Fabrication of antifouling membrane surface by poly(sulfobetainemethacrylate)/polydopamine co-deposition

Rong Zhou 1, Peng-Fei Ren 1, Hao-Cheng Yang, Zhi-Kang Xu n

MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science & Engineering, Zhejiang University,Hangzhou 310027, China

a r t i c l e i n f o

Article history:Received 4 February 2014Received in revised form15 April 2014Accepted 19 April 2014Available online 25 April 2014

Keywords:Poly(sulfobetaine methacrylate)PolydopamineMicroporous polypropylene membraneSurface modificationAntifouling

a b s t r a c t

Poly(sulfobetaine methacrylate) (PSBMA) has been widely employed for the surface modification ofmembranes due to its excellent antifouling property. However, challenges still remain to simplify themodification processes and to increase the utilization efficiency of PSBMA (or sulfobetaine methacrylate,SBMA). In this paper, a simple one-step co-deposition process is introduced to fabricate antifoulingsurfaces for microporous polypropylene membranes (MPPMs) based on the self-polymerization and highadhesion properties of dopamine with the hydrophilicity of PSBMA. Meanwhile, the effect of PSBMAconcentration on the membrane surfaces was studied in detail by ATR/FT-IR, XPS, and FESEM. Significantimprovement is demonstrated for the surface hydrophilicity by results of water contact angle and purewater flux. Dynamic protein filtration experiments confirm the excellent antifouling property of theresulted membranes. Furthermore, the utilization efficiency reaches 9.13 wt% for PSBMA, 10 times higherthan that of SBMA for UV-induced grafting on MPPM. In conclusion, the one-step modification methodprovides a simple and effective approach to construct antifouling surfaces for membranes.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

Membrane fouling usually results in the reduction of separa-tion efficiency and increase of maintenance and operation costs;therefore, it has been regarded as one of the main obstacles forwidespread application of membrane technologies [1]. Variousmethods have been developed to investigate, simulate and controlmembrane fouling [2–5]. Generally, membrane fouling is initiatedby the physical and chemical interactions between membranesurfaces and particulates, colloidal particles or biomacromoleculesin separation solutions [6]. Subsequently, nonspecific adhesion ofbiomacromolecules and microorganisms takes place on the mem-brane surfaces, resulting in diminished or even blocked membranepores and then a sharp decrease of permeation or separationefficiency. The situation becomes even worse for hydrophobicmaterials such as polypropylene, polyethylene and poly(vinylidenefluoride), which are common commercial materials for preparingmicrofiltration and ultrafiltration membranes. To our knowledge,hydrophilic modification is a common method and effectivesolution to fabricate antifouling surfaces for these membranes[4,5,7,8]. The hydrophilic surfaces have been found to form tightly

bounded water layers on the membrane and then repel bioma-cromolecules (such as proteins) adsorption via repulsive hydrationforces [9–11]. Although several methods have been reported forsurface modification, further efforts should be made to developmuch more versatile strategies [12–15].

Water-soluble polymers, typically including poly(ethylene gly-col) (PEG) [16] and poly(vinyl pyrrolidone) (PVP) [17], have beenwidely used as hydrophilization agents in membrane preparationand surface modification. In recent years, zwitterionic polymerssuch as poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC)[18], poly(sulfobetaine methacrylate) (PSBMA) [19], and poly(car-boxybetaine methacrylate) (PCBMA) [20] have been regarded as anew generation of antifouling materials owing to their highresistance to the nonspecific adsorption of proteins and theirreversible adhesion of microorganisms [18–21]. It has beenhypothesized that, with both positively and negatively chargedmoieties, the zwitterionic polymers can bind water moleculesmore strongly via electrostatically induced hydration than PEGchains [22]. Jiang and co-workers introduced PSBMA brushes ontothe gold/glass surfaces by physical adsorption [23] or surface-initiated ATRP [24,25]. They found that these PSBMA surfaces cangreatly reduce protein adsorption. Moreover, PSBMA has beensuccessfully attached on the surfaces of poly(vinylidene fluoride)[26,27], polypropylene [28], polyethersulfone [29] and AnoporeTM

hybrid membranes [30]. “graft from” and “graft to” methodologiesare the most common methods to fabricate PSBMA modified

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Journal of Membrane Science

http://dx.doi.org/10.1016/j.memsci.2014.04.0320376-7388/& 2014 Elsevier B.V. All rights reserved.

n Corresponding author. Tel.: þ86 571 8795 2605.E-mail address: xuzk@zju.edu.cn (Z.-K. Xu).1 The two authors contributed equally to this work.

Journal of Membrane Science 466 (2014) 18–25

surfaces. The “graft-from” method usually immobilizes initiatorsonto the membrane surfaces and then initiates SBMA monomersto polymerize into PSBMA brushes [24,25]. The alternative “graft-to” requires synthesizing PSBMA with reactive group and bindingthese polymer chains onto the membrane surfaces by chemicalreaction [31]. Obviously, both methods require multi-step proce-dures and, therefore, a more convenient process without anylimitation in materials and morphologies or damages to themembrane is still required for attaching PSBMA on the membranesurfaces.

Since 2007, it has been found that dopamine (DPA) can be oxidizedunder alkaline conditions and spontaneously polymerizes to form athin, surface-adhering polydopamine (PDA) film on a wide spectrumof materials with various morphologies [32–37]. Several attempts havealready beenmade to use PDA films for fabricating antifouling surfaceson membranes [38–40]. However, the films were usually formed atfirst and then served as anchoring layers for subsequent graftingamine- [38,39] or thiol-ended polymers [40] using the “graft-to”method. As mentioned above, this two-step process is relativelycomplicated for the demand of synthesizing polymers with amine-or thiol-groups. Most recently, a one-step method has been developedfor the multipurpose surface functionalization of materials by theco-deposition of PDA with other chemicals [41]. These chemicalsinclude tertiary amine for mineralization, quaternary ammonium forantibacterial, ATRP initiator for further grafting, growth factor fortissue regeneration, and polysaccharide for cell adhesion. It is not clearwhether this method is simple and/or versatile for constructingantifouling membrane surfaces with PSBMA. We report in this workthe PDA-assisted deposition of PSBMA onto microporous polypropy-lene membrane (MPPM) to improve the antifouling property. Asschematically shown in Fig. 1, it was simply achieved by immersingMPPM samples in a one-pot mixture of dopamine alkaline solutionwith PSBMA of different concentrations. The PSBMA/PDA co-depositedcoatings on the membrane were characterized by attenuated totalreflectance Fourier transform infrared (ATR/FT-IR) spectroscopy, X-rayphotoelectron spectroscopy (XPS) and field emission scanning electronmicroscopy (FESEM). Moreover, we also investigated the surfacehydrophilicity and antifouling performance of the modified mem-branes. Overall, this work indeed offers a versatile approach for PSBMAimmobilization onto porous membranes for antifouling purpose.

2. Experimental

2.1. Materials

MPPM was a commercial product from Membrana GmbH(Germany) with an average pore size of 0.20 μm, thickness of

160 μm and a relatively high porosity of about 75%. All membranesamples used were cut into rounds with a diameter of 25 mm,washed by acetone overnight to remove impurities adsorbed onthe membrane surfaces and then dried in a vacuum oven at 40 1Cto a constant weight. Dopamine hydrochloride and N-(3-sulfopro-pyl)-N-(methacryloxyethyl)-N,N-dimethyl ammonium betaine(sulfobetaine methacrylate, SBMA, 97%) were purchased fromSigma-Aldrich (USA) and used without further purification. Bovineserum albumin (BSA, pI 4.8, 67 kDa), hemoglobin (Hgb, pI 7.0,65 kDa), lysozyme (Lys, pI 10.8, 14.4 kDa) and Tris (hydroxymethyl)aminomethane (Tris) were acquired from Sinopharm ChemicalReagent Co. Ltd, China. Phosphate-buffered saline (PBS, pH 7.4,ionic strength 10 mM) was prepared from analytical-grade chemi-cals and ultrapure water (18.2 MΩ, produced from an ELGA LabWater system, France). Other reagents were of analytical grade andused as received.

2.2. Synthesis of PSBMA

PSBMA was simply synthesized by free radical polymerization.The aqueous solution containing SBMA (10%, w/v) was placed in awater bath heated to 70 1C and degassed with bubbling N2 for30 min at the same time. Then potassium persulfate (0.4%, w/v)was added to initiate the polymerization for 10 h. The polymeriza-tion mixture was dialyzed against water to remove residualmonomer and oligomers. Finally it was lyophilized to obtain theproduct.

2.3. Dopamine-assisted deposition of PSBMA onto MPPMs

The typical procedure was similar to that reported by Kanget al. [41]. MPPM samples were first dipped into ethanol to makeall membrane pores wet thoroughly. Then they were immersedinto a series of reaction solutions with DPA/PSBMA mixturesdissolved in Tris buffer (pH 8.5, 50 mM). DPA concentration waskept at 2 mg/mL and the molar ratio of PSBMA/DPA (SBMA unit todopamine) varied from 1:0 to 0:11:1, 5:1 and 10:1 in the solutions.These mixtures were put into an open glass dish to provide acontinuous supply of oxygen through air/solution interface. Theco-deposition process was performed under vibration for 18 h at25 1C. Thereafter, the PSBMA/PDA-co-deposited membranes(PSBMA/PDA-modified MPPM) were taken out, rinsed with ultra-pure water and ethanol alternately by gentle shaking. The sampleswere dried under vacuum at 40 1C to a constant weight. Thedeposited density (DD, mg/cm2) was calculated by the followingequation:

DD¼W1�W0

Að1Þ

where W0 is the weight (mg) of the nascent membrane, W1 is theweight (mg) of the modified membrane, A represents the area(4.91 cm2) of the membrane. Each result is an average of at leastthree parallel experiments.

2.4. Characterization of the membrane surface

The membrane surface was characterized by a series of tech-nologies. Vibration spectra were measured by an attenuated totalreflectance Fourier transform infrared spectroscopy (ATR/FT-IR,Nicolet FT-IR/Nexus470, USA) with an ATR accessory (ZnSe crystal,451). Each spectrumwas obtained in the region of 4000–500 cm�1

and collected by cumulating 32 scans at a resolution of 4 cm�1.XPS analyses were performed on an RBD upgraded PHI-5000CESCA system (Perkin Elmer, USA) with Al Kα radiation(hν¼1486.6 eV). In general, the X-ray anode was run at 250 Wand the high voltage was kept at 14.0 kV with a detection angle

Fig. 1. Schematic illustration of the one-step co-deposition of PSBMA/PDA forantifouling membrane surfaces.

R. Zhou et al. / Journal of Membrane Science 466 (2014) 18–25 19

at 541. The base pressure of the analyzer chamber was about5�10�8 Pa. To compensate for surface charging effect, all surveyspectra were referenced to the C 1s hydrocarbon peak at 284.6 eV.Surface morphologies of membranes were observed by a field-emitting scanning electron microscope (FESEM, Hitachi S4800,Japan) after being sputtered with a 10–20 nm gold layer. Watercontact angles (WCA) were determined with CTS-200 contactangle system (MAIST Vision Inspection & Measurement CO. Ltd.,China) at room temperature using the sessile drop method. Adroplet of 2 μL ultrapure water was carefully dropped onto the drymembrane with a micro-syringe, and then images of the waterdroplet were recorded and WCA was calculated with the specificsoftware. At least seven different surface locations of each samplewere measured to obtain a reliable result.

2.5. Antifouling properties measurements

To evaluate the antifouling properties of the modified MPPMsunder flow condition, dynamic protein filtration was performed,using a dead-end stirred-cell filtration system (Millipore 6700P05,USA). The effective area was 4.90 cm2 for each piece of membrane.Firstly, a compaction step was conducted for 30 min by filteringultrapure water at 0.50 MPa. Secondly, the pressure was loweredto 0.10 MPa and the ultrapure water flux (JW) was measured every5 min until reaching a constant value. Thirdly, the water in thevessel was replaced by 1.0 g/L BSA solution and the permeationflux (JP) was recorded until attaining an equilibrium value at0.10 MPa. To investigate the flux recovery property, PBS solutionwas used to clean the membrane for another 30 min at 0.10 MPa.Then, the reservoir was replaced with ultrapure water and thewater flux (JR) was measured again.

The relative flux reduction (RFR) and the flux recovery ratio(FRR) were calculated as follows:

RFR ð%Þ ¼ 1� JpJw

� �� 100 ð2Þ

FRR¼ JRJw

� �� 100 ð3Þ

Overall, membranes with lower RFR and higher FRR are regardedas having better antifouling performance.

The antifouling performance was also evaluated by staticprotein adsorption on the nascent and modified membranesamples. It was obtained from UV absorbance at 280 nm (specificabsorption wavelength for proteins) using a UV–vis spectrophot-ometer (UV-2450, Shimadzu Inc, Japan). BSA, Hgb and Lys werechosen as model proteins. Firstly, the nascent and modifiedmembranes were immersed into ethanol for 0.5 h. Secondly, thesamples were moved into phosphate buffer solution (PBS,pH¼7.0) overnight to exchange ethanol at room temperature.Finally, the samples were exposed to 3 mL protein (BSA/Hgb/Lys)solutions with a concentration of 1.0 mg/mL in PBS (pH¼7.0) for3 h at 25 1C. The protein adsorption capacity of the membranes (Q,mg/g) was defined as the amount of protein (mg) per gram (g) ofthe membrane weight and the value was calculated using thefollowing equation:

Q ¼ ðC0�CÞ � V � 1000=W ð4Þwhere C0 and C are the concentration (mg/mL) of protein solutionbefore and after adsorption, respectively, V is the volume (mL) ofprotein solution and W is the weight (mg) of the membrane.

2.6. Stability of the PSBMA/PDA co-deposited coating

To investigate the stability of the PSBMA/PDA co-depositedcoating, the modified membranes were rinsed by deionized water

in a shaken bath at 60 1C under 150 rpm. The samples were thentaken out, dried, and the weight change and pure water fluxmeasured once every few days. This procedure was continueduntil 27 days. Subsequently, compared with the original values,two parameters – mass retention ratio (MR) and flux retentionratio (FR) – were calculated by the following equations:

MR ð%Þ ¼ Wt

W1� 100 ð5Þ

FR ð%Þ ¼ JtJ1� 100 ð6Þ

whereW1 and J1 are the initial weight (mg) and pure water flux (L/m2 h) of the modified membrane, Wt and Jt are the weight (mg)and pure water flux (L/m2 h) of the membrane after long-termwashing.

3. Results and discussion

3.1. Fabrication and structures of the PSBMA/PDA-modifiedmembranes

PSBMA was synthesized by free radical polymerization, char-acterized by NMR (Fig. S1 in supplementary materials. 1H NMR(400 MHz, D2O, δ): 4.55 (2H, peak c), 3.86 (2H, peak d), 3.64 (2H,peak f), 3.28 (6H, peak e), 3.03 (2H, peak h), 2.32 (2H, peak g), 2.04(2H, peak a), 1.05–1.36 (3H, peak b)) and GPC (Table S1 insupplementary materials). It was mixed with DPA in Tris buffersolution under alkaline condition and then deposited onto thesurfaces of MPPM [41]. Several factors influence the co-depositionprocess, which include pH value, temperature, solution concentra-tion, and deposition time [42–44]. Since DPA tends to form freePDA particles at high temperature [33,42], the co-deposition wasconducted at 25 1C. The pH value was fixed at 8.5 to ensure theself-polymerization of DPA into PDA [44]. We then further inves-tigated the effects of deposition time (Fig. S2 in supplementarymaterials) and DPA concentration (Fig. S3 in supplementarymaterials) on the deposition density (DD) and the membraneproperties. The optimum deposition condition is at the concentra-tion of 2 mg/mL with a reaction time of 18 h. At this condition, themembrane properties can be improved significantly. Nevertheless,the concentration of PSBMA, which is vital to the co-depositionprocess, has not been studied yet. Herein, we studied the influenceof PSBMA concentration on membrane modification by varyingthe molar ratio of PSBMA/DPA (SBMA unit to DPA) from 1:0 to 0:1,1:1, 5:1 and 10:1 in the solution. Fig. 2 shows that when DPA ispresent in the solution, DD increases with the molar ratio ofPSBMA/DPA. However, DD is almost zero when the molar ratio ofPSBMA/DPA is 1:0. PSBMA can be hardly deposited on themembrane surfaces or be easily washed away without the assis-tance of DPA. It demonstrates that the cross-linked structures ofPDA via the self-polymerization of DPA are definitely necessary forPSBMA to participate in the co-deposition process. Furthermore,the effects of deposition time were also studied with the molarratio of PSBMA/DPA as 1:1 (Fig. S4 in supplementary materials).It seems that 18 h is optimal for the pure water flux of themembranes because the deposited PSBMA/PDA will block themembrane pores when DD is too high.

ATR/FT-IR, XPS and FESEM were used to characterize themorphology and chemical composition of the membrane surfaces.ATR/FT-IR and XPS spectra in Fig. 3 confirm the formation ofPSBMA/PDA co-deposited coatings on the membrane surfaces. Ascan be seen from Fig. 3A, there is no change in the spectrum ofMPPM deposited with 1:0 of PSBMA/DPA. The broad peak at about3400 cm�1 is the characteristic adsorption of amine N–H and

R. Zhou et al. / Journal of Membrane Science 466 (2014) 18–2520

phenolic –OH stretching vibrations in the spectra of PDA-modifiedMPPM and PSBMA/PDA-modified MPPM. The peak at 1608 cm�1

is assigned to the overlap of CQC resonance vibration in aromaticring and N–H bending vibrations. Besides, several new adsorptionsignals appear in the spectra of PSBMA/PDA-modified MPPMs,namely O–CQO stretching peak at 1725 cm�1, SQO asymmetricstretching peak at 1160 cm�1 and SQO symmetric stretchingpeak at 1031 cm�1, respectively. Moreover, the relative intensity of

Fig. 4. Effect of PSBMA/DPA molar ratios on PSBMA/PDA molar ratio (A) anddeposited density of PSBMA and PDA (B) for the modified membranes.

Table 1Surface chemical composition of the unmodified and modified MPPMs from XPSspectra (in at%).

Sample DD(mg/cm2)

C 1s(%)

O 1s(%)

N 1s(%)

S 2p(%)

O/C S/N

Nascent MPPM – 97.9 2.1 – – 0.02 –

PDA (theoretic value)a – 72.7 18.2 9.1 – 0.25 –

PDA-modified MPPM 0.259 75.3 18.5 5.4 – 0.25 –

PSBMA (theoreticvalue)a

– 61.1 27.8 5.6 5.6 0.45 1

PSBMA-modified MPPM 0.000 97.0 3.0 – – 0.03 –

PSBMA/PDA-modifiedMPPMs

0.411 68.0 23.2 5.7 3.0 0.34 0.530.453 68.5 23.1 5.3 3.2 0.34 0.600.521 66.4 24.6 5.4 3.6 0.37 0.67

a The composition is calculated by molecular formula.

Fig. 3. ATR/FT-IR (A) and XPS (B) spectra of the nascent MPPM (a), PSBMA-modified MPPM (b), PDA-modified MPPM (c) and PSBMA/PDA-modified MPPMs(d, e, and f) with PSBMA/DPA molar ratios of 1:1, 5:1 and 10:1, respectively.

Fig. 2. Effect of PSBMA/DPA molar ratios on the deposited density for the modifiedmembranes.

R. Zhou et al. / Journal of Membrane Science 466 (2014) 18–25 21

the above characteristic peaks increases with the PSBMA concen-tration, which denotes a corresponding increase of PSBMA in themodified coatings qualitatively. XPS analyses also indicate PSBMAand PDA have been successfully deposited on the membranesurfaces. Typical results are shown in Fig. 3B and Table 1. For thenascent membrane, a major peak at 284.6 eV is ascribed to thebinding energy of C 1s. For PDA-modified MPPM, an additionalpeak is obvious at 401.0 eV for N 1s. Meanwhile, the contentof carbon element decreases from 97.9% to 75.3% on the PDA-modified membrane surface and the percentage of oxygen ele-ment increases dramatically from 2.1% to 18.5%. As a result, the O/Cratio increases distinctly from 0.02 to 0.25. When PSBMA wasadded for co-deposition, the content of oxygen element furtherincreases on the near surface of PSBMA/PDA-modified MPPM,along with the appearance of a new peak for S 2p3. Moreover,the percentage of sulfur element rises gradually with the feedmolar proportion of PSBMA/DPA.

Herein, we propose that the surface composition of thedeposited coating analyzed by XPS is in accordance with that ofthe inner layer. Therefore, we can calculate the amount of PSBMAdeposited onto the membrane by the theoretical ratio of N/C andN/S of PDA and PSBMA, respectively. The result in Fig. 4A showsthe molar ratio of PSBMA to PDA (SBMA unit to dopamine unit) on

the membrane surfaces increases with the proportion of PSBMA toDPA in the solution. DD of PSBMA and PDA shown in Fig. 4B is alsofigured out based on the overall DD and the molar ratio of PSBMAto PDA. PSBMA on the membrane surface can be as much as0.274 mg/cm2 when the PSBMA/DPA proportion was 1:1, andincreases alongside with the proportion. The density of PSBMAequals to those modified by the UV-induced graft polymerizationmethod [28]. And the utilization efficiency of PSBMA is about9.13 wt% in this convenient way, which is 10 times higher than0.65 wt% of SBMA in the grafting method [28]. DD of PDAdecreases a little with PSBMA participation, owing to the fact thatPSBMA could, to some extent, slow down the PDA polymerizationvia non-covalent interaction, which is in agreement with the 1HNMR results (Fig. S5 in supplementary materials).

Fig. 5 shows FESEM images of the surfaces of unmodified (a) andmodified membranes (b–f) with different molar ratios of PSBMA/PDA, respectively. The average pore size is about 0.20 μm for MPPMused in this work (Fig. 5a). No significant change is observed for themembrane deposited with 1:0 of PSBMA/DPA (Fig. 5b). The pore sizeseems to decrease slightly after PDA deposition (Fig. 5c), because themembrane surfaces are wrapped with a thin layer of PDA. Thisphenomenon becomes much obvious when the membrane is co-deposited with PSBMA/PDA (Fig. 5d–f).

Fig. 5. FESEM images of the surface morphologies for the nascent MPPM (a), PSBMA-modified MPPM (b), PDA-modified MPPM (c) and PSBMA/PDA-modified MPPMs (d, e,and f) with different PSBMA/PDA molar ratios (1.1, 1.5, and 2.1).

R. Zhou et al. / Journal of Membrane Science 466 (2014) 18–2522

3.2. Surface hydrophilicity and water flux of the membranes

WCA was used to characterize the hydrophilicity of the mem-brane surfaces. Fig. 6 shows that the nascent membranes have aWCA of 1451, owing to the combination of the intrinsic hydro-phobicity of polypropylene and the high porosity/roughness of themembrane surface. However, it drops sharply below 201 after themembranes are deposited with PDA or PSBMA/PDA. In addition,pure water flux was also measured to evaluate the hydrophilicityof the studied membranes. The flux of nascent MPPMs is917745 L/m2 h at 0.1 MPa. After the membrane was modified byPDA (DD¼0.252 mg/cm2), it reaches a flux of 48987173 L/m2 h,5 times of the nascent one. The water flux even reaches5939787 L/m2 h when the membrane was co-deposited withPSBMA/PDA (DD¼0.521 mg/cm2, PSBMA/PDA¼2.1/1.0). It is dueto the positively and negatively charged moieties carried byPSBMA, which can be strongly hydrated through ionic solvation.Despite the hydrophilicity of the membrane surfaces, pore size andporosity of membranes also affect the pure water flux, whichexplains the phenomenon that the flux increases slightly with theincrease of DD.

3.3. Antifouling properties of the PSBMA/PDA-modified MPPMs

Dynamic protein filtration was performed after pre-filtration byultrapure water to evaluate the antifouling property of the studiedmembranes. BSA was used as the model protein. Table 2 lists theRFRs and FRRs of the membranes. RFR is 76.20% for the nascentMPPM, indicating that a large amount of BSA was absorbed ontothe membrane surfaces due to the strong hydrophobic interac-tions. At the same time, most of the adsorbed BSA is irreversible asillustrated by only 40.30% of FRR. The membranes show a slightincrease in FRR (60.04%) after modification by PDA only, which stilldoes not endow the membranes with good antifouling property.However, RFR drops to 30.47% and FRR rises to 83.52% for the

PSBMA/PDA-modified membrane (DD¼0.453 mg/cm2, PSBMA/PDA¼1.5/1.0). It can be seen that all PSBMA/PDA-modified MPPMsshow excellent antifouling property, while the molar ratio ofPSBMA/PDA has obvious effect on RFR and slight influence onFRR, respectively.

Static protein adsorption was also measured to achieve acomprehensive understanding of the antifouling property for thePSBMA/PDA-modified MPPMs. BSA, Hgb and Lys were used as themodel proteins, each of them was differently charged in PBS (pH7.0). In fact, they have negative (BSA, pI 4.8), neutral (Hgb, pI 7.0),and positive (Lys, pI 10.8) charges in PBS (pH 7.0). Fig. 7 shows theamount of protein adsorbed on the unmodified and modifiedmembranes. It can be seen that the PSBMA/PDA-modified MPPMspresent admirable resistance to the adsorption of BSA and Hgb.However, all membranes, especially the PDA-modified MPPM,show more quantity of adsorbed Lys than those of BSA and Hgb.It is already known that the pI of PDA is close to 4.0 [45]. The PDA-modified MPPMs should be negatively charged in PBS (pH¼7.0).Therefore, the membrane surfaces will adsorb Lys via electrostaticinteraction. It is surprising for us that the influence of PDA stillexists in the case of PSBMA/PDA-modified MPPM with PSBMA/PDA¼1.1/1.0. These membranes do not perform well in proteinresistance to Lys but show great protein resistance against BSAand Hgb.

3.4. Stability of the PSBMA/PDA co-deposited coating

It is important to sustain the antifouling properties duringpractical application for membranes. Therefore, the stability ofPSBMA/PDA co-deposited coating was examined by washing themodified membranes in a shaken water bath at 60 1C for differentperiods. Mass retention ratio (MR) and flux retention ratio (FR),representing the change of mass and water flux of the membranes,respectively, are used to monitor the stability of the coatingand the permeation property of the modified membrane. Fig. 8indicates a small decrease of mass and an increase of flux can be

Table 2Relative flux reduction (RFR) and flux recovery ratio (FRR) of the unmodified and modified MPPMs during BSA protein filtration.

Membrane DD (mg/cm2) PSBMA (mg/cm2) PDA (mg/cm2) RFR (%) FRR (%)

Nascent MPPM – – – 76.20 40.29PDA-modified MPPM 0.252 – 0.252 61.12 60.04PSBMA/PDA-modified MPPM 0.411 0.274 0.137 54.81 86.25

0.453 0.332 0.121 30.47 83.520.521 0.412 0.109 37.16 83.51

Fig. 7. Protein (BSA, Hgb and Lys) adsorption quantity on the nascent MPPM, PDA-modified MPPM and PSBMA/PDA-modified MPPM (PSBMA/PDA¼1.1/1.0).

Fig. 6. Pure water flux (0.1 MPa) and water contact images of the nascent andmodified MPPMs with different deposited densities of PSBMA.

R. Zhou et al. / Journal of Membrane Science 466 (2014) 18–25 23

observed at the beginning. A possible reason is that a smallamount of particles, deposited in the membrane pores, is easilywashed away by hot water. Therefore the membrane pores arebroadened and result in an increase in pure water flux. However,the mass and flux almost remain unchanged during the wholeexperiment, even after the membrane was continuously subjectedto hot water for 27 days. These results demonstrate that PSBMA/PDA-modified MPPM has an excellent stability for long-termoperation in aqueous environment and shows potential usage inpractical applications.

4. Conclusion

Co-deposition of PSBMA in PDA-based coating is put forward asa facile and efficient one-step approach to fabricate antifoulingsurfaces for commercial hydrophobic membranes. This endowsthe membranes with excellent hydrophilicity, low water fluxreduction and high water flux recovery. And the modified mem-branes display distinct adsorption behaviors when faced withdifferent charged proteins, which can be of great potential forapplication in protein separation. Additionally, the PSBMA/PDA co-deposited coatings also show good stability in a long-term wash-ing. In conclusion, this one-step modification method acquiressignificant advantages over the existing ones for constructingantifouling membrane surfaces.

Acknowledgments

This work was supported by a financial support from theNational Natural Science Foundation of China (Grant no.50933006).

Appendix A. Supplementary information

Supplementary data associated with this article can be found inthe online version at http://dx.doi.org/10.1016/j.memsci.2014.04.032.

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