Redox mediators hexacyanoferrate

The choline electrode usually consists of an amperometric transducer and immobilized choline oxidase. The most frequently used electrochemical transducers are hydrogen peroxide electrodes (26-28,33-36). The amperometric signal in this case is due to electrooxidation of hydrogen peroxide, which is the co-product of the enzymatic choline oxidation (equation 2). Oxygen amperometric sensors (Qark-type electrodes) have been also used as basic transducers for choline electrode construction (29, 32, 37). The signal in this case is based on the reduction of molecular oxygen which is the co-reactant in reaction (equation 2). Redox mediators hexacyanoferrate (26), ferrocene derivatives (38) and tetracyanoquinodimethane (39) have also been used in the construction of choline electrodes. 

Using platinum electrodes (167, 238) requires +0.6 V versus SCE to oxidize H2O2. However, this potential precludes selective measurements of uric acid because it is also oxidized at the electrode surface (167). Thus, to improve the selectivity, bienzyme amperometric devices using a redox mediator (hexa-cyanoferrate) have been constructed (239). The enzymes uricase and peroxid ise are immobilized together and the hexacyanoferrate(III) is measured at 0.0 V versus Ag/AgCl. Alternatively, a carbon dioxide selective electrode is used for the detection of the enzymatically liberated CO2 (240, 241). 

The classical approach to a water soluble mediator is the redox pair hexacyanoferrate(II)/ hexacyanoferrate(III). A more convenient approach is the use of the redox pair ferrocene/ ferricinium immobilized to the electrode surface [14,15]. The ferrocene is oxidized at the electrode to regenerate ferricinium ion. The sensor is insensitive to oxygen and operates at relatively low potentials. This technological approach is the basis for the Exadifech glucose sensor being marketed by Baxter Travenol in the United States. 

Clark et al. [32] have published in 1981 a method employing both enzymes in solution and potentiometrically measuring the concentration of the hydrogen peroxide formed. For an amperometric cholesterol assay Yao et al. [33, 34] used immobilized cholesterol esterase and cholesterol oxidase in a reactor assaying the hydrogen peroxide formed at a peroxidase electrode with the redox pair hexacyanoferrate(II)/hexacyanoferrate(III) as a mediator. 

Pauliukaite, R., Florescu, M., Brett, C.M.A. Characterization of cobalt and copper hexacyanoferrate modified carbon film electrodes for redox mediated biosensors. J. Solid State Electrochem. 9, 354—362 (2005)... 

Low-molecular-weight redox mediators have been used in the construction of choline electrodes for hydrogen peroxide detection. A sensor for choline and acetylcholine was described using anodic detection of hexacyanoferrate (III) in solution. Ferrocene and tetracyanoquinodimethane"" were also used in the construction of choline electrodes. The use of redox mediators facilitates the electron transfer but leads to the system complications. 

Electron mediators successfully used with oxidases include 2,6-dichlorophenolindophol, hexacyanoferrate-(III), tetrathiafulvalene, tetracyano-p-quinodimethane, various quinones and ferrocene derivatices. From Marcus theory it is evident that for long-range electron transfer the reorganization energies of the redox compound have to be low. Additionally, the redox potential of the mediator should be about 0 to 100 mV vs. standard calomel electrode (SCE) for a flavoprotein (formal potential of glucose oxidase is about -450 mV vs SCE) in order to attain rapid vectrial electron transfer from the active site of the enzyme to the oxidized form of the redox species. 

A general problem of the potentiometric measurement is the low exchange current density of enzymatic substrate redox couples. Therefore additional mediators have to be applied with a high exchange current densitiy and fast electrode kinetics, which can react with the enzymatic products. Most often, these mediators are soluble low molecular weight substances, e.g. hexacyanoferrate. Until now, redox polymers based containing covalently bound osmium complexes [5 ] were only used in combination with oxidases for amperometric sensors. We could show that this redox polymer (E =... 

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