Scanning electrochemical microscopy for direct imaging of reaction rates

Not only in electrochemistry but also in biology and in membrane transport, localized processes at solid-liquid or liquid-liquid interfaces play an important role at defect sites, pores, or individual cells, but are difficult to characterize by integral investigation. Scanning electrochemical microscopy is suitable for such investigations. After two decades of development, this method is based on a solid theoretical foundation and a large number of demonstrated applications. It offers the possibility of directly imaging heterogeneous reaction rates and locally modifying substrates by electrochemically generated reagents. The applications range from classical electrochemical problems, such as the investigation of localized corrosion and electrocatalytic reactions in fuel cells, sensor surfaces, biochips, and microstructured analysis systems, to mass transport through synthetic membranes, skin and tissue, as well as intercellular communication processes. Moreover, processes can be studied that occur at liquid surfaces and liquid-liquid interfaces.     

Microwave‐Assisted Electroanalysis: A Review

Microwave‐assisted electrochemistry is critically discussed with a focus on the fundamental aspects of the processes involved and its applications in electroanalysis. The concept of direct and nondirect heated electrodes is discussed, and simulation work is evaluated. Microwave‐assisted electrochemistry predominantly results in higher current responses (up to 2 magnitudes higher) due to increased temperature and mass transport to the active electrodes. Temperature gradients at microwave‐affected electrodes may exceed 10⁵ K/cm, with temperature hotspots found in the thin diffusion layers set up at ultramicroelectrodes. Research into microwave‐assisted electroanalysis can lead to enhanced capillary electrophoresis detection, improved stripping voltammetry and development of new high temperature methods.     

Membrane electrochemistry

Electrochemistry and biomembranes are interface science in that both are concerned with the phenomena at, as well as across, the interfaces. Membrane electrochemistry may be defined as the application of electrochemistry to biomembrane studies. Additionally, transport processes within the membrane are involved in biomembranes. Since biomembranes are diverse and are usually not amenable to probing by electrochemicophysical techniques, model membrane systems have been developed for their investigation.

The introduction of experimental bilayer lipid membranes (BLM) technique and its modifications have been instrumental in the development and testing of membrane transport concepts (carriers vs channels) and electronic processes in membranes. Instead merely viewing a biomembrane as a physical barrier containing carriers or channels to carry out ionic processes, an ultrathin lipid or biological membrane can also be considered as a complete ‘electrochemical cell’ with one membrane/solution interface reducing (as a cathode) and the other membrane/solution interface oxidizing (as an anode). It is now possible to understand energy transduction (charge generation, separation, and redox reactions) in terms of ultrathin lipid membranes separating two aqueous solutions.

In this paper, we shall discuss the basic principles of electrochemistry as they are applied to membrane studies. Emphasis will be on experimental bilayer lipid membranes (BLM) which have been extensively investigated as models of biomembranes.     

Electrochemistry of Nanoparticles

Metal nanoparticles (NPs) find widespread application as a result of their unique physical and chemical properties. NPs have generated considerable interest in catalysis and electrocatalysis, where they provide a high surface area to mass ratio and can be tailored to promote particular reaction pathways. The activity of NPs can be analyzed especially well using electrochemistry, which probes interfacial chemistry directly. In this Review, we discuss key issues related to the electrochemistry of NPs. We highlight model studies that demonstrate exceptional control over the NP shape and size, or mass‐transport conditions, which can provide key insights into the behavior of ensembles of NPs. Particular focus is on the challenge of ultimately measuring reactions at individual NPs, and relating the response to their structure, which is leading to imaginative experiments that have an impact on electrochemistry in general as well as broader surface and colloid science.     

Membrane Filtration with Liquids: A Global Approach with Prior Successes,New Developments and Unresolved Challenges

After 70 years, modern pressure‐driven polymer membrane processes with liquids are mature and accepted in many industries due to their good performance, ease of scale‐up, low energy consumption, modular compact construction, and low operating costs compared with thermal systems. Successful isothermal operation of synthetic membranes with liquids requires consideration of three critical aspects or “legs” in order of relevance: selectivity, capacity (i.e. permeation flow rate per unit area) and transport of mass and momentum comprising concentration polarization (CP) and fouling (F). Major challenges remain with respect to increasing selectivity and controlling mass transport in, to and away from membranes. Thus, prediction and control of membrane morphology and a deep understanding of the mechanism of dissolved and suspended solute transport near and in the membrane (i.e. diffusional and convective mass transport) is essential. Here, we focus on materials development to address the relatively poor selectivity of liquid membrane filtration with polymers and discuss the critical aspects of transport limitations. Machine learning could help optimize membrane structure design and transport conditions for improved membrane filtration performance.     

Membrane Filtration with Liquids: A Global Approach with Prior Successes,New Developments and Unresolved Challenges

After 70 years, modern pressure‐driven polymer membrane processes with liquids are mature and accepted in many industries due to their good performance, ease of scale‐up, low energy consumption, modular compact construction, and low operating costs compared with thermal systems. Successful isothermal operation of synthetic membranes with liquids requires consideration of three critical aspects or “legs” in order of relevance: selectivity, capacity (i.e. permeation flow rate per unit area) and transport of mass and momentum comprising concentration polarization (CP) and fouling (F). Major challenges remain with respect to increasing selectivity and controlling mass transport in, to and away from membranes. Thus, prediction and control of membrane morphology and a deep understanding of the mechanism of dissolved and suspended solute transport near and in the membrane (i.e. diffusional and convective mass transport) is essential. Here, we focus on materials development to address the relatively poor selectivity of liquid membrane filtration with polymers and discuss the critical aspects of transport limitations. Machine learning could help optimize membrane structure design and transport conditions for improved membrane filtration performance.     

Electrochemical DNA Probes

Abstract

A report on the status of the electrochemical DNA probes for the detection of a DNA sequence for environmental and clinical studies is presented. The literature on the electrochemistry of DNA in the last ten years is reviewed. Results obtained in this laboratory using an electroactive hybridization indicator for constructing an electrochemical DNA probe are reported.     

Mesostructured carbon-based nanocages: an advanced platform for energy chemistry

The electrochemistry in energy conversion and storage(ECS) not only relies on the active species in catalysts or energy-storage materials, but also involves mass/ion transport around the active species and electron transfer to the external circuit. To realize high-rate ECS process, new architectures for catalysts or energy-storage electrodes are required to ensure more efficient mass/charge transport. 3 D porous mesostructured materials constructed by nanoscale functional units can form a continuous conductive network for electron transfer and an interconnected multiscale pores for mass/ion transport while maintaining the high surface area, showing great promise in boosting the ECS process. In this review, we summarize the recent progress on the design,construction and applications of 3 D mesostructured carbon-based nanocages for ECS. The role of the hierarchical architectures to the high rate performance is discussed to highlight the merits of the mesostructured materials. The perspective on future opportunities and challenges is also outlined for deepening and extending the related studies and applications.     

Synergistic microfluidic and electrochemical strategy to activate and pattern surfaces selectively with ligands and cells

We show a straightforward, flexible synergistic approach that combines microfluidics, electrochemistry, and a general immobilization strategy to activate regions of a substrate selectively for the precise immobilization of ligands and cells in patterns for a variety of cell-based assays and cell migration and cell adhesion studies. We develop microfluidic microchips to control the delivery of electrolyte solution to select regions of an electroactive hydroquinone SAM. Once an electrical potential is applied to the substrate, only the hydroquinone exposed to electrolyte solution within the microfluidic channels oxidizes to the corresponding quinone. The quinone form can then react chemoselectively with oxyamine-tethered ligands to pattern the surface. Therefore, this microfluidic/electrochemistry strategy selectively activates the surface for ligand patterning that exactly matches the channel design of the microfluidic channel. We demonstrate the ease of this system by first quantitatively characterizing the electrochemical activation and immobilization of ligands on the surface. Second, we immobilize a fluorescent dye to show the fidelity of the methodology, and third, we show the immobilization of biospecific cell adhesive peptide ligands to pattern cells. This is the first report that combines microfluidics/electrochemistry and a general electroactive immobilization strategy to pattern ligands and cells. We believe that this strategy will be of broad utility for applications ranging from fundamental studies of cell behavior to patterning molecules on a variety of materials for molecular electronic devices.     

Shaping and exploring the micro- and nanoworld using bipolar electrochemistry

Bipolar electrochemistry is a technique with a rather young history in the field of analytical chemistry. Being based on the polarization of a conducting object which is exposed to an external electric field, it allowed recently the development of new methods for controlled surface modification at the micro- and nanoscale and very original analytical applications. Using bipolar electrodes, analyte separation and detection becomes possible based on miniaturized systems. Moreover, the modified objects that can be created with bipolar electrochemistry could find applications as key components for detection systems. In this contribution, the principles of bipolar electrochemistry will be reviewed, as well as recent developments that focus on the modification of objects at the nano- and microscale and their potential application in miniaturized analytical systems.     

Non-invasive tools for measuring metabolism and biophysical analyte transport: self-referencing physiological sensing

Biophysical phenomena related to cellular biochemistry and transport are spatially and temporally dynamic, and are directly involved in the regulation of physiology at the sub-cellular to tissue spatial scale. Real time monitoring of transmembrane transport provides information about the physiology and viability of cells, tissues, and organisms. Combining information learned from real time transport studies with genomics and proteomics allows us to better understand the functional and mechanistic aspects of cellular and sub-cellular systems. To accomplish this, ultrasensitive sensing technologies are required to probe this functional realm of biological systems with high temporal and spatial resolution. In addition to ongoing research aimed at developing new and enhanced sensors (e.g., increased sensitivity, enhanced analyte selectivity, reduced response time, and novel microfabrication approaches), work over the last few decades has advanced sensor utility through new sensing modalities that extend and enhance the data recorded by sensors. A microsensor technique based on phase sensitive detection of real time biophysical transport is reviewed here. The self-referencing technique converts non-invasive extracellular concentration sensors into dynamic flux sensors for measuring transport from the membrane to the tissue scale. In this tutorial review, we discuss the use of self-referencing micro/nanosensors for measuring physiological activity of living cells/tissues in agricultural, environmental, and biomedical applications comprehensible to any scientist/engineer.     

Transport properties of plasticized polyvinyl chloride membranes based on alkylpyridinium alkylsulfates under conditions of diffusion mass transfer and constant current

Relationships are established for the permeability and flux of ionic surfactants with the concentration of electroactive compounds (EAC), the nature and concentration of solutions adjacent to the membrane, and membrane thickness. Values of permeability and ion fluxes decrease with decreasing EAC concentration and increasing membrane thickness. As the EAC concentration increases, i.e., as the number of charged centers in the membrane increases, permeabilities and ion fluxes also increase proportionally. The quantitative properties of membrane transport are an order of magnitude lower under conditions of diffusion mass transfer than with constant current.     

Membranes and microfluidics: a review

The integration of mass transport control by means of membrane functionality into microfluidic devices has shown substantial growth over the last 10 years. Many different examples of mass transport control have been reported, demonstrating the versatile use of membranes. This review provides an overview of the developments in this area of research. Furthermore, it aims to bridge the fields of microfabrication and membrane science from a membrane point-of-view. First the basic terminology of membrane science will be discussed. Then the integration of membrane characteristics on-chip will be categorized based on the used fabrication method. Subsequently, applications in various fields will be reviewed. Considerations for the use of membranes will be discussed and a checklist with selection criteria will be provided that can serve as a starting point for those researchers interested in applying membrane-technology on-chip. Finally, opportunities for microfluidics based on proven membrane technology will be outlined. A special focus in this review is made on the membrane properties of polydimethylsiloxane (PDMS), since this material is frequently used nowadays in master replication.     

锂离子电池多孔电极理论的回顾与新思考

本文总结了Newman多孔电极理论的基本内容,提出若干改进思路. 提出基于离子-空穴耦合传输机制描述浓电解质中的离子输运过程,在此基础上引入离子-电子耦合转移反应的思想处理电极材料中的离子传输问题,并通过计算嵌锂材料的离子扩散系数验证其合理性. 总结了描述多孔电极多尺度结构的相关理论和技术,表明均质化方法和基于结构重建的介观模拟方法均能给出比较合理的有效输运参数,从而提高多孔电极理论模拟结果的准确性.     

Highlights of 20?years of electrochemical measurements of exocytosis at cells and artificial cells

Advances in electrochemical methodology over the past 30?years have allowed chemical measurements to be made with decreasing amounts of analyte and at smaller spatial dimensions. This has allowed the investigation of single cells and single vesicles in cells either during release of chemical transmitter or separately. The cellular event called exocytosis can be measured with amperometry or cyclic voltammetry as discovered by Wightman and first published in 1990. In addition, the measurement of vesicle contents with electrochemistry is a new approach we have termed electrochemical cytometry. This involves isolation of intact vesicles, separation of the vesicles, and then lysing followed by coulometric analysis of the electroactive vesicle content. In this review, we will highlight work done by us and by others to discuss measurements of exocytosis at single cells and measurements at artificial cell models for studying the biophysical properties of vesicle membrane dynamics and lipid nanotubes connecting artificial cells using electrochemical methods.     

Modified electrodes from hydrophobic alkoxide silica gels—Insertion of electroactive compounds and glucose oxidase

Silica gels issued either from alkoxides or from colloidal silica are well known to be suitable materials for the immobilization of optically active probes or catalytically active chemicals. Recently it has been demonstrated that an enzyme like glucose oxidase could be immobilized into silica gels and glasses and still was able to retain a part of its activity.Recently, we have demonstrated that gels, either from silica or from transition metals, were appropriate mediums to perform electrochemistry, due to their high content of free solvent. It was a natural development of such studies to prepare modified electrodes from gels. We have developped two kinds of system.On one hand modified electrodes starting from hydrophobic DEDMS/TMMS (dimethoxydimethylsilane and trimethoxymethylsilane in various proportions) gels with small organic electroactive compounds incorporated have been prepared in which the electroactive probe is retained due to the hydrophobic balance of the gel. Such gels are adherent to the electrode and they present the electrochemistry of the electroactive probe included. Different conditions of preparation and drying of the gels are presented in relation with their influence on the behaviour of the modified electrode.On the other hand, glucose oxidase has been incorporated with a water soluble ferrocene as a mediator in silica gels made from colloids (NYACOL), and its catalytic activity has been experienced following the electrochemistry of the ferrocene. The electrochemical response in the presence of glucose is typical of the homogeneous response and the influence of the different parameters, governing the catalysis will be discussed.     

Challenges of electrochemistry research in Central Africa: The case of Cameroon

Electrochemistry is defined as the science which permits the transformation of matter using electricity. For many decades, this branch of chemistry has allowed substantial development in western world industries. The most emblematic example is the development of batteries, devices that find applications in almost all domains of our daily life. This review aims to focus on the state of electrochemistry in Central Africa; how electrochemistry is taught and used to improve the daily life of the population in Central Africa are the questions that we will endeavor to answer. We will scrutinize the different research groups in Central Africa having electrochemistry as their first line of research and an evaluation of their achievements so far will be performed, in addition to analyzing their societal impact.     

Electrochemical interfaces for chemical and biomolecular separations

The design of molecularly selective interfaces can lead to efficient electrochemically-mediated separation processes. The fast growing development of electroactive materials has resulted in new electroresponsive adsorbents and membranes, with enhanced selectivity, higher uptake capacities, and improved energy performance. Here, we review progress on the interfacial design for electrochemical separations, with a focus on chemical and biological applications. We discuss the development of new electrode materials and the underlying mechanisms for selective molecular binding, highlighting areas of growing interest such as metal recovery, waste recycling, gas purification, and protein separations. Finally, we emphasize the need for integration between molecular level interface design and electrochemical engineering for the development of more efficient separation processes. We envision that electrochemical separations can play a key role towards the electrification of the chemical industry and contribute towards new approaches for process intensification.     

Electroanalytical sensors for nonconducting media based on electrodes supported on perfluorinated ion-exchange membranes

Electroanalytical sensors, suitable for the analysis and monitoring of electroactive analytes present in gaseous phase or low-conductive liquid media, and based on electrodes in close contact with perfluorinated ion-exchange polymers are reviewed. The basic operative mechanism of these sensors, in which ion-exchange polymers act as solid polymer electrolytes (SPE's), is thoroughly discussed, while stressing the fundamental reasons why their behavior differs from that of conventional membrane electrodes. The procedures for preparing composite working electrodes by coating one side of ion-exchange membranes with stable porous films of conductive materials are described, along with the most common strategies followed to assemble this type of sensors. Useful examples of measurements in electrolyte-free media of inorganic and organic electroactive species of interest mainly for environmental analysis are given. Future prospects for the development of these sensors are also discussed.     

Electrochemical parameters and techniques in drug development, with an emphasis on quinones and related compounds

This review article summarizes recent applications of electrochemical techniques to redox-active drug development and mechanistic studies. It includes a general introduction to the use of electrochemistry in biology, with a focus on how electrochemistry can uniquely provide both kinetic and thermodynamic information. A number of studies are reported from the literature and the authors' laboratories, including the investigation of reactive oxygen species, biooxidative/bioreductive activation of pro-drugs, and DNA alkylation, with a particular emphasis on quinones and related compounds. Data from techniques ranging from traditional cyclic voltammetry to sophisticated single cell studies are presented. The examples herein presented illustrate how electrochemical, biochemical and medical knowledge can be integrated to develop strategies for the design and development of redox-selective therapeutics.