Electrochemical Investigation of Cytochrome c Immobilized onto Self‐Assembled Monolayer of Captopril

The electrochemical behavior of cytochrome c (cyt‐c) that was electrostatically immobilized onto a self‐assembled monolayer (SAM) of captopril (capt) on a gold electrode has been investigated. Cyclic voltammetry, scanning electrochemical microscopy (SECM) and electrochemical impedance spectroscopy were employed to evaluate the blocking property of the capt SAM. SECM was used to measure the bimolecular electron transfer (ET) kinetics (k_BI) between a solution‐based redox probe and the immobilized protein. In addition, the tunneling ET between the immobilized protein and the underlying gold electrode was calculated. A k_BI value of (5.0±0.6)×10⁸ mol^?1 cm³ s^?1 for the bimolecular ET and a standard tunneling rate constant (k⁰) of 46.4±0.2 s^?1 for the tunneling ET have been obtained.     

Mechanism of electrochemical charge transport in individual transition metal complexes

We used electrochemical scanning tunneling microscopy (STM) and spectroscopy (STS) to elucidate the mechanism of electron transport through individual pyridyl-based Os complexes. Our tunneling data obtained by two-dimensional electrochemical STS and STM imaging lead us to the conclusion that electron transport occurs by thermally activated hopping. The conductance enhancement around the redox potential of the complex, which is reminiscent of switching and transistor characterics in electronics, is reflected both in the STM imaging contrast and directly in the tunneling current. The latter shows a biphasic distance dependence, in line with a two-step electron hopping process. Under conditions where the substrate/molecule electron transfer (ET) step is dominant in determining the overall tunneling current, we determined the conductance of an individual Os complex to be 9 nS (Vbias = 0.1 V). We use theoretical approaches to connect the single-molecule conductance with electrochemical kinetics data obtained from monolayer experiments. While the latter leave some controversy regarding the degree of electronic coupling, our results suggest that electron transport occurs in the adiabatic limit of strong electronic coupling. Remarkably, and in contrast to established ET theory, the redox-mediated tunneling current remains strongly distance dependent due to the electronic coupling, even in the adiabatic limit. We exploit this feature and apply it to electrochemical single-molecule conductance data. In this way, we attempt to paint a unified picture of electrochemical charge transport at the single-molecule and monolayer levels.     

Average electron tunneling route of the electron transfer in protein media

We present a new theoretical method to determine and visualize the average tunneling route of the electron transfer (ET) in protein media. In this, we properly took into account the fluctuation of the tunneling currents and the quantum-interference effect. The route was correlated with the electronic factor in the case of ET by the elastic tunneling mechanism. We expanded by the interatomic tunneling currents 's. Incorporating the quantum-interference effect into the mean-square interatomic tunneling currents, denoted as , we could express as a sum of variant Planck's over 2pi(2). Drawing the distribution of on the protein structure, we obtain the map which visually represents which parts of bonds and spaces most significantly contribute to . We applied this method to the ET from the bacteriopheophytin anion to the primary quinone in the bacterial photosynthetic reaction center of Rhodobacter sphaeroides. We obtained 's by a combined method of molecular dynamics simulations and quantum chemical calculations. In calculating , we found that much destructive interference works among the interatomic tunneling currents even after taking the average. We drew the map by a pipe model where atoms a and b are connected by a pipe with width proportional to the magnitude of . We found that two groups of 's, which are mutually coupled with high correlation in each group, have broad pipes and form the average tunneling routes, called Trp route and Met route. Each of the two average tunneling routes is composed of a few major pathways in the Pathways model which are fused at considerable part to each other. We also analyzed the average tunneling route for the ET by the inelastic tunneling mechanism.     

Single-electron tunneling spectroscopy of single C60 in double-barrier tunnel junction

The single-electron tunneling (SET) spectroscopy of C(60) molecule in a double-barrier tunnel junction is investigated by combining the scanning tunneling spectroscopy experiment and the theoretical simulation using the modified orthodox theory. The interplay between the SET effect and the discrete energy levels of C(60) molecule is studied. Three types of SET spectroscopies with different characters are obtained, corresponding to different tunneling processes and consistent with the previous theoretical prediction. Both the charging mode and resonance mode can arouse the current increase in the SET spectroscopy. The resonance mode is realized mainly by two mechanisms, including the resonance when the electron spans the second junction after already spanning the first junction. Some previous confused results have been clarified. Our results show that three types of SET spectroscopies can be together examined to quantitatively determine the frontier orbitals of the nanostructure by identifying the modes of various current increases.     

Using scanning electrochemical microscopy (SECM) to measure the electron-transfer kinetics of cytochrome c immobilized on a COOH-terminated alkanethiol monolayer on a gold electrode

Cytochrome c was electrostatically immobilized onto a COOH-terminated alkanethiol self-assembled monolayer (SAM) on a gold electrode at ionic strengths of less than 40 mM. Scanning electrochemical microscopy (SECM) was used to simultaneously measure the electron transfer (ET) kinetics of the bimolecular ET between a solution-based redox mediator and the immobilized protein and the tunneling ET between the protein and the underlying gold electrode. Approach curves were recorded with ferrocyanide as a mediator at different coverages of cytochrome c and at different substrate potentials, allowing the measurement of k(BI) = 2 x 10(8) mol(-1) cm3 s(-1) for the bimolecular ET and k degrees = 15 s(-1) for the tunneling ET. The kinetics of ET was also found to depend on the immobilization conditions of cytochrome c: covalent attachment gave slightly slower tunneling ET values, and a mixed CH3/COOH-terminated ML gave faster tunneling ET rates. This is consistent with previous studies and is believed to be related to the degree of mobility of cyt c in its binding configuration and its orientation with respect to the underlying electrode surface.     

On the electron transfer through Geobacter sulfurreducens PilA protein

Geobacter sulfurreducens pili composed of the Type IV pili structural peptide PilA have been implicated as efficient electronic conductors. Though investigated experimentally, no detailed theoretical studies have been performed to date that provide quantitative estimation of the transmission spectrum, electron transfer (ET) paths, efficiency of current generation, and other factors needed for understanding possible mechanisms of conductivity. In the present work, we calculate from first principles the possibilities of electron tunneling through 3 PilA fragments which structure was identified recently by NMR. The results indicate that positively charged amino acids, arginines and lysines form electrostatic traps in the middle of the peptide preventing ET at low bias voltages (<～6 V). At higher biases the traps are filled with electrons making possible sequential electron tunneling through the central part of the protein. In addition, leucines and phenylalanines form ET loops facilitating electron stabilization within the protein and sequential ET. Our results indicate that ET through the PilA protein cannot occur by coherent ET, but suggest a sequential (incoherent) mechanism. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1706–1717     

Anomalous temperature-isotope dependence in proton-coupled electron transfer

Motivated by the experiments of Hodgkiss et al. [J. Phys. Chem. (submitted)] on electron transfer (ET) through a H-bonding interface, we present a new theoretical model for proton-coupled electron transfer (PCET) in the condensed phase, that does not involve real proton transfer. These experiments, which directly probe the joint T-isotope effects in coupled charge transfer reactions, show anomalous T dependence in k(H)k(D), where k(H) and k(D) are the ET rates through the H-bonding interface with H-bonded protons and deuterons, respectively. We address the anomalous T dependence of the k(H)k(D) in our model by attributing the modulation of the electron tunneling dynamics to bath-induced fluctuations in the proton coordinate, so that the mechanism for coupled charge transfer might be better termed vibrationally assisted ET rather than PCET. We argue that such a mechanism may be relevant to understanding traditional PCET processes, i.e., those in which protons undergo a transfer from donor to acceptor during the course of ET, provided there is an appropriate time scale separating both coupled charge transfers. Likewise, it may also be useful in understanding long-range ET in proteins, where tunneling pathways between redox cofactors often pass through H-bonded amino acid residues, or other systems with sufficiently decoupled proton and electron donating functionalities.     

1,4‐Benzoquinone Derivatives for Enhanced Bioelectrocatalysis by Fructose Dehydrogenase from Gluconobacter Japonicus: Towards Promising D‐Fructose Biosensor Development

D‐fructose amount in food and beverages should be carefully controlled in order to avoid its overconsumption by human, which can lead to various metabolic diseases. Thus, the development of a low‐cost and portable (bio)sensors, which realize the on‐site monitoring of D‐fructose, is still highly required. In this work, we proposed several reagentless bioelectrochemical systems (BioESs) based on fructose dehydrogenase EC1.1.99.11 from Gluconobacter japonicus (FDH) and on 2‐arylamine‐1,4‐benzoquinone (ABQ) derivatives as a platform for the development of D‐fructose electrochemical biosensor. To design a reliable and easily reproducible BioES, we used the simple physical sorption as the electrode modification strategy for ABQs and FDH immobilization that is suitable for low‐cost mass production of the biosensors by standard microfabrication techniques. To choose the most suitable ABQ compounds for BioESs design, we proposed a novel theoretical approach of potential ET mediator evaluation through its electrochemical properties. A set of newly synthesized and of previously applied ABQs was characterized both electrochemically and using the density functional theory (DFT). It was shown that results obtained by quantum chemical analysis were in a good agreement with the ABQ characteristics obtained in electrochemical measurements. We suggest that the calculated local ionization energy values and theoretical redox potential obtained by DFT are a powerful tool for prediction of ABQs electrochemical properties. The performance of the BioESs with FDH under study was determined by electrochemical and electronic properties of ABQ derivatives and FDH orientation and stabilization on the electrode surface. The most promising BioES was based on carbon paste electrodes and 2‐(3‐nitro(phenyl)amino)‐ cyclohexa‐2,5‐dien‐1,4‐dione as ET mediator, which accelerated FDH catalysis and improved its stability better, then other studied ABQs. The bioelectrocatalytic process was characterized by initial apparent maximum current density of 7.1 μA cm^?2 at the optimal conditions (+400 mV vs. Ag/AgCl, McIlvaine's buffer, pH 5.0 and 20 °C). The findings of this research open new possibilities for development of cost‐effective biosensors with FDH and, potentially, with other redox enzymes.     

Electrochemistry at chemically assembled single-wall carbon nanotube arrays

Single-wall carbon nanotubes (SWNTs) chemically assembled on gold substrates were employed as electrodes to investigate the charge transfer process between SWNTs and the underlying substrates. Cyclic voltammetry (CV) indicates that the assembled SWNTs allow electron communication between a gold electrode and the redox couple in solution, though the SWNTs are linked directly onto the insulating monolayer of 11-amino-n-undecanethiol (AUT) on the Au substrate. An electron transfer (ET) mechanism, which contains an electron tunneling process across the AUT monolayer, is proposed to explain the CV behavior of Au/AUT/SWNT electrodes. Electrochemical measurements show that the apparent electron tunneling resistance, which depends on the surface density of assembled SWNTs, has apparent effects similar to those of solution resistance on CV behavior . The theory of solution resistance is used to describe the apparent tunneling resistance. The experimental results of the dependence of ET parameter psi on the potential scan rate upsilon are in good agreement with the theoretical predictions. Kinetic studies of the chemical assembly of SWNTs by atomic force microscopic (AFM), electrochemical, and Raman spectroscopic methods reveal that two distinct assembly kinetics exist: a relatively fast step that is dominated by the surface reaction, and a successive slow step that is governed by bundle formation.     

Long-range interfacial electron transfer and electrocatalysis of molecular scale <Emphasis Type="Italic">Prussian</Emphasis> Blue nanoparticles linked to Au(111)-electrode surfaces by different chemical contacting groups

We have explored interfacial electrochemical electron transfer (ET) and electrocatalysis of 5–6 nm Prussian Blue nanoparticles (PBNPs) immobilized on Au(111)-electrode surfaces via molecular wiring with variable-length, and differently functionalized thiol-based self-assembled molecular monolayers (SAMs). The SAMs contain positively (?NH₃ ⁺) or negatively charged (–COO–) terminal group, as well an electrostatically neutral hydrophobic terminal group (–CH₃). The surface microscopic structures of the immobilized PBNPs were characterized by high-resolution atomic force microscopy (AFM) directly in aqueous electrolyte solution under the same conditions as for electrochemical measurements. The PBNPs displayed fast and reversible interfacial ET on all the surfaces, notably in multi-ET steps as reflected in narrow voltammetric peaks. The ET kinetics can be controlled by adjusting the length of the SAM forming linker molecules. The interfacial ET rate constants were found to depend exponentially on the ET distance for distances longer than a few methylene groups in the chain, with decay factors (β) of 0.9, 1.1, and 1.3 per CH₂, for SAMs terminated by ?NH₃ ⁺,–COO–, and–CH₃, respectively. This feature suggests, first that the interfacial ET processes follow a tunneling mechanism, resembling that of metalloproteins in a similar assembly. Secondly, the electronic contact of the SAM terminal groups that anchor non-covalently the PBNP are crucial as reported for other types of molecular junctions. Highly efficient PBNP electrocatalysis of H₂O₂ reduction was also observed for the three linker groups, and the electrocatalytic mechanisms analyzed.     

Electron transfer processes in scanning tunneling spectroscopy through small supported particles

Mechanism of the staircase like I–V curve observed recently in scanning tunneling spectroscopy (STS) of a metal fine particle supported on an oxide covered substrate is clarified based on theoretical simulations. It is discussed how the step structures are influenced by the coupling of the fine particle charge to the remaining degrees of freedom, such as induced charge in the surrounding medium. Relation between the characteristic features of single electron tunneling (SET) and the STS of micro-clusters is discussed     

Long-distance electron tunneling in proteins

Long-distance tunneling is the major mechanism of electron transfer (ET) in proteins. For a number of years, a major question has been whether specific electron tunneling pathways exist. This question is still debated in the literature, because the pathways are not observed directly, and interpretation of experimental results on ET rates involves ambiguities. The extremely small tunneling interactions are difficult to calculate accurately. Recently, there has been remarkable progress in the area; however, some problems still remain unresolved. The accurate prediction of the absolute rates of long-distance ET reactions and other biological charge-transfer reactions is a particularly pressing issue. The current theoretical calculations indicate that the specific paths do exist in static protein structures. However, the protein motions can result in significant averaging of the spatial tunneling patterns, and it is not clear how accurately subtle quantum interference effects are described by the present theories. The key to resolving these issues is to perform accurate, first-principles calculations of electron tunneling that include the dynamics of the protein. This paper reviews some of theoretical issues of electron tunneling dynamics in inhomogeneous organic media.     

Interference, fluctuation, and alternation of electron tunneling in protein media. 2. Non-condon theory for the energy gap dependence of electron transfer rate

Developing the quantum transition rate theory of Prezhdo and Rossky (J. Chem. Phys. 1997, 107, 5863), we produced a new non-Condon theory of the rate of electron transfer (ET) which happens through a protein medium with conformational fluctuation. The new theory is expressed by a convolution form of the power spectrum for the autocorrelation function of the electronic tunneling matrix element T(DA)(t) with quantum correction and the ordinary Franck-Condon factor. The new theory satisfies the detailed balance condition for the forward and backward ET rates. The ET rate formula is divided into two terms of elastic and inelastic tunneling mechanisms on the mathematical basis. The present theory is applied to the ET from Bph(-) to Q(A) in the reaction center of Rhodobacter sphaeroides. Numerical calculations of T(DA)(t) were made by a combined method of molecular dynamics simulations and quantum chemistry calculations. We showed that the normalized autocorrelation function of T(DA)(t) is almost expressed by exponential forms. The calculated energy gap law of the ET rate is nearly Marcus' parabola in most of the normal region and around the maximum region, but it does not decay substantially in the inverted region, which is called the anomalous inverted region. We also showed that the energy gap law at the high uphill energy gap in the normal region is elevated considerably from the Marcus' parabola, which is called the anomalous normal region. Those anomalous energy gap laws are due to the inelastic tunneling mechanism which works actively at the energy gap far from zero. We presented an empirical formula for easily calculating the non-Condon ET rate, which is usable by many researchers. We provided experimental evidence for the anomalous inverted region which was basically reproduced by the present theory. The present theory was extensively compared with the previous non-Condon theories.     

Schemes for Single Electron Transistor Based on Double Quantum Dot Islands Utilizing a Graphene Nanoscroll,Carbon Nanotube and Fullerene

The single electron transistor (SET) is a nanoscale switching device with a simple equivalent circuit. It can work very fast as it is based on the tunneling of single electrons. Its nanostructure contains a quantum dot island whose material impacts on the device operation. Carbon allotropes such as fullerene (C₆₀), carbon nanotubes (CNTs) and graphene nanoscrolls (GNSs) can be utilized as the quantum dot island in SETs. In this study, multiple quantum dot islands such as GNS-CNT and GNS-C₆₀ are utilized in SET devices. The currents of two counterpart devices are modeled and analyzed. The impacts of important parameters such as temperature and applied gate voltage on the current of two SETs are investigated using proposed mathematical models. Moreover, the impacts of CNT length, fullerene diameter, GNS length, and GNS spiral length and number of turns on the SET’s current are explored. Additionally, the Coulomb blockade ranges (CB) of the two SETs are compared. The results reveal that the GNS-CNT SET has a lower Coulomb blockade range and a higher current than the GNS-C₆₀ SET. Their charge stability diagrams indicate that the GNS-CNT SET has smaller Coulomb diamond areas, zero-current regions, and zero-conductance regions than the GNS-C₆₀ SET.     

Electron transfer at self-assembled monolayers measured by scanning electrochemical microscopy

New approaches have been developed for measuring the rates of electron transfer (ET) across self-assembled molecular monolayers by scanning electrochemical microscopy (SECM). The developed models can be used to independently measure the rates of ET mediated by monolayer-attached redox moieties and direct ET through the film as well as the rate of a bimolecular ET reaction between the attached and dissolved redox species. By using a high concentration of redox mediator in solution, very fast heterogeneous (10(8) s(-1)) and bimolecular (10(11) mol(-1) cm(3) s(-1)) ET rate constants can be measured. The ET rate constants measured for ferrocene/alkanethiol on gold were in agreement with previously published data. The rates of bimolecular heterogeneous electron transfer between the monolayer-bound ferrocene and water-soluble redox species were measured. SECM was also used to measure the rate of ET through nonelectroactive alkanethiol molecules between substrate gold electrodes and a redox probe (Ru(NH(3))(6)(3+)) freely diffusing in the solution, yielding a tunneling decay constant, beta, of 1.0 per methylene group.     

Nanoscale ordered structure of fullerenes on the surface of highly oriented pyrolytic graphite

The formation of an ordered arrangement of C₆₀ molecules as path-like structures on the surface of highly oriented pyrolytic graphite (HOPG) is reported for the first time with theoretical implementations. Fullerene nucleation and deposition from solutions with different concentrations of C₆₀ is performed under ambient conditions without electrochemical processes. Scanning tunneling microscopy (STM) is used to study the surface topography. The results reveal new aspects of fullerene deposition that can potentially aid in modeling with theoretical simulations.     

Structural correlations in heterogeneous electron transfer at monolayer and multilayer graphene electrodes

As a new form of carbon, graphene is attracting intense interest as an electrode material with widespread applications. In the present study, the heterogeneous electron transfer (ET) activity of graphene is investigated using scanning electrochemical cell microscopy (SECCM), which allows electrochemical currents to be mapped at high spatial resolution across a surface for correlation with the corresponding structure and properties of the graphene surface. We establish that the rate of heterogeneous ET at graphene increases systematically with the number of graphene layers, and show that the stacking in multilayers also has a subtle influence on ET kinetics.     

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.     

The application of copper(II) deactivator on the single‐electron transfer living radical polymerization of tert‐butyl acrylate

We demonstrate the living radical polymerization of tert‐butyl acrylate (tBA) applying the SET mechanism, employing methyl 2‐bromopropionate (MBP) as initiator in dimethyl sulfoxide (DMSO) at ambient temperature. It is observed that introducing copper bromide into the catalyst system is necessary for controlling on the SET‐LRP polymerization of tBA. In this work, we make major investigation for the effect of the different stoichiometry quantity of copper bromide on the polymerization. Experiments show that the polymerization achieves better control with increasing the stoichiometry quantity of copper(II) deactivator. The structural analysis of the resulting polymers by ¹H NMR demonstrates the successful synthesis of poly(tBA)s by SET‐LRP in DMSO. Moreover, this work is helpful to the SET‐LRP of other monomers and is expected to expand the application of SET‐LRP. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2793–2797, 2010     

Iridium–Titanium Textured Electrodes: Production and Investigation by Electrochemical Scanning Tunneling Microscopy and Spectroscopy

A technique for the production of perfect thin-layered Ir coatings on inert Ti supports is developed. The highly textured coatings have some potential uses. Local topography and energy nonuniformness of the surface of such Ir electrodes are studied by electrochemical scanning tunneling microscopy (ESTM) and scanning tunneling spectroscopy (STS). In situ STM images of Ir–Ti textured electrodes with axial texture (111) are obtained with an atomic resolution at potentials of 0.3 to 1.2 V, in 0.05 M H₂SO₄ as well. Energy states of surfaces of Ir–Ti textured electrodes are studied with an atomic resolution using in situ STS by distance and voltage. Dependences of the tunneling current on the tunneling voltage and the tunneling-gap width are measured at Ir-surface potentials of 0.3 to 1.2 V. Effective potential barrier for the electron tunneling is estimated at different potentials of Ir.