Gold nanoparticles, ca. 30 Å in diameter, have been derivatized with specifically deuterated (position 1 and positions 10 to 13) and perdeuterated (positions 2 to 18) octadecanethiols (C 18 SH). The phase behavior of the octadecanethiolate monolayers chemisorbed onto the colloidal gold surface was characterized by differential scanning calorimetry (DSC). The DSC thermograms show that the C 18 SH-derivatized Au nanoparticles undergo distinct phase transitions which can be associated with the reversible disordering of the alkyl chains. Despite the highly curved geometry of these Au particles, there is a remarkable degree of conformational order in the alkanethiolate chains and the thermotropic behavior of the thiol-modified gold nanoparticles is very similar to that of conventional, planar self-assembled monolayers. Both the peak maximum temperature and the enthalpy associated with the DSC transition strongly parallel those of the gel-to-liquid crystalline transition of n-diacylphosphatidylcholine lipid bilayer membranes of equivalent chain length. Restricted chain mobility due to covalent bonding of the sulfur head group to the gold surface does not affect the cooperativity of the transition in terms of the transition temperature and enthalpy. Local chain ordering and dynamics in the deuterated C 18 S/Au nanoparticles have been probed using variable-temperature solid-state deuterium NMR spectroscopy and transmission FT-IR spectroscopy. The temperature dependence of the symmetric CD 2 stretching frequency has confirmed that the DSC-detected phase transition involves a thermallyinduced change from a predominantly all-trans conformation to a chain disordered state. A comparison of the thermal behavior of d 35 -C 18 S/Au and 10, 10,11,11,12,12,13,13-d 8 -C 18 S/Au shows that disordering originates in the chain terminus region and propagates toward the middle of the chain as the temperature increases. Studies of 1,1-d 2 -C 18 S/Au show that the disorder does not extend to the tethered sulfur head group. Deuterium NMR spectroscopy specifically establishes that chain melting arises from an increased frequency of gauche bonds in the alkanethiolate chains. The 2 H NMR line shapes further indicate that the tethered alkanethiolate chains are undergoing rapid transgauche bond isomerization and axial chain rotation.

: Self-assembled monolayers (SAMs) of n-alkanethiolates on gold, silver, and copper have been intensively studied both as model organic surfaces and as modulators of metal surface properties. Sensitivity restrictions imposed by monolayer coverage and the low surface area of planar metal substrates, however, limit the characterization of these films in molecualar terms to surface enhancement techniques. As a result, key aspects such as film dynamics and alkyl chain ordering remain ill-defined. The characterization of the thermal behavior of SAMs is important not only for the design of stable, well-ordered organic superlattices, but also for the fundamental understanding of the factors that drive molecular interactions in two dimensions. Phase properties in SAMs have been addressed here through the synthesis of gold nanoparticles of 20-30 A in diameter and fully covered with alkylthiol chains. These thiolmodified gold nanoparticles with large surface areas have enabled the monolayer film structure to be uniquely characterized by transmission FT-IR spectroscopy, NMR spectroscopy, and differential scanning calorimetry. Our studies reveal that for long-chain thiols ( 2 C,,), the alkyl chains exist predominantly in an extended, all-trans ordered conformation at 25 "C. Furthermore, calorimetry, variable

The oft-cited complexity of tethered ferrocene electrochemistry in single component (FcRS-) or binary (FcRS-/CH3R'S-) self-assembled monolayers (SAMs) on gold has been investigated. The complex voltammetry is shown to be linked to local electrostatics caused by the formation of the ferrocenium ion. This conclusion is reached by studying model effects in binary SAMs, where a cationic alkylthiolate (H3N+ C11S-Au) is mixed with FcC12S-Au. A fitting procedure involving both a Gaussian and a Lorentzian distribution is used for deconvolution of the two peaks which are consistently observed in the SAMs when chi(Fc)surf > or = 0.2. The lower-potential (E degrees ' = 250 mV) and higher-potential (E degrees ' = 350 mV) voltammetric peaks are assigned to Fc moieties in "isolated" and "clustered" states, respectively. Use of this method to better understand SAM structure is demonstrated by distinguishing the degree of homogeneity in two binary SAMs of similar composition.

A revised view of Brust-Schiffrin metal nanoparticle syntheses is presented here. Precursor species of these reactions are identified and quantified for Au, Ag, and Cu systems. Contrary to the assumptions of previous reports, tetraalkylammonium metal complexes are shown to be precursors of the two-phase reactions, whereas M(I) thiolates are shown to be precursors of the one-phase reactions. A new scheme is outlined for the two-phase synthesis, and the implications of this scheme are discussed. A new synthetic strategy employing well-defined precursors is also introduced. Finally, M(I) thiolate formation, and its impact on nanoparticle synthesis, is discussed. It is expected that the results presented here will lead to modifications in the manner in which these important syntheses are conducted.

A polystyrene−poly(ethylene oxide) containing diblock copolymer, when spread at the air/water interface, spontaneously forms surface aggregates. This surface aggregation is shown to be neither compression-induced, associated with micellization in the spreading solvent, nor induced by the Langmuir−Blodgett film transfer process. We have previously found that such two-dimensional surface aggregation occurs for diblock copolymers with a polystyrene block and a hydrophilic block of quaternized poly(vinylpyridine), poly(tert-butyl acrylate), poly(n-butyl acrylate), or poly(dimethylsiloxane). The phenomenon has also been observed in films of polystyrene−poly(methyl methacrylate) by Rice and co-workers. Indeed, whenever an appropriate imaging technique has been used, phase-separated domains with 30−100 nm length scales have been observed, when amphiphilic diblock copolymers are spread at the air/water interface and transferred to solid substrates at appreciable surface pressures. We therefore believe that the formation of surface aggregates (often well defined) is a general phenomenon for hydrophobic−hydrophilic diblock copolymers. The implications of this phenomenon for the study of diblock copolymers at the air/water interface are discussed, particularly in relation to studies using techniques which report properties averaged over the lateral dimensions of the film in question, such as specular neutron reflectivity, specular X-ray reflectivity, and Brewster angle microscopy.

Although self-assembled monolayers (SAMs) of alkylthiols on planar gold (2D SAMs) and on gold nanoparticles (3D SAMs) have been intensely studied, the actual nature of the Au-S bonding remains poorly characterized. Comparison of the X-ray photoelectron spectroscopy (XPS) spectra of 2D and 3D SAMs and "reference" Au(I) complexes, sometimes referred to as Au(I) thiolate polymers, provides detailed insight into this problem. We report high-resolution XPS spectra and Au 4f 7/2 and S 2p 3/2 binding energies (BE) in 2D SAMs, 3D SAMs and the Au(I) thiolate complexes for two short-chain thiols (n-C 4 SH and n-C 5 SH). Sulfur 2p 3/2 BE shifts are used to compare the different states of bonding in the SAM systems and the Au(I) complexes and establish that the S atom in the SAM systems bears a charge of about 0.2e. The 2D and 3D SAMs exhibit similar XPS characteristics and are both distinguishable from the Au(I) complexes. The origins of the observed BE values are discussed in the context of the nature of the gold substrate and the oxidation state of the chemisorbed sulfur atom. Comparison of 13 C NMR chemical shift data and XPS BE data further clarifies the nature of the surface interactions as well as the use of the Au(I) complexes as reference materials.

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A series of polymer-coated Au nanoparticles have been prepared using the "grafting-to" approach. Thiol-terminated polystyrene and poly(ethylene oxide) ligands are found to form dense brushes on the faceted gold nanoparticle surfaces. Depending on the polymer, the ligand grafting densities on the gold nanoparticles are 1.2- to 23.5-fold greater than those available via self-assembled monolayer formation of the corresponding two-dimensional gold surfaces.