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Langmuir 2005, 21, 9524-9528
Control of Gold Nanoparticle Aggregates by Manipulation of Interparticle Interaction Taehoon Kim,† Kangtaek Lee,*,† Myoung-seon Gong,‡ and Sang-Woo Joo*,§ Department of Chemical Engineering, Yonsei University, Seoul 120-749 Korea, Department of Chemistry, Dankook University, Cheonan 330-714, Korea, and Department of Chemistry, Soongsil University, Seoul 156-743 Korea Received February 21, 2005. In Final Form: June 21, 2005 The size of gold nanoparticle aggregates was controlled by manipulating the interparticle interaction. To manipulate the interparticle interaction of gold nanoparticles prepared by citrate reduction, we applied the substitutive adsorption of benzyl mercaptan on the particle surface in the absence of the cross-linking effect. Various experimental techniques such as UV-vis absorption spectroscopy, surface-enhanced Raman scattering, quasi-elastic light scattering, and zeta-potential measurement were used to characterize the nanoparticle aggregates. Our results suggest that the replacement of the trivalent citrate ions adsorbed on the nanoparticle surface with monovalent benzyl mercaptan ions should destabilize the particles, causing aggregation and hence the increase in the size of nanoparticle aggregates. These experimental results were successfully rationalized by the classical DLVO (Derjaguin-Landau-Vervey-Overbeek) theory that describes the interparticle interaction and colloidal stability in solution. Our findings suggest that the control of surface potential is crucial in the design of stable gold nanoparticle aggregates.
1. Introduction There has been recently extensive efforts directed toward the organization of nanoparticles using a controlled self-assembly strategy.1 Gold nanoparticles have attracted much attention in the past decade due to their stability, uniformity, and optical properties.1-9 The size of chemically reduced gold nanoparticles can be controlled by varying the type and amount of the reducing and capping agents.3 Thiol-based organic stabilizers have been widely used as capping agents to produce small gold clusters of 1-5 nm.10-12 For 40 nm gold nanoparticles with selfassembled aliphatic thiols on the surface, the rate of aggregation was found to depend on the solution pH, ionic strength, and chain length and terminal functionality of thiols.13 The adsorption behavior14,15 of aromatic adsorbates on gold nanoparticle surface with mean diameters * To whom correspondence should be addressed. (K.L.) Tel: +82-2-21232760. Fax: +82-2-3126401. E-mail:
[email protected]. (S.-W.J.) Tel: +82-2-8200434. Fax: +82-2-8200434. E-mail: sjoo@ ssu.ac.kr. † Yonsei University. ‡ Dankook University. § Soongsil University. (1) Shipway, A. N.; Katz, E.; Willner, I. ChemPhysChem 2000, 1, 18. (2) Daniel, M.-C.; Astruc, D. Chem. Rev. 2004, 104, 293. (3) Turkevich, J.; Stevenson, P. C.; Hillier, J. Discuss. Faraday Soc. 1951, 11, 55. (4) Faraday, M. Philos. Trans. 1857, 147, 145. (5) Mulvaney, P. Langmuir 1996, 12, 788. (6) Kreibig, U.; Volmer, M. Optical properties of metal clusters; Springer: Berlin, 1995. (7) Link, S.; El-sayed, M. A. J. Phys. Chem. B 1999, 103, 4212. (8) Biggs, S.; Chow, M. K.; Zukoski, C. F.; Grieser, F. J. Colloid Interface Sci. 1993, 160, 511 (9) Chow, M. K.; Zukoski, C. F. J. Colloid Interface Sci. 1994, 165, 97. (10) Yonezawa, T.; Yasui, K.; Kimizuka, N. Langmuir 2001, 17, 271. (11) Teranishi, T.; Kiyokawa, I.; Miyake, M. Adv. Mater. 1998, 10, 596. (12) Chen, S. H.; Kimura, K. Langmuir 1999, 15, 1075. (13) Weisbecker, C. S.; Merritt, M. V.; Whitesides, G. M. Langmuir 1996, 12, 3763. (14) Jang, S.; Park, J.; Shin, S.; Yoon, C.; Choi, B. K.; Gong, M.-s.; Joo, S.-W. Langmuir 2004, 20 1922. (15) Lee, C-r.; Kim, S.; Yoon, C. J.; Gong, M.-s.; Choi, B. K.; Kim, K.; Joo, S.-W. J. Colloid Interface Sci. 2004, 271, 41.
from 6 to 97 nm has been studied by UV-vis absorption spectroscopy and surface enhanced Raman scattering.16 Crystalline colloidal array prepared from nanoparticles has been used as chemical and biological sensors17 and alkanethiol-induced structural rearrangement in goldsilica core-shell type nanoparticle clusters is to be potentially applied to chemical sensor.18 In the utilization of gold nanoparticles as biosensors for DNA detection, controlling the interaction between aggregates plays an important role since the particles exhibit color changes according to the degree of aggregation.19,20 Thus, understanding the interparticle interactions of the functionalized gold nanoparticles should provide the possibilities of new fabrication techniques in the scale of 10-100 nm. The effect of interparticle interaction on the aggregation of gold nanoparticles, however, has not been systematically studied yet. The classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory has been widely employed in colloid science to study particle-particle interactions, colloidal stability, coagulation, sedimentation, filtration, and the behavior of electrolyte solutions.21-23 In the citrate-reduced gold nanoparticle system, electrophoretic mobilities and ion conductances were used to estimate the electrokinetic properties and colloidal stability of particles, and the effect of the citrate concentration on the final particle size was explained by the DLVO theory.9 In this paper, we manipulate the interparticle interaction of gold nanoparticles to control the final aggregate (16) Chang, R. K.; Furtak, T. E. Surface Enhanced Raman Scattering; Plenum Press: New York, 1982. (17) Holtz, J. H.; Asher, S. A. Nature 1997, 389, 829. (18) Osterloh, F. E.; Hiramatsu H.; Porter, R.; Guo, T. Langmuir 2004, 20, 5553. (19) Mirkin, C. A.; Lestinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607. (20) Elghanian, R.; Storhoff, J. J.; Mucic, R. C.; Lestinger, R. L.; Mirkin, C. A. Science 1997, 277, 1078. (21) Hunter, R. J. Foundations of Colloid Science; Clarendon Press: Oxford, U.K., 1992. (22) Verwey, E. J. W.; Overbeek, J. Th. G. Theory of the Stability of Lyophobic Colloids; Dover: Mineola, NY, 2000. (23) Lee, K.; Sathyagal, A. N.; McCormick, A. V. Colloids Surf. 1998, 144, 115.
10.1021/la0504560 CCC: $30.25 © 2005 American Chemical Society Published on Web 09/08/2005
Control of Gold Nanoparticle Aggregates
Langmuir, Vol. 21, No. 21, 2005 9525
Table 1. Final Composition of the Reaction Mixtures sample
[KAuCl4] (10-4M)
[sodium citrate] (10-4M)
[benzyl mercaptan] (10-4 M)
A B C D E
2.65 2.65 2.65 2.65 2.65
11.5 5.02 3.36 2.03 1.42
1.0 1.0 1.0 1.0 1.0
Table 2. Average Diameter and Zeta Potential of the Stable Nanoparticles before and after the Addition of Benzyl Mercaptan before
after
sample
diameter (nm)
zeta potential (mV)
diameter (nm)
zeta potential (mV)
A B C D E
25.1 34.9 39.8 47.1 53.6
-52.72 -43.10 -41.36 -32.65 -29.42
47.10 139.7 148.6 172.0 276.6
-41.38 -34.86 -34.71 -26.48 -26.17
size. First, we prepare differently sized gold nanoparticles by the citrate reduction method. To manipulate the interparticle interaction of these particles, we add benzyl mercaptan ions to replace the citrate ions on the particle surface. The change in the size and surface potential of particles is monitored by various experimental techniques. Note that the change in the size of gold nanoparticle aggregates upon the addition of surfactants has not been quantitatively monitored in the previous studies.14,15 The classical DLVO theory is also employed to rationalize the experimental results. To our knowledge, this is the first study to investigate the formation of gold nanoparticle aggregates with surfactants in real time with the combination of theoretical interpretations. 2. Materials and Methods 2.1. Preparation and Characterization of Gold Nanoparticle Aggregates. Colloidal dispersions of gold nanoparticles were prepared following the procedures reported previously in the literature by Turkevich and co-workers.3 After stable gold nanoparticles were formed, benzyl mercaptan was added to the colloidal suspensions to induce aggregation of gold nanoparticles. A concentrated ethanolic solution of benzyl mercaptan was added to 0.1∼2 mL of gold nanoparticle suspension to a final concentration of ∼10-4 M using a micropipet. The final composition of these mixtures is summarized in Table 1. The diameter and zeta potential of particles were estimated by QELS and zeta potential measurements using ZetaPlus from Brookhaven Instrument Co. UV-vis absorption spectra of colloidal solution were obtained using a Shimadzu UV-3101PC
Figure 2. SERS spectra of ∼10-4 M of benzyl mercaptan on A-E gold particles in the spectral region between 3150 and 50 cm-1. The spectral region between 2850 and 1750 cm-1 was omitted due to the lack of any information. Table 3. Input Parameters Used for DLVO Calculation and the Stability
sample A
Hamaker constant (10-19J)
ionic strength (10-4M)
surface potential (mV)
2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5
6.10 7.10 4.16 5.16 3.66 4.66 3.26 4.26 3.08 4.08
-52.72 -41.38 -43.10 -34.86 -41.36 -34.71 -32.65 -26.48 -29.42 -26.17
before after before after before after before after before after
B C D E
spectrophotometer. To examine the adsorbates on the gold nanoparticle surface, Raman spectra were obtained using a Renishaw confocal system model 1000 spectrometer equipped with an integral microscope (Leica DM LM). The 632.8 nm irradiation from a 35 mW air-cooled HeNe laser (Melles Griot Model 25 LHP 928) with the plasma line rejection filter was used for Raman experiments. 2.2. Calculation of Interparticle Interaction Potentials. The classical DLVO theory states that the total interaction potential between two gold nanoparticles (VT) can be expressed as the sum of the electrostatic repulsion (Velec) and the van der Waals attraction (Vvdw)21-23
VT ) Velec + Vvdw
(1)
Depending on the particle size and the double layer thickness, the electrostatic repulsion potential Velec are usually expressed in the following two different forms21-23
Velec ) 4πψ02
a1a2 ln[1 + exp(-κx)] (in the case of κ a >5) (2) a1 + a2
Velec ) 4πa1a2Y1Y2
(in the case of κ a
