R. C. Salvarezza

The Chemistry of the Sulfur–Gold Interface: In Search of a Unified Model

Over the last three decades, self-assembled molecular films on solid surfaces have attracted widespread interest as an intellectual and technological challenge to chemists, physicists, materials scientists, and biologists. A variety of technological applications of nanotechnology rely on the possibility of controlling topological, chemical, and functional features at the molecular level. Self-assembled monolayers (SAMs) composed of chemisorbed species represent fundamental building blocks for creating complex structures by a bottom-up approach. These materials take advantage of the flexibility of organic and supramolecular chemistry to generate synthetic surfaces with well-defined chemical and physical properties. These films already serve as structural or functional parts of sensors, biosensors, drug-delivery systems, molecular electronic devices, protecting capping for nanostructures, and coatings for corrosion protection and tribological applications. Thiol SAMs on gold are the most popular molecular films because the resulting oxide-free, clean, flat surfaces can be easily modified both in the gas phase and in liquid media under ambient conditions. In particular, researchers have extensively studied SAMs on Au(111) because they serve as model systems to understand the basic aspects of the self-assembly of organic molecules on well-defined metal surfaces. Also, great interest has arisen in the surface structure of thiol-capped gold nanoparticles (AuNPs) because of simple synthesis methods that produce highly monodisperse particles with controllable size and a high surface/volume ratio. These features make AuNPs very attractive for technological applications in fields ranging from medicine to heterogeneous catalysis. In many applications, the structure and chemistry of the sulfur-gold interface become crucial since they control the system properties. Therefore, many researchers have focused on understanding of the nature of this interface on both planar and nanoparticle thiol-covered surfaces. However, despite the considerable theoretical and experimental efforts made using various sophisticated techniques, the structure and chemical composition of the sulfur-gold interface at the atomic level remains elusive. In particular, the search for a unified model of the chemistry of the S-Au interface illustrates the difficulty of determining the surface chemistry at the nanoscale. This Account provides a state-of-the-art analysis of this problem and raises some questions that deserve further investigation.

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.

Surface characterization of sulfur and alkanethiol self-assembled monolayers on Au(111)

In the last two decades surface science techniques have decisively contributed to our present knowledge of alkanethiol self-assembled monolayers (SAMs) on solid surfaces. These organic layers have been a challenge for surface scientists, in particular because of the soft nature of the organic material (which can be easily damaged by irradiation), the large number of atoms present in the molecules, and the complex physical chemistry involved in the self-assembly process. This challenge has been motivated by the appealing technological applications of SAMs that cover many fields of the emerging area of nanotechnology. Sulfur (S) is closely related to alkanethiols and can be used to understand basic aspects of the surface structure of SAMs. In this review we focus on the atomic/molecular structures of S-containing SAMs on Au(111). Particular emphasis is given to the substrate, adsorption sites, chemical state of the S–metal bond and also to the experimental and theoretical tools used to study these structures at the atomic or molecular levels.

Kinetics of passivation and pitting corrosion of polycrystalline copper in borate buffer solutions containing sodium chloride

Abstract The pitting corrosion of copper in borate buffer containing sodium chloride is studied by using potentiostatic and potentiodynamic techniques complemented with scanning electron microscopy and EDAX. The breakdown potential shifts towards more negative values as the sodium chloride concentration increases. During pitting both soluble Cu(I) and Cu(II) species are detected. The first stage of pitting is explained through the competition between the passive layer formation and the nucleation and growth of the CuCl layer in equilibrium with Cu(I)-chloride complexes in solution. When salt nuclei reach the metal surface, pit growth under charge-transfer control is found. In the following stage the kinetics of pit growth changes to a diffusion controlled process when the thick CuCl layer is completed. Secondary breakdown of the salt layer results in copper dissolution through Cu(II) soluble species. The corresponding overall process is discussed in terms of a sum of nucleation and growth processes. The reaction model reproduces the potentiostatic current transients of copper in weakly alkaline borate buffer containing sodium chloride.

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.

The Evaluation of Surface Diffusion Coefficients of Gold and Platinum Atoms at Electrochemical Interfaces from Combined STM‐SEM Imaging and Electrochemical Techniques

A simple method is presented for measuring the surface diffusion coefficients of Au and Pt atoms at electrodispersed electrodes of the same metals in contact with 0.5M H2SO4. The technique is based upon the time dependence of the surface roughness factor of electrodispersed metal overlayers. The method requires a model for the surface roughness of the metal structure. The model is deduced from microscopic measurements by a STM integrated into a conventional SEM microscope. This allows the relationship between the roughness factor and the area of the surface structure to be obtained. For Au and Pt in contact with an electrolyte solution, the values of our diffusion coefficients are higher than those reported in vacuum at the same temperature. The evaluation of surface diffusion coefficients from particle coalescence processes is widely used in solid-state physics. Usually the methods involve the estimation of the time dependence of crystal size by microscopy (1), optical diffraction (2), or ion scattering (3), and the use of the theory developed first by Mullins (4) and extended in recent years by several authors (5, 6). Surface diffusion plays an important role in many electrochemical processes such as electrocrystallization of metals, metallic corrosion, and electrocatalysis. Typical examples of materials whose properties become time dependent due to surface processes, involving the surface diffusion of metal atoms, include the electrochemical behavior (7) and structure (8) of electrodeposits grown under different conditions, the stability of small metallic particles ~ used in dispersed-type metal electrodes as those used for electrochemical energy conversion devices (9), and the surface roughness properties of large active area electrocatalysts (10). Atomic mobility, increased by surface contaminants, also appears to be involved in stress corrosion cracking (11). In spite of the importance of surface diffusion, available diffusion coefficients as determined in vacuum (1) or in air do not account for the presence of the electrolyte. We have recently undertaken a study of the roughness structure of samples produced by an electro-oxidationelectrodeposition process (electrodispersed metal overlayers) (7). The combination of electrochemical measurements together with microscopic measurements by a scanning tunneling microscope (STM) integrated into a conventional SEM (16) has resulted in a model for the roughness structure. This model attributes the large roughness factor of the electrodes to the area of columnar type grains (Fig. 1). The lateral area along the depth of the reduced metal is responsible for the roughness increase. The radius of columns can be measured by STM. The height can be related to both electrochemical and SEM data (7). However, the columnar structure is unstable, decreasing its surface area and, accordingly, the surface roughness factor with the aging time. We propose, in the present paper, a simple method to estimate the surface diffusion coefficients of metal atoms in electrochemical systems based on the time dependence of the surface roughness factor of electrodispersed metal overlayers as measured by conventional voltammetry. The technique proved to be successful for evaluating the surface diffusion coefficients of Au and Pt atoms in contact with aqueous acid solutions at different temperatures. The

The influence of slow Cu(OH)2 phase formation on the electrochemical behaviour of copper in alkaline solutions

Abstract Evidences of a slow Cu(OH) 2 phase formation with data resulting from potentiodynamic potentiostatic and rotating-ring-disc techniques were obtained during the anodization of copper in 0.1 M NaOH. According to the potential and time windows employed in the different runs, electrochemical results can be explained by admitting two limiting complex structures of the anodic layers namely Cu/Cu 2 O (porous inner layer)/CuO (outer layer) and Cu/Cu 2 O (porous inner layer)/CuO/Cu(OH) 2 (outer layer). The formation of the Cu(OH) 2 layer fits a progressive nucleation and 2-D growth under charge transfer control in the −0.175 ⩽ E ⩽ −0.10 V range and an instantaneous nucleation and 2-D growth mechanism under charge transfer control in the −0.20 ⩽ E ⩽ −0.18 V. A well-defined voltammetric peak multiplicity can be distinguished for the electrochemical of each complex anodic layer. These results furnish a reasonable explanation to discrepancies observed in the literature for the electroformation and electroreduction of anodic layers formed on copper in alkaline solutions.

Electrochemical behaviour of copper in aqueous moderate alkaline media, containing sodium carbonate and bicarbonate, and sodium perchlorate

The voltammetric polarization of Cu specimens in Na2CO3, NaHCO3 and NaClO4 solutions (8-12pH range) has been investigated. Voltammetry data were complemented with SEM and electron microprobe analysis. Results are found to be in agreement with the passivation model developed for Cu in plain NaOH solutions. For the latter the process can be described in terms of two steps, namely, at low potentials the initial formation of a Cu2O thin layer followed by the growth of a massive Cu2O layer, and at higher potentials the appearance of a CuO-Cu(OH)2 layer. These processes are accompanied by the formation of soluble Cu species. Beyond a certain potential which increases with the solution pH, copper pitting takes place. This model can be extended to Cu in carbonate/bicarbonate containing solutions by considering that Cu carbonates precipitate as long as soluble ionic Cu species are produced, without interfering appreciably with the formation of Cu oxides. The appearance of copper carbonate species is enhanced when pitting corrosion sets in. The precipitation of Cu carbonates occurs principally around pits. Cu pitting, although it is observed for all solutions, becomes more noticeable at the lowest pH values. At a constant pH, the density of pits increases in the order NaClO4 > NaHCO3 > Na2CO3. The influence of the electrolyte composition on Cu pitting is closely related to the blockage capability for pit nucleation and growth of the corresponding copper salts. Passivation in the Cu2O-Cu(OH)2 region hinders pitting corrosion.

Kinetics of Particle Coarsening at Gold Electrode/Electrolyte Solution Interfaces Followed by In Situ Scanning Tunneling Microscopy

The kinetics of particle coarsening at columnar structured gold electrodeposits immersed in aqueous NaCl containing 0.5 M H 2 SO 4 and plain 0.5 M H 2 SO 4 solutions at 298 K, at a constant potential, has been followed by in situ scanning tunneling microscopy (STM) sequential imaging. For this system large anisotropic gold particles grow at the expense of small ones. The value of r, the particle radius measured for any preset growth direction, increases with time t, the decay time, according to r 4 t, as predicted by a surface diffusion-controlled coarsening mechanism. Coarsening of gold particles occurs without a significant change in the standard deviation of the gold electrodeposit height. The average surface diffusion coefficient of gold atoms derived from STM imaging data agrees with previously reported data derived from electrochemical measurements

Citrate-Capped Silver Nanoparticles Showing Good Bactericidal Effect against Both Planktonic and Sessile Bacteria and a Low Cytotoxicity to Osteoblastic Cells

A common problem with implants is that bacteria can form biofilms on their surfaces, which can lead to infection and, eventually, to implant rejection. An interesting strategy to inhibit bacterial colonization is the immobilization of silver (Ag) species on the surface of the devices. The aim of this paper is to investigate the action of citrate-capped silver nanoparticles (AgNPs) on clinically relevant Gram-positive (Staphylococcus aureus) and Gram-negative (Pseudomonas aeruginosa) bacteria in two different situations: (i) dispersed AgNPs (to assess the effect of AgNPs against planktonic bacteria) and (ii) adsorbed AgNPs on titanium (Ti) substrates, a material widely used for implants (to test their effect against sessile bacteria). In both cases, the number of surviving cells was quantified. The small amount of Ag on the surface of Ti has an antimicrobial effect similar to that of pure Ag surfaces. We have also investigated the capability of AgNPs to kill planktonic bacteria and their cytotoxic effect on UMR-106 osteoblastic cells. The minimum bactericidal concentration found for both strains is much lower than the AgNP concentration that leads to cytotoxicity to osteoblasts. Planktonic P. aeruginosa show a higher susceptibility to Ag than S. aureus, which can be caused by the different wall structures, while for sessile bacteria, similar results are obtained for both strains. This can be explained by the presence of extracellular polymeric substances in the early stages of P. aeruginosa biofilm formation. Our findings can be important to improving the performance of Ti-based implants because a good bactericidal action is obtained with very small quantities of Ag, which are not detrimental to the cells involved in the osseointegration process.