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Talanta
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Synthesisofsurfacemolecularlyimprintedpolymerandtheselectivesolidphaseextractionofimidazolefromitsstructuralanalogs
GuifenZhua,JingFana,∗,YanbuGaoa,XiaGaoa,b,JianjiWanga,∗
SchoolofChemistryandEnvironmentalScience,HenanKeyLaboratoryforEnvironmentalPollutionControl,KeyLaboratoryforYellowRiverandHuaiRiverWaterEnvironmentalandPollutionControl,MinistryofEducation,HenanNormalUniversity,Xinxiang,Henan453007,PRChinab
DepartmentofPublicHealth,XinxiangMedicalUniversity,Xinxiang,Henan453003,PRChina
a
articleinfoabstract
Asurfacemolecularlyimprintedpolymer(MIP)wassynthesizedbyusingimidazoleasthetemplateandmodifiedsilicaparticlesasthesupportmaterial.Thestaticadsorption,solidphaseextraction(SPE)andhigh-performanceliquidchromatography(HPLC)experimentswereperformedtoinvestigatetheadsorptionpropertiesandselectiverecognitioncharacteristicsofthepolymerforimidazoleanditsstructuralanalogs.ItwasshownthatthemaximumbindingcapacitiesofimidazoleontheMIPandthenon-imprintedpolymer(NIP)were312and169molg−1,respectively.Theadsorptionwasfastandtheadsorptionequilibriumwasachievedin30min.Thebindingprocesscouldbedescribedbypseudo-secondorderkinetics.Comparedwiththecorrespondingnon-imprintedpolymer,themolecularlyimprintedpolymerexhibitedmuchhigheradsorptionperformanceandselectivityforimidazole.Theselectiveseparationofimidazolefromamixtureof1-hexyl-3-methylimidazoliumbromide([C6mim][Br])and2,4-dichlorophenolcouldbeachievedontheMIP–SPEcolumn.Therecoveriesofimidazoleand[C6mim][Br]were97.6–102.7%and12.2–17.3%,respectively,but2,4-dichlorophenolcouldnotberetainedonthecolumn.Thesurfacemolecularlyimprintedpolymerpresentedheremayfindusefulapplicationasasolidphaseabsorbenttoseparatetraceimidazoleinenvironmentalwatersamples.Thismayalsoformthebasisforourresearchprogramonthepreparationandapplicationofalkyl-imidazoliumimprintedpolymers.
© 2011 Elsevier B.V. All rights reserved.
Articlehistory:
Received5November2010
Receivedinrevisedform1March2011Accepted8March2011
Available online 16 March 2011Keywords:Imidazole
MolecularlyimprintedpolymerSynthesis
Solidphaseextraction
1.Introduction
Inrecentyears,peopleareincreasinglyinterestedinusingionicliquids,especiallyalkyl-imidazoliumbasedionicliquids,forchem-icalsynthesis,biocatalytictransformation,electrochemicaldevicedesignandanalyticalandseparationprocess,mainlyduetotheir“green”characteristics[1,2].Ionicliquidshavenegligiblevaporpressureandarerarelyflammableorexplosive,suggestingthattheypresentnoairpollutionorenvironmentalriskinclosedpro-cesses.However,ithasbeenshownthatalkyl-imidazoliumbasedionicliquidsaretoxictomammalianzooblast[3,4],microorgan-ism[5,6]andamongothers.Theionicliquidsarejustastoxicasconventionalorganicsolvents,andsometimestwo-to-fourordersofmagnitudemoretoxicthantheorganicsolvents[7].Thetoxicismainlyascribedtothealkyl-imidazoliumcations.Therefore,thewideapplicationofionicliquidswouldinevitablyresultinthelossoftheionicliquidsintowaterecosystems,leadingtopollutionofwater,soilandecologicalenvironment.Accordingly,itisimportant
∗Correspondingauthors.Tel.:+863733325971;fax:+863733326336.E-mailaddress:fanjing@henannu.edu.cn(J.Fan).0039-9140/$–seefrontmatter© 2011 Elsevier B.V. All rights reserved.doi:10.1016/j.talanta.2011.03.015
toassaytracealkyl-imidazoliumbasedionicliquidsinenvironmen-talsamples.
Intraditionalanalysismethodsoforganiccompounds,HPLC[8],spectroscopy[9]andionchromatogram[10]etc.,areusefulforthesystemswithlittleinterfereandhightargetconcentration.How-ever,imidazoleringofcationsoftheionicliquidsisUVactive,andmanyorganicshavesimilarabsorbanceprofilesintheUV–visiblerangewithimidazolering.Thusthesignaloverlappingisaseriousproblemforthedirectdeterminationofalkyl-imidazoliumbasedionicliquids[11].Duetothischaracteristicofionicliquidsandcom-plexoftheenvironmentalsamples,itisdifficulttoassaythetargetcompoundsdirectlybyusingtraditionalanalysismethods.Conse-quently,theestablishmentofeffectivemethodstoseparateandassayalkyl-imidazoliumbasedionicliquidsincomplexmatrixesisveryimportant.
Molecularlyimprintingofsyntheticpolymerisanapproachinwhichfunctionalmonomerandcrosslinkerarecopolymerizedinthepresenceoftemplatemolecule.Removalofthetemplatemoleculefromtheobtainedpolymerbysimplesolventextrac-tionrevealsthecomplementarybindingsitesthatcanrecognizethetemplatemoleculefromtheirstructurallysimilarcompounds[12,13].Duetotheadvantagesofselectiveseparationofthesub-
G.Zhuetal./Talanta84 (2011) 1124–11321125
strates,molecularlyimprintedpolymer(MIP)hasbeendevelopedrapidlyinsamplespre-treatment,especiallyinthepre-treatmentofenvironmentalsamples.Manyconventionalsyntheticapproachestowardimprintedpolymerparticlesinvolvethesynthesisofanimprintedpolymermonolith[14].Themonoliththenhastobegroundandsievedtodeliverafractionofparticlesofdesiredsizefortheintendedapplication.Particlespreparedinthismanneraretypicallyirregularinshapeandsize,andthemasstransferisslow.InordertoovercometheseproblemsandimprovethepropertiesofMIP,differentmethodshavebeenexplored,includingemulsion[15],precipitation[16,17],suspensionandmultistepswellingpoly-merization[18],filmgraftimprinting[19]andsurfaceimprinting[20].Inthesepreparationmethods,surfaceimprintingtechniqueiswelcome.Thepolymerwithbindingsitessituatedatthesurfaceshowsmanyadvantagessuchashighselectivity,enhancedadsorp-tion,moreaccessiblesites,fastmasstransferandbindingkinetics.Inaddition,itisknownthattheimprintedpolymermaterialssyn-thesizedbysurfaceimprintingtechnologyexhibitcontrollablesize,regularshape,bettermechanismintensity,andgoodreuseper-formancecomparedtothetraditionalimprintingmaterials.Thus,thesurfacemolecularlyimprintingtechniquehasbeenthefocusofmoreandmoreresearches[21–25].However,tothebestofourknowledge,nomolecularlyimprintedpolymer,especiallysurfaceimprintedpolymer,hasbeenreportedfortheseparationanddeter-minationofalkyl-imidazoliumbasedionicliquidssofar.
Fortheabovereasons,weprepared,inthiswork,imidazolemolecularlyimprintedpolymerbythesurfaceimprintingtech-niqueusingmodifiedsilicaparticlesasthesupportmaterial.Itwasfoundthattherewereeffectiveimprintedsitesonthesurfaceoftheimprintedpolymer,andthepolymerhasbeenappliedassolid-phaseextractionmaterialfortheselectiveextractionofimidazolefromitsstructuralanalogs.Thisworkisexpectedtobethefirststepforthepreparationofalkyl-imidazoliumimprintedpolymers.
2.Experimental
2.1.Materialsandreagents
Silicaparticles(180–200mindiameter)andaminopropy-ltriethoxysilanewereobtainedfromQingdaoOceanChemicalFactory(Shandong,China)andLancsResearchChemicalsLtd.(NJ,USA),respectively.Acryloylchloride,methacrylicacidandethylenedimethacrylatewerepurchasedfromAcrosCom-pany(NJ,USA).Imidazole,methanol,acetonitrileandchloro-formwereobtainedfromTianjinKermelChemicalReagentCompany(Tianjin,China).1-Methylimidazoleandtheionicliquidsof1-butyl-3-methylimidazoliumchlorideand1-hexyl-3-methylimidazoliumbromidewerepurchasedfromHenanLihuaPharmaceuticalLtd.(Henan,China).Azobisisobutyronitrile,ben-zene,2,4-dichlorophenolandm-dihydroxybenzenewerefromBeijingChemicalReagentCompany(Beijing,China).Allthesechemicalswereofanalyticalreagentgradeexceptforazobi-sisobutyronitrilewhichwasofchemicalpuritygrade.MethanolandacetonitrileofchromatographicpuritywereobtainedfromZhengzhouGuodaChemicalReagentCompany(Henan,China).UltrapurewaterwaspreparedfromMilli-Qpurificationsystem(MilliPore,USA).
Azobisisobutyronitrilewasrecrystallizedfrommethanolandthendriedunderreducedpressurebeforeuse.Methacrylicacidandethylenedimethacrylateweredistilledbeforeuseinordertoremovethepolymerizationinhibitor.Chromatographicpurityreagentswerefilteredthrougha0.22mfilterpriortouse.UltrapurewaterwasusedintheHPLCanalysisanddeionizedwaterwasusedinotherexperiments.
2.2.Instruments
AT6UV-visspectrophotometer(Shanghai,China)andahigh-performanceliquidchromatography(Waters,USA)equippedwithareversed-phasecolumn(C18column,4.6mm×150mm),aC18pre-columnandaPDA2998ultraviolet–visibledetectorwereusedinthepresentwork.Infraredspectra(IR)inKBrwererecordedat4000–400cm−1byusingaPerkin-Elmer983infraredspec-trophotometer(Norwalk,USA).Thescanningelectronmicroscope(SEM)micrographsofthesorbentswereobtainedat20.0kVonaJSM-5610LVscanningelectronmicroscopy(JEOL,Japan).The1HNMRspectrawererecordedbyaAV-400spectrometer(Bruker,Germany).TheASAP2020surfaceareaandporesizeanalyzer(Micromeritics,USA)wasusedtomeasurethespecificsurfacearea,thetotalporevolumeandtheaverageporediametersofthemolec-ularlyimprintingpolymerandnon-imprintedpolymer.2.3.Preparationoftheimidazoleimprintedpolymer
Atwo-stepprocedurewasusedtopreparetheimidazoleimprintedpolymer.TheschematicexpressionforthesyntheticroutewasillustratedinFig.1.Inthefirststep,silicaparti-clesweremodifiedchemicallyasdescribedbelow.Inordertoincreasethecontentof–OH,silicaparticleswereactivatedbydippinginto6molL−1hydrochloricacidfor24h,thenfilteredandwashedrepeatedlywithdistilledwater.Suchtreatedsilicaparticlesweredriedat110◦Ctoconstantweight.Theactivatedsil-icaparticlesweremixedwithaminopropyltriethoxysilane(APTS)inanhydroustolueneinasealedflaskandthenrefluxedfor7h.Theresultingaminopropyltriethoxysilane–silica(APTS–silica)particleswereseparated,andwashedwithanhydroustoluene,ethanolandacetoneinsequence.Thenacryloylchloride(AC)andthecatalyst–triethylamineweremixedwithaminopropy-ltriethoxysilane–modifiedsilicaparticlesinanhydroustoluenesolution.Themixturewasvigorouslystirredfor12hatroomtemperatureunderdrynitrogen.Theproductwasseparatedbyfiltrationandthenwashedsuccessivelywithanhydroustoluene,methanol,ethanolandacetone.Finally,theobtainedacryloylchloride–aminopropyltriethoxysilane–silica(AC–APTS–silica)par-ticlesweredriedundervacuumat40◦Cbeforeuse.
Inthesecondstep,imidazoleimprintedpolymerwasprepared.Forthatpurpose,0.035gofimidazolewasdissolvedin50mLofchloroform,andthen1.0gofAC–APTS–silicaparticlesand0.18mLofmethacrylicacid(MAA)wereadded.Themixturewasshakenatroomtemperaturefor6hforpre-polymerization.Then1.mLofethylenedimethacrylate(EDMA)and0.010gofazobisisobutyroni-trilewereadded.Themixturewassonicated,anddegassedwithnitrogenfor10mintoeliminateoxygen,andthenwassealedforpolymerizationat60◦Cfor24hinaconstanttemperatureincubat-ingshakerwitharateof200rpm.Theresultantswereextractedwithamixedsolventofmethanol/aceticacid(9:1,v/v)for48hinaSoxhletextractortoremovethetemplate.Theobtainedparticleswerewashedwithmethanoltillneutralandthendriedtoconstantweightundervacuumat60◦C.Asacontrol,thenon-imprintedpolymerwaspreparedandtreatedunderthesameconditionsexceptfortheadditionofthetemplate.2.4.Staticadsorptiontests
20.0mgoftheMIPandNIPparticleswereadded,respectively,into2.0mLofacetonitrileinwhichimidazoleoritsstructuralanalogswerecontained.Themixtureswereallowedtostandfor12hatroomtemperaturetofacilitatetheadsorptionofimida-zoleoritsanalogsontotheMIPandNIPparticles.Afterthat,theconcentrationofthesubstratesinthesupernatantsolutionswas
1126G.Zhuetal./Talanta84 (2011) 1124–1132
SiO2APTSACSiO2ONHCC
(I)
HCCH+HCN
HN
OOH
O
OH
H
CCHN
NHCH
O
OH(II)
(I)+(II)
EDMA, AIBNPolymerizationSiO2ONHCC
O
OH
H
CCHN
NHCH
O
OH(III)
(III)
ExtractionRebindingSiO2ONHCC
O
OH
O
OH(IV)
Fig.1.Schematicillustrationforthesyntheticrouteofthemolecularlyimprintedpolymer.
determinedbyaUV-visspectrophotometer.Thebindingcapacityofimidazoleandtheanalogswascalculatedfromtheequation:Q=
(C0−C)V1000
M(1)
whereQstandsforthebindingcapacity(molg−1),C0andCaretheinitialandtheresidualconcentrations(mmolL−1)ofimidazoleortheanalogs,respectively,Visthesolutionvolume(mL),andMistheamount(mg)oftheMIPorNIPparticlesusedfortheadsorptionexperiments.
2.5.Solid-phaseextractionexperiments
Ahomemadeglasscolumn(4.5cm×3mmi.d.)packedwith100.0mgofMIPwasusedassolidphaseextraction(MIP–SPE)col-umn,inwhichasmallportionofglasswoolwaspackedinordertopreventlossofthesorbentduringsampleloading.AftertheMIP–SPEcolumnwaspre-treatedwithmethanolandacetonitrile,1mLofthesamplesolutioncontaining0.2mmolL−1imidazoleoreachofthestructuralanalogs(1-methylimidazole,metronidazoleorbenzene)inacetonitrilewaspassedthroughthecolumnataflowrateof0.5mLmin−1.Then,thecolumnwaswashedbymethanol,andelutedbyamixedsolventofmethanol/water(80:20,v/v).TheeluatewasanalyzedbytheHPLC.TheNIP–SPEcolumnwaspre-paredandtreatedinthesamewayexceptthatthestuffingwastheNIPparticles.
3.Resultsanddiscussion
3.1.InteractionbetweenimidazoleandthefunctionalmonomerTheprincipleofmolecularimprintingliesinthepreser-vationofthepre-polymerizedhost/gueststructureintoapolymermatrix.Thusitisimportantthatthetemplateandthefunctionalmonomercanformstablecomplexesthroughhydrogenbonding,ionicbondingorotherinteractionforcesinthepre-polymerizationmixture[26].Tounder-
standthemolecularrecognitionmechanism,theinteractionbetweentemplateandfunctionalmonomershouldbestud-ied.
Towardtothisend,UVabsorptionspectraofthetemplate(imi-dazole),functionalmonomer(methacrylicacid)andtheirmixtureinacetonitrileweredeterminedandtheresultwasshowninFig.2.Itcanbeseenthatatthewavelengthof215nm,theabsorbancevaluesofimidazoleandmethacrylicacidwere0.236and0.844,respectively.However,theabsorbanceoftheirmixturewas0.1,whichwasmuchlowerthanthesummationof0.236and0.844.Thisdifferencewasresultedfromtheinteractionsofimidazolewithmethacrylicacid.Duetothepre-polymerization,thecomplexofimidazolewithmethacrylicacidwasformedprobablythroughhydrogenbondingand/orionicbondingbetweenN–Hofimidazoleand–COOHofmethacrylicacid.
1.41.21.0
41Absorbance0.80.60.40.20.0-0.2
190200210220230240250260270Wavelength(nm)
Fig.2.UVabsorptionspectraofimidazoleinthepresenceofmethacrylicacidinacetonitrile:(1)imidazole(0.125mmolL−1);(2)methacrylicacid(0.5mmolL−1);(3)themixtureof1and2;(4)thesumofadsorptionvaluesfor1and2.
G.Zhuetal./Talanta84 (2011) 1124–1132
Table1
SomestructuralparametersforMIPandNIP.
Specificsurfaceareaa(m2g−1)
MIPNIP
1127
Totalporevolumeb(cm3g−1)0.290.39
Averageporediameterb(nm)5.225.74
206.04230.54
aMeasuredbyBrunauer–Emmett–Teller(BET)method.b
MeasuredbyBarrett–Joyner–Halenda(BJH)method.
Acomparisonof1HNMRspectraforimidazole,methacrylicacidandtheirmixturewasshowninFig.3.TheprotonofN-1andtheprotonofC-2ontheimidazoleringshifteddownfieldfrom12.101and7.6ppm(spectruma)to12.797and8.025ppm(spectrumc),respectively,inthepresenceofmethacrylicacid.Meanwhile,thedownfieldchemicalshiftofprotonofthecarboxylgroupwasfrom12.388ppm(spectrumb)to12.797ppm(spectrumc)duetotheinteractionbetweenimidazoleandmethacrlicacid.Allthesedatasuggestedtheformationofhydrogenbondingand/orionicbondingbetweenimidazoleandmethacrylicacid.3.2.CharacteristicsofMIPandNIP
ThemorphologyofMIPandNIPobservedfromSEMwasshowninFig.4.Itwasfoundthatshapeofthepolymerswasuniform,andthesizewasclosetothatofthesilicaparticles.Thepossiblereasonwasthatthepolymerizationreactionoccurredonthesurfaceofsilicaparticles,andthecoatingwasverythinsothattheshapeandsizeofsilicaparticlesandthepolymerswereverysimilar.ItwasalsofoundfromthemagnifiedSEMphotos(notshown)thatthemicro-poreonthesurfaceofNIPwaswell-distributedbuttheaverageporediameterwassmallerthanthatofMIP.ThismaybeascribedtotheelutionoftemplatemoleculesinthepreparationofMIP.Table1liststheresultsofnitrogenadsorptionexperimentsforMIPandNIPparticles.Itcanbeseenthatthespecificsurfacearea,thetotalporevolumeandtheaverageporediameterweredifferentforMIPandNIPparticles.ThesedatasupporttheobservationfromSEMphotos.
Fig.5showsIRspectraofthesilicagel-basedsamples.Obviously,APTS–silicadisplayedthecharacteristicpeaksofaminogroupsat1538cm−1,whileAC–APTS–silicaexhibitedtherelativelystrongbandofcarbonylgroupat1665cm−1.Thisobservationrevealsthatthetwo-stepchemicalmodificationwasachievedatthesurfaceofsilicaparticles.Inaddition,thebandataround2959cm−1wasassignedtothestretchingvibrationof–CH2–or–CH3,thebandat
Fig.3.1HNMRspectraofimidazole,methacrylicacidandtheirmixtureinDMSO:(a)imidazole(0.15molL−1);(b)methacrylicacid(0.60molL−1);(c)imidazole(0.15molL−1)+methacrylicacid(0.60molL−1).
1732cm−1wasassignedtotheabsorptionof–COOH,andthebandsat1233and1105cm−1wereascribedtothestretchingvibrationsofC–O–C.TheabsorptionpeaksofMIPandNIPparticlesweresimilar,whichmeansthatnotemplatemoleculeswereretainedontheMIP.3.3.Molarratiooptimizationofthetemplateandthefunctionalmonomer
Althoughthebindingcapacityisaneffectiveparameterfortheperformanceevaluationofthemolecularlyimprintedpolymer,theabilityofselectiveadsorptionismoreimportantfortheirappli-cation.Therefore,itwouldbeverysignificanttoinvestigatetheabilityofselectiverecognitionofthemolecularlyimprintedpoly-merforthetemplatemolecules.Forthispurpose,threedifferentmolarratiosofimidazoletomethacrylicacidwerestudiedinthepreparationofMIPandNIP.Thebindingcapacity(molg−1)ofimi-dazoleonthesepolymerswasdeterminedandusedtocalculatetheimprintingfactor[23]fromtheequation:ˇ=
QMIPQ(2)
NIP
Fig.4.SEMmicrographs:(a)silicaparticle;(b)MIP.
1128G.Zhuetal./Talanta84 (2011) 1124–1132
Fig.5.IRspectraofsilicagel-basedsamples:(1)puresilica;(2)APTS–silica;(3)AC–APTS–silica;(4)MIP;(5)NIP.
Here,ˇistheimprintingfactoroftheMIP,QMIPandQNIPstandforthebindingcapacityofimidazoleontheMIPandNIPmaterial.TheresultsweregiveninTable2.Itcanbeseenthatamongthethreemolarratiosofimidazoletomethacrylicacid,thebindingcapacityofthepolymerspreparedattheratioof1:6wasmaximum,whereasthatattheratioof1:2wasminimum.Thepossiblereasonwasthatthenon-specificbindingsitesincreasedwithincreasingproportionofmethacrylicacid,whichresultedintheincreaseoftheboundamountofimidazole.However,theimprintingfactor(2.29)ofthepolymerspreparedattheratioof1:4wasthehighest.Thissuggeststhatthecomplexofimidazolewithmethacrylicacidwasoptimal,andtheabilityofthepolymerforselectiveadsorptionofimidazolewasthestrongestatthismolarratio.Therefore,1:4ofimidazoletomethacrylicacidwaschosenastherightmolarratioforthepreparationofMIP.
3.4.BindingisothermsofimidazoleontheMIPandNIP
ThebindingisothermcurvesofimidazoleontheMIPandNIPwereplottedinFig.6.Itwasshownthatthebindingcapacityofthepolymersincreasedwithincreasinginitialconcentrationofimi-dazoleintherangeof1.0–10.0mmolL−1,andtheMIPhadhigheraffinityforimidazolethanNIP.Inthehigherconcentrationrange,thebindingcapacitywasclosetobestable.ThebindingdatacanbeanalyzedbyLangmuirequation:Q=
QmaxCeqB+Ceq
(3)
12010080
MIPNIPQ (umol g-1)60402000123456710C (mmol L-1)
Fig.6.AdsorptionisothermsofimidazoleonMIPandNIP.
whereQstandsforthebindingcapacity(molg−1),Qmaxisthemaximumbindingcapacity(molg−1),Ceqisequilibriumconcen-trationofimidazole(mmolL−1),andBisaconstant.InordertocalculatethemaximumbindingcapacityofimidazoleonbothMIPandNIP,thisequationwaschangedintoEq.(4):CeqCeqB
=+QQmaxQmax
(4)
Actually,Eq.(4)showsalinearrelationshipbetweenCeq/QandCeq.Fromtheslopeofthelinearplot,themaximumbindingcapac-Table2
Bindingcapacityandimprintingfactorforthemolecularlyimprintedpolymerspre-paredatdifferentmolarratiosoftemplatetofunctionalmonomer.ni/nma
1:21:41:6
a
ityofimidazoleontheMIPandNIPwascalculatedtobe312and169molg−1,respectively,indicatingthatthemaximumbind-ingcapacityofimidazoleonMIPwas1.84timesofthatonNIP.ThisdifferencesuggestedthatthespatialstructureofMIPandNIPmaterialswasdifferentthoughtheirchemicalcompositionwassimilar.Thereexistedcavitiesandspecificrebindingsitesforimi-dazoleontheMIPbecausethestructureoftemplate–monomercomplex,whichispossiblyresultedfromtheionicbondingorhydrogenbondingbetween–NHand–CH(2-position)intheimi-dazolemoleculesand–COOHinthefunctionmonomer,wasinaccordancewiththeshapeandstructureoftemplatemolecules.However,therewerenothese“memory”sitesinNIP,sothatitsbindingcapacitywaslower.Therefore,itisveryimportanttoformcavitiesandspecificrebindingsitesfortherecognitionofthetem-platemolecules.
Ingeneral,thelargeporevolumeresultsinthelargespecificsurfacearea,thisisadvantageousforthepolymertoadsorbthetemplatemolecules[27].ItwasfoundfromTable1thatthespecificsurfaceareaandaverageporevolumeofNIPwerelargerthanthoseofMIP,butthebindingcapacityofNIPwassmallerthanthatofMIP.ThisindicatesthatMIPhasstrongspecificadsorptionforimidazole.3.5.BindingkineticcurveofimidazoleontheMIP
InFig.7,theadsorptionkineticcurveofimidazoleonMIPwasshownattheimidazoleconcentrationof3.0mmolL−1inacetoni-trile.Itcanbeseenthatthebindingcapacityincreasedrapidlyininitial30min,andthenturnedtoaplateau.BecauseMIPwaspoly-
QMIP(molg−1)13.020.036.0
QNIP(molg−1)7.58.728.5
ˇ1.732.291.26
niandnmarethemolarnumberofimidazoleandmethacrylicacid,respectively.
G.Zhuetal./Talanta84 (2011) 1124–11321129
6050)1-g l40omu( Q302010020406080100120140160180200t (min)
Fig.7.AdsorptionkineticcurveofimidazoleonMIP.
merizedonthesurfaceofsilicaparticles,athinlayerofimprintedmaterialcontainingrecognitionsiteswasonlyonthesurface.Thetemplatecanbeextractedcompletelyfollowingthecreationoftheimprintsitesduringtheimprintingstep.Also,thetemplatemoleculescanreachtheimprintsiteseasilyandquicklyduringtherebindingstep[28].Therefore,MIPshowedgoodsiteaccessibilityforimidazoleandtheequilibriumwasachievedquickly.However,intheMIPsynthesizedbybulkpolymerization,therecognitionsiteswerenotonlyonthesurfacebutalsointheinsideofcrosslinkedpolymernetwork,andthemasstransferoftheobtainedmono-lithpolymerwasslowsothattheadsorptionequilibriumwouldbeachievedinalongperiodoftime.Thesefeaturesarenotwell-suitedfortheanalysisofrealsamples.Theimprintedpolymersynthesizedbysurfaceimprintingtechniqueovercamethesedrawbacks.
Forsolid/solutionsystems,thepseudo-firstorderkineticmodel(5)orthepseudo-secondorderkineticmodel(6):ln(Qe−Qt)=lnQe−kat(5)tt1Qt=Q+(6)
ekbQe2
canbeusedtodescribethebindingprocess[29].Intheseequations,KaandKbaretherateconstantsforthefirstordersorptionandthesecondordersorptionprocesses,respectively.Qtstandsforthebindingcapacity(molg−1)atgiventimet,andQeindicatestheequilibriumbindingcapacity(molg−1).Thepseudo-firstorderkineticmodelisthemostwidelyusedrateequationtodescribetheadsorptionofasolutefromliquidsolution.However,whenthepseudo-firstorderkineticmodelwasappliedtotheexperimentdata,nolinearrelationshipwasobserved,andthismodelhasbeenfoundtobeunsatisfactoryinprovidingaconcretemechanismfortheadsorptionprocessinanequallygoodnumberofcases[30,31].Inthepseudo-secondordermodel,thechemisorptionoftheadsor-bateontheadsorbentwasconsideredtobetherate-limitingstep,andthesolutemoleculesreactedwithtwokindsofadsorptionsites.Ifthepseudo-secondordermodelwasusedtotreattheexperimen-taldata,t/Qtversustyieldedalinearplot(y=0.109+0.01829x)withcorrelationcoefficientof0.9996.Thissuggeststhatthebind-ingprocesscanbedescribedbythepseudo-secondordermodel,andchemisorptionistherate-controllingstep[31].Itisthetwodifferenttypesoffunctionalgroupsofimidazolemoleculewhichformedtwokindsofcomplexeswithmethacrylicacid,leadingtotwokindsofbindingsitesforMIPinmolecularrecognitionprocess.3.6.Selectiveadsorption
Selectiveadsorptionexperimentswerecarriedoutinacetonitrilesolutioncontaining3.0mmolL−1imida-
CHH3NNNNimidazole
1-methylimidazole
benzene
Cl
OH
Cl
OHOH
2,4-dichlorophenol
m-dihydroxybenzene
CN
N
N
4H9
Cl
CH3C6HN13
Br
CH3[C4mim][Cl]
[C6mim][Br]
Fig.8.Structuresofimidazoleanditsanalogs.
zoleoraseriesofstructuralanalogs1-methylimidazole,benzene,2,4-dichlorophenol,m-dihydroxybenzene,1-butyl-3-methylimidazoliumchloride([C4mim][Cl])and1-hexyl-3-methylimidazoliumbromide([C6mim][Br])(theirmolecularstructuresareshowninFig.8),andthebindingcapacityofthesecompoundsontheMIPandNIPwasinves-tigatedbytheequilibriumbindingapproaches.HereweuseQ(Q=QMIP−QNIP)astheparameterfortheevaluationofselectiveadsorptionofthepolymer[32].FromtheresultslistedinTable3,itisobviousthatthebindingcapacityofimidazoleand1-methylimidazoleontheMIPwasmuchhigherthanthatontheNIP,andtheQvaluewasthemaximumforimidazoleexceptforbenzene.Thisresultsuggestedthatthecomplexesbetween–NHand–CH(2-position)intheimidazolemoleculesand–COOHinthefunctionmonomerwereformedbyionicbondingorhydrogenbonding.Intheprocedureofelution,thetemplatemoleculewaswashedout,andthecavitiesmatchedwiththetemplatemoleculesinshapeandsizewerethencreated,whichrecognizedthetemplatemoleculeseffectively.AlthoughthechemicalcompositionofMIPandNIPisthesame,thelatterpolymercanonlybindthetestedcompoundsbynon-specificadsorptionduetotheabsenceofpropercavitiesandrecognitionsitesinthispolymer.1-Methylimidazolehassimilarsizeandstructurewithimidazole,whichledtothehigherbindingcapacitythantheotherstructuralanalogs.However,thesizeof1-butyl-3-methylimidazolium([C4mim][Cl]),1-hexyl-3-methylimidazolium([C6mim][Br])andbenzenederivativeswasmuchlargerthanthatofimidazolethoughtherewere–CH(2-position)in[C4mim][Cl]and[C6mim][Br].Therefore,theirbindingcapacitywassmallerthanthatofimidazole.
Table3
BindingcapacityofdifferentsubstratesonMIPandNIP.Substrates
QMIP(molg−1)QNIP(molg−1)Q(molg−1)Imidazole
46.4621.7124.751-Methylimidazole17.732.5315.2Benzene
225.3572.90152.452,4-Dichlorophenol0.520.000.52m-Dihydroxybenzen4.212.162.05[C4mim][Cl]3.961.2.07[C6mim][Br]
5.05
2.83
2.22
1130G.Zhuetal./Talanta84 (2011) 1124–1132
Table4
Recoveriesofimidazole(IM),1-methylimidazole(1-ME),metronidazole(MET),andbenzene(BEN)fromMIPandNIPcolumns.a
MIPIM
Effluent(%)Firstwash(%)Secondwash(%)Thirdwash(%)Elution(%)
Totalrecovery(%)
a
NIP
1-ME1065.00017.592.5
MET72.38.70019.3100.3
BEN22.570.00010.0102.5
IM27.55.030.035.0097.5
1-ME7500032.5107.5
MET98.42.9003.8105.1
BEN45060.005.0110
015.00087.5102.5
Foreachwashingandelution,5mLofmethanoland5mLofmethanol/water(80:20,v/v)wereused,respectively.
Ontheotherhand,resultsinTable3indicatethatthebindingcapacityincreaseswithdecreasingpolarityofthecompounds.Forexample,thepolarityofbenzeneisthelowestamongthestructuralanalogs,butitsbindingcapacityisthehighest.Thissuggeststhatintherecognitionofbenzene,thenon-specificbindingwasthemainbindingmode.Fromtheseresults,itcanbeconcludedthatboththesizeofthetargetcompoundsandthenon-covalentinteractionswereveryimportantinthebindingprocess.3.7.Solid-phaseadsorptionbehaviorofthepolymers
3.7.1.Effectofflowrateanddesorptionsolvent
Inthesolid-phaseextractioncolumnexperiment,flowrateofsolutionandeluentwasanimportantparameteraffectingthebind-ingofIM.Inordertooptimizethisfactor,theinfluenceofflowrateonIMbindingwasstudiedintherangefrom0.2to2.0mLmin−1anddesorptionsolventwereexaminedbyabatchmodeexperi-ment.Forthispurpose,100.0mgofMIPwasmixedwithmethanolandpackedinaglasscolumn,and5mLofacetonitrilesolutioncontaining10gIMwasallowedtopassthroughthecolumn.Itwasshownthatabout100%ofIMwasboundbyMIPwhentheflowratewaslessthan1.2mLmin−1,buttheretentionofIMwasonly92.8%whentheflowratewasupto1.6mLmin−1.Itiswellknownthatwhentheflowrateistoofast,IMcannotsufficientlybeadsorbedbyMIPinthecolumn.Therefore,wechoose0.5mLmin−1asthebestflowrateinourexperiment.ThenIMretainedinthecol-umnwaselutedbymethanol,acetonitrileandamixedsolventofmethanol/water,respectively.Theresultsuggeststhatthemixedsolventofmethanol/water(80:20,v/v)wasthebestdesorptionsolvent.
3.7.2.SelectivityoftheMIP–SPEandNIP–SPEcolumns
Here,imidazole,imidazole-derivatives(1-methylimidazoleandmetronidazole)andbenzenewerechosentostudytheselectiveadsorptionbehavioroftheMIP–SPEandNIP–SPEcolumns.Afterloadingofsamplesolutions,theremainedconcentrationsofthecompoundsineffluentsandeluentsfromtheMIP–SPEandNIP–SPEcolumnsweredetermined,respectively.Theadsorptionandrecov-eryresultsweregiveninTable4.ItcanbeseenthatthebindingcapacitiesofthesamplesonMIP–SPEcolumnweremuchhigherthanthatonNIP–SPEcolumn,andthatimidazolecouldberetainedcompletelyontheMIP–SPEcolumn,while1-methylimidazole,metronidazoleandbenzenecouldberetainedonlyindifferentextentsonbothofthecolumns.Afterthreetimeswashing,mostoftheimidazole-derivativesandbenzenecouldbewashedofffromthecolumns,butmorethan87%ofimidazolewasstillboundontheMIP–SPEcolumn.ThisindicatesthattheMIPmaterialhasexcellentmolecularrecognitionabilityandhighselectivityforimidazole.Incontrast,theNIPmaterialhasnothisability,andtheinterac-tionbetweentargetsandthepolymerwasnon-specificaffinity.Inaddition,itwasobservedthatbenzenecouldbewashedofffromtheSPEcolumnseasilyaftertwotimeswashing.ThisillustratedagainthatthehighestbindingamountofbenzeneshowninTable3
wasresultedfromitsnon-specificaffinitytothepolymers.Con-sideringthefactthatamongthetestcompounds,thepolarityofbenzeneislowanditsinteractionwiththesolvent(acetonitrile)isweak,benzenewouldbeapttobindwiththesorbentbythenon-selectiveadsorptionduringtheequilibriumbindingprocess.TheseresultssuggestthatitispossibletoselectivelyenrichandseparateimidazolefromitsstructuralanalogsbyusingtheMIPmaterial.3.8.HPLCevaluationoftheselectiveseparation
InordertofurtherinvestigatedwhethertheMIP–SPEcolumncanbeusedfortheselectiveseparationandenrichmentofimi-dazolefromamixtureofitsstructuralanalogs,chromatographicexperimentwascarriedoutbypassing5.0mLofacetonitrilesolution,containing2.0mgL−1imidazole,2.0mgL−11-hexyl-3-methylimidazoliumbromide([C6mim][Br])and2.0mgL−12,4-dichlorophenol,throughthecolumnataflowrateof0.5mL·min−1,where[C6mim][Br]and2,4-dichlorophenolwereusedasinter-ferencespecies.Thenthecolumnwaselutedwith1mLofmethanol–watersolution(80:20,v/v),andtheanalyteintheelu-atewasanalyzedbytheHPLCwithaflowrateof0.5mLmin−1at30◦C.ThemobilephaseusedforHPLCexperimentwasamixtureofmethanolandwater(80:20,v/v),andtheUV–visdetectorwasoperatedat215nm.Thechromatogramsforthemixedsolutionofimidazole,1-hexyl-3-methylimidazoliumbromide([C6mim][Br])and2,4-dichlorophenolinacetonitrilewithandwithoutsolid-phaseextractiontreatmentbyMIPwereshowninFig.9.Inthisfigure,peaks1,2and3wereidentifiedas([C6mim][Br]),imidazoleand2,4-dichlorophenol,respectively.ComparedwiththesignalsshowninFig.9a,itcouldbefoundfromFig.9bthatimidazolecouldbeboundwell,but[C6mim][Br]and2,4-dichlorophenolwereboundpoorlyontheMIP–SPEcolumn.AfterelutionfromMIP–SPEcolumn,therecoveryis97.6–102.7%forimidazoleand12.2–17.3%for[C6mim][Br],but2,4-dichlorophenolintheeluatecouldnotbedeterminedbyHPLCundertheexperimentalconditions.Therefore,itisappropriatetostatethatMIPwouldbeusedasasolid-phasesorbenttoseparateandenrichimidazolefromcomplexmatrix.
OneoftheadvantagesofMIP–SPEsorbentsistheirreusabil-ity.InordertotestthisperformanceoftheMIP–SPEcolumn,themixedsolutioncontainingimidazole,[C6mim][Br]and2,4-dichlorophenolinacetonitrilewaspassedthroughtheMIP–SPEcolumnattherateof0.5mLmin−1,andtheconcentrationofimi-dazoleinthemixturewaskeptat0.2mmolL−1.Afterelutionbyamixedsolventofmethanol–water(80:20,v/v),theeluatewasana-lyzedbyHPLC.Theresultshowedthattherecoveryofimidazolewashigherthan94%,andnosignificantdecreaseinselectivityhadbeenobservedafterfiftytimesofreuseoftheMIP–SPEcolumn.3.9.Applicationtoenvironmentalwatersamples
TodemonstratetheabilityofMIPtoextractIMfromrealsam-ples,thetraceofIMinlakewater,groundwaterandtapwaterweredeterminedundertheoptimizedconditionsbycouplingMIP–SPE
G.Zhuetal./Talanta84 (2011) 1124–11321131
Fig.9.Chromatogramsforamixtureofimidazole,[C6mim][Br]and2,4-dichlorophenol:(a)mixturewithoutextractionbytheMIP–SPEcolumn;(b)eluateofthemixturefromtheMIP–SPEcolumn.ChromatographicconditionsofC18reversed-phasecolumn:detectionwavelength,215nm;mobilephase,methanol–water(80:20,v/v);flowrate,0.5mLmin−1;columntemperature,30◦C.
Table5
DeterminationofimidazoleafterMIP–SPEcolumninrealwatersamples(n=3).SamplesLakewaterLakewaterLakewaterGroundwaterGroundwaterGroundwaterTapwaterTapwaterTapwater
Add(mgL−1)
–0.301.00–0.301.00–0.301.00
Detected(mgL−1)–0.290.99–0.290.94–0.301.00
Recovery(%)–96.799.0–96.794.0–100.0100.0
R.S.D.(%)–4.33.6–3.32.9–5.31.0
andHPLC.Inthepracticalwatersamples,thelakewaterwassam-pledfromMuyeLakeinXinxiang,thetapwaterwastakenfromourlaboratory,andthegroundwaterfromXinxiangCity.Thewatersampleswerefilteredtoeliminatethesolidimpurity,andasuitableamountofwatersamplewaspassedthroughtheMIP–SPEcolumn.TheeluentwascollectedandthecontentofIMintheeluentwasdetermined.ItcanbeseenfromTable5thatnoIMwasdetectedinthesamplesoflakewater,groundwaterandtapwater.When0.3and1.0mgL−1ofIMwereadded,therecoveriesofIMwerefoundtobeintherangefrom94.0to100.0%forthethreewatersam-ples.TheseresultsconfirmedthattheMIPpreparedinthepresentworkcouldbeeffectivelyappliedforthedeterminationofIMinrealsamples.4.Conclusions
Inthiswork,wepreparedanewmolecularlyimprintedpoly-merbysurfacepolymerizationwithimidazoleasthetemplateandsilicaparticlesasthesupportmatrix.Thestructuralandadsorp-
tioncharacteristicsofthispolymerhavebeenstudied.Fromourexperimentalresults,thefollowingconclusionshavebeendrawn:(i)theshapeandsizeofthemolecularlyimprintedpolymerweresimilartothatofthesilicaparticles,andtheimprintingfactorwasthehighestfortheimprintedpolymerpreparedatthemolarratioof1:4ofthetemplatetothefunctionalmonomer;(ii)theMIPhasstrongabilityofselectiverecognitionandspecificadsorptionforimidazole,andcanbeusedasanexcellentsolid-phasesorbenttoseparateandenrichimidazolefromitsstructuralanalogs,sug-gestingthatthecavitiesandspecificrebindingsitesonthesurfaceoftheimprintedpolymerplayanimportantroleintherecogni-tionprocessofsubstrates;(iii)theadsorptionwasfastandtheadsorptionequilibriumwasachievedin30min,andthebindingprocesscouldbedescribedbypseudo-secondorderkinetics;(iv)thepolymercanbereusedwithoutsignificantdecreaseinselec-tivity.Tothebestofourknowledge,thisisthefirstreportonthemolecularlyimprintedpolymerfortheseparationandenrich-mentofimidazole.Theinformationobtainedhereisimportantforthesolidphaseextractionanddeterminationoftraceimidazole
1132G.Zhuetal./Talanta84 (2011) 1124–1132
inenvironmentalwatersamples.Also,thismaylaythebasisforthepreparationofalkyl-imidazoliumimprintedpolymersusedfortheseparationanddeterminationofalkyl-imidazoliumbasedionicliquids.
Acknowledgments
ThisworkwassupportedfinanciallybytheNationalNaturalScienceFoundationofChina(Grantno.20977025)andtheDoc-toralFoundationofMinistryofEducationofChina(Grantno.200804760004).References
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