Kinetics of Heterogeneous Electron Transfer at Liquid/Liquid Interfaces As Studied by SECM

Unraveling Mechanisms of Highly Efficient Yet Stable Electrochemiluminescence from Quantum Dots

With CdSe/CdS/ZnS core/shell/shell quantum dots (QDs) as the model system, time- and potential-resolved spectroelectrochemical measurements are successfully applied for studying the general mechanisms and kinetics of electrochemiluminescence (ECL) generation. The rate constant of electron injection from the cathode into a QD to form a negatively charged QD (QD-) increases monotonically from -0.88 V to -1.2 V (vs Ag/AgCl). Mainly due to the deep LUMO of the QDs, the resulting QD- as the key intermediate for ECL generation is structurally stable and possesses very slow spontaneous deionization channels. The latter (the main non-ECL channels) are usually 3-4 orders of magnitude slower than the rate constant of the successive hole injection from an active co-reactant into a QD-. The kinetic studies quantify the internal ECL quantum yield of ideal QD ECL emitters to be nearly identical to that of photoluminescence, which is near unity for the current system. Identification of the key intermediate, discovery of the related elementary steps, and determination of all rate constants not only establish a general framework for understanding ECL generation but also offer basic design rules for ECL emitters.

Potential-Modulated Ion Distributions in the Back-to-Back Electrical Double Layers at a Polarised Liquid|Liquid Interface Regulate the Kinetics of Interfacial Electron Transfer

Biphasic interfacial electron transfer (IET) reactions at polarisable liquid|liquid (L|L) interfaces underpin new approaches to electrosynthesis, redox electrocatalysis, bioelectrochemistry and artificial photosynthesis. Herein, using cyclic and alternating current voltammetry, we demonstrate that under certain experimental conditions, the biphasic 2-electron O2 reduction reaction can proceed by single-step IET between a reductant in the organic phase, decamethylferrocene, and interfacial protons in the presence of O2. Using this biphasic system, we demonstrate that the applied interfacial Galvani potential differenceΔ o w φ provides no direct driving force to realise a thermodynamically uphill biphasic IET reaction in the mixed solvent region. We show that the onset potential for a biphasic single-step IET reaction does not correlate with the thermodynamically predicted standard Galvani IET potential and is instead closely correlated with the potential of zero charge at a polarised L|L interface. We outline that the appliedΔ o w φ required to modulate the interfacial ion distributions, and thus kinetics of IET, must be optimised to ensure that the aqueous and organic redox species are present in substantial concentrations at the L|L interface simultaneously in order to react.

In Situ Probing Liquid/Liquid Interfacial Kinetics through Single Nanodroplet Electrochemistry

In this study, we report the new application of single nanodroplet electrochemistry to in situ monitor the interfacial transfer kinetics of electroactive species across liquid/liquid interface. Interfacial kinetic information is crucial in drug delivery and membrane transport. However, interfacial information has been mainly studied thermodynamically, such as partition coefficient, which could not manifest a speed of transfer. Herein, we measure the phase-transfer kinetic constant via the steady-state electrochemistry of an extracted redox species in a single nanodroplet. The redox species were transferred from the continuous oil phase to the water nanodroplet by partition equilibrium. The transferred redox species are selectively electrolyzed within the droplet when the droplet contacts with an ultramicroelectrode, while the electrochemical reaction of the redox species outside the droplet (i.e., organic solvent) is effectively suppressed by adjusting the electrolyte composition. The redox species in the water droplets can quickly attain a steady state during electrolysis owing to an extensive mass transfer by radial diffusion, and the steady-state current can be analyzed to obtain kinetic information with help from the finite-element method. Finally, a quick calculation method is suggested to estimate the kinetic constant of phase transfer without simulation.

Electron Transfer Mechanism at the Oil/Water Interface Revealed by Multidimensional Free Energy Calculations

The electron transfer (ET) reaction of ferrocene and ferricyanide at the water-dichloromethane interface, a typical liquid-liquid ET in electrochemistry, was intensively investigated with a three-dimensional free energy surface that fully describes the transport, association, and solvent fluctuation in the ET processes. The calculated free energy surface provides the comprehensive picture of the ET mechanism at the liquid-liquid interface. The present calculation revealed the heterogeneous route of ET that takes place over the interface, rather than the homogeneous one. The present conclusion is found to be consistent with previous results of electrochemical experiments by careful re-examination of the analysis.

Microelectrochemical Techniques for Probing Kinetics at Liquid/Liquid Interfaces

We provide an overview of recent advances in microelectrochemical approaches to investigate the kinetics of various physicochemical processes that occur at the interface between two immiscible electrolyte solutions (ITIES). To place the advances in context, background material on the structure of the ITIES, derived from both experimental studies and computer simulation, is also provided. The main focus of the article is micro-ITIES techniques, single droplet measurements, microelectrochemical measurements at expanding droplets (MEMED) and scanning electrochemical microscopy (SECM). Recent developments in a combined SECM-Langmuir trough technique for probing diffusion processes across Langmuir monolayers at the water/air (W/A) interface are also highlighted, by considering an organic monolayer at a water surface as a special case of a liquid/liquid interface.

Correlating Live Cell Viability with Membrane Permeability Disruption Induced by Trivalent Chromium

Cr(III) is often regarded as a trace essential micronutrient that can be found in many dietary supplements due to its participation in blood glucose regulation. However, increased levels of exposure have been linked to adverse health effects in living organisms. Herein, scanning electrochemical microscopy (SECM) was used to detect variation in membrane permeability of single cells (T24) resulting from exposure to a trivalent Cr-salt, CrCl₃. By employing electrochemical mediators, ferrocenemethanol (FcMeOH) and ferrocenecarboxylic acid (FcCOO^-), initially semipermeable and impermeable, respectively, complementary information was obtained. Three-dimensional COMSOL finite element analysis simulations were successfully used to quantify the permeability coefficients of each mediator by matching experimental and simulated results. Depending on the concentration of Cr(III) administered, three regions of membrane response were detected. Following exposure to low concentrations (up to 500 μM Cr(III)), their permeability coefficients were comparable to that of control cells, 80 μm/s for FcMeOH and 0 μm/s for FcCOO^-. This was confirmed for both mediators. As the incubation concentrations were increased, the ability of FcMeOH to permeate the membrane decreased to a minimum of 17 μm/s at 7500 μM Cr(III), while FcCOO^- remained impermeable. At the highest examined concentrations, both mediators were found to demonstrate increased membrane permeability. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide cell viability studies were also conducted on Cr(III)-treated T24 cells to correlate the SECM findings with the toxicity effects of the metal. The viability experiments revealed a similar concentration-dependent trend to the SECM cell membrane permeability study.

Analysing single live cells by scanning electrochemical microscopy

Scanning electrochemical microscopy (SECM) offers single live cell activities along its topography toward cellular physiology and pathology.

Behaviors of the Interfacial Consecutive Multistep Electron Transfer Controlled by Varied Transition Metal Ions in Porphyrin Cores

Almost all life activities involve the process of multistep electron transfer (ET) which occurs on biomembrane. Metalloporphyrins (MTPPs) are a class of molecules which are closely related to life course. Here, the n-step (n = 1, 2) ET behaviors controlled by different metal ions in porphyrin cores were investigated by thin-layer cyclic voltammetry (TLCV). The bimolecular ET was reacted between the MTPP (M = Fe, Zn, Co, Cu, Ni) and Fe(CN)₆^4- in nitrobenzene and aqueous phase, respectively, and the interface between nitrobenzene and aqueous phase was considered as a bionic membrane. The thin-layer theory, which has been revised, was used to calculate the kinetic constants for each step electron transfer reactions. It was shown that the kinetic data were affected dramatically by the different coordinated ions in porphyrin complexes.

Investigation of proton-driven amine functionalized tube array as ion responsive biomimetic nanochannels

A simple amine embellished tube array was assembled at the liquid–liquid interface to study ion transfer behavior.

A polarized liquid–liquid interface meets visible light-driven catalytic water oxidation

A nanostructured BiVO₄photocatalyst assembled at a chemically polarized liquid–liquid interface generates an efficient amount of O₂with a [Co(bpy)₃]³⁺scavenger in the organic phase.

Novel organic solvents for electrochemistry at the liquid/liquid interface

The 5-nonanone organic phase has been shown to be a stable solvent for use in electrochemistry, ion and electron transfer has been studied at the organic/water interface.

Reaction dynamics at liquid interfaces

The liquid interface is a narrow, highly anisotropic region, characterized by rapidly varying density, polarity, and molecular structure. I review several aspects of interfacial solvation and show how these affect reactivity at liquid/liquid interfaces. I specifically consider ion transfer, electron transfer, and SN2 reactions, showing that solvent effects on these reactions can be understood by examining the unique structure and dynamics of the liquid interface region.

Localized electron transfer and the effect of tunneling on the rates of Ru(bpy)3(2+) oxidation and reduction as measured by scanning electrochemical microscopy

The kinetics of tris(2,2'-bipyridine)ruthenium(II) (Ru(bpy)) oxidation and reduction in acetonitrile were investigated by steady-state voltammetry using scanning electrochemical microscopy (SECM). The SECM setup was placed inside a drybox for carrying out experiments in an anhydrous atmosphere and in the absence of oxygen. The standard rate constant, k°, for Ru(bpy) oxidation at a Pt electrode (radius, a = 5 μm) was 0.7 ± 0.1 cm/s, which is smaller than k° for Ru(bpy) reduction measured under the same conditions (≥3 cm/s). This is attributed to the 2,2'-bipyridine ligands having an electron-transfer (ET) blocking effect on the oxidation of the ruthenium(II) center, as opposed to the reduction, which involves ET to the exposed ligands. Thus, tunneling effects may be important in considering the ET in this molecule.

Beyond the Butler-Volmer equation. Curved Tafel slopes from steady-state current-voltage curves

We report the discovery and analysis of curved Tafel slopes from the electrochemical reduction of hexamminecobalt(III) under steady-state conditions. In order to confirm the existence of the curvature, random assemblies of carbon microelectrodes (RAM™ electrodes) were employed to obtain experimental data over more than three orders of magnitude, without significant double layer charging currents and without ohmic distortion. Since the rate-determining step in the reduction reaction is electron transfer, and no ligand substitution reactions occur on the timescale of experiments, the curvature of the Tafel plot is attributed to the dependence of the symmetry factor on electrode potential.

Effects of cadmium on photosynthetic oxygen evolution from single stomata in Brassica juncea (L.) Czern

Scanning electrochemical microscopy (SECM) was utilized to investigate photosynthetic oxygen evolution from single stomata in leaves of live Brassica juncea (L.) Czern cultured in nutrient solution to which 0.2 or 0.01 mM CdC12 had been added. The bulk leaf surface serves as an insulator normally; therefore, a typical negative feedback was observed on the probe approach curves (PACs) when the probe approached epidermal cells. When the probe tip approached an open stoma, a higher tip current was detected due to the O2 release from this stoma. Thus, SECM can be used to map the O2 concentration profile near the leaf surface and study stomatal complex structure size and density. The oxygen release from single stomata was also analyzed by comparison of experimental PACs with those simulated by COMSOL multiphysics software (version 3.4). In addition to an increase in the stomatal complex size and a decrease in the complex density, the Cd accumulation caused up to a 26% decrease in photosynthetic rate determined at the level of a single stoma. The O2 evolution was also monitored by recording the tip current vs time when a tip sat above the center of a stoma. Periodic peaks in O2 release-time curves were observed, varying from 400 to 1600 s. The opening and closing activities of single stomata were also imaged by SECM.

Scanning electrochemical microscopy

This review describes work done in scanning electrochemical microscopy (SECM) since 2000 with an emphasis on new applications and important trends, such as nanometer-sized tips. SECM has been adapted to investigate charge transport across liquid/liquid interfaces and to probe charge transport in thin films and membranes. It has been used in biological systems like single cells to study ion transport in channels, as well as cellular and enzyme activity. It is also a powerful and useful tool for the evaluation of the electrocatalytic activities of different materials for useful reactions, such as oxygen reduction and hydrogen oxidation. SECM has also been used as an electrochemical tool for studies of the local properties and reactivity of a wide variety of materials, including metals, insulators, and semiconductors. Finally, SECM has been combined with several other nonelectrochemical techniques, such as atomic force microscopy, to enhance and complement the information available from SECM alone.