pubs.acs.org/Langmuir © 2010 American Chemical Society
Rapid Synthesis of Gold Nanorods Using a One-Step Photochemical Strategy Marya Ahmed and Ravin Narain* Department of Chemical and Materials Engineering, University of Alberta, 116th Street and 85th Avenue, Edmonton, Alberta T6G 2G6, Canada Received August 20, 2010. Revised Manuscript Received October 15, 2010 Rapid synthesis of gold nanorods of controlled dimensions is one of the desired aspects of nanotechnology as a result of the potential of these nanomaterials for biomedical applications. The synthesis of gold nanorods has been achieved using a photoinitiator as an instant source of ketyl radicals, which allows the synthesis of gold nanorods in minutes. This is the first report providing a one-step synthesis of nanorods of controlled dimensions in 20-30 min using photoinitiator I-2959 as a source of ketyl radicals. Furthermore, the role of UV intensity, the concentration of silver ions, and the presence of cosolvents and a cosurfactant have been studied in detail in an effort to produce nanorods with controlled dimensions in higher yields. The role of acetone in nanorod synthesis has been explored in detail, and it has been demonstrated that, for the photochemical synthesis of nanorods using a photoinitiator, acetone is not a critical component and can be replaced by other water-miscible solvents, thus the successful synthesis of nanorods in tetrahydrofuran (THF) has been demonstrated. It has also been found that a cosurfactant and an organic solvent are not required for the synthesis of nanorods; however, their presence is found to improve the monodispersity of nanorod samples, in addition to providing a higher yield.
Introduction The rapidly advancing field of nanotechnology has successfully produced a variety of nanomaterials of varying sizes and shapes.1-7 Nanospheres, nanorods, nanoprisms, and nanodisks are just a few examples of nanomaterials that have been produced and are further utilized for various applications.3,6,7 Gold nanomaterials are unique because of their optical and electronic properties in addition to their possible applications for biological purposes as a result of the inert nature of the gold metal itself.1-3,6 For example, the size-dependent uptake of spherical gold nanoparticles by mammalian cells has been explored, and it has been found that the size of gold nanorods plays a role in their gene-delivery efficacy.7-9 In addition to size, the shape of the nanomaterials also defines their interactions with living tissue. The synthesis of gold nanorods has been a focus of current research since their potential applications in gene delivery and cancer therapy have been discovered.2,10-13 It has been found that nanorods in *To whom correspondence should be addressed. E-mail:
[email protected]. Phone: (780) 492-1736. Fax: (780) 492-2881. (1) Perez-Juste, J.; Pastoriza-Santos, I.; Liz-Marzan, M. L.; Mulvaney, P. Coord. Chem. Rev. 2005, 249, 1870–1901. (2) Huang, X.; Neretina, S.; El-Sayed, A. M. Adv. Mater. 2009, 21, 1–31. (3) Cliffel, E. D.; Turner, N. B.; Huffman, J. B. Wiley Interdiscip. Rev. Nanomed. Nanobiotechnol. 2009, 1, 47–59. (4) Jun, Y.-W.; Seo, J.-W.; Cheon, J. Acc. Chem. Res. 2008, 41, 179–189. (5) Prato, M.; Kostarelos, K.; Bianco, A. Acc. Chem. Res. 2008, 41, 61–68. (6) Ray, C. P. Chem. Rev. 2010, 110, 5332–5365. (7) Chithrani, D. B.; Ghazani, A. A.; Chan, W. C. W. Nano Lett. 2006, 6, 662–668. (8) Chithrani, D. B.; Chan, W. C. W. Nano Lett. 2007, 7, 1542–1550. (9) Ahmed, M.; Deng, Z.; Narain, R. ACS Appl. Mater. Interfaces 2009, 1, 1980–1987. (10) Watanabe, K.; Menzel, D.; Nilius, N.; Freund, J.-H. Chem. Rev. 2006, 106, 4310–4320. (11) Guo, R.; Zhang, L.; Qian, H.; Li, R.; Jiang, X.; Liu, B. Langmuir. 2010, 26, 5428–5434. (12) Zhou, W.; Shao, J.; Jin, Q.; Wei, Q.; Tang, J.; Ji, J. Chem. Commun. 2010, 46, 1479–1481. (13) Huang, H.-C.; Barua, S.; Kay, B. D.; Rege, K. ACS Nano 2009, 3, 2941– 2952.
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addition to serving as tumor-targeting agents and gene- and drug-delivery carriers absorb radiation in the near-infrared spectrum and convert this energy into heat energy, thus killing the diseases tissue by photothermal therapy.14 A variety of techniques to synthesize gold nanorods with high yield and controlled dimensions have been studied in the literature.1,15-23 The photochemical method is one of the oldest techniques employed in the synthesis of nanorods.24 Like other methods including chemical and seed-mediated approaches used for the synthesis of nanomaterials, the photochemical method is also well explored with respect to producing nanomaterials with controlled dimensions.1,17,25-28 However, the synthesis of nanorods by the photochemical approach is found to require long reaction times, and up to 30 h is sometimes required for the synthesis.25,28 Niidome et al. provided a facile technique for synthesizing nanorods of varying sizes in a significantly shorter period of time (30 min) by a combination of chemical and photochemical approaches26,29 wherein the gold salt was first reduced by ascorbic acid and was then exposed to UV (14) Li, Z.; Huang, P.; Zhang, X.; Lin, J.; Yang, S.; Liu, B.; Gao, F.; Xi, P.; Ren, Q.; Cui, D. Mol. Pharm. 2010, 7, 94–104. (15) Wu, Y.-H.; Huang, W.-L.; Huang, H. M. Cryst. Growth Des. 2007, 7, 831–835. (16) Zijlstra, P.; Bullen, C.; Chon, M. W. J.; Gu, M. J. Phys. Chem. B 2006, 110, 19315–19318. (17) Ni, W.; Kou, X.; Yang, Z.; Wang, J. ACS Nano 2008, 2, 677–686. (18) Wei, Z.; Zamborini, P. F. Langmuir. 2004, 20, 11301–11304. (19) Guo, L.; Murphy, J. C. Chem. Mater. 2005, 17, 3668–3672. (20) Busbee, D. B.; Obare, O. S.; Murphy, J. C. Adv. Mater. 2003, 15, 414–416. (21) Song, H. J.; Kim, F.; Kim, D.; Yang, P. Chem.;Eur. J. 2005, 11, 910–916. (22) Biswal, J.; Ramnani, P. S.; Tewari, R.; Dey, K. G.; Sabharwal, S. Radiat. Phys. Chem. 2010, 79, 441–445. (23) Qiaoling, L.; Thomas, B.; Hui, C. J. Wuhan Univ. Technol., Mater Sci. Ed. 2010, 25, 104–107. (24) Esumi, K.; Matsuhisa, K.; Torigoe, K. Langmuir. 1995, 11, 3285–3287. (25) Miranda, R. O.; Ahmadi, S. T. J. Phys. Chem. B 2005, 15724–15734. (26) Nishioka, K.; Niidome, Y.; Yamada, S. Langmuir 2007, 23, 10353–10356. (27) Placido, T.; Comparelli, R.; Giannici, F.; Cozzoli, D. P.; Capitani, G.; Striccoli, M.; Agostiano, A.; Curri, L. M. Chem. Mater. 2009, 21, 4192–4202. (28) Kim, F.; Song, H. J.; Yang, P. J. Am. Chem. Soc. 2002, 124, 14316–14317. (29) Niidome, Y.; Nishioka, K.; Kawasaki, H.; Yamada, S. Chem. Commun. 2003, 2376–2377.
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light to produce nanorods of varying sizes depending upon the amount of silver nitrate present in the solution.26,29 The strategy was further evaluated by others to determine the role of ascorbic acid in nanorod synthesis. It was found that an increase in ascorbate concentration increases the length of the nanorods up to a certain critical concentration as a result of the interactions of ascorbate ions with the cationic head groups of surfactants, hence allowing elongated micelle formation. Further increases in the ascorbate ion concentration is found to decrease the length of the nanorods.30 The photochemical synthesis of nanorods has mostly been achieved at a wavelength of 254 nm as a result of the absorption of ketone radicals at this specific wavelength, which is found to be crucial to nanorod synthesis.25,26,28,29 Miranda et al. has studied the synthesis of nanorods at wavelengths of 254 and 300 nm, and the roles of UV intensity and irradiation time have been discussed in detail.25 In addition, it has been found that UV irradiation at a wavelength of 300 nm has the potential to produce longer nanorods compared to those produced at a wavelength of 254 nm.25 However, the nanoparticle synthesis required exceptionally long irradiation times, which is attributed to the slow reduction of gold salt by ketone radicals, in the presence of a surfactant and silver ions.25 One way to address this inefficient synthesis approach is by introducing a source of ketyl radicals with a shorter triplet life span, hence enabling the facile synthesis of nanorods in a significantly shorter period of time by the photochemical approach itself. The use of photoinitiator 4-(2hydroxyethoxy)phenyl-(2-hydroxy-2-propyl)ketone, also known as Irgacure-2959 (I-2959), to synthesize nanomaterials with controlled dimensions in a relatively short period of time is well documented.31-33 The I-2959 photoinitiator is very sensitive to UV radiation at a wavelength of ∼350 nm and rapidly dissociates to provide ketyl radical of high reactivity and a shorter life span, causing the rapid reduction of Au3þ to Au0 metal.31,32 Our group has successfully produced monodisperse gold nanospheres in minutes using this technique, and these spherical nanoparticles are found to serve as potent gene-delivery agents.34,35 The use of photoinitiator I-2959 for the synthesis of nanorods is novel, and to the best of our knowledge, no data has been reported showing the rapid synthesis of nanorods in one step using photochemistry. Furthermore, the successful development of this facile approach to nanorod synthesis is modified to produce nanorods with various aspect ratios. The role of additives in controlling the size and monodispersity of nanorods in high yield is still an active area of research. The role of surfactants, silver nitrate, and the time and intensity of UV radiation in controlling the size of nanorods is now well documented.25,27,33 Herein, we report the rapid and facile synthesis of nanorods with controlled dimensions in 30 min using a photochemical strategy. The roles of organic solvents and cosurfactants and the effects of UV intensity at a wavelength 350 nm and irradiation time on nanorod synthesis have been studied in detail. Compared to the recent literature, where the presence of acetone and prolonged reaction times are found to be critical for nanorod synthesis via the photochemical (30) Miranda, R. O.; Dollahon, R. N.; Ahmadi, S. T. Cryst. Growth Des. 2006, 6, 2747–2753. (31) McGilvray, L. K.; Decan, R. M.; Wang, D.; Scaiano, C. J. J. Am. Chem. Soc. 2006, 128, 15980–15981. (32) Marin, L. M.; McGilvray, L. K.; Scaiano, C. J. J. Am. Chem. Soc. 2008, 130, 16572–16584. (33) Garg, N.; Scholl, C.; Mohanty, A.; Jin, R. Langmuir 2010, 26, 10271–10276. (34) Ahmed, M.; Deng, Z.; Liu, S.; Lafrenie, R.; Kumar, A.; Narain, R. Bioconjugate Chem. 2009, 20, 2169–2176. (35) Housni, A.; Ahmed, M.; Liu, S.; Narain, R. J. Phys. Chem. C 2008, 112, 12282–12290.
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approach alone, we demonstrate that in the presence of photoinitiator I-2959 nanorods of varying dimensions can be synthesized in THF upon irradiation at 350 nm in minutes. The gold nanorods produced are then characterized by UV-vis spectroscopy and transmission electron microscopy (TEM).
Experimental Section Materials. All chemicals were purchased from Sigma-Aldrich and were used without purification. Photoinitiator Irgacure-2959 was obtained from Ciba Chemicals Inc. Nanopure water was obtained from APS Water Services Corporation. Synthesis of Gold Nanorods. All stock solutions were prepared in nanopure water, and all of the reactions were carried out at room temperature. An aqueous solution containing hydrogen tetrachloroaurate (0.25 mL of 0.024 M HAuCl4 3 3H2O), 0.08 M hexadecyltrimethylammonium bromide (CTAB, 3 mL), and different amounts of silver nitrate (AgNO3, 0.01 M) was prepared. The stock solution of photoinitiator Irgacure-2959 (4.8 mM) was prepared in a 4:1 water/methanol mixture and was added to the solution mixture. The mixture was irradiated using a Luzchem photoreactor with 4 UV A lamps while being stirred for 30 min to yield gold nanorods. To investigate the role of additives, a CTAB stock solution was prepared by mixing CTAB (0.08 M) in nanopure water and acetone or tetrahydrofuran (THF) at varying ratios of water to solvents. The CTAB solution was mixed with HAuCl4 3 3H2O and with varying amounts of silver nitrate as described above. I-2959 (4.8 mM) was added, and the solution mixture was irradiated using a Luzchem photoreactor with 10, 6, or 4 UV A lamps with stirring for 10, 20, or 30 min, respectively, to yield gold nanorods of varying lengths. The effects of cosurfactant tetradodecylammonium bromide (TDAB) and organic solvent were studied by adding 50 μL of 0.01 M TDAB and 10 μL of cyclohexane to an aqueous solution of CTAB, HAuCl4 3 3H2O, and AgNO3 to obtain a final concentration of 0.1 mM TDAB in the reaction mixture. The mixture was irradiated in a Luzchem photoreactor while stirring with 4 UV A lamps for 30 min to yield gold nanorods of varying lengths. The nanorods that were produced were centrifuged at 10 000 rpm, and the pellet was redispersed in nanopure water and centrifuged again at 10 000 rpm to remove excess surfactant. The pellet was washed two to three times to ensure the complete removal of excess surfactant. UV-Visible Spectroscopy. UV-visible absorption spectra of an aqueous solution of gold nanorods were recorded from 400 to 900 nm on Jasco V 630 spectrophotometer. Transmission Electron Microscopy (TEM). Transmission electron microscope images of gold nanorod samples were obtained using a Philips transmission electron microscope equipped with a CCD camera. Dynamic Light Scattering (DLS). Dynamic light scattering (DLS) was performed using a Brookhaven Zeta Plus (zeta potential and particle size analyzer) to measure the size of CTAB micelles in aqueous solution containing varying amounts of acetone.
Results and Discussion The photochemical synthesis should be a facile approach to synthesizing gold nanorods with varying aspect ratios. However, on the basis of previous studies, the photochemical synthesis of gold nanorods is found to be very challenging because the process requires long reaction times as a result of the slow excitation and electron transfer of ketone-based radicals to gold salt that further promote the synthesis and growth of gold nanorods and complex one-pot chemistry compared to a seed-mediated stepwise method that is found to provide better control over the reaction conditions.25,36 The seed-mediated method of gold nanorod (36) Gole, A.; Murphy, J. C. Chem. Mater. 2004, 16, 3633–3640.
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synthesis is also found to suffer from long reaction times.36 Recently, a combination of chemical and photochemical methods has been used to eliminate this time lag in nanorod synthesis due to slow excitation and electron transfer by ketyl radicals upon exposure at λ ≈ 254 nm. The approach is similar to the seedmediated two-step synthesis of nanorods in which the reduction of gold salt by ascorbate ions produces gold seed that upon irradiation at 254 nm in the presence of acetone yields nanorods.29 In this article, we discuss a remarkable new photochemical strategy for producing gold nanorods in minutes without prior chemical reduction of the gold salt. Moreover, the synthesis of nanorods in a cosolvent other than acetone is achieved by this one-step approach. Here, an instant source of ketyl radicals is used for the photochemical reduction of gold salt. Photoinitiator 4-(2-hydroxyethoxy)phenyl-(2-hydroxy-2-propyl)ketone, known as I-2959, is used as a source of ketyl radicals, which upon excitation by UV light of λ e 350 nm dissociates into its components. The produced ketyl radicals (via Norrish-type IR cleavage) have a very short triplet lifetime (nanoseconds), which is key to the rapid synthesis.31,32 The ketyl radicals produced by photoinitiator reduce gold and silver salts to their corresponding nanoparticles in the presence of surfactant, resulting in the formation of gold alloys.37 The photochemical approach to synthesize spherical gold nanoparticles using I-2959 as a photoinitiator is well studied, and the chemistry involved in the synthesis of these metallic nanoparticles is well documented.31,32,34,35 The use of the I-2959 photoinitiator to produce gold nanorods in 2030 min in one pot from gold salt is a significant improvement. The formation of gold nanorods in silver-metal-stabilized cylindrical micelles occurs as gold nanoparticles coalesce into rodlike structures. The aspect ratios of nanorods are controlled by the careful monitoring of several parameters including the intensity of UV radiation, the concentration of cosolvents (acetone or THF and cyclohexane), and the concentration of cosurfactant. These parameters are also found to be critical for obtaining nanorods in higher yield and will be discussed in detail here. The formation of gold nanorods is a thermodynamically unfavorable process, and it is expected that control of the monodispersity of gold nanorods can be an issue because of shorter reaction times and the complex one-pot chemistry of this photochemical approach. However, the motivation to synthesize gold nanorods in one pot comes from the work of Scaiano et al.,32,37 where the chemistry of acetone-based ketyl radicals produced in the photochemical process is studied in detail, and it has been emphasized that this technique should be viable for the formation of nanomaterials with various morphologies.32 In an effort to produce gold nanorods with varying aspect ratios and at higher concentration using this strategy, various reaction parameters including the role of UV irradiation intensity at a wavelength of 350 nm, the exposure time, the presence of organic solvent, and the role of cosurfactant were studied. The initial experiments involved the reduction of gold salt using 10 UVA lamps in the presence of silver nitrate, surfactant (CTAB), and photoinitiator I-2959 in nanopure water and in the absence of any organic solvents and cosurfactants. The UV-vis spectra of the resulting solutions showed intense absorbance peaks at ∼520-530 nm that are characteristic of spherical gold nanoparticles and smaller peaks between 600 and 800 nm that confirm the formation of a significantly lower yield of gold nanorods. Under these conditions, a greater amount of silver nitrate does not improve the yield of nanorods versus spherical (37) Gonzalez, M. C.; Liu, Y.; Scaiano, C. J. J. Phys. Chem. C 2009, 113, 11861– 11867.
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nanoparticles (data not shown). These results are found to be consistent with previous studies where the synthesis of nanorods without additives including silver nitrate is found to produce longer nanorods, but TEM images show a significant number of spherical particles with only a few nanorods.24 To improve the synthesis of nanorods further and to achieve varying aspect ratios, many reaction parameters were evaluated. It was found that the addition of cosolvents such as acetone and THF remarkably improves the synthesis of nanorods, possibly by enhancing the formation of short wormlike micellar structures of CTAB in water, thus allowing the synthesis of nanorods.1,33 Effect of Cosolvent. The importance of CTAB in gold nanorod formation has been well studied.38,39 The addition of CTAB to water produces micellar or vesicular structures, the morphology of which is dependent on the concentration of CTAB in water. The synthesis of nanorods is obtained at a CTAB concentration above its critical micellar concentration, which is found to produce wormlike micelles and vesicles in solution.40,41 Moreover, it has been found that the addition of organic solvents to an aqueous CTAB solution tends to change the morphology and size of the particles.40 Therefore, we studied the effect of the addition of acetone on the size of vesicles produced in a CTABwater solution using dynamic light scattering (DLS) (Supporting Information, Table S5). It was found that, with increasing addition of acetone, the size of the particles (wormlike micelles and vesicles) is reduced significantly (from 500 to ∼117 nm). So far, acetone has been reported to be a critical solvent for the synthesis of nanorods, and its role has been well documented.25,26 The addition of acetone to the aqueous solution of surfactant is thought to produce flexible micellar structures that allows the growth of nanorods in addition to serving as an electron-transfer agent to reduce gold salt to gold metal. Therefore, the presence of acetone is reported to be essential for gold nanorod synthesis involving complex chemistry.25,26 For this reason, most of the reactions involving gold nanorod synthesis are limited to UV irradiation at 254 nm because acetone absorbs UV light at this wavelength to produce the excited state, which is found to be involved in nanorod synthesis.25 An alternative approach is introduced in this research where nanorods are synthesized using a photoinitiator as a source of ketyl radicals and the only purpose of acetone is to produce flexible micelles for the growth of nanorods. We have also demonstrated that the production of flexible micelles of CTAB for the growth of nanorods can be achieved by another water-miscible solvent (THF), which further ensures that in the presence of an instant source of ketyl radicals acetone is not required for nanorod synthesis and can be replaced by other solvents. For this reason, gold nanorods are synthesized in varying acetone or THF concentrations and role of these additives in the synthesis of nanorods is studied in detail. The role of acetone in the synthesis of nanorods has been studied previously; however, no insight into this concentrationdependent nanorod synthesis has been reported because no size control of nanorods as a function of silver ion concentration in this process has been achieved.25 Our results suggest that at the critical concentration of silver nitrate (24 μL of 0.01 M solution) the synthesis of nanorods is dependent on the concentration of acetone or THF in solution. A decrease in the amount of acetone in an aqueous solution of CTAB in the presence of gold salt and (38) Smith, K. D.; Korgel, A. B. Langmuir 2008, 24, 644–649. (39) Smith, K. D.; Miller, R. N.; Korgel, A. B. Langmuir 2009, 25, 9518–9524. (40) Anderson, T. M.; Martin, E. J.; Odinek, G. J.; Newcomer, P. P. Chem. Mater. 1998, 10, 1490–1500. (41) Lin, Z.; Cai, J. J.; Scriven, E. L.; Davis, T. H. J. Phys. Chem. 1994, 98, 5984– 5985.
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silver nitrate shows an increasing trend toward nanorod formation as observed from the UV-vis spectra of these solutions (Supporting Information, Table S1 and Figure S1). At a water to acetone ratio of 4.5, an intense transverse surface plasmon resonance (TSPR) peak at around 520 nm revealed the presence of spherical nanoparticles, and the less-intense second longitudinal SPR (LSPR) peak between 600 and 800 nm is characteristic of the presence of some gold nanorods. A decrease in the concentration of acetone is accompanied by a gradual reduction of the intensity of the transverse surface plasmon resonance peak, along with the improved intensity of the LSPR peak, indicating the presence of nanorods in a higher yield compared to that of nanospheres. Hence, UV-vis spectra of nanorods synthesized at a H2O/acetone ratio of g18:1 are shown to possess a dominant population of nanorods (Figure S1, Supporting Information). The data obtained here for the synthesis of nanorods is in agreement with the study reported by Anderson et al. where the micelle formation of CTAB in an aqueous solution in the presence of organic solvents is studied. It has been found that for increased concentrations of organic solvent, micelle formation significantly decreases in aqueous solution and vice versa.40 Hence, we attributed this trend of increased nanosphere formation at higher acetone concentration to the decreased concentration of CTAB micelles in aqueous solution, and at a H2O/acetone ratio of 2:1 a pink solution is formed, which shows a single SPR peak at ∼520 nm in the UV-vis spectra, indicating the presence of gold nanospheres. As discussed above, the role of acetone in this technique is limited to the formation of flexible micellar surfactant templates in aqueous solution. It should be feasible to use an alternative water-miscible solvent to allow for the synthesis of nanorods using the same conditions as determined for acetone. For this purpose, THF is used as an additive in the aqueous solution of CTAB in the presence of gold and silver salt, at a concentration that is found to be optimum for acetone. The synthesis of nanorods in the presence of 4 UVA lamps, in aqueous solutions containing acetone or THF as an additive, is then compared (Supporting Information, Table S7 and Figure S8). The UV-vis spectra and TEM images of nearly monodisperse nanorods produced in the presence of acetone or THF in an aqueous solution of CTAB at a constant concentration of silver nitrate (and hence silver NPs) are shown (Supporting Information Figure S8). The nanorods that are produced are found to be about 15 nm in diameter for H2O to acetone ratio of 18:1 and are 10 nm in diameter for the same H2O/THF ratio, indicating the effect of solvent on the synthesis of nanorods. The nanorods produced under these conditions are 35 nm in length as a result of the same initial concentration of silver nitrate present in both samples. These results further suggest that the diameter of nanorods is controlled as a function of additives in solution and the length is controlled by the feed concentration of silver nitrate (and subsequently silver nanoparticles after photochemical reduction) in solution. Furthermore, the effect of the concentration of the initial silver nitrate added on the length of the nanorods is studied while using THF as an additive in the aqueous solution of gold salt. The results are found to be similar to those described above in the case of acetone. Briefly, as determined by UV-vis spectra and TEM images, the increase in the concentration of silver nitrate (and hence silver NPs) produces a higher aspect ratio for nanorods, and the effect of the feed concentration of silver nitrate and silver NPs on the elongation of nanorods will be discussed in detail later (Supporting Information, Table S4 and Figures S9 and S10). Effect of the Intensity of UV Radiation. The intensity of UV light is another parameter that is found to play a dominant role in controlling the monodispersity and aspect ratio of Langmuir 2010, 26(23), 18392–18399
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nanoparticles. The role of UV intensity in the synthesis of nanospheres of varying sizes has been well studied by Scaiano et al.31 Miranda et al. has also studied the role of UV intensity at a wavelength of 254 and 300 nm in nanorods synthesis, and it has been found that increasing the intensity of UV light increases the concentration of the nanorods; however, no insight into the effect of UV intensity on the sample polydispersity or size of the nanorods has been reported.25 In an effort to obtain nanorods of higher aspect ratios with controlled monodispersity, the role of UV radiation intensity in the synthesis of nanorods is studied, and it has been found that the intensity of UV radiation plays a critical role in controlling the monodispersity and aspect ratio of nanorods. The results suggest that an increase in the intensity of UV light increases the synthesis of metallic gold nanoparticles, as determined by UV-vis spectra and TEM images of nanorod samples irradiated with 10, 6, or 4 UVA lamps under constant reaction conditions (Supporting Information, Figures S6 and S7 and Table S6). The decrease in the intensity of UV radiation from 10 to 4 UVA lamps allows a comparatively slower growth of nanorods, hence improving their monodispersity and aspect ratios. The change in the aspect ratio of nanorods as a function of the feed concentration of silver nitrate and acetone at higher UV intensities is discussed above. It has been found that nanorods synthesized at a lower UV intensity (4 UVA lamps) produce a significant shift in their aspect ratios as a function of the silver NP concentration, which will be discussed in detail. In a photochemical approach involving photoinitiator I-2959, an increase in UV intensity is associated with an increasing trend toward nanospheres formation regardless of the reaction conditions used in the synthesis. In contrast, a trend toward increased numbers of nanorods formed is observed when the reaction solutions are irradiated at a lower UV intensity. This trend toward the synthesis of metallic nanospheres at a higher intensity of UV irradiation is thought to be attributed to the rapid burst of ketyl radicals produced as a result of high UV intensity, causing a rapid reduction of gold salt and its maturation in a very short period of time (10-20 min), thus providing limited time for the growth of gold nanorods. The decrease in UV intensity is associated with the slower rate of photoinitiator dissociation, providing a relatively slow source of active ketyl radicals for the reduction of Au3þ to Au0 and hence allowing the relatively slow synthesis of nanorods with better monodispersity. These results are also supported by Biswal et al., where gold nanorods are produced using γ radiation and an increased intensity of radiation is found to be related to nanosphere formation in solution.22 Moreover, no silver nitrate concentration-dependent size change of gold nanorods has been observed at various concentrations of aprotic solvents, when a high UV intensity is used for the synthesis (data not shown). These results also suggest that at a higher UV intensity the uncontrolled synthesis of metallic nanoparticles occurs, thus favoring the formation of spherical nanoparticles (thermodynamically favored), and sufficient time is not allowed for the formation of nanorods. The radiation time is also found to affect the synthesis of nanorods. It has been observed that nanorod formation occurs in 15 min upon exposure of the reaction sample to UV radiation; however, by increasing the radiation time from 15 to 30 min under same UV intensity, a shift in the absorbance peak in the UV-visible spectra (toward 520 nm) suggests the formation of spherical nanoparticles. These findings are similar to those reported previously, with the exception that 20 h were required to observe this shift in SPR from nanorods to nanospheres in contrast to 30 min reported here.25 This conversion of nanorods to nanospheres is attributed to the removal of gold atoms from the nanorods upon the continuous DOI: 10.1021/la103339g
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Figure 1. TEM images of gold nanorods with varying aspect ratios synthesized using different concentrations of silver nitrate. (A-C) Images of nanorods synthesized in an 18:1 H2O/acetone ratio have a 15 nm diameter with varying lengths depending upon the concentration of silver nitrate. (D-F) Images of nanorods synthesized in a 36:1 H2O/acetone ratio have a 10 nm diameter with different lengths according to the concentration of silver ions. (A-C) Samples MA-03, MA-04, and MA-01, respectively. (D-F) Samples MA-06, MA-07, and MA-08, respectively.
exposure to UV radiation hence shortening the nanorods to produce nanospheres.25 Further irradiation also leads to disappearance of nanospheres and yellow solution, indicating the formation of gold salt. Effect of Silver Nitrate. The critical concentration of cosolvents and the UV intensity required to produce nearly monodisperse nanorods were then used to produce nanorods with a varying aspect ratio as a function of the feed silver nitrate concentration in solution. The role of silver nitrate in the synthesis and size control of nanorods has been well explored.1,27 The growth of nanorods in the absence of silver ions has also been investigated, and it was found that higher-aspect-ratio gold nanorods are produced in the absence of silver ions but in a lower yield. The length of the nanorods in the absence of silver nitrate is fine tuned by manipulating the concentration and nature of the surfactant, the stability of gold seeds, and the temperature.1 In the photochemical approach described here, a minimum concentration of silver nitrate, and hence silver NPs, is required for the synthesis of nanorods, and below this concentration, a pink solution showing the presence of nanospheres alone has been observed. A general trend is that the increase in the feed concentration of silver nitrate in the solution produces the bathochromic shift of the longitudinal surface resonance peak in the UV-vis spectra (Supporting Information, Table S2 and Figure S2-S4). The shift is more pronounced when lower concentrations of acetone (18:1 and 36:1 H2O/acetone) in an aqueous solution of CTAB are used to synthesize nanorods. However, at very low concentrations of acetone (72:1 H2O/acetone) no shift in the longitudinal surface plasmon peak of the nanorods has been observed, indicating the critical conditions required to synthesize nanorods with varying aspect ratios (Supporting Information, Table S3 and Figure S4). The results are in agreement with other reports, where the presence of silver nitrate is shown to be essential to nanorods synthesis and the aspect ratios of the nanorods are controlled by the concentration of silver ions in solution.28,29 18396 DOI: 10.1021/la103339g
It was found that the increase in silver nitrate (and hence silver NP) concentration in solution leads to an increase in the length of the nanorods, which is apparent from the bathochromic shift of the LSPR peak in the UV-vis spectrum of the nanorod solution and from TEM images of corresponding samples (Figures 1 and 2). This increase in the length of nanorods is also optically observed by the color change of the solution from blue to green to brown. However, it should be noted that a very small increase in the aspect ratio causes the remarkable visible color change of the solution. This optical phenomenon of nanorods has also been discussed before.1 It is also apparent from the TEM images of samples 3-8 that the diameter of the nanorods is dependent on the concentration of acetone whereas the length of the nanorods is controlled by silver ions in solution (Figure 1). At a higher concentration of acetone in solution (18:1 H2O/acetone), nanorods of 15 nm diameter are produced with varying lengths depending upon the feed concentration of silver nitrate as shown in Table 1. Nanorods with aspect ratios of 1.5, 2, and 2.5 were obtained for an 18:1 H2O/acetone ratio at different concentrations of silver NPs (after the photochemical reduction of silver nitrate). Aspect ratios of 2, 2.5, and 3 were obtained for a 36:1 H2O/acetone ratio, as determined by TEM images of samples 3-8. Further increases in silver nanoparticles concentration under these conditions led to unclear solutions and the broadening of surface plasmon resonance peaks, indicating the aggregation of nanoparticles in solution. Effect of Cosurfactant. The facile synthesis of gold nanorods with varying aspect ratios in the absence of a cosurfactant and nonpolar solvents has been achieved. It is found that these additives are indeed not required for the synthesis of nearly monodisperse gold nanorods with varying aspect ratios. However, the aspect ratio of nanorods that were produced was found to be limited to 3, and further increases in the aspect ratio of nanorods could not be achieved by varying the concentration of polar aprotic solvents, the silver NP concentration, and the UV intensity. The role of cosurfactant in increasing the length of Langmuir 2010, 26(23), 18392–18399
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Figure 2. UV-vis spectra of gold nanorods with varying aspect ratios synthesized using different concentrations of silver nitrate in 36:1 (left) and 18:1 (right) H2O/acetone ratios. Table 1. Synthesis of Nanorods with Varying Aspect Ratios under Different Concentrations of Silver Nitrate and Acetone as Additives to the Aqueous Solutionb sample ID
[HAuCl4]/ [CTAB]a/[I]
H2O/ acetone
volume of AgNO3 added (0.01 M) (μL)
color of solution
MA-02 1:53:3 18:1 24 blue MA-03 1:53:3 18:1 36 green MA-01 1:53:3 18:1 42 green MA-04 1:53:3 18:1 72 brown MA-06 1:53:3 36:1 36 blue MA-07 1:53:3 36:1 60 green MA-08 1:53:3 36:1 72 brown a CTAB: hexadecyltrimethylammonium bromide. b The reaction solutions were irradiated for 30 min with 4 UVA lamps.
Table 2. Synthesis of Gold Nanorods with Higher Aspect Ratios Using Organic Solvents and a Cosurfactant as Additives in an Aqueous Solution of CTABa [HAuCl4]/ H2O/ volume of AgNO3 sample [CTAB]c/[I]/ acetone/ solution [0.01M] ID [TDAB] cyclohexane added (μL)
sample color
MA-10 1:53:3:0.08 300:6.5:1 45 green MA-11 1:53:3:0.08 300:6.5:1 80 brown b 300:6.5:1 90 reddish brown MA-12 1:53:3:0.08 a The reaction solutions were irradiated for 30 min with 4 UVA lamps. b Sample MA-12 was synthesized using THF as an additive in aqueous solution. c CTAB: hexadecyltrimethylammonium bromide.
nanorods is contradictory and is found to be dependent on the mechanism of nanorod formation. Juste et al. reported that the presence of a cosurfactant reduces the length of the nanorods as a result of the disruption of the micellar structure of CTAB.42 Similarly, Niidome et al. has reported the synthesis of nanorods with aspect ratios greater than 3 in the absence of a cosurfactant.29 The other reports of gold nanorod synthesis emphasize the importance of binary surfactants in solution to achieve higher aspect ratios.1,25,27,31 We also suggest that in a one-step approach involving a photoinitiator the addition of additives (cosurfactant and cyclohexane) is required to achieve higher aspect ratios and to improve the monodispersity of the nanorod samples. However, the concentration of additives required for this approach is much less than in the traditionally used method because of the limited miscibility of organic additives in aqueous solution.25,27,31 The increase in the concentration of the cosurfactant or cyclohexane is (42) Perez-Juste, J; Liz-Marzan, M. L.; Carnie, S.; Chan, C. Y. D.; Mulvaney, P. Adv. Funct. Mater. 2004, 14, 571–579.
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Figure 3. UV-vis spectra of gold nanorods with varying aspect ratios synthesized under different feed concentrations of silver nitrate in the presence of TDAB as a cosurfactant and cyclohexane.
found to precipitate the reactants from the aqueous solution, and nanoparticle formation has not been observed. Once the critical concentration of additives for the growth of nanorods has been achieved, their presence in solution could increase the aspect ratio up to 4, with improved monodispersity and a higher concentration of nanorods being produced as shown by the UV-vis spectra and TEM images of nanorods (Figures 3 and 4). Key Parameters in Nanorod Synthesis. This study therefore provides key parameters for the synthesis of nanorods by this photochemical process. It is clear that nanorod formation occurs in the presence of silver nanoparticles (after the photochemical reduction of silver ions) at critical concentrations of surfactant (0.06 M) and photoinitiator (4.8 mM) and an optimum UV intensity (26 W/m2) with a cosolvent (18:1). The length of the nanorods is controlled by varying the concentration of silver NPs in solution. The presence of cyclohexane and cosurfactant are found to increase the length of the nanorods further but are not crucial for the synthesis of nanorods. Proposed Mechanism of Nanorod Formation. Several mechanisms have been proposed for the synthesis of nanorods and are related to the way that they are synthesized.1,42 It is argued that the formation of nanorods is dependent on the micellar nature of CTAB in aqueous solution regardless of the addition or absence of silver nitrate.1 So far it is not clear how silver salt contributes to the elongation of nanorods. Under certain conditions, the presence of silver salt is critical to the formation of nanorods, as compared to spherical ones.1 In this photochemical DOI: 10.1021/la103339g
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Figure 4. TEM images of gold nanorods with higher aspect ratios synthesized in the presence of a cosurfactant, cyclohexane, and varying amounts of silver nitrate. (A-C) Samples MA-10, MA-11, and MA-12, respectively. Scheme 1. Mechanism of Nanorod Formation in the Presence of Surfactant and Metallic Nanoparticles
process, we found that the presence of silver metal is actually critical to the formation and elongation of nanorods. Therefore, on the basis of this study, we propose a different mechanism for the formation of nanorods by the photochemical approach. The one-step photochemical reduction of gold and silver salts in the presence of I-2959 produces metallic gold and silver nanoparticles, as shown in Scheme 1.The formation of gold and silver 18398 DOI: 10.1021/la103339g
nanoparticles is supported by the work done by Scaiano and co-workers, who have successfully shown the formation of goldsilver nanoalloys using the same photochemical approach in the presence of high concentrations of hexadecyltrimethylammonium chloride (CTAC) surfactant.37 We suggest that the silver nanoparticles that are produced interact with the CTAB micellar structure, providing a template Langmuir 2010, 26(23), 18392–18399
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for gold nanorods growth by stabilizing the cylindrical nature of the micellar structure, as shown in Scheme 1. This stabilization aids in the nucleation of gold nanoparticles within the stabilized cylindrical micelles. This nucleation process has been suggested by El Sayed et al., where gold nanoparticles aggregate and coalesce in CTAB micelles to produce gold nanorods during a photochemical process.1 On the basis of this proposed mechanism, spherical nanoparticles are formed in the absence of silver nitrate, suggesting the instability of cylindrical CTAB micelles during the nucleation of gold nanoparticles. Increasing the concentration of silver nitrate increases the stability of CTAB micelles, thus allowing an increase in the length of nanorods up to a critical concentration.
Acknowledgment. We acknowledge financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC) for this work.
Conclusions We report here a rapid one-pot photochemical strategy that uses a photoinitiator as a source of ketyl radicals for the synthesis of gold nanorods of varying sizes. We describe in detail how the solvent composition, additives, and UV intensity can affect the
Supporting Information Available: Different parameters for the synthesis of gold nanorods, UV-vis spectra,TEM images, and DLS results of CTAB micelles in aqueous solutions. This material is available free of charge via the Internet at http://pubs.acs.org.
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growth of nanorods. The synthesis parameters were carefully monitored for the formation of gold nanorods with varying aspect ratios in high yield. This approach also provides a possible route to synthesize nanorods in a variety of solvents, except acetone, and the role of solvent (THF) in the synthesis of nanorods is explored in detail. Further studies are in progress to synthesize monodisperse nanorods coated with different cationic polymers in one step using the photochemical approach.
DOI: 10.1021/la103339g
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