Electron Transfer Rate Constant Research Articles

Performance Enhancement in Redox Flow Batteries Using Bismuth Based Catalyst

Slow reaction rates of vanadium (V), iron (Fe), chrome (Cr) redox couples are a bottleneck to address for enhancing performance of vanadium and Fe-Cr redox flow battery (VRFB and Fe-Cr RFB) system. For promoting their reaction rates, new catalytic materials based on Bismuth (Bi) are suggested in this study. Reaction reversibility as well as catalytic activity of the catalysts is investigated using cyclic voltammogram (CV) and the results are compared with those of other catalysts. Performances and long term stabilities of VRFB and Fe-Cr RFB using Bi-based catalysts are evaluated by repetitive and cyclic measurements of charge-discharge step. Especially, energy efficiency (EE) and capacity loss rate are mainly inspected to determine superiority of the VRFB and Fe-Cr RFB. Their crystal structure, specific surface area and catalyst morphology are measured by chemical characterizations. The new Bi-based catalysts show high peak current ratio, small peak potential difference and high electron transfer rate constant, confirming that in both catalytic activity and reaction reversibility facets, the Bi-based catalysts play an important role. In terms of charge-discharge curves, VRFB and Fe-Cr RFB including new catalysts indicate high EE, low overpotential and low internal resistance. The results are attributed to role of Bi that activates Fe2+/Fe3 +, Cr2+/Cr3 + and V4+/V5 + redox reactions. Indeed, adoption of the Bi-based catalysts lead to improvement in performances and long term stabilities of VRFB and Fe-Cr RFB.

Role of Carbon Nanostructured Platform in Electron Transfer of Flavin-Dependent Oxidases

Carbon nanostructures, such as carbon nanotubes (CNT) and graphene have gained considerable attention during the last years, especially due to their remarkable electronic and mechanical properties, which have made them extremely attractive for a wide range of sensing applications from structural materials to nanoelectronic components. The ability of carbon nanostructure-modified electrodes to promote electron transfer reactions has been documented in connection with important biomolecules [1]. Our goal was to study the influence of carbon nanostructured platform as an electrode material in facilitating the electron transfer between the enzyme molecule and the electrode surface in bioelectrocatalytic systems for oxidation reaction of glucose and lactate that is of potential utility for bioelectronic devices such as biosensors and biofuel cells. Our research focuses on the direct electrochemical performance of flavin-dependent oxidases such as glucose oxidase (GOx) and lactate oxidase (LOx) immobilized on 4-(pyrrole-1-yl) benzoic acid (PyBA) modified CNT (CNT/PyBA) and/or (for comparison) on hybrid material composed of electrochemically reduced graphene oxide (ERGO) and CNT/PyBA (ERGO/CNT/PyBA). GOx and LOx are enzymes containing tightly bound flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN) redox centers, respectively, embedded deeply in the protein shell [2]. Modification of carbon nanotubes allows to obtain thin and organized films, causing a rapid and effective transfer of electrons between the active center of the biocatalyst (GOx or LOx) and the electrode surface. The presence of PyBA in our hybrid systems significantly improves their stability and introduces new functional groups that have great importance in the enzyme immobilization process onto the modified electrode [3]. Moreover, graphene oxide (GO) has a unique ability to form stable aqueous dispersions, highly bio-compatible and possesses good electrocatalytic properties. In addition, its reductive product graphene is a unique material with potential applications in diverse fields and one of the hottest materials of interest. It was demonstrated that it is possible to prepare a water dispersible GO/CNT hybrid material via non-covalent π-π stacking interactions [4]. Dispersing CNT via non-covalent approaches cannot depress its intrinsic electrical, mechanical, and optical properties. The incorporation of GO with CNT render this hybrid material as a versatile platform for the electrocatalytic applications. Conversely, the insulating property of GO caused by the aliphatic sp3 hybridized domain, limits its conductivity. Therefore for improving the conductance, it can be reduced to graphene either by chemical, thermal or electrochemical methods. Recent studies reveal that the electrochemical reduction of GO to graphene (ERGO) was appreciably improved after the incorporation of CNT. In this work stable immobilization and direct electron transfer of oxidase enzymes were achieved on the hybrid system modified glassy carbon electrode. The resulting electrode gave a well-defined redox peaks with a formal potential of about -458 mV for GOx and -446 mV for LOx (vs. Ag/AgCl) in 0.1 phosphate buffer solution pH=7.0. Furthermore, the method for detecting of glucose and/or lactate was proposed based on the decrease of oxygen caused by the enzyme-catalyzed reaction between enzyme and substrate. The low calculated apparent Michaelis-Menten constants (KM app) were 13.3 mM for GOx and 8.25 mM for LOx, implying high enzymatic activity and affinity of immobilized enzyme for glucose and lactate, respectively. The calculated heterogeneous electron transfer rate constants were twice higher due to the combined contribution of GO with CNT/PyBA. It can reasonably be expected that this observation might hold true for other noble carbon nanostructure-electroactive protein systems, providing a promising platform for the development of biosensors and biofuel cells.

Structure, Electronic Properties and Electrochemical Behavior of a Boron-Doped Diamond/Quartz Optically Transparent Electrode

This presentation will review the morphology, microstructure, chemistry, electronic properties and electrochemical behavior of a boron-doped nanocrystalline diamond (BDD) thin film grown on quartz. Diamond optically transparent electrodes (OTEs) are useful for transmission spectroelectrochemical measurements offering excellent stability during anodic and cathodic polarization and exposure to a variety of chemical environments.1,2 An example will be shown of how the OTE in a spectroelectrochemical measurement can be used to study the redox mechanism of the model analyte, chlorpromazine.1 We will also report on the characterization of the BDD OTE by AFM, optical spectroscopy, Raman spectroscopic mapping, AC Hall Effect measurements, XPS and electrochemical methods. Cyclic voltammetry was used to investigate the background current response as a function of potential, scan rate and electrolyte composition. Cyclic voltammetry and digital simulation were used to determine the heterogeneous electron-transfer rate constants (ko) for IrCl6 2-/3-, Fe(CN)6 3-/4-, ferrocene carboxylic acid, Ru(NH3)6 3+/2+ and methyl viologen. The diamond OTE supports relatively rapid electron transfer for a wide range of redox systems with formal potentials from ca. 0.9 to -1.0 V vs. Ag/AgCl. Capacitance-potential measurements were made in aqueous electrolyte solutions and ionic liquids to better understand the organization of electrified interfaces formed at this electrode. The work constitutes the first comprehensive study of the relationship between the physical and chemical structure and electronic properties of a diamond OTE, and the electrode’s electrochemical activity. 1. Stotter, J; Zak J; Behler Z; Show Y; Swain GM. Optical and Electrochemical Properties of Optically Transparent, Boron-Doped Diamond Thin Films on Quartz. Anal. Chem. 2002, 74, 5924-5930. 2. Stotter J; Show Y; Wang S; Swain GM. Comparison of the Electrical, Optical and Electrochemical Properties of Diamond and Indium Tin Oxide Thin-Film Electrodes. Chem. Mater. 2005, 17, 4880-4888.

Molecular tilt-dependent and tyrosine-enhanced electron transfer across ITO/SAM/[DPPC–Au NP–Tyrosine] Janus nanoparticle junction

Enhanced interfacial electron transfer (ET) across the otherwise insulating indium tin oxide/alkanethiol self-assembled monolayer (SAM)/redox molecule junction was accomplished when a Janus gold nanoparticle (JNP) protected by bioinspired phosphatidylcholine (DPPC) lipid and tyrosine amino acid ligands was anchored on it. In addition to the most theoretical and experimental investigations on the distance-dependent ET across Metal–Organic SAM–Nanoparticle (NP) architectures, the current results succinctly illustrate molecular tilt angle of the SAM and the characteristic of JNP as key factors in expediting the ET rate via electron tunneling. In the absence of JNP, electron tunneling with a tunneling factor β = 1.1 A−1 across the SAM was the rate-limiting step, evidenced from electrochemical impedance spectroscopy (EIS). The apparent electron transfer rate constant (k app 0 ) for anchored SAM was enhanced by at least one order of magnitude than the DPPC-only protected nanoparticle, suggesting the potential role of tyrosine towards the enhanced ET. The asymmetric and biogenic nature of the construct sheds light on a potential bioelectronic device for novel electronic attributes. Entry of TOC .

Intramolecular Electron Transfer from Tryptophan to Guanosyl Radicals in a Linked System as a Model of DNA Repair

Abstract As a model of chemical DNA repair, intramolecular electron transfer from tryptophan to the radical of the purine base guanosine combined into a conjugate by a flexible linker was studied by time-resolved chemically induced dynamic nuclear polarization (CIDNP). The guanosyl radicals were photochemically generated in the quenching reaction of the triplet excited dye 2,2′-dipyridyl. The CIDNP kinetics was obtained by detection of NMR spectra containing anomalously enhanced signals of diamagnetic products that are formed during a variable period after excitation by a laser pulse. The kinetic data obtained for the protons located on the guanosyl and tryptophanyl moieties of the conjugate were compared to those obtained in photoreactions of the molecules containing the same linker, but with only one of the two reactive moieties of the conjugate – tryptophanyl or guanosyl. Strong differences between the CIDNP kinetics of different conjugates were revealed and explained by a rapid intramolecular electron transfer from tryptophan to the guanosyl radical in the conjugate. Model simulations of the CIDNP kinetics allowed for determination of the rate constant of intramolecular electron transfer at (1.0±0.5)×106 s–1 indicating a high potential of chemical repair of the guanosyl radical by means of electron transfer from the tryptophanyl moiety in a surrounding protein pool that can provide rather efficient protection of oxidized DNA bases from pathological damage on a submicrosecond time scale.

Direct Electron Transfer Kinetics of Peroxidase at Edge Plane Sites of Cup-Stacked Carbon Nanofibers and Their Comparison with Single-Walled Carbon Nanotubes.

Electron transfer kinetics at the graphene edge site is of great interest from the viewpoints of application to sensing and energy conversion and storage. Here we analyzed kinetics of direct electron transfer of horseradish peroxidase (HRP) adsorbed through surfactant sodium dodecyl sulfate at cup-stacked carbon nanofibers (CSCNFs), which provide highly ordered graphene edges, and compared it with that at single-walled carbon nanotubes (SWCNTs), which consist of a rolled-up basal plane graphene. The heterogeneous electron transfer rate constant of the Fe(2+/3+) couple of the HRP reaction center at CSCNFs (ca. 34.8 s(-1)) was an order of magnitude larger than that at SWCNTs (ca. 4.7 s(-1)). In addition, the overall rate constant of the electron transfer reaction from CSCNFs to HRP oxidized by H2O2 was higher than that from SWCNTs by a factor of 3. CSCNFs also allowed enhancement of the complex-formation reaction rate of HRP with H2O2, in comparison with that at SWCNTs. CSCNFs would therefore be applied to not only biosensors but also biofuel cells with enhanced performance.

Improved DET communication between cellobiose dehydrogenase and a gold electrode modified with a rigid self-assembled monolayer and green metal nanoparticles: The role of an ordered nanostructuration

Efficient direct electron transfer (DET) between cellobiose dehydrogenase from Corynascus thermophilus (CtCDH) and a novel gold electrode platform, obtained by covalent linking of green AuNPs and AgNPs modified with a dithiol self-assembled monolayer, consisting of biphenyl-4,4′-dithiol (BPDT), was presented. The green AuNPs and AgNPs were synthesized using quercetin as reducing agent at room temperature. TEM experiments showed that the AuNPs and AgNPs were circular in shape with an average diameter of 5 and 8nm, respectively. Cyclic voltammetry of CtCDH immobilized onto the AuNPs/BPDT/AuE and the AgNPs/BPDT/AuE electrode platforms were carried out and compared with naked AuE, BPDT/AuE, AuNPs/AuE, and AgNPs/AuE. A pair of well-defined redox waves in neutral pH solution due to efficient DET of CtCDH was present with both MNPs/BPDT/AuE platforms. No DET communication was found with platforms without MNPs linked to BPDT. The apparent heterogeneous electron transfer rate constants (kS) of CtCDH were calculated to be 21.5±0.8s−1 and 10.3±0.7s−1, for the AuNPs/BPDT/AuE and the AgNPs/BPDT/AuE platforms, respectively.The modified electrodes were successively used to develop an eco-friendly biosensor for lactose detection. The CtCDH/AuNPs/BPDT/AuE based biosensor showed the best analytical performances with an excellent stability, a detection limit of 3µM, a linear range between 5 and 400µM and a sensitivity of 27.5±2.5µAcm−2mM−1. Such performances were favorably compared with other lactose biosensors reported in literature.The biosensor was successively tested to quantify lactose content in real milk and cream samples. No significant interference present in the sample matrices was observed.

Analytical performance of paper electro-biosensor detection platform for point-of-care diagnosis

A whole paper electro-biosensor detection platform for detecting glucose is reported. Glucose oxidase was immobilized on paper-based screen-printed carbon electrodes. The developed biosensor has an excellent electrochemical characteristic and high heterogeneous electron transfer rate constant (ks = 4.23 × 10−4). This new detection platform exhibited good amperometric responses toward glucose, with a wide linear range up to 10 mM with R2 is 0.9973 and the sensitivity is 2.07 μA mM−1. The parallel measurements of glucose by different biosensors had a low relative standard deviation of 4.26 % and demonstrated it had good reproducibility. In the interference studies, there was no significant difference with ascorbic acid or uric acid. For stability, it retained 93.9 % of its initial response after a storage period of 31 days in 4 °C. Besides, the Parkes error grid analysis was indicated that paper-based electrochemical biosensor has a good accuracy on the glucose detection.

Micellar Effects on Photoinduced Electron Transfer in Aqueous Solutions Revisited: Dramatic Enhancement of Cage Escape Yields in Surfactant Ru(II) Diimine Complex/[Ru(NH3)6](2+) Systems.

The effect of cationic micelle incorporation on light induced electron transfer, charge separation and back electron transfer between an aqueous electron donor, [Ru(NH3)6](2+), and a series of Ru(II) diimine complex chromophores/acceptors, is presented. The chromophores have the general formula [(bpy)2Ru(LL)](2+) (LL = bpy; 4-R-4'-methyl-2,2'-bpy, R = pentyl (MC5), terdecyl (MC13), heptadecyl (MC17); 4,4'-di(heptadecyl)-2,2'-bpy (DC17)). Of the five chromophores, the MC13, MC17, and DC17 complexes associate with the added micelle forming surfactant, cetyltrimethylammonium bromide (CTAB). Quenching of the luminescence of the bpy and MC5 complexes by [Ru(NH3)6](2+) is unaffected by addition of surfactant, while rate constants for quenching of the MC13 and MC17 complexes are decreased. Cage escape yields following photoinduced electron transfer to generate [(bpy)2Ru(LL)](+) and [Ru(NH3)6](3+) are approximately 0.1 for all the water-soluble chromophores (excluding DC17) in the absence of added CTAB. In the presence of surfactant, the cage escape yields dramatically increase for the MC13 (0.4) and MC17 (0.6) complexes, while remaining unchanged for [Ru(bpy)3](2+) and the MC5 complex. Back electron transfer of the solvent separated ions is also strongly influenced by the presence of surfactant. For the MC13 and MC17 complexes, back electron transfer rate constants decrease by factors of 270 and 190, respectively. The MC5 complex exhibits two component back electron transfer, with the fast component having a rate constant close to that in the absence of surfactant and a slow component nearly 200 times smaller. Results are interpreted in terms of the partitioning of the 2+ and 1+ forms of the chromophores between aqueous and micellar phases. The extended lifetimes of the radical ions may prove useful in coupling the strong reductants formed to kinetically facile catalysts for reduction of water to hydrogen.

Electrocatalysis of NADH on 3,5-Dinitrobenzoic Acid Encapsulated on Multiwalled Carbon Nanotube-Modified Electrode

We report that glassy carbon electrode (GCE) modified with multiwalled carbon nanotubes (MWCNTs) can encapsulate or entrap 3,5-dinitrobenzoic acid (35DNB) generating a 35DNB-MWCNTGC electrode. After electrochemical reduction in situ of only one nitro group of 35DNB, it turns into the hydroxylamine derivative (R-NHOH), which can be further oxidized to the nitroso derivative (R-NO). Then, R-NO/R-NHOH redox couple was electrogenerated in situ by cycling the potential between 0.20 and −0.20 V vs Ag/AgCl. The very well-defined and persistent redox couple was characterized with a formal potential, E o ’ = −28 mV vs Ag/AgCl at a scan rate of 20 mV s−1. Using the Laviron’s plot, a transfer coefficient, α = 0.45, and an electron transfer rate constant, k s = 10.5 s−1, for the electron transfer of the couple R-NO/R-NHOH, were calculated. This redox reaction results to be a very efficient mediator for electrocatalytic NADH oxidation. The 35DNB-MWCNTGC electrode efficiently catalyzes the oxidation of NADH with a decrease of more than 0.60 V vs Ag/AgCl in the overpotential compared to the bare GCE and a difference of 0.25 V vs Ag/AgCl with respect to the situation without mediator. The preparation of the electrode is very easy and not time-consuming.

Direct electrochemistry and electrocatalysis of cytochrome c based on dandelion-like Bi2S3 nanoflowers

The direct electrochemistry and electrocatalysis of cytochrome c (Cyt c) based on dandelion-like bismuth sulfide (d-Bi2S3) nanoflowers have been developed. The morphologies and composition of the d-Bi2S3 were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDS). Then, the electrochemical behaviors of Cyt c immobilized within the d-Bi2S3/chitosan film and its electrocatalytic ability toward hydrogen peroxide (H2O2) reduction were investigated by cyclic voltammetry. The electron transfer rate constant was estimated to be 13.1 s−1, suggesting that a fast direct electron transfer was realized. The prepared Cyt c/d-Bi2S3/chitosan nanobiocomposite-modified electrode possessed excellent electrocatalytic ability toward H2O2 reduction that showed linearity in the range from 0.5 μM to 1.56 mM with a correlation coefficient of 0.9993. The detection limit was 0.2 μM on signal-to-noise ratio of 3. In addition, the d-Bi2S3 nanoflowers may be also applied to direct electron transfer of other redox proteins.

Development of biofuel cell adopting multiple poly(diallyldimethylammonium chloride) layers immobilized on carbon nanotube as powerful catalyst

A new enzymatic biofuel cell (EBC) adopting multiple glucose oxidase (GOx) and poly(diallyldimethylammonium chloride) (PDDA) immobilized on carbon nanotube (CNT) ([GOx/PDDA]n/CNT) is fabricated and effects of [GOx/PDDA]n/CNT catalysts on EBC performance are investigated using various electrochemical characterizations. Initially, both redox reaction of flavin adenine dinucleotides (FADs) within GOx and stepwise deposition of PDDA and GOx on CNT are estimated to determine optimal [GOx/PDDA]n/CNT catalyst. Also, its electron transfer pathway and reaction mechanism related to oxygen mediator is elucidated. With the optimized [GOx/PDDA]5/CNT catalyst, excellent catalytic activity and EBC performance are measured. When the catalyst is used, its electron transfer rate constant is 15.97 s−1, glucose sensitivity is 17 μA mM−1 cm−2, Michaelis–Menten constant is 1.44 mM and EBC maximum power density (MPD) is 0.79 mW cm−2. It indicates that the values are better than those of other similar structures. Moreover, in a comparison with MPDs of EBCs without provision of glucose, it is proved that PDDA and GOx make key role in improving EBC performance.

Application of nanodisc technology for direct electrochemical investigation of plant cytochrome P450s and their NADPH P450 oxidoreductase.

Direct electrochemistry of cytochrome P450 containing systems has primarily focused on investigating enzymes from microbes and animals for bio-sensing applications. Plant P450s receive electrons from NADPH P450 oxidoreductase (POR) to orchestrate the bio-synthesis of a plethora of commercially valuable compounds. In this report, full length CYP79A1, CYP71E1 and POR of the dhurrin pathway in Sorghum bicolor were reconstituted individually in nanoscale lipid patches, “nanodiscs”, and directly immobilized on unmodified gold electrodes. Cyclic voltammograms of CYP79A1 and CYP71E1 revealed reversible redox peaks with average midpoint potentials of 80 ± 5 mV and 72 ± 5 mV vs. Ag/AgCl, respectively. POR yielded two pairs of redox peaks with midpoint potentials of 90 ± 5 mV and −300 ± 10 mV, respectively. The average heterogeneous electron transfer rate constant was calculated to be ~1.5 s−1. POR was electro-catalytically active while the P450s generated hydrogen peroxide (H2O2). These nanodisc-based investigations lay the prospects and guidelines for construction of a simplified platform to perform mediator-free, direct electrochemistry of non-engineered cytochromes P450 under native-like conditions. It is also a prelude for driving plant P450 systems electronically for simplified and cost-effective screening of potential substrates/inhibitors and fabrication of nano-bioreactors for synthesis of high value natural products.

Mixed Quantum-Classical Liouville Approach for Calculating Proton-Coupled Electron-Transfer Rate Constants.

In this work, we derive a general mixed quantum-classical formula for calculating thermal proton-coupled electron-transfer (PCET) rate constants, starting from the time integral of the quantum flux-flux correlation function. This formula allows for the direct simulation of PCET reaction dynamics via the mixed quantum-classical Liouville approach. Owing to the general nature of the derivation, this formula does not rely on any prior mechanistic assumptions and can be applied across a wide range of electronic and protonic coupling regimes. To test the validity of this formula, we applied it to a reduced model of a condensed-phase PCET reaction. Good agreement with the numerically exact rate constant is obtained, demonstrating the accuracy of our formalism. We believe that this approach constitutes a solid foundation for future investigations of the rates and mechanisms of a wide range of PCET reactions.

Influence of membrane galactolipids and surface pressure on plastoquinone behaviour

In this work biomimetic monolayers of a MGDG, monogalactosyldiacylglycerol, and DGDG, digalactosyldiacylglycerol mixture (MD), in a ratio close to that of the thylakoid membranes of oxygenic photosynthetic organisms, have been prepared. The lipid mixture incorporates plastoquinone-9 (PQ), that is the electron and proton shuttle of the photosynthetic reaction centres. The MD:PQ mixtures have been firstly studied using surface pressure-area isotherms. Langmuir-Blodgett (LB) films of those mixtures have been transferred onto a substrate forming a monolayer that mimics one of the bilayer sides of the thylakoid membranes. These monolayers have been characterized topographically and electrochemically. The results show the influence of PQ in the MD matrix and its partial expulsion when increasing the surface pressure, obtaining two main PQ positions in the MD matrix. The calculated apparent electron transfer rate constants indicate a different kinetic control for the reduction and the oxidation of the PQ/PQH2 couple, being kRapp(I)=0.7·10−6s−1, kRapp(II)=2.2·10−9s−1, kOapp(I)=7.4·10−4s−1 and kOapp(II)=5.2·10−5s−1, respectively. The comparison of the different galactolipid:PQ systems that our group has studied is also presented, concluding that the PQ position in the galactolipid matrix can be tuned according to several controlled variables.

Enhanced photoelectrochemical biosensing performances for graphene (2D) – Titanium dioxide nanowire (1D) heterojunction polymer conductive nanosponges

In this work, an efficient photoelectrochemical (PEC) biosensing platform has been designed and developed based on graphene (G) through modifying it into an electroconductive polymer nanosponge (EPNS) and with the incorporation of titanium dioxide nanowires (TiO2 NW) (designated as TiO2 (G) NW@EPNS). Functioning as an efficient immobilization matrix for immobilization of the enzyme Cytochrome C (Cyt C), TiO2 (G) NW@EPNS delivers features for an efficient PEC biosensor, such as fast kinetics of direct electron transfer (DET) to the electrode and effective separation of photogenerated holes and electrons. TiO2 (G) NW@EPNS exhibited DET to the electrode with a highly heterogeneous electron transfer rate constant of 6.29±0.002s−1. The existence of TiO2, G and EPNS in conjunction facilitates DET between the electrode surface and the protein. The fabricated PEC nitrite ion (NO2−) biosensor showed superior analytical performances such as wide linear range (0.5–9000µM), lowest detection limit (0.225mM) and excellent specificity for NO2− in the presence other interferences at a very low bias potential (−0.11V). This study opens up the feasibility of fabricating a PEC biosensor for any analyte using a matrix comprising of G and a photoactive material and EPNS, because these components synergistically contribute to effective immobilization of on enzyme, DET to the electrode and simple read-out under the light.

Optical Determination of Electron Transfer Dynamics and Kinetics for Asymmetrical [Mo2]–ph–[Mo2] Systems

We report the study, in terms of electronic coupling (EC) and electron transfer (ET), on three asymmetrical Mo2 dimers [Mo2(DAniF)3]2[μ-(NH)OCC6H4CO2] ([NO–ph–OO]) (DAniF = N,N′-di(p-anisyl)formamidinate), [Mo2(DAniF)3]2[μ-(NH)2CC6H4CO2] ([NN–ph–OO]), and [Mo2(DAniF)3]2[μ-(NH)2CC6H4C(NH)O] ([NN–ph–NO]), which are closely related to the three symmetrical analogues [OO–ph–OO], [NO–ph–NO], and [NN–ph–NN] reported earlier. The mixed-valence (MV) complexes [NO–ph–OO]+, [NN–ph–OO]+, and [NN–ph–NO]+ exhibit metal to ligand and ligand to metal charge transfer bands, along with an intervalence (IV) transition absorption in the Near-IR region. The free energy change (ΔG°) for ET is determined by comparing the redox potential splitting (ΔE1/2) and IV transition energy (EIT) with the data for the symmetrical species. The reorganization energy (λ) is estimated from the Hush model (Δν1/2 = [16ln(2)λRT]1/2). Significantly, electrochemical and optical analyses verify EIT = ΔG° + λ, the core energetic relationship underlying the semiclassical theories. With the electronic coupling parameters calculated from the method suggested by Creutz, Newton and Sutin (HMM′ = ∼ 500 cm–1), the adiabatic ET rate constants ket (f) are determined to be ∼1010 s–1 for [NO–ph–OO]+ and [NN–ph–NO]+, smaller than ket (r) for the backward reaction and ket for the symmetrical analogues by 1 order of magnitude, and ∼109 s–1 for [NN–ph–OO]+. This work illustrates that the redox asymmetry in D–B–A systems controls the ET rate and direction.

Modified fractal iron oxide magnetic nanostructure: A novel and high performance platform for redox protein immobilization, direct electrochemistry and bioelectrocatalysis application

A novel biosensing platform based on fractal-pattern of iron oxides magnetic nanostructures (FIOMNs) and mixed hemi/ad-micelle of sodium dodecyl sulfate (SDS) was designed for the magnetic immobilization of hemoglobin (Hb) at a screen printed carbon electrode (SPCE). The FIOMNs was successfully synthesized through hydrothermal approach and characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM) and X-ray diffraction (XRD). In order to provide guidelines for the mixed hemi/ad-micelle formation, zeta-potential isotherms were investigated. The construction steps of the biosensor were evaluated by electrochemical impedance spectroscopy, cyclic voltammetry and Fourier transform infrared spectroscopy. Direct electron transfer of Hb incorporated into the biocomposite film was realized with a pair of quasi-reversible redox peak at the formal potential of −0.355V vs. Ag/AgCl attributing to heme Fe(III)/Fe(II) redox couple. The results suggested that synergistic functions regarding to the hyper-branched and multidirectional structure of FIOMNs and the dual interaction ability of mixed hemi/ad-micelle array of SDS molecules not only induce an effective electron transfer between the Hb and the underlying electrode (high heterogeneous electron transfer rate constant of 2.08s−1) but also provide powerful and special microenvironment for the adsorption of the redox proteins. Furthermore, the biosensor displayed an excellent performance to the electrocatalytic reduction of H2O2 with a detection limit of 0.48µM and Michaelis-Menten constant (Km) value of 44.2µM. The fabricated biosensor represented the features of sensitivity, disposable design, low sample volume, rapid and simple preparation step, and acceptable anti-interferences, which offer great perspectives for the screen-determination of H2O2 in real samples.

A comprehensive heterogeneous electron transfer rate constant evaluation of dissolved oxygen in DMSO at glassy carbon electrode measured by different electrochemical methods

Molecular oxygen radicals are responsible for many problems like aging, cancer, lipid peroxidation, protein denaturation, oxidative stress etc., We present a comprehensive electrochemical investigation of the dissolved oxygen in DMSO containing 0.1M TBAP as the supporting electrolyte. Different electrochemical techniques such as cyclic voltammetry, chronoamperometry and chronopotentiometry were used to extract heterogeneous kinetics and chemically coupled reaction kinetics related to the O2 radicals. The experimentally determined parameters obtained through different methods were further confirmed through digital simulation (DIGISIM). Heterogeneous electron transfer rate constant (k0) was determined by three different methods, Nicholson & Shain, Gileadi, and the Kochi methods. The calculated k0 values are different from the previously cited values.

Turning electron transfer ‘on-off’ in peptides through side-bridge gating

Electrochemical studies are reported on a series of peptides to determine the influence of different side-chains and backbone rigidity on electron transfer, to progress the field of molecular electronics. Specifically, these peptides share either a common helical or β-strand conformation to cover a range of secondary structures, to fully investigate the influence of backbone rigidity. Two types of side-chain tethers, either triazole-containing or alkene-containing, are also compared to investigate these effects on electron transfer. Our results showed that the observed formal potentials (Eo) and electron transfer rate constants (ket) fall into two distinct groups. The peptides constrained via a side-chain tether exhibited high formal potentials and low electron transfer rate constants, whereas the linear peptides displayed low formal potentials and high electron transfer rate constants. This was found to occur irrespective of the backbone conformation, or the nature of the side-chain constraint. The vast formal potential shifts (as much as 482mV) and the large disparity in the electron transfer rate constants (as much as 97%) between the constrained and linear peptides, provides two distinct states (i.e. on/off) with a sizeable differential, which is ideal for the design of molecular switches.