Heterogeneous Electron Transfer Research Articles

Semi-microscopic Theory for the Current Rectification Phenomenon in Nanogap Molecular Devices.

A semi-microscopic theory is developed for heterogeneous electron transfer (HET) kinetics based on the energy level alignment approach at self-assembled monolayer (SAM) covered metal electrodes. Theory provides the electronic and molecular property-dependent equations for the HET rate constant (k0) and the transfer coefficient (α) for potential. k0 is formulated using the activation free energy as a product of the SAM covered metal work function (WF) and fractional electronic charge exchanged at the transition state (attained through the alignment of the frontier molecular orbital (FMO) energy level of the electroactive group with the WF of metal). k0 is a function of the metal jellium electronic screening length and dielectric and of the molecular self-assembly (through its dipole moment, size, and packing density) and the FMO energies of electroactive groups. The operative potential at the transition state is governed by α, which is a function of molecular spacer length and characteristic electronic-dipolar coupling length. The current rectification phenomenon in nanogap molecular devices is theoretically analyzed using equations for k0 and α for SAM covered source and drain electrodes. Theory unravels the LUMO or HOMO dichotomy for a given metal: (i) for the HOMO assisted ET, the metal with a high WF has a high current rectification ratio (RR), while (ii) for the LUMO assisted ET, the metal with a low WF has a high current RR in asymmetrical devices. Theory predicts the reversal in current rectification by altering the dipole moment of the anchoring molecule, the HOMO/LUMO energy of the electroactive groups, and the nature of the metal. Finally, theory shows qualitative and quantitative coherence with the reported experimental current-potential response of molecular device.

Room-Temperature Ionic Liquid Cation Effects on the Structure and Stability of Anionic Lanthanide Complexes.

Room-temperature ionic liquids (RTILs) are a subset of molten salts that are liquids at room temperature and may offer an elegant, low-temperature route to predicting the properties of solvated metal complexes in their high-temperature analogues. This work studied the chemistry of chloride anion-containing RTILs to determine their similarity to inorganic molten chloride salts. The behaviors of complexes of Mn, Nd, and Eu were evaluated in a variety of chloride RTILs by absorption spectrophotometry and electrochemistry to elucidate trends in cation effects on the coordination geometry and redox properties of the solvated species. Spectrophotometric data indicated the metals are present as anionic complex (e.g., MnCl42- and NdCl63-) analogous to those observed in molten chloride salts. Strongly polarizing, charge-dense RTIL cations induced distortions to the symmetry of these complexes, resulting in lower oscillator strengths and red-shifted energies for the observed transitions. Cyclic voltammetry experiments were used to characterize the Eu(III/II) redox couple producing diffusion coefficients on the order of 10-8 cm2 s-1 and heterogeneous electron transfer rate constants ranging between 6 × 10-5 and 2 × 10-4 cm s-1. The E1/2 potentials for Eu(III/II) were also found to shift positively with increasing cation polarization power, stabilizing the Eu(II) oxidation state by removing electron density from the metal center over chloride bond networks. Both the optical spectrophotometry and electrochemistry results suggest that the polarization strength of an RTIL cation plays a major role in the geometry and stability of a metal complex.

Voltammetric Evidence of Proton Transport through the Sidewalls of Single-Walled Carbon Nanotubes.

Understanding ion transport in solid materials is crucial in the design of electrochemical devices. Of particular interest in recent years is the study of ion transport across 2-dimensional, atomically thin crystals. In this contribution, we describe the use of a host-guest hybrid redox material based on polyoxometalates (POMs) encapsulated within the internal cavities of single-walled carbon nanotubes (SWNTs) as a model system for exploring ion transport across atomically thin structures. The nanotube sidewall creates a barrier between the redox-active molecules and bulk electrolytes, which can be probed by addressing the redox states of the POMs electrochemically. The electrochemical properties of the {POM}@SWNT system are strongly linked to the nature of the cation in the supporting electrolyte. While acidic electrolytes facilitate rapid, exhaustive, reversible electron transfer and stability during redox cycling, alkaline-salt electrolytes significantly limit redox switching of the encapsulated species. By "plugging" the {POM}@SWNT material with C60-fullerenes, we demonstrate that the primary mode of charge balancing is proton transport through the graphenic lattice of the SWNT sidewalls. Kinetic analysis reveals little kinetic isotope effect on the standard heterogeneous electron transfer rate constant, suggesting that ion transport through the sidewalls is not rate-limiting in our system. The unique capacity of protons and deuterons to travel through graphenic layers unlocks the redox chemistry of nanoconfined redox materials, with significant implications for the use of carbon-coated materials in applications ranging from electrocatalysis to energy storage and beyond.

Direct Electrochemistry of Glucose Dehydrogenase-Functionalized Polymers on a Modified Glassy Carbon Electrode and Its Molecular Recognition of Glucose.

A glucose biosensor was layer-by-layer assembled on a modified glassy carbon electrode (GCE) from a nanocomposite of NAD(P)+-dependent glucose dehydrogenase, aminated polyethylene glycol (mPEG), carboxylic acid-functionalized multi-wall carbon nanotubes (fMWCNTs), and ionic liquid (IL) composite functional polymers. The electrochemical electrode was denoted as NF/IL/GDH/mPEG-fMWCNTs/GCE. The composite polymer membranes were characterized by cyclic voltammetry, ultraviolet-visible spectrophotometry, electrochemical impedance spectroscopy, scanning electron microscopy, and transmission electron microscopy. The cyclic voltammogram of the modified electrode had a pair of well-defined quasi-reversible redox peaks with a formal potential of -61 mV (vs. Ag/AgCl) at a scan rate of 0.05 V s-1. The heterogeneous electron transfer constant (ks) of GDH on the composite functional polymer-modified GCE was 6.5 s-1. The biosensor could sensitively recognize and detect glucose linearly from 0.8 to 100 µM with a detection limit down to 0.46 μM (S/N = 3) and a sensitivity of 29.1 nA μM-1. The apparent Michaelis-Menten constant (Kmapp) of the modified electrode was 0.21 mM. The constructed electrochemical sensor was compared with the high-performance liquid chromatography method for the determination of glucose in commercially available glucose injections. The results demonstrated that the sensor was highly accurate and could be used for the rapid and quantitative determination of glucose concentration.

Acceptor properties of "carbon nanotubes-redox-active polymer based on bovine serum albumin modified with ferrocenecarboxaldehyde" composite for creating a BOD biosensor with Blastobotrys adeninivorans BKM Y-2677 yeast.

The possibility of using a composite material based on bovine serum albumin (BSA) covalently bonded with ferrocenecarboxaldehyde and containing carbon nanotubes (CNT) for the immobilization of Blastobotrys adeninivorans BKM Y-2677 (B. adeninivorans) yeast is discussed. The optimal ratio of ferrocenecarboxaldehyde to BSA for the redox-active polymer synthesis is 1:2, since the heterogeneous electron transfer constant is 0.45 ± 0.01s-1. When carbon nanotubes (CNTs) are added to this polymer, the heterogeneous electron transfer constant increases: at a CNT specific density of 2.5µg/mm2, it reaches a maximum value of 0.55 ± 0.01s-1. The addition of CNTs into the conducting system leads to increasing of the rate constant of interaction redox species with B. adeninivorans yeast by an order: the rate constant of interaction between B. adeninivorans yeast and electroactive particles in a redox-active polymer is 0.056 ± 0.005 dm3/g × s and in a composite material based on CNTs is 0.51 ± 0.02 dm3/g × s. The yeast specific density at the electrode of 0.1mg/mm2 and electrolyte pH of 6.2 was chosen as the working value for the receptor system operation. Immobilized in a composite material, yeast oxidizes a wider range of substrates compared with a similar receptor element based on the ferrocene mediator. The biosensors formed on the basis of hybrid polymers have a high sensitivity with a lower limit of determined concentrations of 1.5mg/dm3 with an assay time of 5min and a high correlation (R = 0.9945) with the results of the standard method for determining biochemical oxygen demand (BOD) in nine real surface water samples of the Tula region.

Probing Redox Properties of Extreme Concentrations Relevant for Nonaqueous Redox-Flow Batteries

Redox-flow batteries are an emerging energy storage technology that can pair with intermittent renewable energy technologies. There remains a need, however, to understand physicochemical relationships among the solvent, electrolyte salt, and redox-active molecules that comprise catholyte and anolyte solutions. To examine this relationship, we detail a systematic study wherein the concentrations of the redox-active molecule 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) and TBAPF6 electrolyte salts are varied over concentrations of 1 mM to over 1000 mM in acetonitrile. Three series were investigated: (1) varying the concentration of TEMPO while holding the concentration of TBAPF6 constant, (2) varying the concentration of TBAPF6 while holding the concentration of TEMPO constant, and (3) varying both the concentration of TEMPO and TBAPF6 with a 5:1 TBAPF6:TEMPO ratio. Cyclic voltammetry data from macro- and microelectrodes were used to quantify diffusion coefficients and heterogeneous electron transfer rates, and these metrics were connected to the conductivity and viscosity to develop clear trends over the entire concentration range. Fundamental chemical interactions that lead to changes in physical properties were implicated via vibrational spectroscopy and molecular dynamics (MD) simulations. Trends in conductivity and viscosity for systems were inversely related and correlated to trends in diffusion coefficients and heterogeneous electron transfer rates. Intuitively, faster diffusion and electron-transfer rates occurred with lower TEMPO concentrations and higher TBAPF6 concentrations, with the majority of conditions falling in the general proximity of literature values (k0 = 0.1–0.5 cm/s, D ≈ (2.0–4.0) × 10–5 cm2/s). At the highest TBAPF6 concentrations, vibrational spectroscopy and MD simulations show that intermolecular interactions were more nuanced, and solvation and ion-pairing effects begin to influence electrochemical and physical properties. This functional approach including electrochemical and physical characterization paired with MD simulations provides a template for methodically studying systems for redox flow battery applications.

Decoupling Effects of Electrostatic Gating on Electronic Transport and Interfacial Charge-Transfer Kinetics at Few-Layer Molybdenum Disulfide.

The electronic properties of electrode materials play a crucial role in defining their electrochemical behavior in energy conversion and storage devices. The assembly of van der Waals heterostructures and fabrication into mesoscopic devices enable the dependence of an electrochemical response on electronic properties to be systematically interrogated. Here, we evaluate the effect of charge carrier concentration on heterogeneous electron transfer at few-layer MoS2 electrodes by combining spatially resolved electrochemical measurements with field-effect electrostatic manipulation of band alignment. Steady-state cyclic voltammograms and finite-element simulations reveal a strong modulation of the measured electrochemical response for outer-sphere charge transfer at the electrostatic gate voltage. In addition, spatially resolved voltammetric responses, obtained at a series of locations at the surface of few-layer MoS2, reveal the governing role of in-plane charge transport on the electrochemical behavior of 2D electrodes, especially under conditions of low carrier densities.

Electrochemistry at Back-Gated, Ultrathin ZnO Electrodes: Field-Effect Modulation of Heterogeneous Electron Transfer Rate Constants by 30× with Enhanced Gate Capacitance.

We report steady-state voltammetry of outer-sphere redox species at back-gated ultrathin ZnO working electrodes in order to determine electron transfer rate constants kET as a function of independently controlled gate bias, VG. We demonstrate that kET can be modulated as much as 30-fold by application of VG ≤ 8 V. The key to this demonstration was integrating the ultrathin (5 nm) ZnO on a high dielectric constant (k) insulator, HfO2 (30 nm), which was grown on a Pd metal gate. The high-k HfO2 dramatically decreased the required VG values and increased the gate-induced charge in ZnO compared to previous studies. Importantly, the enhanced gating power of the Pd/HfO2/ZnO stack meant it was possible to observe a nonmonotonic dependence of kET on VG, which reflects the inherent density of redox acceptor states in solution. This work adds to the growing body of literature demonstrating that electrochemical kinetics (i.e., rate constants and overpotentials) at ultrathin working electrodes can be tuned by VG, independent of the conventional electrochemical working electrode potential.

Effect of adsorption of pyocyanin on the electron transfer rate at the interface of a glassy carbon electrode

Redox mediators (RM) are an alternative to improve the performance of microbial electrochemical systems (MES) as they help significantly to develop the indirect electron transfer. However, their use is limited when the redox processes between the RM and the electrode surface are not thoroughly understood. The adsorption, the intermolecular interactions with the electrode, and the effects on charge transfer kinetics are important issues in MES such as microbial sensors, microbial electrosynthesis or microbial fuel cells, but are rarely analyzed. Pyocyanin, considered an RM, may present these features due to its chemical structure. Today, the electrochemical studies indicate that pyocyanin can presents adsorption processes on glassy carbon electrodes. However, the effects of adsorption on electron transfer rate and changes as a function of pyocyanin concentration have not yet been studied. Therefore, this work presents an electrochemical study of pyocyanin to determine the effect of concentration and adsorption on the electron transfer rate at a glassy carbon electrode. Analysis of the electrochemical profiles obtained by cyclic voltamperometry (CV) indicates that there is an effect of concentration (C*) on the heterogeneous electron transfer rate constant (k0) since the criterion ΔEp varies as a function of C*. In addition, the peak proportionality parameter Ipa/Ipc and the voltamperometric function Ip/v½ help deduct the adsorption to the electrode. Experimental data obtained by chronocoulometry (CC) and CV, indicate that k0 = 5.7 × 1011 m/s and Rct = 3.40 × 10−16 Ω under the C* = 0.14 mM condition, while k0 = 1.10 × 103 m/s and Rct = 1.60 × 10−7 Ω under the C* = 0.49 mM condition. The differences in k0 and Rct values at different values of C* are due to an accumulation of pyocyanin on the electrode surface in the inner Helmholtz plane (IHP), preventing the electron transfer of bulk-dissolved pyocyanin molecules present in the outer Helmholtz plane (OHP) region. The varying values of α, indicate that a part of the energy of the redox reaction could be employed for the adsorption processes. As for the interactions at the electrode, the values of the Laviron parameter (ν'gθT) indicate that attraction interactions between the pyocyanin molecules and the electrode dominate, since the values of ν'gθT are higher than 0. The results of this study will provide a comprehensive understanding of the redox cycle of pyocyanin. The information will be useful for the future development of electrochemical-microbial systems since that the interactions mediator-electrode may be an operational variable to facilitate their practical applications in environmental, sensing and electrosynthesis areas.

ROS formation driven by pyrite-mediated arsenopyrite oxidation and its potential role on arsenic transformation.

Pyrite-mediated arsenopyrite oxidation is an important process affecting arsenic (As) mobility. The iron sulfides-induced reactive oxidation species (ROS) can exert significant influence on As transformation. However, the impact of pyrite-arsenopyrite association on ROS production and its contribution to As transformation were rarely estimated. Here, ROS formation and the redox conversion of As during the interaction between pyrite and arsenopyrite as function of O2, pH and pyrite surface oxidation were investigated. Pyrite promoted arsenopyrite oxidation and As(III) oxidation due to heterogeneous electron transfer. The electron transfer from arsenopyrite facilitated O2 reduction on pyrite surface with increasing ROS formation. Hydroxyl radical (HO˙), superoxide (O2•)- and hydrogen peroxide (H2O2) were the main reactive species for As(III) oxidation. Iron (hydr)oxides produced from pyrite surface oxidation provided fast electron transfer channels for efficient O2 reduction as evidenced by electrochemical experiment, further verifying the promoted effect of surface-oxidized pyrite (SOP) on arsenopyrite dissolution. However, total As and As(V) obviously decreased during SOP-mediated arsenopyrite oxidation. Iron (hydr)oxides retained appreciable As through adsorption to limit its mobility, and decreased HO˙ production to inhibit As(III) oxidation via decomposing H2O2. This work furthers our understanding of arsenic transformation in the environment which has important implications for mitigating arsenic pollution.

Simultaneous electro-generation/polymerization of Cu nanocluster embedded conductive poly(2,2′:5′,2′′-terthiophene) films at micro and macro liquid/liquid interfaces

Cu nanoparticles (NPs) have been shown to be excellent electrocatalysts, particularly for CO2 reduction – a critical reaction for sequestering anthropogenic, atmospheric carbon. Herein, the micro interface between two immiscible electrolyte solutions (ITIES) is exploited for the simultaneous electropolymerization of 2,2′:5′,2′′-terthiophene (TT) and reduction of Cu2+ to Cu nanoparticles (NPs) generating a flexible electrocatalytic composite electrode material. TT acts as an electron donor in 1,2-dichloroethane (DCE) through heterogeneous electron transfer across the water|DCE (w|DCE) interface to CuSO4 dissolved in water. The nanocomposite formation process was probed using cyclic voltammetry as well as electrochemical impedance spectroscopy (EIS). CV and EIS data show that the film forms quickly; however, the interfacial reaction is not spontaneous and does not proceed without an applied potential. At high [TT] the heterogeneous electron transfer wave was recorded voltammetrically but not at low [TT]. However, probing the edge of the polarizable potential window was found to be sufficient to initiate electrogeneration/electropolymerization. SEM and TEM were used to image and analyze the final Cu NP/poly-TT composites and it was discovered that there is a concomitant decrease in NP size with increasing [TT]. Preliminary electrocatalysis results at a nanocomposite modified large glassy carbon electrode saw a > 2 × increase in CO2 reduction currents versus an unmodified electrode. These data suggest that this strategy is a promising means of generating electrocatalytic materials for carbon capture. However, films electrosynthesized at a micro and ~ 1 mm ITIES demonstrated poor reusability.

Influence of boron doped level on the electrochemical behavior and seawater salinity detection of boron doped diamond film electrodes

Boron doped diamond (BDD) film electrodes have a good application prospect in the field of seawater salinity detection. However, the influence mechanism of boron doping level (given by B/C ratio) on seawater salinity detection limited its application. In this paper, microstructure-phase-electrochemical relationships in BDD film electrodes were systematically investigated, aiming to reveal the effect of boron content on seawater salinity measurement, and lay a foundation for practical application of BDD film electrodes. With the increase of the boron doping, the content of non-diamond phase increased, while the crystallinity and the grain size of diamond decreased. Meanwhile, when the B/C ratio of BDD electrode reached 10000 ppm, they had the largest electroactive surface areas (EASA) and the smallest diffusion resistance, and their electrochemical performance was optimal in this study. In addition, BDD film electrode exhibited fast heterogeneous electron transfer in both internal Fe(CN)6−3/−4 and external Ru(NH3)6+2/+3 redox systems. Under the optimal parameter setting, the maximum signal response of BDD electrode at 40 ‰ was 69 mA/cm2 and the measurement error was less than 0.81 ‰. Appropriate proportion of sp2-C and B-sp3-C phases can significantly improve the detection performance of BDD electrode.

Conductive Metal-Organic Framework Microelectrodes Regulated by Conjugated Molecular Wires for Monitoring of Dopamine in the Mouse Brain.

Herein, we demonstrated a strategy to regulate the conductive metal-organic framework (MOF) surface, by the conjugated molecule wires for selective and sensitive determination of dopamine (DA) in the live brain. The MOFs were decorated at the carbon fiber electrode deposited by Au nanoleaves as the upper electric transducer to provide rich electrocatalytic sites for electron transfer of neurochemicals at the electrode surface, leading to greatly enhanced sensitivity for detection of neurochemicals. On the other hand, the conjugated molecular wire, 4-(thiophen-3-ylethynyl)-benzaldehyde (RP1), was synthesized and assembled as an underlying bridge to regulate the electrochemical processes at the MOF-based electrode, specifically decreasing the reaction Gibbs free energy of DA oxidation, thus selectively promoting the heterogeneous electron transfer of DA from the MOF layer to the electrode surface. Owing to the electrocatalytic activity for DA oxidation, the present microsensor exhibited high selectivity for real-time tracking of DA in a good linear relationship in the range of 0.004-0.4 μM with a detection limit of 1 nM. Eventually, this functionalized electrode was successfully applied for in vivo monitoring of DA in mouse brains with Parkinson's disease (PD) model. The results indicated that the levels of DA were obviously decreased in both acute and subacute PD models. Moreover, the level of DA strongly depended on the amount of uric acid (UA), a physiological antioxidant, which rose as the UA amount was lower than 200 mg kg-1 but was downregulated again after treatment by a higher amount of UA.

In Situ Spectroscopic Identification of the Electron-Transfer Intermediates of Photoelectrochemical Proton-Coupled Electron Transfer of Water Oxidation on Au

Experimental elucidation of the decoupling of electron and proton transfer at a molecular level is essential for thoroughly understanding the kinetics of heterogeneous (photo)electrochemical proton-coupled electron transfer water oxidation. Here we illustrate the electron-transfer intermediates of positively charged surface oxygenated species on Au (Au-OH+) and their correlations with the rate of water oxidation by in situ microphotoelectrochemical surface-enhanced Raman spectroscopy (SERS) and ambient-pressure X-ray photoelectron spectroscopy. At the intermediate stage of water oxidation, a characteristic blue shift of the vibration of Au-OH species in laser-power-density-dependent measurements was assigned to the light-induced production of Au-OH+ in water oxidation. The photothermal effect was excluded according to the vibrational frequencies of Au-OH species as the temperature was increased in a variable-temperature SERS measurement. Density functional theory calculations evidenced that the frequency blue shift is from the positively charged Au-OH species. The photocurrent-dependent frequency blue shift indicated that Au-OH+ is the key electron-transfer intermediate in water oxidation by decoupled electron and proton transfer.

Progress toward a better understanding of the urea oxidation by electromediation of Ni(III)/Ni(II) system in alkaline media

Current treatments of wastewater containing urea are energy intensive and release significant quantities of gaseous nitrous oxide. The anodic oxidation of urea by electromediation of Ni(III)/Ni(II) system in alkaline media is a promising alternative: the nitrogenous pollution is decreased while producing hydrogen at the cathode. Firstly, this study aims to deeply investigate the phenomena occurring at the interface, between electrode and electrolyte, in particular by proposing an original methodology enabling to separately study both steps,i.e. the Ni(II) electro-oxidation and the subsequent indirect heterogeneous urea oxidation. Indeed, the variation the potential scan rate, enables (i) to study the effect of the operating parameters (concentration, temperature, angular velocity of a rotating disk electrode) on the rate of each mechanism step, and to (ii) determine key parameters of the overall process (activation energy, diffusion coefficient, anodic charge transfer coefficient and heterogeneous electron transfer rate constant). In a second time, results of urea electrolysis exhibiting high (∼ 80%) urea conversions, carried out at laboratory scale (1.4 g of urea) are discussed. The effect of some operating parameters (alkalinity, temperature, presence of an anionic separator or not) is examined, thanks to a set of analytical methods developed to establish complete mass balances of the products on the liquid phase. Cyanate, ammonium, carbonate and nitrite ions are identified to represent more than 80 % of the destroyed urea. Their respective quantities appear to be dependent of the operating conditions. This study constitutes an essential preliminary step before designing urea electrochemical process operating at larger scale.

Effects of additives on the performance of a laser-induced graphene sensor modified with ZrO2 nanoparticles for OP detection.

In this study, a simple and portable electrochemical sensor based on laser-induced graphene (LIG) has been developed to systematically investigate the feasibility of LIG as an electrode to detect organophosphorus pesticides (OPs). It proves that the LIG-based electrode has a relatively high electrochemically active surface area (ECSA) and heterogeneous electron transfer (HET) of 0.100 cm2 and 0.000825 cm s-1, respectively. In addition, zirconium dioxide nanoparticles (ZrO2 NPs) have been modified on the electrode with three different binders, β-cyclodextrin (β-CD), chitosan (CS) and Nafion, to improve the adsorption capacity of the electrode toward OPs, and the effect of the binders on the performance of the as-fabricated sensor has been investigated in detail. The results show that β-CD increases not only the electrochemically active surface area of the electrode but also the redox peak current of methyl parathion (MP). To evaluate the sensitivity of the sensor, differential pulse voltammetry (DPV) curves have been tested in solutions containing different concentrations of MP using ZrO2-β-CD/LIG as an electrode, which shows a detection range of 5-200 ng ml-1 and a detection limit of 0.89 ng ml-1. In summary, the LIG-based sensor has a low detection limit, high sensitivity and good interference resistance, and thus has tremendous potential for the detection of pesticides in the environment.

Aqueous OH-/H+ dual-ion gradient assisted electricity effective electro-organic synthesis of 2,5-furandicarboxylic acid paired with hydrogen fuel generation.

The OH-/H+ dual-ion gradient has a hidden electromotive force of 0.82V under standard conditions; however, its non-redox nature completely prevents its direct interconversion as electrical driving force. We show by using organic molecules whose heterogeneous electron transfer is pH dependent, OH-/H+ dual-ion energy can be directly harvested as electrical driving force for performing simultaneous electro-organic synthesis and hydrogen fuel production in an electricity effective manner. To demonstrate this dual-ion gradient assisted electro-organic synthesis, 5-hydroxymethylfurfural (HMF) is chosen as the model molecule because of the immense techno commercial applications of its oxidized products. This dual-ion assisted device only required∼1V to provide a current density of 50mA/cm2 and for achieving the same rate; the traditional state-of-the-art electrolytic cell required a doubling of the applied potential. The dual-ion gradient assisted device can convert biomass-derived HMF to economically important FDCA with∼90% yield and∼87% Faradaic efficiency with simultaneous H2 fuel production at a potential as low as 1V.

Spatio-temporal detachment of homogeneous electron transfer in controlling selectivity in mediated organic electrosynthesis.

In electrosynthesis, electron transfer (ET) mediators are normally chosen such that they are more easily reduced (or oxidised) than the substrate for cathodic (or anodic) processes; setting the electrode potential to the mediator therefore ensures selective heterogeneous ET with the mediator at the electrode, rather than the substrate. The current work investigates the opposite, and counter intuitive, situation for a successful mediated electroreductive process where the mediator (phenanthrene) has a reduction potential that is negative to that of the substrate, and the cathode potential is set negative to both (Eele < EM < Es). Simulations reveal a complex interplay between mass transport, the relative concentrations of the mediator and substrate as well as the heterogeneous and homogeneous rate constants for multiple steps, which under suitable conditions, leads to separation of the homogeneous chemistry in a reaction layer detached from the electrode. Reaction layer detachment is a spatio-temporal effect arising due to opposing fluxes of the mediator radical anion M˙- and the substrate 1, which ultimately prevents 1 from reaching the electrode, thereby affording a different reaction pathway. Simulations representative of unstirred batch (1D) and flow (2D) electrolysis are presented, which qualitatively reproduce the experimental selectivity outcomes for mediated and unmediated electroreductive cyclisation of aryl iodide 1. The potential to use highly reducing homogeneous ET agents, possessing reduction potentials beyond those of the substrates, offers exciting opportunities in mediated electrosynthesis.

Cyclic voltammetry, square wave voltammetry or electrochemical impedance spectroscopy? Interrogating electrochemical approaches for the determination of electron transfer rates of immobilized redox proteins

In this article, we cross-examine three well-established electrochemical approaches, namely cyclic voltammetry (CV), cyclic square-wave voltammetry (SWV) and electrochemical impedance spectroscopy (EIS) to dissect the electron transfer (ET) rate of electrostatically immobilized cytochrome c on Ag electrodes. A detailed analysis supported by simulations of redox transition provided three distinct values for the heterogeneous electron transfer (HET) rate constant of cyt c interfaced on COOH-terminated C10-long alkanethiol, i.e., kHET= 47.8 (±2,91) s−1 in CV, kHET= 64.8 (±1,27) s−1 in SWV, and kHET= 26.5 s−1 in EIS. We discuss the obtained discrepancies obtained from electrochemical methods and compare them with the data from spectro-electrochemical experiments. A comprehensive selection list is created from which the most applicable approach can be chosen for studying proteins of interest. CV is most applicable to study the interfaced proteins exhibiting kHET of ca. 0.5 - 70 s−1, SWV is suitable for a broader range of kHET of 5 – 120 s−1 and EIS for kHET of 0.5 to 5 s−1 if alkanethiols are used as immobilization strategy.

Promoting Electron Transfer Kinetics and Adsorption Capacity for the Detection of Furazolidone in Real Food Samples by Using Ag-core@Fe3O4-Shell-Based Electrochemical Sensing Platform

In this study, the preparation of core/shell Ag@Fe3O4 nanoparticles (NPs) and its potential application toward highly sensitive electrochemical detection of furazolidone (FZD) have been reported. UV–visible spectroscopy, X-ray diffraction, scanning electron microscopy, and Zeta sizer are systematically carried out to confirm the formation, size distribution, and composition of Ag@Fe3O4 NPs. By computing the electrochemical characteristic parameters such as electrochemically active surface area (ECSA), electron-transfer resistance (Rct), standard heterogeneous rate constant (k0), adsorption capacity (Γ), and electron transfer rate constant (ks), the Ag@Fe3O4-modified electrode possessed remarkably enhanced electrochemical sensing performance for FZD determination compared to the unmodified screen-printed electrode (SPE). This enhancement of electrochemical activity can be attributed to the fast electron transfer kinetics and great adsorption capacity that arise from the synergistic coupling between the good electrical conductivity of the core AgNPs and the porosity of the protective Fe3O4 shell. Under optimum conditions, the Ag@Fe3O4-based electrochemical nanosensor exhibited not only high sensitivity toward FZD detection of 1.36 μA μM−1 cm−2 in the linear ranges from 0.5–15 μM and 15–100 μM, and low detection limit of 0.24 μM but also long-term stability, repeatability, and anti-interference ability. The applicability of the proposed sensing platform in honey and milk samples was also investigated.