Quasi-reversible Electron Transfer Research Articles

Convertible electron transfer pathways of cytochrome c at TiO2 quantum electrode

In this work, biofunctional ultrathin TiO2 quantum wires (about 3 nm), self-assembled into flat-cable-like “stacks”, are synthesized by a simple template-free wet chemical method with high uniformity and on a large scale. On the self-assembled TiO2 nanowire (SA-Nw-TiO2) coated electrode, cytochrome c (cyt c) undergoes a fast, reversible (or quasi-reversible) and sensitive electron transfer process that is closely related to the relative ionic strength and the hydrated state of SA-Nw-TiO2 surface. At the suitably hydrated SA-Nw-TiO2 electrode in an optimum bulk ionic environment, electrons are reversibly transferred at the heme edge on the basis of an effective orientation of cyt c. It is interesting to note that in a suitable ionic microenvironment, a convertible electron transfer occurs in the following three steps: electron transfer driven partial unfolding of cyt c, electron transfer, and thermodynamics driven refolding. In addition, the partially unfolded cyt c was proven to exist only in the quasi-reversible electron transfer process, and the multi-step pathway of electron transfer can be converted into the single-step pathway at the fully hydrated SA-Nw-TiO2 electrode.

Syntheses of phenoxo-bridged Zn(II) and metallamacrocyclic Hg(II) complexes of organochalcogen (Se, Te) substituted Schiff-bases: structure and DNA-binding studies of Zn(II) complexes

The coordination of organochalcogen (especially Se and Te) substituted Schiff-bases L1H, L2H, L3H, and L4H toward Zn(II) and Hg(II) has been studied. Reactions of these ligands with ZnCl2 in 1 : 1 molar ratio gave binuclear complexes [{2-[PhX(CH2) n N = C(Ph)]-6-[PhCO]-4-MeC6H2O}2Zn2Cl2] (where X = Se, n = 2 (1); X = Se, n = 3 (2); X = Te, n = 2 (3); and X = Te, n = 3 (4)) with partial hydrolytic cleavage of proligands. In these complexes, two partially hydrolyzed ligand fragments coordinate tridentate (NOO) with two Zn's. Reaction of HgBr2 with L1H and L2H in 1 : 1 molar ratio gave monometallic complexes [C6H2(4-Me)(OH)[2,6-{C(Ph) = N(CH2) n Se(Ph)}2HgBr2]] (n = 2 (5) or 3 (6)) and under similar conditions with L3H and L4H gave bimetallic complexes [C6H2(4-Me)(OH)[2,6-{C(Ph) = N(CH2) n Te(Ph)}2Hg2Br4]] (n = 2 (7) or 3 (8)) in which the ligands coordinate with metal through selenium or tellurium, leaving the imino nitrogen and phenolic oxygen uncoordinated. The proligands L1H, L2H give 14- or 16-membered metallamacrocycles through Se–Hg–Se linkages and L3H, L4H give 16- or 18-membered metallamacrocycles through Te–Hg–Br–Hg–Te linkages. All the complexes were characterized by elemental analyses, ESIMS, FTIR, multinuclear NMR, UV-Vis, and conductance measurements. The redox properties of the complexes were investigated by cyclic voltammetry (CV). Complexes 1–4 exhibited ligand-centered irreversible oxidation processes. Complexes 5 and 6 showed metal-centered quasi-reversible single electron transfer, whereas dinuclear complexes 7 and 8 displayed two quasi-reversible, one-electron transfer steps. A single-crystal X-ray structure determination of 1 showed that the coordination unit is centrosymmetric with Zn(II) in square-pyramidal coordination geometry and the two square pyramids sharing an edge. The Zn ··· Zn separation is 3.232 Å. The DNA-binding properties of 1 and 3 with calf thymus DNA were explored by a spectrophotometric method and CV.

Electrochemistry and biosensing activity of cytochrome c immobilized in macroporous materials

An amperometric biosensor for hydrogen peroxide (H2O2) has been constructed by immobilizing cytochrome c on an indium/tin oxide (ITO) electrode modified with a macroporous material. Cyclic voltammetry showed that the direct and quasi-reversible electron transfer of cytochrome c proceeds without the need for an electron mediator. A surface-controlled electron transfer process can be observed with an apparent heterogeneous electron-transfer rate constant (ks) of 29.2 s−1. The biosensor displays excellent electrocatalytic responses to the reduction of H2O2 to give amperometric responses that increase steadily with the concentration of H2O2 in the range from 5 μM to 2 mM. The detection limit is 0.61 μM at pH 7.4. The apparent Michaelis-Menten constant (Km) of the biosensor is 1.06 mM. This investigation not only provided a method for the direct electron transfer of cytochrome c on macroporous materials, but also established a feasible approach for durable and reliable detection of H2O2.

Bicontinuous gyroidal mesoporous carbon matrix for facilitating protein electrochemical and bioelectrocatalytic performances

A strategy of protein-entrapment in bicontinuous gyroidal mesoporous carbon (BGMC) nanocomposite films is described. Herein, the quasi-reversible electron transfer of redox proteins (such as glucose oxidase and myoglobin) is probed and the associated biocatalytic activity is revealed. The apparent heterogeneous electron transfer rate constant of the immobilized glucose oxidase is up to 9.4s−1, much larger than those in carbon nanotubes and some conventional mesoporous carbons. The BGMC based glucose biosensor enables the determination of glucose at a potential of 0.6V (vs. SCE). Its detection limit is 1.0×10−5M (signal-to-noise ratio, S/N=3), the linear response is up to 7.49mM and the detection sensitivity is 52.5nAmM−1 Furthermore, a series of BGMCs with different pore sizes is designed and synthesized using sucrose or phenol formaldehyde resin to study the influences of pore sizes and carbon sources on the immobilization of redox proteins and on the heterogeneous electron transfer.

Amido functional phthalocyanines as metal ion sensor; synthesis, characterization, spectroscopy, electrochemistry, in-situ spectroelectrochemistry

Tetrasubstituted metal ion sensor amido functional phthalocyanines, PyMA- MPc, {PyMA: [ N 1, N 3-bis(pyridin-2-ylmethyl)malonamide], M = Pd(II), Cu(II) and Co(II)} bearing amido functionality on the periphery were synthesized from the corresponding tetrasubstituted diethylmalonate substituted phthalocyanines, DEM- MPc (DEM: diethylmalonate; M = Pd(II), Cu(II) and Co(II)), and 2-aminopyridine in N, N-dimethylaminoethanol (DMAE) solvent. The new complexes have been characterized by elemental analysis, FT-IR, 1H NMR and mass spectra. Peripheral malonamide groups of the phthalocyanine complexes serve as exocyclic binding sites for metal ion, such as Pd II and also increase the solubility in protic solvents, i.e MeOH. Protonation of the pyridyl groups with HCl formed quaternized salts of the MPc, N 1, N 3-bis(pyridin-2-ylmethyl)malonamide substituted metallophthalocyanines (PyMA-PdPc×8HCl) which are freely soluble in aqueous media. Methylation of M[Pc(CH(COOC 2H 5) 2) 4] with CH 3I gave the octacationic 2(3),9(10),16(17),23(24)-tetrakis-[ N 1, N 3-bis( N-methylpyridin-2-ylmethyl)malonamide]-phthlocyaninato Cu(II) salts which are freely soluble in water. Voltammetric and spectroelectrochemical studies show that while PyMA-Pd(II)Pc and PyMA-Cu(II)Pc give ring-based reversible/ quasi-reversible redox processes, PyMA-Co(II)Pc give both metal and ring-based, reversible/ quasi-reversible electron transfer processes.

Antioxidant activity and redox properties of flavonoids from Limonium narbonense

Limonium spp are perennial plants that are mainly distributed near coasts and in salt marshes, and also on saline and alkaline soils in continental interiors. Some Limonium species are used in folk medicine for the treatment of rheumatism and menstrual disorders. L. narbonense Mill. grows wildly along seashore areas subject to periodical floods such as the Venice lagoon. In the course of our studies we found that the methanolic extracts from roots and aerial parts of L. narbonense Mill., collected near Venice, showed antioxidant activity as has been observed by DPPH· assay. The EC50 determined by the DPPH· assay were 11.8 and 23.1µg/mL for aerial parts and roots, respectively. Phytochemical analysis carried out on methanol extracts led to the identification of several polyphenols including: myricetin glycosides, kaempferol and gallic acid. These and other chemical constituents are reported for the first time for this species. Furthermore, the antioxidant activity of the main isolated flavonoids was examined using the DPPH· test. The knowledge of their redox properties can help to understand the antioxidant activity of the flavonoids isolated from L. narbonense. To this aim, we investigated by cyclic voltammetry the electrochemical properties of myricetin and myricetrin, which were present in higher concentration and, for comparison, also quercetin, rutin and kaempferol. The first oxidation process resulted a quasi-reversible electron transfer, allowing the estimation of its standard potential E°. A good correlation between EC50 and E° has been found, which leads to a possible criteria for predicting the antioxidant ability of a such class of compounds.

Direct electrochemistry of cytochrome c on nanotextured diamond surface

Nanotextured diamond surfaces with geometrical properties close to protein dimensions were used for the realization of direct electron transfer of cytochrome c (cyt c) without any covalent bonding. The peroxidase activity of native and denatured cyt c was also investigated. Cyclic voltammograms of native cyt c show quasi-reversible electron transfer reactions, while no heme redox activity is detected for denatured cyt c. Unfolding (denaturation) of cyt c can be achieved in the presence of hydrogen peroxide. Partially or fully denatured cyt c showed higher peroxidase activity than native cyt c. This is because denatured cyt c loses its tertiary structure and hydrogen peroxide is easier to access the heme redox center. The apparent Michaelis–Menten constant K m for native and denatured cyt c has been determined to be 0.23 mM and 0.08 mM.

Chemically immobilised carbon nanotubes on silicon: Stable surfaces for aqueous electrochemistry

Diazonium ion chemistry has been used to electrochemically graft aminophenyl layers onto p-type silicon (1 0 0) substrates. A condensation reaction was used to immobilise single-walled carbon nanotubes with high carboxylic acid functionality directly to this layer. Electrochemical monitoring of the aminophenyl groups confirmed the formation of an amide linkage between the single-walled carbon nanotubes and the aminophenyl layer. The carbon nanotube electrode showed high stability and good electrochemical performance in aqueous solution. At moderate scan rates the Ru(NH3)6+3/+2 couple exhibited quasi-reversible electron transfer kinetics with a standard heterogenous rate constant of 1.2 × 10−3 cm s−1 at the covalently-linked carbon nanotube surface. The electrode thus combines the advantages of a silicon substrate for easy integration into sophisticated electrical and electronic devices, carbon nanotubes for desirable electrochemical properties, and stability in aqueous medium for future applications in environmental sensing.

Electrochemistry and biosensing of glucose oxidase immobilized on Pt-dispersed mesoporous carbon

A method for immobilizing proteins in a carbon mesoporous material (CMM) containing platinum nanoparticles (Pt-NPs) is demonstrated. Compared to pure CMM or carbon nanotubes, CMM containing Pt-NPs enhances the electron transfer and redox properties of redox enzymes, such as glucose oxidase (GOx), due to a cooperative effect of Pt-NPs and CMM. The quasi-reversible electron transfer of GOx in this system is probed, and the apparent heterogeneous electron transfer rate constants are found to be 66% larger than in pure CMM. The GOx/Pt-CMM based glucose biosensor enables the determination of glucose at a potential of 600 mV (vs. SCE). Its detection limit is 10 times lower, and the sensitivity is 16 times higher than that of the respective biosensor without Pt-NPs.

Isopotential points in square-wave voltammetry of reversible electrode reactions

Isopotential points in square-wave voltammetry are described for the first time. Model calculations and real measurements (performed with UO22+ and Eu3+ in perchlorate and bromide solutions, respectively) indicate that such an intersection could be observed when backward components of the net response, resulting from an increase in frequency or reactant concentration, are presented together. The electrode reaction should be fully reversible because quasireversible or slower electron transfer processes give the isopoints only at increasing reactant concentrations but not at increasing square-wave frequencies. The effect could be used as an additional diagnostic criterion for recognition of reversible electrode reactions where products remain dissolved in the electrolyte solution.

Electrochemistry and biosensing of glucose oxidase based on mesoporous carbons with different spatially ordered dimensions

A strategy of protein entrapment within mesoporous carbon matrices is demonstrated to probe the electrochemistry of glucose oxidase. Large surface area and remarkable electro-catalytic properties of carbon mesoporous materials make them suitable candidates for high loading of protein molecules and the promotion of heterogeneous electron transfer. In this work, two kinds of mesoporous carbon nanocomposite films were designed and prepared with highly ordered two-dimensional (2D) and three-dimensional (3D) structures for the immobilization of glucose oxidase, in which the quasi-reversible electron transfer of the redox enzyme was probed, and the apparent heterogeneous electron transfer rate constants ( k et 0 ) are 3.9 and 4.2 s −1, respectively. Furthermore, the associated biocatalytic activity was also revealed. Highly ordered 3D-mesoporous carbon material exhibited larger adsorption capacity for glucose oxidase and the immobilized enzymes retained a higher bioactivity compared with 2D-mesoporous carbons. The preparation of protein-entrapped mesoporous carbon nanocomposites expands the scope of carbon-based electrochemical devices and opens a new avenue for the development of biosensors.

A new rapid and simple method to determine the kinetics of electrode reactions of biologically relevant compounds from the half-peak width of the square-wave voltammograms

A new method is introduced to determine the kinetic parameters of electron transfer reactions of biologically important compounds, based on the measurements of the half-peak width (Δ E p/2) of the square-wave voltammograms. A simple surface (diffusionless) redox reaction, and a simple electrode reaction occurring from dissolved state are considered as model systems. In the region of quasireversible electron transfer, the half-peak widths of theoretical square-wave voltammograms are linear functions of the logarithm of the dimensionless kinetic parameter ln( K) that characterizes the rate of the electron transfer reaction. The dimensionless kinetic parameter K is defined as K = k s( fD) − 0.5 for the redox reaction taking place from dissolved state, whereas for the surface redox reaction K is defined as K = k s/ f ( k s is the standard rate constant of electron transfer, f is the SW frequency, and D is the diffusion coefficient). A set of linear regression equations for the dependences Δ E p/2 vs. ln( K) are derived, which can be used for rapid and precise determination of the charge-transfer kinetic parameters. The estimated values for the standard rate constants of various biologically relevant redox systems using this approach are in very good agreement with the experimental values determined by other square-wave voltammetric methods. The square-wave voltammetric half-peak width method can be used as a simple and reliable alternative to other voltammetric methods developed for the kinetic characterization of electron transfer rates.

Immobilization of heptyl viologens in DNA strands both to inhibit dimerization and to accelerate quasi-reversible electron transfer reaction

Viologens, mono-molecularly immobilized in DNA grooves, show reversible colour changes without attenuation during potential switching after covering with ionic liquid type polymers.

Photophysical, dynamic and redox behavior of tris(2,6-diisopropylphenyl)phosphine

The title phosphine, Dipp3P, was synthesized using an aryl copper reagent and the structure determined by X-ray crystallography (R = 2.94%): d(P–C) = 1.852(1) A, ∠C–P–C = 111.88(5)°. In hexane solution, the electronic spectrum displays 3 bands [326 (9.3), 254 (8.7), 205 (11.4) nm (log|e|)] and the fluorescence spectrum has a Stokes shift of 129 kJ mol−1. NMR: (δ) 31P = −49.7 ppm in solution and −49.5 in the solid (CP-MAS). Room temperature 1H and 13C spectra reflect D3 symmetry, changing below −30 °C to C3. A variable temperature NMR study provided an activation enthalpy of 49(±1) kJ mol−1 and entropy of 24–27(±5) J mol−1K−1. An energy surface calculation using HF/3-21G theory discovered a single low-energy path describing pyramidal inversion through a transition state that is close to D3 geometry. The B3LYP/6-31G(d) calculated barrier to planarization is 37.5 kJ mol−1. Voltammetric studies employing cyclic, rotating disk, steady state and Fourier Transform ac methods confirm a fully chemically reversible one-electron oxidation of Dipp3P to Dipp3P+˙ at +0.18 (CH3CN–nBu4NPF6) and +0.09 (CH2Cl2–nBu4NPF6) V vs. Fc+/0 (Fc = ferrocene). The diffusion coefficient for Dipp3P is 1.0–1.2 × 105 cm2 s−1. The electrode process displays quasi-reversible electron transfer kinetics [ks ≈ 0.01 (CH2Cl2) to 0.08 (CH3CN) cm s−1]. Optically transparent thin layer electrolysis reversibly generates Dipp3P+˙ in CH2Cl2–nBu4NPF6 [UV-Vis: 498 (3.31), 456 (3.29), 373 (4.04), 357 (3.84), 341 (3.49), 296 (3.78), 385 (3.91), 251 (3.99) nm (log|e|)]. The EPR spectrum of Dipp3P+˙ in solution is a doublet (a(P) = 23.9 mT, g = 2.008), and in frozen solution is axial (a∥ = 42.6 mT, g∥ = 2.0045; a⊥ = 12.7 mT, g⊥ = 2.0085 mT).

A Promising Reconstitution Platform for PQQ-Dependent Apo-Enzymes in Chitosan–Carbon Nanotube Matrices

The prosthetic pyrroloquinoline quinone (PQQ) and PQQ-dependent glucose dehydrogenase (PQQ-GDH) have been chemically bound to chitosan-carbon nanotube (CHIT-CNT) films, respectively, in the presence of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide. The immobilized PQQ-GDH displayed a quasi-reversible electron transfer with a formal potential E°' of -0.110 V vs Ag/AgCl, which is independent of the scan rate. Through ex situ spectrophotometric analysis, we found the bioactivity of the immobilized PQQ-GDH was retained. The enzyme-free bound PQQ exhibits a facile electron transfer with the glassy carbon electrode, suggesting that PQQ-bound CHIT-CNT films hold promise as a platform for the reconstitution of PQQ-dependent apo-enzymes, with potential application in biosensors and biofuel cells.

Effect of heterogeneity on the dc and ac voltammetry of the [Fe(CN) 6] 3−/4− solution-phase process at a highly ordered pyrolytic graphite electrode

The effect of surface heterogeneity in dc and ac voltammetries has been investigated with a variety of dual-electrode materials and electrode reactions for quasireversible electron-transfer reactions in which the substrate and product are present as solutes. Typically, and as theoretically expected, when two distinctly different surfaces are present, the faradaic currents arising from each region combine additively. This leads to a marked effect on the shape of the transient response, particularly in the ac voltammetric case. However, heterogeneity is not possible to detect when the rate constants for the two regions are identical in magnitude (or reversible). At the other extremity where very large differences in rate constants occur, it is possible to detect fully resolved voltammetric process. For example, the presence of a small region of edge planes on an otherwise highly ordered pyrolytic graphite electrode surfaces clearly reveals itself in the case of [Fe(CN) 6] 3− electro-reduction. In this example, the vastly enhanced reversibility of the electron-transfer at defect sites present in the edge planes, compared with the much slower reaction on the highly ordered pyrolytic graphite surface leads to detection of two well resolved [Fe(CN) 6] 3−/4− processes from an experiment at a single electrode.

Extraction of electrode kinetics and transport parameters of ferrocene at a platinum electrode from semiintegral electroanalysis

Abstract Semiintegral electroanalysis for the measurement of the kinetic parameters in case of quasi-reversible electron transfer process was presented. As an experimental example using the electrooxidation of ferrocene at a platinum electrode in acetonitrile was performed. The heterogeneous rate constant, ks was independent of concentration of ferrocene and of the supporting electrolyte chosen (TBAP, TPAP, or TEAP). A linear relationship between the current, I, against the semiintegral of the voltammetric data, M was observed. The simplicity of this relationship makes this a very attractive for kinetic analysis. Values of ΔEp, E1/2, α, and D were also reported Digital simulations of cyclic voltammetric and chronoamperometric curves were performed and good agreement was found with experimental results.

Determination of the kinetic parameters of ferrocene-N-phenylaza-15-crown-5 by methods of convolution electrochemistry

A method of analysis for the determination of the electrochemical parameters in the case of a quasi-reversible electron transfer process followed by a homogeneous chemical reaction (EC irrev) is reported. It is based on methods of convolution electrochemistry and can be applied to chronoamperometric curves. Digital simulation studies are also frequently performed to confirm the results. A procedure for calculation of the values of k c and E 1/2 is reported. As an experimental example, the electrooxidation of ferrocene-N-phenylaza-15-crown-5 at a glassy carbon electrode in 0.1 M TBAP as supporting electrolyte in acetonitrile was examined.

Kinetics of Central Ion Electroreduction in Cerium (IV)-Decatungstate

A general characteristics is given to the kinetics of electroreduction of cerium-decatugstate anions on mercury, polycrystalline gold, and the pyrolytic-graphite basis plane in acetate buffer solutions (pH 3.5-6.0). Based on the analysis of UV absorption spectra, the ratio of two reactant forms differing in the protonation degree in solutions under study is estimated. At a negatively charged mercury electrode, the reduction of the Ce(IV) central ion is shown to proceed as an outer-sphere process with classical manifestations of the psi-prime effect (polarization curves reveal a current minimum which deepens with supporting electrolyte dilution and an increase in the reactant negative charge). On the positively charged surface, the current is observed to increase with an increase in the supporting electrolyte concentration, which is interpreted in terms of the strong adsorption of the reactant and its coadsorption with cations. The gold electrode demonstrates pronounced effects of strong chemisorption. Adsorption complications observed on pyrolytic graphite are shown to become strongther for surfaces with more pronounced nonideal behavior. For low concentrations of atomar steps (apparently, for low coverages of pyrolytic-graphite surface with the adsorbed reactant), the quasireversible electron transfer with the rate increasing with increasing pH is observed.

Complex formation of vanadium(IV) with 1,3,5-triamino-1,3,5-trideoxy-cis-inositol and related ligands.

The complex formation of vanadium(IV) with 1,3,5-triamino-1,3,5-trideoxy-cis-inositol (taci) and 1,3,5-trideoxy-1,3,5-tris(dimethylamino)-cis-inositol (tdci) was studied in aqueous solution and in the solid state. The formation constants of [V(IV)O(taci)](2+), [V(IV)O(tdci)](2+), and [V(IV)(tdci)(2)](4+) and of the deprotonation product [V(IV)(tdci)(2)H(-)(1)](3+) were determined (25 degrees C, 0.1 M KNO(3)). Cyclic voltammetry measurements established a reversible one-electron transfer for the [V(IV)(tdci)(2)H(-)(m)]((4)(-)(m))/[V(III)(tdci)(2)H(-)(n)]((3)(-)(n)) couple (0 <or= n, m <or= 6) with a strongly pH-dependent E(1/2)(obs) (+0.15 V at pH 5, -0.57 V at pH 13.5). Slightly more negative potentials were measured for the taci complexes. An additional quasi-reversible electron transfer at strongly positive potentials (1.2-0.5 V) was assigned to the [V(V)(taci)(2)H(-)(m)]((5)(-)(m))/[V(IV)(taci)(2)H(-)(n)]((4)(-)(n)) couple. The structures of [V(IV)(taci)(2)](SO(4))(2).12H(2)O (1), [V(IV)(tdci)(2)][V(4)(V)O(12)].14.5H(2)O (2), and [V(IV)(tdci)(2)H(-)(1)]Cl(3).15H(2)O (3) were determined by single-crystal X-ray analysis. The cations of 1-3 exhibit a structure of approximate D(3)(d) symmetry. The vanadium centers have an almost regular octahedral geometry. The coordinated oxygen donors are deprotonated, and their protons are transferred to the amino groups which act as internal bases. For both ligands, EPR measurements support the formation of non-oxo V(IV) complexes, with a d(z)2 ground state, in aqueous solution above pH 7. The spectral features are indicative of hexacoordinated complexes with a geometry distorted toward a trigonal prism. Finally, a mechanism is proposed for the decomposition of [V(IV)L(2)H(-)(x)]((4)(-)(x)) (L = taci and tdci) in strongly alkaline media (pH approximately 13).