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J. Phys. Chem. C 2008, 112, 3216-3222
Resonance Energy Transfer from Rhodamine 6G to Gold Nanoparticles by Steady-State and Time-Resolved Spectroscopy 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: August 27, 2007; In Final Form: NoVember 18, 2007
In the present study, we found a pronounced effect on the PL and shortening of the overall lifetime of rhodamine 6G (R6G) when interacting with the spherical, shaped, and capped Au NPs, but there are no measured effects on radiative rate of the dye. The observed Fo¨rster distances (R0) are 105.75, 170, and 124 Å for spherical, shaped, and capped Au nanoparticles, respectively and the distances between the donor and acceptor are 103.56, 194.77, and 134.43 Å. However, the distances between the donor and acceptor are 78.26, 99.04, and 88.43 Å for spherical, shaped, and capped Au 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 findings, we may suggest that surface energy transfer process has a more reasonable agreement with experimental finding. The anisotropy decay of R6G with spherical Au nanoparticles is single exponential with correlation time constant of 240 ps. However, the anisotropy decay of R6G in shaped nanoparticles is biexponential with correlation time constants of 347 ps and 2.46 ns.
Introduction In recent years, the understanding of the Fo¨rster resonance energy transfer (FRET) between two different dyes, QD and dye-labeled biomolecule, and QD and polymers is one of the most important topics.1-10 It is now well-established that QDs have several advantages over dyes in FRET studies in biological system. Dyes are commonly used in biological system for Fo¨rster resonance energy transfer (FRET) study. FRET studies commonly used for biological research to measure molecular distances or donor-to-acceptor proximity.11 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 between donor and acceptor molecules. According to the Fo¨rster theory,12 the rate of energy transfer is given by
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. The Fo¨rster distance (R0) is defined as
R06 )
9000(In10)κ2φD 5
128π Nn
4
∫0∞ FD(λ)A(λ)λ4 dλ
(2)
where φD is the quantum yield of donor in absence of acceptor, N is Avogadro’s number, and n is the refractive index of the medium.τD is the lifetime of the donor in absence of the acceptor, FD(λ) is the corrected fluorescence intensity of the donor in wavelength range from λ to λ + ∆ λ, with the total * Author to whom correspondence should be addressed. E-mail: msap@ iacs.res.in. Phone: (91)-33-2473-4971. Fax: (91)-33-2473-2805.
intensity normalized to unity, A(λ) is the extinction coefficient of the acceptor at λ, which is typically in units of M-1 cm-1, and k2 is the orientation factor of 2 dipoles interacting and is usually assumed to be equal to 2/3. However, FRET technique is restricted on the upper limit of separation of only 80 Å.11 In recent years, surface energy transfer (SET) between dye molecule and metal nanoparticles has gained interest because this technique is capable of measuring distances nearly twice as far as FRET which will help to understand the large scale conformational dynamics of complex biomolecules in macroscopic detail.13-20 SET does not require a resonant electronic transition like the Fo¨rster process because it is originated from the interaction of the electromagnetic field of the donor dipole with the free conduction electrons of the accepting metal. Gersten-Nitzen,21 Persson-Lang,22 and Chance, Prock, and Silbey23 have already demonstrated the mechanism of dye quenching at a metal surface and the separation of donor and acceptor is d-4 dependence. According to their model, the exact form of dipole-surface energy transfer (SET) rate is given by
kSET )
()
1 d0 τD d
4
(3)
According to Persson and Lang’s model,22 the rate of energy transfer is calculated by using a Fermi golden rule calculation for an excited-state molecule depopulating with the simultaneous scattering of an electron in the nearly metal to above the Fermi level. The d0 value can be calculated using their model. SET process is a useful spectroscopic ruler for long distance measurement which will help us to understand the large scale conformational dynamics of complex biomolecules in macroscopic detail. This allows gold nanoparticles to be used as acceptors in biophysical experiments in vitro as well as in vivo. Dulkeith et al.13 reported the change of radiative and nonradiative decay rates of the chemically attached dye molecules with different sized gold nanoparticles. Recently, Strouse et al.
10.1021/jp0768367 CCC: $40.75 © 2008 American Chemical Society Published on Web 02/12/2008
Resonance ET from R6G to Au NPs SCHEME 1: Structures of Rhodamine 6G and Thiosalicylic Acid (TSA)
showed the surface energy transfer (SET) from dye to DNA attached Au-nanoparticle and it follows 1/d4 distance dependence.15-16 Alivisatos and co-workers24 showed the experimental data on the energy transfer from biacetyl donor to Ag (111) surface which matches with Persson model.22 No report has been found on the role of the shape and coated Au nanoparticle on the energy transfer from rhodamine 6G (R6G) to Au nanoparticles, to our knowledge. The understanding of the role of shape and coated Au nanoparticles on the energy transfer is very important. The fundamental question that we are attempting to address in this paper is how the shape of nanoparticle causes any noticeable change on energy transfer process. Here, we present the influence of shape and surface coated Au nanoparticles on the energy transfer from R6G to Au nanoparticles by steady state and time-resolved spectroscopy. Experimental Procedures Chloroauric acid (HAuCl4 0.3 H2O) (S.d.Fine Chem), trisodium citrate (Merck), sodium borohydride (Merck), thiosalicylic acid, cetyltrimethylammonium bromide (CTAB) (Alfa Aesar), ascorbic acid (Aldrich), and R6G (Aldrich) were used in the present study without further purification. The structures of R6G and thiosalicylic acid are shown in Scheme 1. The synthetic procedure was adapted from the method reported by Murphy et al.25-26 Preparation of Spherical Au Nanoparticles. For spherical Au nanoparticles, the following method has been used. A 20 mL aqueous solution containing 0.1 mM HAuCl4 and 0.1 mM tri-sodium citrate was taken in a conical flask and stirred for 5 min. Then, 0.3 mL of ice cold 0.1 M NaBH4 solution was added to the solution under vigorous stirring. The solution color changes from colorless to orange. The colloidal solution was stirred for an additional 10 min. Then, 1.0 mL of 1 µM rhodamin 6G solution was added to 7.0 mL of this Au-colloidal solution. All solutions were kept for 1 day for stabilization. After 1 day, dye containing Au solutions were used for optical study. Preparation of Shaped Au Nanoparticles. The synthetic procedure of anisotropic Au nanoparticles was adapted from the method reported by Murphy et al.25-26 The color of the solution is reddish brown. The solution contains nanorods, spheres, and plates of various shapes of nanoparticles. After centrifugation, the solid part again was redispersed into 10 mL of deionized water. Then, 1 mL of 1 µM R6G solution was added to 7 mL of this solution, and this solution was kept for 1 day for stabilization. After 1 day, dye containing Au solutions were used for optical study. Preparation of Thiosalicylic Acid (TSA) Capped Spherical Au Nanoparticles. A total of 5 mL of 0.2 mM HAuCl4 and 5 mL of 0.2 mM thiosalicylic acid was added together under stirring condition. The pH of the solution was neutralized to ∼7 by adding sodium carbonate, and the solution was further stirred for 5 min. In an another beaker, sodium carbonate was
J. Phys. Chem. C, Vol. 112, No. 9, 2008 3217 added to 15 mL of 0.8 mM thiosalicylic acid until the pH of the solution was ∼7. The temperature of the second solution was cooled to 0°-2 °C. Then 0.005 g of sodium borohydride was added to the cooled solution. Finally, the borohydride solution was added to the previous solution of HAuCl4 dropwise under vigorous stirring until the appearance of a reddish pink color. The colloidal solution was stirred for an additional 10 min for the complete decomposition of sodium borohydride. Then, 1.0 mL of 1 µM R6G solution was added to 7.0 mL of the above Au-colloidal solution. The transmission electron microscopy (TEM) images were taken using a JEOL-TEM-2010 transmission electron microscope with an operating voltage of 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 on 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 +
∑i Ri exp(- t/τi)
(4)
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. For biexponential decays (n ) 2), the average lifetime, 〈τ〉, was calculated from 2
〈τ〉 )
2
Riτi / ∑ Riτi ∑ i)1 i)1 2
(5)
For anisotropy measurements, a polarizer was placed before the sample. The analyzer was rotated by 90° at regular intervals and the parallel (I|) and the perpendicular (I⊥) components for the fluorescence decay were collected for equal times, alternatively. Then, r(t) was calculated using the formula
r(t) )
I|(t) - GI⊥(t) I|(t) + 2GI⊥(t)
(6)
The G value of the setup is 0.55. Results and Discussion Transmission Electron Microscopy. Figure 1a shows the TEM picture of spherical Au nanoparticles. The image clearly shows the nearly spherical shape of the particles with an average diameter of 4.1 ( 0.4 nm. Figure 1b shows the TEM picture of shaped Au nanoparticles having nanorods, spheres, and plates of various shapes of Au nanoparticles. Figure 1c shows the TEM images of Au nanoparticles prepared in presence of TSA and it shows the formation of spherical shaped Au nanoparticles with an average diameter of 5.0 ( 0.2 nm. Steady-State Study. Figure 2 shows the absorption spectra of spherical and shaped gold nanoparticles and photoluminescence (PL) spectra of R6G dye solution without and with
3218 J. Phys. Chem. C, Vol. 112, No. 9, 2008
Sen and Patra
Figure 2. Absorption spectra of (a) spherical and (b) shaped gold nanoparticles and photoluminescence (PL) spectra of (c) rhodamine 6G (R6G) dye solution, (d) shaped Au and 1 µM R6G, (e) spherical Au and 1 µM R6G.
Figure 3. Absorption spectrum of (a) TSA capped spherical gold nanoparticles and photoluminescence (PL) spectra of (b) R6G dye solution in TSA, and (c) TSA capped Au nanoparticles and 1 µM R6G.
Figure 1. TEM micrographs (a) spherical, (b) shaped Au nanoparticles, and (c) TSA capped Au nanoparticles.
spherical and shaped Au nanoparticles. The absorption peaks centered at 524 and 526 nm are due to plasmon band for spherical and shaped Au nanoparticles, respectively. It is reported that the plasmon band appears at 800-900 nm region for 18 aspect ratio gold nanorods.26 In the shaped Au nanoparticles having different shapes of Au nanoparticles did not show any plasmon band at 800-900 nm region. Actually, the absorption band at around 525 nm is interesting to our present study because this band overlaps with the emission band of R6G dye. The PL peak at 550 nm is due to R6G dye. Figure 2 clearly shows that the emission spectrum of pure dye overlaps with the absorption spectrum of Au nanoparticles solution. A drastic quenching in PL intensity of R6G emission in presence of gold
nanoparticles is observed. But, there is no change in emission peak position of dye in presence of gold nanoparticles. The observed quenching of PL intensities are 85 and 48% for spherical and shaped Au nanoparticles which must be related to space interaction of dipole of the donor and the free electrons of metal nanoparticles.15 Figure 3 shows the absorption spectrum of TSA capped Au nanoparticles and photoluminescence spectra of R6G dye solution in TSA and with TSA capped Au nanoparticles. The absorption peak centered at 523 nm is due to plasmon band of TSA capped Au nanoparticles. The PL peak at 551 nm is due to R6G dye in both the cases. Here, we also observed a sharp decrease of PL intensity of R6G in presence of capped Au nanoparticles. In this case, the observed PL quenching is 52%. Figures 4 and 5 show absorption spectra of R6G in absence and in presence of different Au nanoparticles. To understand the origin of the quenching process, we calculate the radiative rate (kr) by using the following relation15
kr ) 3.13 × 10-9ν02
∫ d υj = ν02 f
(7)
where ν0 is the energy in wavenumbers corresponding to the maximum absorption of the dye, is the extinction coefficient, and f is the oscillator strength. The calculated radiative rate of the dye is 2.27 × 108 s-1 in absence of Au nanoparticles. However, the calculated radiative rates are 2.19 × 108 and 2.18 × 108 s-1 for the presence of spherical and shaped gold
Resonance ET from R6G to Au NPs
J. Phys. Chem. C, Vol. 112, No. 9, 2008 3219
Figure 4. Absorption spectra of (a) R6G dye solution, (b) shaped Au and 1 µM R6G, and (c) spherical Au and 1 µM R6G.
Figure 6. Decay curves of (a) rhodamine 6G (R6G) dye solution, (b) spherical Au and 1 µM R6G, and (c) shaped Au and 1 µM R6G.
Figure 5. Absorption spectra of (a) R6G dye in TSA solution and (b) TSA capped Au and 1 µM R6G.
Figure 7. Decay curves of (a) R6G dye in TSA solution and (b) TSA capped Au and 1 µM R6G.
nanoparticles, respectively, indicating very small change in radiative rate (Figure 4). The change in the oscillator strength is 3-4% which is very less compare to quenching of PL intensity (48 and 85%). The calculated radiative rates are 1.58 × 108 and 1.45 × 108 s-1 in absence and presence of dye of TSA capped Au nanoparticles, respectively (Figure 5). The change in the oscillator strength is 8% which is very less compare to PL quenching (52%). Now, we can definitely say that this PL quenching process is due to energy transfer process. Time-Resolved fluorescence Study. The time-resolved fluorescence spectra of aqueous solution of pure R6G dye and in presence of gold nanoparticle are shown in Figure 6. The photoluminescence decay time of the dye solution (1 µM) without Au is single exponential and the value is 3.92 ns. However, the decay times of dye molecules in presence of Au nanoparticles are fitted by biexponential decay (eq 4). The fast and slow components are 1.20 ns (76%) and 3.84 ns (24%) for the dye solution (1 µM) in presence of spherical (Table 1). The fast and slow components are 1.03 (37%) and 3.71 ns (63%) for the dye solution in presence of shaped nanoparticles (Table 1). The average decay times are 1.84 and 2.72 ns for spherical and shaped Au nanoparticles, respectively (eq 5). The timeresolved fluorescence spectra of R6G dye in TSA solution and in presence of TSA capped gold nanoparticles are shown in Figure 7. The decay time of the dye molecule in absence of TSA capped Au nanoparticles is single exponential and the value is 3.62 ns. Similarly, the decay time of dye molecule is
biexponential in presence of TSA capped Au nanoparticles and the fast component is 298 ps (40%) and slow component is 3.53 ns (60%). The average decay time is 2.24 ns (Table 1). It clearly reveals that there is a shortening of the decay time of dye in presence of Au nanoparticles which again confirms the energy transfer from dye to nanoparticles. Dulkeith et al.13 observed similar results in dye containing Au nanoparticles where the fluorescence lifetime decreases in presence of Au nanoparticles. Lifetime measurement is more sensitive than PL quenching efficiency because error comes from the fluctuations in lamp intensity. Therefore, the energy transfer efficiency from dye to Au nanoparticles is calculated by using the relation φET ) 1 - τDA/τD, where τDA is the decay time of dye in presence Au nanoparticles and τD corresponds to the decay time of dye in absence of Au nanoparticles. The calculated energy transfer efficiencies from dye to Au nanoparticles are 53.14, 30.65, and 38.12% for spherical, shaped and capped Au nanoparticles, respectively (Table 1). It reveals that energy transfer is fastest in spherical uncapped Au nanoparticles. Result suggests that the energy transfer efficiency depends on the shape and surface capping of the nanoparticles. Recently, Huang et al.28 reported that phonon frequency in the spherical nanoparticle is almost three times larger than that in the prism. Since the frequencies of the phonon are different in different shaped nanoparticles, thus the rate of energy transfer will undoubtedly be different.
3220 J. Phys. Chem. C, Vol. 112, No. 9, 2008
Sen and Patra
TABLE 1: Time-Resolved Fluorescence Quenching Studies for Different Rh6G-Au Nanoparticles Pairs system
b1
τ1(ns)
Rh6G in blank citrate Rh6G in citrate capped sphericalAu nanoparticles Rh6G in shaped Au Nanoparticles Rh6G in blank TSA Rh6G in TSA capped spherical Au nanoparticles
1 0.76 0.37 1 0.40
3.92 1.20 1.03 3.62 0.30
) (b1τ1+ b2τ2) (ns)
τ2(ns)
b2 0.24 0.63
3.84 3.71
0.60
3.53
3.92 1.84 2.72 3.62 2.24
E(%) 53.14 30.65 38.12
TABLE 2: Energy Transfer Parameters for Different Rh6G-Au Nanoparticles Pairs system
λem( nm)
Rh6G in blank citrate solution Rh6G in citrate capped spherical Au nanoparticles Rh6G in shaped Au nanoparticles Rh6G in blank TSA solution Rh6G in TSA capped spherical Au nanoparticles
549 550 552 550 551
J(λ) (M-1cm-1nm4) 1.00 × 1017 1.72 × 1018 2.89 × 1017
We calculated the distance between donor and acceptor, by using FRET and SET methods. Fo¨rster distance (R0) is calculated from the relation
R0 ) 0.211[κ2n-4φdyeJ(λ)]1/6(in angstroms)
(8)
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, and 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 (Figures 2 and 3) and the values are listed in Table 2. The calculated Fo¨rster distances (R0) are 105.75, 170, and 124 Å for spherical, shaped, and TSA capped Au nanoparticles, respectively. The calculated distance (d) between the donor and acceptor are 103.56, 194.77, and 134.43 Å for spherical, shaped and TSA capped Au nanoparticles, respectively (Table 2), using the efficiency of FRET which depends on the inverse sixth power of the distance of separations between donor and acceptor. The increase in distance between donor and acceptor from 103.56 to 134.43 Å with changing from uncapped to capped Au nanoparticles is definitely due to surface capping of Au nanoparticles. In case of shaped Au nanoparticles, the distance between dye and Au nanoparticles is too high (194.77 Å). It may be due to different kinds of attachment of dye molecules with shaped Au nanoparticles. Further work is necessary to understand this phenomenon. It indicates the shape of particles plays an important role on energy transfer process. It is already reported that FRET based method is restricted on the upper limit of only 80 Å,11 because the energy transfer becomes too weak to be useful. Therefore, we again calculate the distance between donor and acceptor by using surface energy transfer (SET) method. Actually, a large number of theoretical and experimental studies reported on the energy transfer from dye to a metal surface15-27 which follows a 1/d4 distance dependence. We also calculate the d0 value by using Persson model15,23
d0 )
(
3
0.225c Φdye ωdye2ωFkF
)
Φ0D 0.92 0.92 0.92 0.81 0.81
E(%) (PL)
R0 Å)
r Å)
d0 Å)
d Å)
kET (s-1)
85 48
105.75 170
103.56 194.77
80.76 80.76
78.26 99.04
2.9 × 108 1.13 × 108
52
124
134.43
78.34
88.43
1.7 × 108
cm s-1. The calculated d0 values are 80.76, 80.76, and 78.34 Å for spherical, shaped, and TSA capped Au nanoparticles, respectively (Table 2). The quantum efficiency of energy transfer in surface energy transfer process can be written as
φET )
1
()
d 1+ d0
4
(10)
The distances (d) between the donor and acceptor are 78.26, 99.04, and 88.43 Å for spherical, shaped and TSA capped Au nanoparticles, respectively (Table 2), using the efficiency of SET which depends on the inverse fourth power of the distance of separations between donor and acceptor. The increase in distance between donor and acceptor from 78.26 to 88.43 Å with changing from uncapped to capped Au nanoparticles is definitely due to surface capping of Au nanoparticles. In case of shaped Au nanoparticles, the distance between dye and Au nanoparticles is too high (99.04 Å), indicating the attachment of dye molecules varies with changing the shape of Au nanoparticles. It is interesting to note that similar trend is observed in FRET method also. As 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. Time-Resolved Anisotropy Study. The anisotropy decays of R6G in shaped and spherical Au nanoparticles are shown in Figure 8. The anisotropy is primarily determined by rotational motion of the fluorophore and these motions are dependent upon size and shape of the molecules.11 Recently, Veggel et al. studied time-resolved anisotropy for determining the particles shape and size distribution of Eu doped LaF3 nanoparticles.29 For spherical molecule the anisotropy decay is described by a single exponential11
r(t) ) r0e-t/φ
(11)
1/4
(9)
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.15 The d0 value was calculated using φdye ) 0.92 and 0.81, ω ) 3.6 × 1015 s-1, ωF ) 8.4 × 1015 s-1, and kF ) 1.2 × 108 cm-1, and c ) 3 × 1010
where r0 is the anisotropy observed in the absence of other depolarizing processes, t is the time, and φ is the rotational correlation time. The anisotropy decay of R6G with spherical Au nanoparticles is single exponential with correlation time constant of 240 ps. The single-exponential correlation time constant indicates that the shape of the particle is spherical. This correlation time constant may be due to segmental motions of the attached dye molecules to the Au nanoparticles. However, in presence of shaped nanoparticles, the fluorescence anisotropy decay of R6G is found to be very different from that of spherical
Resonance ET from R6G to Au NPs
J. Phys. Chem. C, Vol. 112, No. 9, 2008 3221 constant of 307 ps which is due to segmental motions of the dye molecule attached to the Au nanoparticles. As it gives single-exponential decay, therefore the shape of particles is spherical and result matches with the TEM data. We observed the change in correlation time constant from 240 to 307 ps for uncapped and capped Au nanoparticles indicating the different kinds of attachment of dye molecule for uncapped and capped Au nanoparticles. This result indicates that the time-resolved anisotropy study is essential to unraveling the origin of the motion of the dye molecules with changing with shape of nanoparticles. Conclusions
Figure 8. Fluorescence anisotropy decay curves of (i) spherical Au and 1 µM R6G and (ii) shaped Au and 1 µM R6G.
Figure 9. Fluorescence anisotropy decay curve of TSA capped Au and 1 µM R6G.
nanoparticles. It is reasonable because we observed multiexponential decay in the present case. In shaped nanoparticles for λex ) 405 nm, fluorescence anisotropy decay of R6G is fitted to a biexponential function
r(t) ) r0[a1R exp(t/φ1R) + a2R exp(t/φ2R)]
(12)
The origin of multiple correlation times is due to nonspherical shape. If the particle is not spherical, then there are different rotational rates around each axis. It is important to remember that anisotropic rotations can result in multiexponential decays of anisotropy.11 Here, the anisotropy decay of R6G in shaped nanoparticles is biexponential with correlation time constants of 347 ps and 2.46 ns. This suggests the fast correlation time constant is due to segmental motions of the dye molecule attached to the Au nanoparticles and slow correlation time constant may be due to the motion of dye molecules with superimposed motion of nanoparticles. It again suggests that the motion of the emitting species is hindered, this orientation does not recover to a random distribution. The biexponential correlation times are definitely due to shaped nanoparticles. The anisotropy decay of R6G in TSA capped Au nanoparticles is shown in Figure 9. The anisotropy decay of R6G in TSA capped Au nanoparticles is single exponential with correlation time
In summary, we studied the effect of shape and capped Au nanoparticles on the energy transfer efficiency between dye and Au nanoparticles with steady-state and time-resolved spectroscopy. To the best of our knowledge, this is the first report of resonance energy transfer from R6G to shaped and capped Au nanoparticles. In all the systems, we have obtained a higher value of J(λ); consequently, we obtained a longer Fo¨rster distance (R0) varying from 105.75, 170, and 124 Å for spherical, shaped, and TSA capped Au nanoparticles, respectively. We observed the change in the energy transfer efficiency (48-85%) and energy transfer rate constant (1.7 × 108-2.9 × 108 s-1) for spherical, shape, and capped Au nanoparticles. However, the distance between the donor and acceptor are 78.26, 99.04, and 88.43 Å for spherical, shaped, and capped Au nanoparticles, respectively, using the efficiency of surface energy transfer (SET). The single and biexponential anisotropy decay of R6G are observed for spherical and shaped Au nanoparticles, respectively. This fast correlation time constant may be due to segmental motions of the attached dye molecules to the Au nanoparticles. Acknowledgment. The Department of Science and Technology (NSTI) and “Ramanujan Fellowship” are gratefully acknowledged for financial support. T.S. thanks CSIR for awarding fellowship. References and Notes (1) Medintz, I. L.; Clapp, A. R.; Mattoussi, H.; Goldman, E. R.; Fisher, B.; Mauro, J. M. Nat. Mater. 2003, 2, 630. (2) 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. (3) Peng, H.; Zhang, L.; Kjallman, T. H. M.; Soeller, C.; Sejdic, J. T. J. Am. Chem. Soc. 2007, 129, 3048. (4) Warner, J. H.; Watt, A. R.; Thomsen, E.; Heckenberg, N.; Meredith, P.; Dunlop, H. R. J. Phys. Chem. B 2005, 109, 9001. (5) Dayal, S.; Burda, C. J. Am. Chem. Soc. 2006, 128, 13974. (6) Selmarten, D.; Jones, M.; Rumbles, G.; Yu, P.; Nedeljkovic, J.; Shaheen, S. J. Phys. Chem. B 2005, 109, 15927. (7) Zhou, D.; Piper, J. D.; Abell, C.; Klenerman, D.; Kang, D. J.; Ying, L. Chem. Commun. 2005, 4807. (8) Lu, W.; Tokuhiro, Y.; Umezu, I.; Sugimura, A.; Nagasaki, Y. Appl. Phys. Lett. 2006, 89, 143901-1. (9) Chowdhury, P. S.; Sen, P.; Patra, A. Chem. Phys. Lett. 2005, 413, 311. (10) Montalti, M.; Zaccheroni, N.; Prodi, L.; O’Reilly, N.; James, S. L. J. Am. Chem. Soc. 2007, 129, 2418. (11) Lakowicz, J. R. Principles of Fluorescence spectroscopy, 2nd ed.; Kluwer Academic/Plenum Publishers: New York, 1999. (12) Forster, T. Discuss. Faraday Soc. 1959, 27, 7. (13) Dulkeith, E.; Morteani, A. C.; Niedereichholz, T.; Khar, T. A.; Feldmann, J.; Levi, S. A.; Veggel, F. C. J. M.; Reinhoudt, D. N.; Moller, M.; Gittins, D. I. Phys. ReV. Lett. 2002, 89, 203002-1. (14) Dulkeith, E.; Ringler, M.; Klar, T. A.; Feldmann, J.; Javier, A. M.; Parak, W. J. Nano Lett. 2005, 5, 585. (15) Yun, C. S.; Javier, A.; Jennings, T.; Fisher, M.; Hira, S.; Peterson, S.; Hopkins, B.; Reich, N. O.; Strouse, G. F. J. Am. Chem. Soc. 2005, 127, 3115.
3222 J. Phys. Chem. C, Vol. 112, No. 9, 2008 (16) Jennings, T. L.; Singh, M. P.; Strouse, G. F. J. Am. Chem. Soc. 2006, 128, 5462. (17) Jennings, T. L.; Schlatterer, J. C.; Singh, M. P.; Greenbaum, N. L.; Strouse, G. F. Nano Lett. 2006, 6, 1318. (18) Sen, T.; Sadhu, S.; Patra, A. Appl. Phys. Lett. 2007, 91, 0431041. (19) Govorov, A. O.; Bryant, G. W.; Zhang, W.; Skeini, T.; Lee, J.; Kotov, N. A.; Slocik, J. M.; Naik, R. R. Nano Lett. 2006, 6, 984. (20) Liu, G. L.; Yin, Y.; Kunchakarra, S.; Mukherjee, B.; Gerion, D.; Jett, S. D.; Bear, D. G.; Gray, J. W.; Alivisatos, A. P.; Lee, L. P.; Chen, F. F. Nat. Nanotechnol. 2006, 1, 47. (21) Gersten, J.; Nitzan, A. J. Chem. Phys. 1981, 75, 1139. (22) Persson, B. N.; Lang, N. D. Phys. ReV. B 1982, 26, 5409. (23) Chance, R.; Prock, A.; Silbey, R. AdV. Chem. Phys. 1978, 60, 1.
Sen and Patra (24) Alivisatos, A. P.; Waldeek, D. H.; Harris, C. B. J. Chem. Phys. 1985, 82, 541. (25) Busbee, B. D.; Obare, S. O.; Murphy, C. J. AdV. Mater. 2003, 15, 414. (26) Jana, N. R.; Gearheart, L.; Murphy, C. J. J. Phys. Chem. B 2001, 105, 4065. (27) (a) Bhowmick, S.; Saini, S.; Shenoy, V. B.; Bagchi, B. J. Chem. Phys. 2006, 125 181102. (b) Swathi, R. S.; Sebastian, K. L. J. Chem. Phys. 2007, 126, 234701. (28) Huang, W.; Qian, W.; El-Sayed, M. A.; Ding, Y.; Wang, Z. L. J. Phys. Chem. C 2007, 111, 10751. (29) Bovero. E.; Veggel. F. C. J. M. V. J. Phys. Chem. C 2007, 111, 4529.
