Hexanethiolate Monolayer Protected 38 Gold Atom Cluster Victoria L. Jimenez, Dimitra G. Georganopoulou, † Ryan J. White, ‡ Amanda S. Harper, Allan J. Mills, § Dongil Lee, | and Royce W. Murray* Kenan Laboratories of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290 Received March 19, 2004. In Final Form: May 11, 2004 The nucleation-growth-passivation Brust reaction has been modified so as to enrich the product in useful quantities of a 38-atom gold nanoparticle coated with a hexanethiolate monolayer. Two modifications are described, using -78 °C reduction temperature and a hyperexcess of thiol. Compositional evidence is presented that establishes the product as a Au38(C6)24 hexanethiolate monolayer protected cluster (MPC), based on transmission electron microscopy, laser ionization-desorption mass spectrometry, thermogravi- metric analysis, and elemental analysis. Reverse phase HPLC confirms the relatively good monodispersity of the MPC products, but high-resolution double-column HPLC reveals that the MPCs are a mixture of closely related but chromatographically distinct products. Voltammetry, low energy spectrophotometry, and spectroelectrochemistry reveal, respectively, a 1.6 eV electrochemical energy gap between the first oxidation and the first reduction, an optical HOMO-LUMO energy absorbance edge at 1.3 eV, and a bleaching of optical absorbance near the 1.3 eV band edge that accompanies electrochemical oxidation of the nanoparticle. Introduction Research on nanometer-sized semiconductor and metal particles, where quantum confinement and molecule-like behavior appear, has been active in recent years because of the technological appeal of such materials 1-3 and, more importantly, the need for an improved scientific under- standing of nanoscopic materials of all kinds. The synthesis of nanoscopic materials is always of course a prerequisite to study and discovery of any interesting properties. Except for preparation and study of nanoparticles in the gas phase, 4 it is essentially universal that nanoparticle synthesis is accompanied by coating the nanoparticles with a stabilizing (protective, capping) layer to prevent nanoparticle aggregation. A large literature exists on using different kinds 5-8 of aqueous, nonaqueous, micellar, and two-phase media for the reductive generation of nano- particle cores of different compositions and to provide the required stabilizing chemistry. Stabilizers range from adsorbed polymer coatings 8 to adsorbed small ion layers 6 to explicit monolayers of organic ligands. 9,10 The focus in this paper is on Au nanoparticles. Au colloids have been known since antiquity, and aqueous preparations of ca. 10 nm diameter colloids retain their importance, in for example current bioanalytical method- ology. 11-14 The evolution of size-dependent properties starts, however, only at much smaller Au dimensions, of the order of 100-150 atoms. Schmid et al. 15 in 1981 reported an early example of a molecule-like Au nano- particle, formulated as Au 55 [PPh 3 ] 12 Cl 6 . Subsequent in- vestigations 15 of this interesting material were hampered by its general instability. A new synthetic opening into making small Au nano- particles was provided in 1994 by the Brust two-phase protocol, 9 in which the Au nanoparticle was formed in the presence of alkanethiols, which formed a passivating and protective coating of thiolate ligands. We 10 and others 16-18 were attracted to this synthesis, and considerable sub- sequent work ensued, including use of alkanethiols of different chain lengths, 10 functionalized alkanethiols, 19 and dialkyl disulfides. 20 We label these materials “mono- layer-protected clusters” (MPCs). There has also been related use of one-phase synthetic protocols and of more polar thiols, yielding a range of water-soluble MPCs. 21,22 † Present address: Nanotechnology Institute, Evanston, IL. ‡ Present address: Department of Chemistry, University of Utah, Salt Lake City, UT. § Department of Chemistry, University Liverpool, Liverpool, U.K. | Present address: Department of Chemistry, Western Michigan University, Kalamazoo, MI. (1) Simon, U. Adv. Mater. 1998, 10, 1487. (2) Optical Properties of Metal Clusters; Kreibig, U., Vollmer, M., Eds.; Springer Series in Material Science, Vol. 25; Springer-Verlag: Berlin, 1995. 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