Electrochemistry & Interfaces » Recent Publications Feed (ADMIN)

• Artificial Synapse: Spatiotemporal Heterogeneities in Dopamine Electrochemistry at a Carbon Fiber Ultramicroelectrode

Fri, 02 Jul 2021 10:16:14 GMT

An artificial synapse is developed that mimics ultramicroelectrode (UME) amperometric detection of single cell exocytosis. It comprises the nanopipette of a scanning ion conductance microscope (SICM), which delivers rapid pulses of neurotransmitter (dopamine) locally and on demand at >1000 defined locations of a carbon fiber (CF) UME in each experiment. Analysis of the resulting UME current-space-time data reveals spatiotemporal heterogeneous electrode activity on the nanoscale and submillisecond time scale for dopamine electrooxidation at typical UME detection potentials. Through complementary surface charge mapping and finite element method (FEM) simulations, these previously unseen variations in electrochemical activity are related to heterogeneities in the surface chemistry of the CF UME.

• Microstructural origin of locally enhance CO2 electroreduction activity on gold

Fri, 19 Mar 2021 12:37:18 GMT

Understanding how the bulk structure of a material affects catalysis on its surface is critical to the development of action-able catalyst design principles. Bulk defects have been shown to affect electrocatalytic materials that are important for energy conversion systems, but the structural origins of these effects have not been fully elucidated. Here we use a combi-nation of high-resolution scanning electrochemical cell microscopy and electron backscatter diffraction to visualize the potential-dependent electrocatalytic carbon dioxide ðCO2ÞI electroreduction and hydrogen ðH2ÞI evolution activity on Au elec-trodes and probe the effects of bulk defects. Comparing colocated activity maps and videos to the underlying microstructure and lattice deformation supports a model in which CO2 electroreduction is selectively enhanced by surface-terminating disloca-tions, which can accumulate at grain boundaries and slip bands. Our results suggest that the deliberate introduction of disloca-tions into materials is a promising strategy for improving catalytic properties.

• Lifting the Lid on the Potentiostat: A Beginners Guide to Understanding Electrochemical Circuitry and Practical Operation

Fri, 12 Mar 2021 09:08:25 GMT

Students who undertake practical electrochemistry experiments for the first time, will come face to face with the potentiostat. To many this is simply a box containing electronics which enables a potential to be applied between a working and reference electrode, and a current to flow between the working and counter electrode, both of which are outputted to the experimentalist. Given the broad generality of electrochemistry across many disciplines it is these days very common for students entering the field to have a minimal background in electronics. This article serves as an introductory tutorial to those with no formalized training in this area. The reader is introduced to the operational amplifier, which is at the heart of the different potentiostatic electronic circuits and its role in enabling a potential to be applied and a current to be measured is explained. Voltage follower op-amp circuits are also highlighted, given their importance in measuring voltages accurately. We also discuss digital to analogue and analogue to digital conversion, the processes by which the electrochemical cell receives input signals and outputs data and data filtering. By reading the article, it is intended the reader will also gain a greater confidence in problem solving issues that arise with electrochemical cells, for example electrical noise, uncompensated resistance, reaching compliance voltage, signal digitisation and data interpretation. We also include trouble shooting tables that build on the information presented and can be used when undertaking practical electrochemistry.

• Unveiling the contribution of the reproductive system of individual Caenorhabditis elegans on oxygen consumption by single-point scanning electrochemical microscopy measurements

Tue, 05 Jan 2021 09:30:43 GMT

Metabolic analysis in animals is usually either evaluated as whole-body measurements or in isolated tissue samples. To reveal tissue specificities in vivo, this study uses scanning electrochemical microscopy (SECM) to provide localized oxygen consumption rates (OCRs) in different regions of single adult Caenorhabditis elegans individuals. This is achieved by measuring the oxygen reduction current at the SECM tip electrode and using a finite element method model of the experiment that defines oxygen concentration and flux at the surface of the organism. SECM mapping measurements uncover a marked heterogeneity of OCR along the worm, with high respiration rates at the reproductive system region. To enable sensitive and quantitative measurements, a self-referencing approach is adopted, whereby the oxygen reduction current at the SECM tip is measured at a selected point on the worm and in bulk solution (calibration). Using genetic and pharmacological approaches, our SECM measurements indicate that viable eggs in the reproductive system are the main contributors in the total oxygen consumption of adult Caenorhabditis elegans. The finding that large regional differences in OCR exist within the animal provides a new understanding of oxygen consumption and metabolic measurements, paving the way for tissue-specific metabolic analyses and toxicity evaluation within single organisms.

• Nanoscale Electrochemistry in a Copper/Aqueous/Oil Three-phase System: Surface Structure-Activity-Corrosion Potential Relationships

Tue, 05 Jan 2021 09:29:19 GMT

Practically important metal electrodes are usually polycrystalline, comprising surface grains of many different crystallographic orientations, as well as grain boundaries. In this study, scanning electrochemical cell microscopy (SECCM) is applied in tandem with co-located electron backscattered diffraction (EBSD) to give a holistic view of the relationship between the surface structure and the electrochemical activity and corrosion susceptibility of polycrystalline Cu. An unusual aqueous nanodroplet/oil (dodecane)/metal three-phase is employed, which opens up new prospects for fundamental studies of multiphase electrochemical systems, and mimics the environment of corrosion in certain industrial and automotive applications. In this configuration, the nanodroplet formed at the end of the SECCM probe (nanopipette) is surrounded by dodecane, which acts as a reservoir for oil-soluble species (e.g., O2) and can give rise to enhanced flux(es) across the immiscible liquid-liquid interface, as shown by finite element method (FEM) simulations. This unique three-phase configuration is used to fingerprint nanoscale corrosion in a nanodroplet cell, and to analyse the interrelationship between the Cu oxidation, Cu2+ deposition and oxygen reduction reaction (ORR) processes, together with nanoscale open circuit (corrosion) potential, in a grain-by-grain manner. Complex patterns of surface reactivity highlight the important role of grains of high-index orientation and microscopic surface defects (e.g., microscratches) in modulating the corrosion-properties of polycrystalline Cu. This work provides a roadmap for in-depth surface structure−function studies in (electro)materials science and highlights how small variations in surface structure (e.g., crystallographic orientation) can give rise to large differences in nanoscale reactivity.

• Scanning Ion Conductance Microscopy Reveals Differences in the Ionic Environments of Gram-Positive and Negative Bacteria

Fri, 27 Nov 2020 16:25:42 GMT

This paper reports on the use of scanning ion conductance microscopy (SICM) to locally map the ionic properties and charge environment of two live bacterial strains: the Gram-negative Escherichia coli and the Gram-positive Bacillus subtilis. SICM results find heterogeneities across the bacterial surface and significant differences among the Gram-positive and Gram-negative bacteria. The bioelectrical environment of the B. subtilis was found to be considerably more negatively charged compared to E. coli. SICM measurements, fitted to a simplified finite element method (FEM) model, revealed surface charge values of −80 to −140 mC m–2 for the Gram-negative E. coli. The Gram-positive B. subtilis show a much higher conductivity around the cell wall, and surface charge values between −350 and −450 mC m–2 were found using the same simplified model. SICM was also able to detect regions of high negative charge near B. subtilis, not detected in the topographical SICM response and attributed to the extracellular polymeric substance. To further explore how the B. subtilis cell wall structure can influence the SICM current response, a more comprehensive FEM model, accounting for the physical properties of the Gram-positive cell wall, was developed. The new model provides a more realistic description of the cell wall and allows investigation of the relation between its key properties and SICM currents, building foundations to further investigate and improve understanding of the Gram-positive cellular microenvironment.

• Combined Voltammetric Measurement of pH and Free Chlorine Speciation Using a Micro-Spot sp2 Bonded Carbon–Boron Doped Diamond Electrode

Tue, 17 Nov 2020 11:21:41 GMT

This work demonstrates the use of an sp2-bonded carbon microspot boron doped diamond (BDD) electrode for voltammetric measurement of both pH and analyte concentration in a pH-dependent speciation process. In particular, the electrode was employed for the voltammetric detection of pH and hypochlorite (OCl–) in unbuffered, aerated solutions over the pH range 4–10. Knowledge of both pH and [OCl–] is essential for determination of free chlorine concentration. The whole surface of the microspot BDD electrode was found active toward the voltammetric oxidation of OCl–, with OCl– showing a characteristic response at +1.5 V vs SCE. In contrast, it was only the surface integrated quinones (Q) in sp2-bonded carbon regions of the BDD electrode that were responsible for the voltammetric pH signal. A Nernstian response for pH (gradient = 63 ± 1 mV pH–1) was determined from proton coupled electron transfer at the BDD-Q electrode, over the potential range −0.4–0.5 V vs SCE. By measuring both OCl– and pH voltammetrically, over the pH range 4–10, the OCl– oxidative current was found to correlate extremely well with the predicted pH-dependent [OCl–] speciation profile.

• High pressure high temperature synthesis of highly boron doped diamond microparticles and porous electrodes for electrochemical applications

Tue, 06 Oct 2020 08:43:21 GMT

High pressure high temperature (HPHT) synthesis of crystallographically well-defined boron doped diamond (BDD) microparticles, suitable for electrochemical applications and using the lowest P and T (5.5 GPa and 1200 °C) growth conditions to date, is reported. This is aided through the use of a metal (Fe–Ni) carbide forming catalyst and an aluminum diboride (AlB2) boron source. The latter also acts as a nitrogen sequester, to reduce boron-nitrogen charge compensation effects. Raman microscopy and electrochemical measurements on individual microparticles reveal they are doped to metal-like levels, contain negligible sp2 bonded carbon and display a large aqueous solvent window. A HPHT compaction process is used to create macroscopic porous electrodes from the BDD microparticles. Voltammetric analysis of the one-electron reduction of Ru(NH3)63+ is used to identify the fundamental electrochemical response of the porous material, revealing large capacitive and resistive components to the current-voltage curves, originating from solution trapped within the pores. Scanning electrochemical cell microscopy is employed to map the local electrochemical activity and porosity at the micron scale. Such electrodes are of interest for applications which require the electrochemical and mechanical robustness properties of BDD, e.g. when operating under high applied potentials/currents, but with the additional benefits of a large, electrochemically accessible, surface area.

• Surface microstructural controls on electrochemical hydrogen absorption at polycrystalline palladium

Wed, 30 Sep 2020 12:54:30 GMT

The ease by which hydrogen is absorbed into a metal can be either advantageous or deleterious, depending on the material and application in question. For instance, in metals such as palladium (Pd), rapid absorption kinetics are seen as a beneficial property for hydrogen purification and storage applications, whereas the contrary is true for structural metals such as steel, which are susceptible to mechanical degradation in a process known as hydrogen embrittlement. It follows that understanding how the microstructure of metals (i.e., grains and grain boundaries) influences adsorption and absorption kinetics would be extremely powerful to rationally design materials (e.g., alloys) with either a high affinity for hydrogen or resistance to hydrogen embrittlement. To this end, scanning electrochemical cell microscopy (SECCM) is deployed herein to study surface structure-dependent electrochemical hydrogen absorption across the surface of flame annealed polycrystalline Pd in aqueous sulfuric acid (considered to be a model system for the study of hydrogen absorption). Correlating spatially-resolved cyclic voltammetric data from SECCM with co-located structural information from electron backscatter diffraction (EBSD) reveals a clear relationship between the crystal orientation and the rate of hydrogen adsorption-absorption. Grains that are closest to the low-index orientations [i.e., the {100}, {101}, and {111} facets, face-centered cubic (fcc) system] facilitate the lowest rates of hydrogen absorption, whereas grains of high-index orientation (e.g., {411}) promote higher rates. Apparently enhanced kinetics are also seen at grain boundaries, which are thought to arise from physical deformation of the Pd surface adjacent to the boundary, resulting from the flame annealing and quenching process. As voltammetric measurements are made across a wide potential range, these studies also reveal palladium oxide formation and stripping to be surface structure-dependent processes, and further highlight the power of combined SECCM-EBSD for structure-activity measurements in electrochemical science.

• Nanoscale kinetics of amorphous calcium carbonate precipitation in H2O and D2O

Wed, 30 Sep 2020 12:53:23 GMT

Calcium carbonate (CaCO3) is one of the most well-studied and abundant natural materials on Earth. Crystallisation of CaCO3 is often observed to proceed via an amorphous calcium carbonate (ACC) phase, as a precursor to more stable crystalline polymorphs such as vaterite and calcite. Despite its importance, the kinetics of ACC formation have proved difficult to study, in part due to rapid precipitation at moderate supersaturations, and the instability of ACC with respect to all other polymorphs. However, ACC can be stabilised under confinement conditions, such as those provided by a nanopipette. This paper demonstrates electrochemical mixing of a Ca2+ salt (CaCl2) and a HCO3− salt (NaHCO3) in a nanopipette to repeatedly and reversibly precipitate nanoparticles of ACC under confined conditions, as confirmed by scanning transmission electron microscopy (STEM). Measuring the current as a function of applied potential across the end of the nanopipette and time provides millisecond-resolved measurements of the induction time for ACC precipitation. We demonstrate that under conditions of electrochemical mixing, ACC precipitation is extremely fast, and highly pH sensitive with an apparent third order dependence on CO32− concentration. Furthermore, the rate is very similar for the equivalent CO32− concentrations in D2O, suggesting that neither ion dehydration nor HCO3− deprotonation represent significant energetic barriers to the formation of ACC. Finite element method simulations of the electrochemical mixing process enable the supersaturation to be estimated for all conditions and accurately predict the location of precipitation.