Energy Transfer between Confined Dye and Surface Attached Au

Dec 3, 2009 - Nanoscale architectures have been designed by entrapping rhodamine 6G dye molecules into the channels of mesoporous silica and Au nanopa...
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J. Phys. Chem. C 2010, 114, 707–714

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Energy Transfer between Confined Dye and Surface Attached Au Nanoparticles of Mesoporous Silica 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: September 17, 2009; ReVised Manuscript ReceiVed: NoVember 6, 2009

Nanoscale architectures have been designed by entrapping rhodamine 6G dye molecules into the channels of mesoporous silica and Au nanoparticles anchor onto the surface of the mesoporous matrix. The surface energy transfer between confined dye and Au nanoparticles has been studied by steady state and time-resolved spectroscopy. The appearance of second surface plasmon band at 680 nm with increasing the concentration of mesoporous silica indicates the formation of self-assembled structure of Au nanoparticles which is established by TEM and DLS studies. A mechanism for self-assembled Au nanoparticles is proposed. The PL quenching (76.3% to 27.4%) and energy transfer efficiency (51.8% to 17.4%) can be tuned with changing the arrangement of Au nanoparticles. Analysis reveals that the energy transfer from dye to Au nanoparticles is a surface energy transfer process and it follows 1/d4 distance dependence. This anisotropy decay reveals that the dye molecules are aligned inside the channels of mesoporous silica. Such energy transfer between confined dye and Au nanoparticles could pave the way for designing new optical based materials for the application in chemical sensing or light harvesting system. Introduction Investigations on chromophores confined in nanopores, such as in sol-gel, mesoporous silica, zeolites etc., have opened up new possibilities for the use of nanoporous materials for light harvesting applications.1-10 Dutta et al.1 demonstrated the storage of solar energy by photoelectron transfer in zeolite structure. Control of energy transfer in conjugated polymer immobilized in the mesoporous silica has been studied by Tolbert et al.2 Confinement of the dye molecules into nanochannels not only prevents the dyes from forming aggregates but also improves their photostability. In fact, the hybrid thus formed is proven to be more efficacious as regards the energy transfer. Spatial constraints imposed by the host structure lead to supramolecular organization of the guests which will allow unidirectional energy transfer systems. Calzaferri et al.3 have demonstrated the unidirectional energy transfer in dye-zeolite materials along the channel axis, and it has potential for producing efficient light-harvesting materials. Encapsulation of molecules, nanoparticles in mesoporous materials for potential uses in optoelectronics, biosensing, and energy transfer have received a great attention.4-12 Mesoporous materials are also found to be excellent host due to their large pore diameter and high surface area.9,10 The abundant hydroxyl groups on the pore surface make these materials interact well with the guest molecules. Lee et al. reported the enhanced resonance energy transfer between dyes in polymer nanofibers.5 Scott et al.10 showed the effective energy transfer between coumarin 485 and pyrromethene 567 dyes doped in mesoporous silica thin films. Gao et al.11 reported the doping of multicolor quantum dots in mesoporous materials. Sun et al.12 reported the synthesis of luminescent mesoporous materials MCM-41 co* To whom correspondence should be addressed. E-mail: [email protected]; [email protected]. † Indian Association for the Cultivation of Science. ‡ Jadavpur University.

valently bonded with binary rare-earth complexes. Wang et al.13 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. Therefore, the designing of nanostructures materials with unidirectional energy transfer is the emerging field of research for the application of energy storage system. On the other hand, the research in the field of quantum dots (QD)-based fluorescence energy transfer has recently received a lot of attention in order to find potential applications in the areas of luminescence tagging, imaging, medical diagnostics, multiplexing, and most recently as biosensors.14-16 It is now well established that quantum dots (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.14 Furthermore, the large size of QDs compared to organic dyes also provides the design of such configurations where multiple acceptors could interact with a single donor, which enhances FRET efficiency and thus measurement sensitivity.14 Fo¨rster resonance energy transfer (FRET) is a powerful method to determine the distance between donor and acceptor fluorophores. Initially, Medintz et al.14a reported the potential of luminescent semiconductor quantum dots for development of hybrid inorganic-bio receptor sensing materials. They showed 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 conjugates is discussed as a FRET process. It is known that the FRET technique is restricted on the upper limit of separation of only 80 Å. Therefore, in recent years, surface energy transfer (SET) between dye molecules and metal nanoparticles has gained interest because

10.1021/jp908995j  2010 American Chemical Society Published on Web 12/03/2009

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Sen et al.

this technique is capable of measuring distances nearly twice as far as FRET which will help the large scale conformational dynamics of complex biomolecules in macroscopic detail to be understood.17-20 Thus, the energy transfer between the Au nanoparticle and dye provides a new paradigm for design of an optical-based molecular ruler for long distance measurement. The advantage of gold nanoparticles is that these nanoparticles could be used as acceptors in biophysical experiments in vitro as well as in vivo. Several theoretical studies have been published recently on energy transfer from a dye to metal surface, and it is now well established that the separation of donor and acceptor is d-4 dependent.17-21 Strouse et al.17 showed the surface energy transfer (SET) from dye to a DNA attached Au nanoparticle and the energy transfer process follows 1/d4 distance dependence. Using surface energy transfer phenomenon, Ray et al.18 demonstrated the ultrasensitive detection of mercury in soil, water and fish. Dulkeith et al.19 addressed the important issues how the radiative and nonradiative decay rates of the chemically attached dye molecules are influenced by different sized gold nanoparticles. In our previous work, we examined the influence of different shape and core-shell gold nanoparticles on the efficiency of surface energy transfer process.20 In our previous work, we demonstrated the formation of self-assembled Au nanoparticles by changing the concentration of different thiols containing capping agents and their influence on the surface energy transfer between rhodamine 6G dye and Au nanoparticles.20f Very recently, it was reported that the energy transfer from a dye to graphene is found to be d-4 distance dependence.22 Based on the model,17-21 the exact form of dipole-surface energy transfer (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 using Persson model21a

d0 )

(

0.225c3Φdye ωdye2ωFkF

)

1/4

(2)

where d0 is the distance at which a dye will display equal probabilities for energy transfer and spontaneous emission. φdye is the quantum efficiency of dye, the frequency of the donor electronic transition (ω), and the Fermi frequency (ωF), and Fermi wave vector (kF) of the metal.17 Application of nanoparticle-based fluorescence energy transfer using nanoscopic environment is still in the embryonic stage, further investigations in this field are 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. This study represents an initial attempt to understand 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. To our knowledge, there is no report on the energy transfer between Au nanoparticles with confined dye within MCM-41 mesoporous channels. This paper focuses on how the nanochannels of mesoporous silica and their concentration influence the surface energy transfer between confined dye and surface-anchored Au nanoparticles by steady state and timeresolved spectroscopy.

SCHEME 1: Schematic Representation of Thiol Functionalized MCM-41 and Its Attachment with Au Nanoparticles

Experimental Procedures Materials. Chloroauric acid (HAuCl4.3H2O) (S.d.Fine Chem), Tri-Sodium citrate dihydrate (Merck), and rhodamine 6G dye (Aldrich) were used without further purification. 3-Mercaptopropyl-triethoxysilane (3-MPTS), the cationic surfactant cetyltrimethylammonium bromide (CTAB, 98%), 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 MCM-41. Mesoporous MCM-41 was prepared according to the literature method,23 using C16H33N(CH3)3Br (CTMABr) as the template and tetra-butylammonium bromide (TBABr) with a molar composition of the reactants: 1.0 SiO2: 0.48 CTMA+: 0.96 TBA+: 0.39 Na2O:0.29 H2SO4: 110 H2O. This mixture was stirred for 24 h at room temperature and then was transferred into a Teflon lined autoclave and was statically heated at 100 °C for 4 days. The product was washed with copious amounts of deionized water, collected by filtration, and dried in open air. The collected product was heated at 560 °C for 6 h in nitrogen and then for 6 h in air before using for organic modification. This mesoporous material is designated as MCM-41. Preparation of Organic Modification of MCM-41. To do this 0.1 g of MCM-41 was refluxed with 0.3 g (1.5 mmol) of 3-MPTS in 20 mL of dry toluene at 80 °C for 24 h under N2 atmosphere. The white solid MCM-41-(SiCH2CH2CH2SH)x thus produced was filtered and washed with chloroform and dichloromethane. Anchoring of (3-mercaptopropyl)-trimethoxysilane (3-MPTS) onto MCM-41 has been performed through silicon alkoxide route (Scheme 1). Prepared materials have been characterized by FT-IR spectra (Supporting Information, Figure S1). From the IR spectra of MCM-41 and MCM-41(SiCH2CH2CH2SH)x, it is seen that the S-H and C-H streaching bands in the 2540 and 2929 cm-1 region are present in MCM-41-(SiCH2CH2CH2SH)x sample, and these bands are absent in the case of MCM-41 (S1). This clearly indicates that 3-MPTS has been anchored into the MCM-41 matrix. The content of sulfur was calculated based on the wt % of sulfur in MCM-41-(SiCH2CH2CH2SH)x from elemental analysis. The elemental analysis shows the molar ratio for C:S ≈ 3.1:1 in MCM-41-(SiCH2CH2CH2SH)x, and it contains 1.62 wt % sulfur.

Energy Transfer in Mesoporous Silica Synthesis of Au Nanoparticles and MCM-41 Attached Au Nanoparticles. Gold colloids of fairly uniform size were prepared by using well-known citrate reduction methods.24 Briefly, an aliquot of 47.5 mL of aqueous solution of HAuCl4 (containing 0.005 g of HAuCl4) was heated to boiling. Then 2.5 mL of 1% sodium citrate solution (0.025 g of citrate in 2.5 mL of water) was added to the boiling solution with vigorous stirring. The color of the solution changes from light yellow to deep red through the appearance of bluish gray color which persists in the first 5-10 min. Then the solution was allowed to boil for another 20 min. Finally the solution was removed from the hot plate. However, stirring was continued until the solution cooled down to room temperature. An aqueous suspension of MCM-41 (containing 0.25 mM sulfur) was prepared by dispersing 0.005 g of mercaptofunctionalized MCM-41 in 10 mL of distilled water. 0.5 mL of an aqueous suspension of MCM-41 thus prepared was then added to the deep red gold colloidal solution (10 mL) with stirring. The pH of the solution was adjusted to 3.5 by adding dilute HCl solution and stirring was continued for 6 h. During this time the color of the mixture changes from red to purple. The molar ratio of Au(0) to S used is 1:0.05. The prepared material hereinafter designated as Au-MCM-41. Another two sets of MCM-41 attached Au mixtures were prepared by taking the ratio of 1:0.1 and 1:0.2. At higher concentration of MCM41, Au-MCM-41 imparts a blue color indicating the formation of the aggregates among the gold particles. The color of the final solution becomes blue when the concentration of MCM41 is high (molar ratio of Au(0) to S ) 1:0.2) which indicates the formation of the aggregates among the gold particles. Encapsulation of Dye within MCM-41. For the incorporation of rhodamine 6G dye in MCM-41 channels, 1 mL of 1 µM R6G dye solution was added to 4.0 mL of MCM-41 solution and 4.0 mL of each of the MCM-41 attached colloidal Au solutions. These solutions are stable for 3 days. After 3 days the solid parts settled. The dye-loaded MCM-41 solution may contain two types of dyes, one portion at the outer surface of the MCM-41 which are surface adsorbed and another portion inside the MCM-41 cavities. To remove the surface adsorbed dyes, i.e., to get a homogeneous system, the solid parts were separated out and washed thoroughly several times with distilled water until the supernatant does not show any photoluminescence (PL) spectra of the dye. Finally, the solid parts were redispersed in water. All of the final solutions were kept in air for one day for stabilization, and after 1 day optical studies were done. Characterization. The transmission electron microscopy (TEM) images were taken using a JEOL-TEM-2010 transmission electron microscope with an operating voltage of 200 kV. Dynamic light scattering spectra were taken by using a BI200SM-Goniometer (Ver-2.0) with HeNe laser (Melles Griot). 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 405 nm using a picosecond diode laser (IBH Nanoled-07) in an IBH Fluorocube apparatus. The typical fwhm of the system response using a liquid scatter is about 90 ps. The repetition rate is 1 MHz. The fluorescence decays were collected on a Hamamatsu MCP photomultiplier (C487802). The fluorescence decays were analyzed using IBH DAS6 software. The following expression was used to analyze the experimental time-resolved fluorescence decays, I(t):

J. Phys. Chem. C, Vol. 114, No. 2, 2010 709 n

∑ Ri exp(-t/τi)

I(t) )

(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.25 For biexponential decays (n ) 2), the average lifetime, 〈τ〉, was calculated from 2

∑

〈τ〉 )

i)1

2

Riτi2 /

∑ Riτi

(4)

i)1

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, rotational anisotropy [r(t)] was calculated using the formula

r(t) )

IΙΙ(t) - GI⊥(t) IΙΙ(t) + 2GI⊥(t)

(5)

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

G ) SV/SH

(6)

measured intensity ratio ) IVV/IVH ) (SV/SH)(I|/I⊥) ) G(I|/I⊥)

(7)

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 technique26 to be 0.58. Results and Discussion XRD Characterization of MCM-41. Mesoporous silica MCM-41 was prepared with some modification as described in our earlier publication.23 The prepared material was then subjected for characterization by small-angle X-ray diffraction, N2 sorption study, and transmission electron microscopic study. The MCM-41 sample exhibited a very strong single basal peak for d100 ) 40.4 Å that appears at 2θ ≈ 2.18° which is the characteristic of material possessing a short-range hexagonal ordering (Supporting Information, Figure S2). The sample exhibited another two weaker reflections at 2θ ≈ 3.75° and 4.32° for d110 and d200, respectively, indicating that the material contains quasi-regular arrangement of mesopores with hexagonal symmetry (Supporting Information, Figure S2). In addition we were also able to observe the d300 peak at 2θ ≈ 6.22 and a diffuse d220, d310 peak at 2θ ≈ 7.16 indicating a long-range ordering in the material; these peaks were seldom observed even in the best quality of MCM-41 reported previously.27 The nitrogen adsorption-desorption isotherm study indicates the MCM-41 possesses a BET (Brunner-Emmett-Teller) surface area of ca. 1140 m2 g-1 with a narrow pore size distribution centered at 22.6 Å, and the mesoporous volume (Vp) was ca. 0.61 cm3 g-1 (Supporting Information, Figure S3). TEM and DLS Characterization. A TEM study of MCM41 has been done to understand the morphology. The representative TEM micrograph showed that the prepared mesoporous silica materials feature plate-like particles with good pore ordering (Figure 1). MCM-41 attributes a lamellar type arrangement of hexagonal porous tubules. These plate-like particle ends,

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Figure 1. TEM image of MCM-41 showing good pore ordering in plate-like particles.

which appear to be sealed, are similar to those previously observed in MCM-41.28 The TEM micrograph of MCM-41 viewed along the pore axis reveals a hexagonal array having channel dimension of ca. 4.1 nm which is consistent with the XRD results. In our case the morphology of the tubules in MCM-41 is rectilinear and they are 80-83 nm long. Figure 2 shows the TEM images of Au nanoparticles prepared at different conditions. Figure 2a represents the citrate stabilized Au nanoparticles with well dispersed isolated particles without any aggregation. The average size of these particles is 16.6 ( 0.5 nm. It is interesting to note that TEM image is different in case of Au nanoparticles attached to MCM-41. Figure 2b shows the TEM images of the gold nanoparticles anchored onto MCM41 (Au: S ) 1:0.2). It reveals the spontaneous formation of 2D structures of gold nanoparticles throughout the grid. The assembly formed by the Au nanoparticles attached to MCM41 due to mesoporous silica. Small interparticle spacing is found in all the assembled structures. To establish whether the assemblies are actually forming in the solution and not during the drying process of the sample on the TEM grid, dynamic light scattering (DLS) measurement was performed with pure citrate stabilized gold nanoparticles and after anchoring onto surface of mesoporous silica (Figure 3). It is clearly seen from Figure 3 that the effective hydrodynamic diameter increases with increasing mesoporous silica concentration [curves b-d]. DLS data also reveals that the hydrodynamic diameter of the MCM41 attached assembled Au nanoparticles (Au:S ) 1:0.2) (273.0 nm) [curve d] is much greater than that of isolated citrate stabilized gold nanoparticles (52.4 nm) [curve a]. Analysis suggests that the assembly occurs in the reaction medium and it did not occur during solvent evaporation on the TEM grid. Mercapto-functionalized MCM-41 with -SH end groups can act as soft base while Au nanoparticles being soft acid. Consequently, mercapto-functionalized MCM-41 is expected to be assembled around the Au nanoparticles through soft-acid-softbase interaction. The MCM-41 (mesoporous silica) could be aggregated further through hydrogen bonds between surface hydroxyl (silanol) groups of mesoporous silica. This type of assembly propagates in a 2D plane to form sheets. Additional reinforcement also could be established through -HS · · · · H-O(marcapto- and silanol groups) hydrogen bonds. The above types of hydrogen bonds have already been proposed to occur in similar kinds of mesoporous silica systems.28 Therefore, formation of two-dimensional assembly in mercapto-functionalized

Sen et al. MCM-41 and gold nanoparticles hybrid system explain the observed result in TEM study. Spectroscopic Studies. The dye loaded MCM-41 solution may contain two types of dyes, one portion at the outer surface of the MCM-41 which are surface adsorbed and another portion inside the MCM-41 cavities. Several times washing with distilled water were performed to remove the surface adsorbed dyes, i.e., until the supernatant does not show any photoluimenscence (PL) spectra of the dye. Figure 4 shows the photoluminescence spectra of dye (R6G) containing MCM-41 before washing (curve a) and after several times washing (curves b-g). It is clearly seen that the PL intensity of R6G dye decreases with washing, indicating the removal of surface adsorbed dye molecules. Finally, we stopped washing when we did not observe any change in PL intensity of dye after washing. It indicates that surface adsorbed dyes are removed mostly and dye molecules reside within the channel of mesoporous MCM-41. Again, it is to be noted rhodamine 6G dye is a cationic dye which is expected to interact well with the anionic silicates framework of mesoporous MCM-41 to prevent elution of the dye molecules from the pore channel as reported earlier.13 Figure 5 shows the absorption spectra of the pure citrate stabilized Au nanoparticles [curve a] and Au nanoparticles anchored onto surface of MCM-41 with varying MCM-41 concentration, i.e., the molar ratio of Au(0) to S [curves b-d]. Pure citrate stabilized gold nanoparticles at pH 3.5 display a surface plasmon band at 521 nm [curve a] which is a characteristic of isolated spherical gold nanoparticles. A slight red-shifted plasmon band along with a development of a second plasmon band in a higher-wavelength region at 625 nm is observed at (Au: S ) 1:0.05) lower concentration of MCM-41 [curve b]. The intensity of the transverse band (521 nm) decreases and red-shifts with increasing MCM-41 concentration (Au: S ) 1:0.1) along with the increase in the intensity of the second SPR band shifted to 635 nm from 625 nm [curve c]. The color of the solution changes from deep red to violet. Finally, the transverse band becomes weak and red-shifted to 533 nm. The second SPR band becomes predominant at 670 nm along with a color change of the solution from deep red to blue upon further increasing the MCM-41 concentration (Au: S ) 1:0.2) [curve d]. The appearance of this long-wavelength plasmon band with increasing the MCM-41 concentration suggests the formation of an assembly of Au nanoparticles. During aggregation the plasmon modes interact and form a new plasmon band in the long wavelength region.30 Figure 6 shows the photoluminescence (PL) spectra of the rhodamine 6G (R6G) dye solution in absence [curve a] and in presence of gold nanoparticles anchored onto the surface of MCM-41 with different concentration [Au (0)/S] molar ratio [curves b-d]. The PL intensity of R6G dye is drastically quenched in the presence of gold nanoparticles anchored onto the surface of MCM-41 as seen in Figure 6. It is to be noted that the PL quenching efficiency varies with changing the concentration of mesoporous silica. The observed quenching of PL intensities is 76.3, 63.8, and 27.4% for Au:S ratios 1:0.05, 1:0.1, and 1:0.2, respectively. It reveals that PL quenching of the dye increases with decreasing the concentration of mesoporous silica. This can be explained by taking the relative amount of free Au nanoparticles in the solution under consideration. At a lower concentration of MCM-41, more Au nanoparticles are not attached to MCM-41 and remain free in the solution. These free Au nanoparticles are responsible for higher PL quenching.

Energy Transfer in Mesoporous Silica

J. Phys. Chem. C, Vol. 114, No. 2, 2010 711

Figure 2. TEM images of (a) pure citrate stabilized Au nanoparticles, (b) Au nanoparticles attached with MCM-41 and (c) HRTEM image of Au nanoparticles.

Figure 3. Dynamic light scattering spectra of pure citrate stabilized Au nanoparticles [curve a] and MCM-41 attached Au nanoparticles with changing Au/S ratio [curves b-d] [b ) 1:0.05; c ) 1:0.1; d ) 1:0.2].

Figure 4. Photoluminescence spectra of rhodamine 6G dye containing MCM-41 before washing (curve a) and after successive washing (curves b-g).

Time-Resolved Fluorescence Studies. We measured decay times using pulsed excitation and time-correlated single-photon counting (TCSPC) to understand the decay dynamics of R6G dye solution in the absence and presence of gold nanoparticles attached to MCM-41 with varying [Au (0)/S] molar ratios (Figure 7). The photoluminescence decay times of the aqueous dye solution (1 µM) and dye in the MCM-41 solution (1 µM) without Au nanoparticles are single exponential, and the values are 3.91 and 3.84 ns, respectively. However, the decay times of dye molecules in presence of MCM-41 attached Au nanoparticles having different MCM-41 concentrations are fitted by biexponential decay (eq 3). The decay times of the dye molecules in the absence and presence of MCM-41 attached Au NPs have been enlisted in Table 1. The fast and slow

Figure 5. UV-vis absorption spectra of pure Au [curve a] and MCM41 attached Au nanoparticles with changing Au/S ratio [curves b-d] [b ) 1:0.05; c ) 1:0.1; d ) 1:0.2].

Figure 6. Photoluminescence (PL) spectra of R6G dye solution [curve a] in the absence and presence of MCM-41 attached Au NPs with changing Au/S ratio [curves b-d] [b ) 1:0.05; c ) 1:0.1; d ) 1:0.2]. λexc ) 405 nm.

components are 716 ps (65%) and 3.98 ns (35%) for the dye solution (1 µM) in the presence of MCM-41 attached gold nanoparticles with (Au: S ) 1:0.05). The fast and slow components are 897 ps (57%) and 3.84 ns (43%) for the dye solution in the presence of MCM-41 attached gold nanoparticles with (Au: S ) 1:0.1), and the components are 718 ps (21%) and 3.83 ns (79%) with higher MCM-41 concentration (Au: S ) 1:0.2). The corresponding average decay times are 1.85, 2.16, and 3.17 ns for Au:S ratios of 1:0.05, 1:0.1, and 1:0.2, respectively. 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 wellknown that the lifetime measurement is more sensitive than PL

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Sen et al. TABLE 2: Energy Transfer Parameters for Different Rh6G-Au Systems system pure Rh6G dye solution Rh6G in MCM-41 solution Rh6G in Au-MCM-41 solution (Au:S ) 1:0.05) Rh6G in Au-MCM-41 solution (Au:S ) 1:0.1) Rh6G in Au-MCM-41 solution (Au:S ) 1:0.2)

λem (nm) ΦD0 E(%) (PL) d0 (Å) d (Å) 549 549

0.95 0.91

550

0.91

76.3

80.7

79.3

551

0.84

63.8

80.7

85.9

550

0.84

27.4

80.7

119.1

SCHEME 2: CHEM 3D Simulated Structure of (3-mercaptopropyl)-trimethoxysilane Showing Sulfur to Oxygen Distance Figure 7. Decay curves of rhodamine 6G (R6G) dye solution in the absence and presence of MCM-41 attached Au nanoparticles.

TABLE 1: Decay Times and Energy Transfer Efficiency of Different Au-Dye Systems system

b1

τ1 (ps)

b2

pure Rh6G dye solution 1 Rh6G in MCM-41 solution 1 Rh6G in Au-MCM-41 0.65 716 0.35 solution (Au:S ) 1:0.05) Rh6G in Au-MCM-41 0.57 897 0.43 solution (Au:S ) 1:0.1) Rh6G in Au-MCM-41 0.21 718 0.79 solution (Au:S ) 1:0.2) a

τ2 〈τ〉 ) φET (ns) (b1τ1 + b2τ2) (ns) (%) 3.91 3.84 3.98

3.91a 3.84a 1.85a

3.84

a

43.8

a

17.4

3.83

2.16 3.17

51.8

(10%.

quenching efficiency because error comes from the fluctuations in lamp intensity. The energy transfer efficiency from dye to Au nanoparticles is calculated by using the relation φET ) 1 τDA/τD, where τDA is the decay time of dye in the presence Au nanoparticles and τD corresponds to the decay time of dye in the absence of Au nanoparticles. The calculated energy transfer efficiencies from dye to Au nanoparticles are 51.8%, 43.8%, and 17.4% for Au nanoparticles with increasing MCM-41 concentration (i.e., S contents of 0.05, 0.1, and 0.2). It is worth noting that energy transfer efficiency varies from 17.4% to 51.8% with changing the MCM-41 concentration. It indicates that the self-assembled nanoparticle influences the energy transfer process which may be due to shifting of the plasmon band due to formation of self-assembled structures. The shifting of the plasmon band reduces the spectral overlap between dye and Au nanoparticle, which influences the energy transfer process between dye and Au nanoparticle. This result also provides insight into optimizing nanostructured materials as energy harvesting systems. Using the surface energy transfer process (eq 1), the distance between donor (dye) and acceptor (Au nanoparticles) was calculated. It is already known that the efficiency of surface energy transfer (SET) which depends on the inverse fourth power of the distance of separations between donor and acceptor. The calculated d0 value is 8.07 nm and the distances between donor and acceptor (d) are 7.9, 8.6, and 11.9 nm for Au/S 1:0.05, 1:0.1, and 1:0.2, respectively (Table 2). As the FRET based method is restricted on the upper limit of only 80 Å, therefore, we may suggest that the energy transfer from dye to Au nanoparticles is a SET process in the present study and follows a 1/d4 distance dependence. We try to correlate the distance between the donor (dye) and acceptor (Au nanoparticles) obtained by the SET equation with the distance between donor and acceptor from the structural estimation. By using the SET

equation, the measured distance between dye to Au NP (Au:S ) 1:0.2) is 11.9 nm. The calculated distance between O and -SH of MCM-41 is 6.7 Å (Scheme 2), using Chem 3D Pro software. It is known that the radius of the MCM-41 cavity is equal to 20 Å and the radius of a Au NP is equal to 83 Å (from TEM study). Using these structural values, the estimated distance between the dye and Au nanoparticle is equal to 10.9 nm by considering dye molecules to remain at the center of the cavity. This estimated value is close to the distance 11.9 nm, measured by the SET method. Analysis reveals that the SET method is a useful method to calculate the distance between donor and acceptor for long distance measurements, which is not possible by the FRET process due to limitations up to 80 Å. Time-Resolved Anisotropy Studies. A time-resolved anisotropy study is essential to unraveling the origin of the motion of the dye molecules and the direction of energy transfer. To understand the rotation dynamics of dye molecules inside the MCM-41 channel, a time-resolved anisotropy study is performed. The anisotropy decays of R6G in pure citrate stabilized Au nanoparticle and dye confined MCM-41 attached Au nanoparticles (Au:S ) 1:0.2) are shown in Figure 8. For a spherical molecule, the anisotropy decay is described by a single exponential.16b

r(t) ) r0e-t/φ

(8)

where r0 is the anisotropy observed in the absence of other depolarizing processes, t is the time, and φ is the rotational correlation time. The decay of R6G in pure citrate stabilized Au nanoparticle is single exponential with a correlation time

Energy Transfer in Mesoporous Silica

J. Phys. Chem. C, Vol. 114, No. 2, 2010 713 inside the mesoporous channels. Therefore, such new designing of nanostructured materials could be useful for efficient light harvesting or chemical sensing applications. Acknowledgment. A.P. thanks The Department of Science and Technology (NSTI) and “Ramanujan Fellowship” for generous funding. T.S. and S.J. thank CSIR for awarding fellowship. Financial support from Department of Science and Technology under SERC scheme (to S.K.) is also gratefully acknowledged. Supporting Information Available: FTIR spectra, small angle X-ray diffraction pattern of MCM-41, and N2 adsorption/ desorption isotherms of MCM-41. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 8. Fluorescence anisotropy decay curves of rhodamine 6G (R6G) dye in the presence of citrate stabilized Au NPs [curve (i)] and MCM-41 attached Au nanoparticles (Au:S ) 1:0.2) [curve (ii)].

constant of 230 ps. The single exponential correlation time constant indicates the isotropic energy transfer and the motion of the dye is not hindered. However, for a dye inside the MCM41 mesoporous channels, the fluorescence anisotropy decay of R6G is biexponential with correlation time constants of 235 ps (69%) and 1.51 ns (31%), leading to an average correlation time of 630 ps. Similarly, fluorescence anisotropy decay is reported for C153 dye in γ-CD which is ascribed to formation of linear aggregates of γ-CD.31 This anisotropy decay curve indicates the alignment of dye molecules inside the channels of mesoporous silica which restricts the isotropic motion of dye molecules. The radius of the channels of MCM-41 is 4 nm, whereas the size of the dye is only about 5 Å. Thus, many dye molecules reside inside the cavity. The motion of the central dye molecule will be free. It rotates as free dyes and gives the fast component of 235 ps. The motion of the dyes residing around the periphery inside the cavity is restricted and gives the slow component of the anisotropy decay. Tolbert et al.2 confirmed the unidirectional energy transfer in oriented conjugated polymer-mesoporous silica composites from the initial high value of anisotropy. The initial larger value of the anisotropy may suggest the unidirectional energy transfer in the present hybrid system. Results suggest that the present system is beneficial for an efficient light harvesting system. Therefore, we may say that this type of supramolecular organization of the dyes inside the channels will allow light harvesting within a certain volume of a dye loaded mesoporous materials. Conclusions To the best of our knowledge, this is the first report to study the surface energy transfer between dye confined within mesoporous silica and Au nanoparticles anchored onto the surface of mesoporous silica by steady state and time-resolved spectroscopy. The designing of 2D architectures of Au nanoparticles can be possible with varying concentrations of mesoporous silica which is confirmed by TEM and DLS studies. Quenching efficiency decreases drastically from 76.3% to 27.4%, and the surface energy transfer efficiency varies from 51.8% to 17.4% with changing from isolated to assembled nanoparticles. The distance between dye and gold nanoparticles is 11.9 nm using a SET method which matches well with the structural estimated value. Analysis reveals that the energy transfer from dye to Au nanoparticles is a SET process in the present study, and it follows 1/d4 distance dependence. The anisotropy decay reveals that the dye molecules are aligned

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