of phenanthrene for removal reaction Cyclodextrin-grafted electrospun acetate cellulose nanoﬁbers via“Click” Applied Surface Science

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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,∗

aUNAM-NationalNanotechnologyResearchCenter,BilkentUniversity,Ankara06800,Turkey

bInstituteofMaterialsScienceandNanotechnology,BilkentUniversity,Ankara06800,Turkey

cDepartmentofChemistry,FacultyofArtsandSciences,AmasyaUniversity,Amasya05100,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

a b s t r a c t

Beta-cyclodextrin(␤-CD)functionalizedcelluloseacetate(CA)nanofibershavebeensuccessfullypre- paredbycombiningelectrospinningand“click”reaction.Initially,␤-CDandelectrospunCAnanofibers weremodifiedsoastobeazide-␤-CDandpropargyl-terminatedCAnanofibers,respectively.Then,“click”

reactionwasperformedbetweenmodifiedCDmoleculesandCAnanofiberstoobtainpermanentgraft- ingofCDsontonanofiberssurface.ItwasobservedfromtheSEMimagethat,whileCAnanofibershave smoothsurface,thereweresomeirregularitiesandroughnessatnanofibersmorphologyafterthemodi- fication.Yet,thefibrousstructurewasstillprotected.ATR-FTIRandXPSrevealedthat,CDmoleculeswere successfullygraftedontosurfaceofCAnanofibers.Theadsorptioncapacityof␤-CD-functionalizedCA (CA-CD)nanofiberswasalsodeterminedbyremovingphenanthrene(polycyclicaromatichydrocarbons, PAH)fromitsaqueoussolution.OurresultsindicatethatCA-CDnanofibershavepotentialtobeusedas molecularfiltersforthepurposeofwaterpurificationandwastewatertreatmentbyintegratingthehigh surfaceareaofnanofiberswithinclusioncomplexationpropertyofCDmolecules.

Introduction

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

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

∗ Correspondingauthor.Tel.:+903122903571;fax:+903122664365.

E-mailaddresses:tamer@unam.bilkent.edu.tr,tameruyar@gmail.com(T.Uyar).

relativelyhydrophobicinteriorcavity.Duetotheintriguingmolec- ularstructure,CDsareabletoforminclusioncomplexes(CD-IC) withavarietyofmoleculesalongwithnon-covalentinteractions [14,15].TheinclusioncomplexationwithCD moleculesenhance solubility,stabilityandbioavailabilityofguestmolecules.So,these CD-ICsupramolecularstructuresarequiteapplicableinpharma- ceuticals,foods,cosmetics,home/personalcareandtextilesareas [14–16].Additionally,ﬁltrationandseparationsystemsareanother applicationﬁeldsforCDmoleculesowingtotheircapturingcapa- 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,watersolubilityofCDsrestrictstheiruseforwaterpuriﬁ- 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 includesthelastingattachmentsofCDmoleculesonthefibersur- face.Thus,complexformationpropertyofCDmoleculeswouldbe integratedwiththehighsurfaceareaofpolymericnanofibersina morepermanentwaythatwouldleadtoproductionofpromising filteringmembranes.Actually,thechemicalsurfacemodification withCDmoleculeswasfirstlyperformedonthefiberandfabric surfacesbyusingappropriatecrosslinkingagents[23–30].These functionalizationswereperformedbygraftingCDsontosubstance [23–26]orthesubstanceswerecoveredbycrosslinkedCDpoly- mers[27–30]forthefiltrationofwastemoleculesordeliveryof drugs,antibacterialsetc.Ontheotherhand,wehavefirstlyreported thesurfacemodificationofelectrospunnanofiberswithCDpoly- 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-vinylbenzylchloride)- 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 wereproducedbythegraftingofPNIPAMbrushesonthepoly((3- mercaptopropyl)methylsiloxane)(PMMS)nanofiberswiththeaid of“click”chemistry[36].Inanotherrelatedstudy,Lancuskietal.

developedcarbohydrate-decoratedPCLnanofibersforthespecific proteinadhesionbyapplying“click”reaction[37].In oneofthe associatedstudies,Qianetal.reportedtheintroductionofsaccha- rideresiduestothesurfaceofthepolyphosphazenenanofibrous membraneusing“click”chemistry[38].Mostofthestudiesmen- tionedabovefocusonthebiomedicalapplicationsofnanofibers.

Ontheotherhand,inourstudywehaveapplied“click”reaction forsurfacemodificationofnanofiberstoimprovetheirfiltration performance.

Polycyclicaromatichydrocarbons(PAHs)areoneofthemost widespreadpollutants whicharehighlytoxic,carcinogenic, and theirtoxicity increases withincreasing molecularweight [39].

Moreover,severalstudiesshowthatPAHspollutionscauseseri- 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 ofnanofiberswerecarriedoutbyusingscanningelectronmicro- scope(SEM).The surfacecharacteristicsof thenanofiberswere investigated by attenuated total reflectance-Fourier transform infraredspectroscopy(ATR-FTIR)and x-rayphotoelectronspec- troscopy(XPS).Furthermore,thecomparativemolecularfiltration

performance of ␤-CD-functionalized CAnanofibersand pristine CAnanofiberswereinvestigatedbyremovingphenanthrene(asa modelPAH)fromtheaqueoussolutions.Ourpreliminaryfindings suggestedthat␤-CD-functionalizedCAnanofibershavepotentials tobeusedasmolecularfiltersforthepurposeofwaterpurifica- 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 solutionofCAwaspreparedbydissolv- 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).Theelec- 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-toluenesulfonylchlo- 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 powder 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, itwascooledtoroomtempera- 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-propanolandwatertoremovetheunreactedchem- 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)sulfateanhy- drous(0.052mmol,8.8mg)in1.5mlwaterwerepreparedandboth ofthemaddedintothesolutionwhichincludeazide-␤-CDandCA- propargylnanofibers.Thissystemwasstirredabout24hatroom temperature.Finally, obtainedCA-CD nanofiberswereremoved fromthesolution,washedwithwateranddriedatvacuumoven at40^◦C.

Characterizationsandmeasurements

Themorphologicalcharacterizationandthediametercalcula- 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-rayphotoelectronspectrom- eter(XPS)(ThermoScientific).XPSwasusedbymeansofaflood

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gunchargeneutralizersystemequippedwithamonochromated AlK-␣ x-ray source(h=1486.6eV). Thehighresolution spec- traofCandNwerealsorecordedfortherelatedsamplestoget moredetailedinformation.Highperformanceliquidchromatogra- phy(HPLC)system(Agilent1200Series)wasusedtoinvestigate thephenanthreneremovingperformanceofbothCAandCA-CD nanofibers.Theseparationofphenanthrenewasperformedwith ZorbaxEclipseXDB-C18column(150mm×4.6mm,5␮mparticle size)anditwasdetectedat254nmwavelength.Acetonitrile(100%) wasusedasmobilephaseataflowrateof0.3ml/min.andtheinjec- tionvolumewaskeptat10␮l.Thephenanthrenewassolvedin acetonitrileandthendilutedinwatertocarryoutthemeasure- ments.The0.1gweightednanofiberswereimmersedin1.8ppm phenanthreneincludedwatersolutions(30ml)and0.5mlaliquots weretakenfromthesystematdefinitetimeintervals.Thecalibra- 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.Theexperimentswerecar- riedoutintriplicateandtheresultsweregivenwiththeirstandard deviations.

Resultsanddiscussion Formationofazide-ˇ-CD

The modiﬁcation of the ␤-CD molecules was conﬁrmed by usingFTIRspectraasillustratedin Fig.2a.Asseen, thecharac- 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,toluenesulfonylgroupsignalswasdisappearedinFTIRspec- trumandstretchingfrequencyofN₃becameobviousat2040cm⁻¹ demonstrating asymmetrical azide (–N3) functionality of ␤-CD [44].

Morphologicalcharacterizationofnanoﬁbers

ThemorphologicalbehaviorofCAnanofibersbeforeandafter thesurfacemodificationhavebeencomparedbySEMasdepicted inFig.3.AsitisshownintheSEMimages, somechangeswere occurredatthemorphologyofCAnanofibersaftereachprocess.The uniformandsmoothmorphologywasobservedforun-modifiedCA nanofibers,whereasslightswellingwasobservedbythepropar- gyltreatment(Fig.3aandb).Theroughandirregularappearance wasrecordedafterthe“click”reactionwhichprovedthesuccessful surfacemodificationofCAnanofibers.Thesimilarmorphological changewasalsoobservedinastudyofourresearchgroupinwhich theCDpolymerwasgraftedonthePETnanofibers[31].Theover- allresultssuggestedthat,adoptedproceduredidnotcausetoany deformationandfibrousstructureofnanofiberswaspreserveddur- 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.FTIRspec- trumofCAalsoshowedabsorbancebandat2924and2855cm⁻¹ fortheC–Hstretching.Initially,CAnanofibersweremodifiedwith propargylbromide.Thismodificationwasobviousfromtheappear- 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.Table1sum- marizesthecompositionalpercentagesofnanofiberswhichwere obtainedasaresultofwideenergysurveyscan.Itwasobserved that,C1sandO1saretwointensiveelementsasthemaincompo- 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.

Fig.4.HighresolutionC1sXPSspectraof(a)CA,(b)CA-propargyland(c)CA-CDnanoﬁbers.(d)HighresolutionN1sXPSspectrumofCA-CDnanoﬁbers.

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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.Thecorre- 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,theC1sspec- 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,respectively [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 ofencapsulatingorganiccompoundsduetotheirhydrophobiccav- 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 kepttheirfiberstructureafterthefiltra- tiontest(Fig.6).CAisalready goodcandidatefor thefiltration oforganicpollutantsandtherearealsoreportsintheliterature abouttheuptakingofPAHsfromtheconcernedenvironmentby usingCAbasedmembranes[56–59].Ontheotherhand,tothebest knowledge,thisisfirststudyabouttheinvestigationofmolecular filtrationcapabilityofCAnanofibersanditsCDmodifiedtypeby

“click”chemistry.Fromourresults,itcanbeconcludedthat,thesur- facemodificationofelectrospunCAnanofiberswithCDmolecules improvedthemolecularfiltrationpotentialbyutilizingfromthe inclusioncomplexationpropertyofCDs.The“click”chemistryisa quitenewandpromisingmethodforthefunctionalizationofelec- trospunnanofibers.Inourstudy,betteradsorptionefficiencywas obtainedforCDmodifiedCAnanofiberscomparedtountreatedone duringtheremovingtest.However,theadsorbedamountofPAH orotherorganiccompoundscanbeenhancedbygraftinghigher amountofCDonthenanofibersurfaceusing“click”chemistry.

Conclusion

Inthisstudy,thepermanentgraftingofCDmoleculesonthe electrospunCAnanofiberswasachievedbyusing“click” chemistry.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 graftedCAnanofiberswasinvestigatedbytheremovalofphenan- threnefromitsaqueoussolution.Forcomparison,filtrationtestof pristineCAnanofiberswasalsoperformed.Itwasobservedthat, CA-CDnanofibersadsorbedhigheramountofphenanthrenecom- paredtoCAnanofibersduetotheinclusioncomplexationcapability ofCDmolecules.Wehavealsocheckedthat,thefibrousstructure ofnanofiberswasprotectedafterthefiltrationtest.Inbrief,our

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

Acknowledgements

Dr.T.UyaracknowledgesTUBITAK-TheScientiﬁcandTechno- logicalResearchCouncilofTurkey(project#110M612)forfunding theresearch.Dr.T.UyaralsoacknowledgesEUFP7-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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