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This paper shows that an introduction of thiosulfate anions in place of bromide anions greatly improves both chemical and thermal stability of tetraoctylammonium-protected gold nanoparticles. Tetraoctylammonium thiosulfate [(Oct)4N^+-O3SS]-protected gold nanoparticles are synthesized by the reduction of (Oct)4N^+-AuCl4 to Au(I)-SSO3^-, followed by the addition of sodium borohydride. The presence of thiosulfate anions instead of bromide anions on the surface of gold nanoparticles results in a significant dampening of the surface plasmon band of gold at 526 nm due to the strong interaction between thiosulfate and the gold nanoparticle surface. Cyanide decomposition and heating treatment studies suggest that (Oct)4N^+-O3SS-protected nanoparticles have much higher overall stability compared to (Oct)4N^+-Br-protected gold nanoparticles.

The structural and interfacial properties of self-assembled monolayers (SAMs) on gold derived from the adsorption of a series of 1,1,1-tris(mercaptomethyl)alkanes (i.e., CH3(CH2)mC[CH2SH]3, where m = 9, 11, 13, 15) were investigated. The new SAMs, which possess uniformly low densities of alkyl chains, were characterized by ellipsometry, contact angle goniometry, and polarization modulation infrared reflection absorption spectroscopy. Additional analysis of the SAMs by X-ray photoelectron spectroscopy permitted a direct calculation of the packing densities of the SAMs on gold. The results as a whole, when compared to those obtained on SAMs generated from normal alkanethiols (CH3(CH2)m+2SH), 2-alkylpropane-1,3-dithiols (CH3(CH2)mCH[CH2SH]2), and 2-alkyl-2-methylpropane-1,3-dithiols (CH3(CH2)mC(CH3)[CH2SH]2) having analogous chain lengths, demonstrate that the 1,1,1-tris(mercaptomethyl)alkanes afford SAMs with alkyl chains having the lowest packing density and least conformational order.

The one-pot synthesis of monolayer-protected metal nanoparticles derived from sodium S-dodecylthiosulfate (Bunte salt) in aqueous solution is described. Silver nanoparticles, which were produced by the borohydride reduction of silver nitrate in H2O, were stabilized by the adsorption of S-dodecylthiosulfate followed by the removal of the SO3^- moiety. Temporary stabilization of silver sols by the adsorption of borohydride and borate prevented aggregation of silver nanoparticles in H2O. The syntheses of other metal nanoparticles, including gold, copper, and palladium particles in H2O, were less successful. Gold and copper particles were completely aggregated and precipitated out immediately after the addition of NaBH4, yielding only insoluble clusters. Stable and soluble palladium nanoparticle could be prepared, but the presence of Pd−thiolate complex was also observed. These nanoparticles were characterized using ¹H NMR, UV−vis spectroscopy, FT-IR spectroscopy, and transmission electron microscopy.

We present a systematic study on the synthesis of mixed monolayer-protected clusters (MMPCs) from mixtures of alkanethiols. Our results show that solvation-driven thermodynamic preferential adsorption of precursor ligands governs monolayer composition of MMPCs. In the mixed system HO(CH2)nSH/ CH3(CH2)mSH (n = m or n ≠ m) in toluene, adsorption of the polar component is largely favored because of the poorer solvation of polar tail groups in toluene compared to tetrahydrofuran (THF). When MMPCs are generated from the mixed system HO(CH2)nSH/CH3(CH2)mSH (n = m) in THF, the tail group effect is much less. The MMPC synthesis in THF solutions containing HO(CH2)nSH/CH3(CH2)mSH (n ≠ m) shows large thermodynamic control, which promotes a preferential adsorption of the thiols with a longer alkyl chain onto the surface of clusters. Transmission electron microscopy (TEM) and UV−vis spectroscopy data suggest that the average core dimension of MPCs generated from alkanethiols in THF is slightly smaller than that of MPCs generated from alkanethiols in toluene under the same conditions.

Bimetallic monolayer-protected nanoparticles have been synthesized by the core metal galvanic exchange reaction of dodecylthiolate monolayer-protected metal (Ag, Pd, Cu) clusters with the more noble metal metal thiolate complexes Au^I[SCH2(C6H4)C(CH3)3] and Pd^II[S(CH2)11CH3)2]. The bimetal nanoparticles produced are stable and can be isolated without core aggregation or decomposition. These new materials have been examined by UV−vis spectroscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and elemental analysis. Their optical properties reflect bimetal cluster formation by time-dependent shifts in the surface plasmon resonance absorbance. Transmission electron microscopy results suggest that the core metal replacement can also effect a change in nanoparticle core size. Formation of bimetallic nanoparticles appears to stabilized the less stable member of the metal pair.

Synthesis of mixed monolayer-protected Au clusters (MPCs) from unsymmetrical disulfides (HO(CH2)11SS(CH2)11CH3) yields clusters with a 1:1 composition of HO(CH2)11S− and CH3(CH2)11S− thiolates, whereas products from reaction of a 1:1 mixture of thiols (HO(CH2)11SH and CH3(CH2)11SH) contain mostly (97%) HO(CH2)11S− ligands in the monolayer. The results suggest a thermodynamically preferred ligation of HO(CH2)11SH over CH3(CH2)11SH that is circumvented in the disulfide reaction. The core sizes of MPCs synthesized from alkanethiol and dialkyl disulfides are similar, suggesting similar MPC growth dynamics for thiols and disulfides. The compositions of MPCs synthesized starting from gold(I)−thiolate polymer intermediates suggest that the polymer undergoes immediate breakup upon addition of reductant.

Alkanethiols possessing terminal phenyl groups (C6H5(CH2)nSH, n = 12−15) were adsorbed onto the surface of gold to afford phenyl-terminated self-assembled monolayers (SAMs). The SAMs were characterized by optical ellipsometry, polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS), atomic force microscopy (AFM), and contact angle goniometry. The films generated from the phenyl-terminated alkanethiols exhibited greater thicknesses but similar crystallinities and well-ordered lattice structures when compared to analogous SAMs generated by the adsorption of normal alkanethiols onto gold. Advancing contact angle measurements using water as the test liquid supported the presence of interfacial phenyl moieties. Furthermore, contact angle measurements using the test liquids methylene iodide (MI), dimethyl formamide (DMF), and nitrobenzene (NB) revealed an odd−even effect as a function of the number of methylene units underneath the terminal phenyl groups. The tribological properties of the phenyl-terminated films were characterized by AFM and compared to those of films derived from normal alkanethiols and other materials presenting aromatic hydrocarbon moieties at the interface (i.e., graphite and C60). The phenyl-terminated SAMs exhibited a substantially higher frictional response than graphite, a slightly higher frictional response than normal alkanethiol SAMs, but a much lower frictional response than C60-terminated SAMs.

Sulfonic acid-functionalized monolayer-protected gold clusters (SO3-MPCs) have been synthesized by the reaction of the Bunte (thiosulfate) salt of 2-acrylamido-2-methyl-1-propanesulfonic acid (SO3) with tetrachloroaurate and borohydride in aqueous acetic acid. The nanoparticles produced have an average core diameter, obtained by transmission electron microscopy (TEM), of 2.2 ± 1.1 nm, and a 30 wt % ligand content, obtained by thermogravimetric analysis. These parameters, assuming an ideal closed-shell truncated octahedral nanoparticle, correspond to an average nanoparticle formula of Au314SO3111. Proton NMR, Fourier transform infrared (FTIR), and UV/vis spectroscopies are consistent with nanoparticle formation, and acid/base titrations are consistent with the strong acid character of the SO3-MPC monolayer. These nanoparticles add to a growing family of water-soluble MPCs. Synthesis of a salt of SO3-MPC with a polyether-tailed triethylammonium countercation yields an ionically conductive molten salt, which is studied by alternating current impedance spectroscopy in the solid state (neat melt) and solution form.

A new synthesis for alkanethiolate-functionalized gold clusters derived from sodium S-dodecylthiosulfate is presented. The new clusters are analytically characterized by ¹H NMR spectroscopy, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectroscopy, high-resolution transmission electron microscopy (HRTEM), thermogravimetric analysis (TGA), and UV−vis absorption spectroscopy. These monolayer-protected clusters (MPCs) are indistinguishable in composition, monolayer architecture, and Au−sulfur bonding from those previously prepared from dodecanethiol using the Brust synthesis. One difference is that, for syntheses run under identical conditions, the average core dimensions for MPC products from sodium S-dodecylthiosulfate are somewhat larger. The average core size of MPCs synthesized from sodium S-dodecylthiosulfate is somewhat solvent-dependent, being smaller when they are made using toluene/H2O rather than THF/H2O.

The kinetics of thermal desorption and displacement of self-assembled monolayers (SAMs) on gold derived from the adsorption of 2,2-dipentadecylpropane-1,3-dithiol (d-C17, [CH3(CH2)14]2C[CH2SH]2), 2-pentadecylpropane-1,3-dithiol (m-C17, CH3(CH2)14CH[CH2SH]2), and heptadecanethiol (n-C17, CH3(CH2)16SH) were explored. The kinetics were monitored by optical ellipsometry, contact angle goniometry, and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). Thermal desorption studies of the SAMs in decalin at elevated temperatures demonstrated an enhanced stability for films generated from d-C17 and m-C17 relative to that for the film generated from n-C17. These studies further demonstrated that SAMs adsorbed at elevated temperatures (e.g., 50 °C) are more stable than those adsorbed at room temperature. Upon exposure to ambient laboratory conditions for one month, densely packed SAMs generated from n-C17 and d-C17 underwent no detectable structural changes. In contrast, similar treatment of the SAM generated from m-C17, which possesses a relatively low density of alkyl chains, led to structural change(s), as indicated by a progressive decrease in the values of the hexadecane contact angles. The data from the displacement studies suggest the following trend in the thermodynamic stabilities of the thiol-derived SAMs:  m-C17 > d-C17 ≫ n-C17. The degree of crystallinity of the alkyl chains of the SAMs failed to correlate with the observed trend in stabilities. The strong thermodynamic preference for m-C17 and d-C17 over n-C17 probably originates from the unique ability of the former adsorbates to chelate to the surface of gold and perhaps their decreased tendency toward desorption as a disulfide. The slight thermodynamic preference for m-C17 over d-C17 probably originates from an enhanced conformational flexibility for m-C17 that permits enhanced binding of this adsorbate to the surface of gold.

The kinetics of formation of self-assembled monolayers (SAMs) on gold generated by the adsorption of 2,2-dipentadecylpropane-1,3-dithiol (d-C17, [CH3(CH2)14]2C[CH2SH]2), 2-pentadecylpropane-1,3-dithiol (m-C17, CH3(CH2)14CH[CH2SH]2), and heptadecanethiol (n-C17, CH3(CH2)16SH) from 1 mM solutions in isooctane were explored. A series of systematic coadsorption studies involving these adsorbates was also examined. Similarly, the kinetics of adsorption of the corresponding disulfides, 4,4-dipentadecyl-1,2-dithiolane (d-C17SS, [CH3(CH2)14]2C[CH2S]2), 4-pentadecyl-1,2-dithiolane (m-C17SS, CH3(CH2)14CH[CH2S]2), and diheptadecyl disulfide (n-C17SS, [CH3(CH2)16S]2), were explored. The kinetics were monitored by optical ellipsometry, contact angle goniometry, and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). For the formation of densely packed SAMs, the data showed two kinetic adsorption regimes:  a fast initial regime in which ~80−90% of the monolayer was formed, followed by a slower orientational ordering regime in which the alkyl chains became more densely packed and highly crystalline. In contrast, the formation of loosely packed SAMs exhibited a single rapid adsorption regime with little subsequent change. A comparison of the kinetic and coadsorption data generated from the various adsorption studies was interpreted using a steady-state kinetic model involving an initial steady-state physisorption, followed by a chemisorption step, leading ultimately to complete monolayer formation. The relative rates of adsorption in the slow ordering regime were perhaps additionally influenced by the rates of diffusion of the adsorbates through the partially formed monolayer films.

We report a comparative study of the structure and frictional properties of self-assembled monolayers (SAMs) generated by the adsorption of three homologous 17-carbon alkanethiols&sbd;heptadecanethiol, 2,2-dipentadecyl-1,3-propanedithiol, and 2-pentadecyl-1,3-propanedithiol&sbd;onto the surface of Au(111). The structural properties of these SAMs were characterized by atomic force microscopy, surface infrared spectroscopy, X-ray photoelectron spectroscopy, spectral ellipsometry, and wettability by water and hexadecane. The frictional properties of the SAMs were examined by friction force microscopy. The results demonstrate that the packing density and the related crystalline order of the hydrocarbon chains influence the frictional properties of organic thin films. The origins of the frictional differences measured from these films are discussed in terms of the structure of the films.

Low-density self-assembled monolayers (SAMs) on gold were generated by the adsorption of a series of specifically designed 2-monoalkylpropane-1,3-dithiol derivatives, CH3(CH2)nCH[CH2SH]2, where n = 11, 13, 14. The monolayers were characterized by optical ellipsometry, X-ray photoelectron spectroscopy, contact angle goniometry, polarization modulation infrared reflection absorption spectroscopy, and sum-frequency generation. Comparison of these data to those collected on SAMs generated from normal alkanethiols, CH3(CH2)n+2SH, and 2,2-dialkylpropane-1,3-dithiol derivatives, [CH3(CH2)n]2C[CH2SH]2, of similar chain length suggests that the new “monoalkanedithiol” SAMs are the least crystalline, exposing both methyl and methylene groups at the interface due to the low density of alkyl chains. Further comparison of these low-density SAMs to those obtained on branched and linear polyethylene films suggests that the exposure of interfacial methylene groups is greater for the polymer films.

We report the generation of new multicomponent self-assembled monolayers (SAMs) from the adsorption of unsymmetrical disulfides R(CH2)4SS(CH2)6NC60 (where R = CH3CH2− or CH3O− and C60 = Buckminsterfullerene) onto the surface of gold. These new SAMs expose a high coverage of fullerene moieties as judged by ellipsometric thickness, contact angle measurements, and imaging with a scanning tunneling microscope (STM). The contact angle measurements suggest a more robust coverage than fullerene-terminated SAMs generated from other approaches. Furthermore, the STM measurements represent, to our knowledge, the first direct molecular-scale images of fullerene moieties attached to the surface of a SAM.

The formation of self-assembled monolayers (SAMs) on gold from the adsorption of a series of specifically designed 2,2-dialkylpropanedithiol derivatives, [CH3(CH2)n]2C[CH2SH]2 where n = 0, 3, 7, and 11−15, is reported. The SAMs were characterized by optical ellipsometry, contact angle goniometry, and polarization modulation infrared reflection absorption spectroscopy. These data, when compared to those collected on SAMs generated from normal alkanethiols (CH3(CH2)mSH) of similar chain length, suggest that the new SAMs are highly oriented and well packed. They exhibit, however, slightly less crystallinity than their alkanethiolate-based analogues. A preliminary description of the kinetics of adsorption and desorption of these new SAMs is presented.