Elena E. Karyakina

Electropolymerized Azines: A New Group of Electroactive Polymers

The electropolymerization of different azines from aqueous solutions was investigated. The structure of monomers was systematically varied changing both the nature of the second heteroatom and the substituents in the aromatic rings. Considering the electropolymerization process and the properties of the resulting polymers one can denote polyazines as a new group of electroactive polymers. The electrochemical and spectroelectrochemical investigation of polyazines was done. A hypothesis on azine polymer structure is presented.

ON THE MECHANISM OF H2O2 REDUCTION AT PRUSSIAN BLUE MODIFIED ELECTRODES

Abstract Prussian Blue deposited on the electrode surface under certain conditions is known to be a selective electrocatalyst of hydrogen peroxide (H 2 O 2 ) reduction in the presence of O 2 . The electrocatalyst was stabilized at cathodic potentials preventing its loss from the electrode surface. Hydrodynamic voltammograms of H 2 O 2 reduction indicated the transfer of two electrons per catalytic cycle. The operational stability of Prussian Blue in H 2 O 2 reduction was highly dependent on the buffer capacity of the supporting electrolyte. Since Prussian Blue is known to be dissolved in alkaline solution, it was confirmed that in neutral aqueous solutions the product of H 2 O 2 electrocatalytic reduction is OH − .

Prussian-Blue-based amperometric biosensors in flow-injection analysis

Optimisation of the electrodeposition of Prussian Blue onto mirrored glassy carbon electrodes yielded a modified electrode practically insensitive to oxygen reduction. At the same time the electrode activity towards hydrogen peroxide reduction was extremely high. This allowed the detection of hydrogen peroxide by electroreduction over a wide potential range. Flow-injection investigations of this electrode inserted into a flowthrough electrochemical cell of the confined wall-jet type showed that the response for hydrogen peroxide is limited by diffusion. Glucose and alcohol biosensors were made by immobilisation of glucose oxidase and alcohol oxidase respectively, within a Nafion layer, onto the top of the Prussian-Blue-modified electrodes. By increasing the density of Nafion and decreasing the measuring potential the glucose biosensor was made completely insensitive to both ascorbate and acetominophes.

The electrocatalytic activity of Prussian blue in hydrogen peroxide reduction studied using a wall-jet electrode with continuous flow

Abstract The kinetics of hydrogen peroxide reduction on electrodes modified with specially deposited Prussian blue were investigated using a wall-jet cell with continuous flow. For this aim a new semi-empirical model for the diffusion limited current distribution holding for narrow wall-jet electrodes was described. In spite of the non-uniform accessibility of the wall-jet electrode surface in terms of mass transport the evaluated equation for the total current density ( j ) allowed the separation of the diffusion and kinetic terms of the current through investigating the dependence of j on the volume flow rate ( V ) in (1/ j vs V − 3/4 ) plots. The theoretical conclusions presented were confirmed by kinetic investigations of the electrocatalytic reduction of H 2 O 2 at glassy carbon wall-jet electrodes modified with Prussian blue. The bimolecular rate constant for the reduction of H 2 O 2 on the specially deposited Prussian blue was found to be k cat =3×10 3 M −1 s −1 . Due to the characteristics of the high catalytic activity and selectivity, which were comparable with biocatalysis using peroxidase, the Prussian blue based electrocatalyst is denoted as ‘artificial peroxidase’.

Electroreduction of NAD+ to enzymatically active NADH at poly(neutral red) modified electrodes

The electropolymerisation of neutral red (NR) led to formation of a redox active film on the electrode surface. At negative potentials a current of NAD+ reduction was observed at poly(NR) modified electrodes. To identify the product of NAD+ reduction the alcoholdehydrogenase (ADH) containing Nafion® membrane was put onto the poly(NR) modified electrode surface. In the presence of a small portion of NAD+ (0.1–0.2 mM) the resulting enzyme electrode exhibited a response to addition of acetaldehyde. The cathodic current density was dependent on NAD+ concentration and was more than 10 times higher than that of the acetaldehyde background reduction on a bare poly(NR) modified electrode. Thus, the main product of NAD+ electroreduction at the poly(NR) modified electrode was the enzymatically active NADH.

A High-Sensitive Glucose Amperometric Biosensor Based on Prussian Blue Modified Electrodes

Abstract A first generation amperometric glucose biosensor based on Prussian Blue modified electrodes was developed. Besides exception of noble metals (platinum as usual) the developed biosensor possessed high sensitivity. Linear response dependence on analyte concentration was observed in a range of 10−6 − 5·10−3 M glucose. The current density produced by addition of 10−6 M glucose was 0.18 μ A/cm2. Hydrogen peroxide produced via enzyme reaction was detected by electroreduction. Owing to that reason the biosensor response became independent of the presence of reductants (ascorbate for example). The developed amperometric biosensor was expected to obey requirements for non-invasive diagnostics. There are not principle limits of using other oxidases for Prussian Blue based amperometric biosensor development. Thus one can develop such biosensors for cholesterol, alcohol, glycerol, amino acids ets.

Electropolymerized Azines: Part II. In a Search of the Best Electrocatalyst of NADH Oxidation

A comparative investigation of catalytic activity of different polyazines in NADH electrooxidation is reported. The structure of azine monomers taken for electropolymerization was varied systematically changing both the second heteroatom and the substituents of aromatic rings. It was found that the monomer structure affects catalytic activity of the resulting polymer in the following way: (i) additional substitution of benzene ring by alkyl group reduced the catalytic activity, (ii) polymerized phenoxazine Brilliant Cresyl Blue was a better electrocatalyst than the corresponding phenothiazine (o-Toluidine Blue), (iii) ring substitution with only tertiary nitrogen atoms as ligands provides higher catalytic activity, (iv) higher redox potential of the polymer also provides higher catalytic activity.

THE ELECTROCHEMICAL POLYMERIZATION OF METHYLENE-BLUE AND BIOELECTROCHEMICAL ACTIVITY OF THE RESULTING FILM

Abstract The electrochemical polymerization of methylene blue in aqueous solutions and the properties of the resulting films were investigated. The films possessed monomer-type redox activity, and a new redox couple, responsible for the semiconducting properties of the polymer, was observed. The growth rate of the film increased in basic media. The influence of hydroxide ions can be simulated in terms of both nucleophilic catalysis and a shielding effect. The bioelectrochemical oxidation of glucose at methylene blue semiconductor films in the presence of glucose oxidase was observed.

Potentiometric biosensors based on polyaniline semiconductor films

Abstract Application of polyaniline (PAn) semiconductor films for potentiometric biosensor development provides certain advantages compared with the known systems. For enzyme immobilisation a pH sensitive matrix was used, which could improve sensor sensitivity. Indeed the limiting response value of the PAn based trypsin electrode in 1 mM buffer was 270 mV. The maximal slope of the calibration curve in semilogarithmic coordinates was 180 mV per substrate decade. Using self-doped polyaniline instead of common polymer as pH transducer a stable potentiometric response of 70 mV/pH was obtained, which is higher than for known systems. Taking as an example a glucose biosensor, we showed that the response of the polyaniline based electrode was 3–4 fold increased compared with the glucose-sensitive field-effect transistor. A biosensor for environmental control was made by immobilisation of organophosphate hydrolase (OPH) into self-doped polyaniline film.

Electroactivity of chemically synthesized polyaniline in neutral and alkaline aqueous solutions: Role of self-doping and external doping

The electroactivity of chemically synthesized polyaniline and its copolymer with metanilic acid has been investigated in a wide pH range. It was found that the external doping of PAn with camphorsulfonic acid extends the electroactivity in neutral and alkaline media. Additional ring substitution with sulfo-groups does not further extend the electroactivity, but improves the stability upon cycling in neutral and alkaline media.