. Additionally, functionalization with GO does not have a detrimental

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impact on membrane selectivity, as evidenced by the comparable MWCOs around 90 kDa for both

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pristine and modified membranes (Figure S6B). Taken together, these results indicate that GO

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functionalization does not compromise polysulfone membrane transport properties. Notably,

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transport properties of the PVDF membranes were also not impacted by surface functionalization

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(Figure S7).

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XPS analysis of the GO-functionalized membrane surfaces revealed a slight increase in the -9ACS Paragon Plus Environment


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O/S ratio for the polysulfone membrane from 4.6 to 10.6 and in the O/F ratio for the PVDF

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membrane from 1.0 to 1.9 after the surface functionalization (Table S2). This result is attributable

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to abundant oxygen functional groups of GO-BPh composite originating from pristine GO (Table

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S1). However, the increased content of oxygen in GO-functionalized polysulfone membrane does

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not alter water contact angles of GO100 (~70) and GO500 (~66) membranes compared to those

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of the pristine membrane (~67), indicating that GO functionalization does not affect the

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membrane surface hydrophilicity (Figure S8A). This unchanged hydrophilicity is likely ascribed

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to the partial coverage of the surface by GO nanosheets, which left most unmodified areas exposed.

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Similar phenomena were also observed for the PVDF membranes (Figure S8B). Figure S9 shows

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representative three-dimensional AFM images of polysulfone membranes. Root-mean-square (Rq)

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and average surface roughness (Ra) values of GO100 (Rq: 9.1 nm, Ra: 7.1 nm) and GO500 (Rq: 9.7

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nm, Ra: 7.8 nm) polysulfone membranes were not affected by the grafted GO on the surface. This

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observation is likely due to the atomically thin nanosheets that do not significantly change the

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surface roughness of the relatively smooth pristine polysulfone membrane (Rq: 10.9 nm, Ra: 8.7

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nm) (Table S3).

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GO-Functionalized Membranes Exhibit Enhanced Antibacterial Activity.

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Antimicrobial properties of GO-functionalized membranes were evaluated using the colony-

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forming unit (CFU) enumeration assay.11,17,34 In brief, after the membrane surface was exposed to

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a model bacterium (E. coli) for three hours, live cells were detached from the surface by mild

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sonication, cultured on solid media, and incubated overnight. Photo-grafted GO nanosheets on the

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surface reduced the viability of attached E. coli cells after three hours of exposure by 65% and 75%

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for the GO100 and GO500 polysulfone membranes, respectively (Figure 4A). Similarly,

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functionalized PVDF membranes exhibited strong antibacterial activity, reducing the cell

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viabilities of the GO100 and GO500 PVDF membranes to 20% and 10%, respectively (Figure 4B).

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A live/dead fluorescent staining assay further demonstrated strong antibacterial activity of GO

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nanosheets on the polysulfone membrane surfaces, decreasing viable cells from 84% (pristine

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membrane) to 14% (GO100) and 29% (GO500) (Figure S10A). A lower number of live cells

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(green) and a higher number of dead cells (red) were observed when in contact with GO-

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functionalized surfaces than with the pristine membrane (Figure S10B). This enhanced

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antibacterial effect of GO is in agreement with previous studies where nanosheets were grafted to

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thin-film composite polyamide membranes.15,17 - 10 ACS Paragon Plus Environment

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FIGURE 4

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Bacterial inactivation mechanisms of GO have been attributed to physical disruption and

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chemical oxidation, resulting in loss of cell integrity and proliferation.45,46 Recent studies have

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shown that the edge- and wrinkle-mediated direct contact of GO nanosheets with cells plays a

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critical role in bacterial inactivation and leads to enhanced antibacterial activity.11,39,47 These

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mechanistic insights could explain the higher cell inactivation rate demonstrated for the GO-

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functionalized PVDF membranes with a rougher surface, which could enhance exposure of edges

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and wrinkles on the surface (Figures 4A and 4B). In contrast, on the smooth polysulfone

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membranes, most of GO nanosheets tend to planarly lay down on the surface. Another potential

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reason for this finding is the difference in membrane surface properties, because the CFU results

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for the relevant membrane are determined by not only cytotoxicity of GO but also by the adhesive

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interactions between bacteria and the surface.11,48 Morphological changes of E. coli cells attached

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to the membrane surface were imaged by SEM (Figures 4C-F).34,35 The bacterial cells on GO-

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functionalized polysulfone and PVDF membranes became flattened and shrunk, whereas the

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bacteria on the pristine membranes displayed intact cell integrity, indicating GO functionalization

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significantly damaged the cell membrane and led to cell death.

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In summary, we presented the first demonstration of benzophenone chemistry as a direct

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anchor for surface functionalization of inert water purification membranes with biocidal GO

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nanosheets. GO-functionalized membranes exhibited strong antibacterial activity, which, in turn,

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could increase the resistance of membranes to biofouling by hindering bacterial colonization and

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biofilm formation on the surface. This cytotoxicity of GO can be improved by optimizing its

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physicochemical properties (e.g., sheet size34 or oxidation level11) upon contact with bacteria. Our

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work provides a new platform to impart GO-induced antibacterial activity to inert membrane

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materials and other engineered surfaces.

281 282

SUPPORTING INFORMATION

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The Supporting Information is available free of charge on the ACS Publications website. Materials

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and chemicals, synthesis of benzophenone-functionalized GO nanosheets, membrane surface

285

functionalization with GO-BPh composite, schematic diagram for the synthesis procedure of the

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GO-BPh composite (Figure S1), water flux declines during compaction for polysulfone - 11 ACS Paragon Plus Environment


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membranes (Figure S2), photographs of GO-functionalized membranes (Figure S3), SEM

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micrographs of hydrophobic PVDF membranes (Figure S4), Raman spectrum of hydrophobic

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PVDF membranes (Figure S5), water permeability and PEG rejection of polysulfone membranes

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(Figure S6), water permeability of hydrophilic PVDF membranes (Figure S7), water contact angles

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of polysulfone and hydrophobic PVDF membranes (Figure S8), AFM images of polysulfone

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membranes (Figure S9), cell viability and epifluorescence images of E. coli on polysulfone

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membranes in live/dead fluorescent staining assay (Figure S10), XPS elemental composition of

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GO and GO-BPh composite (Table S1), XPS elemental composition of polysulfone and

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hydrophilic PVDF membranes (Table S2), surface roughness of polysulfone membranes (Table

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S3).

297 298

ACKNOWLEDGMENTS

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The authors acknowledge the financial support received from the United States-Israel Binational

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Agricultural Research and Development Fund (BARD, Project IS-4977-16). The authors

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acknowledge the use of facilities supported by Yale Institute for Nanoscience and Quantum

302

Engineering (YINQE) and NSF MRSEC DMR 1119826. The authors also thank Dr. J. Girard for

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the Raman spectroscopy, Dr. C. Boo for XPS measurements, and C. Fausey for technical advice.

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Figure 1. Schematic diagram of the surface modification procedure using benzophenone

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chemistry. (A) Reaction procedure to synthesize benzophenone-functionalized GO nanosheets.

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GO nanosheets are functionalized with 4-benzoylbenzoic acid (benzophenone) using 1-ethyl-3-[3-

445

(dimethylamino) propyl] carbodiimide hydrochloride (EDC)- and N-hydroxysuccinimide (NHS)-

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mediated activation of carboxyl groups. Native carboxyl groups of GO nanosheets and 4-

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benzoylbenzoic acid are first converted into amine-reactive esters by EDC and NHS.

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Ethylenediamine (ED) is then covalently bonded through the formation of amide bonds to link the

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benzophenone molecule to GO nanosheets. (B) Benzophenone-functionalized GO nanosheets are

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first adsorbed on the membrane surface through hydrophobic interactions. Benzophenone groups

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are then covalently linked to the substrate membranes via photo-induced grafting and crosslinking

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under UV irradiation.


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Figure 2. N1s XPS spectra of (A) control GO and benzophenone-functionalized GO (GO-BPh).

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C1s XPS spectra of (B) control GO and GO-BPh composite.

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Figure 3. Representative SEM micrographs of (A) pristine polysulfone, (B) GO-functionalized

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polysulfone, (C) pristine PVDF, and (D) GO-functionalized PVDF membranes. Raman spectra of

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(E) pristine polysulfone (gray) and GO-functionalized polysulfone membranes (red), and (F)

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pristine PVDF (gray) and GO-functionalized PVDF membranes (red).

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Figure 4. Antibacterial activity of GO-functionalized membranes. Relative number of viable E.

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coli cells after three hours of contact with (A) GO-functionalized polysulfone membranes and (B)

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GO-functionalized PVDF membranes. Values marked with an asterisk (*) are significantly

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different from the value of the control sample (n = 3; Student’s t-test, P < 0.05). Representative

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SEM micrographs of E. coli cells fixed on (C) pristine polysulfone, (D) GO-functionalized

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polysulfone, (E) pristine PVDF, and (F) GO-functionalized PVDF membranes.


Photografting Graphene Oxide to Inert Membrane Materials to Impart

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