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Carbohydrate Polymers
jo u r n al h om ep a ge : w w w . e l s e v i e r . c o m / l o c a t e / c a r b p o l
Electrospun zein nanofibers incorporating cyclodextrins
Fatma Kayaci, Tamer Uyar
∗UNAM-InstituteofMaterialsScience&Nanotechnology,BilkentUniversity,Ankara,06800,Turkey
a r t i c l e i n f o
Articlehistory:
Received26December2011
Receivedinrevisedform16March2012 Accepted22May2012
Available online 30 May 2012
Keywords:
Cyclodextrin Electrospinning Zein
Nanofibers
a b s t r a c t
Zeinnanofiberscontainingcyclodextrins (zein/CD)wereproducedvia electrospinning.Three types ofCDs(␣-CD,-CDand␥-CD)having10%,25%and50%(w/w)wereindividuallyincorporatedinto zeinnanofibers.SEMimagingelucidatedthatthemorphologiesoftheelectrospunzein/CDnanofibers dependedontheCDtypeandweightpercentage.TheincorporationofCDsinzeinimprovedtheelectro- spinnabilityandbead-freenanofiberswereobtainedatlowerzeinconcentrations.Zein/CDnanofibers havingfiberdiameters∼100–400nmwereobtaineddependingonthezeinconcentrations,typesand weightpercentagesofCD.XRDstudiesrevealedthatCDsweremostly distributedwithoutforming crystallineaggregatesforzein/CDnanofiberscontaininglowerweightpercentageofCDs.Thesurface analysesofzein/CDnanofibersbyATR-FTIRandXPSindicatedthatsomeoftheCDswerepresentonthe fibersurface.Thermalanalysesshowedthatzein/-CDnanofibershaveshownhigherglasstransition temperaturesandhigherdegradationtemperaturewithincreasingCDcontent.
© 2012 Elsevier Ltd. All rights reserved.
1. Introduction
Recently,electrospinningtechniquehasgainedagreat inter- est since this technique is quite versatile for fabricating nanofibers/nanowebs from various synthetic or natural poly- mers,polymerblends,sol–gels,ceramics,etc.(Agarwal,Greiner,
&Wendorff,2009;Bhardwaj&Kundu,2010;Ramakrishnaetal., 2006; Teo&Ramakrishna, 2009).Moreover, functional electro- spunnanofibrouscompositestructurescanalsobeproducedby incorporating functional additives and/or nanoparticles in the fiber matrix or on the fiber surface (Andrew & Clarke, 2008;
Anitha,Brabu,Thiruvadigal,Gopalakrishnan,&Natarajan,2012;
Dong, Wang, Sun, & Hinestroza, 2008; He, Hu, Yao, Wang, &
Yu, 2009; Roso, Sundarrajan, Pliszka, Ramakrishna, & Modesti, 2008; Zhang, Shao, et al., 2011).Unique properties of electro- spun nanofibers/nanowebs including a relatively large surface area tovolume ratio and pore sizes within the nanoscale and multi-functionality due to the presence of functional additives andnanoparticlesmakethemfavorablecandidatesinavarietyof applicationareassuchasmembranes/nanofilters,wounddressing, tissue engineering,drugdelivery, nanotextiles,nanocomposites, energy,environment,etc.(Agarwaletal.,2009;Bhardwaj&Kundu, 2010; Chigome,Darko, &Torto, 2011;Lu, Wang,&Wei, 2009;
Ramakrishnaetal.,2006;Teo&Ramakrishna,2009;Thavasi,Singh,
&Ramakrishna,2008;Xie,Li,&Xia,2008;Yoon,Hsiao,&Chu,2008).
∗ Correspondingauthor.Tel.:+903122903571;fax:+903122664365.
E-mailaddresses:tamer@unam.bilkent.edu.tr,tameruyar@gmail.com(T.Uyar).
Inrecentyears,biopolymersfromrenewableresourcessuchas zeinhavegainedattentionforeconomicalandenvironmentalrea- sons(Paraman, &Lamsal,2011;Selling&Woods,2008;Selling, Woods,Sessa,&Biswas,2008).Zein,themajorproteinofcornand aby-productofthebioethanolindustry,isanon-toxic,biocompat- ible,biodegradablepolymerandthispolymercanformfilms.Zein filmsandzeinmicro/nanoparticlescanbeusedforencapsulation ofessentialoils,aromasandflavors, controlledreleaseofactive additives and as an activefood packagingmaterial, etc. (Alkan etal.,2011;Parris,Cooke,&Hicks,2005;Patel,Heussen,Hazekamp, Drost,&Velikov,2012;Sanchez-Garcia,Hilliou,&Lagaron,2010;
Shi,Kokini,&Huang,2009;Zhong,Jin,Davidson,&Zivanovic,2009).
Intherecentyears,electrospinningofzeinnanofibershavereceived muchattentionaswell(Jiang,Reddy,&Yang,2010;Jiang&Yang, 2011;Jiang,Zhao,&Zhu,2007;Miyoshi,Toyohara,&Minematsu, 2005;Sellingetal.,2007,2008;Torres-Giner,Gimenez,&Lagaron, 2008;Yao,Li,&Song,2009).Thesestudiesaremostlyrelatedtothe optimizationoftheelectrospinningparametersofzeinnanofibers (Miyoshietal.,2005;Sellingetal.,2007;Torres-Gineretal.,2008), crosslinkingof zeinnanofibers(Jianget al.,2010;Jiang&Yang, 2011;Sellingetal.,2008)andblendingofzeinwithsomeother typeofbiopolymers(Jiangetal.,2007;Yaoetal.,2009a;Yao,Li, Song,Li,&Pu,2007).Inaddition,-carotenewhichisabioactive antioxidant(Li,Lim,&Kakuda,2009)and(−)-epigallocatechingal- letethatisaplantpolyphenol(Fernandez,Torres-Giner,&Lagaron, 2009)wereincorporatedintoelectrospunzeinnanofibermatrixfor thestabilizationoftheseactiveadditives.
Ourparticularinterestisthefunctionalizationofelectrospun nanofibers with cyclodextrins (CDs). CDs are cyclic oligosac- charides having a toroid-shapedmolecular structure. The most 0144-8617/$–seefrontmatter © 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.carbpol.2012.05.078
Fig.1. (a)Chemicalstructuresof␣-CD,-CDand␥-CDand(b)schematicrepresentationofCD.
commonCDs are named as ␣-CD, -CD and ␥-CD having 6, 7 and8glucopyranoseunits,respectively(Fig.1).Hydrophobiccav- ityof CD actsas a host forthe variousmolecules, and CD can formnon-covalenthost–guestinclusioncomplexes.Thephysical andchemicalpropertiesoftheguestmoleculesaretailoredand becomemorestablewhencomplexedwithCDs(DelValle,2004;
Hedges,1998; Szejtli, 1998), therefore, CDsand their inclusion complexesarequiteapplicableinmanyfieldsincludingpharma- ceuticals,functionalfoods,cosmeticsandhome/personalcareand textiles(DelValle,2004;Hedges,1998;Szejtli,1998,2003).Up todate,severalstudieshavebeencarriedoutdealingwithincor- porationofCDsinelectrospunnanofibersfordifferentpurposes suchascrosslinking offibermatrix(Li&Hsieh,2005), molecu- larfiltration(Uyar,Havelund,Hacaloglu,Besenbacher,&Kingshott, 2010;Uyar,Havelund,Nur,etal.,2010,2009;Zhang,Chen,&Diao, 2011)andCDwasalsousedasareducingandstabilizingagentfor gold(Baietal.,2008)andsilver(Chae,Kim,Yang,&Rhee,2011) nanoparticlesformation.Inourrecentstudies,CDsandCDinclu- sioncomplexes(CD-ICs)ofvolatilefragrancesweresuccessfully incorporatedintoelectrospunnanofibers,andtheseCDfunction- alizedelectrospunnanofiberswereusedasmolecularfilters(Uyar, Havelund,Hacaloglu,etal.,2010;Uyar,Havelund,Nur,etal.,2010, 2009), and CD-ICshaveprovided longershelf-lifeandstabiliza- tionofvolatilefragrancesathighertemperature(Kayaci&Uyar, 2012;Uyar,Hacaloglu,&Besenbacher,2009;Uyar,Hacaloglu,&
Besenbacher,2011;Uyar,Nur,Hacaloglu,&Besenbacher,2009).
Inthisstudy,wereportontheelectrospinningofzeinnanofibers incorporatingCDs.Electrospunzein/CDnanofiberswereobtained byusingthreetypesofCDs;␣-CD,-CDand␥-CDandtheweight loadingsoftheseCDswerevariedfrom10%upto50%(w/w)with respecttozein.WefoundthattheadditionofCDinthepolymer solutionsimprovetheelectrospinnabilityofthezeinnanofibers at lower polymer concentration. The morphological, structural, surfaceand thermal characteristicsof theresultingelectrospun zein/CDnanofibers werecharacterizedby SEM, XRD,ATR-FTIR, XPS,DSCandTGA.Thisstudymainlydealswiththeoptimization ofelectrospinningofzein/CDnanofibersandtheirmorphological, structural,surfaceandthermalcharacterizations.
2. Experimentalpart 2.1. Materials
Zeinfrommaize(Sigma–Aldrich)andN,N-dimethylformamide (DMF, Pestanal, Riedel) were purchased. The alpha-, beta- and gamma-cyclodextrins(␣-CD,-CDand␥-CD)werepurchasedfrom WackerChemieAG(Germany).Allmaterialswereusedas-received withoutanypurification.
2.2. Preparationofthesolutions
First,40%,50%and60%(w/v)zeinweredissolvedinDMFand electrospinningofzeinsolutionswithoutCDswasperformed.For theelectrospinningofzein/CDsolutions,10%,25%and50%(w/w, withrespecttozein)CDs(␣-CD,-CDand␥-CD)weredissolved inDMFandthen,40%,50%and60%zein(w/v,withrespecttosol- ventofDMF)wereaddedtoeachCDsolutionseparatelyandstirred for1hat roomtemperature.Thecompositionsof thesolutions weresummarizedin Table1.Homogeneous andclearsolutions wereobtainedforallofthezein/-CDcompositions.Ontheother hand,thezeinsolutionscontaining25%(w/w)␣-CDwasslightly turbidandalsothesolutionscontaining50%(w/w)␣-CDand␥-CD werehighlyturbid.Theresultingzeinandzein/CDsolutionswere electrospun.
2.3. Electrospinning
Thesolutionswereplacedina3mLsyringefittedwithametal- licneedlehavinginnerdiameterof0.8mm.Thesyringewasfixed horizontallyonthesyringepump(Model:SP101IZ,WPI).Several parameterswereappliedinordertooptimizetheelectrospinning ofthesolutionsandtheoptimalparameterswerechosenasfollows.
Voltageof15kVwasappliedtothemetalneedletipbyusinghigh voltagepowersupply (AUSeries,MatsusadaPrecision Inc.).The polymersolutionwaspumpedwithflowrateof0.5mL/hduring electrospinningandthetip-to-collectordistancewassetto12cm.
Thegroundedstationarycylindricalmetalcollector(height:15cm, diameter:9cm)coveredwithaluminumfoilwasusedforthedepo- sitionoftheelectrospunnanofibers.Theelectrospinningprocess wascarriedoutat24◦Cand30%relativehumidityinanenclosed Plexiglasbox.
2.4. Measurementsandcharacterization
TheviscosityofthesolutionswasmeasuredbyusingAntonPaar PhysicaMCR301Rheometerequippedwithcone/plateaccessory usingthespindletypeCP40-2at22◦Candaconstantshearrateof 100s−1.Theconductivitymeasurementofthesolutionswasper- formedbyusingMultiparametermeterInoLab®Multi720(WTW) atroomtemperature.
Themorphologyandthediameterofthenanofiberswereexam- ined by usingscanning electron microscope (SEM) (FEI-Quanta 200FEG).Thenanofiberswerecoatedwith5nmAu/Pdpriorto SEMimaging.Around100fiberdiametersweremeasuredfromthe SEMimagestodeterminetheaveragefiberdiameter(AFD)ofthe nanofibers.
Table1
Propertiesofzeinandzein/CDsolutionsandtheresultingzeinandzein/CDnanofibers.
Solutions %zein (w/v)a
TypeofCD(%) (w/w)b
Viscosity (Pas)
Conductivity (S/cm)
Fibermorphology Averagefiberdiameter
(AFD)(nm)
zein40 40 – 0.0332 435 Nanofiberswithmanybeads –
zein50 50 – 0.0859 344 Nanofiberswithfewbeads 80±35
zein60 60 – 0.206 264 Bead-freenanofibers 170±30
zein40/␣-CD10 40 ␣-CD,10 0.0421 359 Nanofiberswithmanybeads –
zein40/-CD10 40 -CD,10 0.0428 357 Nanofiberswithmanybeads –
zein40/␥-CD10 40 ␥-CD,10 0.0439 333 Nanofiberswithfewbeads 60±10
zein40/␣-CD25 40 ␣-CD,25 0.0522 270 Nanofiberswithfewbeads 60±20
zein40/-CD25 40 -CD,25 0.0562 283 Nanofiberswithfewbeads 70±20
zein40/␥-CD25 40 ␥-CD,25 0.0732 267 Nanofiberswithfewbeads 60±10
zein40/␣-CD50 40 ␣-CD,50 0.0849 96.8 NanofiberswithbeadsandCDaggregates –
zein40/-CD50 40 -CD,50 0.0727 78.8 NanofiberswithbeadsandCDaggregates –
zein40/␥-CD50 40 ␥-CD,50 0.101 115.6 NanofiberswithbeadsandCDaggregates –
zein50/␣-CD10 50 ␣-CD,10 0.125 286 Bead-freenanofibers 90±20
zein50/-CD10 50 -CD,10 0.171 278 Bead-freenanofibers 100±25
zein50/␥-CD10 50 ␥-CD,10 0.212 268 Bead-freenanofibers 110±30
zein50/␣-CD25 50 ␣-CD,25 0.212 138 NanofiberswithCDaggregates 185±45
zein50/-CD25 50 -CD,25 0.208 167 Bead-freenanofibers 150±30
zein50/␥-CD25 50 ␥-CD,25 0.239 161 Bead-freenanofibers 155±35
zein50/␣-CD50 50 ␣-CD,50 0.39 74.3 NanofiberswithCDaggregates 240±85
zein50/-CD50 50 -CD,50 0.381 97.8 NanofiberswithCDaggregates 360±140
zein50/␥-CD50 50 ␥-CD,50 0.354 126.5 NanofiberswithCDaggregates 265±110
zein60/␣-CD10 60 ␣-CD,10 0.329 211 Bead-freenanofibers 225±30
zein60/-CD10 60 -CD,10 0.292 200 Bead-freenanofibers 185±40
zein60/␥-CD10 60 ␥-CD,10 0.218 189.4 Bead-freenanofibers 170±40
zein60/␣-CD25 60 ␣-CD,25 0.69 89.8 NanofiberswithCDaggregates 375±80
zein60/-CD25 60 -CD,25 0.441 113 Bead-freenanofibers 410±130
zein60/␥-CD25 60 ␥-CD,25 0.664 109.6 Bead-freenanofibers 380±240
zein60/␣-CD50 60 ␣-CD,50 1.56 41.6 Nofiberformation –
zein60/-CD50 60 -CD,50 1.02 85.6 Nofiberformation –
zein60/␥-CD50 60 ␥-CD,50 0.752 85.8 Nofiberformation –
aWithrespecttosolvent(DMF).
bWithrespecttopolymer(zein).
X-raydiffraction(XRD)dataofthenanofiberswerecollectedby usingPANalyticalX’PertPowderdiffractometerwithCuK␣radia- tioninarange2=5–30◦.
Surface characterizations of the nanofibers were performed by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) (Bruker, VERTEX 70) and K-Alpha- monochromated high-performance X-ray photoelectron spec- trometer (XPS) (Thermo Scientific). The ATR-FTIR spectrawere recordedfrom700to4000cm−1witharesolutionof4cm−1bytak- ing64scansforeachsample,andthesespectrawereobtainedwith FTIRspectrometerequippedwithaliquidnitrogencooledmercury cadmiumtelluride(MCT) detectorbyusingATRsetupcontain- ingagermaniumcrystal.XPSwasusedbymeansofafloodgun chargeneutralizersystemequippedwithamonochromatedAlK-
␣X-raysource(h=1486.6eV).Wideenergysurveyscans(WESS) wereobtainedoverthe0–1360eVbindingenergy(BE)rangeata detectorpassenergyof150eVinordertodeterminethesurfaceele- mentalcompositionofthenanofibers.Thehighresolutionspectra wererecordedforC1sregionatpassenergyof50eV.
Thethermalpropertiesofthenanofiberswereinvestigatedby usingdifferentialscanningcalorimetry(DSC)(TAQ2000)andther- mal gravimetric analyzer (TGA) (TA Q500). DSC analyseswere
carriedoutwithabout5mgofsamplesundertheN2 asapurge gas.Initially,thesampleswereequilibratedat25◦Cthentheywere heatedto200◦Cat10◦C/min.TGAwasperformedfromroomtem- peratureto500◦Cataheatingrateof20◦C/minunderthenitrogen atmosphere.
3. Resultsanddiscussion
3.1. Electrospinningofzeinnanofibers
In the literature,the electrospinning of zeinnanofibers was mostlycarriedoutbyusingethanol/watermixturesolventsystem whichresultedinribbon-likefibermorphologyduetotherapid skinformationandcollapseofthefibercorebecauseoftheveryfast evaporationofthesolvent(Miyoshietal.,2005;Sellingetal.,2007;
Torres-Gineretal.,2008).However,round-shapedzeinnanofibers canbeobtainedbyusingsolventsystemshavinghighboilingpoints suchasDMF(Jiangetal.,2007).Inourstudy,theelectrospinningof zeinnanofiberswascarriedoutbyusingDMFasasolventsystem.
ThereasonofchoosingDMFisbecausethinnerandmoreuniform zeinfiberscanbeobtained(Jiangetal.,2007)whencomparedto ethanol/watersystem(Miyoshietal.,2005;Sellingetal.,2007;
Fig.2.RepresentativeSEMimagesofelectrospunzeinnanofibersobtainedfromzeinsolutionsinDMFataconcentrationof(a)40%,(b)50%and(c)60%(w/v).
Torres-Gineretal.,2008),andothermoreimportantreasonisthat DMFisaverygoodsolventforCDs,andthereforewewereableto preparezein/CDhomogeneoussolutionsinmostcases.
Thecharacteristics (composition, viscosityand conductivity) ofthe zeinand zein/CD solutions and themorphologiesof the electrospunnanofibersandtheiraveragefiberdiameter(AFD)are summarizedin Table1. Zeinsolutionshavingdifferentconcen- trationswereelectrospuninorder tofindtheoptimalpolymer concentrationforobtainingbead-freeuniformnanofibers.Therep- resentativeSEMimagesofzeinnanofiberselectrospunfrom40%, 50%and60%(w/v)zeinsolutionsinDMFaredepictedinFig.2.At lowerzeinconcentration(40%,w/v),micronsizeirregularspher- icalbeadedstructureswereobtainedduetothelowviscosityof thepolymersolution.Astheconcentrationofzeinsolutionwas increasedto50%(w/v),thenumberofbeadswasdecreasedsig- nificantly and theshape of beads became more elongated and nanofibershavingAFDof80±35nmwereobtained.Uniformand bead-freezeinnanofibershavingAFDof170±30nmwereobtained when60%(w/v)zeinsolutionwaselectrospunindicatingthat60%
(w/v)istheoptimalzeinconcentrationforproducinguniformzein nanofibersattheappliedelectrospinningconditions.Ourresults correlatewiththeliteraturefindingswherethebead-freeuniform zeinnanofiberswereproducedabove50%(w/v)zeinconcentra- tionwhenDMFwasusedasasolventsystem(Jiangetal.,2007).
Thisbehaviorisverytypicalfortheelectrospinningofpolymeric solutionswherethetransitionfrombeadedstructuretobead-free nanofibersisobservedbyincreasingthepolymerconcentration.
Higherpolymerconcentrationresultedinhighersolutionviscos- ityduetothepresenceofmorepolymerchainentanglementsand thereforethebeadedstructuresareeliminatedsincetheelectrified polymerjetcanbestretchedfullyyieldingbead-freenanofibers (Ramakrishna,Fujihara,Teo,Lim,&Ma,2005;Uyar&Besenbacher, 2008).
3.2. Electrospinningofzein/CDnanofibers
Thezein/CDsolutionswereclearandhomogeneousexceptfor thesolutionscontaininghigherweightpercentageof␣-CDand␥- CD.Thezein/-CDsolutionswereclearinallcompositionswhereas thezein/CDsolutionscontaining50%(w/w)␣-CDand␥-CDwere highlyturbid, andzein/CDsolutioncontaining25% (w/w)␣-CD wasslightlyturbid.Theturbiditywasobservedpossiblybecauseof theprecipitationofthe␣-CDand␥-CDathigher%loading,andit isanticipatedthattheelectrospinningofthesezein/CDsolutions wouldcontainCD aggregatesin thefibermatrix. Inthecase of clearzein/CDsolutions,thehomogeneousdistributionoftheCDsin thefibermatrixisexpectedfortheelectrospunzein/CDnanofibers containingloweramountofCDs.TheSEMimagingoftheelectro- spunzein/CDnanofibersgavesomeinsightfulinformationforthe presenceofCDaggregatesinthefibermatrix.
The representative SEM images of the electrospun zein/CD nanofibersaredepictedinFig.3.Itwasobservedthattheaddition ofCDstozeinsolutionsimprovedtheelectrospinnability,andless beadedstructuresand/orbead-freenanofiberswereobtainedat lowerzeinconcentrationswhencomparedtozeinsolutionswith- outCDs.
The electrospinning of 40% (w/w) zein solutions containing CDs resulted in nanofibershaving much less beaded structure whencomparedto40%(w/w)pristinezeinsolution.Fig.3ashows theSEMimagesofelectrospunzein/CDnanofibersobtainedfrom 40%(w/v)zeinsolutioncontaining10%,25%and50%(w/w,with respecttozein)CDs(␣-CD,-CDand␥-CD).Theelectrospinning of40%(w/w)zeinsolutions containing10%(w/w)CDsresulted inreduction of beadstosomeextent(Fig.3a1–a3). Inthecase ofzein40/␥-CD10sample,theeliminationofbeadsismuchmore pronounced which is possibly because of the higher solution
viscositycompared tozein40/␣-CD10and zein40/-CD10solu- tions.Furthermore,itwasclearlyobservedthattheadditionof25%
(w/w)CDstothe40%(w/v)zeinsolutionsimprovedtheelectro- spinnabilityofthezein/CDsolutions,andyieldednanofiberswith muchlessbeadshavingmoreelongatedstructures(Fig.3a4–a6).
Thisispossiblybecauseofthehighersolutionviscosityofzein/CD systemswherethebeadedstructuresaremostlyeliminateddue tothemorestretchingofelectrifiedsolutionjet.However,even theadditionof50%(w/w)CDsresultedinmoreviscoussolutions, theelectrospinningofthesezein/CDsolutionsyieldednanofibers havingirregularstructures(Fig.3a7–a9)suggestingthatuniform zein/CDnanofiberscannotbeproducedwhenthehighweightper- centageofCDswasused.Theirregularstructuresconsistingofnot onlybeadsbut alsoCDaggregates weremuch moreprominent inzein40/␣-CD50andzein40/␥-CD50nanofiberswhencompared tozein40/-CD50,sincethezein40/␣-CD50and zein40/␥-CD50 solutions werehighlyturbidprior toelectrospinning indicating thatCDaggregateswerealreadypresentinthesolution,andthese CDaggregatespossiblycouldnotbestretchedoutalongthefiber matrixduringtheelectrospinningprocess.Inthecaseofzein40/- CD50,thebeads wereless in number since thezein40/-CD50 solutionwasclear,andsomeCDaggregateswerepossiblyformed duringtheelectrospinningprocesswhenthesolventevaporation tookplace.
The electrospinning of 50% (w/v) zein solution yielded nanofiberswithfew beads as mentioned above(Fig.2b),how- ever, bead-free nanofibers were obtained from 50% (w/v) zein solutionwiththeadditionof10%and25%(w/w)CDs(␣-CD,- CDand␥-CD)exceptforzein50/␣-CD25system(Fig.3b).Inthe caseofzein50/␣-CD25nanofibers,someirregularstructureswere observedwhichisbecauseofthepresenceof␣-CDaggregatesas discussedlaterintheXRDsection.Similarto40%(w/v)zeinsys- tem,theadditionof 50% (w/w)CDsin 50% (w/v)zeinsolution yieldednanofibershavingirregularstructuresduetotheaggre- gationof CD crystalsasconfirmed byXRD results.It wasclear thattheadditionofCDsincertainratios(10%and25%,w/w)to the50%(w/v)zeinsolutionsassistedtoeliminatethebeadforma- tionandprovidedbead-freezein/CDnanofiberswithoutincreasing polymerconcentration.We observedsimilareffectonthemor- phologyoftheelectrospunPS(Uyar,Havelund,Hacaloglu,etal., 2009),PMMA(Uyar,Balan,Toppare,&Besenbacher,2009)andPEO (Uyar&Besenbacher,2009)nanofiberscontainingCDsinourrecent studies.
The electrospinning of 60% zein (w/v) solutions contain- ing 10% and 25% (w/w) CDs resulted in bead-free nanofiber morphology except for zein60/␣-CD25 system (Fig. 3c). The zein60/␣-CD25nanofibershavesomebead-likestructuressimilar tozein50/␣-CD25systemwhichispossiblyduetoCDaggregates.
Furthermore,60%zein(w/v)solutioncontaining50%(w/w)CDs couldnot be electrospundue tothe very highviscosity ofthe solutions.
In brief, the addition of CDs to zein solutions significantly affected the electrospinning, and bead-free nanofibers were obtainedfromlowerzeinconcentrationsforzein/CDsystemswhen comparedtopurezeinsolution.Thisismostlyduetothehigher viscosityofthe zein/CDsolutions,and highersolutionviscosity resultedinmoreuniformfiberswhenelectrospun(Ramakrishna etal.,2005;Uyar&Besenbacher,2008).Inaddition,themorpholo- giesof thezein/CDnanofibers containingdifferentkindof CDs (␣-CD,-CDand␥-CD)haveshownslightvariationsamongeach otherbecauseofthedifferencesinviscosityandconductivityof thesesolutions.ItwasobservedinTable1thattheAFDincreasesas thecontentoftheCDsincreasessincethepresenceofCDscausesa viscosityincreaseofthesolutionswhileitreducestheconductivity ofthesolutions.Therefore,zein/CDsolutionshavinghigherviscos- ityandlowerconductivityvaluesyieldedthickerfibersduetothe
Fig.3.RepresentativeSEMimagesofelectrospunnanofibersof(a1)zein40/␣-CD10,(a2)zein40/-CD10,(a3)zein40/␥-CD10,(a4)zein40/␣-CD25,(a5)zein40/-CD25,(a6) zein40/␥-CD25,(a7)zein40/␣-CD50,(a8)zein40/-CD50and(a9)zein40/␥-CD50;(b1)zein50/␣-CD10,(b2)zein50/-CD10,(b3)zein50/␥-CD10,(b4)zein50/␣-CD25,(b5) zein50/-CD25,(b6)zein50/␥-CD25,(b7)zein50/␣-CD50,(b8)zein50/-CD50and(b9)zein50/␥-CD50;(c1)zein60/␣-CD10,(c2)zein60/-CD10,(c3)zein60/␥-CD10,(c4) zein60/␣-CD25,(c5)zein60/-CD25and(c6)zein60/␥-CD25.
Fig.4.XRDpatternsof(a)(i)zein50,(ii)zein50/␣-CD10,(iii)zein50/␣-CD25,(iv) zein50/␣-CD50and(v)␣-CD;(b)(i)zein50,(ii)zein50/-CD10,(iii)zein50/-CD25, (iv)zein50/-CD50and(v)-CD;(c)(i)zein50,(ii)zein50/␥-CD10,(iii)zein50/␥- CD25,(iv)zein50/␥-CD50and(v)␥-CD.
lessstretchingoftheelectrifiedjet(Ramakrishnaetal.,2005;Uyar
&Besenbacher,2008).
3.3. Structuralcharacterizationofzein/CDnanofibers
TheXRDpatternsofelectrospunzeinnanofibersandzein/CD nanofibers are shown in Fig. 4 and the XRD patterns of as- receivedCDswerealsoshownforcomparison.Zeinnanofibershave showntwobroadpeakshavingmaximaat2=8.99◦(9.8 ˚A)andat
2=19.38◦(4.58 ˚A).Itisreportedthatthelargerd-spacingaround 10 ˚Aisassociatedwiththemeandistanceofapproachofneighbor- inghelices(thespacingoftheinter-helixpackingofzeinchains) whereas theshorter d-spacingat around4.5 ˚Ais relatedtothe averagebackbonedistancewithin␣-helixstructureofzein(Yao, Li,Song,Li,&Pu,2009).
CDs(␣-CD,-CDand␥-CD)arecrystallinematerialshavingdis- tinctdiffractionpatternsat2=5–30◦(Fig.4).CDsgenerallyhave twotypesofcrystalstructures;in‘cage-type’thecavityofeach CD moleculeis blockedby theadjacentCD molecules whereas theCDmolecules arealignedand stackedontopof eachother inthe‘channel-type’structure.TheXRDofas-receivedCDshave showndiffractionpatternsfor‘cage-type’crystallinestructuresas reportedintheliterature(Harata,1998;Rusaetal.,2002;Saenger etal.,1998).
Some structural changes were observed for the zein/CD nanofibersdependingontheweightpercentagesandtypesofCDs.
TheXRDofzein50/␣-CD10nanofibershaveshowntwobroadhalo diffractionpatternscenteredat2=8.94◦(9.9 ˚A)andat2=20.18◦ (4.4 ˚A)whichisverysimilartothezeinnanofibers.Thediffraction peaks for the ␣-CD crystals were absent in this sample indi- cating that ␣-CD molecules were distributed in the zein fiber matrixwithoutforminganyphase separatedcrystalaggregates.
Forzein50/␣-CD25andzein50/␣-CD50nanofibers,thedecreaseof thepeakat2=8.99◦suggestedthatthespacingoftheinter-helix packingofzeinchainswasdisturbedandthezeinmolecularaggre- gatesweresomehowdestroyedwiththepresenceof␣-CDathigher weightpercentages.Moreover,slightly intensediffractionpeaks wereobservedforzein50/␣-CD25nanofiberssuggestingthatsome aggregationof␣-CDcrystalswaspresentinthissample.The␣-CD crystallinepeaksweremuchmorepronouncedforthezein50/␣- CD50sample.ThisfindingcorrelateswiththeSEMimageswhere thebead-likestructuresforzein50/␣-CD25andirregularstructures forzein50/␣-CD50wereobservedfor thesesamplesdue tothe presenceofsome␣-CDaggregates.TheXRDpatternscorrespond tochannel-typepackingof␣-CDsincethesalientdiffractionpeak 2 ∼= 20◦ischaracteristicforthe␣-CDchannel-type(Harata,1998;
Rusaetal.,2002;Saengeretal.,1998).Ingeneral,the‘channel- type’packingofCDisassociatedwiththeinclusioncomplexstate.
For instance, Tonelliet al. reportedthat protein based polymer suchasBombyxmorisilkfibroincanformaninclusioncomplex with␥-CD(Cristian,Bridges,Ha,&Tonelli,2005).However,here wedidnotanticipatetheinclusioncomplexationofzeinwith␣- CD due tothe small size cavity of ␣-CD. As mentioned in the experimentalpart,thezein50/␣-CD25and zein50/␣-CD50solu- tionswereturbidanditismostlikelythat␣-CDprecipitatedas
‘channel-type’crystalsinthezein/DMFsolutionsystem.Wehave alsoobservedsimilarsituationsforelectrospunPScontaining␣- CDwherethe␣-CDprecipitatedas‘channel-type’crystalswithout forminginclusioncomplexation(Uyar,Havelund,Hacaloglu,etal., 2009).Additionally,theXRDofzein50/-CD50andzein50/␥-CD50 havealsoshown somediffractionpeaksdue tothepresenceof someCDcrystallineaggregatesbutthesediffractionsdidnotcorre- spondtochannel-typepackingsuggestingthatCDswerenotinthe complexstatewithzeinchainsinzein50/CD50nanofibers.InXRD, thetypicalchannel-type-CDhastwomajorpeaksat2 ∼= 11.5◦ and 18◦ (Harada,Okada, Li,&Kamachi, 1995), and the charac- teristicdiffractionforchannel-type␥-CDhasonemajorpeakat 2 ∼= 7.5◦withminorreflectionsat2 ∼= 14◦,15◦,16◦,16.8◦and22◦ (Uyar,Hunt,Gracz,&Tonelli,2006).However,forzein50/-CD50 andzein50/␥-CD50nanofibers,thediffractionpeaksweredifferent thanthechannel-typepackingasdiscussedbelow.
InXRD,itwasobservedthatthediffractionpatternsofzein50/- CD10 and zein50/-CD25 nanofiberswere very similar to that of purezeinnanofibers (Fig.4b).In thecase of zein50/-CD50 nanofibers, the intensity of first peak at around 2=9.0◦ was
loweredsignificantlyindicatingthattheinter-helixpackingofzein molecularaggregates were substantially disturbed. In addition, veryweakdiffractionpeaksataround2=6.9◦,13.6◦ and24.4◦ wereobservedforthissamplepossiblybecauseofthepresenceof some-CDaggregatesinthefibermatrix.However,thesepeaksdid notcorrespondtoeithercage-typepackingorchannel-typepack- ingindicatingthattheregularpackingof-CDwasdisturbedbythe zeinchains.Forzein50/-CD10andzein50/-CD25nanofibers,no crystallinepeakswereobservedsuggestingthat-CDmolecules weredistributedinthefibermatrixwithoutforminganycrystal aggregates.
Moreovertheincorporationof␥-CDintozeinfibermatrixhasa verysimilarstructuraleffectasseenforzein/␣-CDandzein/-CD.
TheXRDpatternofzein50/␥-CD10nanofiberswassimilartopure zeinnanofibersshowingtwodistinctbroadhaloataround2=9◦ and2=20◦(Fig.4c).Theincorporationof25%and50%(w/w)␥- CDtofibermatrixresultedindisruptionofinter-helixpackingof zeinchainsasdeducedfromtheXRDpatternsofzein50/␥-CD25 andzein50/␥-CD50sincethepeakataround2=9◦ wasconsid- erablyweakened.Inaddition,certaindiffractionpeaksataround 2=6◦,8.4◦ and17.4◦ wereobservedfor zein50/␥-CD50sample suggestingthatsomecrystalline␥-CDaggregateswerepresentin thissampleasobservedintheSEMimageofthissample.Yet,these peakssomewhatdifferentthanthecage-typepackingorchannel- typepackingsuggestingthat␥-CDpackingwasdisturbedbythe zeinchainswhichwassimilarlyobservedforthezein50/-CD50.
Inbrief,itwasobservedthattheshorterd-spacingca4.5 ˚Acorre- latedtotheaveragebackbonedistancein␣-helixstructureofzein didnotchangewiththeadditionofCDs,whiletheintensityofthe largerd-spacingaround9 ˚Aassociatedwiththemeandistanceof approachofneighboringhelicesdecreasedsignificantlyasthecon- tentofCDincreasedfrom10%to50%(w/w)inzein/CDnanofibers.
Thisresultimpliedthestructuralchangesdependingontheside- chainpackingwereobservedforzein withtheadditionofCDs.
Moreover,XRDdatasuggestedthatCDsweremostlydistributed inthezeinfibermatrixwithoutformingcrystallineaggregatesat lowerweightpercentages(10%of␣-CD,and10%and25%of- CDand␥-CD),but,incorporationof25%␣-CDand50%ofallthree typesofCDsyieldedsomecrystallineCDaggregatesinthezeinfiber matrix.
3.4. Surfacecharacterizationofzein/CDnanofibers
The surface analyses of zein/CD nanofiberswere performed byusingsurfacesensitivetechniques,ATR-FTIRandXPSinorder to corroborate the presence of CDs on the surface of the zein nanofibers.TheATR-FTIRspectraoftheelectrospunzeinnanofibers andzein/CDnanofibersareshowninFig.5a.Zeinproteinback- bonehastwocharacteristicvibrationalbands;amideIandamide II.ThecharacteristicabsorptionbandofamideIcorrespondstothe C Ostretching,whilethatofamideIIcorrespondsN Hbending andC Nstretching(Fernandezetal.,2009;Yaoetal.,2009b).The characteristicabsorptionbandsat1653and1540cm−1indicated thepresenceofamideIandamideII,respectivelyforpurezein nanofibers(Fig.5a1–a3).
ThecharacteristicpeakofcoupledC–C/C–Ostretchingvibra- tionsand the antisymmetric stretchingvibration of the C–O–C glycosidicbridgeofCDwereobservedforzein/CDnanofibersat 1028,1080and1150cm−1,respectively(Uyar,Balan,etal.,2009).
ItwasalsoclearthattheintensityofCDrelatedpeakswasincreased astheCDcontentincreasedfrom10%to50%(w/w)inthezein/CD nanofibers.TheATR-FTIRdataconfirmedthesuccessfulincorpora- tionofCDsinthezeinnanofibersandsomeCDswerepresenton thesurfaceofthezein/CDnanofibers.
InATR-FTIRstudy,itwasobservedthattheamideIandamide IIpeakswereslightlyshiftedtolowerwavenumberforzein/CD
Fig.5.ATR-FTIRspectraofelectrospunnanofibersof(a1)(i)zein50,(ii)zein50/␣- CD10,(iii)zein50/␣-CD25and(iv)zein50/␣-CD50;(a2)(i)zein50,(ii)zein50/- CD10,(iii)zein50/-CD25and(iv)zein50/-CD50;(a3)(i)zein50,(ii)zein50/␥- CD10,(iii)zein50/␥-CD25and(iv)zein50/␥-CD50and(b)overlayofXPSC1sspectra ofthezeinandzein/-CDnanofibersandpure-CDpowder.
Fig.6.(a1)DSCthermograms;(a2)enlargedregionofDSCthermogramsbetween140and190◦Cofelectrospunnanofibersof(i)zein50,(ii)zein50/-CD10,(iii)zein50/-CD25 and(iv)zein50/-CD50and(b)TGAthermogramsofzein50andzein50/-CDnanofibersandpure-CDpowder.
nanofiberswhencomparedtopurezeinnanofibers.Forinstance, theamideIpeakwasobservedat1651,1650and1648cm−1 for zein50/␣-CD10,zein50/␣-CD25andzein50/␣-CD50,respectively.
Similarly,theamideIIpeakwasshiftedtolowerwavenumberas theweightpercentageofthe␣-CDwasincreasedfrom10%to50%.
Thatis,absorptionpeakofamideIIwasobservedat1536,1535 and1520cm−1forzein50/␣-CD10,zein50/␣-CD25andzein50/␣- CD50,respectively.ThepeakshiftofamideIandamideIItolower wavenumbersfor zein/CDnanofiberssuggested thepresence of interactionbetweenzeinand␣-CD,andtheinteractionbecame morepronouncedfornanofibersampleshavinghigherloadingof
␣-CD.Inthecaseofzein/-CDandzein/␥-CDnanofibersamples, theshiftintheamideIwasnotsignificantbuttheamideIIpeakwas shiftedtoaround1535cm−1suggestingthepresenceofinteraction betweenCDmoleculesandzeinchainsforthesesamplesaswell.
But,thepeakshiftforamideIandamideIIwasmuchmoresignif- icantinthecaseofzein/␣-CDnanofiberscomparedtozein/-CD andzein/␥-CDnanofiberspossiblebecause␣-CDhassmallersize whichcaninteractmorewiththezeinchains.
AsobservedintheSEMimaging,theuniformnanofiberswere obtainedinthecaseof-CD,therefore,moredetailedsurfaceanal- ysesandthermalcharacterizationswerecarriedoutforzein/-CD nanofibers.Thein-depthsurface chemistryanalysesfor zein/- CDnanofiberswereperformedbyXPSinordertodetermineto whatextent-CDmoleculesarepresentonthesurfaceofthezein nanofibers.Table2showselementarycompositionsbasedonwide energysurvey spectraof the-CD,zeinnanofibersand zein/- CDnanofibers.Oxygencontentoutersurfaceofthesampleswas increasedwiththeincreasingtheamountof-CD(from10%to
50%,w/w)usedintheelectrospinningofnanofibers.Highenergy resolutionC1sspectrawerealsorecordedtogetmoredetailed informationaboutsurfacechemistryofthezein/-CDnanofibers.
Theoverlay of normalized C 1s spectraof thezein nanofibers, zein/-CDnanofibersand-CDaregiveninFig.5b.Therearethree differentcomponentsforalloftheC1shigh-resolutionspectra.The positionofoneisataround284.5eV,C1,isassignedtoaliphaticcar- bons,C–Cand/orC–H(Shietal.,2009;Uyar,Havelund,Hacaloglu, etal.,2010,2009;Uyar,Havelund,Nur,et al.,2010,2009).It is aprominentpeakforpurezeinnanofibers.ThecomponentC2at about285.7eVisarisenfromeitherC–O–CorC–OH;andC3(cor- relatedtoO–C–O)locatedataround287.7werefoundinbothzein nanofibersand-CD(Shietal.,2009;Uyar,Havelund,Hacaloglu, etal.,2010;Uyar,Havelund,Nur,etal.,2010,2009).Thepeaks aremore distinctivefor -CD,therefore therelativeconcentra- tionsofC2andC3increasedwithincreasingamountofthe-CD usedinthepreparationofzein/-CDnanofibers.Itisfoundthatthe increaseinoxygencontentisduetothepresenceofC–O–C/C–OH and/orO–C–Oonthesurfaceofzeinnanofibers.Thepresenceof
Table2
AtomicconcentrationsgeneratedfromXPSwideenergysurveyscans.
Samples C(%) O(%) N(%)
-CD 36.61 63.39 –
zein50 78.19 13.04 8.77
zein50/-CD10 77.06 15.14 7.8
zein50/-CD25 75.43 16.25 8.32
zein50/-CD50 72.62 18.64 8.74
the-CDonthesurfaceofthezein/-CDnanofibersisconfirmed withthese results. On the otherhand, the-CD concentration oftheprobedvolumeiscalculatedasapproximately4%,7%and 13%forzein/-CD10,zein/-CD25andzein/-CD50,respectively fromtheelementary compositionsin Table2. Thesurface con- tentofCDforallthreezein/-CDsamplesislowerthantheCD contentofthesolutionstheywerepreparedfrom.Thisindicates thatthesomeof theCDmoleculeslocatedonthefibersurface whereassomeCD moleculesareburied inthebulkof thefiber matrix.Zeinisausefulfoodpackagingmaterial(Alkanetal.,2011;
Shietal.,2009),andCDshaveinclusioncomplexationcapability withvarietyofmoleculesincludingaromas,colors,antioxidants, antibacterials,odors,andotherfunctionalingredients(DelValle, 2004;Hedges,1998;Szejtli,1998),therefore,zein/CDnanofibers mayhavethepotentialstobeusedasactivefoodpackaging(López- de-Dicastillo,Gallur, Catalá,Gavara,&Hernandez-Mu ˜noz,2010;
López-de-Dicastillo,Jordá-Beneyto,Catalá,Gavara,&Hernandez- Mu ˜noz,2011)materialsowingtosurfaceassociatedCDmolecules andtheirhighsurfaceareas.
3.5. Thermalcharacterizationofzein/CDnanofibers
Thethermalcharacteristicsofthezein/-CDnanofiberswere studiedbyDSCandTGA.Fig.6adisplaystheDSCthermograms ofzeinnanofibersandzein/-CDnanofiberscontaining10%,25%
and50%(w/w)-CD.Zeinnanofibersandzein/-CDnanofibers haveshownabroadendothermicpeakhavingapeakmaximumat around100◦CintheDSCthermogramindicatingthatthesamples containsomeamountofwater.Theglasstransitiontemperature (Tg)ofthesampleswasalsodetectedfromtheDSCthermograms.
TheTgofthepurezeinnanofiberswasobservedataround154◦C which is in closeagreement withtheTg value reportedin the literatureforzein(Torres-Giner,Gimenez,&Lagaron,2008;Torres- Giner,Ocio,&Lagaron, 2009).TheTg values forzein50/-CD10, zein50/-CD25andzein50/-CD50nanofiberswereobservedat around158◦C,165◦C and172◦C,respectively. Itwasclearthat thehigherTgvalueswereobservedastheweightloadingsof- CD wereincreasedfrom10% to50%.Theaddition ofCD inthe zeinnanofiberscausedanincreaseintheTgvalueswhichispos- sibly due tothe less chain mobility of zein in the presence of CD.
Fig. 6b shows the TGA thermograms of pure -CD, zein nanofibersandzein/-CDnanofibers.TheTGAofpure-CDhas aninitialweightloss(∼12%)below100◦Candmajorweightloss between325and350◦Cowingtowaterlossandmaindegrada- tion of -CD, respectively (Anitha et al., 2012).Similar tozein nanofibers,waterlossforzein/-CDnanofiberswerealsoobserved butthewaterweightpercentagewasaround3–5%(w/w)indicating thatlessamountofwaterwaspresentinthenanofibers.Inaddi- tion,anotherminorweightlossregimebetween125and200◦C wasobservedforzeinnanofibersandzein/-CDnanofibers.This weightlossispossiblyduetothepresenceofremainingsolvent (DMF)inthenanofibersamples.
Themajorweightlossforthezeinnanofiberswasrecordedat around275–350◦Cwhichisconsistentwiththemaindegradation temperaturereportedfortheelectrospunzeinnanofibers(Torres- Giner&Lagaron,2010;Woods,Selling,&Cooke,2009).Sincethe degradationtemperatureforzeinand-CDwasoverlapped,we observedasinglebutbroaderweightlossforzein/-CDnanofibers.
Moreover,weobservedthatthethermaldegradationofzein/CD nanofiberswasshiftedslightlytohighertemperaturewithincreas- ing-CDcontent.Hence,zein/-CDnanofibershaveshownslightly higherdegradationtemperaturecomparedtopurezeinnanofibers indicatingthattheincorporationoftheCDmoleculesinthezein fibermatrixresultedinhigherthermalstability.
4. Conclusion
Zein/CD nanofibers were obtained from electrospinning of zein/CDsolutions inDMF.Threetypes ofCDs(␣-CD,-CD and
␥-CD)usingdifferentweightloadings(10%,25%and 50%,w/w) wereincorporatedinzeinsolutionshavingvariousconcentrations (40%,50%and 60%,w/v),and thesezein/CDsolutionsweresuc- cessfullyelectrospun.WefoundthattheadditionofCDinthezein solutions causedanincrease in solutionviscosityandtherefore resultedinimprovementoftheelectrospinnability,andlessbeaded structuresand/orbead-freezein/CDnanofiberswereobtainedat lowerzeinconcentrationswhencomparedtopristinezeinsolu- tions.Dependingonthezeinconcentration,CDweightpercentage andCDtype,bead-freezein/CDnanofibershavingfiberdiameters in the range of ∼100–400nm were obtained. The morphologi- cal,structural, surfaceand thermalcharacterizationsof zein/CD nanofiberswerestudiedbySEM,XRD,ATR-FTIR,XPS,DSCandTGA.
SEMimagingrevealedthatthemorphologiesoftheelectrospun zein/CDnanofibersweresignificantlyaffectedbytheCD weight percentageandCDtypeaddedinzein/CDsolutions.XRDstudysug- gestedstructuralchangesforzeinchainpackingwherethespacing oftheinter-helixpackingofzeinchainswasdisturbedwiththe additionofCDs,inaddition,itwasfoundthatCDsweremostlydis- tributedinthefibermatrixwithoutformingcrystallineaggregates whenlowerweightpercentagesofCDswereused(10%and25%of
-CDand␥-CDand10%of␣-CD),however,incorporationof50%
(w/w)ofallthreetypesofCDsand25%of␣-CDyieldedcrystalline aggregatesinthezeinfibermatrix.Thethermalanalysescarriedout byDSCandTGAindicatedtheimprovementofthermalproperties forzein/-CDnanofibers,thatis,zein/-CDnanofibershaveshown higherglasstransitiontemperatureandhigherdegradationtem- peraturewithincreasing-CDcontentwhencomparedtopristine zeinnanofibers.ThesurfaceanalysesbyATR-FTIRandXPSshowed thatsomeCDmoleculeswerepresentonthesurfaceofzein/- CDnanofibers.Theseelectrospunzein/CDnanofibersmayhavethe potentialtobeusedasactivefoodpackagingmaterialsowingto veryhighsurfaceareaofzeinnanofibersandsurfaceassociatedCD moleculessinceCDmoleculeshaveinclusioncomplexationcapa- bilitywithvariousmoleculesand thereforefunctionaladditives suchasantioxidants,flavors,aromas,antibacterialagentscanbe complexedwithCDsorremovalofunpleasantodorsfromthesur- roundingscanbeachievedbyCDs.
Acknowledgements
StatePlanningOrganization(DPT)ofTurkeyisacknowledged forthesupportofUNAM-InstituteofMaterialsScienceandNano- technology. Dr. Uyar acknowledges Marie Curie International ReintegrationGrant(IRG) NANOWEB(PIRG06-GA-2009-256428) and The Scientific &Technological Research Council of Turkey (TUBITAK)(project #111M459)for funding. F. Kayaci thanksto TUBITAK-BIDEBfornationalPhDstudyscholarship.
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