Fabrication and Electrochemical Application of Three-Dimensional Gold Nanoparticles: Self-Assembly Ahmed I. Abdelrahman, Ahmad M. Mohammad, † Takeyoshi Okajima, and Takeo Ohsaka* Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Mail Box G 1-5, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan ReceiVed: October 29, 2005; In Final Form: December 23, 2005 Multilayers film of nanostructured citrate-stabilized gold particles (AuNPs) has been fabricated based on the layer-by-layer (LBL) technique using a self-assembled monolayer of 1,4-benzenedimethanethiol (BDMT). The formation of AuNPs and BDMT self-assemblies as alternative multilayers was confirmed by transmission electron microscopy (TEM), X-ray photoelectron spectroscope (XPS), and quartz crystal microbalance (QCM). The formation of uniform AuNP layers with an average monolayer thickness of 5-6 nm was obvious in the TEM images. The existence of BDMT molecules as cross linkers for the AuNPs’ layers was proved by XPS measurements. The greater affinity of AuNPs’ layers to bind BDMT molecules in comparison with the bare Au bulk electrode was revealed by QCM measurements. Electrochemically, the AuNPs’ layers on the electrode surface did not only catalyze the reduction of oxygen (ca. 100-mV positive shift of the reduction peak potential compared with that at the bare Au bulk electrode) but also showed a fascinating nature of working as a renewed activated-electrode surface; a zigzag response was observed for oxygen reduction during alternative immobilization of BDMT and the AuNP layer. The self-assembly of a new AuNPs layer restored the catalytic activity that was entirely blocked by the preceding BDMT layer. 1. Introduction Modification of electrode surfaces with gold nanoparticles (AuNPs) has recently received a considerable interest as a consequence of the rapid progress in nanotechnology both in fundamental studies and technological applications. 1,2 In general, and compared with their bulk materials, nanomaterials, with their size, morphology, large surface area, and possible quantum confinement, exhibit unique physical and chemical properties. 3 In particular, the dependence of the catalytic activity of metallic and metal oxide nanoclusters on the cluster size has become now a fact. 4,5 Interestingly, AuNPs exhibit an outstanding catalytic activity compared to bulk gold, which is unusual considering that bulk gold is chemically inert. 6,7 In fact, AuNPs are considered among the most stable metallic nanoparticles, 1 and they have widely been used in several aspects in chemistry, 8 material science, 9 and biotechnology. 10 In catalysis, the mono- layer-protected core-shell gold/alloy nanoparticle-tailored elec- trodes have been used in the electrocatalytic reduction of oxygen 11 and oxidation of CO, 8 methanol, 2 and methanal. 12 To immobilize nanosized particles on a certain substrate, several approaches such as the electrodeposition and immobilization by electrostatic or covalent interactions have been sought. 13-16 Among these approaches, the layer-by-layer (LBL) self-as- sembly technique has been proved to be facile and flexible to generate ultrathin films with molecular order and stability. 17 In addition, the LBL method was considered as one of the best techniques for the preparation of a three-dimensional structured array of nanoparticles. 18 This technique uses natural and synthetic polyelectrolytes, biomacromolecules, and various polyionic inorganic species to fabricate multilayers. Films prepared by LBL exhibited an independent repeated electro- chemical behavior for the electroactive Os 3+ /Os 2+ and Ru 3+ / Ru 2+ redox centers. 19 Olek et al. 20 have also used the LBL approach to prepare a high concentration of nanotubes with a homogeneous distribution within a polymer. Moreover, the LBL technique could be used to structure covalently linked multi- layers of metallic nanoparticles on (3-aminopropyl) trimethoxy- silane (APTMS)-coated glass, 21 gold, 22 and indium tin oxide (ITO) substrates using different bisthiol cross linkers. 13 These structures were useful for optical and spectroscopic applica- tions. 23 Recently, our group has investigated the electrochemical reduction of oxygen on AuNPs electrochemically deposited on different substrates. 24-26 These nanoparticles exhibited an extraordinary electrocatalytic activity toward oxygen reduction. We have also utilized the self-assemblies of short-chain disulfide with a terminal amino group (cystamine) and aromatic dithiol (1,4-benzenedimethanethiol (BDMT)) for anchoring AuNPs on Au substrate. 27 A significant increase in the cathodic peak current and a 130-mV positive shift in the cathodic peak potential have been observed, 27 indicating that AuNPs achieved a good electrical communication with the underlying electrode surface. In this paper, we expand the idea to fabricate three- dimensional multilayers of AuNPs self-assembled on the surface of a gold electrode using the LBL approach and BDMT as a cross linker. The formation of covalently bonded multilayers of AuNPs and BDMT SAM over the gold electrode was examined by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and quartz crystal microbal- ance (QCM) technique. Next, the electrochemical activity of AuNPs’ layers and their behavior as a renewed electrode surface * To whom correspondence should be addressed. Phone: +81-45-924- 5404. Fax: +81-45-924-5489. E-mail: ohsaka@echem.titech.ac.jp. † Permanent address: Chemistry Department, Faculty of Science, Cairo University, P. O. 12613, Giza, Egypt. 2798 J. Phys. Chem. B 2006, 110, 2798-2803 10.1021/jp056238x CCC: $33.50 © 2006 American Chemical Society Published on Web 01/21/2006