Preparation, Characterization, and Photophysical Properties of

A solution of NaBH4 (38 mg, 0.1 mol) in 1 mL of deionized water was added into the .... All the fluorescence measurements were performed at room tempe...
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Preparation, Characterization, and Photophysical Properties of Alkanethiols with Pyrene Units-Capped Gold Nanoparticles: Unusual Fluorescence Enhancement for the Aged Solutions of These Gold Nanoparticles Tongxin Wang, Deqing Zhang,*,† Wei Xu, Junlin Yang, Rui Han, and Daoben Zhu* Organic Solids Laboratory, Center for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China Received August 13, 2001. In Final Form: November 20, 2001 Gold nanoparticle I and nanoparticle II protected by 10-(1-pyrenyl)-6-oxo-decanethiol and 17-(1-pyrenyl)13-oxo-heptadecanethiol, respectively, were prepared and characterized by 1H NMR, FT-IR, XPS, UV-vis and fluorescence spectroscopies and TEM and elemental analysis. Their compositions were determined to be (C25H27OS)225•Au1601 and (C32H41OS)120•Au1052, respectively. Unusual fluorescence enhancement was observed for the aged solutions of nanoparticle I and nanoparticle II, and a possible explanation for this phenomenon was also discussed. This experimental finding provides further evidence for the chain density gradient and the motion of termini groups of alkanethiolates adsorbed onto the surface of gold nanoparticle.

Introduction Since the important report by Brust et al. in 1994,1 extensive chemical and physical studies have been carried out on alkanethiols-capped gold and other noble metal nanoparticles, which are also referred to three-dimensional monolayers protected clusters. These investigations include new approaches to the syntheses of monodispersed gold nanoparticles,2 preparation of superlattice structures composed of nanoparticles,3 controllable arrangements of nanoparticles with supramolecular chemistry principle,4 in-place exchange reaction,5 and functionallization of nanoparticles.6 For better understanding of the intriguing properties displayed by these nanoparticles, efforts were also taken to probe the molecular arrangements and alkylchain dynamics of alkanethiolate ligands on the surfaces of nanoparticles.7 For instance, Murray et al.8 employed infrared spectroscopy to study the structure of alkanethi* To whom correspondence should be addressed. † E-mail: [email protected]. (1) Brust, M.; Walker, M.; Betheli, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 80. (2) (a) Leff, D. V.; Ohara, P. C.; Heath, J. R.; Gelbart, W. M. J. Phys. Chem. 1995, 99, 7036. (b) Maye, M. M.; Zheng, W.; Leibowitz, F. L.; Ly, N. K.; Zhong, C. Langmuir 2000, 16, 490. (3) (a) Lin, X. M.; Sorensen, C. M.; Klabunde, K. J. Chem. Mater. 1999, 11, 198. (b) Connolly, S.; Fullam, S.; Korgel, B.; Fitzmaurie, D. J. Am. Chem. Soc. 1998, 120, 2969. (c) Wang, Z. L.; Harfenist, S. A.; Whetten, R. L.; Bentley, J.; Evans, N. D. J. Phys. Chem. B 1998, 102, 3068. (d) Martin, J. E.; Wilcoxon, J. P.; Odinek, J.; Provencio, P. J. Phys. Chem. B 2000, 104, 9475. (4) (a) Aherne, D.; Rao, S. N.; Fitzmaurice, D. J. Phys. Chem. B 1999, 103, 1821. (b) Liu J.; Mendoza, S.; Romen, E.; Lynn, M. J.; Xu, R.; Kaifer, A. E. J. Am. Chem. Soc. 1999, 121, 4304. (c) Boal, A. K.; Iihan, F.; DeRouchey, J. E.; Thurn-Albrecht, T.; Russell, T. P.; Rotello, V. M. Nature 2000, 404, 746. (d) Fullam, S.; Rao, S. N.; Fitzmaurice, D. J. Phys. Chem. B 2000, 104, 6164. (5) (a) Brown, L. O.; Hutchison, J. E. J. Am. Chem. Soc. 1997, 119, 12348. (b) Weare, W. W.; Reed, S. M.; Warner, M. G.; Hutchison, J. E. J. Am. Chem. Soc. 2000, 122, 12890. (c) Hostetler, M. J.; Templeton, A. C.; Murray, R. W. Langmuir 1999, 15, 3782. (6) (a) Hostetler, M. J.; Green, S. J.; Stokes, J. J.; Murray, R. W. J. Am. Chem. Soc. 1996, 118, 4212. (b) Templeton, A. C.; Cliffel, D. E.; Murray, R. W. J. Am. Chem. Soc. 1999, 121, 7081. (c) Ingram, R. S.; Hostetler, M. J.; Murray, R. W. J. Am. Chem. Soc. 1997, 119, 9175. (d) Templeton, A. C.; Hostetler, M. J.; Warmoth, E. K.; Chen, S.; Hartshorn, C. M.; Krishnamurthy, V. M.; Forbes, M. D. E.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 4845.

olate monolayers adsorbed onto nanometer-sized gold clusters, and provided evidence that there were defects near the surfaces of the gold nanoclusters and at the outer surface of the ligand skin. Meanwhile, Reven et al.9 used 13C NMR techniques to investigate the structure and dynamic behavior of alkyl chains of alkanethiols confined onto the surfaces of gold nanoparticles, and the results revealed that a significant population of alkyl chains were mobile and conformationally disordered at 25 °C. Due to its special photophysical behaviors exemplified by the excimer emission,10 pyrene has been widely used as a fluorescence probe.11 Thus, it would be interesting to prepare gold nanoparticles capped by alkanethiols containing pyrene units, and hence employ fluorescence spectroscopy to study the molecular arrangements and motion of alkanethiol ligands. Indeed, thiols with pyrene units have been incorporated into the self-assembled monolayer films to study the structures of such systems at molecular level.12 To the best of our knowledge, up to now there have been only two reports13 about the gold nanoparticles with pyrene moieties on their surfaces: one is about the gold nanoparticle capped by 1-aminomethylpyrene and enhanced emission from the fluoroprobe has been observed and the other is about the multivalent receptors based on nanoparticle scaffolds in which the migration of alkanethioate ligands with pyrene units has been suggested upon the addition of guest molecules. In this paper, we wish to describe the synthesis, characterization and photophysical properties of gold nanoparticle (7) Badia, A.; Cuccia, L.; Demers, L.; Morin, F.; Lennox, R. B. J. Am. Chem. Soc. 1997, 119, 2682. (8) Hostetler, M. J.; Stokes, J. J.; Murray, R. W. Langmuir 1996, 12, 3604. (9) Badia, A.; Gao, W.; Singh, S.; Demers, L.; Cuccia, L.; Revens, L. Langmuir 1996, 12, 1262. (10) (a) Fo¨rster, Th.; Kasper, K. Z. Phys. Chem. (Frankfurt Main) 1954, 1, 275. (b) Fo¨rster, Th. Angew. Chem. 1969, 81, 364; Angew. Chem., Int. Ed. Engl. 1969, 8, 333. (11) Dong, D. C.; Winnik, M. A., Photochem. Photobiol. 1982, 35, 17 and references therein. (12) (a) Chen, S. H.; Frank, C. W. Langmuir 1991, 7, 1719. (b) Karpovich, D. S.; Blanchard, G. J. Langmuir 1996, 12, 5522. (13) (a) Thomas, K. G.; Kamat, P. V. J. Am. Chem. Soc. 2000, 122, 2655. (b) Boal, A. K.; Rotello, V. M. J. Am. Chem. Soc. 2000, 122, 734.

10.1021/la0112817 CCC: $22.00 © 2002 American Chemical Society Published on Web 02/08/2002

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Scheme 1. Chemical Structures of Thiol 1 and Thiol 2, and Schematic Representation of Nanoparticle I and Nanoparticle II

I and nanoparticle II protected by 10-(1-pyrenyl)-6-oxodecanethiol (thiol 1) and 17-(1-pyrenyl)-13-oxo-heptadecanethiol (thiol 2), respectively (Scheme 1). We will present the unusual fluorescence enhancement behavior for the aged solutions of these two gold nanoparticles. A possible explanation for this phenomenon will be also discussed. This experimental finding may provide a further evidence for the chain density gradient and the motion of termini groups of alkanethiolates adsorbed onto the surface of gold nanoparticle. Experimental Section Chemicals. The following chemical reagents were purchased from the indicated suppliers and used without purification: 1-pyrenebutanol (Aldrich), 1,5-dibromopentane (Acros), 1,12dibromododecane (Acros), potassium tert-butoxide (Acros), NaBH4 (Aldrich, 98%), HAuCl4•4H2O (Aldrich, 99.999%), tetra-n-octylammonium bromide (TCI), and Aliquat 336 (Acros). Potassium O-tert-butyl dithiocarbonate was prepared according to the literature.14

1-(1-Pyrenyl)-5-oxo-10-bromodecane. A 1.1 g (4.0 mmol) sample of 1-pyrenebutanol was dissolved in 100 mL of anhydrous THF. A 2.88 g (120 mmol) sample of NaH was added to the solution, and the mixture was kept stirring overnight at room temperature under nitrogen atmosphere. A solution of 2.76 g (12 mmol) of 1,5-dibromopentane in 10 mL of dry THF was added to the above vigorously stirred solution via a dropping funnel, and stirring was continued over 3 days. The solvent was removed by vacuum evaporation, and water was carefully added to the reaction mixture to quench the unreacted NaH. The solution was extracted with dichloromethane (4 × 50 mL), and the combined organic layer was washed with 2N HCl, aqueous Na2CO3 (w/w, 10%), and finally with water, and then dried over anhydrous magnesium sulfate overnight. The solvent was removed and the residual was purified by column chromatography [petroleum ether (60-90 °C)/ethyl acetate ) 40:1, v/v] to yield 0.64 g of the bromide (yield, 37.8%). 1H NMR (CDCl3, ppm): 8.3-7.8(m, 9H), 3.5-3.3(m, 8H), 2.0-1.4(m, 10H). 1-(1-Pyrenyl)-5-oxo-17-bromoheptadecane. It was synthesized similarly from 1-pyrenebutanol(0.98 g, 3.58 mmol) and (14) Degami, I.; Fochi, R.; Santi, M. Synthesis 1977, 873.

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1,12-dibromododecane(4 g, 12.2 mmol) as described above (0.7 g, yield, 37.5%). 1H NMR (CDCl3, ppm): 8.3-7.8(m, 9H), 3.53.3(m, 8H), 1.9-1.2(m, 24H). 10-(1-Pyrenyl)-6-oxo-decanethiol (thiol 1, Scheme 1). A 188 mg (1.0 mmol) sample of potassium O-tert-butyl dithiocarbonate was added to 15 mL of deionized water in a three-necked round-bottom flask equipped with a reflux condenser. To this vigorously stirred solution were added 340 mg (0.8 mmol) of the corresponding bromide dissolved in 15 mL of toluene and several drops of Aliquat 336 under nitrogen atmosphere. The resulting solution was stirred for 30 min and then kept at 80 °C for another 30 min. The organic layer was separated and the aqueous phase was further extracted with petroleum ether (60-90 °C) (3 × 30 mL). The combined organic layer was washed with water. The solvent was removed and the residual was subjected to column chromatography on silica gel with the mixture of petroleum ether (60-90 °C) and dichloromethane (2:1, v/v) as eluants to yield 160 mg of the thiol 1 (yield, 53.2%). Anal. Calcd. for C25H28OS: C, 79.95; H, 7.50. Found: C, 79.93; H, 7.62. 1H NMR(CDCl3, ppm): 8.3-7.8 (m, 9H), 3.5-3.3 (m, 6H), 2.5 (q, 2H), 1.9-1.2 (m, 11H). EI-MS (m/z): 376(M+). FT-IR (KBr, cm-1): 3039(m), 2932(s), 2859(s), 2799(w), 1560(m), 1457(m), 1431(m), 1374(m), 1184(m), 1113(s), 974(m), 845(s), 756(m), 711(m), 468 (m). 17-(1-Pyrenyl)-13-oxo-heptadecanethiol (thiol 2, Scheme 1). 17-(1-Pyrenyl)-13-oxo-heptadecanethiol was obtained from 1-(1-pyrenyl)-5-oxo-17-bromoheptadecane (200 mg, 0.38 mmol) similarly (98 mg, yield, 53.8%). Anal. Calcd. for C32H42OS: C, 80.96; H, 8.92. Found: C, 80.59; H, 8.93. 1H NMR(CDCl3, ppm): 8.3-7.8 (m, 9H), 3.5-3.4 (m, 6H), 2.5 (q, 2H), 1.9-1.2 (m, 25H). EI-MS (m/z): 474 (M+). FT-IR(KBr, cm-1): 3043(m), 2925 (s), 2851 (s), 2797(w), 1603(m), 1465(m), 1372(m), 1184(m), 1117(s), 965(w), 847(s), 759(m), 711(m). Nanoparticle I. Nanoparticle I was prepared via a modified Brust’s method.1 In the present case, the molar ratio of thiol:Au was 1.3:1. Briefly, AuCl4- was first transferred from aqueous HAuCl4 solution (30 mmol/L 3 mL) to toluene solution by phase transfer reagent TOAB (50 mmol/L, 8 mL) after stirring at least for 30 min at room temperature. A solution of NaBH4 (38 mg, 0.1 mol) in 1 mL of deionized water was added into the solution quickly via a drop funnel, and the mixture was vigorously stirred under N2 atmosphere for another 30 min. Then, to this vigorously stirred colloid was added a solution of 44 mg (0.12 mmol) of thiol 1 in 1 mL of toluene, and stirring was allowed to proceed overnight for the complete exchange of ligands. The organic layer was separated and then was concentrated to ca. 1 mL using a vacuum evaporator. A 2 mL sample of ethanol was added, and the darkbrown nanoparticle I was precipitated. The precipitate was centrifuged and decanted to remove the soluble fraction. After it was washed thoroughly with acetone to remove the excess thiol, 10 mg of nanoparticle I was obtained in the form of black solid. 1H NMR(CDCl3, ppm): 8.1-7.2 (br, 9H), 3.4-2.8 (br, 6H), 2.0-0.8(br, 10H). FT-IR (KBr, cm-1): 3039(m), 2931(s), 2856(s), 1629(m), 1456(m), 1371(m), 1261(w), 1113(s), 844(s), 804(s), 711(w), 473(w). Elemental analysis. Found: C, 17.22; H, 1.41. Nanoparticle II. It was prepared with a similar approach as that used for nanoparticle I. 1H NMR(CDCl3, ppm): 8.3-7.3 (br, 9H), 3.3-2.7 (br, 6H), 1.9-0.9(br, 24H). FT-IR(KBr, cm-1): 3040(w), 2922 (s), 2851 (s), 1629(m), 1461 (m), 1370 (m), 1114 (s), 1030(s), 844(s), 717 (w), 468 (m). Elemental analysis. Found: C, 27.32; H, 3.04. Characterization Techniques. 1H NMR spectra were recorded on a 300 MHz (Bruck DMX300) spectrometer. Infrared spectra were obtained with a Pekin-Elmer 2000 spectrometer in the form of KBr platelets. EI-MS was performed with AEI-MS50. XPS data were obtained with VG Scientific Escalab 220I-XL. Elemental analysis was measured with Heraeus Chn-Rapid instrument. Transmission electron microscopy (TEM) was preformed on a H-800 (HITACHI) instrument: samples were prepared by evaporating a drop of dichloromethane solutions of nanoparticle I and nanoparticle II on an amorphous carbon-coated Cu TEM grid. Absorption spectra were recorded with a Hitachi (model U-3010) UV-visible spectrophotometer. Fluorescence measurements were carried out with a Hitachi (model F-4500) spectrophotometer in a 1 cm quartz cell. Fluorescence quantum

Wang et al. efficiencies were determined by comparing the integrated fluorescence spectra of the sample with that of a standard (9,10-diphenyl anthracene in cyclohexane, φf ) 1.00). Fluorescence lifetimes were obtained using a time-correlated single photon counting spectrometer (model Horiba NAES-1100). The time resolution of the present system is ca. 100 ps. A hydrogen-filled coaxial flash lamp was used as the excitation source. The fluorescence decay curves were analyzed by the reconvolution procedure using a proper instrument response function, obtained by substituting the sample cell with a light scatter. The decay curves were fitted with an exponential function as I(t) ) ∑ aie(-t/τi), where τi is the fluorescence lifetime of the species in excited state and ai is the preexponential factor. All measurements were done at room temperature. Fluorescence quenching experiments were performed as follows: Determined volumes of the dichloromethane solution of 1,4-dicyanobenzene (0.1 M) were added sequentially to 1 mL of the dichloromethane solution of nanoparticle I (1.5 × 10-2 mg/ mL). After each addition of the quencher, the measuring cell was carefully vibrated for 1 min. and then the fluorescence spectrum was recorded. Similar experiments were carried out for nanoparticle II (1.5 × 10-2 mg/mL) and pyrene (1.0 × 10-2 M).

Results and Discussion Synthesis and Characterization. Nanoparticle I and nanoparticle II were prepared via a modified Brust’s procedure.1 In this case, the reductant NaBH4 was first added to the mixture of HAuCl4 and TOAB, and then the corresponding thiol (thiol 1 and thiol 2, respectively) was added to replace the ammonium ion adsorbed on the surface of gold nanoparticle. As expected, the gold atoms in both nanoparticle I and nanoparticle II are in neutral state, supported by the binding energies of Au(4f7/2) measured with XPS (83.8 eV for both nanoparticle I and nanoparticle II), which are characteristic for Auo. 1H NMR spectrum of nanoparticle I was measured in CDCl3. As compared to those of thiol 1, the signals around 2.5 ppm for -S-CH2 completely disappeared for nanoparticle I due to the broadening of the signals induced by a discontinuity in the diamagnetic susceptibility at the goldhydrocarbon interface and residual dipolar interactions.15 This indicates that all thiols are covalently attached to the surface of nanoparticle I. Besides, as compared to thiol 1, the chemical shifts of pyrene unit in nanoparticle I are upfield-shifted, possibly due to the shielding effect of neighboring pyrene units. The comparison of the 1H NMR spectrum of nanoparticle II with that of thiol II shows similar behavior. The mean diameters of nanoparticle I and nanoparticle II determined by TEM (Figure 1) are 3.75 nm (with a standard deviation σ ) 0.66 nm) and 3.26 nm (with a standard deviation σ ) 1.21 nm), respectively. Taking the gold core as a sphere with density FAu (58.01 atoms/nm3) covered with an outermost layer of hexagonally close-packed gold atoms (13.89 atoms/nm2) with a radius of Rcore - RAu (RAu ) 0.145 nm),16 the model predicts that the average core of nanoparticle I contains 1601 Au atoms, of which 522 lie on the surface. In combination with the elemental results, there are 225 thiols (thiol 1) lie on the surface of each nanoparticle I on average, and hence the composition of nanoparticle I is (C25H27OS)225•Au1601. The coverage ratio (γ) of thiols to surface Au atoms is calculated to be 43% for nanoparticle I. For nanoparticle II, the average core has 1052 Au atoms, of which 384 stay on its surface. Based on the elemental results (see experimental part), the composition of nano(15) Badia, A.; Singh, S.; Demers, L.; Cuccia, L.; Brown, G. R.; Lennox, R. B. Chem. Eur. J. 1996, 2, 359. (16) Terrill, R. H.; Postlethwaite, T. A.; Chen, C.; Poon, C.; Terzis, A.; Chen, A.; Hutchison, J. E.; Clark, M. R.; Wignall, G.; Londono, J. D.; Superfine, R.; Falvo, M.; Johnson, C. S., Jr.; Samulski, E. T.; Murray, R. W. J. Am. Chem. Soc. 1995, 117, 12537.

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Figure 1. TEM images and size distributions of nanoparticle I (left) and nanoparticle II (right). Samples were prepared by placing one drop of the dichloromethane solution of the desired nanoparticle (1.5 × 10-2 mg/mL) on a TEM grid.

particle II was established to be (C32H41OS)120•Au1052, and hence there are 120 thiols (thiol 2) cover its surface. The coverage ratio (γ) was estimated to be 31% for nanoparticle II. Absorption and Fluorescence Spectra of the Fresh Solutions. Figure 2 shows the absorption spectra of the fresh solutions of nanoparticle I and nanoparticle II together with those of thiol 1 and thiol 2 for comparison. Obviously, typical plasmon absorption bands with maximum at 532 and 527 nm are observed for nanoparticle I and nanoparticle II, respectively. Absorption bands in the range of 260-380 nm are attributed to pyrene units, and the spectra in this range for both nanoparticle I and nanoparticle II are similar to those of thiol 1 and thiol 2, respectively. The fluorescence spectra of the fresh solutions of nanoparticle I and nanoparticle II with an excitation wavelength of 340 nm are displayed in Figure 3. For nanoparticle I, two emission bands positioned at 378 and 397 nm, respectively, are observed, which are almost the same as those of thiol 1 (see the inset of Figure 3). But, the emission band around 419 nm for thiol 1, is not distinct for nanoparticle I as shown in Figure 3. In addition, a weak broad emission band ranging from 430 to 600 nm is observed in the fluorescence spectrum of nanoparticle

Figure 2. Absorption spectra of nanoparticle I (1.5 × 10-2 mg/mL in CH2Cl2) and nanoparticle II (1.5 × 10-2 mg/mL in CH2Cl2) together with those of and thiol 1 (inset, 1.0 × 10-4 M in CH2Cl2) and thiol 2 (inset, 1.0 × 10-4 M in CH2Cl2).

I, and it should be due to weak excimer emission of pyrene units. It has been well established10 that face-to-face arrangements of pyrene units facilitate the formation of excimers. Thus, such weak excimer emision implies that most of pyrene units both on the surface of one single gold nanoparticle I and between the neighboring particles are

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Figure 3. Fluorescence spectra of nanoparticle I (1.5 × 10-3 mg/mL in CH2Cl2) and nanoparticle II (1.5 × 10-3 mg/mL in CH2Cl2) together with those of thiol 1 (inset, 1.0 × 10-4 M in CH2Cl2) and thiol 2 (inset, 1.0 × 10-4 M in CH2Cl2) with an excitation wavelength of 340 nm.

not well orientated as face-to-face manner for the fresh solution of nanoparticle I. The relative quantum yield (φf) of a fresh solution of nanoparticle I was determined to be 0.0106. For nanoparticle II, there are two dominant emission bands with the maxima at 377 and 396 nm (Figure 3). Similar to nanoparticle I, weak excimer emission in the range of 430 nm to 600 nm is observed for nanoparticle II. The relative quantum yield (φf) of a fresh solution of nanoparticle II was determined to be 0.0120. The fluorescence emission of the fresh solutions of both nanoparticle I and nanoparticle II exhibits a double exponential decay with the observation and excitation wavelengths of 400 and 340 nm, respectively. The lifetimes of shorter and longer lived species are 4.44 (( 0.13) and 23.2 (( 0.55) ns for nanoparticle I, and 4.15 (( 0.15) and 21.8 (( 0.44) ns for nanoparticle II, respectively. These shorter and longer lived excited species may correspond to two different excited states of pyrene units bound to different ligation sites on the gold nanoparticle surface as suggested by Imahori et al.17 It is noteworthy to mention that the lifetimes of shorter and longer lived species for both nanoparticle I and nanoparticle II are significantly shorter than those of thiol 1 (τ ) 100 ( 0.4 ns) and thiol 2 (τ ) 100 ( 0.4 ns). It is probable that the gold core is responsible for the quenching of these excited states via through-space (alkyl chain folding) energy transfer process, but “concentration quenching” mechanism cannot be ruled out completely although previous results pointed out this process was not important.18 Fluorescence and Absorption Spectra of the Aged Solutions. Interestingly, the fluorescence spectra of nanoparticle I and nanoparticle II are “time-dependent” as indicated in Figure 4, in which the fluorescence spectra of nanoparticle I (up) and nanoparticle II (down) are displayed after their fresh solutions were aged for 17, 41, 71, 209, and 311 h. All the fluorescence measurements were performed at room temperature, but during the interval period the solutions of nanoparticle I and nanoparticle II were carefully sealed and kept in a refrigerator (T = -25 °C). Obviously, the intensity of excimer emission is enhanced for the solutions of both nanoparticle I and nanoparticle II with increasing the aging time, but the variance of excimer emission maxima (λmax ≈ 476 nm) is quite small. The emission intensities for the two bands (17) Imahori, H.; Arimura, M.; Hanada, T.; Nishimura, Y.; Yamazaki, I.; Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 335. (18) Aguila, A.; Murray, R. W. Langmuir 2000, 16, 5949.

Figure 4. Fluorescence spectra of the fresh and aged solutions of nanoparticle I (up, 1.5 × 10-3 mg/mL in CH2Cl2) and nanoparticle II (down, 1.5 × 10-3 mg/mL in CH2Cl2) with an excitation wavelength of 340 nm.

positioned in the short wavelength region are also enhanced by increasing the aging time for the solutions of nanoparticle I and nanoparticle II,19 but the wavelength maxima of these two emission bands almost keep constant. Moreover, for the solution of nanoparticle I, the intensity ratios between the excimer emission and the emission bands in the short wavelength region are increased with the aging time as shown in Figure 5 A, B, where the intensity ratios for the fresh and each aged solutions of four different concentrations versus the aging time are demonstrated. Similar behavior was found for the solution of nanoparticle II. Thus, it can be inferred that molecular arrangements of pyrene units of nanoparticle I and nanoparticle II varied during the aging time and became more and more favorable for the formation of excimers. The emission lifetimes of the solutions of nanoparticle I and nanoparticle II after aged for 144 h were measured with the observation and excitation wavelengths of 400 and 340 nm, respectively. The results are listed in Table 1, and for comparison the corresponding lifetimes of the fresh solutions of nanoparticle I and nanoparticle II are also included in Table 1. For both nanoparticle I and nanoparticle II, the lifetimes of the shorter lived excited species vary little, but it is clear that the lifetimes of the (19) As compared to the respective aged solutions of nanoparticle I, the degree of fluorescence enhancement for the aged solutions of nanoparticle II is relatively small. This is probably due to their differences in size and number of thiols on their surfaces.

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Figure 6. Absorption spectra of the fresh solution and aged solutions of nanoparticle I (1.5 × 10-2 mg/mL in CH2Cl2).

Figure 5. A) and B) The intensity ratios (I476/I378 and I476/I397) between the excimer emission and emission bands at 378 and 397 nm, respectively, versus the aged time for the dichloromethane solution of nanoparticle I. The excitation wavelength is 340 nm. Table 1. Emission Lifetimes (ns) of the Fresh and Aged (144 h) Solutions of Nanoparticle I and Nanoparticle II with the Excitation and Observation Wavelengths of 340 and 400 nm, Respectively τ s of fresh solutions

τ s of aged solutions

nanoparticle I 4.44 ( 0.13 23.2 ( 0.55 4.23 ( 0.11 35.5 ( 0.90 nanoparticle II 4.15 ( 0.15 21.8 ( 0.44 3.84 ( 0.11 29.9 ( 0.81

longer lived excited species are increased after their solutions were aged. The prolongation of emission lifetimes implies the prohibition of some nonradiative processes, which is consistent with the fluorescence enhancement behavior as mentioned above. Figure 6 shows the absorption spectra of the fresh solution of nanoparticle I and its solution after aged for 17, 41, and 71 h. Obviously, in contrast to the fluorescence spectrum, the absorption spectrum of nanoparticle I was almost not altered after its solution was aged. A similar result was found for the fresh and aged solutions of nanoparticle II. To exclude the possible effect of UV light irradiation during fluorescence measurements, fluorescence spectra of the fresh solutions of nanoparticle I and nanoparticle II were measured continuously five times, and no detectable change was found. TEM measurements with the solutions of nanoparticle I and nanoparticle II after aged for 144 h show that both the size and the shape of

Figure 7. TEM images of the dichloromethane solutions of nanoparticle I (up, 1.5 × 10-2 mg/mL) and nanoparticle II (down, 1.5 × 10-2 mg/mL) after aged for 144 h.

nanoparticles (see Figure 7) are not changed by comparing with those of the fresh solutions of nanoparticle I and nanoparticle II (see Figure 1). Thus, the possibility of conglomeration of nanoparticle I and nanoparticle II during the aging time can be ruled out.

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Figure 8. Fluorescence spectra of the aged (144 h) dichloromethane solution of nanoparticle I (1.5 × 10-2 mg/mL) in the presence of different concentrations of the quencher: (a) 0, (b) 2, (c) 4, (d) 6, (e) 8, (f) 10 mM. The excitation wavelength is 340 nm.

Lifetimes and Quenching Studies of Excimer Emissions. To probe the properties of excimers formed in the solutions of nanoparticle I and nanoparticle II, the lifetimes of excimers were measured with the detection and excitation wavelengths of 490 and 340 nm. For both nanoparticle I and nanoparticle II, the fluorescence decay of their fresh solutions is biexponential under this condition. After the solutions of nanoparticle I and nanoparticle II were aged for 144 h, the corresponding fluorescence decay also displays biexponential relation. The resulting excimer lifetimes are summarized in Table 2. The excited species with shorter and longer lifetimes may correspond to excimers of pyrene units bound to different ligation sites on the gold nanoparticle surface.17 For both nanoparticle I and nanoparticle II, lifetimes of the shorter lived excimer species vary little, while lifetimes of the longer lived excimer species are prolonged after the fresh solutions were aged for 144 h. This suggests the deactivation of some nonradiative processes for the excimers, which is again in accordance with the intensity enhancement of exciemer fluorescence as described above. Quenching of the excimer fluorescences was performed by addition of 1,4-dicyanobenzene to the solutions of nanoparticle I and nanoparticle II after aged for 144 h. As an example, Figure 8 shows the gradual decrease of the excimer fluorescence in the presence of 1,4-dicyanobenzene at different concentrations for nanoparticle I. For the solutions of nanoparticle I and nanoparticle II, their optical densities at excitation wavelength of 340 nm remain unchanged in the presence of 1,4-dicyanobenzene since it has no absorption at this wavelength. The bimolecular quenching constants (kq) were estimated separately for nanoparticle I and nanoparticle II using the Stern-Volmer equation (eqs 1 and 2):

Io/I ) 1 + KSV[Q]

(1)

Io/I ) 1 + kqτs[Q]

(2)

Figure 9. Dependence of the bimolecular fluorescence quenching constants of the aged dichloromethane solutions of nanoparticle I (1.5 × 10-2 mg/mL) and nanoparticle II (1.5 × 10-2 mg/mL) as well as the dichloromethane solution of pyrene (1.0 × 10-2 M) on the concentrations of quencher. The fluorescence intensity was monitored at 476 nm.

where KSV is the Stern-Volmer constant, τs is the excimer fluorescence lifetime (37.6 ns and 42.6 ns for nanoparticle I and nanoparticle II, respectively)20 in the absence of the quencher (1,4-dicyanobenzene in the present case), and the other symbols have their conventional meanings. The ratios of the excimer fluorescence intensity, measured at 476 nm, without and with the quencher (Io/I) are plotted against the quencher concentration [Q]. For the aged solutions of both nanoparticle I and nanoparticle II, the Io/I vs [Q] plots are almost linear as indicated in Figure 9. On the basis of the corresponding slope of the plots, the excimer quenching rate constants for the aged solutions of nanoparticle I and nanoparticle II are estimated (Table 2). By using the same procedure, the excimer quenching rate constant of pyrene (τs ) 44.2 ns) is estimated to be 7.4 × 109 M-1 s-1. The excimer quenching rate constants (kq) of the aged solutions of nanoparticle I and nanoparticle II and that of pyrene excimer are in similar range. The little lower values of kq for nanoparticle I and nanoparticle II as compared to that of pyrene excimer may be due to large mass of nanoparticle I and nanoparticle II, which may slow the collision frequencies of excimers of pyrene units attached to their surfaces with quencher molecules.21 Concentration Dependence of the Fluorescence Enhancement Behavior. By varying the concentrations of nanoparticle I and nanoparticle II, it was found that the intensity enhancements for both excimer emission and the emissions in the short wavelength region rely upon their concentrations. As displayed in Figure 5A, B, by decreasing the concentration of the solution of nanoparticle I (from 1.5 × 10-2 to 3.0 × 10-5 mg/mL), the relative enhancement of excimer emission intensity as compared to those for emission bands at 378 and 397 nm became gradually small. A similar concentration dependence of the fluorescence enhancement behavior was observed for nanoparticle II. Such concentration dependence implies that the fluorescence enhancement behavior, in particular for the excimer emission of the aged solutions of nano-

Table 2. Lifetimes and Quenching Rate Constants of the Excimers of Nanoparticle I and Nanoparticle II with the Excitation and Detection Wavelengths of 340 and 490 nm, Respectively lifetime (ns) τ s of fresh solutions nanoparticle I nanoparticle II

1.28 ( 0.14 3.20 ( 0.17

25.5 ( 0.25 31.9 ( 0.97

τ s of aged solutions 4.04 ( 0.22 3.05 ( 0.12

37.6 ( 0.66 42.6 ( 0.77

kq (×109 M-1 s-1) 5.6 4.1

Alkanethiols with Capped Gold Nanoparticles

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Figure 10. Fluorescence spectra of the fresh and aged dichloromethane solutions of nanoparticle I (1.5 × 10-6 mg/ mL). The excitation wavelength is 340 nm.

particle I and nanoparticle II, is related to the intermolecular arrangements of pyrene units between neighboring nanoparticles. To provide further evidence for this assumption, the solutions of nanoparticle I and nanoparticle II were further diluted to 1.5 × 10-6 mg/mL. As an example, Figure 10 shows the fluorescence spectra of the highly diluted solution of nanoparticle I and the same solution after aged for 24, 48, and 72 h. Obviously, the fluorescence spectrum of the fresh solution of nanoparticle I is almost coincident with those of the aged solutions under this condition, and no fluorescence enhancement is observed. Similar phenomenon was found for nanoparticle II. Therefore, it can be concluded that the unusual fluorescence enhancement behavior for nanoparticle I and nanoparticle II is mainly due to interparticle interaction of pyrene units. Tentative Interpretation of the Fluorescence Enhancement Behavior. Previous results7-9 from FTIR, NMR, and other studies indicate that the alkyl chains confined onto the surfaces of gold nanoparticles show chain density gradient; namely, the methylene units of the alkyl chains become progressively less densely packed as one moves further away from the core. This will cause the outermost group to experience substantial freedom of movement. Previous results also reveal that the alkyl chains of neighboring gold nanoparticles can be arranged in interdigitation fashion. On the bases of these observations, the following picture is proposed (Figure 11) to account for the experimental results mentioned above: (1) for the fresh solution of nanoparticle I and nanoparticle II, the pyrene units are not properly orientated and intermolecular arrangements of pyrene units of neighboring gold nanoparticles are not favorable for the formation of excimers; (2) during the aging time, the movement and reorientation of pyrene units lead to generate face-to-face arrangement of pyrene units from neighboring gold nanoparticles (interparticle interaction as mentioned above) through chain interdigitation. As a result, by (20) We tentatively use the lifetimes (see Table 2) of the aged solutions of nanoparticle I (37.6 ns) and nanoparticle II (42.6 ns) for the estimation of their excimer fluorecences quenching constants. (21) On the contrary, it was reported that the quenching rate constants were much higher for clusters of organic molecules than for the monomeric analogue, probably due to the entrapment of the quencher molecules in the clusters (see Kamat, P. V.; et al. Langmuir 2001, 17, 2930). For nanoparticle I and nanoparticle II, such entrapment of quencher molecules (1,4-dicyanobenzene) may be not possible owing to the relatively dense packing of thiols on the surfaces of gold nanoparticles.

Figure 11. Schematic representation for the reorientation of pyrene units adsorbed on the surface of nanoparticle I and nanoparticle II during the aged time.

prolonging the aging time, more and more excimers were formed in the solutions of nanoparticle I and nanoparticle II. This is in accordance with the intensity enhancement of excimer fluorescence as mentioned above. Formation of more and more excimers will result in the generation of more rigid aggregates, and hence the some nonradiative processes (e.g. collision quenching) can be reduced. Thus, the lifetimes of excimers will be prolonged after the solutions of nanoparticle I and nanoparticle II were aged. This prediction is supported by the experimental results as indicated in Table 2. Also because of the favorable formation of excimer from pyrene units, the alkyl chain folding process is attenuated. Hence, the energy transfer process from the excited state of pyrene units to the gold core is weakened. Consequently, the emission intensities for bands in the short wavelength range will be enhanced, which is fully consistent with the experimental result (see above).22

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As anticipated, the absorption spectra of nanoparticle I and nanoparticle II should not be affected by aging the solution, since there is no interaction between two pyrene units (excimer is formed from them) in their ground states. This is again in agreement with the experimental result. In short, the unusual fluorescence enhancement behavior for the aged solutions of nanoparticle I and nanoparticle II can be well explained as above. Conversely, this experimental finding provides a further evidence for the chain density gradient and the motion of termini groups of alkanethiolates adsorbed onto the surface of gold nanoparticle. Therefore, like FT-IR and NMR techniques, fluorescence spectroscopy can also be employed to probe the molecular arrangement and motion of alkanethiols adsorbed onto the surface of gold and other noble nanoparticles.

Wang et al.

acterized, and their compositions were determined to be (C25H27OS)225•Au1601 and (C32H41OS)120•Au1052, respectively, on the bases of the results of TEM and elemental analysis. Unusual fluorescence enhancement was observed after their fresh solutions were aged. A model was proposed to explain this unusual property of nanoparticle I and nanoparticle II. This experimental finding provides a further evidence for the chain density gradient and the motion of termini groups of alkanethiolates adsorbed onto the surface of gold nanoparticle. Further investigations include the influences of solvent, temperature and other parameters on their fluorescence spectra as well as the dynamics of reorientation of pyrene units by using fluorescence spectroscopy. Gold nanoparticles capped by alkanethiols and pyrene-substituted thiols will also be prepared for these studies.

Summary Gold nanoparticle I and nanoparticle II protected by alkanethiols with pyrene units were prepared and char(22) It seems possible to study the chain-length dependence of energy transfer process from excited pyrene units to gold surface with the aged solutions of nanoparticle I and nanoparticle II, but due to the differences in size and composition, such comparative studies with nanoparticle I and nanoparticle II are not well suited. It is difficult for us to obtain nanoparticle I and nanoparticle II with the same size and composition.

Acknowledgment. The present research work was supported by Chinese Academy of Sciences and the Major State Basic Research Development Program. The authors also thank the anonymous reviewers for their excellent suggestions to improve our manuscript. LA0112817