Facile Construction of Long-Lasting Antibacterial Membrane by Using

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Facile construction of long-lasting antibacterial membrane by using an orientated halloysite nanotubes interlayer Xu Liang, Lijuan Qin, Jing Wang, Junyong Zhu, Yatao Zhang, and Jindun Liu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04725 • Publication Date (Web): 20 Feb 2018 Downloaded from http://pubs.acs.org on February 22, 2018

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Industrial & Engineering Chemistry Research

Facile construction of long-lasting antibacterial membrane by using an orientated halloysite nanotubes interlayer Xu Liang a, b, Lijuan Qin a, Jing Wang a, c, Junyong Zhu c, Yatao Zhang a, d*, Jindun Liu a, d ** a

School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou 450001, China.

b

Research Institute of Henan Energy and Chemical Industry Group, Zhengzhou 450046, China

c

Department of Chemical Engineering, KU Leuven, Celestijnenlaan 200F, B-3001 Heverlee,

Belgium. d

Zhengzhou Key Laboratory of Advanced Separation Technology, Zhengzhou University,

Zhengzhou 450001, China

* Corresponding authors: E-mail: [email protected]; Email: [email protected]

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Abstract: Membrane separation technologies have shown a brilliant potential in the field of water treatment, biotechnology and pharmaceutical industry. Surface biofouling featuring inherent complexity and hard treatability severely impedes the development of polymeric membranes, posing a significant decline in their performance and lifespan. Silver nanoparticles (Ag NPs) offer the best solutions to inhibit bacteria growth and proliferation, whereas it remains challenging to confer a long-term bactericidal ability to membranes. In this study, we developed a novel approach to in-situ anchor Ag NPs on membrane surface by implementing natural clay (halloysite nanotubes, HNTs) as an interlayer. The combination of well-aligned HNTs and nanosilver endows the membranes with high dye retention, salt permeation, and water permeability. Most importantly, this novel membrane exhibited a strong, long-lasting antibacterial behavior toward Escherichia coli. This strategy furnishes a new pathway in the rational assembly of Ag/HNTs antibacterial layer for potent dye/salts fractionation.

Keywords: orientated halloysite nanotubes; antibacterial membrane; silver nanoparticles

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1. Introduction As a new separation technology in chemical engineering, membrane separation technology has been extensively applied in many fields except water treatment, such as milk industry, medicinal extraction and pharmaceutical industry.1-4 However, membrane biofouling is still one of the most common and serious problems.5-7 In order to satisfy escalating public and industrial demands on high-quality water sources, membranes with antibacterial activity have emerged and rapidly gained significant attention. Such antibacterial membranes are capable of deactivating pathogens and inhibiting bacterial colonization in untreated water, thus producing pathogen-free water with few by-products.8-9 Besides, bacterial growth and proliferation on membrane surfaces lead to the formation of compact biofilm, which renders bacteria immune to the specific biocides. Furthermore, the existence of biofilm will result in an increased hydraulic resistance to permeation flux and notably shorten membrane lifespan.10-11 Therefore, the most effective route toward long-term antibacterial activities is to hinder the initial cell adhesion and growth onto membrane surfaces. Intensive efforts have been devoted to the design of outstanding anti-biofouling polymeric membranes. Metal based materials, such as metal oxide (ZnO, CuO, TiO2), metal hydroxide (Mg(OH)2, Cu(OH)2) and metal nanoparticles (Cu NPs, Ag NPs), have been verified to possess microbial toxicity.12-18 Among these antibacterial reagents, Ag NPs is one of the most researched bactericides due to its high thermal stability and broad antibacterial spectrum. It can be concluded that both Ag NPs and silver ions contribute to a strong antibacterial activity.19 Ag NPs can interact with the lipid bilayer of bacteria and disrupt the integrity,

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and Ag+ trend to inhibit bacterial growth by binding to essential biological molecules like sulfur, oxygen and nitrogen.20 In the past decades, Ag NPs has been widely incorporated into membrane materials and endows filtration membranes with the antibacterial property. Ag NPs-based composite membranes are typically prepared by either blending ex-situ Ag NPs or in-situ reduction from silver ions, followed by a phase inversion method. For composite membranes, existing limitations are to attain a homogeneous distribution with vast exposure in view of a high silver nanoparticle loading. It is reported that the membranes modified with in-situ formation outperformed those enabled by ex-situ method in terms of antibacterial activity. Despite the feasibility and simplicity of phase inversion, the ability of nanosilver to resist biofouling is greatly restricted owing to being mostly embedded into polymer matrix during this process. Besides, the leaching of silver ions released from Ag NPs not only weakens bactericidal properties, but also induces an adverse damage to surroundings.21 This can also be explained why composite membranes hardly maintain a long-term antibacterial ability. Applicable strategies to sharpen their biocide functionality underscore the paramount importance of surface loading of Ag NPs. Currently, a series of surface functionalization with nanosilver has successfully been conquered via various methods, including in-situ reduction,

dip-coating,

layer-by-layer

assembly,

covalently

binding,

interfacial

polymerization, etc.22-24 The advantages of in-situ formation over other routes lie in an even distribution with few aggregates, a well-controlled Ag NPs size, a low released rate of Ag ions, and strong interfacial force enabled by covalently binding or electrostatic attraction. Ben-Sasson et al. reported a facile procedure for in-situ loading Ag NPs on

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thin-film composite membrane, by using sodium borohydride (NaBH4) as a reducing agent.25 Such technology enables a uniform coverage of Ag NPs, which induces a strong antibacterial activity of this membrane without sacrificing their performance. Liu et al anchored in-situ formed Ag NPs onto hydrolyzed PAN utilizing carbon monoxide gas as a mild reductant.26 These membranes had superior antibacterial resistance for 14 days against the growth of Escherichia coli. It was convinced that Ag NPs on membrane exterior surface can promote the possibility of direct contact with bacteria, thus notably boosting their antibacterial properties. However, in-situ formation of Ag NPs is largely confined to thin-film compact membranes that were used for reverse osmosis and forward osmosis; this is mainly ascribed to the limited leaching of biocides through the dense selective layer. Therefore, despite recent progress in the development of antibacterial membranes, the quest to design and fabricate advanced membranes with extensive applications via in-situ loading remains a high priority. Arduous efforts are still needed to develop antibacterial membrane with an effective and moderate in-situ method, by choosing a porous substrate. Halloysite nanotubes (HNTs) are natural and low-cost clay minerals. Because of its hollow structure, HNTs has been used as nanocontainer for controlling drug release. The tubular structure of HNTs combined with hydroxyapatite nanoparticles could restrict the movability of the sodium alginate polymer chains, which improved drug loading and cumulative release behavior.27-29 Moreover, such one-dimensional nanomaterials have high hydrophilicity, mechanical strength, and have been also explored as the carrier of Ag NPs.30 In this study, to control and avoid Ag NPs leaching, poly (sodium-p-styrenesulfonate)

5



(PSS)-modified HNTs were orientated aligned onto hydrolyzed PAN surface. Despite high density of HNTs, PSS-modified HNTs can be well-dispersed in water for a long time. A coating suspension composed of modified HNTs and PVA solution was dispersed onto hydrolyzed PAN substrate, followed by a gradual solvent evaporation at a relatively low temperature. Afterwards, GA solution was used to crosslink –OH groups between PVA and HNTs and as the reductant for in-situ formation of Ag NPs.31 Compared with AgNO3/NaBH4 reduction method, the method of Tollens’ reagent in this work is more facile. The new method allowed the cross-linking process and the silver reduction process to proceed simultaneously. The preparation process of the Ag NPs@PSS-HNTs/PAN antibacterial membranes is presented in Figure 1.

Figure 1 The fabrication process of Ag/HNTs/PAN membranes.

2. Experimental Materials Silver nitrate (AgNO3, 99.8%, Sinopharm) and Polyacrylonitrile (PAN) membranes (molecular 6


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weight cut-off, MWCO = 50,000 Da) were obtained from Sepro membranes (Beijing, China). Halloysite nanotubes (HNTs) were kindly donated by Henan Xianghu Environmental Protection Technology

Co.,

Ltd.

Polyvinyl

alcohol

(PVA)

(MW

=

67,000

Da)

and

poly

(sodium-p-styrenesulfonate) (PSS) (MW = 70,000 Da) were purchased from Sigma-Aldrich (St. Louis, MO) and J&K Scientific Ltd. (Beijing, China), respectively. Sodium chloride (NaCl, 99.5%), magnesium chloride hexahydrate (MgCl2·6H2O, >98.0%), magnesium sulfate anhydrous (MgSO4,>99.0%), sodium sulfate anhydrous (Na2SO4, >99.0%) sodium hydroxide (NaOH, 96.0%), ammonium hydroxide (NH3·H2O, 25.0-28.0%) and glutaraldehyde (GA, 50% solution in water) were all purchased from Kermal (Tianjin, China). Reactive Red 49 (MW = 576.49 Da) and Reactive Black 5 (MW = 991.82 Da) was purchased from Sunwell Chemicals Co. Ltd. (Hangzhou, China).

Preparation of membranes HNTs modified by PSS were used to enhance the dispersion in water and prepared according to the previous report.32 Briefly, PSS (2 g) was uniformly dispersed in 200 mL water under stirring. Subsequently, HNTs (2 g) were added. The suspension was continued to stir for 48 h and left standing for 1 h to precipitate aggregates and impurities. Then, the solution was collected and centrifuged at 6000 rpm for 10 min, followed by three dispersion/centrifugation cycles (at 8000 rpm for 20min). Finally, the obtained solid, named PSS-HNTs, was dried for 24 h in a vacuum drier at 50°C and crushed into power by mortar. Tollens’ reagent was employed as a source of Ag+ and prepared as followed. AgNO3 (0.1 g) was dissolved in water (100 g) under stirring. NaOH (0.1 M) was not stopped dropping into the Ag+ aqueous under stirring until no new brown precipitate generated. Then, NH3·H2O aqueous solution 7



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(0.1 M) was dropped into the suspension and the amount of precipitate gradually declined. Tollens’ reagent (silver-ammonia complexes) could be obtained when all the precipitate has vanished. Typically, PVA solution (0.2 wt%) was employed as a reagent to fix PSS-HNTs on membrane surface and prepared by dispersing PVA powder (0.2 g) into water (99.8 g) at 95°C under stirring for 3 h. The PAN membrane was immersed into NaOH aqueous solution (1 M) at 50°C for 1 h to hydrolyze before use. The PAN substrate was first coated by the mixture of PSS-HNTs dispersion (10 mg, 2 mL) and PVA solution (0.2 wt%, 1 mL). Then solvent (water) was completely evaporated at 80°C for 1 h. 1 vol% of GA aqueous solution with certain volume (0.2 mL, 0.6 mL, 0.8 mL, 1.0 mL, and 1.4 mL) was poured on the above membrane surfaces, and continued to heat for certain time (1 h, 3 h, 5 h) to allow GA to cross-link with -OH groups of PVA and HNTs. And the membranes were donated as PSS-HNTs/PAN membranes. Finally, Tollens’ reagent (3 mL) was poured on the aforementioned membrane to form PSS-HNTs/PAN membranes decorated with Ag NPs (Ag NPs@PSS-HNTs/PAN membrane) as the Ag NPs can be formed by the GA.31 The Ag NPs@PSS-HNTs/PAN membranes prepared by different amount of GA and reaction time were denoted as Ag-X-Y (X and Y represent the amount of the GA and the cross-linking time, respectively) and the detailed compositions are shown in Table 1. Table 1 Membranes fabricated by controlling the addition of GA and the cross-linking time (1, 3, and 5 h) of GA with PVA and HNTs. The amount of GA (mL)

Sample name

0.2

Ag-0.2-1

Ag-0.2-3

-

0.6

Ag-0.6-1

Ag-0.6-3

-

0.8

Ag-0.8-1

Ag-0.8-3

Ag-0.8-5

1.0

Ag-1.0-1

Ag-1.0-3

-

1.4

Ag-1.4-1

Ag-1.4-3

-

8


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Membranes characterization Attenuated total reflection Fourier transform infrared spectroscopy. ATR-FTIR spectroscopy was performed at 2 cm-1 resolution (Thermo Nicolet IR 200, Thermo Nicolet Corporation, USA) spectra in the range of 400–4000 cm-1 to confirm the successful modification of HNTs. ATR-FTIR spectra of the antibacterial membranes were obtained by a spectrometer (Nicolet IS10, Thermo Scientific, USA) equipped with an attenuated total reflectance (ATR) unit with 10 scan time. Transmission electron microscopy. TEM images were gained by high resolution transmission electron microscopy (JEM-2100, JEOL, and Japan) to observe the morphologies of the HNTs and PSS-HNTs. X-ray diffraction. Morphological changes of the membrane surfaces were X-ray diffraction (XRD) data of the samples were measured at room temperature by using a diffractometer (D8 Advance, Bruker, Germany). The patterns were obtained with Ni-filtered copper radiation at 30 kV and 10 mA with a scanning speed of 2θ= 5 ◦/min. Field emission scanning electron microscopy. The

cross-section

and

surface

morphologies of these membranes were examined by field emission scanning electron microscope (FE-SEM, JEOL JSM-7001F, and Japan). Each sample for SEM mapping was cryo-fractured in liquid nitrogen, and then coated with a layer of sputtered Platinum for 160s to elevate electrical conductivity. The SEM images could obtain under an applied voltage of 10 kV. The cross-section of the Ag-0.8-1 membrane was detected by Energy Dispersive X-ray Spectroscopy (EDS, JEOL JSM-7500F, and Japan) to estimate the distribution of Ag NPs on the membrane.

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Atomic force microscopy. The measurements of atomic force microscopy (AFM, Bruker Dimension Fastscan, USA) were conducted in ScanAsystTM mode in air at ambient temperature using silicon-nitride ScanAsyst Air probes with scan areas of 20 × 20 µm2. The AFM images were further processing via the NanoScope Analysis V.1.5. software (Bruker). Dynamic water contact angle. Water contact angle was employed to measure the hydrophilicity of the membranes. The measurements were conducted by a contact angle goniometer (OCA25, Dataphysics instruments, Germany) with the sessile drop method at ambient temperature. 3 µL water was dropped on the membrane surface and the reported contact angle is the average of at least five measurements. Filtration performances of the membranes. A cross-flow filtration apparatus was applied to evaluate the filtration performance with an effective membrane filtration area of 12.57 cm2.33 In order to reach a stable state before testing, the membranes were held at a pressure of 0.6 MPa for about 30 min with pure water and then the operating pressure was reduced to 0.4 MPa to test. The pure water flux (Jw, L m-2 h-1) was the volume of permeate water (V) per unit time (∆t) in the fixed effective membrane area (A = 12.57 cm2) and calculated by the following equation:

J = AV∆t

(1)

The rejection (R) of salt (NaCl, MgSO4, Na2SO4 and MgCl2, 1g L-1) was evaluated by measuring the concentration of the feed (Cf) and the permeate (Cp) solution. The concentration could be determined by the conductivity meter (DDS-307A conductivity meters) due to the linear relationship between concentration and conductivity.

R = (1 − Cp ) × 100% Cf

(2)

The rejection of Reactive Red 49 (0.5 g L-1) and Reactive Black 5 (0.5 g L-1) was

10


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calculated as the same as the salt rejection. However, different with the determination of salt concentration, the dye concentration was detected by UV-Vis spectrophotometry at the optimal absorption wavelengths (523.5 nm for Reactive Red 49 and 596 nm for Reactive Black 5). Antibacterial activity of membrane and test of Ag NPs release. The antibacterial effects of these membranes in the bacteria (E. coli strain) solution were evaluated by following the previously reported literatures.34 Membrane samples (1 cm × 1 cm) were immersed in the bacterial solution (CFUs/mL ≈ 104, 5 mL) and were cultured at 37˚C for 2 h at a shaking speed of 190 rpm to allow these membranes to contact the bacteria thoroughly. Then, the solution was diluted and spread on nutrient agar plates. These plates were put at 37˚C for 12 h. The number of CFUs on each plate was counted to calculate the antibacterial rate in the solution as follows: N

Arate = (1 − N ) × 100% 0

(3)

Where N is the colony count of the bacteria cultures interacted with the antibacterial membranes and N0 is the colony count of the blank control culture (PAN substrate). The long-lasting antibacterial measurements were conducted by following the above procedure. The PAN substrate was as control and Ag-0.8-1 membrane was tested at two weeks interval. In order to illustrate the immobilization function of HNTs for Ag NPs, Ag NPs/PAN membranes were fabricated as the same as the Ag-0.8-1 membranes except for the addition of PSS-HNTs. Before testing, both Ag-0.8-1 membrane and Ag NPs/PAN membrane (2 cm × 1 cm) were immersed in 50 mL deionized water at Erlenmeyer flasks, followed by sonicating for 5 min, respectively. Then the suspension was taken for tests via the UV-Vis spectrophotometry to determine the release of Ag NPs from the membranes with or without HNTs.

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3. Results and discussion Characterization of PSS-HNTs. As previously mentioned, surface functionalization with poly (sodium-p-styrenesulfonate) (PSS) enables a good dispersion of PSS-HNTs in water. This will facilitate to form a uniform and well-arranged PSS-HNTs coverage on PAN substrate, further providing active sites for in-situ growth of Ag NPs on PSS-HNTs layer. As shown in Figure 2, compared to pristine HNTs, a new peak at 1230 cm-1 originated from the sulfonic groups demonstrated the successful surface modification with PSS.35 The XRD results (Figure S1) showed no apparent difference between HNTs and PSS-HNTs, which indicated that PSS had a little impacted on HNTs structure. Besides, as further confirmed by TEM (Figure 3), HNTs could maintain hollow structure without blocking; this implied that only a small amount of PSS adhered to the HNTs surface via the electrostatic interaction.36 Pristine HNTs PSS-HNTs

Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1230 3693 3624 1200

1033 4000

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber (cm ) Figure 2 FTIR spectra of the pristine HNTs and the PSS-HNTs.

12


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Figure 3 TEM images of the pristine HNTs (a, b) and the PSS-HNTs (c, d).

Characterization of the AgNPs/PSS-HNTs/PAN membranes. The aim of introducing PSS-HNTs interlayer is not only to decrease the average pore size of polymeric substrate, thus notably reducing silver leaching, but also expedite a high silver ion loading via electrostatic adsorption, which boosts the productivity of in-situ formed Ag NPs. ATR-FTIR, XRD and SEM measurements were conducted to confirm the in-situ formation of the Ag NPs on PSS-HNTs/PAN substrates. ATR-FTIR was employed to study surface composition of membranes (Figure 4). The nitrile groups of pristine PAN were partially hydrolyzed to -COOH which was determined by the peaks at 1736 cm-1 and 1663 cm-1. After PSS-HNTs coating, the characteristic peaks of HNTs at 3689 cm-1, 3620 cm-1 and 992 cm-1 clearly emerged, whereas the original peaks of pristine PAN membranes disappeared, which were attributed to the coverage of PSS-HNTs interlayer. PVA can be used to stabilize HNTs coating layer via hydrogen bonds. The HNTs layer was not stable without PVA and the HNTs could re-disperse in water after

13



soaking.37-38 As shown in Figure 4, after the addition of GA that contained carbonyl group, no peak of C=O could be observed and a new peak ascribed to the vibration of C-O-C appeared at 1119 cm-1 for the Ag-0.2-1 membrane, which suggested that the crosslinking reaction of GA with the hydroxyl group occurred. On the other hand, the vibration peak of Si-O exhibited a blue shift from 665 cm-1 to 684 cm-1, indicating that the cross-liking of HNTs and GA also occurred between Si-OH groups on HNTs and C=O on GA.39 Furthermore, the intensity of the peak at 1000 cm-1 (Si-O stretching vibration) enhanced gradually with the increased GA content, demonstrated the elevation of the cross-linking degree between PVA and GA.31

Figure 4. FTIR spectra of the (a) hydrolyzed PAN, (b) PSS-HNTs/PAN, (c) Ag-0.2-1 and (d) Ag-1.4-1 membrane. XRD measurement was used to confirm the formation of Ag NPs (Figure S2). The PSS-HNTs/PAN membrane displayed characteristic peaks at 12.2°, 19.9° and 24.7°, which were consistent with the typical characteristic peaks of pristine HNTs nanomaterial. After in-situ reduction, several new emerging peaks could be readily indexed to face-centered-cubic silver (JCPDS file No.04-0783). Herein, the diffraction peaks at 2θ values of 38.12°, 44.23°, 14


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64.43°,77.47° were ascribed to the reflection of (111), (200), (220), (311) planes of the face-centered-cubic silver.40 Furthermore, the Ag elements almost spread over the cross-section of the membranes according to EDS mapping of the cross-section of the Ag-0.2-1 membrane (Figure S3). These results demonstrated the successful preparation of Ag NPs on the surface of the PAN substrates via a facile in-situ reduction method. Figure 5 presents the surface and cross-section images of the Ag NPs@PSS-HNTs/PAN membranes. As seen from Figure 5, the PSS-HNTs could align on the PAN membrane. The mechanism for the orientated alignment of halloysite nanotube on the PAN membrane has been concluded in our previous study.32, 41 It was original form the structure of HNTs (high axis ratio), the hydrophilic surface of PAN and the capillary flow effect during the evaporation process.32 Besides, the effect of GA amount on surface morphologies of the as-prepared membranes were studied at length through SEM. The in-situ growth of Ag NPs could be verified by the color changes from white (PAN and PSS-HNTs/PAN) to brown (Ag NPs@PSS-HNTs/PAN membranes). In addition, sphere particles appeared on PSS-HNTs surface and continued to increase with color of membranes changing from white to dark brown (insert of Figure 5 a, c, e and g). Furthermore, the thickness of these membranes was also elevated with the increased GA amount. (Figure 5 d, f and h). The morphologies of Ag-0.8-3 and Ag-0.8-5 membranes had no significant changes while the color of the membranes became gradually fainter with increasing the cross-link time (Figure 5 i and k).

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(a)

(b)

(g)

(h)

(c)

(d)

(i)

(j)

(e)

(f)

(k)

(l)

Figure 5. The surface and cross-section SEM images of the HNTs/PAN ((a) and (b)), Ag-0.2-1 ((c) and (d)), Ag-0.8-1 ((e) and (f)), Ag-1.4-1 ((g) and (h)), Ag-0.8-3 ((i) and (j)), and Ag-0.8-5 ((k) and (l)) membranes. Insert: digital graphs of the corresponding membranes. Scale bar: 1 µm. AFM images and analyses were conducted to further study surface roughness affected by GA amount. As presented in Figure 6, it was clear that the PSS-HNTs were aligned on PAN substrate, with Ag NPs being well-distributed on their surfaces, which corroborated result in the SEM images of membrane surface. In view of three-dimensional images, the membranes exhibited a ridge-and-valley morphology, in which the color of light brown represents the ridge and dark brown indicates the valleys. Additionally, the average roughness (Ra) of the membrane slightly declined with the increase of the addition amount of GA and cross-linking time. This was mainly because of the filling effect of the generated Ag NPs. The hydrophilicity of the membrane exhibited the similar tendency with the Ra of the membranes (Figure 6). This indicated the gradually increasing hydrophilicity of the antibacterial membrane

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surface. Liu et al. synthesized novel antibacterial silver nanocomposite membranes based on layer-by-layer method.42 It was found that the incorporation of Ag NPs decreased contact angles and thus enhanced the membrane surface hydrophilicity. Herein, the surface hydrophilicity of the membrane fabricated via in-situ method also enhanced by adding more GA. This was because the increase of the amount of GA would lead to the thoroughly reduction of the Ag+, thus generated more Ag NPs. Highly hydrophilic, swollen, compliant surfaces resist bacterial attachment can effectively disrupt bacterial cell membranes, which caused cell death. Bacterial attachment to a surface occurs through several mechanisms, including hydrophobic and electrostatic interactions. In addition, a hydrophilic surface also facilitated the antibacterial property via decreasing bacterial adhesion.

(c)

(b)

(a)

Ra=51.3 nm

(e)

(f)

Ra=57.2 nm

(d)

Ra=42.8 nm

Ra=49.7 nm

Ra=34.3 nm

70

Water contact angle ( °)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60 50 40 30 20 10 0

HNTs/PAN Ag-0.2-1

Ag-0.6-1

Ag-1.0-1

Ag-1.4-1

Membranes

Figure 6. AFM images of Ag NPs functionalized membranes: Ag-0.2-1 (a), Ag-0.6-1 (b), Ag-1.0-1 (c), Ag-1.4-1 (d) and HNTs/PAN (e); Water contact angles of the Ag 17



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NPs@PSS-HNTs/PAN membranes (f).

Filtration performances of the Ag/HNTs/PAN membranes To understand the filtration performance of the antibacterial membranes, water flux, rejection of salts and dyes were tested, respectively (Figure 7). These membranes exhibited an overall low salt retention in the following order: Na2SO4 > MgSO4 > MgCl2 ≈ NaCl, indicating the negatively charged nature of these membranes.43-44 As GA content increased, the rejection of salts, reactive red 49 and reactive black 5 had no obvious differences, whereas the water flux sharply decreased. It is speculated that the pure water flux declined due to the increase of membrane thickness and the enhanced cross-linking degree of PVA and GA.10 According to the solution diffusion model that describes the osmotic pressure controlled flux, the transport equation through the membrane is as follows:45

V

OSM W

= Lp (∆P − ∆π ) =

k0

µδ

( ∆P − ∆π )

(4)

Where ∆P and ∆π is the pressure differential and the osmotic pressure differential, respectively. V OSM W

is water flux of the membrane, k0 is the traditional Darcy’s permeability, µ is the viscosity of

the permeating solution, and δ is the thickness of the membrane skin. In this work, deionized water was used as the feed solution and all the membranes were tested at 0.4 MPa. Therefore, the main parameters that affect the water flux are k0 and δ. In Figure 5, it was found that the membrane thickness increased and thus δ gradually enhanced as the GA content increased. Meanwhile, k0 decreased due to the increasing compactness of the active layer. These consequences all lead to a declined water flux, which conformed to filtration results. Nevertheless, the increase reaction time merely resulted in a small decline of water flux. Compared with 3 h of cross-linking, dye and salts retention also had no significant changes (Figure 7). In addition, the 18


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antibacterial membranes with orientated halloysite nanotubes layer on the surface of the PAN membrane had higher dye retention rate, with the retention of 90.4% for reactive red 49 and 98.7% for reactive black 5. Such high salt permeation and good dyes retention signifies a loose nanofiltration membrane which has the great potential in dye/salts separation.46-47

Figure 7. The water flux and rejection of salts, Reactive Red 49 and Reactive Black 5 of the antibacterial membranes with the cross-linking time of 1 h (a) and 3 h (b).

Antibacterial study To investigate the antibacterial properties of the composite membranes, Escherichia coli (E. coli) was used as a bacteria model because of a growing concern on the infections worldwide caused by these bacteria.5 Previous studies proved that the direct contact between bacteria and antibacterial nanosilver resulted in a high activity to kill bacteria or inhibit the growth of bacteria.20 In this work, the antibacterial efficiency of the membranes was assessed by plate counting method and the results are shown in Figure S4. It was found that the colonies formation was remarkably suppressed and only a few or no colonies were observed after being treated with antibacterial membranes, which demonstrated a strong antibacterial effect of these Ag NPs modified membranes.48 Increasing the amount of GA, the antibacterial rate gradually elevated. This was 19



mainly due to the enhanced quantity of in-situ formed Ag NPs. The antibacterial rates after being contacted with the Ag-0.2-1, Ag-0.8-1 and Ag-1.4-1 membranes are 97.8%, 99.7% and 100.0%, respectively. The bactericidal efficiency could reach as high as ca.100% when the amount of GA was 0.8 mL. A long-lasting antibacterial property of the Ag NPs@PSS-HNTs/PAN membrane is shown in Figure 8a. It was found that the antibacterial rate of Ag-0.8-1 membrane decreased from 99.7% to 98.6% for 24 weeks, which illustrated the favorable stability of the antibacterial membrane. We expected that an overall high antibacterial activity of these membranes stemmed from an effective sterilization of silver nanoparticles, a uniform distribution of Ag NPs onto PSS-HNTs, a direct contact between the cells and Ag NPs formed on the membrane surface (Figure 9).19

Figure 8 (a) Long-lasting antibacterial rate of the Ag-0.8-1 membranes against E. coli; (b) UV-vis absorption spectra of the Ag membrane and the Ag-0.8-1 membrane.

20


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Figure 9 Proposed antibacterial mechanism of Ag NPs@PSS-HNTs/PAN membrane. Table 2 The antibacterial rate of membranes in the literature and our work. Nanomaterial

Antibacterial rate for

Substrate

References

E. coli (%) ZnO

PAN

90

[12]

Cu(OH)2

RO

99.7±0.1

[15]

Ag

PDMS-b-PMMA

99.4

[16]

Ag

PSf

97.6±2.8

[48]

Ag/Graphene oxide

melamine sponge (MS)

99.8%

[51]

Ag/CoFe2O4

Graphene oxide (GO)

98.8%

[52]

Ag

lysozyme /tannic acid

98.9%

[53]

92.7±1.8

[54]

Ag@SiO2

Polyamide/Poly-sulfone (PA/PSf)

Ag

PDA/polyamide (PA)

42.4 ± 5.7

[55]

Ag

PAN

99.7

Ag-0.8-1 this work

Ag

PAN

100

Ag-1.4-1 this work

To compare the Ag NPs@PSS-HNTs/PAN membranes with other polymer/metal composites membranes, the characteristics of all of these membranes were listed in Table 2. The antibacterial rate of ZnO NPs composite film was only 90%, which was lower than other antibacterial materials. Deng et al reported that GO/AgNPs-MS composite displayed effective antibacterial and antibacterial rate for E. coli was 99.8%(107 CFU/mL) after 30 min incubation. However, GO-MS also showed a little antibacterial ability, which could be attributed to the addition of GO.51 Ag-CoFe2O4-graphene oxide nanocomposite has also

21



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possessed high bactericidal activity with inactivation rates of 98.8% toward E. coli (105 CFU/mL), even at 6.25 g/mL after 2 h incubation.52 Park et al presented a new method to immobilize AgNPs on polyamide membrane. It was believed that the improved hydrophilicity of the membrane could contribute to the antibacterial performance.54 In this study, silver nanoparticles distributed on bactericidal efficiency could reach as high as ca.100% without changing the silver content when the amount of GA was 1.4 mL. The result indicated that the silver content was not the main factor affecting the antibacterial rate, which was contrary to the existing literature.53 It showed that antibacterial performance of the nanocomposite largely depends on the AgNPs content, which increased with an increase in the AgNPs concentration. Although the antibacterial mechanism of nano silver has not been fully clarified, it is no doubt that AgNPs plays an important role in antibacterial treatment. One of the mechanism of Ag NPs toxicity against various bacteria was that Ag NPs naturally interact with the membrane of bacteria and destroy the integrity the bacterial membrane and prevent bacterial growth.49 Ag-1.4-1 with only 1% silver content has best antibacterial activity. It is clearly that more Ag NPs formed on membrane surface could substantially contribute to achieving better antibacterial capacities with the increase of the amount of GA. Silver losses from the membrane surface are inevitable and widely reported by numerous studies. Recently, it was found that nanomaterial such as multi-walled carbon nanotubes, montmorillonite, and graphene oxide could preferably restrain the leakage of Ag NPs.22,

50

In this work, the

capability of PSS-HNTs to limit Ag NPs leaching were evaluated. To compare the performance of silver loss, the Ag membrane without HNTs was prepared by the same method with the Ag-0.8-1 membrane. Figure 8b shows the UV absorption spectra of the suspension after exposing the

22


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membrane to bath sonication. For the Ag membrane, two absorption peaks can be observed, 285 nm for Ag ions and 415.5 nm for Ag NPs respectively (Figure 8b).56,57 This demonstrated that the release of silver from the Ag membrane can happened in the form of silver ions and Ag NPs simultaneous. No absorption peaks in the visible region can be found in the suspension of Ag-0.8-1 membrane, which implied membranes with PSS-HNTs layer could restraint the silver loss effectively.

4. Conclusion A novel antibacterial Ag NPs@PSS-HNTs/PAN membrane was fabricated via a simple and facile in-situ reduction method. The PAN membrane was first coated by HNTs/PVA solution to form HNTs support layer and then GA solution was poured on the coating, followed by the addition of Tollens’ reagent. The GA was not only crosslinking agent but also reducing agent of Ag+. It was found that the amount of the GA significantly affected the thickness of the coating layer and the cross-linking degree between GA and PVA. Thus, an ideal membrane can be designed and manufactured by controlling the amount of GA and crosslinking time. Furthermore, these Ag NPs@PSS-HNTs/PAN membranes exhibited an excellent antibacterial property, bacterial viability which can be zero due to the direct contact of Ag NPs and bacteria and a strong capability of HNTs to fix Ag NPs on the membranes surface. We anticipate that these membranes could separate dye/salt solution, have the capability of antibacterial to extend these membranes life and the in-situ reduction method can give some guidance fabricating composite materials decorated Ag NPs. Moreover, this in-situ reduction method on coating layer can be also applied in fabricating functional surfaces for many fields. Acknowledgements 23



This work is financially sponsored by the National Natural Science Foundation of China (No. U1704139 and 21476251) and Training Plan for Young Backbone Teachers in Universities of Henan Province (2017GGJS002). Supporting Information The supporting information is available: Figure S1 shows the XRD pattern of the pristine HNTs and the PSS-HNTs. Figure S2 presents the XRD patterns of the PSS-HNTs/PAN membrane and the Ag-0.2-1 membrane. Figure S3 reveals the EDS mapping of the cross-section of the Ag-0.2-1 membrane. Figure S4 illustrates the digital graphs of the antibacterial effect of the PAN (a), Ag-0.2-1 (b), Ag-0.8-1 (c), Ag-1.4-1 (d), Ag-0.8-3 (e) and Ag-0.8-5 (f) membranes.

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Facile Construction of Long-Lasting Antibacterial Membrane by Using

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