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Journal of Membrane Science 349 (2010) 258–267

Contents lists available at ScienceDirect

Journal of Membrane Science

journa l homepage: www.e lsev ier .com/ locate /memsci

thermo-responsive affinity membrane with nano-structured pores and graftedoly(N-isopropylacrylamide) surface layer for hydrophobic adsorption

ao Meng, Rui Xie, Yong-Chao Chen, Chang-Jing Cheng, Peng-Fei Li, Xiao-Jie Ju, Liang-Yin Chu ∗

chool of Chemical Engineering, Sichuan University, Chengdu, Sichuan 610065, PR China

r t i c l e i n f o

rticle history:eceived 18 September 2009eceived in revised form3 November 2009ccepted 24 November 2009vailable online 29 November 2009

eywords:hermo-responsive membranesydrophobic adsorptionoly(N-isopropylacrylamide)ano-structured poresurface wettability

a b s t r a c t

A novel thermo-responsive affinity membrane with nano-structured pores and grafted poly(N-isopropylacrylamide) (PNIPAM) surface layer is successfully fabricated for hydrophobic adsorptionthrough the membrane surface wettability change tuned by environmental temperature. Shirasu porousglass (SPG) membranes with mean pore size of 1.8 �m are used as substrate membranes. Nano-structuredpore surfaces are formed by depositing 125 nm SiO2 nano-particles onto the SPG membrane pore surfaces.PNIPAM brushes are then grafted on the nano-structured pore surfaces of membranes by plasma-inducedgrafting polymerization method. The formation and microstructures of the prepared membranes areinvestigated systematically by employing XPS, SEM, contact angle instrument, and mercury intrusionmethod. The results show that SiO2 nano-particles and PNIPAM-grafted layer are formed homoge-neously on the SPG membrane pore surfaces. When the environmental temperature is 20 ◦C (belowthe lower critical solution temperature, LCST), the PNIPAM-grafted nano-structured membranes presentvery hydrophilic surfaces with water contact angle of 0◦; on the other hand, when the environmen-tal temperature is 40 ◦C (above the LCST), the PNIPAM-grafted nano-structured membranes present veryhydrophobic surfaces with water contact angle of 130◦. The thermo-responsive hydrophilic/hydrophobicsurface wettability change of the prepared membranes is reversible and reproducible. Temperature-

controlled hydrophobic-adsorption performance of the prepared membranes is investigated by studyingthe adsorption/desorption behavior of Bovine serum albumin (BSA) on the membrane surfaces withchanging the environmental temperature across the LCST. The PNIPAM-grafted nano-structured mem-branes show satisfactory “adsorbing at temperature above the LCST – desorbing at temperature below theLCST” performance, and the desorption efficiency is as high as about 90%. The nano-structured architec-tures of the membrane pore surfaces are verified to be beneficial for the thermo-responsive hydrophobic

les.

adsorption of BSA molecu

. Introduction

Affinity membranes are membranes that can identify and sepa-ate specific molecules. In the fields of separation and purificationf protein, enzyme, chiral substance, hydrophobic solutes ando on, affinity membranes have been widely studied [1–7]. Itas been reported that thermo-responsive affinity membranesith poly(N-isopropylacrylamide) (PNIPAM) functional surface

ayers can be used for hydrophobic adsorption and separationf hydrophobic solutes [8], because the surfaces of PNIPAM-rafted membranes can change from hydrophilic to hydrophobichen the environmental temperature increases across the lower

∗ Corresponding author at: School of Chemical Engineering, Membrane ScienceFunctional Materials Laboratory, Sichuan University, Chengdu, Sichuan 610065,

R China. Tel.: +86 28 8546 0682; fax: +86 28 8540 4976.E-mail address: chuly@scu.edu.cn (L.-Y. Chu).

376-7388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.memsci.2009.11.058

© 2009 Elsevier B.V. All rights reserved.

critical solution temperature (LCST) of PNIPAM and vice versa.Choi et al. [8] showed that PNIPAM-grafted porous polypropy-lene membranes could reversibly adsorb nonionic surfactantpoly(oxyethylenenonylphenyl ether) (NP-10) molecules at temper-atures above the LCST of PNIPAM, and desorb them at temperaturesbelow the LCST. The results show a simple and promising approachfor affinity separation of hydrophobic solutes by using PNIPAM-grafted thermo-responsive membranes.

Recently, it has been verified that the micro- and nano-structures on the surfaces of some natural plants such as lotusleaves contribute significantly to their surface superhydropho-bicity [9]. Inspired by such natural phenomena, micro- andnano-structures have been applied to achieve artificial superhy-

drophobic surfaces [9]. For PNIPAM-modified functional surfaces,it has been found that micro- and/or nano-structured sur-faces could enhance the thermo-responsive wettability changebetween hydrophobicity and hydrophilicity [10–13], i.e., micro-and/or nano-structured surface architectures could make the

brane

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T. Meng et al. / Journal of Mem

NIPAM-modified surfaces more hydrophilic at temperatureselow the LCST but more hydrophobic above the LCST. For example,urface-initiated atom-transfer radical polymerization was usedo fabricate thermo-responsive PNIPAM thin film on both flat andano-structured silicon substrates, and the results showed that theater contact angle of the flat surface increased from 63◦ to 93◦

hen the temperature changed from 25 ◦C to 40 ◦C, while the waterontact angle values of the nano-structured surface were about 0◦

elow 29 ◦C and about 150◦ above 40 ◦C [10]. Such results showpromising approach for improving the hydrophobic-adsorptionerformance of PNIPAM-grafted thermo-responsive membranes.hat is, if we introduce nano-structures onto the membrane sur-aces and membrane pore surfaces before grafting the PNIPAMurface layer, the reversible hydrophobic-adsorption/hydrophilic-esorption performance could be improved by the enhancedwitching of thermo-responsive hydrophilic/hydrophobic sur-ace wettability and the nano-structured surface architectures.owever, such attempt to improve the hydrophobic-adsorptionerformance of PNIPAM-grafted thermo-responsive membranesas not been reported yet.

Herein, we report on the fabrication and characterizationf a novel thermo-responsive affinity membrane with nano-tructured pores and grafted PNIPAM surface layer for hydrophobicdsorption. The schematic illustration of preparation and thermo-esponsive surface property change of the proposed membrane ishown in Fig. 1. Shirasu porous glass (SPG) membranes are used ashe substrate membranes, SiO2 nano-particles are deposited ontohe SPG membranes to construct the nano-structured surfaces of

embrane pores, and then PNIPAM brushes are grafted onto theano-structured membrane pores. SPG membranes are obtained

y phase separation of a primary CaO–Al2O3–B2O3–SiO2 type glass,ade of Shirasu (volcanic ash from the southern part of Kyushu

sland, Japan), calcium carbonate and boric acid [14]. SPG mem-ranes are a special kind of porous inorganic glass membranes, soiO2 nano-particles can be deposited on hydroxylated SPG mem-

ig. 1. Schematic illustration of preparation of the thermo-responsive affinity membrandsorption. (a) Hydroxylated substrate SPG membrane, (b) nano-structured SPG membraneiO2 particles layer by layer, (d) PNIPAM-grafted nano-structured SPG membrane with htructured SPG membrane with hydrophilic surface at temperature below the LCST.

Science 349 (2010) 258–267 259

brane surfaces by chemical deposition. When the nano-particlesare deposited on the SPG membranes, the smooth membranesurfaces turn to nano-structured concavo-convex rough surfaces.The proposed nano-structured membrane surface in this studycould bring three main advantages for the thermo-responsivehydrophobic-adsorption/hydrophilic-desorption performance ofBovine serum albumin (BSA) molecules by using PNIPAM-graftedmembranes. First, the nano-structured membrane surface canenhance the surface hydrophilicity of PNIPAM-grafted membraneat temperature below the LCST by the capillary effect from thenano-concave-spaces between adjacent SiO2 nano-particles asnano-capillaries. Second, the nano-structured membrane surfacecan provide more adsorption surface because of the increaseof the total specific surface area. Third, the nano-structuredconcavo-convex surface can provide a BSA molecule with two orthree stereo sites for its adsorption on the membrane surface byhydrophobic interaction, which enhances the thermo-responsivehydrophobic-adsorption performance of PNIPAM-graftedmembranes.

In the present study, SiO2 nano-particles with average diameterof 125 nm are deposited onto the SPG membranes to generate uni-form and compact nano-structured membrane surfaces, and PNI-PAM is grafted on the surfaces and in the pores of nano-structuredSPG membranes by plasma-induced grafting polymerization. Theformation and microstructures of the prepared membranes areinvestigated systematically by employing XPS, SEM, water contactangle instrument, and mercury intrusion method. The thermo-responsive hydrophobic-adsorption/hydrophilic-desorption per-formance of membranes is studied by investigating thermo-responsive adsorption and desorption behavior of BSA on the

prepared membranes with changing the environmental tem-perature across the LCST. The results show that the preparedPNIPAM-grafted nano-structured SPG membranes possess satis-factory thermo-responsive hydrophobic-adsorption/hydrophilic-desorption performance for BSA molecules.

e with nano-structured pores and grafted PNIPAM surface layer for hydrophobicby depositing SiO2 particles once, (c) nano-structured SPG membrane by depositing

ydrophobic surface at temperature above the LCST, and (e) PNIPAM-grafted nano-

2 brane

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60 T. Meng et al. / Journal of Mem

. Experimental

.1. Materials

The N-isopropylacrylamide (NIPAM) was kindly provided byohjin Co., Ltd., Japan, and was used after purification by recrys-

allization in hexane and acetone and then drying in vacuumt room temperature. Chlorotrimethylsilane ((CH3)3SiCl3, CTMS,hengdu Kelong Chemical Industrial Reagent Co., China) was used

or hydrophobic modification. SPG membranes with the mean poreize of 1.8 �m and membrane thickness of 1 mm were purchasedrom SPG Technology Co., Ltd., Miyazaki, Japan. BSA (moleculareight of 68 kDa, Shanghai Bio Life Science & Technology Co. Ltd.,hina) was used as a model protein for the thermo-responsiveydrophobic-adsorption experiments. Deionized water (18.2 M�,illi-Q, Millipore) was used throughout the experiments.

.2. Fabrication of nano-structured hydrophilic and hydrophobicat glass surfaces

To investigate the effect of nano-structured architecture or theano-particle effect on the surface wettability of glass substrate,ano-structured flat glass surfaces were fabricated by depositingiO2 onto the glass substrates. The nano-structured glasses wereabricated in a two-step process. First, hydroxyl groups were gen-rated on the surfaces of the glasses by treating them with a slightlyoiled piranha solution (7:3 mixture of 98% H2SO4 and 30% H2O2)15] at 90 ◦C for 90 min, and then rinsing with abundant amount ofater and blowing with nitrogen gas, and finally drying in vacuum

ven at 50 ◦C for 12 h. Then, all the nano-structured glasses wereade by dip-coating the SiO2 nano-particle solution on the glass

urfaces, using a withdrawal speed of approximately 14 cm/min16]. The SiO2 nano-particle solution was prepared by the well-nown Stober method [17], and the recipe and the proceduredopted were briefly as follows: 3.5 ml of deionized water, 46.5 mlf ethanol and 1 ml of ammonia (25 wt%) were mixed well first,nd afterward 3 ml of tetraethyl orthosilicate (TEOS, Si(OC2H5)4,iO2 content ≥28%) were added with agitation, and then the reac-ion mixture was stirred at 25 ◦C for 48 h. Subsequently, the treatedlasses were calcinated in an oven at 210 ◦C for 2 h, and theninsed with abundant amount of water and blown with nitro-en gas, and finally dried in vacuum oven at 50 ◦C for 12 h. Thushe glass surfaces with nano-structured hydrophilic surface layerere obtained. Nano-structured layer with uniform and compact

iO2 nano-particles deposited on the surfaces could be obtainedy repeating the nano-particle deposition process [15]. To getano-structured hydrophobic surfaces for the glass, CTMS mono-

ayer was generated onto the deposited SiO2 nano-particles byydrophobic modification with the following procedure [16]. Theano-structured hydrophilic glasses were treated with a commer-ial CTMS solution (0.2 M CTMS in hexane) at room temperature forh, and then washed with ethanol to remove the redundant CTMSolecules and dried in vacuum oven at 50 ◦C for 12 h.

.3. Preparation of PNIPAM-grafted nano-structured SPGembranes

The proposed PNIPAM-grafted nano-structured SPG mem-ranes were fabricated in a three-step process. In the first step,ydroxyl groups were generated on the pore surfaces of SPG mem-ranes by treating the SPG substrate membranes with a slightly

oiled piranha solution at 90 ◦C for 90 min (Fig. 1a). In order to fillhe piranha solution in the membrane pores, the SPG membranesere immersed into piranha solution with ultrasonic treating. Theiranha solution was the mixture of 98 wt% H2SO4 and 30 vol% H2O2v:v = 7:3) [15]. The treated membranes were rinsed with abundant

Science 349 (2010) 258–267

water and blew with nitrogen gas, and then dried in vacuum ovenat 50 ◦C for 12 h.

In the second step, the SPG membranes with nano-structuredpores were fabricated by depositing SiO2 nano-particles ontothe membrane pore surface (Fig. 1b). In order to fill the nano-particle solution in the membrane pores, the SPG membraneswere immersed into SiO2 nano-particle solution with ultrasonictreating. The SiO2 nano-particle solution was prepared by theabove-mentioned Stober method [17]. The recipe and procedurefor preparing the SiO2 nano-particle solution were the same asmentioned above for treating flat glasses. After ultrasonic treating,the SPG substrate membranes were removed from the SiO2 nano-particle solution and calcined in an oven at 210 ◦C for 2 h, and thenrinsed with abundant amount of water and blew with nitrogen gas,and finally dried in vacuum oven at 50 ◦C for 12 h. As a result, SiO2nano-particles were deposited onto the pore surfaces of the SPGmembranes. Nano-structured SPG membranes with uniform andcompact SiO2 nano-particles deposited on the pore surfaces couldbe obtained by repeating the nano-particle deposition process [15](Fig. 1c). In this study, the nano-particle deposition process wasoperated thrice.

In the third step, plasma-graft pore-filling polymerization wasemployed to graft the linear PNIPAM chains onto nano-structuredSPG membrane pores according to the method described previously[18–25]. Briefly, the nano-structured SPG membrane was placedin a transparent glass tube, which was filled with argon gas andevacuated to a pressure of 10 Pa beforehand, and then was treatedby plasma at 50 W for 60 s. After that, the membrane was dunkedinto the monomer solution, and the grafting polymerization tookplace in a shaking constant-temperature bath at 40 ◦C for a prede-termined time. The feed weight ratio of NIPAM and deionized waterwas 1:100, and the polymerization time was 6 h to get membraneswith thin PNIPAM film. The grafted membrane was rinsed in well-deionized water under vibration in a constant-temperature bath(30 ◦C) for 24 h to remove any unreacted monomer and homopoly-mer, and then was dried in a vacuum oven at 50 ◦C overnight.The PNIPAM-grafted nano-structured SPG membrane could thenexhibit thermo-responsive surface property (Fig. 1d and e). Thegrafting yield (YPNIPAM) of PNIPAM onto the membrane was mea-sured by weighing the ungrafted and grafted membranes withan electronic balance (PR 5002, Mettler Toledo, Germany). Here,YPNIPAM of the membrane is defined as the weight increase per-centage of membrane after grafting and is calculated according tothe following equation:

YPNIPAM = Wg − W0

W0(1)

where Wg and W0 stand for the mass of membrane after and beforegrafting PNIPAM surface layer respectively [mg].

2.4. Characterization of modified flat glasses and membranes

The microstructure and surface wettability of the modified flatglasses were investigated by scanning electron microscope (SEM,JSM-5900LV, JEOL, Japan) and contact angle instrument (DSA100,Krüss, Germany). All the samples were gilt before SEM observing.To study the surface wettability, a small drop of water (3 �l) wasdripped onto the glass surface at 20 ◦C, the whole process wasrecorded by the contact angle instrument. The contact angle val-ues of the examined glasses in this study were all the arithmeticaverage values of five repetitive tests on the same glass sample.

The microscopic configurations of the substrate and PNIPAM-grafted nano-structured SPG membranes were investigated by SEM(JSM-5900LV, JEOL, Japan). To observe the inner surfaces of mem-branes, the substrate and PNIPAM-grafted nano-structured SPGmembranes were fractured mechanically before sticking to the

brane

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T. Meng et al. / Journal of Mem

ample holder. All the samples were gilt before observing. Theean pore sizes and the total pore surface area of the membranes

efore and after modification were measured by the mercury intru-ion method (AutoPore IV 9500, Micromeritics Instrument, USA).PS (XSAM800, KRATOS, UK) was employed to study the chem-

cal composition of membrane surfaces before and after graftingNIPAM.

At temperatures below and above the LCST of PNIPAM respec-ively, the surface wettability of the prepared thermo-responsiveffinity membrane with nano-structured pores and grafted PNIPAMurface layer were investigated by measuring the water contactngle on the membrane surface with a contact angle instrumentDSA100, Krüss, Germany). Before the test, the examined mem-rane was pre-wetted without any obvious water on the surfacend then fixed on the sample holder, which was heated up to aredetermined temperature. After the temperature of membranequilibrated at the predetermined temperature, a small drop ofater (3 �l) was dripped onto the surface of membrane. Simul-

aneously, the whole process was recorded by high-speed videoamera. The contact angle value of the examined membrane in theaper was the arithmetic average value of five repetitive tests onhe same membrane. The temperatures were selected as 20 ◦C and0 ◦C respectively, which were lower and higher than the LCST ofNIPAM (around 32 ◦C).

.5. Thermo-responsive hydrophobic adsorption

Thermo-responsive hydrophobic-adsorption performance of

he PNIPAM-grafted nano-structured SPG membrane was stud-ed by carrying out dynamic adsorption experiments using BSAs solute [26,27]. In the adsorption experiments, the environmen-al temperatures were changed across the LCST of PNIPAM (20 ◦Cnd 40 ◦C respectively). The adsorption experiment started imme-

ig. 2. Effect of nano-structured architecture on the surface wettability of glass substraarticles for one time (a2) and three times (a3), (b) water contact angle of substrate glasimes (b3) before hydrophobic modification, and (c) water contact angle of substrate glaimes (c3) after hydrophobic modification with CTMS. The volume of water droplet for m

easurement is 20 ◦C.

Science 349 (2010) 258–267 261

diately after immersing the examined membrane into 15 ml ofBSA aqueous solution (BSA concentration 1.0 mg ml−1), which wasconstantly at 40 ◦C (above the LCST) and well stirred. At fixedtime intervals, a small amount of solution was taken out and itsabsorbance was analyzed by UV-visible spectrophotometer (UV-9600, Rayleigh) at a wavelength of 280 nm. After the membranereached the adsorption equilibrium at 40 ◦C, the temperature of BSAsolution was promptly changed to 20 ◦C (below the LCST). After themembrane reached the adsorption equilibrium at 20 ◦C, the tem-perature changed again for another thermo-responsive adsorptionrun. From the adsorption of BSA on the membrane with the envi-ronmental temperature changing as 40 ◦C → 20 ◦C → 40 ◦C → 20 ◦C,thermo-responsive hydrophobic-adsorption performance of theprepared membrane could be obtained.

3. Results and discussion

3.1. The effect of nano-structured architecture on the surfacewettability of glass substrate

Fig. 2 shows the nano-particle effect on the surface wettabilityof flat glass substrate. Before hydrophobic modification with CTMS,the glass surfaces are hydrophilic due to the hydroxyl groups onthe surfaces. Compared with that of the substrate glass, the watercontact angles on the glass surfaces deposited with SiO2 nano-particles are smaller (Fig. 2b). Because of the capillary effect fromthe nano-concave-spaces between adjacent SiO2 nano-particles asnano-capillaries, the nano-structured glass surfaces become more

hydrophilic. When the glass surface is treated by nano-particledeposition thrice, the nano-particles form on the glass surfacemore compactly than that only treated once (Fig. 2a), the nano-capillary effect between the particles becomes more significant;as a result the glass surface becomes more hydrophilic than that

tes. (a) SEM images of substrate glass (a1) and glasses deposited with SiO2 nano-s (b1) and glasses deposited with SiO2 nano-particles for one time (b2) and threess (c1) and glasses deposited with SiO2 nano-particles for one time (c2) and three

easuring the contact angle is 3 �l, and the temperature during the contact angle

262 T. Meng et al. / Journal of Membrane Science 349 (2010) 258–267

F AM-gs icles is

tcmwoetmbto

3m

asanwmogsoei1mgdpPbmu

ig. 3. SEM images of inner surfaces of substrate SPG membrane (a and b) and PNIPize of substrate SPG membrane is 1.8 �m. The average diameter of SiO2 nano-part

reated by nano-particle deposition only once (Fig. 2b). On theontrary, the SiO2 nano-particles deposited on the glass surfacesake the surfaces more hydrophobic after modifying the surfacesith CTMS (Fig. 2c), because the 2-D nano-structured architectures

n the surfaces constructed by the deposited nano-particles couldnlarge the water contact angle on the surfaces [9]. In summary,he nano-structured architecture makes the hydrophilic surface

ore hydrophilic and the hydrophobic surface more hydropho-ic. That is, the nano-structured architecture is beneficial for thehermo-responsive hydrophilic/hydrophobic wettability switchingf PNIPAM-grafted surfaces.

.2. Morphological analysis and chemical characterization ofembranes

Fig. 3 shows SEM images of inner surfaces of the substratend PNIPAM-grafted nano-structured SPG membranes. For theubstrate SPG membrane (Fig. 3a and b), pore surface is rel-tively smooth; on the other hand, for the PNIPAM-graftedano-structured SPG membrane (Fig. 3c and d), SiO2 nano-particlesith average diameter of 125 nm are obviously observed on theembrane pore surfaces. From the above results on the effect

f nano-structured architecture on the surface wettability of flatlass substrate, it is found that the more the nano-particle depo-ition times, the more compact the SiO2 nano-particles formedn the glass surface, and as a result the more significant theffect of nano-structured architecture on the surface wettabil-ty. For the modification of SPG membranes with pore size of.8 �m, it is improper to deposit the SiO2 nano-particles tooany times, because too many nano-particles might cause clog-

ing of the membrane pores. In this study, the nano-particleeposition is operated thrice, which is optimized from certain

rimary experimental results. It is clearly seen that the pores ofNIPAM-grafted nano-structured SPG membrane are not cloggedy the deposited SiO2 nano-particles and grafted PNIPAM poly-ers, and the stereo-structures of membrane pores are almost

nchanged after depositing the SiO2 nano-particles and grafting

rafted nano-structured SPG membrane (YPNIPAM = 0.1 wt%) (c and d). The mean pore125 nm.

the PNIPAM polymers. The SiO2 nano-particles are found to beuniformly and compactly covered on the membrane pore sur-face.

The mercury intrusion measurement results show that thecalculated mean pore size of substrate SPG membrane and thatof PNIPAM-grafted nano-structured SPG membrane are 1.57 �mand 1.56 �m respectively, and the specific surface areas are5.668 m2 g−1 and 6.814 m2 g−1 respectively. It is verified againthat the mean pore size of substrate SPG membrane and that ofthe PNIPAM-grafted nano-structured SPG membrane is almost thesame. On the other hand, the total surface area of PNIPAM-graftednano-structured SPG membrane increases by 36% compared withthat of substrate SPG membrane, which indicates that the nano-structured SPG membrane could provide more adsorption surfacearea for the hydrophobic adsorption than the substrate mem-brane.

The XPS spectra of nano-structured SPG membranes beforeand after grafting PNIPAM (YPNIPAM = 0.1 wt%) are shown in Fig. 4.For the C1s spectra of the ungrafted nano-structured SPG mem-brane (Fig. 4a), there is only one peak with binding energy of284.715 eV (C atom in C–H bond). On the other hand, for theC1s spectra of the PNIPAM-grafted nano-structured SPG mem-brane (Fig. 4b), two new peaks appear with binding energies of286.860 eV (C atom in C–N bond) and 288.370 eV (C atom in C Obond), respectively. Similarly, there is only one peak in the O1sspectra of ungrafted nano-structured SPG membrane (Fig. 4c) withbinding energy of 532.619 eV (O atom in Si−O bond), while anew peak with binding energy of 531.538 eV (O atom in C Obond) appears in O1s spectra of PNIPAM-grafted nano-structuredSPG membrane (Fig. 4d). For the N1s spectra, there is no peakappears for the ungrafted membrane (Fig. 4e), while a new peakappears with the binding energy of 399.626 eV (N atom in C–N

bond) for the PNIPAM-grafted nano-structured SPG membrane(Fig. 4f). Compared with those of the ungrafted membrane, thecomponents of C and N elements of the PNIPAM-grafted membraneincrease from 11.79% to 58.49% and from 0.00% to 6.29% respec-tively, while the component of O element decreases from 47.08%

brane

tbs

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obP4SPfiisbhbCs

T. Meng et al. / Journal of Mem

o 20.43%. From the above data, we can verify that PNIPAM haseen successfully grafted onto the nano-structured SPG membraneurface.

.3. Thermo-responsive surface wettability change ofNIPAM-grafted nano-structured SPG membranes

Fig. 5 shows the snapshots of dynamic contacting processesf water droplets onto the surfaces of substrate SPG mem-rane at 20 ◦C, nano-structured SPG membrane at 20 ◦C, andNIPAM-grafted nano-structured SPG membranes at 20 ◦C and0 ◦C respectively. All the water contact angles of the substratePG membrane, ungrafted nano-structured SPG membrane andNIPAM-grafted nano-structured membrane at 20 ◦C become 0◦

nally, although the time periods for the water droplet spread-ng and/or disappearing processes are different. Because theubstrate SPG membrane, ungrafted nano-structured SPG mem-

rane and PNIPAM-grafted nano-structured membrane at 20 ◦C areydrophilic and porous, the water droplets spread fast on the mem-rane surfaces and finally are wicked into the membrane pores.ompared with the substrate SPG membrane (Fig. 5a), the nano-tructured architecture of the nano-structured SPG membrane

Fig. 4. XPS spectra of ungrafted nano-structured SPG membrane (a, c and e) and PNIP

Science 349 (2010) 258–267 263

(Fig. 5b) promotes the water droplet spreading and disappearingprocess. The water contact angle of the substrate SPG membranebecomes 0◦ at 2 s, while that of the nano-structured SPG membranebecomes 0◦ in a time period as short as 24 ms. For the PNIPAM-grafted nano-structured membrane at 20 ◦C (Fig. 5c), although thenano-particle effect is still working, the grafted PNIPAM chains areswollen in the membrane pores; as a result the time period forthe water droplet being totally wicked into the membrane poresis relatively longer (being 8 s). However, for the PNIPAM-graftednano-structured membrane at 40 ◦C (Fig. 5d), although the mem-brane is also porous, the water contact angle on the membranesurface is not changed with time and constantly keeps as 130◦.

Under steady states, for the surface of the PNIPAM-grafted nano-structured SPG membrane, the water contact angles are about0◦ at 20 ◦C (below the LCST) and about 130◦ at 40 ◦C (above theLCST). It is well known that thermo-responsive PNIPAM polymerchains are in the swollen and hydrophilic state at temperatures

lower than the LCST, while they become shrunken and hydropho-bic at temperatures higher than the LCST. It has been reported thatnano-structures on hydrophilic surface will enhance the surfacehydrophilicity by the capillary effect from nano-capillaries [28,29].On the surface of as-prepared PNIPAM-grafted nano-structured

AM-grafted nano-structured SPG membrane with YPNIPAM = 0.1 wt% (b, d and f).

264 T. Meng et al. / Journal of Membrane Science 349 (2010) 258–267

F to difm and

SnpvOhoi

Prwstwaab

ig. 5. Snapshots of dynamic contacting processes of water droplets (3 �l each) onembrane, and (c and d) PNIPAM-grafted nano-structured SPG membranes at 20 ◦C

PG membrane, the nano-concave-spaces between adjacent SiO2ano-particles might act as nano-capillaries. Therefore, the pre-ared PNIPAM-grafted nano-structured SPG membrane exhibitsery hydrophilic surface wettability (contact angle = 0◦) at 20 ◦C.n the other hand, at 40 ◦C, the nano-structures will enhance theydrophobicity of the membrane surfaces because of the existencef nano-particles on the surfaces [9], and the water contact angles as high as 130◦.

The thermo-responsive surface wettability change of theNIPAM-grafted nano-structured SPG membrane is highlyeversible and repeatable. Fig. 6 shows the thermo-responsiveater contact angle changes of the PNIPAM-grafted nano-

tructured SPG membrane surface by repeatedly changing the◦ ◦

emperature between 20 C and 40 C. Within 10 runs, the average

ater contact angle on the membrane surface at 20 ◦C is alwaysbout 0◦ and that at 40 ◦C is always about 130◦. That is to say, thes-prepared PNIPAM-grafted nano-structured SPG membrane cane used repeatedly.

ferent membrane surfaces. (a) Substrate SPG membrane, (b) nano-structured SPG40 ◦C respectively. “CA” means contact angle.

3.4. Thermo-responsivehydrophobic-adsorption/hydrophilic-desorption performance

The thermo-responsive hydrophobic-adsorption/hydrophilic-desorption performance of PNIPAM-grafted nano-structured SPGmembranes is studied by investigating the dynamic adsorptionbehavior of BSA on the membrane with temperatures changingacross the LCST. The amount of BSA adsorbed on the membranesurface (Qm, mg m−2) is calculated by the following equation:

Qm = (C0 − Ct) · V

Am(2)

where C0 is the initial concentration of BSA aqueous solution at

beginning of the dynamic adsorption experiment (t = 0) [mg ml−1];Ct is the BSA concentration in solution at different time inter-vals (t = t) [mg ml−1]; V is the volume of BSA solution [ml];and Am is the membrane surface area [m2]. Both C0 and Ct

could be measured by UV-visible spectrophotometer, and the

T. Meng et al. / Journal of Membrane Science 349 (2010) 258–267 265

Fig. 6. Reversibly temperature-responsive change of water contact angle data ofPdT

mm

asPaa2t4s(vttmmcat

F(

NIPAM-grafted nano-structured SPG membrane surfaces. The water contact angleata are the average values of both inside and outside surfaces of the membrane.he volume of water droplet for measuring the contact angle is 3 �l.

embrane surface area is measured by the mercury intrusionethod.The experimental results of temperature-dependent dynamic

dsorption and desorption of model protein BSA on the sub-trate SPG membrane, the nano-structured SPG membrane and theNIPAM-grafted nano-structured SPG membrane (YPNIPAM = 0.1%)re shown in Fig. 7. The substrate SPG membrane nearly does notdsorb BSA molecules whether the environmental temperature is0 ◦C or 40 ◦C. Because the BSA molecules are macromolecules withhe molecular weight of 68 kDa and the molecular dimension ofnm × 4 nm × 14 nm [30], the smooth pore surface of SPG sub-

trate membrane is difficult to trap the protein macromoleculeFig. 8a). As for the ungrafted nano-structured membrane, the Qm

alue increases to about 0.5 mg m−2 within 3 h at beginning andhen stays the same no matter how the environmental tempera-ure changes. Compared with the substrate SPG membrane, BSA

olecules are easier to be adsorbed on the nano-structured SPG

embrane, because the nano-structured concavo-convex surface

an provide a BSA molecule with two or three stereo sites for itsdsorption on the membrane surface and also can provide lagerotal specific surface area for adsorption (Fig. 8b). However, it is

ig. 8. Schematic illustration of the adsorption of BSA molecules onto membranes with dc) PNIPAM-grafted nano-structured SPG membrane at temperature below the LCST, and (

Fig. 7. Temperature-dependent dynamic adsorption and desorption of BSA on thesubstrate SPG membrane, the nano-structured SPG membrane and the PNIPAM-grafted nano-structured SPG membrane.

obvious from Fig. 7 that there is no thermo-response for the adsorp-tion of BSA on the ungrafted nano-structured SPG membrane.

For the PNIPAM-grafted nano-structured SPG membrane, theequilibrium Qm value at 40 ◦C is about 2.95 mg m−2, whichis much larger than that of ungrafted nano-structured SPGmembrane (0.5 mg m−2). It is attributed to the hydrophobicPNIPAM-grafted surface of the PNIPAM-grafted nano-structuredSPG membrane at temperature higher than the LCST, becauseBSA adsorption is mainly resulted from the hydrophobic interac-tion. More importantly, the PNIPAM-grafted nano-structured SPGmembrane exhibits excellent thermo-responsive hydrophobic-adsorption/hydrophilic-desorption characteristics toward BSA. At

40 ◦C, the Qm value of the PNIPAM-grafted nano-structured SPGmembrane increases sharply to about 2.95 mg m−2 within the first6 h, and stays almost the same in the next 7 h. With decreasingthe environmental temperature to 20 ◦C, the Qm value goes down

ifferent surfaces. (a) Substrate SPG membrane, (b) nano-structured SPG membrane,d) PNIPAM-grafted nano-structured SPG membrane at temperature above the LCST.

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66 T. Meng et al. / Journal of Mem

apidly to about 0.25 mg m−2 within 6 h, and then stays almost theame in next 4 h. When the environmental temperature is heatedack to 40 ◦C, the Qm value goes up to about 2.95 mg m−2 again.hen the temperature is decreased to 20 ◦C once more, and the

m value also decreases again. The desorption performance of BSAolecules becomes a little worse in the second run mainly because

he membrane has been fouled by BSA molecules in the first run.ot all the BSA molecules adsorbed on the membrane in the first

un can desorb in the next run.It takes time for the adsorption and desorption to reach

quilibrium, which is due to the following two main reasons.n the one hand, BSA molecules are macromolecules with theolecular weight of 68 kDa and the molecular dimension ofnm × 4 nm × 14 nm [30]. Such a large dimension prevents the BSAolecules to move/adsorb/desorb fast. It has been reported that

he time for reaching adsorption/desorption equilibrium of BSAolecules on nonporous materials is usually in the range of 1–10 h,

nd the maximum is 16 h [31,32]. On the other hand, the SPG mem-rane is a kind of homogeneous porous membrane, i.e., the poresre of almost the same dimensions and are interconnected through-ut the whole membrane thickness [33]. The nano-structured andNIPAM-grafted surface layer has been introduced on the pore sur-aces throughout the whole thickness of the SPG membrane. Thats, the adsorption sites for BSA molecules are distributed through-ut the whole thickness of the membrane. It takes time for theSA molecules with large dimensions to diffuse through the zigzagores and adsorb to and/or desorb from those adsorption sites thatar away from the membrane surface.

In a word, the as-prepared PNIPAM-grafted nano-structuredPG membrane exhibits a satisfactory “adsorbing at temperaturebove the LCST–desorbing at temperature below the LCST” perfor-ance for BSA molecules, which is a simple and efficient mode

or the adsorption/desorption of hydrophobic solutes. At tem-erature higher than the LCST (40 ◦C in this study), the graftedNIPAM polymer chains on the nano-structured SPG membranere in shrunken and hydrophobic state; as a result, the BSAolecules with hydrophobic groups are easily adsorbed on theembrane pore surface by the hydrophobic interaction (Fig. 8d).n the other hand, at temperature lower than the LCST (20 ◦C

n this study), the grafted PNIPAM polymer chains on the nano-tructured SPG membrane become swollen and hydrophilic state,nd the nano-structured membrane pore surfaces could enhancehe surface hydrophilicity as mentioned above; therefore, thedsorbed BSA molecules on the membrane surface desorb fromhe very hydrophilic membrane pore surface due to the absencef hydrophobic sites on the membrane surface (Fig. 8c).

In this study, for the PNIPAM-grafted nano-structured SPGembrane with grafting yield of YPNIPAM = 0.1%, the difference

etween the maximum Qm value at 40 ◦C and the minimum Qm

alue at 20 ◦C is about 2.7 mg m−2. That is, more than 90% of BSAroteins adsorbed on the as-prepared membrane at 40 ◦C can beesorbed just by cooling the environmental temperature down to0 ◦C, which means a convenient and efficient approach for thedsorption/desorption of BSA proteins.

. Conclusions

In summary, thermo-responsive affinity membranes withano-structured pore surfaces and grafted PNIPAM surface layer

or hydrophobic adsorption have been successfully developed.

he nano-structured membrane pore surfaces are generated byepositing SiO2 nano-particles onto the SPG membrane pore sur-aces, and PNIPAM brushes are grafted on the nano-structuredore surfaces of membranes by plasma-induced grafting polymer-

zation method. SEM, XPS and mercury intrusion measurements

[

[

Science 349 (2010) 258–267

show that SiO2 nano-particles and PNIPAM-grafted layer areformed homogeneously on the SPG membrane pore surfacesand the stereo-structures of the membrane pores are nearly notaffected by the treatments. The nano-structures on the mem-brane pore surfaces are verified to be beneficial for improvingnot only the hydrophobicity at temperature above the LCST butalso the hydrophilicity at temperature below the LCST and ofPNIPAM-grafted membranes. The as-prepared PNIPAM-graftednano-structured membranes present very hydrophobic surfaceswith water contact angle of 130◦ at 40 ◦C but very hydrophilicsurfaces with water contact angle of 0◦ at 20 ◦C. Such thermo-responsive hydrophilic/hydrophobic surface wettability change ofthe prepared membranes is perfectly reversible and reproducible.Because of such a fantastic thermo-responsive surface property, theas-prepared PNIPAM-grafted nano-structured membranes exhibitsatisfactory “adsorbing at temperature above the LCST – desorbingat temperature below the LCST” performance for BSA molecules.The BSA adsorption capability of the prepared membrane (withgrafting yield of YPNIPAM = 0.1%) is about 2.95 mg m−2 in this study,and the desorption efficiency is higher than 90% by just decreasingthe environmental temperature from 40 ◦C to 20 ◦C. There shouldstill be some spaces for improving the performance indexes by opti-mizing the nano-structures on the membrane pore surfaces and thegrafting yields of PNIPAM. The results in this study provide valuableguidance for designing efficient thermo-responsive membranes forhydrophobic adsorption.

Acknowledgements

The authors gratefully acknowledge support from the NationalScience Fund for Distinguished Young Scholars (20825622), theNational Basic Research Program of China (2009CB623407),the National Natural Science Foundation of China (20674054,20806049), the Specialized Research Fund for the Doctoral Pro-gram of Higher Education by the Ministry of Education of China(200806101045), and Sichuan Youth Science and TechnologyFoundation for Distinguished Young Scholars (08ZQ026-042). Theauthors gratefully acknowledge the help of Ms. X.-Y. Zhang ofAnalytical and Testing Center at Sichuan University for the SEMmicrographs, Ms. J.-H. Li at Sichuan University for contact anglemeasurements, Mr. C. Xin at Chinese Academy of Science for XPSmeasurements, and also thank the Kohjin Co., Ltd., Japan, for kindlysupplying the N-isopropylacrylamide.

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