Au ZnO Core Shell Nanoparticles Are Efficient Energy Acceptors with Organic Dye DonorsAu,ZnO复合

Au@ZnO Core -Shell Nanoparticles Are Ef?cient Energy Acceptors with Organic Dye Donors

Krishna Kanta Haldar,Tapasi Sen,and Amitava Patra*

Department of Materials Science and Center for Ad V anced Materials,Indian Association for the Culti V ation of Science,Kolkata 700032,India

Recei V ed:April 11,2008;Re V ised Manuscript Recei V ed:May 26,2008

The present study highlights the ef?cient ?uorescence resonance energy transfer from Rhodamine 6G dye to Au@ZnO core -shell nanoparticle by steady state and time-resolved spectroscopy.The calculated energy transfer ef?ciencies from dye to nanoparticles are 41.3,52.6,and 72.6%for Au,mixture of Au and ZnO,and core -shell Au@ZnO nanoparticles,respectively.There is a pronounced effect on the PL quenching and a shortening of the lifetime of the dye in the presence of Au@ZnO core -shell nanoparticle which is associated with high charge storage capacity.The nonradiative decay rates are 2.80×108,3.90×108and 7.67×108s -1for pure Au,mixture of Au and ZnO and core-shell Au@ZnO nanoparticles,respectively,indicating the resonance energy transfer process.The calculated Fo ¨rster distances (R 0)are 135.0and 144.4?for Au,and core-shell Au@ZnO nanoparticles,respectively and corresponding the calculated distances (d )between the donor and acceptor are 143.05,and 123.5?.Considering the interactions of one acceptor and several donors,the calculated average distances (r n )between the donor and acceptor are 89.2and 77.2?for Au and core-shell Au@ZnO nanoparticles,respectively.However,the distances between the donor and acceptor are 88.2and 67.6?for Au and core-shell Au@ZnO nanoparticles,respectively,using the ef?ciency of surface energy transfer which follows a 1/d 4distance dependence between donor and acceptor.On the basis of these ?nding,we may suggest that surface energy transfer process has a more reasonable agreement with experimental ?nding.

Introduction

Over the past few years,the study of energy transfer between quantum dots (QD)and their molecular conjugations has proven to be a very useful tool in many biological studies.1–5Quantum dots (QD’s)are excellent donors in ?uorescence resonance energy transfer (FRET)based applications due to their narrow emission and broad excitation spectra to reduce background.Medintz et al.1a reported the potential of luminescent semicon-ductor quantum dots for development of hybrid inorganic-bio receptor sensing materials.They also reported that the nonra-diative quenching of the QD’s emission by proximal Au nanoparticle is due to long-distance dipole-metal interactions.1c They have examined the use of luminescent CdSe-ZnS QD’s as energy acceptors in FRET based assays with organic dyes as energy donors in QD’s-dye labeled protein conjugates.Burda et al.5a observed non-Fo ¨rster type energy transfer behavior in QD-phthalocyanine conjugates and they also reported the surface effects on QD-based energy transfer.5b In most cases,the energy transfer in QD conjugates is discussed as a FRET process.Fo ¨rster resonance energy transfer (FRET)is a powerful method to determine the distance between donor and acceptor ?uoro-phores.FRET occurs when the electronic excitation energy of a donor ?uorophore is transferred to a nearby acceptor molecule and the transfer ef?ciency increases with increasing the spectral overlap between the donor emission and acceptor absorption.FRET occurs through the dipole -dipole interactions between an excited donor (D)molecule and an acceptor (A).The ef?ciency of FRET depends on the distance of separation

between donor and acceptor molecules.According to the Fo ¨rster

theory,the rate of energy transfer is given by

6k T (r ))

1τD (R 0

r

)

6

(1)

where τD is the lifetime of the donor in the absence of the acceptor,r is the distance between the donor and acceptor,and

R 0is known as the Fo

¨rster distance,the distance at which the transfer rate k T (r )is equal to the decay rate of the donor in absence of the acceptor.Length scale for detection is limited by the nature of dipole -dipole mechanism in FRET based method.Recently the energy transfer between Au nanoparticle and dye provides a new paradigm for design of optical based molecular ruler for long distance measurement.Therefore,gold nanoparticles are used to be acceptors in biophysical experiments in vitro as well as in vivo.Several studies on theoretical and experimental have been published on energy transfer from a dye to metal surface and they have demonstrated the mechanism of dye quenching at a metal surface and the separation of donor and acceptor is d -4dependence.7–10According to their model,9the exact form of dipole-surface energy transfer (SET)rate is given by

k SET )

1τD (d 0

d

)

4

(2)

Therefore,SET process is a useful spectroscopic ruler for long distance measurement which will help to understand the large scale conformational dynamics of complex biomolecules in macroscopic detail.Dulkeith et al.8showed the change of radiative and nonradiative decay rates of the chemically attached dye molecules with different sized gold nanoparticles.Strouse

*To whom correspondence should be addressed.E-mail:msap@iacs.res.in.Phone:(91)-33-2473-4971.Fax:(91)-33-2473-2805.

J.Phys.Chem.C 2008,112,11650–11656

1165010.1021/jp8031308CCC:$40.75 2008American Chemical Society

Published on Web 07/11/2008

et al.7reported the surface energy transfer(SET)from dye to DNA attached Au-nanoparticle and the energy transfer process follows1/d4distance dependence.7,8We also reported the energy transfer process from dye to different shaped Au nanoparticle by steady state and time-resolved spectroscopy.10To the best of our knowledge,there has been no study on the energy transfer from dye to core-shell nanoparticle.Such core-shell(metal-semiconductor)multifunctional nanoparticles may have great potentials for optical-based molecular ruler because this core-shell nanoparticle showed unusual charge storage behavior.Recently, core-shell nanoparticle,particularly metal-semiconductor nano-composite has received signi?cant attention for photocalatytic properties,storage element and solar energy conversion.11,12 Important progress has been made in chemical synthesis of core-shell-type Ag/TiO2,Au/TiO2,Au/SnO2,Fe2O3/Au for potential applications.12–17Kotov et al.18reported the biologically inspired superstructures made from metal NPs and semiconduc-tor NWs.In particular,to the best of our knowledge,there has been no study on the energy transfer from Rhodamine6G to Au@ZnO core-shell nanoparticle.The main motivation for this work is to prepare water soluble core-shell type Au@ZnO nanoparticle and study their energy transfer from Rhodamine 6G to Au@ZnO core-shell nanocomposite by steady state and time-resolved spectroscopy.Such Au@ZnO multifunctional nanoparticles should have great potentials for optical-based molecular ruler because this core-shell nanoparticle showed high electronic storage capacity at Au nanoparticle.We are addressing the following issues:Can these core-shell nano-particles are ef?cient in energy transfer?What kind of mech-anism(static or dynamic)is involved?How are the radiative and nonradiative decay rates of the dye molecule changed? Finally,we compare their ef?ciency with pure gold and a mixture of Au and ZnO nanoparticles.All investigations are done in aqueous solution in order to match biological conditions. Of particular interest to our research program is how the physical properties vary with core-shell structure with the hope that such knowledge will enable us to construct ef?cient nanomaterials for the applications in chemical sensing or biological imaging. Experimental Procedures

Zinc nitrate hexahydrate puri?ed(from Merck),sodium hydroxide pellets puri?ed(from Merck),and tri-Sodium citrate dehydrate(from Merck)were used as received. Preparation of Water Soluble Au@ZnO Core-Shell.To prepare water soluble and stable Au@ZnO core-shell nano-composite,a novel colloidal approach is presented.A total of 0.03g of zinc nitrate hexahydrate(10mM)was dissolved into 10mL of distilled water and formed a white precipitate of zinc hydroxide[Zn(OH)2]after adding2mL(2M)of sodium hydroxide solution.Then,an excess amount of sodium hydrox-ide solution was added to dissolve the white precipitate to form sodium zincate(Na2ZnO2).The reaction equations3and4are given in below.

Zn(NO3)2·6H2O+NaOH f Zn(OH)2+NaNO3+H2O

(3)

Zn(OH)2+NaOH f Na

2ZnO

2

+H

2

O(4)

Then,1mL of sodium zincate(10mM)solution was diluted to5mL.This solution was added to as prepared0.2mM Au colloid10a(reported in our previous work)under stirring condi-tion for3-4min.Finally,this solution was kept at90-95°C for30min under stirring condition to prepare Au@ZnO core-shell nanocomposites(eq5).Au-citrate+Na

2

ZnO

2

+CO

2

(from air)f Au@ZnO core-shell+Na2CO3(5) A total of1mL of1μM Rhodamine6G aqueous dye solutions was added to3mL of Au@ZnO core-shell nano-particle solution,and the solution was kept for1day for stabilization.Similarly,1mL of1μM Rhodamine6G aqueous dye solutions was added to3mL of0.2mM Au nanoparticle solution.A total of1mL of1μM Rhodamine6G aqueous dye solutions was added to a mixture of1.5mL(0.2mM)of Au nanoparticle and1.5mL(0.5mM)of ZnO nanoparticle solution The mixture of Au and ZnO solution is stable for few hours. All dye containing solutions were used for optical study. The transmission electron microscopy(TEM)images were taken using a JEOL-TEM-2010transmission electron micro-scope operating voltage at200kV.Room temperature optical absorption spectra were obtained with an UV-vis spectropho-tometer(Shimadzu).The emission spectra of all samples were recorded in a Fluoro Max-P(HORIBA JOBIN YVON)Lumi-nescence Spectrometer.For the time correlated single photon counting(TCSPC)measurements,the samples were excited at 405nm using a picosecond diode laser(IBH Nanoled-07)in an IBH Fluorocube apparatus.The typical fwhm of the system response using a liquid scatter is about90ps.The repetition rate is1MHz.The?uorescence decays were collected at a Hamamatsu MCP photomultiplier(C487802).The?uorescence decays were analyzed using IBH DAS6software.The following expression was used to analyze the experimental time-resolved ?uorescence decays,P(t):

P(t))b+∑

i

n

R

i

exp(-t/τi)(6)

Here,n is the number of discrete emissive species,b is a baseline correction(“dc”offset),and R i andτi are the pre-exponential factors and excited-state?uorescence lifetimes associated with the i th component,respectively.

Results and Discussion

Transmission Electron Microscopy.Figure1a shows the TEM picture of Au@ZnO core-shell nanoparticles which represents the surface coating of nanocrystals.It is clearly seen from Figure1b that Au nanoparticle is coated with ZnO and the measured thickness of the shell is2.4nm.The FFT pattern (Figure1c)for shell con?rms the plane(101)of ZnO and the FFT pattern(Figure1d)con?rms the plane(111)of Au nanoparticle.The HTEM image(Supporting Information,Figure S1a)was taken from a mixture of Au and ZnO nanoparticles. It is seen from the picture that Au and ZnO particles are separated out as indicated in the picture(S1).Again it is con?rmed(Figure S1b)that there is no surface coating on Au nanoparticle.

Steady-State Study.Figure2shows the absorption spectra of aqueous solution of pure Au,pure ZnO,a mixture of Au and ZnO nanoparticles,and Au@ZnO core-shell nanoparticles. The solution of Au and ZnO nanoparticles mixture is stable for few hours whereas the solution of Au@ZnO core-shell nanoparticle in aqueous solution is stable for few months.The plasmon band centered at518,518,and537nm are for pure Au,a mixture of Au and ZnO nanoparticles,and Au@ZnO nanoparticles,respectively.The band position is same for pure Au and mixture of Au and ZnO nanoparticles solution.However, the plasmon band is shifted from518to537nm for Au@ZnO nanoparticles.The absorption peaks at367,367,and351nm

Au@ZnO Core-Shell Nanoparticles J.Phys.Chem.C,Vol.112,No.31,200811651

are due to excitonic band 11c for pure ZnO nanoparticles,a mixture of Au and ZnO nanoparticles,and Au@ZnO core -shell nanoparticles,respectively.Again,this band position is same for pure ZnO and mixture of Au and ZnO nanoparticles.The blue shifting of the excitonic band (ZnO)with respect to pure ZnO (367nm)is obviously due to the thin shell of ZnO on Au nanoparticles.As shown earlier,the high dielectric constant of the TiO 2and SnO 2shell causes a red shift in the plasmon absorption of the Au core.11a,12In the core -shell structure,the plasmon peak position of the metal core is given by

λ){λp [εR +2n H 2O 2+2g (n ZnO 2-n H 2O 2)/3]

}1?2

(7)

where n is refractive index of the surrounding medium,εR is high frequency of the core metal,g is the volume fraction of shell layer,λis the estimated peak position of metal core,and λp is bulk plasma wavelength which is de?ned by

λp )[4π2c 2m eff ε0/Ne 2]1?2)130.9nm for Au

(8)

where the m eff is effective mass of the free electron of the metal and N is electron density of metal core.The refractive index of ZnO is 1.92which is greater than water (n )1.33),and the plasmon absorption shows a red shift.In the case of Au@ZnO core -shell nanoparticles,the calculated value is 539.4nm using volume fraction of shell layer (g)unity.The calculated value is close with the observed value (537nm).This shifting of plasmon absorption band is exploited to monitor the concentration of electrons in the Au core.This shifting is due to storage of electrons within the core,rather than within the semiconductor shell.

It is well-known that the Fermi levels of two components equilibrate when metal nanoparticles come in contact with a

charge semiconductor.19The Fermi level of Au is more positive (E F )0.4V vs NHE)than the conduction band energy of ZnO (E CB )-0.5V vs NHE),therefore,the charge transfer from the excited ZnO to Au nanoparticles would be thermodynami-cally favorable.The processes that lead to storing of electrons in the Au core are summarized below.

ZnO f ZnO (e +h )

(9)ZnO (e )+Au f ZnO +Au (e )(10)

Therefore,Au -ZnO nanocomposite shows unusual charge storage behavior.In order to estimate the capacitance of the core -shell particles,16the following equation is used:

C )4πε0ε(r /d )(r +d )

(11)

where ε0is the permittivity of the free space,εis the static dielectric constant of the shell which is taken to be 8.87.From the TEM images,the calculated r Au value is 4.0nm and d ZnO value is 2.4nm,and the capacitance value is 11.0aF (attofarad)for Au@ZnO core -shell nanoparticle,indicating more electrons are needed to raise Fermi level.Lee et al.16reported the change of capacitance from 0.31to 3.42aF with increasing the core size of Au.

The emission spectrum of aqueous solution of unbound R6G dye (pure)overlaps very well with the absorption spectra of Au nanoparticles containing solution,as shown in Figure 3.It is well-known 6that the energy transfer depends on the spectral overlap between donor emission and acceptor absorption.The photoluminescence (PL)peak at 551nm is due to R6G dye.A drastic quenching (50%)in PL intensity of R6G emission in presence of gold nanoparticles is observed,which must

be

Figure 1.Low resolution TEM images of Au@ZnO core -shell (a),HRTEM of Au@ZnO core -shell (b),FFT pattern of ZnO shell (c),and FFT pattern of Au core

(d).

Figure 2.Absorption spectra of pure ZnO (a),Au and ZnO mixture (b),Au@ZnO core -shell (c),and pure Au

(d).

Figure 3.Photoluminescence (PL)spectra of Rhodamine 6G (1μM R6G)dye solution (a),pure Au and 1μM R6G (b),Au and ZnO mixture and 1μM R6G nanoparticles (c),Au@ZnO core -shell with 1μM R6G (d),and the absorption spectrum of pure Au (e).

11652J.Phys.Chem.C,Vol.112,No.31,2008Haldar et al.

related to space interaction of dipole of the donor and the free electrons of metal nanoparticles.7In presence of aqueous solution of Au and ZnO nanoparticle mixture,58%PL quench-ing of R6G is observed.The shift of the PL peak to 554nm is observed.However,75%PL quenching of dye is observed in the presence of Au@ZnO core -shell nanoparticles which reveals that the signi?cant PL quenching is observed in the presence of Au@ZnO core -shell nanoparticles.To understand the quenching process,the measurement of ?uorescence inten-sity with varying quencher (particle)concentration (1.14×10-8to 6.84×10-8M)is important.The concentration of quencher (particle)in aqueous solution was determined from the total amounts of Au and ZnO and taking into account their sizes.20A gradual quenching of the ?uorescence intensity of Rhodomine 6G with increasing the concentration of Au@ZnO core -shell nanoparticle (quencher)is seen in Figure 4a.Based on the relationship between collisional quenching of excited states and quencher concentration,the Stern -Volmer equation is given by 6

F 0

F

)1+K q τ0[Q ])1+K SV [Q ](12)

where F 0and F are the relative ?uorescence intensity in absence and presence of quencher,respectively.K SV is the Stern-Volmer dynamic quenching constant and [Q]is the concentration of the quencher.Figure 4b shows the Stern -Volmer plots,F 0/F versus quencher concentration i.e.[Au@ZnO nanoparticle].After careful analyzing by the Stern -Volmer kinetic model,

both dynamic and static mechanisms to the dye quenching were found.The intensity Stern-Volmer plot for quenching by Au@ZnO nanoparticles shows clear upward curvature.The static and dynamic quenching constants can be determined by a plot of K app versus concentration of quenchers.The slope (S )and intercept (I )were found to be 1.20×1014M -2and 32.95×106M -1,respectively (as shown in insert Figure 4b).Therefore,the dynamic and static quenching constants are 2.88×107and 4.15×106M -1,respectively,for Au@ZnO nanoparticles.Similarly,the dynamic and static quenching constants are 9.6×106and 3.8×106M -1,respectively for Au nanoparticles (Supporting Information,Figure S2),which is consistent with an earlier report.20

Hereafter,we also estimate the ef?ciency of the quenching process by the value E )1-F DA /F D ,where F DA is the integrated ?uorescence intensity of dye in presence of Au@ZnO core -shell nanoparticles and F D corresponds to the integrated ?uorescence intensity of dye in absence of Au@ZnO core -shell nanoparticles.Figure 5shows the ef?ciency of the quenching of dye as a function of the ratio between the molar concentra-tions of donors and acceptor (x )C D /C A )in water solution.It is clearly seen from the Figure 5that the ef?ciency of quenching process increases monotonically with decreasing in x .It is noteworthy that the very effective quenching of dye character-ized by E )0.75was observed for assemblies with x equal to 14.It reveals that one acceptor Au@ZnO core -shell nanopar-ticle assembles with 14donor dye molecules.Figure 6shows the ef?ciency of the quenching of dye as a function of the ratio between the molar concentrations of donor and acceptors (Au only)in water solution.In presence of Au nanoparticles,the maximum ef?ciency obtained when the x value is equal to 17.It found that the nanoparticles (core -shell or pure)form different assemblies comprising several donors per acceptor,probably due to Coulomb attraction which may vary with changing the charges of nanoparticles as suggested by Nabiev et al.20The presence of several donor molecules associated with each Au nanoparticle contributes signi?cantly to the high quenching.

Time-Resolved Fluorescence Study.Figure 7shows the time-resolved ?uorescence spectra of aqueous solution of pure R6G dye and in presence of Au nanoparticles,Au and ZnO nanoparticles mixture and Au@ZnO nanoparticles.The

photo-

Figure 4.Quenching of photoluminescence emission of R6G Dye (donors)with Au@ZnO core -shell (acceptor)for different donor to acceptor (x)ratios (a)and the corresponding Stern-Volmer plot (b).Insert picture shows a plot of K app versus concentration of quenchers.The concentration of donor (R6G)was maintained constant (1μ

M).

Figure 5.Dependence of the ef?ciency of R6G dye photoluminescence by Au@ZnO core -shell particles,E )1-F /F 0as a function of the R6G dye to Au@ZnO core -shell ratio (C D /C A ).Squares are E values obtained in the experiment where the concentration of the donor (R6G)was maintained as a constant (1μM)and the concentration of the acceptor (core -shell)was varied.Triangles correspond to the experi-ment where the concentration of the acceptor was maintained as a constant (6.84×10-8M)and the concentration of the donor was varied.The solid line is ?tting data in a ?rst order exponential decay.

Au@ZnO Core -Shell Nanoparticles J.Phys.Chem.C,Vol.112,No.31,200811653

luminescence decay of aqueous solution of pure R6G dye (1μM)without Au is monoexponential and the decay time is 3.91ns,which is for unbound dye molecules.However,the ?uorescence decay of dye in presence of Au nanoparticle to a biexponential function was used instead of a stretched expo-nential because it provided better agreement (Table 1).If the intensity decays are multiexponential then it is important to use an average decay time which is proportional to the steady-state intensity.6The average values are given by the sum of the ∑b i τi products.The fast and slow components are 1.31(61.3%)and 3.87ns (38.7%),respectively for the dye solution in presence of Au nanoparticles and the average decay time is 2.30ns.We attribute the slow component (3.87ns)is due to unbound dye molecules and fast component (1.31ns)is attributed to bound dye molecules with Au nanoparticles.8The decay rate in presence of acceptor will only remain a single exponential if there is a single donor -acceptor distance.In presence of a mixture of Au and ZnO nanoparticles,the components are 0.42(12.5%),3.65(48.3%),and 97.8ps (39.2%)and the average decay time is 1.85ns (Table 1).Nabiev et al.20also showed three exponentials PL decay curves during the energy transfer between CdSe/ZnS QDs and Au nanorods.Three components were identi?ed in the decay dynamics of lissamine dye molecules when chemically attached with Au nanoparticles and it was explained due to bound and unbound dye molecules.8In the present study,the slow component decay time 3.65ns is attributed to unbound dye and slow components 0.42ns and 97.8ps are attributed to bound dye molecules to Au and ZnO nanoparticles i.e.heterogeneity of the two different nanopar-

ticles.The shortening of the decay time of dye in presence of Au or mixture of Au and ZnO nanoparticles again con?rms the dynamic quenching process.This quenching process is either energy transfer or electron transfer process.The decay compo-nents are 1.25(44.4%),3.42(13.8%),and 98.2ps (41.8%)for the dye in presence of core -shell Au@ZnO nanoparticles and the average decay time is 1.07ns.Similarly,the decay component 3.42ns is due to unbound dye molecules and the fast components 1.25and 98.2ps are due to bound dye molecules for different sizes or heterogeneity of two different nanoparticles.Moreover,NP samples are quite polydisperse as seen from TEM picture and the gold nuclei are not located in the center of the ZnO shell which as a consequence has non constant thickness.Therefore,?uorophores bound to the surface may ?nd extremely diverse conditions and form a complex system which is dif?cult to understand at this moment.In case of pure Au,the energy transfer takes place from dye f nanoparticles,whereas in Au@ZnO nanocomposites,the energy transfer takes place from dye f core -shell nanoparticles.The energy transfer ef?ciency from dye to pure Au or Au@ZnO nanocomposite is estimated accordingly φET )1-τDA /τD ,where τDA is the decay time of dye in presence of nanoparticles and τD corresponds to the decay time of dye in absence of nanoparticles.The calculated energy transfer ef?ciencies from dye to nanoparticles are 41.3,52.6,and 72.6%for Au,a mixture of Au and ZnO,and core -shell Au@ZnO nanoparticles,respectively (Table 1).It is interesting to note that the most ef?cient energy transfer occurs in core -shell nanoparticles,which is an important ?nding in this study.The Fo ¨rster formalism has been widely used to describe nonradiative energy transfer between dyes.6However,the change radiative and nonradiative decay rates of lissamine dye molecules,chemically attached to Au nanoparticles,were reported.8Here,we also estimated the radiative and nonradiative decay rate of R6G dye molecules in the presence of Au nanoparticles by time-resolved spectroscopy.It is interesting to note that the radiative decay rates are 1.50×108,1.50×108,and 1.68×108s -1for pure Au,mixture of Au and ZnO and core -shell Au@ZnO nanoparticles,respectively (Table 2),indicating there is no change in radiative decay rate in presence of core -shell Au@ZnO nanoparticles.However,a signi?cant modi?cation of nonradiative decay rate is observed.The nonradiative decay rates are 2.80×108,3.90×108,and 7.67×108s -1for pure Au,mixture of Au and ZnO and core -shell Au@ZnO nanoparticles,respectively (Table 2).Most pronounced effect on nonradiative decay rate is obtained for core -shell Au@ZnO nanoparticles due to dipole -metal inter-actions.Analysis suggests that the PL quenching of dye is mainly due to nonradiative decay channel by Au nanoparticles without any signi?cant modi?cation of the radiative rate which con?rms the resonant energy transfer process.6

To estimate the distance between donor and acceptor,we used both FRET and SET methods.Fo ¨rster distance (R 0)is calculated

from the relation

6R 0)0.211[κ2n -4 dye J (λ)]1?6

(in ?)(13)

where k 2is the orientation factor,φdye is the quantum ef?ciency of dye,J (λ)is the overlap integral between the absorption peak of acceptor and emission peak of donor,n is the refractive index of the medium.We calculated the overlap integral [J (λ)]from the overlap of emission spectra of donor (dye)and absorption spectra of the acceptor and the values are listed in Table 3.The overlap integral increases from 4.30×1017to 6.47×1017for pure Au and core-shell Au@ZnO

nanoparticles,

Figure 6.Dependence of the ef?ciency of R6G dye photoluminescence

by Au particles,E )1-F /F 0as a function of the R6G dye to pure Au ratio.E values obtained in the experiment where the concentration of donor (R6G)was maintined as a constant (1μM)and the concentration of the acceptor (Au)was varied.The solid line is the ?tting of data in a ?rst orfer exponential

decay.

Figure 7.Decay curves of (a)Rhodamine 6G (R6G)dye solution,(b)Au and 1μM R6G,(c)the Au and ZnO nanoparticle mixture and 1μM R6G,and (d)Au@ZnO core -shell with 1μM R6G.

11654J.Phys.Chem.C,Vol.112,No.31,2008Haldar et al.

respectively,indicating more ef?cient energy transfer occurs in core-shell nanoparticle.The calculated Fo¨rster distances(R0) are135.0106.4,and144.4?for Au,Au-ZnO,and core-shell Au@ZnO nanoparticles,https://www.docsj.com/doc/755679563.html,ing eq1,the calculated distances(d)between the donor and acceptor are143.05,103.63, and123.5?for Au,a mixture of Au and ZnO,and core-shell Au@ZnO nanoparticles,respectively(Table3),using the ef?ciency of FRET which depends on the inverse sixth power of the distance of separations between one donor and one acceptor.In the present study where(Figures5and6)one acceptor nanoparticle(core-shell or pure)can interact with several donors brought in close proximity simultaneously,and for this complex interactions,the ef?ciency(E)can be ex-pressed21as E)nR06/nR06+r n6,where r n is the average donor-acceptor distance and where n is acceptor/donor ratio. The calculated average distances(r n)between the donor and acceptor are89.2and77.2?for Au,and core-shell Au@ZnO nanoparticles,respectively(Table3).The average distance between donor-acceptor values are close to ef?cient FRET range.It is already reported that FRET based method is restricted6on the upper limit of only80?because the energy transfer becomes too weak to be useful.Furthermore,we estimate the distance between donor and acceptor by using surface energy transfer(SET)method.We estimate the d0value by using Persson model9

d 0)(0.225c3Φdye

ω2dyeω

F

k

F)

1?4

(14)

where d0is the distance at which a dye will display equal probabilities for energy transfer and spontaneous emission.φdye is the quantum ef?ciency of dye,the frequency of the donor electronic transition(ω),and the Fermi frequency(ωF),and Fermi wave vector(k F)of the metal.7The d0value was calculated usingφdye)0.92,ω)3.6×1015s-1,ωF)8.4×1015s-1,k F)1.2×108cm-1,and c)3×1010cm s-1.The calculated d0values are80.76,80.76and80.76?for Au, Au-ZnO,and core-shell Au@ZnO nanoparticles,respectively (Table3).The distances(d)between the donor and acceptor are88.2,74.5,and67.6?for Au,Au-ZnO,and core-shell Au@ZnO nanoparticles,respectively(Table3),using the ef?ciency of SET(eq2)which depends on the inverse fourth power of the distance of separations between donor and acceptor. It is interesting to note that the calculated average distance(r n) between the donor and acceptor are89.2and77.2?for Au, and core-shell Au@ZnO nanoparticles,which are very close to the calculated values from SET method.As the FRET based method is restricted on the upper limit of only80?,therefore, we may suggest that the energy transfer from dye to Au nanoparticles is surface energy transfer(SET)process in the present study and it follows1/d4distance dependence.The short distance between donor-acceptor in case of core-shell nano-particle indicates the ef?cient energy transfer from donor(dye) to the acceptor(core-shell nanoparticles),which is a strong evidence of highly ef?cient resonance energy transfer.The pronounced quenching of dye in presence of core-shell nanoparticles is an important?nding.This allows core-shell nanoparticles to be used as acceptors for SET experiments. Conclusion

To the best of our knowledge,this is the new report of energy transfer from R6G dye to Au@ZnO core-shell nanoparticle. The calculated energy transfer ef?ciencies from dye to nano-particles are41.3,52.6,and72.6%for Au,mixture of Au and ZnO,and core-shell Au@ZnO nanoparticles,respectively.The obtained values of quenching constant indicate the dynamic quenching process.Most pronounced effect on nonradiative decay rate is obtained for core-shell Au@ZnO nanoparticles. Analysis suggests that the PL quenching of dye is mainly due to nonradiative decay channel without any signi?cant modi?ca-tion of the radiative rate which con?rms the resonant energy transfer process.The presence of several donor molecules associated with each Au nanoparticle contributes signi?cantly to the high quenching.The calculated Fo¨rster distances(R0)are 135.0106.4and144.4?for Au,Au-ZnO,and core-shell Au@ZnO nanoparticles,respectively and corresponding the calculated distances(d)between the donor and acceptor are 143.05,103.63and123.5?.The calculated average distance (r n)between several donors and one acceptor are89.2and77.2?for Au,and core-shell Au@ZnO nanoparticles,respectively. However,the distances between the donor and acceptor are88.2,

TABLE1:Time Resolved Fluorescence Quenching Studies of Rh6G in the Presence of Au,the Au and ZnO mixture,and

Au@ZnO Core-Shell Nanoparticles

system b1(%)τ1a(ps)b2(%)τ2a(ns)b3(%)τ3a(ns)?τ?∑b iτi(ns) 2E(%) Rh6G Dye(1μM)100 3.91 3.91 1.02

Au(0.2mM)+Rh6G Dye61.3 1.3138.7 3.87 2.30 1.0741.3 Au(0.2mM)+ZnO(0.5mM)Rh6G39.297.812.50.4248.3 3.65 1.85 1.1052.6 Au(0.2mM)+ZnO(0.5mM)+Rh6G41.898.244.4 1.2513.8 3.42 1.07 1.1972.6 a(5%(error).

TABLE2:Radiative and Nonradiative Decay Rate of Dye

in Presence of Au,the Au and ZnO mixture,and Au@ZnO

Core-Shell Nanoparticles

systemΦD radiative

rate

(s-1)

nonradiative

rate

(s-1)

Au+R6G Dye0.34 1.50×108 2.80×108

Au+ZnO+R6G Dye0.27 1.50×108 3.90×108

Au@ZnO+R6G Dye0.18 1.68×1087.67×108

TABLE3:Energy Transfer Parameters for Different Rh6G-Au Nanoparticles Pairs

systemλem(nm)J(λ)(M-1cm-1nm4)ΦD0E(%)(PL)R0(?)r(?)r n(?)a d0(?)d(?) Rh6G Dye5510.915

Rh6G+Au nanoparticle551 4.30×10170.91550135.0143.089.2b80.7688.2 Rh6G+Au+ZnO nanoparticles554 1.03×10170.91558106.4103.6380.7674.5 Rh6G+Au@ZnO nanoparticle558 6.47×10170.91575144.4123.577.2c80.7667.6

a E)nR06/n R06+r n6.n)acceptor/donor.

b n)0.057.

c n)0.068.

Au@ZnO Core-Shell Nanoparticles J.Phys.Chem.C,Vol.112,No.31,200811655

74.5,and67.6?for Au,Au-ZnO,and core-shell Au@ZnO nanoparticles,respectively,using the ef?ciency of surface energy transfer which follows a1/d4distance dependence between donor and acceptor.Therefore,such multifunctional core-shell nanoparticles should have great potentials for optical-based molecular rulers and it could pave the way for designing new optical based materials for the application in chemical sensing or biological imaging.

Acknowledgment.The Department of Science and Technol-ogy(NSTI)and“Ramanujan Fellowship”are gratefully ac-knowledged for?nancial support.T.S.thanks CSIR for awarding a fellowship.

Supporting Information Available:HTEM pictures of Au and ZnO nanoparticles mixture(a)and uncoated Au nanopar-ticles(b)are shown in Figure S1.Figure S2shows quenching of photoluminescence emission of R6G Dye(donors)with pure Au(acceptor)for different donor to acceptor ratios.The insert picture shows the corresponding Stern-Volmer plot.This material is available free of charge via the Internet at http:// https://www.docsj.com/doc/755679563.html,.

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