22 research outputs found

    Ion atmosphere relaxation controlled electron transfers in cobaltocenium polyether molten salts

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    A room-temperature redox molten salt for the study of electron transfers in semisolid media, based on combining bis(cyclopentadienyl)cobalt with oligomeric polyether counterions, [Cp2Co](MePEG350SO3), is reported. The transport properties of the new molten salt can be varied (plasticized) by varying the polyether content. The charge transport rate during voltammetric reduction of the ionically conductive [Cp2Co](MePEG350SO3) molten salt exceeds the actual physical diffusivity of [Cp2Co]+ because of rapid [Cp2Co]+/0 electron self-exchanges. The measured [Cp2Co]+/0 electron self-exchange rate constants (kEX) are proportional to the diffusion coefficients (DCION) of the counterions in the melt. The electron-transfer activation barrier energies are also close to those of ionic diffusion but are larger than those derived from optical intervalent charge-transfer results. Additionally, the [Cp2Co]+/0 rate constant results are close to those of dissimilar redox moieties in molten salts where DCION values are similar. All of these characteristics are consistent with the rates of electron transfers of [Cp2Co]+/0 (and the other donor−acceptor pairs) being controlled not by the intrinsic electron-transfer rates but by the rate of relaxation of the ion atmosphere around the reacting pair. In the low driving force regime of mixed-valent concentration gradients, the ion atmosphere relaxation is competitive with electron transfer. The results support the generality of the recently proposed model of ionic atmosphere relaxation control of electron transfers in ionically conductive, semisolid materials

    Electrochemical mapping reveals direct correlation between heterogeneous electron-transfer kinetics and local density of states in diamond electrodes

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    Conducting carbon materials: A multi-microscopy approach shows that local heterogeneous electron-transfer rates at conducting diamond electrodes correlate with the local density of electronic states. This model of electroactivity is of considerable value for the rational design of conducting diamond electrochemical technologies, and also provides key general insights on electrode structure controls in electrochemical kinetic

    Intrinsic electrochemical activity of single walled carbon nanotube–Nafion assemblies

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    The intrinsic electrochemical properties and activity of single walled carbon nanotube (SWNT) network electrodes modified by a drop-cast Nafion film have been determined using the one electron oxidation of ferrocene trimethyl ammonium (FcTMA+) as a model redox probe in the Nafion film. Facilitated by the very low transport coefficient of FcTMA+ in Nafion (apparent diffusion coefficient of 1.8 × 10−10 cm2 s−1), SWNTs in the 2-D network behave as individual elements, at short (practical) times, each with their own characteristic diffusion, independent of neighbouring sites, and the response is diagnostic of the proportion of SWNTs active in the composite. Data are analysed using candidate models for cases where: (i) electron transfer events only occur at discrete sites along the sidewall (with a defect density typical of chemical vapour deposition SWNTs); (ii) all of the SWNTs in a network are active. The first case predicts currents that are much smaller than seen experimentally, indicating that significant portions of SWNTs are active in the SWNT–Nafion composite. However, the predictions for a fully active SWNT result in higher currents than seen experimentally, indicating that a fraction of SWNTs are not connected and/or that not all SWNTs are wetted completely by the Nafion film to provide full access of the redox mediator to the SWNT surface

    Development and Applications of Nanoscale Scanning Electrochemical Microscopy

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    DEVELOPEMENT AND APPLICATIONS OF NANOSCALE SCANNING ELECTROCHEMICAL MICROSCOPY Jiyeon Kim, PhD University of Pittsburgh, 2012 After more than 20 years of basic nanoscience research, advances in nanotechnology have opened up unprecedented possibilities and opportunities in electrochemistry. Especially, fabrication, characterization, modification and the understanding of various electrochemical interfaces or electrochemical processes at the nanoscale have led to applications of electrochemical methods to novel technologies. Nanoscale characterization and theoretical analysis of electrochemical interfaces and reactions can lead to the understanding of these complicated chemical systems at the molecular level. This is not only scientifically interesting, but also crucial for the controlled applications of electrochemistry in nanotechnology. A theme of my PhD work is to seek the better understanding of important nanosystems such as single walled carbon nanotube (SWNT) and nanopores in biological as well as artificial nanoporous membrane. The understanding of the electrochemistry of carbon nanotubes as an attractive electrode material for electroanalysis and electrocatalysis is fundamentally and practically important. Also, the greater understanding of nucleocytoplasmic transport through the nuclear pores in nuclear envelope is highly significant because of its critical roles as a regulator of gene expression, a gateway for gene delivery, and a model of biomimetic transport systems. In addition, the quantitative understanding of membrane permeability at a single nanopore level is a prerequisite for the development and the application of nanoporous membrane for nanofiltration, biomedical devices, nano fluidics, and biomimetic membrane transport. To achieve these goals, I developed scanning electrochemical microscopy (SECM) as a powerful nanoscale tool and applied this technology to the kinetic study and high-resolution imaging of heterogeneous reactions at various interfaces. Therefore, this thesis is based on two sections. In the first section, I summarize the application of nanoscale SECM to the study of a few different nanostructures and the substantial findings. The second section is concerned about the development of nanoscale SECM. Based on these achievements, the capacity of nanoscale SECM will be greatly increased to characterize and understand various nanomaterials and interfaces at the nanoscale

    New approaches to the study of biophysicochemical processes

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    This thesis is concerned with the study of biophysicochemical processes using electrochemistry and related techniques. The first part of the thesis discusses the electrochemical detection of biological species, and characterisation of the electrode materials employed. A comparison of two novel forms of carbon electrode, namely carbon nanotubes and polycrystalline boron doped diamond (pBDD), with more conventional carbon electrode materials reveals their enhanced characteristics for bioelectrochemistry, with improved sensitivity and resistance to fouling. These materials are further characterised using novel high-resolution electrochemical imaging methods, to determine heterogeneous electron transfer rates for a number of different redox species. The kinetic rate constants are determined from measured electrochemical currents using finite element method (FEM) modelling, which proves to be a powerful technique for the quantitative analysis of intrinsic system parameters that cannot be studied directly. The electrochemical response of isolated regions of pristine SWNTs is investigated using scanning electrochemical cell microscopy, demonstrating high electrochemical activity at the nanotube sidewalls. A similar analysis of the different facets of pBDD is performed using intermittent contact scanning electrochemical microscopy coupled with FEM simulations, revealing that the electroactivity is strongly influenced by the local density of states of the material. New techniques are also presented for the investigation of transport processes at membrane interfaces. A new method of bilayer formation is developed, which overcomes many of the limitations of current techniques, and is used to investigate the permeation rates of a series of aliphatic carboxylic acids. Using confocal laser scanning microscopy (CLSM) with a pH-sensitive uorophore, the pH change as a weak acid permeates across the bilayer can be visualised, and the permeation coefficient determined by comparison with FEM simulations. This reveals a trend of increasing permeability with lipophilicity. Finally, CLSM is used to study the lateral diffusion of protons at lipid bilayers and other surfaces. Protons are generated galvanostatically by a UME positioned close to the substrate, altering the local pH which can be visualised by means of a pH-sensitive uorophore. The uorescence profile is again compared to FEM simulations, allowing the lateral diffusion coefficient to be determined

    Electrochemical Metabolic Profiling of Live Cells and Pancreatic Islets

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    Electron transport dynamics in room temperature redox molten salts and chemistry of monolayerprotected AU nanoparticles

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    Chapter One briefly introduces the basics of redox molten salts and monolayer-protected Au nanoparticles. Chapter Two examines the mass transport of counterions in molten salts of ruthenium complexes where the counterions are mixtures of perchlorate and iodide ions. The average diffusion coefficients of the counterions were obtained by ionic conductivity impedance measurements, while that of iodide (as a surrogate for perchlorate ion transport) was measured directly using iodide voltammetry. Agreement between the conductivity-based and Faradaic counter ion transport data provides a quantitative validation of previous use of ionic conductivity data in electron transfer dynamics study in redox semi-solids. Chapter Three focuses on the ligand-exchange reaction of phenylethanethiolate (SC2Ph) ligands on 1.1 nm Au nanoparticles with varied amounts of triphenylphosphine. The results from UV-Vis, NMR and electrochemistry suggest that the reaction liberate AuISC2Ph complexes (accompanying core size change), as opposed to SC2Ph thiols, the common out-coming ligands in thiolat-to-thiolate ligand exchange reaction on Au nanoparticles. Chapter Four explores a room-temperature Au38 nanoparticle polyether melt in which voltammetric and chronoamperometric, and impedance measurements have been made, respectively, of the rates of electron and ion transport in the melt. The measured rates of electron and of electrolyte ion transport are very similar, as are their thermal activation energy barriers, observations that are consistent with a recently introduced ionic atmosphere relaxation model for control of electron transfer in redox polyethers. Chapter Five describes ferrocenated imidazolium molten phases where ferrocene is chemically linked to various dialkylimidazoliums. The physical properties, including density, fluidity, and ionic characteristics, are discussed. The electrochemical results are presented in Chapter Six. It is observed that the electron diffusion in structurally-different ferrocenated imidazoliums is more efficient than the physical transport of redox ions. The rate of electron transfer is linearly correlated to the counterion diffusion, the first observation from imidazolium-based redox semi-solids consistent with the counterion relaxation control of electron transfer model. Chapter Seven investigates the surface properties of Au nanoparticle films contacted with imidazlium ionic liquids. A dynamic contact angle change is observed and explained on the basis of anion penetration which is further compared to the formally similar electrowetting phenomenon

    Development and application of pipet-based electrochemical imaging techniques

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    This thesis describes the development of an electrochemical scanned probe microscope, SECCM, outlining the need for such a development, by highlighting previous techniques and their limitations. SECCM consists of a double barrel capillary pulled to small dimensions, filled with electrolyte solution and a redox mediator of choice, with a QRCE is inserted into each channel. A potential is applied between the QRCEs, whilst modulating the pipet normal to the surface. The probe is translated towards the surface and once contact is established, a modulation in the ion current arises due to the physical oscillation of the probe, which is then used as a feedback parameter for imaging. The potential at the working electrode substrate is also controlled. SECCM is introduced using a model test substrate, gold bands on glass, showing that the probe is able to track topographical features, making simultaneous electrochemical measurements. Ion conductance measurements between the two QRCEs, are shown to be sensitive to the nature of the substrate investigated. The fundamental electrochemical behaviour of CVD graphene and SWNT is investigated. A multimicroscopy approach is used for CVD graphene studies, correlating surface structure and activity, deducing heterogeneous electron transfer kinetics through simulation. The SWNT samples are studied in two different morphologies: as 3D forests; and, as a 2D network. In the forests, the probe is positioned at the ends and sidewalls, making spot measurements. The voltammetric behaviour shows very similar responses, whilst in the network, a nanosized probe is scanned across the surface, showing relatively uniform activity across an entire tube. These new insights indicate that SWNTs are highly active electrode materials. The fabrication and characterisation of SECM-SICM probes, in a straightforward manner is also presented. These types of probes were found to be ideal for the investigation of biological samples, being extremely easy and quick to fabricate
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