Efficient Energy Transfer between Confined Dye and Y-Zeolite

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J. Phys. Chem. C 2010, 114, 19667–19672

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Efficient Energy Transfer between Confined Dye and Y-Zeolite Functionalized Au Nanoparticles Tapasi Sen,† Sreyashi Jana,‡ Subratanath Koner,‡ and Amitava Patra*,† Department of Materials Science, Indian Association for the CultiVation of Science, Kolkata 700 032, India, and Department of Chemistry, JadaVpur UniVersity, Kolkata 700 032, India ReceiVed: August 31, 2010; ReVised Manuscript ReceiVed: October 13, 2010

A new optical-based nanostructured materials having coumarin 480 dye confined within Y-zeolite and Y-zeolite functionalized Au nanoparticles has been designed for light-harvesting system. Unprecedented PL quenching (94%) of confined dye and efficient energy transfer (70%) between confined dye and Au nanoparticles are obtained. The enhanced fluorescence and large blue shift (9 nm) in the emission maximum of C480 in zeolite confirm the confinement of the probe dye molecules inside the zeolite cavity. The anisotropy study reveals that this type of supramolecular organization of the dyes inside the zeolite channels will allow light harvesting. Such nanostructured materials could be useful for efficient light-harvesting or chemical-sensing applications. Introduction The designing of nanostructured materials with unidirectional energy transfer is the emerging field of research for the application of energy-storage system. Unidirectional energy transfer in dye-zeolite materials for potential use in efficient light-harvesting materials has been demonstrated by Calzaferri and co-workers.1 Dutta et al.2 demonstrated the storage of solar energy by photoelectron transfer in zeolite structure. Spatial constraints imposed by the host structure lead to supramolecular organization of the guest molecules inside the nanochannels which will allow light-harvesting system to be built. Similarly, Tolbert et al.3 reported controlled energy transfer in conjugated polymers immobilized in the channels of mesoporous silica. Scott et al.4 showed the effective energy transfer between coumarin 485 and pyrromethene 567 dyes doped in mesoporous silica thin films. Wang et al.5 reported the effect of different mesoporous hosts on energy transfer between dyes. It has been found that, at low dye concentration, confined dye molecules in nanosized cavity showed enhanced resonance energy-transfer efficiency. They have used cationic dyes as guests in anticipation of the anionic silicates framework in different mesoporous hosts, which would prevent the elution of the dye molecules from the pore channel. Very recently, we have reported the energy transfer between confined rhodamine 6G dye within MCM-41 mesoporous materials with Au nanoparticles anchored onto the surface of mesoporous silica MCM-41.6 Among all potential hosts, the zeolite host seems to be the most important ones because it has a rigid structure with a three-dimensional framework forming channels and/or cages with molecular dimension, and it is an optically transparent material.7 Zeolite NaY has a spherical cavity of diameter 13 Å with four tetrahedrally arranged pore openings of 7.4 Å.8 Therefore, a large number of molecules of appropriate dimension can be encapsulated within the supercages of zeolite Y and form host-guest supramolecular structures. * Author to whom correspondence should be addressed. E-mail: [email protected]; Phone: (91)-33-2473-4971; Fax: (91)-33-2473-2805. † Indian Association for the Cultivation of Science. ‡ Jadavpur University.

Recently, the research in the field of quantum dots (QD)based fluorescence energy transfer has been paid great attention to find out potential applications in the areas of luminescence tagging, imaging, medical diagnostics, multiplexing, and most recently, biosensors.9-11 In the recent years, there has been a growing focus on utilizing QD in fluorescence resonance energy transfer. This interest stems from their tunable electronic structure, narrow emission and broad excitation spectra.9 Fo¨rster resonance energy transfer (FRET) is a powerful method to determine the distance between donor and acceptor fluorophores. In most cases, the energy transfer in QD conjugates is discussed as a FRET process. It is known that FRET technique is restricted on the upper limit of separation of only 80 Å. Therefore, 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 big as those measured by FRET, which will help understand the large-scale conformational dynamics of complex biomolecules in macroscopic detail.12-16 Thus, the energy transfer between Au nanoparticle and dye provides a new paradigm for the design of optical-based molecular ruler for long-distance measurements. The biggest advantage of gold nanoparticles is that these nanoparticles could be used as acceptors in biophysical experiments in vitro as well as in vivo. According to the Persson model,12-16 the exact form of dipole SET rate is given by

kSET )

()

1 d0 τD d

4

(1)

where τD is the lifetime of the donor in the absence of the acceptor and d is the distance between the donor and acceptor. The d0 value is calculated by using the Persson model16

d0 )

(

0.225c3Φdye ω2dyeωFkF

)

1/4

(2)

where d0 is the distance at which a dye will display equal probabilities for energy transfer and spontaneous emission. φdye

10.1021/jp108262f  2010 American Chemical Society Published on Web 11/02/2010

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is the quantum efficiency of dye, ω is the frequency of the donor electronic transition, ωF is the Fermi frequency, and kF is the Fermi wave vector of the metal.11-16 Application of Au-nanoparticle-based fluorescence energy transfer using nanoscopic environment is still in the embryonic stage; further investigations in this field are necessary for indepth understanding of the phenomenon. To the best of our knowledge, there is no report on the energy transfer between Y-zeolite functionalized Au nanoparticles with confined dye within Y-zeolite cavity. In this letter, we first demonstrate the designing of Y-zeolite functionalized Au nanoparticles and study the energy transfer between Au nanoparticles and confined coumarin 480 dyes within Y-zeolite by using steady-state and time-resolved spectroscopy.

Sen et al. SCHEME 1: Schematic Presentation for the Fabrication of NaY Zeolite-Attached Au Nanoparticles

Materials and Methods Materials. NaY was procured from Tosoh Company, Japan. The ratio of SiO2 to Al2O3 (mol) of the NaY used in this study was 5.7. Chloroauric acid (HAuCl4 · 3H2O) (S.d.Fine Chem), Tri-Sodium citrate dihydrate (Merck), and Coumarin 480 dye (Aldrich) were used without further purification. (3-Mercaptopropyl)-triethoxysilane (3-MPTS) and all other reagents were purchased either from Sigma-Aldrich/Fluka or Alfa-Aesar. These chemicals were used as received without further purification. Preparation of Organic Modification of Zeolite. At first, NaY (0.1 g) was refluxed with a dry toluene solution of 3-MPTS (16 mL, 5 × 10-8 N) for 12 h at 80 °C under N2 atmosphere. The white solid NaY-(SiCH2CH2CH2SH)x thus produced was filtered and washed with toluene and dichloromethane. Prepared materials have been characterized by FT-IR. The content of sulfur was calculated on the basis of the wt% of sulfur in NaY-(SiCH2CH2CH2SH)x from elemental analysis. The elemental analysis shows the molar ratio for C:S ≈ 3.1:1 in NaY-(SiCH2CH2CH2SH)x, and it contains 0.52 wt% sulfur. Synthesis of Au Nanoparticles and Zeolite-Attached Au Nanoparticles. Gold colloids of fairly uniform size were prepared by using the modified method reported by Graber et al.17 Briefly, 50 mL aqueous solution of HAuCl4 (1 mM) was heated to boiling with vigorous stirring. Then, 2.5 mL of 50 mM sodium citrate solution was added to the boiling solution with vigorous stirring. The color of the solution changes from light yellow to deep red within 5-10 min of heating. Then, the solution was allowed to boil for another 10 min. Finally, the solution was removed from the hot plate. However, stirring was continued until the solution reached room temperature. The zeolite sample we have used contains 0.52 wt% sulfur. Thus, zeolite aqueous dispersion containing 300 µM S was prepared by dispersing 0.0184 g zeolite in 10 mL distilled water. For the attachment of as prepared Au NPs to this zeolite, 1 mL of aqueous suspension of zeolite was added to 5 mL of the asprepared Au-nanoparticles solution with stirring. The mixture solution was stirred for 24 h. The final concentration of S in Au-attached zeolite is 50 µM. Here, the ratio of Au nanoparticle to S is 1:4000. Functionalization of NaY-zeolite with Au nanoparticles is represented in Scheme 1. Encapsulation of Dye within Zeolite. To this zeoliteattached Au-nanoparticles solution (3 mL), 10 µM Coumarin 480 dye solution (1 mL) was added. This mixture solution was kept for one day for the encapsulation of the dye molecules into the cavity of the zeolite. For the preparation of dye confined in the cavity of zeolite without Au nanoparticles, 1 mL of 10 µM C480 dye solution was added to 3 mL of zeolite dispersion having 50 µM S concentration. This solution was also kept for one day for

inclusion of dye in the zeolite cavity. Another two sets of pure aqueous dye solution and dye solution in presence of Au nanoparticles without zeolite were prepared by keeping the final concentration of Au and dye the same as before. Optical properties of all these solutions were taken after one day. The transmission electron microscopy (TEM) images were taken by using a JEOL-TEM-2010 transmission electron microscope with an operating voltage of 200 kV. Roomtemperature optical absorption spectra were obtained with an UV-vis spectrophotometer (Shimadzu). FTIR spectra of pure zeolite and Au nanoparticles functionalized zeolite were recorded with Nicolet 380 FTIR by mixing with KBr. 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 405 nm by using a picosecond diode laser (IBH Nanoled-07) in an IBH Fluorocube apparatus. The typical fwhm of the system response obtained by 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 by 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)1

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.18 For anisotropy measurements, a polarizer was placed before the sample. The analyzer was rotated by 90° at regular intervals, and the parallel (III) and the perpendicular (I⊥) components for the fluorescence decay were collected for equal times, alternatively. Then, r(t) was calculated by using the formula

r(t) )

IΙΙ(t) - GI⊥(t) IΙΙ(t) + 2GI⊥(t)

(4)

Energy Transfer between Confined Dye and Y-Zeolite Au NPa

Figure 1. TEM images of (a) pure citrate-stabilized Au nanoparticles, (b) and (c) Au nanoparticles attached with zeolite, and (d) SAED picture of zeolite-attached Au nanoparticles.

G factor in anisotropy measurement is the ratio of the sensitivities of the detection system for vertically and horizontally polarized light:

G ) SV/SH

(5)

Measured intensity ratio ) IVV/IVH ) (SV/SH)(I|/I⊥) ) G(I|/I⊥) (6) The magnitude of G, the grating factor of the emission monochromator of the TCSPC system, was found by using a coumarin dye in methanol and following longtime tail matching technique19 to be 0.58. Results and Discussion TEM Characterization. Figure 1 shows the TEM images of citrate-stabilized Au nanoparticles and zeolite-attached Au

J. Phys. Chem. C, Vol. 114, No. 46, 2010 19669 nanoparticles. Figure 1a represents the TEM images of citratestabilized Au nanoparticles with well dispersed isolated particles. The average size of these particles is 14 ( 0.5 nm. Figure 1b shows the TEM image of the Au nanoparticles attached with zeolite. The image clearly shows that all the Au nanoparticles are immobilized on the surface of mercapto- functionalized zeolite materials. It is clearly seen from Figure 1c that the Au NPs remain isolated after attachment on the surface of zeolite. Figure 1d gives the selected area electron diffraction (SAED) pattern recorded from the Au nanoparticles shown in Figure 1b. The diffraction pattern indicates the hexagonal Au cell structure. TEM image of the pure zeolite is shown in the Supporting Information, Figure S1. The binding of Mercaptofunctionalized zeolite with Au nanoparticles through Au-S bond formation is confirmed by FTIR spectra. The FTIR spectra of zeolite and Au nanoparticles attached to zeolite is shown in the Supporting Information, Figure S2. The S-H stretching band20 around 2540 cm-1 in pure zeolite vanishes after adding Au nanoparticles, indicating the formation of Au-S bond. Spectroscopic Studies. The absorption spectra of pure citratestabilized Au nanoparticles (shown in Figure 2a) display a surface plasmon band at 521 nm which is a characteristic of isolated spherical Au nanoparticles. The intensity of this band (521 nm) decreases after attachment with zeolite. Figure 2b shows the emission spectra of C480 dye in water and within NaY zeolite in a water suspension. A blue shift of the emission peak of C480 in water [curve (i)] from 488 to 479 nm is observed after incorporation of dye in zeolite [curve (ii)]. The enhanced fluorescence and large blue shift (9 nm) in the emission maximum of C480 in zeolite confirm the confinement of the probe dye molecules inside the zeolite cavity which has less polarity compared to bulk water.21 A time-resolved anisotropy study is performed to understand the rotational dynamics of dye molecules inside the zeolite channel. Fluorescence anisotropy decay study reveals the reorientational dynamics of the excited fluorophore which directly help understand structural information. Figure 3 shows the anisotropy decay of C480 in zeolite. Single-exponential anisotropy decay of C480 (monitored at 479 nm) with time constant of 100 ( 20 ps is observed in bulk water. However, the fluorescence anisotropy decay of C480 in zeolite is fitted to a biexponential function15c

Figure 2. (a) UV spectra of citrate-stabilized Au nanoparticles [curve (i)] and zeolite-attached Au nanoparticles [curve (ii)]. (b) Emission spectra of C480 dye in water [curve (i)] and in zeolite dispersion [curve (ii)]. λex) 405 nm.

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Sen et al.

Figure 3. Fluorescence anisotropy decay of C480 dyes in zeolite dispersion. λex) 405 nm.

Figure 5. Stern-Volmer plots for C480 dye in presence of Au nanoparticles (a) with zeolite and (b) without zeolite. Inset of Figure 4b shows the corresponding [1 - (I/I0)]/[Q] vs I/I0 plot.

Figure 4. Emission spectra of C480 dye [curve (i)] in water, in zeolite dispersion [curve (ii)], in the presence of citrate-stabilized Au nanoparticles without zeolite [curve (iii)], and in the presence of immobilized Au nanoparticles on the surface of zeolite [curve (iv). λex) 405 nm.

r(t) ) r0[a1R exp(t/φ1R) + a2R exp(t/φ2R)]

(7)

Here, r(t) is the rotational time, and φR represents the rotational correlation time constant. The anisotropy decay of C480 in zeolite exhibits a biexponential behavior with a fast component of 221 ps (68%) and a slow component of 1.93 ns (32%), leading to an average correlation time of 768 ps. This higher correlation time implies the restricted rotation of the dye molecules because of the confinement of the dyes in the zeolite cavity. It reveals that this type of supramolecular organization of the dyes inside the zeolite channels will allow light harvesting. From the PL spectra (shown in Figure 4), we can see that the PL intensity of C480 dye [curve (ii)] is drastically quenched in presence of Au nanoparticles immobilized on the surface of zeolite [curve (iv)]. The emission maximum of the dye molecules in zeolite remains the same (at 479 nm) both in the absence and in the presence of Au nanoparticles, which indicates that the dye molecules are in confined state in both cases. The observed PL quenching efficiency is 94% which is unprecedented. However, the quenching efficiency of C480 dye is 70% in the presence of citrate-stabilized Au nanoparticles without zeolite. It reveals that the quenching efficiency is much higher in the case of confined dye molecules in zeolite and surface-

Figure 6. Decay curves of C480 dye in zeolite dispersion [curve (i)] and in the presence of Au nanoparticles immobilized on the surface of zeolite [curve (ii)]. λex) 405 nm.

immobilized Au nanoparticles. This is due to the unidirectional energy transfer from aligned dye molecules in the cavity of zeolite to the immobilized Au nanoparticles on the surface of the zeolite. To understand the quenching mechanism, we studied the fluorescence quenching of C480 dye with and without zeolite, by changing the quencher (Au nanoparticle) concentration. On the basis of the relationship between collisional quenching of excited states and quencher concentration, the Stern-Volmer equation is given by22

Energy Transfer between Confined Dye and Y-Zeolite Au NPa

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TABLE 1: Decay Times and Energy-Transfer Efficiency of Confined Dye-Au-Zeolite system

b1

τ1a (ps)

b2

τ2a (ns)

b3

τ3a (ns)

〈τ 〉a ) ∑ibiτi (ns)

φET (%)

C480 dye in zeolite C480 dye in Au-zeolite

0.64

216

0.06 0.10

2.98 1.09

0.94 0.26

5.89 5.55

5.71 1.69

70.5

a

(10%.

I0 ) 1 + KSV[Q] I

(8)

Here, I0 and I are the relative fluorescence intensity in the absence and the presence of quencher, respectively, and [Q] is the concentration of the quencher. KSV is the Stern-Volmer quenching constant which measures the efficiency of quenching. Figure 5a,b shows the Stern-Volmer plots, I0/I versus [Au nanoparticles] with zeolite and without zeolite, respectively. Figure 5a shows the linear plot, which implies the involvement of only one type of quenching. In the present system, donor dye molecules are confined in the cavity of zeolite, and the quencher Au nanoparticles are attached onto the surface of zeolite. Therfore, the interaction between dye and Au nanoparticles may be static. From the slope of the linear plot, we found the quenching constant to be 1.25 × 109 M-1. The plot shown in Figure 5b deviates positively from linearlity, indicating the involvement of both static and dynamic quenching. The static and dynamic quenching constants can be determined by using the sphere of action static quenching model.23 According to the model, the modified form of Stern-Volmer plot is as follows:

I0 ) {1 + KSV[Q]}/W I

(9)

The additional factor W is expressed as

W ) exp(-V[Q])

(10)

Here, V is the static quenching constant, which represents an active volume element surrounding fluorophore in its excited state. W depends on the quencher concentration [Q]. Thus, eq 9 can be rewritten as

[

1-

]

()

I I /[Q] ) KSV + (1 - W)/[Q] I0 I0

average decay time is 5.71 ns. However, the decay time of dye molecules in the presence of Au nanoparticles immobilized on the surface of the zeolite is fitted by tri-exponential decay. The decay times of the dye molecules in the absence and in the presence of immobilized Au NPs are listed in Table 1. The components are 216 ps (64%), 1.09 ns (10%), and 5.55 ns (26%), leading to an average decay time of 1.69 ns. It clearly reveals that there is a shortening of the decay time of dye in the presence of Au nanoparticles, which confirms the energy transfer from dye to nanoparticles. It is well known that the lifetime measurement is more sensitive than PL quenching efficiency because errors come from the fluctuations in lamp intensity. The energy-transfer efficiency from confined dye to Au nanoparticles is calculated by using the relation φET ) 1 τDA/τD, where τDA is the decay time of dye in the presence of Au nanoparticles and τD corresponds to the decay time of dye in the absence of Au nanoparticles. The calculated energytransfer efficiency from dye to Au nanoparticles is 70.5%. This efficient energy transfer also provides for fabrication of energyharvesting system. To the best of our knowledge, this is the first report to study the energy transfer between confined dye within zeolite and Au nanoparticles by using time-resolved spectroscopy. It is already well recognized that the energy transfer between dye and Au nanoparticles is a SET process, which depends on the inverse fourth power of the distance of separations between donor and acceptor. By using the SET process (eq 1), the distance between donor (dye) and acceptor (Au nanoparticles attached with zeolite) was calculated. The calculated d0 value is 68.7 Å, and the distance between dye and Au nanoparticles attached with zeolite (d) is 55.3 Å. The anisotropy decay reveals that the dye molecules are aligned inside zeolite channels. Therefore, such new designing of nanostructured materials could be useful for efficient lightharvesting or chemical-sensing applications. Conclusions

(11)

The inset of figure 5b shows the [1 - (I/I0)]/[Q] versus I/I0 plot. The linear fitting of the plot gives the slope and intercept, from which the static quenching constant (V) and the dynamic quenching constant (KSV) can be calculated. The dynamic and static quenching constants are found to be 1.4 × 108 M-1 and 1.3 × 107 M-1 for 12 nM Au concentration, respectively. Thus, the quenching constant of Au nanoparticles with zeolite is almost 10 times higher than the quenching constant of Au nanoparticles without zeolite. To understand the decay dynamics of C480 dye solution in pure zeolite and in the presence of Au nanoparticles immobilized on the surface of the zeolite, we measured the decay time by using pulsed excitation and TCSPC. Figure 6 shows the decay spectra of C480 dye confined in zeolite and in the presence of Au nanoparticles immobilized on the surface of the zeolite. The photoluminescence decay time of dye in zeolite without Au nanoparticles is bi-exponential, with a fast component of 2.98 ns (6%) and a slow component of 5.89 ns (94%). Thus, the

To the best of our knowledge, this is the first report to study the energy transfer between Y-zeolite functionalized Au nanoparticles with confined dye within Y-zeolite cavity by steadystate and time-resolved spectroscopy. The designing of Au nanoparticles immobilized onto the surface of Y zeolite is confirmed by TEM study. The enhanced fluorescence and large blue shift (9 nm) in the emission maximum of C480 in zeolite confirm the confinement of the probe dye molecules inside the zeolite cavity. The quenching efficiency increases from 70 to 94% in the case of confined dye molecules in zeolite and surface-immobilized Au nanoparticles, which is unprecedented. The anisotropy study reveals that this type of supramolecular organization of the dyes inside the zeolite channels is possible. Therefore, such new designing of nanostructured materials could be useful for efficient light harvesting. Acknowledgment. A.P. acknowledges DST, CSIR, and “Ramanujan Fellowship” for generous funding. T.S. thanks CSIR for awarding fellowship. S.K. thanks Department of Science and Technology under SERC scheme for financial support.

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Supporting Information Available: TEM images of pure zeolite and FTIR spectra of zeolite before and after attachment with Au nanoparticles. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Calzaferri, G.; Huber, S.; Maas, H.; Minkowski, C. Angew. Chem., Int. Ed. 2003, 42, 3732. (b) Ruiz, A. Z.; Li, H.; Calzaferri, G. Angew. Chem., Int. Ed. 2006, 45, 5282. (2) Borja, M.; Dutta, P. K. Nature 1993, 362, 43. (3) Nguyen, T. Q.; Wu, J.; Doan, V.; Schwartz, B. J.; Tolbert, S. H. Science 2000, 288, 652. (4) Scott, B. J.; Bartl, M. H.; Wirnsberger, G.; Stucky, G. D. J. Phys. Chem. A 2003, 107, 5499. (5) Wang, L.; Liu, Y.; Chen, F.; Zhang, J.; Anpo, M. J. Phys. Chem. C 2007, 111, 5541. (6) Sen, T.; Jana, S.; Koner, S.; Patra, A. J. Phys. Chem.C 2010, 114, 707. (7) (a) Scaiano, J. C.; Garcı´a, H. Acc. Chem. Res. 1999, 32, 783. (b) Guerrero-Martı´nez, A.; Fibikar, S.; Pastoriza-Santos, I.; Liz- Marza´n, L. M.; Cola, L. D. Angew. Chem., Int. Ed. 2009, 48, 1266. ´ lvaro, M.; Garcı´a, H.; Pillai, M. N. Eur. J. Org. Chem. (8) Casades, I.; A 2002, 2074. (9) (a) Medintz, I. L.; Clapp, A. R.; Mattoussi, H.; Goldman, E. R.; Fisher, B.; Mauro, J. M. Nat. Mater. 2003, 2, 630. (b) Clapp, A. R.; Medintz, I. L.; Uyeda, H. T.; Fisher, B. R.; Goldman, E. R.; Bawendi, M. G.; Mattoussi, H. J. Am. Chem. Soc. 2005, 127, 18212. (10) Peng, H.; Zhang, L.; Kjallman, T. H. M.; Soeller, C.; Sejdic, J. T. J. Am. Chem. Soc. 2007, 129, 3048.

Sen et al. (11) (a) Montalti, M.; Zaccheroni, N.; Prodi, L.; O’Reilly, N.; James, S. L. J. Am. Chem. Soc. 2007, 129, 2418–2419. (b) Boulesbaa, A.; Issac, A.; Stockwell, D.; Huang, Z.; Huang, J.; Guo, J.; Lian, T. J. Am. Chem. Soc. 2007, 129, 15132–15133. (12) Jennings, T. L.; Singh, M. P.; Strouse, G. F. J. Am. Chem. Soc. 2006, 128, 5462. (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) Darbha, G. K.; Ray, A.; Ray, P. C. ACS Nano 2007, 1, 208. (15) (a) Sen, T.; Haldar, K. K.; Patra, A. J. Phys. Chem. C 2010, 114, 11409. (b) Haldar, K. K.; Sen, T.; Patra, A. J. Phys. Chem. C 2010, 114, 4869. (c) Sen, T.; Patra, A. J. Phys. Chem. C 2009, 113, 13125. (d) Sen, T.; Sadhu, S.; Patra, A. Appl. Phys. Lett. 2007, 91, 043104-1. (16) (a) Gersten, J.; Nitzan, A. J. Chem. Phys. 1981, 75, 1139. (b) Persson, B. N.; Lang, N. D. Phys. ReV. B 1982, 26, 5409–5415. (17) Grabar, K. C.; Freeman, R. G.; Hommer, M. B.; Natan, M. J. Anal. Chem. 1995, 67, 735. (18) Lakowicz, J. R. Principles of Fluorescence spectroscopy, 3rd ed.; Springer publisher: New York, 2006. (19) O’Connor, D. V.; Phillips, D. Time Correlated Single Photon Counting; Academic Press: London, 1984. (20) Procopio, A.; Das, G.; Nardi, M.; Oliverio, M.; Pasqua, L. ChemSusChem 2008, 1, 916. (21) Franco, M.; Rosenbach, N., Jr.; Ferreira, G. B.; Guerra, A. C. O.; Kover, W. B.; Turci, C. C.; Mota, C. J. A. J. Am. Chem. Soc. 2008, 130, 1592. (22) Thipperudrappaa, J.; Biradarb, D. S.; Hanagodimath, S. M. J. Lumin. 2007, 124, 45. (23) Bhattacharyya, S.; Sen, T.; Patra, A. J. Phys. Chem. C 2010, 114, 11787.

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