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NANOEXPRESSOpenAccessSilver(I)-directedgrowthofmetal-organiccomplexnanocrystalswithbidentateligandsofhydroquinineanthraquinone-1,4-diyldiethersaslinkersatthewater-chloroforminterfaceYingTang1,Hui-TingWang1,MengChen1,Dong-JinQian1*,LiZhang2andMinghuaLiu2AbstractImmiscibleliquid-liquidinterfacesprovideuniquedoublephaseregionsforthedesignandconstructionofnanoscalematerials.
Here,wereportedAg(I)-directedgrowthofmetal-organiccomplexnanocrystalsbyusingAgNO3asaconnectorintheaqueoussolutionandbidentateligandof1,4-bis(9-O-dihydroquininyl)anthraquinone[(DHQ)2AQN]anditsenantiomerof(DHQD)2AQNinthechloroformsolutionsaslinkers.
TheAg-(DHQ)2AQNandAg-(DHQD)2AQNcomplexnanocrystalswereformedattheliquid-liquidinterfacesandcharacterizedbyusingUV-visabsorptionandfluorescencespectroscopyandX-rayphotoelectronspectroscopy,aswellasbyusingscanningelectronmicroscopy.
Screw-likenanocrystalswereformedattheinitial30minaftertheinterfacialcoordinationreactionstarted,thentheygrewintonanorodsafterseveraldays,andfinallybecamecubicmicrocrystalsafter2weeks.
Thepureligandshowedtwoemissionbandscenteredatabout363and522nminthemethanolsolution,thesecondoneofwhichwasquenchedandshiftedtoabout470nmintheAg-complexnanocrystals.
TwocouplesofreversibleredoxwaveswererecordedfortheAg-complexnanocrystals;onecenteredatabout0.
25V(vs.
Ag/AgCl)wasdesignatedtooneelectrontransferprocessofAg(DHQ)2AQNandAg(DHQ)2AQN+,andtheotheronecenteredatabout0.
2VwasdesignatedtooneelectrontransferprocessofAg(DHQ)2AQNandAg+(DHQ)2AQN.
Keywords:Metal-organiccomplexnanocrystal;Liquid-liquidinterface;Morphology;Fluorescence;ElectrochemistryBackgroundSelf-assemblyofnanostructuralmaterialsatthefluidin-terfaceshasrecentlyreceivedgrowingattentionbecausetheinterfaceregionshaveadoublephasethicknessoftensofnanometersdependingonthenatureofthesol-ventsandspecieswithinthem,adimensionofwhichiscomparabletothatofthenanostructuralmaterials[1].
Thatis,thefluidinterfaceprovidesauniqueregionforthegrowthofmicro-ornanoscalematerialsandfortheconstrainedchemicalreactions[2,3].
Ithasbeenfurtherfoundthatthosematerialsproducedatthefluidinter-facesarehighlymobileandcanrapidlyachieveanequi-libriumassemblywiththereactantsineachphase.
Thedynamicprocessofspeciesorparticlesacrossinterfaceusuallydominatecomposition,morphology,andstruc-tureofthematerialsproduced[4,5].
Therapiddiffusionofnanoparticlesandreagentsineitherfluidphaseleadstoveryefficientinterfacialchemistry,includinginter-facialchemicalreactionsandmolecularassembly.
Immisciblesolutionsareoftenusedtoformthefluidinterfacesincesuchaninterfacecanprovideadefect-freejunctionthathasanimportancefortheproductswithhighpurity[6].
Manyone-dimensional(1D)nano-wiresandnanotubesand2Dnanosheetsandnanocombsofmetals,metaloxide,metalsulfide,andcomplexeshavebeendesignedandconstructedinthepastdecades[7-9].
Ourpreviousworkhasrevealedthatthespecificfeaturesofthemetalions(suchasthegeometry)andcoordinationnumbersoftheanionicionsorligandstakeanimportantroleingoverningthecrystalstructureoftheproducts[10-12],thoughacomplexinterplayofvanderWaals,*Correspondence:djqian@fudan.
edu.
cn1DepartmentofChemistry,FudanUniversity,220HandanRoad,Shanghai200433,ChinaFulllistofauthorinformationisavailableattheendofthearticle2014Tangetal.
;licenseeSpringer.
ThisisanOpenAccessarticledistributedunderthetermsoftheCreativeCommonsAttributionLicense(http://creativecommons.
org/licenses/by/4.
0),whichpermitsunrestricteduse,distribution,andreproductioninanymedium,providedtheoriginalworkisproperlycredited.
Tangetal.
NanoscaleResearchLetters2014,9:488http://www.
nanoscalereslett.
com/content/9/1/488electrostatic,magnetic,molecular,andentropiceffectsneedstobeconsidered.
Besidestheinorganiccompounds,polymercrystallinenanomaterialscouldalsobecon-structedattheliquid/liquidinterfaces.
Forinstance,Matsuiandcoworkerssynthesizedsinglecrystallinecon-ductingpolymer,poly(3,4-ethylenedioxythiophene),withthefastconductanceswitchingproperty[13].
TheyfurtherpreparedsinglecrystallinenanoneedlesofpolyanilineandpolypyrroleviaaninterfacialpolymerizationinducedbyFeCl3[14];theproductshaveafastconductanceswitchingtimebetweentheinsulatingandconductingstatesintheorderofmilliseconds.
Layeredporouspoly(4-vinylpyridine)(P4VP)filmscouldalsobeformedatthewater-oilinterfaceswiththeporousdiametersintherangefromhundrednanometerstose-veralmicrometers[15].
Liuandcoworkershavefurtherdevelopedthismethodtopreparemicrocapsulesandfoamfilms,whichwereusedasplatformstoformvariouscompositeinorganicnanomaterials.
Examplesincludedgoldnanoparticle-dopedpoly(2-vinylpyridine)andpoly(N-vinylcarbazole)composites[16,17]andsilver-orgold-dopeddiblockcopolymerofpoly(t-butylmethacrylate)-block-poly(2-vinylpyridine)composites[18].
Thesemetal-dopedcompositesshowedhighcatalyticactivityanddurabilityonthereductionoforganiccompoundssuchasnitrobenzene,4-nitrophenol,and4-nitrobenzoicacid[19].
Inthepresentwork,Ag(I)-directedmetal-organiccomplexnanocrystalswerefabricatedatthewater-chloroforminterfacebyusingAgNO3asaconnectorandbidentatechiralligandof1,4-bis(9-O-dihydroqui-ninyl)anthraquinone[(DHQ)2AQN]anditsenantiomerof(DHQD)2AQNaslinkers.
OurpreviousworkhasrevealedthatAgNO3coulddirectformationofchiralcoordinationpolymers(CPs)attheair-waterinterface[20].
Atomicforcemicroscopic(AFM)imagesforthetransferredCPsrevealedirregularaggregatesthatwerecomposedofmanyroundparticles.
Theseparticleswereconnectedtogethertoformwireswithparticularorien-tation.
However,duetothelimitationofthereactionspecieswithintheLangmuirmonolayersattheair-waterinterface,theorientationofthoseaggregateswasnotclear.
Hence,toclarifymorphologiesoftheseag-gregates,thecoordinationreactionwasperformedhereatthewater-chloroforminterfacewhereintheirregu-laraggregatesmaygrowintorelativelylargermacro-/nanocrystalssincetherewereenoughinorganicmetalionsandligandsineachphase.
Theas-preparedAg-organiccomplexnanocrystalswerecharacterizedbyusingUV-visabsorptionspectroscopyandX-raypho-toelectronspectroscopy(XPS)andscanningelectronmicroscopy(SEM).
Finally,theluminescentbehaviorsandelectrochemicalpropertiesofthenanocrystalswereinvestigated.
MethodsMaterialsChiralligandofhydroquinineanthraquinone-1,4-diyldi-etherof(DHQ)2AQNanditsenantiomer(DHQD)2AQN(Figure1)werepurchasedfromSigma-AldrichCo.
(St.
Louis,MO,USA).
ChloroformwasfromAlfaAesar(Beijing,China).
AgNO3wasfromShanghaiChemicalRe-agentCo.
(Shanghai,China).
Allchemicalswereusedasreceivedwithoutfurtherpurification.
Ultrapurewater(18.
2ΩMcm)waspreparedwithaRephiLefiltrationunit(RephiLeBioscienceLtd,Shanghai,China).
GrowthofAg-complexnanocrystalsatthewater-chloroforminterfaceInterfacialself-assemblyoftheAg-(DHQ)2AQNorAg-(DHQD)2AQNcomplexnanocrystalswasperformedasfollows:20mL10mMAgNO3aqueoussolutionwasslowlyaddedontothesurfaceof30mL(DHQ)2AQNor(DHQD)2AQNchloroformsolutioninabeaker.
Thereactionsystemwasleftundisturbedatroomtempe-raturefromseveralminutesto2weeks.
Asacontrolex-periment,interfacialphenomenonof(DHQ)2AQNor(DHQD)2AQNattheinterfaceofpurewaterandligandchloroformsolutionwasalsoinvestigated.
TransferofAg-complexnanocrystalsontosubstratesurfacesLayersofAg-(DHQ)2AQNorAg-(DHQD)2AQNcom-plexnanocrystalsgrownattheinterfaceweretransferredontosubstratesurfaceswiththeuseofadipperfromFigure1Structureoftheligandsusedinthepresentwork.
Tangetal.
NanoscaleResearchLetters2014,9:488Page2of9http://www.
nanoscalereslett.
com/content/9/1/488KSV5000minitrough(KSVInstrumentCo.
,Helsinki,Finland)orfromJML04C2trough(Powereach,Shanghai,China).
Thesubstratewasfirstlyimmersedintotheliquid-liquidinterfacebeforetheinterfacialreactionstarted,andafteragiventimewaitingforthecoordinationreactionandformationofAg-complexnanocrystals,thesubstratewasverticallywithdrawnfromtheinterface.
Thedippingratewaskeptat1mm/min.
InstrumentsUV-visspectraweremeasuredwiththeuseofaShimadzuUV-2550UV-visspectrophotometer(Shimadzu,Kyoto,Japan).
Steady-statefluorescencespectrawererecordedbyusingaShimadzuRF-5300PCspectrophotometer.
XPSspectrawererecordedbyusingaVGESCALABMKIImultifunctionspectrometer(VGScientific,EastGrinstead,UK),withnonmonochromatizedMg-KαX-raysastheexcitationsource.
Thesystemwascarefullycali-bratedbyFermiedgeofnickel,Au4f2/7,andCu2p2/3bindingenergy.
Passenergyof70eVandstepsizeof1eVwerechosenwhentakingspectra.
Intheanalysis,chamberpressuresof1~2*107Pawereroutinelymaintained.
ThebindingenergiesobtainedintheXPSanalysiswerecorrectedbyreferencingtheC1speakto284.
60eV.
Scanningelectronmicroscopic(SEM)measurementswereperformedonaPhilipsXL30electronmicroscope(Philips,Amsterdam,TheNetherlands).
ThesamplesweredepositedontheSisubstratesurface.
High-resolutiontransmissionelectronmicroscopeimagewasacquiredonaJEOLJEM-2010transmissionelectronmicroscope(JEOLLtd.
,Akishima-shi,Japan)operatingatanaccel-eratingvoltageof200kV.
Thesamplewasdepositedontoa230-meshcoppergridcoveredwithFormvar.
Cyclicvoltammogram(CV)wasmeasuredbyusinganelectrochemicalanalyzer(CHI601b,CHInstruments,Inc.
,Shanghai,China).
APtwireandAg/AgClelectrodewereusedastheauxiliaryandreferenceelectrodes,respectively,andtheindiumtinoxide(ITO)electrodecoveredwithlayersofAg-(DHQ)2AQNorAg-(DHQD)2AQNnanocrys-talswasusedastheworkingelectrodewitha10mmol/LHClO4solutionastheelectrolyte.
FortheCVmeasurementoftheligandredoxreaction,aninitialpotentialof0.
5Vwasappliedfor2s,followedbycyclicscanstoafinalpo-tentialof0V.
ForthemeasurementoftheAg(I)redoxre-action,theinitialpotentialof0.
1Vwasappliedfor2s,followedbycyclicscanstothefinalpotentialof0.
5V.
AllCVmeasurementsweredonefor10cyclesunderanArat-mosphereatroomtemperature.
ResultsanddiscussionGrowthofAg-complexnanocrystalsatthewater-chloroforminterfaceInterfacialreactionbetweenthesilverionandligandof(DHQ)2ANQor(DHQD)2ANQoccurredquickly.
Duringexperiments,wefoundthatthereactionratewascloselydependentontheconcentrationofAgNO3inwaterandthatoftheligandinchloroformaswellasthetempe-rature.
Similartothosereportedintheliterature[21],strongerconcentrationofthereactantsandhigherre-actiontemperaturecouldresultinaquickformationofmetal-organiccomplexnanocrystals.
Thesenanocrystalswerethentransferredonvarioussubstratesurfacesbytheverticaldippingmethodforthecharacterizationofmorphologies,absorptionandemissionspectra,XPS,andelectrochemistry.
MorphologycharacterizationMorphologiesoftheAg-directedcomplexnanocrystalswerecharacterizedbyusingSEMtechnique.
Thesena-nocrystalsweredepositedonthefreshlycleanedSisubstratesurfacebyverticaldippingmethod.
Similarmorphologieswereobtainedforthetwoligands,soasanexample,Figure2showsseveralSEMimagesoftheAg-(DHQ)2AQNcomplexnanocrystalsformedatthewater-chloroforminterfacefromtheinitial5minto2weeks,whichrevealedthefollowingfeatures.
Firstly,manyscrew-likenanocrystalswereformedattheinitial30minwithalengthofseveralhundrednanome-tersanddiametersoftensofnanometers(Figure2A,B).
Thesenanocrystalsformedaggregatespossiblyduetoastronginteractionbetweeneachscrew-likenanocrystal.
Suchkindofinteractionsbetweenadjacentnanocrystalshasbeenusedtocontrolthegrowthoflarge-scaleorcol-loidalnanocrystalbuildingblocksintheorganicsolutions[22].
Asithasbeenreportedthateachsilverionmayco-ordinatewithtwopyridylgroups[20,23],sowemaysug-gestthateachsilverioncoordinatedwithtwoligandstoformAg-directedCPnanocrystals.
Thescrew-likefeatureofthenanocrystalsmaybeduetothefactthattheligandwasachiralmolecule,whichdominatedgenerallythefor-mationofuniquesupramolecularaggregatesornanocrys-talsashavingbeenreportedbyseveralresearchgroups[24-26].
Secondly,thescrew-likenanocrystalstransformedintonanorodsaftertheinterfacialreactiontimeincreasedtoabout30min.
AsshowninFigure2C,D,thelengthofnanorodsincreasedtobeaboutseveralmicrometerswiththediametersabouttensofnanometers,alittlein-creasedascomparedwiththoseofthescrew-likenano-crystals.
Thisincreasemaybeattributedtothefollowingreasons:(1)thecoordinationreactionofthesilverionsandbidentateligandscontinuedonthesurfaceofthescrew-likenanocrystals,and(2)thenanocrystalsformedattheinitialtimewereofhighlyactivesurfaceenergythatresultedinastronginteractionbetweeneachnano-crystal.
Asaresult,thesmallnanocrystalsformedlargerparticlesasthoseoftenoccurredintheair-organicsol-ventinterfaces[27].
Tangetal.
NanoscaleResearchLetters2014,9:488Page3of9http://www.
nanoscalereslett.
com/content/9/1/488Thirdly,whentheinterfacialreactioncontinueduptoseveraldays,thenanorodsfurthergrewintocubicma-crocrystalswiththelengthofasideabouthundredsofnanometers(Figure2E,F).
Thisprocesswassimilartothatwehaveobservedformetal-mediatednano-crystalsofmultiporphyrinarrays[10],whereintheshapesoftheproductswerecloselydependentonthegeometriesofthecentralmetalions.
Here,theAg+ionsweretetrahedrallycoordinatedwithbidentateligandof(DHQD)2AQN,soscrew-likeornanorodswerefirstlyformed,thentheygrewintolargercubicparticles.
Acomparisonofthesizeofthenanorodswiththatofthecubicparticlescouldfurtherfindthatthecubiclengthwasshorterthanthatofthenanorods;thisphenomenonmaybeattributedtoaslowkineticprocessofthecrys-talgrowingattheearlierstage,thentoathermodyna-micprocessafterseveraldays.
Theotherpossiblereasonmaybethelowestsurfaceactiveenergyofthecubiccrys-tals;thatis,thenanocubesmaybemorestablethanthenanorods.
SimilarTEMimageswereobservedfortheAg-(DHQ)2AQNcomplexnanocrystals,butthescrew-likenano-crystalsformedattheinitialtimewerenotverystableunderthehighbeamenergyofTEM.
Asanexample,Figure2GshowsaTEMphotooftheAg-complexnano-crystalsatthereactiontimeof2h,whichrevealedthatitwascomposedofmanynanorodsanddot-likeag-gregatesandinagreementwiththatobservedfromtheFigure2SEMandTEMimagesofAg-(DHQ)2AQNnanocrystalsgrownatwater-chloroforminterfaceafterdifferentreactiontimes.
SEMimagesafter(A)30min,(B)30min(enlargedphoto),(C)2h,(D)12h,(E)3days,and(F)15days;(G)TEMimageafter2h;(H)electrondiffractionpatternofthenanocrystals.
Tangetal.
NanoscaleResearchLetters2014,9:488Page4of9http://www.
nanoscalereslett.
com/content/9/1/488SEMphotos.
ElectrondiffractionpatternofthepresentAg-complexnanocrystalsrevealedmanyirregulardots(Figure2H),indicatingthattheywerepolycrystalline.
X-rayphotoelectronspectroscopyElementcompositionsfortheAg-directedcomplexnano-crystalsweredetectedbyusingtheXPStechnique.
Alsoasanexample,Figure3showsthehigh-resolutionXPSbandsfortheAg-(DHQ)2AQNcomplexnanocrystals,whichrevealedfourpeaksinthebindingenergyfrom100to600eVexceptfortheSielementfromsubstratesurface.
Thebindingenergyofthesefourpeakswasasfollows:284.
6,368.
6/374.
4,399.
4~403,and532.
6eV,whichcouldbeassignedtotheelementsofC(1s),Ag(3d),N(1s),andO(1s),respectively.
TheC,partofN,andOelementswerefromtheligandof(DHQ)2AQN,whiletheelementsofAg,partofN,andOwerefromAgNO3.
Thus,theseXPSdataconfirmedformationofAg-(DHQ)2AQNcom-plexnanocrystals[20].
AbsorbanceandfluorescenceemissionofthenanocrystalsAbsorptionandemissionfeaturesoftheAg-complexnanocrystalsas-preparedwereinvestigatedonthequartzsurfaces.
Asanexample,Figure4showsabsorptionspec-trafortheAg-(DHQ)2AQNnanocrystalstransferredfromthewater-chloroforminterfaceatdifferentreactiontimes,togetherwithaspectrumoftheligandinthedilutemethanolsolution.
Threeabsorptionbandswererecordedandappearedatabout230~238,323~334,and416nmfortheligandof(DHQ)2AQNinthesolution,whichcanbedesignatedtotheelectrontransitionofquinuclidine,quinine,andanthraquinonesubstituents.
Ourpreviousworkhasrevealedthatthesepeaksshiftedtoabout240,337,and416nminitsLangmuir-Blodgett(LB)film[20].
WhentheligandwascoordinatedwithAg+ionstoformtheLBfilmofAg-(DHQ)2AQNcoordinationpolymers,theseabsorptionbandsappearedatabout245,342,and413nm,respectively[20].
Thatis,aredshiftwasrecordedfortheformertwopeakswhentheligandwascoordinatedwithAg+ions.
Here,forthefilmsofAg-(DHQ)2AQNcomplexnano-crystals,abroadabsorptionbandwasrecordedwiththemaximumatabout270~296nm.
Butitishardtodis-tinguisheachpeakasthoseinthesolutionsandLBfilms[20].
ThisdifferencecanbeattributedtothefactthattheLBfilmwasalmosttransparentanddidnotscatterthelightduringtheabsorptionmeasurements;however,thepresentfilmofAg-(DHQ)2AQNcomplexnanocrys-talswasnot.
Thedetectedlightwasstronglyscatteredbythenanoparticles,resultinginabroadenbandfrom200tonearly600nm.
Moreover,withincreasingthere-actiontime,theaveragesizesofthenanocrystalsin-creased(asshownintheSEMimagesinFigure2).
Asaresult,themainabsorptionbandalittleredshifted,thefeatureofwhichwasinagreementwiththatobservedintheaggregatesofinorganiccomplexesormacrocycliccompoundsduetoastrongmolecularinteractioninthelargeraggregatesortoastrongerlightscatteredbythelargeraggregates[28,29].
Basedonthechemicalstructureofthechiralligandsused,wecanfindthattheycontainbothquinineandan-thraquinonesubstituents,bothofthemareimportantlight-harvestingunits.
Theycannotonlyabsorbultravio-letlightbutalsogiveoffemissioninthenearultravioletandvisibleregion[30,31],sotheyhavepotentialapplica-tionsinthefieldsofoptical,electroluminescentmaterialsandlight-emittingdiodes.
ThelightenergyabsorbedbytheligandscanbefurthertransferredtosomemetalionslikeEu3+andTb3+asluminophores,sensors,andorganiclight-emittingdiodes[32,33].
Here,luminescentemission280284288366372378396402408BindingEnergy(eV)528534540Figure3High-resolutionXPSspectraoftheAg-(DHQ)2AQNnanocrystals.
2003004005006000.
00.
10.
20.
30.
40.
5cbAbsorbance(arb.
unit)Wavelength(nm)aFigure4Absorptionspectra.
(a)Ligandof(DHQ)2AQNinthemethanolsolutionand(b)Ag-(DHQ)2AQNnanocrystalsdeposited2hand(c)Ag-(DHQ)2AQNnanocrystalsdeposited3daysaftertheinterfacialreaction.
Tangetal.
NanoscaleResearchLetters2014,9:488Page5of9http://www.
nanoscalereslett.
com/content/9/1/488propertiesfortheAg-(DHQ)2AQNcomplexnanocrystalswereinvestigatedaftertheyweretransferredonthequartzsubstratesurfaces.
Figure5showsemissionspectrumfortheAg-(DHQ)2AQNcomplexnanocrystalsonthequartzsubstratesurface,togetherwithanemissionspectrumoftheligandinthemethanolsolution.
Theexcitedwavelengthwas317nm.
Theseemissionspectrarevealedthefollowingfeatures.
Firstly,twobroademissionpeakswererecordedandcen-teredatabout363and520to530nmfortheligand(DHQ)2AQNinthemethanolsolution.
Thefirstpeakmaybedesignatedtotheemissionfromthehydroquininesub-stituentsandthesecondonetothatoftheanthraquinone[20].
Secondly,theAg-(DHQ)2AQNcomplexnanocrystalsshowedalsotwobroademissionpeaks;thefirstoneap-pearedatabout360nm(verysimilartothatinthemetha-nolsolution),whilethesecondone'blue'shiftedtoabout470nm.
Previously,wehavefoundthatthefluorescentemissionfeaturesfortheligandsinthecastingfilmsweresimilartothoseintheLBfilms;thatis,thefirstoneap-pearedatabout365nmwhilethesecondoneslightlyredshiftedtotherangeof530~560nm.
Thisredshifthasbeenattributedtoacloselypackedarrangementofthemoleculesintheorganizedultrathinfilmsasoftenobservedforthemacrocyclicmoleculessuchasporphyrinsandinorganiccomplexes[10-12].
Theblueshiftphenomenonofthesecondemissionbandwasalsoobservedinthelayer-by-layermultilayersofPd-(DHQ)2ANQandPd-(DHQD)2ANQcoordinationpolymers[34],whichwasattributedtotheformationofthePd-(DHQ)2AQNandPd-(DHQD)2ANQcomplexes.
Duringexperiments,wemeasuredtheemissionspectraforthemixturesof(DHQ)2ANQandAgNO3atthemolarratiosfrom1:0to1:10inthemethanolsolution.
AsshowninFigure6,theemissionatapproximately360nmdidnotshowasignificantdifferencewiththeincreaseoftherelativemolarfractionsofAgNO3.
How-ever,theemissionat525nmgraduallyweakenedwhentheAgNO3solutionwasadded.
WhenthemolarratiosofAgNO3relativetotheligandincreasedto10,aweakemissionpeakwasobservedatapproximately470nm,whichwasinagreementwiththatobservedintheAg-(DHQ)2AQNcomplexnanocrystals.
Theseresultssug-gestedthattheblueshiftandquenchingofthesecondemissionpeakmaybeattributedtotheformationofmetal-ligandcomplexesinthenanocrystals[34].
VoltammetricpropertiesBesidestheinterestingopticalandchiralbehaviors,(DHQ)2AQNand(DHQD)2AQNarealsoelectroactivecompoundsbecausetheycontaintheanthraquinonesubstituents,whichhavebeenwidelyusedaselectroac-tivematerialseitherforthefundamentalresearchesonelectrochemistryorforthepotentialapplicationsinthesensors,electrochromism,andorganicbatteries[32,33].
Here,thecyclicvoltammogramsofAg-complexnano-crystalsontheITOelectrodeswereinvestigatedandcomparedwiththoseoftheligandinthecastingfilms.
Figure7AshowstheCVcurvesfortheITOelectrodecoveredbytheAg-(DHQ)2AQNcomplexnanocrystalsinthe0.
01mol/LHClO4electrolytesolutionsinthepo-tentialrangeof0.
5to0Vatthescanratesfrom0.
05to0.
6V/s.
Onecoupleofredoxwavewasrecordedwiththecathodic(Epc)andanodic(Epa)potentialsataround0.
27~0.
29and0.
22~0.
20Vvs.
Ag/AgCl,respect-ively.
Basedontheliterature[35],thisredoxcouplewasdesignatedtotheelectrontransferprocessof(DHQ)2AQNand(DHQ)2AQN.
ThepotentialdifferenceΔE(ΔE=Epa–Epc)was0.
05Vwhenthescanratewas0.
05V/s,whichslightlyincreasedto0.
09Vwhenthescanratewas0.
6V/s.
Thereductioncurrentintensitywasabout15.
6μA,whichwasalsoclosetothatofthe3504004505005506000306090120EmissionIntensity(arb.
unit)Wavelength(nm)Figure5Fluorescencespectra.
Ag-(DHQ)2AQNnanocrystals(solidline)andtheligandinthemethanolsolution(dashedline).
350400450500550600080160240Ag+:(DHQ)2ANQ1:01:21:51:10Absorbance(arb.
unit)Wavelength(nm)Figure6Fluorescencespectraforthemixturesof(DHQ)2AQNandAgNO3inthemethanolsolutions.
Tangetal.
NanoscaleResearchLetters2014,9:488Page6of9http://www.
nanoscalereslett.
com/content/9/1/488oxidationcurrentintensity(12.
6μA)whenthescanratewas0.
6V/s.
TheseCVfeaturessuggestedthattheredoxprocessoftheligandsinthenanocrystalswasreversible.
Figure7BshowstheCVcurvesfortheAg-(DHQ)2AQNnanocrystalsinthe0.
01mol/LHClO4electrolytesolutionsinthepotentialrangeof0.
1to0.
5Vatthescanratesfrom0.
05to0.
6V/s.
Onecoupleofredoxwavewasrecordedwiththecathodicandanodicpoten-tialsataround0.
13~0.
07and0.
28~0.
32V,whichwasattributedtotheredoxreactionoftheconnectorofAg+ions[36],withtheelectrontransferprocessofAg-(DHQ)2AQNandAg+-(DHQ)2AQN.
Thepotentialdiffer-enceΔEwas0.
35Vwhenthescanratewas0.
05V/s,whichincreasedto0.
45Vwhenthescanratewas0.
6V/s.
Relationoftheredoxcurrentintensityofthemodifiedelectrodetothescanrateandtherootofthescanratewascalculated.
Figure8AshowsplotsofthecurrentintensityforthereductionreactionoftheligandAg-(DHQ)2AQN→Ag-(DHQ)2AQNtothescanrateandtherootofthescanratefortheITOelectrodemodifiedbythefilmofAg-(DHQ)2AQNnanocrystals.
Basedonthesedata,wecanfindthatthecurrentintensitywasproportionaltotherootofthescanratesratherthanthatofthescanrates,whichindicatedthattheelectroac-tivethicknessofthenanocrystalswasthickerthanthatofthediffusionlayer.
ThiswasreasonablebecausethefilmwascomposedofirregularAg-(DHQ)2AQNcom-plexnanocrystalswiththesizesintherangeofhundredsofnanometers(Figure2).
Acloseinspectionofthefig-urecouldfurtherfindthatthislinedidnotgothroughthezeropoint.
Thisfeaturesuggestedthat,besidesthediffusionlayer,theelectrontransferprocessbetweentheAg-(DHQ)2AQNnanocrystalsandelectrodesurfacemaybealsoinfluencedbysomeotherissues,suchasthein-terfacialresistancebetweenthenanocrystalsandelec-trodesurfaceandtheconnectorsofAgNO3betweentheligandsandelectrodesurface[36].
-0.
5-0.
4-0.
3-0.
2-0.
10.
0-80-4004080120CurrentIntensity(A)Potential(VvsAg/AgCl)(A)-0.
10.
00.
10.
20.
30.
40.
5-300-200-1000100200300CurrentIntensity(A)Potential(VvsAg/AgCl)(B)Figure7CyclicvoltammogramsoftheAg-(DHQ)2AQNnanocrystals.
(A)Potentialrangeof0.
5to0Vand(B)potentialrangeof0.
1to0.
5Vatthescanratesof0.
05,0.
1,0.
2,0.
3,0.
4,0.
5,and0.
6V/sinthe0.
01mol/LHClO4electrolytesolutions.
(A)(B)Figure8Plotsofcurrentintensitytothescanrates(blackcircles)androotofscanrates(blacksquares).
(A)ReductionreactionofAg-(DHQ)2AQN→Ag-(DHQ)2AQNand(B)reductionreactionofAg+-(DHQ)2AQN→Ag-(DHQ)2AQNcomplexnanocrystals.
Tangetal.
NanoscaleResearchLetters2014,9:488Page7of9http://www.
nanoscalereslett.
com/content/9/1/488Figure8BshowsplotsofthecurrentintensityforthereductionreactionofthesilverionsinthecomplexnanocrystalsofAg+-(DHQ)2AQN→Ag-(DHQ)2AQNtothescanrateandtherootofthescanrate.
DifferentfromthatobservedinFigure8Aforthereductionreac-tionoftheligands,here,thecurrentintensitywasnotproportionaltoneitherthescanratenortherootofthescanrate.
Accordingtothetheoryoffilm-modifiedelec-trode[37,38],thisphenomenoncouldbeattributedtothattheelectroactivethicknessofthenanocrystalswasnotverythinnerorverythickerascomparedwiththatofthediffusionlayer.
ConclusionsSilver(I)-directedmetal-organiccomplexnanocrystalshavebeenpreparedusingAgNO3asaconnectorandchiralbidentateligandsaslinkersatthewater-chloroforminterface.
Screw-likeAg-directedcomplexnanocrystalsformedattheinitialreactiontime;then,theygrewintonanorodsandfinallybecamecubicnano-ormicrocrystals.
Theas-preparedAg-complexnanocrystalsshowedstrongluminescentemissionsaswellasreversibleredoxproper-ties,whichmayhavepotentialinterestsinthefundamen-talresearchesandapplicationsinthefieldsofchemicallymodifiedelectrodesandoptoelectronicdevices.
CompetinginterestsTheauthorsdeclarethattheyhavenocompetinginterests.
Authors'contributionsYT,HTW,MC,andLZcarriedoutthesynthesisandcharacterizationsofthematerialsanddraftedthemanuscript.
MLandDJQcontributedinthedesignanddiscussionofthisworkandintherevisionofthemanuscript.
Allauthorsreadandapprovedthefinalmanuscript.
AcknowledgementsTheauthorsaregratefultotheNationalScienceFoundationofChina(91027042)andtheProgramforChangjiangScholarsandInnovativeResearchTeaminUniversity(IRT1117).
Authordetails1DepartmentofChemistry,FudanUniversity,220HandanRoad,Shanghai200433,China.
2BeijingNationalLaboratoryforMolecularScience,CASKeyLaboratoryofColloid,InterfaceandChemicalThermodynamics,InstituteofChemistry,ChineseAcademyofSciences,No.
2ZhongguancunBeiyijie,Beijing100190,China.
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