Metal Conjugated Semiconductor Hybrid Nanoparticle-Based

Mar 2, 2010 - Author to whom correspondence should be addressed: electronic mail, [email protected]; phone, (91)-33-2473-4971; fax, (91)-33-2473-2805. ...
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J. Phys. Chem. C 2010, 114, 4869–4874

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Metal Conjugated Semiconductor Hybrid Nanoparticle-Based Fluorescence Resonance Energy Transfer Krishna Kanta Haldar, Tapasi Sen, and Amitava Patra* Department of Materials Science, Indian Association for the CultiVation of Science, Kolkata, 700 032, India ReceiVed: NoVember 30, 2009; ReVised Manuscript ReceiVed: February 15, 2010

In the present study, we demonstrate a pronounced effect on the photoluminescence (PL) quenching and shortening of decay time of CdSe quantum dots (QDs) during interaction with Au nanoparticles in a Au-BSA conjugated CdSe QD system. A systematic blue shift of the excitonic band of CdSe QDs and the red shifting of a plasmon band of Au nanoparticles are observed in a Au-BSA conjugated CdSe QDs system. Strong evidence of size dependent efficient resonance energy transfer between CdSe QDs and Au nanoparticles is observed. The PL quenching values are 60%, 40%, and 30% for 5.0 nm CdSe QDs, 5.4 nm CdSe QDs, and 5.8 nm CdSe QDs, respectively. The energy transfer efficiencies are 40.9%, 30%, and 19.2% for 5.0 nm CdSe, 5.4 nm CdSe, and 5.8 nm CdSe QDs, respectively. Using the FRET process, the measured distances (d) between the donor and acceptor are 95.3, 102.2, and 110.3 Å for Au-BSA conjugated 5.0 nm CdSe, Au-BSA conjugated 5.4 nm CdSe, and Au-BSA conjugated 5.8 nm CdSe QDs, respectively. These results are well matched with the structural estimated value, transmission electron microscopy data, and data from dipole approximation. Such energy transfer between QDs and Au nanoparticles provides a new paradigm for design of an optical based molecular ruler for the application in chemical sensing. Introduction Research in the field of quantum dot (QD)-based fluorescence energy transfer has recently received great attention in the search for potential applications in the areas of luminescence tagging, imaging, medical diagnostics, multiplexing, and most recently biosensors.1–3 It is now well established that QDs are used in fluorescence resonance energy transfer (FRET) because of several advantages, i.e., their narrow emission and broad excitation spectra to reduce background.1 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.1 Fo¨rster resonance energy transfer is a well-known powerful method to determine the distance between donor and acceptor fluorophores.4 In the FRET process, 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 between donor and acceptor molecules. According to the Fo¨rster theory, 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 * Author to whom correspondence should be addressed: electronic mail, [email protected]; phone, (91)-33-2473-4971; fax, (91)-33-2473-2805.

transfer rate kT(r) is equal to the decay rate of the donor in the absence of the acceptor. On the other hand, 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.5 Recently, semiconductor-metal hybrid nanomaterials have drawn a lot of attention because of their potential applications for the fabrication of light-emitting devices and solar cells, etc., due to exciton-plasmon interactions.6–8 Kotov at al. demonstrated plasmon-exciton interactions between metal and semiconductor nanoparticles assembled into superstructures which could be useful for a wavelength-based biodetection tool.6a Again, they reported that Au-conjugated CdTe nanowires showed 5-fold enhancement of luminescence intensity and a blue shift for the emission peak as compared to unconjugated CdTe nanowires.6b However, they did not observe noticeable shift of the exciton peak in the case of a Au-NPCdTe-NP assembly.6c Mattoussi et al. reported strong fluorescence quenching of CdSe-ZnS quantum dots in the presence of Au nanoparticles.7a,b They demonstrated that nonradiative quenching of the QD emission by Au nanoparticles is due to long distance dipole-metal interactions which is beyond the classical Fo¨rster range. Very recently, Kanemitsu et al. showed the direct and stepwise energy transfer from excitons to plasmons in close-packed metal and semiconductor nanoparticle monolayer films.7c Recently, Kotov et al. and Bagchi et al. have done extensive work on the theoretical understanding of plasmon-exciton coupling interaction between semiconducting nanoparticles and metal nanoparticles.8 The rate of energy transfer is proportional to 1/d6 when the separation between donor and acceptor is larger than the radius of the nanoparticle. However, the rate of energy transfer varies as 1/dσ, where σ lies between 3 and 4 when the separation, d ) a and d ) 4a. Application of metal-semiconductor hybrid nanoparticle-based

10.1021/jp911348n  2010 American Chemical Society Published on Web 03/02/2010

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fluorescence energy transfer using nanoscopic environment is still in the embryonic stage, further investigation in this field is necessary for in-depth understanding of the phenomenon. The tunability of these highly organized materials offers fascinating new possibilities for exploring energy transfer phenomena for developing new challenging photonic devices. The aim of the work focuses on the fabrication of Au-BSA conjugated CdSe QDs and study the influence of the size of CdSe QD on the energy transfer between CdSe QDs and Au nanoparticles by steady-state and time-resolved spectroscopy. The distance between donor and acceptor is estimated using FRET method and this distance can be correlated with the data measured by dipole approximation and TEM data. All investigations are done in aqueous solution in order to match biological conditions. Such metal conjugated QDs multifunctional nanoparticles should have great potentials for optical-based molecular ruler. Experimental Section Materials. Chloroauric acid (HAuCl4 · 3H2O, S. d. Fine Chem), bovine serum albumin (BSA, Sigma), and CdO (99.5%), Se powder (99.99%), stearic acid (99.99%), 1-octadecene, 3-mercaptopropionic acid (3-MPA), 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC), Rhodamine 6G, and Rhodamine B dye were purchased from Aldrich and sodium borohydride (NaBH4), chloroform (chemical grade), n-hexane (analytical grade), diethyl ether (analytical grade), ethyl acetate (analytical grade), methanol (analytical grade), and alcohol (analytical grade) were purchased from Merck India Ltd. All of the reagents were used as received. Synthesis of BSA Conjugated Gold Nanoparticles. BSA conjugated gold nanoparticles were prepared using our previous method.9 For the preparation of BSA conjugated Au nanoparticles at pH 7.0, 0.1 µmol of BSA (0.0066 g) was dissolved in 9.5 mL of PBS buffer solution of pH 7.0. Then, 0.5 mL of 10 mM HAuCl4 solution was added into the BSA buffer solution. After 5 min of stirring, 100 µL of ice cold NaBH4 aqueous solution (0.001 g/mL) was added to the BSA solution slowly under constant stirring. The solution color changes from light yellow to reddish brown. The colloidal solutions were stored at 4 °C prior to use. The concentration of the as-prepared Au nanoparticles solution was calculated to be 1.24 × 10-7 M under the assumption that all Au3+ ions converted to Au0. No separation technique to extract the excess protein in the bioconjugates was used because this may disrupt the original conformation of the protein in the bioconjugates.9 Synthesis of CdSe QDs. A Se precursor solution was prepared by dissolving Se powder (0.15 mmol, 0.019 g) in 2 mL of 1-octadecene at 200 °C (above the melting temperature of Se) under vigorous stirring. A 0.0127 g (0.1 mmol) portion of CdO, 0.114 g (0.4 mmol) of stearic acid (SA), and 8 mL of 1-octadecene were mixed in a three-neck flask, and the temperature were gradually increased to 150 °C under argon flow for the preparation of Cd precursor solution. After the solution became clear, the temperature was raised to 250 °C and then 1.94 g of TOPO was added to the reaction mixture. Now 2 mL of a solution that contained Se precursors was quickly injected into the Cd precursor solution with vigorous stirring, and the growth temperature was kept at 250 °C.10 The size of CdSe nanocrystals was controlled by varying reaction time. Aliquots were taken at different time intervals, 5 min (greenish-yellow), 10 min (yellow), and 15 min (orange), and immediately put into n-hexane to prevent further growth. The nanocrystals were separated from the n-hexane solution by the addition of ethanol and centrifuged, and the washing was carried

Haldar et al. SCHEME 1: Schematic Representation for the Synthesis of Au-BSA Conjugated CdSe QDs

out several times. After purification, finally the TOPO-capped nanocrystals greenish-yellow (GY), yellow (Y), and orange (Or) were dispersed in chloroform for further study. Preparation of Water-Soluble Quantum Dots. For preparation of water-soluble QDs with functional groups, TOPO on the surface of as-synthesized QDs must be exchanged with MPA.11,12 Now 0.5 mL of MPA was dissolved in 10 mL of double distilled water and subsequently the pH of the solution was increased to 10 by adding 5 mM sodium hydroxide (the NaOH solution was added to the MPA solution in order to deprotonate the thiol groups of MPA), and then 10 mL of TOPO-capped QD nanocrystals (in chloroform) was added to this solution under stirring, resulting in a two-phase system (water above chloroform). Upon mild stirring, the NCs were transferred into the water phase. After phase transferring, 4 mL of diethyl ether and 4 mL of ethyl acetate were added to the mixture and by adding 10 mL of methanol. After centrifugation the precipitate was washed by methanol and redissolved in 10 mL of double distilled water, the purification was repeated at least three times to remove the free MPA ligands. Preparation of Au-BSA Conjugated QDs. Two milliliters of 1.5 × 10-3 M as-prepared MPA-capped CdSe quantum dots and 3 mL of 1.04 × 10-5 M 1-(3-(dimethylamino)propyl)-3ethylcarbodiimide hydrochloride (EDC) were added to 3 mL of 0.10 mol · L-1 phosphate buffer (pH 7.4). After the mixture was stirred magnetically for 30 min at room temperature, the solution was incubated at 4 °C for 12 h. Now 3 × 10-6 M BSA protein conjugated colloidal gold nanoparticles suspension was added to the EDC containing QDs and the mixture was kept at 4 °C for 4 h. This solution was used for optical studies. A scheme representation for the synthesis of Au-BSA conjugated CdSe QD’s is given in Scheme 1. Photoluminescence quantum yields (QY) were obtained by comparison with reference dye (Rhodamine 6G, and Rhodamine B in water), using eq 213

QYs ) (FsArns2QYr)/(FrAsnr2)

(2)

where Fs and Fr are the integrated PL spectra of the sample and the reference, 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 ) 90%),14 respectively. The refractive indices of the solvents used for the preparation of the sample and reference are given by ns (1.33) and nr (1.33), respectively (here both solvents are water). 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

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Figure 1. TEM image of Au-BSA-MPA-GYCdSe QDs.

edge luminescence peak was integrated (any other luminescence bands, such as defect associated luminescence or solvent fluorescence were discard as background). 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 a UV-vis spectrophotometer (Shimadzu). The emission spectra of all samples were recorded in a fluoro Max-P (Horiba Jobin Yvon) luminescence spectrophotometer. For the time correlated single photon counting (TCSPC) measurements, the samples were excited at 370 nm using a picoseconds diode laser (IBH Nanoled-07) in an IBH Fluorocube apparatus. The typical full width at half-maximum (fwhm) of the system response using a liquid scatter is about 162 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, I (t) n

I(t) )

∑ Ri exp(-t/τi)

(3)

i

Here, n is the number of discrete decay components and Ri and τi are the pre-exponential factors and excited-state fluorescence lifetimes associated with the ith component, respectively. Results and Discussion Figure 1 shows the TEM image of Au conjugated CdSe QD nanoparticles. It is seen from the TEM image that there is no aggregation of particles which indicates the formation of QD conjugated Au nanocomposites. Analysis suggests that Au nanoparticle is surrounded by CdSe nanoparticles. From the TEM image, it is seen that the ratio of Au NP to CdSe QD is 1:5. The measured distance between CdSe and Au nanoparticles is 9.6 nm. Figure 2 shows the absorption spectra of three different sizes of CdSe QDs and Au-BSA conjugated CdSe nanoparticles. Strong excitonic bands at 492, 499, and 525 nm were observed for three different sized CdSe QDs, i.e., greenish-yellow (GY), yellow (Y), and orange (Or), respectively. Using the standard method,15 the particle sizes have been estimated and the estimated particles sizes are 5.0, 5.4, and 5.8 nm for greenishyellow (GY), yellow (Y), and orange (Or), respectively. A systematic red shift in the excitonic band with increasing the

particles size is obviously due to the quantum size effect of CdSe QDs. It is interesting to note that a systematic blue shift of the excitonic band of CdSe QDs is observed when CdSe QDs are conjugated with Au-BSA nanoparticles. The blue shifting of the excitonic band is observed, i.e., from 492 to 457 nm, 499 to 468 nm, and 525 to 485 nm for greenish-yellow (GY), yellow (Y), and orange (Or), respectively. This shifting varies from 30 to 40 nm, indicating the interaction between Au-BSA nanoparticles and CdSe QDs. Agarwal et al.16 observed a blue shift in emission spectra of CdTe NPs due to reduction of Coulomb binding energy of an exciton in the presence of Au nanoparticles. The shifting of plasmon band of Au nanoparticles is also observed, which is consistent with the previous work.17 In the pure BSA conjugated Au NP, an absorption peak appears at 515 nm. The surface-plasmon resonance of the Au NP band centered at 560, 575, and 590 nm are for Au-BSA conjugated 5.0 nm CdSe, 5.4 nm CdSe, and 5.8 nm CdSe, respectively. The emission spectrum of aqueous solution of unconjugated CdSe QDs (pure) overlaps very well with the absorption spectra of Au nanoparticles containing solution, as shown in Figure 3. It is well-known18 that the energy transfer depends on the spectral overlap between donor emission and acceptor absorption. It is seen from Figure 3 that the spectral overlap region decreases with increasing the size of the CdSe (from greenishyellow to orange). The photoluminescence (PL) peak at 552 nm is due to greenish-yellow (GY) CdSe. Figure 4 shows the photoluminescence (PL) spectra of CdSe QDs without and with Au conjugated. A significant PL quenching of CdSe nanoparticles is observed after conjugation with Au nanoparticles. The PL quenching values are found to be 60%, 40%, and 30% for 5.0 nm CdSe, 5.4 nm CdSe, and 5.8 nm CdSe, respectively which vary with changing the size of particle. Therefore, the PL quenching is sensitive to the size of CdSe QDs. Figure 5 shows the time-resolved fluorescence spectra of aqueous solution of CdSe QDs and corresponding Au conjugated CdSe QDs. The luminescence decay of pure 3-MPA capped GY-CdSe is well fitted with triexponential (Table 1). If the intensity decays are multiexponential, then it is important to use an average decay time which is proportional to the steadystate intensity.18 The average values are given by the sum of the ∑biτi products. The components are τ1 ∼200 ps (81.1%), τ2 ∼ 2.88 ns (15.6%), and τ3 ∼13.42 ns (3.3%) and the average decay time is 1.08 ns. For the Au conjugated GY-CdSe, the components are τ1 ∼187 ps (72.4%), τ2 ∼ 2.53 ns (24.8%), and τ3 ∼12.68 ns (2.80%) and the average decay time is 0.640 ns. The decay time (τ3) is roughly equal to the excitons lifetime in unbound CdSe QDs in solution, and this originates from radiative recombination within CdSe QDs in the solution. As the Au NP conjugated to CdSe QDs, the amplitudes of two slower components (τ2 ∼ 2.88 ns and τ3 ∼13.42 ns) decrease and the amplitude of the fast decay component (τ1 ∼200 ps) becomes dominant. These results indicate that the quenching is caused by the energy transfer from CdSe QDs to Au NPs for Au conjugated CdSe QDs. Kanemitsu et al.8 also reported three exponential decay curves during the energy transfer from excitons to plasmons in close-packed metal (Au) and semiconductor (CdSe) nanoparticle monolayer films. They described the three decay rates are due to the direct energy transfer between the nearest-neighbor CdSe QDs and Au nanoparticles (CdSe f Au), the stepwise energy transfer from CdSe to CdSe to Au nanoparticles (CdSe f CdSe f Au), and the radiative recombination in CdSe QDs. The energy transfer efficiency from CdSe QDs to Au nanoparticles is estimated accordingly φET ) 1-

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Figure 2. (a) The absorbance spectra of (i) GY-CdSe, (ii) Y-CdSe, and (iii) Or-CdSe and (b) (i) Au conjugated GY-CdSe, (ii) Au conjugated Y-CdSe, and (iii) Au conjugated Or-CdSe.

Figure 3. The absorption spectrum of pure Au (a) and photoluminescence (PL) spectra (b) GY- CdSe QDs, (c) Y- CdSe QDs, and (d) OrCdSe QDs.

Figure 4. Photoluminescence (PL) spectra of (a) GY-CdSe, (a′) Au-BSA conjugated GY-CdSe QDs; (b) Y-CdSe, (b′) Au-BSA conjugated Y-CdSe QDs; (c) Or-CdSe, (c′) Au-BSA conjugated OrCdSe QDs.

τDA/τD, where τD is the decay time without Au conjugated CdSe QDs and τDA corresponds to the decay time with Au conjugated CdSe QDs. The calculated energy transfer efficiency from GYCdSe to Au nanoparticles is 40.9%. The time-resolved fluorescence decay of aqueous solution of pure Y-CdSe is triexponential, the components are 165 ps (89.5%), 3.19 ns (8.7%), and 17.46 ns (1.8%), and the average decay time is 0.739 ns. However, the fluorescence decay components of CdSe after conjugation with Au nanoparticles are 147 ps (92.7%), 2.95 ns (6.4%), and 18.54 ns (0.9%) and the corresponding average decay time is 0.517 ns. The calculated energy transfer efficiency from Y-CdSe QDs to Au nanoparticles is 30%. The shortening of decay time in the presence of Au nanoparticles suggests the energy transfer process from CdSe QDs to Au nanoparticles. Time-resolved PL spectroscopy clarified that the PL quenching of CdSe NPs occurs through rapid energy transfer from excitons in CdSe NPs to plasmons in Au NPs. For the unconjugated OrCdSe QDs, the decay components are 642 ps (82.5%), 3.55 ns (15.3%), and 17.44 ns (2.20%) and the average decay time is 1.46 ns. For the Au conjugated Or-CdSe QDs, the decay

Figure 5. Decay curves of (a) GY-CdSe and Au conjugated GY-CdSe QDs, (b) Y-CdSe and Au conjugated Y- CdSe QDs, and (c) Or-CdSe and Au conjugated Or CdSe QDs.

components are 351 ps (81.6%), 2.88 ns (15.4%), and 15.30 ns (3.0%) and the average decay time is 1.18 ns. Here, the energy transfer efficiency from Or-CdSe QDs to Au nanoparticles is 19.2%. Thus, the energy transfer efficiency values are 40.9%, 30%, and 19.2% for GY-CdSe, Y-CdSe, and Or-CdSe nanoparticles, respectively. It reveals the energy transfer process depends upon the size of CdSe nanoparticles. Therefore, the quantum size plays an important role on energy transfer process which is a new observation in the present study. Here, the size of CdSe QD (donor) is being varied and the size of Au (acceptor) is being fixed. It is already demonstrated that dipole moment of CdSe QDs linearly depends on the radius of the QD.19 Thus, electronic coupling and Coulombic interaction are to be increased with increasing the size of particles. As a result the energy transfer rate should be increased with increasing the size. However, in the present study we observed reverse results.

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TABLE 1: Time-Resolved Fluorescence Studies for Different Sized CdSe QDs and Au-BSA Conjugated CdSe Nanoparticles Pairs system

λem (nm)

τ1a (ps) (b1, %)

τ2a (ns) (b2, %)

τ3a (ns) (b3, %)

〈τ〉a ) ∑biτi (ns)

pure CdSe

552 552

pure CdSe

562

Au-CdSe

562

pure CdSe

578

Au-CdSe

578

2.88 (15.6) 2.53 (24.8) 3.19 (8.7) 2.95 (6.4) 3.55 (15.3) 2.88 (15.4)

13.42 (3.3) 12.68 (2.8) 17.46 (1.8) 18.54 (0.9) 17.44 (2.2) 15.30 (3.0)

1.084

Au-CdSe

187 (81.1) 203 (72.4) 165 (89.5) 147 (92.7) 642 (82.5) 351 (81.6)

a

0.640

E (%)

40.9

0.739 0.517

30.0

1.460 1.180

19.2

(10%.

TABLE 2: Energy Transfer Parameters for Different CdSe Nanoparticles Pairs

a

system

λem (nm)

J(λ) (M-1 cm-1 nm4)

E (%) (PL)

R0 (Å)

r (Å)

rna (Å)

Au-CdSe Au-CdSe Au-CdSe

552 562 578

2.43 × 1018 2.15 × 1018 1.94 × 1018

60.7 44.5 34.5

117.33b 112.02 104.01

124.5 129.3 136.3

95.3 101.2 110.3

E ) nR06/(nR06 + rn6), n ) acceptor/donor. b n ) 0.2.

Here, it is seen that the spectral overlap integral decreases with increasing the size, indicating the energy transfer rate decreases with size which matches with our results. Another reason is the surface area which increases with decreasing the size. Recently, it is reported2e that attachment rate constant of dye molecules increases with increasing surface area and the rate of energy transfer for quantum dots is much higher than that for quantum rods. These two reasons may explain why energy transfer rate increases with decreasing the size of CdSe QDs in Au-BSA conjugated CdSe QDs system. The distance between donor and acceptor has been estimated using the FRET method and the Fo¨rster distance (R0) is calculated from the relation18

R0 ) 0.211[κ2n-4φQ-dotJ(λ)]1/6

(in Å)

(4)

where k2 is the orientation factor, φQ-dot is the quantum efficiency of CdSe QD, 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 (CdSe) and absorption spectra of the acceptor (Au) and the values are listed in Table 2. The overlap integral values are 2.43 × 1018, 2.15× 1018, and 1.94× 1018 M-1 cm-1 nm4 for GY-CdSe, Y-CdSe, and Or-CdSe nanoparticles, respectively, which changes with size. The calculated Fo¨rster distances (R0) are 117.33, 112.02, and 104.4 Å for Au-BSA conjugated GYCdSe, Au-BSA conjugated Y-CdSe, and Au-BSA conjugated Or-CdSe nanoparticles, respectively. Using eq 1, the calculated distances (d) between the donor and acceptor are 95.32, 102.23, and 110.31 Å for Au-BSA conjugated GY-CdSe, Au-BSA conjugated Y-CdSe, and Au-BSA conjugated Or-CdSe nanoparticles, respectively (Table 2), using the efficiency of FRET which depends on the inverse sixth power of the distance of separations between one donor and one acceptor. Furthermore, the large size of QDs compared to organic dyes also provides the design of such a configuration where multiple acceptors could interact with a single donor, which enhances FRET efficiency and thus measurement sensitivity.1 It is already seen

from a TEM image (Figure 1) that one acceptor (Au) nanoparticle can interact with five donor (CdSe) nanoparticles brought in close proximity simultaneously, and for this complex interaction, the efficiency (E) can be expressed1a as

E)

nR06

(5)

nR06 + rn6

where rn is the average donor-acceptor distance and where n is the acceptor/donor ratio. The calculated average distances (rn) between the donor and acceptor are 95.32, 102.23, and 110.31 Å for Au-BSA conjugated GY-CdSe, Au-BSA conjugated Y-CdSe, and Au-BSA conjugated Or-CdSe nanoparticles, respectively (Table 2). It is interesting to note that the calculated average distances (rn) between the donor and acceptor are 95.32, 102.23, and 110.31 Å for the three Au conjugated CdSe systems, which are matched with TEM data from Figure 1. In the present study, the geometric CdSe QD-to-Au-NP center-to-center separation distances (r) for the three different sized QDs were estimated using the QD and Au-NP radii, the chain length of MPA molecule, and the hydrodynamic radius of BSA protein at pH 7.0. The reported20 length of one MPA molecule is 0.262 nm, and the reported hydrodynamic radius of BSA protein at pH 7.0 is 4.5 nm.9 In this present study, the radius of Au-NP is 2.7 nm. Thus, the center-to-center separation distances (r) are 9.96, 10.46, and 10.96 nm, for three different sized CdSe QDs having radii of 2.5, 2.7, and 2.9 nm, respectively. These results are well matched with the calculated data from FRET process. Again, we calculate the distance between donor and acceptor using dipole approximation. With the dipole approximation,7c,21 the energy transfer rate from exciton of semiconductor NP to plasmon in a metal NP is given below

γnonrad,metal(ωPL) )

3bRRMNC3c3 6

5/2

2d εb τradωPL

3

[

Im

ωγ(ωPL) - εb εγ(ωPL) + 2εb

]

(6)

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TABLE 3: Distance Calculation from CdSe QDs to Au Nanoparticles Using Different Methods system

λem (nm)

FRET (nm)

geometric distance (nm)

dipole approximately (nm)

Au-CdSe Au-CdSe Au-CdSe

552 562 578

9.5 10.2 11.0

9.9 10.5 10.9

9.6 10.2 11.4

where γnonrad,metal, bR, d, RMNP, τrad, εb, and ωPL are the rate of energy transfer, geometrical factor, the interparticle distance, radius of metal NP, radiative decay time, background dielectric constant, dielectric constant of metal, and PL frequency, respectively. In the above equation, 1/d6 dependence corresponds to the FRET theory. Equation 6 provides the lower limits of the energy transfer rate because the dipole approximation is valid for the condition d . R (MNP radius). At large separations compared to the radius of the nanopaticle, the rate of energy transfer follows FRET theory. When the surface to surface distance (∆) is very much less than the MNP radius (between ∆ ) R and ∆ ) 4R) then the rate of energy transfer follows 1/dσ dependence, where σ lies between 3 and 4.8 The calculated average distance (d) between the metal and semiconductor NP is 9.6 nm for Au-BSA conjugated GY-CdSe using these parameters b ) 2, RMNP ) 2.7 nm, τrad ) 0.64 ns, εb ) 1.8, and εγ (2.3 eV) ) -0.2.9337 + 2.621i.22 The calculated average distances between the metal and semiconductor NP are 10.18 and 11.40 nm for Au-BSA conjugated Y-CdSe and Or-CdSe, respectively. Here, the parameters b ) 2, RMNP ) 2.7 nm, τrad ) 1.16 ns, εb ) 1.8, and εγ (2.2 eV) ) -5.842 + 2.113i and b ) 2, RMNP ) 2.7 nm, τrad ) 0.443 ns, εb ) 1.8, and εγ (2.1 eV) ) -8.113 + 1.663i are used, respectively.21 It is worth noting that the calculated distance agrees very well with the TEM result as well as FRET data (Table 3). This distance also nicely matches with the geometric CdSe QD-to-Au-NP center-to-center separation distance. Analysis suggests that the quantum dot based FRET process is a very useful technique to measure the molecular distance between donor (CdSe) and acceptor (Au).

Conclusions To the best of our knowledge, this is the first report to study the resonance energy transfer between different sized CdSe QDs and Au nanoparticles using steady state and timeresolved spectroscopy. There is a shortening of the decay time and PL quenching of CdSe QDs in the presence of Au nanoparticles indicating the efficient FRET between donoracceptor molecules. We have demonstrated that the energy transfer from CdSe QDs and Au nanoparticles varies with changing the size of QDs. With the FRET process, the calculated distances (d) between the donor and acceptor are 95.3, 102.2, and 110.3 Å for Au-BSA conjugated GY-CdSe, Au-BSA conjugated Y-CdSe, and Au-BSA conjugated OrCdSe nanoparticles, respectively. These results are well matched with the structural estimated value, TEM data, and data from dipole approximation. In conclusion, we demonstrated that strong evidence of size-dependent efficient resonance energy transfer between CdSe QDs and Au nanoparticles is observed. The observed properties of the

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