ContentslistsavailableatScienceDirect
Carbohydrate
Polymers
jo u r n al h om ep age :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
Fast
ultrasound
assisted
synthesis
of
chitosan-based
magnetite
nanocomposites
as
a
modified
electrode
sensor
T.M.
Freire
a,
L.M.U.
Dutra
b,d,
D.C.
Queiroz
b,
N.M.P.S.
Ricardo
b,
K.
Barreto
b,
J.C.
Denardin
c,
Frederik
R.
Wurm
d,
C.P.
Sousa
e,
A.N.
Correia
e,
P
de
Lima-Neto
e,
P.B.A.
Fechine
a,∗aGroupofChemistryofAdvancedMaterials(GQMAT)-DepartmentofAnalyticalChemistryandPhysical-Chemistry,FederalUniversityofCeará-UFC,
CampusdoPici,CP12100,CEP60451-970Fortaleza,CE,Brazil
bDepartmentofOrganicandInorganicChemistry,FederalUniversityofCeará-UFC,CampusdoPici,CP12100,CEP60451-970Fortaleza,CE,Brazil cDepartmentofPhysical,UniversityofSantiagodeChile,USACH,Av.Ecuador3493,Santiago,Chile
dMaxPlanckInstituteforPolymerResearch,Ackermannweg10,55128Mainz,Germany
eGroupofElectrochemistryandCorrosion(GELCORR)-DepartmentofAnalyticalChemistryandPhysical-Chemistry,FederalUniversityofCeará-UFC,
CampusdoPici,CP12100,CEP60451-970Fortaleza,CE,Brazil
a
r
t
i
c
l
e
i
n
f
o
Articlehistory:
Received28January2016
Receivedinrevisedform25May2016 Accepted26May2016
Availableonline3June2016 Keywords: Nanocomposites Magnetite Chitosan Superparamagnetic Ultrasound
a
b
s
t
r
a
c
t
Chitosan-basedmagnetitenanocompositesweresynthesizedusingaversatileultrasoundassistedinsitu methodinvolvingonequickstep.Thissyntheticrouteapproachresultsintheformationofspheroidal nanoparticles(Fe3O4)withaveragediameterbetween10and24nm,whichwerefoundtobe superparam-agneticwithsaturationmagnetization(Ms)rangesfrom32–57emug−1,dependingontheconcentration. TheincorporationofFe3O4intochitosanmatrixwasalsoconfirmedbyFTIRandTGtechniques.Thishybrid nanocompositehasthepotentialapplicationaselectrochemicalsensors,sincetheelectrochemicalsignal wasexcepitionallystable.Inaddition,theinsitustrategyproposedinthisworkallowedustosynthesize thenanocompositesysteminashorttime,around2minoftime-consuming,showinggreatpotentialto replaceconvencionalmethods.Herein,theprocedurewillpermitafurtherdiversityofapplicationsinto nanocompositematerialsengineering.
©2016ElsevierLtd.Allrightsreserved.
1. Introduction
Development of organic-inorganic nanocomposites has attracted attention of many researchers, since these materials canbeappliedindifferentareasofscience.Magneticnanoparticles (MNPs),suchas magnetite (Fe3O4), have shown hugepotential for differentapplications: enzyme immobilization,waste water treatment,targeteddeliveryofdrugsandbiosensors(Limetal., 2015; Wang,Li, Luo,Wang,&Duan, 2016; Zhanget al., 2010), owing to their superparamagnetic behavior and low toxicity. However,thelargesurfacearea-to-volume ratioinFe3O4 MNPs exhibitsahighsurfaceenergyandtheytendtoaggregate,limiting
∗ Correspondingauthor.
E-mailaddresses:tiagomfreire.ufc@gmail.com(T.M.Freire),
lmudutra@hotmail.com(L.M.U.Dutra),daniloqueiroz46@gmail.com(D.C.Queiroz),
nagilaricardo@gmail.com(N.M.P.S.Ricardo),kambarreto@yahoo.com.br
(K.Barreto),juliano.denardin@usach.cl(J.C.Denardin),wurm@mpip-mainz.mpg.de
(F.R.Wurm),pinheiro.cs@gmail.com(C.P.Sousa),adriana@ufc.br(A.N.Correia),
pln@ufc.br(P.deLima-Neto),fechine@ufc.br(P.B.A.Fechine).
theirrangeofapplications(Gregorio-Jaureguietal.,2012;Yallapu etal.,2011).
Herein,controllingand/orpreventingparticlesagglomeration makestheirproductionatrulychallengeintoindustryfield.Thus, organiccompoundshavebeenusedtocoverMNPs,promoting sta-bilityandsurfacefunctionalization(Nicolásetal.,2013;Zhangetal., 2012).Severalworksprovedthatmagnetitenanoparticlescoated bychitosan(Sheteetal.,2014), poly(vinylalcohol)(Mahmoudi, Simchi,&Imani,2009),poly(acrylicacid)(Xu,Zhuang,Lin,Shen, &Li,2013),DNA(Navarathne,Ner,Jain,Grote,&Sotzing,2011), orproteins(Bayrakcietal.,2014Bayrakci,Gezici,Bas,Ozmen,& Maltas,2014),promoteswhetherelectrostaticorstericrepulsion, stabilizingMNPsdispersion.Chitosanhasbecomeoneofmostused naturalpolymer,beingbiocompatible,biodegradableand bioac-tive.Additionally,thispolyaminosaccharidehasintrinsicchemical properties,whichitsfreeaminogroupsallowtheformationof posi-tivelychargedcomplexes,providingreactivesitestobindFe-based MNPs(Shen,Shen,Wen,Wang,&Liu,2011;Zhangetal.,2010).
Somestudieshavebeendemonstratedthatnanosizeand sur-face effects display significantrules in NP’smagnetic behavior.
http://dx.doi.org/10.1016/j.carbpol.2016.05.095
Innumerous strategies have been developed in order to mod-ulate a hybrid functional metal oxide with desired properties, suchas co-precipitation (Silva et al., 2013), hydrothermal syn-thesis(Gyergyek,Drofenik,&Makovec,2012)andsonochemistry (Barkade,Pinjari,Nakateetal.,2013;Barkade,Pinjari,Singhetal., 2013;Theerdhalaetal.,2010).However,thesemethodsareenergy andtime-consuming,andthusitisimportanttohighlighttheuse ofultrasoundforsynthesisinvolvingawiderangeof nanomateri-als,suchassilvernanoparticles(Heetal.,2014),magnesiumferrite (Chen,Li,zhang,&Kang,2013)andironoxide(Dolores,Raquel,& Adianez,2015).Thetechniquehasbecomeaneasyandsimpletool, thatmaypromotesultrahighsurfacearea(organic/inorganic)with synthesis ofinorganicmaterialduring process(in situmethod), whichtheformationofamonodispersecantailorhybridmaterials, controllingthecrystalgrowthandformingparticleswith nanome-tersizedistribution(Barkade,Pinjari,Singhetal.,2013; Carroll, Ulises-Reveles,Shultz,Khanna,&Carpenter,2011).
Ingeneral,chitosan-basedmagnetitesynthesisresultsintwo typesofdifferentstructures:asinglemagnetitecoresurroundedby achitosanlayer,and/ormagnetite“multi-cores”dispersedina chi-tosanmatrix(Patiletal.,2014;Reddy&Lee,2013;Safari&Javadian, 2015).Recently, Safariand Javadiansynthesized Fe3O4-chitosan nanoparticlesbyco-precipitationmethodunderultrasound irra-diationintwodifferentsteps.Firstly,theyobtainedthemagnetic nanoparticlesbychemicalco-precipitation,followedbya subse-quentfunctionalizationwithchitosanbyaidofultrasound.Even though,cavitationphenomenonprovidesanincreaseinreactions rate,theultrasoundirradiation(US)isusedonlyinthesecondstep. Thereby,theadaptedmethodisstillbeingtime-consuming,since thesynthesisiscarriedoutintwosequentialsteps,namelyparticle precipitationandfunctionalization(Safari&Javadian,2015).
In this work, we present a rapid ultrasonic assisted in situ routetoproducemagneticFe3O4-chitosannanocomposites,where thechemicalco-precipitationandfunctionalizationoccuratthe same time, i.e., in one quick step. Besides the in situ method synthesizesMNPswithlowcrystallitesize,highcrystallinityand saturationmagnetization,thenanocompositematerialhasshown optimalelectrochemicalresponseduetodevelopmentofmodified electrodes.Furthermore,ourmethodminimizestheoxidationof magnetiteanddrasticallydecreasesthesynthesistimefrom sev-eralminutesand/orhoursfor theconventional co-precipitation methodtoafewminutes(∼2min).Additionally,inthesenseof electrochemicalsensorsapplication,theproposedrouteis poten-tiallyidealtoproducealargeamountofmaterial,i.e.,scalingupto industriallevel,sincesensorsofhybridmaterialsbasedonchitosan andFe3O4havebeendevelopedfordetectionofglucose(Kaushik etal.,2008),urea(Kaushiketal.,2009)andassensorsfordiseases diagnosis(Tranetal.,2011).
2. Experimentalprocedure
2.1. Materials
Chitosan(DPN−DeltaProdutosNaturaisLtda)was character-izedregardingtomolecularweight(Mw),31kDa,anddeacetylation degree (DD), 92%, by gel permeation chromatography (GPC) and potenciometry, respectively. Ferric chloride hexahydrate (FeCl3·6H2O, 97%), potassium ferrocyanide (K4Fe(CN)6, 98.5%), potassiumferricyanide(K3Fe(CN)6,99%),potassiumchloride(KCl, 99%)and glacial acetic acid(CH3COOH, 99.7%)were purchased fromVETEC.Ferroussulfateheptahydrate(FeSO4·7H2O,99%)and ammonium hydroxide solution (NH4OH, 30%) were purchased fromDINAMICA, and glutaraldehyde solution 25%from Sigma-Aldrich.Allusedreagentsareanalyticalgrade.
2.2. Preparationofchitosan/Fe3O4nanocomposites
Themagneticchitosannanocompositesweresynthesizedas fol-low. Firstly,differentamounts ofchitosan (0.1,0.05and 0.01g) weredissolvedin15mLofaceticacid1%(v/v)understirringat 50◦Cfor10min.Subsequently,10mLofFesolution0.33molL−1 (FeCl3.6H2O/FeSO4.7H2OFeCl3·6H2O/FeSO4·7H2O (Fe3+:Fe2+, 2:1 molarratio)wasaddedintochitosansolutions.After homogeniza-tion,2mLofNH4OHwasslowlyaddedunderUSirradiationfor 2min(50%amplitude,inapulseregime20son–10soff)usinga G.HeinemannUltraschall−undLabortechinikSonifier.The resul-tantprecipitatedchitosan-basedmagnetitewasremovedfromthe solutionbymagneticdecantation.Then,obtainedchitosan/Fe3O4 nanoparticleswerewashedseveraltimeswithdistilledwaterand redispersedin aglutaraldehyde (GL)solution(chitosan: GL,2:1 molarratio)at50◦Cfor2hundermechanicalstirring.Thefinal productwaswashedseveraltimeswithdistilledwateranddried inanovenat50◦Cfor48h.Thesampleswerecollectedasablack powderandlabeledasChM0.1,ChM0.05andChM0.01,where thenumbers0.1,0.05and0.01correspondtothemassofchitosan usedinthesynthesis.Thesamestrategywasusedinthesynthesis of“naked”Fe3O4nanoparticles.
2.3. Characterizationmethods
X-ray Powder Diffraction (XRPD) analysis was performed to confirm the crystalline structures of magnetite present in chitosan/Fe3O4 nanocomposites.The sampleswereanalyzedby X-raypowder diffractometerXpertPro MPD(Panalytical)using Bragg–Brentanogeometryintherangeof15◦–70◦witharateof 1◦min−1. CuK␣radiation (k=1.54059Å)wasusedand thetube operatedat40kVand30mA.
Fourier Transform Infrared Spectroscopy (FTIR)analysis was carried out in a PerkinElmer 2000 spectrophotometer used to recordspectraintherangebetween4000and400cm−1.Previously measurements,thesamplesweredriedandgroundedtopowder andpressed(∼10mgofsampleto100mgofKBr)indiskformat.
ThemorphologicstudywasperformedbyTransmission Elec-tronMicroscopy(TEM).TheimageswererecordedbyusingaJEOL JEM-1400electronmicroscopeoperatingatanacceleratingvoltage of120kV.Thesampleswerepreparedbydilutingnanoparticles dispersionindistillatedwater. Then,one dropletofthesample was placed on300mesh carbon-coated cooper gridsand dried overnight under ambient conditions. The size distribution was determinedbymeasurementof50randomlyselectedparticlesin differentregionsoftheexpandingTEMimage.
Forthermogravimetricanalysis(TG),5mgofnanoparticleswere carriedoutinnitrogenatmospherebyemployinga Thermogravi-metricAnalyzerQ50V20.Thelossofmasswasmonitoredheating upsamplesfrom25to900◦Cinarateof10◦Cmin−1.Thezerotime for thethermaldegradation studywastakenaftertemperature stabilization.
MagneticpropertieswereinvestigatedbyaVibratingSample Magnetometer(VSM)Mini5TfromCryogenicLtd.Previously,the VSMwascalibratedusingaYIGsphere,andaftermeasuringthe massofeachsample,themagnetizationwasgiveninemug−1.The zero-field-cooled(ZFC)curvewasobtainedbycoolingthesamples (300–5K)intheabsenceofanexternalfield,uponreaching tem-peratureanexternalfieldisappliedandmeasuredasafunctionof theincreasingtemperature.Thefield-cooled(FC)curveisobtained followingthesameZFCprocedure;instead,thesamplesarecooled inthepresenceofexternalfield.
Cyclic voltammetry measurements were carried out with a potentiostatic(Autolab PGSTAT101Metrohn-EcoChemie) con-trolledbyapersonalcomputerwithNovaversion1.11.2software usingconventionalthree-electrodesystem.Thisiscomposedbya
modifiedglassycarbonelectrode(GCE)(3mmofdiameter,BASi) asthe workingelectrode, a Ptsheet asauxiliary electrode and anAg(s)/AgCl(s)/Cl−(aq)(saturatedKCl)asreferenceelectrode.All measurementswereperformedinasolutionof1.0×10−3molL−1 Fe(CN)63−/4−containing0.1molL−1KClindifferentscanrates.The GCEwas carefullypolished with3m diamondpasteand was ultrasonicatedinethanolandpurifiedwater.ThemodifiedGCEwas preparedbyaddinga5Lofsampledispersion1mgmL−1 inBr bufferpH6ontothetopoftheelectrode.Thesolventwasallowed toevaporateatroomtemperature(23±1◦C)for60min.Whennot inuse,themodifiedelectrodewasstoredinadesiccatoratroom temperature.
3. Resultsanddiscussion
3.1. Synthesisofchitosan/Fe3O4nanocomposites
Chitosan-Fe3O4nanocompositeshavebeenpreparedby chem-icalco-precipitationmethodunderUSirradiation.Theprocedure schemeandidealizedobtainedstructureareshowninFig.1(a)and (b),respectively.AspreviouslydescribedinSection1,thework pro-posaltouseultrasoundassistedinsituchemicalco-preciptation method has several advantages over conventional procedures. Since,thesynthesisiscarryingoutunderUSirradiation,the soni-cationwavesthemselvespromoteahomogenousdispersionwith anultrahighsurfaceareabetweenchitosanandironions.Inthe firststage,Fe2+andFe3+ionsarecapturedbytheaminogroups ofchitosantoformthechitosan-complexedFe.Asaresultofthis chelationeffect,thechelatedaminogroupscanhindertheironions diffusion,controllingthegrowthofmagnetitecrystals(Wang,Li, Zhou,&Jia,2009).Inthesecondstage,whilethebaseisadded,the cavitationprocessimprovesthesystemhomogeneity,avoidingthe formationofagglomeratesduringthecrystalgrowth.Theinsitu magnetitemineralizationunderUSirradiationcanbeobservedin Eq.(1).
Fe2++2Fe3++8OH− )))→Fe
3O4+4H2O (1)
Ultrasoundassistedmethodshavebeenanupsurgeinthestudy ofhybridnanocomposites,owingtointerestingproperties(Patil etal.,2014;Reddy&Lee,2013;Safari&Javadian,2015;Sheteetal., 2014).Generally,themagneticnanoparticle(Fe3O4)isfirstly syn-thesized,andchitosanmatrixisformedsubsequently.However, ourmethodologypresentsonequickstepinsitusynthesisto pro-ducethechitosan/Fe3O4nanocomposite,wherethesonicationtime hasanimportantroleinthesynthesis,aswellasinfinalproduct properties.SincetheprocedureleadswithchitosanduringUS irra-diation,somelimitationshavetobeconsidered,therebytoavoid somenegativeeffects.Forinstance,duringthepulseonthe col-lapseofthebubblesprovidesahighshearforce,occurringcleavage ofchitosanmolecules.Inthiswork,evenwithalow pulse-time-regimeof2min,thechitosanmolecularweight(Mw)waschecked aftersynthesisbyGPC.Asexpected,thedecreaseofchitosanMw wasslightlysignificant,consideringthattheobtainedMwvalues areinthesamerangeofchitosanbeforesonicationstep,30–10kDa foreachchitosansolution(Ch0.01,0.05and0.1)(Wu,Zivanovic, Hayes,&Weiss,2008).Anotherrelevantpointisthe inhomoge-neousoutputpower,causedbytheheatingupoftheprobe,ifthe ultrasoundisusedforalongtime.Besidestheheatingupofthe probemaydecreasetheamplitude%,hightemperaturesprovide oxidationofFe(II)toFe(III),decreasingsubstantiallythesaturation magnetizationoftheMNPs,mayinfluencenegativelyinthe super-paramagneticproperty,andalsotheefficiencyofthesynthesis.
Otherpointthatshouldbeconsideredisthereticulationstep after the synthesis of the nanocomposite. For example, adsor-bentandresin materialsrequirebiopolymersresistanttoacidic medium,andthathasadirectrelationwithcrosslinkingreactions. Considering achitosan-basedhybrid material,few studieshave reportedinstabilityofchitosaninstronglyacidicmedium(Sinha etal.,2004).Thereby,crosslinkeragentsasglutaraldehydehave beenusedtostabilizationpurposes,formingSchiffbaselinkages, inwhichmayacquiregoodstabilitytonanocomposites.Thus, pro-ducingaversatileandresistancechitosan-basedmagnetiteunder mildconditions,whereitcanbeappliedindifferentusages.
3.2. Characterizationofchitosan/Fe3O4nanocomposites 3.2.1. Morphologicalandstructuralcharacterization
XRPDtechniquewasusedtoidentifythecrystallinestructureof themagneticnanoparticlesdispersedinchitosanmatrix,asshown in Fig.2(a).All samplespresented main characteristic peaksat 30.2,35.6,43.2,53.6,57.2,62.8◦,whichcanbeindexedto(220), (311),(400),(422),(511)and(440)planes,respectively,by com-parison withinorganic crystal structure database(ICSD, file n◦ 01-086-1340).Thesepeaksareassociatedwithcubicspinelphase ofmagnetite(Fe3O4),andpatternspeaksobservedinallobtained nanocompositesshowed the crystallinestructure of magnetite. However,itwaspossibletoobservethatthesampleChM0.1 pre-sentedthebroadeningathalfthehighermaximumintensitywhen comparedtotheothersamples.Thisincreasingmaybeassociated withtheinfluenceofamorphousstructureofchitosan.This influ-encewasnotobservedintheChM0.05and ChM0.01 samples, suggestingthattheinterferenceofchitosanisonlyobservedwhen themolarratioofchitosanmonomerandironionsishigherthan 0.1902.
AlldiffractogramswererefinedbyusingRietveldmethod.The latticeparameters(a=b=c)forsamplesFe3O4,ChM0.01,ChM0.05 andChM0.10werefoundtobe8.3625,8.3534,8.3510and8.3517, respectively.Theseresultsindicateastronginteractionbetween chitosanandmagnetiteoccasionedbycontractionsoftheunitcell ofmagnetite.Thecrystallitesizewascalculatedaccordingto Scher-rer’sequation,whichisrepresentedas
Tc= K
ˇcos (2)
whereTcistheaveragediameterofcrystallite;Kisthedimensional factor;istheX-raywavelength;isthebroadeningathalfofthe maximumintensityandistheBragg’sangle.Diametervaluesof thesamplesFe3O4,ChM0.01,ChM0.05andChM0.1werefound tobe11.65±0.11, 12.67±0.10,11.79±0.16 and8.37±0.18nm,
respectively.Gregorio-Jaureguiandcoworkersobtained chitosan-basedmagnetitenanoparticlesbyco-precipitationmethod,which weremixed FeCl3.6H2O andFeCl2.4H2Owithdifferentchitosan mass in a reactor (Gregorio-Jauregui et al., 2012), and it was observed a crystallite sizesimilar tofound in this work. How-ever, theyusedlarger amountof chitosan and thesynthesis is time-consuming,more than30min.Clearly,associatedwiththe USirradiation,itisobservedchitosanactingasacontrollerofthe crystallitesize,and thatcanbeassociatedwiththeFe-chitosan complexformedprior tothenucleationoftheparticles.Indeed, theformationofFe-chitosancomplexpositivelyinterferesinthe crystalgrowthprocessbycontrollingtheironionsdiffusion, lead-ingtotheformationofcrystalswithanarrowsizedistributionand smalldiameter(Wangetal.,2009).
TEManalysisofChM0.05sample (referencesample,onceit hasshownbetterresults)atdifferentamplificationsisshownin
Fig.2(b) and(c).Despite nanoparticleswereagglomerated dur-ingthebrieflydryingprocess,areasonablehomogenizationwith anarrowsizedistributionwasnoticed,alsoshowingaspheroidal morphology.Instead,chitosanlayerwasnotvisibleintheseimages, owingtolowcontrastofchitosanonthegrid.Fig.2(b)showsthe correspondinghistogram (inset).The size distributionwas also determinedusinganimageanalysisprogramfromdifferent micro-graphs.Asitcanbeseen,thediameterofthenanoparticlesranges from10to25nm,smallerthanmagneticnanocompositesprepared byconventionalco-precipitationdescribedintheliterature(Li,Jia, Zhou,Hu,&Cai,2006; Yallapuetal.,2011).Sheteand cowork-ersalsoobtainedchitosan-basedmaterialswithparticlesizeabout 15nm(Sheteetal.,2014).Otherauthors,usinginsitusynthesis, reporteddiametervaluesevensmallerthanthoseobtainedinthis method(Gregorio-Jaureguietal.,2012;Wangetal.,2009),butit cannotbeexcludedthatin thisultrasound assistedmethodthe oxidationofmagnetitemaydecrease,oncetheprocedureis carry-ingoutin∼2min.Forcomparison,valuescalculatedbyTEMwere similartoparticlessizeestimatedbyScherrer’sequation(2).
Fig.2.(a)XRPDpatternsofthe(I)standardFe3O4(ICSD,filen◦01-086-1340),(II)Fe3O4,(III)ChM0.01,(IV)ChM0.05and(V)ChM0.1;(b)and(c)TEMimagesofthesample
Fig. 3(a) shows FTIR spectra to Fe3O4, chitosan, and ChM-NPssynthesizedwithdifferentchitosanamounts.ForFe3O4 and nanocomposites,bandsaround580cm−1 wereassignedtoFe–O deformationin octahedraland tetrahedral sites,confirmingthe presenceofFe3O4intonanocompositesstructure(Donadeletal., 2008). Absorption related to stretching vibration C O of ether and primaryalcohol in thechitosan spectrum wasobserved in 1027and1085cm−1,respectively.Interestingly,thesebandswere shiftedto1031and1068cm−1innanocompositesspectra,where wasalsoobservedbandsat1629,1631and1635cm−1,whichcan beassociatedtothestretchingof thelinkingC N oftheimine (Schiffbase)fromthecrosslinkedreaction(Kim,Shin,Spinks,Kim, & Kim, 2005). Additionally, the absorption band at 1151cm−1 is characteristic ofthe asymmetricstretching of C O C bridge (Kyzas&Lazaridis,2009).Inchitosanspectrum,anabsorptionat
1658cm−1wasalsoobserved,correspondingtostretchingC Oof amidegroup(Donadeletal.,2008).Thesamebandin nanocompos-itesspectracannotbeobserved,owingtooverlapontheabsorption of the C N imine. In ChMNPs spectra, theabsorption around 3452cm−1,referringtothestretchingofchelatedOHgroups,was overlappedonthestretchingoftheN Haminegroup.However, ChM0.1sampleshowedabandat1563cm−1 correspondingto theangulardeformationofthelinkingN Hfromamine.In chi-tosanspectrum,thesamebandwasalsoobserved,butshiftedtoa regionoflowerwavenumber.AccordingtoGregorio-Jaureguiand coworkers,couldbeconcludedthatallchitosanin nanocompos-itesstructuresarechemicallyboundedtothemagnetite,sincethe ChMNPswerewashedseveraltimesandseparatedbymagnetic decantation(Gregorio-Jaureguietal.,2012).
Fig.3.(a)FTIRspectraofthesamples(I)Fe3O4,(II)chitosan,(III)ChM0.1,(IV)ChM0.05and(V)ChM0.01;(b)and(c)TGandDTGcurvesofthesamplesCh(chitosan),ChM
TGanalysiswasusedtoconfirmtheformationofthe nanocom-posite and to quantify the relative magnetite amount in the nanocomposites.TGandDTGcurvesofchitosanand nanocompos-itesareshownin Fig.3(b) and(c),respectively.Thermalevents aswellasquantitiesofmagnetiteinthenanocomposites(mgof chitosan per g of nanocomposite)are given in Table 1. In chi-tosancurveisshowingfourthermalevents,thefirstoneoccursat thetemperaturerangefrom25to110◦C,whereitwasassigned tothedehydration ofthe material.Thesecond eventoccurs in the temperature range around 200–400◦C, related to polymer degradationsuchasdehydrationreactions,deamination, deacety-latedandbreakingofglycoside(Ziegler-Borowska,Chełminiak,& Kaczmarek, 2015). Thermalevents continueseven after 400◦C, occurringaround410–750◦Cand760–856◦Cwithaweightloss of 29.88and 10.29%, mayrelated toopeningof thepyran ring inthepresenceoforganicacids(Marroquin,Rhee,&Park,2013). Fornanocompositescurves,thefirstthermaleventoccursaround 25–120◦C,italsoattributedtodehydration.Interestingly,wecould observethatthenanocompositewiththebiggestamountof chi-tosanshowedthehighestweightlossinthistemperaturerange. Inthissense,theweightlosscorrespondingtotheevaporationof waterseemstodependontheamountofwatermoleculesadsorbed inthechitosanpolymer(deBritto&Campana-Filho,2004).The sec-ondeventoccursaround170–440◦C,wheretemperatureonthe maximumrateoftheweightlosswasfoundtobe253,244and 264◦CforsamplesChM0.01,ChM0.05andChM0.10(Fig.3(c)).In otherwords,thenanocompositespresentedtheseeventsatlower temperaturesthanpurechitosan.Thismayberelatedtothe cooper-ativelossofthehydrogenbondalongthechitosanskeleton,dueto changesinthedonorandreceptorssitesofhydrogenoccasionedby crosslinkingreactionwithglutaraldehyde,asareshowninFig.3(d) (Poon,Wilson, &Headley,2014).The thermaldegradation con-tinueseveninsmallerproportionuntil800◦C,correspondingto thebreakdownofthemainchitosanchainscoveringtheformed magnetite (Table1).As canbeobservedin TGand DTGcurves (Fig.3(b) and(c),respectively),theweightlossformagnetiteis showedaround6%inthedecompositionstagecorrespondingto thetemperaturerange25–800◦C.AsreportedbyGregorio-Jauregui andcoworkers,thisweightlosscanbeattributedtotheremoval of freeand physically adsorbedwater (Gregorio-Jaureguiet al., 2012).Therefore,thetotalweightlossobservedintheChMNP sam-plescorrespondstotherelativeamountofchitosanimmobilized onthenanocomposite,andtheresidualmasscorrespondstothe percentageofFe3O4.
Fromtheseresults, therelativequantityofmagnetiteonthe nanocompositecanbeestimatedusingPeniche’smethodbyEq.
(3)(Peniche,Osorio,Acosta,delaCampa,&Peniche,2005). R= Rq
Wq∗WC+M
(3) whereRqistheresidualpercentageat856◦Cofpurechitosan,Wq andWCarethetotalweightlossinpercentagebetween200and 400◦C,Misthewt.%ofthemagnetiteinnanocomposites,andRis theresidualmassat856◦C.AsshowninTable1,theamountof magnetitefoundintonanocompositestructureincreasedwiththe
decreasingofchitosanmassusedinthesynthesis.Thus,asinitially proposed,assumingthatthenanocompositewaswashedseveral timesaftersynthesisprocedure,thefoundmagnetiteandchitosan intonanocompositearechemicallybounded.
3.2.2. Magneticmeasurements
Themagneticpropertiesofthenanocompositessynthetizedin thisworkweremeasuredasafunctionoftheexternalmagnetic fieldandtemperature.Theresultsofmagnetizationcurves mea-suredat300KareshowninFig.4(a).Absenceofhysteresisloops werefoundforallsamples,sincecoercivityandremanencevalues werezero,whichcanbeattributedtosuperparamagneticbehavior (Freireetal.,2013).Thesaturationmagnetization(Ms)forsamples Fe3O4,ChM0.01,ChM0.05andChM0.1arefoundasbeing56.78, 54.35,49.48e36.0emug−1,respectively.ThedecreaseintheMs valuesofchitosan/Fe3O4 nanocompositecanbeexplainedbythe decreaseintheamountofmagneticmomentsperunitweightdue tothediamagneticcontributionofchitosancoating(Kalkan,Aksoy, Aksoy,&Hasirci,2012).Sheteandcoworkersprepared chitosan-basedmagnetitenanocompositebyimmobilizationofmagnetite inchitosanmatrix,whichobtainedsimilarMsvaluescomparedto ours(Nicolásetal.,2013).Themagnetizationcurveatroom tem-peratureofthesesamplescanbedescribedbyLangevinfunction
M M0 = coth
KH BT −KBT H (4)whereisthemagneticmoment,Htheexternalmagneticfield, TthetemperatureandKB theBoltzmannconstant.The parame-tera=/KB,whichisassociatedwiththediameteroftheparticle asa=4(d/2)3M
0/3KBwithdbeingthediameterofparticle.Thus, adjustingthefitting,itwasfoundforparametersamong1and1.6, theaveragediameter:11.6,12.4,13.2and13.5nmforFe3O4,ChM 0.01,ChM0.05andChM0.1,respectively.Theseresultsare con-sistentwiththeresultsobtainedbyXRPD,exceptforthesample ChM0.1,whichbyXRPDhadasmallerparticlesize.InTEMimages, a largerparticlesizewasobserved.However,this canbeeasily explainedsincebothXRPDandVSMestimateparticlesize consid-eringonlythecrystallineandmagneticpartofthenanoparticle.
Cordenteandcoworkersnormalizedmagnetizationcurvesper gramofNitodeterminetherealMsonNinanoparticles(Cordente et al.,2001).Similarly, we normalized thecurve of magnetiza-tion of nanocompositesper gramof magnetite (takenfrom TG curves).Thecurves,showedinFig.4(b),werealsodescribedby Langevinfunction,wheredeterminedaveragesizediameterwere 11.3,11.6e11.9nmforChM0.01,ChM0.05, ChM0.10, respec-tively.Theseresultssuggestthattheadditionofthechitosanacts asaparticlesizecontroller.TheMsforsamplesChM0.01,ChM 0.05,ChM0.10wasfoundtobe61.12,72.96and60.84emuper gram ofFe3O4, respectively. It is important tonotethat Ms of chitosan/Fe3O4sampleswerehigherthaninpuremagnetite. How-ever,itisstillremainedbelowthebulk(92emug−1)(Guardiaetal., 2007).Despitesomeauthorssuggestthatcoordinationofamino ligands onthesurfaceof nanoparticlesshouldnotreduce their magneticproperties(Pick&Dreyssé,2000).Theliteraturereports amagneticallydeadlayer(MDL)forvariousmagnetic nanoparti-Table1
ThermaleventsobtainedfromTGandDTGcurves,andmagnetiteamountintoMNPs.
Weightloss(%) Fe3O4inthenanocomposites(mgg−1)
1ststage 2ndstage 3rdstage 4thstage
Chitosan 11.56 47.86 29.88 10.29 –
ChM0.01 2.44 8.30 – – 893.05
ChM0.05 4.78 17.15 10.15 – 676.90
ChM0.1 8.04 19.21 13.82 – 588.30
Fig.4. (a)Magnetizationcurvesnormalizedpergramofthesamples;(b)MagnetizationcurvesnormalizedpergramofFe3O4.Asdiscussedinthetext,thedatahasbeen
fitted(solidlines)throughaLangevinfunction;(c)CurvesofMsagainstMDLthickness;(d)IllustrationoftheMDLofthenanocomposites.
clesthatisresponsebydecreaseoftheMs(Ikedaetal.,2010).This
MDLisresultantofthesurfaceeffect,duetoadisorderofthe mag-neticmoments.Thus,thedifferenceinMsbetweensamplesand thediscrepancyofbulkindicatesthatchitosaninterferinginthe thicknessoftheMDLformedinthemagnetite.TheMDLcanbe estimatedusingEq.(5).(He,Zhong,Au,&Du,2013)
Ms(P)=Ms(B)
1−6t d (5) WhereMs(p)andMs(B)aretheMsforthesampleandbulk, respec-tively,dis theaverageparticlediameter andt is thethickness ofMDL(Fig.4(d)). For samplesChM0.01, ChM0.05,ChM0.10 andFe3O4werefoundthatthethicknessoftheMDLis0.66,0.39, 0.63and0.74nm,respectively.Fig.4(c)showsthedecreaseofthe magnetizationsaturationin functionof MDLthickness. Despite observedthatthechitosandecreasesthethicknessoftheMDLof allnanocompositescomparingtopureFe3O4.Thisdecreasedoes notconstantwiththeincreaseofchitosanamountinthereaction medium.TheseresultsareconsistentwiththosefoundbyXRPD, sinceitwasobservedthatthemagnetiteintonanocompositeshad acontractionofitscrystallattice.Fig.5showsthezerofield-cooled(ZFC)/field-cooled(FC)curves measuredat a field of 50Oe for samples Fe3O4,ChM0.01 and ChM0.1.ThesuperpositionoftheZFCandFCcurvesat300Kwas observed,featuringasuperparamagneticsystem.Itwaspossible toverify that theFe3O4 shows a blockingtemperature around 179K.ComparingtoChM0.01andChM0.1curves,itcanbeseen
thatthesamplewithhighestamountofchitosanshowedsmallest blocking temperature,169 and 155K, respectively. This behav-iorisassociatedwithadecreasein dipolarinteractionbetween nearbynanoparticlesduetothecontributionofchitosancoating (Meiorin,Muraca,Pirota,Aranguren,&Mosiewicki,2014).Indeed, theultrasoundmethodproposedinthisworkasingleandfaststep, consideringtimeandpulseregimeconditions,hasbeenshowinga goodalternativeofsynthesis,oncethehybridmaterialpresents superparamagnetic behavior, even coated with chitosan, small crystallitesandhighcrystallinity.
3.3. Electrochemicalmeasurements(modifiedelectrodes)
Inthedevelopmentofmodifiedelectrodesisessentialto evalu-atethestabilityofmodificationwithelectrochemicalactivityofthe analyteofinterest(Hasanzadeh,Shadjou,&delaGuardia,2015). Therefore,thestudyoftheelectrochemicalbehaviorofcommon electroactivespeciesasaredoxcouple[Fe(CN)6]3−/4−areappliedto monitorthesurfacestatusandthebarrierofthemodifiedelectrode (Wangetal.,2015).
The electrochemical behavior of [Fe(CN)6]3−/4− was investi-gatedbycyclicvoltammetry,asshown inFig.6.ChM0.01/GCE exhibitsapairofredoxpeakswiththeanodicpeakpotential(Epa) at303mVandthecathodicpeakpotential(Epc)at74mV,giving apeak-to-peakseparation(Ep)of229mV.Remarkably,forChM 0.05/GCEtheEpa andEpcare251and117mV,respectively,then theirpeak-to-peakseparationiscalculatedtobe134mV.ForGCE
Fig.5.Zerofield-cooled/field-cooledcurvesofsamplesFe3O4,ChM0.01andChM
0.10.
modifiedwithChM0.1,Epa andEpcare244and122mV, respec-tively,andtheEpiscalculatedtobe122mV.Thereductionof Epdemonstratesafastelectrontransfer(Dengetal.,2014),in whichwaspromotedbytheincorporationofchitosaninthe func-tionalization(Yang,Ren,Tang,&Zhang,2009;Yu,Gou,Zhou,Bao, &Gu,2011).
Furthermore, cyclic voltammograms of ChM 0.01/GCE, ChM 0.05/GCE, ChM 0.1/GCE reveal well-defined symmetric peaks at different scan rates (
); meanwhile, both Ipa and Ipc increase as the square root of the scan rate grows from 10 up to 150mVs−1 (data not shown). The peak currents vary linearly withv
1/2, whose linear regression equations are Ipa (A)=−1.40×10−6+1.87×10−6v
1/2 (R=0.98064, n=6), Ipc (A)=4.75×10−7−1.50×10−6v
1/2 (R=0.97268, n=6) for ChM 0.01/GCE. For ChM 0.05/GCE, ChM 0.1/GCE the equa-tions are Ipa (A)=−1.52×10−6+2.99×10−6v
1/2 (R=0.97284, n=6), Ipc (A)=2.09×10−6−2.63×10−6v
1/2 (R=0.97496, n=6), and Ipa (A)=−4.95×10−6+3.43×10−6v
1/2 (R=0.97763, n=6), Ipc(A)=3.89×10−6−2.89×10−6v
1/2(R=0.97778,n=6), respec-tively.ThemagnitudeofcurrentresponseforChM0.1/GCEincreases incomparisontoChM0.01/GCE,andissimilarforChM0.05/GCE. ThismaybeduetotheinducedmagnetizationofFe3O4magnetic domainsunderanelectricfield.Itseemsthattheelectricfieldgiven directioninducesalignmentofmagneticnanoparticlesand facili-tatestheflowofelectrons,resultinginanincreaseofthecurrent value(Kaushiketal.,2009).
In order to elucidate the good conductivity, improving the electronictransportefficiencyandlargesurfacearea,we quanti-tativelydetectedtheelectro-activesurfaceareaforallmodified electrodesbyrecordingcyclicvoltammetryatdifferentpotential scanrates[Fe(CN)6]3−/4−servingasredoxprobes.Theelectroactive surfaceareawascalculatedaccordingtoRandles–Sevcikequation
Fig.6.Cyclicvoltammetryof( )GCE,( )ChM0.01/GCE,( )ChM0.05/GCEand ( )ChM0.1/GCEsamples,inasolutionof1.0×10−3Fe(CN)63−/4−and0.1molL−1
ofKClatascanrateof50mVs−1.InsertFiveCyclicvoltammetryofChM0.05/GCE, inasolutionof1.0×10−3Fe(CN)63−/4−and0.1molL−1KClatascanrateof50mVs −1.
(Bard&Faulkner,2000)Ip=2.69×105AD1/2 n3/2
v
1/2C,inwhich A is the electrode area, Dis the diffusion coefficient (at25◦C, D=7.60×10−6cm2s−1),nisthenumberofelectronstransferred intheredoxreaction(n=1),Cistheconcentrationofthereactant (1mMFe(CN)63−/4−),Ipreferstotheredoxcurrentpeakandv
isthe scanrateofthecyclicvoltammetrymeasurement.UsingIpav−1/2 indicatedthatChM0.1/GCE(3.42×10−3±5.36×10−4cm2)owns alargerelectro-activesurfaceareacomparedwiththesurfacearea (1.37×10−3±1.57×10−4cm2)ofbareglassycarbonelectrode.The increase of electroactive area is not significant when compared to theChM 0.05/GCE (3.31×10−3±5.16×10−4cm2). However,itispossibletoobservethatbyincreasingthe electroac-tiveareaoflowerconcentration(1.88×10−3±1.05×10−4cm2)is 1.82timessmallerthanthemodificationChM0.1/GCE,duetothe smallincreaseincurrentandelectroactivearea.Thereby,ChM0.05 nanocompositeisrecommendedforfutureelectrochemical appli-cations.Inaddition,theincreasingintheelectroactiveareaand theimprovementofelectron transfer,anotheradvantageof the ChMNPsistogenerateanelectrochemicalsignalexceptionally sta-ble,whereboththepeakcurrentandthepotentialremainalmost unchangedevenafter5cycles(Fig.6).
4. Conclusion
The US assisted in situ synthesis proposed in this work introduces a new fast wayto obtain chitosan-basedmagnetite nanocomposite.TEMimagesconfirmthatChMNPs hadan aver-agediameterintherangeof10–25nm,whencomparedtothose obtained from XRPD (crystallite size, 8–13nm) and magnetic measurements(11.3–11.9nm).FTIRandTGanalysisprovedthat chitosanwaschemicallyboundedintonanocompositestructure, furthermoreTGresultsshowedthatthequantityofchitosaninto ChMNPincreases accordingtotheincrease of chitosan amount usedinthesynthesis. Allsamplespresentedsuperparamagnetic behavior,andaccordingtotheseresultsthesynthesized nanopar-ticleshadsimilarcharacteristicscomparingtothoseproducedby othermethodsrelatedintheliterature.Additionally,besidesthe fastpreparationofthesamples(around2min)mayminimizethe oxidation ofmagnetite, drasticallydecreasesthesynthesis time whencomparedtotraditionalmethods.Nevertheless,the synthe-sizednanocompositesshowedagreatpotentialaselectrochemical sensor,hencedemonstratedelectrochemicalsignalexceptionally stable,and theinsertion ofchitosan increasedtheelectroactive area.
Acknowledgements
ThisworkwaspartlysponsoredbyCAPESandCNPq(Brazilian agencies).ThesupportfromFondecyt1140195andCONICYTBASAL CEDENNAFB0807aregratefullyacknowledged(Chileanagencies).
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