Spontaneous Formation of Gold Nanoparticles in Poly(ethylene oxide

The colloidal stability of gold particles formed in aqueous solutions is excellent for .... Figure 3 Absorbance at ∼540 nm due to the surface plasmo...
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Langmuir 2005, 21, 8019-8025

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Spontaneous Formation of Gold Nanoparticles in Poly(ethylene oxide)-Poly(propylene oxide) Solutions: Solvent Quality and Polymer Structure Effects Toshio Sakai and Paschalis Alexandridis* Department of Chemical and Biological Engineering, University at Buffalo, The State University of New York, Buffalo, New York 14260-4200 Received March 22, 2005. In Final Form: May 17, 2005 We report here on the effects that the solution properties of poly(ethylene oxide)-poly(propylene oxide)poly(ethylene oxide) block copolymers have on the reduction of hydrogen tetrachloroaurate(III) hydrate (HAuCl4‚3H2O) and the size of gold nanoparticles produced. The amphiphilic block copolymer solution properties were modulated by varying the temperature and solvent quality (water, formamide, and their mixtures). We identified two main factors, (i) block copolymer conformation or structure (e.g., loops vs entanglements, nonassociated polymers vs micelles) and (ii) interactions between AuCl4- ions and block copolymers (attractive ion-dipole interactions vs repulsive interactions due to hydrophobicity), to be important for controlling the competition between the reactivities of AuCl4- reduction in the bulk solution to form gold seeds and on the surface of gold seeds (particles) and the particle size determination. The particle size increase observed with increased temperature in aqueous solutions is attributed to enhanced hydrophobicity of the block copolymer, which favors AuCl4- reduction on the surface of seeds. The lower reactivity and higher particle sizes observed in formamide solutions are attributed to the shielding of ion-dipole interaction between AuCl4- ions and block copolymers by formamide, which overcomes the beneficial effects of formamide on the block copolymer conformation (lower micelle concentration).

Introduction Selection of a solvent is one of the main challenges in the preparation of metal particles.1-16 The solvent should enhance particle formation, prevent particle aggregation * To whom correspondence should be addressed. Phone: (716) 645-2911, ext. 2210. Fax: (716) 645-3822. E-mail: palexand@ eng.buffalo.edu. (1) Pastoriza-Santos, I.; Liz-Marzan, L. M. Formation and Stabilization of Silver Nanoparticles through Reduction by N,N-Dimethylformamide. Langmuir 1999, 15 (4), 948-951. (2) Osakada, K.; Taniguchi, A.; Kubota, E.; Dev, S.; Tanaka, K.; Kubota, K.; Yamamoto, T. New Organosols of Copper(II) Sulfide, Cadmium Sulfide, Zinc Sulfide, Mercury(II) Sulfide, Nickel(II) Sulfide and Mixed Metal Sulfides in N,N-Dimethylformamide and Dimethyl Sulfoxide. Preparation, Characterization, and Physical Properties. Chem. Mater. 1992, 4 (3), 562-570. (3) Murakoshi, K.; Hosokawa, H.; Saitoh, M.; Wada, Y.; Sakata, T.; Mori, H.; Satoh, M.; Yanagida, S. Preparation of Size-Controlled Hexagonal CdS Nanocrystallites and the Characteristics of their Surface Structures. J. Chem. Soc., Faraday Trans. 1998, 94 (4), 579-586. (4) Chen, D. H.; Huang, Y. W. Spontaneous Formation of Ag Nanoparticles in Dimethylacetamide Solution of Poly(ethylene glycol). J. Colloid Interface Sci. 2002, 255 (2), 299-302. (5) Liz-Marzan, L. M.; Lado-Tourino, I. Reduction and Stabilization of Silver Nanoparticles in Ethanol by Nonionic Surfactants. Langmuir 1996, 12 (15), 3585-3589. (6) Linnert, T.; Mulvaney, P.; Henglein, A.; Weller, H. Long-Lived Nonmetallic Silver Clusters in Aqueous Solution: Preparation and Photolysis. J. Am. Chem. Soc. 1990, 112 (12), 4657-4664. (7) Yonezawa, Y.; Sato, T.; Kuroda, S.; Kuge, K. Photochemical Formation of Colloidal Silver-Peptizing Action of Acetone Ketyl Radical. J. Chem. Soc., Faraday Trans. 1991, 87 (12), 1905-1910. (8) Yeung, S. A.; Hobson, R.; Biggs, S.; Grieser, F. Formation of Gold Sols Using Ultrasound. J. Chem. Soc., Chem. Commun. 1993, No. 4, 378-379. (9) Hirai, H.; Nakao, Y.; Toshima, N. Preparation of Colloidal Transition-Metals in Polymers by Reduction with Alcohols or Ethers. J. Macromol. Sci., Chem. 1979, A13 (6), 727-750. (10) Zhang, L.; Yu, J. C.; Yip, H. Y.; Li, Q.; Kwong, K. W.; Xu, A.-W.; Wong, P. K. Ambient Light Reduction Strategy to Synthesize Silver Nanoparticles and Silver-Coated TiO2 with Enhanced Photocatalytic and Bactericidal Activities. Langmuir 2003, 19 (24), 10372-10380. (11) Hirai, H.; Nakao, Y.; Toshima, N. Preparation of Colloidal Rhodium in Poly(vinyl-alcohol) by Reduction with Methanol. J. Macromol. Sci., Chem. 1978, A12 (8), 1117-1141.

during the synthesis process, and allow for stable dispersion of the particles. For example, N,N-dimethylformamide (DMF) is a typical polar organic solvent with uses that include the preparation of metal and semiconductor colloids.1-3 DMF can act as a reducing agent under suitable conditions;17,18 e.g., it can reduce Ag+ ions even at room temperature.1 Other solvents have also been used for metal particle synthesis. Silver particles can spontaneously form in dimethylacetamide solutions of poly(ethylene oxide) (EOx).4 Ethanol can be advantageous because it is a solvent for both metal ions and surfactants,5 and has been used in conjunction with γ,6 UV,7 or ultrasound8 irradiation or prolonged reflux.9 For example, silver particles can be obtained from [Ag(NH3)2]+ with the aid of ambient light illumination in ethanol solutions of poly(ethylene oxide)poly(propylene oxide)-poly(ethylene oxide) block copolymer (EO20PO79EO20).10 Methanol is another solvent that (12) Huang, Z.-Y.; Mills, G.; Hajek, B. Spontaneous Formation of Silver Particles in Basic 2-Propanol. J. Phys. Chem. 1993, 97 (44), 11542-11550. (13) Esumi, K.; Hosoya, T.; Suzuki, A.; Torigoe, K. Formation of Gold and Silver Nanoparticles in Aqueous Solution of Sugar-Persubstituted Poly(amidoamine) Dendrimers. J. Colloid Interface Sci. 2000, 226 (2), 346-352. (14) Esumi, K.; Hosoya, T.; Suzuki, A.; Torigoe, K. Spontaneous Formation of Gold Nanoparticles in Aqueous Solution of SugarPersubstituted Poly(amidoamine) Dendrimers. Langmuir 2000, 16 (6), 2978-2980. (15) Esumi, K.; Nakamura, R.; Suzuki, A.; Torigoe, K. Preparation of Platinum Nanoparticles in Ethyl Acetate in the Presence of Poly(amidoamine) Dendrimers with a Methyl Ester Terminal Group. Langmuir 2000, 16 (20), 7842-7846. (16) Esumi, K.; Kameo, A.; Suzuki, A.; Torigoe, K. Preparation of Gold Nanoparticles in Formamide and N,N-Dimethylformamide in the Presence of Poly(amidoamine) Dendrimers with Surface Methyl Ester Groups. Colloids Surf., A 2001, 189 (1-3), 155-161. (17) Yu, J. Y.; Schreiner, S.; Vaska, L. Homogeneous Catalytic Production of Hydrogen and Other Molecules from Water-DMF Solutions. Inorg. Chim. Acta 1990, 170 (2), 145-147. (18) Hugar, G. H.; Nandibewoor, S. T. Kinetics of Oxidation of Dimethylformamide (DMF) by Diperiodatonickelate(IV) (DPN) in Aqueous Alkaline Medium. Indian J. Chem. 1993, 32A (12), 10561059.

10.1021/la050756h CCC: $30.25 © 2005 American Chemical Society Published on Web 07/23/2005

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has been used for the synthesis of silver particles.11,12 Esumi et al. have investigated metal particle formation in the presence of poly(amideamine) dendrimers in various polar solvents (e.g., water,13,14 ethyl acetate,15 formamide, and DMF16). Appropriate solvent selection can confer further advantages in metal particle applications such as catalysts19,20 and as optical materials with properties that depend on the surrounding medium and on the modification of the particle surface.21-23 We have recently reported a facile and effective method for synthesis of gold particles via autoreduction of hydrogen tetrachloroaurate(III) (AuCl4-) in aqueous media containing poly(ethylene oxide)-poly(propylene oxide)poly(ethylene oxide) (EOxPOyEOx) block copolymers.24,25 The AuCl4- reduction is facilitated via metal ion complexation with EOxPOyEOx block copolymers (formation of pseudo crown ether structures) induced by ion-dipole interaction.24-28 The particle formation and growth are controlled by the amphiphilic (dual-nature) character of the EOxPOyEOx block copolymers: EOx-enhanced AuCl4reduction and POy-enhanced block copolymer adsorption on the surface of gold particles.24,25 This results in a competition between AuCl4- reduction in the bulk solution (which leads to an increase in the number of particles) and that on the particle surface (which causes an increase in particle size).25 We expect that the metal ion complexation with EOxPOyEOx block copolymers will be affected by the temperature and solvent. Furthermore, the competition between AuCl4- reduction in the bulk solution and on the surface of gold particles could be modulated by the solvent because several properties of EOxPOyEOx block copolymers (e.g., solubility, interfacial activity, micelle formation, micelle structure) are strongly affected by the temperature and solvent quality.29-32 These provide an opportunity to (19) Andrews, M. P.; Ozin, G. A. Liquid-Phase Agglomeration of Silver Atoms in Olefinic and Ether Media: Electrocatalytic Application. 2. J. Phys. Chem. 1986, 90 (13), 2929-2938. (20) Nakao, Y.; Kaeriyama, K. Adsorption of Surfactant-Stabilized Colloidal Noble Metals by Ion-Exchange Resins and their Catalytic Activity for Hydrogenation. J. Colloid Interface Sci. 1989, 131 (1), 186191. (21) Linnert, T.; Mulvaney, P.; Henglein, A. Photochemistry of Colloidal Silver Particles: The Effects of Nitrous Oxide and Adsorbed Cyanide Ion. Ber. Bunsen-Ges. Phys. Chem. 1991, 95 (7), 838-841. (22) Mulvaney, P.; Giersig, M.; Henglein, A. Surface Chemistry of Colloidal Gold: Deposition of Lead and Accompanying Optical Effects. J. Phys. Chem. 1992, 96 (25), 10419-10424. (23) Mulvaney, P. Surface Plasmon Spectroscopy of Nanosized Metal Particles. Langmuir 1996, 12 (3), 788-800. (24) Sakai, T.; Alexandridis, P. Single-Step Synthesis and Stabilization of Metal Nanoparticles in Aqueous Pluronic Block Copolymer Solutions at Ambient Temperature. Langmuir 2004, 20 (20), 84268430. (25) Sakai, T.; Alexandridis, P. Mechanism of Metal Ion Reduction, Nanoparticle Growth and Size Control in Aqueous Amphiphilic Block Copolymer Solutions at Ambient Conditions. J. Phys. Chem. B 2005, 109 (16), 7766-7777. (26) Longenberger, L.; Mills, G. Formation of Metal Particles in Aqueous Solutions by Reactions of Metal Complexes with Polymers. J. Phys. Chem. 1995, 99 (2), 475-478. (27) Liu, K.-J. Nuclear Magnetic Resonance Studies of Polymer Solutions. V. Cooperative Effects in the Ion-Dipole Interaction between Potassium Iodide and Poly(ethylene oxide). Macromolecules 1968, 1 (4), 308-311. (28) Yanagida, S.; Takahashi, K.; Okahara, M. Metal-Ion Complexation of Noncyclic Polyoxyethylene Derivatives. I. Solvent Extraction of Alkali and Alkaline Earth Metal Thiocyanates and Iodides. Bull. Chem. Soc. Jpn. 1977, 50 (6), 1386-1390. (29) Alexandridis, P.; Nivaggioli, T.; Hatton, T. A. Temperature Effects on Structural Properties of Pluronic P104 and F108 PEO-PPOPEO Block Copolymer Solutions. Langmuir 1995, 11 (5), 1468-1476. (30) Lin, Y.; Alexandridis, P. Temperature-Dependent Adsorption of Pluronic F127 Block Copolymers onto Carbon Black Particles Dispersed in Aqueous Media. J. Phys. Chem. B 2002, 106 (42), 10834-10844. (31) Alexandridis, P.; Yang, L. Micellization of Polyoxyalkylene Block Copolymers in Formamide. Macromolecules 2000, 33 (9), 3382-3391.

Sakai and Alexandridis

control the reactivity and particle size. We selected formamide to examine solvent effects on the basis of its unique properties (higher dipole moment and dielectric permittivity than those of water)33 and the practical reason that the solution properties of EOxPOyEOx block copolymers in formamide are well established.31,32 In the present work, we discuss how block copolymer properties, modulated by temperature and solvent, affect the reactivity of AuCl4- reduction and the size, shape, and crystallinity of the resulting gold particles. While several reports have examined various solvents in the synthesis of metal particles,1-16 most of them capitalize on the reducing function of the solvents themselves. The solvent quality-controlled mechanism that we consider here is different in that it addresses the amphiphilic character of the block copolymers that we utilize as reductants and stabilizers. Experimental Section Gold particles were prepared by simply mixing an aqueous 10 × 10-3 mol L-1 hydrogen tetrachloroaurate(III) hydrate (HAuCl4‚ 3H2O; 99.9+%, Aldrich) solution with formamide (HCONH2; 99.5+%, Aldrich) or aqueous (18.2 MΩ cm, Millipore-filtered water) solutions containing EO37PO56EO37 block copolymer (Pluronic P105; molecular weight 6500, BASF Corp.). To examine the effect of polymer hydrophobicity, EO136 homopolymer (PEG6000; molecular weight 6000, Fluka Biochemika) was used to synthesize gold particles in the same solvents. The final AuCl4concentration was 2 × 10-4 mol L-1. The polymer concentrations reported here are those of the polymer solution prior to mixing with the HAuCl4 solution. The mixing ratio was such that the final polymer concentration was 2% lower than that before mixing. Following agitation by a vortex mixer for ∼10 s, the solutions were left standing at ambient conditions (∼25 °C) for 2 days for the reaction to proceed. For the investigation of temperature dependence, the solutions were left standing at ∼50 and ∼100 °C for 30 min for the reaction to proceed following the initial mixing (in the case of EO37PO56EO37 in water, the solutions were left standing at ∼90 °C so as not to exceed the polymer cloud point34). The colloidal stability of gold particles formed in aqueous solutions is excellent for over a year. Gold particles formed in formamide start to precipitate within a few days after particle formation because of the larger particle size (but the solution retains the red color originating from dispersed gold particles for a couple of weeks). The formation of gold particles was monitored by spectroscopy.25,35 The size and shape of the obtained gold particles were observed by electron microscopy.25

Results and Discussion Relation between the AuCl4- Reduction Mechanism and Gold Particle Size. We have proposed that gold particle formation via AuCl4- reduction in EOxPOyEOx block copolymer aqueous solutions involves three steps:25 (1) (nucleation) AuCl4- reduction through complexation with EOxPOyEOx block copolymers in the bulk solution, resulting in the formation of gold seeds. (2) (growth) adsorption of EOxPOyEOx block copolymers on the surface of gold seeds, accompanied by AuCl4- reduction (32) Yang, L.; Alexandridis, P. Polyoxyalkylene Block Copolymers in Formamide-Water Mixed Solvents: Micelle Formation and Structure Studied by Small-Angle Neutron Scattering. Langmuir 2000, 16 (11), 4819-4829. (33) Samii, A. A.; Karlstrom, G.; Lindman, B. Phase Behavior of Poly(ethylene oxide)-Poly(propylene oxide) Block Copolymers in Nonaqueous Solution. Langmuir 1991, 7 (6), 1067-1071. (34) Alexandridis, P. Poly(ethylene oxide)/Poly(propylene oxide) Block Copolymer Surfactants. Curr. Opin. Colloid Interface Sci. 1997, 2, 478489. (35) Link, S.; El-Sayed, M. A. Spectral Properties and Relaxation Dynamics of Surface Plasmon Electronic Oscillations in Gold and Silver Nanodots and Nanorods. J. Phys. Chem. B 1999, 103 (40), 8410-8426.

Formation of Au Nanoparticles in PEO-PPO Solutions

at that location and particle growth. (3) (stabilization) stabilization of gold particles by EOxPOyEOx block copolymers. Particle formation and growth are controlled by the amphiphilic (dual-nature) character of the EOxPOyEOx block copolymers: EOx-enhanced AuCl4- reduction and POy-enhanced block copolymer adsorption on the surface of gold particles.25 This results in a competition between AuCl4- reduction in the bulk solution (step 1) and that on the surface of gold seeds (step 2).25 This competition becomes important for particle size determination. For example, if the reactivity of AuCl4- reduction in the bulk solution to form gold seeds (step 1) were higher (faster) than that on the surface of the gold seeds (step 2), new particle formation (number increase) would be more significant than particle growth (size increase). Alternately, if the reactivity of AuCl4- reduction on the surface of gold seeds (step 2) were higher (faster) than that in the bulk solution (step 1), then particle growth (size increase) would be more significant. For a given initial concentration of metal ions, the average volume per metal particle formed, V, is inversely proportional to the number of particles, N: V ∝ 1/N. Since the number of particles is equal to that of the seeds (initial reaction sites), an important factor for particle size control is the number of reaction sites. This number is related to the reactivity of AuCl4- reduction in the bulk solution (step 1), which in the case of aqueous EOxPOyEOx block copolymer solutions depends on the (i) block copolymer conformation or structure (e.g., loops vs entanglements and unimers vs micelles) and (ii) interactions between AuCl4- ions and block copolymers (i.e., attractive iondipole interactions vs repulsive interactions due to polymer hydrophobicity). The polymer loops, unimers, and attractive ion-dipole interactions are expected to enhance the complexation of AuCl4- ions with polymers, while polymer entanglements, micelles, and repulsive interactions due to polymer hydrophobicity should have the opposite effect.24-28,36 Since all these are controllable by the solution structure of EOxPOyEOx block copolymers, we address here relationships among the block copolymer structure, AuCl4reduction reactivity, and resulting particle size. In particular, we examine the effects of temperature and solvent since these strongly affect the block copolymer solution properties.29-32 Temperature Effects on Gold Particle Formation. Let us first consider the effects of temperature on gold particle synthesis in aqueous EOxPOyEOx solutions. Heating is known to increase the hydrophobicity of EOxPOyEOx due to dehydration of the PEO and PPO blocks, increase its interfacial activity, and decrease the critical micelle concentration (cmc) (increase in the number of micelles).29-32 Thus, the reactivity of AuCl4- reduction in solution (step 1) is expected to decrease as a result of a decrease in the number of unimers and an increase in repulsive interactions between EOxPOyEOx block copolymers and AuCl4- ions due to an increase in polymer hydrophobicity.25 Consequently, the number of gold seeds (reaction sites) would decrease, and the particle size would increase. Furthermore, since polymer adsorption on gold particles (step 2) is promoted at higher temperature,30 the increase in particle size should be enhanced by heating. TEM images of gold particles synthesized in 10 wt % EO37PO56EO37 aqueous solutions indicate that the particle diameter increased from ∼20 to ∼40 nm with an increase (36) Elliott, B. J.; Willis, W. B.; Bowman, C. N. Polymerization Kinetics of Pseudocrown Ether Network Formation for Facilitated Transport Membranes. Macromolecules 1999, 32 (10), 3201-3208.

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Figure 1. TEM images of gold particles synthesized in 10 wt % EO37PO56EO37 aqueous solutions at ∼25 °C (upper image) and at ∼90 °C (bottom image) for 30 min. The scale bar represents 100 nm.

in temperature from ∼25 to ∼90 °C (Figure 1). This is consistent with the considerations outlined in the previous paragraph. No remarkable difference in reactivity between ∼25 and ∼90 °C was observed; the reaction was completed within 30 min at both temperatures. To test the possibility of AuCl4- reduction being caused by heating alone, absorption spectra of AuCl4- aqueous solution left standing at ∼90 °C for 30 min were recorded in the absence of EO37PO56EO37 block copolymer. No noticeable surface plasmon band at ∼540 nm was observed, indicating that the AuCl4- reduction observed at elevated temperature is not due to an energy input by heating but rather due to the EO37PO56EO37 block copolymers. Having established that the particle size increase upon heating is most likely due to changes in the solution properties of the amphiphilic block copolymers,31,32 the role of the hydrophobic PPO blocks appears prominent. To further examine the effects of polymer hydrophobicity, we performed gold particle synthesis in 10 wt % EO136 homopolymer aqueous solutions at different temperatures (∼25 and ∼100 °C). Heating causes an increase in the EOx homopolymer hydrophobicity, but this would be small compared to that of EOxPOyEOx block copolymers. The effects of heating on the homopolymer conformation (release of entanglements and increase in the number of coils) should be more significant. Thus, the reactivity of AuCl4- reduction in the solution (step 1) would increase, and lead to an increase in the number of gold seeds

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Figure 3. Absorbance at ∼540 nm due to the surface plasmon band of gold nanoparticles prepared in water/formamide mixtures containing EO37PO56EO37 block copolymers 2 days after the reaction was initiated at ∼25 °C, plotted as a function of the formamide volume fraction in water. The EO37PO56EO37 concentration is (4) 0, (9) 0.1, (0) 0.5, (b) 1.0, and (O) 5.0 wt %.

Figure 2. TEM images of gold particles synthesized in 10 wt % EO136 aqueous solutions at ∼25 °C for 2 days (upper image) and at ∼100 °C for 30 min (bottom image). The scale bar represents 100 nm.

(reaction sites). As a result, smaller particles were formed in EO136 homopolymer aqueous solution at elevated temperature. While gold particles with a diameter of 50-200 nm formed in 10 wt % EO136 aqueous solutions at ∼25 °C, much smaller particles with a diameter of 560 nm) compared to that (∼520 nm) in aqueous solutions (see the middle panel of Figure 6), suggesting the formation of larger particles in formamide.35 The longer fwhm observed in formamide solutions (see the bottom panel of Figure 6) also suggests the formation of larger particles. The fwhm increases with an increase in particle size for particles with diameter >∼20 nm because of

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Figure 8. High-resolution TEM image (left) and electron diffraction patterns (right) of gold particles synthesized in 10 wt % EO37PO56EO37 formamide solutions at ∼25 °C for 2 days. Also shown is the SEM image of gold particles synthesized in 10 wt % EO37PO56EO37 formamide solutions at ∼100 °C for 30 min.

Figure 7. TEM images of gold particles synthesized in 10 wt % EO37PO56EO37 formamide solutions at ∼25 °C for 2 days (upper image) and at ∼100 °C for 30 min (bottom image). The scale bar represents 100 nm.

extrinsic size effects, while for particles with diameter