Langmuir 2008, 24, 9215-9218
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Architecture of Linear Arrays of Fluorinated Co-oligomeric Nanocomposite-Encapsulated Gold Nanoparticles: A New Approach to the Development of Gold Nanoparticles Possessing an Extremely Red-Shifted Absorption Characteristic Masaki Mugisawa and Hideo Sawada* Department of Frontier Materials Chemistry, Graduate School of Science and Technology, Hirosaki UniVersity, Hirosaki 036-8561, Japan ReceiVed May 15, 2008. ReVised Manuscript ReceiVed June 25, 2008 Fluoroalkyl end-capped co-oligomeric nanoparticles, which were prepared by the reaction of fluoroalkanoyl peroxide with 2-acrylamido-2-methylpropanesulfonic acid (AMPS) and 1-hydroxy-5-adamantylacrylate (Ad-HAc), were applied to the preparation of novel fluorinated co-oligomeric nanocomposite-encapsulated gold nanoparticles. These fluorinated gold nanocomposites were easily prepared by the reductions of gold ions with poly(methylhydrosiloxane) (PMHS) in the presence of the corresponding fluorinated nanoparticles and tri-n-octylamine (TOA) in 1,2-dichloroethane (DE) at room temperature. These fluorinated gold nanoparticles were isolated as wine-red powders and were found to exhibit good dispersibility in a variety of traditional organic solvents such as DE, methanol, and t-butyl alcohol to afford transparent wine-red solutions. The morphology and stability of these fluorinated co-oligomeic nanocompositeencapsulated gold nanoparticles were characterized using transmission electron microscopy (TEM), dynamic light scattering measurements (DLS), and UV-vis spectroscopy. DLS measurements and UV-vis spectroscopy showed that these particles are nanometer-size-controlled very fine nanoparticles (185-218 nm) that exhibit a plasmon absorption band at around 530 nm. TEM images also showed that gold nanoparticles are tightly encapsulated into fluorinated co-oligomeric nanoparticle cores. Interestingly, these fluorinated co-oligomeric nanocomposites-encapsulated gold nanoparticles were found to afford linear arrays of these fluorinated nanoparticles with increases in the feed amounts of TOA. More interestingly, these fluorinated gold nanoparticles were able to afford the extremely red-shifted plasmon absorption band at around 960 nm.
Introduction Nanometer-size-controlled metal particles are attractive materials because of their enhanced electrical, magnetic, optical, and catalytic properties.1 In these metal nanoparticles, gold nanoparticles are the most stable and have been widely studied from a higher applicable viewpoint in a variety of fields such as catalysis, electronics, biomedicine, and optical materials.2 The functionalization of gold nanoparticles has also been developed through specific binding by the use of thiol groups and electrostatic interactions via layer-by-layer deposition.3 In particular, polymercoated metal nanoparticles offer interesting prospects in a wide spectrum of applications, ranging from catalysis to cosmetics, inks, and paints.4 In fact, there have been a variety of reports on the formation of metal nanoparticles on the surface of matrix polymers and the incorporation of metal nanoparticles into a polymer matrix.5 Block copolymers and star-shaped copolymers have also been reported to be potential encapsulators of gold * Corresponding author. Tel: +81-172-39-3578. Fax: +81-172-39-3578. E-mail:
[email protected]. (1) (a) Sun, Y.; Xia, Y. Science 2002, 298, 2176–2179. (b) Alivisatos, A. P. Science 1996, 271, 933–937. (c) Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989–1992. (d) Van Dijk, M. A.; Lippitz, M.; Orrit, M. Acc. Chem. Res. 2005, 38, 594–601. (e) Campbell, C. T. Science 2004, 306, 234–235. (f) Crooks, R. M.; Zhao, M.; Sun, L.; Chechik, V.; Yeung, L. K. Acc. Chem. Res. 2001, 34, 181–190. (2) (a) Remediakis, I. N.; Lopez, N.; Norskov, J. K. Angew. Chem., Int. Ed. 2005, 44, 1824–1826. (b) Thomas, K. G.; Kamat, P. V. Acc. Chem. Res. 2003, 36, 888–898. (3) (a) Daniel, M.-C.; Astruc, D. Chem. ReV. 2004, 104, 293–346. (b) Gittins, D. I.; Caruso, F. AdV. Mater. 2000, 12, 1947–1949. (4) Caruso, F., Ed. Colloids and Colloid Assemblies; Wiley-VCH: Weinheim, Germany, 2004. (5) (a) Forster, S.; Antonietti, M. AdV. Mater. 1998, 10, 195–217. (b) Chen, C. W.; Serizawa, T.; Akashi, M. Chem. Mater. 2002, 14, 2232–2239. (c) Pothukuchi, S.; Li, Y.; Wong, C. P. J. Appl. Polym. Sci. 2004, 93, 1531–1538.
nanoparticles and to be efficient templates for stabilizing gold nanoparticles, respectively.6,7 However, it is well known that fluorinated polymers, in particular, fluoroalkyl end-capped oligomers, are attractive functional materials because they exhibit various unique properties such as high solubility, surface-active properties, biological activity, and nanometer-size-controlled selfassembled molecular aggregates that cannot be achieved by the corresponding nonfluorinated, random, or block-type fluoroalkylated polymers and low-molecular-weight fluorinated surfactants.8 In addition, very recently, we succeeded in preparing fluoroalkyl end-capped co-oligomeric nanoparticles containing betaine-type and adamantyl segments.9 Therefore, it is of particular interest to prepare new fluorinated oligomeric nanocomposite-encapsulated gold nanoparticles in oligomeric particle cores from the developmental viewpoint of new fluorinated metal nanocomposite materials. In this letter, we would like to demonstrate the preparation of new fluoroalkyl end-capped co(6) (a) Kang, Y.; Taton, T. A. Angew. Chem., Int. Ed. 2005, 44, 409–412. (b) Kang, Y.; Erickson, K. J.; Taton, T. A. J. Am. Chem. Soc. 2005, 127, 13800– 13801. (7) (a) Filali, M.; Meier, M. A. R.; Schubert, U. S.; Gohy, J. F. Langmuir 2005, 21, 7995–8000. (b) Lowe, A. B.; Sumerlin, B. S.; Donovan, M. S.; McCormick, C. L. J. Am. Chem. Soc. 2002, 124, 11562–11563. (8) (a) Sawada, H. Chem. ReV. 1996, 96, 1779–1808. (b) Sawada, H. J. Fluorine Chem. 2000, 105, 219–220. (c) Sawada, H. Prog. Polym. Sci. 2007, 32, 509–533. (d) Sawada, H. Polym. J. 2007, 39, 637–650. (9) Fluoroalkanoyl peroxide: (RFCOO)2 can decompose homolytically with three-bond radical fission to afford an electron-poor RF radical. Under our preparative conditions for the RF-(AMPS)x-(Ad-HAc)y-RF co-oligomer, in which the concentration of the peroxide (3.0 mmol) was almost the same as that of the AMPS monomer (1.9 mmol) or the Ad-HAc monomer (17.7 mmol), mainly AMPS-Ad-HAc co-oligomer with two fluoroalkyl end groups can be obtained via primary radical termination through the electron-poor RF radical or radical chain transfer to the peroxide. See (a) Mugisawa, M.; Ohnishi, K.; Sawada, H. Langmuir 2007, 23, 5848–5851, and ref 8a.
10.1021/la8015035 CCC: $40.75 2008 American Chemical Society Published on Web 08/05/2008
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Scheme 1. Preparation of RF-(AMPS)x-(Ad-HAc)y-RF/Au Nanocomposites
oligomeric nanocomposite-encapsulated gold nanoparticles in these oligomeric cores. In particular, we have also found that these fluorinated co-oligomeric nanoparticles can form linear arrays of oligomeric nanoparticles by the encapsulation of gold nanoparticles into these particle cores to exhibit an extremely red-shifted plasmon absorption characteristic related to the presence of gold nanoparticles. We believe that this is the first example of the extremely red-shifted plasmon absorption characteristic of gold nanoparticles.
Results and Discussion Fluoroalkyl end-capped co-oligomeric nanoparticles containing both betaine-type and adamantyl segments [RF-(AMPS)x-(AdHAc)y-RF], which were prepared by the reaction of fluoroalkanoyl peroxide with 2-acrylamido-2-methylpropanesulfonic acid (AMPS) and 1-hydroxy-5-adamantylacrylate (Ad-HAc),9 was able to afford the corresponding fluorinated co-oligomeric nanocomposites-encapsulated gold nanoparticles [RF-(AMPS)x-(AdHAc)y-RF/Au nanocomposites] in good isolated yields: 72-86% by using PMHS as a reducing agent and TOA as a catalyst in DE as shown in Scheme 1. Interestingly, fluorinated gold nanocomposites thus obtained could be quantitatively recovered as wine-red powders from their dispersed DE solutions at room temperature by simple centrifugal separation (2000 rpm/30 min), and the particle powders were easily redispersed not only in DE but also in a variety of organic solvents such as methanol and t-butyl alcohol to afford the winered solutions, respectively. The contents of gold nanoparticles in these nanocomposites in Scheme 1 were estimated to be 6-9% by the use of thermogravimetric analysis. DLS measurements showed that the size of these nanocomposites was found to increase from 167 nm (number-average diameter of the parent fluorinated co-oligomeric nanoparticles) to 218-243 nm in methanol by the composite reactions, indicating that gold nanoparticles should be effectively encapsulated into fluorinated oligomeric cores. UV-vis spectrum of the obtained RF-(AMPS)x(Ad-HAc)y-RF/Au nanocomposites (run 1 in Scheme 1) in methanol showed a well-defined sharp plasmon absorption band around 533 nm related to the formation of stable gold nanoparticles (Figure 1a). It is well known that the aggregates of gold nanoparticles, which were prepared by the use of ethylenediamine as a (10) Rautaray, D.; Kavathekar, R.; Sastry, M. Faraday Discuss. 2005, 129, 205–217.
bifunctional linker, could afford the longer-wavelength absorption band around 640 nm due to excitation of the plasmon vibration from these aggregates.10 We have very recently reported that the size of the parent RF-(AMPS)x-(Ad-HAc)y-RF co-oligomeric nanoparticles is extremely sensitive to the solvent dielectric constant () and temperature changes.9 Therefore, it is strongly expected that the plasmon absorption band of encapsulated gold nanoparticles into RF-(AMPS)x-(Ad-HAc)y-RF co-oligomeric particle cores would be effectively shifted by a change in the solvent dielectric constant or temperature. In fact, we prepared well-dispersed RF-(AMPS)x-(Ad-HAc)y-RF/Au nanocomposites in a variety of solvents possessing different dielectric constants (DE, ) 10.4; t-butyl alcohol, ) 12.2; methanol, ) 32.6), and the UV-vis spectra of these well-dispersed nanocomposite solutions were measured (Figure 2). Unexpectedly, we could not detected any red- or blue-shifted plasmon absorption bands in these solvents at all, although dramatic changes in the size of these fluorinated nanocomposites were observed from 144 nm (in methanol) to 167 nm (in t-butyl alcohol) or 390 nm (in DE) (Figure 2). We also reported that RF-(AMPS)x-(Ad-HAc)y-RF co-oligomeric nanoparticles could exhibit the lower critical solution temperature (LCST) characteristic around 60 °C in organic media such as t-butyl alcohol to afford a dramatic increase in fluorinated co-oligomeric nanoparticle size around the LCSTs compared to
Figure 1. UV-vis spectra of RF-(AMPS)x-(Ad-HAc)y-RF/Au nanocomposites (1.67 g/dm3) in MeOH solutions (a) run 1, (b) run 2, (c) run 3, (d) run 4, and (e) run 5 in Scheme 1.
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Figure 4. Photographs of methanol solutions of RF-(AMPS)x-(Ad-HAc)yRF/Au nanocomposites (1.67 g/dm3) (a) run 1, (b) run 2, (c) run 3, (d) run 4, and (e) run 5 in Scheme 1.
Figure 2. UV-vis spectra of RF-(AMPS)x-(Ad-HAc)y-RF/Au nanocomposites in a variety of solvents (0.5 g/dm3).
Figure 3. UV-vis spectra of t-butyl alcohol solutions of 2 g/dm3 RF(AMPS)x-(Ad-HAc)y-RF/Au nanocomposites at 30 °C (a), 80 °C (b), and 30 °C after cooling (c).
that below the LCSTs.9 Thus, we have studied the relationship between the plasmon absorption bands of RF-(AMPS)x-(AdHAc)y-RF/Au nanocomposites and the temperature changes (Figure 3). A dramatic increase in the size of RF-(AMPS)x-(Ad-HAc)yRF/Au nanocomposites (run 2 in Scheme 1) from 167 to 380 nm in t-butyl alcohol was observed with increasing temperature from 30 to 80 °C; however, UV-vis spectra of the composites at 80 °C showed the same plasmon absorption band at 533 nm as that of the composites at 30 °C. In addition, no changes in the plasmon absorption band at 533 nm were observed even after cooling from 80 to 30 °C. These findings suggest that encapsulated gold nanoparticles should be well-dispersed and almost uniform size in the oligomeric particle cores to exhibit the intense sharp plasmon absorption band around 530 nm even for changes in environmental parameters such as the solvent dielectric constant and temperature. Interestingly, an extremely red-shifted plasmon absorption band around 960 nm was newly observed by increasing the amount of TOA from 67 to 100 or 300 µmol (Figure 1c,d), and this absorption band around 960 nm was remarkably decreased by the addition of 400 µmol of TOA (Figure 1e). These dramatically red-shifted absorption bands in Figure 1 are also evident from the difference in color of methanol solutions of Figure 1a,b (transparent wine-red solution) and Figure 1c,d (transparent, slightly blue solutions) (Figure 4).
Figure 5. TEM (transmission electron microscopy) images of RF(AMPS)x-(Ad-HAc)y-RF/Au nanocomposites in methanol.
To clarify this interesting behavior, we recorded TEM (transmission electron microscopy) photographs of methanol solutions of these RF-(AMPS)x-(Ad-HAc)y-RF/Au nanocomposites, and these results are shown in Figure 5. The electron micrograph also shows the formation of RF(AMPS)x-(Ad-HAc)y-RF/Au nanocomposite fine particles (with mean diameters of (a) 253, (b) 212, (c) 175, (d) 206, and (e) 236 nm in Figure 5). These values are quite similar to those of DLS measurements (number-average diameters of (a) 218, (b) 185, (c) 188, (d) 243, and (e) 241 nm in Scheme 1). In the feed amounts of TOA ((a) 50 and (b) 67 µmol in Figure 2), TEM images showed that gold nanoparticles encapsulated in fluorinated co-oligomeric nanoparticle cores were well dispersed independently and homogeneously to exhibit an intense, sharp plasmon absorption band around 530-533 nm related to the formation of stable gold nanoparticles. Surprisingly, in the cases of the feed amounts of TOA (100 and 300 µmol), we could observe the self-association of RF(AMPS)x-(Ad-HAc)y-RF/Au nanocomposites to afford stable fluorinated nanocomposite linear arrays (Figure 1c,d). In particular, it is suggested that these encapsulated gold nano-
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particles in oligomeric cores should afford the architecture of nanorod-like self-assemblies of gold nanoparticles derived from nanocomposites without any composite particle agglomeration. Moderate amounts (100-300 µmol) of TOA are essential for the architecture of such nanorod-like assemblies because TOA should act as a colloidal stabilizer of gold nanoparticles to conjugate RF-(AMPS)x-(Ad-HAc)y-RF/Au nanocomposites to each other. Such nanorod-like self-assemblies of gold nanoparticles should afford the extremely red-shifted plasmon absorption band around 960 nm due to the abnormally extended gold nanoparticle assemblies. Therefore, an additional amount (400 µmol) of TOA should afford the agglomeration of RF-(AMPS)x-(Ad-HAc)yRF/Au nanocomposites to decrease the plasmon absorption band remarkably around 960 nm. It is well known that gold nanorods rather than gold nanoparticles exhibit a long-wavelength plasmon band in the 600-900 nm region.11 However, our self-assemblies of gold nanoparticles are quite different from the traditional gold nanorods, and each gold nanoparticle is arranged regularly to construct a linear arrays of gold nanoparticles that are similar to gold nanorods. Quite recently, Lassiter et al. reported that individual nanoparticle dimers in directly adjacent pairs and touching geometries show a new long-wavelength plasmon mode arising from charge-transfer oscillations.12 Therefore, it is suggested that our present extremely red-shifted peak corresponds to the plasmon mode arising from charge transfer oscillations when adjacent nanoparticles are located at distances as close as the linear arrays. Hitherto, the formation of extremely long fibrous aggregates (average width 90 nm, length 12 mm) of 1H,1H,2H,2H(11) (a) Karg, M.; Pastoriza-Santos, I.; Perez-Juste, J.; Hellweg, T.; Liz-Marzan, L. M. Small 2007, 3, 1222–1229. (b) Jana, N. R. Small 2005, 1, 875–882. (c) Das, M.; Mordoukhovski, L.; Kumaceva, E. AdV. Mater. 2008, 20, 23712375. (d) Wei, Q.; Ji, J.; Shen, J. Macromol. Rapid Commun. 2008, 29, 645–650. (12) Lassiter, J. B.; Aizpurua, J.; Hernandez, L. I.; Brandl, D. W.; Romero, I.; Lal, S.; Hafner, J. H.; Nordlander, P.; Halas, N. J. Nano Lett. 2008, 8, 1212– 1218.
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perfluorodecanethiol-stabilized silver nanoparticles has already been reported, and these aggregates showed a considerably broadened red-shifted peak from 420 to 450 nm.13 3-Heptadecafluorooctylsulfonylaminopropyltrimethylammonium iodine was also used as an assembling agent to form dendrimer-like gold nanoparticle assemblies with a red-shifted plasmon absorption band at around 670 nm.14 However, to the best our knowledge, our finding is the first example of the extremely red-shifted plasmon absorption behavior of gold nanoparticles. Further studies on the preparation and properties of fluoroalkyl end-capped cooligomeric nanocomposites-encapsulated gold nanoparticles are actively in progress.
Experimental Section A typical procedure for the preparation of fluoroalkyl end-capped co-oligomeric nanocomposites consisting of [RF-(AMPS)x-(AdHAc)y-RF]-encapsulated gold nanoparticles is as follows: To a 1,2dichloroethane (DE) (25 mL) solution of RF-(AMPS)x-(Ad-HAc)yRF co-oligomeric nanoparticles (RF ) CF(CF3)OC3F7; 50 mg; 167 nm average particle size determined by DLS), which were prepared according to our previously reported method,9 was added HAuCl4 (4.0 µmol DE solution, 5 mL). The mixture was stirred with a magnetic bar at room temperature for 12 h. Poly(methylhydrosiloxane) (30 µmol) and tri-n-octylamine (50 µmol) were mixed with this solution, and then the mixture was stirred for 3 h at room temperature to afford the wine-red solutions. The UV-vis spectrum of this solution afforded a plasmon absorption band at around 530 nm that was related to the formation of gold nanoparticles. We could easily isolate the wine-red gold nanocomposite powders after the centrifugal separation (2000 rpm/30 min) of this solution at room temperature, and the plasmon absorption peak was not detected at all in the supernatant solution. These composite powders were easily redispersed into not only DE but also traditional organic media such as methanol and t-butyl alcohol to afford a similar wine-red solution with the plasmon absorption band at around 530 nm. LA8015035 (13) Yonezawa, T.; Onoue, S.; Kimizuka, N. AdV. Mater. 2001, 13, 140–142. (14) Pang, S.; Kondo, T.; Kawai, T. Chem. Mater. 2005, 17, 3636–3641.
