J. Phys. Chem. C 2010, 114, 3891–3897
3891
Size Dependent Resonance Energy Transfer between Semiconductor Quantum Dots and Dye Using FRET and Kinetic Model Suparna Sadhu, Krishna Kanta Haldar, and Amitava Patra* Department of Materials Science, Indian Association for the CultiVation of Science, Kolkata-700 032, India ReceiVed: December 14, 2009; ReVised Manuscript ReceiVed: January 21, 2010
In the present study, we demonstrate the size dependent resonance energy transfer from CdSe QDs (donor) to Nile Red dye (acceptor) using steady state and time-resolved spectroscopy. A strong evidence of size dependent efficient resonance energy transfer between CdSe QDs and dye molecules is observed. Using the Fo¨rster theory, the calculated energy transfer efficiencies from QD to dye are 8.4, 13.8, and 51.2% for 2.4 nm CdSe, 2.9 nm CdSe, and 3.3 nm CdSe, respectively. A stochastic model for the kinetics of energy transfer from CdSe QDs to Nile Red dye molecules has been proposed to understand the interaction between excited states of CdSe QDs with dye molecules. By analyzing time-resolved fluorescence decay curves of CdSe QDs in the absence and in the presence of Nile Red dye, the values of the rate constant (kq) for energy transfer per one dye molecule and the efficiency (φET) of quenching have been calculated. The estimated energy transfer rates are 0.002, 0.016, and 0.038 ns-1 for 2.4 nm CdSe, 2.9 nm CdSe, and 3.3 nm CdSe QDs, respectively, which are well matched with FRET data. Introduction Fluorescence resonance energy transfer (FRET) is widely used for biological research to measure the molecular distances or donor-to-acceptor proximity.1 FRET occurs through the dipoledipole interactions between an excited donor (D) molecule and an acceptor (A). FRET has been used to determine submicroscopic distances in organic molecules,2 and a wide variety of macromolecular assemblies such as biological membranes3 and proteins.4 Over the past years, quantum dot (QD)-based fluorescence energy5-10 transfer has been paid great attention because of their narrow emission and broad excitation spectra to reduce background. Furthermore, the large size of QDs compared to organic dyes also provides the design of such configuration where multiple acceptors could interact with a single donor, which enhances FRET efficiency and thus measurement sensitivity.11 The potential applications are in the areas of luminescence tagging, imaging, medical diagnostics, multiplexing, and most recently as biosensors.11-14 As these potential applications are still very much in the design phase, further fundamental research in the field of QD-based fluorescence energy transfer remains a challenge. Medintz et al.11 reported the potential of luminescent semiconductor quantum dots for development of hybrid inorganic-bio receptor sensing materials. They demonstrated the use of luminescent CdSe-ZnS QDs as energy donors in FRET based assays with organic dyes as energy acceptors in QDs-dye labeled protein conjugates. In most cases, the energy transfer in QD based systems is analyzed by using Fo¨rster theory assuming the donor and acceptor as points in the interaction space. The shape-dependent resonance energy transfer between nanoparticles and dye has been reported.10 Schrier et al.15 theoretically demonstrated the shape dependence of resonance energy transfer between semiconductor nanocrystals. Again, it is reported16 that the composition of quantum dots plays a significant role in energy transfer between QD donor and proximal dye acceptors, because the spectral * Author to whom correspondence should be addressed. E-mail: msap@ iacs.res.in. Phone: (91)-33-2473-4971. Fax: (91)-33-2473-2805.
overlap varies with changing composition without changing the particle size. Recently, we proposed a stochastic model for energy transfer from CdS quantum dots and rods to dye.17 To our knowledge, there is no study on the size dependence of resonance energy transfer between CdSe QD and dye molecules by steady state and time-resolved spectroscopy. The distribution of acceptor molecules around QDs is essential to understand the kinetics of energy transfer by using a suitable model because this would be a governing factor for efficient energy transfer. It could be expected that photoexcitation, radiationless decay in the QD, dye to QD back transfer, vibrational energy redistribution, photoluminescence quenching by trap states present on the surface of QD, etc., take place during the quantum dot based energy transfer process. In the present study, we try to understand the interaction between the excited state of quantum dot (QD) of CdSe with dye molecules and present quantitative estimation of quenching parameters and compared with the experimental data. Experimental Section Materials. Stearic acid (95%, Aldrich), cadmium oxide (Merck), 1-octadecene (90%, Aldrich), trioctyl phosphine (90%, Aldrich), oleylamine (70%, Aldrich), and selenium powder (100 mesh, Aldrich) were used as received. Nile Red dye (Aldrich) was used as an acceptor for energy transfer study. The spectroscopic grade solvents (chloroform, toluene, methanol, and ethanol) were used for optical study. Preparation of CdSe QDs. A solution of TOP-Se was prepared by dissolving selenium (1 mmol, 0.078 g) in a mixture of trioctylphosphine (TOP) (0.5 mL) and 1-octadecene (0.5 mL) at 50 °C temperatures under vigorous stirring. CdO (1 mmol) and stearic acid (2.5 mmol) were mixed in 1-octadecene (ODE) (5 mL) and heated to 250 °C under Ar flow. After the solution became clear, the temperature was reduced to 150 °C and 1 mL of TOP-Se solution was injected into it. The temperature was raised to 175 °C, and the reaction mixture was kept at this temperature for the required time. The reaction was quenched by injection of a small amount of hot reaction mixture in a large
10.1021/jp911801m 2010 American Chemical Society Published on Web 02/12/2010
3892
J. Phys. Chem. C, Vol. 114, No. 9, 2010
Sadhu et al.
volume of toluene at room temperature. The size of CdSe nanocrystals was controlled by varying the reaction time. The nanocrystals were separated from the toluene solution by the addition of ethanol and centrifuged, and the washing was carried out for several times. After the purification, the nanocrystals were treated with oleylamine (0.2 mL in 5 mL of chloroform) to stabilize it in chloroform. Physical Characterization. The transmission electron microscopy (TEM) images were taken using a JEOL-TEM-2010 transmission electron microscope operating voltage at 200 kV to analyze the shape, size, size distribution, and structure of the resulting QDs. Samples for TEM were prepared by making a clear solution of samples in choloroform and placing a drop of the solution on a carbon coated copper grid. Optical Measurements. All optical measurements were carried out promptly (within 24 h after the synthesis) on samples in order to minimize reabsorption and avoid absorption saturation. Absorption and fluorescence spectra of CdSe QDs in choloroform (Spectroscopic grade) solution were taken at room temperature with a Shimadzu UV-2450 UV-vis spectrometer and a Horiba Jobin Yvon Fluoro Max-P fluorescence spectrometer, respectively. Photoluminescence quantum yields (QY) were obtained by comparison with standard dye (rhodamine 6G dye in deionized water), using the following equation9
QYs ) (Fs × Ar × ns2 × QYr)/(Fr × As × nr2)
(1)
where Fs and Fr are the integrated fluorescence emission of the sample and the standard, respectively. As and Ar are the absorbance at the excitation wavelength of the sample and the reference, respectively, and QYs and QYr are the quantum yields of the sample and the reference (QYr ) 95%), respectively. The refractive indices of the solvents in which the sample and reference are prepared are given by ns (1.445) and nr (1.0), respectively. The values of Fs and Fr are determined from the photoluminescence spectra corrected for the instrumental response, by integrating the emission intensity over the desired spectral range. Only the band-edge luminescence peak was integrated (any other luminescence bands, such as defect associated luminescence or solvent fluorescence, were discarded as background). For the time correlated single photon counting (TCSPC) measurements, all samples were excited at 375 nm using a nano LED in an IBH Fluorocube apparatus. The pulse duration is about 1.2 ns. The repetition rate is 500 kHz. The fluorescence decays were collected at a Hamamatsu MCP photomultiplier (C487802). The fluorescence decays were analyzed using IBH DAS6 software.
Figure 1. (a) Absorption spectra of CdSe QDs: (1) 2.4 nm CdSe, (2) 2.9 nm CdSe, and (3) 3.3 nm CdSe. (b) TEM image of CdSe QDs.
Results and Discussion
respectively. Figure 1b shows the TEM image of particles of the 546 nm CdSe QDs. Results reveal uniform spherical QDs with an average diameter of 3.1 ( 0.2 nm. This TEM data matches well with the calculated value. Figure 2 shows the normalized absorption spectrum of Nile Red dye in chloroform and photoluminescence (PL) spectra of CdSe QDs of different sizes. The emission peaks are at 530, 545, and 565 nm for CdSe nanoparticles under excitation at 370 nm for 2.4 nm CdSe, 2.9 nm CdSe, and 3.3 nm CdSe QDs. The absorption spectrum of Nile Red dye shows a peak centered at 538 nm. As shown in Figure 2, the absorption spectrum of dye overlaps with the emission spectrum of CdSe QDs. Generally, FRET rates depend strongly on the overlap between donor emission and acceptor absorption spectra and the spectral overlap is constant for a given donor/acceptor system. Pons et al.18 demonstrated the effect of size heterogeneity on the spectral
Steady-State Study. Figure 1a displays the absorption spectra of three different sizes of CdSe QDs. The first excitonic absorption peaks are 508, 522, and 546 nm. The average size of the CdSe nanocrystals was calculated from the first excitonic absorption peak of the corresponding samples using the formula7
∆E ) Eeff g - Eg )
π2p2 2µR2
(2)
where R is the particle radius, µ is the effective reduced mass, Eg is the bulk bandgap energy (2.5 eV), and Egeff is the effective band gap energy. The average sizes are 2.4, 2.9, and 3.3 nm for the first excitonic absorption peaks 508, 522, and 546 nm,
Figure 2. Emission spectra of CdSe QDs: (1) 2.4 nm CdSe, (2) 2.9 nm CdSe, (3) 3.3 nm CdSe, and absorption spectrum (4) of Nile Red dye in choloroform.
Size Dependent Resonance Energy Transfer
J. Phys. Chem. C, Vol. 114, No. 9, 2010 3893
overlap between QD donors and proximal dye acceptors in the FRET process. The overlap functions are defined as1
J(λ) )
∫0∞ FD(λ)εAλ4 dλ ∫0∞ FD(λ) dλ
(3)
where FD(λ) is the normalized emission spectrum of the donor (CdSe) and εA(λ) is the absorption coefficient of the acceptor (Nile Red dye) at wavelength λ (λ in nanometers). The values of overlap integrals are 2.476 × 1015, 3.377 × 1015, and 5.01 × 1015 M-1 cm-1 nm4 for 2.4 nm CdSe, 2.9 nm CdSe, and 3.3 nm CdSe QDs, respectively. It is interesting to note that the overlap integral value increases with increasing size of particles. Nile Red dye has negligible absorption at 370 nm; therefore, the CdSe-dye complexes were excited at this wavelength to excite the QDs selectively. It is known from the previous study19 that van der Waals type interactions between CdSe nanocrystals and dye molecules contribute to the binding of the energy acceptor to the nanocrystal’s surface. A systematic quenching in PL intensity of QDs with changing dye concentrations is observed (Figure 3). Photoluminescence intensity of CdSe QDs decreases and dye emission intensity increases as the molar ratio of Nile Red dye to CdSe nanoparticles increases, which is evidence of the energy transfer process. This PL quenching is due to the energy transfer from semiconducting nanoparticles to dye molecules. On the basis of the relationship between the quenching of excited states and quencher concentration, the Stern-Volmer equation is given by1
F0 ) 1 + kqτ0[Q] ) 1 + KSV[Q] F
(4)
where F0 and F are the PL intensity in the absence and presence of quencher, respectively. KSV is the Stern-Volmer quenching constant, and [Q] is the concentration of the quencher. In the present study, the dye concentration varies from 1 to 12 µM, because Stern-Volmer theory is only valid for diluted solution. The Stern-Volmer plot of F0/F versus quencher concentration is given in Figure 4. By the linear fitting of the data, the slope of the plot, KSV, was found to be 0.059 × 106, 0.102 × 106, and 0.013 × 106 M-1 for 2.4 nm CdSe, 2.9 nm CdSe, and 3.3 nm CdSe, respectively. The estimated rate constant (kq) values are 4.97 × 1012, 7.01 × 1012, and 0.52 × 1012 M-1s-1 for 2.4 nm CdSe, 2.9 nm CdSe, and 3.3 nm CdSe, respectively. Time-Resolved Fluorescence Study. To confirm the energy transfer from semiconducting nanoparticles to dye molecules, time-correlated single-photon counting (TCSPC) study was performed because decay time measurements are more sensitive than PL quenching efficiencies where errors come from the fluctuations in the lamp intensity. We used pulsed excitation (375 nm) to measure the decay times of these nanoparticles at their maximum PL peak. Figure 5 shows the time-resolved fluorescence decay curves of three different sized CdSe QDs without and with Nile Red dye solution. The decay profiles are well fitted with two-exponential function, I(t) ) R1 exp(-t/τ1) + R2 exp(-t/τ2). The average decay times are 11.84, 15.03, and 24.86 ns for 2.4 nm CdSe, 2.9 nm CdSe, and 3.3 nm CdSe, respectively, in the absence of Nile Red dye, and the average decay times are 10.85, 12.95, and 12.12 ns for 2.4 nm CdSe, 2.9 nm CdSe, and 3.3 nm CdSe QDs, respectively, in the presence of dye (Table 1). It clearly reveals that there is a
Figure 3. Quenching of photoluminescence emission of CdSe QDs: (a) 2.4 nm CdSe, (b) 2.9 nm CdSe, and (c) 3.3 nm CdSe at different CdSe/Nile Red dye ratios.
shortening of the decay time of nanocrystals in the presence of dye which is one of the hallmarks of efficient FRET between donor-acceptor molecules. The decrease in lifetime further confirms the energy transfer from CdSe nanoparticles to Nile Red dye. The energy transfer efficiency from nanoparticles to dye can be calculated using eq 5
φET ) 1 - τDA /τD
(5)
where τDA is the decay time of CdSe in the presence of dye and τD corresponds to the decay time in the absence of dye. The calculated energy transfer efficiencies from nanoparticles to dye are 8.4, 13.85, and 51.25% for 2.4 nm CdSe, 2.9 nm CdSe, and 3.3 nm CdSe QDs, respectively (Table 2). The
3894
J. Phys. Chem. C, Vol. 114, No. 9, 2010
Sadhu et al.
Figure 4. Stern-Volmer plots of (a) 2.4 nm CdSe, (b) 2.9 nm CdSe, and (c) 3.3 nm CdSe QDs.
distance between donor and acceptor has been estimated using the FRET method. Fo¨rster distance (R0) is calculated from the relation
R0 ) 0.211[κ2n-4φDJ(λ)]1/6
(in angstroms)
(6)
where k2 is the orientation factor of two dipoles interacting and is usually assumed to be equal to 2/3, φD is the quantum efficiency of the donor, J(λ) is the overlap integral between the absorption peak of the acceptor and emission peak of the donor, and n is the refractive index of the medium. The estimated Fo¨rster distances (R0) are 50.61, 54.36, and 51.30 Å for 2.4 nm CdSe, 2.9 nm CdSe, and 3.3 nm CdSe QDs, respectively (Table 2). Alphande´ry et al.20 reported a Fo¨rster distance of ∼3-3.5 nm between CdTe and rhodamine B dye. Kagan et al.21 reported the same order of magnitude for high efficient FRET between QDs having different sizes. Medintz and co-workers22 reported a Fo¨rster distance in the range 42-55 Å for QD with peptide/
Figure 5. Time-resolved fluorescence decay curves of CdSe QDs: (a) 2.4 nm CdSe, (b) 2.9 nm CdSe, and (c) 3.3 nm CdSe in the absence (1) and presence (2) of Nile Red dye molecules. The fitted curves are shown in red.
TABLE 1: Decay Parameters for CdSe QDs without and with NR Dye in Choloroform system
τ1 (ns)
τ2 (ns)
R1
R2
〈τ〉 (ns)
2.4 nm CdSe 2.4 nm CdSe + NR dye 2.9 nm CdSe 2.9 nm CdSe + NR dye 3.3 nm CdSe 3.3 nm CdSe + NR dye
1.03 1.05 0.98 1.01 3.27 1.22
12.15 11.44 15.34 13.42 25.26 12.34
24.99 39.78 25.48 34.37 11.81 16.92
75.01 60.22 74.54 65.63 88.19 83.08
11.84 10.85 15.03 12.95 24.86 12.12
dye conjugates. In the present study, the estimated distances (r) between QDs and dye are 75.37, 73.73, and 50.86 Å for 2.4 nm CdSe, 2.9 nm CdSe, and 3.3 nm CdSe QDs, respectively (Table 2), considering one donor and one acceptor interaction, using the efficiency of FRET. Now, the rate of energy transfer is given by
Size Dependent Resonance Energy Transfer
kT(r) )
( )
1 R0 τD r
J. Phys. Chem. C, Vol. 114, No. 9, 2010 3895 ∞
6
(7)
I(t, m) ) I0 ) I0
where τD is the lifetime of the donor in the absence of the acceptor. The calculated energy transfer rates are 0.008, 0.011, and 0.042 ns-1 for 2.4 nm CdSe, 2.9 nm CdSe, and 3.3 nm CdSe QDs, respectively (Table 2), which is strong evidence of size dependent resonance energy transfer between CdSe QDs and dye. It is already demonstrated that the dipole moment of CdSe QDs linearly depends on the radius of the QD.23 Thus, electronic coupling and Coulombic interaction are to be increased with increasing size of particles.15 This will explain the energy transfer rate increases with increasing size of the particle. For understanding the interaction between the excited state of CdSe nanoparticles with dye molecules, it is essential to know the distribution of acceptor molecules around the nanocrystals because this is a governing factor for efficient energy transfer. We assume that the energy transfer occurs in competition with unimolecular decay processes:
k0
QDn* 98 QDn
nkq
QDn* 98 QDn
∑ (mn/n!) exp(-m) exp[-(k0 + nkq)t]
n)0
∞
) I0 exp(-k0t - m)
∑ {[m exp(-kqt)]n/n!}
n)0
) I0 exp(-k0t - m) exp[m exp(-kqt)] ) I0 exp{-k0t - m[1 - exp(-kqt)]}
(11) The above kinetic model is a simplified version of the model developed by Tachiya26 for luminescence quenching in micelles. His general model is described by eq 1 in ref 26, and the decay curve of the excited CdSe nanoparticles is given by his eq 2′. Along with the acceptor dye molecules, there exist some unidentified traps on the surface of the nanocrystals and these are also taken into account. If the distribution of the number of unidentified traps on the surface of CdSe nanoparticles follows a Poisson distribution with the average number (mt), the decay curves of the excited state of CdSe nanoparticles in the absence and presence of dye molecules are described by
I(t, 0) ) I0 exp{k0t - mt[1 - exp(-kqtt)]}
(8)
(12)
I(t, m) ) I0 exp{-k0t - mt[1 - exp(-kqtt)] m[1 - exp(-kqt)]} (13)
(9)
where QDn* stands for excited state CdSe QD with n dye molecules attached, while QDn stands for ground state CdSe QD with n dye molecules attached. k0 is the decay constant due to the unimolecular process of the excited state donor in the absence of acceptor molecules, and kq is the rate constant for energy transfer per one dye molecule. When a nanoparticle with n dye molecules is excited, the rate constant of the excited state decay for that nanoparticle is given by k0 + nkq and the total energy transfer rate constant is nkq. In this kinetic model,17 it is assumed that the distribution of the number of dye molecules attached to one CdSe nanoparticle follows a Poisson distribution,24 namely,
Φ(n) ) (mn /n!) exp(-m)
∑ Φ(n) exp[-(k0 + nkq)t]
n)0 ∞
(10)
where m is the mean number of dye molecules attached to one QD. Therefore, the ensemble averaged decay curve of the excited QDs with the mean number of dye molecules attached is given by25
where the quenching rate constant (kqt) by unidentified traps may be different from that (kq) by dye molecules. We have determined the values of the parameters mt, kqt, k0, m, and kq by fitting eqs 12 and 13 to the decay curves in the absence and presence of dye molecules. Figure 5 shows the time-resolved fluorescence decay curves of three different sized CdSe QDs in the absence and presence of Nile Red dye molecules, and red curves show the result of fitting the curves with eqs 12 and 13. It is seen that the model describes the decay curves reasonably well. The quenching parameters are summarized in Table 3. The quenching rate constant (kqt) due to unidentified traps on the surface of the nanocrystals are the same even after addition of dye because we consider the surface trap states to be the same. It is seen from Table 3 that the average number of unidentified trap states increases with addition of dye. Unfortunately, there are still many unknowns in nanocrystal excitation dynamics and these will require more complex models and much larger data sets to unravel the phenomenon. It is seen that the mean number of dye molecules attached to QD increases with increasing size of the QD. The estimated energy transfer rates are 0.002, 0.016, and 0.038 ns-1 for 2.4 nm CdSe, 2.9 nm CdSe, and 3.3 nm CdSe, respectively. The efficiency of quenching given by eq 5 can also be calculated on the basis of the same kinetic model. According to this model,
TABLE 2: Energy Transfer Parameters for Different Sizes of CdSe QDs and NR Dye in Choloroform Using the FRET Method system
λem (nm)
J(λ) (M-1 cm-1 nm4)
Φ0D
E (%)
R0 (Å)
r (Å)
kT(r) (ns-1)
2.4 nm CdSe + NR dye 2.9 nm CdSe + NR dye 3.3 nm CdSe + NR dye
530 545 565
2.476 × 1015 3.377 × 1015 5.01 × 1015
0.50 0.56 0.27
8.4 13.85 51.25
50.61 54.36 51.30
75.37 73.73 50.86
0.008 0.011 0.042
3896
J. Phys. Chem. C, Vol. 114, No. 9, 2010
Sadhu et al.
φET ) 1 - I/I0 where I/I0 is given by R
R
∑∑
n)0 n′)0
(
I ) I0
)(
n′ -mt mne-m mt e n! n′!
)
(
1 kq kqt 1 + n + n′ k0 k0
mtn′e-mt kqt n′)0 n! 1 + n′ k0 R
∑
(
)
) (14)
The quenching efficiencies are found to be 2.3, 10.8, and 53.6% for 2.4 nm CdSe, 2.9 nm CdSe, and 3.3 nm CdSe, respectively, and the FRET data are 8.4, 13.8, and 51.2% for 2.4 nm CdSe, 2.9 nm CdSe, and 3.3 nm CdSe, respectively, which are nicely matched. It reveals that the size of the QDs played an important role in the energy transfer process. Finally, we have plotted the rate of quenching (Figure 6a) and the efficiency of quenching (Figure 6b) with the particle size of CdSe nanocrystals. This figure shows an approximately linear dependence on the size and the quenching rate estimated from the kinetic model is slightly faster or slower than expected from the FRET method. Moreover, it is interesting to note that the FRET data are nicely matched with kinetic model data. Therefore, this kinetic model can be used to estimate the exact number of acceptor molecules that participate in the energy transfer process which will be helpful in various biological and biomedical applications.
Figure 6. Plots of (a) the rate of quenching vs the particle size of CdSe QDs and (b) the efficiency of quenching vs the particle size of CdSe QDs.
Conclusions To the best of our knowledge, this is the first report to understand the size dependent resonance energy transfer study from CdSe QDs to Nile Red dye using FRET and kinetic model. Steady state and time-resolved spectroscopies are used to understand the quenching process. There is a shortening of the decay time and PL quenching of QDs in the presence of dye, which indicates efficient FRET between donor-acceptor molecules. We have demonstrated that the energy transfer from QD to dye varies with changing the size of QDs. Using the FRET process, the estimated distances (r) between nanocrystals and dye are 75.37, 73.73, and 50.86 Å for 2.4 nm CdSe, 2.9 nm CdSe, and 3.3 nm CdSe QDs, respectively, considering one donor and one acceptor interaction. We have proposed a kinetic model for the kinetics of energy transfer from quantum dots of CdSe to Nile Red dye molecules assuming the Poisson distribution of the dye molecules around QDs. The estimated energy transfer rates are 0.002, 0.016, and 0.038 ns-1 for 2.4 nm CdSe, 2.9 nm CdSe, and 3.3 nm CdSe, respectively, which nicely matched with FRET data. The quenching efficiencies are found to be 2.3, 10.8, and 53.6% for 2.4 nm CdSe, 2.9 nm CdSe, and TABLE 3: Overview of the Values of Quenching Parameters Using the Kinetic Model system
k0 (ns-1)
mt
kqt (ns-1)
2.4 nm CdSe 2.4 nm CdSe + NR dye 2.9 nm CdSe 2.9 nm CdSe + NR dye 3.3 nm CdSe 3.3 nm CdSe + NR dye
0.035 0.035 0.02 0.02 0.012 0.012
2.27 2.95 1.76 2.41 0.79 1.13
0.35 0.35 0.44 0.44 0.29 0.29
m
kq (ns-1) E (%)
0.21
0.002
2.3
0.28
0.016
10.87
1.23
0.038
53.6
3.3 nm CdSe, respectively, varying with changing the size of the QDs. Analysis suggests that the size of the QDs has played an important role in the energy transfer process. The observed properties of the nanoassemblies are promising for their potential applications to the development of FRET based nanosensors. Acknowledgment. A.P. thanks The Department of Science and Technology (NSTI) and “Ramanujan Fellowship” for generous funding. S.S. thanks CSIR for awarding a fellowship. The authors thank Prof. M. Tachiya for helping us to develop this kinetic model. References and Notes (1) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Plenum: New York, 1983. (2) Stryer, L. Annu. ReV. Biochem. 1978, 47, 819–846. (3) Gorbenko, G. P.; Domanov, Y. A. J. Biochem. Biophys. Methods 2002, 52, 45–58. (4) Scholes, G. D. Annu. ReV. Phys. Chem. 2003, 54, 57–87. (5) Clapp, A. R.; Medintz, I. L.; Mattoussi, H. ChemPhysChem 2006, 7, 47–57. (6) Rogach, A. L.; Klar, T. A.; Lupton, J. M.; Meijerink, A.; Feldmann, J. J. Mater. Chem. 2009, 19, 1208–1221. (7) Chowdhury, P. S.; Sen, P.; Patra, A. Chem. Phys. Lett. 2005, 413, 311–314. (8) Curutchet, C.; Franceschetti, A.; Zunger, A.; Scholes, G. D. J. Phys. Chem. C 2008, 112, 13336–13341. (9) Sadhu, S.; Patra, A. ChemPhysChem 2008, 9, 2052–2058. (10) Artemyev, M.; Ustinovich, E.; Nabiev, I. J. Am. Chem. Soc. 2009, 131, 8061–8065. (11) Medintz, I. L.; Clapp, A. R.; Mattoussi, H.; Goldman, E. R.; Fisher, B.; Mauro, J. M. Nat. Mater. 2003, 2, 630–638. (12) Clapp, A. R.; Medintz, I. L.; Mauro, J. M.; Fisher, B. R.; Bawendi, M. G.; Mattoussi, H. J. Am. Chem. Soc. 2004, 126, 301–310.
Size Dependent Resonance Energy Transfer (13) Peng, H.; Zhang, L.; Kjallman, T. H. M.; Soeller, C.; Sejdic, J. T. J. Am. Chem. Soc. 2007, 129, 3048–3049. (14) Zhou, D.; Piper, J. D.; Abell, C.; Klenerman, D.; Kang, D. J.; Ying, L. Chem. Commun. 2005, 4807–4809. (15) Schrier, J.; Wang, L. W. J. Phys. Chem. C 2008, 112, 11158–11161. (16) Sadhu, S.; Patra, A. Appl. Phys. Lett. 2008, 93, 183104–1. (17) Sadhu, S.; Tachiya, M.; Patra, A. J. Phys. Chem. C 2009, 113, 19488–19492. (18) Pons, T.; Medintz, I. L.; Sykora, M.; Mattoussi, H. Phys. ReV. B 2006, 73, 245302-1–245302-7. (19) Dayal, S.; Lou, Y.; Samia, A. C. S.; Berlin, J. C.; Kenney, M. E.; Burda, C. J. Am. Chem. Soc. 2006, 128, 13974–13975. (20) Alphande´ry, E.; Walsh, L. M.; Rakovich, Y.; Bradley, A. L.; Donegan, J. F.; Gaponik, N. Chem. Phys. Lett. 2004, 388, 100–104.
J. Phys. Chem. C, Vol. 114, No. 9, 2010 3897 (21) Kagan, C. R.; Murray, C. B.; Bawendi, M. G. Phys. ReV. B 1996, 54, 8633–8643. (22) Medintz, I. L.; Clapp, A. R.; Brunel, F. M.; Tiefebrunn, T.; Uyeda, H. T.; Chang, E. L.; Deschamps, J. R.; Dawson, P. E.; Mattoussi, H. Nat. Mater. 2006, 5, 581–589. (23) Blanton, S. A.; Leheny, R. L.; Hines, M. A.; Guyot-Sionnest, P. Phys. ReV. Lett. 1997, 79, 865–868. (24) Tachiya, M. Chem. Phys. Lett. 1975, 33, 289–292. (25) Koole, R.; Luigies, B.; Tachiya, M.; Pool, R.; Vlugt, T. J. H.; Donega, C. D. M.; Meijerink, A.; Vsanmaekelbergh, D. J. Phys. Chem. C 2007, 111, 11208–11215. (26) Tachiya, M. J. Chem. Phys. 1982, 76, 340–348.
JP911801M
