Quenching of Confined C480 Dye in the Presence of Metal

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Quenching of Confined C480 Dye in the Presence of Metal-Conjugated γ-Cyclodextrin Tapasi Sen, Krishna Kanta Haldar, and Amitava Patra* Department of Materials Science, Indian Association for the CultiVation of Science, Kolkata 700 032, India ReceiVed: April 9, 2010; ReVised Manuscript ReceiVed: June 2, 2010

Here we demonstrate the designing of new optical-based materials having nanotubular γ-CD aggregates linked by coumarin 480 dyes: Au nanoparticles for light harvesting system. It is found that the fluorescence quenching of the coumarin 480 dye increases from 79 to 99% due to confinement of the dye inside γ-CD. The formation of the nanotubular structure of C480/CD complex is confirmed by fluorescence anisotropy decay and TEM study. The average decay times of the C480 dye inside γ-CD in the absence and presence of attached Au nanoparticles are 4.77 and 1.91 ns, respectively, and the rate constant of ET is found to be 3.13 × 108 s-1. Introduction 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 photonics and biophotonics.1-3 Metal nanoparticles display very interesting optical properties, related to surface plasmon resonance bands, which give rise to its versatile applications in sensors, biosensors, and many emerging areas of nanotechnology.4 Recently, Freeman5 demonstrated the FRET-based competitive assay using β-cyclodextrin-modified CdSe/ZnS QDs as sensors and chiroselective sensors. Tang et al.6 demonstrated the formation of Au nanoparticles (NP) assembled with β-cyclodextrin (β-CD) and fluorescein (FL). They suggested the use of this Au NPs-β-CDs-FL assembly as a fluorescent probe for the sensing of cholesterol. Investigations on chromophores confined in nanochannels, have opened up new possibilities for the use of nanoporous materials for light harvesting applications. Dutta et al.7a 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.8 Calzaferri et al.7b,c have demonstrated the unidirectional energy transfer in dye-zeolite materials along the channel axis for producing efficient lightharvesting materials. Confinement of the dye molecules into nanochannels not only prevent the dyes from forming aggregate, but also improve their photostability.9 Among all potential hosts, the cyclodextrins seem to be the most important ones because of several advantages. γ -Cyclodextrins (CD) are cyclic oligosaccharides compounds in which eight (γ-CD) glucose units are linked to form a truncated conical structure. The interior cavity of γ-CD is hydrophobic in nature, having a dimension of 7.5-8.5 Å and the height of ∼8 Å.10-12 Therefore, a large number of organic molecules can be encapsulated in its hydrophobic cavity and form host-guest supramolecular structures. A large numbers of studies exist on dynamics of organic fluorophores confined in a CD cavity. Fleming et al.13 studied the solvation dynamics of two dyes C480 and C460 inside the γ-CD cavity. They found 1:1 complex formation between C480 and γ-CD. Bhattacharyya et al.14 reported the electron transfer between DMA to C480 dye inside the cavity of hydroxypropyl γ-cyclodextrin (hpCD). * To whom correspondence should be addressed. Phone: (91)-33-24734971. Fax: (91)-33-2473-2805. E-mail: [email protected].

To our knowledge, there is no report on the energy transfer between γ-cyclodextrin-functionalized Au nanoparticles with confined dye within γ-cyclodextrin cavity. Application of nanoparticle-based fluorescence energy transfer using a nanoscopic environment is still in the embryonic stage, and further investigations in this field are necessary for in-depth understanding of the phenomenon. In this letter, we first demonstrate the design of γ-cyclodextrin-functionalized Au nanoparticles and study their electron transfer between Au nanoparticles and confined coumarin 480 dye within γ-cyclodextrin using steady state and ultrafast spectroscopy. Experimental Section Coumarin 480 (C480; Sigma-Aldrich), γ-cyclodextrin (γ-CD; Fluka), chloroauric acid (HAuCl4 · 3H2O; S.d.Fine Chem), sodium borohydride (NaBH4; Merck), glutathione (GSH; Aldrich), N-hydroxysulphosuccinamide (NHS; Aldrich), and 3-aminophenylboronic acid (Aldrich) were used as received. Preparation of Glutathione (GSH)-Capped Gold Nanoparticles. In 10 mL of 10 mM phosphate buffer solution (pH ) 7.2), 0.008 g HAuCl4 · 3H2O was added with stirring. After 5 min of stirring, 0.008 g glutathione was added to that solution. After 10 min, NaBH4 aqueous solution (25 mM) was added dropwise with constant stirring. The color of the solution becomes dark brown, indicating the formation of small Au nanoparticles. The concentration of as-prepared gold nanoparticle solution was 7.1 µM. Then glutathione acid-capped Au nanoparticles were chemically attached with γ-cyclodextrin. Attachment of GSH-Capped Au NPs with γ-Cyclodextrin. To the 5 mL of the above GSH-capped Au nanoparticles solution, 1 mL of N-hydroxysulphosuccinamide (NHS) solution (1 mg/mL) was added and the solution was stirred for 15 min. A total of 1 mL of 3-aminophenylboronic acid stock solution (1 mg/mL) was added to that solution with stirring. After 1.5 h of stirring, 1 mL of 80 mM γ-cyclodextrin solution was added to that solution and stirred for 18 h. Final concentration of γ-cyclodextrin solution was 10 mM. A schematic representation is given in Scheme 1. Now, 0.5 mL of 10 µM C480 solution was added to each of 2 mL of as-prepared γ-CD-capped Au nanoparticles solution, 2 mL of 10 mM γ-CD solution in the absence of Au nanoparticles, 2 mL of GSH-capped Au nanoparticles solution, and 2 mL of phosphate buffer solution at pH 7.2. All these samples were kept under airtight conditions for 24 h. After 1 day, these samples were used for optical studies.

10.1021/jp103192r  2010 American Chemical Society Published on Web 06/15/2010

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SCHEME 1: Schematic Presentation for the Fabrication of γ-Cyclodextrin Attached Au Nanoparticles

The transmission electron microscopy (TEM) images were taken using a JEOL-TEM-2010 transmission electron microscope with an operating voltage of 200 kV. The emission spectra of all samples were recorded in a fluoro Max-P (HORIBA JOBIN YVON) luminescence spectrophotometer. For the timecorrelated 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. 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 using the formula

r(t) )

IΙΙ(t) - GI⊥(t) IΙΙ(t) + 2GI⊥(t)

confirm that the probe dye molecules are in lower polarity and a more hydrophobic environment inside the γ-CD cavity than that in bulk water, which is consistent with previous results.15 To obtain the stoichiometry and the equilibrium constant of such a cyclodextrin-dye complex, the widely used double reciprocal plot is being used.14 If the complex between C480 and γ-CD has a 1:1 stoichiometry, then the binding constant (Kb) of C480 to γ-CD corresponds to the following equilibrium: Kb

C480 + CD a [C480/CD]

(2)

If the emission quantum yields of C 480 in the presence and absence of CD are φf and φ0, respectively, then ∆φf () φf φ0) is given by

(1)

The G value of the setup is 0.704. The femtosecond upconversion set up (FOG 100, CDP) is described earlier.14 The sample was excited at 405 nm using the second harmonic of a modelocked Ti-sapphire laser (Tsunami, Spectra Physics), pumped by a 5 W Millennia (Spectra Physics). The femtosecond data were fitted by using IGOR Pro 6.04 software. Results and Discussion Figure 1a shows the fluorescence spectra of C480 dye in phosphate buffer and in 10 mM γ-CD. The emission peak maxima of C480 is observed at 486 nm in phosphate buffer solution. A blue shifting of the emission peak of C480 is observed after addition of γ-CD and the amount of shifting increases with an increase in the concentration of γ-CD (shown in Figure 1b). The peak appears at 470 nm in the presence of 10 mM γ-CD (Figure 1a). It is seen from Figure 1b, the emission intensity of C480 rises very steeply as the concentration of the γ-cyclodextrin increases, indicating a very strong association between C480 and γ-CD.15c The quantum yield of C480 in 10 mM γ-CD becomes 0.96. The enhanced fluorescence and large blue shift (16 nm) in the emission maximum of C480 in γ-CD

Figure 1. (a) Normalized fluorescence spectra of C480 dye (i) in phosphate buffer and (ii) in 10 mM γ-CD; (b) fluorescence spectra of C480 dye in phosphate buffer and in γ-CD solution with increasing the concentration of γ-CD from 1 to 10 mM. λex ) 405 nm. Inset in (b) shows the double reciprocal plot of ∆φf vs concentration of γ-CD.

Quenching of Confined C480 Dye

1 1 1 ) + ∆φf (φ∞ - φ0) (φ∞ - φ0)Kb[CD]

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(3)

where, φ∞ denotes the emission quantum yields of C480 when completely bound to CD. The value of Kb can be determined from a double reciprocal plot of ∆φf against [CD].14 If the complex really has a 1:1 stoichiometry, then according to eq 3, the double reciprocal plot will show nice linearity. However, the double reciprocal plot (inset of Figure 1b) shows nonlinearity in the present system, indicating the complex between C480 and γ-CD is not 1:1. Because there are many steps in the formation of nanotubular aggregates, the double reciprocal plot approximation cannot be applied in this case.15c Time-resolved anisotropy study is essential to unraveling the origin of the motion of the dye molecules. To understand the rotation dynamics of dye molecules inside the γ-cyclodextrin cavity, time-resolved anisotropy study is performed. A fluorescence anisotropy decay study reveals the reorientational dynamics of the excited fluorophore, which directly help to understand structural information. Figure 2 shows the anisotropy decay of C480 in 10 mM γ-CD solution. In bulk water, the anisotropy decay of C480 (monitored at 470 nm) is single exponential with time constant of 100 ( 20 ps with no residual anisotropy decay. In the presence of 10 mM γ-CD, the fluorescence anisotropy decay of C480 (monitored at 470 nm) exhibits a biexponential decay with a fast time constant of 680 ps (25%) and a slow component of 20.6 ns (75%). Thus, the average correlation time constant is 15.6 ns. The fast component of the anisotropy decay of 680 ps may be ascribed to restricted rotation of the C480 dye molecules, which is due to encapsulation of dye inside the γ-CD cavity. It is seen from the anisotropy plot that the residual anisotropy does not decay even over 23 ns, indicating the larger hydrodynamic size. The hydrodynamic size is calculated using Stokes-Einstein equation

τrot

ηV ) kBT

Figure 2. Fluorescence anisotropy decay of C480 dye in 10 mM γ-CD (λex ) 405 nm).

Figure 3. TEM image of γ-cyclodextrin attached Au nanoparticles.

(4)

where τrot is the rotational time constant, η is the viscosity of the medium, and V is the volume of the complex. Here, τrot ) 23 ns, V is calculated to be 94585.2 Å3. The C480/γ-CD complex is an ellipsoid with semiaxes a, b, and c and volume V ) 4πabc/3. In this case, a ) b ) 9 Å (same as that of γ-CD), while the length is 2c. Thus, c ) 278.9 Å and the length of the complex () 2c) is 557.8 Å. The reported height of the γ-CD cavity is ∼8 Å. Thus, the C480 and γ-CD form a linear aggregate having more than 557.8/8-70 cyclodextrins, which is unprecedented. Thus, the nanotubular aggregates will be at least ∼70 nm long. It is seen from the TEM image (Figure 3) that the large elongated structure is formed by linking up the small rodlike aggregates of C480-γ-CD. From the TEM image, the average length of these rodlike aggregates is found to be 84 nm, which nicely matches with the length obtained by using the anisotropy decay. The formation of this nanotubular structures is based on the contribution of two forces. One is the van der Waals force of attraction between C480 and the interior of the CD cavity; the other is the H-bonding interaction between the ring OH groups of the cyclodextrin moieties, resulting in the self-association of γ-CDs. Kamachi et al.16 demonstrated the supermolecular assemblies of R-CD units and McGown et al.17 reported the formation of rigid molecular nanotube aggregates of β-CD and γ-CD. Chattopadhyay et al.18 reported the formation of cyclodextrin-based elongated super-

Figure 4. Emission spectra of pure C480 (i) in phosphate buffer, (ii) in 10 mM γ-CD, (iii) in the presence of glutathione-capped Au nanoparticles without attachment with γ-CD, and (iv) in the presence of γ-CD attached Au nanoparticales (λex ) 405 nm).

structure anchored by a biologically active probe molecule. The quenching efficiency of C480 dye is 79% in the presence of glutathione-capped Au nanoparticles only (Figure 4). However, the PL intensity of the coumarin 480 dye confined in the γ-CD nanocavity is drastically quenched in the presence of γ-CD attached Au nanoparticles (Figure 4) and the quenching of PL intensity is 99%. It reveals that the quenching efficiency strongly enhanced during the confinement of dye in γ-CD. We measured decay time using pulsed excitation and timecorrelated single-photon counting (TCSPC) to understand the decay dynamics of C480 dye solution in γ-CD without Au nanoparticles and in γ-CD attached Au nanoparticles (Figure

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1 1 + kET ) τ τ0

(5)

where τ0 and τ denote lifetime of the donor C480 dye in the absence and presence of Au nanoparticles. Because the rise component of C480 dye without Au nanoparticles is due to solvation dynamics, we used the long component of decay to get the average decay. However, the average decay of C480 dye in Au attached γ-CD is being calculated considering all components. The average decay time of C480 dye in γ-CD in the absence and presence of Au nanoparticles are 4.77 and 1.91 ns, respectively. Thus, the rate constant of ET is found to be 3.13 × 108 s-1. Bhattacharyya et al.14 studied the electron transfer rate from dimethylaniline (DMA) to four coumarin dyes (C151, C481, C153, and C480) inside the cavity of hydroxypropyl γ-cyclodextrin (hpCD) using femtosecond upconversion study. They found 33% PL quenching for C480 dye and the ET rate constant is found to be 0.21 × 107 s-1. They also demonstrated that the ET rate depends on the nature of the dye molecules. In the present study, PL quenching of C480 dye is ∼99% in presence of Au nanoparticles, which is unprecedented. The calculated ET rate (3.13 × 108 s-1) is 100 times higher than the reported ET rate. Analysis suggests that Au nanoparticles influence on this very fast electron transfer process. Figure 5. (a) Picosecond transients of C480 dye (i) in 10 mM γ-CD and (ii) in Au nanoparticles attached γ-CD solution; (b) Femtosecond transients of C480 (λem ) 470 nm) (i) in 10 mM γ-CD and (ii) Au nanoparticles attached γ-CD solution (λex ) 405 nm). The initial part of decay (i) is shown in the inset (b).

TABLE 1: Fluoresence Decay Parameters of C480 Dye in γ-CD and Au Attached γ-CD (λem ) 470 nm) system

a1

τ1a (ps)

a2

τ2a (ns)

a3

τ3a (ns)

a (ns)

C480 dye in γ-CD C480 dye in Au attached γ-CD

-0.25 0.3

1 1.25

0.34 0.53

2.62 2.07

0.91 0.17

5.57 4.81

4.77 1.92

a

(10%.

5a). It clearly shows from the decay curves that there is a shortening of lifetime of C480 dye in presence of Au nanoparticles. The long component of the decay as well as the ultrafast part of the decay was affected by Au nanoparticles. We measured the ultrafast part of the decay using femtosecond up-conversion set up. It is seen from the ultrafast study (Figure 5b) that there is a rise of decay time of the confined dye in γ-CD, which is given in the inset of Figure 5b. The rise originates from the solvation dynamics of C480 dye.14 However, in the case of γ-CD attached gold nanoparticles, we observed shortening of ultrafast decay time. The absence of the rise component and shortening of decay time reveal the electron transfer process, which is faster than solvation process. Thus, the rise is masked by the ultrafast decay of the quenched emission of the dyes. The ultrafast decay component and the long decay component of C480 dye in the absence and presence of Au nanoparticles are shown in Table 1. The rate constant of the electron-transfer process (kET) was calculated by using the following equation14

Conclusions In conclusion, this is the first report to study the electron transfer between confined dye within γ-cyclodextrin and Au nanoparticles using ultrafast spectroscopy. The PL intensity of the coumarin 480 dye confined in γ-CD nanocavity is drastically quenched in the presence of γ-CD attached Au nanoparticles, and the quenching of PL intensity is 99%. It reveals that the quenching efficiency is strongly enhanced during the confinement of dye in γ-CD. It is found from the fluorescence anisotropy measurement that the nanotubular aggregates of C480 anchored cyclodextrin contain at least 70 γ-CD units, which nicely matches with TEM data. The calculated ET rate (3.13 × 108 s-1) is 100 times higher than the reported ET rate. Analysis suggests that Au nanoparticles influence this very fast electron transfer process. Such electron 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. Acknowledgment. A.P. thanks The Department of Science and Technology and “Ramanujan Fellowship” for generous funding. T.S. and K.K.H. thank CSIR for awarding fellowship. Authors thank Prof. K. Bhattacharyya, IACS, for the generous use of the Femto facility. References and Notes (1) (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. (2) (a) Zhou, D.; Piper, J. D.; Abell, C.; Klenerman, D.; Kang, D. J.; Ying, L. Chem. Commun. 2005, 4807. (b) Sadhu, S.; Patra, A. ChemPhysChem 2008, 9, 2052. (c) Haldar, K. K.; Sen, T.; Patra, A. J. Phys. Chem. C 2010, 114, 4869. (3) (a) Zhang, C.-Y.; Yeh, H.-C.; Kuroki, M. T.; Wang, T.-H. Nat. Mater. 2005, 4, 826. (b) Lu, H.; Scho¨ps, O.; Woggon, U.; Niemeyer, C. M. J. Am. Chem. Soc. 2008, 130, 4815. ´ lvarez-Puebla, R. A.; Contreras-Ca´ceres, R.; Pastoriza-Santos, I.; (4) A Pe´rez-Juste, J.; Liz-Marza´n, L. M. Angew. Chem., Int. Ed. 2009, 48, 138.

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