One-step synthesis of size-tunable Ag nanoparticles incorporated in electrospun PVA/cyclodextrin nanofibers

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Carbohydrate

Polymers

j ourna l h o m e pa g e :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

One-step

synthesis

of

size-tunable

Ag

nanoparticles

incorporated

in

electrospun

PVA/cyclodextrin

nanoﬁbers

Asli

Celebioglu

a

_,

_Zeynep

_Aytac

a

_,

_Ozgun

_C.O.

_Umu

a

_,

_Aykutlu

_Dana

a

_,

Turgay

Tekinay

a,b,c

_,

_Tamer

_Uyar

a,∗

a_{UNAM-Institute}_of_Materials_Science_&_{Nanotechnology,}_Bilkent_University,_Ankara_06800,_Turkey b_Life_Sciences_Application_and_Research_Center,_Gazi_University,_Ankara_06830,_Turkey

c_Gazi_University,_Polatlı_Science_and_Literature_Faculty,_Ankara_06900,_Turkey

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received10May2013

Receivedinrevisedform18July2013 Accepted27August2013

Available online 7 September 2013 Keywords:

Electrospinning Nanoﬁbers

Silvernanoparticles(Ag-NP) Polyvinylalcohol(PVA) Cyclodextrin

a

b

s

t

r

a

c

t

One-stepsynthesisofsize-tunablesilvernanoparticles(Ag-NP)incorporatedintoelectrospunnanofibers wasachieved.Initially,insitureductionofsilversalt(AgNO3)toAg-NPwascarriedoutinaqueous solutionofpolyvinylalcohol(PVA).Here,PVAwasusedasreducingagentandstabilizingpolymer aswellaselectrospinningpolymericmatrixforthefabricationofPVA/Ag-NPnanofibers.Afterwards, hydroxypropyl-beta-cyclodextrin(HP␤CD)wasusedasanadditionalreducingandstabilizingagentin ordertocontrolsizeanduniformdispersionofAg-NP.ThesizeofAg-NPwas∼8nmandsomeAg-NP aggregateswereobservedforPVA/Ag-NPnanofibers,conversely,thesizeofAg-NPdecreasedfrom∼8nm downto∼2nmwithinthefibermatrixwithoutaggregationwereattainedforPVA/HP␤CDnanofibers. ThePVA/Ag-NPandPVA/HP␤CD/Ag-NPnanofibersexhibitedsurfaceenhancedRamanscattering(SERS) effect.Moreover,antibacterialpropertiesofPVA/Ag-NPandPVA/HP␤CD/Ag-NPnanofibrousmatswere testedagainstGram-negative(Escherichiacoli)andGram-positive(Staphylococcusaureus)bacteria.

1. Introduction

Electrospinning has become an attractive and a practical technique for the production of nanofibers and nanofibrous non-woven mats because of its versatilityand a cost-effective setup(Ramakrishna,2005;Wendorff,Agarwal,&Greiner,2012). Electrospun nanofibrous mats have very high specific surface area,nanoscaleporousstructuresanduniquechemical,physical, mechanical and surface properties (Greiner & Wendorff, 2007; Ramakrishna, 2005; Ramakrishna et al., 2006; Wendorff et al., 2012).Electrospinninghasanexceptionaladvantageover conven-tionalfiberproductiontechniquessincenanofiberscanbereadily producedfromavarietyofpolymers,polymerblends,sol-gels, sus-pensions,emulsionsandcompositestructures.Ithasbeenshown that electrospunnanofibers and theirnanofibrous matscan be particularlyusefulinfiltration,tissueengineering,drugdelivery, energy,sensors,electronicsandenvironmentapplicationsdueto theirdistinctivepropertiesandspecificfunctionalities(Greiner& Wendorff,2007;Guiping,Dawei,Yang,Xiaodan,&Jun,2012;Li& Xia,2004;Ramakrishna,2005;Ramakrishnaetal.,2006;Wendorff etal.,2012).

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

Electrospun nanofibers have design flexibility for particular functionalization which can be performed during the electro-spinning process or by applying post-treatment methods. For instance, functional composite nanofibers can be produced by incorporation of metal nanoparticles (NP), such as silver or gold,intoelectrospunpolymeric nanofibers(Hang, Tae,&Park, 2010; Xiao, Xu, Ma,& Fang, 2012; Zhuet al., 2012).However, thesizeandhomogeneousdistributionofmetalNPthroughthe nanofibermatrix shouldbetaken intoconsideration toachieve highefficiencyandeffectiveperformancefromthesenanofibrous composite materials. For example, silver nanoparticles (Ag-NP) haveattractedconsiderableattentionduetotheiruniqueoptical, electronic, catalytic and antibacterial properties (Arvizo et al., 2012;Fouda,El-Aassar,&Al-Deyab,2013;Rycengaetal.,2011). SeveralstudiesaimedatincorporationofAg-NPwithinelectrospun nanofiberswhichcanenableproductionoffunctionalnanofibrous compositesbycombiningtheuniquepropertiesofnanofiberswith thatofAg-NP(Mahanta&Valiyaveettil,2012;Nametal.,2010;Shi etal.,2011;Xiaoetal.,2012).Yet,thesizecontrolanduniform dis-tributionofAg-NPwithoutaggregationinthepolymericnanofiber matrixischallenging,therefore,anumberofdifferentapproaches werefollowedforobtainingelectrospunpolymer/Ag-NP compos-itenanofibers(Jinetal.,2007;Lietal.,2006;Patel,Li,Wang,Zhang, &Wei2007;Xiaoetal.,2012;Xuetal.,2006).However,inthese approachessilversaltprecursorwasdirectlyaddedtothepolymer solution and the synthesis of Ag-NP was carried out through 0144-8617/$–seefrontmatter © 2013 Elsevier Ltd. All rights reserved.

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Fig.1.(a)SchematicviewandchemicalstructureofHP␤CDmolecule.(b)SchematicrepresentationandthephotographsofPVA/AgNO3,PVA/Ag-NP,PVA/HP␤CD-25%/AgNO3

andPVA/HP␤CD-25%/Ag-NPsolutions.(c)SchematicrepresentationoftheelectrospinningandthephotographofPVA/HP␤CD-25%/Ag-NPnanoﬁbrousmatwiththe repre-sentativeSEMandTEMimages.

thermal(Jinetal.,2007;Pateletal.,2007),chemical(Xiaoetal., 2012;Xuetal.,2006)andphotoreductive(Lietal.,2006) post-treatmentoftheelectrospunnanoﬁbers.Sotheseabovementioned methodsareoftencomplex,time-consuming,andmostlyrequire reducing andstabilizing chemicalswhich aresometimeshighly toxic.

Therefore,simpleandenvironmentallyfriendlyapproachesare essentialforthepractical applicationsofelectrospunnanofibers incorporatingAg-NP. For example,one-step synthesis of Ag-NP incorporatedinpolyethyleneoxidenanofibers(Saquing,Manasco, & Khan, 2009) or polyvinyl alcohol nanofibers (Mahanta & Valiyaveettil,2012)wasachievedwherethepolymersolutionwas usedasbothareducingandprotectingagentforAg-NPandan elec-trospinningtemplate.Inotherstudies,polymer/Ag-NPcomposite nanofiberswereproducedinone-stepbyusingtheelectrospinning solventasareducingagentandtheelectrospinningpolymermatrix asa stabilizing/protectiveagent(Shietal.,2011; Wang,Bai,Li, Zhang,&Zhang,2012).

Theuseoftoxicreducingandstabilizingagentsforthesynthesis ofAg-NP werereplacedwithso-called “green”substances such as natural biomolecules including cellulose, chitosan, polyphe-nols,ascorbicacidandcyclodextrins(Cai,Kimura,&Kuga,2009; Laudenslager,Schiffman,&Schauer,2008;Ng,Yang,&Fan,2008; Ravi, Christena, SaiSubramanian, & Anthony 2013; Wu et al., 2011).As well,“green” practices started to taketheir place in theelectrospinning process bythe useof natural and nonhaz-ardouspolymers(Mahanta&Valiyaveettil,2012).Veryrecently, polymer/cyclodextrin/AgNO3 mixtures were electrospun into polymer/Ag-NPcompositenanoﬁberswherethecyclodextrinwas usedasastabilizingandreducingagentfortheformationofAg-NP inthepolymermatrix(Chae,Kim,Yang,&Rhee,2011;Wang,Bai, Li,&Zhang,2012).However,polymerusedinthesestudieswas dissolvedindimethylformamide(DMF)which isanundesirable solventtype from thepoint of biomedical applications.On the otherhand,cyclodextrins (CD)arenatural and non-toxic cyclic oligosaccharides which areproduced byenzymatic degradation ofstarch.CDhavea truncatedcone-shapedmolecularstructure whichenablesthemtoformhost-guestinclusioncomplexeswith a variety of compoundsvia non-covalent interactions (Fig.1a). Therefore,CDareusedinanumberofindustrialareassuchas phar-maceuticals,food,chromatography,cosmeticsandtextiles(Chung, Guo, Priestley, & Kwak, 2011; Harada, Kobayashi, Takashima, Hashidzume,& Yamaguchi, 2010; Hedges, 1998; Szejtli, 1998). In addition to that, being a non-toxic and naturally occurring material, CD are also very promising candidates for use as

reducing and stabilizing agent for the formation of metal NP (Alvarez,Liu,Román, &Kaifer,2000; Huang,Meng, &Qi,2009; Kochkar,Aouine,Ghorbel,&Berhault,2011;Liuetal.,2012;Ng etal.,2008).

Inthisstudy,wereportaone-stepsynthesisofAg-NP incorpo-ratedintoelectrospuncompositenanofibers.Initially,weuseda biocompatiblepolymer,polyvinylalcohol(PVA),asreducingagent toconvertsilversalt(AgNO3)intoAg-NPandthenweobtained PVA/Ag-NPcompositenanofibersviaelectrospinning.Inaddition tothat,wealsousedhydroxypropyl-beta-cyclodextrin(HP␤CD) as both reducing and stabilizing agent to control the size and uniformdispersionofAg-NPwithintheelectrospunnanofibers. ThePVA/HP␤CDsolutionhavingdifferentamountofHP␤CDwere preparedtoinvestigatetheeffectofHP␤CDamountonthesize of Ag-NP and their dispersion within the nanofiber matrix. It wasobservedthatthesizeofAg-NPdecreasessignificantlyand homogeneous distribution of Ag-NP without aggregation were achievedintheelectrospunPVA/HP␤CDnanofibers.Thedetailed morphological and structural characterizations of the samples wereperformed byscanning electronmicroscope(SEM), trans-missionelectronmicroscope(TEM),X-raydiffractometer(XRD), UV-Vis-NIRspectroscopy,X-rayphotoelectronspectroscopy(XPS) and Raman spectroscopy. Surface enhanced Raman scattering (SERS) properties of these electrospun nanofibers incorporat-ing Ag-NP have been investigated. The antibacterial property of Ag-NP is well-known and widely studied (Guzman, Dille, & Godet, 2012; Morones et al., 2005; Rai, Yadav,& Gade, 2009). Hence, we have also performedantibacterial tests for PVA/Ag-NP andPVA/HP_␤CD/Ag-NPcomposite nanofibrousmatsagainst Gram-negative(Escherichiacoli)andGram-positive(Staphylococcus aureus)bacteria.

2. Experimental 2.1. Materials

The hydroxypropyl-beta-cyclodextrin (HP_␤CD, substitution: 0.6–0.9)waspurchasedfromWackerChemieAG(Germany) com-mercially. The polyvinyl alcohol (PVA, Scientiﬁc Polymer, 88% hydrolyzed, Mw 125,000),silvernitrate(AgNO3,Sigma Aldrich, ≥99.5%)andsodiumhydroxide(NaOH,Fluka,≥98%,smallbeads) werepurchased. Thedeionized waterwasused fromthe Milli-poreMilli-QUltrapureWaterSystem.Allthematerialswereused withoutanypuriﬁcation.

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Fig.2. TherepresentativeSEMimagesofelectrospun(a)PVA,(b)PVA/HP␤CD-25%,(c)PVA/Ag-NP,(d)PVA/HP␤CD-7.5%/Ag-NP,(e)PVA/HP␤CD-15%/Ag-NPand(f) PVA/HP␤CD-25%/Ag-NPnanoﬁbers.

2.2. Electrospinning

First,the aqueousPVA solutionwasprepared byusing 7.5% (w/v,withrespecttosolvent)concentration.Ontheotherhand, HP␤CDwasaddedtotheaqueousPVAsolution(7.5%,w/v)atthree differentconcentrations(7.5%,15%and25%,w/v,withrespectto solvent).Afterobtainingaclearandhomogeneousaqueous solu-tionsofPVAandPVA/HP␤CD,AgNO3wasaddedtoeachofthese solutionsandtheconcentrationofAgNO3wasadjustedaccordingly tohave1%(w/w,withrespecttototalPVAorPVA/HP␤CD con-centration)elementalAgintheelectrospunnanofibers.Afterthe AgNO3wasdissolvedcompletely,pHofthesystemswereadjusted to∼8.5byadding1MNaOHtothesolutions.ThepHofPVA solu-tionwithoutHP␤CDwasalsoadjustedtothesamepHlevelto keep themedium similar toeach other.As thesolutions were stirredovernight,thedarkbrownsolutionswereobtained indi-catingtheformationofAg-NPintheelectrospinningsolutions.For comparison,PVA(7.5%,w/v)andPVA(7.5%,w/v)/HP␤CD(25%,w/v) solutionswithoutcontainingAgNO3werealsopreparedfor elec-trospinning.Eachpreparedsolutionwasloadedinto3mLsyringes (metallicneedlewith0.6innerdiameter)andpositioned horizon-tallyonthesyringepump(Model:SP101IZ,WPI).Theelectrodeof thehighvoltagepowersupply(MatsusadaPrecision,AUSeries)was clampedtothemetalneedletipofthesyringeandthecylindrical aluminumcollectorwasgrounded.Theelectrospinningofthe solu-tionswasperformedatthefollowingparameters:appliedvoltage: 15kV,tip-to-collectordistance:15cmandthesolutionflowrate: 0.5mL/h.Electrospuncompositenanofibersweredepositedona groundedstationarycylindricalmetalcollectorcoveredbyapiece ofaluminumfoil.Theelectrospinningapparatuswasenclosedina Plexiglasboxandtheelectrospinningwascarriedoutat25◦Cand 25%relativehumidity.

2.3. Measurementsandcharacterization

A rheometer (AntonPaar Physica CR 301) equipped with a cone/plateaccessory(spindletypeCP40-2)wasusedtomeasure therheologicalbehaviorof theelectrospinning solutions inthe range of 0.1–100s−1 shear rate. The conductivity of the solu-tionswasmeasured witha Multiparameter InoLab® _Multi ₇₂₀ (WTW)atroomtemperature.Themorphologicalcharacterizations

of nanofibers were carried out by using the scanning electron microscope(SEM)(Quanta200FEG,FEI).Samplesweresputtered with5nmAu/Pd(PECS-682)andtheaveragefiberdiameter(AFD) wascalculated fromthe SEMimagesby analyzing atleast 100 fibers.Transmissionelectronmicroscope(TEM)(FEI-TecnaiG2F30) wasusedforthedetectionofAg-NPin thenanofiberstructure. ForTEMimaging, HC200gridswereattachedonthealuminum foilandthenanofibersamplesweredirectlyelectrospunontothe grids.TheaverageparticlesizesofAg-NPweredeterminedfrom GATANdigital micrographprogram. X-raydiffractometer(XRD) (X’Pert powder diffractometer, PANalytical) wasused to deter-minetheX-raydiffractionpatternofthenanofibersampleswith CuK␣radiationintherangeof2=25–80◦.TheUV-Vis-NIR spec-trophotometer(VarianCary5000,USA)wasusedinthewavelength rangeof400–800nmtoobservecharacteristicabsorptionofAg-NP. TheUV–visspectrawereobtainedbydissolvingthenanofibersin water.Thebackgroundwascorrectedwiththeaqueoussolution ofpurePVAandPVA/HP␤CDnanofiberswithoutAg-NP.TheX-ray photoelectronspectraofthenanofiberswererecordedbyusing X-rayphotoelectronspectrometer(XPS)(ThermoScientific).XPSwas usedbymeansofafloodgunchargeneutralizersystemequipped witha monochromated AlK␣ X-raysource(h

=1486.6eV). In ordertoobtaineddetailedinformation,thehighresolution spec-tra were recorded for the spectral regions relating to silver at passenergyof50eV.Ramanmeasurementsareperformedusing aWITECAlpha300Ssystem.Adiode-pumpedsolid-state532nm wavelengthlaser is usedfor excitation in theRaman measure-ments.Laserpowerhasbeencalibratedusingasiliconphotodiode atsampleplane.

2.4. Antibacterialtest

Theantibacterialactivitiesofthenanofiberswereperformed againstEscherichiacoliRSHM888(RSHM,NationalTypeCulture CollectionLaboratory,Ankara,Turkey)asaGram-negativebacteria andStaphylococcusaureusRSHM96090/07035(ATCC25923)asa Gram-positivebacteria.Thenanofibrousmatswerecutintocircular discshavingdiameterof0.8cm.Thediskagardiffusionmethodwas conducted.150_␮Loftheovernightgrowncultures(_∼101_cfu/mL_of E.coliand∼109_cfu/mL_of_S._aureus)_were_spreaded_on_{Luria-Bertani} (LB)agar.Thenanofibrousmatswereplacedontopoftheagar

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Table1

ThepropertyoftheelectrospinningsolutionsandtheaverageﬁberdiameterandtheaverageparticlesizeofAg-NPpresentintheelectrospunnanoﬁbers.

Sample Conductivity (␮S/cm) Viscosity (Pas) Averageﬁber diameter(nm) Averageparticle sizeofAg-NP(nm) PVA 525 0.50 290±75 – PVA/HP␤CD-25% 830 0.60 500±140 – PVA/Ag-NP 1415 0.35 235±40 8.0±0.5 PVA/HP␤CD-7.5%/Ag-NP 1630 0.60 360±70 2.7±0.5 PVA/HP␤CD-15%/Ag-NP 1625 0.63 400±70 2.6±0.5 PVA/HP␤CD-25%/Ag-NP 1635 0.70 485±100 1.8±0.4

plate.ThePetridisheswereincubatedat37◦Cfor24h.Thetests wererepeatedthreetimesforeachofbacteria.Thezoneswhere thebacterialgrowthwasnotobservedwererecordedasinhibition zonesanddiametersweremeasured.

3. Resultsanddiscussion

3.1. Electrospinningandmorphologicalcharacterizationsof PVA/Ag-NPandPVA/HPˇCD/Ag-NPcompositenanoﬁbers

Here, in situ reduction of silver nitrate (AgNO3) into silver

nanoparticle (Ag-NP) was achieved in polyvinyl alcohol (PVA) aqueoussolutionwherePVAwasactedasbothreducingagentand stabilizingagentaswellaselectrospinningpolymermatrixforthe fabricationofPVA/Ag-NPcompositenanoﬁbers.PVAcanreduce theAg+_ions_into_Ag-NP_due_to_the_hydroxyl_groups_on_the_polymer

backbone(Mahanta&Valiyaveettil,2012)andalsostabilizethe Ag-NPbykeepingthemfromaggregation.Yet,inordertocontrol

thesizeandensureuniformdispersionofAg-NPinPVAsolution, we also used hydroxypropyl-beta-cyclodextrin (HP␤CD) as a supplementaryreducingandstabilizingagent.Thecarbohydrates (glucose, starch, etc.) have high potential for the reduction of AgNO3totheAg-NP;howevertobemoreefficientintheambient conditions,NaOHcouldbeaddedtothesystemtoenhanceand acceleratethereductionofAg1+_to_Ag0_by_releasing_the_electrons fromglucosemolecules(Shervani,&Yamamoto,2011),otherwise, theseagentsrepresentlimitedreducing effect.So,wehave pre-paredthealkalineconditionsbyusingNaOH(pH∼8.5)tobenefit fromthereducingpotentialofHP␤CDmoleculesthathavevery similarchemicalstructurewithothertypesofcarbohydrates.The concentrationof PVA inaqueoussolutionwasadjusted as7.5% (w/v, withrespecttosolvent) in orderto obtainbead-freeand uniformnanofibersbyelectrospinning.ForPVA/HP␤CDblend solu-tions,theconcentrationofPVAwaskeptat7.5%(w/v)andthree differentconcentrationsofHP␤CDwasused;7.5%,15%and25% (w/v).Previously,wehaveshownthatHP␤CDcanbeelectrospun

Fig.3.TherepresentativeTEMimagesofelectrospun(a)PVA/Ag-NP,(b)PVA/HP␤CD-7.5%/Ag-NP,(c)PVA/HP␤CD-15%/Ag-NPand(d)PVA/HP␤CD-25%/Ag-NPnanoﬁbers. TheHR-TEMimageofasingleAg-NPindicatingthed-spacingbetweenAg{111}planesasinsetﬁgure.

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geneousaqueoussolutionsofPVAandPVA/HP␤CDandthedark brownsolutionswereobtainedafterovernightmixingindicating theAg-NPformationintheelectrospinningsolutions(Fig.1b).In each electrospinning solution,the concentrationof AgNO3 was adjustedaccordinglytohave1%(w/w,withrespecttototalPVAor PVA/HP_␤CDconcentration)Ag-NPintheelectrospunnanoﬁbers.

TheelectrospinningofPVA/HP␤CD/Ag-NPnanofiberswas illus-tratedinFig.1c.TheelectrospunPVA/HP␤CD/Ag-NPcomposite nanofibrousmatshavethecharacteristiccolorofAg-NPandthese materialsareflexiblewhichcanbeeasilyhandledasafree-standing mat(Fig.1c).TherepresentativeSEMimagesofPVA,PVA/HP␤CD, PVA/Ag-NPandPVA/HP␤CD/Ag-NPnanofiberswereshownatFig.2 andtheiraveragefiberdiameters(AFD)weregiveninTable1.Inall cases,bead-freeanduniformnanofiberswereobtainedelucidating thattheconcentrationandviscosityoftheelectrospinning solu-tionswereattheoptimallevel.However,theAFDofthenanofibers weredifferentfromeachotherbecauseofthedifferencesinthe viscosityand conductivity values of the solutions (Table 1). In electrospinning,typicallyhighsolutionviscosityorlowsolution conductivityyieldthickerfibersbecauseofthelessstretchingof theelectrospinningjet(Ramakrishna,2005;Wendorffetal.,2012). Onthecontrary,electrospinningofsolutionshavinglowerviscosity orhighersolutionconductivityresultedinthinnerfibersbecauseof themorestretchingoftheelectrospinningjet(Ramakrishna,2005; Wendorffetal.,2012).TheelectrospunPVAnanofibershaveAFD of290±75nmandwhenHP␤CDwasaddedtothePVAsolution atthehighestHP␤CDconcentration(25%,w/v),PVA/HP␤CD-25% nanofibershavingAFDof500±140nmwereobtained.Although thesolutionconductivityincreasedabit,highsolutionviscosity ofPVA/HP␤CD-25%yieldedthickerfibers.Inthecase of electro-spunPVA/Ag-NP nanofibers,the AFD decreased to 235±40nm whencomparedtoPVAnanofibersandthiswasowingtothe pres-enceofAg-NPwhichcontributedtohighersolutionconductivity (Saquingetal.,2009)andtheviscosityofthesolutionwasalso low-eredwhichresultedinmorestretchingoftheelectrospinningjet. ThePVA/HP␤CD/Ag-NPsolutionshavehigherviscosityand con-ductivityvalues, in addition,the higheramount ofHP␤CD,the highertheviscosityofthesolutionswereobtainedandthesolution conductivityofthePVA/HP␤CD/Ag-NPsystems wereveryclose toeachother.Therefore,thickerfiberswereexpectedtobe pro-ducedastheamountofHP␤CDincreasedfrom7.5%(w/v)through 25%(w/v).Asexpected,weobservedthattheAFDofPVA/HP ␤CD-7.5%/Ag-NP,PVA/HP␤CD-15%/Ag-NPandPVA/HP␤CD-25%/Ag-NP nanofibershasAFDof360±70nm,400±70nmand485±100nm, respectively.

The representative TEM images of PVA/Ag-NP and PVA/HP␤CD/Ag-NP nanofibers were shown in Fig. 3 and the averageparticlesize(APS)ofAg-NPpresent inthefibermatrix wasgiven inTable1.Ag-NPwereseenasblack sphericalspots in TEM images of the fiber samples. For PVA/Ag-NP nanofiber sample,theAPSoftheAg-NPwas8.0±0.5nmbutsomeAg-NP aggregationswerealsoobservedforthissample(Fig.3a).Onthe otherhand,itwasapparentthatAg-NPhavehomogenous distri-butionthroughthefibermatrixfor PVA/HP␤CD/Ag-NPsamples (Fig.3b–d).Additionally,thesizeofAg-NPdecreasessignificantly in the PVA/HP␤CD/Ag-NP nanofiber samples as the amount of HP␤CDincreasesinthefibermatrix.TheAPSoftheAg-NPwas 8.0±0.5nmforHP␤CDfreePVA/Ag-NP nanofiberswhereasthe size of the Ag-NP decreased to 2.7±0.5nm, 2.6±0.5nm and 1.8±0.4nmforPVA/HP␤CD-7.5%/Ag-NP,PVA/HP␤CD-15%/Ag-NP andPVA/HP_{␤CD-25%/Ag-NP}nanofibersamples,respectively.

Owingtohydroxylgroups,PVApolymerhastheabilitytoreduce AgsaltsintoAg-NP13_,_however,_with_the_addition_of_HP_␤CD_in_the

Fig.4. (a)XRDspectraofPVA,PVA/HP␤CD-25%,PVA/Ag-NP, PVA/HP␤CD-7.5%/Ag-NP, PVA/HP␤CD-15%/Ag-NP andPVA/HP␤CD-25%/Ag-NPnanofibrous mats.(b) UV–vis spectratakenfrom thedissolved PVA/Ag-NP,PVA/HP␤CD-7.5%/Ag-NP, PVA/HP␤CD-15%/Ag-NP and PVA/HP␤CD-25%/Ag-NP nanofibers in water. (c) The high resolution XPS of electrospun PVA/Ag-NP, PVA/HP␤CD-7.5%/Ag-NP, PVA/HP␤CD-15%/Ag-NPandPVA/HP␤CD-25%/Ag-NPnanofibers.

PVAsolutionsignificantdecreasewasobservedforthesizeof Ag-NPduetothehighlyefficientstabilizingandreducingpropertiesof CDmolecules(Kochkaretal.,2011;Ngetal.,2008).Furthermore, Ag-NPdidnotaggregateorcoagulatelocallyduetothestabilizing effectofHP␤CDthatleadstouniformdistributionofAg-NPwithin thefibermatrix.Moreimportantly,thesizeAg-NPcanbecontrolled byvaryingtheamountofHP␤CD,that is,highertheamountof HP␤CD,smallerthesizeofAg-NP.

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Fig.5.(a)RamanspectraofPVAandPVA/HP␤CD-25%nanofiberswhenilluminatedwith532nm,10mWpower,20×objective.(b)TimedependentRamanspectraofpristine PVAnanofibers.(c)TimedependentRamanspectraofPVA/HP␤CD-25%nanofibers.WhenAg-NParepresentin(d)PVA/Ag-NPand(e)PVA/HP␤CD-7.5%/Ag-NPnanofibers, timedependentRamanspectrashowfluctuationsevenatlowpowers(532nm,0.5mW,20×objective),typicalindicationofhighsurfaceenhancementRamanscattering (SERS).Duetohighabsorptioncoefficientandhighthermalinsulationoffreestandingfibers,absorptioncausesthermaldamagetosampleathighpowers(e.g.10mW).

3.2. StructuralcharacterizationofPVA/Ag-NPand PVA/HPˇCD/Ag-NPcompositenanoﬁbers

Thecharacteristicsof Ag-NPwereinvestigated byusing HR-TEMandXRDmeasurements.TherepresentativeHR-TEM,given inFig.3aasaninsetfigure,showsthelatticefringesofAg-NPin PVA/Ag-NPcompositenanofibersandthed-spacingwasmeasured tobe0.235nmfromthelatticefringesthatcorrespondedtothe latticespacingofthe(111)planesofthefccAg(Zengetal.,2012). TheXRDpatternsoftheelectrospunnanofibrousmatsweregiven inFig.4a.ThePVA/Ag-NPsamplehasdiffractionpeaksat2=38.4◦, 44.4◦,64.6◦ and77.6◦ whichbelongto(111),(200),(220),and (311)crystalplanesofAg,respectively(Lietal.,2007;Wei,Han, Walker,Fuller,&Grzybowski,2012).ForPVA/HP␤CD/Ag-NP sam-plessamecharacteristicpeaksofelementalAgwereobserved,but, therewerealsoadditionalpeaksat2=28.0◦,32.4◦,46.4◦,55.0◦ and57.9◦whichcorrespondtoexistingofoxidizedAg(Chenetal., 2006;Singh,Mehta,Joshi,Kruis,&Shivaprasad,2007;Weietal., 2012).Thisispossiblyoriginatedfromtheoxidationofunsaturated outersurfaceofAg-NPbythehydroxylgroupsofHP␤CDmolecules (Porramezan &Eisazadeh,2011).ForPVA/HP␤CD-25%/Ag-NP,it wasalsoobservedthatthediffractionpeakintensityforAgwas decreasedsignificantly andAg2Opeaksbecameprominent sug-gestingthat Ag-NPhashigheroxizedcontentsincethis sample containsthehighestamountofHP␤CD.

TheUV–visabsorptionmeasurementsof thePVA/Ag-NPand PVA/HP␤CD/Ag-NP were obtained from the aqueous solution by dissolving the nanofibrous mats in water (Fig. 4b). For all samples,the spectra showan absorption bandat the range of 400–450nmwavelengthsduetothecharacteristicsurface plas-monic resonance (SPR) band for Ag-NP (Ng et al., 2008; Wu et al., 2011). For PVA/Ag-NP nanofibers the particle size was 8.0±0.5nmandthemaximumabsorptionvalueexistsat421nm. On the other hand, the SPR peak for PVA/HP␤CD-7.5%/Ag-NP (APS=2.7±0.5nm)wasobservedat439nm.AstheHP␤CDcontent increased,blue-shiftofabsorptionpeaksto435nmand 428nm forPVA/HP␤CD-15%/Ag-NP(APS=2.6±0.5nm)andPVA/HP ␤CD-25%/Ag-NP(APS=1.8±0.4nm)wereobserved,respectivelydueto thedecreasingsizeofAg-NPinthefibermatrix.Eventhoughthe Ag-NPparticlesizeisdistinctivelysmallerforthePVAnanofibers containingHP␤CDcomparedtoHP␤CDfreePVAnanofibers,the

absorption peak of the spectrum ﬁrstly shows red-shift to the higherwavelengthandbroadeningforPVA/HP␤CD/Ag-NPsystems. ThiscanbeexplainedbytheexistenceAg2OontheAg-NPthatwas alsoprovedbytheXRDmeasurements.ForthePVA/HP ␤CD/Ag-NP samples, the Ag2O layer on the Ag-NP surface results an absorption shoulder at 450–500nm range and it also causes broadeningatthecharacteristicabsorptionbandofAg-NP(Chen et al., 2006).Fig. 4c shows theXPS spectra of PVA/Ag-NP and PVA/HP␤CD/Ag-NP nanoﬁbers. Thedoublet peaks at 368.2 and 374.2eVareassignedtothebindingenergies3d5/2and3d3/2ofAg 3dcorelevel,respectively(Suh,Moon,Lee,&Jang,2006).Thereare merelydifferencesbetweenthebindingenergyofAg0_and_Ag1+_, sotheionictypeexistencecannotbeeasilynoticedfromAg3d corelevelwhichwassupportedbyXRDmeasurements(Lietal., 2007).

3.3. SurfaceenhancedRamanscattering(SERS)propertiesof PVA/Ag-NPandPVA/HPˇCD/Ag-NPcompositenanoﬁbers

SurfaceenhancedRamanscattering(SERS)propertiesofAg-NP surface-decoratedelectrospunpolymericnanofibershasbeen pre-viouslydemonstrated,resultinginflexibleSERSactivesubstrates (Zhangetal.,2012).WeinvestigatedthepresenceofSERSeffects in Ag-NP loadedPVA and PVA/HP␤CD nanofibers as shown in Fig.5.WithoutAg-NP,thePVAand PVA/HP␤CDnanofiber sam-plesexhibitedRamanspectra(Fig.5a),thatarestableintime,even underhighexcitationpowersof10mW(Fig.5bandc).However, PVA/Ag-NP(Fig.5d)andPVA/HP␤CD/Ag-NP(Fig.5e)nanofibers exhibitedfluctuatingRamanspectraevenatlowpowers(<1mW) due tothepresence ofAg-NP, typicalindicationof strongSERS effect.TimedependentRamanspectraofPVA/HP␤CD-7.5%/Ag-NP samplewasgivenasexampleinFig.5e,but,PVA/HP ␤CD-15%/Ag-NPandPVA/HP␤CD-25%/Ag-NPsampleshavealsoshownsimilar characteristics(datanotshown).It hasbeenalsoobservedthat (datanotshown),highexcitationpowersresultedinthermal dam-agetoPVA/Ag-NPandPVA/HP␤CD/Ag-NPnanofibrousmatsdue topoorthermalconductionofnanofibersandhighabsorptionof Ag-NP.FluctuationsoftheSERSsignalfromAg-NPincorporated nanofiberssuggestthatathermallyactivatedmechanismislikely tobethesourceofobservedblinkingbehavior.Inshort,PVA/Ag-NP

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Fig.6. Thephotographsofantibacterialtestingofnanoﬁbrousmatswhichwereperformedagainst(a)E.coliand(b)S.aureus.

andPVA/HP␤CD/Ag-NPsamplesexhibitedSERSeffectwhichcanbe usefulforsensingapplication(Zhangetal.,2012).

3.4. AntibacterialpropertiesofPVA/Ag-NPand PVA/HPˇCD/Ag-NPcompositenanoﬁbers

Here,we investigated theantibacterialeffects of PVA/Ag-NP andPVA/HP_␤CD/Ag-NPnanoﬁbrousmatsagainstGram-negative (E.coli)and Gram-positive (S.aureus)bacteria.Theelectrospun PVAandPVA/HP␤CDwithoutAg-NPwerealsotestedfor compar-ison.Fortheantibacterialtest,threesamplestakenfromdifferent locations of the same nanoﬁbrous mat were placed on E. coli andS. aureusspreaded agarplates and visualizedafter incuba-tionfor24h. Theplateswerecheckedforthepresenceandsize ofthe inhibition zones(Fig.6 and Table 2).We observed that,

PVAand PVA/HP␤CDnanofibersdidnotshowanyantibacterial activity.On theotherhand,PVA/Ag-NP nanofibershave shown antibacterialpropertydue tothepresence ofAg-NP.Ag-NP has antibacterialproperty,which is sizedependent(Morones etal., 2005).So,inthecaseofPVA/HP␤CD/Ag-NPnanofibers,asthesizeof Ag-NPdecreaseddependingontheHP␤CDamount,theinhibition zonebecamelargerindicatingenhancedantibacterialactivity com-paredtoPVA/Ag-NPnanofibers(Fig.6).AmongPVA/HP␤CD/Ag-NP samples,thePVA/HP␤CD-25%/Ag-NPhasshownbetter antibacte-rialactivitysincetheAg-NPhasthesmallestsizeinthissample (Table 1).This may be due to the fact that the release of the Ag-NPbecomeseasierastheparticlesizedecreases,sothat Ag-NP can more effectively reach the bacteria regionand contact withthebacteria.In addition,owingtothesmallerdimensions, highersurfacetovolumeratioswereobtainedwhichalsoenhances

Table2

Theinhibitionzoneresultstakenafter24hforPVA,PVA/HP␤CD-25%,PVA/Ag-NP,PVA/HP␤CD-7.5%/Ag-NP,PVA/HP␤CD-15%/Ag-NPandPVA/HP␤CD-25%/Ag-NPnanoﬁbers againstE.coliandS.aureus.

Samples E.coli S.aureus

0h(cm) 24h(cm) 0h(cm) 24h(cm) PVA 0.8 0.8 0.8 0.8 PVA/HP␤CD-25% 0.8 0.8 0.8 0.8 PVA/Ag-NP 0.8 1.32±0.00 0.8 1.41±0.15 PVA/HP␤CD-7.5%/Ag-NP 0.8 1.55±0.07 0.8 1.62±0.16 PVA/HP␤CD-15%/Ag-NP 0.8 1.60±0.03 0.8 1.73±0.21 PVA/HP␤CD-25%/Ag-NP 0.8 1.72±0.16 0.8 1.85±0.07

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the antibacterial activity of Ag-NP (Martinez-Castanon, Nino-Martinez,Martinez-Gutierrez,Martinez-Mendoza,&Ruiz,2008; Mollahosseini, Rahimpour, Jahamshahi, Peyravi, & Khavarpour, 2012).Similarly,Moronesetal.reportedthatbactericidal proper-tiesoftheAg-NParesizedependentandsmallerAg-NPweremuch moreeffectiveagainstGram-negativebacteriasinceAg-NPhaving adiameterof∼1–10nmpreferentiallypresentadirectinteraction withthebacteria(Moronesetal.,2005).Moreover,weobserved thattheincreaseintheinhibitionzonewasmoredistinctivefor S.aureuscomparedtoE.coliastheAg-NPsizegetsmallerandthis couldbeattributedtothecellularwallcontentdifferencesbetween Gram-positiveandGram-negativebacteriaasdiscussedbyThiel etal.(2007).

4. Conclusion

Here,wehaveachievedone-stepsynthesisofAg-NPby reduc-tionof AgNO3 byusing PVA and PVA/HP␤CD aqueoussolution as reducing and stabilizing medium as well as the electro-spinningmatrix.For thePVA/Ag-NPsystem,Ag-NPwithanAPS of8.0±0.5nmwasobtainedandasmallnumberofAg-NP aggre-gationwasobservedwithinthePVAfibermatrix.ThesizeofAg-NP decreasedsignificantlywiththeadditionofHP␤CDtothePVA solu-tionandhomogeneousdistributionofAg-NPwithoutaggregation wasachievedforthePVA/HP␤CD/Ag-NPnanofibers.Byincreasing theamountofHP␤CDin thePVAsolution,thesize-tunable Ag-NPsynthesiswassuccessfulowingtotheefficientreducingand stabilizingpropertiesofHP␤CD.ThesizeoftheAg-NPin PVA/Ag-NP nanofibers was decreased to 2.7±0.5nm, 2.6±0.5nm and 1.8±0.4nmforPVA/HP␤CD-7.5%/Ag-NP,PVA/HP␤CD-15%/Ag-NP and PVA/HP␤CD-25%/Ag-NP nanofibers, respectively. We have observed that, multifunctional PVA/Ag-NP and PVA/HP ␤CD/Ag-NP nanofibersexhibited SERSeffect which mightbe applicable insensingapplications,inaddition,thesenanofibrousmatshave shown antibacterialeffect againstE.coli andS. aureusbacteria. WhencomparedtoPVA/Ag-NP,PVA/HP␤CD/Ag-NPsampleshave shownbetterantibacterialefficiencyduetosmallersizeofAg-NP. Inaddition,thePVA/HP␤CD-25%/Ag-NPhavingthesmallerAg-NP hasshownbetterantibacterialefficiencyamongPVA/HP ␤CD/Ag-NPsamples.Inbrief,ourapproachisa“green”andfacilemethod forthefabricationofAg-NPincorporatedfunctionalnanofibrous matshavingSERSeffectandantibacterialproperties.HP␤CDisa bio-compatibleandnon-toxicoligosaccharideandtheadditional useofhazardousreducing/stabilizingagentfortheformationof Ag-NPcanbeeliminatedbyusingHP␤CD.Furthermore,polymer matrixsuchasPVAisalsoknownforitsbiocompatiblenatureand suitableforbiomedicalapplications.Therefore,theseelectrospun PVA/HP␤CD/Ag-NPnanofibrousmatscanbequiteapplicableasa woundhealingmaterial,insensingorotherbiomedicaluses. Acknowledgements

StatePlanningOrganization(DPT)ofTurkeyisacknowledged forthesupportofUNAM-InstituteofMaterialsScience& Nano-technology. Dr T. Uyar acknowledges EU FP7-Marie Curie-IRG forfundingNANOWEB(PIRG06-GA-2009-256428).A.Celebioglu acknowledgesTUBITAK-BIDEBforNationalPhDScholarship. References

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