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J. Phys. Chem. B 2006, 110, 3990-3994
Quantitation of Metal Content in the Silver-Assisted Growth of Gold Nanorods Christopher J. Orendorff and Catherine J. Murphy* Department of Chemistry and Biochemistry, UniVersity of South Carolina, 631 Sumter Street, Columbia, South Carolina 29208 ReceiVed: December 6, 2005; In Final Form: January 5, 2006
The seed-mediated approach to making gold nanorods in aqueous surfactant solutions has become tremendously popular in recent years. Unlike the use of strong chemical reductants to make spherical gold nanoparticles, the growth of gold nanorods requires weak reducing conditions, leading to an unknown degree of gold reduction. The metal content of gold nanorods, made in high yield in the presence of silver ion, is determined by inductively coupled plasma atomic emission spectroscopy. Through the use of the known gold concentration in nanorods, molar extinction coefficients are calculated for nanorods of varying aspect ratios from 2.0 to 4.5. The extinction coefficients at the longitudinal plasmon band peak maxima for these nanorods vary from 2.5 × 109 to 5.5 × 109 M-1 cm-1, respectively, on a per-particle basis. Many of the gold ions present in the growth solution remain unreacted; insights into the growth mechanism of gold nanorods are discussed.
Introduction There has been significant interest in metallic nanoparticles over the past several years because of their unique shape- and size-dependent physical properties that have been exploited for a plethora of applications including optical sensing, catalysis, and nanoscale electronics.1 Much of the focus on spectroscopic applications of metal nanoparticles is devoted to those of gold and silver because they have intense plasmon absorption bands in the visible wavelength regime. Visible plasmon absorption makes gold and silver nanoparticles attractive substrates for surface-enhanced Raman scattering (SERS),2,3 surface-enhanced fluorescence (SEF),4 two-photon luminescence,5 and enhanced surface plasmon resonance (SPR) spectroscopy.6 Recently, our laboratory has developed seed-mediated synthetic procedures to prepare gold nanoparticles of anisotropic shapes including stars, tetrapods, triangles, cubes, and rods.7,8 Rod-shaped gold nanoparticles are optically interesting because the two principle plasmon absorption bands are tunable with aspect ratio through much of the visible and near-IR wavelength regime. Despite the enormous popularity of these materials as SERS substrates,9-11 biorecognition platforms,12,13 and their potential use as components for solid-state electronics,14 there are many unanswered questions about the growth mechanisms of these materials using the seed-mediated approach and the role of additive ions, such as Ag+, in these preparations.15 In the conventional preparation of spherical gold nanoparticles via chemical reduction, strong reducing conditions are used such that gold ions are completely reduced from Au3+, or Au+, to Au0. In the preparation of anisotropic gold nanorods, weak reducing agents are used and the assumption of 100% gold reduction does not hold under these conditions.16 Therefore, quantifying the amount of gold reduced during these reactions is important for understanding the growth mechanism of these materials. In addition, as these materials are used for increasing numbers of applications, a simple means to quantify nanorod concentrations becomes increasingly critical. Since gold nano* Author to whom correspondence should be addressed. E-mail:
[email protected].
rods have strong plasmon absorption bands in the visible spectrum, the use of their extinction coefficients may be the easiest way to measure nanorod concentrations. Determining extinction coefficients for anisotropic nanomaterials, in general, can be challenging because of variability in nanoparticle dimensions, low yield of the desired shape/size, and side products of unwanted nanoparticle shapes. Perhaps the most critical challenge is in determining the quantity of gold that actually makes it into the nanorods because the assumption of 100% gold reduction is likely not valid. The use of Ag+ to assist in the growth of gold nanorods is common using the seed-mediated approach in surfactants17-20 and has also been used in the photochemical synthesis of gold nanorods.21 While the presence of Ag+ appears to be critical for generating high yields of rod-shaped gold nanoparticles with controllable aspect ratios, the final form of silver that exists in these gold nanorods is unknown. Through the use of the seeding approach, it was initially proposed that the presence of Ag0 is less likely than Ag+ simply because the weak reducing agent, ascorbate, is too weak to reduce Ag+ at low pH.17,18 Under these reaction conditions, the amounts of Ag+ and Br- (from the shape-directing surfactant cetyltrimethylammonium bromide, CTAB) are high enough that AgBr precipitate should be formed and may deposit on the nanorod surface, terminating nanorod growth.17,19 Recently, Guyot-Sionnest described how Ag0 underpotential deposition (UPD) on the growing gold nanorods contributes to the role of silver in gold nanorod growth.20 Even though ascorbic acid is not strong enough to reduce Ag+ at low pH, Ag+ could be reduced at the growing gold nanorod surface at a potential less than its standard reduction potential to form monolayers or submonolayers of Ag0.20 In addition to knowing the oxidation state of silver present in gold nanorods, determining the amount of silver in nanorods would also be useful to further understand the growth mechanism of nanorods. In this report, elemental analysis for Au and Ag are determined for gold nanorods that are uniform in size and have aspect ratios from 2.0 to 4.5. Through the use of the gold concentration, wavelength-dependent extinction coefficients are calculated for both transverse and longitudinal absorption bands
10.1021/jp0570972 CCC: $33.50 © 2006 American Chemical Society Published on Web 02/03/2006
Ag-Assisted Growth of Au Nanorods of nanorods. These data will be used to develop an understanding between nanorod aspect ratio and extinction coefficient. In addition, quantitation of silver in or on the nanorod may also give some insight into the role of silver ions in the preparation of these anisotropic nanoparticles. Experimental Section Materials. Chloroauric acid, ascorbic acid, sodium borohydride, and silver nitrate were purchased from Aldrich. Cetyltrimethylammonium bromide (CTAB) was purchased from Sigma. Deionized ultrafiltered (DIUF) water was purchased from Fisher Scientific. All reagents were used as received. Prior to use, all glassware was washed with aqua regia (3:1 (v/v) concentrated HCl/concentrated HNO3) and rinsed with copious amounts of deionized water. Methods. Gold nanorods, aspect ratios 2.0 ( 0.5 (length 36.0 ( 6.5 nm, width 18.2 ( 2.9 nm), 2.9 ( 0.5 (length 41.1 ( 6.7 nm, width 14.3 ( 2.4 nm), 3.5 ( 0.8 (length 42.0 ( 9.5 nm, width 12.3 ( 2.4 nm), and 4.5 ( 0.6 (length 52.5 ( 6.3 nm, width 11.8 ( 1.8 nm), were prepared from a seed-mediated surfactant-directed approach described previously.17-19 Briefly, spherical gold seed particles were prepared by reducing 2.5 × 10-4 M chloroauric acid in 10 mL 0.1 M CTAB with 600 µL ice cold 0.01 M NaBH4. After 2 h, 10 µL of this seed solution was added to 10 mL of growth solution containing 5 × 10-4 M chloroauric acid, 15-100 µL of 0.01 M silver nitrate, 0.1 M CTAB, and 55 µL of 0.1 M ascorbic acid. Spherical gold nanoparticles (diameter 23 ( 2 nm) were prepared by boiling 2.5 × 10-4 M aqueous chloroauric acid, followed by the addition of 10 mL of 1% sodium citrate and continued boiling for ∼1 h, as described by Frens.22 Gold nanoparticles were separated from unreacted gold and silver ions in solution by centrifuging 10 × 1 mL aliquots of solution at 14 000 rpm for 5-10 min, collecting the supernatant, and redispersing nanoparticles in 10 mL of deionized water. Purified gold nanorods (10 mL) were digested by the addition of 2 mL of aqua regia. Gold and silver ion concentrations determined using either a Perkin-Elmer Plasma 400 or a Liberty II Axial inductively coupled plasma (ICP) atomic emission spectrometer. A minimum of three ICP measurements were made for each nanorod sample. Absorption spectra of nanorod solutions were acquired using a Cary 500 Scan UV-vis-nearIR spectrometer. Transmission electron micrographs were acquired using a Hitachi-8000 transmission electron microscope. Results and Discussion Determining Gold and Silver Concentrations in Nanorods. To quantify the amount of gold in nanorods, particles were separated from unreacted gold ions in solution by centrifugation, removing the supernatant containing unreacted gold ions and redispersing nanorods in 10 mL of deionized water. Purified and isolated nanorods were digested in aqua regia. The gold concentration, for gold atoms in nanorods, was determined to be ∼7 × 10-5 M for aspect ratios from 2.0 to 4.5 ([Au]initial ) 5 × 10-4 M) by ICP, as shown in Table 1. A minimum of three measurements were made for each nanorod sample. From these data, we observe that only ∼15% of the initial gold in the growth solution is reduced to form nanorods, and this amount is generally independent of nanorod aspect ratio. As a control, the unreacted gold concentrations in the supernatants were measured to be 4.15 × 10-4 and 4.19 × 10-4 M for two samples containing aspect ratio 2.0 and 4.5 gold nanorods, respectively. In both cases, the total gold concentration measured for unreacted gold ions and reduced gold in the form of gold
J. Phys. Chem. B, Vol. 110, No. 9, 2006 3991 TABLE 1: Gold and Silver Concentrations in Nanoparticle Samples (10 mL Total Volume) nanorod aspect ratio
[Au]initiala (M)
[Ag]initialb (M)
[Au]npc (M)
[Ag]npd (M)
2.0 ( 0.5 2.9 ( 0.5 3.5 ( 0.7 4.5 ( 0.6 spheres
5.0 × 10-4 5.0 × 10-4 5.0 × 10-4 5.0 × 10-4 2.5 × 10-4
1.5 × 10-5 3.0 × 10-5 5.0 × 10-5 8.0 × 10-5 NAe
7.1 ( 0.8 × 10-5 7.5 ( 0.4 × 10-5 7.1 ( 0.6 × 10-5 7.3 ( 0.5 × 10-5 2.4 ( 0.2 × 10-4
1.8 ( 0.2 × 10-6 2.4 ( 0.1 × 10-6 2.6 ( 0.6 × 10-6 3.3 ( 0.1 × 10-6 NAe
a [Au]initial ) initial gold ion concentration in solution prior to reduction. b [Ag]initial ) initial silver ion concentration in solution prior to reduction c [Au]np ) gold concentration in nanoparticles after reduction, purification, and digestion in aqua regia measured by ICP. d [Ag]np ) silver concentration in nanoparticles after reduction, purification, and digestion in aqua regia measured by ICP. e Not applicable for spherical gold nanoparticles.
nanorods using ICP are ∼4.8 × 10-4 M, >95% of the initial 5 × 10-4 M gold concentration. The analogous experiment was performed on a sample of spherical gold nanoparticles prepared by boiling citrate reduction.22 Through the use of these strong reducing conditions, >95% of the initial gold in aqueous solution is reduced to form spherical particles (Table 1). Through the use of this seeding approach to make gold nanorods, it is surprising that such a small fraction of the initial gold in solution is actually reduced to form rods. Stoichiometric reduction of Au3+ to Au0 with ascorbic acid requires a molar ratio of 1:1.5 Au3+ to ascorbic acid. In this preparation, the molar ratio of Au3+ to ascorbic acid is approximately 1:1. Then, the expected amount of Au3+ reduced to Au0 is ∼70%; however, the actual amount of reduced Au in nanorods is significantly less (∼15%). While this large excess of unreacted gold appears to be wasteful, it is essential for preparing nanorods with a narrow size distribution.18,19 If the initial growth solution gold ion concentration is 95% gold ion reduction under stronger reducing conditions to prepare spherical particles. Incomplete gold reduction is additional evidence that nanorod growth under these conditions is a kinetically controlled process. These data provide additional insight into the growth mechanism of gold nanorods and are consistent with the mechanisms described by GuyotSionnest et al.20 and Perez-Juste et al.23 Through the use of the gold concentration in nanorods, molar extinction coefficients
3994 J. Phys. Chem. B, Vol. 110, No. 9, 2006
Figure 4. (a) Plot of extinction coefficient versus aspect ratio for gold nanorods with aspect ratios from 2.0 to 4.5. The y-axis error bars correspond to error in the ICP measurement, and the x-axis error bars correspond to the error in the measured dimensions of nanorods by TEM. (b) Plot of extinction coefficient versus longitudinal plasmon peak maximum for gold nanorods with aspect ratios from 2.0 to 4.5. The y-axis error bars correspond to error in the ICP measurement.
of gold nanorods with aspect ratios from 2.0 to 4.5 are calculated. The extinction coefficients of the longitudinal plasmon band increase as the plasmon band peak maximum shifts to longer wavelengths for nanorods increasing in aspect ratio. Extinction coefficients will allow for the facile determination of nanorod concentration in solution. Acknowledgment. We gratefully acknowledge S. R. Goode and W. E. Brewer for the use of the ICP atomic emission spectrometers and the University of South Carolina for funding. References and Notes (1) El-Sayed, M. A. Acc. Chem. Res. 2001, 13, 57-64. (2) Nie, S.; Emory, S. R. Science 1997, 275, 1102-1106. (3) Kneipp K.; Wang, Y.; Kneipp, H.; Perelman, L. T.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Phys. ReV. Lett. 1997, 78, 1667-1670. (4) Parfenov, A.; Gryzczynski, I.; Malicka, J.; Geddes, C. D.; Lakowicz, J. R. J. Phys. Chem. B 2003, 107, 8829-8833. (5) Wang, H.; Huff, T. B.; Zweifel, D. A.; He, W.; Low, P. S.; Wei, A.; Cheng, J. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 15752-15756.
Orendorff and Murphy (6) Lyon, L. A.; Musick, M. D.; Natan, M. J. Anal. Chem. 1998, 70, 5177-5183. (7) Jana, N. R.; Gearheart, L.; Murphy, C. J. J. Phys. Chem. B 2001, 105, 4065-4067. (8) Sau, T. K.; Murphy, C. J. J. Am. Chem. Soc. 2004, 126, 86488649. (9) Nikoobakht, B.; El-Sayed, M. A. J. Phys. Chem. A 2003, 107, 3372-3378. (10) Orendorff, C. J.; Gole, A.; Sau, T. K.; Murphy, C. J. Anal. Chem. 2005, 77, 3261-3266. (11) Orendorff, C. J.; Gearheart, L.; Jana, N. R.; Murphy, C. J. Phys. Chem. Chem. Phys. 2006, 8, 165-170. (12) Caswell, K. K.; Wilson, J. N.; Bunz, U. H. F.; Murphy, C. J. J. Am. Chem. Soc. 2003, 125, 13914-13915. (13) Chang, J.-Y.; Wu, H.; Chen, H.; Ling, Y.-C.; Tan, W. Chem. Commun. 2005, 1092-1094. (14) Mieszawska, A. J.; Jalilian, R.; Sumanasekera, G. U.; Zamborini, F. P. J. Am. Chem. Soc. 2005, 127, 10822-10823. (15) Murphy, C. J.; Sau, T. K.; Gole, A. M.; Orendorff, C. J.; Gao, J.; Gou, L.; Hunyadi, S. E.; Li, T. J. Phys. Chem. B 2005, 109, 13857-13870. (16) Gou, L.; Murphy, C. J. Chem. Mater. 2005, 17, 3668-3672. (17) Jana, N. R.; Gearheart, L.; Murphy, C. J. AdV. Mater. 2001, 13, 1389-1393. (18) Nikoobakht, B.; El-Sayed, M. A. Chem. Mater. 2003, 15, 19571962. (19) Sau, T. K.; Murphy, C. J. Langmuir 2004, 20, 6414-6420. (20) Liu, M.; Guyot-Sionnest, P. J. Phys. Chem. B 2005, 109, 2219222200. (21) Kim, F.; Song, J. H. Yang, P. J. Am. Chem. Soc. 2002, 124, 1431614317. (22) Frens, G. Nature 1973, 241, 20-22. (23) Perez-Juste, J.; Liz-Marzan, L. M.; Carnie, S.; Chan, D. Y. C.; Mulvaney, P. AdV. Funct. Mater. 2004, 14, 571-579. (24) Rodriguez-Fernandez, J.; Perez-Juste, J.; Mulvaney, P.; Liz-Marzan, L. M. J. Phys. Chem. B 2005, 109, 14257-14261. (25) Johnson, C. J.; Dujardin, E.; Davis, S. A.; Murphy, C. J. Mann, S. J. Mater. Chem. 2002, 12, 1765-1770. (26) Sanchez, C. G.; Del Popolo, M. G.; Leiva, E. P. M. Surf. Sci. 1999, 421, 59-72. (27) Rojas, M. I.; Sanchez, C. G.; Del Popolo, M. G.; Leiva, E. P. M. Surf. Sci. 2000, 453, 225-228. (28) Link, S.; Wang, Z. L.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 3529-3533. (29) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 84108426. (30) Maye, M. M.; Han, L.; Kariuki, N. N.; Ly, N. K.; Chan, W. B.; Luo, L.; Zhong, C. J. Anal. Chim. Acta 2003, 496, 17-27. (31) Pastoriza-Santos, I.; Gomez, D.; Perez-Juste, J.; Liz-Marzan, L. M.; Mulvaney, P. Phys. Chem. Chem. Phys. 2004, 6, 5056-5060. (32) Kim, Y.; Johnson, R. C.; Li, J.; Hupp, J. T.; Schatz, G. C. Chem. Phys. Lett. 2002, 352, 421-428. (33) Nikoobakht, B.; Wang, J.; El-Sayed, M. A. Chem. Phys. Lett. 2002, 366, 17-23. (34) Yu, Y.-Y.; Chang, S.-S.; Lee, C.-L.; Wang, C. R. C. J. Phys. Chem. B 1997, 101, 6661-6664. (35) Chang, S.-S.; Shih, C.-W.; Chen, C.-D.; Lai, W.-C.; Wang, C. R. C. Langmuir 1999, 15, 701-709. (36) Liao, H.; Hafner, J. H. Chem. Mater. 2005, 17, 4636-4641. (37) Jensen, T.; Kelly, L.; Lazarides, A.; Schatz, G. C. J. Cluster Sci. 1999, 10, 295-317. (38) Kelly, K. L.; Jensen, T. R.; Lazarides, A. A.; Schatz, G. C. In Metal Nanoparticles: Synthesis, Characterization, and Applications; Feldheim, D. L., Foss, C. A., Jr., Eds.; Marcel Dekker: New York, 2002; pp 89118. (39) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668-677.
