Electrode biomimetic

Figure 8.2 shows an electrochemical system - a model of a catalase-biomimetic sensor, consisting of the reference electrode (Ag/AlCl/Cl ) and biomimetic electrode. In this system, the electrochemical potential changed as a result of mimetic electrode interaction with... 

For the purpose of determining low hydrogen peroxide concentrations, the authors have designed the most cost-effective and simple to use potentiometric-biomimetic sensors based on immobilized catalase mimics. These sensors possess high hydrodynamic properties and the fastest speed of response. Figure 8.3 shows experimental data on catalase activity of biomimetic electrode in 0.03% aqueous H202. For the sake of comparison, catalase activities of aluminum electrode and aluminum electrode with applied adhesive are also shown. 

Figure 8.3 Time dependence of electrochemical potential in the system. T = 20 °C, 0.03 wt.% H202 (1 biomimetic electrode 2 aluminum electrode 3 aluminum electrode with applied adhesive).

It is common knowledge that hydrogen peroxide is a soft dibasic acid. Therefore, catalase activity of biomimetic electrodes may change the pH of the H202 solution by both... 

These data present an experimental proof of process (8.1) and (8.2) run on the biomimetic electrode and the consequences ... 

A definite quantity of oxygen molecules accumulated on the surface of the biomimetic electrode (catalase reaction) must diffuse to the volume of the adhesive layer, toward the electrode surface. Hence, the specific requirements to the adhesive follow on the one hand, it must provide strong enough adhesion of a mimic to electrode on the other hand, it must possess low oxygen adsorption ability. 

Figure 8.8 Apparent mechanism of catalase-biomimetic electrode operation in electrocatalytic mode.

Thus, two reactions (catalase and electrochemical) are implemented in the biomimetic electrode-H202-Cr-AgCl-Ag system. In the case of inorganic support, it is not the kinetic but the diffusion (external and internal) factor that is predominant, owing to which the electrochemical reaction is of a self-oscillation type [7, 8],... 

Studies on ethanol trace detection in aqueous solutions were carried out in an electrochemical cell of peroxidase-mimetic sensor of the potentiometric type, consisting of a reference electrode (Ag/AgCl) and a biomimetic electrode. Redistilled water was used for the background solution. The biomimetic electrode was prepared by adhering hematin-containing meroxidase mimetic to aluminum foil with Pattex adhesive [7, 8],... 

The authors devoted their investigations to the development of a peroxidase-biomimetic sensor for determining trace quantities of ethyl alcohol in various solutions. In all tests the reaction system represented a mixture of microamounts of hydrogen peroxide and ethyl alcohol in an aqueous medium. The task was to determine the effect of the H202 C2H50H ratio on the detection ability of the biomimetic electrode. 

By analogy with the mechanism of the catalase reaction, the probable mechanism of the peroxidase reaction is considered (Figure 8.12). Note that a proton transferred to the active site of the biomimetic electrode can be replaced by H+ from the reaction mixture volume. The mechanisms of catalase and peroxidase reactions provide an insight into the ways of their realization in the electrochemical mode. The ratio of products synthesized in both reactions (02 and CH3CHO) depends on the ratio of the H202 and CH3CHO interaction rates with the surface intermediate. 

Of course, this fact outlines ways to improve the physicochemical parameters of biomimetic sensor design and, consequently, the technological modernization of biomimetic electrode preparation. As concluded from the data in Figure 8.14, the sensitivity threshold of the designed sensor is very high (10 xwt. % aqueous ethyl alcohol) [14],... 

H., and Lecomte, S. (2007) Kinetics of the electron transfer reaction of cytochrome c(552) adsorbed on biomimetic electrode studied by time-resolved surface-enhanced resonance Raman spectroscopy and electrochemistry. European Biophysics Journal, 36,1039-1048. 

So far, certain biomimetic catalysts (1 and 2b in Fig. 18.17) have been shown to reduce O2 to H2O under a slow electron flux at physiologically relevant conditions (pH 7,0.2-0.05 V potential vs. NHE) and retain their catalytic activity for >10" turnovers. Probably, only the increased stability of the turning-over catalyst is of relevance to the development of practical ORR catalysts for fuel cells. In addition, biomimetic catalysts of series 1,2,3, and 5, and catalyst 4b are the only metalloporphyrins studied in ORR catalysis with well-defined proximal and distal environments. For series 2, which is by far the most thoroughly studied series of biomimetic ORR catalysts, these well-defined environments result in an effective catalysis that seems to be the least sensitive among all metalloporphyrins to the electrode material (whether the catalyst is adsorbed or in the film) and to chemicals present in the electrolyte or in the O2 stream, including typical catalyst poisons (CO and CN ). 

A biomimetic sensor was created with electrodes from aluminum wire (2 mm thick) and aluminum foil (size 20 X 10 X 1 mm), to which the working element was applied by two methods ... 

The change of electrode potential (E) of the catalase reaction with time was measured by a voltmeter. pH and E values for aqueous hydrogen peroxide were determined simultaneously for possible correlations between pH metric and potentiometric results of enzymatic activity of catalase-biomimetic sensors. The electrochemical unit was also equipped with a magnetic mixer. 

Biomimetic sensors, prepared from catalase adsorbed on diasorb and A1203, treated with trypsine and adhered to an aluminum electrode surface using 7.5% polyacrylamide gel of... 

Biomimetic sensors, prepared by catalase adsorption on diasorb and agarose (treated with trypsine) and adhered to an aluminum electrode surface by Pattex adhesive, displayed an abrupt decrease of the electrode potential. Sensors prepared by catalase adsorption on A1203 (without trypsine treatment) and adhesion to the aluminum electrode with Pattex adhesive displayed a high oscillation of the electrode potential, which induces extreme instability of the operation. Hence, it should be noted that sensor operation was always better in the case of enzyme treatment with trypsine. 

Another way considered as of biomimetic inspiration and that was shown to be efficient for enzyme attachment, it consists in using the very strong and specific interaction of the small protein avidin for the biotin [61,62]. The tetrameric structure of avidin permits itself to interact with four different molecules of biotin at the same time. Various proteins and enzyme could be easily biotinylated, and this mode of enzyme grafting has already been used for electrodes production as well as for membranes made up of conducting fibers. 

Beyond the complete assembly of biomimetic membranes, interfacial supramolec-ular assemblies which incorporate biocomponents represent an important approach to replicating the biological functions outside of living systems. For example, the ability to link or wire otherwise electro-inactive enzymes to electrodes so that they can efficiently transport electrons allows sensitive and selective sensors to be developed for important bioactive molecules, e.g. glucose, lactate, urea, etc. 

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