Cyclodextrin-grafted electrospun cellulose acetate nanofibers via "Click" reaction for removal of phenanthrene

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ContentslistsavailableatScienceDirect

Applied

Surface

Science

j o ur na l ho me pa g e :w w w . e l s e v i e r . c o m / l o c a t e / a p s u s c

Cyclodextrin-grafted

electrospun

cellulose

acetate

nanoﬁbers

via

“Click”

reaction

for

removal

of

phenanthrene

Asli

Celebioglu

a,b

_,

_Serkan

_Demirci

a,c

_,

_Tamer

_Uyar

a,b,∗

a_{UNAM-National}_{Nanotechnology}_Research_Center,_Bilkent_University,_Ankara_06800,_Turkey b_Institute_of_Materials_Science_and_{Nanotechnology,}_Bilkent_University,_Ankara_06800,_Turkey c_Department_of_Chemistry,_Faculty_of_Arts_and_Sciences,_Amasya_University,_Amasya_05100,_Turkey

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received23January2014

Receivedinrevisedform17March2014 Accepted21March2014

Availableonline29March2014 Keywords: Electrospinning Nanoﬁbers Cyclodextrin “Click”reaction Phenanthrene Filtration

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Beta-cyclodextrin(␤-CD)functionalizedcelluloseacetate(CA)nanofibershavebeensuccessfully pre-paredbycombiningelectrospinningand“click”reaction.Initially,␤-CDandelectrospunCAnanofibers weremodifiedsoastobeazide-␤-CDandpropargyl-terminatedCAnanofibers,respectively.Then,“click” reactionwasperformedbetweenmodifiedCDmoleculesandCAnanofiberstoobtainpermanent graft-ingofCDsontonanofiberssurface.ItwasobservedfromtheSEMimagethat,whileCAnanofibershave smoothsurface,thereweresomeirregularitiesandroughnessatnanofibersmorphologyafterthe modi-fication.Yet,thefibrousstructurewasstillprotected.ATR-FTIRandXPSrevealedthat,CDmoleculeswere successfullygraftedontosurfaceofCAnanofibers.Theadsorptioncapacityof␤-CD-functionalizedCA (CA-CD)nanofiberswasalsodeterminedbyremovingphenanthrene(polycyclicaromatichydrocarbons, PAH)fromitsaqueoussolution.OurresultsindicatethatCA-CDnanofibershavepotentialtobeusedas molecularfiltersforthepurposeofwaterpurificationandwastewatertreatmentbyintegratingthehigh surfaceareaofnanofiberswithinclusioncomplexationpropertyofCDmolecules.

Introduction

Electrospunnanofibers/nanowebspossessseveralunique prop-ertiesthatmakethemgoodcandidateforthefiltration,separation andcleaningapplications,suchas;largespecificsurfacearea,highly porousstructure withnanosizerange,highdegreeof intercon-nectionandmodifiablenature[1–5].Thepotentialofnanofibrous structureforfiltrationpurposeshasbeenreportedinliteratureby showingseparationoftinyparticles,filtrationofliquidmedium [4–6]andwastevaportreatment[3,7,8].Even,production variabil-ity,low-costandhighout-putofthistechniquemakepossiblethe filtrationperformancetoenterintocompetitionwithconventional filtrationsystems.Moreover,electrospunnanofibersfacilitatefor chemical/physicalfunctionalizationsthatcanleadstobetteruptake performanceduringthefiltrationprocess[9–13].

Cyclodextrins(CDs)arenaturalcyclicoligosaccharideswhich areregeneratedbytheenzymaticdegradationofstarch.Thereare threenativetypesofCDmolecules;␣-CD,␤-CDand␥-CDwhichare consistedofsix,sevenandeightglucopyranosesubunits, respec-tively[14,15].CDshavetoroid-shapedmolecularstructurewitha

∗ Correspondingauthor.Tel.:+903122903571;fax:+903122664365. E-mailaddresses:tamer@unam.bilkent.edu.tr,tameruyar@gmail.com(T.Uyar).

relativelyhydrophobicinteriorcavity.Duetotheintriguing molec-ularstructure,CDsareabletoforminclusioncomplexes(CD-IC) withavarietyofmoleculesalongwithnon-covalentinteractions [14,15].TheinclusioncomplexationwithCD moleculesenhance solubility,stabilityandbioavailabilityofguestmolecules.So,these CD-ICsupramolecularstructuresarequiteapplicablein pharma-ceuticals,foods,cosmetics,home/personalcareandtextilesareas [14–16].Additionally,ﬁltrationandseparationsystemsareanother applicationﬁeldsforCDmoleculesowingtotheircapturing capa-bilityofhazardousorganicmoleculesbyinclusioncomplexation [17–20].

CD moleculesare commonly utilizedin theform ofpowder orcrosslinkedpolymericgranules[14–16,20].Unfortunately,this statecancauselimitationduringtheirusage.So,tobenefitfrom CDsuniquepropertiesmoreefficientlyandrenderthemintomore applicableform,theycanbecombinedwithpolymericmatrix.In ourpreviousstudies,wehavephysicallyblendedCDmoleculesinto polymericnanofibersbyelectrospinning[7,21,22].Itwasobserved that,whilemostoftheCDmoleculeswereembeddedinsidethe nanofibers,someofthemwerelocatedonthefibersurfaceand theseaccessibleCDenabletheremovaloforganicwastemolecules frombothvaporphase[7]andwater-basedenvironment[21,22]. However,watersolubilityofCDsrestrictstheiruseforwater purifi-cation purposes,becauseofa probableleachingfromnanofiber http://dx.doi.org/10.1016/j.apsusc.2014.03.138

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582 A.Celebiogluetal./AppliedSurfaceScience305(2014)581–588 surfacethatcanbeoccurredduringﬁltration.Therefore,another

approachshouldbeadoptedforthemodificationofnanofibersthat includesthelastingattachmentsofCDmoleculesonthefiber sur-face.Thus,complexformationpropertyofCDmoleculeswouldbe integratedwiththehighsurfaceareaofpolymericnanofibersina morepermanentwaythatwouldleadtoproductionofpromising filteringmembranes.Actually,thechemicalsurfacemodification withCDmoleculeswasfirstlyperformedonthefiberandfabric surfacesbyusingappropriatecrosslinkingagents[23–30].These functionalizationswereperformedbygraftingCDsontosubstance [23–26]orthesubstanceswerecoveredbycrosslinkedCD poly-mers[27–30]forthefiltrationofwastemoleculesordeliveryof drugs,antibacterialsetc.Ontheotherhand,wehavefirstlyreported thesurfacemodificationofelectrospunnanofiberswithCD poly-merin ourpreviousstudy[31]. Here,citric acidwasusedas a crosslinkingagentfortheformationofCD(␣-CD,␤-CDand␥-CD) polymer(CDP)andafterthesurfacemodificationofpolyethylene terephthalate (PET) nanofibers, the molecularfiltration perfor-manceofPET/CDPwasinvestigatedaswell[31].

Surfacemodifiedelectrospunnanofibersareofgreat interest duetotheirhigherpotentialfortheapplicationofaforementioned fields.Nanofibersfunctionalizedinthiswaycouldbeexpectedto increasetheirperformanceforthedesiredapplications,sincethe availabilityof more activesidesontheirsurface.“Click” chem-istrycanbeanalternativewaytomodifysurfaceofnanofibers, because“click”reactionsshowhighyields andexceptional tol-erancetowardsa wide rangeof functional groupsand reaction conditions in thematerial science [32,33]. Very recent studies have also been reported in the literature about the modifica-tionofelectrospunnanofibersvia“click”reaction.For instance, Fu et al. formed thermal-sensitive poly-N-isopropylacrylamide (PNIPAM)brushesonthesurfaceofpoly(4-vinylbenzyl chloride)-block-poly(glycidylmethacrylate)(PVBC-b-PGMA)nanofibersby using“click”reaction[34].InanotherstudyofChangetal.,“click” wasused for thefunctionalization of polyimide nanofibers via alkyne-terminatedpoly(methylmethacrylate)chains[35].Inthe studyofYangetal.nanofibershavingthermallysensitivesurface wereproducedbythegraftingofPNIPAMbrushesonthe poly((3-mercaptopropyl)methylsiloxane)(PMMS)nanofiberswiththeaid of“click”chemistry[36].Inanotherrelatedstudy,Lancuskietal. developedcarbohydrate-decoratedPCLnanofibersforthespecific proteinadhesionbyapplying“click”reaction[37].In oneofthe associatedstudies,Qianetal.reportedtheintroductionof saccha-rideresiduestothesurfaceofthepolyphosphazenenanofibrous membraneusing“click”chemistry[38].Mostofthestudies men-tionedabovefocusonthebiomedicalapplicationsofnanofibers. Ontheotherhand,inourstudywehaveapplied“click”reaction forsurfacemodificationofnanofiberstoimprovetheirfiltration performance.

Polycyclicaromatichydrocarbons(PAHs)areoneofthemost widespreadpollutants whicharehighlytoxic,carcinogenic, and theirtoxicity increases withincreasing molecularweight [39]. Moreover,severalstudiesshowthatPAHspollutionscause seri-oushealthproblemsforhumanandlivingorganisms[39,40].For thesereasons,varietiesofadsorbentssuchasnanoﬁbers,silicagel, porousnanoparticlesetc.,havebeendevelopedfortheremovalof PAHs[31,41,42].

In this study, ␤-CD-functionalized CA nanofibers were suc-cessfullyproducedbycombinationofelectrospinningand“click” chemistry(Fig.1).Thatis,␤-CDwasgraftedontoelectrospunCA nanofibersvia“click”reaction.Themorphologicalcharacterization ofnanofiberswerecarriedoutbyusingscanningelectron micro-scope(SEM).The surfacecharacteristicsof thenanofiberswere investigated by attenuated total reflectance-Fourier transform infraredspectroscopy(ATR-FTIR)and x-rayphotoelectron spec-troscopy(XPS).Furthermore,thecomparativemolecularfiltration

performance of ␤-CD-functionalized CAnanofibersand pristine CAnanofiberswereinvestigatedbyremovingphenanthrene(asa modelPAH)fromtheaqueoussolutions.Ourpreliminaryfindings suggestedthat␤-CD-functionalizedCAnanofibershavepotentials tobeusedasmolecularfiltersforthepurposeofwater purifica-tionand/orwastewatertreatmentbyintegratingthehighsurface areaoftheelectrospunnanofiberswithinclusion complexation propertyofCDmolecules.

Materialsandmethods

Materials

Celluloseacetate (CA, Mw: 30000,39.8wt. %acetyl, Sigma– Aldrich) dichloromethane (DCM, _≥99% (GC), Sigma–Aldrich), methanol (≥99.7% (GC), Sigma–Aldrich), beta-cyclodextrin (␤-CD) (Wacker Chemie AG), sodium hydroxide (NaOH, Fluka, ≥98%, small beads), acetonitrile (99.9%, Sigma–Aldrich), p-toluenesulfonyl chloride (puriss., ≥99.0%, Sigma–Aldrich) dimethyformamide (≥99% (GC), Sigma–Aldrich), sodium azide (ReagentPlus,≥99.5%,Sigma–Aldrich),sodiumhydride(60% dis-persioninmineraloil,Aldrich),propargylbromidesolution(80%in toluene,Fluka),acetone(≥99%(GC),Sigma–Aldrich),2-propanol (≥99.5%(GC),Sigma–Aldrich),coppersulfate(anhydrous,≥99.0%, Sigma–Aldrich), l-ascorbic acid (reagent grade, Sigma–Aldrich) phenanthrene(98%,Sigma–Aldrich) were purchased.The water usedwasfromaMilliporeMilli-QUltrapureWaterSystem.Allthe materialswereusedwithoutanypuriﬁcation.

ElectrospinningofCAnanoﬁbers

Theelectrospinning solutionofCAwaspreparedby dissolv-ingpolymerinaDCM/methanol(4/1(v/v))binarysolventmixture ata12%(w/v)polymerconcentration.TheclearCAsolutionwas then placed in a 5mL syringe fitted with a metallic needle of a0.4mminnerdiameter.Thesyringewasfixedhorizontallyon thesyringepump(modelKDS-101,KDScientific,USA).The elec-trodeofthehigh-voltagepowersupply(Spellman,SL30,USA)was clampedtothemetalneedletip,andtheplate-shapedaluminum collectorwasgrounded.Electrospinningparameterswereadjusted asfollows:feedrateofsolutions=1mL/h,appliedvoltage=15kV, tip-to-collectordistance=10cm.Theelectrospinningprocesswas performedat25◦Cat20%relativehumidityinPlexiglasbox.After theelectrospunnanofibersweredepositedonthegroundedmetal collectorcoveredwithaluminumfoil,theywerekeptinvacuum oven(40◦C)foralmost12htoremovethesolventresidualinthe nanofibers.

Synthesisoftheazide-ˇ-cyclodextrin

␤-CD(63g,35.2mmol)wasdispersedin500mLofwaterand bytheadditionofNaOHsolution(5.6gin20mlwater),CDswere completelydissolved.Afterstirring1h,thep-toluenesulfonyl chlo-ride solution(9.5g in 30ml acetonitrile) was dropped into CD solution slowly. The suspension was stirred vigorously for 6h and kept in refrigerator overnight. The precipitate white pow-der was ﬁltered and dried under vacuum (12g TsO-␤-CD). In the second step, the TsO-_␤-CD (6g) powder was dissolved in DMF (50ml) and sodium azide (NaN3, 2.75g) was added into

solution.Thissystem wasstirredat 80◦C forabout 24hunder nitrogenatmosphereandthen, itwascooledtoroom tempera-ture.Finally,thesolutionwasdroppedintocoldacetone(600ml) andthewhiteprecipitateoftheproductwasobtainedafterthe ﬁltration.

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Fig.1.(a)SchematicrepresentationofelectrospinningofCAnanofibers.(b)Schematicviewandchemicalstructureof␤-CD,schematicviewofazide-␤-CDsynthesisand CA-propargylnanofibersformation.(c)TheschematicrepresentationofthemodificationofCA-propargylnanofiberswithazide-␤-CDby“click”reaction.

Graftingofazide-ˇ-CDontoCAnanofibersby“click”chemistry Underanitrogenatmosphere,CAnanofibers(1.0equiv.)and 2-propanolsolutionof NaH(1.2equiv.)were addedtoa round bottomedflaskat0◦Candstirredforfewminutes.Thereaction mixturewasgraduallywarmedtoroomtemperaturefor2h,and propargylbromide(1.8equiv.)wassubsequentlyaddeddropwise. Theresulting mixturewasstirred atroomtemperature for 6h. TheCAnanofiberswererecoveredfromthereactionmixtureand washedwith2-propanolandwatertoremovetheunreacted chem-icals,anddriedundervacuumat30◦C.Theazide-␤-CDobtained intheformerstep(0.6mmol,0.7g)wasfirstlydissolvedin20ml water and CA nanofiberhaving propargylmoiety wasputinto thisCDsolution.Meanwhile,thefreshsolutionsofl-ascorbicacid (0.12mmol,21mg)in1.5mlwaterandcopper(II)sulfate anhy-drous(0.052mmol,8.8mg)in1.5mlwaterwerepreparedandboth ofthemaddedintothesolutionwhichincludeazide-␤-CDand CA-propargylnanofibers.Thissystemwasstirredabout24hatroom temperature.Finally, obtainedCA-CD nanofiberswereremoved fromthesolution,washedwithwateranddriedatvacuumoven at40◦C.

Characterizationsandmeasurements

Themorphologicalcharacterizationandthediameter calcula-tionoftheCA,CA-propargylandCA-CDnanofiberswereperformed byusingscanningelectronmicroscope(FE-SEM)(FEI,Quanta200 FEG).Samplesweresputteredwith5nmAu/Pd (PECS-682)and around100fiberdiametersweremeasuredfromtheSEMimages tocalculatetheaveragefiberdiameterofeachsample.Theinfrared spectraof theCDswere obtainedbyusing a Fouriertransform infrared spectrometer (FTIR) (Bruker-VERTEX 70). The samples weremixedwithpotassiumbromide(KBr)andpressedaspellets. Thescans(64scans)wererecordedbetween4000and400cm−1 ataresolutionof4cm−1.TheAttenuatedtotalreflectance-Fourier transforminfrared(ATR-FTIR)wasusedforthesurfacestructural analysisof nanofibers.ATR-FTIRspectraofthenanofibers were obtainedusingaThermoNicolet6700spectrometerwithaSmart Orbit attenuated total reflection attachment. The spectra were takenataresolution4cm−1 after128scanaccumulationforan acceptablesignal/noiseratio.Thex-rayphotoelectronspectraof nanofiberswererecordedbyusingx-rayphotoelectron spectrom-eter(XPS)(ThermoScientific).XPSwasusedbymeansofaflood

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584 A.Celebiogluetal./AppliedSurfaceScience305(2014)581–588 gunchargeneutralizersystemequippedwithamonochromated

AlK-␣ x-ray source(h

=1486.6eV). Thehighresolution spec-traofCandNwerealsorecordedfortherelatedsamplestoget moredetailedinformation.Highperformanceliquid chromatogra-phy(HPLC)system(Agilent1200Series)wasusedtoinvestigate thephenanthreneremovingperformanceofbothCAandCA-CD nanofibers.Theseparationofphenanthrenewasperformedwith ZorbaxEclipseXDB-C18column(150mm×4.6mm,5␮mparticle size)anditwasdetectedat254nmwavelength.Acetonitrile(100%) wasusedasmobilephaseataflowrateof0.3ml/min.andthe injec-tionvolumewaskeptat10␮l.Thephenanthrenewassolvedin acetonitrileandthendilutedinwatertocarryoutthe measure-ments.The0.1gweightednanofiberswereimmersedin1.8ppm phenanthreneincludedwatersolutions(30ml)and0.5mlaliquots weretakenfromthesystematdefinitetimeintervals.The calibra-tioncurveofphenanthrenewaspreparedbyusingstocksolutions in4differentconcentrations;1.8␮g/ml,0.9␮g/ml0.45␮g/ml,and 0.23␮g/ml.It showedlinearity andacceptabilitywithR2_≥_0.99.

Themeasurementresultswereadaptedtothiscalibrationcurve intermsof peakareaundercurves.Theexperimentswere car-riedoutintriplicateandtheresultsweregivenwiththeirstandard deviations.

Resultsanddiscussion

Formationofazide-ˇ-CD

The modiﬁcation of the ␤-CD molecules was conﬁrmed by usingFTIRspectraasillustratedin Fig.2a.Asseen, the charac-teristicabsorption bands of ␤-CD for the given three samples, appeared at around 1030,1080, and 1155cm−1 corresponding to the coupled C–C/C–O stretching vibrations and asymmet-ric stretching vibration of the C–O–C glycosidic bridge. After p-toluenesulfonyl chloride treatment, beside the ␤-CD signals, toluenesulfonylgroupcharacteristicbandswerealsoobservedas aromaticC Cstretchingat1599cm−1,S Ostretchingat1366cm−1 andS–O–Arstretchingat838cm−1 [43].Asa resultofthenext step,toluenesulfonylgroupsignalswasdisappearedinFTIR spec-trumandstretchingfrequencyofN3becameobviousat2040cm−1

demonstrating asymmetrical azide (–N3) functionality of ␤-CD

[44].

Morphologicalcharacterizationofnanoﬁbers

ThemorphologicalbehaviorofCAnanofibersbeforeandafter thesurfacemodificationhavebeencomparedbySEMasdepicted inFig.3.AsitisshownintheSEMimages, somechangeswere occurredatthemorphologyofCAnanofibersaftereachprocess.The uniformandsmoothmorphologywasobservedforun-modifiedCA nanofibers,whereasslightswellingwasobservedbythe propar-gyltreatment(Fig.3aandb).Theroughandirregularappearance wasrecordedafterthe“click”reactionwhichprovedthesuccessful surfacemodificationofCAnanofibers.Thesimilarmorphological changewasalsoobservedinastudyofourresearchgroupinwhich theCDpolymerwasgraftedonthePETnanofibers[31].The over-allresultssuggestedthat,adoptedproceduredidnotcausetoany deformationandfibrousstructureofnanofiberswaspreserved dur-ingthechemicaltreatments.Theaveragefiberdiameters(AFD) weredeterminedas675±160,960±190and1520±370forCA, CA-propargylandCA-CDnanofibers,respectively.Theincreaseof AFDcouldbeoriginatedfromtheswellingofnanofibersthrough themodificationand/orirregularpartsyieldedasaresultofCD grafting.

Fig.2. (a)FTIRspectraof␤-CD,TsO-␤-CDandazide-␤-CDpowder,(b)ATR-FTIR spectraofCA,CA-propargylandCA-CDnanoﬁbers.

Structuralsurfacecharacterizationofnanoﬁbers

TheATR-FTIRcharacterizationwasperformedtoprovetheCD modificationonthenanofibersurface(Fig.2b).Thecharacteristic bandofCAwasobservedat1739and1221cm−1duetotheC Oand C–Ostretching,respectively.Thebroadbandat3700–3100cm−1 indicatesthepresenceofOHgroupintheCAstructure.FTIR spec-trumofCAalsoshowedabsorbancebandat2924and2855cm−1 fortheC–Hstretching.Initially,CAnanofibersweremodifiedwith propargylbromide.Thismodificationwasobviousfromthe appear-anceofC Cbandat2019cm−1intheATR-FTIRspectrum(Fig.2b). Then, azide-␤-CD was attached to the CA-propargyl nanofiber surface by a “click” reaction and accordingly, the C C bandat 2019cm−1andN3at2040cm−1disappeared[45,46].Furthermore,

allcharacteristicbandsofCAandCDwereobservedfortheCA-CD nanoﬁbers(Fig.2b).

ThesurfaceoftheCA,CA-propargylandCA-CDnanofiberswere alsocharacterized byusing XPSwide scan and highresolution scanstoverifythefunctionalizationofthesesamples.Table1 sum-marizesthecompositionalpercentagesofnanofiberswhichwere obtainedasaresultofwideenergysurveyscan.Itwasobserved that,C1sandO1saretwointensiveelementsasthemain compo-sitionsofnanofibers.Forun-modifiedCAnanofibers,theratioofC 1s:O1sis62.77:37.23(%),whereasforCA-propargyl,theintensity ofC1sincrease(C1s:O1sis71.21:28.79(%))duetocontribution

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Fig.3.RepresentativeSEMimagesof(a)CA,(b)CA-propargyland(c)CA-CDnanoﬁbers.Theinsetsshowhighermagniﬁcationimages.

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586 A.Celebiogluetal./AppliedSurfaceScience305(2014)581–588 Table1

AtomicconcentrationsofnanoﬁberswhichwereobtainedfromXPSwideenergy surveyscans.

Samples C(%) O(%) N(%)

CAnanoﬁbers 62.77 37.23 –

CA-propargylnanoﬁbers 72.21 28.79 –

CA-CDnanoﬁbers 72.75 25.58 1.67

of CH2C CH group in the ﬁrst step of modiﬁcation [47]. After

“click”reaction,N1swasalsorecordedasoneofthecomponent whichindicatesthesuccessfulformationoftrizoleringbetween CAnanofibersurfaceandCDmolecules[48].HighresolutionC1s scanwasperformedtogetmoredetailedinformationaboutthe chemicalstateofnanofibers’surface.Fig.4a–cshowsC1sspectra ofun-modifiedCA,CA-propargylandCA-CDnanofiberswiththeir subpeaksobtainedbyfitting.Inaddition,thehighresolutionscanof N1sisgiveninFig.4dthatbelongstoCA-CDnanofibers.The corre-spondingpositionsofpeakbindingenergiesandtheirvalues(%area ratio)werealsolistedinTable2.Forun-modifiedCAnanofibers,C 1sspectrumisdeconvolutedintofoursubpeaksassignedtoC–(C, H)at284.62eV,C–Oat286.28eV,O–C–Oat287.62eVandO C–O at289.22eV[49].Afterthefirststepofmodification,theC1s spec-trum(Fig.4b)clearlyshowsincreaseofC–(C–H)peakratiofrom 30.45%to43.21%anddecreaseofotherpeaks(Table2)duetothe graftingofCH2C CHmoiety[47].Thisevidencemadeitpossible

togotonextstepofCAnanofibersfunctionalization.Inthecaseof azide-␤-CDgrafting,thepeakratiosofC–OandO–C–Osituatedat 286.66and287.91,respectivelyincreased,ontheotherhand,the chemicalstateofO C-Oat289.13eVdecreasedsignificantlyowing tothelocationof␤-CDonthenanofibersurface(Fig.4c,Table2). InadditiontoC1speak,theN1speakwasalsodetectedatabout 400eVforCA-CDnanofibersoriginatedfromthetriazolegroupas aresultof“click”reaction[48].Fortriazolering,theN1score-level peak can be curve-fitted into two components having binding energyat398.4and399.7eVattributedtoC–NandN N, respec-tively [48,50]. From XPS measurements, it was also confirmed thatthesurfacemodificationofCAnanofiberswithCDmolecules wasachievedbyusing“click”chemistry.Inaddition,thegrafting densityofCDmoleculesontoCAnanofiberswerecalculatedfrom highresolutionXPSspectraofC1s.Forthis,O–C–Opeakoriginated frombothCAand CD,and O C-Opeak onlyexisting intheCA structurewerechosenandused.Thepeakratio(O–C–O/O C-O) belongstoCAnanofiberwascalculatedas0.72anditisrelatively close to the theoretical values (0.80) calculated from atomic compositionofCAnanofibers.Ontheotherhand,O–C–O/O C–O ratiowasdeterminedas4.55forCA-CDnanofibers.Asitisknown, each ␤-CD molecules have 7 glucopyranose subunits and after theclickreaction,O–C–Opeakarearatioincreasedby6.32times, whichmeansthateach␤-CDmoleculewasapproximatelybound toonerepeatunitsoftheCApositionedatnanofibersurface.

Fig.5.Thetimedependentdecreaseofphenanthreneconcentrationinaqueous solutionwhichcontainsCAandCA-CDnanoﬁberswebs.

MolecularﬁltrationcapabilityofCAandCA-CDnanoﬁbers

PAHs are important organic pollutants because of their mutagenicand carcinogenicpotentials.However,thelow-water solubilityofthesecomponentslimitstheremediationprocessof contaminatedwaterandsoil[39,40].Asitisknown,CDsarecapable ofencapsulatingorganiccompoundsduetotheirhydrophobic cav-ityandtherearemanystudiesreportedcomplexationbetweenCDs andPAHsmolecules[51–55].Phenanthreneisthemostcommonly knownexamplethroughotherhydrocarbons,sointhisstudy,it waschosenasamodelPAHtoexaminethemolecularfiltration potentialofCAandCA-CDnanofibers.Fig.5depictsthecumulative decreaseofphenanthreneconcentration(%)againstprogressing time intervalswhileCAand CA-CDnanofiberswere beingkept intothis organiccompoundaqueoussolution.Asit isseen,the adsorptionofphenanthrenewasachievedbybothCAandCA-CD nanofibers.Even,inthefirst30min,whileCAnanofibersremoved 50%ofphenanthrenefromthesolution,thisratioreachedto64% for CD-CA nanofibers.Towards theend of experiment, the dif-ferencesofadsorbedamountbetweenCAandCA-CDnanofibers increase,therefore phenanthreneconcentrationdecreased more significantlyfor CA-CDnanofiberscompared toun-modifiedCA nanofibers.ThehigherremovingefficiencyofCD-CAnanofibersis probablyoriginatedfromtheinclusioncomplexationpropertyof CDmoleculeswhichwerelocatedonthesurfaceofnanofibersand leadedtohigheradsorptionoforganiccompoundfromaqueous medium.Itisknownthat,hydrophobicinteractionsaretherelation typebetweenCDscavityandphenanthrenemoleculeduringthe inclusion complexation.Besides,repulsive interactions between Table2

FittingparametersoftheC1sXPSspectraofCA,CA-propargylandCA-CDnanoﬁbers.

Samples Fittingpeaks Bonds Peakbindingenergy(eV) Arearatio(%)

CAnanofibers C1s#1 C–(C–H) 284.62 30.45 C1s#2 C–O 286.28 20.32 C1s#3 O–C–O 287.62 20.61 C1s#4 O C–O 289.22 28.62 CA-propargylnanofibers C1s#1 C–(C–H) 284.73 43.21 C1s#2 C–O 286.66 20.29 C1s#3 O–C–O 287.91 5.37 C1s#4 O C-O 289.13 13.74 CA-CDnanofibers C1s#1 C–(C–H) 284.80 43.13 C1s#2 C–O 286.41 45.09 C1s#3 O–C–O 287.99 9.66 C1s#4 O C–O 289.30 2.12

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Fig.6. RepresentativeSEMimagesof(a)CAand(b)CA-CDnanoﬁbersaftertheﬁltrationtest.

thehydrophobicguestandtheaqueousenvironment,andmore

favorableinteractionsbetweenhydrophobicguestandapolarCD

cavity arethedriving forcesfor theremoving ofphenanthrene

moleculesfromtheaqueousenvironment[16–53].Inthecaseof

CDgraftingonto nanofiberssurface, onlyCDmoleculesbecome moreapplicablecomparedtotheirpowderformbythelocation onastablecarriermatrix.However,itdoesnotcauseanychange at the entrapment and removing mechanism of phenanthrene moleculesbyCDs.Here,itwasalsoobservedthat,bothCAand CA-CDnanofibersstill kepttheirfiberstructureafterthe filtra-tiontest(Fig.6).CAisalready goodcandidatefor thefiltration oforganicpollutantsandtherearealsoreportsintheliterature abouttheuptakingofPAHsfromtheconcernedenvironmentby usingCAbasedmembranes[56–59].Ontheotherhand,tothebest knowledge,thisisfirststudyabouttheinvestigationofmolecular filtrationcapabilityofCAnanofibersanditsCDmodifiedtypeby “click”chemistry.Fromourresults,itcanbeconcludedthat,the sur-facemodificationofelectrospunCAnanofiberswithCDmolecules improvedthemolecularfiltrationpotentialbyutilizingfromthe inclusioncomplexationpropertyofCDs.The“click”chemistryisa quitenewandpromisingmethodforthefunctionalizationof elec-trospunnanofibers.Inourstudy,betteradsorptionefficiencywas obtainedforCDmodifiedCAnanofiberscomparedtountreatedone duringtheremovingtest.However,theadsorbedamountofPAH orotherorganiccompoundscanbeenhancedbygraftinghigher amountofCDonthenanofibersurfaceusing“click”chemistry.

Conclusion

Inthisstudy,thepermanentgraftingofCDmoleculesonthe electrospunCAnanofiberswasachievedbyusing“click” chem-istry.First,␤-CDwasmodifiedsoastobeazide-␤-CD.Atthesame time,CAnanofiberswereproducedviaelectrospinningandthey weretreatedchemicallytobepropargyl-terminatedCAnanofibers. Then,“click”reactionwasperformedtograftthe␤-CDmoleculeson thesurfaceofCAnanofibers.Themorphologicalcharacterizations ofnanofiberswerecarriedoutbySEMtechnique.Itwasrevealed that,theCDmodifiedCAnanofibershave rougherandirregular surfacewhenitwascomparedwithpristineCAnanofibers.The existenceoftheCDmoleculesonthenanofibersurfacewasproved byusingATR-FTIRandXPSanalyses.ThefiltrationcapabilityofCD graftedCAnanofiberswasinvestigatedbytheremovalof phenan-threnefromitsaqueoussolution.Forcomparison,filtrationtestof pristineCAnanofiberswasalsoperformed.Itwasobservedthat, CA-CDnanofibersadsorbedhigheramountofphenanthrene com-paredtoCAnanofibersduetotheinclusioncomplexationcapability ofCDmolecules.Wehavealsocheckedthat,thefibrousstructure ofnanofiberswasprotectedafterthefiltrationtest.Inbrief,our

resultsindicatethatCDfunctionalizedCAnanofiberswouldhave potentialtobeusedasmolecularfiltersforthepurposeofwater purificationandwastewatertreatmentbyintegratingthehigh sur-faceareaof nanofiberswithinclusioncomplexationpropertyof CDmolecules.Moreover,“click”chemistrywouldbeapromising candidateforthemodificationofnanofiberssurfacewithvarious functionalgroupsandmoietiestobenefitfromthepotentialsof nanofibersmoreefficientlyintheirapplications.

Acknowledgements

Dr.T.UyaracknowledgesTUBITAK-TheScientiﬁcand Techno-logicalResearchCouncilofTurkey(project#110M612)forfunding theresearch.Dr.T.UyaralsoacknowledgesEU FP7-PEOPLE-2009-RG Marie Curie-IRG(NANOWEB, PIRG06-GA-2009-256428) and TheTurkishAcademyofSciences–OutstandingYoungScientists AwardProgram (TUBA-GEBIP)for partialfunding. A.Celebioglu acknowledgesTUBITAK-BIDEBforthenationalPh.D.scholarship.

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