Journal of Colloid and Interface Science 280 (2004) 506–510 www.elsevier.com/locate/jcis Formation of gold nanoparticles in microreactor composed of helical peptide assembly in water Hiroyuki Nishikawa a , Tomoyuki Morita a , Junji Sugiyama b , Shunsaku Kimura a,∗ a Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Kyoto-Daigaku-Katsura, Nishikyo-ku, Kyoto 615-8510, Japan b Institute ofWood Science, Kyoto University, Gokanosho, Uji, Kyoto 611-0011, Japan Received 19 April 2004; accepted 4 August 2004 Available online 11 September 2004 Abstract A novel microreactor was prepared by self-assembly of an amphiphilic block copolymer composed of a hydrophobic helical peptide unit with a naphthyl group at the C terminal and a hydrophilic poly(ethylene glycol) unit. The copolymer formed a self-assembly in water, taking a vesicular structure. Noticeably, when the copolymer was dispersed in an Au 3+ aqueous solution, gold nanoparticles were formed without addition of any reducing reagent. The naphthyl groups, which are located at the inner surface of the vesicular assembly, promoted the reduction of Au 3+ ions with accompanying pH decrease.  2004 Elsevier Inc. All rights reserved. Keywords: Gold nanoparticles; Helical peptide; Self-assembly; Dipole moment; Vesicles; Microreactor 1. Introduction Metal nanoparticles have attracted much attention due to their unique properties in various aspects associated with their quantum-scale dimensions [1–6]. In particular, gold nanoparticles have been extensively studied on their optical and electronic properties, and various preparative methods were reported. Chemical methods generally use reducing agents for Au 3+ as well as protective molecules such as dendrimers [7], micelles [8,9], star-shaped molecules [10], and inorganic polymers [11] to avoid intensive aggregation and keep the particle size in the nanometer range. How- ever, it is challenging to obtain gold nanoparticles without protective molecules at the surface but keeping a homoge- neous particle size, which should provide an active surface for various purposes. The particle size seems to be deter- mined by the balance between the reduction reaction and the covering of the active surface by the protective mole- * Corresponding author. Fax: +81-75-383-2401. E-mail address: shun@scl.kyoto-u.ac.jp (S. Kimura). cules. Usage of amine or alcohol compounds as reducing agents tends to afford large particles, because Au 3+ is eas- ily reduced by them. The particle size may be controlled more precisely when the reductive reaction is carried out under milder conditions. In the present study, vesicles com- posed of helical peptides (peptosome) [12,13] were chosen as microreactors for Au 3+ reduction for the following rea- sons: (i) the size of gold particle may be limited by amount of Au 3+ incorporated in a small volume of inner aqueous phase of peptosome, (ii) aggregation of gold nanoparticles should be suppressed by encapsulation in peptosome, and (iii) reduction reactions may be facilitated in peptosomes if reducing agents in peptosomes are strengthened by an elec- trically negative atmosphere generated by the large dipole moments of helical peptides. A naphthyl group was cho- sen as a chromophore, which may help the reduction of Au 3+ . Especially, naphthyl groups attached at the C termi- nals of helical peptides should possess an enhanced reducing ability due to the negative end of the large helical dipole moment. A hydrophilic poly(ethylene glycol) (PEG) seg- ment was connected to the N terminal of the hydrophobic helical peptide (Fig. 1, BPl24NAB). The amphiphilic block 0021-9797/$ – see front matter  2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.08.013