Au@ZnO Core−Shell Nanoparticles Are Efficient Energy Acceptors

the calculated average distances (rn) between the donor and acceptor are 89.2 and 77.2 Å for Au and core- ...... Persson, B. N.; Lang, N. D. Phys. Re...
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11650

J. Phys. Chem. C 2008, 112, 11650–11656

Au@ZnO Core-Shell Nanoparticles Are Efficient Energy Acceptors with Organic Dye Donors Krishna Kanta Haldar, Tapasi Sen, and Amitava Patra* Department of Materials Science and Center for AdVanced Materials, Indian Association for the CultiVation of Science, Kolkata 700 032, India ReceiVed: April 11, 2008; ReVised Manuscript ReceiVed: May 26, 2008

The present study highlights the efficient fluorescence resonance energy transfer from Rhodamine 6G dye to Au@ZnO core-shell nanoparticle by steady state and time-resolved spectroscopy. The calculated energy transfer efficiencies 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 × 108 and 7.67 × 108 s-1 for 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 (R0) are 135.0 and 144.4 Å 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 Å. Considering the interactions of one acceptor and several donors, the calculated average distances (rn) between the donor and acceptor are 89.2 and 77.2 Å for Au and coreshell Au@ZnO nanoparticles, respectively. However, the distances between the donor and acceptor are 88.2 and 67.6 Å for Au and core- shell Au@ZnO nanoparticles, respectively, using the efficiency of surface energy transfer which follows a 1/d4 distance dependence between donor and acceptor. On the basis of these finding, we may suggest that surface energy transfer process has a more reasonable agreement with experimental finding. 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.1–5 Quantum dots (QD’s) are excellent donors in fluorescence 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 semiconductor quantum dots for development of hybrid inorganic-bio receptor sensing materials. They also reported that the nonradiative 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 fluorophores. FRET occurs when the electronic excitation energy of a donor fluorophore is transferred to a nearby acceptor molecule and the transfer efficiency 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 efficiency of FRET depends on the distance of separation * To whom correspondence should be addressed. E-mail: [email protected]. Phone: (91)-33-2473-4971. Fax: (91)-33-2473-2805.

between donor and acceptor molecules. According to the Fo¨rster theory, the rate of energy transfer is given by6

kT(r) )

( )

1 R0 τD 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 R0 is known as the Fo¨rster distance, the distance at which the transfer rate kT(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-4 dependence.7–10 According to their model,9 the exact form of dipole-surface energy transfer (SET) rate is given by

kSET )

()

1 d0 τD 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.8 showed the change of radiative and nonradiative decay rates of the chemically attached dye molecules with different sized gold nanoparticles. Strouse

10.1021/jp8031308 CCC: $40.75  2008 American Chemical Society Published on Web 07/11/2008

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et al.7 reported the surface energy transfer (SET) from dye to DNA attached Au-nanoparticle and the energy transfer process follows 1/d4 distance dependence.7,8 We also reported the energy transfer process from dye to different shaped Au nanoparticle by steady state and time-resolved spectroscopy.10 To the best of our knowledge, there has been no study on the energy transfer from dye to core-shell nanoparticle. Such core-shell (metalsemiconductor) 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 nanocomposite has received significant 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.12–17 Kotov et al.18 reported the biologically inspired superstructures made from metal NPs and semiconductor NWs. In particular, to the best of our knowledge, there has been no study on the energy transfer from Rhodamine 6G 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 nanoparticles are efficient in energy transfer? What kind of mechanism (static or dynamic) is involved? How are the radiative and nonradiative decay rates of the dye molecule changed? Finally, we compare their efficiency 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 efficient nanomaterials for the applications in chemical sensing or biological imaging.

(from

air) f Au@ZnO core-shell + Na2CO3 (5)

A total of 1 mL of 1 µM Rhodamine 6G aqueous dye solutions was added to 3 mL of Au@ZnO core-shell nanoparticle solution, and the solution was kept for 1 day for stabilization. Similarly, 1 mL of 1 µM Rhodamine 6G aqueous dye solutions was added to 3 mL of 0.2 mM Au nanoparticle solution. A total of 1 mL of 1 µM Rhodamine 6G aqueous dye solutions was added to a mixture of 1.5 mL (0.2 mM) of Au nanoparticle and 1.5 mL (0.5 mM) 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-2010 transmission electron microscope operating voltage at 200 kV. Room temperature optical absorption spectra were obtained with an UV-vis spectrophotometer (Shimadzu). The emission spectra of all samples were recorded in a Fluoro Max-P (HORIBA JOBIN YVON) Luminescence Spectrometer. For the time correlated single photon counting (TCSPC) measurements, the samples were excited at 405 nm 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 about 90 ps. The repetition rate is 1 MHz. The fluorescence decays were collected at a Hamamatsu MCP photomultiplier (C487802). The fluorescence decays were analyzed using IBH DAS6 software. The following expression was used to analyze the experimental time-resolved fluorescence decays, P(t): n

P(t) ) b +

∑ Ri exp(-t/τi)

(6)

i

Here, n is the number of discrete emissive species, b is a baseline correction (“dc” offset), and Ri and τi are the pre-exponential factors and excited-state fluorescence lifetimes associated with the ith component, respectively. Results and Discussion

Experimental Procedures Zinc nitrate hexahydrate purified (from Merck), sodium hydroxide pellets purified (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 nanocomposite, a novel colloidal approach is presented. A total of 0.03 g of zinc nitrate hexahydrate (10 mM) was dissolved into 10 mL of distilled water and formed a white precipitate of zinc hydroxide [Zn (OH)2] after adding 2 mL (2M) of sodium hydroxide solution. Then, an excess amount of sodium hydroxide solution was added to dissolve the white precipitate to form sodium zincate (Na2ZnO2). The reaction equations 3 and 4 are given in below.

Zn(NO3)2 · 6H2O + NaOH f Zn(OH)2 + NaNO3 + H2O (3) Zn(OH)2 + NaOH f Na2ZnO2 + H2O

Au-citrate + Na2ZnO2 + CO2

(4)

Then, 1 mL of sodium zincate (10 mM) solution was diluted to 5 mL.This solution was added to as prepared 0.2 mM Au colloid10a (reported in our previous work) under stirring condition for 3-4 min. Finally, this solution was kept at 90-95 °C for 30 min under stirring condition to prepare Au@ZnO core-shell nanocomposites (eq 5).

Transmission Electron Microscopy. Figure 1a shows the TEM picture of Au@ZnO core-shell nanoparticles which represents the surface coating of nanocrystals. It is clearly seen from Figure 1b that Au nanoparticle is coated with ZnO and the measured thickness of the shell is 2.4 nm. The FFT pattern (Figure 1c) for shell confirms the plane (101) of ZnO and the FFT pattern (Figure 1d) confirms 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 confirmed (Figure S1b) that there is no surface coating on Au nanoparticle. Steady-State Study. Figure 2 shows 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 at 518, 518, and 537 nm 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 from 518 to 537 nm for Au@ZnO nanoparticles. The absorption peaks at 367, 367, and 351 nm

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Haldar et al.

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).

are due to excitonic band11c 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 (367 nm) is obviously due to the thin shell of ZnO on Au nanoparticles. As shown earlier, the high dielectric constant of the TiO2 and SnO2 shell causes a red shift in the plasmon absorption of the Au core.11a,12 In the core-shell structure, the plasmon peak position of the metal core is given by

λ ) {λp εR + 2nH2O2 + 2g(nZnO2 - nH2O2)/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, λ is the estimated peak position of metal core, and λp is bulk plasma wavelength which is defined by

λp ) [4π c meffε0/Ne 2 2

2 1⁄2

]

) 130.9 nm for Au

(8)

where the meff is effective mass of the free electron of the metal and N is electron density of metal core. The refractive index of ZnO is 1.92 which 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 539.4 nm using volume fraction of shell layer (g) unity. The calculated value is close with the observed value (537 nm). 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

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

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

ZnO f ZnO(e + h) ZnO(e) + Au f ZnO + Au(e)

(9) (10)

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

C ) 4πε0ε(r/d)(r + d)

(11)

where ε0 is the permittivity of the free space, ε is the static dielectric constant of the shell which is taken to be 8.87. From the TEM images, the calculated rAu value is 4.0 nm and dZnO value is 2.4 nm, and the capacitance value is 11.0 aF (attofarad) for Au@ZnO core-shell nanoparticle, indicating more electrons are needed to raise Fermi level. Lee et al.16 reported the change of capacitance from 0.31 to 3.42 aF 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-known6 that the energy transfer depends on the spectral overlap between donor emission and acceptor absorption. The photoluminescence (PL) peak at 551 nm 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

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Figure 5. Dependence of the efficiency of R6G dye photoluminescence by Au@ZnO core-shell particles, E ) 1 - F/F0 as a function of the R6G dye to Au@ZnO core-shell ratio (CD/CA). Squares are E values obtained in the experiment where the concentration of the donor (R6G) was maintained as a constant (1 µM) and the concentration of the acceptor (core-shell) was varied. Triangles correspond to the experiment where the concentration of the acceptor was maintained as a constant (6.84 × 10-8 M) and the concentration of the donor was varied. The solid line is fitting data in a first order exponential decay.

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 Kapp versus concentration of quenchers. The concentration of donor (R6G) was maintained constant (1 µM).

related to space interaction of dipole of the donor and the free electrons of metal nanoparticles.7 In presence of aqueous solution of Au and ZnO nanoparticle mixture, 58% PL quenching of R6G is observed. The shift of the PL peak to 554 nm is observed. However, 75% PL quenching of dye is observed in the presence of Au@ZnO core-shell nanoparticles which reveals that the significant PL quenching is observed in the presence of Au@ZnO core-shell nanoparticles. To understand the quenching process, the measurement of fluorescence intensity with varying quencher (particle) concentration (1.14 × 10-8 to 6.84 × 10-8 M) 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.20 A gradual quenching of the fluorescence 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 by6

F0 ) 1 + Kqτ0[Q] ) 1 + KSV[Q] F

(12)

where F0 and F are the relative fluorescence intensity in absence and presence of quencher, respectively. KSV is the Stern-Volmer dynamic quenching constant and [Q] is the concentration of the quencher. Figure 4b shows the Stern-Volmer plots, F0/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 Kapp versus concentration of quenchers. The slope (S) and intercept (I) were found to be 1.20 × 1014 M-2 and 32.95 × 106 M-1, respectively (as shown in insert Figure 4b). Therefore, the dynamic and static quenching constants are 2.88 × 107 and 4.15 × 106 M-1, respectively, for Au@ZnO nanoparticles. Similarly, the dynamic and static quenching constants are 9.6 × 106 and 3.8 × 106 M-1, respectively for Au nanoparticles (Supporting Information, Figure S2), which is consistent with an earlier report.20 Hereafter, we also estimate the efficiency of the quenching process by the value E ) 1 - FDA/FD, where FDA is the integrated fluorescence intensity of dye in presence of Au@ZnO core-shell nanoparticles and FD corresponds to the integrated fluorescence intensity of dye in absence of Au@ZnO core-shell nanoparticles. Figure 5 shows the efficiency of the quenching of dye as a function of the ratio between the molar concentrations of donors and acceptor (x ) CD/CA) in water solution. It is clearly seen from the Figure 5 that the efficiency of quenching process increases monotonically with decreasing in x. It is noteworthy that the very effective quenching of dye characterized by E ) 0.75 was observed for assemblies with x equal to 14. It reveals that one acceptor Au@ZnO core-shell nanoparticle assembles with 14 donor dye molecules. Figure 6 shows the efficiency 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 efficiency 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.20 The presence of several donor molecules associated with each Au nanoparticle contributes significantly to the high quenching. Time-Resolved Fluorescence Study. Figure 7 shows the time-resolved fluorescence 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-

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

Figure 6. Dependence of the efficiency of R6G dye photoluminescence by Au particles, E ) 1 - F/F0 as 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 (1 µM) and the concentration of the acceptor (Au) was varied. The solid line is the fitting of data in a first orfer exponential decay.

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

luminescence decay of aqueous solution of pure R6G dye (1 µM) without Au is monoexponential and the decay time is 3.91 ns, which is for unbound dye molecules. However, the fluorescence decay of dye in presence of Au nanoparticle to a biexponential function was used instead of a stretched exponential 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.6 The average values are given by the sum of the ∑ biτi products. The fast and slow components are 1.31 (61.3%) and 3.87 ns (38.7%), respectively for the dye solution in presence of Au nanoparticles and the average decay time is 2.30 ns. We attribute the slow component (3.87 ns) is due to unbound dye molecules and fast component (1.31 ns) is attributed to bound dye molecules with Au nanoparticles.8 The 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 97.8 ps (39.2%) and the average decay time is 1.85 ns (Table 1). Nabiev et al.20 also showed three exponentials PL decay curves during the energy transfer between CdSe/ZnS QDs and Au nanorods. Three components were identified 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.8 In the present study, the slow component decay time 3.65 ns is attributed to unbound dye and slow components 0.42 ns and 97.8 ps are attributed to bound dye molecules to Au and ZnO nanoparticles i.e. heterogeneity of the two different nanopar-

Haldar et al. ticles. The shortening of the decay time of dye in presence of Au or mixture of Au and ZnO nanoparticles again confirms the dynamic quenching process. This quenching process is either energy transfer or electron transfer process. The decay components are 1.25 (44.4%), 3.42 (13.8%), and 98.2 ps (41.8%) for the dye in presence of core-shell Au@ZnO nanoparticles and the average decay time is 1.07 ns. Similarly, the decay component 3.42 ns is due to unbound dye molecules and the fast components 1.25 and 98.2 ps 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, fluorophores bound to the surface may find extremely diverse conditions and form a complex system which is difficult 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 efficiency 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 efficiencies 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 efficient energy transfer occurs in core-shell nanoparticles, which is an important finding in this study. The Fo¨rster formalism has been widely used to describe nonradiative energy transfer between dyes.6 However, the change radiative and nonradiative decay rates of lissamine dye molecules, chemically attached to Au nanoparticles, were reported.8 Here, 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 × 108 s-1 for 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 significant modification of nonradiative decay rate is observed. The nonradiative decay rates are 2.80 × 108, 3.90 × 108, and 7.67 × 108 s-1 for 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 interactions. Analysis suggests that the PL quenching of dye is mainly due to nonradiative decay channel by Au nanoparticles without any significant modification of the radiative rate which confirms the resonant energy transfer process.6 To estimate the distance between donor and acceptor, we used both FRET and SET methods. Fo¨rster distance (R0) is calculated from the relation6

R0 ) 0.211[κ2n-4φdyeJ(λ)]1⁄6

(in Å)

(13)

where k2 is the orientation factor, φdye is the quantum efficiency 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 × 1017 to 6.47 × 1017 for pure Au and core- shell Au@ZnO nanoparticles,

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TABLE 1: 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 (%)

Rh6G Dye (1 µM) Au (0.2 mM) + Rh6G Dye Au (0.2 mM) + ZnO (0.5 mM) Rh6G Au (0.2 mM) + ZnO (0.5 mM) + Rh6G a

39.2 41.8

τ1a (ps)

b2 (%)

τ2 a (ns)

b3 (%)

τ3 a (ns)

〈τ〉 ∑ biτi (ns)

χ2

E(%)

97.8 98.2

61.3 12.5 44.4

1.31 0.42 1.25

100 38.7 48.3 13.8

3.91 3.87 3.65 3.42

3.91 2.30 1.85 1.07

1.02 1.07 1.10 1.19

41.3 52.6 72.6

(5% (error).

TABLE 2: Radiative and Nonradiative Decay Rate of Dye in Presence of Au, the Au and ZnO mixture, and Au@ZnO Core-Shell Nanoparticles

system

ΦD

radiative rate (s-1)

Au + R6G Dye Au + ZnO+ R6G Dye Au@ZnO + R6G Dye

0.34 0.27 0.18

1.50 × 108 1.50 × 108 1.68 × 108

nonradiative rate (s-1) 2.80 × 108 3.90 × 108 7.67 × 108

respectively, indicating more efficient energy transfer occurs in core-shell nanoparticle. The calculated Fo¨rster distances (R0) are 135.0 106.4, and 144.4 Å for Au, Au-ZnO, and core-shell Au@ZnO nanoparticles, respectively. Using eq 1, the calculated distances (d) between the donor and acceptor are 143.05, 103.63, and 123.5 Å for Au, a mixture of Au and ZnO, and core-shell Au@ZnO nanoparticles, respectively (Table 3), using the efficiency 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 (Figures 5 and 6) one acceptor nanoparticle (core-shell or pure) can interact with several donors brought in close proximity simultaneously, and for this complex interactions, the efficiency (E) can be expressed21 as E ) nR06/nR06 + rn6, where rn is the average donor-acceptor distance and where n is acceptor/donor ratio. The calculated average distances (rn) between the donor and acceptor are 89.2 and 77.2 Å for Au, and core- shell Au@ZnO nanoparticles, respectively (Table 3). The average distance between donor-acceptor values are close to efficient FRET range. It is already reported that FRET based method is restricted6 on the upper limit of only 80 Å 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 d0 value by using Persson model9

d0 )

(

0.225c3Φdye ω2dyeωFkF

)

1⁄4

(14)

where d0 is the distance at which a dye will display equal probabilities for energy transfer and spontaneous emission. φdye is the quantum efficiency of dye, the frequency of the donor electronic transition (ω), and the Fermi frequency (ωF), and Fermi wave vector (kF) of the metal.7 The d0 value was calculated using φdye ) 0.92, ω ) 3.6 × 1015 s-1, ωF ) 8.4 × 1015 s-1, kF ) 1.2 × 108 cm-1, and c ) 3 × 1010 cm s-1. The

calculated d0 values are 80.76, 80.76 and 80.76 Å for Au, Au-ZnO, and core-shell Au@ZnO nanoparticles, respectively (Table 3). The distances (d) between the donor and acceptor are 88.2, 74.5, and 67.6 Å for Au, Au-ZnO, and core-shell Au@ZnO nanoparticles, respectively (Table 3), using the efficiency of SET (eq 2) 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 (rn) between the donor and acceptor are 89.2 and 77.2 Å 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 only 80 Å, 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 follows 1/d4 distance dependence. The short distance between donor-acceptor in case of core-shell nanoparticle indicates the efficient energy transfer from donor (dye) to the acceptor (core-shell nanoparticles), which is a strong evidence of highly efficient resonance energy transfer. The pronounced quenching of dye in presence of core-shell nanoparticles is an important finding. 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 efficiencies 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. 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 significant modification of the radiative rate which confirms the resonant energy transfer process. The presence of several donor molecules associated with each Au nanoparticle contributes significantly to the high quenching. The calculated Fo¨rster distances (R0) are 135.0 106.4 and 144.4 Å 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, 103.63 and 123.5 Å. The calculated average distance (rn) between several donors and one acceptor are 89.2 and 77.2 Å for Au, and core- shell Au@ZnO nanoparticles, respectively. However, the distances between the donor and acceptor are 88.2,

TABLE 3: Energy Transfer Parameters for Different Rh6G-Au Nanoparticles Pairs system

λem (nm)

Rh6G Dye Rh6G +Au nanoparticle Rh6G+Au +ZnO nanoparticles Rh6G +Au@ZnO nanoparticle

551 551 554 558

a

J(λ) (M-1 cm-1 nm4) 4.30 × 1017 1.03 × 1017 6.47 × 1017

E ) nR06/nR06 + rn6. n ) acceptor/donor. b n ) 0.057. c n ) 0.068.

ΦD0

E(%) (PL)

R0 (Å)

r (Å)

rn (Å)a

d0 (Å)

d (Å)

0.915 0.915 0.915 0.915

50 58 75

135.0 106.4 144.4

143.0 103.63 123.5

89.2b

80.76 80.76 80.76

88.2 74.5 67.6

77.2c

11656 J. Phys. Chem. C, Vol. 112, No. 31, 2008 74.5, and 67.6 Å for Au, Au-ZnO, and core-shell Au@ZnO nanoparticles, respectively, using the efficiency of surface energy transfer which follows a 1/d4 distance 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 Technology (NSTI) and “Ramanujan Fellowship” are gratefully acknowledged for financial support. T.S. thanks CSIR for awarding a fellowship. Supporting Information Available: HTEM pictures of Au and ZnO nanoparticles mixture (a) and uncoated Au nanoparticles (b) are shown in Figure S1. Figure S2 shows 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:// pubs.acs.org. References and Notes (1) (a) Medintz, I. L.; Clapp, A. R.; Mattoussi, H.; Goldman, E. R.; Fisher, B.; Mauro, J. M. Nat. Mater. 2003, 2, 630. (b) Clapp, A. R.; Medintz, I. L.; Fisher, B. R.; Anderson, G. P.; Mattoussi, H. J. Am. Chem. Soc. 2005, 127, 1242. (c) Pons, T.; Medintz, I. L.; Sapsford, K. E.; Higashiya, S.; Grimes, A. F.; English, D. S.; Mattoussi, H. Nano Lett. 2007, 7, 3157. (d) Goldman, E. R.; Medintz, I. L.; Whitley, J. L.; Hayhurst, A.; Clapp, A. R.; Uyeda, H. T.; Deschamps, J. R.; Lassman, M. E.; Mattoussi, H. J. Am. Chem. Soc. 2005, 127, 6744. (2) (a) Peng, H.; Zhang, L.; Kjallman, T. H. M.; Soeller, C.; Sejdic, J. T. J. Am. Chem. Soc. 2007, 129, 3048. (b) Boulesbaa, A.; Issac, A.; Stockwell, D.; Huang, Z.; Huang, J.; Guo, J.; Lian, T. J. Am. Chem. Soc. 2007, 129, 15132. (c) Chowdhury, P. S.; Sen, P.; Patra, A. Chem. Phys. Lett. 2005, 413, 311. (3) Warner, J. H.; Watt, A. R.; Thomsen, E.; Heckenberg, N.; Meredith, P.; Dunlop, H. R. J. Phys. Chem. B 2005, 109, 9001.

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