18
J. Phys. Chem. B 2002, 106, 18-21
Photoinduced Charge Separation in a Fluorophore-Gold Nanoassembly Binil Itty Ipe and K. George Thomas* Photochemistry Research Unit, Regional Research Laboratory (CSIR), TriVandrum 695 019, India
Said Barazzouk,† Surat Hotchandani,† and Prashant V. Kamat*,‡ Notre Dame Radiation Laboratory, Notre Dame, Indiana 46556 ReceiVed: September 20, 2001
We report the first spectroscopic demonstration of direct electron transfer between a gold nanoparticle and a surface-bound fluorophore induced by pulsed laser irradiation. Binding of pyrene thiol directly to the gold nanoparticle results in quenching of its singlet excited state. The suppression of S1-T1 intersystem crossing process as well as the formation of pyrene radical cation confirm the excited-state interaction between the metal nanoparticle and the surface-bound fluorophore. The charge separation is sustained for several microseconds before undergoing recombination.
Unique electronic and chemical properties of metal nanoparticles have drawn the attention of chemists, physicists, biologists, and engineers who wish to use them for a new generation of nanodevices.1-10 Electrochemical studies have established the electron-storing properties of gold nanoparticles and their ability to act as an electric relay on a given nanotemplate structure.11-14 Modification of the gold nanoparticles with fluorophores is important for the development of biological tracers as well as optoelectronic devices.12,15,16 Gold nanoparticles themselves show limited photoactivity under UV-visible irradiation, although photoinduced fusion and fragmentation have been observed under laser irradiation.17-22 Binding of a photoactive fluorophore such as pyrene to a gold nanoparticle renders the organic-inorganic hybrid nanoassemblies suitable for light-harvesting and optoelectronic applications.23,24 Direct binding of a fluorophore to the metal surface often results in the quenching of excited states.25-28 Both energytransfer and electron-transfer processes are considered to be major deactivation pathways for excited fluoroprobes on metal surface. Most of these studies are limited to bulk gold surfaces modified with self-assembled monolayers. Indirect evidence for electron transfer between the chromophore and the gold surface has been obtained from photocurrent measurements.29,30 Obtaining insight into such processes using spectroscopic measurements is important to improve the charge separation efficiencies in gold-fluorophore nanoassemblies. We now report transient absorption studies that relate the excited-state quenching of the surface-bound fluorophore to an electron-transfer process in the pyrene thiol-bound gold (Au-SR-Py) nanoassemblies (Scheme 1) suspended in THF solutions. Preparation of pyrene thiol and its binding to gold nanoparticles and experimental details are described in the Supporting Information. Addition of dodecane thiol during the surface modification ensured the dispersion of pyrene moieties uniformly on the gold surface without inducing any ground stateaggregation. TEM micrographs (inset of Figure 1A) have † Permanent Address: Groupe de Recherche en Energie et Information Biomole´culaires, Universite´ du Que´bec a` Trois Rivie`res, Trois Rivie`res, Que´bec, Canada G9A 5H7. ‡ Email:
[email protected]. WWW: http://www.nd.edu/∼pkamat.
Figure 1. A. Absorption spectra of (a) pyrene thiol and (b) Au-SRPy in THF. The inset shows the Transmission Electron Micrograph of Au-SR-Py deposited on a copper grid. B. (a) Emission spectrum of Au-SR-Py in THF (λexc) 325 nm). (b) and (c) Excitation spectra of Au-SR-Py in THF at monitoring wavelengths 390 and 500 nm, respectively.
confirmed the particle diameter of these gold nanoparticles to be 2-3 nm. Repeated precipitation using ethanol and resuspending the particles in THF ensured the removal of any unbound fluorophores from the suspension. Figure 1A,B show the absorption and emission spectra of Au-SR-Py suspended in THF solutions. The broad absorption band around 530 nm arises from the surface plasmon absorption
10.1021/jp0134695 CCC: $22.00 © 2002 American Chemical Society Published on Web 12/08/2001
Letters
J. Phys. Chem. B, Vol. 106, No. 1, 2002 19
SCHEME 1. Au-SR-Py Nanoassembly
of gold nanoparticles. This absorption band is significantly damped because of small particle size and the organic capping by thiols. As indicated in earlier studies, the evolution of plasmon absorption as a prominent band is seen only for particles of diameter >5 nm.31,32 The absorption in the 300350 nm region show three distinct absorption bands (313, 328, and 345 nm) corresponding to the absorption of pyrene. The emission spectrum shows maxima at 376, 382 and 395 nm and a broad band in the 475 nm region. The intensity of these three peaks in the absorption and emission spectra often serves to sense the polarity of the microenvironment.33 The prominence of peak III over peak I (395 nm emission band over 376 nm band in Figure 1B) suggests the immediate surroundings of the gold-fluorophore nanoassembly to be highly nonpolar. While the 390 nm band is characteristic of monomer pyrene emission, the broad emission at longer wavelength arises from the excimer emission. The fluorescence spectrum of Au-SRPy is similar to that of unbound pyrene thiol in THF solution, but exhibits noticeably lower yields. Because of the overlap of strong gold absorption in the UV region, accurate determination of fluorescence quantum yield of Au-SR-Py was not possible. The decreased fluorescence yield confirms that a large fraction of excited pyrene molecules are quenched by the gold nanocore. The emission spectra presented in Figure 1B represent the excited states that survive the deactivation by the metal surface. The observation of excimer formation indicates that the excited pyrene moieties bound to gold nanoparticles are flexible enough to interact with neighboring pyrene moieties. The excitation spectra recorded by monitoring the emission in the monomer (390 nm) and excimer band (500 nm) confirm the origin of both emission bands to be the same, viz., excitation of the pyrene moieties in the Au-SR-Py nanoassemblies. The
SCHEME 2
excitation spectra show spectral features that are characteristic of pyrene monomer bands (315, 327, and 343 nm). This further ensures that the nanoassemblies have well dispersed pyrene moieties without undergoing interactions in the ground state. The excimer emission observed in the Au-SR-Py nanoassemblies arises from the excited-state dimerization of the pyrene moieties and not from the excitation of ground-state aggregates. Scheme 2 illustrates pathways with which the excited pyrene undergoes deactivation in the Au-SR-Py system. The excited pyrene emission at 390 nm was further analyzed by monitoring its emission decay. The emission decay was modeled by a biexponential kinetic fit (see Figure 2 of the Supporting Information). We obtain lifetimes of 2.6 and 6.6 ns for the free pyrene thiol and 1.6 and 6.4 ns for Au-SR-Py (Py ) pyrene) in THF solutions. Quenching events that occur in the subnanosecond time scale were impossible to resolve in the present experimental setup.34 The obvious interest would be to probe the fate of the quenched excited state. There are various examples in the literature in which the excited-state quenching has been attributed to energy transfer to gold nanoparticles.25,35 Drexhage and co-workers have observed distance-dependent quenching of excited state at the metal
20 J. Phys. Chem. B, Vol. 106, No. 1, 2002
Letters to note that this charge separation is sustained for several microseconds before undergoing recombination. The results demonstrate for the first time the feasibility of employing fluorophore-bound gold nanoparticles as light-harvesting assemblies. There have been significant efforts in the literature to facilitate photoinduced charge transfer in donor-acceptortype dyad molecules. Quick charge recombination in these systems is often considered to be a limiting factor in improving charge separation efficiency. The presence of a gold nanocore can significantly improve the charge separation in such dyad systems by acting as a buffer to hold the charges. Further experiments are underway to utilize functionalized goldfluorophore nanoassemblies in artificial light-harvesting systems.
Figure 2. (a) Transient absorption spectra of (a) degassed THF solution of pyrene thiol, (b) degassed THF solution of Au-SR-Py, and (c) oxygenated THF solution of Au-SR-Py. All spectra were recorded 2 µs after 337 nm laser pulse excitation. Inset shows the absorptiondecay profiles recorded at 390 and 340 nm.
surface.36,37 Since metals in the nanoparticle dimension are more electronegative than the bulk material, they can also participate in an electron-transfer process. We further probed the fate of the pyrene quenching using transient absorption spectroscopy. Figure 2 shows the transient absorption spectra of pyrene thiol and Au-SR-Py in degassed and air-saturated THF solutions. The difference absorption spectrum recorded following 337 nm laser pulse excitation of pyrene thiol in degassed THF solutions exhibits a maximum around 425 nm. This absorption band is characteristic of triplet-triplet absorption of pyrene and is readily quenched in oxygenated solutions. On the other hand, the transient absorption spectrum recorded following 337 nm laser pulse excitation of Au-SR-Py shows the formation of a transient with absorption maximum at 400 nm. Time-resolved spectra recorded at different times confirm the presence of a single transient, which decays with a lifetime of 4.5 µs. The presence of O2 in the solution has no effect on the formation or decay of the transient (spectrum c in Figure 2). Thus, the properties of the transient observed with Au-SR-Py are distinctly different from those recorded with pyrene thiol solution. The pyrene cation radical has been shown to absorb strongly in the 400-450 nm regions. Evidence in the literature suggests strong dependence of the maximum on the medium and substituent.38 Pyrene cation radical formed in polynorbornene end-labeled with pyrene has been shown in this study to exhibit absorption maximum at 400 nm. Hence, we attribute the 400-nm absorption band to the formation of an oxidation product, viz., a radical cation formed by the interaction of excited pyrene with the gold nanocore. The absence of triplet excited state in spectrum b (Figure 2) confirms the role of metal nanocore in suppressing the intersystem crossing and/or total quenching of the triplet excited state in the Au-SR-Py system. The decay of the transient absorption at 390 nm and recovery of bleaching at 340 nm (see absorption-time profiles in the inset of Figure 2) show a similar first-order kinetics with lifetimes of 4.7 and 4.4 µs, respectively. The similarities between these two processes suggest that the decay of the pyrene cation radical corresponds to the recovery of parent fluorophore via a back electron-transfer process (reaction 2).
Au(e)-SR-Py+• f Au-SR-Py
(2)
As shown earlier, thiol-capped gold nanoparticles are capable of holding the charge in a quantized fashion.12,39 The ability to accept charge from the excited pyrene makes the charge separation possible in our Au-SR-Py system. It is interesting
Acknowledgment. The work described herein is supported by the Office of the Basic Energy Sciences of the U.S. Department of Energy and the Council of Scientific and Industrial Research, and the Department of Science and Technology (India). S.H. and S.B. acknowledge the support of Natural Sciences and Engineering Research Council of Canada. This is contribution no. 4319 from the Notre Dame Radiation Laboratory and RRLT-PRU-138 from RRL, Trivandrum. Supporting Information Available: Figure 1: 1H NMR spectra of (a) pyrenethiol, (b) dodecanethiol, and (c) a mixture of pyrenethiol and dodecanethiol (30:70) on the surface of gold clusters. Figure 2: Emission decay of (1) pyrene thiol and (2) Au-S-Py in THF. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer: Berlin, 1995. (2) Mulvaney, P. Spectroscopy of Metal Colloids. Some Comparisons with Semiconductor Colloids. In Semiconductor Nanoclusters-Physical, Chemical and Catalytic Aspects; Kamat, P. V., Meisel, D., Eds.; Elsevier Science: Amsterdam, 1997; p 99. (3) Goia, D. V.; Matijevic, E. New J. Chem. 1998, 22, 1203. (4) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27. (5) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 8410. (6) Zhao, X. M.; Xia, Y. N.; Whitesides, G. M. J. Mater. Chem. 1997, 7, 1069. (7) Badia, A.; Lennox, R. B.; Reven, L. Acc. Chem. Res. 2000, 33, 475. (8) Whetten, R. L.; Shafigullin, M. N.; Khoury, J. T.; Schaaff, T. G.; Vezmar, I.; Alvarez, M. M.; A., W. Acc. Chem. Res. 1999, 32, 397. (9) Hodak, J.; Henglein, A.; Hartland, G. V. J. Phys. Chem. B 2000, 104, 9954. (10) Haynes, C. L.; Van Duyne, R. P. J. Phys. Chem. B 2001, 105, 5599. (11) Baum, T.; Brust, M.; Bethell, D.; Schiffrin, D. J. Langmuir 1999, 15, 866. (12) Chen, S.; Ingram, R. S.; Hostetler, M. J.; Pietron, J. J.; Murray, R. W.; Schaaff, T. G.; Khoury, J. T.; Alvarez, M. M.; Whetten, R. L. Science 1998, 280, 2098. (13) Chen, S.; Murray, R. W. J. Phys. Chem. B 1999, 103, 9996. (14) Fishelson, N.; Shkrob, I.; Lev, O.; Gun, J.; Modestov, A. D. Langmuir 2001, 17, 403. (15) Hickman, J. J.; Ofer, D.; Laibinis, P. E.; Whitesides, G. M.; Wrighton, M. S. Science 1991, 252, 688. (16) Elghanian, R.; Storhoff, J. J.; Mucic, R. C.; Letsinger, R. L.; Mirkin, C. A. Science 1997, 277, 1078. (17) Kamat, P. V.; Flumiani, M.; Hartland, G. J. Phys. Chem. B 1998, 102, 3123. (18) Kurita, H.; Takami, A.; Koda, S. Appl. Phys. Lett. 1998, 72, 789. (19) Fujiwara, H.; Yanagida, S.; Kamat, P. V. J. Phys. Chem. B 1999, 103, 2589. (20) Link, S.; Burda, C.; Mohamed, M. B.; Nikoobakht, B.; El-Sayed, M. A. J. Phys. Chem. A 1999, 103, 1165. (21) Takami, A.; Kurita, H.; Koda, S. J. Phys. Chem. B 1999, 103, 1226. (22) Ah, C. S.; Soo Han, H. S.; Kim, K.; Jang, D. J. J. Phys. Chem. B 2000, 104, 8153.
Letters (23) Shipway, A. N.; Katz, E.; Willner, I. Phys. Chem. Phys 2000, 1, 18. (24) George Thomas, K.; Kamat, P. V. J. Am. Chem. Soc. 2000, 122, 2655. (25) Avouris, P.; Persson, B. N. J. J. Phys. Chem. 1984, 88, 837. (26) Saito, K. J. Phys. Chem. B 1999, 103, 6579. (27) Pagnot, T.; Barchiesi, D.; Tribillon, G. Appl. Phys. Lett. 1999, 75, 4207. (28) Makarova, O. V.; Ostafin, A. E.; Miyoshi, H.; Norris, J. R.; Meisel, D. J. Phys. Chem. B 1999, 103, 9080. (29) Enger, O.; Nuesch, F.; Fibbioli, M.; Echegoyen, L.; Pretsch, E.; Diederich, F. J. Mater. Chem. 2000, 10, 2231. (30) Imahori, H.; Norieda, H.; Yamada, H.; Nishimura, Y.; Yamazaki, I.; Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 100. (31) Alvarez, M. M.; Khoury, J. T.; Schaaff, T. G.; Shafigullin, M. N.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. B 1997, 101, 3706.
J. Phys. Chem. B, Vol. 106, No. 1, 2002 21 (32) Henglein, A. Langmuir 1998, 14, 6738. (33) Thomas, J. K. J. Phys. Chem. 1987, 91, 267. (34) Single photon counting measurements have confirmed the presence of an additional fast decay component of lifetime 0.2 ns in the Au-S-Py in THF. (35) Rossetti, R.; Brus, L. E. J. Chem. Phys. 1982, 76, 1146. (36) Drexhage, K. H.; Flock, M.; Kuhn, H.; Shafer, F. P.; Sperling, W. Ber. Bunsen-Ges. Phys. Chem. 1969, 73, 1179. (37) Drexhage, K. H.; Kuhn, H.; Shafer, F. P. Ber. Bunsen-Ges. Phys. Chem. 1968, 72, 329. (38) Possum, R. D.; Fox, M. A. J. Phys. Chem. B 1997, 101, 6384. (39) Hicks, J. F.; Templeton, A. C.; Chen, S.; Sheran, K. M.; Jasti, R.; Murray, R. W.; Debord, J.; Schaaff, T. G.; Whetten, R. L. Anal. Chem. 1999, 71, 3703.
