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Liposome Induced Self-Assembly of Gold Nanoparticles into Hollow Spheres Xiaohong Li,†,‡ Yunchao Li,†,‡ Chunhe Yang,† and Yongfang Li*,† Key Laboratory of Organic Solids, Center for Molecular Science, Institute of Chemistry, and Graduate School, Chinese Academy of Sciences, Beijing 100080, China Received October 9, 2003. In Final Form: February 13, 2004 Gold nanoparticles were prepared by the reduction of [(C7H15)4N]+[AuCl4]- with 3,4-ethylenedioxythiophene (EDOT) as reductant in toluene solution. The employed stabilizers include 3,3′-thiodipropionic acid (TDPA), 1-dodecanethiol (DDT), (()-10-camphorsulfonic acid (CSA), and 11-mercaptoundecanoic acid (MUA). The reaction processes were tracked by UV-vis and FT-IR spectroscopy, and the as-prepared gold nanoparticles were characterized by scanning electron microscopy, transmission electron microscopy, energydispersive spectroscopy, and X-ray photoelectron spectroscopy measurements. When TDPA and MUA, which possess both -S- and -COOH groups, were used as the stabilizer in the preparation, the asprepared nanoparticles could self-assemble into hollow spheres. While when DDT with a -SH group or CSA with a -SO3H group was used as the protecting agents, only discrete gold nanoparticles were observed. The results show that the groups of both -S- and -COOH in the stabilizer play an important role in forming the hollow nanospheres. It is proposed that the formation mechanism of the hollow spheres is a liposome that formed between -COO- and [(C7H15)4N]+ could act as a template to induce the self-assembly of the gold nanoparticles into the hollow spheres.
1. Introduction Intensive studies have been devoted to assembling nanoparticles into ordered two- or three-dimensional superstructures due to their significant electronic1 and optical properties2 and their promising applications in information storage devices.3 The properties of the assembled nanoparticles are mostly different from those of the individual nanoparticles or their macroscopic equivalents. Nanoscaled hollow spheres composed of nanoparticles are very useful for encapsulation, drug delivery, development of artificial cells, and protection of biologically active agents.4 The ability to synthesize ordered assembly configuration of the nanoparticles could provide a new horizon to study the collective properties and develop future nanoscaled optical, electronic, and information storage devices. Shape control of nanoparticles has been successfully achieved through use of a template method. The used templates include porous alumina,5 polycarbonate membranes,6 carbon nanotubes,7 and micelles.8 As for hollow nanospheres, one strategy is also to employ a template, such as polymer beads9 or emulsions,10 to produce the nanostructures, then the hollow spheres could be obtained after removing the template. But the homogeneity of the * Corresponding author. E-mail:
[email protected]. † Key Laboratory of Organic Solids, Center for Molecular Science, Institute of Chemistry. ‡ Graduate School. (1) (a) Xia, Y.; Kim, E.; Mrksich, M.; Whitesides, G. M. Chem. Mater. 1996, 8, 601. (b) Kamar, A.; Whitesides, G. M. Science 1994, 263, 601. (2) (a) Mullen, K.; Ben-Jacob, E.; Jaklevic, R.C.; Schuss, Z. Phys. Rev. B 1988, 37, 98. (b) Grabert, H., Devoret, M. H., Eds. Single Charge Tunneling; Plenum Press: New York, 1992. (3) Jin, B. Y.; Ketteren, J. B. Adv. Phys. 1988, 38, 189. (4) (a) Mathlowitz, E.; Jacob, J. S.; Jong, Y. S.; Carino, G. P.; Chichering, D. E.; Chaturvedl, P.; Santos, C. A.; Vijayaraghavan, K.; Montegometry, S.; Bassett, M.; Morrrell, C. Nature 1997, 386, 410. (b) Huang, H.; Resen, E. E. J. Am. Chem. Soc. 1999, 121, 3805. (c) Suhorukov, L.; Da¨hne, J.; Hartmann, E.; Donath, H. M. Adv. Mater. 2000, 12, 112. (5) (a) Martin, B. R.; Dermody, D. J.; Reiss, B. D.; Fang, M. M.; Lyon, L. A.; Natan, M. J.; Mallouk, T. E. Adv. Mater. 1999, 11, 1021. (b) van der Zande, B.M. I.; Bohmer, M. R.; Fokkink, L. G. J.; Schonenberger, C. Langmuir 2000, 16, 451.
obtained spherical structure was often affected, especially for hollow polymer spheres. The other strategy is to utilize functional molecules, such as diblock polymer11 or biomolecules,12 by which a self-assembled monolayer can be formed. Recently, calcium phosphate and silica nanoshells prepared by vesicle-directed growth were reported by Schmidt et al.13 and Hubert et al.14 respectively. The preparation of gold nanoparticles in aqueous and organic media has been extensively studied. Various morphologies, such as nanorods,15 nanorings,16 nanoribbons,17 and different self-assembly structures, such as double helical and single-chain array,18 were obtained. However, to our knowledge, few reports regarding the (6) (a) Martin, C. R. Chem. Mater. 1996, 8, 1739. (b) Cepak, V. M.; Martin, C. R. J. Phys. Chem. B. 1998, 102, 9985. (c) Schonenberger, C.; van der Zande, B. M. I.; Fokkink, L. G. J.; Henny, M.; Schmid, C.; Kruger, M.; Bachtold, A.; Huber, R.; Birk, H.; Staufer, U. J. Phys. Chem. B 1997, 101, 5497. (7) (a) Sloan, J.; Wright, D. M.; Woo, H. G.; Bailey, S.; Brown, G.; York, A. P. E.; Coleman, K. S.; Hutchison, J. L.; Green, M. L. H. Chem. Commun. 1999, 699. (b) Kyotani, T.; Tsai, L. F.; Tomita, A. Chem. Commun. 1997, 701. (c) Govindaraj, A.; Satishkumar, B. C.; Nath, M.; Rao, C. N. R. Chem. Mater. 2000, 12, 202. (d) Pradhan, B. K.; Kyotani, T.; Tomita, A. Chem. Commun. 1999, 1317. (8) (a) Pileni, M. P.; Gulik-Krzywichi, T.; Tanori, J.; Filankembo, A.; Dedieu, J. C. Langmuir 1998, 14, 7359. (b) Pileni, M. P.; Ninham, B. W.; Gulik-Krzywichi, T.; Tanori, J.; Lisiecki, I.; Filankembo, A. Adv. Mater. 1999, 11, 1358. (c) Qi, L. M.; Ma, J. M.; Cheng, H. M.; Zhao, Z. G. J. Phys. Chem. B 1997, 101, 3460. (d) Li, M.; Schnablegger, M. H.; Mann, S. Nature 1999, 402, 393. (e) Heywood, B. R.; Mann, S. Adv. Mater. 1994, 6, 9. (9) (a) Zhong, Z.; Yin, Y.; Gates, B.; Xia, Y. Adv. Mater. 2000, 12, 206. (b) Santos, I.; Scho¨ler, B.; Caruso, F. Adv. Funct. Mater. 2001, 11, 122. (c) Caruso, F.; Spasova, M.; maceira, V. S.; Marza´n, M. L. Adv. Mater. 2001, 13, 1090. (10) Huang, J.; Xie, Y.; Li, B.; Liu, Y.; Qian, Y.; Zhang, S. Adv. Mater. 2000, 12, 808. (11) (a) Spatz, J. P.; Herzog, T.; Mo¨ssmer, S.; Ziemann, P.; Mo¨ller, M. Adv. Mater. 1991, 11, 149. (b) Putlitz, B.; Landfester, K.; Fischer, H.; Antonietti, M. Adv. Mater. 2001, 13, 500. (c) Zhang, L.; Eiserberg, A. J. Am. Chem. Soc. 1996, 118, 3168. (12) (a) Fuhrhop, J. H.; Helfrich, W. Chem. Rev. 1993, 93, 1565. (b) Hubert, D. H. W.; Jung, M.; German, A. L. Adv. Mater. 2000, 12, 1291. (13) Schmidt, H. T.; Ostafin, A. E. Adv. Mater. 2002, 14, 532. (14) Hubert, D. H. W.; Jung, M.; Frederik, P. M.; Bomans, P. H. H.; Meuldijk, J.; German, A. L. Adv. Mater. 2000, 12, 1286. (15) Busbee, B. D.; Obare, S. O.; Murphy, C. J. Adv. Mater. 2003, 15, 414. (16) Sun, Y. G.; Xia, Y, N. Adv. Mater. 2003, 15, 695.
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formation of nanoscaled hollow spheres have emerged until now. Here, we report the hollow spheres formed by selfassembly of the gold nanoparticles, which were prepared by the reduction of [(C7H15)4N]+AuCl4- in toluene solution with EDOT as the reductant and TDPA as the stabilizer. TDPA had been used as stabilizer to prepare nanoparticles in aqueous solution, and tiny and discrete metal nanoparticles were obtained.19 In our case, the liposome formed in the preparation process acted as a template to achieve the self-assembly of gold nanoparticles into hollow spheres. The results and forming mechanism are reported in this paper. 2. Experimental Section Chemicals. Hydrogen tetrachloroaurate(III) (HAuCl4), tetraheptylammonium bromide ((THA)-Br), 1-dodecanethiol (DDT), (()-10-camphorsulfonic acid (CSA), 11-mercaptoundecanoic acid (MUA), and toluene were purchased from Aldrich and used without further purification. 3,4-Ethylenedioxythiophene (EDOT) was obtained from Bayer Company (Germany) and purified by distillation under reduced pressure before use. 3,3′-Thiodipropionic acid (TDPA) was purchased from TCI (Tokyo) and used as received. Synthesis of Gold Colloids. Au(III) in toluene phase was prepared by the method similar to that described by Brust et al.,20 employing (THA)-Br as the phase transfer agent. An aqueous HAuCl4 solution (10 mL of 0.02 M) was mixed with (THA)-Br in toluene (10 mL of 0.02 M). A [(C7H15)4N]+[AuCl4]solution in toluene (0.02 M) was formed and used as the precursor for the synthesis of Au nanoparticles. The reaction was performed at 120 °C under stirring and refluxing, unless otherwise stated. Preparation of Au Colloids with TDPA as Stabilizer. The synthesis of gold nanoparticles was carried out by the chemical reduction of [(C7H15)4N]+[AuCl4]- with EDOT as the reductant and TDPA as the stabilizer. Four methods with different molar ratios of EDOT/[AuCl4]-/TDPA were performed in the preparation. The detailed preparation processes are as follows: In each method, 2 mL of [(C7H15)4N]+[AuCl4]- solution was dissolved in 18 mL of toluene containing different amounts of TDPA for different methods. Then the mixtures were heated to 120 °C under refluxing and stirring and kept at that temperature for 15 min to ensure that the color of the reaction mixture turned from brown into colorless. To the heated mixture, EDOT was added quickly. Then the mixture was continuously heated for 5 h under stirring. The molar ratios of EDOT/[AuCl4]-/TDPA for methods I, II, III, and IV were 5:2:5, 10:2:5, 15:2:5, and 5:2:2, respectively. A yellow gold colloid solution stabilized by TDPA was obtained at the end of each reaction. Preparation of Au Colloids with MUA, DDT, and CSA as Stabilizer. The Au colloids stabilized by DDT, MUA, and CSA were prepared with the same method as method I, where the molar ratio of EDOT/[AuCl4]-/ DDT (CSA or MUA) is 5:2:5. Instrumentation. Scanning electron microscopy (SEM) measurement was performed on a JEOL JSM-6700F field emission scanning electron microscope. transmission electron microscopy (TEM) observation was undertaken with a JEOLJEM-100CX II transmission electron microscope operated at 100 kV. Energy-dispersive X-ray spectroscopic (EDS) analysis and selected area electron diffraction (SAED) were acquired with a JEOL-JEM 2010F transmission electron microscopy operated at 200 kV. UV-vis absorption spectroscopy was performed with a Hitachi U-3010 UV-Vis scanning spectrophotometer. Infrared spectra, in the region 4000-1100 cm-1, were recorded on a Brucker EQUINOX55 infrared spectrophotometer with 4 cm-1 resolution. X-ray photoelectron spectroscopy (XPS) data were obtained on a VG-Scientific ESCA Lab 220i-XL spectrometer equipped with a hemisphere analyzer, and an Al KR X-ray source (17) Swami, A.; Kumar, A.; Selvakannan, P. R.; Mandal, S.; Pasricha, R.; Sastry, M. Chem. Mater. 2003, 15, 17. (18) Fu, X. Y.; Wang, Y.; Huang, L. X.; Sha, Y. L.; Gui, L. L.; Lai, L. H.; Tang, Y. Q. Adv. Mater. 2003, 15, 902. (19) Tan, Y. W.; Dai, X. H.; Li, Y. F.; Zhu, D. B. J. Mater. Chem. 2003, 5, 1069. (20) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Commun. 1994, 801.
Figure 1. (a) SEM image. (b) TEM image. (c) Enlarged view of the self-assembled hollow spheres of gold nanoparticles synthesized with method I. The inset in Figure 1b presents the SAED pattern of gold nanoparticles. at 1486.6 eV. Peak positions were internally referenced to the C 1s peak at 284.6 eV.
3. Results and Discussion 3.1. Preparation of Hollow Nanospheres with 3,3′Thiodipropionic Acid (TDPA) as Stabilizer. The Au colloids prepared with method I were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The formation of uniform spheres was observed from the SEM image as shown in Figure 1a. Figure 1b shows the TEM image of the formed spheres with sharp contrast between the dark edge and the pale center, which indicates that the aggregates are hollow spheres with diameters of ca. 160 nm and a shell thickness about 20-50 nm. An enlarged view of the hollow sphere (see Figure 1c) shows that the hollow spheres are composed of small gold nanoparticles (ca. 2 nm). The formation of small-sized nanoparticles could be ascribed to the weak reductant of EDOT in comparison with the strong reductants of pyrrole21 and o-anisidine22 in the preparation of gold nanoparticles. The corresponding SAED is also displayed in the inset of Figure 1b, which shows the crystalline property of the nanoparticles. These spherical nanostructures composed of such tiny particles are expected to have potential applications in material science. To investigate the influence of EDOT concentration on the hollow spherical structures, the concentration of EDOT in the reactive mixture was varied as described in method I, method II, and method III. In all the three cases, hollow spherical structures composed of tiny gold nanoparticles (21) Selvan, S. T.; Hayakawa, T.; Nogami, M.; Mo¨ller, M. J. Phys. Chem. B 1999, 103, 7441. (22) Dai, X. H.; Tan, Y. W.; Xu, J. Langmuir 2002, 18, 9010.
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Figure 3. Elemental analysis of a gold nanoparticle in the hollow spheres by EDS. The sample was prepared with TDPA as stabilizer.
Figure 2. TEM images of the gold colloids prepared with (a) method II, (b) method III, and (c) method IV.
were observed (see Figure 1a, Figure 2a, and Figure 2b, respectively). The results indicate that the concentration of the EDOT reductant has little influence on the selfassembled hollow spheres. The hollow spheres were also observed when changing the concentration of TDPA in the reaction mixture. One of the representative TEM images is shown in Figure 2c in order to avoid verbosity of the text. This is to say that such hollow spheres are characteristic of this preparation system, insensitive to the concentrations of EDOT and TDPA. The chemical composition of the as-prepared gold nanoparticles which build up the hollow spherical nanostructures was investigated by EDS analysis. The peaks of Au, S, O, Cu, and C are observed from the EDS spectrum, as shown in Figure 3. The peaks of C and Cu are due to the carbon-coated copper grid used for the TEM measurement. The appearance of the peaks of Au further indicates the formation of gold nanoparticles. In addition, the peaks of S and O imply that polymer PEDOT, which was formed by oxidation polymerization of EDOT under the concurrent formation of gold nanoparticles, were coated on the surfaces of the as-prepared gold nanoparticles. When EDOT was added into the toluene solution containing only 2 mM [(C7H15)4N]+[AuCl4]-, the solution turned quickly into green and precipitation was formed, which indicates that EDOT was oxidized to form PEDOT with the reduction of gold ions. Obviously the formed PEDOT could not act as an efficiency stabilizer. To further probe the nature of S and confirm the formation of gold nanoaprticles, the sample was investigated by XPS measurement as shown in Figure 4. Figure 4a shows the peaks of Au 4f, S 2p, O 1s, and C 1s, which is consistent with the result of EDS measurement. Among these peaks, the peak of S 2p was analyzed through fitting treatment. Figure 4b presents the fitting result on the peak of S 2p, indicating the presence of two kinds of S. The binding energy of 164.0
Figure 4. XPS spectra of the gold nanoparticles prepared with TDPA as stabilizer: (a) XPS survey spectrum; (b) the fitting result of the high-resolution XPS for the S region. The fitting results are listed in the inset of part b.
eV for S 2p1/2 and 532.7 eV for O 1s could be respectively ascribed to those of thiophene and CsOsC units in EDOT,23 which indicates that no ring opening of EDOT occurred during the preparation process, and as-formed PEDOT was only coated on the surface of the gold nanoparticles. The binding energy of S 2p1/2 at 161.9 eV was assigned to metal sulfide,24 which confirms the (23) Nacer, S.; Salah, A.; Jean-Jacques, A.; Mohamed, J.; Jean, C. L.; Pierre-Camille, L. Langmuir 1999, 15, 2566.
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Figure 5. TEM images of the gold nanoparticles synthesized with the stabilizers of (a) DDT and (b) CSA.
formation of S-Au bonds. The appearance of the metal sulfide S peak indicates that the self-assembled gold nanoparticles were stabilized by TDPA. The binding energy of 83.6 eV for Au 4f is similar to that of pure Au.25 3.2. Influence of Functional Groups in the Stabilizer on the Self-Assembled Hollow Spheres. To study the influence of the functional groups in the stabilizer (such as -COOH and -S- groups) on the self-assembled hollow spheres, DDT and CSA were also used as the stabilizer instead of TDPA to prepare the Au colloid solution. When the gold colloid was prepared with DDT and CSA as the stabilizer, as described in the Experimental Section, only dispersive and small gold nanoparticles (1-3 nm) could be obtained (see Figure 5). Comparing the stabilizers, we can find that TDPA contains both the -SO3H and -S- groups, while DDT contains only the -S- group and CSA contains only the -COOH group. Obviously, the existence of both -COOH and -S- groups in the stabilizer is very important for the self-assembly of the gold nanoparticles into the hollow spheres. 3.3. Mechanism of the Self-Assembling of Gold Nanoparticles into Hollow Nanospheres. The selfassembly of nanoparticles has attracted much interest in both colloidal and material sciences. Many methods have been exploited to establish force action to obtain the selfassembled structures, such as electronic actions,26 covalent linkage,27 and sorption interactions between “building blocks”.28 Recently, Hubert et al. reported that nanoscaled liposomes could be used as a kind of template for the deposition of inorganic materials to prepare a hollow nanoshell.13 Thus, successful cooperation of template and growing nanoparticles is necessary to finish some sort of inducing growth. Moreover, TDPA has been reported as a kind of capping agent to prepare metal nanoparticles (24) Tan, Y. W.; Li, Y. F.; Zhu, D. B. Langmuir 2002, 18, 3392. (25) Hu¨fner, S. Photoelectron Spectroscopy, 2nd ed.; SpringerVerlag: New York, 1996. (26) Shipway, A. N.; Lahav, M.; Gabai, R.; Willner, I. Langmuir 2000, 16, 8789. (27) Boal, A. K.; Ilhan, F.; Derouchey, J. E.; Thurn-Albrecht, T.; Russell, T. P.; Rotello, V. M. Nature 2000, 404, 746. (28) Jian, J.; Tomokazu, I.; Chang, S. C.; Song, Y. L.; Jiang, L.; Li, T. J.; Zhu, D. B. Angew. Chem., Int. Ed. 2001, 11, 40.
Figure 6. UV-vis spectra of 20 mL of toluene solution containing 2 mM [(C7H15)4N]+[AuCl4]- and 5 mM TDPA before refluxing, after refluxing for 5 min and 15 min before 5 mM EDOT was added, and after refluxing for 5 h after 5 mM EDOT was added. An enlarged view is shown in the inset.
(Pt, Pd, Ag, and Au) with potassium bitartrate as reductant in aqueous solutions.19 But the forming mechanism was not discussed. Herein, we carried out the measurements of UV-vis and IR spectra for illuminating the reaction processes and discussed the formation mechanism of the hollow gold spheres in the following. UV-vis Spectroscopy. For the sake of studying the mechanism of the formed hollow spheres, the forming process of the gold nanoparticles was tracked with UVvis spectrum, as shown in Figure 6. Twenty milliliters of toluene solution containing 2 mM [(C7H15)4N]+[AuCl4]and 5 mM TDPA shows a peak at 325 nm due to the ligandto-metal charge-transfer transition of the [AuCl4]- ions. After the solution was refluxed for 5 min at 120 °C, the peak at 325 nm was reduced. Then after continuous refluxing for 10 min more, the solution turned from brown to colorless, and the absorption peak completely disappeared. It must be stated that when this colorless solution was continuously heated at 120 °C, its UV-vis spectrum remained unchanged. It is known that, in the case of the
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Figure 8. The formed liposome observed by TEM before EDOT was added.
Figure 7. Comparison of the FT-IR spectra of pure TDPA (curve a), the colorless preparation solution before adding EDOT (curve b) and the resulting gold colloid (curve c): (A) the whole spectra between 4000 and 800 cm-1, (B) the high-resolution spectra between 1800 and 800 cm-1.
alkanethiol reaction with [AuCl4]-, the reactive mixture turned colorless with the reduction of Au(III) to Au(I) to form [-Au(I)-SR-]n polymers as reported by Hostetler.29 In our case, TDPA-induced reduction of Au(III) to Au(I) may occur. But the reaction could not continuously realize the reduction of Au(I) to Au(0). The produced Au(I) was further confirmed by XPS. The formed product may be Au(I)-SCH2CH2COOH. This result was consistently confirmed by FT-IR spectroscopy, which will be discussed later. For the preparation of the gold nanoparticles, EDOT was added into the colorless solution, and the mixture was continuously refluxed for 5 h. Finally, a yellow solution was obtained at the end of the reaction. The gold nanoparticles were prepared under concurrent polymerization of EDOT to yield PEDOT. From the UV-vis spectrum, only Mie scattering from the nanoparticle suspensions was observed as shown the curve “reaction 5 h” compared with the curve “refluxing 15 min” in Figure 6. The results show that a vast majority of the gold nanoparticles are smaller than 2 nm,30 which is consistent with the result of the TEM image of Figure 1c. In addition, PEDOT with shorter conjugation chain also formed corresponding to the absorption peak at 367 nm in the UV-vis spectrum,23 as shown in the inset of Figure 6. (29) Hostetler, M. J.; Wingate, J. E.; Zhong, C.-J.; Harris, J. E.; et al. Langmuir 1998, 14, 17. (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 9. The TEM image of Au colloid prepared with MUA as stabilizer.
FT-IR Spectroscopy. Figure 7 shows the FT-IR spectra of pure TDPA (curve a), the colorless solution before adding EDOT (curve b), and the final colloid solution (curve c). In curve a, it can be seen that there is a wide band between 3400 and 2500 cm-1, which is ascribed to the vibrations of C-H and O-H in the solid TDPA.31 A band at ca. 930 cm-1 is also due to the vibration of O-H in TDPA.31 Compared with curve a, in curve b the vibration of O-H bands at 930 cm-1 is reduced, but did not disappear. Meanwhile, the vibrations of C-H of groups CH2 and CH3 in [(C7H15)4N]+ are also probed at 30002800 cm-1 in curve b. However, the characteristic vibration of COO- is not detected from the two curves (see Figure (31) Nakanishi, K.; Solomon, P. H. Infrared Absorption Spectroscopy, 2nd ed.; Holden-Day, Inc., San Francisco, CA, 1977; pp 214-216.
Hollow Spheres of Gold Nanoparticles Scheme 1. Proposed Mechanism for the Formation of the Assembled Hollow Spheres of Gold Nanoparticles Prepared with TDPA as Stabilizer
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both end groups of -COOH and -S is necessary for causing such liposomes to act as a template to finish the selfassembly of gold nanoparticles into hollow spheres in the organic solution. The result was further confirmed by employing MUA, which possesses both -S- and -COOH groups, as stabilizer. When MUA was used as the stabilizer, the hollow spheres composed of small Au nanoparticles were also observed, as shown in Figure 9. The SAED (see the inset of Figure 9) shows the crystalline property of the gold nanoparticles. Proposed Mechanism of the Self-Assembly of gold Nanoparticles into Hollow Spheres. On the basis of the above results, the mechanism for the formation of the hollow spheres is proposed as Scheme 1. AuCl4- ions in the aqueous solution are transferred into toluene through a phase transfer agent ((THA)-Br) and [(C7H15)4N]+[AuCl4]- is formed. The toluene solution containing 2 mM [(C7H15)4N]+[AuCl4]- and 5 mM TDPA becomes colorless after refluxing for 15 min at 120 °C, which indicates the formation of Au(I), and the produced compound is Au(I)-SCH2CH2COOH. Subsequently, the reductant EDOT is added into the colorless solution. First of all, the COOH in Au(I)-SCH2CH2COOH and cationic [(C7H15)4N]+ in THA-Br could spontaneously connect to form liposome vesicles. The formed liposome is possibly composed of the bilayer membranes: the exterior layer is THA, whose longtailed alkyl spreads into the solution, the interior layer is TDPA connected with Au(I) through -S- and with THA through COOH. Next, the added EDOT reacts with Au(I) at the interior surface of the liposome. At the same time, the as-prepared gold nanoparticles are stabilized at this moment. Meanwhile, the produced PEDOT through the oxidation polymerization of EDOT also coats on the surface of the Au nanoparticles. Thus, the formed liposome acts as a template to induce the formation of the hollow spheres. Obviously, both -S- and -COOH groups in the stabilizer play a very important role in the proposed mechanism of the self-assembly of the gold nanoparticles. Similarly, the forming mechanism could be applied to the self-assembled hollow gold nanospheres prepared with MUA as stabilizer. Conclusion
7B).31 A conclusion could be made that COOH did not react with [(C7H15)4N]+ to form COO- at this stage, and the reduced vibration of O-H in COOH may be ascribed to the moiety broken off from TDPA after reaction with AuCl4-. So we could conclude that Au(I)-SCH2CH2COOH was possibly formed when the solution became colorless. In Figure 7B, curve c shows two new vibration bands of COO- at 1600-1575 cm-1,31 in comparison with curves a and b. Meanwhile, the vibration band of O-H at 930 cm-1 is almost gone. The result indicates that COOH in Au(I)-SCH2CH2COOH reacted with [(C7H15)4N]+ to produce liposome. The formed liposome was observed by TEM image before EDOT was added, as shown in Figure 8, which may act as a kind of template to induce the selfassembly of gold nanoparticles into the hollow spheres. When DDT acted as the stabilizer instead of TDPA, the -SH group in DDT could react with AuCl4- to form polymer (-Au-SR-)n and the solution also turned colorless,28 but hollow spheres were not obtained. When CSA was used as the stabilizer instead of TDPA, -SO3H in CSA could react with [(C7H15)4N]+ to form liposome as reported by Dai,22 but hollow spherical nanostructures were not formed either. Thus, the stabilizer containing
Gold colloid solutions stabilized by TDPA were prepared through the reduction of [(C7H15)4N]+[AuCl4-] by EDOT in toluene solution. TEM images show that the as-obtained gold nanoparticles (∼2 nm) self-assembled into hollow spheres with different sizes from 50 to 200 nm. The forming process of such hollow spheres was tracked with UV-vis and FT-IR spectroscopy, and the reaction mechanism was proposed. It was shown that Au(III) reacted with TDPA to form Au(I)-CH2CH2COOH at initial reaction stage. Hereafter, -COOH in Au(I)-CH2CH2COOH connected with [(C7H15)4N] + to form liposome and the Au(I) was reduced by EDOT to produce gold nanoparticles at the interior surface of the formed liposome. The formed liposome acted as a template to induce the self-assembly of gold nanoparticles into the hollow spheres. Thus, both the -S- and -COOH groups in the stabilizer are important for the self-assembly of the gold nanoparticles into the hollow spheres. In summary, we present a facile chemical reaction strategy to prepare gold hollow spheres. The method may be extended to the preparation of other material hollow spheres. Acknowledgment. This work was supported by the Key Basic Research Project of the Ministry of Science and Technology of China (No. 2001CB610507). LA035884P
