Thecrystallinestructures ofthe Eu-dopedSi-HA were char-acterizedbyXRDD8 Advance,Bruker,Bremen,Germany.The microstructureof the Eu-doped Si-HA wasdetermined using a fieldemissionscanningel
Trang 1j ou rn a l h o m epa g e :w w w e l s e v i e r c o m / l o c a t e / m s e b
Cao Xuan Thang, Vuong-Hung Pham∗
Keywords:
Nanosilicon
Luminescence
Hydroxyapatite
Europium
Nanophosphors
Nanobiophosphors
a b s t r a c t
Thispaperreportsthefirstattemptforthesynthesisofeuropium-dopedSi-substitutedhydroxyapatite (HA)nanostructuretoachievestrongandstableluminescenceofnanobiophosphor,particularly,by addi-tionofdifferentEudopants,Sisubstitutions,andapplicationofoptimumannealingtemperaturesofup
to1000◦C.Thenanobiophosphorwassynthesizedbythecoprecipitation,microwave,andhydrothermal methods.Thenanoparticlesdemonstratedananowiretoaspindle-likemorphology,whichwas depend-entonthemethodofsynthesis.Thephotoluminescence(PL)intensityofthesampleincreaseswith theincreaseinSisubstitutionsandEudopants.Theluminescentnanoparticlesalsoshowedthetypical luminescenceofEu3+centeredat610nm,whichwasmoreefficientfortheannealedEu-dopedSi-HA nanoparticlesthanfortheas-synthesizednanoparticles.Amongthedifferentsynthesismethods,the hydrothermalmethodrevealsthebestlightemissionrepresentedbyhighPLintensityandnarrowPL spectra.TheseresultssuggestthepotentialapplicationofEu-dopedSi-HAinstableandbiocompatible nanophosphorsforlightemissionandnanomedicine
©2015ElsevierB.V.Allrightsreserved
1 Introduction
Thedevelopmentofnewmaterialsandsynthesistechniquesfor
improvingthelightemissionperformancesandlong-termstability
ofdevicesisoneofthemostactiveresearchareasinoptoelectronic
industriesandnanomedicine[1,2].Fundamentally,thelight
emis-sionabilityofmaterialsisstronglyaffectedbytheirhostmatrices,
activators,defects,andcrystallinityofmaterials[3,4].This
encour-agedscientistsandengineerstoexplorenewmethodsofdesigning
materialsby tailoringthephysical and chemical characteristics
ofmaterials,forexample,bydopingwithstrongandstablelight
emissionmaterialsanddesigningthesuitablehostmaterialsfor
stimulatingenergytransferfromthehostmatrixtotheactivator
[5,6]
Inordertoenhancethelightemissionperformanceofthedevice
anditslifetimeforagricultureproductsandnanomedicine,it is
necessarytocombinetheadvantagesofbiocompatibleproperties
withluminescence[7,8].Asbiocompatiblematerials,silicon(Si)
andhydroxyapatite(HA)havereceivedconsiderableattentionas
hostmaterialsinthedesignofluminescentmaterials[9,10]
Intro-ductionofSitoHAresultedinsubstitutionofphosphateion(PO4 −)
bysilicate ion(SiO4 −)in theHA crystal structure,and silicon-substitutedHA(Si-HA)hasbecomeasubjectofgreatattentionin biomedicalresearch[11].Similarly,europiumisasuitable activa-torfordopingintocalcium-basedmaterialsbecausetheyexhibit significantadvantagescomparedwithavailablephosphorsuchas lowertoxicities,photostabilities,highthermalandchemical stabil-ities,highluminescencequantumyield,andsharpemissionband [12,13]
Themicrowaveandhydrothermalmethods,beingpowerfulwet chemistrysynthesismethods,havebeenwidelyusedtosynthesize nanostructuredparticlessuchassemiconductorsandmetaloxide forimprovingsurfacecharacteristic,grainsize,andcrystallinityas wellasperformanceofmaterials[14,15].Thesetechniqueshave beenintroducedto biomedicalengineering for thesynthesis of nanorod,nanospider-likeparticles,whichdemonstratedpositive effectsontheinvitrobiocompatibilityofHA[16,17].Althoughthe physicochemicalpropertiesofHAandSi-HAarewelldocumented, thusfar,only afew papers have reportedontheluminescence
of europium-doped HA [18,19],and, to thebest of our knowl-edge,noattemptshavebeenmadetosynthesizeEu-dopedSi-HA witha well-crystalline structure bythe microwaveand hydro-thermalmethods,whichwouldopenupnewavenuesfordesigning strongandstablelightemissionforagricultureandnanomedicine Recently,Eu-dopedsilicon-substitutedSi-HAwassynthesized suc-cessfullyinourlaboratorybythecoprecipitationmethod[20].In
http://dx.doi.org/10.1016/j.mseb.2015.02.014
Trang 2theluminescenceoftheEu-dopedSi-HA.Toexpandthisresearch,
wehereinreportthefirstattempttosynthesizeEu-dopedSi-HA
bymeansofwetchemistrysynthesismethodssuchasthe
copre-cipitation,microwave,and hydrothermalmethods.Theeffectof
thesiliconsubstitutions,europiumdopants,andannealing
tem-peratureaswellasthesynthesis methodonthelight emission
ofEu-doped Si-HAwascloselyexamined.Thecrystalline
struc-turesoftheEu-dopedSi-HAwerecharacterizedbyX-raydiffraction
(XRD).ThemicrostructureoftheEu-dopedSi-HAwascharacterized
byscanningelectronmicroscopy(SEM)andtransmissionelectron
microscopy(TEM),respectively.ChemicalbondingofthepureHA,
Si-HA,andEu-dopedSi-HAwasdeterminedbyinfrared(IR)
absorp-tionspectroscopy.Theluminescencewasalsodeterminedusinga
photoluminescence(PL)spectrometer
2 Experimental procedure
Eu-dopedSi-HAwassynthesizedthroughawetchemical
syn-thesismethod,asfollows:10.620gofCa(NO3)2·4H2O(99%purity,
Aldrich,SaintLouis,MO, USA) and5.625gof (NH4)2HPO4 (99%
purity,Aldrich,SaintLouis,MO,USA) weredissolvedin100mL
ofdistilled water (DW)and theconcentrationsof thesolutions
werefixedin allsetsof experiments.Forstudyingtheeffectof
Siconcentration,afixedamountof0.7gofEu(NO3)3dissolvedin
50mLofDWwasmixedwiththeabovementionedCa(NO3)2·4H2O
at25◦Cfor60mintoformsolutionA.Varyingamounts(2,4,and
6g)ofSi(OCH2CH3)4 (tetraethyl orthosilicate,TEOS;99%purity;
Merck,Darmstadt,Germany)weredilutedin50mLofDWandthen
mixedwiththeabovementionedsolutionof(NH4)2HPO4at25◦C
for60mintoformsolutionB.Finally,solutionBwasaddedatan
additionrateof20mLmin−1intosolutionAat80◦C.Afterthe
addi-tion,thereactionmixturewasfurtherstirredfor0.5hat80◦Cand
thepHwasadjustedto11byusinganaqueousammoniasolution
(DucGiangChemicals,Hanoi,Vietnam).Forcomparisonpurposes,
un-dopedHAwasalsopreparedwiththatmentionedabove
with-outadditionofTEOSorEu(NO3)3at80◦CandpH11.Thesample
withdifferentSisubstitutionswaspreparedaccordingtotheabove
procedurewithafixedamountof Eu(NO3)3 (0.7g)and varying
amountsofTEOS(2,4,and6g),hereafterdesignated0.7Eu:2Si-HA,
0.7Eu:4Si-HA,and0.7Eu:6Si-HA.Similarly,thesamplewith
differ-entEudopantswaspreparedwithafixedamountofTEOS(4g)and
varyingamountsofEu(NO3)3(0,0.5,0.7,and1g),hereafter
desig-nated0.5Eu:4Si-HA,0.7Eu:4Si-HA,andEu:4Si-HA.Formicrowave
synthesis,theabovementionedsolutionsAandBweremixed,and
theirpHwasmaintainedat6byadditionofammoniasolutionata
temperatureof25◦C.Afterthereactionmixturewasstirredfor0.5h
atatemperatureof25◦C,themixturewassubjectedtomicrowave
apparatus(LGMS3840SR;80W;LGMicrowave;Seoul,Korea)for
30min.Forhydrothermalsynthesis,thereactionsolutionprepared
inthesamemanneratpH6andatatemperatureof25◦Cwas
trans-ferredintoa200-mLTeflon-linedautoclave,andthentheautoclave
wassealedandmaintainedat150◦Cfor12h.Theresulting
pre-cipitateswerewashedthrice,andthendriedat100◦Cfor6h.A
fractionofeachsamplewastreatedat1000◦Cwiththeheating
rateof6◦min−1for1hinair
Thecrystallinestructures ofthe Eu-dopedSi-HA were
char-acterizedbyXRD(D8 Advance,Bruker,Bremen,Germany).The
microstructureof the Eu-doped Si-HA wasdetermined using a
fieldemissionscanningelectronmicroscope(JSM-6700F,JEOLLtd.,
Tokyo,Japan)andatransitionelectronmicroscope(JEOL,JEM1010,
JEOLTechniques, Tokyo,Japan), respectively Toinvestigatethe
chemical bondingof theEu-dopedSi-HA, IRabsorption spectra
wererecordedinthewave-numberrangefrom4000to400cm−1
withaPerkin-ElmerSpectrumBXspectrometerusingKBrpellets
PLtestswereperformedtoevaluatetheopticalpropertiesofthe Eu-dopedSi-HA.ANANOLOGspectrofluorometer(Horiba,Edison,
NJ,USA)equippedwitha450-WXearclampanddouble-excitation monochromatorswasused.ThePLspectrawererecorded automat-icallyduringthemeasurements
3 Results and discussion
3.1 Phasecharacterization Fig.1shows theXRD diagramoftheas-synthesizedHA and Eu-dopedSi-HApreparedbythecoprecipitation,microwaveand hydrothermalmethodsandtheirthermalannealingsamples.The coprecipitation specimenshowed peaks matchingthe standard patternsofCa10(PO4)6(OH)2,calciumHA(PDF01-084-1998).On theotherhand,amixtureofHA and-TCP (PDF09-0169)was observedinthemicrowaveandhydrothermalspecimen(Fig.1(B) and(C)).Whenthermalannealingwasused,allofthethreeXRD patternsshowedamixtureofHAand-TCPwithgoodcrystallinity (Fig.1(D)–(F)).Thissuggeststhatthephasecharacteristicof Eu-dopedSi-HAcanbecontrolledbychangingthesynthesismethod
orbyapplyingthermalannealing.Itiscanalsobeseenthatthe XRD diagrams obtainedfor all of theEu-doped Si-HA samples
do notrevealthepresence ofanyphasesrelated tosiliconand othereuropiumspecies,suggestingthesuccessfulpreparationof europium-dopedSi-HA.Basedonspecificapplication,controlling thephasecompositionofcalciumphosphateisofparticular inter-est.ThemixtureofHA and-TCPfoundonthespecimens may suggestabetterperformanceinthefieldsofoptoelectronicsand nanomedicinebecauseofthestrongluminescence,high resorba-bility properties for -TCP, and good osteoconductivity for HA [20–22]
Fig.2showstheXRDdiagramoftheas-synthesizedEu-doped
Si-HApreparedbycoprecipitationwithdifferentTEOSandEu(NO3)3 concentrations.Allofthespecimensshowedatypicalpatternof
HA(Fig.2(A)–(E)).ThissuggeststhatthephasecharacteristicofHA remainsstableinthisstudyonintroductionofdifferentTEOSand Eu(NO3)3concentrationsinthereactionsolution
Trang 3Fig 2. XRD patterns of the Eu-doped Si-HA prepared by the coprecipitation method
3.2 Scanningelectronanalysis
ThemicrostructuralvariationsinEu-dopedSi-HAwere
exam-ined by SEM as shown in Fig 3(A)–(F) It can be seen that
theEu-doped Si-HAnanoparticles have a wireshape and their
aspectratiosdropwhentheTEOSconcentrationincreasesinthe
0.7Eu:2Si-HA, 0.7Eu:4Si-HA, and 0.7Eu:6Si-HA samples,
respec-tively(Fig.3(A)–(C)).ThereductionintheaspectratiosofSi-HA
withincreaseinTEOSconcentrationobservedinthepresentand
otherstudiescanbeexplainedintermsofthehighernucleation
densityduringtheprecipitationprocess[23].However,theeffect
ofEu(NO3)3concentrationonthemorphologyofEu-dopedSi-HA
wasmuchstrongercomparedtothatoftheTEOSconcentration
ThesamplepreparedatlowEu(NO3)3concentration(the
0.5Eu:4Si-HAsample)consistsofwell-dispersednanowires(Fig.3(D)).When
the Eu(NO3)3 concentration increased (the Eu:4Si-HA sample), thesynthesizedparticleassumed arod-likeshape witha parti-clesizeofabout30nmandalengthof<100nm(Fig.3(E)).The significantmorphology differences with varying Eu(NO3)3 con-centrationsinthespecimenssuggestthatEu(NO3)3concentration playsanimportantroleincontrollingthegrowthand crystalliza-tionprocess The ionicradius ofEu3+ ion is documentedtobe smallerthanthatforCa2+[24,25].Thismaybeattributedtothe smallersizeandnarrower-sizeddistributionobservedbyusand othersinEu-dopedHA-basedcompounds[7,13,25].Similarly,itcan
beseenthatthetrendlinesofthechangeinresultsfortheeffect
ofEu(NO3)3concentrationandsynthesismethodwerethesame, namelytheaspectratiosoftheparticledecreasedinthesample pre-paredbyahydrothermalmethod(Fig.3(F)).Itiswelldocumented thatthetemperatureplaysanimportant rolein controllingthe morphologyofnanoparticlesinthesolution-basedgrowthprocess Thehomogeneousheatingfromthemicrowaveorhydrothermal methodcanenhancetheprobabilityofnucleargrowthandeach facetofthenucleihasalmostthesameprobability,resultingin weaklyanisotropicgrowth;thatis,thespecimenshavealowaspect ratio,asshowninFig.3(F)[26,27].It isgenerallyacceptedthat tailoringthemorphologyofHA-basedmaterialsoftenrequiresthe useofsurfactants(e.g.cetyltrimethylammoniumbromide(CTAB)), whichisknowntobecytotoxictocertaincellsevenatlow lev-els[7,28].ThelowaspectratiomorphologyoftheEu-dopedSi-HA synthesizedherehaspotentialapplicationinbiomedicineasthis lowaspectratiofacilitateseasierentryintocellscomparedtoother morphologiesofparticles[18]
3.3 IRanalysis Fig.4(A)–(D)showsthetypicalIRspectraofthepureHA,Si-HA, andEu-dopedSi-HApreparedbythecoprecipitationmethodwith differentTEOSconcentrations.TheIRspectraofEu-dopedSi-HA presentcharacteristicsofHA.AllIRspectraillustratethe charac-teristicofPO4 −at1102,1029,962,603,and563cm−1associated withHA[23,29].Theappearanceofthebandat868cm−1isrelated
totheSiO4 −group,henceregardedasaspecificfeatureofSi-HA [11,23].Thepeakat632cm−1 isassignedtothepresenceofthe
Trang 40.7Eu:4Si-Fig 4.IR spectra of (A) pure HA, (B) 4Si-HA, (C) 0.7Eu:2Si-HA, and (D) 0.7Eu:4Si-HA
OH− groupin thesample[30].Thesmallbandlocatedatabout
804cm−1 may account for the presence of a small amount of
HPO4 −in thecrystalofHA [22].Thepeak atabout1383cm−1
is attributed tothe vibration modeof NO3− and is introduced
duringtheprecipitationprocess[31].Inthis experiment, dilute
HNO3 was used asa reagentto dissolve Eu2O3,and allof the
startingmaterialsarenitratesalts.TheappearanceofNO3−inthe
IRspectrummightbecausedbytheinclusionoftheresidualNO3−
inthelatticeof thesample.Thebroad bandaround3449cm−1
andtheabsorptionbandat 1634cm−1 areassigned totheOH−
ionsandtheO–HvibrationoftheH2Omoleculeabsorbedinthe
sample.TheintensitiesoftheOH−groupdecreasewithincreasing
TEOSconcentration.Theseresultsindicatethatthephosphatesite
wassubstitutedbySiintheHAstructure
3.4 EffectofTEOSconcentrations
Fig.5(A)and(B)showstheemissionspectraofun-dopedHA
withdifferentsynthesis methodsandEu-doped Si-HA
nanopar-ticlesmonitoredat393nm.Alltheun-dopedHAshowedstrong
visibleemissionpeaksappearingatabout500nmwithabroadband
spectrum(Fig.5(A)).Inaddition,itcanbeseenthattheintensity
ofthePLincreasedwiththesamplepreparedbythemicrowave andhydrothermalmethods.Itiswelldocumentedthat lumines-cencefromanactivator(suchasrareearthion)showedthetypical narrowspectrum.However,inthisstudy,thebroadbandspectrum canbeattributedtothedefectsorelectronicstateasaluminescent centerfortheself-activatedluminescenceinthematrixoftheHA [32,33].Fig.5(B)showstheemissionspectraoftheEu-dopedSi-HA preparedbythecoprecipitationmethodwithdifferentTEOS con-centrations.AlltheEu-dopedSi-HAshowedstrongvisibleemission peaksappearingatabout590,616,650,and700nm,whichcanbe attributedtothe5Do–7F1,5Do–7F2,5Do–7F3,and5Do–7F4 transi-tionswithinEu3+ ion,respectively.However,itshouldbenoted thattherelativePLintensityofthesampleincreasedbyincreasing theinitialTEOSconcentrationinthereactionsolution.Itisgenerally notedthatthepresenceofsiliconmayacceleratethebreakdown processintheOH−groupinHA,resultinginincreasingPL inten-sities[13,19,23,34].Theobservedhigherluminescentemissionof Eu-dopedSi-HAwithdifferentTEOSconcentrationsisrelatedto theabsenceoftheOH−groupthatconstitutespossiblequenching centersfortheluminescence,whichcanbeconfirmedbyIRresults 3.5 EffectofEu(NO3)3concentrations
Fig.6(A)and(B)showstheemissionspectraofun-dopedSi-HA withdifferentsynthesismethodsandEu-dopedSi-HA nanoparti-cleswithvaryingEu(NO3)3concentrationsmonitoredat393nm Similartotheeffectofsynthesisontheluminescenceofun-doped
HA,theluminescenceofun-dopedSi-HAalsoshowedstrong vis-ibleemissionpeaksappearingatabout500nmwithabroadband spectrum(Fig.6(A)),whichmayberelatedtodefectsorelectronic stateintheSi-HAmatrix.Furtherexperimentsareunderwayto elucidate theexact luminescencemechanism inSi-HA.Fig.6(B) showstheemissionspectraofEu-dopedSi-HAnanoparticleswith varyingEu(NO3)3concentrationsmonitoredat393nm.Itcanbe seenthattheintensityofthePLincreaseswithincreasingEu(NO3)3 concentrationsinthereactionsolution.Thisresultsuggestedthat enoughdistanceexistsbetweenneighboringEuionsinthehost matrixattheseEuconcentrations,whichpreventsnonradioactive concentrationquenchingtherebyenhancingPL
3.6 Effectofsynthesizedmethods Fig.7showstheemissionspectraofEu-dopedSi-HA nanopar-ticlesmonitoredat393nmwithdifferentsynthesis methods.It
Trang 5Fig 6. Photoluminescence spectra of un-doped 4Si-HA and Eu-doped Si-HA with different Eu(NO 3 ) 3 concentrations: (A) un-doped Si-HA with different synthesis methods
canbeseenthattheintensityofthePLincreasedwiththesample
preparedby the microwave and hydrothermal methods It has
beenwelldocumentedthatthecoprecipitationmethodgenerally
creates amorphous or less crystalline materials, whereas the
microwaveandhydrothermaltechniquesallowforthecreationof
awell-crystallinestructureviahighertemperature[23,27,35].The
increaseinPLintensitiesshouldbemainlyduetotheirdifferent
crystallinities, which wasalso consistent with theXRD results
(Fig.1)
ThePLoftheEu-dopedSi-HAwithvarioussynthesismethods
wasfurthercharacterizedbythermalannealingupto1000◦Cin
air,asshowninFig.8.SimilartothePLemission characteristic
ofEu-dopedSi-HAsynthesizedbythreemethodswithoutthermal
annealing,theluminescentemissionintensitywasincreased
sig-nificantlyonapplicationofthermalannealingupto1000◦C.Among
thedifferentsynthesismethods,thehydrothermalmethodsreveals
thebestlightemissionrepresentedbyhighPLintensityandnarrow
PLspectra.Itisgenerallyacceptedthatwhentheenergytransfer
fromthematrixtotheactivatorisoptimized,thephosphorwill
emitstrongemission withasharp andnarrow bandspectrum;
otherwise,theywillemitrelativelyweakintensitywithabroad
emissionband[1,20]
Fig 9 illustrates the morphology of the power in the as-synthesizedstateandthermalannealingof1000◦C
The coprecipitation specimen showed a long nanowire microstructure(Fig.9(A))withasmoothsurface,asisoftenthe case with HA synthesized without surfactants [36,37] On the otherhand,alargenumberofrelativelyrectangulargrainswere observedonthemicrowavespecimen(Fig.9(B)).Withoutthermal annealing,thehydrothermalspecimenshowedspindle-like mor-phology,aspresentedinFig.9(C).Ontheotherhand,whenthermal annealing wasapplied, a change froma spindle-liketo a rela-tivelyroundedmorphologyandanincreaseinitsgrainsizewere observed(Fig.9(D)).Thissuggeststhattheparticlemorphology
oftheEu-dopedSi-HAcanbecontrolledbychangingthe synthe-sismethodorapplyingthermalannealing.It iswellknownthat thelowaspectratioofparticle(sphericalornear-spherical) mor-phologyofthematerialsissuitableforhighbrightnessandhigh resolutionbecauseofthehighpackingdensityandreductionof lightscattering,whichofferthepossibilityofenhancing lumines-cence[12,18,38].Here,thesamplemorphologychangedfromlong nanowireparticletorectangularandnear-sphericalparticlewhen thesamplesweresynthesizedbythemicrowaveandhydrothermal methods,respectively.TheincreaseinPLintensitiesshouldalsobe
Trang 6Fig 9.TEM image of the 0.7Eu:4Si-HA sample prepared by (A) coprecipitation, (B) microwave, (C and D) hydrothermal methods, (A–C) before and (D) after annealing at
duetothenear-sphericalmorphology.However,otherfactorssuch
ascrystallinityandphasecompositionofspecimensshouldalsobe
takenintoaccount
4 Conclusions
Threesynthesismethodssuchascoprecipitation,microwave,
andhydrothermalprocesseshavebeenproposedforthe
synthe-sisofluminescentEu-dopedSi-HA.Themorphologyandparticle
sizesoftheEu-dopedSi-HAcanbetunedbythesynthesismethod
orthermalannealingupto1000◦C.ThePLintensityofthe
sam-pleincreaseswithincreasingTEOSandEu(NO3)3 concentrations
inthereactionsolutionwiththecharacteristicemissionofEu3+
Amongthedifferentsynthesismethods,thehydrothermalmethod
revealsthebestlightemission,whichwasmoreefficientforthe
annealedEu-dopedSi-HAnanoparticlesthanfortheas-synthesized
nanoparticles.ThisenhancementofthePLwasmainlyattributed
totheparticlemorphologyandwell-crystalline materialviathe
hydrothermalmethod.Thesephosphorsshowpotential
applica-tionin agriculture suchasplant cultivationand nanomedicine,
whichrequireacombinationofbiocompatibilityandlight
emis-sion
Acknowledgment
Thisresearchis funded by VietnamNationalFoundation for
Scienceand Technology Development (NAFOSTED) under grant
number103.99-2013.05.
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