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Journal of Membrane Science, 401-402, 15 May 2012, pp. 89-96, 2012-02-06

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Chiral separation with molecularly imprinted polysulfone-aldehyde

derivatized nanofiber membranes

Sueyoshi, Yuuki; Utsunomiya, Akira; Yoshikawa, Masakazu; Robertson,

Gilles P.; Guiver, Michael D.

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ContentslistsavailableatSciVerseScienceDirect

Journal

of

Membrane

Science

j our na l h o me p ag e :w w w . e l s e v i e r . c o m / l o c a t e / m e m s c i

Chiral

separation

with

molecularly

imprinted

polysulfone-aldehyde

derivatized

nanofiber

membranes

夽夽

Yuuki

Sueyoshi

_,

_Akira

_Utsunomiya

_,

_Masakazu

_Yoshikawa

_,

_Gilles

_P.

_Robertson

_,

_Michael

_D.

_Guiver

a_{DepartmentofBiomolecularEngineering,KyotoInstituteofTechnology,Matsugasaki,Kyoto606-8585,Japan}

b_{InstituteforChemicalProcessandEnvironmentalTechnology,NationalResearchCouncilofCanada,Ottawa,Ontario,CanadaK1A0R6}

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received15December2011

Receivedinrevisedform25January2012 Accepted28January2012

Available online 6 February 2012 Keywords:

Electrospraydeposition Chiralseparation Membrane Molecularimprinting

Molecularlyimprintednanofiber membrane

Nanofiberfabric Opticalresolution

a

b

s

t

r

a

c

t

Molecularly imprinted membranes (MIPMs) and molecularly imprinted nanofiber membranes (MINFMs)were prepared from polysulfone with aldehyde (PSf-CHO-05 or PSf-CHO-10) and N-␣-benzyloxycarbonyl-d-glutamicacid(Z-d-Glu)orN-␣-benzyloxycarbonyl-l-glutamicacid(Z-l-Glu)as aprintmolecule.Thosetwotypesofmolecularlyimprintedmembrane,suchasMIPMsandMINFMs, incorporatedtheenantiomer,ofwhichabsoluteconfigurationwassameasthatoftheprintmolecule, inpreferencetothecorrespondingantipode.Inotherwords,themembranesimprintedbythed-isomer preferentiallyadsorbedthed-isomerandviceversa.Thosetwotypesofmembraneshowedchiral sep-arationabilitybymembranetransport.Againstexpectation,transportoftheenantiomerpreferentially adsorbedinthemembranewasretarded,inotherwords,theenantiomerlessadsorbedinthe mem-branewasselectivelytransported.Thefluxesthroughthemolecularlyimprintednanofibermembranes gaveonetotwoordersofmagnitudehigherthanthoseofusualmolecularlyimprintedmembranes with-outdepressionofpermselectivity.Thepresentstudydemonstratedthatmolecularlyimprintednanofiber membranegavehighfluxwithoutdepressionofpermselectivity.Abreakthroughinmembraneseparation wouldberealizedbyadoptingmolecularlyimprintednanofibermembranes.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Membraneseparation(transport)isexplainedbyacoupleof mechanism,suchasincorporationofpermeantintoagiven mem-braneanddiffusion(migration)ofpermeantwithinthemembrane fromthefeedsidetothepermeateside [1–3].Inthecase of a membranetransportthroughanon-porousmembrane,such trans-port mechanism is called solution-diffusion mechanism, while thatthroughaporousmembrane,ifapproved,partition-diffusion mechanism.Diffusionofpermeantwithinthemembranewould bedominantlydeterminedbyitsdimensionand/orshape.From this,thecontrol ofpermselectivityby usingdifferencein diffu-sivity wouldbeintrinsically limited. Againstthis, incorporation ofpermeantintoagivenmembraneistheoreticallyexpectedto varyfromnaughttoinfinity.Inordertoimprovepermselectivity ofmembranes,introductionofmolecularrecognitionsites,which rigorouslydiscriminatethetargetsubstrateandothers,intoagiven membraneiscrucial.

Atime-consuming,costly,andawkwardexperimentalworkis requiredtointroducemolecularrecognitionsitesintopolymeric

夽 NRCCPublicationNo.53064. ∗ Correspondingauthor.

E-mailaddress:masahiro@kit.ac.jp(M.Yoshikawa).

membranes.Against this,molecularimprintingtechnique [4–7]

wouldintroducemolecularrecognitionsitesintopolymeric mem-braneswithoutsuchaneffortmentionedaboveandisperceived tobeoneofthemostfacilewaystogivepolymericmembranes substrate specificity [8–11]. Among molecularimprinting tech-niques, an alternative molecular imprinting is a facile way to introducemolecularrecognitionsitesintopolymericmembranes. Thealternativemolecularimprintingcanbetakenasanextension ofMichales’study[12],whichwasthefirstapplicationof molec-ularimprintingtomembraneseparation.Applyinganalternative molecularimprinting,anypolymericmaterials,suchassynthetic polymers [13], oligopeptide derivatives [14,15], derivatives of natural polymers[16], and naturalpolymers[17] were directly convertedintomolecularlyimprintedmembranes.Inmembrane separation,notonlypermselectivitybutalsofluxisanimportant membraneperformance.Inasense,thedevelopmentofseparation membraneswithhighfluxismoreimportantthanthatwithhigh permselectivity.However,inmembraneseparation,afluxandthe correspondingpermselectivityoftenshowatrade-offrelationship. Membranologistshaveperceivedthatitisanunsolvedproblem inmembraneseparationtosimultaneouslyenhancebothfluxand permselectivity.

Membraneformofnanofibermat,whichisfabricatedby elec-trospraydeposition,isexpectedtosolveatrade-offrelationship inmembraneseparation,sincenanofibermatsgivehighersurface

0376-7388/$–seefrontmatter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2012.01.033

90 Y.Sueyoshietal./JournalofMembraneScience401–402 (2012) 89–96

areaandhigherporosity[18–22].Electrosprayednanofiber mem-braneswithmolecularrecognitionsiteswerefirstfabricatedfrom polyallylamine and poly(ethylene terephthalate) adopting 2,4-dichlorophenoxyaceticacidasaprintmoleculebyanalternative molecularimprinting[23].Thisprovedthatanalternative molecu-larimprintingisasuitablemethodtoobtainnanofibermembranes bearingmolecularrecognitionsitesdirectlyfrompolymeric mate-rials.Simultaneouslyapplyinganelectrospraydepositionandan alternativemolecularimprinting,molecularlyimprintednanofiber membranescanbefabricatedfromjustpolymersolutionwitha printmoleculewithease.Nanofibermembraneswithmolecularly imprintednanoparticleswerealsofabricatedbyelectrospray depo-sitionofpoly(ethyleneterephthalate)andmolecularlyimprinted nanoparticles[24,25].Thosematerialswereapplicableto solid-phaseextractorandsensorchipforthedetectionofagiventarget molecule.

The recognition of a target molecule by nanofiber fabrics with molecular recognition sites corresponds to the incorpo-ration of the permeant into the membrane in a membrane transport system. If the membrane diffusion process within the membrane and dissociation of a target molecule from a molecularrecognitionsite or release of it from the membrane are accompanied with molecular recognition process, a mem-brane transport process witha higher flux would be attained. To this end, nanofiber membranes with molecular recognition sites, which are called molecularly imprinted nanofiber mem-branes,werefabricatedbysimultaneouslyapplyinganalternative molecular imprinting and an electrospray deposition [26–28]. Those studies revealed that molecularly imprinted nanofiber membranes gave high flux without depression of permselec-tivity. A breakthrough in membrane separation was attained, in other words,there wasfound one of promising and simple ways to solve a trade-off relationship in membrane separa-tion.

In theprevious study[26],molecularly imprinted nanofiber membranesfrompolysulfonewithcarboxylicacidasafunctional moietybearingoxygenatomwasinvestigated.Aldehydegroupis anotherfunctionalmoietybearingoxygenatom.Itisinterestingto studytheeffectoffunctionalmoietyonchiralseparationability. To this end, in the present study, polysulfone with aldehyde group (PSf-CHO) was adopted as membrane materials. In the presentstudy,molecularlyimprintednanofibermembranesand usual molecularly imprinted membranes were fabricated from PSf-CHOand printmolecule, suchas N-␣-benzyloxycarbonyl-d-glutamic acid (Z-d-Glu) or N-␣-benzyloxycarbonyl-l-glutamic acid (Z-l-Glu), and their membrane performances of adsorption selectivity, permselectivity, and flux were studied.

2. Experimental 2.1. Materials

Polysulfoneswithaldehyde(PSf-CHO),withdegreeof substitu-tionof0.50(PSF-CHO-05)and1.00(PSf-CHO-10)(Scheme1)were preparedbythemodificationofPSfUdelP-3500(BPAmoco)as reportedpreviously[29].

N-␣-benzyloxycarbonyl-d-glutamic acid (Z-d-Glu) or N-␣-benzyloxycarbonyl-l-glutamic acid (Z-l-Glu), purchased from WatanabeChemicalIndustries,Ltd.(Hiroshima,Japan),wasused asaprintmoleculewithoutfurtherpurification.d-Glutamicacid (d-Glu), l-glutamic acid (l-Glu), sodium azide, copper sulfate, and ethanol were obtainedfrom commercialsources and used as received. N,N-Dimethylformamide (DMF) and tetrahydrofu-ran(THF)werepurifiedbytheconventionalmethod[30].Water

Scheme1.Chemicalstructuresofpolysulfonewithaldehydegroup(PSf-CHO)and printmolecule(Z-d-GluorZ-l-Glu).

purifiedwithanultrapurewatersystem(SimpliLab,MilliporeS.A., Molsheim,France)wasused.

2.2. Preparationofmolecularlyimprintedmembranes

Usualmolecularlyimprintedmembraneswerepreparedfrom THFsolution,containingtheimprintingcomponents.Moleratio of0.50forprintmoleculetoconstitutionalrepeatingunitof PSf-CHOinthemembranepreparationprocesswasstudied.A200mg ofquantityofPSf-CHOandaprescribedamountofprintmolecule, Z-d-GluorZ-l-Glu,wasdissolvedin3.0cm3_{ofTHF.Theamountof} Z-Gluwas61.61mgforthepreparationofmolecularlyimprinted membrane from PSf-CHO-05 and 59.78mg for that from PSf-CHO-10,respectively.TheTHFsolutionthuspreparedwaspoured into 89mm diameter flat laboratorydish, and the solvent was allowedtoevaporateatambienttemperaturefor24h.The result-ingmembranewasfurtherdriedat50◦_{Cfor2h.Afterdrying,the} printmoleculewasextractedfromtheresultantmembranebya largevolumeof50vol.%aqueousethanolsolutionuntiltheprint moleculewashardlydetectableinaqueousethanolsolutionbyUV analysis.Inthepresentstudy,mostofaddedprintmoleculewas leachedfromthemembranes.

2.3. Fabricationofmolecularlyimprintednanofibermembranes Inthepresentstudy,themolecularimprintingratio,whichwas themoleratioofprintmoleculetoconstitutionalrepeatingunitof PSf-CHOinthemembranepreparationprocesswasfixedtobe0.50 sothattheresultscanbecomparedwiththoseofusualmolecularly imprintedmembranes.MixtureofDMFandTHF(DMF/THF=2/1 (vol./vol.))was adoptedas solventand the polymer concentra-tionwasfixedat12.0wt.%inthepresentstudy.EsprayerES-2000 (FuenceCo.Ltd.,Japan)wasadoptedastheelectrospray deposi-tiondevice.Polymersolutioncontainingeither oneoftheprint moleculeswaselectrosprayedatambienttemperatureusingan appliedvoltageof25kVformolecularlyimprintednanofiber mem-branesor15kVforcontrolnanofibermembranes.Thesyringeused inthepresentstudyhadacapillarytipof0.52mmdiameter.The feedingratewas3.0mm3_{forthemolecularlyimprintedmembrane} and5.0mm3_{forthecontrolnanofibermembrane,respectively.A} groundedaluminumfoilusedasacounterelectrodewasplaced 10cmfromthetipofthecapillary.

Themorphology,diameter,andthicknessoftheelectrosprayed nanofibermembranesweredeterminedwithHitachiS-3000 scan-ningelectronmicroscope(SEM).Asmallsectionofthemembrane was placed on the SEM sample holder. The fiber diameter of nanofibermembranewasdeterminedusingImageJsoftware pro-grambymeasuringatleast30fibersfromeachSEMimage. 2.4. Adsorptionselectivity

Themembranesampleswereimmersedinaracemicmixture ofGlu,whichwasthesameracemicmixturestudiedinthe mem-branetransportability(i.e.,a50vol.%aqueousethanolsolutionof racemicGlu,withconcentrationof1.0×10−3_moldm−3_)andthe mixturewasallowedtobeequilibratedat40◦_{C;0.02wt.%sodium} azide was added asa fungicide. Quantitative measurements of aliquotsofthesolutionattheinitialstageandafterequilibrium hadbeenreachedweremadeusingHPLCemployingaChiralpak MA(+)column(50mm×4.6mm(i.d.))(DaicelChemicalInd.Ltd.). TheamountofGluinthesupernatantsubtractedfromtheamount initiallyinthesolutiongavetheamountofGluadsorbedinthe membrane.

TheadsorptionselectivitySA(i/j)isdefinedas SA(i/j)= (i-Glu)/(j-Glu)

[i-Glu]/[j-Glu] (1)

where(i-Glu)and[i-Glu]aretheamountofi-Gluadsorbedinthe membraneandconcentrationinthesolutionafterequilibriumhad beenreached,respectively.

2.5. Adsorptionisothermsofd-Gluandl-Glu

Themembranesampleswereincontactwithvarious concentra-tionsofpured-Gluorl-Glusolutionandallowedtobeequilibrated at40◦_{C.Thequantitativeanalysesweredoneasdescribedabove.} TheconcentrationofGluin themembrane[i-Glu]m or[j-Glu]m (i=D,j=Lori=L,j=D)wasdeterminedadoptingtheamountof Gluadsorbedinthemembraneandthevolumeofmembranephase, includingthatofmembraneandthatofsolutioninthemembrane.

2.6. Enantioselectivemembranetransport

Amembranewithanareaof3.0cm2_{wastightlysecuredwith} Parafilmbetween two chambersof a permeationcell. The vol-umeofeachchamberwas40.0cm3_{.AracemicGlusolutionwas} placedintheleft-handsidechamber(L-side)anda50vol.% aque-ousethanolsolutionintheright-handsidechamber(R-side).Each concentrationofracemicGluwas1.0×10−3_moldm−3_.Transport experimentswerecarriedoutat40◦_{Cwithstirring.Analiquotwas} drawnfromthepermeatesideateachsamplingtime.Theamount ofd-andl-Glu transportedthroughthemembranewere deter-minedbyHPLCasdescribedabove.

Theflux,J(molcmcm−2_h−1_{),isdefinedas} J=Qı

At (2)

whereQ[mol]istheamountoftransportedGlu,ı[cm]the mem-branethickness,A[cm2_{] theeffectivemembranearea,andt [h]} meanstime.

Thepermselectivity˛i/jisdefinedasthefluxratio,Ji/Jj,divided bytheconcentrationratio[i-Glu]/[j-Glu]

˛i/j= Ji/Jj

[i-Glu]/[j-Glu] (3)

Fig.1.SEMimagesofZ-d-Gluimprintednanofibermembrane(MINFM-10-D)(a), controlnanofibermembrane(MINFM-10-C)(b),andZ-l-Gluimprintednanofiber membrane(MINFM-10-L)(c).

3. Resultsanddiscussion

3.1. Morphologyofmolecularlyimprintednanofibermembranes TheSEMimagesoftheelectrosprayednanofibermembranes formPSf-CHO-10areshowninFig.1.ThosefromPSf-CHO-05also

92 Y.Sueyoshietal./JournalofMembraneScience401–402 (2012) 89–96

gavesimilarSEMimages.Inallnanofibermembranesinthepresent study,beadsarehardlyobserved.

Table1 summarizesmembrane thicknessofeach membrane studiedinthepresentstudyandfiberdiameterofeachnanofiber membrane. In the membrane code, MINFM denotes “molecu-larly imprinted nanofiber membrane” and MIPM “molecularly imprintedmembrane”,respectively.Inthemiddletwodigitinthe membrane code,05 denotes that themembrane was prepared fromPSf-CHO-05,in otherwords,thedegree ofsubstitution of PSf-CHObeing0.50,and 10wasfromPSf-CHO-10,respectively. Asforthelastalphabetinthemembranecode,D,C,andLmean d-isomerimprinted,control,andl-isomerimprintedmembrane, respectively.

3.2. Adsorptionselectivity

Adsorptionselectivityofmolecularlyimprintedandmolecularly imprintednanofiber membranes wasstudied adopting racemic mixtureofGlu’sasmodelracemates.Resultsaresummarizedin

Table2.Asexpectedfrompreviousstudies[13–16,26,27],the mem-branes imprintedby Z-d-Glu,incorporated d-Gluin preference tothe corresponding l-Glu and vice versa. In other words,the membranesimprintedbythel-isomerpreferentiallyadsorbedthe l-isomer.Asanticipated,controlmembranes,whichwereprepared fromPSf-CHOwithoutprintmolecule,hardlyshowedadsorption selectivity.Fromtheseresults,itwasconcludedthatZ-d-Gluand Z-l-Gluworkedasaprintmoleculeinthepresentstudy,aswell. Theadsorptionselectivityofthepresent molecularlyimprinted membranesmightbeexpressedbymolecularrecognitionsitesin themembrane,whichwereconstructedbythepresenceofprint moleculeduringthemembranepreparationprocess.Inthenext section,substratespecificityofthosemolecularrecognitionsites wasstudied.

3.3. Adsorptionisotherms

Itisinterestingtostudythesubstratespecificityofthe molecu-larrecognitionsitesinthemolecularlyimprintedandmolecularly imprintednanofibermembranes.Tothisend,adsorptionisotherms ofd-Glu and l-Glu for thosemembranes wereinvestigated. As anexample,theadsorptionisotherms ofmolecularlyimprinted nanofibermembranesfromPSf-CHO-10areshowninFig.2.The adsorptionisothermofl-GlufortheZ-d-Gluimprintednanofiber membrane(MINFM-10-D)andthatof d-Gluforthe MINFM-10-Lare straight lines passingthrough origin,implying that those enantiomerswereadsorbedinthemembranewithoutanyspecific interactionwiththemembrane,inotherwords,thosewere non-specificallyadsorbedinthemembranes.Adsorptionisothermsfor othermembranes,suchasmolecularlyimprintednanofiber mem-branesfromPSf-CHO-05,andmolecularlyimprintedmembranes fromPSf-CHO-05and PSf-CHO-10, alsogave similaradsorption isothermsasshowninFig.2.

TheadsorptionisothermofGlunon-specificallyadsorbedinthe membranecanberepresentedbythefollowingequation:

[j-Glu]_m=kA[j-Glu] (4)

where[j-Glu]mdenotes theconcentrationofj-Gluin the mem-brane,whichwasnon-specificallyadsorbedinthemembrane,kA isadsorptionconstant,and[j-Glu]meanstheconcentrationof j-Gluinthesolutionequilibratedwiththemembrane.Ontheother hand,theadsorptionisothermofd-GluinMINFM-10-Dandthat ofl-Glu inMINFM-10-L, whichwere preferentiallyadsorbedin themembrane, gave complicated profiles. Those isotherms are dualadsorptionisotherms,whichconsistofnon-specific adsorp-tionandadsorptiononthespecificrecognitionsites,whichwere

constructedduringthemembranepreparationprocessbythe pres-enceof a print moleculeof Z-d-Gluor Z-l-Glu.Theadsorption isothermforGluspecificallyincorporatedintothemembranecan berepresentedbythefollowingequation:

[i-Glu]_m=kA[i-Glu]+KS[Site]0[i-Glu]

1+_KS[i-Glu] (5)

where[i-Glu]mdenotes theconcentrationof i-Gluin the mem-brane,whichwasspecificallyincorporatedinthemembrane,KSis theaffinityconstantbetweenmolecularrecognitionsiteandi-Glu, [i-Glu]meanstheconcentrationofi-Gluinthesolutionequilibrated withthemembrane.

TwoparametersinEqs.(4)and(5),whichweredeterminedtofit eachadsorptionisothermbest,aresummarizedinTable3together withthoseforothersstudiedinthepresentstudy.Asforthe con-centrationofmolecularrecognitionsites,whichwereconstructed bytheprint molecules,inthemembrane, thoseformolecularly imprintednanofibermembranesareslightlylowerthanthosefor usualmolecularlyimprintedmembranes.Thisleadstothe spec-ulationthat the amountof print molecule, which workedas a printmoleculetomemorizemolecularmemoryduringmembrane preparationprocess,forthepreparationofmolecularlyimprinted nanofibermembranewasslightlylowerthanthatforusual molecu-larlyimprintedmembrane.Suchphenomenawerealsoobservedin thestudyofmolecularlyimprintednanofibermembranesfrom cel-luloseacetate(CA)[27].ComparingPSf-CHOmolecularlyimprinted nanofibermembraneswiththoseofCAones,moreamountofprint moleculeworkedwellintheelectrospraydepositionprocessof PSf-CHOthanthatofCA.Inthepresentstudy,smallamountofprint moleculewaselectrosprayedsolelytowardthecounterelectrodeof groundedaluminumfoilaccompanyingnoPSf-CHOmolecule.This mightbeduetothedifferenceinaffinityconstantKSforthose mem-branes.Moreprecisediscussionaboutthispointwillbedescribed later.

Inthepreviousstudy[27,31],theaffinityconstantwasobserved toincrease withthedecrease in molecularimprintingratio; in otherwords,atthelowermolecularimprintingratio,more func-tionalgroupsinthecandidatepolymerinteractedwithoneprint molecule,resultinginahigheraffinityconstant.Similar phenom-ena were also observed in the present study, and the affinity constants for the molecularly imprinted nanofiber membranes gavehighervaluesthanthoseoftheusualmolecularlyimprinted membranes.

Asdescribedintheintroduction,apartofthemotivationtostart thepresentstudywashowthedifferenceinfunctionalgroupwould affectthemembrane performances. Tothis end, in thepresent study,polysulfonewithaldehydegroup(PSf-CHO)insteadof car-boxylicacidwasadoptedasamembranematerial.Thedetermined parameters for adsorption equations for PSf-CHO molecularly imprintedmembranesareagainsummarizedinTable4together withthoseforPSf-COOHmolecularlyimprintedmembranes.Inthis table,theparametersforthemolecularlyimprintedmembranes fromPSf-CHO-10withDSof1.00aresummarized,sincethedegree ofsubstitutionofPSf-COOHmembranesbeing0.88[13].Inthe con-structionofmolecularrecognitionsitesfromPSf-COOH,carboxylic acidmoietyandsulfonegroupmightcontributetorecognize align-mentofthefunctionalmoietiesoftheprintmoleculeviahydrogen bond.InthecaseofPSf-CHO,justsulfonegroupmightcontributeto recognizethealignmentofthefunctionalmoieties.Ifsuch specu-lationmentionedaboveisaproperone,themolecularrecognition abilityoftheformedrecognitionsitesfromPSf-CHOislessthan thatfromPSf-COOH.Againstanticipation,theaffinityconstantsof PSf-CHOmembranewerehigherthanthoseofPSf-COOH.Inthe presentcase,shapememoryabilityofPSf-CHOshouldbebetter thanthatofPSf-COOH,whichmightbedominantlydependenton vander Waalsinteraction. Thisledtothedifferencein affinity

Table1

Membranethicknessandfiberdiameterofvariousmembranesinthepresentstudy.a

DS Membrane Membranethickness(␮m) Fiberdiameter(nm)

0.50 MINFM-05-D 200 269±170 MIPM-05-D 51 MINFM-05-C 175 564±333 MIPM-05-C 30 MINFM-05-L 129 280±218 MIPM-05-L 30 1.00 MINFM-10-D 135 177±64 MIPM-10-D 50 MINFM-10-C 50 189±85 MIPM-10-C 30 MINFM-10-L 100 165±105 MIPM-10-L 58

a_{MINFM,molecularlyimprintednanofibermembrane;MIPM,molecularlyimprintedmembrane.}

Table2

Adsorptionselectivityofmolecularlyimprintedandmolecularlyimprintednanofibermembranes.a

DS Membrane (d-Glu)/mem.(mol/g-mem.) (l-Glu)/mem.mol/g-mem. SA(D/L) SA(L/D)

0.50 MINFM-05-D 2.26×10−4 _1.63×10−4 _{1.39 0.72} MIPM-05-D 8.92×10−5 _7.06×10−5 _{1.26 0.79} MINFM-05-C 1.48×10−4 _1.48×10−4 _{1.00 1.00} MIPM-05-C 6.77×10−5 _6.79×10−5 _{1.00 1.00} MINFM-05-L 1.44×10−4 _1.85×10−4 _{0.78 1.28} MIPM-05-L 6.95×10−5 _8.34×10−5 _{0.83 1.20} 1.00 MINFM-10-D 3.03×10−4 _2.31×10−4 _{1.31 0.76} MIPM-10-D 9.10×10−5 _7.01×10−5 _{1.30 0.77} MINFM-10-C 1.97×10−4 _1.97×10−4 _{1.00 1.00} MIPM-10-C 6.92×10−5 _6.92×10−5 _{1.00 1.00} MINFM-10-L 2.61×10−4 _4.10×10−4 _{0.64 1.57} MIPM-10-L 7.32×10−5 _9.69×10−5 _{0.76 1.32}

a_{Molecularimprintingratio,(Z-Glu)/(PSf-CHO)=0.50.}

Fig.2.Adsorptionisothermsofd-Gluandl-GluinthenanofibermembraneimprintedbyZ-d-Glu(MINFM-10-D)(a)andthenanofibermembraneimprintedbyZ-l-Glu (MINFM-10-L)(b).

Table3

Parametersofadsorptionisothermsformolecularlyimprintedmembranes(MIPM)andmolecularlyimprintednanofibermembranes(MINFM).a Z-d-Gluimprintedmem. Z-l-Gluimprintedmem.

MIPM-10-D MINFM-10-D MIPM-10-L MINFM-10-L

kA 5.3×102 7.3×102 5.4×102 6.9×102

[Site]0(moldm−3) 2.0×10−1 1.5×10−1 1.6×10−1 1.5×10−1

KS(mol−1dm3) 1.3×104 2.2×104 1.7×104 2.0×104

Z-d-Gluimprintedmem. Z-l-Gluimprintedmem.

MIPM-05-D MINFM-05-D MIPM-05-L MINFM-05-L

kA 5.4×102 _6.5×102 _5.7×102 _5.6×102

[Site]0(moldm−3) 1.5×10−1 _1.5×10−1 _1.6×10−1 _1.4×10−1

KS(mol−1dm3) 1.4×104 _1.9×104 _1.4×104 _1.8×104 a_{Themolecularimprintingratios(PrintMolecule)/(PSf-CHO),forthosemembraneswerefixedtobe0.50.}

94 Y.Sueyoshietal./JournalofMembraneScience401–402 (2012) 89–96 Table4

Parametersofadsorptionisothermsformolecularlyimprintedmembranes(MIPM)fromPSF-CHO-10andPSf-COOH.a

Z-d-Gluimprintedmem. Z-l-Gluimprintedmem.

MIPM-10-D PSf-COOHb _{MIPM-10-L PSf-COOH}b

kA 5.3×102 _1.7×103 _5.4×102 _1.8×103

[Site]0(moldm−3) 2.0×10−1 _4.2×10−1 _1.6×10−1 _4.2×10−1

KS(mol−1dm3) 1.3×104 7.9×103 1.7×104 7.9×103 a_{Themolecularimprintingratios(PrintMolecule)/(PSf-CHO)or(PrintMolecule)/(PSf-COOH),forthosemembraneswerefixedtobe0.50.}

b _{DS=0.88;citedfromthepreviousstudy.}

constantKS forthosetwotypesofmolecularlyimprinted mem-brane(Table5).

3.4. Chiralseparation

Fromtheadsorptionstudy,thosetwotypesofmembrane in thepresentstudywereexpectedtoshowchiralseparation abil-ity. To this end, chiral separation ability for those membranes wasinvestigated.Inthepresentstudy,concentrationgradientwas adoptedas a driving force for membrane transport. The trans-port of racemic Glu’s through the control membranes, suchas MIPM-10-Cand MIPM-05-C, washardlyobserved. Other mem-branes transported racemic mixture of Glu’s. As an example, time-transportcurvesofd-Gluandl-Gluthroughthemolecularly imprintednanofibermembranes,MINFM-10-DandMINFM-10-L, areshown inFig.3.Againstexpectation basedontheprevious results[26,27],MINFM-10-Dmembrane,whichwasimprintedby thed-isomerandpreferentiallyadsorbedthed-isomer,transported l-Gluinpreferencetothecorrespondingd-Gluandviceversa.Such transport phenomena were oftenobserved in chiral separation

[15,32–37].RetardedtransportofGlupreferentiallyincorporated intothemembranemightbebecauseofarelativelystrong interac-tionbetweenthemembraneandGluselectivelyadsorbed.

Inmembraneseparation,notonlypermselectivitybutalsoflux isanimportantmembraneperformance.Inasense,development ofamembranewithhighfluxismoreimportantthanthatwith highpermselectivity.Fromthisitisnecessarytocompare mem-braneperformancesofthepresentmembraneseachother.Inorder tocomparemembrane performanceofflux,themolarmobility, u(molcmcm−2_J−1_h−1_{),forGluforeachmembraneisasuitable} parameter.FromNernst–Planckequation,afluxcanberepresented bythefollowingequation[38]:

J=−_ucRT















v







whereJdenotesthesumofd-Gluandl-Glufluxes,cisthe concen-trationofeachGluinthefeedside,Rmeansuniversalgasconstant, T denotes absolute temperature, dc/dx is the concentration gradientatthatpoint,zisvalenceofpermeant,FmeanstheFaraday constant,d/dxdenoteselectricalpotentialgradientatthatpoint,

v

gradientatthatpoint,Inthepresentstudy,membranetransport experimentswerecarriedoutunderisothermalandisobar condi-tions;andaconcentrationgradientwasadoptedasadrivingforce formembranetransport.Fromthese,thefluxinthepresentstudy canberepresentedbyjustfirstterminEq.(6).Thereforethemolar mobilitywasdeterminedbythefollowingequation:

u=− J/cRT

d lnc/dx (7)

Themolar mobility isdefined as themobility and issimply thefluxperunitmembranearea,perunitmembranethickness, perunitconcentration,andperunitdrivingforce.Inthe calcula-tionofthechemicalpotentialduetotheconcentrationgradient, theconcentrationof Glu in thepermeateside wasdetermined tobe 1.0×10−8_moldm−3_{,since thelowest limit of the} detec-tionofGluinthepresentstudywastheconcentrationofaround 1.0×10−8_moldm−3_{.Inthetable,arelativemobilityisalsoshown} forconvenience.Intheupperpartofthetable,ofwhichdataare membraneperformancesforthemembranesfromPSf-CHO-10,the relativemolarmobility,whichisrelativetoMIPM-10-L,isgiven; andinthelowerpartofthetable,therelativemolarmobility,which isrelativetoMIPM-05-L,isalsogiven.

Fluxvaluesforthemolecularlyimprintednanofibermembranes wereonetotwoordersofmagnitudehigherthanthoseforusual molecularlyimprintedmembranes.Comparingwiththeprevious resultsof molecularly imprinted nanofiber membranes [26,27], thepresent nanofiber membranesgave similarmolar mobility. Inthecaseofmolecularlyimprintednanofibermembranesfrom

Fig.3. Time–transportcurves of racemic Glu’sthrough the molecularly imprinted nanofiber membranes,MINFM-10-D (a)and MINFM-10-L (b) ([d-Glu]L,0=

Table5

Resultsofchiralseparationwithmolecularlyimprintednanofibermembranes(MINFMs)andmolecularlyimprintedmembranes(MIPMs).

Membrane Z-d-Gluimprintedmembrane Z-l-Gluimprintedmembrane

˛L/D uc ˛D/L uc MINFM-10a _{1.24 1.15×10}−9 _{1.20 1.67×10}−9 (28) (41) MIPM-10a _{1.20 4.20}×10−11 _{1.20 4.10}×10−11 (∼1) (1) MINFM-05b _{1.12 7.00×10}−9 _{1.20 2.20×10}−9 (231) (72) MIPM-05b _{1.25 6.64}×10−11 _{1.16 3.05}×10−11 (2.2) (1)

a_{Figuresinparenthesesaretherelativevalues;theuvalueforMIPM-10imprintedbyZ-l-Glubeingsetasunity.} b_{Figuresinparenthesesaretherelativevalues;theuvalueforMIPM-05imprintedbyZ-l-Glubeingsetasunity.} c _{u=(−J/C)/(d/dx)[{(molcmcm}−2_h−1_)/(molcm−3_)}/(Jmol−1_cm−1_)=molcmcm2_J−1_h−1_].

PSf-COOH, the molar mobility was determined to be around 5.3×10−9_(molcmcm2_J−1_h−1_).

The permselectivities for the present molecularly imprinted nanofiber membraneswere not sohighcomparingwith mem-branespreviouslyreported[39–41].Thepermselectivityisthought to be greatly dependent on adsorption selectivity. From this, enhancementofadsorptionselectivityisindispensablesothata givenmolecularlyimprintednanofibermembranecangivehigher permselectivity.Theremightbefollowing plausiblemethodsto enhancepermselectivity:(1)anincreaseinsurfacearea (surface-to-volumeratio)bynarrowingdiameterofmolecularlyimprinted nanofibermembrane,(2)localizationofmolecularrecognitionsites onthesurfaceofnanofiber,whichwouldbefabricatedbyadopting coaxial,two-capillaryspinneret[19,42–46],or(3)applyingahigher molecularimprintingratio.Thosethreetypesofmethodwilllead toanincreaseinconcentrationofmolecularrecognitionsiteinthe membrane.

In the membrane separation, a flux and the corresponding permselectivityoftenshowatrade-offrelationship.Againstthis,as reportedinthepreviousstudies[26,27],theenhancementofflux withoutdepressionofpermselectivitywasrevealedinthepresent study.Inotherwords,molecularlyimprintednanofibermembranes havepotentialtosimultaneouslyenhancethroughput(flux)and permselectivity.

4. Conclusions

Molecularly imprinted membranes (MIPMs) and molecu-larlyimprintednanofiber membranes(MINFMs) wereprepared from polysulfone with aldehyde (PSf-CHO-05 or PSf-CHO-10) and N-␣-benzyloxycarbonyl-d-glutamic acid (Z-d-Glu) or N-␣-benzyloxycarbonyl-l-glutamicacid(Z-l-Glu)asaprintmolecule. Those two types of molecularly imprinted membrane, such as MIPMsandMINFMs,incorporatedtheenantiomer,ofwhich abso-luteconfigurationwassameasthatoftheprintmolecule,in prefer-encetothecorrespondingantipode.Inotherwords,themembranes imprintedbythed-isomerpreferentiallyadsorbedthed-isomer andviceversa.Thosetwotypesofmembraneshowed permselectiv-ity.Againstexpectation,transportoftheenantiomerpreferentially adsorbedinthemembranewasretarded,inotherwords,the enan-tiomerlessadsorbedinthemembranewasselectivelytransported. Thiscanbeexplainedbyarelativelystronginteractionbetween membraneandtheenantiomerpreferentiallyincorporatedintothe membrane.The fluxesthoughthemolecularly imprinted mem-branesgave one totwoordersof magnitudehigherthan those of usualmolecularly imprintedmembraneswithout depression ofpermselectivity.Asprovedinthepreviousstudies,thepresent studyalsorevealedthat molecularlyimprintednanofiber mem-branesgavehighfluxwithoutdepressionofpermselectivity.The

emergenceofmolecularlyimprintednanofibermembranewould solveatrade-offrelationshipinmembraneseparation.

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