An electrochemical immunosensor using p-aminophenol redox cycling by NADH on a self-assembled monolayer and ferrocene-modified Au electrodes

Redox cycling of enzymatically amplified electroactive species has been widely employed for high signal amplification in electrochemical biosensors. However, gold (Au) electrodes are not generally suitable for redox cycling using a reducing (or oxidizing) agent because of the high background current caused by the redox reaction of the agent at highly electrocatalytic Au electrodes. Here we report a new redox cycling scheme, using nicotinamide adenine dinucleotide (NADH), which can be applied to Au electrodes. Importantly, p-aminophenol (AP) redox cycling by NADH is achieved in the absence of diaphorase enzyme. The Au electrodes are modified with a mixed self-assembled monolayer of mercaptododecanoic acid and mercaptoundecanol, and a partially ferrocenyl-tethered dendrimer layer. The self-assembled monolayer of long thiol molecules significantly decreases the background current of the modified Au electrodes, and the ferrocene modification facilitates easy oxidation of AP. The low amount of ferrocene on the Au electrodes minimizes ferrocene-mediated oxidation of NADH. In sandwich-type electrochemical immunosensors for mouse immunoglobulin G (IgG), an alkaline phosphatase label converts p-aminophenylphosphate (APP) into electroactive AP. The amplified AP is oxidized to p-quinoneimine (QI) by electrochemically generated ferrocenium ion. NADH reduces QI back to AP, which can be re-oxidized. This redox cycling enables a low detection limit for mouse IgG (1 pg mL(-1)) to be obtained.     

Electron transfer and interfacial behavior of redox proteins

This paper reviews the recent progress in the electron transfer and interfacial behavior of redox proteins. Significant achievements in the relevant fields are summarized including the direct electron transfer between proteins and electrodes, the thermodynamic and kinetic properties, catalytic activities and activity regulation of the redox proteins. It has been demonstrated that the electrochemical technique is an effective tool for protein studies, especially for probing into the electron transfer and int...     

Organic redox couples and organic counter electrode for efficient organic dye-sensitized solar cells

A series of organic thiolate/disulfide redox couples have been synthesized and have been studied systematically in dye-sensitized solar cells (DSCs) on the basis of an organic dye (TH305). Photophysical, photoelectrochemical, and photovoltaic measurements were performed in order to get insights into the effects of different redox couples on the performance of DSCs. The polymeric, organic poly(3,4-ethylenedioxythiophene) (PEDOT) material has also been introduced as counter electrode in this kind of noniodine-containing DSCs showing a promising conversion efficiency of 6.0% under AM 1.5G, 100 mW·cm(-2) light illumination. Detailed studies using electrochemical impedance spectroscopy and linear-sweep voltammetry reveal that the reduction of disulfide species is more efficient on the PEDOT counter electrode surface than on the commonly used platinized conducting glass electrode. Both pure and solvated ionic-liquid electrolytes based on a thiolate anion have been studied in the DSCs. The pure and solvated ionic-liquid-based electrolytes containing an organic redox couple render efficiencies of 3.4% and 1.2% under 10 mW·cm(-2) light illumination, respectively.     

SnS-quantum dot solar cells using novel TiC counter electrode and organic redox couples

Low-cost quantum-dot sensitized solar cells (QDSSCs) were fabricated by using the earth-abundant element SnS quantum dot, novel TiC counter electrodes, and the organic disulfide/thiolate (T(2)/T(-)) redox couple, and reached an efficiency of 1.03?%. QDSSCs based on I(-)/I(3)(-), T(2)/T(-), and S(2-)/S(x)(2-) redox couples were assembled to study the role of the redox couples in the regeneration of sensitizers. Charge-extraction results reveal the reasons for the difference in J(SC) in three QDSSCs based on I(-)/I(3)(-), T(2)/T(-), and S(2-)/S(x)(2-) redox couples. The catalytic selectivity of TiC and Pt towards T(2)/T(-) and I(-)/I(3)(-) redox couples was investigated using Tafel polarization and electrochemical impedance analysis. These results indicated that Pt and TiC show a similar catalytic selectivity for I(-)/I(3)(-). However, TiC possesses better catalytic activity for T(2)/T(-) than for I(-)/I(3)(-). These results indicate the great potential of transition metal carbide materials and organic redox couples used in QDSSCs.     

Electron transfer studies of redox probes in bovine milk

In this work, we show that milk can act as an electrolytic medium to study electrochemical processes in the absence of any supporting electrolyte. The electron transfer properties of three different redox systems in bovine homogenized whole milk, skimmed milk, and reconstituted milk powder have been studied by cyclic voltammetry and impedance spectroscopy using a three-electrode system with a gold disk working electrode, a platinum sheet counter electrode, and a standard calomel reference electrode. It has been shown that the milk incredibly sustains the redox reactions in the absence of any supporting electrolyte and the electrochemical responses are comparable to those obtained when the same reactions were carried out in standard solvent preparations containing supporting electrolytes. The study clearly demonstrates the potential of developing new innovative techniques based on the intricate concepts of electrochemistry to study various aspects of milk that may help in the development of analytical sensors for the diary industry.     

Electron transfer reactions of redox cofactors in spinach photosystem I reaction center protein in lipid films on electrodes

Thin film voltammetry was used to obtain direct, reversible, electron transfer between electrodes and spinach Photosystem I reaction center (PS I) in lipid films for the first time. This reaction center (RC) protein retains its native conformation in the films, and AFM showed that film structure rearranges during the first several minutes of rehydration of the film. Two well-defined chemically reversible reduction-oxidation peaks were observed for native PS I in the dimyristoylphosphatidylcholine films, and were assigned to phylloquinone, A(1) (E(m) = -0.54 V) and iron-sulfur clusters, F(A)/F(B) (E(m) = -0.19 V) by comparisons with PS I samples selectively depleted of these cofactors. Observed E(m) values may be influenced by protein-lipid interactions and electrode double-layer effects. Voltammetry was consistent with simple kinetically limited electron transfers, and analysis of reduction-oxidation peak separations gave electrochemical rate constants of 7.2 s(-)(1) for A(1) and 65 s(-)(1) for F(A)/F(B). A catalytic process was observed in which electrons were injected from PS I in films to ferredoxin in solution, mimicking in vivo electron shuttle from the terminal F(A)/F(B) cofactors to soluble ferredoxin during photosynthesis.     

Electron transfer in retrospect and prospect 1: Adiabatic electrode processes

The development of the theory of adiabatic electrode processes leading to its current state is briefly reviewed.     

Syntheses of molecular wires containing redox center： Reversible redox property and good energy level matching with Au electrode

Molecular wires with tetrathiafulvalene （TTF） as redox center were synthesized and characterized. UV-vis spectra and cyclic voltammetry showed these wires had good reversible redox behavior under ambient conditions and their HOMO energy levels （--5.0 eV） matched well with the Fermi level of Au （--5.1 eV）.     

Control of quinone redox potentials in photosystem II: Electron transfer and photoprotection

In O(2)-evolving complex Photosystem II (PSII), an unimpeded transfer of electrons from the primary quinone (Q(A)) to the secondary quinone (Q(B)) is essential for the efficiency of photosynthesis. Recent PSII crystal structures revealed the protein environment of the Q(A/B) binding sites. We calculated the plastoquinone (Q(A/B)) redox potentials (E(m)) for one-electron reduction with a full account of the PSII protein environment. We found two different H-bond patterns involving Q(A) and D2-Thr217, resulting in an upshift of E(m)(Q(A)) by 100 mV if the H bond between Q(A) and Thr is present. The formation of this H bond to Q(A) may be the origin of a photoprotection mechanism, which is under debate. At the Q(B) side, the formation of a H bond between D2-Ser264 and Q(B) depends on the protonation state of D1-His252. Q(B) adopts the high-potential form if the H bond to Ser is present. Conservation of this residue and H-bond pattern for Q(B) sites among bacterial photosynthetic reaction centers (bRC) and PSII strongly indicates their essential requirement for electron transfer function.     

Electrochemical oxidation of glucose at Hg adatom-modified Au electrode in alkaline aqueous solution

Electrochemical oxidation of glucose at Hg adatom-modified Au polycrystalline electrode was examined in alkaline aqueous solutions using cyclic voltammetry. Two oxidation current peaks for glucose were observed on Hg adatom-modified Au electrode at almost the same potentials as those observed on a bare Au electrode. The oxidation peak currents were much larger than those on a bare Au electrode in the concentration range from 0.5 to 20 mM. The observed enhancement of the glucose oxidation was considered to be due to the increase in the amount of the adsorbed OH⁻ on the Hg adatom-modified Au electrode. The reaction was catalyzed through the pairing of glucose and the intermediate in the oxidation to the large amount of the adsorbed OH⁻ on the Hg adatom-modified Au electrode.     

Electron transfer of heme proteins on the ITO electrode in polymer solvents

The redox reaction of poly(ethylene oxide) (PEO)-modified hemoglobin (PEO–Hb) was analyzed in PEO oligomers with cyclic voltammetry. The PEO–Hb was made soluble in PEO with molecular weight of 200 (PEO₂₀₀) containing 0.5 M KCI. Quasi-reversible redox signals of PEO–Hb were obtained by using an indium tin oxide (ITO) glass working electrode. PEO–Hb, cast on the ITO electrode, also showed the redox response in PEO with molecular weight of 400 (PEO₄₀₀). The peak current of PEO–Hb on the ITO electrode gradually increased during potential cycling. The effect of the scan rate on the quantity of electricity (Q) was analyzed after the peak current reached a constant value. The constant Q value was observed at the scan rate ranging from 30 to 500 mV/sec. The number of reactive PEO–Hb molecules was estimated from this constant Q-value. It was suggested that the electron transfer was carried out at the first layer of the PEO–Hb which was in direct contact with the ITO electrode. The quantity of electricity of PEO–Hb increased when the ITO electrode was first washed in an aqueous medium with ultrasonicator. This strongly suggested that the more effective surface area of the ITO electrode turned to be covered with PEO–Hb when the microporous region of the ITO particles was more hydrated.     

Theory of electrode reactions of redox couples confined to electrode surfaces at monolayer levels: Part II. Cyclic voltammetry and ac impedance measurements

The expression of the current-potential relationship derived in Part I for simple one-step surface redox-electrode reactions of the species confined to electrode surfaces is applied to cyclic voltammetry and the method of faradaic impedance measurements. A method to obtain cyclic voltammograms is presented for a quasi-reversible or general case and equations for reversible and irreversible cyclic voltammograms are derived. The effect of the interaction parameters, W/RT and ΔW^≠/RT, kinetic parameters, Λ and α, and coordination number z on the waveform of the cyclic voltammograms is discussed. An interesting feature in the voltammograms, i.e. the appearance of two peaks, is predicted when W/RT < −2.19 for z = 6 in spite of the simple one-step redox process. Furthermore, equations for the faradaic resistance and capacitance are derived and it is shown that the faradaic impedance is independent of the frequency of ac perturbation.     

Electron transfer between cytochrome c and copper enzymes

The electron transfer between cytochrome c and ascorbate oxidase or laccase from Coriolus hirsutus was investigated using both an electrochemical and a spectrophotometric method. A quasi-reversible cyclic volammogram of cytochrome c was observed on a gold electrode modified with 4,4′-dithiodipyridine. The addition of laccase resulted in the appearance of a catalytic current due to the regeneration of ferricytochrome c by laccase in the presence of oxygen. The second-order rate constant of the reaction between cytochrome c and laccase is calculated to be 9.2 × 10³ M⁻¹ s⁻¹ in 50 mM phosphate buffer of pH 5.8. The reaction rate with ascorbate oxidase is almost three orders of magnitude slower. The difference in the redox potential is considered to be the driving force of the reaction between cytochrome c and the copper proteins investigated.     

Electron transfer reactions between pentafluorobenzoyl peroxide and dimethoxybenzenes

Pentafluorobenzoyl peroxide (FBP) reacted rapidly with dimethoxybenzenes in F₁₁₃ (CCl₂FCClF₂) with kinetics of first order in each component. High yields of ring-substituted esters of $\text{m}$-dimethoxybenzene ($\text{m}$-DMB) and $\text{p}$-dimethoxybenzene ($\text{p}$-DMB) were obtained, whereas for 2,5-di-$\text{t}$-butyl-1,4-dimethoxybenzene (DBDMB), the $\text{t}$-Bu group was simultaneously eliminated. For 2,5-dimethyl-1,4-dimethoxybenzene (DMDMB), the benzylic hydrogen was substituted. Rate and product studies both indicate a rate-determining electron transfer step leading to radical ion pairs which collapse to products.     

Electron transfer between protonated and unprotonated phenoxyl radicals

The reaction of phenoxyl radicals with acids is investigated. 2,4,6-Tri-tert-butylphenoxyl radical (13), a persistent radical, deteriorates in MeOH/PhH in the presence of an acid yielding 4-methoxycyclohexa-2,5-dienone 18a and the parent phenol (14). The reaction is facilitated by a strong acid. Treatment of 2,6-di-tert-butyl-4-methylphenoxyl radical (2), a short-lived radical, generated by dissociation of its dimer, with an acid in MeOH provides 4-methoxycyclohexa-2,5-dienone 4 and the products from disproportionation of 2 including the parent phenol (3). A strong acid in a high concentration favors the formation of 4 while the yield of 3 is always kept high. Oxidation of the parent phenol (33) with PbO(2) to generate transient 2,6-di-tert-butylphenoxyl radical (35) in AcOH/H(2)O containing an added acid provides eventually p-benzoquinone 39 and 4,4'-diphenoquinone 42, the product from dimerization of 35. A strong acid in a high concentration favors the formation of 39. These results suggest that a phenoxyl radical is protonated by an acid and electron transfer takes place from another phenoxyl radical to the protonated phenoxyl radical, thus generating the phenoxyl cation, which can add an oxygen nucleophile, and the phenol (eq 5). The electron transfer is a fast reaction.     

Electron transfer at different electrode materials: Metals,semiconductors, and graphene

Electron transfer reactions are the most important processes at electrochemical interfaces. They are determined by the interplay between the interaction of the reactant with the solvent and the electronic levels of the electrode surface. Theoretical treatments only based on Density Functional Theory calculations are not sufficient. This review emphasizes mainly the effect of the electronic structure of the electrode material on electron transfer under different kinetic regimes. Our goal is to understand experimental results in the framework of a theory valid for arbitrary strengths of electronic coupling.     

Electron transfer kinetics with both reactant and product attached to the electrode surface

An open circuit charge injection (“coulostatic”) technique was employed to measure electron transfer kinetics for reactants confined to the surface of graphite electrodes. Iron protoporphyrin IX exhibited kinetics that were too fast to measure but the rate constant for the reduction of 9,10-phenanthrenequinone at its formal potential in 1 M HClO₄ was evaluated as 320 s^?1 at 22°C. Measurements of the rate constant at temperatures between 5 and 50°C produced a linear Arrhenius plot with an intercept which was rationalizable in terms of a frequency factor near kT/h. This result is utilized in a discussion of the factors that control the electrochemical reactivity of reactants in attached and unattached states.