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May 8, 2007 - A novel carbazolyldiacetylene derivative of 5-[1,2]dithiolan-3-ylpentanoic acid 1,6-bis((N-carbazol-3-yl)methyl)-2,4-hexadiyne ester (DC...
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6754

Langmuir 2007, 23, 6754-6760

Unusual Fluorescence Enhancement of a Novel Carbazolyldiacetylene Bound to Gold Nanoparticles Cuihong Li,†,‡ Xiaofeng Liu,†,‡ Mingjian Yuan,†,‡ Junbo Li,†,‡ Yanbing Guo,†,‡ Jialiang Xu,†,‡ Mei Zhu,†,‡ Jing Lv,†,‡ Huibiao Liu,† and Yuliang Li*,† Beijing National Laboratory for Molecular Sciences (BNLMS), CAS Key Laboratory of Organic Solids, Center for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, P.R. China, and Graduate UniVersity of Chinese Academy of Sciences, Beijing 100080, P.R. China ReceiVed January 31, 2007. In Final Form: March 2, 2007 A novel carbazolyldiacetylene derivative of 5-[1,2]dithiolan-3-ylpentanoic acid 1,6-bis((N-carbazol-3-yl)methyl)2,4-hexadiyne ester (DCHD-HS) was synthesized. DCHD-HS showed unusual fluorescence enhancement when it was bound to gold nanoparticles. The studies also showed that varied solvents and ratios of ligand/gold particles that served as the driving forces to control aggregation resulted in emission enhancement. A rational explanation for this phenomenon was discussed and elucidated.

Introduction In recent years, fluorescent organic materials have attracted a great deal of attention because of their broad applicability in fluorescent biological labels, sensors, and light-emitting diodes.1 Meanwhile, noble metal clusters on the nanoscale display widely unique physicochemical properties that have gained increased scientific interest from photochemists and photobiologists for organic functionalization of metal nanoparticles.2 Chromophorefunctionalized gold nanoparticles (AuNPs) with applications in photovoltaics and light-mediated binding have emerged as one of the most exciting areas of scientific endeavor in this decade.3 However, the noticeable properties of the fluorophore molecules when bound to metal surfaces include the decreased singlet lifetime as a result of energy transfer from excited dye molecules to bulk metal films.4,5 Total quenching of the singlet excited states of the chromophores can limit the applications of chromophore-labeled metal nanoparticles in optoelectronic devices and photonic materials. Thus, the fluorescent enhancement of chromophore-labeled metal nanoparticles plays a pivotal role in the development of materials with sophisticated functions. It is well-known that AuNP aggregation-induced analytes have been demonstrated for DNA, several metal ions, and so on.6 Moreover, various organic and biological molecules, such as sulfur-based compounds,7 protein,8 DNA,9 enzymes,10 and * To whom correspondence should be addressed. E-mail: ylli@ iccas.ac.cn. † Chinese Academy of Sciences. ‡ Graduate University of Chinese Academy of Sciences. (1) (a) Murata, H.; Kafafi, Z. H.; Uchida, M. Appl. Phys. Lett. 2002, 80, 189. (b) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Dos Santos, D. A.; Bre´das, J. L.; Lo¨gdlund, M.; Salaneck, W. R. Nature 1999, 397, 121. (c) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. ReV. 2000, 100, 2537. (d) Chen, F.; Gerion, D. Nano Lett. 2004, 4, 1827. (2) (a) Fo¨rster, T.; Selinger, B. K. Z. Naturforsch. 1964, A19, 38. (b) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039. (c) Bertolotti, S. G.; Zimmerman, O. E.; Cosa, J. J.; Previtali, C. M. J. Lumin. 1993, 55, 105. (d) Ghosh, S. K.; Pal, A.; Kundu, S.; Mandal, M.; Nath, S.; Pal, T. Langmuir 2004, 20, 5209. (3) (a) Hens, Z.; Tallapin, D. V.; Weller, H. Appl. Phys. Lett. 2002, 81, 4245. (b) Hainfeld, J. F.; Powell, R. D. J. Histochem. Cytochem. 2000, 48, 471. (c) Nam, J. M.; Park, S. J.; Mirkin, C. A. J. Am. Chem. Soc. 2002, 124, 3820. (4) (a) Ishida, A.; Sakata, Y.; Majima, T. J. Chem. Soc., Chem. Commun. 1998, 57. (b) Saito, K. J. Phys. Chem. B 1999, 103, 6579. (c) Pagnot, T.; Barchiesi, D.; Tribillon, G. Appl. Phys. Lett. 1999, 75, 4207. (5) Jenekhe, S. A.; Osaheni, J. A. Science 1994, 265, 765.

cyclodextrin,11 are used to functionalize AuNPs. Among them, carbazolyl moieties have attracted considerable research interest due to their hole-transporting properties, which can be further enhanced by proper functionalization and can induce photoconduction or electroluminescence.12 In addition, carbazolyldiacetylenes have significantly contributed to the development of materials with implemented nonlinear optical properties.13,14 The combination of carbazolyldiacetylenes and AuNPs is expected to reveal many significant functions. (6) (a) Storhoff, J. J.; Mirkin, C. A. Chem. ReV. 1999, 99, 1849. (b) Mirkin, C. A.; Letsinger, R. L.; Mucic, C. A.; Storhoff, J. J. Nature 1996, 382, 607. (c) Kim, Y.; Johnson, R. C.; Hupp, J. T. Nano Lett. 2001, 1, 165. (d) Lin, S.-Y.; Liu, S.-W.; Lin, C.-M.; Chen, C.-h. Anal. Chem. 2002, 74, 330. (e) Thanh, N. T. K.; Rosenzweig, Z. Anal. Chem. 2002, 74, 1624. (f) Otsuka, H.; Akiyama, Y.; Nagasaki, Y.; Kataoka, K. J. Am. Chem. Soc. 2001, 123, 8226. (7) (a) Fitzmaurice, D.; Rao, S. N.; Preece, J. A.; Stoddart, J. F.; Wenger, S.; Zaccheroni, N. Angew. Chem., Int. Ed. 1999, 38, 1147. (b) Schroedter, A.; Weller, H. Angew. Chem., Int. Ed. 2002, 41, 3218. (c) Kang, Y.; Taton, T. A. Angew. Chem., Int. Ed. 2005, 44, 409. (d) Tshikhudo, T. R.; Demuru, D.; Wang, Z.; Brust, M.; Secchi, A.; Arduini, A.; Pochini, A. Angew. Chem., Int. Ed. 2005, 44, 2913. (8) (a) Chan, W. C. W.; Nie, S. Science 1998, 281, 2016. (b) Maxwell, D. J.; Taylor, J. R.; Nie, S. J. Am. Chem. Soc. 2002, 124, 9606. (c) Mattoussi, H.; Mauro, J. M.; Goldman, E. R.; Anderson, G. P.; Sundar, V. C.; Mikulec, F. V.; Bawendi, M. G. J. Am. Chem. Soc. 2000, 122, 12142. (9) (a) Tkachenko, A. G.; Xie, H.; Coleman, D.; Glomm, W.; Ryan, J.; Anderson, M. F.; Franzen, S.; Feldheim, D. L. J. Am. Chem. Soc. 2003, 125, 4700. (b) Sato, K.; Hosokawa, K.; Maeda, M. J. Am. Chem. Soc. 2003, 125, 8102. (c) Liu, J.; Lu, Y. J. Am. Chem. Soc. 2004, 126, 12298. (d) Storhoff, J. J.; Lazarides, A. A.; Mucic, R. C.; Mirkin, C. A.; Letsinger, R. L.; Schatz, G. C. J. Am. Chem. Soc. 2000, 122, 4640. (e) Jin, R.; Wu, G.; Li, Z.; Mirkin, C. A.; Schatz, G. C. J. Am. Chem. Soc. 2003, 125, 1643. (10) (a) Kanaras, A. G.; Wang, Z.; Bates, A. D.; Cosstick, R.; Brust, M. Angew. Chem., Int. Ed. 2003, 42, 191. (b) Pardo-Yissar, V.; Katz, E.; Wasserman, J.; Willner, I. J. Am. Chem. Soc. 2003, 125, 622. (c) Yun, C. S.; Khitrov, G. A.; Vergona, D. E.; Reich, N. O.; Strouse, G. F. J. Am. Chem. Soc. 2002, 124, 7644. (d) Pen˜a, S. R. N.; Raina, S.; Goodrich, G. P.; Fedoroff, N. V.; Keating, C. D. J. Am. Chem. Soc. 2002, 124, 7314. (11) (a) Sylvestre, J. P.; Kabashin, A. V.; Sacher, E.; Meunier, M.; Luong, J. H. T. J. Am. Chem. Soc. 2004, 126, 7176. (b) Banerjee, I. A.; Yu, L.; Matsui, H. J. Am. Chem. Soc. 2003, 125, 9542. (c) Liu, J.; Alvarez, J.; Ong, W.; Roma´n, E.; Kaifer, A. E. J. Am. Chem. Soc. 2001, 123, 11148. (d) Liu, J.; Alvarez, J.; Ong, W.; Kaifer, A. E. Nano Lett. 2001, 1, 57. (e) Liu, J.; Xu, R.; Kaifer, A. E. Langmuir 1998, 14, 7337. (f) Crespo-Biel, O.; Dordi, B.; Reinhoudt, D. N.; Huskens, J. J. Am. Chem. Soc. 2005, 127, 7594. (g) Huskens, J.; Deij, M. A.; Reinhoudt, D. N. Angew. Chem., Int. Ed. 2002, 41, 4467. (h) Nijhuis, C. A.; Huskens, J.; Reinhoudt, D. N. J. Am. Chem. Soc. 2004, 126, 12266. (12) Giorgetti, E.; Sottini, S.; DelRosso, T.; Margheri, G.; Alloisio, M.; Dellepiane, G. Synth. Met. 2004, 147, 271. (13) (a) Margheri, G.; Giorgetti, E.; Sottini, S.; Toci, G. J. Opt. Soc. Am. B 2003, 20, 751. (b) Alloisio, M.; Sottini, S.; Riello, P.; Giorgetti, E.; Margheri, G.; Cuniberti, C.; Dellepiane, G. Surf. Sci. 2004, 68, 554. (14) (a) Menzel, H.; Horstman, S.; Mowery, M. D.; Cai, M.; Evans, C. E. Polymer 2000, 41, 8113. (b) Kim, T.; Chan, K. C.; Crooks, R. M. J. Am. Chem. Soc. 1997, 119, 189.

10.1021/la070110k CCC: $37.00 © 2007 American Chemical Society Published on Web 05/08/2007

Fluorescence Enhancement of DCHD-HS Bound to AuNPs

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Figure 1. Chemical structures of carbazolyldiacetylenes: DCHD-HS and R-DCHD.

In our studies, a novel carbazolyldiacetylene of 5-[1,2]dithiolan3-ylpentanoic acid 1,6-bis((N-carbazol-3-yl)methyl)-2,4-hexadiyne ester (DCHD-HS) and another carbazolyldiacetylene derivative of 5-[1,2]dithiolan-3-ylpentanoic acid 6-(N-carbazole)2,4-hexadiynyl ester (R-DCHD) containing disulfide units (Figure 1) were synthesized and employed for the surface modification of gold particles. Interestingly, in our observations, the binding of DCHD-HS to AuNPs can enhance the molecular fluorescence; however, R-DCHD exhibited usual quenching of fluorescence upon binding to AuNPs. Importantly, the research results enable us to rigidly understand that the unique origin of enhanced fluorescence is attributed to the effect of restricted intramolecular rotation resulting from DCHD-HS capped AuNP aggregation. This work makes it more beneficial to design and build novel functional gold nanoparticles with organic molecules and polymers. Here, we would like to report the results of controlling the morphologies of gold nanoparticles of organic/inorganic hybrid materials and the unusual fluorescence enhancement of DCHD-HS bound to gold nanoparticles. Experimental Section General Information. Chemicals and solvents were reagent grade and were purchased from Lancaster, Aldrich, and ACROS Chemical Co. 1H and 13C NMR spectra were obtained with Bruker ARX300 and ARX400 spectrometers using tetramethylsilane (TMS) as the internal standard. The chemical shifts (δ) are given in parts per million (ppm) relative to TMS. The abbreviations used are as follows: s ) singlet, d ) doublet, t ) triplet, and m ) multiplet. UV spectra were recorded using a Hitachi U-3010 spectrophotometer. Fluorescent spectra were obtained on a Hitachi F-4500 fluorimeter. Transmission electron microscopy (TEM) images were taken using a JEOL JEM-2010 microscope. Dynamic light scattering (DLS) measurements were carried out on a ZetaPALS particle sizing analyzer purchased from Brookheaven Instrument Corp. Matrixassisted laser desorption ionization time-of-flight (MALDI-TOF) spectra were obtained from a Bruker Biflex III MALDI-TOF spectrometer. Synthesis of N-Propargyl-carbazole (2).15 To a solution of carbazole 1 (1.67 g, 10 mmol) in 30 mL of acetone, K2CO3 (2.76 g, 20 mmol) was added. After the mixture was stirred for 15 min, 3-bromopropyne (2.25 mL of an 80 wt % solution in toluene, 15 mmol) was added dropwise for ∼30 min. After it was stirred for 3 h at room temperature, the mixture was filtered, the filtrate was rotoevaporated, and the crude product was separated by silica gel column chromatography (petroleum ether) to give 2 (yield: 1.64 g, 80%) as a white solid. 1H NMR (CDCl3, 400 MHz, ppm): δ 2.24 (s, 1H); 5.04 (d, 2H); 7.27 (t, 2H); 7.48 (t, 4H); 8.09 (d, 2H). MS (EI): (205, M+). Synthesis of N-Propargyl-carbazole-3-carbaldehyde (3).16 2 (4.1 g, 20 mmol) was dissolved in 40 mL of dry dimethylformamide (15) Gan, H. Y.; Liu, H. B.; Li, Y. J.; Zhao, Q.; Li, Y. L.; Wang, S.; Jiu, T. G.; Wang, N.; He, X. R.; Yu, D. P.; Zhu, D. B. J. Am. Chem. Soc. 2005, 127, 12452.

(DMF), and then POCl3 (9.3 mL, 100 mmol) was added dropwise for 1 h at 0 °C. The mixture turned yellow after the addition of POCl3. It was stirred for another 2 h and then heated at 90 °C for 36 h. Upon completion of the reaction, the mixture was poured into ice water and then extracted with dichloromethane. The organic layer was collected and rotoevaporated, and then the red residue was obtained. The crude product was separated and purified by silica gel column chromatography (petroleum ether/ethyl acetate 10:1) to give 3 (yield: 2.33 g, 50%) as a white solid. 1H NMR (CDCl3, 400 MHz, ppm): δ 2.31 (s, 1H); 5.09 (s, 2H); 7.34 (t, 1H); 7.57 (m, 3H); 8.03 (d, 1H); 8.17 (d, 1H); 8.62 (s, 1H); 10.11 (d, 1H). MS (EI): (233, M+). Synthesis of 1,6-Bis(N-carbazole-3-carbaldehyde)-2,4-hexadiyne (4).17 PdCl2(PPh3)2 (49.3 mg, 0.07 mmol) and CuI (27.2 mg, 0.14 mmol) were added into a solution of 3 (822.5 mg, 3.53 mmol) in tetrahydrofuran (THF) (2 mL) and NEt3 (2 mL). The reaction mixture was stirred for 1 h under air atmosphere, and then Et2O (10 mL) and H2O (15 mL) were added. The phases were separated, and the aqueous phase was extracted with Et2O. Drying over anhydrous Na2SO4 and purification by silica gel column chromatography (petroleum ether/ethyl acetate 1:1) afforded 4 (yield: 491.4 mg, 30%) as a yellow solid. 1H NMR (CDCl3, 400 MHz, ppm): δ 5.07 (s, 4H); 7.30 (t, 2H); 7.50 (m, 6H); 8.01 (d, 2H); 8.15 (d, 2H); 8.60 (s, 2H); 10.08 (d, 2H). MS (EI): (464, M+). Synthesis of 1,6-Bis(N-carbazole-3-methanol)-2,4-hexadiyne (5).18 4 (482.6 mg, 1.04 mmol) was dissolved in 20 mL of methanol. Subsequently, sodium borohydride (160 mg, 2.0 mmol) was added and the reaction mixture was stirred for 1 h at room temperature under an argon atmosphere. After addition of Et2O, the organic layer was washed twice with water. The mixture was dried over anhydrous Na2SO4 and evaporated in Vacuo. The crude product was purified by silica gel column chromatography (dichloromethane/ethyl acetate 1:1) and finally afforded 5 (yield: 97.4 mg, 20%). 1H NMR (CDCl3, 400 MHz, ppm): δ 5.03 (s, 4H); 5.22 (s, 4H); 7.20 (s, 2H); 7.36 (m, 4H); 7.41 (m, 4H); 8.01 (d, 4H). MS (MALDI-TOF): (468, [M+]). Synthesis of 5-[1,2]Dithiolan-3-ylpentanoic Acid 1,6-Bis((Ncarbazol-3-yl)methyl)-2,4-hexadiyne Ester (DCHD-HS) (6).18 5 (23.4 mg, 0.05 mmol) and thioctic acid (24.8 mg, 0.12 mmol) were dissolved in 6 mL of dichloromethane (DCM). The mixture was stirred for 15 min at 0 °C under an argon atmosphere. Then, a solution of 1,3-dicylcohexylcarbodiimide (DCC) (28.8 mg, 0.14 mmol) and 4-(dimethylamino)pyridine (DMAP) (3.6 mg, 0.028 mmol) in 4 mL of DCM was added and stirred for another 15 min. After stirring for an additional 3 h at room temperature, the reaction was completed. The reaction mixture was washed three times with water, dried over anhydrous Na2SO4, and evaporated in Vacuo. Purification by column chromatography on silica gel (petroleum ether/DCM 1:1) yielded pure 6 (yield: 37.98 mg, 90%). 1H NMR (CDCl3, 400 MHz, ppm): (16) Chowdhury, B. K.; Saha, C.; Podder, G.; Bhattacharyya, P. Indian J. Chem. Sect. B 1990, 29, 5, 405. (17) Beny, J. P.; Dhawan, S. N.; Kagan, J.; Sundlass, S. J. Org. Chem. 1982, 47, 2201-2204. (18) Herrikhuyzen, J. V.; Janssen, R. A. J.; Meijer, E. W.; Meskers, S. C. J.; Schenning, A. P. H. J. J. Am. Chem. Soc. 2006, 128, 686.

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Li et al. Scheme 1. Synthesis of DCHD-HS and r-DCHD

δ 1.44-2.0 (m, 16H); 2.38 (t, 4H); 3.09 (m, 4H); 3.55 (m, 2H); 5.05 (s, 4H); 5.27 (s, 4H); 7.23 (s, 2H); 7.44 (m, 4H); 7.52 (m, 4H); 8.01 (d, 4H). 13C NMR (CDCl3, 75 MHz, ppm): δ 173.7; 143.3; 140.4; 129.4; 127.7; 127.1; 123.7; 123.6; 123.2; 121.1; 120.9; 109.2; 108.9; 72.5; 68.4; 61.9; 56.3; 40.2; 38.5; 34.6; 34.5; 34.2; 33.2. MS (MALDITOF): (844.8, M+). Calcd for C48H48N2O4S4: 844.3. Synthesis of 6-(N-Carbazolyl)-2,4-hexadiyn-1-ol (7).15 To a 100 mL flask was added 25 mL of THF, 25 mL of methanol, and 2 mL of butylamine. The solution was thoroughly purged with nitrogen and then cooled down to -20 °C. Next, 0.1 g of CuCl was added, and the solution turned blue. The color disappeared with the addition of 2 g of hydroxylamine hydrochloride. N-Propargyl-carbazole (1.03 g) in 5 mL of THF/methanol (1:1, v/v) was then added. After stirring for 10 min, 0.7 g of 3-bromo-2-propyn-1-ol in 5 mL of THF/methanol (1:1, v/v) was added dropwise. The mixture was stirred for 30 min before it was allowed to reach room temperature. After stirring for 6 h, the solution was poured into 4 N hydrochloric acid (100 mL) and then extracted with ether. The ether layer was washed with saturated sodium hydrogen carbonate and water, dried, and rotoevaporated. The crude product was separated by column chromatography (petroleum ether/ethyl acetate 1:1) to give 7 (yield: 0.342 g, 26%) as a white solid. 1H NMR (CDCl3, 400 MHz, ppm): δ 4.27 (s, 2H); 5.13 (s, 2H); 7.28 (d, 2H); 7.48 (q, 4H); 8.09 (d, 2H). MS (EI): (259, M+). Synthesis of 5-[1,2]Dithiolan-3-ylpentanoic Acid 6-(N-Carbazolyl)-2,4-hexadiynyl Ester (8).15 8 was prepared in a similar way as 6. Purification by column chromatography on silica gel (petroleum ether/ethyl acetate 10:1) yielded pure 8 (yield: 100.8 mg, 90%).1H NMR (CDCl3, 400 MHz, ppm): δ 1.44-2.0 (m, 8H); 2.38 (t, 2H); 3.10 (m, 2H); 3.52 (m, 1H); 4.65 (s, 2H); 5.08 (s, 2H); 7.28 (m, 2H); 7.49 (q, 4H); 8.09 (d, 2H). 13C NMR (CDCl3, 75 MHz, ppm): δ 173.1; 125.1; 122.6; 120.1; 118.9; 111.1; 106.2; 73.2; 71.1; 70.8; 65.3; 56.3; 52.2; 41.2; 40.2; 35.2; 33.0; 31.5; 25.0; 24.6. MS (EI): (448, M+). Synthesis of DCHD (DCHD-HS, R-DCHD) Capped Gold Nanoparticles (Ligand Exchange Method).19 The required volume of TOAB-AuNPs (1.02 × 10-2 M in toluene) was taken and mixed with various volumes of specific concentrations of DCHD in toluene. To the well-mixed resulting mixtures was added various solvents (THF, methanol, or toluene) before testing (solvents used in these studies contained only 0.5% toluene). (19) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc. Chem. Commun. 1994, 801.

Results and Discussion Synthesis. DCHD-HS and R-DCHD were synthesized according to the previously reported procedure shown in Scheme 1. First, 1,6-bis(N-carbazole-3-carbaldehyde)-2,4-hexadiyne (4) was obtained with a 30% yield by the self-coupling reaction of N-propargylcarbazole-3-carbaldehyde (3) in the presence of PdCl2(PPh3)2 and CuI in THF/NEt3. Then, 1,6-bis(N-carbazole3-carbaldehyde)-2,4-hexadiyne (4) was reduced with sodium borohydride and afforded 1,6-bis(N-carbazole-3-methanol)-2,4hexadiyne (5). 6-(N-Carbazolyl)-2,4-hexadiyn-1-ol (7) was synthesized using the Curtis coupling reaction. The final product was readily prepared by condensation with thioctic acid with a high yield (90%). Further, TOAB capped AuNPs (1.02 × 10-2 M in toluene) were prepared via a modified Brust’s procedure (see Supporting Information) and DCHD functionalized AuNPs were obtained through ligand exchange.19 Unusual Fluorescence Enhancement of DCHD-HS Bound to Gold Nanoparticles. The absorption spectra of AuNPs (5.1 × 10-5 M in toluene) exhibited a surface plasmon absorption band (max. at 513 nm in toluene). The position and shape of the plasmon absorption of metal nanoclusters are strongly dependent on the particle size, dielectric medium, and surface adsorbed species.20 Transmission electron microscopy (TEM) provides direct information about the shape, size, and size distribution of ligand capped gold nanoparticles. A typical TEM image showed numerous discrete tetra-n-octylammonium bromide capped gold nanoparticles (TOAB-AuNPs) with an average diameter of 3-5 nm (see Supporting Information). Upon addition of DCHD-HS into the gold suspension (5.1 × 10-5 M in THF, THF used in these studies contained only 0.5% toluene), UV absorption spectra exhibited dampening and broadening of the gold surface plasmon absorption band with a red shift from 513 to 529 nm (trace b, Figure 2A), which was (20) (a) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer: Berlin, 1995. (b) Kreibig, U.; Gartz, M.; Hilger, A.; Hovel, H. In Fine Particles Science and Technology; Pelizzatti, E., Ed.; Kulwer Academic Publishers: Boston, MA, 1996; p 499. (c) Mulvaney, P. In Semiconductor NanoclusterssPhysical, Chemical and Catalytic Aspects; Kamat, P. V., Meisel, D., Eds.; Elsevier Science: Amsterdam, 1997; p 99.

Fluorescence Enhancement of DCHD-HS Bound to AuNPs

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Figure 2. (A) UV absorption spectra of TOAB-AuNPs (5.1 × 10-5 M in THF) before (trace a) and after (trace b) addition of DCHD-HS (1.36 × 10-5 M in THF). (B) Fluorescence spectra of DCHD-HS (1.36 × 10-5 M in THF) before (trace a) and after (trace b) addition of TOAB-AuNPs (5.1 × 10-5 M in THF). (C) UV absorption spectra of TOAB-AuNPs (5.1 × 10-5 M in THF) before (trace a) and after (trace b) addition of R-DCHD (8.1 × 10-5 M in THF). (D) Fluorescence spectra of R-DCHD (8.1 × 10-5 M in THF) before (trace a) and after (trace b) addition of TOAB-AuNPs (5.1 × 10-5 M in THF). Inset scale bar represents 50 nm.

evident as these molecules complexed with the gold surface.21 The red shift can be interpreted in terms of the Mie scattering theory and results from the coupling of the transition from dipoles associated with the surface plasmon band of metal particles at close distance.21 In the UV absorption spectra of DCHD-HS (1.36 × 10-5 M in THF), the strong absorption below 300 nm was related to the high-energy transition of the carbazolyldiacetylene chromophore. The double bands at 333 and 346 nm showed the characteristics of the carbazolyl group.22 The fluorescence spectra possessed well-defined and weak emission bands (e.g., at 333 nm) with maxima at 354 and 368 nm (trace a, Figure 2B). Interestingly, when AuNPs were added to the solution of DCHD-HS (1.36 × 10-5 M in THF), the fluorescence intensity (max. at 354 nm) increased by ∼3 times compared to the initial solution free of AuNPs after their surface complexation for 25 min (trace b, Figure 2B). No such enhanced emission could be observed when we added a THF solution of DCHD-HS containing TOAB and treated with NaBH4. However, no such enhanced emission could be observed when we added the suspension of AuNPs into the solution of R-DCHD (8.1 × 10-5 M in THF) as shown in Figure 2D, which only showed the usual excited-state quenching (trace b, Figure 2D) as a result of energy transfer from the excited molecules to the gold cores.4,5 Hitherto, a few cases of enhanced emission in specific organic molecules bound to AuNPs have been reported and interpreted. Binding of the amine group of Py-CH2NH2 to the gold surface (21) Herrikhuyzen, J. V.; Janssen, R. A. J.; Meijer, E. W.; Meskers, S. C. J.; Schenning, A. P. H. J. J. Am. Chem. Soc. 2006, 128, 686. (22) Cuniberti, C.; Dellepiane, G.; Piaggio, P.; Franco, R.; Musso, G. F. Chem. Mater. 1996, 8, 708.

led to an increase in the efficiency of radioactive deactivation of the singlet excited state and provided prominent emission enhancement.23 Furthermore, unusual fluorescence enhancement was observed from the aged solution of AuNPs capped by alkanethiols with pyrene units. Face-to-face arrangement of pyrene units resulted in the reduction of some of the nonradiative processes and therefore resulted in fluorescence enhancement.24 In our studies, no obvious wavelength changes in the absorption and fluorescence spectra were observed, which reflected that there was no alteration of the electronic properties of DCHD-HS as it bound to the gold nanoparticles.23 It also indicated the absence of the electron-transfer process and intermolecular J-aggregation in DCHD-HS-AuNPs.23-25 Therefore, most probably, the enhanced emission of DCHD-HS was attributed to the effect of restricted intramolecular rotation resulting from DCHD-HS capped AuNP aggregation. We suppose that this enhancement originates from the aggregation mechanisms. The aggregation is driven by the van der Waals force between the nanoparticles, when the repulsive interaction is greatly reduced by complex formation (Au-ligandAu) on their surfaces. The attractive force works from a distance and leads to the aggregation.26 Importantly, the change of the emission intensity suggests that it has a similar origin of the intramolecular interactions in the aggregated state. Further support (23) (a) George Thomas, K.; Zajicek, J.; Kamat, P. V. Langmuir 2002, 18, 3722. (b) George Thomas, K.; Kamat, P. V. J. Am. Chem. Soc. 2000, 122, 2655. (24) Wang, T. X.; Zhang, D. Q.; Xu, W.; Yang, J. L.; Han, R.; Zhu, D. B. Lamgmuir 2002, 18, 1840. (25) Chen, J.; Xie, Z.; Lam, J. W. Y.; Law, C. C. W.; Tang, B. Z. Macromolecules 2003, 36, 1108. (26) (a) Sato, K.; Hosokawa, K.; Maeda, M. J. Am. Chem. Soc. 2003, 125, 8102. (b) Ipe, B. I.; Mahima, S.; Thomas, G. K. J. Am. Chem. Soc. 2003, 125, 7174. (c) Yu, C.; Wong, K. M. C.; Chan, K. H.; Yam, V. W.-W. Angew. Chem., Int. Ed. 2005, 44, 791.

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Figure 3. (A) Fluorescence emission spectra of DCHD-HS (1.36 × 10-5 M) in CHCl3 (black) and in methanol (red). (B-D) Time-dependent fluorescence spectra of DCHD-HS (1.36 × 10-5 M) with the addition of AuNPs (5.1 × 10-5 M) in different solvents: (B) methanol, (C) toluene, and (D) THF. (E) DLS data for the evolution of spherical size of DCHD-HS-AuNPs as a function of time in three different solvents. (F) UV spectra of DCHD-HS-AuNPs in different solvents: (a) TOAB-AuNPs in toluene, (b) DCHD-HS-AuNPs in THF, (c) DCHD-HSAuNPs in toluene, and (d) DCHD-HS-AuNPs in methanol. [DCHD-HS] ) 1.36 × 10-5 M, [Au] ) 5.1 × 10-5 M.

for such enhancement came from solid-state emission studies.27 Although the origins of these enhanced emissions are still in debate, it is assumed that the unique fluorescence change is more or less related to the effects of intramolecular interactions or a specific aggregation (H- or J-aggregation). Intramolecular effects on fluorescence enhancement are simply explained by the conformational changes of the chromophores. It is supposed that the twisted conformations of chromophores in solution tend to suppress the radiative process, whereas the planar conformations of chromophores induced in the solid state activate the radiation process.28 Effects on fluorescence changes by intermolecular interactions are correlated with the aggregation morphology, for example, H-type or J-type aggregation. The formation of (27) (a) Lam, J. W. Y.; Tang, B. Z. Acc. Chem. Res. 2005, 38, 745. (b) Lam, J. W. Y.; Tang, B. Z. J. Polym. Sci., Part A: Polym. Chem. 2003, 41, 2607. (c) Lam, J. W. Y.; Dong, Y. P.; Law, C. C. W.; Dong, Y. Q.; Cheuk, K. K. L.; Lai, L. M.; Li, Z.; Sun, J.; Chen, H.; Zheng, Q.; Kwok, H. S.; Wang, M.; Feng, X.; Shen, J.; Tang, B. Z. Macromolecules 2005, 38, 3290. (d) Dong, Y. P.; Lam, J. W. Y.; Peng, H.; Cheuk, K. K. L.; Kwok, H. S.; Tang, B. Z. Macromolecules 2004, 37, 6408.

H-aggregates is characterized by blue-shifted absorption bands with respect to those of the isolated chromophore.24 In contrast, J-aggregates where the molecules are arranged in a head-to-tail direction induce a relatively high fluorescence efficiency with a bathochromic shift of the UV absorption maximum. In our system, DCHD-HS consists of two carbazole units with a diacetylene moiety as the bridging group. The disulfide units are respectively located on both ends of the molecule. The binding of the disulfide units onto the gold nanoparticles induced the aggregation of AuNPs by strong complexation, which promoted the surface binding of DCHD-HS molecules close to each other. Being in closer proximity with each other increased the packing degree of DCHD-HS as a result of restricting the rotation of the two carbazole rings. DCHD-HS molecules between the coupled particles became more rigid after complexation and stacking, and they were expected to weaken the competitive quenching (28) (a) Souza, M. M.; Rumbles, G.; Gould, I. R.; Amer, H.; Samuel, I. D. W.; Moratti, S. C.; Holmes, A. B. Synth. Met. 2000, 111-112, 539-543. (b) Gruszecki, W. I. J. Biol. Phys. 1991, 18, 99-109.

Fluorescence Enhancement of DCHD-HS Bound to AuNPs

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Figure 4. TEM images of DCHD-HS-AuNPs in different solvents. Inset scale bar represents 50 nm.

by the energy transfer between DCHD-HS and the AuNPs. Therefore, the enhanced fluorescence emission in DCHD-HS was explained in terms of the intramolecular effects. That is, the aggregation which resulted from the two-point complex interaction between the disulfide units and the surfaces of the AuNPs may restrict the torsional dynamics of the ligand and lead to enhanced fluorescence. It was consistent with the dimensional coordination of DCHD-HS with two disulfide units forming a closer packed array than that of R-DCHD, which had one sulfide unit and held greater spatial freedom for the rotation of the carbazole rings when it complexed with the surfaces of the AuNPs. Obviously, the aggregation resulting from the complexation interactions is weak and is unable to lead to the shift of the spectra wavelength as aggregation induced emission in the solid state. It can also be concluded from these studies that the nature of the linker group provides topographical control of the various interactions. Effect of Solvents on the Fluorescence Enhancement of DCHD-HS Bound to Gold Nanoparticles. Different enhancement rates of the DCHD-HS-AuNPs were observed in different solvents. As shown in Figure 3A, the fluorescence of DCHD-HS was weak in organic solvents and was partially quenched in methanol compared to that in CHCl3, which resulted from intermolecular interactions in the excited state in polar solvents. However, with the addition of the gold suspension into the solution of DCHD-HS (1.36 × 10-5 M in methanol, methanol used in these studies contained only 0.5% toluene), a clear colorimetric change to purple (representing aggregation) was immediately observed. The emission intensity was remarkably enhanced without significant changes in the wavelength of the maximum emission. The fluorescence intensity (max. at 354 nm) increased by ∼6 times compared to that of the initial solution free of AuNPs after their surface complexation for 25 min as shown in Figure 3B. The time-dependent increase showed the surface complexation and aggregation process of DCHD-HS-AuNPs. In addition, different enhancement rates of emission in different solvents were observed in the fluorescence spectra. As shown in Figure 3B-D, the fluorescence intensity was enhanced 6 times in methanol, whereas it was only enhanced 3 times in THF, in contrast to the initial solution of DCHD-HS free of AuNPs with the same concentration. Obviously, the emission intensity of DCHD-HS-AuNPs in methanol was increased more remarkably than in THF. Such different enhancement behaviors were considered to be attributed to the different aggregation abilities of DCHD/HS-AuNPs in different solvents. Hydrophobic DCHDHS capped gold nanoparticles aggregated much easier in polar solvents (e.g., methanol) and induced a faster aggregation process, which resulted in further enhancement in a polar solvent than in THF. Our assignment was also supported by dynamic light scattering (DLS) experiments that revealed the formation of denser aggregates of spherical objects with an average diameter of 895

nm in methanol, compared to an average diameter of 58 nm in THF, which indicated that DCHD-HS-AuNPs aggregated much easier in methanol (see the Supporting Information). The diameter of the spherical assembly of DCHD-HS-AuNPs was plotted against the mixture time in three different solvents (Figure 3E). Remarkably, the transition from isolated hybrid particles to aggregated particles indicated a cooperative nature for the aggregation and emission enhancement process. Thus, solvents can be regarded as an important factor affecting the emission enhancement of the fluorophore on the particle surface. DCHD-HS-AuNPs were coupled through disulfides, which resulted in aggregation because of multiple displacements of DCHD-HS on each particle. However, the aggregates of R-DCHD-AuNPs were unable to lead to fluorescence enhancement, showing that the structures of the ligands were crucial for fluorescence enhancement. Fluorescence increased as a result of the coupling of the particles, indicating that a particle-dyeparticle sandwich structure could be an efficient approach to improve sensitivity in biological detection assays.29 Moreover, the more favorable aggregation of DCHD-HSAuNPs in methanol was observed by the dampening and broadening of the surface plasmon band with the red-shift of a maximum band from 513 to 575 nm in the UV spectra (trace d, Figure 3F). TEM provided direct information on different aggregation behaviors in solvents. As shown in Figure 4, TEM exhibited spherical particles with diameters from 2 to 5 nm. Numerous discrete gold nanoparticles with an average diameter of 3.5 ( 1.2 nm in a THF solution of DCHD-HS-AuNPs were observed. In addition, well-defined three-dimensional spherical aggregates of 100-150 nm diameter were observed in toluene, which could presumably change the driving force for aggregation. At higher magnification, TEM revealed that the spherical aggregates had an internal structure. One spherical aggregate was comprised of 20 000-70 000 individual gold particles of 3-4 nm diameter. However, in methanol, the aggregates exhibited that DCHD-HS-AuNPs were in close proximity to the individual particles. Branched-rod images with an average diameter of 80 nm comprised of individual gold particles of 2-3 nm diameter showed denser aggregation. TEM images displayed a more compact aggregation of DCHD-HS-AuNPs in methanol than in THF (see Supporting Information). Effect of the [Au]/[DCHD-HS] Ratios on the Fluorescence Enhancement of DCHD-HS Bound to Gold Nanoparticles. The enhancement resulting from the aggregation of DCHDHS-AuNPs was further confirmed by the fluorescence changes at various [Au]/[DCHD-HS] ratios. It has been well-established that different ligand/gold ratios can have an effect on the aggregation behavior of AuNPs.30 The fluorescence spectral (29) Zhang, J.; Malicka, J.; Gryczynski, I.; Lakowicz, J. R. J. Phys. Chem. B 2005, 109, 7643.

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with the increase in the degree of aggregation of DCHD-HSAuNPs and gave further evidence to the enhancement resulting from the aggregation of DCHD-HS-AuNPs.

Conclusions

Figure 5. Fluorescence emission spectra of different [Au]/[DCHDHS] ratios in toluene after complexation for 10 min. [DCHD-HS] ) 1.04 × 10-5 M.

changes of DCHD-HS were investigated by different [Au]/ [DCHD-HS] ratios at a constant DCHD-HS concentration (2.04 × 10-5 M in toluene). A high concentration of DCDH-HS around a single Au nanoparticle leads to the formation of small clusters, which further undergo interparticle clustering. By increasing the [Au]/[DCHD-HS] ratio, the comparatively decreasing amount of ligand compared to AuNPs will result in weaker aggregation of ligand capped gold nanoparticles. The fluorescence intensity was recorded after the surface complexation and aggregation for 10 min (Figure 5) at various [Au]/[DCHD-HS] ratios (3.75, 7.5, and 15). For the relatively low ratio of gold/ligand at 3.75, the fluorescence intensity increased by ∼2 times compared to that of the initial solution free of AuNPs. However, for the relatively high ratio of gold/ligand at 15, the fluorescence intensity increased by only 1.4 times compared to that of the initial solution. This small increase is mainly due to weaker aggregation at high [Au]/ [DCHD-HS] ratios. Thus, emission enhancement was consistent (30) Maye, M. M.; Luo, J.; Lim, I-Im. S.; Han, L.; Kariuki, N. N.; Rabinovich, D.; Liu, T.; Zhong, C. J. J. Am. Chem. Soc. 2003, 125, 9906.

We have demonstrated the ability to control the morphologies of gold nanoparticles of organic/inorganic hybrid materials and showed the unusual fluorescence enhancement of DCHD-HS bound to gold nanoparticles. Varied solvents and ligand/gold ratios that served as the driving forces to control aggregations resulted in emission enhancement. With relatively strong binding to the surfaces of gold nanoparticles, DCHD-HS-AuNPs underwent a substantial enhancement of fluorescence when forced into tight packing (aggregations) via surface binding on AuNPs. This fluorescence enhancement behavior is mainly due to restricted intramolecular rotation in the packed nanoclusters. DCHD-HS-AuNPs are expected to be new fluorophore probes for potentially useful applications in the fabrication of biological nanosensors, biochemical labeling, and optoelectronic nanodevices. The present work will provide a novel concept for fabricating chromophore labeled metal nanoparticles with unusual fluorescence enhancement. Acknowledgment. This work was supported by the National Natural Science Foundation of China (20531060, 10474101, 20418001, 20473102) and the Major State Basic Research Development Program (2006CB806200, 2006CB932100). Supporting Information Available: Synthesis and UV and fluorescence spectra of TOAB-AuNPs, DLS size distribution data of DCHD-HS capped AuNPs in different solvents, and additional TEM micrographs of DCHD-HS capped AuNPs. This material is available free of charge via the Internet at http://pubs.acs.org. LA070110K