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Formation of Gold Nanoparticles in the Presence of o-Anisidine and the Dependence of the Structure of Poly(o-anisidine) on Synthetic Conditions Xinhua Dai,† Yiwei Tan,‡ and Jian Xu*,† State Key Laboratory of Polymer Physics and Chemistry and Laboratory of Organic Solids, Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China Received May 9, 2002. In Final Form: August 14, 2002 Gold nanoparticles were prepared in an aqueous solution and a 50:50 N-methyl-2-pyrrolidone (NMPD)/ toluene mixed solvent using o-anisidine as the reducing agent, respectively. The reaction is conducted by choice of the temperature and medium. Transmission electron microscopy (TEM) and UV-vis absorption spectroscopy measurements show that the size of Au particles is dependent on the synthetic protocols. At the same time, the oxidative polymerization of o-anisidine was achieved. Moreover, the structure of the polymer varied with the preparative conditions. Cross-linked poly(o-anisidine) and poly(o-anisidine) in its doping state (in the form of the emeraldine salt) were obtained, respectively, supported by UV-vis absorption spectra, infrared spectra, and X-ray photoelectron spectroscopy (XPS).
Introduction A recent focal point in nanoscale material research has involved the physical and chemical properties of metal and semiconductor nanoparticles, such as specific catalytic, magnetic, electronic, and photophysical properties, depending on their size (quantum size effect).1-7 This is ascribed to the reduced sizes and dimensions of nanophase materials being at the basis of unique applications in construction of microdevices, catalysts, ultrathin films, and so forth. When the electrons and holes are confined within the three-dimensional potential well, the continuity of states in the conduction and valence bands is broken down into discrete states with an energy spacing, relative to the band edge, which is approximately inversely proportional to the square of the particle size. Nanoparticles have a characteristic high surface-tovolume ratio, providing sites for the efficient adsorption * To whom corresponence should be addressed. Fax: 86-1062559373. E-mail:
[email protected]. † State Key Laboratory of Polymer Physics and Chemistry. ‡ Laboratory of Organic Solids. (1) Henglein, A. J. Phys. Chem. 1993, 97, 5457. (2) (a) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 4212. (b) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 8410. (c) Hodak, J. H.; Martini, I.; Hartland, G. V.; Link, S.; El-Sayed, M. A. J. Chem. Phys. 1999, 108, 9210. (3) (a) Francois, L.; Mostafavi, M.; Belloni, J.; Delouis, J. F.; Delaire, J.; Feneyrou, P. J. Phys. Chem. B 2000, 104, 6133. (b) Bigioni, T. P.; Whetten, R. L.; Dag, O ¨ . J. Phys. Chem. B 2000, 104, 6983. (c) Jensen, T. R.; Malinsky, M. D.; Haynes, C. L.; Van Duyne, R. P. J. Phys. Chem. B 2000, 104, 10549. (4) (a) Lin, X.; Ramer, N. J.; Rappe, A. M.; Hass, K. C.; Schneider, W. F.; Trout, B. L. J. Phys. Chem. B 2001, 105, 7739. (b) Doty, R. C.; Yu, H.; Shih, C. K.; Korgel, B. A. J. Phys. Chem. B 2001, 105, 8291. (c) Park, S.; Wasileski, S. A.; Weaver, M. J. J. Phys. Chem. B 2001, 105, 9719. (5) Rao, C. N. R.; Kulkarni, G. U.; Thomas, P. J.; Edwards, P. P. Chem.sEur. J. 2002, 8, 29 and references therein. (6) (a) Hirai, H.; Nakao, Y.; Toshima, N. J. Macromol. Sci., Chem. 1978, A12, 1117. (b) Bo¨nnemann, H.; Brijoux, W.; Siepen, K.; Hormes, J.; Franke, R.; Pollmann, J.; Rothe, J. Appl. Organomet. Chem. 1997, 11 (10, 11), 783. (c) Chen, C. W.; Serizawa, T.; Akashi, M. Chem. Mater. 1999, 11, 1381. (d) Gao, H. R.; Angelici, R. J. J. Am. Chem. Soc. 1997, 119, 6973. (7) (a) Hirai, H.; Chawanya, H.; Toshima, N. React. Polym. 1985, 3, 127. (b) Toshima, N.; Takahashi, T.; Yonezawa, T.; Hirai, H. J. Macromol. Sci., Chem. 1988, A25 (5-7), 669.
of the reacting substrates leading to unusual sizedependent chemical reactivity.8 For example, colloidal palladium nanoparticles serve as a good catalyst for Suzuki-, Heck-, and Stille-type coupling reactions for carbon-carbon bond formation.9 Platinum and gold nanoparticles catalyze the oxidation of L-sorbose and carbon monoxide, respectively.10,11 The colloidal particles of 1-3 nm mean diameter with narrow size distributions showed high activity and selectivity for the hydrogenation of olefins and dienes,6 the hydration of acrylonitrile,7a and the light-induced hydrogen generation from water.7b Noble metal particles have also found application in proteincolloid conjugates, where they are used as markers and tracers in optical and electron microscopy to help locate and quantitate antibody-antigen binding sites in cells and tissues.12 The preparation of size-selected metal nanoparticles such as chemical reduction of a metal salt in an organic or aqueous medium has thus motivated a vast amount of work. Meanwhile, the achievement of accurate control of the stability and reactivity of the quantum particles is also required to allow the attachment to the surface substrates or other particles without leading to coalescence and hence losing their size-induced electronic properties. One design strategy that shows tremendous potential as a viable route by which to produce metal nanoparticles with improved size control and colloidal stability is to vary the particle growth medium. Examples of such growth medium include solvent, template (i.e., using micelles as (8) (a) Siegel, R. Nanostruct. Mater. 1993, 3, 1. (b) Boakye, E.; Radovic, L. R.; Osseo-Asare, K. J. Colloid Interface Sci. 1994, 163, 120. (9) (a) Pathak, S.; Greci, M. T.; Kwong, R. C.; Mercado, K.; Prakash, G. K. S.; Olah, G. A.; Thompson, M. E. Chem. Mater. 2000, 12, 1985. (b) Li, Y.; El-Sayed, M. A. J. Phys. Chem. B 2001, 105, 8938. (10) Sidorov, S. N.; Volkov, I. V.; Davankov, V. A.; Tsyurupa, M. P.; Valetsky, P. M.; Bronstein, L. M.; Karlinsey, R.; Zwanziger, J. W.; Matveeva, V. G.; Sulman, E. M.; Lakina, N. V.; Wilder, E. A.; Spontak, R. J. J. Am. Chem. Soc. 2001, 123, 10502. (11) (a) Vargaftik, M. N.; Zagorodnikov, U. P.; Stolarov, I. P.; Chuvilin, A. L.; Zamaraev, K. I. J. Mol. Catal. 1989, 53, 315. (b) Haruta, M.; Kobayashi, T.; Sano, H.; Yamada, N. Chem. Lett. 1987, 405. (12) (a) Horrisberger, M. In Preparation of Biological Specimens for Scanning Electron Microscopy; Murphy, J. A.; Roomans, G. M., Eds.; Scanning Electron Microscopy, Inc.: Chicago, IL, 1984; p 315. (b) DeMey, J. EMSA Bull. 1984, 14, 54.
10.1021/la025926u CCC: $22.00 © 2002 American Chemical Society Published on Web 10/16/2002
Formation of Gold Nanoparticles
the microreactor), and specialized polymer matrix. Alternatively, a careful choice of the reducing agents and capping agents to control the relative rate between nucleation and growth opens new ways to such production. The control of the size and/or polydispersity of the particles is carried out using various colloidal systems such as reverse micelles (i.e., water-in-oil (W/O) microemulsions),12,13 Langmuir-Blodgett films,14 organometallic techniques,15 and simple single-phase or two-phase liquidliquid systems.16,17 Although the history of the preparation of colloidal gold can be traced back to Faraday’s times, the past several decades have witnessed great developments of the production of gold nanoparticles.17a,18 Among these, two highlighted advances are the aqueous Turkevich method18a and the “phase transfer catalyst” method,17a which yield uniform and very stable Au nanoparticles in aqueous solution and organic solvent, respectively. These approaches provide convenient access to gold nanoparticles. However, so far, few synthetic approaches adapting to preparing both water and oil-soluble gold nanoparticles are available. In this study, we prepared Au nanoparticles in both aqueous and organic systems by reducing gold salts with o-anisidine as the reductant through judicious choice of the reaction media. The suitable reducing environment permits reduction at more moderate conditions and results in higher colloid uniformity than usual methods. Meanwhile, cross-linked poly(o-anisidine) or doped poly(o-anisidine) is produced, whose structure is confirmed by UV-vis absorption, FT-IR, and XPS spectra. Experimental Section Materials. Hydrogen tetrachloroaurate(III), (()-10-camphorsulfonic acid (CSA), and tetraheptylammonium bromide ((THA)Br) were purchased from Aldrich and used without further purification. o-Anisidine, N-methyl-2-pyrrolidone (NMPD), and toluene were supplied by Beijing Chemical Reagent Co. oAnisidine was purified by distillation under nitrogen gas prior to use. The water used throughout this work was degassed high(13) (a) Pileni, M. P. J. Phys. Chem. 1993, 97, 6961. (b) Pileni, M. P.; Taleb, A.; Petit, C. J. Dispersion Sci. Technol. 1998, 19 (2 & 3), 185. (c) Zhang, Z.; Patel, R. C.; Kothari, R.; Johnson, C. P.; Friberg, S. E.; Aikens, P. A. J. Phys. Chem. B 2000, 104, 1176. (d) Chiang, C. L. J. Colloid Interface Sci. 2001, 239, 334. (e) Pileni, M. P. J. Phys. Chem. B 2001, 105, 3358 and references therein. (f) Pileni, M. P. Langmuir 2001, 17, 7476 and references therein. (g) Lo´pez-Quintela, M. A.; Rivas, J. J. Colloid Interface Sci. 1993, 158, 446. (h) Chang, C. L.; Fogler, H. S. Langmuir 1997, 13, 3295. (14) (a) Heath, J. R.; Knobler, C. M.; Leff, D. V. J. Phys. Chem. B 1997, 101, 189. (b) Chen, S. Langmuir 2001, 17, 2878. (15) (a) Hambrock, J.; Becker, R.; Birkner, A.; Weib, J.; Fischer, R. A. Chem. Commun. 2002, 68. (b) Hyeon, T.; Lee, S. S.; Park, J.; Chung, Y.; Na, H. B. J. Am. Chem. Soc. 2001, 123, 12798. (c) Li, Y.; Liu, J.; Wang, Y.; Wang, Z. L. Chem. Mater. 2001, 13, 1008. (16) a) Lin, X. M.; Jaeger, M. H.; Sorensen, C. M.; Klabunde, K. J. J. Phys. Chem. B 2001, 105, 3353. (b) Chaki, N. K.; Sudrik, S. G.; Sonawane, H. R.; Vijayamohanan, K. Chem. Commun. 2002, 76. (c) Enustun, B. V.; Turkevich, J. J. Am. Chem. Soc. 1963, 85, 3317. (17) (a) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Commun. 1994, 801. (b) Sarathy, K. V.; Raina, G.; Yadav, R. T.; Kulkarni, G. U.; Rao, C. N. R. J. Phys. Chem. B 1997, 101, 9876. (c) Chen, S.; Huang, K.; Stearns, J. A. Chem. Mater. 2000, 12, 540. (d) Walker, C. H.; St. John, J. V.; Neilson, P. W. J. Am. Chem. Soc. 2001, 123, 3846. (18) (a) Enustun, B. V.; Turkevich, J. J. Am. Chem. Soc. 1963, 85, 3317. (b) Yee, C. K.; Jordan, R.; Ulman, A.; White, H.; King, A.; Rafailovich, M.; Sokolov, J. Langmuir 1999, 15, 3486. (c) Templeton, A. C.; Chen, S.; Gross, M.; Murray, R. W. Langmuir 1999, 15, 66. (d) Han, M. Y.; Quek, C. H.; Huang, W.; Chew, C. H.; Gan, L. M. Chem. Mater. 1999, 11, 1144. (e) Bronstein, L. M.; Chernyshov, D. M.; Valetsky, P. M.; Wilder, E. A.; Spontak, R. J. Langmuir 2000, 16, 8221. (f) Zhou, Y.; Itoh, H.; Uemura, T.; Naka, K.; Chujo, Y. Chem. Commun. 2001, 613. (g) Yonezawa, T.; Yasui, K.; Kimizuka, N. Langmuir 2001, 17, 271. (h) Shon, Y.-S.; Wuelfing, P. W.; Murray, R. W. Langmuir 2001, 17, 1255.
Langmuir, Vol. 18, No. 23, 2002 9011 purity water processed by distillation of deionized water. All the solvents used in this work were purging with dry argon for 30 min. Procedures. Two aqueous solutions of HAuCl4 at a concentration of 0.5 mM and 0.01 M, respectively, a HAuCl4 solution in NMPD (0.02M), and a [(C7H15)4N]+[AuCl4]- solution in toluene (0.02 M) were respectively used as the precursor for the synthesis of Au nanoparticles in an aqueous solution and a NMPD/toluene (50:50 (v/v)) mixed solvent. Au(III) in toluene phase was prepared by following a method similar to that described by Brust et al.,17a employing (THA)Br as the phase transfer agent. An aqueous HAuCl4 solution (8 mL of 0.01 M) was mixed with (THA)Br in toluene (4 mL of 0.015 M). o-Anisidine was used as the reductant. Without giving an additional statement, the reaction was performed at room temperature. The following reduction methods were subsequently employed. Preparation of Au Colloids in Aqueous Solution. Method I. A 19 mL sample of aqueous solution of o-anisidine (2.5 mg) prepared in a 50 mL conical flask was mixed fully with 1 mL of an aqueous CSA solution (0.02 M) under stirring to produce a reaction mixture. To the reaction mixture, 20 mL of an aqueous HAuCl4 solution (0.5 mM) was added quickly with vigorous stirring. Hardly had a drop of HAuCl4 solution been added when a clear light pink hydrosol appeared, indicating the reduction reaction occurring. A red solution was obtained finally. Method II. A 2.5 mg (0.02 mmol) amount of o-anisidine and 19.6 mg (0.04 mmol) of (THA)Br were added to 20 mL of degassed water. The mixture was maintained at 60 °C under a nitrogen environment for 0.5 h to ensure that the additives were totally dissolved. Subsequently, to the solution was added rapidly 1 mL of an aqueous HAuCl4 solution (0.01 M) and the stirring was continued for 5 min. A deep red Au colloid solution was obtained at the end of the reaction. Method III. In a typical procedure, 2.5 mg (0.02 mmol) of o-anisidine and 19.6 mg (0.04 mmol) of (THA)Br were added to 18 mL of degassed water. Under a nitrogen atmosphere, the mixture was maintained at 60 °C for 0.5 h to ensure that the additives were totally dissolved. To the solution was subsequently added 2 mL of aqueous CSA solution (0.02 M), and almost immediately the solution turned turbid and ivory white in color, which is indicative of inducing the formation of large-scale liposomes due to the interaction between CSA and (THA)Br. After the solution was cooled to ambient temperature, 20 mL of an aqueous HAuCl4 solution (0.5 mM) was added rapidly and the stirring was continued for 5 min. A pink-red Au colloid dispersion in liposomes was obtained in the end of the reaction. Preparation of Au Colloids in Organic Solvent. Method I. A 0.154 g amount of o-anisidine and 0.29 g of CSA were dissolved in 48 mL of a cosolvent of 23:25 NMPD/toluene (v/v) under continuous stirring, and then 2 mL of a HAuCl4 solution in NMPD (0.02 M) was added dropwise. The reaction mixture was cooled to 5 °C and kept at that temperature over a period of 5 days while rapidly stirring. The reaction mixture changed to a green (after 3 days) and subsequent dark green solution without any precipitate (after 5 days). Method II. A 61.6 mg amount of o-anisidine and 116.1 mg of CSA were added to 48 mL of a cosolvent of 23:25 NMPD/toluene (v/v), and then the reaction mixture was brought to 100 °C under a stream of nitrogen while continuously stirring. A 2 mL volume of a HAuCl4 solution in NMPD (0.02 M) was added quickly. A change of color from light yellow to orange-brown was gradually observed, suggesting the formation of Au nanoparticles. Method III. In a typical procedure, 2 mL of a [(C7H15)4N]+[AuCl4]- solution in toluene (0.02 M) was used as the precursor for the production of Au nanoparticles but under otherwise conditions identical with those of method II. An orange solution is obtained finally. UV-Vis Absorption Spectroscopy. UV-vis absorption spectra were recorded on a Shimadzu UV1601 PC double beam spectrometer, by scanning the Au-containing solution in a 1 cm quartz cell. The scanning range was from 190 to 1100 nm. Absorption from the solvent was subtracted from each spectrum. Transmission Electron Microscopy. Transmission electron microscopy (TEM) observation was performed on a JEOL-JEM100 CX II transmission electron microscope operated at a 100kV accelerating voltage. High-resolution TEM (HRTEM) images
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Table 1. Mean Diameter and Standard Deviation of Au Nanoparticles Synthesized by Various Methods methods preparation in aq soln mean diameter (nm) std devn (nm)
preparation in org medium
I
II
III
I
II
III
3.02 0.56
3.77 1.45
4.89 0.83
13.35 2.81
1.82 0.62
1.38 0.48
and selected area electron diffraction (SAED) data were acquired with a JEOL-JEM 2010F transmission electron microscopy operated at 200 kV. The high-resolution digitized images with atomic resolution were enlarged through an image intensifier and recorded in a computer. Specimens for TEM were prepared by spreading a small drop (2 µL) of the colloidal solution onto a 400 mesh copper grid that had been coated with a thin amorphous carbon film. In the case of water or 50:50 NMPD/toluene (v/v) as the solvent, the drop was dried almost completely in air at room temperature for nearly 1 h or 1 day, respectively. The specimens were placed on a grid holder. The average diameter and size distribution histograms of the gold particles was determined from the diameters of 300 nanoparticles found in several arbitrarily chosen areas in enlarged microphotographs. Infrared Spectroscopy. Infrared spectra, in the region 4000-400 cm-1, were performed on a Perkin-Elmer system 2000 FT-IR spectrometer at a resolution of 2 cm-1. The aqueous solution of poly(o-anisidine) was repeatedly spread on a silicon crystal slice to form a cast film and dried.
Results and Discussion
Figure 1. UV-vis spectra recorded from the gold colloids in an aqueous solution prepared with o-anisidine as the reductant by (a) method I, (b) method II, and (c) method III. Experimental conditions: (a) [o-anisidine] ) 1 mM, [CSA] ) 1 mM, [HAuCl4] ) 0.5 mM; (b) [o-anisidine] ) 1 mM, [(THA)Br] ) 2 mM, [HAuCl4] ) 0.01 M; (c) [o-anisidine] ) 1 mM, [CAS] ) 2 mM, [(THA)Br] ) 2 mM, [HAuCl4] ) 0.5 mM. The molar ratios of o-anisidine/CSA/HAuCl4, o-anisidine/THAB/HAuCl4, and oanisidine/THAB/CSA/ HAuCl4 for methods I-III are 2:2:1, 2:4: 1, and 2:4:4:1, respectively.
In this study, because the reducing agent (o-anisidine) is soluble in both water and organic solvent, we employed an aqueous phase or a single organic phase system for the synthesis of gold nanoparticles, which allowed us to investigate a wide range of possibilities to control mean size. All the resulting samples obtained are quite stable. The Au nanoparticle dispersions in aqueous solution and NMPD/toluene can be kept for several weeks and even 1 year, respectively. The average sizes and the corresponding standard deviations of all the samples prepared by various methods are summarized in Table 1. Formation of Au Nanoparticles in Aqueous Solution. Similar to pyrrole,19 o-anisidine can easily reduce Au(III) to elemental Au. Meanwhile, the corresponding monomer undergoes oxidative polymerization. Figure 1 portrays the characteristic UV-vis absorption at ca. 500 nm of Au nanoparticles prepared in aqueous medium. Curves a-c correspond to the samples prepared by methods I-III, respectively. A continuous rising background toward higher energy can be seen for all the samples, which is due to the Mie scattering from the nanoparticle suspension. For absorbance bands a and c, a broad surface plasmon band around 500 nm of Au particles which is ascribed to a collective oscillation of conduction electrons in response to optical excitation superimposes on the background. This is in good agreement with what has already been observed for smaller gold clusters with a similar size.20 A red-shift of the λmax of the absorbance band b compared to that of the absorbance bands a and c manifests that the mean size of the nanoparticles obtained by method II increases, although not dramatically. The high absorbance intensity of the peak from the sample prepared by method II is probably due not so much to the increased average diameter of the particles but to the presence of a small
number of strongly absorbing larger particles (4.5-6.5 nm in diameter). Absorbance band b in addition shows somewhat a tail absorption at 600-700 nm, which is generally regarded as an indication of either particle elongation or agglomeration.21,22 In the present case the origin of the weak tail absorption is probably due to the presence of large and small Au nanoparticles and a partial aggregation between them as seen in the TEM images (see below, Figures 3 and 4). However, as one can see, no other absorption at the wavelength longer than 550 nm, which is contributed to the polaron band transition23 (corresponding to the localization of electron) of the polymer in its doped state after the oxidation and polymerization of o-anisidine or to exciton transition of the quinoid ring of the polymer in its base form, can be found. In addition, the π-π* absorption peak of benzenoid rings appears at ∼330 nm for all the samples prepared in an aqueous solution. The typical bright-field TEM images and the corresponding histogram of size distribution of Au nanoparticles prepared by method I in an aqueous solution are shown in Figures 2 and 4a, respectively. The micrographs of the sample prepared by method I clearly illustrate that a coexistence of a large number of spherical micelles (ca. 40 nm in diameter), which mutually interact and form networks, and some wormlike micelles with a cross section of 35 nm formed after the polymerization of o-anisidine (see Figure 2a). It can be inferred that the polymer coils self-assemble into spherical micelles, some of which grow
(19) (a) Selvan, S. T. Chem. Commun. 1998, 351. (b) Henry, M. C.; Hsueh, C. C.; Timko, B. P.; Freund, M. S. J. Electrochem. Soc. 2001, 148, D155. (20) Hostetler, M. J.; Wingate, J. E.; Zhong, C.-J.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; London, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17.
(21) Shon, Y. S.; Gross, S. M.; Dawson, B.; Porter, M.; Murray, R. W. Langmuir 2000, 16, 6555. (22) Sawant, P.; Kovalev, E.; Klug, J. T.; Efrima, S. Langmuir 2001, 17, 2913. (23) (a) Stafstrom, S.; Breadas, J. L.; Epstein, A. J.; Woo, H. S.; Tanner, D. B.; Huang, W. S.; Macdiarmid, A. G. Phys. Rev. Lett. 1987, 59, 1464. (b) Cao, Y.; Smith, P.; Heeger, A. J. Synth. Met. 1989, 32, 263.
Formation of Gold Nanoparticles
Figure 2. (a) Representative TEM image of poly(o-anisidine) micelles decorated by Au nanoparticles prepared in an aqueous solution by method I. (b) Enlargement of a part of (a) to permit closer examination of the Au nanoparticles wrapped in the poly(o-anisidine) micelles. The inset in part a presents the SAED pattern for a small Au nanoparticle in a poly(o-anisidine) micelle.
into cylinders; these cylinders lengthen into semiflexible worms, which then entangle in solution. One would anticipate that the interactions between micelles gradually strengthen as the concentration of micelles is increased while evaporating the solvent during the preparation of TEM specimens, which results in the aggregation of micelles into networks. Moreover, both spherical and wormlike micelles are decorated with a number of tiny spherical Au nanoparticles with a narrow size distribution, as shown in Figure 2b (an enlarged view of the indicated area in Figure 2a). The average size of these small Au particles is less than 4 nm. The structure of the Au nanoparticles is confirmed by the small area electron diffraction patterns obtained from several small particles wrapped in a polymer micelle, which is presented in the inset (upper left) in Figure 2a. The fine discrete spot patterns suggest the highly crystalline features of the tiny Au nanoparticles. This coating of nanoparticles with polymer is analogous to a previous report by Caruso and co-workers.24 Recent studies by Netz and co-workers have modeled the wrapping phenomena of polyelectrolytes around particles.25,26 Calculations showed that by taking into account the polymer length, polymer stiffness, and added salts (leading to finite screening length), certain conditions would result in a strongly bound, wrapped, polyelectrolyte-sphere complex. An analysis given by Caruso et al. in the previous study pointed out that a low (24) Gittins, D. I.; Caruso, F. J. Phys. Chem. B 2001, 105, 6846. (25) Kunze, K.-K.; Netz, R. R. Phys. Rev. Lett. 2000, 85, 4389. (26) Netz, R. R.; Joanny, J.-F. Macromolecules 1999, 32, 9026.
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Figure 3. Representative TEM micrographs of (a) Au nanoparticles synthesized by method II and (b) Au nanoparticles in CSA-induced liposomes prepared by method III. The insets in part b present the SAED pattern (upper left) for a Au nanoparticle in a CSA-induced liposome, an enlarged view of the ensemble of Au particles in a liposome (lower right), and a HRTEM image for a single Au nanoparticle (middle), respectively.
molecular weight polymer and a low salt concentration would be advantageous to successful coating small spheres.24 Figure 3a presents a representative micrograph of the Au nanoparticles synthesized by method II. The image shows that all the particles are almost spherical. Furthermore, a few aggregates formed between large and small Au nanoparticles can be found in the TEM micrograph. In Figure 4b, the particle size analysis shows that the Au nanoparticles have a slightly wide size distribution ranging from 0.3 to 9.1 nm. The TEM observation is identical with the conclusions obtained by the UV-vis spectra. Figure 4b displays that large particles (>4 nm) have a comparably large percentage (12%) although small particles (95%; see Figure 7) are obtained, which is consistent with a recent optical study that shows the surface plasmon band is essentially unidentifiable for crystallites of less than 2 nm effective diameter.30 The size of Au nanoparticles prepared from different methods was examined by TEM. The representative TEM micrographs of Au nanoparticles prepared in the 50:50 NMPD/toluene mixed solvent are shown in Figure 6. The statistical analyses of the dimensions of these Au nanoparticles are also shown in Figure 7. Figure 6a presents a TEM image of Au nanoparticles prepared by slow reduction process at lower temperature, which leads to the particle growth occurring over a longer period versus (30) Alzarez, M. M.; Khoury, J. T.; Schaaff, T. G.; Shafigullin, M. N.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. B 1997, 101, 3706.
Figure 7. Size distribution histograms of Au nanoparticles prepared in a 50:50 NMPD/toluene mixed solvent by (a) method I, (b) method II, and (c) method III.
nucleation to yield large particles. The large Au nanoparticles (a mean size of 13.35 nm) with hexagonal (or pseudohexagonal) and spherical outlines can be identified. The two different two-dimensional shapes correspond for example to icosahedrons and polyhedrons. The facet particles are occasionally observed for weak reducing agent where growth takes place over a longer period.31 The TEM photographs also display the polymer matrix that was obtained by the oxidative polymerization of o-anisidine. Au nanoparticles produced by fast reduction at high temperature are shown in Figure 5b,c. Small Au nanoparticles with a rather narrow size distribution are obtained by methods c and III, as evidenced by TEM observation and particle size analysis (see Figure 7b,c). For methods II and III, the average size of Au nanoparticles (31) Goia, D. V.; Matijevic, E. New J. Chem. 1998, 22, 1203.
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Figure 8. FT-IR spectra of poly(o-anisidine) film cast from its dedoped hydrosol prepared in an aqueous system by method I.
decreases to 1.82 and 1.38 nm, respectively. From the viewpoint of the particle nucleation and growth, a fast reduction rate of Au(III) is needed to generate the smaller Au nanoparticles with narrower distribution because nucleation predominates over growth. In analogy to method III in aqueous system described above, the Au particles are homogeneously distributed in the spherical micelles when [(C7H15)4N]+[AuCl4]- is employed as the precursor for the synthesis of Au nanoparticles, as illustrated in Figure 6c. The inset of Figure 6c more clearly displays the Au nanoparticles within a micelle. The formation of micelles may be attributed to the trace water resulting from the process of phase transfer. As a comparison of Figure 7b with Figure 7c, it can also be found that the size distribution becomes more uniform than that of Au nanoparticles obtained by method II, because the growth of particles is expected to be confined to the single micelle core. Structures of Poly(o-anisidine). To determine the structures of the polymers without electronic absorption at the wavelength longer than 550 nm, we convert the water-soluble poly(o-anisidine)/CSA nanocomposite obtained by method I into its base form by adding excess 0.1 M NH4OH to the solution (the pH of this solution is 9.5), and then we purified this solution by dialysis against pure water to remove all of the remaining salts. The FT-IR of the dedoped poly(o-anisidine) film cast from the aqueous solution is shown in Figure 8. It is noticeable that the strong N-H stretching bands at 3300-3500 cm-1 are converted to a weak absorption band. In addition, the oxidation state of nitrogen element is characterized by X-ray photoelectron spectroscopy (XPS) (the figures are not shown to avoid verbosity in the text). A prominently strong peak at ∼399.4 eV in the N1s spectrum, which is ascribable to the amine nitrogen,32 appears. The ratio of imine to amine is about 0.25 by fitting the N1s spectrum.
Dai et al.
All these results are similar to that described by Lee et al. in ref 33. Therefore, it is supposed that a cross-linked structure of poly(o-anisidine) is formed under strong oxidation conditions. Analogically, poly(o-anisidine) with the same structure is obtained in organic medium for methods II and III. In contrast, during the slow oxidation process, the change of the solution color to green as well as the presence of bipolaron band as shown in Figure 5 manifests the formation of fully doped poly(o-anisidine) with CSA. Thus, the mild oxidation process is favorable for the formation of emeraldine salt (the conducting state). The position of the polaron band is consistent with the proposal of the “compact coil” conformation for the polymer chains.34 In the case of methods II and III for the preparation of Au nanoparticles in aqueous solution, the lower molecular weight poly(o-anisidine) is expected to be obtained in the absence of dopant (CSA) (method II) or when H+ is used as the counterions of micelles (method III), because the molecular weight of poly(o-anisidine) is sensitive to pH of the solution. Low acidity results in the low molecular weight of a conducting polymer, for example, polyaniline.35 Conclusions In this study, we present a series of approaches to preparing Au nanoparticles in both aqueous and organic medium. All the Au colloids produced show a long-term stability. In a fast reduction reaction of Au(III), small Au nanoparticles with a narrow size distribution are obtained as determined by the TEM and UV-vis measurements. Meanwhile, the polymerization of o-anisidine occurs and leads to the formation of cross-linked structure of poly(o-anisidine). In aqueous solution, the size distributions of Au nanoparticles is well controllable due to the production of poly(o-anisidine) micelles and liposomes induced by the interaction between CSA and o-anisidine. However, in a slow reduction reaction of Au(III) that proceeds at low temperature, large Au particles (the average size greater than 13 nm) are yielded for the particle growth over a long period of time. Furthermore, poly(o-anisidine) chains protonated with CSA are expected to be formed. Because the reducing agent (o-anisidine) can be dissolved both in water and organic solvent, the reaction medium can be used as a parameter to adjust the diameter and distribution of Au nanoparticles. Acknowledgment. This project is supported by the National Natural Science Foundation of China (NSFC) No. 20204017. LA025926U (32) Kang, E. T.; Neoh, K. G.; Tan, K. L. Adv. Polym. Sci. 1993, 106, 135. (33) Lee, Y. M.; Kim, J. H.; Kang, J. S.; Ha, S. Y. Macromolecules 2000, 33, 7431. (34) (a) Su, S. J.; Kuramoto, N. Macromolecules 2001, 34, 7249. (b) Su, S. J.; Kuramoto, N. Chem. Mater. 2001, 13, 4787. (35) Cao, Y.; Andreatta, A.; Heeger, A. J.; Smith, P. Polymer 1989, 30, 2305.
