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 Au3+ 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 Au3+ ions with accompanying pH decrease. (C) 2004 Elsevier Inc. All rights reserved.