Stabilized Gold Nanoparticles by Reduction Using 3,4-Ethylenedioxythiophene-polystyrenesulfonate in Aqueous Solutions: Nanocomposite Formation, Stability, and Application in Catalysis Shanmugam Senthil Kumar, Chinnaiah Siva Kumar, Jayaraman Mathiyarasu, and Kanala Lakshminarasimha Phani* Electrodics and Electrocatalysis DiVision, Central Electrochemical Research Institute, Karaikudi 630 006, India ReceiVed October 27, 2006. In Final Form: December 12, 2006 Herein, we report a one-pot synthesis of highly stable Au nanoparticles (AuNPs) using 3,4-ethylenedioxythiophene (EDOT) as a reductant and polystyrene sulfonate (PSS - ) as a dopant for PEDOT and particle stabilizer. The synthesis demonstrated in this work entails the reduction of HAuCl 4 using EDOT in the presence of PSS - . The formation of AuNPs with concomitant EDOT oxidation is followed by UV-vis spectroscopy at various time intervals. Absorption at 525 nm is due to the surface plasmon band of AuNPs (violet), and broad absorption above 700 nm is due to oxidized PEDOT that was further characterized to be in its highly oxidized (doped) state, using FT-Raman spectroscopy. Transmission electron microscopy shows a polydisperse nature of the particles, and the selected area electron diffraction pattern reveals the polycrystalline nature of AuNPs. With stabilizers such as sodium dodecylsulfate (SDS) (green) and polyvinylpyrrolidone (PVP) (blue), the absorbance around 525 nm was found to be negligibly small, while PSS - showed high absorbance at 525 nm (violet) and above 700 nm (oxidized PEDOT). PSS - also allows complete oxidation of EDOT and serves as an effective dopant for PEDOT. While AuNPs covered by PEDOT alone cannot be dispersed in aqueous solutions, PSS - renders Au-PEDOT water soluble. The hydrodynamic diameter of the nanocomposite estimated from the dynamic light scattering (DLS) measurements increases in the order Na-PSS < SDS < PVP. Interestingly, the color of the Au nano -PEDOT/PSS - aqueous dispersion changed reversibly between violet and blue and vice versa on addition of NaOH and HCl, respectively. This reversible color change appears to be a combination effect of acid/base on the properties of PEDOT, in turn changing the environment around the embedded AuNPs. The nanoparticle dispersion also exhibited very high stability in presence of 3.0 M NaCl. Remarkably, the nanocomposite Au nano -PEDOT/PSS - was found to function as an effective catalyst to activate the reduction of 4-nitrophenol to 4-aminophenol in the presence of excess NaBH 4 , and the calculated apparent rate constant value of 4.39 × 10 -2 s -1 is found to be higher than those obtained using other nanocomposites with SDS and PVP and comparable to the values reported in the case of other encapsulants. Introduction The synthesis and use of Au nanoparticles constitute a major research area that attracts both academic and industrial interest. 1 Many applications require these particles to be water dispersible and to remain suspended in water with no loss of physical or chemical properties over extended periods of time. 2-3 Moreover, the usefulness of these particles in biological applications, however, will require much more than mere water solubility. The stability of these nanoparticles in high ionic strength environments becomes very important. Nevertheless, water based synthesis of nanoparticles is beset with problems as a result of ionic interactions, which are typically overcome by using low reactant concentrations 4 (about 5 × 10 -4 M) or because the synthesis is carried out in the presence of stabilizers that are subsequently difficult to remove. 5 In contrast, particles synthesized in organic solvents can be made at relatively high concentrations 6 (up to 1 M reactant) with a predefined size and shape 7,8 and with improved monodispersity as compared to those prepared in aqueous solutions. Such particles are, however, water immiscible, which limits their range of application. Hence, phase transfer to aqueous solutions was used. Several routes for chemical reduction of hydrogen tetrachloroaurate (III) for preparing gold colloids have been evolved in the past few decades. The classic citrate reduction method has been extensively used for the generation of aqueous solutions of gold colloids with very narrow size distribution. Biphasic syntheses were performed to produce organic soluble gold colloids (<5 nm). Recently, Au nanoparticles with a high colloidal uniformity were prepared in aqueous solution and organic systems by reducing gold salts with o-anisidine. 9 In addition, a variety of Au nanoparticles was synthesized using ascorbic acid, oxalic acid, or hydrazine as the reducing agents, and it was found that the reactivity and concentration of the reducing agents have a significant effect on the size and dispersity of the metal nanoparticles. 1 Many molecules such as pyrrole, 10 aniline, 11 thiophene, or substituted molecules 12,13 can be oxidized * Corresponding author. Tel.: +914565227555. Fax: +914565227779. E-mail: kanalaphani@gmail.com. (1) Daniel, M.-C.; Astruc, D. Chem. ReV. 2004, 104, 293. (2) (a) Davis, S. C.; Klabunde, K. J.; Chem. ReV. 1982, 82, 153. (b) Lewis, L. N. Chem. ReV. 1993, 93, 2693. (3) Fendler, J. H. Nanoparticles and Nanostructured Films; Wiley: Weinheim, 1998. (4) Turkevich, J.; Stevenson, P. C.; Hillier, J. Discuss. Faraday Soc. 1951, 55. (5) Goia, D. V.; Matijevic, E. New J. Chem. 1998, 22, 1203. (6) Bonnemann, H.; Brijoux, W. In AdVanced Catalysts and Nanostructured Materials; Moser, W., Ed.; Academic Press, New York, 1996; p 165. (7) Green, M.; O’Brien, P. Chem Commun. 1999, 2235. (8) Pileni, M. P. New J. Chem. 1998, 22, 693. (9) Dai, X.; Tan, Y.; Xu, J. Langmuir 2002, 18, 9010. (10) Selvan, S. T. J. Chem. Soc., Chem. Commun. 1998, 351. (11) (a) Pillalamarri, S. K.; Blum, F. D.; Tokuhiro, A. T.; Bertino, M. F. Chem. Mater. 2005, 17, 5941. (b) Tan, Y; Li, Y; Zhu, D. Synth. Met. 2003, 847, 135. (12) Youk, J. H.; Locklin, J.; Xia, C.; Park, M.-K.; Advincula, R. Langmuir 2001, 17, 4681. 3401 Langmuir 2007, 23, 3401-3408 10.1021/la063150h CCC: $37.00 © 2007 American Chemical Society Published on Web 02/07/2007