Water soluble carbon nanoparticles Hydrothermal synthesis and excellent photoluminescence properties

ColloidsandSurfacesB:Biointerfaces87 (2011) 326–332

ContentslistsavailableatScienceDirect

ColloidsandSurfacesB:Biointerfaces

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Watersolublecarbonnanoparticles:Hydrothermalsynthesisandexcellentphotoluminescenceproperties

XiaodieHea,1,HaitaoLia,1,YangLiua,HuiHuanga,ZhenhuiKanga, ,Shuit-TongLeea,b

InstituteofFunctionalNano&SoftMaterialsandJiangsuKeyLaboratoryforCarbon-BasedFunctionalMaterials&Devices,SoochowUniversity,Suzhou,Jiangsu215123,Chinab

CenterofSuper-DiamondandAdvancedFilms(COSDAF)andDepartmentofPhysicsandMaterialsScience,CityUniversityofHongKong,HongKong,China

a

article

info

abstract

Articlehistory:

Received17March2011

Receivedinrevisedform17May2011Accepted20May2011

Available online 27 May 2011

Keywords:

CarbonnanoparticlesPhotoluminescence

Up-conversionphotoluminescence

Visible–nearinfraredphotoluminescence

Water-solublecarbonnanoparticles(CNPs)werefabricatedbyafacile,onestephydrothermalsyn-theticrouteusingacid/alkaliasadditives.TheseCNPsemitbrightphotoluminescence(PL)coveringtheentirevisible-nearinfrared(NIR)spectralrange.PLmeasurementscon rmedthattheCNPshaveup-conversionofPLproperties,andthattheNIRPLoftheCNPscanalsobeobservedbyNIRexcitation.ControlexperimentsindicatedthatdifferentadditivescanstronglyaffectthePLpropertiesoftheCNPs.WithacombinationoffreedispersioninwaterandattractivePLproperties,theseCNPsholdpromiseforapplicationsinnanotechnology.

© 2011 Elsevier B.V. All rights reserved.

1.Introduction

Animmenseinteresthasbeenshownrecentlyonthephoto-luminescentnanostructuresduetotheirpromisinganddiverseapplicationsrangingfromoptoelectronicstobiology,especiallyintherapidlygrowing eldonbionanotechnology[1].More-over,thedemandforphotoluminescentnanostructuresemittinginvisible-to-nearinfrared(NIR)spectralrangeisrapidlyincreas-ing[1,2].ComparedtoconventionalmeasurementsmadeintheUV–visibleregion,spectro uorimetrywithinthetherapeuticwin-dowof700–1200nmhasmanyadvantages,suchaslowerlevelsofbackgroundinterferenceanddeeperlightpenetrationoflivingtissues[2].Consequently,NIRphotoluminescence(PL)obtainedunderNIRexcitationholdsgreatpotentialfortheinvivousesatasigni cantdepthinthebiologicalmediaandthedevelopmentofnoninvasivediagnostictechniques[1–4].

WhilesomesemiconductorscanexhibitNIRPL,theyaretypi-callyexcitedbyUV,visibleorNIRlight,whichraisesconcernsonhealth,environmentandhighcost(mostassociatedwithClassAelement:Cd,Pb,InandHg;andClassBelement:SeandAs),thuslimitingtheirinvitroandinvivoapplications[3].Tomaintaina

Correspondingauthor.Tel.:+8651265880957;fax:+8651265882846.E-mailaddresses:yangl@(Y.Liu),zhkang@(Z.Kang).1

Theseauthorscontributedequallytothiswork.

benignenvironment,low-toxicitysiliconandcarbonnanostruc-turesarepreferredinmanyapplications.WhileSiquantumdotshavebeenappliedinimmuno uorescentcellimagingtoactascellularprobes,theycannotyieldNIRPL[4].Recently,graphenequantumdots[4]andoxygen-containingcarbonnanoparticles(CNPs)withPLpropertieshavebeenpreparedbylaserablationandelectrochemicaloxidationofgraphite[5,6],electrochemicalsoakingofcarbonnanotubes[7],thermaloxidationofsuitablemolecularprecursors[8],vapordepositionofsoot[9],proton-beamirradiationofnanodiamonds[10],microwavesynthesisandwetchemicalmethod[11].TheCNPsreportedsofaremitef -cientlyonlyinthevisiblerange,andnoneofthemhasbeenreportedtogenerateNIRPL.Although,NIRPLunderNIRexcitationhasbeenobservedinsingle-walledcarbonnanotubes(SWCNTs)[12],howevertheysufferfromthedrawbacksofuncontrollablespeci edchiralityandexpensivepreparationand/orseparationprocedures.

Carbohydratesarethemostabundantclassoforganiccom-poundsfoundinlivingorganisms.Theformulasofmanycarbohydratescanbewrittenascarbonhydrates,Cm(H2O)n,andtherehavebeenmanytrialsintheliteraturetoreplicatecarbonformationfromcarbohydrates[13].TheCNPswerepreparedbyasimplehydrothermalmethodbyusingacarbonsource[13d].Herein,wereportafacileandeffectivesynthesisofCNPsthroughthehydrothermaltreatmentofthreecommoncarbohydrates(glu-cose,sucroseandstarch).Theobtainedwater-solubleCNPscan

0927-7765/$–seefrontmatter© 2011 Elsevier B.V. All rights reserved.doi:10.1016/j.colsurfb.2011.05.036

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Scheme1.FabricationofCNPscapableofvisibleandNIRemissionbyacid/alkali-assistedhydrothermaloxidation(carbohydraterepresentglucose,sucroseandstarch).

Fig.1.Typical(a)TEMand(b)SEMimagesofCNPssamplespreparedfromcarbohydrates.

Fig.2.TypicalPLimages(allscalebarsare20 m)ofCNP’ssamplespreparedfromstarchwithexcitationwavelengthsat(a)365nm,(b)455nm,(c)545nmandcollectionwavelengthsat(a)>470nm,(b)>515nm,(c)600±40nm,respectively.

emitbrightandcolourfulPLcoveringtheentirevisible–NIRspectralrange.Signi cantly,theNIRPLcanbeexcitedbyNIRlight,andup-conversionPLpropertyisalsoobservedintheCNPssamples.

2.Materialsandmethods

2.1.Materials

Glucose,sucrose,starch,HCl,NaOH,hexamine,ethanol(analyticalpurity,SinopharmChemicalReagentsLimitedCompany).Theywereusedasreceived.Alltheaqueoussolutionswerepreparedusingde-ionizedwater.

2.2.Methods

CNPsweresynthesizeddirectlyfromacid/alkali-assistedhydrothermaloxidationofcarbohydrates(glucose/sucrose/starch)inwater.Carbohydrates(glu-cose/sucrose/starch)wereusedascarbonsourceandacid/alkaliasanadditive.Inatypicalprocedure,1gglucoseand0.1gNaOH(oranyothermoderateadditive)wereaddedin15mLwaterunderstirring.Thesolutionwasthentransferredintoa20mLTe on-linedstainless-steelautoclaveandwasheatedataconstanttemperatureof160 Cfor4h.Theresultingsolutionwascooledatroomtemperatureandtheup-layersolutioncontainingproductwasobtainedaftercentrifugation.Topurify,thissolutionwasheatedat100 Ctoallowtheresidualsodiumhydroxidetodissolveout(Incase,HClbeingusedastheadditive,theresidualHClisevaporatedat100

C).

Fig.3.TypicalPLspectraofCNPsobtainedfromglucose(curvea),sucrose(curveb)andstarch(curvec)withexcitationat350nm.

328X.Heetal./ColloidsandSurfacesB:Biointerfaces87 (2011) 326–332

Thiswasfollowedby ltrationofthesuspensionandthesolidwasremoved,leavingthesolutioncontainingCNPs.ThesolutionwasdriedundervacuumandtheCNPssampleobtainedwas nallykeptindeionizedwaterintheformofatransparentsolution.Here,theCNPspreparedfromglucose,sucroseandstarchwerenamedasglu-,suc-,andsta-CNPs,respectively.

ThesizeandmorphologyoftheCNPssampleswereexaminedwithaPhilipsXL30FEGscanningelectronmicroscope(SEM)andaFEI/PhilipsTechal12BioTWINtransmissionelectronmicroscope(TEM).Thestructureandchemicalcomposi-tionwerefurtherinvestigatedbyX-raydiffraction(XRD),RamanandElementaryanalyzer.Thephotoluminescence(PL)spectrawereobtainedatroomtempera-turebyusingaPerkin-ElmerLuminescencespectrometerLS50B.The uorescenceimagesofCNPswereinvestigatedunder uorescentmicroscope(LEICADM4000M).Theexcitationwavelengthswere365,455,545nmandthecorrespondingcollec-tionwavelengthsare>470nm,>515nmand600±40nm,respectively.TheUV–visabsorptionspectrawereacquiredbyaLambda750spectrophotometer.FTIRspectrawereperformedusingaNicolet360spectrometer.

Werepeatedthesynthesisandthemeasurementabovefor vetimes,andallthemeasurementdatacanbereplicatedverywell.

3.Resultsanddiscussion

Inourexperiment,afacileandone-stephydrothermalsyntheticroute(typicallywithHClorNaOHasadditives)wasemployedforthefabricationof uorescentCNPs.AsillustratedinScheme1,theCNPssampleswerepreparedfromthreedifferentcarbonsources(glucose/sucrose/starch)astheyallexhibitstrongPLinvisiblespec-tralrange,whereas,NIRPLcanonlybeobservedinthesamplessynthesizedfromsucroseandstarch.Notably,NIRemissioncanbeobtainedbyNIRexcitation.

Investigationindicatesthatallofthesamplespreparedfromdifferentcarbohydratesshowsimilarmorphology.Fig.1depictsthetransmissionelectronmicroscopy(TEM)andscanningelectronmicroscopy(SEM)imagesofCNPspreparedfromglucose,indicat-ingthattheCNPsaresphericalwithdiametersof70–100nm.The

Fig.4.VisiblePLspectraof(aandb)glu-CNPs,(candd)suc-CNPs,and(eandf)sta-CNPspreparedwithusing(a,cande)HCland(b,dandf)NaOHasadditives.

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HRTEMimageoftheCNPsisshowninFig.S1,fromwhichitcanbeseenthatthereisnolatticestructureobserved,indicatingitsamorphousnature.TheXRDpatternoftheCNPsispresented(inFig.S2).Thebroadpeaknear20 witharelativelylowintensityisattributedtoamorphouscarbon[13e]whichisconsistentwiththeresultofHRTEM.

ThePLimageofCNP’swasinvestigatedunder uorescentmicro-scope.Typicalspecimenforopticalmicroscopywaspreparedbyplacingadropoftheaqueoussolutiononacoverglassandevapo-ratingthewater.Fig.2showsthe uorescenceimagesofthesamplepreparedfromstarch,whichshowthattheCNPsarestronglyemissiveinthevisiblerangeunderUVandvisibleexcitation.Lumi-nescenceofdifferentcolours(blue,yellowandred)andbrightnessisobtainedby365nm,455nmand545nmexcitation,respectively.ThesimilarPLphenomenoncanalsobeobservedinthesamplespreparedfromglucoseandsucrose(seeSupportingInformation,Figs.S3andS4).

Tofurtherexploretheiropticalproperties,thePLspectraofas-preparedCNPswereassessed.AllPLspectraofCNPssynthesized

fromglucose,sucroseandstarchexhibitthevisibleemissionscov-eringblue-to-redwavelengthrangeinthesamesampleunderUVandvisibleexcitation.Fig.3showsthetypicalPLspectraofCNPsobtainedfromglucose(curvea),sucrose(curveb),andstarch(curvec)withexcitationat350nm.AclosedobservationshowsthatthePLemissioncanbeextendedintoNIRwavelengthrangeintheCNPssamplesobtainedfrombothsucrose(curveb)andstarch(curvec).Thesharppeaksnear700nmarethesecondorderdiffractionoftheexcitationlight.Moreover,furtherPLstudyshowsthattheseNIRPLemissionscanalsobeobtainedunderNIRexcitation(seethefollowingdiscussion).ItshouldbenotedthatNIRPLemissionsexcitedbyNIRexcitationareparticularlysigni cantandusefulforbionanotechnologybecauseofthetransparencyofbodytissuesintheNIR“waterwindow”.

InordertofurtherinvestigatetheopticalpropertiesoftheCNPs,thedetailedPLstudywascarriedoutbyusinglightatdifferentwavelengths(300,350,400,450,500nm)asexcitation.Fig.4showsthePLspectraofglu-CNPs(aandb),suc-CNPs(candd)andsta-CNPs(eandf)byusingHCl(a,cande)andNaOH(b,dandf)asadditive

Fig.5.Up-conversionPLspectraof(aandb)glu-CNPs,(candd)suc-CNPsand(eandf)sta-CNPspreparedbyusingHCl(a,cande)andNaOH(b,dandf)asadditives.

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Fig.6.PLspectraofCNPswithexcitationandemissioninNIRband.(aandb)suc-CNPsand(candd)sta-CNPspreparedwithusingHCl(aandc)andNaOH(bandd)asadditives.

reagents.Wecanseethatunderdifferentexcitation,alloftheCNPsexhibit uorescenceemissioninvisiblespectralrange.AsshowninFig.4a,candeandFig.4b,dandf,theadditives(HClorNaOH)canchangethedistributionoftheemissioncoloursoftheCNPs.Thatis,HClleadstotheincreasingdistributionofCNPsemittingatlongerwavelengthsorwarmercolours,whereas,NaOHleadstotheincreaseindistributionofCNPsemittingatshorterwavelengthsorcoldercolours.Itisreasonabletoassumethatacid/basecanintro-ducethedifferentdefectsonthecarbonparticlesurfaceactingasexcitationenergytrapsandleadingtothedifferentPLproperties.Also,wethinkthatthedifferentPLintensitywithdifferentexci-tationwavelengthsshouldbeattributedtothedifferentquantumyields(SeeSupportingInformation,TableS1).

Theup-conversionPLmaterialscanconvertalongerwave-lengthradiation(e.g.,NIRlight)toshorterwavelength uorescence(e.g.,visiblelight),whichpossessprominentpotentialsinbiologicalandclinicalapplications.Mostoftheup-conversion uorescencematerialsreportedwereinorganiccrystalsdopedwithrare-earthelements[14]orencapsulatingorganicdyesorquantumdotsinthesilicashell[15].Untilnow,therearefewmaterialswhichcandisplaybothvisibleemissionandup-conversionPL.Here,theCNPspreparedcanbeexcitedbyNIRwavelengthlightandemitbrightPLinvisiblespectralband,whichsuggestthattheseCNPshavetheup-conversionPLproperty.Fig.5showstheup-conversion uorescencespectraofglu-CNPs(aandb),suc-CNPs(candd),andsta-CNPs(eandf).Forglu-CNPsandsuc-CNPsobtainedbyusingHClasadditive,theup-conversionPLspectrashowtwopeaks(at480,580nm)whentheyareexcitedby800and850nm,whileonepeak(520nm)isseenbyexcitationat900nm(a–c).However,whenglu-CNPsandsuc-CNPspreparedbyusingNaOHasadditiveisused,thereisonlyonepeakintheup-conversionPLspectra

(b–d)withallexcitationwavelengthsinNIRspectralband.TheupconversionPLspectraofSta-CNPs(obtainedbyusingHClandNaOHasadditives)alsoshowsonlyonepeakat530nmwithNIRexcitation(Fig.5eandf).

FurtherdetailedstudyshowsthattheNIRemissionofsuc-CNPsandsta-CNPscanbeobtainedbyusingNIRlight(700and750nm)asexcitation.Fig.6showstheNIRemissionspectraofsuc-CNPs(aandb)andsta-CNPs(candd).Moredetailedandcarefullycon-ductedPLtestexperimentscon rmthattheglu-CNPscannotgiveNIRemissionwitheithervisibleorNIRexcitation.ItshouldbenotedthattheNIRemissionexcitedbyNIRexcitationofthesuc-CNPsandsta-CNPsshowgreatpotentialapplicationinbionanotechnol-ogyandbioimaging.AlthoughthemechanismforthePLdifferencesinCNPspreparedfromdifferentcarbohydratesisnotfullyunder-stood,itisbelievedthatthestructuredifferencesinglucose,sucroseandstarchisthemainreason.Glucoseisasimplesugarandmainlyexistsasthesix-memberedringcontainingahemiacetalgroupinasolution.Sucroseisadisaccharidederivedfromthecondensationofglucoseandfructose,andtheyarelinkedviaanetherbondcalledaglycosidiclinkage.Starchisacarbohydrateconsistingofalargenumberofglucoseunitsjoinedtogetherbyglycosidicbonds.Thus,underthehydrothermalcondition,theoxidationandcarboniza-tionofglucose,sucroseandstarcharedifferentfromeachotherduetotheirdifferentstructures(withorwithoutglycosidiclink-age),whichaffectthePLpropertiesoftheCNPsobtained.Thatis,CNPspreparedfromtheprecursorswithglycosidiclinkage(sucroseandstarch)haveNIRemissionwithNIRexcitation.While,CNPspreparedfromtheprecursorswithoutglycosidiclinkage(glucose)cannotgivetheNIRemission.Ontheotherhand,aspreviouslyreported,thePLofcarbonor“carbogetic”dotshasbeenattributedtopassivateddefectsonthecarbon–oxygeninterfaceactingas

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Fig.7.Photographsofsuc-CNPsundersunlightandUVlamp(365nm)withdifferentadditives(hexamine:aandd;NaOH:bande;HCl:candf,respectively).

excitationenergytraps.Inourexperiment,theglycosidiclink-agecanprovideapre-placedactionfortheformationofCNPs,whichmayleadtodifferentdefectsonthecarbon–oxygeninter-facecomparedtotheprecursorswithoutglycosidiclinkage.Thus,itisreasonablethatthedifferentprecursorsleadtodifferentPLpropertiesofCNPs.

ThechemicalcompositionofCNPshadbeenmeasuredbyele-mentalanalysis.Elementalanalysis:C59.2%,H4.2%,O(calculated)36.6%.TheCNPscontainedmainlyelementalcarbonandoxygen.TheRamanspectrum(Fig.S5)showsaDbandatabout1340cm 1andGbandatabout1580cm 1,ascribedtodisorderedamorphouscarbonandcrystallinegraphite,respectively.

Fig.S6depictsthetypicalUV–visible(UV–vis)absorptionspec-traoftheCNPs.Asshown,theCNPssamplespreparedfromdifferentcarbonsourceswithbothHClandNaOHasadditivesgivethesim-ilarabsorption.Thepeakat250–300nmisthetypicalabsorptionofanaromaticpisystem,whichissimilartothatofpolycyclicaro-matichydrocarbons[16].TheextendedconjugationinthestructureofCNPsleadstotheredshiftofthe – *transition[17].

Thetypicalinfrared(IR)spectraofCNPsareshowninFig.S7.Thepeaksaround3000,1600and1500cm 1correspondtotheCCstretchofthecarbonskeletonofcarbonnanoparticles,whichareconsistentwiththeUVspectra.Thepeaksatabout1700cm 1indicatetheexistenceofcarbonyl(Cgroups,whilethepeaksatabout1720,1200,and1080cm 1areduetocarboxylicgroups.Thepeakat3346cm 1correspondstothe–OHstretchmode[18].ThefunctionalcarboxylicandhydroxylgroupsofCNPsleadtothegoodwaterdispersibility,whichcanplayanimportantroleinfurtherapplicationforbionanotechnology[4e–g,5–11]

.

Fig.8.PLspectraofsuc-CNPsobtainedwithdifferentadditives:blue,green,andredemissionforadditivesofhexamine,NaOH,andHCl,respectively.

Asdiscussedabove,paredtoNaOH,HClleadstotheincreaseindistributionofwarmercolours.Infurtherstudies,wecarriedoutaseriesofcon-trolexperimentswithdifferentadditives,suchasHCl,NaOHandhexamine.Notably,asshowninFig.7,theemissionofsuc-CNPscanbetunedbydifferentadditives:blue,greenandredemissioncanbeobtainedwhenhexamine,NaOHandHClareusedasadditives,respectively.ThecorrespondingPLspectraofabovethreesuc-CNPssamplesareshowninFig.8.ThesimilarPLtunedbyaboveadditivescanalsobeobservedinglu-CNPsandsta-CNPssamples.

Inthepresentreactionsystem,theCNPswerepreparedfromcarbohydratebyahydrothermaltreatmentwithacid/alkaliasaddi-tives.WethinkthatthegrowthofCNPsmaysufferfromtheprocessesofcarbohydratepolymerization,carbonizationandtheformationofCNPs,whichissimilartothatreportedbySunetal.[8a]andshouldconformtotheLaMermodel[19].Themecha-nismofthePLpropertiesofCNPsisstillanopenquestion.Ithasbeenpreviouslysuggestedthatthemodi erpassivatesthesur-faceofthecarbon-basednanoparticleshelpingtogenerateenergytrapsthatemitlightwhenstimulated[8a].Besidesthismechanism,theformationofseveraldifferentpolycyclicaromaticcompoundswithintheCNPsmayalsoexplainthePLproperties[8b,9a].Inaddi-tion,theconceptandthebasicideaoftwo-photonexcitation rstdescribedbyGöppert-Mayer[20]maybesuggestiveforexplainingtheup-conversionPLbehavioroftheCNPs.Also,itcanbeexpectedthatabetteradditive(e.g.,HNO3,H2SO4,KOH,andvarioussurfac-tants,etc.)and/ormoresuitablereactionconditions(e.g.,ultrasonicpower,reactiontemperatureandhydrothermaltreatment)mayfurtherimprovethepresentreactionprocessandthePLpropertyoftheCNPs.

4.Conclusions

Insummary,thepresentworkdevelopedageneralandfacilehydrothermaloxidationstrategyforthesynthesisofCNPs,whichexhibitadequatelystable(>6months)andstrongPLinvisiblebandrange,biningtheirfreedispersioninwater(withoutanysurfacemodi cations)andattractiveup-conversionPLproperties,these uorescentCNPsareexpectedtoholdpromiseintheapplicationssuchasnewtype uorescencemarker,bio-sensors,drugdelivery,andbio-imagingoftissuesatmillimeterdepthsandtrackingofbiologicaleventsinvivo.

Acknowledgements

ThisworkissupportedbytheNationalBasicResearchProgramofChina(973Program)(No.2010CB934500),NationalNaturalScienceFoundationofChina(NSFC)(No.21073127,21071104,20801010,20803008),aFoundationfortheAuthorofNationalExcellentDoctoralDissertationofPRChina(FANEDD)(No.200929),aProjectFundedbythePriorityAcademicProgramDevelopmentof

332X.Heetal./ColloidsandSurfacesB:Biointerfaces87 (2011) 326–332

JiangsuHigherEducationInstitutions(PAPD)andRGC-CRFproject(No.9041313).

AppendixA.Supplementarydata

Supplementarydataassociatedwiththisarticlecanbefound,intheonlineversion,atdoi:10.1016/j.colsurfb.2011.05.036.

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