Surface Engineering of Thin Film Composite Polyamide Membranes

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Surface Engineering of Thin Film Composite Polyamide Membranes with Silver Nanoparticles through Layer-byLayer Interfacial Polymerization for Antibacterial Properties Zhongyun Liu, Longbin Qi, Xiaochan An, Caifeng Liu, and Yunxia Hu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12314 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 9, 2017

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ACS Applied Materials & Interfaces

Surface Engineering of Thin Film Composite Polyamide Membranes with Silver Nanoparticles through Layer-by-Layer Interfacial Polymerization for Antibacterial Properties

Zhongyun Liu1,2#, Longbin Qi2#, Xiaochan An2, Caifeng Liu3, Yunxia Hu1,2* 1. State Key Laboratory of Separation Membranes and Membrane Processes, Tianjin Polytechnic University, Tianjin 300387, China 2. CAS Key Laboratory of Coastal Environmental Processes and Ecological Remediation， Yantai Institute of Coastal Zone Research, Chinese Academy of Sciences, Yantai 264003, Shandong Province, China. 3. College of Chemistry and Chemical Engineering, Yantai University, Yantai, 264000, Shandong Province, China.

*Corresponding author, Tel: +86-22-83955129; E-mail: [email protected]

Keywords:

TFC polyamide membrane, Silver nanoparticles, Layer-by-layer, Interfacial

polymerization, Fouling, Antibacterial property

ACS Paragon Plus Environment


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Abstract In this study, we developed a simple and facile approach to covalently immobilize Ag nanoparticles (NPs) onto polyamide surfaces of thin film composite membranes through layer-by-layer interfacial polymerization (LBL-IP) for biofouling mitigation. Stable and uniform bovine serum albumin (BSA) capped Ag NPs with an average diameter of around 20 nm were synthesized using BSA as a template under the assistance of sonication, and Ag NPs incorporated thin film composite (TFC) polyamide membrane was then fabricated by LBL-IP on

a

nanoporous

polysulfone

(PSf)

substrate

upon

sequential

coating

with

m-phenylenediamine (MPD) aqueous solution, trimesoyl chloride (TMC) hexane solution, and finally with BSA capped Ag NPs aqueous solution, respectively. The influence of Ag NPs incorporation was investigated on the surface physicochemical properties, water permeability, and salt rejection of TFC polyamide membrane. Our findings show that Ag NPs functionalized membrane exhibited excellent antibacterial properties without sacrificing their permeability and rejection, and Ag NPs incorporation affected very little surface roughness and charge of polyamide layer. Moreover, the incorporated Ag NPs presented a low release rate and excellent stability on polyamide surface in cross-flow condition. Given the simplicity and versatility of this approach, our study provides a practicable avenue for direct incorporation of various surface-tailored nanomaterials on polyamide surface to develop high-performance TFC membranes with fouling-resistant properties on a large scale.


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1. INTRODUCTION As an emerging and promising membrane-based process, forward osmosis (FO) technology uses the osmotic pressure as the driving force to introduce spontaneous water transportation across a semipermeable membrane, and attracts increasing attention for gaining fresh water from wastewater and seawater to alleviate freshwater crisis worldwide.1, 2 In FO system, the exploited membrane as a key component plays a critical role in the energy consumption and costs of the separation process. Thin-film composite (TFC) polyamide membrane, consisting of a non-porous, highly crosslinked polyamide selective layer and an underlying porous polysulfone (PSf) support layer, exhibits superior water permeability, high salt rejection, and wide-range pH stability compared to other types of desalination membranes, and has been widely used in FO process working as state-of-the-art desalination membrane.3-6 However, owing to the intrinsically physiochemical properties of polyamide layer, TFC membrane is highly susceptible to irreversible biological fouling, which significantly compromises the water flux, produced water quality and separation efficiency of FO process.7, 8 Therefore, surface engineering of polyamide layer surface is essential to suppress biofilms formation on TFC membrane. Incorporation of antibacterial nanomaterials onto the polyamide surface of TFC membrane has been considered as an effective and efficient approach to mitigate membrane biofouling. The incorporated nanomaterials such as silver nanoparticles (Ag NPs),9-13 copper nanoparticles (Cu NPs),14-16 TiO2 nanoparticles,17-19 and graphene oxide (GO) nanosheets20-28 have been demonstrated to not only prohibit the growth of microorganisms effectively but also affect little on the membrane permeability and selectivity.29-32 To date, two major



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approaches including ex-situ and in-situ methods have been developed to incorporate these nanomaterials onto polyamide surface. The ex-situ method generally involves two steps including the fabrication of TFC polyamide membranes first and then incorporation of nanomaterials onto the membrane surface via surface coating or covalent attachment.30 For example, biocidal GO nanosheets were grafted onto the prepared TFC polyamide surface via amide coupling between carboxyl groups of GO nanosheets and carboxyl groups of the polyamide

layer

using

1-ethyl-3-[3-(dimethylamino)

ethylenediamine propyl]

(ED)

carbodiimide

as

cross-linking

hydrochloride

(EDC)

agent, and

N-hydroxysuccinimide (NHS) as catalysts.21 For this strategy, however, it often requires multiple reactions, making this modification process time- and cost-consuming. In addition, the ex-situ method mostly need an extra post-treatment process, which is generally disengaged from the interfacial polymerization manufacturing process of polyamide membrane, thus hampering its scalability and practical application on a large scale. The in-situ method is preferred to directly immobilizing biocidal nanomaterials onto the polyamide layer of TFC membrane during the interfacial polymerization process via blending them into organic phase or aqueous phase.8 For instance, Ag NPs were dispersed in the TMC organic phase and then immobilized into polyamide layer during the interfacial polymerization process. The functionalized membranes exhibited sharply reduced biofouling against P. aeruginosa.33 However, one of the most challenges for this approach is inevitable agglomeration and non-uniform dispersion of nanoparticles in the polyamide layer due to the poor dispersion quality of nanoparticles in organic solvents, which even results in the defects formation of polyamide layer. Although dispersing nanoparticles into aqueous phase can


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reduce their agglomeration to some extent, it leads to only a small number of nanoparticles leaving in the substrate because most nanoparticles are removed together with excessive amine aqueous solution during interfacial polymerization, thus limiting mass loading of nanomaterials in polyamide layer.34 Furthermore, a large fraction of the incorporated nanomaterials was encapsulated in the bulk of the formed polyamide layer rather than on polyamide surface, thus further comprising their antibacterial efficiency. These limitations motivate continual search for simple and versatile strategies to incorporate nanomaterials onto the polyamide surface of TFC membranes for membrane biofouling mitigation. Here, we develop a simple and facile layer-by-layer interfacial polymerization (LBL-IP) method to immobilize Ag NPs onto the polyamide surface of TFC membrane for biofouling mitigation. Stable and uniform bovine serum albumin (BSA) capped Ag NPs with an average diameter of around 20 nm were synthesized using BSA as a template under the assistance of sonication. and Ag NPs incorporated TFC polyamide membrane was then fabricated by LBL-IP on a nanoporous PSf substrate upon sequential coating with m-phenylenediamine (MPD) aqueous solution, trimesoyl chloride (TMC) hexane solution, and finally BSA capped Ag NPs aqueous solution, respectively, as shown in Scheme 1. The influence of Ag NPs incorporation was investigated on the surface physicochemical properties, water permeability, and salt rejection of the prepared TFC membranes. Furthermore, the antibacterial properties and stability of the incorporated silver nanoparticles on the polyamide surface of TFC membrane were also evaluated. Given the simplicity and versatility of this approach, our research paves a new way to develop TFC membranes with outstanding biofouling-resistant performances.



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Scheme 1. Incorporation of silver nanoparticles onto polyamide surface of thin film composite membrane through layer-by-layer interfacial polymerization.

2. EXPERIMENT Materials and chemicals. Polysulfone (PSf) (Mn: 22,000 Da), 1,3-phenylenediamine (MPD, >99%), 1,3,5-benzenetricarbonyl trichloride (TMC, 98%), bovine serum albumin (BSA), sodium borohydride and propidium iodide (PI) were purchased from Sigma-Aldrich (St. Louis, MO, USA). SYTO®9 green fluorescent nucleic acid stain was received from Invitrogen (Eugene, Oregon, USA). Silver nitrate and other chemicals were obtained from Sinopharm Chemical Reagent Beijing Co., Ltd., China and used as received. Escherichia coli (E. coli, DH5α) was purchased from Beijing Dingguo Changsheng Biotechnology Co., Ltd., China. Synthesis of BSA/Ag NPs. BSA was chosen as a template to prepare Ag NPs for the following reasons. First, BSA is capable of coordinating silver ions by its various functional groups such as –NH, –OH, and –SH, and facilitating a straightforward synthesis of highly stable and dispersed Ag NPs in aqueous solution.35-37 Second, and more importantly, abundant residual carboxyl groups and amino groups on BSA enable BSA/Ag NPs to be covalently bonded on the membrane surface.38 Also, the previous study has demonstrated incorporation


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of BSA onto polyethersulfone (PES) membrane surface could effectively reduce the protein adsorption due to the surface hydrophilicity increase and negative surface charge of the modified membrane.39 In addition, BSA, as a common commercially available protein, is cheap in price. Herein, BSA/Ag nanoparticles were synthesized using the reported protocol.40 In a typical synthesis procedure, 1 mL of 0.423 M silver nitrate was added to 40 mL BSA solution (0.0225 g/mL) at 23 ºC and sonicated for 10 min under the sonicator KQ5200DE (Kun Shan Ultrasonic Instruments Co., Ltd, China). Then 1 mL of 0.423 M sodium borohydride was added dropwise into BSA solution under 30 min sonication to reduce silver ions. The resulting solution was dialyzed in deionized (DI) water to remove unreacted silver nitrate or sodium borohydride using the dialysis bag with 8,000 ~ 14,000 molecular weight cutoff (Beijing Solarbio Science & Technology Co., Ltd., China) for 24 h and then freeze dried to collect nanoparticle powder. For control samples, silver nanoparticles were prepared by mixing equimolar amounts of silver nitrate and sodium borohydride in the absence of BSA under the same conditions as above, and another control sample was prepared by mixing a solution of BSA and sodium borohydride without adding silver nitrate. The absorption spectra of all the above samples were collected to determine the formation of Ag NPs from 200 nm to 600 nm using a UV–Vis spectrometer (TU-1810, Persee, China). Characterization of BSA/Ag nanoparticles. The morphology of BSA/Ag nanoparticles was observed using scanning electron microscopy (SEM, S-4800, Hitachi, Japan) under 3 keV energy after loading the dry nanoparticle powder on the electric conductive adhesive stuck on the sample stage, and the chemical composition of BSA/Ag nanoparticles was determined using EX-350 Energy Dispersive X-ray Microanalyzer (EDX, Horiba, Tokyo,



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Japan) with 15 keV energy. The Ag content in BSA/Ag NPs was quantified using thermo-gravimetric analysis (TGA) and inductively coupled plasma mass analysis (ICP-MS), respectively. For TGA analysis, 10.0 mg of dry nanoparticle powder was loaded in the TGA instrument (Mettler 5MP/PF7548/MET/400W, Mettler-Toledo, Switzerland) and heated from 30 ºC to 100 ºC at a heating rate of 10 ºC/min, followed by 30 min incubation at 100 ºC to remove any moisture, and then heated further to 900 ºC at 10 ºC/min under air atmosphere. For ICP-MS analysis, 1.0 mg of dry nanoparticle powder was dispersed in 11 mL of DI water with 1 mL of 68% nitric acid in a glass vial. After 48 h incubation in a shaker (THZ-82A, Kexi Instrument Ltd., China), the mixed solution was filtered through the Millex-GV syringe filter with a 0.22 µm PVDF membrane (EMD Millipore Corporation, USA) and silver ions in the leach solution were subsequently quantified with an ICP-MS spectrometer (ELAN DRC II, PerkinElmer (Hong Kong) Ltd.), which was calibrated by internal standard with standard curve of

107

115

In and a

Ag. The luminescence of the as-prepared BSA/Ag NPs solution was

detected using a fluorescence spectrophotometer (LS55, Perkin Elmer, USA) under the excitation of 380 nm, and the color of BSA/Ag NPs solution was observed under UV light. The average particle size and zeta potential of BSA/Ag NPs were determined using ZetaPALS (Brookhaven Instruments Corporation, USA) at 25 °C. Fabrication of TFC polyamide membranes with and without grafting Ag NPs. TFC polyamide membrane was fabricated via interfacial polymerization on a nanoporous PSf support according to the reported protocol with some modifications.41,

42

Typically, PSf

support was prepared on PET non-woven fabric by non-solvent induced phase separation. The homogenous PSf casting solution (12 wt.%) was obtained by dissolving PSf beads in N,


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N-Dimethylformamide (DMF) under 8 h stirring and degassing in a desiccator for 12 h. Prior to casting, PET non-woven fabric was attached to a clean glass plate using waterproof adhesive tape and N-methyl-2-pyrrolidone (NMP) was applied to prewet the PET fabric. The excess NMP was removed using Kimwipes (Kimberly-Clark, USA). PSf solution was then poured into the PET fabric covered glass plate and cast using a casting knife (Tianjin Hong Ju Li Experimental Equipment Factory, China) with a gate height of 200 µm at an ambient temperature of 25 ± 1 °C and humidity of 40 ± 5 RH%. After that, the glass plate was immediately immersed into deionized (DI) water coagulation bath for 10 min. PSf membrane was formed and transferred to fresh DI water for storage. TFC polyamide membranes were fabricated by interfacial polymerization of MPD in the aqueous phase and TMC in organic phase on top of PSf membrane. A wet PSf membrane was taped to a clean glass plate and then immersed in a 30-mL aqueous solution of 3.4 wt.% MPD for 2 min. After removing excess MPD solution from membrane surface using a rubber roller, the MPD-containing PSf membrane was then immersed in a 0.15 wt % trimesoyl chloride solution in hexane for 1 min to form a polyamide film. Finally, the TFC composite membrane was cured in DI water at 95 °C for 2 min. After a thorough rinse with DI water, the prepared TFC FO membrane was stored in DI water at 4 °C before characterization and measurement. The fabrication procedure of Ag NPs grafted TFC membranes was presented in Scheme 1, and the difference from the pristine TFC membranes fabrication process is that after draining the excess TMC organic solution, the nascent polyamide layer was then immersed in Ag NPs aqueous solution (1.0 wt.% in DI water with pH value of 6.8) for 2 min to immobilize Ag NPs via the second interfacial reaction between primary amine groups of BSA on Ag NPs and acyl



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chloride groups on the polyamide layer surface. The membrane surface was rinsed three times with DI water to remove unbound Ag NPs before curing in a DI water bath at 95 °C for 2 min. Membrane Characterization. The morphologies of the pristine and Ag NPs grafted TFC polyamide membranes were observed using SEM (S-4800, Hitachi) with 3 keV energy. All samples were dried in vacuum oven at 50 °C for 12 h and then coated with platinum (Pt) for 100 s utilizing an EMITECH SC7620 sputter coater before SEM observation. The elemental compositions of the polyamide layers of membranes were detected using X-ray photoelectron spectroscopy (XPS, Thermo Esca Lab 250Xi, Thermo Fisher Scientific, USA) with monochromatic Al Kα X-ray source (hv = 1486.6 eV). For XPS analysis, all binding energies (BEs) were referenced to that of the neutral C 1s hydrocarbon peak at 284.6 eV. The XPS wide scan spectra of pristine and Ag NPs grafted membranes were collected in the region from 200 to 1200 eV with step size of 1 eV, and the high-resolution XPS spectra of Ag peak for Ag NPs grafted membrane were obtained by narrow scan between 360 and 382 eV with step size of 0.1 eV. The membrane surface charge was measured using Anton Paar SurPASSTM 2 electrokinetic analyzer (Anton Paar, Austria) with pH of the test solution (1.0 mM KCl) varied from 2 to 10. Surface roughness of membranes was measured via atomic force microscopy (AFM, Multi-Mode 8, Veeco, US) under tapping mode in air using TESP tips (Sharp silicon probe, 42 N/m, 320 kHz, tip radius 8 nm, no coating, Bruker, USA), and nanoscope analysis software was used to calculate various roughness parameters. Membrane transport properties. The performances of pristine and Ag NPs grafted TFC membranes were tested using a cross-flow FO test system reported in our previous work.11 The effective membrane surface area was 8 cm2. The feed solution was DI water and the draw


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solution was 1 M NaCl solution. The temperature of feed and draw solution was kept constant at 25.0 ± 0.5 °C. The cross-flow rate of both feed and draw solutions was maintained at 8.5 cm/s. The membranes were operated under AL-DS (active layer facing draw solution) mode. The water flux was calculated as the water weight gain of the draw side per hour and per area of the membrane, and the reverse salt flux was calculated as the salt weight gain of the feed side per hour and per area of the membrane. The water weight gain was monitored using the balance (ME3002, Mettler-Toledo, Switzerland), and the salt weight gain was monitored using the conductivity meter (CON2700, Eutech, USA). Antibacterial performances of Ag NPs grafted TFC polyamide membranes. The colony forming unit (CFU) counting method was used for evaluating the antibacterial activities of Ag NPs grafted membranes against Gram-negative Escherichia coli (E. coli, DH5α) following the reported protocol.11, 14, 43 Briefly, an overnight culture of bacteria (1 mL) in Luria-Bertani (LB) broth was diluted in 20 mL fresh LB broth and then grew for 2-3 h to reach the mid-exponential growth phase. The bacteria culture was centrifuged for 1 min at 5000 rpm to remove supernatant, washed with PBS twice, and then re-suspended in a sterile physiological saline solution (0.15 M NaCl, pH 7.0, 20 mM NaHCO3) to initial optical cell density at 600 nm (OD600) of 0.15 ± 0.09. Circular membrane coupons with 1.6 cm in diameter were placed in sterile Erlenmeyer flask with 20 mL of the prepared bacteria solution. After 5 h incubation at 37 °C, the membranes were taken out of the bacteria solution and rinsed gently three times with DI water to remove unattached bacteria. Membrane coupons were then put into sterile glass vials with 10 mL of physiological saline solution and sonicated in a bath sonicator (KQ5200DE, Kun Shan Ultrasonic Instruments Co., Ltd, China) for 7 min



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at 25 °C to remove the attached bacteria. The obtained suspension was serially diluted 100 times, and 100 µL of the bacterial solution was taken to plate on LB agar plates. Then bacteria colonies were counted after overnight incubation. The antibacterial efficiency was measured from Eq. (1). (1)

Where Np and Nm are the numbers of colonies corresponding to the pristine and Ag NPs modified membranes, respectively. To observe the bacteria adhesion and morphological characteristics on membrane surfaces using SEM,44, 45 the membrane coupons were incubated in the bacteria solution with initial optical cell density at 600 nm (OD600) of 0.15 ± 0.09 for 4 h at 37 °C, following the aforementioned method. After being taken out of the bacteria solution and rinsed gently three times with DI water, the membrane coupons were subsequently fixed in 3 % (v/v) aqueous glutaraldehyde at 4 °C for 30 min. Then the serial dehydration of membrane coupons in 20, 40, 60, 80 and 100 % of ethanol was performed for 10 min each. After drying in vacuum oven at 30 °C, the membranes were sputter-coated with platinum (Pt) for 100 s utilizing an EMITECH SC7620 sputter coater and imaged by SEM. The live and dead bacteria attached on the membrane coupons were also imaged using confocal laser scanning microscopy (Fluo View FV1000, Olympus, Japan). SYTO®9 is an excellent green-fluorescent nuclear and chromosome counterstain that is permeant to both live and dead cell membranes, and PI is a red-fluorescent dye but is membrane impermeant and generally excluded from viable cells. Therefore, SYTO®9 stain and PI are commonly used for identifying live and dead cells, respectively. For fluorescence imaging, circular membrane


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coupons were incubated in the bacteria solution with initial optical cell density at 600 nm (OD600) of 0.15 ± 0.09 for 1 h at 37 °C based on the aforementioned approaches. Then the circular membrane coupons were placed into a 12-well tissue culture plate and covered with 1 mL of 3.34 µM SYTO®9 solution and 1 mL of 20 µM PI solution for 15 min at 37 °C in dark. After that, the membranes were rinsed twice gently with DI water and imaged with a confocal laser scanning microscopy (SYTO®9, excitation with an argon laser at 488 nm and emission at 519 nm; PI, excitation with an argon laser at 559 nm and emission at 619 nm). Dynamic biofouling experiments in cross-flow FO test system. The dynamic biofouling measurements were carried out in a lab-scale FO cross-flow set-up reported in our previous work.11 The effective membrane surface area was 8 cm2. The dynamic biofouling procedures were operated following the protocols described in previous publications.20, 29, 46 Before each measurement, the FO set-up was disinfected through sequentially circulating bleach, 5 mM EDTA solution and then ethanol for 1 h. DI water was used as feed solution and draw solution to stabilize the FO set-up. When the water flux reached 0 approximately for 20 min, the synthetic wastewater stock was added into the feed solution side and adjusted its pH to 7.5 ± 0.2, and NaCl stock was added into the draw solution side. The volumes of both feed solution and draw solution were kept at 2 L. Once the water flux was reached 15 L m-2 h-1, E. coli suspension was added into the feed solution to reach the bacteria concentration of 6.0 × 107 CFU L−1 approximately. During each measurement, both feed and draw solutions were maintained with the cross-flow rates of 0.5 L/min using two gear pumps (WT3000-1FA, Longer Pump, China) separately and kept at the temperatures of 25±0.5 °C by a temperature-controlling water bath (SDC-6, Ningbo Scientz Biotechnology CO., LTD, China).



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The permeate flux was continuously monitored by the weight gain of the draw solution using the electronic balance (ME3002, Mettler, Switzerland), and the reverse salt flux was monitored by the salt weight gain of the feed solution using the conductivity meter (CON2700, Eutech, US). The membranes were operated under FO mode. To eliminate the effects of the draw solution dilution and feed solution concentration during the fouling experiments, baseline experiments for each membrane were conducted following the same procedure as that for the fouling experiments except that no bacteria was added to the feed solution. Silver ions release measurement. To evaluate the stability of the incorporated Ag NPs on membrane surface, the release of silver ions for the modified membranes was measured in a cross-flow FO test system. Typically, the modified membrane with effective membrane surface area of 8 cm2 was loaded into the membrane cell and the feed solution and draw solution were both DI water with volume of 1 L. The temperature and cross-flow rate of both feed and draw solution were kept constant at 25.0 ± 0.5 °C and 8.5 cm/s, respectively. The system was operated for 24 h, and at predetermined time intervals, 10 mL solution was taken out from feed solution for ICP-MS analysis. Before ICP-MS tests, 0.1 mL of 68% nitric acid was added to acidify solution. The accumulative released silver mass was calculated according to the obtained concentration of silver ions and the volume of feed solution.

3. RESULTS AND DISCUSSION 3.1 Synthesis and characterization of BSA/Ag NPs. In this study, BSA capped Ag NPs were synthesized in aqueous solution under ultrasonication using sodium borohydride as a reducing agent, and the synthesis procedure was illustrated in Figure 1A. To determine the


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crucial role of BSA in the preparation of Ag NPs, the reaction process of a mixture solution of AgNO3 and sodium borohydride was monitored in the presence and absence of BSA. As shown in Figure S1, the color of the mixture solution of BSA/AgNO3 turned from yellowish to brown with adding sodium borohydride and did not form precipitation in the solution; however, when equimolar amounts of sodium borohydride were added into AgNO3 solution without BSA, the color of the solution did not change much and gray precipitation was observed at the bottom of the reaction flask, which may because the freshly formed Ag NPs rapidly agglomerated and finally precipitated in the absence of BSA. The UV–Vis absorption spectroscopy was used to detect the formation of Ag NPs by tracking the characteristic absorbance peaks of the surface plasmon resonance of Ag NPs at around 415 nm.36 As shown in Figure 1B, a prominent absorption peak centered at 420 nm confirmed the formation of Ag NPs in the mixture solution of BSA/AgNO3/NaBH4.Whereas there was no characteristic absorbance peak of Ag NPs observed in the reaction system of AgNO3/NaBH4 and BSA/NaBH4, it further demonstrated the predominant role of BSA during the synthesis of Ag NPs. The BSA coated Ag NPs were uniformly spherical (Figure 1C) with a narrow size distribution having an average diameter of 21.9 nm. The element compositions of the prepared Ag NPs were examined using energy-dispersive X-ray spectrometry (EDX) and strong signals of silver, sulfur, nitrogen, oxygen, and carbon elements were observed as shown in Figure 1D, indicating the prepared Ag NPs were capped by BSA. The silver content in the BSA capped Ag NPs was quantified by TGA and deduced to be 3.91 ± 0.42 % by subtracting the residual mass of pristine BSA from the residual mass of BSA/Ag NPs (Figure 1 E). Based on the silver content, the molar ratio of silver to BSA in BSA/Ag NPs was



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calculated to be (25.26 ± 2.72) : 1, which is generally consistent with the theoretical molar ratio of silver to BSA (31.22 : 1), also agreeing with the previously reported Ag content in BSA/Ag NPs.40 Furthermore, the silver content in the nanoparticles was further determined by inductively coupled plasma mass spectrometry (ICP-MS) and the results showed that the silver content in the nanoparticle was 4.04 ± 0.33 wt.% by mass, which is very close to TGA findings.

Figure 1. Characterization of BSA/Ag NPs: the synthesis procedure of silver nanoparticles

using BSA as a template (A); UV-vis absorption spectra of different reaction systems (B);


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SEM morphology of BSA/Ag NPs and size distribution of BSA/Ag NPs (insert) (C); EDX spectrum of BSA/Ag NPs (D) and TGA curves of BSA/Ag NPs and pure BSA under air atmosphere (E).

To assess the antibacterial activities of the as-prepared BSA/Ag NPs, the growth profiles of E. coli incubated with various concentrations of BSA/Ag NPs were measured as well as the width of the zone of inhibition (ZOI) of BSA/Ag NPs on agar plates. As shown in Figure S1, an apparent decline in bacterial density was observed with increasing the Ag concentration from 6 µg/mL to 60 µg/mL in the bacterial culture suspension, and no visible bacterial growth was observed when Ag concentration was up to 30 µg/mL, representing the minimum inhibitory concentration value for BSA/Ag NPs. This concentration was comparable to the reported Ag NPs with diameter of 10 nm.47 The antibacterial abilities of the synthesized BSA/Ag NPs were further evaluated with an oxford cup method. The results revealed that compared with sterile DI water and BSA solution, the BSA/Ag NPs solution showed apparent ZOI on agar plates where the bacteria were killed or stopped from growing, and the width of ZOI clearly increased with the increase of BSA/Ag NPs solution concentration. When the concentration of BSA/Ag NPs solution was upto 30 µg/mL, the width of ZOI was mearsured to be about 12 mm which was much larger than that of BSA/Ag NPs solution with concentration of 6 µg/mL and 12 µg/mL. Based on our findings, the synthesized BSA/Ag NPs exhibit outstanding antibacterial activities. 3.2 Preparation, surface morphology and chemical composition of Ag NPs incorporated TFC membranes. The Layer-by-layer interfacial polymerization (LBL-IP) method was used to immobilize Ag NPs onto membrane surface via the introduction of



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BSA/Ag NPs solution to react with free acyl chloride groups in nascent polyamide surface freshly formed by MPD and TMC. To determine that BSA/Ag NPs were grafted onto membrane surfaces, the luminescence of polyamide layer was detected with laser confocal microscopy, considering that the prepared BSA/Ag NPs could emit a green luminescence with an emission peak at 520 nm under UV excitation (Figure S3 A). As shown in Figure S3 B and C, intense green luminescence was observed on the polyamide layer of TFC membrane upon the immobilization of BSA/Ag NPs, but no luminescence can be observed for the pristine membrane, which confirmed the successful immobilization of BSA/Ag NPs on the polyamide surface of TFC membrane. The presence of BSA/Ag NPs on the membrane surface was further confirmed by surface morphology observation. As shown in Figure 2A and B, both pristine and BSA/Ag NPs functionalized polyamide surfaces presented characteristic ridge-and-valley morphologies, but densely populated small particles with about 20 nm in average diameter were clearly observed on the modified TFC membrane surfaces, demonstrating the successful incorporation of BSA/Ag NPs. Meanwhile, the individual distribution of BSA/Ag NPs on the membrane surface without visible particle agglomeration highlights the advantage of our method over the conventional in-situ method, in which Ag NPs were blended into MPD or TMC solution and the aggregation of Ag NPs on the formed polyamide layer was inevitable.34 Furthermore, the chemical elemental compositions of polyamide layers of TFC membranes were determined by XPS, and the XPS wide scans spectra of the pristine and BSA/Ag NPs modified membranes presented in the Figure 2C revealed three characteristic signals of carbon, nitrogen and oxygen elements at 284 eV, 399 eV, and 531 eV, respectively, consistent


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with the chemical compositions of polyamide layer of TFC membranes. The spectra of the BSA/Ag NPs modified membranes (red in Figure 2C) showed the appearance of binding energy (BE) peak associated with silver, and the high-resolution XPS spectra (Figure 2D) exhibited two contributions, Ag 3d3/2 and Ag 3d5/2 located at 374 eV and 368 eV, respectively, further confirming the presence of Ag NPs on the modified membrane surface. The Ag loading mass was also determined by ICP-MS analysis via soaking the membrane coupons in 3.5% HNO3 solution for 48 h to dissolve Ag NPs completely, and the results showed that the Ag loading mass was 1.4 ± 0.4 µg/cm2 on the membrane surfaces.

Figure 2. Characterization of TFC membrane polyamide surfaces: SEM micrographs before

(A) and after immobilizing BSA/Ag NPs (B); XPS wide scans spectra before (black) and after immobilizing BSA/Ag NPs (red) (C); High resolution XPS spectra of Ag peaks for Ag NPs containing TFC membrane polyamide surfaces in the region between 360 and 380 eV (D).



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3.3 The effects of the incorporated BSA/Ag NPs on membrane surface roughness and charge. It is noteworthy that the membrane biofouling propensity highly depends on their surface physicochemical properties, and therefore the impacts of BSA/Ag NPs immobilization were investigated on the surface roughness, charge and hydrophilicity of TFC membrane polyamide layer.3, 48 As shown in Figure 3A and B, although the root-mean-square (RMS) roughness of polyamide layer exhibits a slightly increase from 75.3 ± 6.7 nm to 82.9 ± 3.0 nm upon grafting BSA/Ag NPs, the RMS value of the modified membrane is comparable or even lower compared to those of the reported TFC membranes.20, 30, 42, 49 The hydrophilicity of pristine and Ag NPs modified polyamide layer surfaces was also determined and the water contact angles were measured to be 69.7 ± 5.7° and 87.4 ± 2.1° for pristine and BSA/Ag NPs modified membrane, respectively, showing a slight increase upon incorporation of Ag NPs.

Figure 3. AFM images (A, B), water contact angle (C) and zeta potential (D) of TFC


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polyamide membranes before and after immobilizing BSA/Ag NPs. The water contact angle values of membranes are an average of five different samples for each membrane type. Error bars represent the standard deviation.

The zeta potentials of pristine and modified membrane surfaces were also measured with the pH value ranging from 3 to 10, and the results are presented in Figure 3C. The isoelectric point (IEP) of BSA/Ag NPs modified TFC membrane showed an increase from 3.7 to 4.6, very close to the IEP of BSA 4.7, which indicates the membrane surface was fully covered with BSA/Ag NPs. It can be also observed that the surface charge of BSA/Ag NPs modified membrane displayed only a slight increase compared to that of the pristine membrane over the entire examined pH range, and remained negatively charged at pH higher than 5, demonstrating that the incorporation of BSA/Ag NPs did not alter membrane surface charge significantly at neutral or mild alkali conditions. As a contrast, the reported incorporation of nanoparticles capped by amine functional group-containing polymers such as PEI coated copper nanoparticles onto the TFC membrane surface, resulted in an obvious increase of membrane surface charge from negative to positive at neutral or mild alkali conditions,13 which may promote the bacterial adhesion due to the electrostatic attraction between positive membrane surface and negative surface charge of microorganism. From this aspect, this approach is preferable in decreasing the biofouling propensity of TFC membranes due to the negative charge of BSA/Ag NPs modified TFC membrane at neutral or mild alkali conditions. 3.4 The influence of the Ag NPs incorporation on the separation performance of TFC membrane. The separation performances of the membranes including water flux and reverse salt flux were tested using a custom lab-scale cross-flow forward osmosis test system in



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AL-DS (active layer facing draw solution) operational mode. Figure 4 shows that the water flux of BSA/Ag NPs containing TFC membranes (30.2 ± 0.8 L m−2 h−1) was even increased a little compared to that of the pristine membrane (28.3 ± 1.7 L m−2 h−1), implying the incorporation of BSA/Ag NPs did not increase the water molecular transport resistance. Furthermore, the reverse salt flux decreased a little from 3.90 ± 0.92 g m−2 h−1 for the pristine membrane to 2.89 ± 0.12 g m−2 h−1 for Ag NPs containing membranes, presumably due to the surface charge effects from the incorporated BSA/Ag NPs on the polyamide layer. Based on the above results, our findings demonstrate that the incorporation of BSA/Ag NPs has a beneficial impact on membrane transport properties without decreasing the intrinsic water permeability. Although extensive efforts have been devoted to grafting antimicrobial polymer brushes, peptides or lysozyme onto TFC membranes for antibacterial properties, these modification strategies tend to significantly decrease the water fluxes of membranes as a consequence of the additional resistance to water transport imparted by the grafted polymer brushes, peptides or lysozyme.50-52 In this study, the incorporated BSA/Ag NPs are individually and uniformly anchored on the membrane surface other than forming a dense layer due to the three-dimensional globular-shaped structures and high steric hindrance of nanoparticles.26,27 Therefore, antibacterial nanomaterials present superior advantages of imparting biofouling properties to TFC polyamide membranes without sacrificing their filtration performances.


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BSA/Ag NPs

Figure 4. Water flux and reverse salt flux of TFC FO membranes before and after the

incorporation of Ag NPs, tested in FO unit with the AL-DS operation mode using 1 M NaCl solution as draw solution and deionized water as feed solution. Results are an average of the measurements from three different membrane coupons. Error bars are given as standard deviation.

3.5 Antibacterial activities of Ag NPs modified TFC FO membranes. To evaluate the antibacterial properties of Ag NPs modified TFC FO membranes, the bacterial adhesion and morphological characteristics of E. coli (DH5α) on the pristine and Ag NPs functionalized membranes surface were investigated using SEM. As shown in Figure 5, after exposing the polyamide layer surface to bacteria suspension for 4 h, a large number of bacterial cells with characteristic and intact morphology of E. coli adhered individually or in small clusters onto the pristine membrane surface, while few bacterial cells with morphological integrity were observed on the BSA/Ag NPs modified TFC membrane. It may because Ag NPs killed the adhered bacteria and the dead bacteria were easily washed out under hydrodynamic shear forces during the incubation.24, 53 The numbers of viable E. coli cells attached on the pristine and Ag NPs modified membrane surfaces were counted via the CFU counting method, and



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the results shown in Figure 6 illustrate that the numbers of attached live E. coli on the Ag NPs containing membranes decreased by 96.4 ± 3.4% compared to the pristine membranes, further confirming the excellent antibacterial properties of Ag NPs containing membrane. Previous studies have demonstrated that the mechanism of antibacterial action of Ag NPs is due to the aerobic release of silver ions, where the released sliver ions cause the death of bacteria with complex mechanisms, such as Ag+-induced cell membrane damage, Ag+-promoted generation of reactive oxygen species (ROS) and Ag+-based disruption of ATP production and DNA replication.54, 55 In our study, the excellent antibacterial behavior of Ag NPs functionalized membrane surface was mainly ascribed to the small particle size of the incorporated Ag NPs (shown in Figure 1 and 2), which leads to efficient exposure of bacteria to Ag NPs and a fast rate of silver ions release, as reported in previous studies.47, 56

Figure 5. SEM micrographs of polyamide surfaces of TFC membranes before (A and B) and after grafting BSA/Ag NPs (C and D) upon 4 h incubation with bacteria suspension.


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1.2 Normalized CFU

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BSA/Ag NPs

Figure 6. CFUs of attached live E. coli. bacteria on polyamide surfaces of TFC membranes before and after grafting BSA/Ag NPs.

Furthermore, a confocal laser scanning microscopy (CLSM) analysis was performed to intuitively assess the attachment and distribution of the viable and dead bacteria on the pristine and Ag NPs functionalized TFC FO membranes, and the representative fluorescence microscopic images are given in Figure 7, where the green fluorescence and red fluorescence present the live and dead bacteria, respectively. After 1 h incubation of membranes in E. coli suspension, intense green fluorescence and weak red fluorescence were observed for the pristine membranes, demonstrating the viable bacteria were predominant on the pristine membrane surface. In comparison, the red fluorescence was much stronger on the Ag NPs modified membranes than on the pristine membranes, suggesting the dead bacterial rather than the live bacteria were predominant on the Ag NPs modified membranes. These results further demonstrate the incorporation of BSA/Ag NPs imparted outstanding antimicrobial



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properties to the TFC membranes.

Merge

Figure 7. Representative laser confocal microscopy images of E. coli on polyamide surfaces of TFC membranes before and after grafting BSA/Ag NPs. Live and dead bacterial cells were stained with SYTO®9 (green), and PI (red), respectively.

3.6 The dynamic biofouling behaviors of pristine and Ag NPs modified membranes. The water flux of the pristine and Ag NPs functionalized TFC membranes was monitored to investigate their biofouling behaviors in the cross-flow FO operation. The water flux decline during the fouling process is attributed to both the formation of biofouling layer on the membrane surface and the draw solution dilution and feed solution concentration. To eliminate the effects of the latter two factors during the fouling experiments, baseline experiments for each membrane were conducted following the same procedure as that for the fouling experiments except that no bacteria was added to the feed solution. The corrected water flux decline profiles for pristine and Ag NPs functionalized TFC membranes are shown


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in Fig 8A as a function of the cumulative permeation volume. When the cumulative permeation volume of water reached 250 L/m2, Ag NPs modified membranes exhibited 8.38% water flux decline, which is much less than 31.7% of pristine membrane, indicating the improved biofouling resistance in cross-flow operation condition due to the incorporation of Ag NPs. 3.7 The stability of the incorporated Ag NPs. The stability of Ag NPs on the polyamide layer surface was investigated in a cross-flow FO test system and the released silver ions mass was quantified via ICP-MS. As shown in Figure 8B, in the first 1 h, the accumulative released silver ions amount was 32.7 ng/cm2, exhibiting a initial fast release behavior, and then the silver release rate decreased dramatically with the test time ranging from 2 h to 24 h. This trend for silver ions release with highly initial release rate is also observed with unknown reason in the previous studies.13, 57 The accumulative released silver ions amount after 24 h operation in the cross-flow system was 40.4 ng/cm2, which accounted for only 2.88% of the total Ag loading mass on the membrane surfaces considering that the total Ag loading mass measured by ICP-MS was 1.4 ± 0.4 µg/cm2. Therefore, our results elucidated that the Ag NPs were immobilized on the polyamide layer surfaces firmly and had an excellent stability. To achieve the long lasting antibacterial activity of Ag NPs modified membrane in practical application, the total loading mass of Ag NPs on the membrane surface could be further optimized by adjusting the concentration of BSA/Ag NPs solution, controlling the reaction time between Ag NPs and the acyl chloride groups and tailoring the number of LBL-IP process.


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Normalized Water Flux (%)


Figure 8. The normalized water flux decline of the TFC membranes as a function of the cumulative permeate volume in the cross-flow FO operation with bacteria suspension as feed solution (A) and the silver release profile of Ag NPs modified TFC membranes (B) during the cross-flow FO operation.

4. CONCLUSIONS Our work presents a simple and facile LBL-IP approach to covalently graft Ag NPs onto the polyamide surfaces of TFC membranes during the fabrication process of membranes. The incorporation of Ag NPs onto the polyamide surfaces of TFC membranes did not change their surface roughness and charge much, but imparted excellent antibacterial properties to the membranes without compromising their water flux and reverse salt permeability. The incorporated Ag NPs exhibited a slow release behavior and had an excellent stability on polyamide surface in the cross-flow running condition. Considering the simplicity and versatility, this approach exhibits promising potentials in the practical fabrication of antibacterial TFC membranes on a large scale. Our study also provides a practicable avenue for the direct incorporation of various antifouling and surface-tailoring nanomaterials on TFC membranes surface for antifouling performances through this LBL-IP process.


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ACKNOWLEDGEMENTS The authors gratefully acknowledge the funding support from National Natural Science Foundation of China (No. 21476249), Key Science and Technology Program of Shandong Province (No. 2014GHY115021), and Key Science and Technology Program of Yantai City (No. 2015ZH063).

ASSOCIATED CONTENT Supporting Information The photographs of different reaction systems after 30 min sonication treatments (Figure S1); Growth profile and inhibition zone of E. coli in the presence of varying concentrations of BSA/Ag nanoparticles (Figure S2); Fluorescence emission profile and photograph of BSA/Ag nanoparticles, and laser confocal microscopy images of polyamide layers of TFC membranes before and after grafting BSA/Ag NPs (Figure S3). This information is available free of charge via the Internet at http://pubs.acs.org/. AUTHOR INFORMATION Corresponding Authors *Yunxia Hu,

E-mail: [email protected].

Author Contributions # Zhongyun Liu and Longbin Qi have contributed equally to this work. Notes The authors declare no competing financial interest.



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TOC


Surface Engineering of Thin Film Composite Polyamide Membranes

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