Preparation of Size-Controlled, Highly Populated, Stable, and Nearly Monodispersed Ag Nanoparticles in an Organic Medium from a Simple Interfacial Redox Process Using a Conducting Polymer Arnab Dawn, Pratap Mukherjee, and Arun K. Nandi* Polymer Science Unit, Indian Association for the CultiVation of Science, JadaVpur, Kolkata - 700 032, India ReceiVed NoVember 4, 2006. In Final Form: February 22, 2007 The reducing property of an organically soluble conducting polymer (poly(o-methoxyaniline), POMA) is used to prepare monodisperse, size-controlled, highly populated, and highly stable silver nanoparticles in an organic medium through an interfacial redox process with an aqueous AgNO 3 solution. The transition of emeraldine base (EB) to the pernigraniline base (PB) form of POMA occurs during nanoparticle formation, and the nitrogen atoms of POMA(PB) stabilize Ag nanoparticles by coordination to the adsorbed Ag + on the nanoparticle surface. The conductivity of the nanocomposite is on the order of 10 -11 S/cm, indicating that no doping of POMA occurs under the preparation conditions. The nanoparticles are free of excess oxidant and external stabilizer particles. The POMA (EB) concentration tailors the size of nanoparticles, and at its higher concentration (0.01% POMA with 0.01 N AgNO 3 ), very dense Ag nanoparticles (6 × 10 15 particles/m 2 ) of almost uniform size and shape are produced. The rate constant and Avrami exponent values of the nanoparticle formation are measured from the time-dependent UV-vis spectra using the Avrami equation. The Avrami exponent (n) values are close to 1, indicating 2D athermal nucleation with the circular shape of the nuclei having diffusion-controlled growth. The rate constant values are almost independent of AgNO 3 concentration but are strongly dependent on POMA concentration. The higher rate constant with increasing POMA- (EB) concentration has been attributed for the lowering of nanoparticle size due to increased nucleation density. Introduction Nanoparticles in organic media are interesting for their applications in catalytic processes, for surface modifications with organic functional groups, and in printed electronics. 1-5 Water- based methods for the preparation of metal nanoparticles are easy to use because of their ability to solubilize a variety of ions and stabilizer molecules (e.g., surfactants, polymers, and coordinating ligands 3,6 ). In spite of the great advantages of water- based methods, there are some inherent problems (e.g., removing the stabilizer residue after synthesis, difficulties in surface modifications of nanoparticles, the use of low reactant concen- trations, etc. 7,8 ). Also, the tendency of nanoparticle agglomeration hinders their preparation at high concentration in aqueous media. 5b Therefore, the preparation of Ag nanoparticles in organic media might be a way to achieve very dense nanoparticles because the hydrophobic stabilizing agents prevent the coagulation of nanoparticles very effectively. A silver nanoparticle-based localized surface plasmon resonance (LSPR) nanosensor is used in an ultrasensitive biodetection technique, and for this purpose, a nearly monodisperse silver organosol is necessary. 5b Mainly, there are two methods of dispersing the nanoparticles in organic media: 1-5 (i) synthesizing and stabilizing them in organic media and (ii) transferring the nanoparticles from an aqueous medium to an organic medium. The first method has difficulty in controlling the size and shape of nanoparticles, but the second method is easier because it is possible to use a phase-transfer catalyst 3 or to cap the nanoparticles with thiols or amines, which facilitate the transfer of nanoparticles from aqueous to organic media. 5,9 However, the later procedure leads to an organosol with contaminated phase-transfer and caping agents. Here we report a new method of preparing Ag nanoparticles in organic media using a single-step interfacial process with a conducting polymer, POMA, as the reductant. This method yields Ag nanoparticles that are free of external stabilizers and phase- transfer agents. Also, the interfacial reduction process has the benefit of control the size of the nanoparticles because it is usually governed by the rate of the reduction process, which may be slow because of the slow diffusion of reactants at the interface. Conducting polymers are important polymers for their semiconducting and optoelectronic properties. 10 Among the conducting polymers, polyaniline (PANI) is an important member because it is cheap and easy to synthesize. It is now known that metal ions (e.g., Cu 2+ ) can oxidize polyaniline from the emeraldine base (EB) to the pernigraniline base (PB) form. 11 The oxidizing ability of metal ions depends on their standard reduction potentials, and Ag + has a higher value (+0.8 V) than Cu 2+ (+0.34 V). Therefore, polyaniline can be used as a reductant to prepare Ag nanoparticles, but PANI has the disadvantage of being insoluble either in aqueous or organic media. 10 We have searched a derivative of PANI, poly(o-methoxyaniline) (POMA), which is highly soluble in an organic medium, 12 so POMA can be used to prepare metal nanoparticles from metal ions with high reduction potentials if the metal ions are kept in contact with * Corresponding author. E-mail: psuakn@mahendra.iacs.res.in. 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