Ferrocene derivatives, mediated electron transfer

The response of a surface antigen-antibody reaction can also be mediated by an electron transfer reagent. This has been demonstrated by Robinson et al. in their studies on electrochemical immunoassays for hCG and thyroxine. In the analysis of hCG, a two-site amperometric immunoassay was developed in which monoclonal capture antibodies were immobilised on the surface of a glassy carbon electrode. A second antibody against hCG was labelled with glucose oxidase. The electrode was used both to separate free from bound enzyme-antibody conjugate and to assay the enzyme activity electrochemically by use of dimethylaminomethyl ferrocene as an electron transfer mediator. This method was found to correlate well with an immunoradiometric assay. In the analysis of thyroxine, another ferrocene derivative, namely ferrocenemonocarboxylic acid, was used as the electron transfer mediator. ... 

The second-generation 02" biosensors are mainly based on the electron transfer of SOD shuttled by surface-confined or solution-phase mediators, as shown in Scheme 2(b). In 1995, Ohsaka et al. found that methyl viologen could efficiently shuttle the electron transfer between SOD and the glassy carbon electrode and proposed that such a protocol could be useful for developing 02 biosensors [125], Recently, Endo et al. reported an 02, biosensor based on mediated electrochemistry of SOD [148], In that case, ferrocene-carboxaldehyde was used as the mediator for the redox process of SOD. The as-developed 02 biosensor showed a high sensitivity, reproducibility, and durability. A good linearity was obtained in the range of 0 100 pM. In the flow cell system, tissue-derived 02 was measured. 

Mizutani et al. [16] have demonstrated that ferrocene derivatives, attached by means of covalent bonds to the surface of bovine serum albumin, have been able to mediate the electron transfer between the glucose oxidase and the electrode through the osemium complex. 

However, because of the mostly very slow electron transfer rate between the redox active protein and the anode, mediators have to be introduced to shuttle the electrons between the enzyme and the electrode effectively (indirect electrochemical procedure). As published in many papers, the direct electron transfer between the protein and an electrode can be accelerated by the application of promoters which are adsorbed at the electrode surface [27], However, this type of electrode modification, which is quite useful for analytical studies of the enzymes or for sensor applications is in most cases not stable and effective enough for long-term synthetic application. Therefore, soluble redox mediators such as ferrocene derivatives, quinoid compounds or other transition metal complexes are more appropriate for this purpose. 

Attempts to reduce interference and minimize the effect of variations in oxygen tension have resulted in the development of biosensors with improved linear ranges which operate at lower electrode potentials. They incorporate artificial electron acceptors, called mediators, to transfer electrons from the flavoenzyme (e.g. glucose oxidase) to the electrode and thus are not dependent on oxygen. Ferrocene (bis(i75-cyclopentadienyl)iron) and its derivatives are examples of redox mediators for flavoenzymes. The reaction now becomes... 

Photoswitchable electrical communication between enzymes and electrodes has also been achieved by the application of photoisomerizable electron-transfer mediators [195, 199]. DilTusional electron mediators (viologen or ferrocene derivatives) were functionalized with photoisomerizable spiropyran/merocyanine units. These mediators can be reversibly photoisomerized from the spiropyran state to the merocyanine state (360 < A < 380 nm) and back (A > 475 nm). An enzyme multilayer array composed of glutathione reductase or glucose oxidase was electrically contacted only when the photoactive group linked to the redox relay (viologen or ferrocene derivative, respectively) was in the spiropyran state. 

Ferrocene modified flexible polymeric electron transfer systems Ferrocene and its derivatives are readily available and commonly used organometalUc redox mediators, so it is quite natural that they were selected first to synthesize mediator modified polymeric electron transfer systems. Siloxane pol5uners are flexible but aqueous insoluble pol3nmers. As previously indicated, a flexible polymer backbone allows close contact between the redox center(s) of the enzyme and the mediator, and the water insoluble property of the polymer prevents not only redox polymer from leaching into bulk media but also prevents enzyme diffusion away fi-om the electrode surface by entrapping it in the polymer/carbon paste matrix. Therefore, ferrocene and... 

A novel principle for accelerating the electron transfer has been the direct chemical modification of GOD by electron-mediating groups such as ferrocene derivatives (Heller and Degani, 1987). The distance between the mediator molecules was at most 1 nm and the relays had to be attached in the vicinity of the prosthetic group. The binding of ferrocene to GOD was therefore conducted in 2 mol/1 urea. After refold-... 

Substituted ferrocenes also form monoanions at very negative potentials electron addition is genuinely associated with the ferrocene nucleus rather than with an electroactive substituent. The E° value for [Fe(f/-C5H4Ph)2] is — 2.62 V, and [FeCp2] itself shows a quasi-reversible reduction at —2.93 V in dmf (452), with a peak separation of 250 mV at — 37°C(v = 1 Vsec-1). Exhaustive electrolytic reduction of ferrocene derivatives yields solutions containing the substituted cyclopentadienide anions the latter may be used in the syntheses of other cyclopentadienylmetal complexes (453). Ferrocenes are also finding use as mediators in electron-transfer reactions, especially at electrode surfaces (454-456). 

The reaction scheme used in the first commercial electrochemical test strip from MediSense (now Abbott Diabetes Care) is shown later. Electron transfer rates between the reduced form of glucose oxidase and ferricinium ion derivatives are very rapid compared with the unwanted side reaction with oxygen (Cass et al. 1984 Forrow et al. 2002). The Abbott Diabetes Care Precision QID strip includes the l,l -dimethyl-3-(2-amino-l-hydroxyethyl) ferrocene mediator, which has the desirable characteristics of high solubility in water, fast electron-shuttling (bimolecular rate constant of 4.3 X 10 M s ), stability, and pH independence of the redox potential (Heller and Feldman 2008). Electrochemical oxidation of the ferrocene derivative is performed at 0.6 V. Oxidation of interferences, such as ascorbic acid and acetaminophen present in blood, are corrected for by measuring the current at a second electrode on the strip that does not contain glucose oxidase. 

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