Химия и химические технологии/ 8. Кинетика и катализ

Kurbanalieva S. K., Zaitseva A.S., Arlyapov V.A.

Tula state university

The study of the process of electron transfer in the system "substrate – D. hansenii yeast –mediator ferrocene – carbon-paste electrode"

Currently, biosensors are widely used in ecology and medicine. Mainly mediator biosensors are used for biosensor development. Mediators are low molecular weight redox couple, transfer electrons from the active center of the enzyme (or the enzymes of cells) to the indicator electrode surface.

The advantages of using these compounds in the development of electroanalytical devices are independence of the electrode reaction from the oxygen partial pressure. Ferrocene is one of the often used mediators due its non-toxicity, furthermore ferrocene, as a hydrophobic compound, allows modify the graphite paste and create non-reagent mediator biosensors. Cells of the yeast D. hansenii was used in this work are the microorganisms, having the wide range of oxidizable substrates and the stabile enzymatic systems under stress conditions, making them are promising in the development of sensors. Carbon-paste electrodes was used due to their developed working surface and high adsorption capability towards organic and inorganic substances.

In order to control such parameters as the sensitivity and accuracy of the electroanalytical devices, for example, biosensors, it is necessary to establish the electrochemical pattern transfer of electron, therefore the aim of this work is to study the physico-chemical and electrochemical factors of the processes in the "substrate – yeast D. hansenii – the mediator ferrocene – carbon-paste electrode" system.

Cyclic voltammetry method was used for the studies of the system. After modification of carbon-paste electrode by ferrocene, anodic peak appears on the voltammogram (Fig.1), due to the transfer of electrons from reduced ferrocene form to its oxidized form in linear potential sweep. Similarly, the cathodic peak is formed.

Fig.1. Voltammogram of the carbon-paste electrode modified with ferrocene (4% by weight of graphite powder) at different scan rates.

In the studied system with increasing scan rate, the cathodic and anodic peaks go separate ways after the ferrocene modification, which indicates the irreversibility of electron transfer. This may be due to the significant difference between the rate of direct and reverse reactions, therefore electron transfer coefficients were obtained by the Tafel equation (1).

(1),

where (for cathodic process).

Electron transport coefficients was 0,82±0,03 for anodic process and 0,27±0,02 for cathodic process. Thus, the rate of oxidation is significantly greater than the reduction.

It may be assumed that in this system, adsorbed mediator molecules provides the electrochemical process, and the current linearly depends on scan rates in accordance with equation (2).

(2).

Connection of current limit (I) from scan rate (ν) for the oxidation and reduction process at various mediator concentrations (from 1% to 10% of graphite paste) is linear, which confirms that the electron transfer is due to the adsorbed molecules.

Heterogeneous rate constant of electron transfer for adsorption processes can be found by using the equations of Laviron (3-4).

(3),

(4), where

k is the heterogeneous rate constant of the electrochemical system (cm s^-1), k_s is the rate constant for electron transfer (s^-1), A - area of electrode (cm²), V - volume of graphite powder (cm³), α is the transfer coefficient for cathode process, (1-α) is the transfer coefficient for anodic process, ν is the scan rate (V s^-1), R is the universal gas constant (J·mol K^-1) T - temperature (K), n is the number of electrons, F – Faraday constant (C·mol^-1); E_p is the difference between the anodic and the cathode potential (V).

Heterogeneous rate constant in the system "ferrocene-carbon-paste electrode" was 0.4±0.1 cm s^-1. After the addition in the system of biological material, the electron transfer becomes more complex due to the biochemical reactions of the biomaterial interaction with mediator. The cyclic voltammetry method allows calculate the constant of biomaterial interaction with the mediator using the equation of Nicholson and Shain (5).

(5),

where I_k is the maximum current in the substrate presence, I_d is the maximum current in the substrate absence, k_Ox is the constant of biomaterial interaction with the mediator; ν - scan rate (V/s), R is the universal gas constant (J·mol/K) T - temperature (K), F – Faraday constant (C·mol^-1), [E] - initial concentration of the biomaterial (mg/dm³).

After plotting a curve ratio maximum current in the presence and maximum current in the absence of substrate from 1/ν^1/2 constant of biomaterial interaction with the mediator was obtained by the coefficient slope, which was 0,024±0,003 dm³/(g•s).

Thus, it is possible to conclude: the process of electron transfer in the system is irreversible, the oxidation rate is higher than the recovery rate, the electrochemical process is carried out by the adsorbed mediator molecules, the heterogeneous rate constant in the system "ferrocene - carbon-paste electrode" was 0.4±0.1 cm/s, and the constant interaction of the biomaterial and the mediator was 0,024±0,003 dm³/(g•s).

This work was supported by a grant from the RFBR and the Government of the Tula region № 16-48-710959 p_a and a grant from the President of the Russian Federation for state support of young Russian scientists - candidates of science, contract № 14.Z56.16.5425-MK.

References

Ponamoreva O. N., Reshetilov A. N., V. A. Alferov Biosensors. Principles of operation and practical application. Textbook. 2007. Tula State University. (ISBN 978-5-7679-1189-9) 256p.

Сatalytic oxidation and voltammetric determination of cysteine at an electrode modified by films of osmium hexacyanocobaltate or osmium hexacyanoruthenate L.G. Shaidarova, A.V. Gedmina, E.R. Zhaldak, I.A. Chelnokova, H.C. Budnikov // Kazan State University. 2013. №4.

Laviron E. General expression of the linear potential sweep voltammogram in the case of diffusionless electrochemical systems //Journal of Electroanalytical Chemistry and Interfacial Electrochemistry. 1979. V. 101. №. 1. PP. 19-28.