9704
Langmuir 2006, 22, 9704-9711
Effect of Hydrophobicity inside PEO-PPO-PEO Block Copolymer Micelles on the Stabilization of Gold Nanoparticles: Experiments Shu Chen,†,‡ Chen Guo,*,†,‡ Guo-Hua Hu,§,| Jing Wang,†,‡ Jun-He Ma,†,‡ Xiang-Feng Liang,†,‡ Lily Zheng,†,‡ and Hui-Zhou Liu*,†,‡ Laboratory of Separation Science and Engineering, State Key Laboratory of Biochemical Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100080, and Graduate School of the Chinese Academy of Sciences, Beijing 100039, People’s Republic of China, Laboratory of Chemical Engineering Sciences, CNRS-ENSIC-INPL, 1 rue GrandVille, BP 20451, 54001 Nancy Cedex, France, and Institut UniVersitaire de France, Maison des UniVersite´ s, 103 BouleVard Saint-Michel, 75005 Paris, France ReceiVed April 21, 2006. In Final Form: August 14, 2006 In this paper we present the effect of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymer micelles and their hydrophobicity on the stabilization of gold nanoparticles. Gold nanoparticles were prepared by a method developed by Sakai et al. (Sakai, T.; Alexandridis, P. Langmuir 2004, 20, 8426). An absorption centered at 300-400 nm in time-dependent UV spectra provided evidence that the very first step of the synthesis was to form primary gold clusters. Then the gold clusters grew in size and were stabilized by block copolymer micelles. The stabilization capacities of the micelles were modulated by tuning the block copolymer concentration and composition and by adding salts. With good stabilization, gold particles were spherical and uniform in size with a diameter of 5-10 nm. Otherwise they were aggregates with irregular shapes such as triangular, hexagonal, and rodlike. The presence of a small amount of NaF significantly increased the stabilization capacity of the micelles and consequently modified the quality of the gold particles. Using FTIR and 1H NMR spectroscopy, micellization of the block copolymers and hydrophobicity of the micelles were proven very important for the stabilization. A higher hydrophobicity of the micelle cores was expected to favor the entrapment of primary gold clusters and the stabilization of gold nanoparticles.
Introduction Gold nanoparticles have attracted great attention due to their fascinating size-related electronic, magnetic, and optical properties and their applications in catalysis and biology.1,2 A general route to preparing gold nanoparticles is the reduction of a gold(III) precursor by various techniques in an appropriate solvent. However, nanoparticles often are fairly unstable in solution. Thus, stabilization is an important and challenging task to prevent them from aggregation and to finely control their size and shape.3-8 Recently, Sakai et al.9-12 reported on a single-step synthesis and stabilization of gold nanoparticles from hydrogen tetrachloroaureate(III) hydrate (HAuCl4‚3H2O) in aqueous poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEOPPO-PEO) block copolymer (Pluronic) solutions at ambient temperature, where the block copolymer proved very efficient both as a reductant and as a colloidal stabilizer. That novel procedure was simple, environmentally benign, and economic * To whom correspondence should be addressed. Fax: +86-10-62554264. E-mail:
[email protected] (H.-Z.L.);
[email protected] (C.G.). † Chinese Academy of Sciences. ‡ Graduate School of the Chinese Academy of Sciences. § CNRS-ENSIC-INPL. | Maison des Universite ´ s. (1) Roucoux, A.; Schulz, J.; Patin, H. Chem. ReV. 2002, 102, 3757. (2) Daniel, M. C.; Autruc, D. Chem. ReV. 2004, 104, 293. (3) Schmid, G.; Chi, L. F. AdV. Mater. 1998, 10, 515. (4) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. ReV. Mater. Sci. 2000, 30, 545. (5) Brust, M.; Kiely, C. J. Colloids Surf., A 2002, 202, 175. (6) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; El-Sayed, M. A. Science 1996, 272, 1924. (7) Sun, Y. G.; Xia, Y. N. Science 2002, 298, 2176. (8) Pileni, M. P. Langmuir 1997, 13, 3266. (9) Sakai, T.; Alexandridis, P. Langmuir 2004, 20, 8426. (10) Sakai, T.; Alexandridis, P. J. Phys. Chem. B 2005, 109, 7766. (11) Sakai, T.; Alexandridis, P. Langmuir 2005, 21, 8019. (12) Sakai, T.; Alexandridis, P. Nanotechnology 2005, 16, S344.
and thus a promising method to provide gold nanoparticles as building blocks for fabricating various types of nanomaterials.13 It was suggested that the formation of gold nanoparticles from AuCl4- comprised three main steps: (1) reduction of metal ions by block copolymers in solution and formation of gold seeds, (2) adsorption of block copolymers on gold clusters and reduction of metal ions on the surfaces of those gold clusters, and (3) stabilization of metal particles by block copolymers.10-12 The colloidal stabilization of gold nanoparticles by the Pluronic block copolymer (step 3) is most likely related to their amphiphilic character and their ability to form micelles in solution and/or on the surface of particles.9-12 Lately, micelles14-17 and reversed micelles18,19 of amphiphilics in selective solvents have been used for stabilizing nanoparticles. This strategy has distinct advantages of controlling the growth and stabilization of dispersed particles well by tuning their characteristics and/or solvent quality. For example, Deng et al.15 reported that the addition of pyrene in aqueous sodium dodecyl sulfate (SDS) micellar solution resulted in the formation of gold nanoparticles in smaller size and narrower distribution. To explore the full potential of micelles as stabilizers, the underlying mechanism that controls their stabilization capacity should be better understood. In fact, the characteristics of Pluronic micelles can easily be varied by varying the polymer composition and concentration and temperature20-25 or by modifying the solvent quality with the addition of cosolutes such as salts.26-29 (13) Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. Nature (London) 2005, 437, 121. (14) Mandal, M.; Ghosh, S. K.; Kundu, S.; Esumi, K.; Pal, T. Langmuir 2002, 18, 7792. (15) Deng, J. P.; Wu, C.; Yang, C. H.; Mou, C. Y. Langmuir 2005, 21, 8947. (16) Kuo, P. L.; Chen, C. C.; Jao, M. W. J. Phys. Chem. B 2005, 109, 9445. (17) Liu, S.; Weaver, J. V. M.; Save, M.; Armes, S. P. Langmuir 2002, 18, 8350. (18) Chen, F.; Xu, G. Q.; Hor, T. S. A. Mater. Lett. 2003, 57, 3282. (19) Zhang, R.; Liu, J.; Han, B.; He, J.; Liu, Z.; Zhang, J. Langmuir 2003, 19, 8611.
10.1021/la061093m CCC: $33.50 © 2006 American Chemical Society Published on Web 10/11/2006
Au Nanoparticle Stabilization by Copolymer Micelles
Langmuir, Vol. 22, No. 23, 2006 9705
Table 1. Properties of the Pluronic Block Copolymers Used in This Study20
Pluronic
nominal formula
MW
[PEO] (wt %)
critical micellization temp for 5 wt % Pluronic (°C)
F108 F88 F68 F38 P105
(EO)132(PO)50(EO)132 (EO)103(PO)39(EO)103 (EO)76(PO)29(EO)76 (EO)44(PO)17(EO)44 (EO)37(PO)56(EO)37
14600 11400 8400 4800 6500
80 80 80 80 50
24.5 30.5 38 60 20
These characteristics have been successfully applied to tailor the properties of nanoparticles.19,30-33 Consequently, more insight into the relationship between gold colloidal stability and microenvironments of Pluronic micelles in Sakai’s method is helpful for further development of this promising synthetic method. Moreover, it may provide possible pathways to improve the stabilization of other types of nanoparticles. This work is aimed at addressing the importance of micellar microenvironments on stabilization. Using Sakai’s method, gold nanoparticles were prepared at relatively high Pluronic concentrations. Under such conditions, the reduction of metal ions took place very rapidly10-12 and stabilization was expected to be the dominant step for the quality of gold nanoparticles. The polymer concentration and composition and addition of salt were tuned to examine the stabilization capacity of micelles. FTIR and 1H NMR were used to probe the hydrophobicity inside Pluronic micelles to shed light on its effect on the stabilization of gold nanoparticles. In a forthcoming paper, dissipative particle dynamics (DPD) simulation,34 a mesoscale simulation method, will be applied to simulate the Pluronic-gold-water systems that were experimentally studied in this work. That type of simulation will provide detailed mesoscale information on the structure and dynamic process of gold colloids stabilized by Pluronic micelles, which are otherwise difficult to observe by experiment. Experimental Section Materials. PEO-PPO-PEO block copolymers were supplied by BASF. Table 1 gathers some of the properties of the copolymers used in this study. HAuCl4‚3H2O, 99.9+%) and NaF salt (analytical grade) were purchased from Aldrich. All materials were used as received without further purification. Synthesis of Gold Nanoparticles. Similar to Sakai’s method,9 gold nanoparticles were prepared by mixing 0.4 mL of aqueous 2 × 10-3 mol L-1 HAuCl4 solution with 4 mL of aqueous PEOPPO-PEO block copolymer solution with a desired concentration. (20) Alexandridis, P.; Holzwarth, J. F.; Hatton, T. A. Macromolecules 1994, 27, 2414. (21) Guo, C.; Wang, J.; Liu, H. Z.; Chen, J. Y. Langmuir 1999, 15, 2703. (22) Su, Y. L.; Wang, J.; Liu, H. Z. Langmuir 2002, 18, 5370. (23) Yang, L.; Alexandridis, P.; Steytler, D. C.; Kositza, M. J.; Holzwarth, J. F. Langmuir 2000, 16, 8555. (24) Goldmints, I.; von Gottberg, F. K.; Smith, K. A.; Hatton, T. A. Langmuir 1997, 13, 3659. (25) Goldmints, I.; Yu, G.; Booth, C.; Smith, K. A.; Hatton, T. A. Langmuir 1999, 15, 1651. (26) Su, Y. L.; Liu, H. Z.; Wang, J.; Chen, J. Y. Langmuir 2002, 18, 865. (27) Alexandridis, P.; Holzwarth, J. F. Langmuir 1997, 13, 6074. (28) Mao, G.; Sukumaran, S.; Beaucage, G.; Saboungi, M. L.; Thiyagarajan, P. Macromolecules 2001, 34, 552. (29) Jorgensen, E. B.; Hvidt, S.; Brown, W.; Schille’n, K. Macromolecules 1997, 30, 2355. (30) Meier, W. Curr. Opin. Colloid Interface Sci. 1999, 4, 6. (31) Lin, Y.; Alexandridis, P. J. Phys. Chem. B 2002, 106, 10834. (32) Lai, J.; Shafi, K. V. P. M.; Ulman, A.; Loos, K.; Lee, Y.; Vogt, T.; Lee, W.; Ong N. P. J. Phys. Chem. B 2005, 106, 15. (33) Karanikolos, G. N.; Alexandridis, P.; Itskos, G.; Petrou, A.; Mountziaris, T. J. Langmuir 2004, 20, 550. (34) Groot, R. D.; Warren, P. B. J. Chem. Phys. 1997, 107, 4423.
After agitation by a vortex mixer for about 10 s, the solutions were kept at 40 °C till the formation of gold nanoparticles had gone to completion, namely, the absorbance of the spectra did not vary any more. That took about 2 h. The polymer concentrations reported in this paper were those of the aqueous polymer solutions prior to mixing with the HAuCl4- solution. Salt solutions were prepared by dissolving salts in aqueous Pluronic solutions before mixing with HAuCl4 solution. Characterization of Gold Nanoparticles. Morphologies of the freshly prepared gold particles (obtained after 2 h of reaction, at which the reaction had gone to completion) were observed with a transmission electron microscope of type TECNAI 20 (FEI) operated at 100 keV. Samples were prepared by placing a drop of gold colloids on a Formvar-covered copper grid. Particle size distributions, numberaverage diameters, and standard derivations (SDs) were calculated by measuring the diameters of more than 300 particles for samples A, B, C, and c or 100 particles for the other samples from the corresponding transmission electron microscopy (TEM) micrographs. The optical properties of freshly prepared gold colloidal solutions were characterized with a Lambda Bio 40 UV-vis spectrometer (Perkin-Elmer) with a resolution of 2 nm. For the 450-800 nm wavelength regions, the UV-vis spectra were deconvoluted and then curve-fitted with Gaussian bands using Bruker OPUS software. FTIR and 1H NMR Spectra of Pluronic Solutions. FTIR spectra were recorded on a Bruker Vecter 22 FTIR spectrometer with a deuteriotriglycine sulfate detector and a resolution of 2 cm-1. The effect of temperature was studied from 2 to 50 °C. The temperature of the samples was measured by a thermocouple inserted in a stainless steel block that hosted a sample cell. The time for reaching the temperature equilibrium was about 2 min. 1H NMR experiments were conducted on a Bruker Avance 600 spectrometer at a Larmor frequency of 600.13 MHz for protons. 2,2-Dimethyl-2-silapentane-5-sulfonate sodium salt (DSS; g97%, purchased from Aldrich) was used as an external standard to ensure that the microenvironments of block copolymers in water and the effect of NaF salt could be correctly assessed. Chemical shifts (δ) were relative to the peak for the external standard DSS.
Results and Discussion The PEO-PPO-PEO block copolymer solutions used were chosen with the aim of studying the effects of micelle properties on the stabilization of gold nanoparticles. Under relatively high Pluronic concentrations, the reduction of metal ions took place very rapidly,10-12 as indicated by the disappearance of the absorption band centered at 220 nm. The stabilization was expected to be the dominant step for the quality of gold nanoparticles. It was concluded that the reduction of metal ions (step 1) was controlled primarily by the PEO content.10 The adsorption of the block copolymer on the initial clusters and the reduction of metal ions on the surface of those gold clusters (step 2) were related to the PPO content. Since Pluronic F108, F88, F68, and F38 had the same PEO/PPO ratio, their solutions with the same weight concentration had the same PPO and PEO contents. Therefore, steps 1 and 2 for those systems were expected to be similar, but the ability for those block copolymers to form micelle block copolymers was different.20 This provided an opportunity to study the effect of micelle characteristics on the stabilization step. Samples A-E were prepared using 20, 10, 5, 3, and 1 wt % P105 aqueous solution, respectively, and samples F-I were prepared using 5 wt % F108, F88, F68, and F38 aqueous solutions, respectively. It was reported26-29 that addition of salt to Pluronic solutions could significantly promote micellization and affect the microenvironments inside the micelles. Samples a-i were the analogues of samples A-I except that 0.7 M NaF was present in the former. They were used for studying the effect of a salt, NaF.
9706 Langmuir, Vol. 22, No. 23, 2006
Chen et al.
Figure 2. Photographs taken at room temperature with a digital camera of samples A-E (20, 10, 5, 3, and 1 wt % P105), F-I (5 wt % F108, F88, F68, and F38), and a-i (A-I in the presence of 0.7 M NaF).
Figure 1. Time-dependent UV-vis absorption spectrum of sample C at room temperature.
Evidence of the Three-Step Gold Nanoparticle Formation Mechanism. Figure 1 shows the evolution of the time-dependent UV-vis absorption spectrum of sample C that was typical of all other samples. The absorption band centered at 220 nm originated from the gold(III) chloride solution formed during the reaction16 and that at 538 nm from the surface plasma resonance (SPR) of the gold nanoparticles.35 As the reaction proceeded, the concentration of the gold(III) chloride decreased and that of the spherical gold colloids increased. An absorption centered at 325 nm also appeared after 2 min of reaction. It is better shown in the inset. The absorbance increased rapidly at the initial stage of the reaction (curves 1-3) and then disappeared after 20 min of reaction (curves 4 and 5). Since the absorption of the 5 wt % P105 aqueous solution was centered at about 280 nm, that of sample C at about 325 nm might correspond to gold clusters composed of only a few gold atoms that were formed in the initial stage of the nanoparticle formation process. Mosseri et al.36 and Longenberger et al.37 observed similar phenomena when they synthesized gold nanoparticles using different methods. The absorption centered around 300∼400 nm was direct evidence of the formation of primary gold clusters involved in the three-step formation mechanism proposed by Sakai et al.10 In what follows, we show the effects of the concentration and molecular weight of the copolymer (Pluronic) and that of the presence of salt on the morphology (shape and size) of the gold nanoparticles, the micelle microenvironments, and the mechanism of stabilization of gold particles inside the micelles. Various methods were used to assess those effects to have as complete a picture as possible on the systems under study. Color and Stability of the Colloidal Solutions. Figure 2 shows the photographs of all the fresh samples prepared in this work. From samples A-E, as the copolymer concentration decreased the color of the sample gradually shifted from red to purple, implying that the gold particle size increased. The latter was confirmed by UV-vis and TEM results, as will be shown later. After 2 days of storage at room temperature, precipitates were observed for samples D and E. In those samples, the initial P105 concentrations were the lowest, 3 and 1 wt %, respectively. Freshly prepared samples F and G were red, sample H was purple, and sample I was gray, implying that decreasing the (35) Liz-Marzan, L. M. Langmuir 2006 22, 32. (36) Mosseri, S.; Henglein, A,; Janata, E. J. Phys. Chem. 1989, 93, 6791. (37) Longenberger L.; Mills, G. J. Phys. Chem. 1995, 99, 475.
molecular weight of the copolymer seemed to have an effect similar to that of decreasing its concentration. After 2 days of storage at room temperature, cyan precipitates were observed for samples H and I. When NaF was added, the color of the samples was different. Apart from samples e and i, all other samples were red and were similar to sample A. Moreover, the samples with NaF were much more stable than those without NaF. In fact, no precipitates were observed even after 9 months of storage at ambient temperature when NaF was present. The formation of precipitates after 2 days of storage was direct evidence of poor stabilization in samples D, E, H, and I. The SPR bands in UV spectra of those samples slightly shifted to longer wavelength and became broader after precipitation. To get rid of the effect of the precipitation, all UV-vis analyses and TEM characterizations were performed using freshly prepared samples, i.e., after 2 h of reaction where the formation of gold nanoparticles had gone to completion.9-12 At the end of the particle formation, the UV absorbance remained almost unchanged and no precipitation appeared at that time. Figure 1 shows a typical time-dependent UV spectrum of the process. Structures of the Colloidal Particles Assessed by UV-Vis Spectroscopy. Figure 3 shows the UV-vis absorption spectra of samples A-E (a) and those of samples a-e (b). The absorbance around 540 nm is known to originate from the gold SPR, the position and shape of which are a function of the geometry of the particles.35 In the case of samples A and B, there was a symmetrical band centered at about 530 nm, indicating the formation of dispersed spherical nanoparticles with a narrow size distribution. As the concentration in P105 decreased, the absorbance red-shifted from ∼520 to ∼570 nm, suggesting an increase in diameter from ∼5 to 100 nm.35 Meanwhile, the shape of the SPR became more and more unsymmetrical and broader, likely because of particle aggregation and formation of anisotropic particles.38-41 Figure 4 shows the UV-vis absorption spectra of samples F-I (a) and those of samples f-i (b). For sample F, a band centered at ∼550 nm was observed. It was symmetrical, indicating that the nanoparticles were spherical and uniform. For samples H and I, in addition to a band centered at ∼550 nm, a new shoulder band appeared at a longer wavelength and was broad. The one centered at ∼550 nm originated from the transverse plasma resonance of the spherical nanoparticles with ∼30 nm size. The new band might have resulted from the longitudinal plasmon resonance of the gold particles that deviated from a spherical geometry.35 Yu et al. attributed the longitudinal plasmon (38) Wang, L.; Chen, X.; Zhan, J.; Chai, Y.; Yang, C.; Xu, L.; Zhuang, W.; Jing, B. J. Phys. Chem. B 2005, 109, 3189. (39) Jiang, C.; Markutsya, S.; Tsukruk, V. V. Langmuir 2004, 20, 882. (40) Cheng, W.; Dong, S.; Wang, E. Angew. Chem. 2003, 115, 465. (41) Yu, Y. Y.; Chang, S. S.; Lee, C. L.; Wang, C. R. C. J. Phys. Chem. B 1997, 101, 6662.
Au Nanoparticle Stabilization by Copolymer Micelles
Langmuir, Vol. 22, No. 23, 2006 9707
Figure 5. UV-vis spectrum of sample C (solid curve) in the range of 450-800 nm and its deconvolution (dotted curves). The peaks of the resulting three deconvoluted curves were located at 532, 580, and 660 nm. Table 2. Peak Area Fractions of Single Gold Particles (FS) in the Samples
Figure 3. UV-vis absorption spectra of samples A-E (a) and a-e (b).
Figure 4. UV-vis absorption spectra of samples F-I (a) and f-i (b).
resonance to gold nanorods.41 Shipaway et al. reported that gold nanoparticle aggregation might also result in longitudinal plasmon resonance due to interparticle plasmon coupling.42 Consequently, besides spherical particles, large anisotropic aggregates or rodlike
S
FS
S
FS
S
FS
S
FS
A B C D E
1.00 0.86 0.69 0.10 0.08
a b c d e
1.00 1.00 0.88 0.86 0.40
F G H I
0.73 0.44 0.47 0.20
f g h i
1.0 0.89 0.87 0.27
particles might also have existed in samples H and I, as will be confirmed by TEM images to be shown later. The situation of particles in sample G would be between those of particles in samples F and H (or I). When NaF was added, the SPR positions of all the samples a-d and f-h were shifted to ∼530 nm. Moreover, their shapes became much more symmetrical and were similar to that of the SPR of sample A. This infers that when a small amount of NaF was added to the Pluronic solutions, the quality of the resulting colloidal solutions was significantly improved and the colloidal particles were much more spherical and uniform in size. This was confirmed by TEM micrographs, as will be shown later. In other words, the presence of a small amount of NaF in the Pluronic solutions significantly improved the stabilization capacity of the Pluronic micelles. Consequently, spherical and uniform gold particles were obtained from Pluronic solutions whose copolymer concentrations were relatively low (samples c and d) or whose copolymer molecular weights were relatively low (samples g and h). To obtain more quantitative information on the ratio of single particles to aggregates in the samples, the UV-vis spectra were deconvoluted to possible band components. The unsymmetrical spectra of poorly stabilized samples contained two or three peaks in the wavelength range from 450 to 800 nm. They were assigned to single gold particles (the peak centered at ∼530 nm) and aggregates (the peak centered at ∼580 nm or/and longer wavelength), respectively. As an example, Figure 5 shows the UV spectrum of sample C and its deconvolution. Table 2 shows the area fractions of the peaks corresponding to single gold particles (FS) in the samples. Since the extinction coefficients of those systems were unknown, the area fractions were not equal but proportional to the population ratios of the single gold particles in the samples. They were also indicative of the degree of aggregation. From these data, it is obvious that the higher the stabilizing efficiency of Pluronic micelles, the higher the fraction of the single gold particles. This is in agreement with the TEM images and the size distributions of the particles shown in the subsequent section. (42) Shipway, A. N.; Lahav, M.; Gabai, R.; Willner, I. Langmuir 2000, 16, 8789.
9708 Langmuir, Vol. 22, No. 23, 2006
Chen et al.
Figure 6. TEM images and size distributions of samples A, B, C, and D (20, 10, 5, and 3 wt % P105).
Figure 7. TEM images and size distributions of samples F, G, H, and I (5 wt % F108, F88, F68, and F38).
Structures of the Colloidal Particles Assessed by TEM. TEM images provided direct visualization of the structures of the colloidal particles. Figure 6 shows the TEM images and size distributions of samples A-D. When the P105 concentration was 20 wt % (sample A) or 10 wt % (sample B), the resulting gold particles were spherical and uniform with number-average diameters of ∼6.0 nm. At lower P105 concentrations, part of the gold particles were aggregated. The aggregates ranged from 30 to 50 nm in diameter (sample C). In sample D, the particles were triangular or hexagonal platelike. Their equivalent diameters were about 70-100 nm. Figure 7 shows the TEM images and size distributions of samples F-I. In the case of F108 (sample F), gold particles were spherical and nicely dispersed. Their diameter ranged from 10 to 20 nm. In the case of F88 and F68 (samples G and H), most of the particles were still spherical and their average diameters were about 30 nm. However, some of them formed large anisotropic aggregates ranging from 50 to 80 nm or rodlike structures. As for F38 (sample I), the particles were highly
irregular in shape (rods, triangular plates, etc.) and polydisperse in size (ranging from 10 to 100 nm). Figure 8 shows the TEM images and size distributions of samples c, d, and f-i. Keep in mind that the gold particles of their analogues (samples C, D, and F-I) were either more or less aggregated or highly irregular in shape. Upon addition of a small amount of NaF, apart from sample i the gold particles of all the other samples were spherical, uniform in size, and nicely dispersed. Their average diameter was about 13 nm, which could not be obtained otherwise without NaF, except for sample A. In the latter, the copolymer concentration (P105) was very high, the highest used in this work (20 wt %). Thus, adding NaF provided a simple new and yet very efficient method to improving the state of dispersion, stability, and morphology of colloidal particles. However, this method was not always efficient. For example, in samples I and i gold particles were all strongly aggregated and highly irregular in shape. The former was without NaF and the latter with NaF. In summary, all the experimental results presented above have consistently shown that the stabilization capacities of different
Au Nanoparticle Stabilization by Copolymer Micelles
Langmuir, Vol. 22, No. 23, 2006 9709
Figure 9. Temperature-dependent evolution of the wavenumber of the C-O-C stretching vibration band in FTIR spectra of (a) 1, 3, 5, 10, and 20 wt % aqueous P105 solutions and (b) 5 wt % F38, F68, F88, and F108 aqueous solutions without (open symbols) or with (closed symbols) NaF.
Figure 8. TEM images and size distributions of samples c, d, and f-i.
Pluronic micelles were different. Efficient stabilization was necessary for obtaining reduced gold nanoparticles with uniform size and shape in a controlled manner. Otherwise irregular aggregates were formed. As the block copolymer concentration and/or molecular weight increased, its micelles were more efficient at stabilizing gold nanoparticles. The presence of a small amount of NaF greatly improved the stability of the colloidal solutions and modified the quality of the gold nanoparticles. Its efficiency was higher when the copolymer concentration or molecular weight was lower. The underlying mechanism that controls the colloidal stabilization will be discussed in the subsequent paragraph. Micelle Microenvironments Assessed by FTIR and 1H NMR. Our early studies21,22,26 showed that the frequency of the C-O-C stretching vibration band in FTIR spectra of the PEOPPO-PEO block copolymer was sensitive to the local polarity and conformation of block copolymer chains. Moreover, its variations with temperature could be indicative of micellization. The curves with solid symbols in Figure 9 show the temperaturedependent variation of the wavenumber of the C-O-C band in FTIR spectra of the aqueous P105 solutions with different P105
9710 Langmuir, Vol. 22, No. 23, 2006
concentrations (a) and that of 5 wt % F38, F68, F88, and F108 aqueous solutions (b). The curves with open symbols correspond to the above solutions to which 0.7 M NaF was added. As the temperature increased, the C-O-C stretching band shifted to a higher wavenumber and showed a reverse sigmoid dependence on temperature. The lower wavenumber of the band (∼1085 cm-1) indicates that the copolymers were dissolved in water as unimers, while the higher wavenumber of the band (∼1094 cm-1) suggests that the copolymers aggregated to form stable micelles. The presence of NaF significantly shifted the band to higher frequency, implying that it greatly promoted micelle formation. On the basis of Figure 9, at the temperature at which the gold particles were synthesized (40 °C), there was no micellization in the Pluronic solutions used in the preparation of samples D, E, H, I, and i. Therefore, the gold particles obtained from those solutions were expected to be of poor quality (aggregation and irregular shapes). This is consistent with the above results. However, samples A-C, F, G, and a-h prepared from Pluronic solutions with stable micelles still showed very different stabilities. The reason might be related to the microenvironments inside the micelles. It was known that PEO-PPO-PEO block copolymer would form spherical micelles with relatively compact cores in hydrophobic PO blocks surrounded by a corona or shell of EO segments with strong hydration.20-25 In this study, the protons of the PO -CH3 groups were investigated by 1H NMR to probe the microenvironment and conformation of such micelles.43-45 An external standard (DSS) was used in NMR experiments to ensure that the microenvironments of the block copolymers in water and the effect of NaF salt could be correctly assessed. The chemical shift of DSS was calibrated to 0 ppm, and the sharp peak around 1.16 ppm was assigned to the protons of the PO -CH3 groups. Figure 10 shows the PO -CH3 chemical shifts in 1H NMR of aqueous Pluronic solutions used in samples A-I (closed symbols) and those used in samples a-i (open symbols) at 40 °C. As the P105 concentration or the molecular weight of the Pluronic increased, the chemical shifts of the PO -CH3 protons decreased drastically. Numerous studies showed that the formation of C-H‚‚‚O hydrogen bonds enhanced the deshielding effect of protons and led to 1H downfield chemical shifts. Therefore, the upfield chemical shift of C-H protons might be related to the breakdown of the intermolecular hydrogen bonds between the C-H protons in PO segments and water oxygen atoms.46,47 The lower chemical shift of PO -CH3 protons indicates more hydrophobic microenvironments in Pluronic micelle cores. The addition of NaF significantly decreased the chemical shifts of PO -CH3 protons, suggesting that the block copolymers formed more hydrophobic micelle cores. Moreover, the -CH3 chemical shifts of the copolymer solutions that showed good stabilization capacities (samples A, B a-d, and f-h in which gold particles were spherical and in uniform size) were all decreased to a minimum of about 1.09 ppm. This suggested that the micelle cores in those solutions were the most hydrophobic. The wavelengths of the SPR of the gold colloids were also plotted on the right axes of Figure 10. They followed the same trends as the PO -CH3 chemical shifts. (43) Nivaggioli, T.; Tsao, B.; Alexandridis, P.; Hatton, T. A. Langmuir 1995, 11, 119. (44) Wanka, G.; Hoffmann, H.; Ulbricht, W. Macromolecules 1994, 27, 4145. (45) Zeghal, M.; Auvray, L. Eur. Phys. J. E 2004, 14, 259. (46) Friebolin, H. P. Basic One- and Two-Dimensional NMR Spectroscopy; VCH Publishers: New York, 1991. (47) Jeffrey, G. A. An Introduction to Hydrogen Bonding; Oxford University Press: Oxford, U.K., 1997.
Chen et al.
Figure 10. 1H NMR -CH3 chemical shifts of Pluronic solutions at 40 °C and wavelength of the SPR in UV-vis spectra of the resulting gold colloidal solutions: (a) 1, 3, 5, 10, and 20 wt % aqueous P105 solutions and (b) 5 wt % F38, F68, F88, and F108 aqueous solutions in the absence (open symbols) or presence (closed symbols) of 0.7 M NaF.
The above results show that micellization of the block copolymers and hydrophobic microenvironments of the micelles were crucial for the stabilization of gold nanoparticles. Since the hydrophobic PO blocks of the polymer tended to adsorb onto the surface of the primary gold clusters,10-12 a higher hydrophobicity inside the micelle cores was more favorable for the entrapment of those gold clusters and thus resulted in a better stabilization of the gold nanoparticles.
Conclusions We have reported on the effects of PEO-PPO-PEO block copolymer micelles on the stabilization of gold nanoparticles prepared by a method developed by Sakai et al. Real-time UVvis spectra indicated the formation of primary gold clusters composed of a few gold atoms, evidence that supported the threestep gold nanoparticle formation mechanism they proposed. The stabilization capacities of the micelles were modulated by tuning the block copolymer concentration and composition and by adding salts. As the block copolymer concentration and/or molecular weight increased, the micelles became more efficient in stabilizing gold nanoparticles. When the stabilization was good, gold nanoparticles were spherical and uniform in size (5-10 nm in diameter). Otherwise, gold particles were aggregated and irregular in shape such as triangular, hexagonal, and rodlike. The presence of a small amount of NaF salt in the Pluronic solutions significantly improved the stabilization capacity of the micelles. As a result, spherical and uniform gold nanoparticles were also obtained for Pluronic solutions whose Pluronic concentration or molecular weight was relatively low. This method provided an economic way to improve the dispersion and stabilization of gold nanoparticles.
Au Nanoparticle Stabilization by Copolymer Micelles
Using FTIR and 1H NMR spectroscopy, micellization of the block copolymers and hydrophobicity of the micelles were proved very important for the stabilization of gold nanoparticles. The higher the hydrophobicity inside the micelle cores, the more efficient the stabilization and thus the more spherical and uniform the gold nanoparticles. It is hoped that that the results will be useful for stabilizing or modifying other types of nanoparticles with a Pluronic-type block copolymer.
Langmuir, Vol. 22, No. 23, 2006 9711
Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 20221603, 20273075, and 20490200), the Scientific Research Foundation for the Returned Overseas Chinese Scholars of the State Ministry of Education, and the Chinese Academy of Sciences for international cooperation. LA061093M
