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Aspect-ratio-controlled synthesis of high-aspect-ratio gold nanorods in high-yield
Current Applied Physics 9 (2009) e140–e143
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Aspect-ratio-controlled synthesis of high-aspect-ratio gold nanorods in high-yield Won Min Park a, Yun Suk Huh b, Won Hi Hong a,* a b
Department of Chemical and Biomolecular Engineering, KAIST, Daejon 305-701, Republic of Korea Sibley School of Mechanical and Aerospace Engineering, Cornell University, Ithaca, NY 14853, United States
a r t i c l e
i n f o
Article history: Available online 12 March 2009 PACS: 81.16.Be 81.07.Bc 78.67.Bf Keywords: Gold nanorods Aspect ratio Seed-mediated synthesis Plasmon
a b s t r a c t We describe a modified seed-mediated synthesis of high-aspect-ratio gold nanorods controlling the aspect ratio with variation of pH in the growth solution. By adding various amounts of sodium hydroxide, pH in the growth solution was controlled from 1.29 to 7.06 in the presence of nitrate anions. At various pH of the growth solution, the yield of high-aspect-ratio gold nanorods was significantly improved by the role of nitrate anions, which reduce the formation of spherical nanoparticles and triangular nanoplates. The synthesized high-aspect-ratio gold nanorods have diameters of 23–25 nm at all pH conditions and the mean lengths are from 589 to 469 nm with variation of pH. On increasing pH, the mean aspect ratio of gold nanorods decreased from 25.6 to 20.0. This result shows that the aspect ratio of gold nanorods follows a trend as a function of pH in the growth solution. The intensity of the plasmon band of wavelength 780 nm, which resulted from triangular nanoplates, gradually decreased with increasing pH. This observation indicates that pH of the growth solution also influences the formation of triangular nanoplates. A transverse plasmon band of gold nanorods appeared at a wavelength of 510 nm, and a longitudinal plasmon band was observed over 2000 nm in UV–vis–NIR absorption spectra. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Anisotropic gold nanomaterials have interesting properties, such as strong surface-enhanced Raman scattering (SERS) [1] and localized surface plasmon resonance (LSPR) strongly dependent on the aspect ratio [2,3], upon which some plasmonic and electronic applications are based [4–9]. An important requirement in the synthesis of anisotropic gold nanomaterials, particularly gold nanorods, is control over the size and aspect ratio in reasonable quantities [10,11]. In principle, the synthetic growth of gold nanorods can be achieved by the formation of rod-like micelles in seedmediated growth [12,13], and the aspect ratio can be manipulated by the size and nature of the seeds [14]. Also, additives such as silver ions for the nanorod growth play an important role to enhance the yield of gold nanorods by up to nearly 100%; however, the proportion of gold nanorods of high-aspect-ratio is low in the presence of silver ions [15]. High-aspect-ratio gold nanorods (25) have been made by the seed-mediated growth approach, focusing on the improvement of yield because high-yield is an important requirement for further applications of long gold nanorods. Murphy et al. [16] reported that the growth of gold nanorods at slightly higher pH enabled production of gold nanorods of high-aspect-ratio in 90% yield, with simple purification. Huang et al. [17,18] recently proposed a modified
* Corresponding author. E-mail address: [email protected] (W.H. Hong). 1567-1739/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2009.03.007
seed-mediated synthetic method using nitric acid as an additive that enhances the yield of long gold nanorods. Here, we report a seed-mediated synthesis of high-aspect-ratio gold nanorods with some modifications to control the aspect ratio. In the presence of nitric acid, high-yield synthesis of long nanorods and fine control of aspect ratio based upon pH variation in the growth solution were achieved. 2. Experimental The experimental procedures for the synthesis of high-aspectratio gold nanorods reported by Murphy et al. [16] and Huang et al. [17,18] were followed, with some modifications. For the preparation of gold seeds, 0.25 ml of 0.01 M HAuCl4 (Aldrich, P99.9%) in deionized water was added to 9.2 ml of 0.1 M cetyltrimethylammonium bromide (CTAB) (Sigma, 99%) in water. 1.0 10 4 mol of NaBH4 (Aldrich, 99%) was added in 10 ml of ice-cold water and gently stirred for 10 s. Five hundred microliter of the NaBH4 solution was slowly added to the prepared HAuCl4/CTAB solution. The solution initially became colorless, and then the color changed to faint dark brown. The growth solution containing 2.5 10 4 M HAuCl4, 0.1 M CTAB and 5.0 10 4M L-ascorbic acid (Aldrich, P99%) in deionized water was prepared and transferred into 8 conical flasks labeled A, B, C1, C2, C3, C4, C5, and C6, respectively. One milliliter of the prepared gold seeds was added to 9 ml of the growth solution A and stirred for 10 s. Next, 1.0 ml of the mixture of gold seeds and growth solution A was transferred to 9 ml of the
W.M. Park et al. / Current Applied Physics 9 (2009) e140–e143
growth solution B and stirred for 15 s. From this mixture solution, 1.0 ml was added to 9 ml of growth solutions in various pH conditions (C1, C2, C3, C4, C5, C6), which were prepared by adding 30 ll of 0.1 M nitric acid (Dae Jung) and various amounts of 0.1 M sodium hydroxide (Junsei) solution in water (0, 20, 40, 60, 80, 100 ll) into the initially prepared growth solution. Then, the growth solutions (C1–C6) were allowed to sit overnight after gentle stirring for 15 s. After 18 h, each of the supernatant solutions was removed and 5 ml of water was added to redisperse the precipitates. The redispersed solutions mostly containing long gold nanorods were centrifuged at 2000 rpm for 20 min (Hanil Supra 22 K), and each of the supernatants was carefully removed again. One microliter of the concentrated nanorods was added on the silicon substrate and evaporated for imaging by scanning electron microscopy (SEM). SEM images were obtained by using a FEI Sirion model. Ten microliter of each precipitate was redispersed in 1 ml of water, and the UV–vis absorption spectra of gold nanorod solutions were recorded on a Mecasys Optizen 3200 UV. For the near-IR absorption spectra, a Jasco V-600 spectrophotometer was used. 3. Results and discussion In this modified seed-mediated synthesis of gold nanorods, nitric acid served as an additive to improve the percent yield of high-aspect-ratio gold nanorods. Huang et al. recently demonstrated that nitrate ions in the growth solution could play a key role to grow gold nanorods exceptionally long [17,18]. SEM images
e141
of gold nanorods shown in Fig. 1(A–F) indicate that high-yield synthesis of long gold nanorods was achieved in the presence of nitrate ions in various pH conditions. They self-assembled in ordered three-dimensional arrays on the silicon substrate, aligning end-to-end and side-by-side when the SEM sample was prepared. For all the gold nanorods grown with variation of the pH conditions, each percent yield calculated in the SEM images was 90%, and it is clear that variation of pH in the growth solution does not influence the yield of long nanorods. Each percent yield of gold nanorods whose aspect ratio is higher than 15 is shown in Fig. 2. The high-aspect-ratio gold nanorods had a mean width of 23– 25 nm; however, the mean length of the gold nanorods was shortened with an increase of pH in the growth solution. At pH 1.29, with no additional sodium hydroxide, the mean length of the gold nanorods was 589 ± 44 nm, and this decreased to 469 ± 44 nm as pH in the growth solution was increased to 7.06. The width, length, pH in the growth solutions, and aspect ratio of the gold nanorods are displayed in Table 1. It can be seen from Table 1 that the variation in length of the gold nanorods is dependent on the pH, whereas their width is not influenced by pH in the growth solution. In addition, the aspect ratio of the gold nanorods follows a trend as a function of pH. Fig. 2 shows the variation of the aspect ratios of the gold nanorods grown at different pH. It is interesting to note that there is an exponential decay in the aspect ratio with an increase of pH. As pH in the growth solutions was slightly increased from 1.29 to 1.84, the averaged aspect ratio of the gold nanorods was considerably reduced from 25.6 ± 2.4 to 20.0 ± 1.9, whereas
Fig. 1. (A–F) SEM images of high-aspect-ratio gold nanorods synthesized in growth solutions at different pH: (A) 1.29, (B) 1.55, (C) 1.84, (D) 2.33, (E) 4.25, (F) 7.06. The scale bar in each image measures 1 lm.
e142
W.M. Park et al. / Current Applied Physics 9 (2009) e140–e143
30
100
0.30
80
24
60
22 40
20
Percent yield (%)
Aspect ratio
26
Absorbance (arb. units)
28
0.25 pH 1.09
0.20 0.15
pH 7.06
0.10
(A) (B) (C) (D) (E) (F)
0.05
18 20 percent yield (%) aspect ratio
16
0.00 400
14 2
4
6
8
800
1000
Fig. 3. UV–vis absorption spectra of the gold nanorods synthesized in growth solutions at different pH: (A) 1.29, (B) 1.55, (C) 1.84, (D) 2.33, (E) 4.25, (F) 7.06.
pH in the growth solution Fig. 2. Aspect ratio and percent yield of high-aspect-ratio gold nanorods grown at pH 1.29–7.06. The solid line is an exponential fit to the data. The vertical lines at the data points are error bars.
Table 1 Averaged width, length, and aspect ratio of the long nanorods synthesized in growth solutions at different pH. pH Width (nm) Length (nm) Aspect ratio
600
Wavelength (nm)
0
1.29 23.0 ± 1.0
1.55 24.0 ± 1.2
1.84 25.0 ± 2.4
2.33 24.6 ± 0.8
4.25 24.6 ± 4.1
7.06 23.5 ± 0.9
589 ± 44
557 ± 40
547 ± 66
517 ± 32
479 ± 60
469 ± 44
25.6 ± 2.4
23.2 ± 2.3
21.9 ± 2.8
21.1 ± 1.3
19.5 ± 1.9
20.0 ± 1.9
gold nanorods appeared at a wavelength of 510 nm, and a broad band at a wavelength of 780 nm was observed, which can be attributed to triangular nanoplates 100–120 nm in width [18]. It is noticeable that this absorbance gradually decreased as pH in the growth solution increased. The variation of the plasmon absorption band of the triangular nanoplates can be pronounced as a function of pH, and an increase of pH is believed to reduce the formation of the triangular nanoplates. The longitudinal plasmon absorption band of the gold nanorods, which were dried on the microscope cover glass in order to overcome absorption interference from water, was observed over 2000 nm; however, it could not be accurately recorded, due to the plasmon coupling induced by the alignment of the gold nanorods. 4. Conclusion
there was only a moderate reduction of the aspect ratio for a further increase of pH. What is the mechanism for the reduction of the aspect ratio of the gold nanorods induced by pH in the growth solution? Other workers found that an increase of the pH from 2.8 to 3.5, which represents an increase in the fraction of ascorbate monoanion, resulted in high-aspect-ratio nanomaterials, and ascorbate is an effective reductant in the presence of CTAB [16]. However, it can be seen from the results in Fig. 1 that the proposed role of the pH is not valid in the presence of nitrate ions, since no trends between pH and the proportion of gold nanorods were found, except in that some short nanorods were found at pH 1.84 and 4.25. Wang et al. [19] have recently reported that adjusting the pH value of the reaction medium can lead to the synthesis of multiple shapes of gold nanostructural architectures using gold nanorods as seeds through seed-mediated growth. They explained in their study that the relatively large amounts of OH ions in the growth solution at high pH might form ion pairs with CTA+ through electrostatic force reducing the amount of CTA+ packed on the {1 1 0} face of the gold nanorods. We believe that, in our experiment, a destabilization of the CTAB bilayer on the {1 1 0} face of the gold nanorods might exist in the growth as a result of ion pair formation between CTAB and OH ions. At the same time, the effect of nitrate ions that can assist the formation of elongated micelles [18] seems dominant, since there was no significant decrease in the proportion of long nanorods. Therefore, we postulate that the nitrate ions mainly enhance the formation of rod-like CTAB micelles and there might be an effect of OH ions that may destabilize the CTAB bilayer during the growth of gold nanorods and reduce their aspect ratio. Fig. 3 shows UV–vis spectra of the long gold nanorods dispersed in deionized water. The transverse plasmon absorption band of the
In summary, high-aspect-ratio gold nanorods with controllable aspect ratios were synthesized by a modified seed-mediated method in the presence of nitric acid varying the pH in the growth solution. It is believed that the variation of the aspect ratio as a function of pH is possibly based on the destabilization of the CTAB bilayer during the growth of the gold nanorods, although nitrate ions in the growth solution assist the formation of elongated CTAB micelles and improve the percent yield of long gold nanorods. Also, the formation of triangular nanoplates seems to be reduced by an increase of pH. The most useful aspect ratio of long gold nanorods has not been reported yet, since application is only just beginning. With fine control of the aspect ratio of long gold nanorods, their use may become more practical in nanostructure fabrication and plasmonic biosensing. Acknowledgements This work was supported in part by the IT Leading R&D Support Project from the Ministry of Knowledge Economy through IITA. References [1] B. Nikoobakht, J. Wang, M.A. El-Sayed, Chem. Phys. Lett. 366 (2002) 17. [2] K.L. Kelly, E. Coronado, L.L. Zhao, G.C. Schatz, J. Phys. Chem. B 107 (2003) 668. [3] S.-S. Chang, C.-W. Shih, C.-D. Chen, W.-C. Lai, C.R.C. Wang, Langmuir 15 (1999) 701. [4] S.M. Marinakos, S. Chen, A. Chilkoti, Anal. Chem. 79 (2007) 5278. [5] C.-D. Chen, S.-F. Cheng, L.-K. Chau, C.R.C. Wang, Biosens. Bioelectron. 22 (2007) 926. [6] C. Yu, J. Irudayaraj, Anal. Chem. 79 (2007) 572. [7] N.R. Jana, T. Pal, Adv. Mater. 19 (2007) 1761. [8] R.S. Norman, J.W. Stone, A. Gole, C.J. Murphy, T.L. Sabo-Attwood, Nano Lett. 8 (2008) 302.
W.M. Park et al. / Current Applied Physics 9 (2009) e140–e143 [9] [10] [11] [12]
U. Yogeswaran, S.-M. Chen, Sensors 8 (2008) 290. N.R. Jana, L. Gearheart, C.J. Murphy, J. Phys. Chem. B 105 (2001) 4065. C.J. Murphy, N.R. Jana, Adv. Mater. 14 (2002) 80. C.J. Murphy, T.K. Sau, A.M. Gole, C.J. Orendorff, J. Gao, L. Gou, S.E. Hunyadi, T. Li, J. Phys. Chem. B 109 (2005) 13857. [13] N.R. Jana, L. Gearheart, C.J. Murphy, Adv. Mater. 13 (2001) 1389. [14] A. Gole, C.J. Murphy, Chem. Mater. 16 (2004) 3633.
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[15] T.K. Sau, C.J. Murphy, Langmuir 20 (2004) 6416. [16] B.D. Busbee, S.O. Obare, C.J. Murphy, Adv. Mater. 15 (2003) 414. [17] H.-Y. Wu, H.-C. Chu, T.-J. Kuo, C.-L. Kuo, M.H. Huang, Chem. Mater. 17 (2005) 6447. [18] H.-Y. Wu, W.-L. Huang, M.H. Huang, Cryst. Growth Des. 7 (2007) 831. [19] C. Wang, T. Wang, Z. Ma, Z. Su, Nanotechnology 16 (2005) 2555.
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Aspect-ratio-controlled synthesis of high-aspect-ratio gold nanorods in high-yield Won Min Park a, Yun Suk Huh b, Won Hi Hong a,* a b
Department of Chemical and Biomolecular Engineering, KAIST, Daejon 305-701, Republic of Korea Sibley School of Mechanical and Aerospace Engineering, Cornell University, Ithaca, NY 14853, United States
a r t i c l e
i n f o
Article history: Available online 12 March 2009 PACS: 81.16.Be 81.07.Bc 78.67.Bf Keywords: Gold nanorods Aspect ratio Seed-mediated synthesis Plasmon
a b s t r a c t We describe a modified seed-mediated synthesis of high-aspect-ratio gold nanorods controlling the aspect ratio with variation of pH in the growth solution. By adding various amounts of sodium hydroxide, pH in the growth solution was controlled from 1.29 to 7.06 in the presence of nitrate anions. At various pH of the growth solution, the yield of high-aspect-ratio gold nanorods was significantly improved by the role of nitrate anions, which reduce the formation of spherical nanoparticles and triangular nanoplates. The synthesized high-aspect-ratio gold nanorods have diameters of 23–25 nm at all pH conditions and the mean lengths are from 589 to 469 nm with variation of pH. On increasing pH, the mean aspect ratio of gold nanorods decreased from 25.6 to 20.0. This result shows that the aspect ratio of gold nanorods follows a trend as a function of pH in the growth solution. The intensity of the plasmon band of wavelength 780 nm, which resulted from triangular nanoplates, gradually decreased with increasing pH. This observation indicates that pH of the growth solution also influences the formation of triangular nanoplates. A transverse plasmon band of gold nanorods appeared at a wavelength of 510 nm, and a longitudinal plasmon band was observed over 2000 nm in UV–vis–NIR absorption spectra. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Anisotropic gold nanomaterials have interesting properties, such as strong surface-enhanced Raman scattering (SERS) [1] and localized surface plasmon resonance (LSPR) strongly dependent on the aspect ratio [2,3], upon which some plasmonic and electronic applications are based [4–9]. An important requirement in the synthesis of anisotropic gold nanomaterials, particularly gold nanorods, is control over the size and aspect ratio in reasonable quantities [10,11]. In principle, the synthetic growth of gold nanorods can be achieved by the formation of rod-like micelles in seedmediated growth [12,13], and the aspect ratio can be manipulated by the size and nature of the seeds [14]. Also, additives such as silver ions for the nanorod growth play an important role to enhance the yield of gold nanorods by up to nearly 100%; however, the proportion of gold nanorods of high-aspect-ratio is low in the presence of silver ions [15]. High-aspect-ratio gold nanorods (25) have been made by the seed-mediated growth approach, focusing on the improvement of yield because high-yield is an important requirement for further applications of long gold nanorods. Murphy et al. [16] reported that the growth of gold nanorods at slightly higher pH enabled production of gold nanorods of high-aspect-ratio in 90% yield, with simple purification. Huang et al. [17,18] recently proposed a modified
* Corresponding author. E-mail address: [email protected] (W.H. Hong). 1567-1739/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2009.03.007
seed-mediated synthetic method using nitric acid as an additive that enhances the yield of long gold nanorods. Here, we report a seed-mediated synthesis of high-aspect-ratio gold nanorods with some modifications to control the aspect ratio. In the presence of nitric acid, high-yield synthesis of long nanorods and fine control of aspect ratio based upon pH variation in the growth solution were achieved. 2. Experimental The experimental procedures for the synthesis of high-aspectratio gold nanorods reported by Murphy et al. [16] and Huang et al. [17,18] were followed, with some modifications. For the preparation of gold seeds, 0.25 ml of 0.01 M HAuCl4 (Aldrich, P99.9%) in deionized water was added to 9.2 ml of 0.1 M cetyltrimethylammonium bromide (CTAB) (Sigma, 99%) in water. 1.0 10 4 mol of NaBH4 (Aldrich, 99%) was added in 10 ml of ice-cold water and gently stirred for 10 s. Five hundred microliter of the NaBH4 solution was slowly added to the prepared HAuCl4/CTAB solution. The solution initially became colorless, and then the color changed to faint dark brown. The growth solution containing 2.5 10 4 M HAuCl4, 0.1 M CTAB and 5.0 10 4M L-ascorbic acid (Aldrich, P99%) in deionized water was prepared and transferred into 8 conical flasks labeled A, B, C1, C2, C3, C4, C5, and C6, respectively. One milliliter of the prepared gold seeds was added to 9 ml of the growth solution A and stirred for 10 s. Next, 1.0 ml of the mixture of gold seeds and growth solution A was transferred to 9 ml of the
W.M. Park et al. / Current Applied Physics 9 (2009) e140–e143
growth solution B and stirred for 15 s. From this mixture solution, 1.0 ml was added to 9 ml of growth solutions in various pH conditions (C1, C2, C3, C4, C5, C6), which were prepared by adding 30 ll of 0.1 M nitric acid (Dae Jung) and various amounts of 0.1 M sodium hydroxide (Junsei) solution in water (0, 20, 40, 60, 80, 100 ll) into the initially prepared growth solution. Then, the growth solutions (C1–C6) were allowed to sit overnight after gentle stirring for 15 s. After 18 h, each of the supernatant solutions was removed and 5 ml of water was added to redisperse the precipitates. The redispersed solutions mostly containing long gold nanorods were centrifuged at 2000 rpm for 20 min (Hanil Supra 22 K), and each of the supernatants was carefully removed again. One microliter of the concentrated nanorods was added on the silicon substrate and evaporated for imaging by scanning electron microscopy (SEM). SEM images were obtained by using a FEI Sirion model. Ten microliter of each precipitate was redispersed in 1 ml of water, and the UV–vis absorption spectra of gold nanorod solutions were recorded on a Mecasys Optizen 3200 UV. For the near-IR absorption spectra, a Jasco V-600 spectrophotometer was used. 3. Results and discussion In this modified seed-mediated synthesis of gold nanorods, nitric acid served as an additive to improve the percent yield of high-aspect-ratio gold nanorods. Huang et al. recently demonstrated that nitrate ions in the growth solution could play a key role to grow gold nanorods exceptionally long [17,18]. SEM images
e141
of gold nanorods shown in Fig. 1(A–F) indicate that high-yield synthesis of long gold nanorods was achieved in the presence of nitrate ions in various pH conditions. They self-assembled in ordered three-dimensional arrays on the silicon substrate, aligning end-to-end and side-by-side when the SEM sample was prepared. For all the gold nanorods grown with variation of the pH conditions, each percent yield calculated in the SEM images was 90%, and it is clear that variation of pH in the growth solution does not influence the yield of long nanorods. Each percent yield of gold nanorods whose aspect ratio is higher than 15 is shown in Fig. 2. The high-aspect-ratio gold nanorods had a mean width of 23– 25 nm; however, the mean length of the gold nanorods was shortened with an increase of pH in the growth solution. At pH 1.29, with no additional sodium hydroxide, the mean length of the gold nanorods was 589 ± 44 nm, and this decreased to 469 ± 44 nm as pH in the growth solution was increased to 7.06. The width, length, pH in the growth solutions, and aspect ratio of the gold nanorods are displayed in Table 1. It can be seen from Table 1 that the variation in length of the gold nanorods is dependent on the pH, whereas their width is not influenced by pH in the growth solution. In addition, the aspect ratio of the gold nanorods follows a trend as a function of pH. Fig. 2 shows the variation of the aspect ratios of the gold nanorods grown at different pH. It is interesting to note that there is an exponential decay in the aspect ratio with an increase of pH. As pH in the growth solutions was slightly increased from 1.29 to 1.84, the averaged aspect ratio of the gold nanorods was considerably reduced from 25.6 ± 2.4 to 20.0 ± 1.9, whereas
Fig. 1. (A–F) SEM images of high-aspect-ratio gold nanorods synthesized in growth solutions at different pH: (A) 1.29, (B) 1.55, (C) 1.84, (D) 2.33, (E) 4.25, (F) 7.06. The scale bar in each image measures 1 lm.
e142
W.M. Park et al. / Current Applied Physics 9 (2009) e140–e143
30
100
0.30
80
24
60
22 40
20
Percent yield (%)
Aspect ratio
26
Absorbance (arb. units)
28
0.25 pH 1.09
0.20 0.15
pH 7.06
0.10
(A) (B) (C) (D) (E) (F)
0.05
18 20 percent yield (%) aspect ratio
16
0.00 400
14 2
4
6
8
800
1000
Fig. 3. UV–vis absorption spectra of the gold nanorods synthesized in growth solutions at different pH: (A) 1.29, (B) 1.55, (C) 1.84, (D) 2.33, (E) 4.25, (F) 7.06.
pH in the growth solution Fig. 2. Aspect ratio and percent yield of high-aspect-ratio gold nanorods grown at pH 1.29–7.06. The solid line is an exponential fit to the data. The vertical lines at the data points are error bars.
Table 1 Averaged width, length, and aspect ratio of the long nanorods synthesized in growth solutions at different pH. pH Width (nm) Length (nm) Aspect ratio
600
Wavelength (nm)
0
1.29 23.0 ± 1.0
1.55 24.0 ± 1.2
1.84 25.0 ± 2.4
2.33 24.6 ± 0.8
4.25 24.6 ± 4.1
7.06 23.5 ± 0.9
589 ± 44
557 ± 40
547 ± 66
517 ± 32
479 ± 60
469 ± 44
25.6 ± 2.4
23.2 ± 2.3
21.9 ± 2.8
21.1 ± 1.3
19.5 ± 1.9
20.0 ± 1.9
gold nanorods appeared at a wavelength of 510 nm, and a broad band at a wavelength of 780 nm was observed, which can be attributed to triangular nanoplates 100–120 nm in width [18]. It is noticeable that this absorbance gradually decreased as pH in the growth solution increased. The variation of the plasmon absorption band of the triangular nanoplates can be pronounced as a function of pH, and an increase of pH is believed to reduce the formation of the triangular nanoplates. The longitudinal plasmon absorption band of the gold nanorods, which were dried on the microscope cover glass in order to overcome absorption interference from water, was observed over 2000 nm; however, it could not be accurately recorded, due to the plasmon coupling induced by the alignment of the gold nanorods. 4. Conclusion
there was only a moderate reduction of the aspect ratio for a further increase of pH. What is the mechanism for the reduction of the aspect ratio of the gold nanorods induced by pH in the growth solution? Other workers found that an increase of the pH from 2.8 to 3.5, which represents an increase in the fraction of ascorbate monoanion, resulted in high-aspect-ratio nanomaterials, and ascorbate is an effective reductant in the presence of CTAB [16]. However, it can be seen from the results in Fig. 1 that the proposed role of the pH is not valid in the presence of nitrate ions, since no trends between pH and the proportion of gold nanorods were found, except in that some short nanorods were found at pH 1.84 and 4.25. Wang et al. [19] have recently reported that adjusting the pH value of the reaction medium can lead to the synthesis of multiple shapes of gold nanostructural architectures using gold nanorods as seeds through seed-mediated growth. They explained in their study that the relatively large amounts of OH ions in the growth solution at high pH might form ion pairs with CTA+ through electrostatic force reducing the amount of CTA+ packed on the {1 1 0} face of the gold nanorods. We believe that, in our experiment, a destabilization of the CTAB bilayer on the {1 1 0} face of the gold nanorods might exist in the growth as a result of ion pair formation between CTAB and OH ions. At the same time, the effect of nitrate ions that can assist the formation of elongated micelles [18] seems dominant, since there was no significant decrease in the proportion of long nanorods. Therefore, we postulate that the nitrate ions mainly enhance the formation of rod-like CTAB micelles and there might be an effect of OH ions that may destabilize the CTAB bilayer during the growth of gold nanorods and reduce their aspect ratio. Fig. 3 shows UV–vis spectra of the long gold nanorods dispersed in deionized water. The transverse plasmon absorption band of the
In summary, high-aspect-ratio gold nanorods with controllable aspect ratios were synthesized by a modified seed-mediated method in the presence of nitric acid varying the pH in the growth solution. It is believed that the variation of the aspect ratio as a function of pH is possibly based on the destabilization of the CTAB bilayer during the growth of the gold nanorods, although nitrate ions in the growth solution assist the formation of elongated CTAB micelles and improve the percent yield of long gold nanorods. Also, the formation of triangular nanoplates seems to be reduced by an increase of pH. The most useful aspect ratio of long gold nanorods has not been reported yet, since application is only just beginning. With fine control of the aspect ratio of long gold nanorods, their use may become more practical in nanostructure fabrication and plasmonic biosensing. Acknowledgements This work was supported in part by the IT Leading R&D Support Project from the Ministry of Knowledge Economy through IITA. References [1] B. Nikoobakht, J. Wang, M.A. El-Sayed, Chem. Phys. Lett. 366 (2002) 17. [2] K.L. Kelly, E. Coronado, L.L. Zhao, G.C. Schatz, J. Phys. Chem. B 107 (2003) 668. [3] S.-S. Chang, C.-W. Shih, C.-D. Chen, W.-C. Lai, C.R.C. Wang, Langmuir 15 (1999) 701. [4] S.M. Marinakos, S. Chen, A. Chilkoti, Anal. Chem. 79 (2007) 5278. [5] C.-D. Chen, S.-F. Cheng, L.-K. Chau, C.R.C. Wang, Biosens. Bioelectron. 22 (2007) 926. [6] C. Yu, J. Irudayaraj, Anal. Chem. 79 (2007) 572. [7] N.R. Jana, T. Pal, Adv. Mater. 19 (2007) 1761. [8] R.S. Norman, J.W. Stone, A. Gole, C.J. Murphy, T.L. Sabo-Attwood, Nano Lett. 8 (2008) 302.
W.M. Park et al. / Current Applied Physics 9 (2009) e140–e143 [9] [10] [11] [12]
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