Selective Initiation of Corrosion Pits in Stainless Steel Using Scanning Electrochemical Cell Microscopy.

Scanning electrochemical cell microscopy is a useful technique for determining variations in corrosion behavior across a surface. However, the numerous options for experimental parameters and little understanding of their effect on the corroding system render comparisons of results between studies difficult. Herein, we explore changes in corrosion behavior of two martensitic stainless steels, a cast CA6NM and a wrought S41500, as a result of the chosen experimental parameters, including scan rate, approach potential, surface oil immersion, and tip aperture diameter. The study demonstrates that these experimental parameters can be controlled to probe oxide passivation kinetics and single pitting events by changing the surface state and cathodic currents. We measured the pitting and repassivation kinetics of a single pit and determined the compositional change of the Al2O3 inclusion site initiation point. Hundreds of data points were measured within 17 h of experimental time on the stainless steel samples, allowing statistical averages of corrosion and pitting values. This work will open new avenues for fine-tuning various corrosion aspects at the microscale, thereby contributing to a deeper understanding of the corrosion processes and mechanisms of diverse materials.

Evaluation of Quatsome Morphology, Composition, and Stability for Pseudomonas aeruginosa Biofilm Eradication.

Biofilm infections are a major cause of food poisoning and hospital-acquired infections. Quaternary ammonium compounds are a group of effective disinfectants widely used in industry and households, yet their efficacy is lessened when used as antibiofilm agents compared to that against planktonic bacteria. It is therefore necessary to identify alternative formulations of quaternary ammonium compounds to achieve an effective biofilm dispersal. Quaternary ammonium amphiphiles can form vesicular structures termed "quatsomes" in the presence of cholesterol. In addition to their intrinsic antimicrobial properties, quatsomes can also be used for the delivery of other types of antibiotics or biomarkers. In this study, quatsomes were prepared from binary mixtures of cholesterol and mono- or dialkyl-quaternary ammonium compounds; then, the integrity and stability of their vesicular structure were assessed and related to monomer chain number and chain length. The quatsomes were used to treat Pseudomonas aeruginosa biofilms, showing effective antibiofilm abilities comparable to those of their monomers. A systematic liquid chromatography-mass spectrometry method for quantifying quatsome vesicle components was also developed and used to establish the significance of cholesterol in the quatsome self-assembly processes.

Fitting Kinetics from Scanning Electrochemical Microscopy Images of Finite Circular Features.

Scanning electrochemical microscopy (SECM) is a powerful technique for imaging the electrochemical reactivity of a surface. Unfortunately, SECM images are mainly used qualitatively. Kinetics of reactions at the surface are almost exclusively obtained from the microelectrode current as it approaches the surface, called an approach curve. The approach curve method is excellent when the reaction at the surface has the same kinetics everywhere, but was not designed to fit the kinetics of finite-sized reactive features. We propose a method for extracting kinetics, feature area, and microelectrode tip-to-substrate distance from SECM images by fitting with simulated images of reactive discs using the Levenberg-Marquardt algorithm. The area of experimental reactive features can be fit to within 10% if the underlying feature is roughly disc-shaped. When the reaction at simulated reactive features is activation-limited, the rate constant can be fit to within 15% of the true value. This work heralds the beginning of quantifying kinetics from SECM images.

Correlating Corrosion to Surface Grain Orientations of Polycrystalline Aluminum Alloy by Scanning Electrochemical Cell Microscopy.

The study of grain-dependent corrosion behaviors of practical polycrystalline metals remains challenging due to the difficulty in eliminating the influences of other microstructural features, such as intermetallic particles and grain boundaries. In this work, we performed thousands of microscopic potentiodynamic polarization measurements on a polycrystalline aluminum alloy AA7075-T73 using the spatially resolved oil-immersed scanning electrochemical cell microscopy measurement. Data were extracted only from grain interior areas excluding intermetallic particles and grain boundaries. Based on the multiple potentiodynamic polarization measurements, the differences between grains can be revealed. Cathodic currents exhibited a strong grain orientation dependence with a decreasing order of {101} > {001} > {111}, agreeing with the prediction from the order of atomic planar density. By contrast, the dependence of anodic currents on grain orientation was weak, and pitting was independent of grain orientation, which could be due to the limited mass transport of ions within the surface oxide film. This work highlights the capability of oil-immersed scanning electrochemical cell microscopy in resolving small electrochemical differences, which will greatly promote the study of grain-dependent behaviors of practical polycrystalline samples.

Controlling Surface Contact, Oxygen Transport, and Pitting of Surface Oxide via Single-Channel Scanning Electrochemical Cell Microscopy.

In single-channel scanning electrochemical cell microscopy, the applied potential during the approach of a micropipette to the substrate generates a transient current upon droplet contact with the substrate. Once the transient current exceeds a set threshold, the micropipette is automatically halted. Currently, the effect of the approach potential on the subsequent electrochemical measurements, such as the open-circuit potential and potentiodynamic polarization, is considered to be inconsequential. Herein, we demonstrate that the applied approach potential does impact the extent of probe-to-substrate interaction and subsequent microscale electrochemical measurements on aluminum alloy AA7075-T73.

Quantitative Feedback Referencing for Improved Kinetic Fitting of Scanning Electrochemical Microscopy Measurements.

Scanning electrochemical microscopy (SECM) has matured as a technique for studying local electrochemical processes. The feedback mode is most commonly used for extracting quantitative kinetic information. However, approaching individual regions of interest, as is commonly done, does not take full advantage of the spatial resolution that SECM has to offer. Moreover, fitting of experimental approach curves remains highly subjective due to the manner of estimating the tip-to-substrate distance. We address these issues using negative or positive feedback currents as a reference to calculate the tip-to-substrate distance directly for quantitative kinetic fitting of approach curves and line profiles. The method was first evaluated by fitting simulated data and then tested experimentally by resolving negative feedback and intermediate kinetics behavior in a spatially controlled fashion using (i) a flat, binary substrate composed of Au and SiO2 segments and (ii) a dual-mediator system for live-cell measurements. The methodology developed herein, named quantitative feedback referencing (QFR), improves fitting accuracy, removes fitting subjectivity, and avoids substrate-microelectrode contact.

Formation of Oxidation- and Acid-Sensitive Assemblies from Sterols and a Quaternary Ammonium Ferrocene Derivative: Quatsome- and Onion-like Vesicles and Extended Nanoribbons.

Quatsomes are a class of nonphospholipid vesicles in which bilayers are formed from mixtures of quaternary ammonium (QA) amphiphiles and sterols. We describe the formation of oxidation and acid-sensitive quatsome-like vesicles and other bilayer assemblies from mixtures of a ferrocenylated QA amphiphile (FTDMA) and several cholesterol derivatives. The influence of the sterol and the preparation method (extrusion or probe sonication) on the stability and morphology of the resulting vesicles is explored; a variety of structures can be obtained from small (ca. 30 nm) spherical unilamellar and oligolamellar quatsome-like vesicles to large (ca. 200 nm) multilamellar onion-like vesicles to extended nanoribbons many micrometers long. FTDMA-sterol vesicles undergo drastic shifts in vesicle and membrane structure when treated with a chemical oxidant (Frémy's salt), a feature previously observed in liposomes containing FTDMA and now confirmed in nonphospholipid vesicles. Size distributions of spherical quatsome-like vesicles obtained from cryo-TEM are examined to estimate the membrane bending rigidity, and a hypothesis is presented to explain the underlying mechanism of the profound membrane alterations observed as a consequence of ferrocene oxidation.

Tunable Assembly of Protein Enables Fabrication of Platinum Nanostructures with Different Catalytic Activity.

Proteins are promising biofunctional units for the construction of nanomaterials (NMs) due to their abundant binding sites, intriguing self-assembly properties, and mild NM synthetic conditions. Tobacco mosaic virus coat protein (TMVCP) is a protein capable of self-assembly into distinct morphologies depending on the solution pH and ionic strength. Herein, we report the use of TMVCP as a building block to organize nanosized platinum into discrete nanorings and isolated nanoparticles by varying the solution pH to modulate the protein assembly state. Compared with a commercial Pt/C catalyst, the TMVCP-templated platinum materials exhibited significant promotion of the catalytic activity and stability toward methanol electrooxidation in both neutral and alkaline conditions. The enhanced catalytic performance is likely facilitated by the protein support. Additionally, Pt nanorings outperformed isolated nanoparticles, although they are both synthesized on TMVCP templates. This could be due to the higher mechanical stability of the protein disk structure and possible cooperative effects between adjacent nanoparticles in the ring with narrow interparticle spacing.

High-Throughput Strategy for Glycine Oxidase Biosensor Development Reveals Glycine Release from Cultured Cells.

Glycine is an important biomarker in clinical analysis due to its involvement in multiple physiological processes. As such, the need for low-cost analytical tools for glycine detection is growing. As a neurotransmitter, glycine is involved in inhibitory and excitatory neurochemical transmission in the central nervous system. In this work, we present a 10 µM Pt-based electrochemical enzymatic biosensor based on the flavoenzyme glycine oxidase (GO) for localized real-time measurements of glycine. Among GO variants at position 244, the H244K variant with increased glycine turnover was selected to develop a functional biosensor. This biosensor relies on amperometric readouts and does not require additional redox mediators. The biosensor was characterized and applied for glycine detection from cells, mainly HEK 293 cells and primary rat astrocytes. We have identified an enzyme, GO H244K, with increased glycine turnover using mutagenesis but which can be developed into a functional biosensor. Noteworthy, a glycine release of 395.7 ± 123 µM from primary astrocytes was measured, which is ∼fivefold higher than glycine release from HEK 293 cells (75.4 ± 3.91 µM) using the GO H244K biosensor.

EDTA-Gradient Loading of Doxorubicin into Ferrocene-Containing Liposomes: Effect of Lipid Composition and Visualization of Triggered Release by Cryo-TEM.

Efficient delivery of therapeutic compounds to their sites of action has been a ubiquitous concern throughout the history of human medicine. The tumor microenvironment offers a variety of endogenous stimuli that may be exploited by a responsive nanocarrier, including heterogeneities in redox potential. In the early stages of the design of such responsive delivery systems, it is necessary to develop a comprehensive understanding of the biophysical mechanism by which the stimulus response occurs, as well as how the response may change from the inclusion of cargo compounds. We describe the optimization of lipid compositions for liposomes containing synthetic ferrocene-appended lipids to achieve highly efficient loading of doxorubicin via an ethylenediaminetetraacetic acid (EDTA) gradient. Liposomes containing ferrocenylated phospholipid are shown to be unstable to the loading conditions, while those including a ferrocenylated alkylammonium amphiphile obtain a near-quantitative loading efficiency. Calorimetric studies demonstrate that this instability is the consequence of the relative degree of lipid hydrolysis that occurs under the acidic loading conditions. Drug-loaded liposomes of the optimized composition are studied by cryo-TEM; the presence of doxorubicin aggregates is observed inside vesicles, and doxorubicin release, as well as the changes in membrane structure resulting from oxidant treatment, is also observed by cryogenic transmission electron microscopy (cryo-TEM). These results further demonstrate the potential of ferrocene lipids in the design of redox-responsive nanocarriers and begin to explore their possible role as probes of membrane dynamics.

Ag+ Interference from Ag/AgCl Wire Quasi-Reference Counter Electrode Inducing Corrosion Potential Shift in an Oil-Immersed Scanning Micropipette Contact Method Measurement.

Quantitative scanning micropipette contact method measurements are subject to the deleterious effects of reference electrode interference. The commonly used Ag/AgCl wire quasi-reference counter electrode in the miniaturized electrochemical cell of the scanning micropipette contact method was found to leak Ag+ into the electrolyte solution. The reduction of these Ag+ species at the working electrode surface generates a faradaic current, which significantly affects the low magnitude currents inherently measured in the scanning micropipette contact method. We demonstrate that, during the microscopic corrosion investigation of the AA7075-T73 alloy using the oil-immersed scanning micropipette contact method, the cathodic current was increased by the Ag+ reduction, resulting in positive shifts of corrosion potentials. The use of a leak-free Ag/AgCl electrode or an extended distance between the Ag/AgCl wire and micropipette tip droplet eliminated the Ag+ contamination, making it possible to measure accurate corrosion potentials during the oil-immersed scanning micropipette contact method measurements.

Quantitative measurements of free and immobilized RgDAAO Michaelis-Menten constant using an electrochemical assay reveal the impact of covalent cross-linking on substrate specificity.

Challenges facing enzyme-based electrochemical sensors include substrate specificity, batch to batch reproducibility, and lack of quantitative metrics related to the effect of enzyme immobilization. We present a quick, simple, and general approach for measuring the effect of immobilization and cross-linking on enzyme activity and substrate specificity. The method can be generalized for electrochemical biosensors using an enzyme that releases hydrogen peroxide during its catalytic cycle. Using as proof of concept RgDAAO-based electrochemical biosensors, we found that the Michaelis-Menten constant (Km) decreases post immobilization, hinting at alterations in the enzyme kinetic properties and thus substrate specificity. We confirm the decrease in Km electrochemically by characterizing the substrate specificity of the immobilized RgDAAO using chronoamperometry. Our results demonstrate that enzyme immobilization affects enzyme substrate specificity and this must be carefully evaluated during biosensor development.

Enhancing Electrochemical Biosensor Selectivity with Engineered d-Amino Acid Oxidase Enzymes for d-Serine and d-Alanine Quantification.

d-Amino acid oxidase (DAAO) enzymes bind a range of d-amino acids with variable affinity. As such, the design of selective DAAO-based enzymatic biosensors remains a challenge for real-world biosensor application. Herein, a methodology for developing biosensors with varying substrate selectivity is presented. First, we address DAAO-based biosensor selectivity toward d-serine by introducing point mutations into DAAO using rational design. Next, the wild-type yeast DAAO (RgDAAO WT) and variants human DAAO W209R and yeast M213G are characterized for their selectivity and activity toward d-serine and d-alanine, the preferred DAAO substrates. The DAAO enzymes have been immobilized for final biosensor design, where they demonstrate selectivity comparable to free DAAO. The cross-linking procedure impacts on DAAO structure and function and the use of a regeneration strategy allows the biosensor response to be improved.

Recent Advances in Bio-Templated Metallic Nanomaterial Synthesis and Electrocatalytic Applications.

Developing metallic nanocatalysts with high reaction activity, selectivity and practical durability is a promising and active subfield in electrocatalysis. In the classical "bottom-up" approach to synthesize stable nanomaterials by chemical reduction, stabilizing additives such as polymers or organic surfactants must be present to cap the nanoparticle to prevent material bulk aggregation. In recent years, biological systems have emerged as green alternatives to support the uncoated inorganic components. One key advantage of biological templates is their inherent ability to produce nanostructures with controllable composition, facet, size and morphology under ecologically friendly synthetic conditions, which are difficult to achieve with traditional inorganic synthesis. In addition, through genetic engineering or bioconjugation, bio-templates can provide numerous possibilities for surface functionalization to incorporate specific binding sites for the target metals. Therefore, in bio-templated systems, the electrocatalytic performance of the formed nanocatalyst can be tuned by precisely controlling the material surface chemistry. With controlled improvements in size, morphology, facet exposure, surface area and electron conductivity, bio-inspired nanomaterials often exhibit enhanced catalytic activity towards electrode reactions. In this Review, recent research developments are presented in bio-approaches for metallic nanomaterial synthesis and their applications in electrocatalysis for sustainable energy storage and conversion systems.

Oil-Immersed Scanning Micropipette Contact Method Enabling Long-term Corrosion Mapping.

This work reports the development of an oil-immersed scanning micropipette contact method, a variant of the scanning micropipette contact method, where a thin layer of oil wets the investigated substrate. The oil-immersed scanning micropipette contact method significantly increases the droplet stability, allowing for prolonged mapping and the use of highly evaporative saline solutions regardless of ambient humidity levels. This systematic mapping technique was used to conduct a detailed investigation of localized corrosion taking place at the surface of an AA7075-T73 aluminum alloy in a 3.5 wt % NaCl electrolyte solution, which is typically challenging in the conventional scanning micropipette contact method. Maps of corrosion potentials and corrosion currents extracted from potentiodynamic polarization curves showed good correlations with the chemical composition of surface features and known galvanic interactions at the microscale level. This demonstrates the viability of the oil-immersed scanning micropipette contact method and opens up the avenue to mechanistic corrosion investigations at the microscale level using aqueous solutions that are prone to evaporation under noncontrolled humidity levels.

Operando Tracking of Solution-Phase Concentration Profiles in Li-Ion Battery Positive Electrodes Using X-ray Fluorescence.

The trade-off between energy density and power capabilities is a challenge for Li-ion battery design as it highly depends on the complex porous structures that holds the liquid electrolyte. Specifically, mass-transport limitations lead to large concentration gradients in the solution-phase and subsequently to crippling overpotentials. The direct study of these solution-phase concentration profiles in Li-ion battery positive electrodes has been elusive, in part because they are shielded by an opaque and paramagnetic matrix. Herein we present a new methodology employing synchrotron hard X-ray fluorescence to observe the concentration gradient formation within Li-ion battery electrodes in operando. This methodology is substantiated with data collected on a model LiFePO4/Li cell using a 1 M LiAsF6 in 1:1 ethylene carbonate/dimethyl carbonate (EC/DMC) electrolyte under galvanostatic and intermittent charge profiles. As such, the technique holds great promise for optimization of new composite electrodes and for numerical model validation.

Charge Storage in Graphene Oxide: Impact of the Cation on Ion Permeability and Interfacial Capacitance.

The charge storage and membrane applications of graphene oxide (GO) materials are dictated by its intrinsic material properties. Structure-function relationships correlating periodic parameters, such as the hydrated ion radius and ion-GO interactions, are currently lacking yet are needed to provide insight on the charge storage and ion transport mechanism. We report the use of scanning ion conductance microscopy to measure the ion permeability of GO films and evaluate its relationship with the measured capacitance. We demonstrate that species (namely K+) with strong electrostatic interactions with the oxygen functionalities of GO provide the benefit of higher capacitance but suffer from inhibited ion mobility due to constriction of the GO interlayer spacing.

Polymers with intrinsic microporosity (PIMs) for targeted CO2 reduction to ethylene.

CO2 reduction offers an attractive alternative green synthetic route for ethylene, especially where CO2 could be sourced from industrial exhausts and in combination with green power sources. However, practical applications are currently limited due to the unfortunately low selectivity of cathode materials towards ethylene. This work uses polymers with intrinsic microporosity (PIMs) to improve the performance of copper gas diffusion electrodes for CO2 reduction to ethylene. We report an improved selectivity and activity towards ethylene with the addition of a thin PIMs layer, which is seen as improved Faradaic efficiency, increased stability and a shift in the reduction to lower overpotential. This improvement is highly dependent on the thickness of the added polymer layer, with too thick a layer having a detrimental impact on the hydrophobicity of the gas diffusion layer. With a compromise in loading, PIMs can be used to enhance the activity and selectivity of catalysts for targeted CO2 reduction to ethylene.

Super-resolution Scanning Electrochemical Microscopy.

To achieve super-resolution scanning electrochemical microscopy (SECM), we must overcome the theoretical limitation associated with noncontact electrochemical imaging of surface-generated species. This is the requirement for mass transfer to the electrode, which gives rise to the diffusional broadening of surface features. In this work, a procedure is developed for overcoming this limitation and thus generating "super-resolved" images using point spread function (PSF)-based deconvolution, where the point conductor plays the same role as the point emitter in optical imaging. In contrast to previous efforts in SECM towards this goal, our method uses a finite element model to generate a pair of corresponding blurred and sharp images for PSF estimation, avoiding the need to perform parameter optimization for effective deconvolution. It can therefore be used for retroactive data treatment and an enhanced understanding of the structure-property relationships that SECM provides.

Corrosion of One-Step Superhydrophobic Stainless-Steel Thermal Spray Coatings.

As most superhydrophobic coatings are made of soft materials, the need for harder, more robust films is evident in applications where erosional degradation is of concern. The work herein describes a methodology to produce superhydrophobic stainless-steel thermal spray coatings using the high-velocity oxygen fuel technique. Due to the use of a kerosene fuel source, a carbon-rich film is formed on the surface of the thermal spray coatings, lowering the surface energy of the high-energy metallic substrates. The thermal spray process generates a hierarchical micro-/sub-micro-structure that is needed to sustain superhydrophobicity. The effect of spray parameters such as particle velocity and temperature on the coating's hydrophobicity state was explored, and a high particle velocity was shown to cause superhydrophobic characteristics. The coatings were characterized using scanning electron microscopy, profilometry, X-ray photoelectron spectroscopy, static water contact angle measurements, water droplet roll-off measurements, and water droplet bouncing tests. The corrosion behavior of the coatings was studied using potentiodynamic polarization measurements in order to correlate water repellency with corrosion resistance; however, all coatings demonstrated active corrosion without passivation. This study describes an interesting phenomenon where superhydrophobicity does not guarantee corrosion resistance and discusses alternative applications for such materials.