C. Alonso

Corrosion inhibition of powder metallurgy Mg by fluoride treatments

Pure Mg has been proposed as a potential degradable biomaterial to avoid both the disadvantages of non-degradable internal fixation implants and the use of alloying elements that may be toxic. However, it shows excessively high corrosion rate and insufficient yield strength. The effects of reinforcing Mg by a powder metallurgy (PM) route and the application of biocompatible corrosion inhibitors (immersion in 0.1 and 1M KF solution treatments, 0.1M FST and 1M FST, respectively) were analyzed in order to improve Mg mechanical and corrosion resistance, respectively. Open circuit potential measurements, polarization techniques (PT), scanning electrochemical microscopy (SECM) and electrochemical impedance spectroscopy (EIS) were performed to evaluate its corrosion behavior. SECM showed that the local current of attacked areas decreased during the F(-) treatments. The corrosion inhibitory action of 0.1M FST and 1M FST in phosphate buffered solution was assessed by PT and EIS. Under the experimental conditions assayed, 0.1M FST revealed better performance. X-ray photoelectron spectroscopy, energy dispersive X-ray and X-ray diffraction analyses of Mg(PM) with 0.1M FST showed the presence of KMgF(3) crystals on the surface while a MgF(2) film was detected for 1M FST. After fluoride inhibition treatments, promising results were observed for Mg(PM) as degradable metallic biomaterial due to its higher yield strength and lower initial corrosion rate than untreated Mg, as well as a progressive loss of the protective characteristics of the F(-)-containing film which ensures the gradual degradation process.

Study of the electrooxidation of n-propanol on an Au electrode in basic medium

Abstract This paper describes the oxidation mechanism of n-propanol on polycrystalline gold electrodes in basic solutions. The oxidation takes place after adsorption of n-propanol molecules on the AuOH coverage and continues only up to the formation of n-propanol under the working conditions used. A coherent explanation for every experimental fact has been found. This was accomplished by using a modified Temkin isotherm to describe the AuOH coverage and non-activated adsorption for the n-propanol molecules on the AuOH coverage. Further steps give rise to n-propanoic acid as a final product.

The surface diffusion of gold atoms on gold electrodes in acid solution and its dependence on the presence of foreign adsorbates

Abstract The surface diffusion coefficient ( D s ) of Au atoms in 0.5 M H 2 SO 4 at 298 K has been determined from the time dependence of the surface roughness factor of electrodispersed Au electrodes in 0.5 M H 2 SO 4 . The value of D s for 0.5 M H 2 SO 4 is 5 × 10 −14 cm 2 s −1 , a figure which depends considerably on the presence of adsorbable species in solution. Thus, the adsorption of pyridine produces a remarkable decrease of D s , Whereas the adsorption of Cl − ion results in the opposite effect. The corresponding values of D s are 1.2 × 10 −14 cm 2 s −1 for 0.5 M H 2 SO 4 + 0.1 M pyridine, and 2.5 × 10 −13 cm 2 s −1 for 0.5 M H 2 SO 4 + 5 × 10 −4 M KCl. These results are interpreted through the influence of the adsorbate on the Au surface atom diffusion mechanism.

Nanostructured rough gold electrodes for the development of lactate oxidase-based biosensors.

The design and characterization of a lactate biosensor using a nanostructured rough gold surface as a transducer is reported. The biosensor is developed by immobilization of lactate oxidase (LOx), on a rough gold electrode modified with a self-assembled monolayer of dithiobis-N-succinimidyl propionate (DTSP). This bifunctional reagent preserves the rough gold structure and allows further covalent immobilization of the enzyme through the terminal succinimidyl groups. The rough gold electrode is characterized using field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM). The preferential orientation and average crystallite size are obtained by X-ray diffraction (XRD). The resulting lactate oxidase monolayers are characterized by electrochemical impedance spectroscopy (EIS). This nanostructured transducer allows higher mediated electrocatalytic activity than polycrystalline ones. The biosensor response to increasing lactate concentrations, using hydroxymethylferrocene as a redox mediator in solution, is linear up to 1.2 mM with a sensitivity of 1.49 microA mM(-1).

STM-SEM combination study on the electrochemical growth mechanism and structure of gold overlayers: A quantitative approach to electrochemical metal surface roughening

Abstract The growth mode and structure of gold overlayers resulting from the electroreduction of thick oxide films are studied using potentiodynamic and potentiostatic techniques combined with ex-situ STM and SEM. Electroreduction of the thick gold oxide at low overpotentials, that is slowly grown gold overlayers, results in a close-packed array of grains exhibiting a low roughness factor. Grains in the order of 100 nm of radii are formed by aggregation of small monomers. At higher overpotentials, fast grown gold overlayers, we suggest that the monomer growth results in a columnar structure terminated on rounded domes with radii ranging between 10 and 20 nm. This overlayer exhibits a roughness factor which increases according to electrodeposit height. The columnar structure is unstable decreasing its surface free energy by coalescence of small columns to form large units leading to a drastic decrease in the surface area with ageing time. A mechanism for the growth mode is proposed where the crystallite size depends on the diffusion length of the electrodepositing particles which is controlled by the applied overpotential.

The mechanism of silver(I) oxide formation on polycrystalline silver in alkaline solution. Determination of nucleation and growth rates

Abstract The nucleation and growth of Ag(I) oxide layers on polycrystalline Ag electrodes is studied by potentiodynamic and potentiostatic current transients in 0.1 M NaOH. A model involving the diffusion controlled growth of a basal layer followed by the nucleation and growth of a secondary layer is proposed. Various nucleation parameters are estimated by comparing experimental data with simulated transients, and the results are discussed in terms of the atomistic theory of electrochemical phase formation. The influence of the nucleation parameters on the shape of the current transients is analysed and some controversial results in the literature are explained.

Influence of organic adsorbates on the under and overpotential deposition of copper on polycrystalline platinum electrodes

Abstract The deposition of copper onto a polycrystalline platinum surface has been studied in the underpotential ( upd ) and overpotential ( opd ) regions using cyclic voltammetry in the presence of organic co-adsorbates including pyridine, pyrazine, 2,2′-bipyridine, 4,4′-bipyridine, 4-phenylpyridine, 1,2-bis(4-pyridyl)ethane, 2-mercaptopyridine and 4-mercaptopyridine dissolved in either 0.5 M H 2 SO 4 or water. Adsorbates that bind primarily through a ring nitrogen atom delay both the upd and opd processes, depending on the bond strength of the adsorbed layer. These adsorbates also show a significant pH dependence due to protonation of the binding nitrogen atom. On the other hand, the sulfur containing adsorbates completely inhibit the underpotential deposition but not the overpotential deposition process. In addition, a monolayer of copper electrodeposited onto a platinum surface is oxidized upon exposure of the electrode to a solution of 2-mercaptopyridine which is, in turn, reduced.

Influence of macroporous gold support and its functionalization on lactate oxidase-based biosensors response.

A general bioanalytical platform for biosensor applications was developed based on three-dimensional ordered macroporous (3DOM) gold film modified electrodes using lactate oxidase (LOx) as a case study, within the framework of developing approaches of broad applicability. The electrode was electrochemically fabricated with an inverted opal template, making the surface area of the 3DOM gold electrode up to 18 times higher than that of bare flat gold electrodes. These new electrochemical transducers were characterized by using Field Emission Scanning Electron Microscopy (FE-SEM), Atomic Force Microscopy (AFM) and the X-ray diffraction (XRD). The biosensor was developed by immobilization of lactate oxidase (LOx), on a 3DOM gold electrode modified with a self-assembled monolayer of dithiobis-N-succinimidyl propionate (DTSP). The resulting lactate oxidase biosensor was characterized by electrochemical impedance spectroscopy (EIS). The 3DOM gold electrode not only provides a good biocompatible microenvironment but also promotes the increase of conductivity and stability. Thus, the developed lactate oxidase bioanalytical platforms showed higher mediated bioelectrocatalytic activity compared to others previously described based on polycrystalline gold transducers. The response to varying lactate concentrations has been obtained in the presence of hydroxymethylferrocene as redox mediator in solution. Under these conditions, the bioanalytical platform response for DTSP covalently bound enzyme was improved with respect to that obtained in absence of DTSP.

Study of the interaction of inorganic and organic compounds of cell culture medium with a Ti surface.

The interaction between Ti and each component of Dulbeccos modified Eagles medium was studied in depth using different techniques, such as the measurement of the corrosion potential, electrochemical impedance spectroscopy and polarization curves. The characterization of metal surfaces was carried out by scanning electron microscopy and X-ray photoelectron spectroscopy (XPS). The adsorption process of each component was studied using the quartz crystal balance (QCM). The QCM and XPS results reveal that the adsorption kinetics for phosphate and calcium ions is slow. However, the bovine serum albumin (BSA) totally covers the Ti surface rapidly. Because the passive film (titanium oxide) has acidic hydroxyl groups, the calcium ions would have a bridging effect on the electrostatic adsorption of phosphate ions as well as that of BSA. The polarization curves reveal that the adsorbed glucose permits the ionic diffusion of the oxygen to the electrode, while the BSA and fetal bovine serum (FBS) adsorbed after 7 days of immersion act as a diffusive barrier. The impedance measurement and data fitting to the electrical equivalent circuit model show that the resistance of the proteins/TiO(2) interface, for Ti immersed in FBS, is higher than those obtained for BSA, due to the proteins present in the solution as well as the fact that the adsorbed proteins on the surface are greater.

Scanning tunneling microscopy (STM) and scanning electron microscopy (SEM) of electrodispersed gold electrodes

Abstract Images of large active surface (electrodispersed) gold electrodes obtained by electroreduction of an oxide layer grown by applying a fast periodic square wave potential to polycrystalline specimens, have been obtained by STM and SEM. For the first time a correlation of imaging data of both microscopes can be established which allows one to find different structural details of these electrodes. The roughening increase produced by the electrochemical activation is explained tentatively through a generalized structural model consisting of an overlayer of sticking spheres of about 10 nm average diameter leaving inner interconnected channels of nearly the same average diameter and penetrating in the overlayer structure. The size of the spherical units is comparable to that of metal clusters involving the optimal ratio of surface to bulk atoms associated with the greatest catalytic activity.