Photothermal Reshaping of Gold Nanorods Depends on the

Aug 30, 2008 - Department of Applied Chemistry, Graduate School of Engineering, Kyushu UniVersity, Moto-oka,. Fukuoka 819-0395, Japan, and Department ...
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Langmuir 2008, 24, 12026-12031

Photothermal Reshaping of Gold Nanorods Depends on the Passivating Layers of the Nanorod Surfaces Yukichi Horiguchi,† Kanako Honda,‡ Yuichi Kato,† Naotoshi Nakashima,† and Yasuro Niidome*,† Department of Applied Chemistry, Graduate School of Engineering, Kyushu UniVersity, Moto-oka, Fukuoka 819-0395, Japan, and Department of Biological and EnVironmental Chemistry, Kinki UniVersity-Kyushu, 11-6 Kayanomori, Iizuka 820-8555, Japan ReceiVed March 14, 2008. ReVised Manuscript ReceiVed July 29, 2008 Photothermal reshaping of gold nanorods was triggered by pulsed-laser irradiation. The efficiency of the reshaping was strongly dependent on the surface conditions of the gold nanorods. When the gold nanorods were dispersed in concentrated hexadecyltrimethylammonium bromide (CTAB), the gold nanorods were efficiently transformed into a φ-shape. By comparison when poly(styrene sulfonate), poly(vinylpyrrolidone), poly(ethylene glycol), or phosphatidylcholine layers were used, the CTAB layers were found to be a better thermal insulator that helped to enhance the photothermal reshaping of the gold nanorods.

Introduction Gold nanorods have been an attractive research subject due to their two distinctive extinction bands that are assignable to transverse and longitudinal modes of surface plasmon (SP) oscillation.1-3 The transverse SP band is located around 520 nm, while the longitudinal SP band is observed in the near-infrared (near-IR) regions. Because of the longitudinal SP bands in the near-IR regions, the gold nanorods are expected to be novel functional materials for analytical,4-11 photofunctional,2,12-14 and biochemical15-24 applications. The properties of the SP bands * To whom correspondence should be addressed. E-mail: ynidotcm@ mail.cstm.kyushu-u.ac.jp. † Kyushu University. ‡ Kinki University-Kyushu. (1) Yu, Y.-Y.; Chang, S.-S.; Lee, C.-L.; Wang, C. R. C. J. Phys. Chem. B 1997, 101, 6661. (2) van der Zande, B. M. I.; Bo¨hmer, M. R.; Fokkink, L. G. J.; Scho¨neberger, C. J. Phys. Chem. B. 1997, 101, 852. (b) van der Zande, B. M. I.; Koper, G. J. M.; Lekkerkerker, H. N. W. J. Phys. Chem. B 1999, 103, 5754. (c) van der Zande, B. M. I.; Pages, L.; Hikmet, R. A. M.; Blaaderen, A. J. Phys. Chem. B 1999, 103, 5761. (3) (a) Link, S.; El-Sayed, M. A. Int. ReV. Phys. Chem. 2000, 19, 409. (b) Pe´rez-Juste, J.; Pastoriza-Santos, I.; Liz-Marza´n, L. M.; Mulvaney, P. Coord. Chem. ReV. 2005, 249, 1870. (c) Ni, W.; Kou, X.; Yang, Z.; Wang, J. ACS Nano 2008, 2, 677. (4) Nikoobakht, B.; Wang, J.; El-Sayed, M. A. Chem. Phys. Lett. 2002, 366, 17. (5) Nikoobakht, B.; El-Sayed, M. A. J. Phys. Chem. A 2003, 107, 3372. (6) Suzuki, M.; Niidome, Y.; Terasaki, N.; Inoue, K.; Kuwahara, Y.; Yamada, S. Jpn. J. Appl. Phys. 2004, 43, L554. (7) Niidome, Y.; Takahashi, H.; Urakawa, S.; Nishioka, K.; Yamada, S. Chem. Lett. 2004, 33, 454. (8) Hu, X.; Cheng, W.; Wang, T.; Wang, Y.; Wang, E.; Dong, S. J. Phys. Chem. B 2005, 109, 19385. (9) Rex, M.; Hernandez, F. E.; Campiglia, A. D. Anal. Chem. 2006, 78, 445. (10) Orendorff, C. J.; Gearheart, L.; Jana, N. L.; Murphy, C. J. Phys. Chem. Chem. Phys. 2006, 8, 165. (11) Ueno, K.; Juodkazis, S.; Mino, M.; Mizeikis, M.; Misawa, H. J. Phys. Chem. C 2007, 111, 4180. (12) Pe´rez-Juste, J.; Rodrı´guez-Gonza´lez, B.; Mulvaney, P.; Liz-Marza´n, M. AdV. Funct. Mater. 2005, 15, 1065. (13) Niidome, Y.; Urakawa, S.; Kawahara, M.; Yamada, S Jpn. J. Appl. Phys 2003, 42, 1749. (14) (a) Das, M.; Sanson, N.; Fava, D.; Kumacheva, E. Langmuir 2007, 23, 196. (b) Shiotani, A.; Mori, T.; Niidome, T.; Niidome, Y.; Katayama, Y. Langmuir 2007, 23, 4012. (15) (a) Takahashi, H.; Niidome, Y.; Yamada, S. Chem. Commun. 2005, 2247. (b) Horiguchi, Y.; Niidome, T.; Yamada, S.; Nakashima, N.; Niidome, Y. Chem. Lett. 2007, 36, 952. (16) Takahashi, H.; Niidome, Y.; Niidome, T.; Kaneko, K.; Kawasaki, H.; Yamada, S. Langmuir 2006, 22, 2.

of gold nanorods have been extensively studied by El-Sayed and co-workers.25-30 They proposed theoretical models of the SP oscillation of gold nanorods and indicated that discrete dipole approximation was a good model to fit the experimental data.30,31 Photothermal dynamics of gold nanorods after pulsed-laser irradiation have been studied using ultrafast transient absorption spectroscopies.25,27,28 It was shown that photoexcitation of SP bands generated excited states of free electrons in the gold nanorods and then the electron excited states relaxed into phonons. The time constant of the electron-phonon relaxation was a few picoseconds.25,26 Then, the phonon relaxed and diffused as heat within 100 ps. Even though the nanorods showed photoluminescence in the visible region, their efficiency was low (10-3-10-4).22,29 That is, almost all the photons absorbed in the gold nanorods were transformed into heat. The heat then induced (17) (a) Takahashi, H.; Niidome, T.; Nariai, A.; Niidome, Y.; Yamada, S. Chem. Lett. 2006, 35, 500. (b) Takahashi, H.; Niidome, T.; Nariai, A.; Niidome, Y.; Yamada, S. Nanotechnology 2006, 17, 4431. (18) Huang, X.; El-Sayed, I. H.; Qian, W.; El-Sayed, M. A. J. Am. Chem. Soc. 2006, 128, 2115. (19) Chen, C.-C.; Lin, Y.-P.; Wang, C.-W.; Tzeng, H.-C.; Wu, C.-H.; Chen, Y.-C.; Chen, C.-P.; Chen, L.-C.; Wu, Y.-C. J. Am. Chem. Soc. 2006, 128, 3709. (20) Wang, C.; Ma, Z.; Wang, T.; Su, Z. AdV. Funct. Mater. 2006, 16, 1673. (21) (a) Niidome, T.; Yamagata, M.; Okamoto, Y.; Akiyama, Y.; Takahashi, H.; Kawano, T.; Katayama, Y.; Niidome, Y. J. Controlled Release 2006, 114, 343. (b) Niidome, T.; Akiyama, Y.; Shimoda, K.; Kawano, T.; Mori, T.; Katayama, Y.; Niidome, Y. Small 2008, 4, 1001. (22) (a) Wang, H.; Huff, T.-B.; Zweifel, D.-A.; He, W.; Low, P.-S.; Wei, A.; Cheng, J.-X. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 15752. (b) Huff, T. B.; Hansen, M. N.; Zhao, Y.; Cheng, J.-X.; Wei, A. Langmuir 2007, 23, 1596. (23) (a) Liao, H.; Hafner, J. H. Chem. Mater. 2005, 17, 4636. (b) Huff, T. B.; Tong, L.; Zhao, Y.; Hansen, M. N.; Cheng, J.-X.; Wei, A. Nanomedicine 2007, 2, 125. (24) (a) Takahashi, H.; Niidome, T.; Kawano, T.; Niidome, Y.; Yamada, S. J. Nanopart. Res. 2007, 10, 221. (b) Hauck, T. S.; Ghazani, A. A.; Chan, W. C. W. Small 2008, 4, 153. (25) Link, S.; Burda, C.; Nikoobakht, B.; El-Sayed, M. A. Chem. Phys. Lett. 1999, 315, 12. (26) (a) Link, S.; Mohamed, M. B.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 3073. (b) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 2005, 109, 10531. (27) (a) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 8410. (b) El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257. (28) Link, S.; Burda, C.; Mohamed, M. B.; Nikoobakht, B.; El-Sayed, M. A. Phys. ReV. B 2000, 61, 6086. (29) Mohamed, M. B.; Volkov, V.; Link, S.; El-Sayed, M. A. Chem. Phys. Lett. 2000, 317, 517. (30) (a) Lee, K.-S.; El-Sayed, M. A. J. Phys. Chem. B 2005, 109, 20331. (b) Jain, P. K.; Eustis, S.; El-Sayed, M. A. J. Phys. Chem. B 2006, 110, 18243. (31) Brioude, A.; Jiang, X. C.; Pileni, M. P. J. Phys. Chem. B 2005, 109, 13138.

10.1021/la800811j CCC: $40.75  2008 American Chemical Society Published on Web 08/30/2008

Photothermal Reshaping of Gold Nanorods

photothermal reshaping of the gold nanorods into φ-shaped or spherical nanoparticles.32-34 The time constant of the reshaping was reported to be about 35 ps.25,28 Computer simulations indicated that the temperature should be increased to over 300-500 K to induce the reshaping of the gold nanorods.35 Further intense laser irradiation on gold nanorods induced the evaporation of gold33,36 and resulted in the fragmentation of gold into smaller spherical particles. Chang et al. estimated that simultaneous absorption of 4 or 5 photons was needed to fragment gold nanorods into smaller spherical particles.32 Thus, the transiently accumulated heat in or just around gold nanorods induced the photothermal reshaping. Recently, this type of transient heating has been utilized to induce the controlled release of DNA,15 to phototrigger gene expression,18 to photoinduce cell death,17,18 and to store optical data.37 Surface modification is a key technique to realize the practical applications of gold nanorods. Because micellar solutions of hexadecyltrimethylammonium bromide (CTAB) are essential to synthesize the gold nanorods, the gold nanorods are passivated with bilayers of CTAB.38 The CTAB-passivated gold nanorods can disperse for long periods without forming aggregates. This indicates that the CTAB bilayers can give good stability of the colloidal dispersion to the gold nanorods. In order to obtain a functionalized gold nanorod, the CTAB layers can be replaced with functional materials. However, removal of the CTAB layers tends to induce aggregation or precipitation of the gold nanorods. Previously, several procedures have been investigated for replacing the CTAB with functional molecules. For example, silica layers have been shown to be a stable protective layer that can suppress aggregation even in dried conditions.39 Poly(ethylene glycol) has been used to obtain biocompatible gold nanorods.21 Polyanionic and polycationic polymers have also been used for wrapping gold nanorods with electrostatic interactions, and the cytotoxicities of those gold nanorods were evaluated using living cells.24 Biorelated materials, such as lipids40 and peptides,18,41 were also used to obtain biofunctional gold nanorods. Thus, surface modification is necessary in order to give the various gold nanorods their different functionalities. In this work, we found that photothermal reshaping of gold nanorods depended on their type of surface modification. The correlation between the reshaping and surface modifications of gold nanorods will be discussed.

Experimental Section Materials. Gold nanorods were synthesized based on a photochemical method42 in a joint research project between Mitsubishi Materials Corp. and Dai-Nippon-Toryo Co., Ltd. The average sizes of the nanorods were 44 ( 8 nm and 10 ( 1 nm (aspect ratio ) ∼4.4, the content of gold atoms was 1.52 mM). Poly(ethylenesulfonate hydrochloride) (PSS, MW ) 70 000, Scientific Polymer Production (32) Chang, S.-S.; Shih, C.-W.; Chen, C.-D.; Lai, W.-C.; Wang, C. R. C. Langmuir 1999, 15, 701. (33) Link, S.; Burda, C.; Mohamed, M. B.; Nikoobakht, B.; El-Sayed, M. A. J. Phys. Chem. A 1999, 103, 1165. (34) Link, S.; Wang, Z. L.; El-Sayed, M. A. J. Phys. Chem. B 2000, 104, 7867. (35) (a) Wang, Y.; Dellago, C. J Phys. Chem. B 2003, 107, 9214. (b) Wang, Y.; Teitel, S.; Dellago, C. J. Comput. Theor. Nanosci. 2007, 4, 282. (36) Takami, A.; Kurita, H.; Koda, S. J. Phys. Chem. B 1999, 103, 1226. (37) (a) Chon, J. W. M.; Bullen, C.; Zijlstra, P.; Gu, M. AdV. Funct. Mater. 2007, 17, 875. (b) Zijlstra, P.; Chon, J. M.; Gu, M. Opt. Express 2007, 15, 12151. (38) (a) Nikoobakht, B.; El-Sayed, M. A. Langmuir 2001, 17, 6368. (b) Gao, J.; Bender, C. M.; Murphy, C. J. Langmuir 2003, 19, 9065. (39) Pastoriza-Santos, I.; Pe´rez-Juste, J.; Liz-Marza´n, M. Chem. Mater. 2006, 18, 2465. (40) Niidome, Y.; Honda, K.; Higashimoto, K.; Kawazumi, H.; Yamada, S.; Nakashima, N.; Sasaki, Y.; Ishida, Y.; Kikuchi, J. Chem. Commun. 2007, 3777. (41) Jain, P. K.; El-Sayed, I. H.; El-Sayed, M. A. Nano Today 2007, 2, 18. (42) Niidome, Y.; Nishioka, K.; Kawasaki, H.; Yamada, S. Chem. Commun. 2003, 2376.

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Figure 1. Schematic illustration of the laser irradiation.

Inc.), poly(allylamine hydrochloride) (PAH, MW ) 15 000, Aldrich), thiol-terminated poly(ethylene glycol) (mPEG5000-SH, MW ) 5000, NOF Corporation), poly(vinylpyrrolidone) (PVP, MW ) 10 000, Kishida Chemical Co., Ltd.), and phosphatidylcholine (PC, from egg yolk, Nakarai Tesque, Inc.) were commercially available as indicated and used without further purification. PSS-Passivated Gold Nanorod (PSS-rod) and PVP-Passivated Gold Nanorod (PVP-rod). (PSS-rod). The modification processes were the same as those from a previous report.39 An as-prepared gold nanorod solution (20 mL, 1.52 mM Au atoms) was centrifuged (15 000g, 10 min), and the precipitate was redispersed in 20 mL of water. The redispersed solution was centrifuged again, and the precipitate was dispersed in 20 mL of a PSS solution (2 mg/mL, 6 mM NaCl) and stirred for 1 h (PSS-rod solution, 1.68 mM Au atoms). The ζ-potential of the PSS-rods was -27.6 mV. (PVP-rod). A PSS-rod solution (20 mL) was centrifuged (10 000g, 10 min), and the precipitate was dispersed in 20 mL of PAH solution (2 mg/mL, 6 mM NaCl) and stirred for 1 h. The stirred solution was centrifuged (10 000g, 10 min), and the precipitate was dispersed in 20 mL of PVP solution (4 mg/mL) and stirred overnight (PVP-rod solution, 1.08 mM Au atoms). The ζ-potential of the PVP-rod was +37.9 mV. PEG-Passivated Gold Nanorod (PEG-rod). The preparation of the PEG-rod was reported previously.21 An as-prepared gold nanorod solution (16 mL) was centrifuged at 14 000g for 10 min, and the precipitate was dispersed in water such that it contained 1 mM (Au atoms) gold nanorod solution. In a solution of mPEG-SH (0.2 mL, 5 mM), 1 mL of the centrifuged gold nanorods was added. PC-Passivated Gold Nanorod (PC-rod). The preparation of the PC-rod was reported previously.15,16,43 The as-prepared gold nanorod solution (20 mL) was shaken with 10 mL of a PC chloroform solution (10 mg/mL). After phase separation, the organic phase was discarded. This process was repeated two more times. The water phase was centrifuged (10 000g, 10 min), and the precipitate was dispersed in the same volume of water. The gold nanorods in water were centrifuged again, and the precipitate was dispersed in 1 mL of water (PC-rod, 1.27 mM Au atoms). Laser Irradiation and Autoclave Heating. Pulsed-laser light (∼10 ns, 870 nm, 16 mJ/pulse, 10 Hz, ∼3 mm φ) from a Q-switched Cr-LiSAF laser (Indeco) irradiated the nanorod solution (4 µL) in a plastic tube. The laser light was perpendicularly introduced without focusing in the bottom of the tube (Figure 1). The laser-irradiated samples were diluted to 300 µL and then put in a micro-optical cell whose optical path length was 1 cm. The focused laser light irradiated 20 µL of a sample solution, and then the sample was diluted to 300 µL. The surface-modified gold nanorods in a glass bottle (1 mL) were heated at 121 °C for 3 h in an autoclave (SX-500, TOMY). The glass bottle was sealed with aluminum foil, and no noticeable dilution was observed before and after the autoclave treatments. The absorption spectra of the gold nanorod solutions were obtained using a spectrophotometer (V570, JASCO). Zeta (ζ)-potential measurements were performed using a Zetasizer Nano ZS (Marvern) instruments. For transmission electron microscopic observations, a JEM 2010 (JEOL, operated at 120 kV) microscope was employed. (43) Honda, K.; Kawazumi, H.; Yamada, S.; Nakashima, N.; Niidome, Y. Trans. Mater. Res. Soc. Jpn. 2007, 32, 421.

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Figure 2. Absorption spectra of gold nanorod solutions irradiated by the laser light (∼10 ns, 870 nm, 16 mJ/pulse, 10 Hz, 4 mm φ) before (thin solid line) and after 30 s (dotted line), 60 s (dashed line), 120 s (dot-dashed line), and 300 s (thick solid line) of laser irradiation. (a) As-prepared gold nanorods, (b) once-centrifuged gold nanorods, (c) twice-centrifuged gold nanorods.

Results and Discussion Figure 2 shows the spectral changes of three kinds of gold nanorod solutions that were irradiated by the near-IR pulsedlaser light (870 nm, 16 mJ/pulse). Figure 2a presents the spectral changes of an as-prepared gold nanorod solution. Figure 2b and c presents those of once- and twice-centrifuged gold nanorod solutions, respectively. Thus, the CTAB concentrations of these solutions were very different; the as-prepared solution contained 480 mM CTAB, but the centrifuged nanorod solutions contained much less CTAB. In every case, the extinction spectra before the laser irradiation (solid lines) showed two extinction peaks at 520 and 900 nm which were the transverse and longitudinal modes of the SP oscillation of the gold nanorods.1-3,30 These were representative of typical spectra of gold nanorods without forming aggregates.44 Macroscopic temperatures of the nanorod solutions rose about 20 °C after laser irradiation (see Supporting Information). Centrifugation did not affect the temperature changes, indicating that the spectral changes in Figure 2a did not originate from the heating of the solution. Pulsed-laser irradiation induced remarkable spectral changes on the as-prepared nanorod solution (a). The longitudinal SP band at 900 nm decreased after the laser irradiation, while a new peak appeared at 830 nm. We confirmed that the initial gold nanorod solution contained no gold ions; no gold nanoparticles were formed in a supernatant of a centrifuged gold nanorod solution, even when NaBH4 was added to it. This indicated that the spectral changes in Figure 2a did not originate from the photochemical reduction of gold ions. Temperature changes of the laser-irradiated solutions were very simple; that is, if the spectroscopic properties of the gold nanorods would not be affected by the laser irradiation, the temperature of the gold nanorod solution significantly increased. However, under our experimental conditions, the laser irradiation did not cause the solution to boil. This indicated that the temperature changes were not a key factor in the photoinduced reshaping of the gold nanorods. With these results, we concluded that the photothermal reshaping of gold nanorods was the origin of the spectral changes. Because the spectral changes showed an isosbestic point, the photoreaction occurring within 300 s of the laser irradiation probably gave a unique product. After 600 s of laser irradiation on an as-prepared gold nanorod solution (Figure 3), three SP bands were observed: a transverse SP band at 520 nm, a new SP band at 830 nm, and a SP band at around 980 nm. The transverse SP bands at 520 nm did not change their peak positions and bandwidth after the laser irradiation. It indicated that the size in the transverse direction of the gold nanorods was almost retained even after the laser irradiation. It should be noted that the peak (44) Gloudenis, M.; Colby, A; Foss, J. J. Phys. Chem. B 2002, 106, 9484.

Figure 3. Absorption spectra of an as-prepared gold nanorod solution (a) before, which is identical to those in Figure 2a, and after (b) 300 s and (c) 600 s of the laser irradiation.

positions of the transverse SP bands were not as sensitive to the shape changes of the gold nanorods in comparison with those of the longitudinal SP bands.26,30,31 On the other hand, the new SP band at 830 nm was attributable to the gold nanorods that were the product of the photothermal reshaping. The SP band located at 980 nm did not seem to be a new band generated by the laser irradiation because the decreasing lines toward the longer wavelength region (1050-1300 nm) were the same as those of the initial SP band (thin solid line). The SP band in the longer wavelength region probably came from longer gold nanorods that retained their shapes even after the laser irradiation because of a smaller absorbance at the laser wavelength (870 nm). The centrifuged nanorod solutions (Figure 2b and c) showed small spectral changes, which included peak shifts to shorter wavelength regions, but the degrees of the changes were not as remarkable as those of the as-prepared gold nanorod solution (Figure 2a). It was clear that removal of the excess CTAB in the gold nanorod solution suppressed the photothermal reshaping of the gold nanorods. Spectral changes of the polymer- (PSS-, PVP-, and PEG-rod) and lipid-passivated gold nanorods are shown in Figure 4. The PSS-rods (Figure 4a) did not show remarkable spectral changes, even after 300 s of laser irradiation. The PVP- and PEG-rods (Figure 4c and d) presented relatively larger spectral changes than those observed for the PSS-rod solution. In the case of the PC-rods (Figure 4d), the laser irradiation induced considerably more spectral changes than in the case of the polymer-passivated gold nanorods, but a new SP band at 830 nm was not generated by the laser irradiation. Thus, the order of spectral changes was as follows: as-prepared nanorod . PC-rod > PEG-rod > PVProd > PSS-rod ≈ once-centrifuged ≈ twice-centrifuged. In order to reveal the final shape of the photothermal reshaping, as-prepared gold nanorods were irradiated by focused-laser light (16 mJ/pulse). Figure 5 shows the spectral changes of an as-

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Figure 6. TEM images of the laser-irradiated gold nanorods. (a) Before laser irradiation and (b-d) after pulsed laser irradiation of (b) φ-shaped gold nanorods, (c) spherical particles and shortened gold nanorods, and (d) elongated gold nanorods.

Figure 4. Absorption spectra of polymer- and lipid-passivated gold nanorods before (dotted line) and after (solid line) the laser irradiation. Laser irradiation: ∼10 ns, 870 nm, 16 mJ/pulse, 10 Hz, 4 mm φ, (a) PSS-modified gold nanorods, (b) PVP-modified gold nanorods, (c) PEGmodified gold nanorods, and (d) PC-passivated gold nanorods.

Figure 7. Extinction spectra of gold nanorod solutions before (a) and after (c,d) heating in an autoclave. As-prepared (a,b), once-centrifuged (c), and twice-centrifuged (d) gold nanorod solutions were heated at 121 °C for 3 h.

Figure 5. Spectral changes of an as-prepared gold nanorod solution irradiated by focused laser light (16 mJ/pulse). (a) Before laser irradiation and (b) irradiated for 60 s and (c) 300 s.

prepared gold nanorod solution (20 µL) irradiated by the focusedlaser light. The SP band at 900 nm disappeared after 300 s of the focused laser irradiation, and a new SP band was observed at around 790 nm. Because the spectral changes were same as those observed in Figure 3, the new band could be assigned to the final products of the photothermal reshaping. Transmission electron microscopy (TEM) images of the gold nanoparticles in an as-prepared gold nanorod solution and in the laser-irradiated solution are shown in Figure 6a and b-d, respectively. Without the laser irradiation (Figure 6a), the TEM image indicated the typical shape of the gold nanorods. After 300 s of laser irradiation, there were three kinds of products in the solution (Figure 6b-d). The most frequently observed particles were φ-shaped gold nanorods (Figure 6b) with a yield of 59% (300 particles were counted). The same φ-shaped gold nanorods were also reported in previous studies.32-34 The yield of the shorter nanorods (Figure 6c) was 11%. The yield of the original gold nanorods whose shape was retained even after laser irradiation was 6%; those were frequently observed (Figure 6c). In addition, fused nanorods in the longitudinal direction (Figure 6d) were obtained in 8% yield. The remaining particles (16%) were spherical or amorphous nanoparticles. Because the same percentage of spherical and amorphous particles was found in an as-prepared nanorod solution, they were not the result of the photothermal reshaping of the

Figure 8. Model of gold nanorod wrapped with a CTAB bilayer.

gold nanorods. Thus, the SP band at around 790 nm shown in Figure 5 was assignable to the φ-shaped (59%) and shorter nanorods (11%) that were the major products of the specific laser irradiation conditions. Figure 7 shows spectral changes of the gold nanorod solutions that were heated in an autoclave at 121 °C for 3 h. After heating, the longitudinal SP bands of every nanorod solution decreased and shifted to the shorter wavelength region. The decrease of the SP bands could be assigned to the precipitation of aggregated gold nanorods, and the blue shifts strongly suggested that shorter gold nanorods stably dispersed even at the high temperature. The spectral changes in Figure 7 indicated that thermal reshaping of gold nanorods into spherical particles was negligible in every case. In addition, the as-prepared gold nanorods (Figure 7b) showed a larger SP band in the near-IR region than the centrifuged gold nanorod solutions (Figure 7c and d). Thus, the presence of excess CTAB (480 mM) suppressed the aggregation and precipitation of gold nanorods at the high temperature in an autoclave. A previous paper indicated that PVP and poly(ethyleneimine) (PEI) were also suitable for preventing aggregation

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of gold nanorods when they were heated at 100-250 °C.45 Thus, the order of aggregation of the gold nanorods, induced by conventional heating in an autoclave, was as follows: twicecentrifuged > once-centrifuged > as-prepared ≈ PVP-rod ≈ PEI-passivated nanorods. This order was very different from that of the photoinduced reshaping discussed above. It was shown that conventional heating affected the stability of the colloidal dispersion of the gold nanorods but it was negligible in the transient heating from the pulse-laser irradiation. Our experimental data indicated that the concentrated CTAB solution (480 mM) accelerated the photothermal reactions while it suppressed the aggregation in an autoclave. It has been described that, at 480 mM, a CTAB solution has a micellar phase while hexagonal, cubic, or lamellar phases are not formed.46,47 There was no large scale molecular assembly, such as liquid crystals, in the CTAB solution. Thus, the suppression of the aggregation probably comes from the properties of the CTAB layers on the gold nanorod surfaces. Previous papers indicated the presence of CTAB bilayers on gold nanorods.37 Because bromide ions strongly adsorb on a gold surface,48 hexadecyltrimethylammonium (CTA+) likely forms a stable and static monolayer. Onto the first layer, another CTAB layer would be adsorbed through the hydrophobic interactions of the methylene chains. The stable bilayers would be effective in suppressing the aggregation of the gold nanorods in the CTAB solutions. Chon and co-workers discussed the photothermal reshaping of pulsed-laser irradiated gold nanorods in silica shells.37 They revealed that a simple heat-diffusion model explained the photothermal reshaping of gold nanorods. Their model indicted that the heat diffusion to the silica shells was competitive to the reshaping of gold nanorods.37 Under our experimental conditions, the densely packed CTAB multilayers probably acted as protective layers for heat diffusion. It should be noted that the thermal conductivity of water (Kw, 0.598 W/m K, at 20 °C) is greater than that of hexadecane (Khd, 0.140 W/m K).49 The pulsed-laser irradiation provided a lot of heat to the gold nanorods, and the heat was encapsulated by the CTAB multilayers. A model of thermal diffusion for a gold nanorod irradiated by pulsed-laser light is shown in Figure 8. The thermal diffusion was estimated by the simplest model where heat in a nanorod (nanocolumn) diffuses through the CTAB bilayer on the nanorod surface into the bulk water, but the temperature of the bulk water does not change. The temperature (T) of the nanorod at time t is

(

T(t) ) T(t)0) exp -

)

KhdSAu t dhdFAuVAuCAu

(1)

where dhd is the thickness of the CTAB (hexadecane) bilayer, SAu, FAu, VAu, and CAu are surface area, density, volume, and specific heats of a gold nanorod, respectively. We assume a gold nanorod is 10 × 50 nm2 in size (Figure 8) and use SAu ) 1727 nm2, FAu ) 19 300 kg/m3, VAu ) 3925 nm3, and CAu ) 0.1291 J/g K. The value for dhd was assumed to be 3.9 nm.50 With these values, eq 1 indicates that the heat relaxation time is about 150 (45) (a) Pastoriza-Santos, I.; Gomez, D.; Pe´rez-Juste, J.; Liz-Marza´n, L. M.; Mulvaney, P. Phys. Chem. Chem. Phys. 2004, 6, 5056. (b) Petrova, H.; Pe´rezJuste, J.; Pastoriza-Santos, I.; Hartland, G. V.; Liz-Marza´n, L. M.; Mulvaney, P. Phys. Chem. Chem. Phys. 2005, 8, 814. (46) Wa¨rnheim, T.; Jo¨nsson, A. J. Colloid Interface Sci. 1988, 125, 627. (47) Auvray, X.; Petipas, C.; Anthore, R.; Rico, I.; Lattes, A. J. Phys. Chem. 1989, 93, 7458. (48) Waters, C. A.; Mills, A. J.; Johnson, K. A.; Schiffrin, D. J. Chem. Commun. 2003, 540. (49) Lide, D. R., Ed. Handbook of Chemistry and Physics, 82nd ed.; CRC Press: Washington, D.C., 2001. (50) Jana, N. R.; Gearheart, L. A.; Obare, S. O.; Johnson, C. J.; Edler, K. J.; Mann, S.; Murphy, C. J. J. Mater. Chem. 2002, 12, 2909.

ps if the gold nanorod is wrapped with the CTAB bilayer. The theoretical calculation of the heat relaxation of a gold nanoparticle in water (in the absence of the CTAB bilayer) was presented by Bloemer et al.51 They indicated that the heat relaxation time depended on the size and surface area of the gold nanoparticles. If Bloemer’s model was applied to a spherical particle (19.6 nm in diameter) that had the same volume as that of a gold nanorod, its heat relaxation time could be estimated as 48 ps. Because a gold nanorod has a larger surface area than that of spherical particles, the heat relaxation time of a gold nanorod should be less than 48 ps. This time constant is competitive with the photothermal reshaping of a gold nanorod (∼35 ps). It is more difficult for a gold nanorod which is dispersed in water to accumulate the heat provided by pulsed-laser irradiation. Chon et al. reported surface melting of gold nanorods in silica shells induced by pulsed-laser irradiation.37 and estimated the heat relaxation time of gold nanorods in silica shells to be 20 ps.37 The fast heat diffusion suppressed the fragmentation and reshaping of the gold nanorods into spherical particles. However, they used femtosecond pulsed-laser light, which has a much higher fluence than that of the nanosecond pulsed-laser light that was used in this work. They also showed that gold nanorods in poly(vinyl alcohol) (PVA) film thoroughly transformed into spherical particles by the femtosecond laser irradiation. The slow heat relaxation time in PVA, which was estimated to be 135 ps, contributed to the efficient reshaping of the gold nanorods.37 These results were consistent with those reported here. Thus, the CTAB layer was an effective heat barrier to maintain the nanorod at a higher temperature, thereby inducing the photothermal reshaping. When the excess CTAB was removed by the centrifugation, the bilayers on the nanorod surfaces would likely be unstable. The CTAB molecules in the outer layer were frequently replaced with free CTAB molecules from the bulk water phase. That is, the CTAB bilayers on the centrifuged gold nanorods were dynamic and unstable. Consequently, rapid heat diffusion would occur through the thin and dynamic bilayer and suppress the photothermal reshaping of the nanorods. The PSS- and PVP-rods were prepared using twice-centrifuged gold nanorods; the innermost layers on the nanorod surfaces were CTAB bilayers, and the polymers adsorbed on the CTAB bilayers.38 The polymer layers were effective in stabilizing the colloidal dispersion of the gold nanorods; however, the laser irradiation experiments (Figure 4) indicated that they were not as effective in suppressing the heat diffusion. In the case of the PSS-rods, the spectral changes (Figure 4a) were as small as those of the centrifuged gold nanorods (Figure 3b and c). Because a lot of water molecules were contained in the PSS layers, they could not be barriers for heat diffusion. PVP-rods (Figure 4b) were passivated with multilayered polymer assemblies (PSSPAH-PVP) and had more drastic photothermal reshaping than that found for the PSS-rods. The multilayered polymer assembly would be a moderate barrier of thermal diffusion. In the case of the PEG- and PC-rods, some CTAB molecules on the nanorod surfaces were replaced with PEG or PC molecules.15,16,21 Pulsedlaser irradiation on PEG- and PC-rods also induced spectral changes (Figure 4c and d) that were more drastic than those of the centrifuged gold nanorods, but not as remarkable as those of the as-prepared nanorod solution (Figure 2a). This may be because, in contrast to the densely packed CTAB layers, PEG and PC are amphiphilic molecules that form spontaneous (51) Bloemer, M. J.; Haus, J. W.; Ashley, P. R. J. Opt. Soc. Am. B 1990, 7, 790.

Photothermal Reshaping of Gold Nanorods

molecular assemblies which in turn could act as barriers and affect the photothermal reshaping.

Conclusions Bilayers of CTAB effectively accelerated the photothermal reshaping of the gold nanorods due to their low heat diffusion constant. The polymers and PC were not effective heat barriers. On the other hand, the CTAB layers suppressed aggregation and precipitation of the gold nanorods by conventional heating. It was shown that the molecular assemblies on the surfaces of the gold nanorods affected the photothermal reshaping of the gold nanorods. Various surface modifications with surfactants, polymers, and inorganic materials will be able to tune the thermal properties of the gold nanorods in a transiently heated state through pulsed-laser irradiation. In this work, we used a nanosecond pulsed-laser light. Use of a femtosecond laser-light would give definitive information on the two competitive paths that included photothermal reshaping and heat diffusion into the

Langmuir, Vol. 24, No. 20, 2008 12031

water phase. In addition, we have not investigated the use of transient absorption spectroscopy that will provide quantitative dynamics of the transient states. On the basis of the results described in this paper, further studies to reveal the dynamics of gold nanorods are underway. Acknowledgment. This study was supported by a Grant-inAid for Scientific Research (No. 15350085), KAKENHI (Grantin-Aid for Scientific Research) on Priority Area “Strong PhotonMolecule Coupling Fields (No. 470)”, and a Grant-in-Aid for the Global COE Program, “Science for Future Molecular Systems”, from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government. Supporting Information Available: Temperature changes of the laser-irradiated gold nanorod solutions. This material is available free of charge via the Internet at http://pubs.acs.org. LA800811J