Arkady A. Karyakin

Prussian Blue and Its Analogues: Electrochemistry and Analytical Applications

This article reviews fundamental aspects of deposition, structure and electrochemistry of Prussian Blue and its analogues. Special attention is given to the metal hexacyanoferrates with potential analytical applications. Prussian Blue and its analogues as advanced sensing materials for nonelectroactive ions are discussed. In contrast to common ‘smart materials’, the sensitivity and selectivity of metal hexacyanoferrates to such ions is provided by thermodynamic background. Prussian Blue itself is recognized as the most advantageous low-potential transducer for hydrogen peroxide over all known systems. Both high sensitivity (ca. 1 A M−1 cm−2) and selectivity in relation to oxygen reduction are more than three orders of magnitude higher, than for platinum electrodes. Biosensors based on different transducing principles containing enzymes oxidases are compared, and the devices operated due to hydrogen peroxide detection with the Prussian Blue based transducer are shown to be the most advantageous ones. The future prospects of chemical and biological sensors based on metal hexacyanoferrates are outlined.

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.

Self-doped polyanilines electrochemically active in neutral and basic aqueous solutions

Abstract To obtain polyaniline (PAn) films that are electrochemically active in neutral aqueous solutions, the electropolymerization of three substituted anilines (anthranilic acid, m-aminobenzoic acid and m-aminobenzenesulphonic acid) was studied. Cyclic voltammograms of copolymers of substituted and unsubstituted anilines (monomer ratio 1 : 1) showed redox activity in neutral and basic aqueous solutions up to pH 10. The decrease in redox activity occurred more than 5 pH units higher than in the case of the usual PAn. Among aniline derivatives taken for the synthesis, a meta position and a sulpho group were preferable to produce the optimal self-doped PAn. A new redox couple in the negative potential region (E°′ = 0.24 V, Ag/AgCl, pH = 1) was observed. Self-doped polyanilines were also investigated by means of impedance spectroscopy.

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.

Electropolymerization of phenothiazine, phenoxazine and phenazine derivatives : characterization of the polymers by UV-visible difference spectroelectrochemistry and Fourier transform IR spectroscopy

Abstract The discrepancies observed between the redox potential values of poly(phenothiazines or phenoxazines) obtained from cyclic voltammetry and from spectroelectrochemical measurements (curves of Δ A vs. E) suggests that the polymer films are not homogeneous, but include some amount of monomeric species which tend to detach from the electrode surface after electrochemical scanning. The polymerization of the phenazine neutral red, however, gave rise to a homogeneous film. On the contrary, the stability of the polymer film was found to be strongly dependent on the nature of the parent monomer. A model for the structure of poly(phenothiazines or phenoxazines) is proposed on the basis of their optical and vibrational spectra. In every case, the mechanism of electropolymerization seems to involve the formation of a cation-radical species after release of one hydrogen atom from the monomer at high positive potentials. When the monomer has a primary or secondary amino group as ring substituent, the polymer may be composed of phenothiazine units linked through one secondary or tertiary amine moiety. However, when the monomer has only tertiary amino groups as ring substituents (as methylene blue), the cation-radical species are only formed at extremely positive potentials, close to the potential of oxygen evolution, and it would be possible that at least one of the tertiary amino groups could be oxidized before the formation of the cation-radical yielding an oxime derivative of the monomer and formaldehyde as subproduct. However, the great similarity of the spectra of poly(thionine), poly(azur A) and poly(methylene blue) suggests that the kind of linkage between monomer units is the same in all these polymers.