Tuning the Hydrophilicity of Gold Nanoparticles Templated in Star Block Copolymers Charles-Andre ´ Fustin, ² Catheline Colard, ² Mariam Filali, ² Pierre Guillet, ² Anne-Sophie Duwez, ‡ Michael A. R. Meier, § Ulrich S. Schubert, § and Jean-Franc ¸ ois Gohy* ,² Unite ´ de Chimie des Mate ´ riaux Inorganiques et Organiques (CMAT) and Research Center in Micro- and Nano-Materials and Electronic DeVices (CeRMiN), UniVersite ´ catholique de LouVain, Place Pasteur 1, 1348 LouVain-la-NeuVe, Belgium, Unite ´ de Chimie et de Physique des Hauts Polyme ` res (POLY) and CeRMiN, UniVersite ´ catholique de LouVain, Place Croix du Sud 1, 1348 LouVain-la-NeuVe, Belgium, Laboratory of Macromolecular Chemistry and Nanoscience, EindhoVen UniVersity of Technology and Dutch Polymer Institute, P.O. Box 513, 5600 MB EindhoVen, The Netherlands ReceiVed March 21, 2006. In Final Form: May 10, 2006 We report on a simple procedure to tune the hydrophilicity of hybrid gold nanoparticles. The nanoparticles have been prepared in the core of a poly(ethylene glycol)-block-poly(ǫ-caprolactone) (PEG-b-PCL) five-arm star block copolymer. A hydrophilic corona was then added to these hybrid gold nanoparticles by direct chemisorption of trithiocarbonate-containing poly(acrylic acid) chains. These polymers were synthesized by RAFT polymerization with a trithiocarbonate as the chain-transfer agent. The efficiency of the grafting was evidenced by TEM, AFM, and DLS and by the successful transfer of these nanoparticles from organic solvent to water. Introduction Nanoparticles, and in particular gold nanoparticles (AuNP’s), are a rapidly growing area of material science because of their numerous applications in various fields ranging from chemical separations 1 and sensing, 2-7 to applications in the medical community such as the diagnosis and treatment of some cancers. 8-11 The unique properties of AuNP’s are directly related to their size and are significantly different from those of the corresponding bulk materials. 12 Besides precise control over the size, size polydispersity, and shape, the colloidal stability of AuNP’s is an extremely important issue. AuNP’s have been synthesized using a variety of methods including citrate reduction, 13 two-phase synthesis, 14 and one- phase synthesis in organic solvents. 15 Low molecular weight (MW) surfactants and thiol end-functionalized molecules have also been used in the synthesis of AuNP’s to promote steric stabilization of the accordingly formed particles. 16 Because low MW surfactants are self-organized in micellar nanocontainers, a templating effect could occur provided that the nucleation and growth processes of AuNP’s are limited to one specific locus, most often the core of the micelles. Such a templating effect has been widely investigated because it allows the simultaneous tuning of the size and shape of the resulting AuNP’s as well as their stabilization. 16 The use of polymers has been considered to be a valuable approach to the synthesis of gold nanoparticles because polymers can act as reducing and/or stabilizing agents. 17 For example, gold nanoparticles have been prepared in one step by mixing (in water or organic solvents) a gold precursor, a reducing agent, and a polymer made by reversible addition-fragmentation chain transfer (RAFT) polymerization. By virtue of the polymerization mechanism, this polymer bears a dithioester moiety at one end of the chain. 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