New Insight into the Chemical Nature of the Plasmonic Nanostructures Synthesized by the Reduction of Au(III) with Sulfide Species.

We have studied the products of the controversial synthesis of HAuCl4 with Na2S, which include gold nanostructures (Au NSs) that absorb in the near-infrared (NIR) region and are highly promising for photothermal therapies and other nanomedical applications. From high-resolution transmission electron microscopy, X-ray absorption spectroscopy, and small-angle X-ray scattering, we have found that only metallic Au NSs are formed as a result of this synthesis, with no detectable amount of gold sulfide or other oxidized gold species that could account for the NIR absorption. Different sulfur species are adsorbed on the Au NSs, mainly sulfides (monomeric sulfur) and polysulfides, similar to what is found on the planar gold surfaces, therefore precluding the idea that thiosulfate or other oxidized species are the actual reducing agents for Au(III) ions. The presence of strongly adsorbed S species, which are difficult to remove from the gold surface, is of great importance for their applications as regards toxicity and use of postfunctionalization strategies to anchor biomolecules and/or to increase circulation time after administration.

Electrochemistry of Os(2,2'-bpy)2ClPyCH2NHCOPh tethered to Au electrodes by S-Au and C-Au junctions.

Osmium pyridine-bipyridine redox centers have been tethered to Au electrodes by chemical modification through Au-S and Au-C bonds respectively. 4-Mercapto benzoic acid and the reduction product of the aryl diazonium salt of 4-amino benzoic acid were reacted on Au surfaces, with further post-functionalization by chemical reaction of the osmium complex amino-pyridine derivative with the surface carboxylates. The resulting modified Au surfaces were characterized by polarization modulated infrared reflection absorption spectroscopy (PM-IRRAS), scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), resonant raman spectroscopy and cyclic voltammetry.

"Naked" gold nanoparticles supported on HOPG: melanin functionalization and catalytic activity.

Reductive electrodesorption has been used to produce "naked" gold nanoparticles (AuNPs) 3 nm in size on HOPG from different thiolate-capped AuNPs. The clean AuNPs transform the electrocatalytic inert HOPG into an active surface for hydrogen peroxide electroreduction, causing a lowering of the cathodic overpotential of 0.25 V with respect to the Au(111) surface. Compared to the plain gold substrates, the nanostructures promote only a slight increase in the hydrogen evolution reaction. In a second modification step a ∼1 nm thick melanin-iron coating is electrochemically formed around the AuNPs. This ultrathin melanin-iron coating largely improves the catalytic activity of the bare AuNPs for both hydrogen peroxide electroreduction and hydrogen evolution reaction. This strategy, which integrates electrochemistry and nanotechnology, can be applied to the preparation of efficient "naked" AuNPs and organic-iron capped AuNPs catalysts.

Thiol with an unusual adsorption-desorption behavior: 6-mercaptopurine on Au(111).

A multitechnique study of 6-mercaptopurine (6MP) adsorption on Au(111) is presented. The molecule adsorbs on Au(111), originating short-range ordered domains and irregular nanosized aggregates with a total surface coverage by chemisorbed species smaller than those found for alkanethiol SAMs, as derived from scanning tunneling microscopy (STM) and electrochemical results. X-ray photoelectron spectroscopy (XPS) results show the presence of a thiolate bond, whereas density functional theory (DFT) data indicate strong chemisorption via a S-Au bond and additional binding to the surface via a N-Au bond. From DFT data, the positive charge on the Au topmost surface atoms is markedly smaller than that found for Au atoms in alkanethiolate SAMs. The adsorption of 6MP originates Au atom removal from step edges but no vacancy island formation at (111) terraces. The small coverage of Au islands after 6MP desorption strongly suggests the presence of only a small population of Au adatom-thiolate complexes. We propose that the absence of the Au-S interface reconstruction results from the lack of significant repulsive forces acting at the Au surface atoms.

Spontaneous adsorption of silver nanoparticles on Ti/TiO2 surfaces. Antibacterial effect on Pseudomonas aeruginosa.

Titanium is a corrosion-resistant and biocompatible material widely used in medical and dental implants. Titanium surfaces, however, are prone to bacterial colonization that could lead to infection, inflammation, and finally to implant failure. Silver nanoparticles (AgNPs) have demonstrated an excellent performance as biocides, and thus their integration to titanium surfaces is an attractive strategy to decrease the risk of implant failure. In this work a simple and efficient method is described to modify Ti/TiO(2) surfaces with citrate-capped AgNPs. These nanoparticles spontaneously adsorb on Ti/TiO(2), forming nanometer-sized aggregates consisting of individual AgNPs that homogeneously cover the surface. The modified AgNP-Ti/TiO(2) surface exhibits a good resistance to colonization by Pseudomonas aeruginosa, a model system for biofilm formation.

Self-assembled monolayers of thiols and dithiols on gold: new challenges for a well-known system.

Self-assembled monolayers (SAMs) of alkanethiols and dialkanethiols on gold are key elements for building many systems and devices with applications in the wide field of nanotechnology. Despite the progress made in the knowledge of these fascinating two-dimensional molecular systems, there are still several "hot topics" that deserve special attention in order to understand and to control their physical and chemistry properties at the molecular level. This critical review focuses on some of these topics, including the nature of the molecule-gold interface, whose chemistry and structure remain elusive, the self-assembly process on planar and irregular surfaces, and on nanometre-sized objects, and the chemical reactivity and thermal stability of these systems in ambient and aqueous solutions, an issue which seriously limits their technological applications (375 references).

Restricted surface mobility of thiolate-covered metal surfaces: a simple strategy to produce high-area functionalized surfaces.

We have studied the self-assembly of thiol monolayers on high-area nanostructured gold surfaces. These surfaces are highly irregular with a fractal dimension close to 2.5. Auger electron spectroscopy and voltammetric data indicate that thiol self-assembly with a maximum surface coverage approximately 1/3 takes place, the same result as that found for smooth gold surfaces. Therefore, neither curvature effects, which would promote higher coverage, nor excluded volume effects, which would result in lower coverage, are present in these irregular surfaces. The high surface area of the bare electrodes exhibits a rapid surface decay in different liquid media that is hindered by alkanethiolate chemisorption. The presence of thiolate SAMs reduces markedly the mass transport surface diffusion of gold adatoms, hindering surface area decay and freezing the system in a metastable state for days. This effect cannot be explained by considering only hydrocarbon-hydrocarbon chain interactions, because it is also observed for ordered arrays of adsorbed S atoms. Therefore, interactions between ordered chemisorbed species at high coverage seem to be responsible for the observed behavior. The thiol-covered high-area metallic substrates can be used to efficiently anchor a large number of molecules, biomolecules, or nanostructures, improving the performance of SAM-based optical and electrochemical devices.

Immobilization of methylene blue on self-assembled iodine monolayers on gold.

The immobilization of methylene blue (MB) on iodine-covered Au(111) is studied by electrochemical techniques, scanning tunneling microscopy (STM), Auger Electron Spectroscopy (AES), and Raman spectroscopy. Results show that MB species are efficiently adsorbed on the square root of 3 x square root of 3 R30 degrees I lattice on Au(111). The electrochemical behavior of the adsorbed MB molecules is reversible, indicating a relatively fast electron transfer from the Au(111) surface to the immobilized MB species through the iodine layer. STM images with molecular resolution are consistent with adsorption of MB dimers on a square root of 3 x square root of 3 R30 degrees I lattice placed atop of the Au(111) substrate. Results are compared to those obtained for MB immobilized on Au(111) covered by S(n) (n = 3-8) surface structures.

Self-assembled monolayers of alkanethiols on Au(111): surface structures, defects and dynamics.

The surface structures, defects and dynamics of self-assembled monolayers (SAMs) on Au(111) are reviewed. In the case of the well-known c(4 x 2) and radical 3 x radical 3 R30 degrees surface structures, the present discussion is centered on the determination of the adsorption sites. A more complex scenario emerges for the striped phases, where a variety of surface structures that depends on surface coverage are described. Recently reported surface structures at non-saturation coverage show the richness of the self-assembly process. The study of surface dynamics sheds light on the relative stability of some of these surface structures. Typical defects at the alkanethiol monolayer are shown and discussed in relation to SAMs applications.

Role of surface heterogeneity and molecular interactions in the charge-transfer process through self-assembled thiolate monolayers on Au(111).

A comparative study of charge-transfer processes from/to methyl-terminated and carboxylate-terminated thiolate-covered Au(111) surfaces to/from immobilized methylene blue (MB) molecules is presented. Scanning tunneling microscopy images with molecular resolution reveal the presence of molecular-sized defects, missing rows, and crystalline domains with different tilts that turn the thickness of the alkanethiolate SAM (the spacer) uncertain. The degree of surface heterogeneity at the SAMs increases as the number of C units (n) in the hydrocarbon chain decreases from n = 6. Defective regions act as preferred paths for MB incorporation into the methyl-terminated SAMs, driven by hydrophobic forces. The presence of negative-charged terminal groups at the SAMs reduces the number of molecules that can be incorporated, immobilizing them at the outer plane of the monolayer. Only MB molecules incorporated into the SAMs close to the Au(111) surface (at a distance < 0.5 nm) are electrochemically active. MB molecules trapped in different defects explain the broad shape and humps observed in the voltammogram of the redox couple. The heterogeneous charge-transfer rate constants for MB immobilized into methyl-terminated thiolate SAMs are higher than those estimated for carboxylate- terminated SAMs, suggesting a different orientation of the immobilized molecule in the thiolate environment.

Following adsorption kinetics at electrolyte/metal interfaces through crystal truncation scattering: sulfur on Au(111).

Combining electrochemical methods, in situ scanning tunneling microscopy, and surface x-ray diffraction allowed study of the structure and kinetics of S/Au(111) electrodes in aqueous electrolytes under potential control. Integrated intensities of a particular crystal truncation rod at anti-Bragg conditions were used to trace the sulfur adsorption and desorption as a function of electrode potential in real time. The S desorption is a first order process and the adsorption follows a Langmuir isotherm. A weakly bound S layer is found on the surface before charge transfer, and then specific adsorption occurs.