Mario A. Alpuche-Aviles

Comparative cyclic voltammetry and surface analysis of passive films grown on stainless steel 316 in concrete pore model solutions

The study of passive layers grown on AISI 316 stainless steel in two model solutions, saturated Ca(OH)2 and cement extract (CE) solution, show that each solution simulates the concrete pore environment in a different way. A more resistant passive layer is formed in CE solution due to its distinctive composition, homogeneity, thickness and porosity. The CE-exposed sample withstood several hours of cathodic polarization before the characteristic cyclic voltammetry peaks of the non-passivated AISI 316 reappeared. This is in contrast to the saturated Ca(OH)2-exposed sample, which showed the characteristic CV peaks after several minutes of cathodic polarization. XPS spectra of the passive layer formed in saturated Ca(OH)2 indicate that the amount of Ca is as much as ten times larger than that in the passive-layer formed in the CE solution. Films formed in CE solution contained Si and S, which are part of the cement mixture composition. Topographical dissimilarities in the passive films are also found in the AFM and optical microscopy images. The passive layer formed in CE solution is homogeneous and covers the metal surface completely. The film formed in saturated Ca(OH)2 is rough, crystalline, and nonhomogeneous. The differences in the composition and electrochemical behavior of passive layers formed on AISI 316 in saturated Ca(OH)2 solution or CE model solution suggest that CE solutions are superior for simulating concrete pore environments and that CE solutions create a more resistive passive film. In addition, passive films formed without polarization were superior to films formed on AISI 316 under anodic polarization. # 2002 Elsevier Science B.V. All rights reserved.

Screening of oxygen evolution electrocatalysts by scanning electrochemical microscopy using a shielded tip approach

Oxygen evolution electrocatalysts in acidic media were studied by scanning electrochemical microscopy (SECM) in the substrate generation-tip collection (SG-TC) imaging mode with a 100 microm diam tip. Pure IrO2 and Sn(1-x)Ir(x)O2 combinatorial mixtures were prepared by a sol-gel route to form arrays of electrocatalyst spots. The experimental setup has been developed to optimize screening of electrocatalyst libraries under conditions where the entire array is capable of the oxygen evolution reaction (OER). The activity of individual spots was determined by reducing the interference from the reaction products of neighboring spots diffusing to the tip over the spot of interest. A gold layer deposited on the external wall of the SECM tip was used as a tip shield. In this study the shield was kept at a constant potential to reduce oxygen under mass transfer controlled conditions. The tip shield consumes oxygen coming from the neighbor spots in the array and enables the tip to correctly detect the activity of the spot below the tip. Simulations and experimental results are shown, demonstrating the effectiveness of the tip shield with the SG-TC setup in determining the properties of the composite materials and imaging arrays.

Observation of individual semiconducting nanoparticle collisions by stochastic photoelectrochemical currents.

We describe a method to detect individual semiconducting nanoparticles (NPs) using the photoelectrochemical (PEC) current measured at an ultramicroelectrode (UME). We use photooxidation of MeOH by TiO2 NPs as a model system of photocatalysis in solution. NPs suspended in MeOH under constant illumination produce valence-band holes that oxidize MeOH. The electrons are collected at the UME, and the current-versus-time data show discrete current changes that are assigned to particle-by-particle interactions of the NPs with the UME. The stepwise changes in the photocurrent denote irreversible attachment of NPs to Pt UMEs (<30 μm diameter). We found that accumulation of electrons in the conduction band by the NPs is not enough to explain the stochastic PEC currents. We propose that the observed anodic steps have a PEC nature and are due to photooxidation of MeOH by the NPs at the electrode surface.

Scanning electrochemical microscopy. 59. Effect of defects and structure on electron transfer through self-assembled monolayers.

Electron transfer (ET) rate kinetics through n-alkanethiol self-assembled monolayers (SAMs) of alkanethiols of different chain lengths [Me(CH2)nSH; n=8, 10, 11, 15] on Au and Hg surfaces and ferrocene (Fc)-terminated SAMs (poly-norbornylogous and HS(CH2)12CONHCH2Fc) on Au were studied using cyclic voltammetry and scanning electrochemical microscopy (SECM). The SECM results allow determination of the ET kinetics of solution-phase Ru(NH3)63+/2+ through the alkanethiol SAMs on Au and Hg. A model using the potential dependence of the measured rate constants is proposed to compensate for the pinhole contribution. Extrapolated values of koML for Ru(NH3)63+/2+ using the model follow the expected exponential decay (beta is 0.9) for different chain lengths. For a Fc-terminated poly-norbornyl SAM, the standard rate constant of direct tunneling (ko is 189+/-31 s(-1)) is in the same order as the ko value of HS(CH2)12CONHCH2Fc. In blocking and Fc SAMs, the rates of ET are demonstrated to follow Butler-Volmer kinetics with transfer coefficients alpha of 0.5. Lower values of alpha are treated as a result of the pinhole contribution. The normalized rates of ET are 3 orders of magnitude higher for Fc-terminated than for blocking monolayers. Scanning electron microscopy imaging of Pd nanoparticles electrochemically deposited in pinholes of blocking SAMs was used to confirm the presence of pinholes.

Imaging of metal ion dissolution and electrodeposition by anodic stripping voltammetry-scanning electrochemical microscopy.

We have developed a new imaging method for scanning electrochemical microscopy (SECM) employing fast-scan anodic stripping voltammetry (ASV) to provide sensitive and selective imaging of multiple chemical species at interfaces immersed in solution. A rapid cyclic voltammetry scan (100 V/s) is used along with a short preconcentration time (300-750 ms) to allow images to be acquired in a normal SECM time frame. A Hg-Pt film electrode is developed having an equivalent Hg thickness of 40 nm that has good sensitivity at short preconcentration times and also retains thin-film behavior with high-speed voltammetric stripping. Fast-scan anodic stripping currents are shown to be linear for 1-100 microM of Pb (2+) and Cd (2+) solutions using a preconcentration time of 300 ms. SECM images showing the presence of Pb (2+) and Cd (2+) at concentrations as low as 1 microM are presented. In addition, a single ASV-SECM image is shown to produce unique concentration maps indicating Cd (2+) and Pb (2+), generated in situ from a corroding sample, while simultaneously detecting the depletion of O 2 at this sample. The transient voltammetric response at the film electrode is simulated and shows good agreement with the experimental behavior. We discuss the behavior of images and concentration profiles obtained with different imaging conditions and show that mass-transport limitations in the tip-substrate gap can induce dissolution. ASV-SECM can thus be used to detect and study induced dissolution not only at bulk metal surfaces but also on underpotential deposition layers, in this case Cd and Pb on Pt. In addition, we discuss how surface diffusion phenomena may relate to the observed ASV-SECM behavior.

Selective Insulation with Poly(tetrafluoroethylene) of Substrate Electrodes for Electrochemical Background Reduction in Scanning Electrochemical Microscopy

We describe a wet process for the fabrication of poly(tetrafluoroethylene) (PTFE)-covered electrodes in which arrays of holes ( approximately 200 microm) are formed. The PTFE coating provides electrical insulation of most of the electrode surface with selected regions exposed for electrochemical experiments. The arrays of microholes can be controllably patterned and filled with precursor solutions using a piezoelectric dispenser. A micrometer spot of electrocatalyst is produced after reduction of the precursor. The application is tested for scanning electrochemical microscopy (SECM) in the tip generation-substrate collection (TG-SC) studies of electrocatalysts. The method is shown to reduce the substrate background currents that are included in the electrochemical signal read from the local perturbation induced with the SECM tip to the substrate in the TG-SC mode of SECM. This background current reduction is consistent with the decrease in the exposed area of the electrode. The general methodology for the fabrication of the substrate electrodes and two proof-of-concept applications in the TG-SC SECM modality are described.

Boronic esters: a simple route to discotic liquid crystals that are electron deficient

The formation of boronic esters lowers the LUMO energy of hexamethoxytriphenylene by ∼1 V, converting the discotic core archetype from electron-rich to electron-deficient. Cyclic voltammetry gives E1/2 reduction values (ca. −1.2 V vs. Fc/Fc+) that are comparable to that of C60, the standard electron acceptor in organic electronics. Suitable peripheral functionalization imparts large mesophase temperature ranges. Homeotropic, columnar hexagonal phases with good charge carrier mobilities (up to 8 × 10−2 cm2 V−1 s−1) are observed.

Simultaneous Electrochemical Speciation of Oxidized and Reduced Glutathione. Redox Profiling of Oxidative Stress in Biological Fluids with a Modified Carbon Electrode

The simultaneous electrochemical quantification of oxidized (GSSG) and reduced glutathione (GSH), biomarkers of oxidative stress, is demonstrated in biological fluids. The detection was accomplished by the development of a modified carbon electrode and was applied to the analysis of biological fluids of model organisms under oxidative stress caused by lead intoxication. Nanocomposite molecular material based on cobalt phthalocyanine (CoPc) and multiwalled carbon nanotubes functionalized with carboxyl groups (MWCNTf) was developed to modify glassy carbon electrodes (GCE) for the detection of reduced and oxidized glutathione. The morphology of the nanocomposite film was characterized by scanning electron microscopy (SEM) and profilometry. The electrochemical behavior of the modified electrode was assessed by cyclic voltammetry (CV) to determine the surface coverage (Γ) by CoPc. The electrocatalytic behavior of the modified electrode toward reduced (GSH) and oxidized (GSSG) forms of glutathione was assessed by CV studies at physiological pH. The obtained results show that the combined use of CoPc and MWCNTf results in an electrocatalytic activity for GSH oxidation and GSSG reduction, enabling the simultaneous detection of both species. Differential pulse voltammetry reveals detection limits of 100 μM for GSH and 8.3 μM for GSSG, respectively. The potential interference from ascorbic acid, cysteine, glutamic acid, and glucose was also studied, and the obtained results show limited effects from these species. Finally, the hybrid electrode was used for the determination of GSH and GSSG in rat urine and plasma samples, intoxicated or not by lead. Both glutathione forms were detected in these complex biological matrixes without any pretreatment. Our results portray the role of GSH and GSSG as markers of oxidative stress in live organisms under lead intoxication.

Stochastic electrochemistry and photoelectrochemistry of colloidal dye-sensitized anatase nanoparticles at a Pt ultramicroelectrode

We report the stochastic interactions between dye sensitized anatase nanoparticles, suspended in a colloid, and a Pt ultramicroelectrode (UME) that result in step-wise behavior in the current vs. time response. The stochastic currents are observed in the dark and under illumination. In the dark, the currents are anodic, consistent with the oxidation of the dye N719 at the Pt surface. The electrochemical behavior of the dye was investigated in MeOH and MeCN with a quasireversible cyclic voltammogram (CV) observed at 1 V s-1. The anodic currents observed in the dark due to nanoparticles (NPs) at the Pt surface are consistent with the CVs in MeOH and MeCN. Under illumination cathodic steps are observed and assigned to the reduction of the oxidized form of the dye generated after electrons are injected into the TiO2 NPs. The colloidal behavior is a strong function of the history of the colloid with illumination time increasing the size of the agglomerates and with larger agglomerates being less photoelectrochemically active. Agglomerates of ca. 100 nm in diameter are proposed to be photoactive entities with a higher probability of detection that contribute to the staircase photocurrent response.

Expanding the scope of peropyrenes and teropyrenes through a facile InCl3-catalyzed multifold alkyne benzannulation

Herein, we report a facile synthesis of bay-region-functionalized peropyrenes and teropyrenes through an InCl3-catalyzed double or quadruple benzannulation reaction of alkynes. This method also allows for the formation of persistently twisted, and thus chiral, peropyrenes. In contrast to using Bronsted acids to facilitate the alkyne benzannulation reaction, these InCl3-catalyzed reactions are capable of cyclizing electron-rich, electron-poor, and even alkyl-substituted alkynes; this drastically improves the scope of products that can be accessed. X-ray crystallographic analysis showed that the substituents in the bay-region play an important role, not only in the crystal packing, but also for their chirality in the solid-state. The excellent solubility and broad scope of both the peropyrenes and teropyrenes obtained by this method, enable us to fully study their interesting optical and electrochemical properties for the first time.