Self-Assembled Stable Silver Nanoclusters and Nanonecklace Formation: Poly(methylhydrosiloxane)-Mediated One-Pot Route to Organosols † Bhanu P. S. Chauhan* and Rajesh Sardar Polymers and Engineered Nanomaterials Laboratory, Department of Chemistry and Graduate Center, City University of New York at the College of Staten Island, 2800 Victory Boulevard, Staten Island, New York 10314 Received February 16, 2004 Revised Manuscript Received May 5, 2004 Nanometer-sized particles of metals and semiconduc- tors have been investigated intensively in recent years. 1 In this context, silver nanoparticles are of great interest due to their role in photographic processes, 2 their utility as substrates for surface-enhanced Raman spectroscopy (SERS), 3 and also their application in catalysis. 4 Silver nanocrystallites, mostly hydrosols, have been widely studied because of the ease of their preparation. Since most of the catalytic reactions are performed in organic solvents, it is desirable to design synthetic methods which lead to the stabilization of metal nanoparticles in such solvents. Colloidal dispersions of silver in nonaqueous liquids (organosols) are rare and more difficult to prepare and stabilize. It has been observed that the stability, particle size, and properties of metal colloids strongly depend on the specific method of preparation and the experimental conditions applied. In most cases, nanoparticles are stabilized with strong coordinating surfactants to prevent the agglomeration and to provide specific surface properties. 5 On the other hand, utility and activity of such particles are compro- mised due to the difficulty in ligand exchange reactions in catalytic processes. In this context it is desirable to develop synthetic strategies to fabricate nanoparticles which provide flexibility to functionalize nanoparticles according to the need and hence tailoring of nanoparticle surfaces. Moreover, for catalytic applications such con- jugates may have superior activity and selectivity over the nanoparticles passivated by strong coordinating ligands. Nanosized colloidal metal nanoparticles protected by polymers exhibit intriguing optical, catalytic, or elec- tronic properties due to the “size effect” and additionally provide the option of influencing the materials’ proper- ties by selection of the polymeric matrix. 6,7 A large number of preparative methods for these colloidal metal dispersions involve the presence of the polymer during the reduction from the metal precursors. In this way, the polymer can profoundly influence the particle features of the resulting metal colloids as well as their long-term colloidal stability. Monomeric hydrosilanes are known reducing agents and have been successfully used for the generation of Pt, Pd, and Rh nanosized particles in the context of metal-catalyzed hydrosilyla- tion of alkenes. 8 On the other hand, investigations of polymeric analogous such as polyhydrosilanes as reduc- ing agents for the generation and in-situ stabilization of nanosized metal particles have not been explored. Though, their property profile may provide the means of directing metallic particles into specific physicochem- ical environments 9a in addition to their utility as reduc- ing agents. Moreover, combining the ease of process- ability of polysiloxane polymers with the improved mechanical and optical properties of metal nanoparticles is of practical use for the fabrication of many new devices. 9b In this Communication, we describe a versatile method and first example of polyhydrosiloxane-induced genera- tion and stabilization of functionalizable monodisperse silver sols (Scheme 1). This method enables routine formation of stable nanosilver reservoirs, avoiding particle aggregation during the storage as well as nucleation and growth process. We also demonstrate the utility of such reservoirs in grafting the surface proper- ties of nanosized silver particles by exchange reactions with trioctylamine. In an exploratory experiment, when poly(methylhy- drosiloxane) (PMHS; 0.024 mL, 0.4 mmol, M w ∼ 2000, 33-35 Si-H units) was added to the 50 mL toluene suspension of silver acetate (0.032 g, 0.2 mmol), the mixture turned faint yellow and showed a very broad peak at 445 nm in UV-vis spectra, indicating formation of silver nanoparticles. 10 But, it was observed that the reduction process was very slow (24 h) and was ac- companied by particles precipitation. To accelerate the reduction reaction, an amine cata- lyst was added to the reaction mixture. Amines are known to polarize the Si δ+ -H δ- bonds via intermediate formation of hypercoordinated silicon species. 11 Thus, in an optimized procedure, AgOAc (0.032 g, 0.2 mmol) was suspended in 50 mL of toluene and PMHS (0.072 mL, 1.2 mmol) was added while stirring gently at room temperature under nitrogen. The solution was stirred † Part of the Ph.D. Thesis of R.S. This research was presented in 36th Organosilicon Symposium in Akron, OH. See: Chauhan, B. P. S.; Sardar, R.; Tewari, P.; Sharma, P. 2003 (May 29-31), P-31. * Corresponding author. E-mail: chauhan@postbox.csi.cuny.edu or chauhan@mail.csi.cuny.edu. Scheme 1. Synthetic Strategy to Polysiloxane-Stabilized Silver Sols and Their Surface Grafting 5136 Macromolecules 2004, 37, 5136-5139 10.1021/ma0496798 CCC: $27.50 © 2004 American Chemical Society Published on Web 05/26/2004