Barrier For Electron Transfer Research Articles

Electrochemiluminescent immunoassay for the lung cancer biomarker CYFRA21-1 using MoOx quantum dots.

Molybdenum oxide quantum dots (MoOx QDs) were synthesized by a one-pot method and used as a versatile probe in an electrochemiluminescent (ECL) immunoassay of the non-small cell lung cancer biomarker cytokeratin 19 fragment21-1 (CYFRA21-1) as a model analyte. The MoOx QDs exhibited stable and strong cathodic green ECL, with an emission peak at 535nm, in the presence of K2S2O8 within the potential range of -2.0 to 0V. On exposure toCYFRA21-1, theECL decreases because of the immunoreaction between CYFRA21-1 and its antibody which generates a barrier for electron transfer. The determination of CYFRA21-1 with favorable analytical performances was successfully realized under the optimal conditions. ECL decreases linearly in the 1pgmL-1 to 350ngmL-1 CYFRA21-1 concentration range, and the detection is as low as 0.3pgmL-1. Excellent recoveries from CYFRA21-1-spiked human serum indicate that the assay can be operated under physiological conditions. Graphical abstractSchematic representation of the fabrication of molybdenum oxide quantum dots (MoOx QDs) and the electrochemiluminescent (ECL) immunoassay based on the use of theMoOx QDs ECL probe for cytokeratin 19 fragment21-1 (CYFRA21-1).

Polarizability of the Active Site in Enzymatic Catalysis: Cytochrome c.

Anisotropic polarizability of the heme in cytochrome c is found to be a major factor in suppressing the activation barrier of protein electron transfer (catalytic effect). Polarizability couples to the electric field of protein and water to enhance fluctuations of the electron-transfer energy gap and the corresponding variance reorganization energy λvar. The reorganization energy observable by kinetic measurements λr = (λSt)2/λvar is composed of λvar and the Stokes-shift reorganization energy λSt. It is lowered compared to the usually reported λSt due to polarizability of the active site leading to λvar > λSt. The coupling of electrostatic protein-water fluctuations to the polarizable active site is accounted for here by empirical valence-bond diagonalization of the active-site Hamiltonian along the simulation trajectory. We show that recent simulations employing this technique, which failed to find the effect of polarizability on electron-transfer kinetics, were erroneous in neglecting the diagonal dipole moments in the Hamiltonian matrix and failing to rotate the electric field produced by the protein-water medium into the molecular frame of the active site. We find that anisotropy of the tensor of polarizability difference in the two oxidation states of the heme matches anisotropy of the second-rank tensor constructed from the electric field at the active site. Exposure of the heme to water from only one side carries significant catalytic function, directly leading to the field anisotropy and the corresponding depression of the activation barrier.

Regulation of electron-hole recombination kinetics on uniform metal-semiconductor nanostructures for photocatalytic hydrogen evolution

Photocatalytic hydrogen evolution has garnered considerable attention as a potential technology for the conversion of solar energy to chemical energy to replace fossil fuels with the development of hydrogen energy infrastructure. Semiconductors have been intensively studied as photocatalysts due to their tunable bandgap, eco-friendly reaction mechanism, photochemical stability, and ease of reusability. To achieve highly efficient photocatalysts, regulation of exctions, which are photoinduced electrons and holes in photocatalysts, is necessary. Semiconductor nanoparticles have been applied in this purpose because of their confined exciton pathways and differentiated catalytic characteristics depending on their size, shape, and morphology. In addition, metal cocatalysts have been decorated with semiconductor nanoparticles because the metal cocatalyst not only provides efficient shuttling of photoinduced electrons and proper reaction sites for the hydrogen evolution but also controls exciton pathways via fast electron transfer kinetics from semiconductor to metal. This research update reviews recent advances in representative metal-semiconductor hybrid nanostructures of core-shell and tipped nanorods for photocatalysts with a focus on the exciton pathways. The metal at semiconductor core-shell nanostructures has shown extraordinary photocatalytic stability via passivation of the metal by a semiconductor. In photocatalytic hydrogen evolution, the semiconductor shell hinders electron transfer to water. Hence, various core-shell related metal-semiconductor nanostructures such as yolk-shell, core-island shell, and double shell hollow structures have been proposed in efforts to overcome the electron transfer barrier to water. Metal tipped nanorods are another versatile nanostructure to control and monitor exciton pathways. The correlation between exciton pathways and photocatalytic efficiencies was demonstrated by monitoring metal tipped semiconductor nanorods with different composition, morphology, and surface structure. The insights reported here suggest a rational and versatile design strategy of metal-semiconductor hybrid nanostructures for developing highly efficient photocatalysts for hydrogen evolution.

Defect Engineering of Palladium-Tin Nanowires Enables Efficient Electrocatalysts for Fuel Cell Reactions.

The defect engineering of noble metal nanostructures is of vital importance because it can provide an additional yet advanced tier to further boost catalysis, especially for one-dimensional (1D) noble metal nanostructures with a high surface to bulk ratio and more importantly the ability to engineer the defect along the longitudinal direction of the 1D nanostructures. Herein, for the first time, we report that the defect in 1D noble metal nanostructures is a largely unrevealed yet essential factor in achieving highly active and stable electrocatalysts toward fuel cell reactions. The detailed electrocatalytic results show that the Pd-Sn nanowires (NWs) exhibit interesting defect-dependent performance, in which the defect-rich Pd4Sn wavy NWs display the highest activity and durability for both the methanol oxidation reaction (MOR) and the oxygen reduction reaction (ORR). Density functional theory (DFT) calculations reveal that a large number of surface vacancies/agglomerated voids are the driving forces for forming surface grain boundaries (GBs) within Pd4Sn WNWs. These electronic active GB regions are the key factors in preserving the number of Pd0 sites, which are critical for minimizing the intrinsic site-to-site electron-transfer barriers. Through this defect engineering, the Pd4Sn WNWs ultimately yield highly efficient alkaline ORR and MOR. The present work highlights the importance of defect engineering in boosting the performance of electrocatalysts for potentially practical fuel cells and energy applications.

Improving Amorphous Carbon Anodes for Na Ion Batteries by Surface Treatment of a Presodiated Electrode with Al2O3.

Disordered carbons are promising anode materials for sodium ion batteries. However, a major drawback of these materials is their low coulombic efficiency in the first cycles, which indicates parasitic reactions. Such reactions can be suppressed by alumina coating on the surface of the anodic materials; more ions are then available for electrochemical activity, and less electrolyte solution is lost. On the other hand, some pores and surface edge sites are passivated by the coating and are no longer available for reversible reaction with sodium ions; hence, their contribution is eliminated, leading to reduction in specific capacity. We show herein that electrochemical insertion of sodium ions into carbon anodes prior to alumina coating has a double positive effect on anode perfomances, meaning preventing passivation and maintaining high specific capacity. We show that the artificial layer still prevented parasitic reactions, while the pores and surface edge sites retained electrochemical activity. The capacity values were thus restored and even became higher as a result of preventing the development of a surface layer. Ultraviolet photoelectron spectroscopy measurements assessed the energetic states of the electrodes and confirmed that the alumina coating forms a barrier for interfacial electron transfer from the electrode to the solution at any polarization stage.

Ni3+-Induced Hole States Enhance the Oxygen Evolution Reaction Activity of NixCo3–xO4 Electrocatalysts

This work reports a systematical study on the relationship of electronic structure to oxygen evolution reaction (OER) activity of NixCo3–xO4 (x = 0–1) mixed oxides. The specific OER activity is substantially increased by 16 times from 0.02 mA cm–2BET for pure Co3O4 to 0.32 mA cm–2BET for x = 1 at an overpotential of 0.4 V and exhibits a strong correlation with the amount of Ni ions in the +3 oxidation state. X-ray spectroscopic study reveals that inclusion of Ni3+ ions upshifts the occupied valence band maximum (VBM) by 0.27 eV toward the Fermi level (EF), and creates a new hole (unoccupied) state located ∼1 eV above the EF. Such electronic features favor the adsorption of OH surface intermediates on NixCo3–xO4, resulting in enhanced OER. Furthermore, the emerging hole state effectively reduces the energy barrier for electron transfer from 1.19 to 0.39 eV, and thereby improves the kinetics for OER. The electronic structure features that lead to a higher OER in NixCo3–xO4 can be extended to other transitio...

Performance Improvement of Dye-Sensitized Solar Cell- (DSSC-) Based Natural Dyes by Clathrin Protein

Dye-Sensitized Solar Cell (DSSC) is a solar cell device that works using electrochemical principles in which sensitive dyes are absorbed in the TiO2 photoelectrode layer. The problem of DSSC-based natural dyes is the lower efficiency than silicon solar cells. This low efficiency is due to the barrier of electron transfer in the TiO2 semiconductor layer. In this study, the addition of clathrin protein to the TiO2 layer was used to increase electron transfer in the semiconductor layer resulting in improved DSSC performance. Clathrin is a protein that plays a role in the formation of transport vesicle membrane in eukaryotic cells. The method used in this study is clathrin protein with a concentration of 0%, 25%, 50%, and 75% added to TiO2 in DSSC structure. Photovoltaic characteristics of DSSC were measured using a data logger to determine the performance of DSSC, layer morphology was analyzed using Scanning Electron Microscopy (SEM), the element content in DSSC was analyzed using Energy-Dispersive X-ray Spectroscopy (EDS), and functional groups in DSSC layers were analyzed using Fourier-Transform Infrared Spectroscopy (FTIR). The result of this study is the addition of clathrin protein can improve DSSC performance, which resulted in the highest performance of DSSC on 75% clathrin protein addition with efficiency=1.465%, Isc=5.247 mA, and Voc=657 mV. From the results of SEM analysis, it appears that clathrin protein molecules fill the cavities in TiO2 molecules. EDS analysis shows an increase in carbon, oxygen, and phosphorus content in TiO2 layers with increasing clathrin protein concentration. FTIR analysis shows an increasingly sharp absorption in the FTIR spectrum of protein-forming functional groups by increasing clathrin protein concentration in DSSC.

Interfacial Defect Engineering for Improved Portable Zinc–Air Batteries with a Broad Working Temperature

AbstractAtomic‐thick interfacial dominated bifunctional catalyst NiO/CoO transition interfacial nanowires (TINWs) with abundant defect sites display high electroactivity and durability in the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR). Density functional theory (DFT) calculations show that the excellent OER/ORR performance arises from the electron‐rich interfacial region coupled with defect sites, thus enabling a fast‐redox rate with lower activation barrier for fast electron transfer. When assembled as an air‐electrode, NiO/CoO TINWs delivered the high specific capacity of 842.58 mAh gZn−1, the large energy density of 996.44 Wh kgZn−1 with long‐time stability of more than 33 h (25 °C), and superior performance at low (−10 °C) and high temperature (80 °C).

Interfacial Defect Engineering for Improved Portable Zinc-Air Batteries with a Broad Working Temperature.

Atomic-thick interfacial dominated bifunctional catalyst NiO/CoO transition interfacial nanowires (TINWs) with abundant defect sites display high electroactivity and durability in the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR). Density functional theory (DFT) calculations show that the excellent OER/ORR performance arises from the electron-rich interfacial region coupled with defect sites, thus enabling a fast-redox rate with lower activation barrier for fast electron transfer. When assembled as an air-electrode, NiO/CoO TINWs delivered the high specific capacity of 842.58 mAh gZn -1 , the large energy density of 996.44 Wh kgZn -1 with long-time stability of more than 33 h (25 °C), and superior performance at low (-10 °C) and high temperature (80 °C).

(Invited) Engineering of Enzymatic Bioelectrochemical Interface for In Vivo Analysis

Bioelectrocatalysts have enabled a rapid development of sensing methods and devices for in vivo quantification of chemical species, which is crucial for understanding the molecular basis of life. However, these sophisticated functional units face a set of challenges when operated in vivo. One inevitable issue is the kinetic barrier for heterogeneous electron transfer at the electrode-enzyme interface, especially when direct electron transfer (DET) is preferred as toxic and unstable mediators are not suitable for in vivo applications. Different strategies have been explored to improve DET efficiency, and lately, a new method for regulating the molecular orientation of laccase on single-walled carbon nanotubes was developed based on the principle of surface wetting. Electrochemical and vibrational spectroscopic investigation drew a mechanistic picture of how enzymes behave in the presence of wetting reagents (such as ethanol), offering a new guidance for the design of bioelectrochemical interfaces. Another issue associated with in vivo biosensing rises from interference of intrinsically existing substrates (other than the analytes) for enzymes, e.g., oxygen and NAD+ that function as electron acceptors for commonly employed oxidases and dehydrogenases. In our recent studies, we have explored a new enzyme candidate, ferredoxin-dependent glutamate synthase (Fd-GltS), as a bioelectrocatalyst. Containing multiple redox centers, Fd-GltS can be fined tuned with various mediators to enable either bioelectrosynthsis of glutamate or bioelectrochemical oxidation of glutamate. By this means, we can construct a new bioelectrochemical interface devoid of oxygen and cofactor interference, thus providing a new option for the development of biosensing platform for glutamate-mediated neurotransmission.

110th Anniversary: Unleashing the Full Potential of Quinones for High Performance Aqueous Organic Flow Battery

Aqueous organic flow batteries (AOFBs) are promising energy storage solutions to counteract the intermittent and fluctuating nature of renewable energy. However, we have limited options of electrolyte chemistry and we discarded many organic compounds because of their sluggish electrochemical kinetics, which would compromise the power capability of an AOFB. Here, exemplified by 2,5-dihydroxy-3,6-dimethyl-1,4-benzoquinone (DMBQ), we present two approaches including engineering the molecular structure and utilizing an inexpensive catalyst to enhance the electrochemical kinetics of benzoquinones with the ultimate purpose of diminishing the electron transfer barrier thereby increasing the power capability of the AOFB. We show that, by exploiting these strategies, the electron transfer resistance could be reduced by 48.1%, or 55.8%, respectively, thereby leading to a 49.4% or 60.7% increase in the peak power density of a flowing cell. We believe our strategy could be extended to the full family of quinones and ...

Effect of ZrO2 barrier layers on the photovoltaic parameters of rose bengal dye-sensitized TiO2 solar cell

Dye-sensitized solar cells (DSSCs) based on thin porous TiO2 and ZrO2 coated TiO2 thin films with rose bengal as the sensitizer were investigated. The TiO2 as well as ZrO2 coated TiO2 photo-electrodes were characterized by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy and UV–vis spectroscopy. The electrochemical behavior of TiO2, TiZr5, TiZr10 and TiZr15 photo-electrodes were studied using electrochemical impedance spectroscopy. A facile successive ionic layer adsorption reaction (SILAR) method was used to deposit a thin overlayer of ZrO2 on TiO2 photo-electrode film to minimize interfacial recombination losses as the ZrO2 layer acts as a barrier for electron transfer at the interfaces. Thin ZrO2 layers were deposited by alternate dipping TiO2 film in 0.1 M Zr(SO4)2 and 0.1 M NaOH solutions, maintained at room temperature, followed by annealing at 450 °C. The cells fabricated using these films showed good correlation between current/voltage analyses. The introduction of ZrO2 overlayers with five SILAR cycles increases the overall cell efficiency to 0.052% from 0.030% of the pure TiO2 photo-electrode film. The improved solar conversion efficiency exhibited by TiZr5 makes the technique as a novel promising method to enhance efficiency of DSSC.

Microfluidic-enhanced 3-D photoanodes with free interfacial energy barrier for photoelectrochemical applications

The sluggish reaction kinetics and poor interfacial mass transfer seriously limit the industrial applications of planar photoelectrocatalytic devices. Here, the principle of 3-D flow-through photoanodes with free interfacial barrier for electron transfer and microfluidic channels for reactant transportation was demonstrated. Owing to the epitaxial growth of anisotropic ZnO nanorods with internal electrostatic field onto carbon cloth (CC), the spatial separation of photo-induced charge carriers was realized. Experimental characterizations confirmed the formation of Schottky-barrier-free interface with low electrical resistance for electron transfer, resulting in the significantly decreased onset potential. Compared to traditional planar FTO/ZnO photoanodes, flow-through CC/ZnO photoanodes exhibited 4 times (for Rhodamine B) and 3 times (for bisphenol A) higher degradation kinetics. Fluid dynamics simulation suggested that the flow-through mode greatly enhanced the microscale velocity magnitude and the mass transfer of reactants. Thus, this work presents an ideal platform for the design of 3-D microfluidic-enhanced system for photoelectrochemical applications.

Long distance electron transfer through the aqueous solution between redox partner proteins

Despite the importance of electron transfer between redox proteins in photosynthesis and respiration, the inter-protein electron transfer rate between redox partner proteins has never been measured as a function of their separation in aqueous solution. Here, we use electrochemical tunneling spectroscopy to show that the current between two protein partners decays along more than 10 nm in the solution. Molecular dynamics simulations reveal a reduced ionic density and extended electric field in the volume confined between the proteins. The distance-decay factor and the calculated local barrier for electron transfer are regulated by the electrochemical potential applied to the proteins. Redox partners could use electrochemically gated, long distance electron transfer through the solution in order to conciliate high specificity with weak binding, thus keeping high turnover rates in the crowded environment of cells.

Energy Level Evolution and Oxygen Exposure of Fullerene/Black Phosphorus Interface

The heteroepitaxial growth of fullerene (C60) on single-crystal black phosphorus (BP) has been studied using low-energy electron diffraction, X-ray and ultraviolet photoelectron spectroscopy, and density functional theory simulation. The occupied orbital features from C60 observed in the photoelectron spectra for C60/BP interface are slightly broadened at higher coverages of C60 and exhibit no direct evidence of hybridization, demonstrating that the C60/BP interaction is physisorption. Oxygen exposure of interface leads to obvious oxidation of BP in which C60 bridges the large electron-transfer barrier from BP to oxygen and plays an important role for the production of O2- and oxidation of BP. Our findings suggest that C60 does not form an ideal protection layer as the other n-type semiconductors. With the assistance of density functional theory calculations, the oxidized phosphorus at the interface prevents further charge transfer from BP to C60.

Mixed‐Valent Molecular Triple Deckers

AbstractTwo phenothiazine (PTZ) moieties were connected via naphthalene spacers to a central arene to result in stacked PTZ‐arene‐PTZ structure elements. Benzene and tetramethoxybenzene units served as central arenes mediating electronic communication between the two PTZ units. Based on cyclic voltammetry, UV/Vis‐NIR absorption, EPR spectroscopy, and computational studies, the one‐electron oxidized forms of the resulting compounds behave as class II organic mixed‐valence species in which the unpaired electron is partially delocalized over both PTZ units. The barrier for intramolecular electron transfer depends on the nature of the central arene sandwiched between the two PTZ moieties. These are the first examples of rigid organic mixed‐valent triple‐decker compounds with possible electron‐transfer pathways directly across a stacked structure, and they illustrate the potential of oligo‐naphthalene building blocks for long‐range electron transfer and a future molecular electronics technology.

Mixed-Valent Molecular Triple Deckers.

Two phenothiazine (PTZ) moieties were connected via naphthalene spacers to a central arene to result in stacked PTZ-arene-PTZ structure elements. Benzene and tetramethoxybenzene units served as central arenes mediating electronic communication between the two PTZ units. Based on cyclic voltammetry, UV/Vis-NIR absorption, EPR spectroscopy, and computational studies, the one-electron oxidized forms of the resulting compounds behave as class II organic mixed-valence species in which the unpaired electron is partially delocalized over both PTZ units. The barrier for intramolecular electron transfer depends on the nature of the central arene sandwiched between the two PTZ moieties. These are the first examples of rigid organic mixed-valent triple-decker compounds with possible electron-transfer pathways directly across a stacked structure, and they illustrate the potential of oligo-naphthalene building blocks for long-range electron transfer and a future molecular electronics technology.

Structure Sensitivity of Electrochemical Reactions from First Principles: Applications to Nitrogen and Water Cycles

Nitrogen cycle electrochemistry is an emerging area of interest in the electrocatalysis community, with applications ranging from removal of nitrates from wastewater streams to the development of fundamental understanding of NO electrochemistry. In spite of a significant amount of fundamental research for this chemistry on single crystal surfaces, however, the mechanistic details of the reactions are not fully known, and even such basic information as the nature of the rate-limiting step is not understood. In this work, we apply some of the innovations in theoretical electrochemistry that have emerged over the past 5-10 years to the study of NO and nitrate reduction on transition metal surfaces. We begin with a simple Density Functional Theory analysis of direct NO reduction in acid on Pt(111), Pt(100), and Pt(211) surfaces, and we identify potential- and rate-determining steps on each surface, using extensive ab-initio molecular dynamics simulations to estimate the impact of the aqueous environment on surface thermodynamics and kinetics (Ang. Chem. Int. Ed. 2015). We next introduce a novel approach that couples DFT calculations with rigorous kinetic Monte Carlo (kMC) simulations to understand, in unprecedented detail, the effect of higher adsorbed NO coverages on these chemistries, and we demonstrate that these coverage-dependent effects are essential for describing experimentally measured polarization curves (ACS Catalysis 2017). We subsequently generalize our analysis by developing universal Brønsted-Evans-Polanyi relationships that rigorously relate barriers for proton-coupled electron transfer to surface species to the corresponding thermodynamics across a large space of different transition metals and potentials. These relationships, combined with the potential- and coverage-dependent energetics determined on platinum, provide a basis to extrapolate our mechanistic analyses to other transition metals, including PtSn and related alloys. We employ these techniques to propose design strategies to tune selectivity of NO electroreduction to desired products, including N2 and hydroxylamine, on these different transition metals and alloys. If time permits, we will briefly describe some additional work on the development of structural and catalytic reactivity models at the interface between thin hydroxyoxide films and precious metal platinum substrates. We will examine the kinetics of the hydrogen evolution reaction as a function of the structure and oxidation state of these interfaces and will demonstrate that the bifunctional nature of the interfaces plays a key role in their electrocatalytic properties.

Electron Transfer in Spacer-Free DNA Duplexes Tethered to Gold via dA10 Tags.

Electrical properties of DNA critically depend on the way DNA molecules are integrated within the electronics, particularly on DNA-electrode immobilization strategies. Here, we show that the rate of electron transport in DNA duplexes spacer-free tethered to gold via the adenosine terminal region (a dA10 tag) is enhanced compared to the hitherto reported DNA-metal electrode tethering chemistries. The rate of DNA-mediated electron transfer (ET) between the electrode and methylene blue intercalated into the dA10-tagged DNA duplex approached 361 s-1 at a ca. half-monolayer DNA surface coverage ΓDNA (with a linear regression limit of 670 s-1 at ΓDNA → 0), being 2.7-fold enhanced compared to phosphorothioated dA5* tethering (6-fold for the C6-alkanethiol linker representing an additional ET barrier). X-ray photoelectron spectroscopy evidenced dA10 binding to the Au surface via the purine N, whereas dA5* predominantly coordinated to the surface via sulfur atoms of phosphothioates. The latter apparently induces the DNA strand twist in the point of surface attachment affecting the local DNA conformation and, as a result, decreasing the ET rates through the duplex. Thus, a spacer-free DNA coupling to electrodes via dA10 tags thus allows a perspective design of DNA electronic circuits and sensors with advanced electronic properties and no implication from more expensive, synthetic linkers.

Eliminating light soaking effect of inverted polymer solar cells functionalized with a conjugated macroelectrolyte electron-collecting interlayer

To overcome the light soaking and hysteresis characteristics in indium tin oxides (ITO)/poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) based bulk-heterojunction polymer solar cells, a smart gate dielectrics nano-interfacial layer was incorporated by spin-coating an ultrathin polyetherimide (PEI) between ITO and PFN electron extraction layer. The ultrathin PEI micromolecule can effectively fill the surface defects of ITO and surmount electron transport barrier upon forming attached dipoles. The passivation surface decreased the total interfacial electron localized state and electron transfer barrier, which could re-modulate the interfacial property of buffer layer and facilitate electron collection. The additional insulating layer can influence the photoconductivity response and consequently restrain photocurrent hysteresis phenomena, leading to the improved power conversion efficiency. This finding would be helpful to achieve large-scale and low-cost polymer solar cells without light soaking effect.