In Situ Reduction of Silver by Polydopamine: A Novel Antimicrobial

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In situ reduction of silver by polydopamine: a novel anti-microbial modification of thin-film composite polyamide membrane Zhe Yang, Yichao Wu, Jianqiang Wang, Bin Cao, and Chuyang Y. Tang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b01867 • Publication Date (Web): 01 Aug 2016 Downloaded from http://pubs.acs.org on August 8, 2016

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Environmental Science & Technology

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In situ reduction of silver by polydopamine: a novel anti-microbial modification

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of thin-film composite polyamide membrane

3 Zhe Yanga, Yichao Wub,c, Jianqiang Wanga, Bin Caob,c, Chuyang Y. Tanga,*

4 5 6

a

Department of Civil Engineering, the University of Hong Kong, Pokfulam, Hong Kong

7

b

School of Civil and Environmental Engineering, Nanyang Technological University, 50 Nanyang Avenue,

8

Singapore 639798

9

c

Singapore Centre for Environmental Life Sciences Engineering, Nanyang Technological University, 60 Nanyang

10

Avenue, Singapore 637551

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*

Corresponding Author. Tel: +852 2859 1976, Fax: +852 2559 5337, E-mail address: [email protected]

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ACS Paragon Plus Environment


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Abstract

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We report a facile method for the anti-microbial modification of a thin-film composite polyamide reverse osmosis (RO) membrane. The membrane surface was first coated with polydopamine (PDA), whose reducing catechol groups subsequently immobilized silver ions in situ to form uniformly-dispersed silver nanoparticles (AgNPs) inside the coating layer. Agglomeration of AgNPs was not observed despite a high silver loading of 13.3 ± 0.3 µg/cm2 (corresponding to a surface coverage of 18.5% by the nanoparticles). Both diffusion inhibition zone tests and colony formation unit tests showed clear anti-microbial effects of the silver loaded membranes on model bacteria Bacillus subtills and Escherichia coli. Furthermore, the silver immobilized membrane had significantly enhanced salt rejection compared to the control PDA coated membrane, which is attributed to the preferential formation of AgNPs at defect sides within the PDA layer. This self-healing mechanism can be used to prepare anti-microbial RO membranes with improved salt rejection without scarifying the membrane permeability, which provides a new dimension for membrane surface modification.

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TOC art

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

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Water scarcity is a grand challenge to human beings.1 This challenge can be

36

potentially addressed by desalination using reverse osmosis (RO) technology.2 A key

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obstacle of the RO technology is membrane biofouling, i.e., the development of a

38

cohesive biofilm on membrane surfaces. Severe biofouling can lead to significant loss

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of water permeability and product water quality.

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issues, physicochemical pretreatment methods and disinfection (e.g., chlorine or UV)

41

of feed water are applied.4 Although membrane biofouling can be mitigated by

42

chlorinating the feed water, some microorganisms can survive through the

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chlorination treatment and eventually colonize on membrane surfaces.5 In addition,

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chlorine-based disinfectants can cause chemical damage to RO membranes.6, 7 For

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example, it has been well documented that thin-film composite (TFC) polyamide (PA)

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RO membranes8 have a low tolerance to free chlorine (< 0.1 ppm).9 Therefore,

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alternative disinfection methods are needed to avoid membrane damage.

INTRODUCTION

3

To mitigate membrane biofouling

48 49

In recent years, researchers have developed a variety of anti-adhesion10, 11 and anti-

50

microbial12-15 surface modification methods. Among the various anti-microbial agents

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used, silver nanoparticles (AgNPs) have attracted growing interest due to their strong

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disinfection power.16 AgNPs are commonly blended with polymers or monomers

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during membrane formation.17, 18 An inherent disadvantage of this approach is the

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agglomeration of AgNPs, which can lead to the formation of defects and even the loss



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of membrane rejection at high loading of particles.19 Yin et al.12 reported the grafting

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of AgNPs onto a TFC membrane surface by using a bridging chemical agent

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(cysteamine) to form covalent bonding with AgNPs. Nevertheless, the chemical

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modification led to a reduced membrane rejection, which is attributed to the swelling

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of the rejection layer. Ben-Sasson et al.15 prepared a uniform coverage of AgNPs on a

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TFC RO membrane by reducing silver ions from a solution phase. Although this

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approach avoids the agglomeration of AgNPs, a strong reducing agent (sodium

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borohydride) has to be used. Greener and more environmentally benign methods for

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AgNPs incorporation are yet to be developed.

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We are inspired by the recent literature on using polydopamine (PDA) for the

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preparation of antifouling membranes.20, 21 PDA is a polymer derived from dopamine,

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a “bio-glue” that has been isolated from mussels and that is responsible for their

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attachment to different surfaces in natural environment (e.g., rock).20 The sticky

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nature of dopamine and its ability to form polymers enable researchers to prepare

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PDA coating on a variety of solid substrates for applications in environmental, energy,

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and biomedical fields.22, 23 Furthermore, PDA is highly hydrophilic due to its catechol,

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quinone, and amine functional groups24, which makes PDA an ideal surface

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modification agent for preparing anti-adhesion membranes.25, 26 In a recent study on

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synthesizing metal/organic hybrid nanomaterials, Hong et al.21 presented a simple and

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elegant method to fix silver in situ by exposing PDA to an AgNO3 solution. Silver


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ions were reduced spontaneously by the catechol groups of PDA to form well-

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dispersed AgNPs on a polymer nanofiber substrate. A similar approach has been

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reported by Tang et al. for biofouling control for a porous ultrafiltration (UF)

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membrane.27 These authors demonstrated strong anti-adhesive and anti-biofouling

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properties of PDA/Ag coated UF membranes. Up to date, such PDA/Ag coating has

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not been applied to membranes with dense rejection layers (such as RO). Compare to

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porous membranes, a PDA/Ag coating on RO will likely exhibit very different impact

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on the membrane’s separation properties.

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These existing studies prompt us to use PDA as a bioinspired scaffold to prepare anti-

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microbial RO membranes. Specifically, PDA-AgNPs composite coatings were

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prepared on a commercial RO membrane, and their effects on the separation

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performance and anti-microbial behavior are systematically.

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

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Materials and chemicals.

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A commercial RO membrane (XLE), received from Dow FilmTec, was used as the

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base membrane for further modification. XLE is a brackish water TFC RO

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membrane.28 Its dense polyamide rejection layer is supported by a porous polysulfone

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(PSF) layer, and a polyester (PET) nonwoven fabric is used to provide the required

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mechanical strength.28

MATERIALS AND METHODS



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Unless specified otherwise, all chemicals were of ACS reagent grades. All solutions

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were prepared using deionized water supplied from a Milli-Q system (Millipore,

100

Billerica,

MA).

Dopamine-hydrochloride

powder,

Tris

(hydroxymethyl)

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aminomethane (Tris-HCl, ≥ 99.0 %), and hydrochloric acid (HCl) were obtained from

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Sigma Aldrich and were used for the preparation of PDA coatings. Silver nitrate

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(AgNO3, Sigma Aldrich) was used as the silver source for the in situ formation of

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AgNPs. Sodium chloride (NaCl, Sigma Aldrich) was used in membrane rejection

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tests. N,N-Dimethylformamide (DMF, ReagentPlus®, ≥99%, Sigma Aldrich) was

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used as a solvent for dissolving the PSF support to prepare isolated polyamide films

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used for transmission electron microscopy (TEM) characterization.

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PDA coating and in situ formation of AgNPs on TFC RO membranes.

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The main surface modifications in the current study include a PDA coating step

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followed by in situ formation of AgNPs (Figure 1). Prior to PDA coating, the base

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membrane XLE was thoroughly rinsed with deionized water and was dried in air. A

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membrane coupon of 20 × 12 cm was placed in a custom-made container in such a

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way that only its rejection layer was exposed to the coating solution. The coating

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solution was prepared by adding 0.4 g dopamine hydrochloride into a 200 ml buffer

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solution of 10 mM Tris-HCl. The solution pH was adjusted to 8.5 to allow optimized

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self-polymerization of PDA.26, 29 During the entire coating step, the coating solution


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was continuously stirred to minimize the aggregation of PDA. The PDA-coated

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membrane is denoted as PDAn, where n is the duration of coating (n = 0.5, 1, and 2 h

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in the current study).

121 122 123 124 125

Fig. 1. Schematic diagram of in situ formation of AgNPs on a thin film composite membrane. The base membrane XLE was first coated with polydopamine, followed by in situ reduction of silver by immersing the PDA coated membrane into an AgNO3 solution.

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The freshly coated membrane was rinsed with deionized water for 30 minutes before

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it was placed in a 200 ml solution of 4 gL-1 AgNO3 for the immobilization of silver.

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Briefly, the PDA-coated rejection layer was exposed to the silver solution in the same

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custom-made container that was used for PDA coating. The container was wrapped

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with an aluminum foil to avoid sunlight and was placed on a shaking bath under room

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temperature for 5 h (adopted based on existing literature30). The resulting silver

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incorporated membrane is named as PDAnAg.

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Membrane characterization.

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X-ray photoelectron spectroscopy (XPS) was performed using an SKL-12



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spectrometer (Leybold, Sengyang, China) modified with a VG CLAM 4 MCD

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electron energy analyzer. An Al Kα gun (1496.3 eV) operated at 10 kV and 15 mA

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was used as the x-ray source. Membrane samples were thoroughly rinsed several

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times and dried before XPS characterization. Survey spectra over 0-1000 eV were

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obtained at a scanning resolution of 0.1 eV.

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Membrane structure and morphology were characterized by scanning electron

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microscopy (SEM) and TEM. To isolate the membrane rejection layer for TEM

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characterization, the polyester fabric layer was first removed and the PSF layer was

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subsequently dissolved using DMF as the solvent. The use of DMF for isolating

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rejection layers of RO and NF membranes has been reported in the existing

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literature.31 The isolated rejection layer was float on the DMF solvent and was picked

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up by a carbon-coated copper TEM grid. The sample was allowed to dry in air at

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room temperature. TEM sample characterization was performed with Philips CM100

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TEM (Philips, Eindhoven, Netherlands) operating at 100 kV.

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SEM characterization was conducted using a Field Emission Gun Scanning Electron

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Microscope (LEO1530 FEG SEM, UK) equipped with an energy dispersive

155

spectroscopy (EDS) detector. Vacuum dried membrane samples of approximately 0.5

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cm × 0.5 cm were sputter-coated with a uniform layer of gold and platinum (SCD

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005, BAL-TEC, NYC) to avoid sample charging. SEM micrographs were obtained at


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an accelerating voltage of 5 kV. EDS was also performed to determine the elemental

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composition of the membrane sample. For EDS analysis, a voltage of 20 kV was

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used.

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Water contact angle measurements were performed using a goniometer equipped with

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a video capture device (Powereach®, China). Prior to each test, a membrane sample

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was dried in vacuum at room temperature for 24 h. Each deionized water droplet with

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a volume of approximately 5 µL was introduced to the membrane surface, and a

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stabilizing time of 10 seconds was allowed. For each membrane coupon, contact angle

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was measured at five different locations and the average value was reported.

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Image analysis.

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An image analysis software (MediaCybernetics, Inc.) was utilized to analyze the size

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distribution of AgNPs and their surface coverage over the silver incorporated

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membranes. Prior to the analysis, TEM micrographs were first saved as 8-bit

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greyscale images in Tagged-Image File Format (TIFF). These greyscale images were

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then converted into binary black-and-white images. Background noise was alleviated

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by applying threshold value adjustment following a previous study.32 In addition to

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the particle size determination, the percentage of membrane area covered by AgNPs

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in the 2D images were determined and reported as the surface coverage. The analysis

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was performed based on three replicate measurements.



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Separation performance tests.

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A high pressure cross-flow filtration system, similar to the one reported by Tang et

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al.33, was used to evaluate water flux and solute rejection of membranes under a

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constant pressure mode. The temperature was maintained constant at around 25 °C

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using an immersion thermostat (J.P. Selecta S.A., Barcelona, Spain). For each test, a

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membrane coupon with an effective area of 42 cm2 was placed in a cross-flow cell

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(CF042, Delrine acetal, Sterlitech). The coupon was pre-compacted using deionized

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water at the set pressure of 2 MPa for 24 h in order to achieve a stable water flux. The

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pure water flux was then determined by measuring the mass of the permeate water

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collected over a specified time interval according to Equation (1):

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Jv 

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where Jv (Lm-2h-1) is the water flux, w (kg) is the mass of permeate water collected

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over a time period of t (h), A (m2) is the effective membrane area, and  (kgL-1) is

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the density of permeate water.

w t  A  

(1)

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Salt rejection was measured using a 2000 ppm NaCl solution as the feed water. An

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Ultrameter II (Myron L company, Carlsbad, CA) was used to determine the

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conductivity of the feed water (Cf) and that of the permeate (Cp), respectively.

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Membrane rejection R was calculated by:


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R  (1 

Cp Cf

) 100%

(2)

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Antibacterial assessment.

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All membranes were immersed in deionized water for 24 h before the assessment of

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their anti-microbial activity. A Gram-positive Bacillus subtilis 168 (ATCC 27370) and

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a Gram-negative Escherichia coli K12 (ATCC 10798) were as the model bacteria

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Membrane coupons were immersed in the cell suspension that was incubated on a

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rotary shaker (150 rpm) at room temperature (10 h for B. subtilis and 24 h for E. coli).

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Viable cells in the suspension were quantified using the colony forming unit (CFU)

208

method (CLSI M07-A935). Experimental results were obtained from three

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independent replicates.

34

.

210 211

Diffusion inhibition zone (DIZ) tests were also performed to assess the bactericidal

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effects of the membranes following a modified CLSI M07-A9 method.35 Briefly,

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aliquots (100 µL) of bacterial culture were spread onto an LB agar plate. Membrane

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disk samples (diameter = 12.7 mm) were then placed onto the plate with their active

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layers in contact with the agar surface. After incubation at optimal temperature (37 ºC

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for E. coli and 30 ºC for B. subtilis) for 24 h, the colonies formed under the membrane

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samples were examined.

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Silver loading quantification and silver leaching tests.

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To measure the total amounts of silver immobilized on the membrane, AgNPs

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incorporated membrane coupons (3.8 cm2) were placed in glass vials containing 0.2

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ml 70% HNO3 in 20 ml deionized water. The vials were placed on a reciprocal

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shaking bath (Unitronic Reciprocal 6032011, J.P. Selecta, S.A., Barcelona, Spain)

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under 200 rpm for one day. Subsequently, the dissolved silver concentration was

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quantified using an inductive coupled plasma optical emission spectrometer (ICP-

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OES, Optima 8x00, Perkin Elmer).

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Ag leaching tests were performed to evaluate the stability of the AgNPs. Membrane

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coupons (3.8 cm2) were immersed in 20 ml deionized water agitated at 200 rpm. The

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soaking solution was replaced daily, and the collected water samples were filtered

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through a 0.45 m membrane and acidified by 1% HNO3 before ICP-OES analysis.

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

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Membrane characterization.

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Figure 2 presents the SEM micrographs of the control membrane XLE, the coated

236

membranes PDAn, and the silver-immersed membranes PDAnAg. The control

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membrane XLE had a ‘ridge and valley’ roughness structure. PDA coating did not

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have obvious effect on the surface morphology, except for the relatively long coating

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duration of 2 h at which some aggregates of PDA were apparent. Similar phenomena

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of PDA aggregation have been observed at relative long coating duration in the

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existing literature.36-39

RESULTS AND DISCUSSION

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243 244 245 246 247 248

Fig. 2. SEM micrographs (plan view) of control membrane XLE, the PDA coated membranes (PDA0.5, PDA1 and PDA2), and the silver incorporated membranes (PDA0.5Ag, PDA1Ag, and PDA2Ag). For the silver incorporated membranes, the corresponding EDS spectrum is shown in the insert of each micrograph. The scale bar of all micrographs is 1 µm.

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250

After the PDA-coated membranes were immersed in the AgNO3 solution, additional

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fine particles appeared on the membrane surfaces (Figure 2). Elemental analysis by

252

EDS (inserts of Figure 2) and XPS (Supporting Information S1) suggests that these

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nano-sized particles were AgNPs. The spontaneous immobilization of silver can be

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attributed to the reducing catechol groups contained in PDA.21 According to Yang et

255

al.24, electrons released by the oxidation of catechol to quinone can reduce silver ions

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in the solution phase. At the same time, the O- and N-based ligand sites in PDA could

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serve as anchors for the resulting AgNPs.40, 41

258 259

Figure 3a-d presents the TEM micrographs (plan view) of the control membrane XLE

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and the silver incorporated membranes. Consistent with the SEM micrographs (Figure

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2), the TEM micrographs also show the ‘ridge and valley’ structure of the polyamide

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rejection layer. For the silver incorporated membranes, spherical AgNPs with

263

relatively uniform size can be clearly observed. These particles were homogeneously

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distributed over the membrane surface. In addition, increasing the coating time of

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PDA led to increased loading of AgNPs, which can be attributed to the enhanced PDA

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deposition and thus the increased concentration of catechol groups. TEM cross-

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sections of XLE, PDA2, and PDA2Ag are presented in Figure 3e-g (see additional

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cross sections in Supporting Information SI3). The PDA coating thickness was on the

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order of 20-30 nm after 2-h coating, which agrees with the literature20 that the coating

270

layer grows at an initial rate of approximately 10 nm/h. We were not able to observe


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major changes in PDA thickness or conformation on the basis of TEM cross-sections.

272 (e)

(f)

(g)

273 274 275 276 277

Fig. 3. TEM micrographs. Plan view (a-d): control membrane XLE (a) and the silver incorporated membranes PDA0.5Ag (b), PDA1Ag (c), and PDA2Ag (d). Crosssectional view (e-g): control membrane XLE (e), PDA-coated membrane PDA2 (f), and silver incorporated membrane PDA2Ag (g). The scale bar of all micrographs is 200 nm.



278

The TEM micrographs were further analyzed to determine the size and quantity of the

279

AgNPs immobilized on the membrane surface. As shown in Figure 4a, these in situ

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reduced AgNPs had a relatively narrow size distribution, with particle size generally

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on the order of 10-20 nm. The particle number density, i.e., the number of particles

282

found within a unit area, increased from 6.8 × 107 to 12.9 × 107 #/cm2 as the PDA

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coating time increased from 0.5 to 2 h. The increase in particle number density led to

284

a significantly increased surface coverage by AgNPs (from 7.9 ± 1.9 % for PDA0.5Ag

285

to 18.5 ± 0.6 % for PDA2Ag, see Figure 4b). Such high surface coverage by AgNPs

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has been seldom reported for conventional silver loading methods including simple

287

blending.17

288 289 290 291

Fig. 4. Characterization of AgNPs. The size distribution of AgNPs (a) was obtained by image analysis of the corresponding TEM micrograph (shown in the insert). The particle number density and surface coverage of AgNPs (b) were also based on image


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analysis of TEM micrographs. The elemental percentage of silver and the silver mass loading (c) were obtained by EDS analysis and silver dissolution experiments, respectively.

295 296

Further characterization of silver loading was performed using EDS analysis and

297

silver dissolution tests (Figure 4c). Consistent with TEM observations, the silver

298

dissolution tests revealed a greatly increased mass loading of silver (from 4.5 ± 0.2

299

µg/cm2 for PDA0.5Ag to 13.3 ± 0.3 µg/cm2 for PDA2Ag). A similar trend was also

300

identified on the basis of the EDS elemental analysis.

301 302

Membrane hydrophilicity and separation performance.

303

The membrane contact angle was strongly affected by the PDA coating and silver

304

loading (Figure 5a). A 2-h PDA coating effectively reduced the contact angle from

305

45.1 ± 2.2º (XLE) to 24.7 ± 5.4º (PDA2), which can be attributed to the hydrophilic

306

catechol, quinone, and amine groups in PDA.42,

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AgNPs further reduced the contact angle (13.5± 2.4º for PDA2Ag). The effect of

308

AgNPs on improving membrane hydrophilicity has been reported in the literature.18,

309

44, 45

43

The in situ immobilization of

According to a recent review on nanocomposite membranes,19 the inclusion of

310

AgNPs can significantly improve the wettability of membrane surface as a result of

311

their hydrophilic nature.

312



313 314 315 316 317 318

Fig. 5. Effect of PDA coating and silver loading on membrane properties. (a) Water contact angle, (b) water flux and salt rejection. Membrane filtration was performed at 20 MPa. Water flux was obtained using deionized feed water, and rejection tests were performed for feed solutions containing 2000 ppm NaCl, (c) The resulting membrane contains uniformly dispersed silver nanoparticles.

319

No major effect by the PDA coating was observed with short duration ( 1 h) within

320

the experimental uncertainty (Figure 5b). However, increasing coating time to 2 h

321

resulted in a flux reduction of nearly 20%, which is likely due to the formation of a

322

relatively thick PDA layer. The excessive formation of PDA aggregates (Figure 2)

323

may also partially explain the reduced membrane flux. The incorporation of AgNPs

324

into the PDA layer decreased water flux from 54.3 ± 1.4 L/m2h (PDA2) to 32.1 ± 2.8

325

L/m2h (PDA2Ag). In comparison, the effect of silver loading on water flux was much

326

milder at lower PDA coating time. The trend observed in the current study may be


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explained by the competing effects of (1) the increased loading of impermeable

328

AgNPs (Figure 4b) that blocks water pathways and (2) the improved membrane

329

hydrophilicity (Figure 5a) that promotes water passage.

330 331

The PDA coating reduced the NaCl rejection, with greater loss observed at longer

332

coating time (Figure 5b). Although charge interaction is a plausible cause for the

333

reduced rejection, there were no major differences in the measured zeta potential of

334

the control and PDA-coated membranes (Supporting Information S4). The current

335

result suggests that the PDA layer was more permeable to NaCl compared to the base

336

membrane, which is consistent to the existing literature reports on the use of PDA as

337

nanofiltration (NF) rejection layers.46 Indeed, the combination of a relatively salt-

338

permeable PDA layer and a high rejection polyamide layer can lead to the

339

accumulation of rejected NaCl within the PDA layer and thus the loss of salt rejection,

340

a phenomenon known as cake enhanced concentration polarization.47 Regardless of

341

the PDA coating time, the AgNPs incorporated membranes consistently showed

342

improved rejection compared to their respective counterpart without silver loading.

343

Such enhancement of rejection implies a reduction of salt flux in addition to the loss

344

of water flux, which is likely due to the simultaneous blockage of salt flux and water

345

pathways (i.e., defects in the PDA layer) by the impermeable AgNPs. The effect of

346

defect sealing upon in situ immobilization of silver can be explained by a self-healing

347

mechanism (Figure 5c): a defect region that is more permeable to NaCl is also more



348

accessible to silver ions, which leads to enhanced localized AgNPs formation to heal

349

the defect.

350 351

To facilitate the further understanding of the rejection behavior of the coated

352

membrane, a simple resistance-in-series model is developed in Supporting

353

Information S5. According to the model, whether the coating improves or decreases

354

the rejection of the coated membrane depends on the A/B value of the coating in

355

comparison to the A/B value of the base membrane, where A is the water permeability

356

and B is the salt permeability. Rejection improves if (A/B)coating > (A/B)base. Sealing the

357

defects in the coating layer can greatly increase (A/B)coating due to greatly decreased B

358

value of the coating, thus leading to improved overall rejection. On the other hand,

359

excessive silver loading may lead to severe loss of water permeability (low Acoating),

360

leading to a reduced (A/B)coating and thus a reduction in rejection compared to the base

361

membrane (e.g., PDA2Ag vs. XLE).

362 363

Antibacterial tests.

364

Diffusion inhibition zone tests clearly demonstrate the antibacterial effect of the

365

AgNPs incorporated membranes (Figure 6). For both B. subtilis and E. coli, colonies

366

developed under the silver-free control membranes XLE and PDA2. In contrast, no

367

apparent bacterial growth under the membrane was observed for all the silver

368

incorporated membranes (PDA0.5Ag, PDA1Ag, and PDA2Ag). CFU tests also


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showed compelling antibacterial effect of the silver loaded membranes (Figure 7).

370

The exposure to PDA2Ag led to a reduction of 62.7 ± 9.3% for viable B. subtilis and

371

42.4 ± 5.7% for E. coli.

372

373 374 375 376 377

Fig. 6. Diffusion inhibition zone test for Gram-positive B. subtilis and Gram-negative E. coli. Membrane disks (diameter = 12.7 mm, rejection layer facing downwards) were placed onto agar plate spread with bacterial culture. A 24 h incubation time was used in the tests.

378



379 380 381 382 383 384

Fig. 7. Colony forming units (CFU) tests for Gram-positive B. subtilis and Gramnegative E. coli. Membrane disks (diameter = 12.7 mm) were placed into cell suspension (cell density of approximately 3000 cells/mL for B. subtilis and 2.0 × 108 cells/mL for E. coli). After culturing at room temperature (10 h with B. subtilis or 24 h with E. coli), the cultivated cells in suspension were quantified using CFU method. The reported data is the average value of three replicates.

385 386


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Implications.

388

We developed a green and facile method to prepare anti-microbial RO membrane.

389

Silver ions from an AgNO3 solution was spontaneously immobilized by the reducing

390

catechol groups in a PDA coating layer without the need to use additional strong

391

reductants and aggressive chemicals. The ability of PDA to coat onto a wide variety

392

of substrates20 implies that this method can be potentially used to modify membranes

393

with different surface chemistries and morphological features. Future studies may

394

further explore its use for modifying other membrane types (e.g., nanofiltration and

395

pervaporation membranes) as well as different membrane base materials (e.g.,

396

inorganic membranes in addition to polymeric membranes). The conditions for PDA

397

coating and silver incorporation may be further optimized to provide a fine control on

398

the silver loading (e.g., the size and number density of AgNPs). The recharge of

399

AgNPs upon their depletion can be a potential challenge. Future studies shall address

400

this problem, for example, by the use of catechol-containing polymers that can

401

sacrificed and re-coated to allow silver recharge. In addition, long-term stability of

402

separation performance needs to be assessed.

403 404

We further report a self-healing mechanism that involves the preferential formation of

405

AgNPs at the defect sites of a PDA coating layer due to their greater accessibility to

406

silver ions. The sealing of the salt pathway is responsible for the enhanced membrane

407

salt rejection. At the meantime, the water permeability of the PDA-AgNPs modified



408

membranes was not significantly affected at short PDA coating time (e.g., 0.5 h). The

409

current study demonstrated the feasibility of using surface modification to achieve

410

improved salt rejection without sacrificing the water permeability. This method may

411

be further extended to direct defects sealing of RO and NF membranes where

412

damages can be repaired locally to maintain the membrane integrity,48 and a wide

413

range of other metals or metal oxides based nanoparticles can be potentially utilized

414

for such purpose.

415 416



417

Support Information. S1. X-ray photoelectron spectroscopy; S2. Silver leaching

418

tests; S3. TEM cross sectional images; S4. Zeta potential measurements; S5.

419

Resistance-in-series model. This material is available free of charge via the Internet at

420

http://pubs.acs.org.

ASSOCIATED CONTENT

421 422



423

Corresponding Author

424

*Phone: (+852) 2859 1976; e-mail: [email protected]

425

Notes

426

The authors declare no completing financial interest.

AUTHOR INFORMATION

427


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Page 25 of 29


428



429

The study receives financial support from the Innovation and Technology

430

Commission of the Hong Kong Government (Project # ITS/208/14) and the General

431

Research Fund of the Research Grants Council (Project # 17207514). The partial

432

funding support from Strategic Research Theme on Clean Energy at the University of

433

Hong Kong is also appreciated. We acknowledge the help on ICP-OES measurements

434

by Ms. Hanlu Yan and Dr. Kaimin Shih at the Department of Civil Engineering, the

435

University of Hong Kong. The XPS used in this work was supported by the Institute

436

for Advanced Materials (IAM) with funding support by the Special Equipment Grant

437

from the University Grants Committee of the Hong Kong Special Administrative

438

Region, China (SEG_HKBU06).

ACKNOWLEDGMENTS

439



440

Reference

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