Stable graphene based membrane with pH-responsive gates for

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Stable graphene based membrane with pHresponsive gates for advanced molecular separation Lina Zhang, Abdul Ghaffar, Xiaoying Zhu, and Baoliang Chen Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b03662 • Publication Date (Web): 07 Aug 2019 Downloaded from pubs.acs.org on August 8, 2019

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Environmental Science & Technology

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Stable graphene based membrane with pH-responsive gates for

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advanced molecular separation

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Lina Zhang1,2, Abdul Ghaffar1,2, Xiaoying Zhu1,2,* and Baoliang Chen1,2

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1. Department of Environmental Science, Zhejiang University, Hangzhou, Zhejiang 310058, China.

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2. Zhejiang Provincial Key Laboratory of Organic Pollution Process and Control, Hangzhou 310058, China.

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*Corresponding author:

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Dr. Xiaoying Zhu, [email protected]

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Tel.: +86-571-88982651

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Fax: +86-571-88982651

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Co-authors:

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Lina Zhang, [email protected]

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Abdul Ghaffar, [email protected]

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Baoliang Chen, [email protected]

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ACS Paragon Plus Environment


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TOC

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Abstract

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Graphene based stable pH-responsive membranes (GPMs) were developed by alternative

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deposition of graphene oxide (GO) with polyethylenimine (PEI) in a layer-by-layer manner.

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Different from the conventional pore-blocking pH-responsive membranes, the size-adjustable gaps

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among the GO sheets were firstly designed to response to the surrounding pH. Atomic force

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microscopy (AFM) was used to dynamically explore the internal structure altering of GPM in the pH

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range from 3 to 11. It was found that the PEI molecules not only cross-linked the GO sheets through

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amide bonds to ensure the membrane stability but also reversibly altered the gate size of GPM in a

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certain extent according to the surrounding pH. In filtration, the gates of GPM were widening with

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the decreasing pH of the feed and vice-versa. As a result, the permeate flux of GPM was increasing

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with the decreasing feed pH. More importantly, the molecular weight cut-off (MWCO) of GPM

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could be continuously regulated by the feed pH in a certain range; in filtration of the PVP and PEO2

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mixed solution, only PVP (58 kDa) could penetrate GPM at pH: 11, while the left PEO2 (600 kDa)

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would penetrate GPM at pH: 3. The controlled penetration through GPM led to a complete

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separation and recovery of the molecules in different sizes, which is highly desirable for advanced

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molecular separation in environmental applications.

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Keywords:

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pH-responsive membrane; graphene oxide; polyethylenimine; layer-by-layer assembly; molecular

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separation

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1. Introduction

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The stimuli-responsive membranes are of great interests in both academia and industry, whose

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performances such as sieving effect and fouling resistance are changing with the surrounding

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conditions such as pH, temperature, light irradiation, electric field, concentration of chemical

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species, and ionic strength of the feed.1-4 As an example, pore size of a thermal responsive membrane

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prepared by assembling graphene oxide (GO) sheets grafted with poly(N-isopropylacrylamide) was

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adjusted by temperature of the feed.5 However, the consumption of energy will be tremendous, if

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either the feed solution or the membrane module was heated up from 25 oC to 50 oC in a typical

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wastewater treatment plant (50,000 m3/day). On the other hand, pH adjusting is a simple and routine

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procedure in water and wastewater treatments.4 Thus, the membrane which is responsive to pH rather

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than the other stimuli indicated much greater potential to be practically applied in plants. It has been

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reported that polyelectrolytes containing weak acidic or basic functional groups were incorporated

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into polymeric membranes through blending, grafting and coating to achieve the pH-responsive

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function.4 Since pH value of the feed solution determines ionization degree of weak acidic or basic

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groups in the polyelectrolyte chain, the polymeric chain conformation which affects channel size and

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surface property of the prepared membrane would be changed accordingly.4, 6 However, stability of

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the pH-responsive membranes containing only polyions is of the main concern that swelling of

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polyions is uncontrollable in these cases. Even cross-linking might improve the membrane stability,

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the pH-responsive effect of the membrane will be limited because of the disordered and intertwined

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chains in the polymeric matrix. In addition, pore blocking caused by conformation change of

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pH-responsive polymeric chains was the mechanism of the sieving effect altering in the reported

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pH-responsive membranes, which however may result a wide pore size distribution because of the

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high flexibility of the polymeric chains.

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Graphene oxide (GO) is one of the most important graphene derivatives which not only inherits

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the desirable properties such as atomic thickness, superior chemical and mechanical stability from

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graphene but also contains abundant oxygen-containing functional groups such as hydroxyl, epoxy

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and carboxyl providing more possibilities of further modification and application.7-10 Moreover, GO

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nanosheets can be easily prepared in large scale, which makes the fabrication of GO based

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membrane (GOM) for practical applications possible.9, 11-14 GOMs were usually prepared by stacking 4


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GO nanosheets through vacuum filtration,15,

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layer-by-layer (LbL) assembly,17,

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liquid crystalline.12 As a versatile method, the LbL assembly could well control thickness, interlayer

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space and interaction of GOMs.19-21 GOMs with controlled nanochannel size by spacers were widely

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applied in molecular separation during gas, organic solvent and water purifications.13, 22-24 However,

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the sizes of the sieving channels of the reported GOMs were usually constant and not changing with

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the environmental stimuli.

or casting GO

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As a typical weak acidic group, carboxylic group is abundant in the edges and defects of GO

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sheets.8 Obviously, GOMs may be endowed with the GO initiated pH-responsive feature.25 However,

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GOMs constructed from GO only were suffering the same stability issue as polymeric pH-responsive

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membranes.26, 27 Thus, GO was usually collaborated with polycations to prepare more stable GOMs

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or control interlayer space.21 It has been reported that the ionization of carboxylic groups in GO took

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response for the channel size shifting to a very limited extend in a GO/poly(allylamine

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hydrochloride) multilayer membrane system because only very limited amine groups existed in the

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GOM.17 Polyethylenimine (PEI) contains high density weakly basic amine groups which not only

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may react with carboxylic group to form stable amide bond but also may change the conformation of

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the PEI chain with the surrounding pH.28-30 It has been reported that PEI could either improve

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stability of GOM31 or shift surface charge of GOM from negative to positive.32 It is of great interest

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to study the polycation initiated pH-responsive feature of GOM. However, the pH-responsive feature

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of the spacer molecule has never been adopted in any GOM systems.

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In this study, novel graphene based pH-responsive membranes denoted as GPMs were

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constructed by alternatively assembling GO nanosheets with two types of PEI in LbL manner,

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respectively. To study the PEI initiated pH-responsive effect of GPM, both branched and linear PEI

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molecules were used in the membrane preparation. The prepared GPMs were characterized by SEM,

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XPS and FTIR. Afterwards, AFM and XRD were adopted to reveal the internal structure altering of

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GPM in the pH range from 3 to 11. Subsequently, the pH-responsive performances such as the

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permeate flux and the sieving of GPM were studied in the pH range from 3 to 11. The barrier of

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GPM regulated by pH was applied to separate molecules with different molecular weights. It was

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found that one single piece of GPM may respectively separate a series of molecules with different

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molecular weights by simply adjusting pH of the mixture, which indicated great potential for

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advanced molecular separation in practical applications such as water purification. 5



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2. Methods

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2.1 Materials

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Natural graphite flakes (325 meshes, 99.8%) were purchased from Alfa-Aesar. Poly(vinylidene

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fluoride) (PVDF, Mw: ~530,000) and Poly(2-ethyl-2-oxazoline) (PEOX, Mw: ~50,000, 1H NMR

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(D2O) δH: 3.4 ppm (m), 2.3 ppm (m) and 1 ppm (m)) were provided by Sigma-Aldrich. Branched

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polyethylenimine (PEI, Mw: ~10,000 Da), polyvinylpyrrolidone (PVP, Mw: ~58,000 Da),

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N-N-dimethylacetamide (DMAc, >99%) were obtained from Aladdin Reagent Company (Shanghai,

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China) and polyethylene oxides (PEO1, Mw: ~300,000 Da and PEO2, Mw: ~600,000 Da) were

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provided by Macklin. Silver nanoparticles (Ag NP, size: ~50nm) were provided by Xianfeng Nano

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Company (Nanjing, China). All chemicals were used without further purification.

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2.2 Synthesis and characterization of the linear PEI

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The linear PEI was synthesized by hydrolyzing PEOX (Figure S1). In this process, PEOX (1 g)

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was added to a 50 mL round flask containing 20 mL of HCl solution (6 mol/L); the mixture was

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heated to 100 oC for 4 h in a microwave digester (Hanon SH230). After cooling the mixture to room

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temperature, the generated white turbid suspensions were collected and purified through dialysis

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(Scientific Research Special, Mw: 3500) against DI water for a few days. Finally, the pure linear PEI

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molecules were concentrated and freeze dried for further use.

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Synthesis of the linear PEI was characterized by Nuclear Magnetic Resonance (NMR, Agilent

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600) and Fourier Transform Infrared Spectrometer (FTIR, Thermo, Nicolet 6700). Molecular weight

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of the linear PEI was characterized with gel permeation chromatography (GPC, Waters 1525/2414).

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The chromatography conditions were as followed: using water as solvent and eluent; column:

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PL1149-6830, PL1149-6840 and PL1149-6850 in series; flow rate: 0.8 mL/min; temperature: 30 oC.

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Information of the linear PEI was summarized below [GPC determined Mw: 8956 Da. 1H NMR

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(D2O) δH: 2.9 ppm (s); FTIR: 3425 cm-1, 1609 cm-1, 1139 cm-1 and 755 cm-1].

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2.3 Preparation and characterization of GPMs

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GO nanosheets was synthesized from natural graphite flakes by a modified Hummers’ method

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and detailed earlier.33, 34 The support membrane was prepared by a phase inversion technique. First,

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GO (100 mg) was sonicated in 40.9 g of DMAc for 30 min to obtain a homogeneous dispersion.

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Thereafter, PVDF (8 g) and PVP (1 g) were added and dissolved in the dope solution at 70 °C under

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continuous stirring in sequence to achieve a homogeneous solution. After centrifuging (3000 rpm, 5

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min) to degas, the dope solution was uniformly casted on glass plates with a thickness of 200 µm by

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using an automatic film applicator (BEVS1811) and a doctor blade (BEVS 1806/150). Subsequently,

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the glass plates were immediately immersed in a coagulation bath containing DI water. The

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solidified membranes were stored in a fresh pure water bath for overnight to ensure a complete phase

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inversion and then dried in air.

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For LbL deposition, a GO solution (1 mg/mL) was prepared by dissolving GO in DI water and

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sonicating for 30 min to get a homogeneous solution; the branched and linear PEI solutions (1

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mg/mL) were prepared by dissolving PEI in DI water. The pH values of the GO and PEI solutions

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were all adjusted to 3.5 using NaOH (1 mol/L) and HCl (1 mol/L) solutions, respectively. Firstly, the

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negatively charged support membrane was immersed in the PEI solution for 10 min; subsequently,

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the membrane was taken out and rinsed with DI water to remove residual PEI. After drying under

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nitrogen steam, the above membrane was immersed into the GO solution for 10 min; and then the

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membrane was taken out and rinsed with DI water to remove residual GO; afterwards the membrane

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was dried again under nitrogen stream. This cycle was repeated seven times to obtain a membrane

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with seven bilayers of GO&PEI. Finally, the membrane was heated in an oven at 60 oC for overnight

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to obtain the GPM membrane (Figure 1). The LbL assembly was also applied on a silicon wafer

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surface for the subsequent AFM characterizations. The silicon wafer was pretreated with oxygen

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plasma at 80 w for 2 min (Saot (Beijing) Optoelectronic Technology Co, Ltd.), resulting negatively

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charged surface (Figure S2) to initiate the LbL assembly. GPMs prepared from the branched PEI

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(Mw: 6639 Da determined by GPC) and the linear PEI (Mw: 8956 Da determined by GPC) will be

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donated as MB and ML, respectively.

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Surface morphology of the membranes was observed using a field emission scanning electron

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microscope (FESEM, Hitachi S-4800, Tokyo Japan) and atomic force microscopy (AFM, Bruker

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Dimension Icon). Membrane surface scanning by AFM was performed in air using the Bruker

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ScanAsyst mode using a probe (ScanAsyst in air) provided by Bruker. Surface roughness of

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membrane was calculated from the AFM height images (1×1 μm) through the software (NanoScope 7



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Analysis, Bruker).

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Thickness of the LbL film on silicon wafer was characterized by AFM. A scratch made by a

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clean and sharp blade on the silicon wafer with the LbL film was scanned in the ScanAsyst air mode

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using probe (ScanAsyst in air) in air and the ScanAsyst fluid mode using tip (ScanAsyst in fluid) in

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liquid. Thickness of the LbL films can be determined by measuring the height of the step from the

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silicon wafer substrate to top of the film in the AFM images. To minimize the influence of the

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membrane surface unevenness in the thickness determination, the lowest points of the higher and

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lower steps were selected to measure their vertical distance which was considered as the membrane

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thickness. For the measurements in liquid, the membrane was fixed in a tank filled with liquid and

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thickness shifting was monitored by scanning the same scratched step while changing the immersion

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solutions at different pH values (from 3 to 11 and back to 3). The thickness was determined by the

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average of five measurements at different positions for each membrane.

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Water contact angle of GPMs was evaluated through the captive bubble method in liquid. A

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surface analyzer (OSA200 Optical) provided by Ningbo NB Scientific Instruments Co., Ltd. was

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used for the contact angle measurements. A membrane sample (5 cm × 1cm) was fixed in a rack and

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immersed in a cubic tank filled with the HCl and NaOH water solutions at pH 3, 6 and 11 for 30 min

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until equilibrium, respectively. Subsequently, an air bubble was released by a syringe with a hook

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needle to the membrane surface; image of the bubble was taken and analyzed by the software

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(Surface Meter) to give the water contact angle of the sample at a certain pH. Five measurements

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were taken for each membrane at different positions, and the average value was recorded as the

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contact angle of the membrane at the specific pH.

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Surface chemical compositions of the membrane samples were analyzed by X-ray photoelectron

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spectroscopy (XPS, Thermo-Fisher Scientific) and FTIR (Thermo, Nicolet 6700). Interlayer spacing

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of the dry GPM membranes were analyzed by X-ray diffraction (XRD, Bruker D8 Advance),

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equipped with Cu Kα radiation source (λ = 0.15418 nm). The interlayer spacing was calculated based

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on the Bragg's law. ζ potentials of GPMs were measured using a Surpass 3 electro-kinetic analyzer

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from Anton Paar (GmbH, Austria). GPMs were cut into 1×2 cm sliders and attached to the sample

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holders. A 1 mM KCl aqueous background solution was used for determination of the ζ potential of

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the membranes at different pH values from 3 to 11 adjusted by HCl (0.05 mol/L) or NaOH (0.05

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mol/L). 8


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2.4 Filtration tests of GPM at different pH values

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The filtration experiments were carried out by using a dead-end filtration system. The detailed

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setup of the system can be found in our pervious paper.28 A serial of working solutions were

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prepared by adjusting pH of DI water to 3, 5, 7, 9 and 11, using the HCl and the NaOH solutions (1

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M). In the membrane flux determination, each membrane disk (diameter: 2.2 cm) was equilibrated in

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the working solutions for 30 min at pH: 3, 5, 7, 9 and 11, respectively, before filtration. In between

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the filtrations at different pHs, the membrane sample was rinsed with DI water. For each filtration,

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the membrane was pressed at 20 psi for 10 min; and then, the flux was recorded for 30 min when the

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transmembrane pressure was stabilized at 14.5 psi (1 bar). The stable flux in the last 10 min filtration

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was recorded as the membrane flux. Flux of each type of membrane was the average stable fluxes of

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three membrane samples filtration at the specific pH.

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Stability of the GPM was evaluated through measuring TOC of the permeate during the

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filtration of the working solutions at pH: 3, 5, 7, 9, and 11. One piece of GPM was set in the filtration

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cell that, firstly, the side with the GO/PEI multilayer was facing up; the volume of permeate

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collected at each pH point was about 10 mL; subsequently, the same piece of GPM was turned over

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with the GO/PEI multilayer side facing down; the same working solution series was filtrated through

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the membrane again. TOC of the collected permeates were measured using a total organic carbon

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analyzer (TOC-Vcph, SHIMADZU). Three membrane samples were tested for each GPM.

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In the single component retention experiments, each of the feed solutions including 100 mg/L

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PVP (58 kDa, its chemical structure can be seen in Figure S3a), 100 mg/L PEO1 (300 kDa, its

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chemical structure can be seen in Figure S3b), 100 mg/L PEO2 (600 kDa) and 100 mg/L silver

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nanoparticles (Ag NP, size: 50 nm) were prepared using the work solutions at pH: 3, 5, 7, 9 and 11,

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respectively. The prepared solutions (10 mL) at different pH values were filtrated through the

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membrane sample, while the permeates were collected and the solute concentrations were

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determined accordingly. Three membrane samples were tested for each GPM to determine the

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retention rate to the specific object (PVP, PEO1, PEO2 or Ag NP) at the specific pH value (3, 5, 7, 9

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or 11).

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The concentration of PVP and Ag NP was analyzed using a UV-visible spectrophotometer

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(UV-2550 SHIMADZU) at wavelength of 214.5 and 408 nm, respectively. The concentration of 9



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PEOs was measured through a total organic carbon analyzer (TOC-Vcph, SHIMADZU).

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Retention rate (R) of a membrane was calculated using the following equation:

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R= (C0-Cp)/C0*100%

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where C0 is the solute concentration of the feed solution and Cp is the solute concentration of

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the permeate solution.

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In the molecular mixture separation experiment, the feed solution was prepared by dissolving

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both PVP (58 kDa) and PEO2 (600 kDa) in the working solution at pH: 11 to give concentrations

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both at 100 mg/L. After equilibrium for 30 min, half volume (5 mL) of the feed was filtrated through

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the membrane; subsequently, another 5 mL of the working solution at pH: 11 was added into the

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feed; the PVP and PEO2 concentrations of the permeate were constantly monitored; the filtration and

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refill cycle was repeated until the PVP concentration in the permeate dropped to 0. In the second

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phase, pH of the feed was adjusted to 3 using a diluted HCl solution (1 M) and volume of the feed

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was fixed at 10 mL; after equilibrium for 30 min, half volume (5 mL) of the feed was filtrated

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through the membrane; subsequently, another 5 mL of the working solution at pH: 3 was added into

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the feed; the same filtration and refill cycle was repeated until the PEO2 concentration dropped to 0.

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Three membrane samples were tested for each GPM.

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3. Results and discussion

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3.1 Construction of stable GPMs with controllable gates

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GPMs were constructed through LbL deposition of GO nanosheets and PEI molecules on a

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negatively charged supporting membrane (Figure S2) which was prepared by imbedding GO sheets

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in the bulk PVDF matrix, as shown in Figure 1. Obviously, GO and PEIs bore opposite charges

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(Figure S4) at pH: 3.5, which ensured the surface charge inversion in every LbL deposition cycle.

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Since GO sheets framed the main structure of GPM, it is necessary to integrate more GO sheets in

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GPMs to ensure the membrane functions and reduce defects. It has been reported that the quantity of

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weak polyelectrolyte in each layer could be precisely controlled by tuning pH of the dipping solution

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during assembly.35, 36 Since the carboxylic groups of GO were only partially ionized at pH: 3.5, more

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GO nanosheets were needed to compensate the positive charge of the previous PEI layer. Thus,

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GPMs with less defects were purposely assembled at pH: 3.5.

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The GO sheets framed structure of GPM, while the PEI molecules were playing two important 10


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roles including cross-linker and spacer in the GPM system. To study the contributions of PEIs in

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different shapes to the membrane stability and pore tuning, two types of PEI molecules in branched

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and linear shapes were used in the GPM fabrication. The branched PEI (Mw: 6639 Da, determined

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by GPC) was commercially available, while the linear PEI was synthesized by hydrolyzing PEOX

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(Figure S1, more information can be seen in Figure S5 and S6). The GPM membrane prepared from

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GO and the linear PEI or the branched PEI was denoted as ML or MB, respectively.

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Figure 1. Schematic diagram of the stable GPMs.

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Morphologies of the GPM membranes were studied through SEM and AFM. As shown in

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Figure 2a and 2c, the surfaces of the supporting membrane (Figure S7) were homogeneously covered

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by the GO/PEI assembly. In addition, due to the flexibility of GO sheets, the assembled GO sheets

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were folded to show wrinkles (Figure S8). These wrinkles would be the entrances and the buffering

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spaces of the permeation species during filtration, resulting enhanced permeability of the GO based

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membrane.37 Similarly, the AFM images of MB (Figure 2b and S9a) and ML (Figure 2d and S9b)

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showed smooth surfaces with low roughness (Ra) at 4.8 nm and 5.2 nm, respectively. This could be

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attributed to the coverage of smooth planar GO nanosheets on the supporting membrane. In addition,

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the cross-sections of MB and ML were also observed through SEM. As shown in Figure S10, the

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dense skinny separation layer and the sponge-like supporting structure were presented both in the

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MB and ML membranes. Owing to the strong connection between the GO/PEI multilayer and the

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supporting membrane, it is difficult to identify the boundary between them from the SEM images

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(Figure S10). Since the ending layers of MB and ML were both GO, the ζ potential curves of the MB

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and ML membranes indicated negative charge in the pH range from 3 to 11 (Figure S11).

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Figure 2. SEM (above) and AFM (below) images of (a)-(b) MB and (c)-(d) ML.

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To examine the interaction between GO and PEI, chemical compositions of MB and ML were

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characterized by FTIR and XPS. As shown in the FTIR spectra (Figure 3a), the C=O stretching

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vibration at 1720 cm-1 of GO completely disappeared, while new peaks at 1654, 1572 and 1433 cm-1

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which could be assigned to the amide bond (HNC=O), the N-H bond and the stretching of C-N bond,

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respectively, showed up in the spectra of MB and ML.38 These results indicated the formation of

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amide bonds and the existence of amine groups in MB and ML. Surface chemical compositions of

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MB and ML were further analyzed by XPS. The XPS scans of MB and ML in Figure 3b both

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presented the N1s peak (400 eV) rather than GO (Figure S12), indicating the introduction of

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nitrogen-containing functional groups to GPM. In addition, the C1s spectrum of MB in Figure 3c

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presented two new peaks at 285.5 eV and 288.6 eV belonging to C-N and N-C=O, respectively, as

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compared with GO (Figure S12). The N1s spectrum of MB indicated two peaks at 400.1 eV and

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398.4 eV belonging to the NC=O and C-N group, respectively, as shown in Figure S13a.33, 39, 40 The

287

C1s and N1s spectra of ML were similar to those of MB, as shown in Figure 3d and S13b, 12


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respectively. Obviously, the XPS spectra confirmed the amide bond formation between GO and PEI

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in MB and ML. The formation of covalent amide bond or cross-linking ensured the GPM membranes

290

could sustain the pH range from 3 to 11 in application. Even the amide bond formation may consume

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a certain amount of amine groups; there were still abundant amine groups in GPM indicated by both

292

FTIR and XPS data. In addition, after calculation ratios of carbon containing functional groups based

293

on the XPS spectra of MB and ML, it was found that the percentages of carbon in NC=O were

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10.3% and 9.3% in MB and ML, respectively, as shown in Table S1. It seems the branched PEI

295

formed more amide bonds with GO nanosheets than the linear PEI. It is possible that MB with more

296

anchoring points might present a narrower extension range by responding to the pH stimuli than ML.

297

Stability of GPMs was evaluated by measuring TOC of the permeate during filtration of the

298

working solutions in the pH range from 3 to 11. As shown in Figure S14a, after filtrating the serial

299

working solutions through MB and ML with the multilayer side facing up (feed), the TOC data of the

300

permeates were very close to DI water, when pH of the feed was in the range from 3-9; TOC of the

301

permeates were still below 1.5 mg/L, when pH of the feed was raised to 11. To further investigate

302

the membrane stability, both MB and ML were filtrated again by the above working solutions but

303

with the multilayer side facing down (permeate). Most of the TOC values measured in the tested pH

304

range were lower than 1 mg/L, as shown in Figure S14b. In general, GO nanosheets without

305

cross-linking were extremely unstable at high pH because of the ionization of their carboxylic

306

groups.41 Fortunately, both the MB and ML membranes indicated highly stable structures in the

307

working pH range from 3 to 11.

308

Stability of GPM is an important concern of this study, which was determined in two aspects:

309

first, the interaction between the supporting membrane and the LbL assembly; second, the bonding

310

among the layers of the LbL assembly. Besides the negative charges provided by both PVDF and

311

GO might attract PEI, GO contains abundant carboxylic groups which may easily react with amine

312

groups of PEI to form stable amide bond.28, 31 Both the chemical composition and the permeate TOC

313

data proved that, in the bottom-up GPM system, the GO sheets blended in the supporting membrane

314

were functioning as the anchoring points to firmly fix the multilayer GPM on top of the support;

315

furthermore, the GO nanosheets were the bricks for the GPM construction, while the PEI molecules

316

acted as the glue and spacer in the laminated GPM structure.

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Figure 3. (a) FTIR and (b-d) XPS spectra of MB and ML.

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3.2 Characterization of GPMs in the varied pH conditions

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The pH-responsive performance of GPM during filtration is the main concern of this study. The

321

permeate fluxes which may be affected by the internal structure altering of GPMs were evaluated in

322

the working pH range from 3 to 11. As illustrated in Figure 4a, the permeate fluxes of MB and ML

323

were both linearly decreasing with the increasing pH of the feed solution. The flux of MB was 30.9 ±

324

2.1 L m-2 h-1 bar-1 at pH: 3, which dropped to 4.3 ± 0.6 L m-2 h-1 bar-1 at pH: 11. In addition, the flux

325

of ML was decreasing from 47.5 ± 2.8 L m-2 h-1 bar-1 to 5.8 ± 0.5 L m-2 h-1 bar-1, when pH of the feed

326

was adjusted from 3 to 11. Obviously, the permeate flux of GPM was responding to the pH changing

327

of the feed. It seems the water channels of GPM were linearly enlarged according to the pH decrease

328

of the feed during filtration. Moreover, the flux of ML was higher than that of MB at pH: 3, but

329

similar to MB at pH: 11. To investigate reversibility of the pH-responsive flux shifting of GPM, the

330

permeate flux of GPM was measured, when pH of the feed was adjusted from 11 to 3 and back to 11

331

for four cycles. As shown in Figure 4b, in the four filtration cycles, the flux variation of MB at pH: 3 14


Page 15 of 24


332

and 11 were 7% and 7%, respectively; the flux variation of ML at pH: 3 and 11 were 2% and 4%,

333

respectively. Obviously, in the four pH adjusting cycles, the permeate fluxes of GPMs either at pH: 3

334

or at pH: 11 were relatively consistent. In short, GPM indicated stable structure and reversible

335

pH-responsive permeate flux shifting performance, which ensured reliability of the membrane in the

336

subsequently molecular separation applications.

337

To reveal mechanism of the GPM's pH-responsibility, AFM collaborated with XRD was used to

338

statically and dynamically explore the internal structure of GPM in dry and wet states. The

339

thicknesses of MB and ML in dry form, directly measured through AFM (Figure S15), were 16.1 ±

340

0.6 nm and 20.4 ± 0.4 nm (Figure 4d), respectively. Since the GPM system contained seven bilayers

341

of PEI and GO, the average thickness of each bilayer for MB and ML was 2.3 nm or 2.9 nm,

342

respectively. In addition, the thickness of the GO nanosheets was about 1 nm (Figure S16), which

343

was also determined through AFM. After deduction, the average distance among the assembled GO

344

sheets of MB was 1.3 nm which is shorter than that of ML at 1.9 nm. Interlayer distances of the

345

laminated GPMs were also determined via XRD. As shown in Figure 4c and S17, the intense 2θ

346

peaks of GO were at 9.92o for MB and 9.49o for ML, indicating d-values of 0.89 nm and 0.94 nm,

347

respectively. Even the interlayer distances estimated from the results of XRD were lower than the

348

average values measured through AFM, these data ensured that the interlayer space created by

349

inserting the branched PEI molecules among the GO sheets was smaller than that created by

350

inserting the linear PEI molecules. Moreover, thickness shifting of GPM in liquid was monitored by

351

scanning the same scratched step on the sample using AFM while changing pH of the immersion

352

solution. As shown in Figure 4d, when the pH was varied from 3 to 11, the thickness of MB or ML

353

was changing from 25.6 ± 1.4 nm to 20.3 ± 1.1 nm or from 32.2 ± 2.1 nm to 22.7 ± 2.0 nm,

354

respectively; in addition, when the pH was decreased from 11 to 3 again, the thickness of GPM

355

increased to the initial value as well. Figure S18 and S19 indicated the representative thicknesses of

356

ML and MB at pH 3 and 11, respectively.

357

Interestingly, besides the thickness of GPM, the protonation extent of the amine groups from

358

PEI were also increasing with the decreasing pH. Obviously, the conformation change of PEI was

359

linked to the pH variation through the amine group protonation extent.30 It seems that the shape of

360

the PEI chain determined by the surrounding pH manipulated the internal structure of GPM

361

reversibly. The GPM system was constructed from GO and PEI, while the PEI molecules were not 15



Page 16 of 24

362

only the cross-linkers to ensure the structure stability but also the spacers to control the movement of

363

the GO sheets vertically and horizontally in the laminated structure. The PEI molecule would extend

364

at low pH because of the electrostatic repulsion contributed by its protonated amine groups, which

365

might result the enlarged distance among the adjacent GO sheets. On the contrary, the PEI chain

366

would compress at high pH, which might reduce the distance among the adjacent GO sheets,

367

showing in Figure 4e. It is highly possible that the reversible thickness altering as well as water flux

368

changing of GPM triggered by surrounding pH could be attributed to the reversible transformation of

369

PEI determined by ionization/deionization of the amine groups. Even the carboxylic groups of GO

370

may be responsive to pH as well, they were not taking the main roles in the GPM system. This might

371

be attributed to the low quantity of carboxylic group in GPM and the long distance among the GO

372

sheets, which may restrict the electrostatic effects.42

373

The conformation change of PEI might make the GPM internal structure altering and water flux

374

changing possible; furthermore, the formed stable amide bonds between PEI and GO ensured the

375

vertical and horizontal movement of the GO sheets in GPM was limited in a certain range. The

376

thickness data indicated that MB was thinner than ML both in dry and wet conditions; moreover, the

377

thickness of MB could only change in a narrower range than that of ML. These phenomena should

378

be attributed to the different structures (branched and linear) of the PEI molecules. The XPS spectra

379

of MB and ML indicated that the branched PEI formed more amide bonds with GO nanosheets than

380

the linear PEI. It is possible that more anchoring points were formed in the MB membrane than ML.

381

Consequently, MB not only indicated a more compacted structure but also can swell to a smaller

382

extent than ML because the existence of more anchoring points in the MB system. These results

383

proved the flux data showing wider water flux range of ML than MB.

16


Page 17 of 24


384 385

Figure 4. pH-responsive performances of GPMs, (a) permeate flux of MB and ML in the pH range

386

from 3 to 11; (b) permeate flux of MB and ML when pH was shifting from 11 to 3 and back to 11 for

387

4 cycles; (c) XRD spectra of MB and ML; (d) film thickness of MB and ML at different pH values;

388

(e) schematic of potential conformation change of GPM at pH: 3 and 11.

389 390

Surface hydrophilicity of MB and ML at different pH values were evaluated through the captive

391

bubble method for the equilibrium membrane samples in the working solutions. As shown in Figure

392

5, the water contact angles of MB at pH: 3, 6 and 11 were 48 ± 2o, 56 ± 3o and 50 ± 2o, respectively;

393

40 ± 5o, 45 ± 3o and 41 ±5o were the water contact angles of ML at pH: 3, 6 and 11, respectively. 17



Page 18 of 24

394

Even the contact angle variation of ML at pH: 3, 6 and 11 was not very significant, MB indicated the

395

highest water contact angle (statistically significant) or the lowest hydrophilicity at pH: 6, but more

396

hydrophilic surface of MB was observed in both acidic and basic conditions. This could be attributed

397

to the ionization of the amine groups at low pH or the carboxylic groups at high pH on the GPM

398

surface. It has been reported that the protonation of amine groups in acidic condition might promote

399

the hydrophilicity of PEI.43 On the other hand, the deprotonation of GO's carboxylic groups in basic

400

condition would enhance GO's hydrophilicity as well.44 In addition, the ML membrane showed more

401

hydrophilic surface than MB at the three tested pH values, which should be contributed by the lower

402

cross-linking extent of ML (indicated by XPS data) leaving more available hydrophilic functional

403

groups. It is well known that the hydrophilicity enhancement may promote the membrane's permeate

404

flux,45 which however was not the main contributor as compared with the membrane physical

405

structure changing to the permeate flux shifting in the GPM system.

406

In short, owing to the reversible internal structure altering at varied pH values, the gates of

407

GPM could be regulated by pH accordingly for permeate flux control and precise molecular

408

separation.

409

410 411

Figure 5. Contact angle of MB and ML at different pH values.

412

3.3 pH-responsive sieving of GPMs

413

Besides the permeate flux, the sieving effect of GPM was also evaluated in the working pH

414

range from 3 to 11. Three macromolecules with different molecular weights including PVP (58 kDa),

415

PEO1 (300 kDa) and PEO2 (600 kDa) as well as one silver nanoparticle (Ag NP, 50 nm), which are 18


Page 19 of 24


416

stable in the working pH range from 3 to 11, were adopted as the indicators in different sizes to

417

evaluate the retention and separation performances of GPMs. The highly hydrophilic substances

418

were purposely chosen to prevent the potential adsorption of the indicators to GPM. As shown in

419

Figure 6, the retention rates for the different indicators were all increasing with pH of the feed during

420

filtrations using GPM. When pH of the feed solution was changed from 3 to 11, the retention rates

421

for PVP using MB and ML were increasing from < 1% to 45.5% and from 1.5% to 26.4%,

422

respectively; the retention of PEO1 was changing from 29.6% to 88.7% and from 32.6% to 81.7%

423

for MB and ML, respectively; the retained PEO2 was shifting from 56.7% to 96.6% and from 53.9%

424

to 97.4% for MB and ML, respectively. For the largest Ag NP, both MB and ML could reach

425

retention rates which were higher than 90% in the whole working pH range from 3 to 11. The high

426

retention and recovery rate of Ag NP implied the gate size of GPM was smaller than 50 nm in the pH

427

range. The retention results further indicated that the sieving effects of ML and MB were similar and

428

both responding to pH of the feed that the gaps of the barriers to retain the indicators were narrowing

429

when the feed pH was increasing. This trend is consistent with the permeate flux changing of GPM

430

in the working pH range. Since some of the penetrated components were larger than the average

431

interlayer distance among the assembled GO sheets in GPM, the 3D channels formed by stacking the

432

2D edge-to-edge gaps beside the GO sheets in each layer, as shown in Figure 4e, were the barriers

433

for sieving, which was also reported in the literature.46 Nonetheless, both the interlayer spaces and

434

the edge-to-edge gap channels were the water channels taking response for the permeate flux of

435

GPM, as shown in Figure 4e. It seems the gates of GPM including the water channels and the

436

barriers were following the same changing trend with the surrounding pH. However, the shape

437

difference of PEI (branched and linear) resulted varied permeate fluxes but similar retention

438

performance of GPMs. Moreover, the results indicated that MB could only retain PEO2 at pH: 5,

439

while PEO1 could be well retained by MB at pH: 11. In other words, GPM may retain molecules

440

with a lower molecular weight at a higher pH. It is possible that size of the gates for sieving was

441

linearly decreasing with the increasing pH of the feed. Based on the retention rates of PEO1 and

442

PEO2, the estimated molecular weight cut-off (MWCO) range of GPM was from 750 kDa to 150

443

kDa, when pH of the feed was changing from 3 to 11. Therefore, theoretically, MWCO of GPM

444

could be continuously adjusted in the above certain range by pH of the feed. In other words, one

445

piece of GPM might separate/release substances with different sizes in the certain range respectively 19



Page 20 of 24

446

by simply adjust pH of the mixture, which is desirable advanced molecular separation that cannot be

447

achieved by conventional membranes with fixed pore sizes.

448 449

Figure 6. Retention rate of single components (Ag NP, PEO2, PEO1 and PVP) by (a) MB and (b)

450

ML.

451

To evaluate the performance of GPM in molecular separation, a mixture containing PVP and

452

PEO2 was filtrated through ML at pH: 11 and subsequently at pH: 3. As shown in Figure 7, only the

453

PVP molecules could penetrate GPM, when pH of the mixture was at 11; after nine cycles of half

454

volume (5 mL) filtration and refill at pH: 11, the dosed PVP was fully recovered (recovery rate:

455

107%) (Table S2). Subsequently, the PEO2 molecules begun to permeate, when pH of the feed was

456

adjusted to 3; similarly, PEO2 from the mixture was fully recovered (recovery rate: 111%) after ten

457

cycles of half volume (5 mL) filtration and refill. These results confirmed the sieving effect of GPM

458

was responding to pH that the gaps of the barrier were widening with the decreasing pH. The

459

components (PVP and PEO2) with different molecular weights in the feed would penetrate GPM in

460

sequence, when pH of the feed was adjusted from 3 to 11, as shown in Figure 7. The molecule

461

release controlled by pH led to a complete separation of the PVP and PEO2 mixture. In addition, the

462

complete recovery of PVP and PEO2 proved the retention of PVP and PEO was contributed by

463

sieving rather than adsorption of GPM. Theoretically, the penetration of any molecules with sizes in

464

the tunable MWCO range of GPM (150-750 kDa) could be wisely controlled by the feed pH. In

465

other words, separation of multiple components with different sizes from a complicated mixture,

466

which was only possible by filtrating the mixture through numbers of membranes with fixed pore

467

sizes in series, could be accomplished by using one single piece of GPM.

468 20


Page 21 of 24


469 470

Figure 7. Concentration of PVP and PEO2 in permeate at pH: 11 and pH: 3 during filtration of their

471

mixture through ML (the above image is a schematic diagram of the molecular separation).

472 473

In this study, the novel pH-responsive membranes (GPMs) were constructed by alternatively

474

assembling the GO nanosheets with the PEI molecules on a supporting PVDF membrane imbedded

475

with GO. The PEI molecules not only cross-linked the GO sheets through amide bonds to ensure the

476

membrane stability but also determined the pH-responsive effects of GPM. The reversible

477

conformation change of PEI contributed by the protonation of amine groups at different pH values

478

made the GPM internal structure altering possible; furthermore, the formed stable amide bonds

479

between PEI and GO ensured the vertical and horizontal movement of the GO sheets in GPM was

480

limited in a certain range. It seems the gates of GPM including the water channels and the barriers

481

were narrowing with the increasing pH of the feed and vice-versa. As a result, the permeate flux

482

governed by the water channels was decreasing but the retention rate determined by the barriers was

483

increasing, when the feed pH was increasing. Nevertheless, the branched PEI formed more amide

484

bonds with GO resulting a more compacted membrane (MB) which could swell to a smaller extent

485

than ML prepared from the linear PEI and GO in the pH range from 3 to 11. Thus, the permeate flux

486

of MB was lower than ML, when the PEI molecules were in the extended state at pH: 3. Moreover,

487

the MWCO of one GPM may be continuously regulated by the feed pH in a certain range, which was

488

adopted to allow the penetration of PVP (58 kDa) at pH: 11 and then PEO2 (600 kDa) at pH: 3 from

489

their mixture resulting a completely separation and recovery. Theoretically, the penetration of any 21



Page 22 of 24

490

molecules with sizes in the tunable MWCO range of GPM could be wisely controlled by the feed

491

pH, which indicated a sustainable and efficient way of advanced molecular separation in

492

environmental applications such as water purification and resource recovery.

493 494

Supporting Information

495

Chemical bond content of MB and ML based on the XPS spectra, concentration of PVP and

496

PEO2 in permeate at pH:11 and pH:3 after different filtration cycles using ML, synthesis of the

497

linear PEI, ζ potential of the substrates and the membranes, chemical structure of PVP and PEO, ζ

498

potential of PEI and GO, NMR and FTIR spectra of linear PEI and PEOX, SEM and AFM images of

499

the membranes, XPS spectra of GO, MB and ML, TOC of the permeate after filtration of the

500

working solutions at different pHs, thickness image determined of GO, MB and ML by AFM, XRD

501

spectra of MB and ML.

502 503

Acknowledgments

504

This work was supported by the National Natural Science Foundation of China (21425730,

505

21621005 and 21607124), the National Key Research and Development Program of China

506

(2017YFA0207000) and the Qianjiang River Talent Plan.

507 508

Reference

509 510 511 512 513 514 515 516 517 518 519 520 521 522 523

1.

Nunes, S. P.; Behzad, A. R.; Bobby Hooghan; Sougrat, R.; Karunakaran, M.; Pradeep, N.; Vainio, U.; Peinemann,

K.-V., Switchable pH-responsive polymeric membranes prepared via block copolymer micelle assembly. ACS Nano 2011, 5, 3516-3522. 2.

Dutta, K.; De, S., Smart responsive materials for water purification: an overview. J. Mater. Chem. A 2017, 5, (42),

22095-22112. 3.

Wandera, D.; Wickramasinghe, S. R.; Husson, S. M., Stimuli-responsive membranes. J. Membr. Sci. 2010, 357, (1-2),

6-35. 4.

Zhao, C.; Nie, S.; Tang, M.; Sun, S., Polymeric pH-sensitive membranes—A review. Prog. Polym. Sci. 2011, 36, (11),

1499-1520. 5.

Liu, J.; Wang, N.; Yu, L. J.; Karton, A.; Li, W.; Zhang, W.; Guo, F.; Hou, L.; Cheng, Q.; Jiang, L.; Weitz, D. A.; Zhao, Y.,

Bioinspired graphene membrane with temperature tunable channels for water gating and molecular separation. Nat Commun 2017, 8, (1), 2011. 6.

Lee, D.; Nolte, A. J.; Kunz, A. L.; Rubner, M. F.; Cohen, R. E., pH-Induced hysteretic gating of track-etched

polycarbonate membranes: swelling/deswelling behavior of polyelectrolyte multilayers in confined geometry. J. Am. Chem. Soc. 2006, 128, 8521-8529. 22


Page 23 of 24

524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567

7.


Sun, P.; Zhu, M.; Wang, K.; Zhong, M.; Wei, J.; Wu, D.; Xu, Z.; Zhu, H., Selective ion penetration of graphene oxide

membranes. ACS Nano 2013, 7, 428-437. 8.

Perreault, F.; Fonseca de Faria, A.; Elimelech, M., Environmental applications of graphene-based nanomaterials.

Chem. Soc. Rev. 2015, 44, 5861-5896. 9.

Joshi, R. K.; Carbone, P.; Wang, F. C.; Kravets, V. G.; Su, Y.; Grigorieva, I. V.; H. A. Wu; Geim, A. K.; Nair, R. R.,

Precise and ultrafast molecular sieving through graphene oxide membranes. Science 2014, 343, 752-754. 10. Ji Won Suk, R. D. P., Jinho An, and Rodney S. Ruoff, Mechanical Properties of Monolayer Graphene Oxide. ACS Nano 2010, 4, 6557-6564. 11. Sun, P.; Zheng, S.; Zhu, M.; Song, Z.; Wang, K.; Zhong, M.; Wu, D.; Little, R. B.; Xu, Z.; Zhu, H., Selective trans-membrane transport of alkali and alkaline earth cations through graphene oxide membranes based on cation-π interactions. ACS Nano 2014, 8, 850-859. 12. Akbari, A.; Sheath, P.; Martin, S. T.; Shinde, D. B.; Shaibani, M.; Banerjee, P. C.; Tkacz, R.; Bhattacharyya, D.; Majumder, M., Large-area graphene-based nanofiltration membranes by shear alignment of discotic nematic liquid crystals of graphene oxide. Nat Commun 2016, 7, 10891. 13. Nair, R. R.; Wu, H. A.; Jayaram, P. N.; Grigorieva, I. V.; Geim, A. K., Unimpeded permeation of water through helium-leak-tight graphene-based membranes. Science 2012, 335, (6067), 442-4. 14. Hu, W.; Peng, C.; Luo, W.; Lv, M.; Li, X.; Li, D.; Huang, Q.; Fan, C., Graphene-based antibacterial paper. ACS Nano 2010, 4, 4317-4323. 15. Xu, W. L.; Fang, C.; Zhou, F.; Song, Z.; Liu, Q.; Qiao, R.; Yu, M., Self-Assembly: A facile way of forming ultrathin, high-performance graphene oxide membranes for water purification. Nano Lett 2017, 17, 2928-2933. 16. Han, Y.; Xu, Z.; Gao, C., Ultrathin graphene nanofiltration membrane for water purification. Adv. Funct. Mater. 2013, 23, (29), 3693-3700. 17. Oh, Y.; Armstrong, D. L.; Finnerty, C.; Zheng, S.; Hu, M.; Torrents, A.; Mi, B., Understanding the pH-responsive behavior of graphene oxide membrane in removing ions and organic micropollulants. J. Mem. Sci. 2017, 541, 235-243. 18. Song, X.; Zambare, R. S.; Qi, S.; Sowrirajalu, B. N.; James Selvaraj, A. P.; Tang, C. Y.; Gao, C., Charge-gated ion transport through polyelectrolyte intercalated amine reduced graphene oxide membranes. ACS Appl. Mater. Interfaces 2017, 9, (47), 41482-41495. 19. Xiao, F. X.; Pagliaro, M.; Xu, Y. J.; Liu, B., Layer-by-layer assembly of versatile nanoarchitectures with diverse dimensionality: a new perspective for rational construction of multilayer assemblies. Chem. Soc. Rev. 2016, 45, (11), 3088-121. 20. Richardson, J. J.; Bjornmalm, M.; Caruso, F., Multilayer assembly. Technology-driven layer-by-layer assembly of nanofilms. Science 2015, 348, (6233), aaa2491. 21. Mi, B., Graphene oxide memranes for ionic and molecular sieving. Science 2014, 343, 740-742. 22. Sun, P.; Wang, K.; Zhu, H., Recent developments in graphene-based membranes: structure, mass-transport mechanism and potential applications. Adv. Mater. 2016, 28, 2287-2310. 23. Lee, C. S.; Choi, M. K.; Hwang, Y. Y.; Kim, H.; Kim, M. K.; Lee, Y. J., Facilitated water transport through graphene oxide membranes functionalized with aquaporin-mimicking peptides. Adv Mater 2018, 30, (14), e1705944. 24. Huang, L.; Chen, J.; Gao, T.; Zhang, M.; Li, Y.; Dai, L.; Qu, L.; Shi, G., Reduced graphene oxide membranes for ultrafast organic solvent nanofiltration. Adv Mater 2016, 28, (39), 8669-8674. 25. Huang, H.; Mao, Y.; Ying, Y.; Liu, Y.; Sun, L.; Peng, X., Salt concentration, pH and pressure controlled separation of small molecules through lamellar graphene oxide membranes. Chem. Commun. 2013, 49, (53), 5963-5965. 26. Zhu, X.; Yang, K.; Chen, B., Membranes prepared from graphene-based nanomaterials for sustainable applications: a review. Environ. Sci.: Nano 2017, 4, (2267), 2267-2285. 27. Zheng, S.; Tu, Q.; Urban, J. J.; Li, S.; Mi, B., Swelling of graphene oxide membranes in aqueous solution: 23



568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606

Page 24 of 24

characterization of interlayer spacing and insight into water transport mechanisms. ACS Nano 2017, 11, (6), 6440-6450. 28. Zhang, L.; Chen, B.; Ghaffar, A.; Zhu, X., Nanocomposite membrane with polyethylenimine-grafted graphene oxide as a novel additive to enhance pollutant filtration performance. Environ. Sci. Technol. 2018, 52, (10), 5920-5930. 29. Sharma, K. P.; Choudhury, C. K.; Srivastava, S.; Davis, H.; Rajamohanan, P. R.; Roy, S.; Kumaraswamy, G., Assembly of polyethyleneimine in the hexagonal mesophase of nonionic surfactant: effect of pH and temperature. J. Phys. Chem. B 2011, 115, (29), 9059-9069. 30. Choudhury, C. K.; Roy, S., Structural and dynamical properties of polyethylenimine in explicit water at different protonation states: a molecular dynamics study. Soft Matter 2013, 9, (7), 2269. 31. Nam, Y. T.; Choi, J.; Kang, K. M.; Kim, D. W.; Jung, H. T., Enhanced stability of laminated graphene oxide membranes for nanofiltration via interstitial amide bonding. ACS Appl. Mater. Interfaces 2016, 8, 27376-27382. 32. Nan, Q.; Li, P.; Cao, B., Fabrication of positively charged nanofiltration membrane via the layer-by-layer assembly of graphene oxide and polyethylenimine for desalination. Appl. Surf. Sci. 2016, 387, 521-528. 33. Wang, J.; Chen, Z.; Chen, B., Adsorption of polycyclic aromatic hydrocarbons by graphene and graphene oxide nanosheets. Environ. Sci. Technol. 2014, 48, 4817-4825. 34. Chen, X.; Chen, B., Direct observation, molecular structure, and location of oxidation debris on graphene oxide nanosheets. Environ. Sci. Technol. 2016, 50, 8568-8577. 35. Zhu, X.; Janczewski, D.; Guo, S.; Lee, S. S. C.; Velandia, F. J. P.; Teo, S. L.-M.; He, T.; Puniredd, S. R.; Vancso, G. J., Polyion Multi layers with Precise Surface Charge Control for Antifouling. ACS Appl. Mater. Interfaces 2015, 7, (1), 852-861. 36. Zhu, X.; Guo, S.; He, T.; Jiang, S.; Janczewski, D.; Vancso, G. J., Engineered, robust polyelectrolyte multilayers by precise control of surface potential for designer protein, cell, and bacteria adsorption. Langmuir 2016, 32, 1338-1346. 37. Qi, B.; He, X.; Zeng, G.; Pan, Y.; Li, G.; Liu, G.; Zhang, Y.; Chen, W.; Sun, Y., Strict molecular sieving over electrodeposited 2D-interspacing-narrowed graphene oxide membranes. Nat. Commun. 2017, 8, (1), 825. 38. Erno, P.; Philippe, B.; Martin, B., Structure determination of organic compounds. 2009. 39. Liu, H.; Kuila, T.; Kim, N.; Ku, B.; Lee, J., In situ synthesis of the reduced graphene oxide–polyethyleneimine composite and its gas barrier properties. J. Mater. Chem. A 2013, 1, 3739-3746. 40. Crist, B. V., Handbook of The Elements and Native Oxides. 1999. 41. Yeh, C. N.; Raidongia, K.; Shao, J.; Yang, Q. H.; Huang, J., On the origin of the stability of graphene oxide membranes in water. Nat. Chem. 2014, 7, (2), 166-170. 42. Amadei, C. A.; Montessori, A.; Kadow, J. P.; Succi, S.; Vecitis, C. D., Role of oxygen functionalities in graphene oxide architectural laminate subnanometer spacing and water transport. Environ. Sci. Technol. 2017, 51, 4280-4288. 43. Xia, F.; Feng, L.; Wang, S.; Sun, T.; Song, W.; Jiang, W.; Jiang, L., Dual-responsive surfaces that switch between superhydrophilicity and superhydrophobicity. Adv. Mater. 2006, 18, (4), 432-436. 44. Wan, S.; Pu, J.; Zhang, X.; Wang, L.; Xue, Q., The tunable wettability in multistimuli-responsive smart graphene surfaces. Appl. Phys. Lett. 2013, 102, (1), 011603. 45. Zhu, X.; Loo, H.; Bai, R., A novel membrane showing both hydrophilic and oleophobic surface properties and its non-fouling performances for potential water treatment applications. J. Membr. Sci. 2013, 436, 47-56. 46. Wei, N.; Peng, X.; Xu, Z., Understanding water permeation in graphene oxide membranes. ACS Appl. Mater. Interfaces 2014, 6, (8), 5877-5883.

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Stable graphene based membrane with pH-responsive gates for

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