Langmuir 2004, 20, 575-578
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Reversible Generation of Gold Nanoparticle Aggregates with Changeable Interparticle Interactions by UV Photoactivation Anjali Pal,† Sujit Kumar Ghosh,‡ Kunio Esumi,§ and Tarasankar Pal*,‡ Department of Chemistry and Department of Civil Engineering, Indian Institute of Technology, Kharagpur-721302, India, and Department of Applied Chemistry, Tokyo University of Science, Kagurazaka, Shinjuku- ku, Tokyo 162-8601, Japan Received October 21, 2003. In Final Form: December 16, 2003 Evolution of exclusively spherical tiny gold nanoparticle aggregates is reported by UV photoactivation of aqueous HAuCl4 solution in cetyltrimethylammonium chloride micelles. In the photoactivation process, citrate is introduced for the first time to obtain aggregates of interacting particles with a tight size distribution. It is seen that the spectrum is substantially altered from the usual Au plasma resonance while the particles are present in the aggregate. Due to the interparticle interaction, the solution renders a blue color. Bluepink color transformation of the gold sol has been observed in relation to the change in the interparticle distances without any change in the aspect ratio of the particles.
Introduction Evolution of gold nanoparticles has attracted tremendous attention for several reasons.1-3 Among the innumerable applications of nanoparticles, gold has become a promising candidate in the biomedical field.4 For biomedical applications, the need for a suitable procedure for the evolution of monodisperse, stable, and tiny gold nanoparticles still exists. Evolution of small metal particles with a tight size distribution is important. Again, assembly of individual nanoparticles into ensembles has recently become a widely pursued objective.5 Therefore, it is interesting to produce and study the interparticle interactions while the particles are held by weak forces in an aggregate.4 We have achieved success through the use of UV-photoactivation technique to produce gold nanoparticles with a tight size distribution in cetyltrimethylammonium chloride (CTAC) micelles. The method is simple and produces gold particles in the ∼3-5 nm size range. Interestingly enough, this is the first report of citratemediated evolution of tiny gold nanoparticles through UV photoactivation. Citrate (as sodium citrate) has been widely used to produce gold sols6 of variable size from boiling aqueous HAuCl4 solution. It is well documented in the literature that the assembly of nanoparticles in various states of aggregation influences the plasma resonance.3 However, in the present report a deep blue * To whom correspondence should be addressed. E-mail: tpal@ chem.iitkgp.ernet.in. † Department of Civil Engineering, Indian Institute of Technology. ‡ Department of Chemistry, Indian Institute of Technology. § Department of Applied Chemistry, Tokyo University of Science. (1) Clusters and Colloids from Theory to Applications; Schmid, G., Ed.; VCH: Weinheim, 1994. (2) Bohren, C. F.; Huffman, D. R. Absorption and Scattering of Light by Small Particles; Wiley: New York, 1983. (3) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer: Berlin, 1995. (4) (a) Nam, J. M.; Park, S.-J.; Mirkin, C. A. J. Am. Chem. Soc. 2002, 124, 3820. (b) Hainfeld, J. M.; Powell, R. D. J. Histochem. Cytochem. 2000, 48, 471. (c) Hirsch, L. R.; Stassord, R. J.; Bankson, J. A.; Sershen, S. R.; Rivera, B.; Price, R. E.; Hazle, J. D.; Halas, N. J.; West, J. L. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 13549. (5) Grabar, K. C.; Smith, P. C.; Musick, M. D.; Davis, J. A.; Walter, D. G.; Jackson, M. A.; Guthrie, A. P.; Natan, M. J. J. Am. Chem. Soc. 1996, 118, 1148. (6) Frens, G. Nature 1973, 241, 20.
color (λmax ∼ 650 nm) gold sol is obtained at room temperature in the presence of citrate in an alkaline pH (8-12) range. Different types of surfactants, reagent concentrations, and pH conditions (using NaOH7) were used to achieve success in producing monodisperse gold nanoparticle aggregates. The objective of this work to produce spherical gold nanoparticle aggregates through UV photoactivation where the interparticle distance is uniformly lower than the size of the evolved particles. Experimental Section Cetyltrimethylammonium chloride (C16TAC) and other cationic surfactants of different chain length (C10, C12, C14, C18) of the same homologous series were obtained from Kokusan Kagaku Kogyo and recrystallized twice from acetone. The anionic surfactant sodium dodecyl sulfate (SDS) and the nonionic surfactant poly(oxyethylene) isooctylphenyl ether (Triton X-100 or TX-100) were used as received from Aldrich. Chloroauric acid (HAuCl4) was kindly supplied by Tanaka Kikinzoku Kogyo. The water used in this study was purified through a Milli-Q Plus system. The transition from spherical to rodlike micelles due to change in C16TAC concentration in aqueous solution was estimated by static light scattering. UV irradiation was carried out with a 200 W low-pressure mercury lamp of wavelength 253.7 nm. UV-vis spectra of the colloidal solutions were recorded on a Hewlett-Packard 8452A diode array spectrophotometer. Electron micrographs of the metal colloids were taken with a Hitachi H-9000NAR transmission electron microscope, operating at 200 kV. The samples were prepared by mounting a drop of the solutions on a carbon-coated Cu grid and allowing it to dry in air. In a typical set, an aqueous solution (2.2 mL) containing HAuCl4 (0.23 mM), C16TAC (4.5 mM), sodium citrate (2.3 mM), and NaOH (4.5 mM) were mixed in a rectangular quartz cell. Upon UV exposure for 10 min, the solution turned blue. Absorbance measurement of this blue solution shows the appearance of a broad band at ∼650 nm. Transmission electron microscopic (TEM) studies of the UV-exposed solution showed that the gold particles are in the size range of ∼3-5 nm.
Results and Discussion Under standard conditions (with C16TAC and citrate at a pH of 8-12 with respect to NaOH), UV photoactivation produced blue sol. Interestingly, UV-visible spectropho(7) Caswell, K. K.; Bender, C. M.; Murphy, C. J. Nano Lett. 2003, 3, 667.
10.1021/la035961g CCC: $27.50 © 2004 American Chemical Society Published on Web 01/10/2004
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Figure 1. Evolution of UV-visible spectra during the formation of gold nanoparticle aggregates. Conditions: [HAuCl4] ) 0.23 mM, [C16TAC] ) 4.5 mM, [sodium citrate] ) 2.3 mM, [NaOH] ) 4.5 mM. Irradiation time: (a) 0, (b) 5, (c) 10, and (d) 15 min. The irradiation was carried out with a 200 W UV light.
Figure 2. TEM images of the gold nanoparticle aggregates. Conditions: [HAuCl4] ) 0.23 mM, [C16TAC] ) 4.5 mM, [sodium citrate] ) 2.3 mM, [NaOH] ) 4.5 mM, irradiation time ) 10 min. The irradiation was carried out with a 200 W UV light.
tometry reveals the absence of any peak near the 520 nm region (characteristic region for dispersed small gold particles); instead a distinct peak appeared at ∼650 nm (Figure 1). This is very unusual and has not been reported earlier from photoactivation studies. TEM studies of the UV-exposed solution showed that the tiny particles (∼3-5 nm size) are in a closely packed assembly (Figure 2). The optical properties of small particles in the nanometer size regime are mainly determined by two contributions:8(i) the particles acting individually as an isolated sample and (ii) the collective properties of the whole ensemble. But in the closely packed aggregates of an ensemble of a large number of particles (as seen in our case), the isolated-particle approximation breaks down and electromagnetic interactions between the particles become important and affect the optical spectra enormously. Therefore, in the aggregates of gold nanoparticles, it is very difficult to observe the properties of the isolated sample as interparticle interactions generally overpower the single-particle properties. The color of the solution in such aggregates depends on nanoparticle spacing. Nanoparticle aggregates of gold with interparticle distances substantially greater than the average particle diameter appear red with λmax ∼ 520 nm, but as the interparticle distances in the aggregates decrease to less than approximately the average particle diameter, the color (8) Kreibig, U.; Genzel, L. Surf. Sci. 1985, 156, 678.
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becomes blue.4 The blue color of the solution in the present case indicates that the interparticle distances in the aggregate are obviously less than the average particle diameter which is also evident from the TEM studies. The appearance of a red-shifted band at ∼650 nm can be attributed to the surface plasmon resonance of gold organized into an aggregate structure. Excellent reproducibility of the particle evolution as ensembles of small particles was noticed from UV-vis spectra and also from TEM studies. Since the particles are not discrete in the solution, the color of the solution does not seem to be red and consequently no band is observed at ∼520 nm. The evolution of particle aggregates is pH dependent. Generation of spherical particles with a perfect blue tinge is observed at pH > 8. The particle evolution at pH < 8 was not reproducible, and the solution becomes pink and sometimes bluish pink. However, wide variation of pH in the 8-12 range does not affect the required particle evolution while all other conditions remain unaltered. But higher pH (>12) accelerates the precipitation of particles. The precipitated particles are dried under a vacuum and are subjected to redispersion in the aqueous phase, whenever needed. The aqueous medium with the redispersed particles shows the usual spectral profile (λmax ∼ 650 nm) and blue color. Cationic surfactant, C16TAC, has already been introduced in the UV-photoactivation process to produce anisotropic rod-shaped gold nanoparticles.9 Herein, we report the evolution of the ensemble of spherical particle aggregates in C16TAC by UV photoactivation. We have noted the effect of C16TAC concentrations (0-6.8 mM) and also the effect of chain lengths of several other but homologous cationic surfactants in this Au nanoparticle evolution process. The prescribed concentration range (0.23-4.5 mM) of C16TAC has been found to generate exclusively spherical particles (∼3-5 nm, Figure 2) with blue color. Lower concentrations (0-0.23 mM) of C16TAC generate spherical particles of variable size, whereas higher C16TAC concentrations (5.0-6.8 mM) produce a mixture of particles of variable size and shape. A similar observation has been reported earlier.10 A pink sol is produced above and below the prescribed concentration range. In the former case, gold particles exhibit a sharp plasmon absorption band (maximum absorbance at a wavelength of 520 nm) as the surfactant acts as a stabilizer only but did not allow the particles to form aggregates.9 On the other hand, in the latter case the spectrum is broadened and λmax is red shifted which is probably due to the increase in filling factor in the aggregate at higher surfactant concentration.8 There exists a threshold concentration of surfactant above which the blue sol is produced for always. However, in the absence of any surfactant spherical particles with wide variation in size are produced but the solution is always pink with a λmax at 520 nm. In addition, the chain length of cationic surfactants (4.5 mM) has a profound influence on the evolution of particle geometry and also on their aggregation. Surfactants with smaller chain (C10TAC and C12TAC) lengths produce smaller particles and impart stability to the particles in the aqueous phase and hence the particles get precipitated. Surfactants with smaller chain lengths give rise to higher concentrations of micelles with a lower aggregation (9) Kameo, A.; Suzuki, A.; Torigoe, K.; Esumi, K. J. Colloid Interface Sci. 2001, 241, 289. (10) Mandal, M.; Ghosh, S. K.; Kundu, S.; Esumi, K.; Pal, T. Langmuir 2002, 18, 7792.
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Figure 3. Citric acid induced corrosion of blue gold sol. Conditions: [HAuCl4] ) 0.23 mM, [C16TAC] ) 4.5 mM, [citric acid] ) 2.3 mM, irradiation time ) 10 min. The irradiation was carried out with a 200 W UV light.
number,11 and hence smaller micellar structures encompass a smaller amount of gold ions. Hence evolution of smaller Au particles is understandable. However, smaller chain lengths of the surfactants are not compatible enough to render stability to the evolved Au particles through hydrophobic repulsion; that is, they cannot provide steric stability.12 A surfactant containing a longer chain (C18TAC) helps to generate nonspherical particles as is observed in the case of higher C16TAC concentrations. In this case, the number of Au(III) ions per micelle increases which stimulates the growth of nonspherical particles.9 However, C14TAC has also been found to be suitable for the purpose and behaves similarly to C16TAC. Thus, both C16TAC and C14TAC have been found to suit the purpose of Au particle evolution under the standard procedure. Photoactivation in the presence of other types of surfactants (anionic and nonionic) did not generate blue sol. Sodium citrate generates Au particles from boiling aqueous HAuCl4 solution.6 In the proposed method, sodium citrate is employed in the photoactivation process at room temperature under ambient conditions. Here citrate has been found to act as a reducing agent under UV. It was observed that CTAC alone (devoid of citrate) cannot effectively influence the Au particle evolution in ∼10 min,9 but on the contrary the reducing action of citrate has been observed even if the reaction medium is devoid of any surfactant. Spherical Au(0) particles are photogenerated after UV exposure of the reaction mixture for a longer time in the absence of citrate ion which show the usual plasma resonance at ∼520 nm.13 It is well-known from the literature that spheroids or rod-shaped particles with a high aspect ratio always exhibit two peaks due to longitudinal and transverse vibration.14 However, UV activation produced a pink (λmax ) 520 nm) sol when citric acid (to see the effect at lower pH) was introduced in lieu of sodium citrate. Higher concentrations of citric acid (i.e., at very low pH) corrode the particles as is evident from TEM studies (Figure 3). Sodium citrate (2.3 mM) and NaOH offer perfect pH ()8) and salt concentrations to (11) Dorrance, R. C.; Hunter, T. F. J. Chem. Soc., Faraday Trans. 1 1974, 70, 1572. (12) Belloni, J.; Mostafavi, M.; Remita, H.; Marignier, J. L.; Delcourt, M. O. New J. Chem. 1998, 22, 1239. (13) Esumi, K.; Matsuhisa, K.; Torigoe, K. Lamgmuir 1995, 11, 3285. (14) Jana, N. R.; Gearheart, L.; Murphy, C. J. J. Phys. Chem. B 2001, 105, 4065.
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Figure 4. TEM images of spherical gold particles produced by a 40 W UV light. Conditions: [HAuCl4] ) 0.23 mM, [C16TAC] ) 4.5 mM, [sodium citrate] ) 2.3 mM, [NaOH] ) 4.5 mM, irradiation time ) 60 min.
render stability to the particle ensembles. Again, after the formation of the blue sol, if it is treated with citric acid to decrease the pH up to ∼6 the blue sol turned pink and from the TEM measurements it seemed that the particles did not have any particular shape (Figure 3). Again regeneration of the blue color from the pink solution is possible by the increase of pH and subsequent UV exposure. This authenticates the reversible blue-pink color transformation. From the UV-vis spectra, it is seen that the blue sol leaves no peak at 520 nm; instead a peak appears at the ∼650 nm region, but at low pH conditions (with citric acid) the 650 nm band disappears with the evolution of a 520 nm peak. Repetitive UV exposure of the pink sol at higher pH regenerates the peak at 650 nm. This spectral change in this nanoparticle system can be attributed to the reversible formation and dissociation of aggregates.4 A decrease in the pH of the solution results in the increase in interparticle distances within the aggregate without dispersing the gold nanoparticles in solution, and therefore the solution shows pink color. It is possible to obtain tiny particles (∼5 nm) of gold employing very powerful (200 W) UV light. The particle evolution becomes complete in 10 min (Figure 1). Kinetic studies for gold particle evolution reveal the complete reduction of Au(III) ions in 10 min while the concentration of Au(III) ion is kept at ∼0.23 mM. On the other hand, low-power UV (40 W) produces blue sol containing comparatively larger particles (10-12 nm) (Figure 4) but requires a longer time for reduction. Whether it is high or low UV, the blue sol is always produced without any rod, triangle, or ellipsoidal particles as was observed earlier.13 The photoproduced gold nanoparticles remain homogeneously dispersed for 3-4 h, but after that the particles get precipitated. However, the particles remain stable for months together in a vacuum desiccator if isolated from the reaction mixture by centrifugation. In the proposed method, CTAC concentration supersedes citrate concentration and hence CTAC covers the negatively charged Au surfaces forming a bilayer. Actually, bilayer formation has been recognized when cationic surfactants are adsorbed on solid surfaces.15 To have conclusive proof, studies were conducted with variable concentrations of C16TAC and a fixed concentration of citrate in one case and in the other with variable citrate (15) Esumi, K. Structure-Performance Relationship in Surfactants, 2nd ed.; Esumi, K., Ueno, M., Eds.; Marcel Dekker: New York, 2003; Chapter 17.
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Scheme 1. Schematic Presentation of Gold Nanoparticle Aggregates with CTAC and Negatively Charged Citrate Ions
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of the particle surfaces. Thus, interparticle interaction becomes pronounced in the citrate case causing the weak aggregation of the gold particles. Hence a blue solution results. The stabilization of gold nanoparticle aggregates with cationic surfactant and negatively charged citrate ions has been shown in Scheme 1. Conclusion
and a fixed concentration of C16TAC. It was observed that lower concentrations (0-0.23 mM) of C16TAC evolved pink and higher concentrations (0.23-4.5 mM) produced blue Au sol. However, variation of citrate ion concentrations with a higher C16TAC concentration always evolves blue sol. So, the importance of C16TAC for the bilayer formation is conceived beyond doubt for the generation of blue sol. At low pH, citric acid weakly interacts with the surfactant-covered gold surfaces, whereas at high pH citrate ions strongly interact resulting in a charge neutralization
An effective photochemical method has been described for the stabilization of aggregates of Au nanoparticles. Reversible blue-pink color transformation, that is, reversible formation and dissociation of aggregates, by repetitive UV exposure is possible without any shape change of the Au nanoparticles. At high cationic surfactant concentrations, stabilization of gold particle aggregates has been attributed to a plausible bilayer formation. Influence of the alkyl chain length of cationic surfactants on the aggregate formation has been described. LA035961G
