Ferrocene mediator

Di Gleiia K, Hill HAO, McNeil CJ, Green MJ (1986) Homogeneous ferrocene mediated amperometiic biosensors. Anal Chem 58 1203-1205... 

A. Cass, G. Davis, G. Francis, H.A. Hill, W. Aston, J. Higgins, E. Plotkin, L. Scott, and A.P. Turner, Ferrocene-mediated enzyme electrode for amperometric determination of glucose. Anal. Chem. 56, 667-671 (1984). 

K. di Gleria, H.A. Hill, C.J. McNeil, and M.J. Green, Homogeneous ferrocene-mediated amperometric immunoassay. Anal. Chem. 58, 1203-1205 (1986). 

Since the first report on the ferrocene mediated oxidation of glucose by GOx [69], extensive solution-phase studies have been undertaken in an attempt to elucidate the factors controlling the mediator-enzyme interaction. Although the use of solution-phase mediators is not compatible with a membraneless biocatalytic fuel cell, such studies can help elucidate the relationship between enzyme structure, mediator size, structure and mobility, and mediation thermodynamics and kinetics. For example, comprehensive studies on ferrocene and its derivatives [70] and polypy-ridyl complexes of ruthenium and osmium [71, 72] as mediators of GOx have been undertaken. Ferrocenes have come to the fore as mediators to GOx, surpassing many others, because of factors such as their mediation efficiency, stability in the reduced form, pH independent redox potentials, ease of synthesis, and substitutional versatility. Ferrocenes are also of sufficiently small size to diffuse easily to the active site of GOx. However, solution phase mediation can only be used if the future biocatalytic fuel cell... 

Ferrocene-mediated enzyme sensors have usually been obtained by adsorption of the mediator onto the electrode surface because of this insolubility in aqueous solution. Due to the good solubility of ferrecinium cations in aqueous solution complications arise from leakage of the mediator from the electrode surface. 

To overcome the poor stability of ferrocene-mediated enzyme sensors, mediator-modified electrodes have been used. In the case of glucose oxidase, the cofactor FAD is deeply buried within the protein matrix. The depth of the active center is estimated to be 0.87 nm. Therefore, one cannot expect that the mediator covalently attached to the electrode surface via a short spacer retain the possibility of closely approaching the cofactor of the enzyme. 

Ferrocene mediators, in biosensing, 12, 594 Ferrocene polyaromatic dyads, preparation, 6, 189—190 Ferrocene polymer compounds, characteristics, 12, 449 Ferrocene—ruthenocene compounds, preparation, 6, 635 Ferrocenes... 

Polymeric alkynylmercury derivatives, preparation, 2, 427 Polymeric ferrocene mediators, in biosensing, 12, 594 Polymeric (isocyanide)gold(I) aryl complexes, characteristics, 2, 283... 

The process implies a first electrochemical step with a very fast conversion of the ferroceno/ferrocinium couple, due to the short length of the alkyl chain, and a second chemical step with a simple electron transfer between the iron complex in solution and that of the monolayer. Moreover, the thiols block the gold surface in such a way that the Fe(CN)g- oxidation will take place due solely to the ferrocene mediation at the monolayer, and with a very high efficiency (i.e., the catalytic way is observed at potentials 500 mV lower than those corresponding to a gold electrode with a C6SH monolayer). 

Figure 5. Schematic of sensing layer at electrode surface. Ferrocene mediator is indicated by circled cubic structures.

In conclusion, a ferrocene-mediated glucose sensor was constructed by all-chemical means. They are easily and quickly constructed (<2 h) and show excellent linearity and stability. Also, this construction method is applicable to any graphite electrode, regardless of shape or size. Future work will focus on applying electron mediators to ultramicrobiosensors. 

Hillard EA, Vessieres A, Thouin L, Jaouen G, Amatore C (2006) Ferrocene-mediated proton-coupled electron transfer in a series of ferrocifen-type breast cancer drug candidates. Angew Chem Int Ed Engl 45 285-290... 

The flexibility of the sol-gel process allows multiple approaches to enzymatic activity monitoring. The formation of hydrogen peroxide can be followed by optical measmements using another enzymatic reaction in which the oxidation of an organic dye is catalyzed by a peroxidase (HorseRadish Peroxidase HRP), coentrapped with GOD. The presence of glucose in the solution can also be detected via electrochemical means by following the redox reactions at the active site of GOD. However, because of the steric hindrance of the protein molecule, a ferrocene mediator has to be used in order to transfer electrons from the hidden active site to the electrode. Alternatively, oxygen consumption can be measmed with a Clark electrode. The hybrid gel is deposited on a Pt cathode... 

Figure 16. Optical electron transfer excitation of [2]rotaxane [46]. Shuttling does not take place because the direct and ferrocene-mediated back electron transfer reactions are much faster than the motion of the ring.

It was also possible to bind the redox active ferrocene mediator directly to the lysine amino groups of the enzyme GPO via an oligo(ethylene glycol) anchor (Fig. 27). The catalytic activity was demonstrated convincingly by cyclic voltammetry (Fig. 28) [140,141]. 

Pyruvate oxidase (Pyox) is a FAD- and thiamine diphosphate (ThDP)-dependent enzyme that catalyzes the reaction of pyruvate to give acetyl phosphate or vice versa (see Fig. 15). If used in the oxidative way, it can be activated and reactivated under nonaerobic conditions using ferrocene mediators. Kinetic parameters of the indirect electrochemical process using the enzyme incorporated into a biomimetic supported bilayer at a gold electrode have been reported [142]. Similarly, FAD-dependent amino oxidases may also be applied. 

Figure 7.2. (a) Ferrocene-mediated glucose sensor (b) calibration curves recorded in N2, air, and 02-saturated solutions. 

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