Comparative Efficiency of Energy Transfer from CdSe-ZnS Quantum Dots or Nanorods to Organic Dye Molecules

July 11, 2017 | Autor: Igor Nabiev | Categoría: Semiconductors, Nanoparticles, Quantum Dots, FRET, THEORETICAL AND COMPUTATIONAL CHEMISTRY
Share Embed


Descripción

DOI: 10.1002/cphc.201100552

Comparative Efficiency of Energy Transfer from CdSe–ZnS Quantum Dots or Nanorods to Organic Dye Molecules Marya Hardzei,[a] Mikhail Artemyev,*[a] Michael Molinari,[b] Michel Troyon,[b] Alyona Sukhanova,[b] and Igor Nabiev[b] Fluorescence resonance energy transfer (FRET) in conjugates of CdSe–ZnS semiconductor nanocrystals of different shapes (FRET donors) and an Alexa Fluor organic dye (FRET acceptors) is examined. The dye molecules are chemically conjugated with quantum dots (QDs) or nanorods (NRs) in dimethyl sulfoxide colloidal solutions, and FRET efficiency in the purified conjugates is measured. The FRET from NR to a single dye mole-

cule is less efficient than that of the QD–dye conjugates and this effect is explained in terms of distance-limited energytransfer rate in the case of a point-like acceptor and extended donor dipoles. However, the larger surface area of NRs allows for many more dye acceptors to be bound, and the total FRET efficiency in NR–dye conjugates approaches those of QD–dye conjugates.

1. Introduction Luminescent sensors based on Fçrster (or fluorescence) resonance energy transfer (FRET) between highly luminescent semiconductor quantum dots (QDs) and dye molecules attract significant interest in practical applications due to their high sensitivity and low background signal.[1–3] The QD and dye molecule may both serve either as FRET donor or acceptor depending on the sensor design.[1–11] Additionally, since elongated semiconductor nanoparticles (or nanorods, NRs) show molar absorption coefficients an order of magnitude larger than those of spherical QDs and absorb and emit linearly polarized photoluminescence (PL), they are of especial interest for sensing applications based on resonant energy transfer.[12] Unlike spherical QDs with randomly oriented optical dipoles, elongated NRs have linearly oriented optical dipoles which allow realization of FRET donor–acceptor pairs with a well-defined geometry of dipole–dipole orientation. To understand how to fully explore these interesting optical properties of NRs, the energytransfer efficiency within the [dye (donor)–QD or NR (acceptor)] and [QD or NR (donor)–dye (acceptor)] complexes must be investigated. Recently, we examined the efficiency of FRET from dye molecules to spherical (QD) or elongated (NR) CdSe–ZnS core–shell nanocrystals and proposed a model explaining the observed phenomena based on the existence of only a highly spatially limited region of the NR to which efficient FRET from dye molecule can be realized.[13] Here we examine the reverse FRET scheme, in which a CdSe–ZnS QD or NR nanocrystal acts as a donor transferring its energy to chemically conjugated dye molecules, which permits comparison of relative efficiencies of energy transfer within conjugates of organic dyes with nanocrystals of different shape. In principle, such optical coupling should be more suitable for practical applications, since semiconductor nanocrystals have broad optical absorption and narrow PL bands in the UV/Vis region. The spectral position of the PL band can be easily matched with the absorption band of an organic dye by

330

varying the size, shape or chemical composition of nanocrystal. Here, we selected a pair of CdSe–ZnS QDs and NRs emitting at about 600 nm and Alexa Fluor 647 dye (Invitrogen) as FRET acceptor having an absorption band spectrally overlapping with PL bands of our QDs and NRs. The semiconductor nanocrystals are ideal FRET donors for efficient background-free FRET, since they can be optically excited far away from the optical absorption of dye molecules (FRET acceptors). FRET applications of semiconductor CdSe–ZnS NRs studied here are of special interest due to their much larger molar absorption coefficients compared to spherical QDs and an extended optical dipole with well-defined linear orientation.

2. Results and Discussion Figure 1 shows the room-temperature optical absorption and PL spectra of CdSe–ZnS core–shell QDs and NRs and their conjugates with Alexa Fluor 647 dye. The concentrations of dye molecules and nanoparticles were derived from the molar extinction coefficients for Alexa Fluor 647 at l = 660 nm and the first excitonic transitions at l  590 nm for QDs and NRs used in the work. The molar extinction coefficient of CdSe–ZnS NRs of 4 nm in diameter and 23 nm in length was calculated from the known extinction coefficient for CdSe–ZnS QDs of the same diameter by exploiting the fact that the extinction coefficient is propor[a] M. Hardzei, Dr. M. Artemyev Institute for Physico-Chemical Problems Belarussian State University 220030, Minsk (Belarus) Fax: (+) 375-172-264696 E-mail: [email protected] [b] Dr. M. Molinari, Prof. M. Troyon, Dr. A. Sukhanova, Prof. I. Nabiev Universit de Reims Champagne-Ardenne 51100 Reims (France)

 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

ChemPhysChem 2012, 13, 330 – 335

Energy Transfer from CdSe–ZnS Nanocrystals to Organic Dye absorption and PL bands for Alexa Fluor 647 in DMSO when compared with its aqueous solution was taken into account. Table 1 summarizes both optical and concentration parameters for all studied samples. From the data of Table 1 the average number of dye molecules per nanocrystal for each conjugate can be derived as 2.7 molecules per QD and 12 molecules per NR. The difference in the number of dye molecules bound to the surface of QD or NR roughly corresponds to the difference in surface areas of a single QD and NR (including tips), and thus confirms the high efficiency of the conjugation procedure. The data of Figure 1 and Table 1 also demonstrate very efficient FRET in QD–Alexa Fluor and NR–Alexa Fluor conjugates: strong PL quenching of donors (QDs or NRs) is always accompanied by simultaneous enhancement in PL emission of acceptor (Alexa Fluor dye). The data show that for almost equal concentration of QDs in solutions of free QDs and QD–dye conjugate, the magnitude of PL peak for the QD–dye conjugate is 40 times less than that of the free QDs. A similar, but slightly weaker, effect was found upon comparison of the PL intensity for free NRs and NR– Alexa Fluor conjugates (Table 1). We emphasize here that the reference QDs and NRs without conjugated dye (spectra 1 and 2 in Figure 1 A and solid lines in Figure 1 C and D) were prepared in the same way as the corresponding conjugates, including the purification by dialysis and washing steps. This was done to take into account possible variations in PL quantum yield and PL band shape of QDs and NRs alone due to the dialysis and washing procedures. The only difference between the preparation of conjugates and the corresponding reference samples is in the addition of the dye to the colloidal solutions of nanocrystals in the former case. No additives or coupling agents were used for the conjugation procedure. The FRET efficiency in QD–dye and NR–dye conjugates can be determined according to Equation (1) E ¼ 1F DA =F D

Figure 1. Room-temperature absorption (a, b) and PL (c, d) spectra of CdSe– ZnS QDs, NRs and their conjugates with Alexa Fluor 647 dye in DMSO. Dotted curves in (b) correspond to the fitted Alexa Fluor absorption spectra without the nanoparticle contribution. Curves 3 and 4 in (a) correspond to absorption and PL spectra of free Alexa Fluor dye in DMSO. Dotted curves in (c) and (d) correspond to PL spectra of Alexa Fluor 647 dye conjugated with QDs and NRs, respectively. PL excitation wavelength lex = 450 nm.

tional to the molecular weight of the nanoparticle.[13] The molar extinction coefficient of Alexa Fluor 647 in DMSO was calculated by using the extinction coefficient of this dye in aqueous solution provided by the supplier (www.invitrogen. com) and the absorption spectra of equimolar Alexa Fluor 647 solutions in water and in DMSO. A red shift of about 13 nm for ChemPhysChem 2012, 13, 330 – 335

ð1Þ

where FDA and FD are the fluorescence intensities of FRET donors with and without acceptor, respectively, normalized to the nanoparticle concentration.[14] Based on the data of Figure 1 and Table 1 we determined the FRET efficiencies E for QD–dye and NR–dye conjugates to be 0.96 and 0.87, respectively. Taking into account the five times larger number of acceptor molecules per NR than per QD, this result indicates much lower efficiency of FRET from NRs to dye compared to QD–dye conjugates. Another quantitative parameter characterizing the efficiency of energy transfer, the Fçrster distance R0 at which 50 % FRET efficiency occurs, also can be derived from the experimental data [Eq. (2)] R ¼ R0 ð1=E1Þ1=6

ð2Þ

where R is the physical distance between the donor and the acceptor dipoles. In case of multiple acceptors per donor, this should be modified to Equation (3)[15]

 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemphyschem.org

331

M. Artemyev et al. ration of conjugates with different dye/nanoparticle ratios was the same as that described above, and only the amount of dye used for Molar Molar absorption co- Concentration [mol L1] Integrated PL conjugation was varied. Using Equation (1) we weight efficient [cm1 m1] intensity [cps][d] further determined the FRET efficiency within QDs 120 000[a] 300 000[b] 1.3  106 1.4  108 prepared conjugates, and these results are preNRs 1 000 000[a] 1 800 000[c] 2.5  107 2.04  108 sented in Figure 2 in the form of FRET efficiency 6 5 dye 1 300 215 000 2.1  10 2.0  10 E versus the average number of dye molecules QD–dye 4.4  106 120 000 300 000 1.4  106 per NR or QD. conjugate (QD paramTwo important results can be immediately deeters) rived from Figure 2. Firstly, for the case of NR– 6 7 QD–dye 3.4  10 1 300 215 000 3.8  10 dye conjugates, the dependence of FRET efficonjugate ciency on the dye/NR ratio begins to be non(dye parameters) linear above five dye molecules per NR and satNR–dye 1 000 000 1 800 000 2.0  107 2.2  107 urates for ratios above ten dye molecules per conjugate NR. Secondly, the E values for QD–dye and NR– (NR paramdye conjugates prepared at 1/1 dye/nanopartieters) NR–dye 1.07  108 1 300 215 000 2.5  106 cle ratios and derived from the corresponding conjugate fitting curves indicates two times lower FRET ef(dye paramficiency from a single NR to a single dye moleeters) cule (E = 0.18) compared to a QD–dye pair (E = [a] Calculated from the geometrical size of nanocrystals, taking QD as ideal sphere and NR 0.42). Using Equation (2) we also determined R0 as ideal cylinder. [b] W. W. Yu, L. Qu, W. Guo, X. Peng, Chem. Mater. 2003, 15 2854–2860; for conjugates prepared at 1/1 dye/nanoparticle Chem. Mater. 2004, 16, 560. [c] Estimated value (see text for details). [d] PL excitation wavelength lex = 450 nm. ratios, fixing the dipole–dipole distance to R = 4 nm between the center of CdSe core and the dye molecule located on the surface of the nanoparticle ZnS shell. Excited states in semiconductor QDs or NRs are delocalized over their whole volð3Þ R ¼ R0 ðn=EnÞ1=6 umes, so that the exact value of R in our case is not known. Nevertheless, the value of R = 4 nm used in the calculations is reasonable, as it is equal to the sum of CdSe core radius, the where n is the number of acceptors per donor. If the Fçrster length of the dye linker molecule (aminoethanethiol) and the distance R0 is much larger than the real distance between the thickness of the ZnS shell. donor and the acceptor dipoles one should expect very effiThe value of R0 (in angstrom) can also be calculated from cient FRET. However, in case of multiple acceptors one should the emission quantum yield of the donor QD, the overlap intetake into account possible non-linear effects, like FRET saturagral J(l), which determines the degree of spectral overlap betion. To study the saturation effect in QD–dye and NR–dye contween the bands of donor emission and the acceptor absorpjugates we prepared a series of conjugates with the average tion, and the factor k2, which determines the relative spatial number of Alexa Fluor 647 molecules varying from 1.5 to 27 orientation of the donor and the acceptor dipoles [Eq. (4)][16] per NR and from 1.5 to 3.4 per QD. The protocol for the prepaTable 1. Optical parameters and concentrations of QDs, NRs, Alexa Fluor 647, and QD–Alexa Fluor 647 and NR–Alexa Fluor 647 conjugates in DMSO solution.

R0 ¼ 0:211½k2 n4 QD JðlÞ1=6

Figure 2. FRET efficiency E determined by Equation (1) versus the number of Alexa Fluor 647 dye molecules per CdSe–ZnS QD (!) or NR (&). The circle indicates the zero point (no dye/no FRET) used for the fitting procedure. Dotted curves are first-order exponential decay fits for QD–dye and NR–dye conjugates. Arrows correspond to the interpolated values of FRET efficiency for a single dye molecule per single QD or NR.

332

www.chemphyschem.org

ð4Þ

We calculated the FRET efficiency factor E using the integrated PL intensities of QDs and NRs before and after conjugation with Alexa Fluor 647. In these calculations the integrated PL intensity of free nanoparticles was normalized onto their relative molar concentration in the conjugates. The PL quantum yield of free QDs and NRs was determined relative to Rhodamine 6G solution in ethanol.[16] To determine the value of the overlap integral J(l), the data of Figure 1 have been redrawn in such a way that the PL peaks of donors are normalized to unity, and absorption spectra of acceptor represented in absolute values of molar absorption coefficients (Figure 3). The value of J(l) was determined for QDs and NRs by integrating over the shaded areas in Figure 3 A and B, respectively, and results were expressed in units of m1 cm1 nm4.[16] The most intriguing parameter in such calculations is a value of k2

 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

ChemPhysChem 2012, 13, 330 – 335

Energy Transfer from CdSe–ZnS Nanocrystals to Organic Dye

Figure 3. Optical absorption spectra (g) of free Alexa Fluor 647 dye and PL spectra (c) of free CdSe/ZnS QDs (a) and NRs (b) in DMSO solution. The PL intensities of both samples were normalized to unity, and the absorption spectrum of the dye is represented in the absolute units of m1 cm1.

The experimentally determined value of R0 for the NR–dye conjugate was found to be smaller than that of the QD–dye conjugate, as a consequence of the reduced FRET efficiency of the NR–dye pair. Almost equal values of calculated R0 for NR– dye and QD–dye presented in Table 2 result from close values of their experimentally determined overlap integrals J(l) and the PL quantum yields. However, the calculated value of R0 for QD–dye is much smaller than determined experimentally, which may point to the limited accuracy of theoretical calculations for the case of FRET donor–acceptor pair of point-like acceptor (dye molecule) and extended donor (NR or QD). Conventional point-dipole FRET theory needs to be modified when FRET-based configurations involving extended oriented dipoles are concerned. A number of sophisticated FRET theories and models have been developed for systems of interacting extended and point-like optical dipoles,[17, 19, 20] but the goal of our work is not detailed analysis of these theories and calculations of theoretical FRET parameters in a system of interacting extended and point-like optical dipoles, but the determination of experimental FRET parameters. The fact that the energytransfer rate is distance-limited in case of the energy transfer between extended and point-like dipoles reduces the NR region from which the efficient FRET may occur to only a few nanometers.[13, 17] For example, one-dimensional origin of the optical dipole present in the FRET system (Figure 4) may cause slower spatial decay of energy transfer rate g ~ 1/R5, as compared to g ~ 1/R6 for conventional dipole–dipole interaction.[17] In this case the Equation (2) transforms into Equation (5)

R ¼ R0 ð1=E1Þ1=5 which depends on the relative orientation of the donor and acceptor dipoles. In the case of QD–Alexa Fluor 647 conjugate the orientation of optical dipoles in QDs is random, while in the dye molecules it is fixed. This situation gives rise to k2 around 1/2.[14] For NR–Alexa Fluor 647 conjugate the situation is more interesting, since both dipoles are fixed. While the optical dipole in the NR is always parallel to the nanorod main axis, the relative orientation of the dipole in the Alexa Fluor 647 dye molecule is unknown and may vary between orthogonal (k2 = 0), parallel (k2 = 1) and collinear (k2 = 4) orientations.[16] The last case can be realized for dye molecules attached to the tips of NR and having the optical dipole parallel to the NR long axis. A priori, we can not expect an appreciable portion of dye molecules to be attached to the NR tips, due to their small area compared to the sides. Since quite a strong FRET effect is observed for the NR–Alexa Fluor 647 conjugate and very strong FRET is measured for the QD–Alexa Fluor 647 conjugate, the assumption of a value of k2 = 2/3 (random orientation of dipoles) for both QD–Alexa Fluor 647 and NR–Alexa Fluor 647 conjugates is reasonable. Note that possible variation of k2 value far above zero will not strongly influence R0, since the sixth root of k2 is taken in Equation (4). Finally, we fixed the value of refractive index to n = 1.4. All calculated data are presented in Table 2.

ChemPhysChem 2012, 13, 330 – 335

ð5Þ

Table 2. FRET parameters for QD–Alexa Fluor 647 and NR–Alexa Fluor 647 conjugates in DMSO. Overlap integral J(l) [m1 cm1 nm4]

Experimental FRET distance R0 [] for conjugates prepared at 1/1 dye/nanoparticle ratios and that calculated with Equation (4) (in parentheses)[b]

0.42

4.5  1014

38 (25)

0.18

5.7  1014

31 (27)

Experimental FRET efficiency E for conjugates prepared at 1/1 dye/nanoparticle ratios (data of Figure 2)

QDs NRs Dye QD– dye NR– dye

PL quantum yield, lex = 450 nm

0.14 0.17 0.015[a]

[a] The optical absorption of Alexa Fluor 647 at lex = 450 nm is negligible. [b] Calculated for ensemble of dye molecules, n = 1.4 (see text).

 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemphyschem.org

333

M. Artemyev et al.

Experimental Section

Figure 4. Coupled NR–dye and QD–dye systems (conjugates) involved in the resonance energy transfer process. Arrows show the FRET from an NR or QD to organic dye. In this unidirectional energy transfer, only highly spatially limited parts of the nanorod may ensure efficient FRET from the nanoparticle to organic dye, whereas the other parts of the nanoparticle are inactive in the energy-transfer process.

which gives R0 = 29  calculated for the NR–dye conjugate prepared at 1/1 dye/NR ratio. This value is close to that of R0 = 27  calculated by using Equation (2) and presented in Table 2. The difference between the values of R0 calculated by Equations (2) and (5) is not remarkable. However, since the FRET efficiency E is proportional to R60 in the standard model or R50 in a modified one,[17] small variations in R0 may result in significant difference in the value of E for QDs and NRs donors. We can not determine precisely at this stage which scenario matches better our experimental results. The origin of FRET saturation for NR–dye conjugates in Figure 3 is not clear. We carefully purified our conjugates from unbound dye molecules, so that the saturation is not due to the error in determination of the concentration of dye molecules in NR conjugates. Possibly, increasing dye–dye interaction with increasing amount of dye molecules bound to the surface of each NR causes reorientation of optical dipoles of dye molecules toward less favourable FRET conditions.

Hydrophobic CdSe–ZnS core–shell QDs (4 nm in diameter) and NRs (4 nm in diameter and 23 in length) were synthesized according to previously published procedure based on high-temperature reaction between organometallic precursors in coordinating solvents.[18] The nanocrystals were solubilized in water with a mixture of aminoethanethiol and dimethylaminoethanethiol. Briefly, dry nanocrystals (ca. 5 mg) were dissolved in chloroform (2 mL), and methanol (3 mL) was then added. The solid phase was centrifuged, washed with methanol and dissolved in chloroform (3 mL). Next, an equimolar mixture of aminoethanethiol hydrochloride (Aldrich, 0.3 mL) and dimethylaminoethanethiol hydrochloride (Aldrich, 0.3 mL) in methanol (0.1 mmol mL1 each) was added to the chloroform solution of nanocrystals and the mixture stirred for 20 min to complete the binding of thiol-containing molecules to the surface of the nanocrystals. The solid phase containing the nanocrystals with a monolayer of thiols on their surfaces was centrifuged, washed two times with methanol and dissolved in anhydrous dimethyl sulfoxide (DMSO, 3 mL). Alexa Fluor 647 carboxylic acid succinimidyl ester (Invitrogen) was dissolved in anhydrous DMSO at a concentration of 0.1 mm, and 100 mL of this solution was added to the nanocrystal solution in DMSO. The mixture was stirred in the dark at room temperature for 2 h in order to complete the reaction between the succinimidyl ether groups of Alexa Fluor 647 and amino groups of aminoethanethiol on the surface of nanoparticles. The conjugates of nanocrystals with dye molecules were dialyzed against doubly distilled water. During dialysis the conjugates formed aggregates, which were removed from solution by centrifugation (5000 rpm, 3 min). The solid phase was washed several times with doubly distilled water to remove unbound dye molecules and re-dispersed in a fresh portion of DMSO (3 mL) in an ultrasound bath. The reference samples of free QDs and NRs without dye molecules were prepared in the same way, including the solubilization, dialysis and washing steps, in order to take into account possible variations in the PL quantum yield from QDs and NRs alone due to the dialysis and washing procedures. Solutions of QDs, NRs, Alexa Fluor 647 and their conjugates were placed in 10 mm quartz cells for fluorescence analysis. The PL spectra of all solutions were recorded at room temperature with a Jobin-Yvon Fluoromax-2 spectrofluorimeter (lex = 450 nm, lem = 500–800 nm), and optical absorption spectra were recorded with an Ocean Optics HR 2000 spectrophotometer.

3. Conclusions We compared the efficiency of resonance energy transfer in conjugates of CdSe–ZnS core–shell QDs or NRs (FRET donors) and Alexa Fluor 647 organic dye molecules (FRET acceptors). Due to the larger surface area of NRs compared to QDs, many more dye molecules can be attached to an NR. This results in the total FRET efficiency in the dye–NR system (0.87) close to that in the dye–QD conjugate (0.96). However, the relative FRET efficiency per acceptor molecule in NR–dye conjugate (0.18) is is about two times lower than that of QD–dye (0.42). We attribute the less efficient FRET in NR–dye conjugate to a distance-limited interaction between the extended oriented optical dipole of an NR and the point-like dipole of a dye molecule. An increased number of dye molecules per NR causes saturation of the FRET efficiency. The determination of correct FRET efficiency in NR–dye conjugates requires interpolation of experimental data to a single dye molecule per NR in order to overcome the saturation effect.

334

www.chemphyschem.org

Acknowledgements This work was supported by the “CONVERGENCE” program (Belarus), by NATO grant SfP-983207 and by the French National Research Agency (Agence Nationale de Recherche, ANR) under the grant ANR-08-BLAN-0185. Supports of the Region ChampagneArdenne through project HYNNOV (to I.N. and M.M.) and EU NMP-2009-4.0-3-246479 project NAMDIATREAM (to I.N.) are also acknowledged. M.A. acknowledges visiting Professor fellowship to University of Reims. Keywords: dyes/pigments · FRET · nanoparticles · quantum dots · semiconductors

[1] M. Suzuki, Y. Husimi, H. Komatsu, K. Suzuki, K. T. Douglas, J. Am. Chem. Soc. 2008, 130, 5720 – 5725.

 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

ChemPhysChem 2012, 13, 330 – 335

Energy Transfer from CdSe–ZnS Nanocrystals to Organic Dye [2] I. L. Medintz, T. Pons, S. A. Trammel, A. F. Grimes, D. S. English, J. B. Blanco-Canosa, P. Dawson, H. Mattoussi, J. Am. Chem. Soc. 2008, 130, 16745 – 16756. [3] E. R. Goldman, I. L. Medintz, J. L. Whitley, A. Hayhurst, A. R. Clapp, H. T. Uyeda, J. R. Deschamps, M. E. Lassman, H. Mattoussi, J. Am. Chem. Soc. 2005, 127, 6744 – 6751. [4] M. Tomasulo, I. Yildiz, F. M. Raymo, J. Phys. Chem. B 2006, 110, 3853 – 3855. [5] D. M. Willard, T. Mutschler, M. Yu, J. Jung, A. V. Orden, Anal. Bioanal. Chem. 2006, 384, 564 – 571. [6] A. Sukhanova, A. S. Susha, A. Bek, S. Mayilo, A. L. Rogach, J. Feldmann, V. Oleinikov, B. Reveil, B. Donvito, J. H. M. Cohen, I. Nabiev, Nano Lett. 2007, 7, 2322 – 2327. [7] I. Nabiev, A. Sukhanova, M. Artemyev, V. Oleinikov in Colloidal Nanoparticles in Biotechnology (Ed.: A. Elaissari), Wiley-VCH, New York, 2008, pp. 133 – 168. [8] A. R. Clapp, I. L. Medintz, H. T. Uyeda, B. R. Fisher, E. R. Goldman, M. G. Bawendi, H. Mattoussi, J. Am. Chem. Soc. 2005, 127, 18212 – 18221. [9] T. Pons, I. L. Medintz, X. Wang, D. S. English, H. Mattoussi, J. Am. Chem. Soc. 2006, 128, 15324 – 15331. [10] Q. Zhang, T. Atay, J. R. Tischler, M. S. Bradley, V. Bulovic, A. V. Nurmikko, Nat. Nanotechnol. 2007, 2, 555 – 559.

ChemPhysChem 2012, 13, 330 – 335

[11] H. Lu, O. Schçps, U. Woggon, C. M. Niemeyer, J. Am. Chem. Soc. 2008, 130, 4815 – 4827. [12] J. Hu, L.-S. Li, W. Yang, L. Manna, L.-W. Wang, A. P. Alivisatos, Science 2001, 292, 2060 – 2063. [13] M. Artemyev, E. Ustinovich, I. Nabiev, J. Am. Chem. Soc. 2009, 131, 8061 – 8065. [14] B. W. van der Meer, G. Coker, S.-Y. Chen, Resonance Energy Transfer: Theory and Data, Wiley-VCH, New York, 1994. [15] A. R. Clapp, I. L. Medintz, J. M. Mauro, B. R. Fisher, M. G. Bawendi, H. Mattoussi, J. Am. Chem. Soc. 2004, 126, 301 – 310. [16] J. R. Lakowicz, Principles of Fluorescence Spectroscopy, Springer, New York, 2006. [17] P. L. Hernandez-Martinez, A. O. Govorov, Phys. Rev. B 2008, 78, 0353141 – 035314-7. [18] Z. A. Peng, X. J. Peng, J. Am. Chem. Soc. 2001, 123, 1389 – 1395. [19] B. P. Krueger, G. D. Scholes, G. R. Fleming, J. Phys. Chem. B 1998, 102, 5378 – 5386. [20] S. Sadhu, M. Tachiya, A. Patra, J. Phys. Chem. C 2009, 113, 19488 – 19482. Received: July 18, 2011 Revised: September 30, 2011 Published online on November 7, 2011

 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemphyschem.org

335

Lihat lebih banyak...

Comentarios

Copyright © 2017 DATOSPDF Inc.