ContentslistsavailableatScienceDirect
Journal
of
Hazardous
Materials
j ou rn a l h om epa ge :w w w . e l s e v i e r . c o m / l o c a t e / j h a z m a t
Poly-cyclodextrin
cryogels
with
aligned
porous
structure
for
removal
of
polycyclic
aromatic
hydrocarbons
(PAHs)
from
water
Fuat
Topuz
a,∗,
Tamer
Uyar
a,b,∗∗aUNAM-NationalNanotechnologyResearchCenter,BilkentUniversity,06800Ankara,Turkey
bInstituteofMaterialsScience&Nanotechnology,BilkentUniversity,06800Ankara,Turkey
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•Polycyclodextrin (polyCD) cryogels
weresuccessfullysynthesizedusing
PEGdiepoxidecross-linkers.
•The cryogels displayed an aligned
porousnetworkstructureduetothe
directionalfreezingofthematrix.
•The polyCD cryogels showed very
highPAHsorptioncapacitiesvarying
between105and1250gpergram
material.
•Thematerialscouldberecycledand
reusedwithoutanysignificantlossin
PAHadsorptioncapacity.
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Articlehistory:
Received15November2016
Receivedinrevisedform5April2017
Accepted6April2017
Availableonline7April2017
Keywords: Cyclodextrin Cryogel Poly-cyclodextrin PAH Waterremediation
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Cyclodextrins(CDs)aresugar-basedcyclicoligosaccharides,whichforminclusioncomplexeswithsmall
guestmoleculesthroughtheirhydrophobiccavity.Herewesuccessfullysynthesizedhighly porous
poly-cyclodextrin(poly-CD)cryogels,whichwereproducedundercryogenicconditionsbythe
cross-linkingofamine-functionalCDswithPEG-baseddiepoxidecross-linker.Thepoly-CDcryogelsshowed
alignedporousnetworkstructuresowingtothedirectionalfreezingofthematrix,ofwhichtheporesize
andarchitectureexposedvariationsdependingonthecompositionofthereactants.Thecryogelswere
employedfortheremovalofgenotoxicpolycyclicaromatichydrocarbons(PAHs)fromaqueous
solu-tions.TheyreachedPAHsorptioncapacitiesashighas1.25mgPAHpergramcryogel.Thishighsorption
performanceisduetointeractionsbetweenPAHsandthecompleteswollennetwork,andthus,isnot
restrictedbyinterfacialadsorption.Giventhatthehydrophilicnatureofthecomponents,thesorption
performancecouldonlybeattributedtotheinclusioncomplexformationofCDswithPAHmolecules.
Thepoly-CDcryogelscouldberecycledwithanexposuretoethanolandreusedwithoutanysignificant
lossinthesorptioncapacityofPAHs.
©2017ElsevierB.V.Allrightsreserved.
∗ Correspondingauthor.
∗∗ Correspondingauthorat:UNAM-NationalNanotechnologyResearchCenter,
BilkentUniversity,06800Ankara,Turkey.
E-mailaddresses:fuat.topuz@rwth-aachen.de(F.Topuz),
tamer@unam.bilkent.edu.tr(T.Uyar).
1. Introduction
Cyclodextrins (CDs) are cyclic oligosaccharides produced by enzymatic conversion of starch. CDs have a distinct molecular structurelikeatruncatedconewithapartiallyhydrophobic cav-ity,whichallowsnon-covalenthost-guestinclusioncomplexation withalargevarietyofhydrophobicmoleculesinwater,andthis complexationistriggeredbyenthalpicand entropicfactors[1].
http://dx.doi.org/10.1016/j.jhazmat.2017.04.022
NativeCDs(˛-CD,ˇ-CDand-CD)havecertainlimitationsinthe senseoftheirlow water-solubility;thewatersolubilityof˛-CD and-CDisabout145g/Land 232g/L,respectively,whileˇ-CD hasthelowestwatersolubility(18.5g/L)amongstnativeCDsdue tohighnumberofintramolecularhydrogenbondsamongst sec-ondary hydroxylgroups [2]. Onthe otherhand,thechemically modifiedCDs,suchashydroxypropyl-CDandmethylated-CDhave muchhigherwatersolubility(above600g/L)[3].Thus,numerous attemptshavealreadybeenmadetowardsthesynthesisof water-solubleCDderivativesforfunctionalmaterialplatforms[4–7].In thatcontext,amine-modifiedCDshavetakenaconsiderable inter-est as theypresent accessible and highly reactive nucleophilic amines,whichletfurtherchemicalligationswithvariouschemical groups under mild conditions. Particularly, using such amine-modifiedCDsforthesynthesisofporousmaterialswouldbean interestingapproach fora widerangeof applications,including waterremediation.
Cryogelsareporousmaterialsgeneratedundercryogenic con-ditions, which take place in a non-frozen liquid of precursors existing in a macroscopically frozen sample [8]. These highly porousnetworkshavebeenproducedbyeithercovalentorphysical cross-linkingofadditivestowardsmacroscopicplatforms,which providesinter-linkedporesalongwithsurroundedthickpolymer walls[9,10].Theseplatformsstructurallymimicspongeswiththeir inter-linkedmacroporousnetworks,andunliketheirhydrogel ana-logues, suchscaffolds offer many intrinsic benefits in terms of mechanicaltoughness(i.e.,theabilityofdissipateenergy),ahighly porousnetworkstructure,fasterresponse,rapidwatertransport (water uptake and release) and many otherstructural benefits
[11,12].However,tothebestoftheauthors’knowledge,todate therehasbeennostudyonaCD-basedcryogelplatform,which waspreparedwithoutusinganypolymersupport.
PAHsarealargechemicalfamilyoffusedbenzeneringswith their potent carcinogenic, teratogenic and mutagenicactivities
[13,14].Theyarelipophiliccompoundsandubiquitouslypresent inwatersources.Thus,theycanaccumulateontissues,andexert itslong-termeffectsinvivo.PAHshavepotentialtoformPAH-DNA adductsaftermetabolicactivationbycytochromep450enzymes, whichleadtothegeneticmisleading,andtherefore,theytransform anormalcellintoacancercell[15,16].Oneimportantsourceof PAHuptakeispolluteddrinkingwater,particularlyfordeveloping countries,wheretheyareoftenhamperedbythelackoflimited enforcementofwater qualitystandardsand available technolo-giesforeffectivewater-remediation.Thelong-termPAHexposure inducessignificanttoxicity,andthus,severalpolymericplatforms havealreadybeenreportedfortheremovalofPAHs(Table1).Dueto itsuniquecone-typestructurethatcanformsupramolecular com-plexeswithguestmolecules,CDshavebeenusedasmoleculartraps toremoveorganicpollutantsthataresmallenoughtofitintothe innercavityofCD,suchasPAHs[17,18].Duetothewater-solubility ofCDs,theirdirectimplementationaswaterfilteringmaterialsfor theremovalofpollutantsfromwaterisnotpractical.Therefore, CD-basedplatformshavebeenproducedwithappropriate cross-linkingroutes[19,20],orfunctionalizationorstabilizationofCDs onsolidmaterials[21–25].
In this paper, we synthesize poly-cyclodextrin (poly-CD) cryogels from amine-functional CDs and by its cryogenic gela-tionthroughpoly(ethyleneglycol)(PEG)-baseddiepoxidelinkers
towards functional materials with a highly porous network
architecture.Thisprocessgraduallyhappensbecauseoflow tem-peraturealongwithlowerkinetics,whichcausesslowcross-linking ofCDmolecules.Further,suchcryogelspresentahydrophilic mate-rialwithhydrophobicCDcavities,whereeachCDcavityactsasa moleculartrapfortheentrapmentofpollutantsfromaqueous sys-tems.Thecross-linker(i.e.,poly(ethyleneglycol)diglycidylether, PEG-DGE)isahydrophiliccompound,anditwillthusnot
inter-ferewithhydrophobicpollutantsoverhydrophobicassociationsso thatthetotalremovalofpollutantscouldonlybeattributedtothe inclusion-complexformationwithorganicpollutants.Beyondthe noveltyofthesynthesizedpoly-CDcryogel,thisstudyalsoreveals thesorptionperformanceofpoly-CDcryogels,particularlyforthe removal of genotoxic polycyclicaromatic hydrocarbons(PAHs), withthe advantageof having a highly poroushydrophilic net-workforthewatertreatment.Variouscharacterizationtoolswere employedtoelucidatethestructure-propertyrelationshipofthe cryogels.Thepoly-CDcryogelswerelateremployedforthe elim-inationof severalPAH molecules fromwater, and thesorption capacitieswerecalculated.Thematerialscouldberecycledwith anethanolexposure,andthereafterretreatedwithPAHsolutions withoutshowing anysignificantreduction inthe PAH-sorption capacities.
2. Experimentalsection
2.1. Materials
PAHmolecules(i.e.,pyrene,anthracene,phenanthrene,fluorene andfluoranthene),methanol(MeOH,>99%)andpoly(ethylene gly-col) diglycidylether(PEG-DGE, Mn=500g/mol)werepurchased
fromSigma-Aldrich.3-Aminoproyltrimethoxysilane(APTMS)was kindly provided fromEvonik(Germany), andhydroxypropyl ˇ-cyclodextrin(Cavasol®HP-W7)wasreceivedasagiftbyWacker Co.(USA).
2.2. Synthesisofamine-functionalbeta-cyclodextrins (NH2-ˇ-CDs)
Amine-functionalCDs(NH2-ˇ-CDs)weresynthesizedthrough
silane-hydroxyl reaction using an aminosilane coupler
(3-aminoproyl trimethoxysilane, APTMS) in methanol over 5days. APTMS(2mL)wasmixedwithHP-ˇ-CD(2g)inmethanol(100mL) undercontinuousstirringfor5days.Thereafter,thesolutionwas subjectedto90◦Cfor2handpurifiedbyprecipitationincold ace-tone.The product wasdried at vacuumovenat 60◦C, and the NH2-ˇ-CDwasobtainedaswhitepowder.1HNMRspectraofthe
NH2-ˇ-CDandHP-ˇ-CDwereshowninFig.1.
Thereactionofpolysaccharideswithsilanemoleculesis pre-viouslyreported,wherethereactionproceedsbetweenhydroxyls andsilanolgroups,formingsilylether(Si O C)linksuponheat exposure[26,27].Thissol-gelprocessbetweensilanesandvarious typesofpolysaccharideswithouttheadditionofanorganicsolvent andacatalystledthejellificationandtheformationofmonolithic hydrogels[27].ThesolubilityofnativeCDsinalcoholsis consid-erablylimited,andtherefore,thesynthesiswasperformedusing solubleCDderivative,HP-ˇ-CD.Theamine-modificationof HP-ˇ-CDwasperformedbythesilane-treatmentinmethanolover5days. AstheCDshavesomewatercontent,sothatsilanegroupswere sus-ceptibletoslowhydrolysistoyieldsilanols(Si-OH).Afterward,the solutionsubjectedto90◦Cfor2h.Theheat-treatmentfor2hcan drivethechemicalconjugationbetweenthesilaneandCD. 2.3. Synthesisofmacroporouspoly-cyclodextrin(poly-CD) cryogels
Amine-modifiedˇ-CDcompoundsweredissolvedinwaterand thereaftermixedwithPEG-DGE.Thereactiontookplacebetween theepoxygroupofPEGDGEandtheaminegroupoftheNH2
-ˇ-CD (seechemical structures ofthereactants inFig.S1). During thesynthesesofthecryogels,theconcentrationsofbothCDand cross-linker(PEG-DGE)weresystematicallyvaried;theCDcontent boostedfrom10to20%(w/v)attheidenticalcross-linker
concen-Table1
Materialsystemsusedfortheclean-upofPAHsfromwater.
MaterialComposition Materialform MaterialPolarity Sorptionmechanism Sorptioncapacity
(g/g)
Specialfeatures Refs.
ModifiedSilica Gels Apolar Hydrophobicinteractions 200–300 Cytocompatible Halletal.[28]
Hemoglobinimmobilized
onmesoporoussilica
Particles Apolar Hydrophobicinteractions N.D. Cytocompatible Laveilleetal.[29]
Polyphenol Particles Apolar Hydrophobic
interactions,-
interactions
750 RequiredtoxicH202 Chenetal.[30]
Poly(ethylene glycol)-b-poly(lactic
acid)(PEG-b-PLA)
copolymers
Nanoparticles Polar-Apolar Hydrophobicinteractions 310 Cytocompatible,problem
withscalability
Brandletal.[31]
CD-functionalcellulose
acetate
Fibers Apolar-Polar Hydrophobicinteractions 540 Cytocompatible Celebiogluetal.[25]
Polypropylene Fibers Apolar Hydrophobicinteractions 615 Cytocompatible Ceylanetal.[32]
Poly-CDcryogelsa Macroporousgels Polar Host-guest 105–1250 Cytocompatible&
Biodegradable
Presentstudy
Cyclophanes Crystals Apolar Donor-acceptor
interactions
N.D. Possibletoxicity,
problemwithscalability
Barnesetal.[33]
Butylrubber Macroporouscryogels Apolar Hydrophobicinteractions 721 Cytotoxic Ceylanetal.[32]
N.D.;notdetermined.
aThepresentstudy.
Fig.1. 1HNMRspectrumoftheNH
2-ˇ-CDinD2O.Insetshowsthe1HNMRoftheHP-ˇ-CD.
tration(0.35mM),orthePEG-DGEcontentincreasedfrom0.17to 0.35mMattheconstantCDcontentat20%(w/v).Thereafter,the solutionsweretransferredintoplasticdisposablesyringes(inner diameter(d)=4.7mm)andkeptat−20◦Cforthecross-linking reac-tionsover5days.Followingthisprocedure,theformationofhighly opaquegelswasobserved.Thegelsampleswerecutwitha cold-razorbladeandputinwaterforfewhourstogetridofunbound precursorsfromthegelmatrices. Followingthisprocedure, the poly-CDcryogels wereproducedatvariouscompositionsofthe precursors.Theporesizedistributionofthepoly-CDcrogelswas estimatedusingImageJsoftware.
Fig. 2 illustrates the synthesis scheme of poly-CD cryogels, wherebothprecursors(NH2-ˇ-CDandPEG-DGE)weremixedand
exposedtoliquidnitrogenandimmediatelykeptat−20◦Ctofreeze
watermoleculestowardsicecrystals.Thispromptfreezingstep wasusedtopreventundesiredcross-linkingreactionsthatcanlead ahydrogelnetwork.Asthewatermoleculestransformedintoice crystals,theconcentratedCDzonesremainedliquiddistrictsareas inbetween.TheCDaggregatesgraduallyreactedtowardsaporous cross-linkedmatrix.Thisprocess wasendedup witha
sponge-likesystemwiththickpolymerwalls,whileinnermatrixhadan irregularinter-linkedporousnetwork.
2.4. Characterizationofmaterials
FT-IRspectraofthedriedpoly-CDcryogelsandNH2-ˇ-CD
pow-derwererecordedusingaBruker-VERTEX70spectrometer.The spectraweretakenataresolutionof4cm−1withanaccumulation of128scans.
TheXPS spectraof thedried poly-CDcryogel sampleswere recordedbyusinganX-rayphotoelectronspectrometer(Thermo Fisher Scientific, U.K.). As an X-ray source, Al K-alfa X-ray monochromator(0.1eVstepsize,12kV,2.5mA,spotsize400m) wasusedatanelectrontake-offangleof90◦.Foreachsample, sur-veyspectrumwastaken5timeswith50msdwelltime(passenergy 200eV).AllN1s,O1s,C1sandSi2pspectraweretaken10times with50msdwelltime(passenergy30eV).Thebindingscalewas referencedtothealiphaticcomponentofC1sspectraat284.85eV.
1HNMRspectrawererecordedonaBrukerDPX-400
Fig.2. ThesynthesispathwayoftheNH2-ˇ-CD(a),andacartoonillustrationofthecryogelationofNH2-ˇ-CDaggregateswithPEG-DGE(b).Opticalphotoshowsthe
poly-CDcryogelsjustafterthesynthesis,revealinghighlyopaquenetworks(c).Thecompositionofeachcryogelinthephotoisasfollows;(i)cNH2-ˇ-CD=20%(w/v)and
cPEG-DGE=0.17mM,(ii)cNH2-ˇ-CD=20%(w/v)andcPEG-DGE=0.26mM,(iii)cNH2-ˇ-CD=20%(w/v)andcPEG-DGE=0.35mM,(iv)cNH2-ˇ-CD=10%(w/v)andcPEG-DGE=0.35mM,(v)
cNH2-ˇ-CD=15%(w/v)andcPEG-DGE=0.35mM,and(vi)cNH2-ˇ-CD=20%(w/v)andcPEG-DGE=0.35mM.
thespectrumwasrecordedat400MHzand512scanswere per-formed.
For themolecular weightanalysis of theNH2-ˇ-CD,Agilent
Technologies6530Accurate-MassQ-TOFLC–MSandZorbax SB-C8columnwereused.Solventswerewater(0.1%formicacid)and acetonitrile(ACN)(0.1%formicacid).LC–MSwasrunfor25minfor eachsample,anditstartedwith2%ACNand98%H2Ofor5min.
Afterward,ACNconcentrationreachedto100%for20min. There-after,theconcentrationwasdroppedto2%,anditkeptrunningfor 5min.Thesolventflowsetto0.65mL/min,and5Lsamplewas injected.m/z:1506,1621.68,and1742.
Theinner-morphologyofthefreeze-driedpoly-CDcryogelswas exploredbySEM(Quanta200FEG,FEI)aftergold-sputtering.The averageporediameter(<D>)andtheirdistributionwerecalculated byanalyzingca.100poresfromSEMimagesusingImageJsoftware (NIH,Bethesda,USA).Energy-dispersiveX-ray(EDX)spectroscopy wasusedformonitoringtheelementalcompositionofthematerial at30kVand4.5mAoncuppertapesaftergold-sputtering(Gatan 682PrecisionandCoatingSystem(PECS)).
FluorescencespectraofthePAHsbeforeandaftertreatments with the poly-CDcryogels were recorded on a Cary 100 fluo-rescencespectrophotometerusingfour-transparentfacedquartz cuvettes.The excitation wavelengthswerecalculated usingthe
maximumabsorbanceobserved fromUV-spectrum(Figs. S2–3).
Emissionwavelengthrangewasfrom260to600nm.Theexcitation wavelengthsforPAHsareasfollows:260nm(forphenanthrene), 264nm(forfluoranthene),250nm(foranthracene),260nm(for pyrene)and260nm(forfluorene).
TheadsorptionspectraofthePAHsweregatheredonaCary 100spectrophotometer.PAHs weredissolvedinwater, andthe
respectivespectrawerecollectedbetween200and800nmusing two-transparentfacedquartzcuvettes.Theinitialconcentrationof PAHsarerespectivelyasfollows: 0.20g/mL (forphenanthrene andpyrene),0.40g/mL(forfluoranthene),0.086g/mL(for fluo-rene)and0.027g/mL(foranthracene).
2.5. PAHsorptionexperiments
PAH molecules (i.e.,pyrene,anthracene, phenanthrene, fluo-reneand fluoranthene)weretreatedwiththepoly-CDcryogels (cNH2-ˇ-CD=20%(w/v)andcPEG-DGE=0.35mM)asafunctionoftime
(3,6and9h).Thecryogelswerecutintocylindricaldiskswitha razorbladeandtransferredintoanaqueousPAHsolution(20mL). The initial concentrations of PAHs are as follows: 0.20g/mL (for phenanthrene and pyrene), 0.40g/mL (for fluoranthene), 0.086g/mL(forfluorene)and0.027g/mL(foranthracene).After shakingfor3,6and12hat25◦Cusingaheat-controlledincubator (Fig.S4),3mLofthissolutionwastransferredintovialsandthe flu-orescencemeasurementswereperformed.3mLwaterwasadded tokeepthetotalvolumeconstant.Notethatthecryogelsremained aswhitecylindricalmassesinvialssuchthattheaqueouspartcould easilybetakenout.Thus,noparticularstepwasusedtoseparate thecryogelsfromPAHs.Theamountofcryogels(i.e.afterthe syn-thesis)usedineachexperimentvariedbetweenca.50and60mg. TheemissionspectraoftherespectivePAHmoleculesafterhaving treatedwiththepoly-CDcryogelswererecorded,and the sorp-tioncapacityforeachPAHmoleculewascalculatedusingstandard curvesoftherespectivePAHmolecule(Figs.S5–9),andthedata havegivenpergramdrycryogel.Theexperimentswereperformed intriplicate,andthemeandatawerepresented.
Recyclabilityexperimentswereperformedafteranexposureto ethanolof20mLbythreetimesover5minforeach. Thereafter, thesamplewastreatedwithwater(20mL)for10min.Theamount ofcryogelsissimilartothematerialusedforPAHsorptiontests (∼50mg).Thereafter,thefreshlypreparedsamplesweretreated againwithPAHsolutionswithindifferenttimeintervals,andthe supernatantpartwasmeasuredwithafluorescence spectropho-tometertomonitorvariationsinPAHcontents.
3. Resultsanddiscussion
The LC–MS spectrum of the product revealed the
success-ful formation of amine-functional CDs(NH2-ˇ-CDs), where the
peaks related to the NH2-ˇ-CDs appeared in the range of
1400–1900gmol−1 (Fig. S10). Note that pristine HP-ˇ-CD has anaveragemolecularweightof∼1400gmol−1.Thus,thepeaks appearedbetween1400and1900gmol−1 canbeassignedtothe silaneconjugation.Interestingly,nodimerortrimerformationwas obervedafterthereaction.However,notethatHP-ˇ-CDisalsoa polydispersemolecule,andthus,itsreactionwithsilanesleadsto variationsinthemolecularweightoftheproduct.Thesynthesisof theNH2-ˇ-CDwasfurtherconfirmedby1HNMRanalysis,where
thecharacteristicpeaksofCDprotonswereobservedbetween3 and6ppm(Fig.1).Thepresenceofmethylprotonsboundsilicon at0.56ppmreferstothesilaneconjugation.1HNMRshowsthe
methylprotonsadjacenttotheamineat2.90ppm,whilethemethyl protonsofthepropylgroupofCDsappearedat1.15ppm.The pro-tonsofCH2-CH2-NH2wereobservedasmultipletat1.60ppm.The
XRDpatternoftheNH2-ˇ-CDrevealedtheamorphousstructureas
similartoHP-ˇ-CD(Fig.S11).
Fig.2(inset)showstheopticalphotosofthepoly-CDcryogels producedatvariouscompositionsofprecursors.Forallconditions, theformationofacryogelmatrixsuggeststhatthecryogelation wassuccessfulatvariousconcentrationsofCDandPEG-DGE(Fig.2, insetphoto).Ontheotherhand,nogelformationwasobservedin
theabsenceofCDmoieties.Allcryogelswereopaquewhile main-tainingtheircylindricalforms, suggestinghighlyheterogeneous networkstructures.Thisopaquenessisanintrinsiccharacteristic ofcryogelsystemsbecauseofthenetworkheterogeneityinduced byirregularinterlinkedmacropores.Thewatercontentsatfreshly synthesizedgelsamplesafterfreeze-dryingwerecalculatedinthe rangeof67–76%dependingontheusedprecursorcontent, demon-stratingthatnearlyallCDswerechemicallyboundtothescaffolds withcorrespondinggelfractions(Wg)over0.98(98%).
Theinnermatricesofthecryogelswereexploredbyscanning electronmicroscopy (SEM), revealedaligned macroporesrather than irregular pores.Whereas, thegel surface displayed thick-polymer walls withcollapsedpores (Fig. 3).This is due tothe directionalfreezingofwater,whichledtothegrowthofice crys-talsandtheorientationfromthesurfacetotheinterior.Generally, suchakindofporestructurerequiresparticularfreezing-setups, i.e.,one-sideofthechamberexposedtocoolingwithaso-called method“unidirectionalfreezing”[34–36].Theporesize distribu-tionwasshowninFig.3(f),wherethemeanporesizewascalculated as2.53m.Thealignmentoftheporescouldbeascribedtothe directionalfreezingofwatermoleculesonthewayofthe tempera-turegradient.Thiscanalsobeseenontheorientationofthepores fromthesurfacetothecore(Fig.3a).Theporesizewasdirectly affectedbytheconcentrationoftheprecursors.Forinstance, well-aligned poreswere observed athigh concentrations of CD and cross-linker while lowering PEG-DGE concentration led to col-lapsedpores(Figs.4andS12).Thismightbeattributedthatoncethe concentrationishighenough,thestructuralintegrityofthematrix ispreserved.
Theatomiccompositionofthepoly-CDcryogels,whichwere synthesizedatvariouscompositionsofprecursors,wasstudiedby X-rayphotoelectronspectroscopy(XPS).AlthoughXPSismainly usedfor the compositionalanalysis of surfacesrather thanthe matrix,herethesamplesarehomogenouslyformedbythemixed solutionoftheNH2-ˇ-CDandPEG-DGE.Thoughthesamplesshow
Fig.3. Morphologicalanalysesofthepoly-CDcryogel.SEMimagesofthecryogel(cNH2−ˇ-CD=20%(w/v)andcPEG-DGE=0.35mM)displayhierarchicalalignedpores(a,b,c)
Fig.4.SEMimagesofthepoly-CDcryogelssynthesizedatvariouscompositionsofprecursorsshowalignedporousstructures.(a–c)SEMimagesofthecryogelsproduced
attheconstantCDcontent(20%(w/v))andvariouscross-linkerconcentrationsindicated(i.e.,0.35mM(a),0.26mM(b),and(c)0.17mM).Theporesizedistributionofthe
respectivecryogelsystems.
differentporearchitecturebetweenthesurfaceandinnermatrix, itisnotexpectedtohaveanysignificantvariationsinthe chemi-calcompositionbetweenthesurfaceandmatrix.Fig.5showsthe XPSsurveyspectraofthecryogelswherethecryogelsweremade upsubstantiallybycarbon(C)andoxygen(O)andsubordinately nitrogen(N)andsilicon(Si).Thepresenceofthenitrogen(N)and silicone(Si)atomscouldbeattributedtotheamine-modifiedCDs. Withanincrease ofCDratiointheformulationofthecryogels, bothNandSicontentssubstantiallyrise.DeconvolutedC1sspectra showthatwithanincreaseofCD,theC Ccarbon(284.8eV)peak rises,whiletheadditionofmorecross-linkerinducesasignificant increaseinC Opeak(286eV)(Fig.S13).NotethattheHP-ˇ-CD hasahighamountofC Obond,butaftermodificationwithsilane moieties,itsproportiondecreasesintheoverallcomposition.On theotherhand,thecross-linkerPEG-DGEhasconsiderableC O bonds,andthus,theincreaseofCcontentcouldbeattributedto highercross-linkercontent.WithanincreaseoftheCDcontentby
two-foldfrom10to20%(w/v),nitrogen(N)contentinthesample rises.
Table2summarizestheatomiccompositionsofthecryogels andaswellasNH2-ˇ-CDmolecules.TheHP-ˇ-CDhasnotanyN
andSi,whiletheNH2-ˇ-CDhasconsiderableamountsofNand
Si.Therefore,thecryogelsdisplayconsiderableproportionsofthe respectiveelementsinoverallcomposition.TheNcontentinthe NH2-ˇ-CDisabout5.84%,whiletheSicontent(asSi2sandSi2p)is
foundas9.37%,suggestingthatNH2functionalCDmoleculeshave
substantialSicontent.Thus,thematerialformedbytheNH2
-ˇ-CDshouldalsopossessquantifiableamountsofbothatoms(Nand Si).WithanincreaseofCDcontentfrom10to20%(w/v),N con-tentincreasesfrom1.06to1.84,whileSicontentrisesfrom3.94 to6.68.Ontheotherhand,withanincreaseofPEGDGEcontentby two-foldfrom0.17to0.35mM,Ncontentdecreasesfrom2.22to 1.84%.PEGDGEdoesnothaveanySiandNatomswhileithasmore C(64.68%)thantheO(35.32%)(seeTable2).Thechemical
composi-Fig.5.XPSsurveyspectraoftheNH2-ˇ-CD(a),PEGDGE(b)andpoly-CDcryogels(c-f),whichweresynthesizedatvariousconcentrationsofprecursors.Thecompositionof
precursorsineachgelasfollows:[c]20%(w/v)CD,0.17mMPEGDGE;[d]20%(w/v)CD,0.35mMPEGDGE;[e]15%(w/v)CD,0.35mMPEGDGEand[f]10%(w/v)CD,0.35mM
PEGDGE.
Table2
XPSatomiccompositionsofthepoly-CDcryogels,NH2-ˇ-CDandPEG-DGE.
[a]NH2-ˇ-CD [b]PEG-DGE [c](20%(w/v)CD –0.17mM) [d](20%(w/v)CD –0.35mM) [e](15%(w/v)CD –0.35mM) [f](10%(w/v)CD –0.35mM) C1s(285eV) 56.52 64.68 63.06 65.31 65.60 66.21 O1s(581eV) 28.77 35.32 28.21 26.17 29.63 29.79 N1s(399eV) 5.84 0 2.22 1.84 0.90 1.06 Si2p(101eV) 5.07 0 3.71 3.16 2.86 1.60 Si2s(152eV) 4.30 0 2.81 3.52 1.01 1.34
[a]–[f]denotetothesamplenumbersinFig.5.
tionsofthecryogelswerealsoanalyzedbyFT-IR,wherethetypical stretchingpeakofC Hbondappearedat2932cm−1(Fig.S14).A broadpeakat3408cm−1couldbeascribedtotheO Hvibration, andbut,itoverlapswithN Hvibrationofprimaryandsecondary amines.TheC Hstretchingofepoxyringshouldnormallyappear at∼840and910cm−1,andbothsamplesdonotshowanypeak intherespectiveregion,suggestingthatepoxyringsreactedwith amines.Thereactionbetweenepoxyandaminegroupsthrougha ring-openingmechanismledtoacross-linkedgelnetwork.
Fig.S15 (see Supporting Information) shows the PAH sorp-tion performances of the cryogels after 6h treatment, where significantreduction inthefluorescenceintensitywasobserved for all PAH molecules. For some PAH molecules (pyrene, fluo-rantheneand phenanthrene),almostcomplete removalofPAHs was observed, for other PAHs (fluorene and anthracene), very small peaks associated residual PAHs were detected, suggest-ingtheefficientremovalofPAHsbypoly-CDcryogels.Likewise, time-dependentPAHremovalexperimentsrevealedthat theno significantvariationwasobservedbetween3and9htreatments (Fig.6).ThefluorescencespectraofthePAHsafter3hPAH treat-mentrevealedasubstantialdecreaseinthefluorescenceintensity. Furtherincreasingexposuretimedidnotreducetheamountof tracePAHs.
ThePAH-sorptioncapacitiesofthecryogelsafter6htreatment wereshown inFig.7,wherethesamplesshowPAHscavenging capacities intherange of 0.6–6.22M PAH moleculepergram poly-CD cryogel. The highest sorption capacity was found for fluoranthene(6.22M/gdry cryogel)while thelowestonewas
observed for anthracene (0.59M/g). The photo of thecryogel sampleshows thatthestructural integrity ofthematerialafter 6h treatment with fluoranthene. The sorption values are high enoughandcomparabletoothergoodPAH scavengingmaterial systems(Table1),andfurther,thisperformancecanbeattributed tothecomplete network morethan interfacial adsorption.The totalremovalofPAHsinpercentwasfoundover94%,suggesting thehighefficiencyofthesematerialsinPAHremoval(seeFig.7, inset).Thewater-solubilityrangeofthePAHs(i.e.phenanthrene, anthracene, fluorene, fluoranthene and pyrene)varies between 0.044and1.9mgperliter.Eventhoughhavinglessringnumber, thewatersolubilityofanthracene(0.044mg/L)ismuchlowerthan thefluoranthene(0.265mg/L).SincePAHsarepoorlysoluble com-pounds,verylowamountofpoly-CDcryogelscoulddecreasetheir initialcontentupto97%.
Oneofthemainadvantagesoftheproposedsystemisits recycla-bilitywithanexposuretoethanol.ThesolubilityofPAHinethanolis muchhigherthanitssolubilityinwater.Thus,supramolecular com-plexesbetweenPAHsandCDswillbebrokenduetotheentropic reasons.Theuseofethanol forthePAHseparation from differ-entsources,includingsoilswaspreviously reported,wherethe ethanoltreatmentsignificantlyreducedPAHconcentration[37]. Afterethanolexposureandthesubsequentuseofpoly-CD cryo-gels,thematerialsrevealedalmostidenticalsorptionperformance forallPAHmolecules(Fig.S16),suggestingtherecyclabilityofthe presentedsystem.Asthepoly-CDgelsarechemicallycross-linked networks,ethanolexposuredoesnotcauseanysubstantialchange onthematerialmorphology.
Fig.6. Time-dependentfluorescencespectraofthePAHsbeforeandaftertreatmentswithpoly-CDcryogelsfor3,6,and9h(blacklines).
Fig.7.ThePAHsorptioncapacitiesofthepoly-CDcryogels(cNH2-ˇ-CD)=20%(w/v)
&cPEGDGE=0.35mM)after1stand2nduse.Insetshowsthe“Percent(%)–Removal”
oftherespectivePAHcompounds.Theinsetphotoshowsthepoly-CDcryogelafter
the6htreatmentoffluoranthene.
4. Conclusion
ThesynthesisofCD-basedcryogelsthroughthecryogenic gela-tionoftheNH2-ˇ-CDwithPEGDGEwithoutusinganypolymer
supportwassuccessfullyshown. Thefabricatedgelspossessan aligned porous network due to the directional freezing of the samplesduringthe cross-linkingof precursorsunder cryogenic conditions at which thepore architecture couldbe tailoredby variationsintheformulationparameters.Thecryogelsdisplay rel-ativehighsorptioncapacitiesfortheremovalofPAHmolecules (e.g., pyrene, anthracene, phenanthrene, fluorene and fluoran-thene)withinarangeof0.6–6MPAHmoleculepergpoly-CD.
Thishighsorptionperformanceisduetobothinterfacialadsorption andthevolume-basedscavengingmechanism.Further,the cross-linkednetworkstructureallowsrecyclingthematerialswithan exposuretoethanol,andthematerialscouldbeusedrepeatedly withoutanysignificantlossinthesorptionperformance.Beside theiruseasasorbent,suchfunctionalporousplatformsalsohave highpotentialfordrugdelivery,wheretheCDcavitiescanactas drugcarriers.
Acknowledgements
F.T.thankstoTUBITAKCo-Funded BrainCirculationScheme
(project number: 116C031). T. U. acknowledges The Turkish
AcademyofSciences–OutstandingYoungScientistsAward Pro-gram (TUBA-GEBIP)-Turkey for partial funding of the research. Authorsthank toDr.Kugalur S. Ranjith forthe XPSanalysisof PEG-DGEmolecule.
AppendixA. Supplementarydata
Supplementarydataassociatedwiththisarticlecanbefound,in theonlineversion,athttp://dx.doi.org/10.1016/j.jhazmat.2017.04. 022.
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