A model for the propagation of a redox reaction through microcrystals

Chronoamperometry of reversible redox reactions with the insertion of cations into solid particles immobilised at an electrode surface is analysed theoretically using a semiinfinite planar diffusion model. A coupled diffusion of electrons and ions within the crystal lattice is separated in two differential equations. The redox reaction is initiated by the polarisation of the three-phase boundary, where the crystal is in contact with both the electrode and the solution. From this contact line the redox reaction advances on the surface and into the crystal body by the diffusion of ions and conductance of electrons. The effects of the geometry and conductivity of the particles on the current are discussed. Received: 28 December 1996 / Accepted: 17 February 1997     

Electrocatalytic oxidation of some biologically important compounds at conducting polymer electrodes modified by metal complexes

Conducting poly(3-methylthiophene) electrodes were electrochemically prepared. The resulting polymer films were modified with an inorganic complex, ferrocene. The incorporation of the ferrocene/ferrocenium moiety into the polymer film resulted in enhanced charge transfer towards the oxidation of some organic molecules of biological interest. The electrochemical response of the complex-containing polymer electrode was compared to that of the unmodified polymer electrode and that of the substrate. Apparent diffusion coefficients of the redox species were estimated from the cyclic voltammetric data for different biological molecules at the ferrocene-containing polymer electrode. Infra-red spectroscopic measurements for the “as-grown” films revealed the presence of the inorganic complex within the polymer. The modified polymer electrode showed noticeable enhancement for the charge transfer across the film interface and can be used as an electrochemical sensor for biological compounds. Received: 3 June 1997 / Accepted: 7 July 1997     

A model for the coupled transport of ions and electrons in redox conductive microcrystals

Coupled diffusion of ions and electrons in microcrystals of insertion compounds immobilized at an electrode surface is theoretically analysed by a lattice-gas model without interactions. The transport in the direction perpendicular to the electrode surface depends on Wagner's factor for electrons, while the transport parallel to the electrode depends on this factor for ions. The iso-concentration profiles may depend on the orientation of the particle on the electrode surface. Chronoamperometric responses of volume and surface redox reactions are calculated. Received: 5 June 1998 / Accepted: 22 August 1998     

An amperometric biosensor using toluidine blue as an electron transfer mediator intercalated in α-zirconium phosphate-modified horseradish peroxidase immobilization matrix

A novel H₂O₂ biosensor was constructed employing α-zirconium phosphate as a new support substrate to hold an electron shuttle toluidine blue between a glassy carbon electrode and horseradish peroxidase. Toluidine blue was intercalated into α-zirconium phosphate-modified horseradish peroxidase immobilization matrix cross-linked on a glassy carbon electrode surface via bovine serum albumin-glutaraldehyde. This co-immobilization matrix of the mediator and the enzyme was formed from the α-zirconium phosphate (α-ZrP)-toluidine blue (TB) inclusion colloid in which horseradish peroxidase (HRP) was dissolved. Intercalation of TB in layered α-ZrP was investigated by scanning electron microscopy (SEM), X-ray powder diffraction (XRD) and electrochemical measurements. TB immobilized in this way underwent a quasi-reversible electrochemical redox reaction at the electrode. Cyclic voltammetry and amperometric measurements demonstrated good stability and efficiently-shuttled electrons between HRP and the electrode. The sensor responded rapidly to H₂O₂ with a detection limit of 3.0 × 10^–7 mol/L. Received: 1 July 1997 / Revised: 13 October 1997 / Accepted: 21 October 1997     

Steady-state electrolysis at a grooved or ridged plane

This study concerns an infinite plane whose smoothness is marred by a single defect: either a groove or a ridge. The blemished plane serves as an electrode supporting a diffusion-controlled steady-state process. By using a convenient coordinate transformation, the local current density at all points on the surface is determined exactly. The results are found to confirm intuitive expectations. Thus, compared with normal values on the plane remote from a groove, the electron transfer rate is diminished within the groove but enhanced along its margins. Similarly, an abnormally large transfer rate is encountered high on the ridge but the rate is subnormal on its lower flanks. The total current is demonstrated to be unchanged by the presence of the blemish. Received: 27 September 1996 / Accepted: 11 March 1997     

Crystal habit of fcc metal particles controlled by electrode potential in solution

The crystal habit of fcc metal particles formed on an amorphous carbon film electrode in solution at different electrode potentials is discussed. The fcc metal particles have different crystallographic habits depending on applied electrode potential; that is, icosahedral and/or decahedral particles are formed at lower potentials, and fcc single-crystalline or polycrystalline particles at higher potentials. It was found that decahedra and icosahedra of Cu-Au alloy particles are formed in the potential region of underpotential deposition (UPD) of Cu at which only fcc Au single-crystalline particles and Au polycrystalline particles appear. This is attributed to the charge transfer from the UPD Cu ions to the Au overlayer of Cu-Au alloy particles. The formation of decahedral and icosahedral Cu-Au alloy particles depends on the composition of the Cu-Au alloy. On the basis of these results it was deduced that the contraction of the surface lattice of the growing particles is responsible for the formation of icosahedral and decahedral particles. Received: 25 February 1997 / Accepted: 21 April 1997     

Association and phase behavior of hydrophobically modified photoresponsive poly(acrylamide)s in the presence of ionic surfactant

 The extent of association between the cationic surfactant TTAB and a series of hydrophobically modified polyacrylamides (HPAMs) containing an N-n-alkyl and substituted azobenzene hydrophobic sidegroup has been studied utilizing a cationic surfactant-selective membrane electrode. Binding of TTAB to the polymer hydrophobes is found to increase with increasing hydrophobicity of the hydrophobe. In the presence of electrolyte, aqueous solutions of HPAMs and ionic surfactant exhibit an associative phase separation. The temperature or clearing point (CP) at which the system goes from a one phase to two-phase system are reported. The area of the two-phase region is found to increase with increasing electrolyte concentration, hydrophobicity of the hydrophobe for the high molecular weight HPAMs, and decreasing hydrophobicity for low molecular weight HPAMs. Exposure of HPAMs containing an azobenzene hydrophobe to UV light results in a decrease in interaction between the hydrophobe and surfactant and a corresponding decrease in the CP due to conversion of azobenzene from the more hydrophobic trans form to the less hydrophobic cis isomer. Received: 23 September 1996 Accepted: 11 March 1997     

A mechanistic study of electron transfer from the distal termini of electrode-bound, single-stranded DNAs

Electrode-bound, redox-reporter-modified oligonucleotides play roles in the functioning of a number of electrochemical biosensors, and thus the question of electron transfer through or from such molecules has proven of significant interest. In response, we have experimentally characterized the rate with which electrons are transferred between a methylene blue moiety on the distal end of a short, single-stranded polythymine DNA to a monolayer-coated gold electrode to which the other end of the DNA is site-specifically attached. We find that this rate scales with oligonucleotide length to the -1.16 ± 0.09 power. This weak, approximately inverse length dependence differs dramatically from the much stronger dependencies observed for the rates of end-to-end collisions in single-stranded DNA and through-oligonucleotide electron hopping. It instead coincides with the expected length dependence of a reaction-limited process in which the overall rate is proportional to the equilibrium probability that the end of the oligonucleotide chain approaches the surface. Studies of the ionic strength and viscosity dependencies of electron transfer further support this "chain-flexibility" mechanism, and studies of the electron transfer rate of methylene blue attached to the hexanethiol monolayer suggest that heterogeneous electron transfer through the monolayer is rate limiting. Thus, under the circumstances we have employed, the flexibility (i.e., the equilibrium statistical properties) of the oligonucleotide chain defines the rate with which an attached redox reporter transfers electrons to an underlying electrode, an observation that may be of utility in the design of new biosensor architectures.     

Potential dependence of the kinetics of thiol self-organization on Au(111)

The self-assembly of thiol molecules from ethanolic solution on Au(111) depends significantly on the electrode potential. Especially at cathodic potentials, chemisorption of thiol molecules and the development of the highly ordered structure are slowed down significantly. At potentials near the point of zero charge, first a disordered thiol film of already high thiol density is formed, and then domains of the highly ordered phase develop and grow together. At cathodic potentials, first a disordered film of very low density of predominant flat adsorbed thiol molecules is formed; the formation of ordered domains takes time three orders of magnitude longer than at potentials near the point of zero charge. Received: 27 May 1997 / Accepted: 8 September 1997     

Resonance capture of electrons by molecules of substituted pyrans

Negative-ion mass spectrometry in the mode of resonance capture of electrons and photoelectron spectroscopy in combination with quantum-chemical calculations showed that the formation of the resonance states of negative molecular ions in the reaction of electrons with molecules of the mechanism of intershell Feshbach resonance with the consecutive excitation of an electron from several higher occupied MO to one vacant MO. In a low-energy region, the resonance at 1.4 eV is a resonance of form and the resonance at 3–4 eV is the usual electron exciting Feshbach resonance with a parent triplet state (π.π*)³. The one and the same vacant π*_CC MO is “active” in all the resonances mentioned. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1892–1894, October, 1997.     

Preparation of glass carbon electrode modified with nanocrystalline nickel-decorated carbon nanotubes and electrocatalytic oxidation of methanol in alkaline solution

Nanocrystalline nickel with an average diameter of about 16 nm and a face-centered cubic (fcc) structure was uniformly attached to the surface of carbon nanotubes (CNT) by wet chemistry. The sample was characterized by X-ray powder diffraction and transmission electron microscopy (TEM). A glass carbon electrode modified with nickel-modified multi-wall carbon nanotubes (MWCNTs-Ni/GCE) was prepared. The electrochemical behavior of the MWCNTs-Ni/GCE and the electrocatalytic oxidation of methanol at the MWCNTs-Ni/GCE were investigated by cyclic voltammetry in 1.0 mol/L NaOH solution. The cyclic voltammograms showed that the electron transfer between β-Ni(OH)₂ and β-NiOOH is mainly a diffusion-controlled quasireversible process, and that the electrode has high catalytic activity for the electrooxidation of methanol in alkaline medium, revealing its potential application in alkaline rechargeable batteries and fuel cells. __________ Translated from Chinese Journal of Applied Chemistry, 2007, 24(5): 503–506 [译自: 应用化学]     

Direct and mediated electron transfer catalyzed by anionic tobacco peroxidase

The properties of anionic tobacco peroxidase (TOP) adsorbed on graphite electrode have been studied in direct and mediated electron transfer in a wall-jet flow injection system. The percentage of tobacco peroxidase molecules active in directelectron transfer is about 83%, which is higher than that for horeradish peroxidase (40–50%). This observation is explained in terms of the lower degree of glycosylation of TOP compared with horseradish peroxidase and, therefore, a reduced in terference from the oligosaccharide chains with direct electron transfer. Calcium ions cause an 11% drop in the reaction rate constant toward hydrogen peroxide. The detection limit of calcium chloride has been estimated as 5 m M. The results obtained by means of bioeletrochemistry, stopped-flow kinetics, and structural modeling provide evidence for the interaction between calcium cations and negatively charged residues at the distal domain (Glu-141, heme propionates, Asp-79, Asp-80) blocking the activesite. The observation that both soluble and immobilized enzyme under go conformational changes resulting in the blockade of the active site indicates that the immobilized enzyme preserves conformational flexibility. An even stronger suppressing effect of calcium ions on the rate constant for mediated electron transfer was observed. In the case of direct electron transfer, this couldmean that there is nodirect contact between the electrode and the active site of TOP. The electrons are shuttled from the active site to the surface of the electrode through electron transfer pathways in the protein globule that are sensitive to protein conformational changes.     

Molecular recognition: Supramolecular, polymeric and biomimetic coatings for chemical sensors and chromatographic columns

Complete control of the selective and reversible interaction of molecules from the gas or liquid phase at complementary recognition sites is of increasing interest for both basic science and practical applications. This recognition may occur at the surface or in the bulk of optimized chemically sensitive coatings. It is either monitored discontinuously by chromatography or continuously by a suitable sensor. The latter contains the optimized coating and converts the chemical information about concentrations of certain molecules by means of a certain transducer into an electronic signal. Generally speaking, these transducers form the essential part of ‘chemical sensors’; they monitor the molecular interactions at the chemically sensitive layer by changes in resistivity, impedance, mass, capacitance, work function, heat, electrochemical potential, optical thickness, or optical absorption in a certain spectral range. Three selected case studies of such molecular recognition devices which utilize supramolecular, polymeric, and biomimetic coatings are presented. Examples are given for both gas and liquid sensing devices. For simplification, because of its general applicability and its easy absolute calibration, particular emphasis is put on signal transduction via quartz crystal oscillators. The measurement principle is based on frequency changes which are directly correlated with mass changes and thus provide a particularly suitable signal transduction. The examples presented here concern systematic variations in the design of supramolecular cages, of selective interaction sites in polymeric matrices, and of covalently attached biomimetic recognition sites to monitor antibodies or enzyme interactions. Received: 11 August 1997 / Revised: 3 March 1998 / Accepted: 3 March 1998     

Molecular recognition: Supramolecular, polymeric and biomimetic coatings for chemical sensors and chromatographic columns

Complete control of the selective and reversible interaction of molecules from the gas or liquid phase at complementary recognition sites is of increasing interest for both basic science and practical applications. This recognition may occur at the surface or in the bulk of optimized chemically sensitive coatings. It is either monitored discontinuously by chromatography or continuously by a suitable sensor. The latter contains the optimized coating and converts the chemical information about concentrations of certain molecules by means of a certain transducer into an electronic signal. Generally speaking, these transducers form the essential part of ‘chemical sensors’; they monitor the molecular interactions at the chemically sensitive layer by changes in resistivity, impedance, mass, capacitance, work function, heat, electrochemical potential, optical thickness, or optical absorption in a certain spectral range. Three selected case studies of such molecular recognition devices which utilize supramolecular, polymeric, and biomimetic coatings are presented. Examples are given for both gas and liquid sensing devices. For simplification, because of its general applicability and its easy absolute calibration, particular emphasis is put on signal transduction via quartz crystal oscillators. The measurement principle is based on frequency changes which are directly correlated with mass changes and thus provide a particularly suitable signal transduction. The examples presented here concern systematic variations in the design of supramolecular cages, of selective interaction sites in polymeric matrices, and of covalently attached biomimetic recognition sites to monitor antibodies or enzyme interactions. Received: 11 August 1997 / Revised: 3 March 1998 / Accepted: 3 March 1998     

Electrochemical preparation of oxide materials for electrochemistry and electronics

Electrochemical formation of barium tungstate (BaWO₄) was studied as a model case of electrochemical formation of an advanced oxide material for electronics. BaWO₄ is formed on the surface of tungsten electrode during oxidation in alkaline media (pH > 12) containing a corresponding cation. The analysis of electrochemical as well as electrochemical quartz crystal microbalance (EQCM) data taken during these experiments identifies at least three qualitatively different steps composing the electrode process. Effects of the potential, applied current density and alkaline earth metal cation concentration are demonstrated using cyclic voltammetry and galvanostatic experiments. Specific constraints of the ECC formalism for the electrochemical oxide deposition following from the galvanostatic data are discussed. Received: 2 October 1997 / Accepted: 4 December 1997     

Voltammetric properties at pH 14 of electrodes modified by azobenzene polymers

Azobenzene polymers were prepared by condensation of p-phenylazobenzoyl chloride and poly(ethylenimine). Their loadings in electroactive sites range from 5 to 95%. They were adsorbed on a glassy carbon electrode or on a hanging mercury drop electrode (HMDE) whose area could be expanded after the adsorption. The voltammetric behavior of the polymeric films is described at pH 14. The azobenzene sites which are in the vicinity of the electrode surface are electroactive, but the electrochemical reaction does not propagate to the bulk of the coating. When the loading of the polymer is not too high, the expansion of the HMDE causes an increase in the number N of azobenzene double bonds which are reduced, N remaining proportional to the drop area A, because-the film is sufficiently flexible to cover the new surface which appears (soap bubble effect). For the highly loaded polymers (loading larger than about 50%), conversely, N remains nearly constant, owing to the rigidity of the film, which causes it to break up when the drop is expanded. The reversibility of the electrochemical reaction depends both on the loading and on the expansion, which could be due to changes in the orientation of the azobenzene molecules at the surface of the electrode.     

Trans-membrane electron transfer in red blood cells immobilized in a chitosan film on a glassy carbon electrode

We have studied the trans-membrane electron transfer in human red blood cells (RBCs) immobilized in a chitosan film on a glassy carbon electrode (GCE). Electron transfer results from the presence of hemoglobin (Hb) in the RBCs. The electron transfer rate (k _s) of Hb in RBCs is 0.42 s^?1, and <1.13 s^?1 for Hb directly immobilized in the chitosan film. Only Hb molecules in RBCs that are closest to the plasma membrane and the surface of the electrode can undergo electron transfer to the electrode. The immobilized RBCs displayed sensitive electrocatalytic response to oxygen and hydrogen peroxide. It is believed that this cellular biosensor is of potential significance in studies on the physiological status of RBCs based on observing their electron transfer on the modified electrode.

Caffeic acid modified glassy carbon electrode for electrocatalytic oxidation of reduced nicotinamide adenine dinucleotide (NADH)

A new modified electrode was prepared by electrodeposition of caffeic acid (CFA) at the surface of an activated glassy carbon electrode. Cyclic voltammetry was used to investigate the redox properties of this electrode at various solution pH values and at various scan rates. The pH dependence of the electrode response was found to be 58.5 mV/pH, which is very close to the expected Nernstian value. The electrode was also employed to study electrocatalytic oxidation of reduced nicotinamide adenine dinucleotide (NADH), using cyclic voltammetry, chronoamperometry and rotating disk voltammetry as diagnostic techniques. It was found that the modified electrode exhibits potent and persistent electrocatalytic properties toward NADH oxidation in phosphate buffer solution (pH 7.0) with a diminution of the overpotential of about 450 mV compared to the process at an unmodified electrode. The electrocatalytic current increases linearly with NADH concentration in the range tested from 0.05 to 1.0 mM. The apparent charge transfer rate constant and transfer coefficient for electron transfer between the electrode surface and immobilized CFA were calculated as 11.2 s⁻¹ and 0.43, respectively. The heterogeneous rate constant for oxidation of NADH at the CFA-modified electrode surface was also determined and found to be about 3 × 10³ M⁻¹ s⁻¹. Finally, the diffusion coefficient of NADH was calculated as 3.24 × 10⁻⁶ cm² s⁻¹ for the experimental conditions, using chronoamperometric results. Received: 6 January 1999 / Accepted: 11 May 1999     

Mediator-free amperometric determination of glucose based on direct electron transfer between glucose oxidase and an oxidized boron-doped diamond electrode

A mediator-free glucose biosensor, termed a “third-generation biosensor,” was fabricated by immobilizing glucose oxidase (GOD) directly onto an oxidized boron-doped diamond (BDD) electrode. The surface of the oxidized BDD electrode possesses carboxyl groups (as shown by Raman spectra) which covalently cross-link with GOD through glutaraldehyde. Glucose was determined in the absence of a mediator used to transfer electrons between the electrode and enzyme. O₂ has no effect on the electron transfer. The effects of experimental variables (applied potential, pH and cross-link time) were investigated in order to optimize the analytical performance of the amperometric detection method. The resulting biosensor exhibited fast amperometric response (less than 5 s) to glucose. The biosensor provided a linear response to glucose over the range 6.67×10⁻⁵ to 2×10⁻³ mol/L, with a detection limit of 2.31×10⁻⁵ mol/L. The lifetime, reproducibility and measurement repeatability were evaluated and satisfactory results were obtained.