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aSchool of Chemistry and Chemical En

Technology, Guangzhou 510641, China. E-mbDivision of Polymer and Composite M

Technology and Engineering, Chinese Acad

E-mail: tao.chen@nimte.ac.cn; zhangjiawei@cFaculty of Materials Science and Chemical

315211, China

† Electronic supplementary informa10.1039/c4ra02826k

Cite this: RSC Adv., 2014, 4, 22759

Received 31st March 2014Accepted 13th May 2014

DOI: 10.1039/c4ra02826k

www.rsc.org/advances

This journal is © The Royal Society of C

Polymer brush functionalized Janus grapheneoxide/chitosan hybrid membranes†

Di Han,ab Peng Xiao,b Jincui Gu,b Jing Chen,b Zhiqi Cai,*a Jiawei Zhang,*b

Wenqin Wangc and Tao Chen*b

A robust and simple method is reported to prepare polymer brush

functionalized Janus graphene oxide (GO)/chitosan hybrid

membranes via the combination of interface self-assembly of GO and

chitosan, with subsequent self-initiated photografting and photo-

polymerization (SIPGP) from both sides of the GO/chitosan composite

membrane.

Named aer the two faced Roman God Janus, the concept ofJanus particles comprising multiple compositions and func-tionalities was introduced by P. G. de Gennes in his Nobellecture in 1991.1 The term of Janus has thus been extended todescribe materials having different properties at opposite sides,which attempt to mimic the behavior of certain molecules, suchas surfactants bearing opposite functionalities, leading tofascinating self-assembled behaviors.2 Over the past few years,Janus materials have attracted tremendous attention due totheir fantastic properties and potential applications in manyelds.3 Yang et al. have developed a series of facile approachesfor fabricating inorganic Janus nanosheets, which can serve assolid surfactants to stabilize the emulsion droplets.4–6 Zhaoet al. prepared Janus particles which could be anchored at theair–water interface and act as a exible barrier for preventingcoalescence of water droplets.7 Baraban et al. fabricated Janusparticles with magnetic caps which were capable of acquiringdirected motion by an external magnetic eld.8

Furthermore, chemistry in two dimensions differs signi-cantly from chemistry in three-dimensions, which results in therecent interest in thin free-standing two dimensional (2D) Janus

gineering, South China University of

ail: cezqcai@scut.edu.cn

aterials, Ningbo Institute of Material

emy of Science, Ningbo 315201, China.

nimte.ac.cn

Engineering, Ningbo University, Ningbo

tion (ESI) available. See DOI:

hemistry 2014

materials, such as nanosheets or nanomembranes. These nanomaterials even could be transported from one environment toanother one without losing their structural integrity, yetproviding new opportunities for 2D chemistry. Golzhauser et al.presented the selective chemical functionalization of free-standing nanomembrane with amino or thiol chemical func-tionalities on different sides to achieve a 2D Janus nano-membrane by modifying this nanosheet with differentuorescent dyes, respectively.9 Liu et al. reported the prepara-tion of Janus graphene by asymmetrically functionalizing gra-phene with halogen and phenyl groups.10 Sharma et al. havereported a methyltrimethoxysilane based route to growing aJanus silica lm at the oil–water interface, which shows aniso-tropic wetting by water on its two surfaces.11 These novel Janusstructures provide a platform for studying 2D chemistry andgraphene devices with multiple functions. However, despitemany desirable applications, fabrication of functional self-supporting 2D Janus nanosystems is still a dream of chemists. Amore robust and general approach is still highly required.

With abundant oxygen-containing groups on the basalplanes and edges, graphene oxide (GO) has been making aprofound impact in many areas of science and technologybecause of its remarkable physicochemical properties andpotential applications.12–14 Enormous effort has been focusedonto functionalizing GO nanosheets with chemical groups,15

polymer chains,16,17 nanoparticles and nanoplates.18,19 Chenet al. obtained modied GO with nonvolatile rewritable memoryeffect by attaching triphenylamine-based polyazomethine to GOsurface.20 Maser et al. graed poly(vinyl alcohol) to GO sheetsleading to tough GO based materials.21 Functionalization of GOnanosheets have created unexpected properties for advancedpotential applications. As excellent candidate for 2D chemistry,Janus asymmetric functions will bring us more surprises invarious applications. However, to the best of our knowledge,asymmetrically covalently attaching polymer chains to bothsides of the hybrid GO thin sheets has not been reported.

Recently, it was demonstrated that polymer brushes could beprepared even without a surface-bound initiator by self-initiated

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photograing and photopolymerization (SIPGP).22,23 Herein, wereport a robust and facile method to fabricate a polymer brushesgraed Janus graphene oxide/chitosan hybrid membrane whichis outlined in Fig. 1. GO/chitosan composite membrane wasobtained rstly by interface self-assembly of GO and chitosaninduced by electrostatic interaction,24 poly(styrene) (PS) andpoly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) werethen graed from the photoactive sites of upper surface and thelower surface of the GO/chitosan membrane by SIPGP,respectively.

GO nanosheets can be easily synthesized by exfoliatinggraphite according to a modied Hummers' method,25 and bedispersed uniformly in water for a long time. The thickness of1.2 � 0.2 nm and the diameters of 1 to several micrometers ofGO nanosheets was determined by atom force microscope(AFM) (Fig. S1†). With negative charge, GO nanosheets behavelike negatively charged polyelectrolytes that can interact withpositively charged polyelectrolytes to form stable complex. GO/chitosan composite membrane was thus prepared throughinterface self-assembly of GO with positively charged chitosansolution (Fig. S2†).

There are abundant hydroxyl, carboxyl and amine groups onthemembrane, which provides the possibility as photo active sitefor SIPGP to grow polymer brushes.22,23 By graing polymersthrough SIPGP from both sides of GO/chitosan membrane, apolymer brushes functionalized Janus GO/chitosan hybridmembrane was achieved. PS brushes were graed rstly fromthe upper side of silicon substrate supported GO/chitosanmembrane. Aer photopolymerization, the membrane wasrigorously rinsed with toluene to remove physisorbed polymerand monomer. The PS graed GO/chitosan was then etchedusing KOH solution (0.1 g mL�1) at 80 �C to achieve a free-standing membrane (Fig. 1a–c). Aer a reversal transfer (Fig. 1d),

Fig. 1 Schematic procedure of fabricating polymer brushes func-tionalized Janus graphene oxide/chitosan hybrid membrane. (a) GO/chitosan composite membrane is transferred from air–liquid interfaceto silicon wafer after the interface self-assemble. (b) PS brushes weregrafted fromGO/chitosan composite by SIPGP. (c) Themembranewasreleased from the substrate by heating in KOH solution. (d) Free-standing membrane is turned over and transferred onto silicon wafer.(e) PDMAEMA brushes were finally grafted from opposite side of PSbrushes to achieve the polymer brushes grafted Janus grapheneoxide/chitosan hybrid membrane.

22760 | RSC Adv., 2014, 4, 22759–22762

PDMAEMA brushes were subsequently graed from another side(Fig. 1e). The free-standing polymer brushes graed Janus GO/chitosan hybrid membrane was nally achieved aer the sameetching process (Fig. 2a). The membrane is strong enough to bemanipulated with tweezers and transferred to silicon wafer.Through asymmetrically bifacial cograing of hydrophilicPDMAEMA and hydrophobic PS on the opposite sides, Janusmembrane is supposed to exhibit anisotropic surface wettability.Water contact angle (CA) test was performed to investigatesurface CA evolution during the fabrication of polymer brushesgraed Janus GO/chitosan membrane. Once being graed withPS brushes, the CA was changed from original 60� (upper side ofGO/chitosan membrane) to �87� (Fig. 2b). Graing PDMAEMAbrushes from the lower surface leads to water CA decreasing from�65� (lower side of GO/chitosan membrane) to�17� (Fig. 2b). Toexhibit Janus membrane's anisotropic surface wettability moredirectly, two drops of water were applied to two sides of the Janusmembrane respectively. The difference of CA reaches as high as�70� which demonstrates dramatic disparity on surface proper-ties of two sides (Fig. 2b).

In order to investigate the surface morphology evolutionduring the fabrication of polymer brushes functionalized JanusGO/chitosan hybrid membrane, atom force microscope (AFM)and scanning electronic microscope (SEM) were used. AFMimages show that many irregular wrinkles exist on surfaces ofthe GO/chitosan membrane (Fig. S3†). Consistent with theresults of AFM, SEM image show that many wrinkles on uppersurface are highly crumpled to form ber-like structure(Fig. S4†), and crumpled wrinkles are also observed on the lowersurface (Fig. S5†). Some distinct raised structures are observedon both sides of GO/chitosan membrane. On the upper surface,the raised structures are isolated to each other with average sizeof dozens of nanometers (Fig. 3a). Hair-like structures arecovered on the lower surface (Fig. 3b). The size and shape ofthese raised structures can be tuned by varying the concentra-tion of chitosan solution. Aer modication, the highly crum-pled wrinkles on upper surface are not so obvious and island-like structures disappear (Fig. 3c). Compared with unmodiedstate, the lower surface becomes relatively smooth. The wrinklesand hair-like structures on it are not observed aer PDMAEMAbrushes are graed (Fig. 3d).

The membrane turned black from brown aer polymergraing,26 which can be ascribed to partial reduction of GO

Fig. 2 (a) Optical photographs of free-standing polymer brushesfunctionalized Janus GO/chitosan hybridmembrane and (b) two dropsof water on opposite sides of the Janus hybrid membrane.

This journal is © The Royal Society of Chemistry 2014

Fig. 3 AFM images of morphology of (a) upper and (b) lower surfacesof GO/chitosan membrane, (c) PS brushes and (d) PDMAEMA brushesgrafted from upper and lower surfaces of GO/chitosan membranes,respectively.

Fig. 5 (a) Cross-sectional SEM image of a free-standing polymerbrushes grafted Janus GO/chitosan hybrid membrane. (b) XPS spectraof PDMAEMA grafted surface and PS grafted surface of Janusmembrane. (c) C1s spectrum of PDMAEMA grafted surface. The insertis labeling of the different carbon moieties in a PDMAEMA molecule.(d) C1s spectrum of PS grafted surface.

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sheets by UV. Raman spectroscopy was performed to investigateGO reduction. As shown in Fig. 4, Raman spectra of originalGO/chitosan membrane and polymer brushes graed Janusmembrane exhibit prominent D and G bands at 1345 cm�1 and1600 cm�1, respectively. Aer modication of polymer brushesby SIPGP, the ID/IG ratio increases from 0.93 to 0.99, indicatingGO was partially reduced.

In order to further understand the structure of polymerbrushes graed Janus GO/chitosan hybrid membrane, cross-sectional SEM was applied to obtain the cross prole of thehybrid membrane. As shown in Fig. 5a, a distinct three-layer“sandwich” structure can be observed by SEM. The upper layeris homogeneous and has an obvious boundary with the middlelayer. Its thickness is about 350 nm, which is consistent withthickness increase aer photopolymerization. The middle layeris mainly composed of GO/chitosan sheets, and they are stackedin a layered fashion whose surfaces are rough due to encapsu-lation of chitosan. The bottom layer has a homogeneous

Fig. 4 (a) Raman spectra of original membrane and polymer brushesgrafted Janus hybrid membrane. (b) ID/IG ratio of original membraneand polymer brushes grafted Janus hybrid membrane.

This journal is © The Royal Society of Chemistry 2014

structure just like the upper layer which has a thickness of 250nm. X-ray photoelectron spectroscopy (XPS) was used to verifythe Janus structure of the membrane. The maximum of detectdepth for XPS is about 10 nm, and the thickness of the graedlayers are 200–400 nm. Therefore, the composition of the twolayers can be characterized with XPS without interference. TheXPS spectra of both surfaces of Janus membrane are shown inFig. 5b. XPS survey spectrum of PDMAEMA graed surfacereveals N1s peak at 398 eV, which can be assigned to the tertiaryamine group in PDMAEMA. The O1s peak and Si peaks in theXPS spectra are attributed to contamination of SiO2 duringetching process. N is the characteristic element to distinguishPS and PDMAEMA, so the two sides can be determined bycontent of N. C1s spectra for both surfaces of Janus membraneare shown in Fig. 5c and d. The C1s spectrum of PDMAEMAgraed surface is deconvoluted into ve component peaks.27

The intensity ratios of these deconvoluted peaks are in goodagreement with the stoichiometric ratio of the correspondingcarbon atoms in chemical structure of PDMAEMA as[A] : [B] : [C] : [D] : [E] ¼ 2 : 1 : 3 : 1 : 1. The C1s spectrum of PSgraed surface includes a main hydrocarbon peak at bindingenergy of 284.8 eV and p–p* shake-up satellites at 391.5 eV.28

These results strongly conrm that PS and PDMAEMA weresuccessfully graed from the upper and lower surfaces ofGO/chitosan membrane, respectively.

Conclusions

We represent a robust and simple approach to fabricate free-standing polymer brushes functionalized Janus GO/chitosanhybrid membrane. GO/chitosan composite are constructedrstly through interface self-assembly induced by electrostaticinteraction. PS and PDMAEMA brushes are respectively graedfrom photoactive sites of the upper and lower surfaces of the

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composite membrane through SIPGP to obtain a distinct Janusstructure. SIPGP is a facile polymerization method in whichsurface-bound initiator is not required, and many vinyl mono-mers can be graed from photo active site on the surface ofGO/chitosan membrane by SIPGP. Therefore, the compositionsof Janus membrane surfaces are alternative in a wide range andthe surface properties of GO/chitosan can be tuned in a denitedirection by selecting appropriate vinyl monomers. Moreover,the thickness of graed layer can be modulated by adjusting theirradiation time.29 The new asymmetric polymer brushes func-tionalized Janus GO/chitosan hybrid membrane opens the wayto many potential applications, including separation, sensingand as the versatile material platform for the hybrid withmetallic nanoparticles for catalysis applications.

Acknowledgements

We thank Chinese Academy of Science for Hundred TalentsProgram, Chinese Central Government for Thousand YoungTalents Program and the National Natural Science Foundationof China (51303195, 21304105), and Excellent Youth Founda-tion of Zhejiang Province of China (LR14B040001).

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