Electrochemical Signal Enhancement via Redox Cycling Involving Iron Oxide Magnetic Particles (Adaptable, Reversible Redox Reservoirs) and Its Application in Sensitive Cu2+ Detection
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Electrochemical signals may be affected by the presence of iron oxide magnetic particles (MPs) on an electrode owing to their distinct magnetic and redox properties. Recognizing their significance in electrochemical detection, we investigated the changes in electrochemical signals in the presence of MPs and their underlying causes. In the presence of MPs, the cyclic voltammograms of (quasi)reversible redox species (e.g., Fe(CN)64– and Ru(NH3)63+) exhibit different current-enhancing behaviors, depending on their formal potentials. Several redox species, such as Fe(CN)63– and Os(2,2′-bipyridyl)2Cl2, display non-zero initial currents at non-oxidizing or -reducing applied potentials in the presence of MPs. These findings are primarily attributed to the rapid redox reaction between the redox species and MP rather than the enhancement of mass transfer via magnetoconvection. The reaction between a redox species and an MP leads to a positive or negative shift in the equilibrium potential of the MP, which depends on the formal potential of the redox species. This enables MPs to act as adaptable, reversible redox reservoirs, facilitating current enhancement via redox cycling involving the MPs at specific potentials or during anodic and cathodic scanning. We applied signal enhancement via redox cycling to electrochemical Cu2+ detection. Cu2+ is rapidly reduced to Cu+ by the MPs during incubation, and Cu+ is then measured by using redox cycling. The calculated detection limit is approximately 15 nM, which is ∼100-fold lower than that observed without using MPs.Keywords:
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Redox titrations using robust aromatic oxidants allow for a quantitative analysis of redox and optical properties of organic electron donors in their oxidized states. Unlike spectroelectrochemistry, redox titrations can be performed without added electrolyte in relatively nonpolar solvents, affording quick access to the redox and optical properties of a given electron donor without the need of a complex electrochemical setup. However, the redox potentials obtained by the two methods are not the same. To establish the direction and magnitude of this discrepancy, we have performed a systematic case study using a set of tetraarylethylene donors and a tetrasubstituted hydroquinone ether cation radical ( T HEO +• ) as a stable aromatic oxidant. We show that redox potentials (especially second and higher oxidation potentials) measured by electrochemical methods are systematically lower compared with the redox potentials obtained by redox titrations in the absence of electrolyte, because of the enhanced stabilization of dications and polycations by electrolyte. We have also uncovered that the smaller cation radicals (e.g., a para‐hydroquinone or ortho‐hydroquinone ether cation radicals) are much more effectively stabilized when compared with the cation radicals in which charge is delocalized over larger area (e.g., tetraarylethylene cation radicals) in the presence of electrolyte because of increased ionic strength of the solution. Copyright © 2015 John Wiley & Sons, Ltd.
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Abstract Redox potential ( E h ) is an intensity measure of the reducing or oxidizing conditions in a system and reflects the tendency of ions or molecules in a solution to donate or accept electrons. It is an easily measured parameter to evaluate water chemistry. However, there is a large discrepancy between theoretical definition and measured E h values, leading to difficulties in correlating the measured E h with specific redox reactions or redox species in water. This is mainly because redox systems in natural environments are seldom at equilibrium, in addition to the presence of multiple redox components and many redox species are not electroactive. Thus, E h measurement only gives a qualitative indication of the electron richness or poorness of a system. If handled carefully by following proper procedures, E h data can indicate the progress of a system toward reduction or oxidation, delineate redox gradients in a stratified system, and assist in further determination of redox reactions. A review and summary of theoretical predictions, measurement, and the applications of E h in aqueous chemistry are presented.
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A simple theoretical expression for the redox potential of a redox couple (X/X−• or X•/X−) has been presented by using a modified Born equation and has been successfully applied to the organic and inorganic redox couples the reliable standard redox potentials of which are directly measured electrochemically, and to polarographic half-wave redox potentials of organic compounds. The expression has also been applied to inorganic redox couples the provisional standard redox potentials of which are estimated indirectly by using thermodynamic cycles, to examine the validity of the provisional potentials. At least, the provisional redox potentials, with which the potentials calculated approximately agree, seem to be reliable.
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This chapter contains sections titled: Solvent Effects on Various Types of Redox Reactions Fundamentals of Redox Reactions Solvent Effects on Redox Potentials and Redox Reaction Mechanisms Dynamical Solvent Effects on the Kinetics of Redox Reactions Redox Properties of Solvents and Potential Windows Redox Titrations in Non-Aqueous Solutions Titrations with Oxidizing Agents Titrations with Reducing Agents
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A redox flow lithium-oxygen battery (RFLOB) by using soluble redox catalysts with good performance was demonstrated for large-scale energy storage. The new device enables the reversible formation and decomposition of Li2O2 via redox targeting reactions in a gas diffusion tank, spatially separated from the electrode, which obviates the passivation and pore clogging of the cathode.
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Sulfonated carbon nanotubes (CNTs) was prepared by direct hydrothermal treatment in chlorosulfonic acid and employed as electrocatalysts for V3+/V2+ redox reaction for vanadium redox flow battery. The sulfonic groups introduced on the surface of CNTs not only significantly promote the accessibility of vanadium electrolyte, but also provide more active sites for V3+/V2+ redox reaction. Therefore, a series of sulfonated CNTs exhibit higher electrochemical activity and reversibility toward V3+/V2+ redox reaction compared with pristine CNTs. In particular, sulfonated CNTs treated for 10 hrs (CNTs-10) demonstrate the best electrocatalytic performance. The cell using CNTs-10 as negative catalysts achieves excellent charge-discharge performance with voltage efficiency and energy efficiency of 68.34% and 65.56% at a current density of 120 mA cm−2, respectively, which are higher than those (60.74% and 60.50%) for the pristine cell. The excellent electrocatalytic performance of sulfonated CNTs toward V3+/V2+ redox reaction is ascribed to the promoted electrochemical kinetic process of V3+/V2+ redox reaction and the accelerated mass transfer of vanadium ions.
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Fundamental differences between synthetic manganese clusters and the biological water oxidizing catalyst are demonstrated in the modulation of their redox potential by redox-inactive cations.
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Journal of the Chemical Society Faraday Transactions 1 Physical Chemistry in Condensed Phases (1989)
An electrochemical model based on the Wagner–Traud additivity principle has been used to predict the kinetics of catalysis of a redox reaction involving a Nernstian reduction reaction (Ox1+n1 e–→ Red1) coupled to an irreversible oxidation reaction (Red2→ Ox2+n2 e–). The mixture current (imix) flowing through a redox catalyst may be diffusion-controlled, partly diffusion-controlled or activation-controlled depending upon the mixture potential adopted by the redox catalyst. Kinetic equations are derived for each of these cases and predictions are made about the mixture current as a function of [Ox1], [Red1], catalyst surface area and temperature. In addition,xs a general method for reconstructing [Ox1]vs. time decay curves is described.
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