Electrode mediator-modified

Self-Assembly of Mediator-Modified Redox Enzymes on the Porous Gold Electrode Surface... 

In contrast to the molecular wire of molecular interface, electron mediators are covalently bound to a redox enzyme in such a manner as an electron tunneling pathway is formed within the enzyme molecule. Therefore, enzyme-bound mediators work as molecular interface between an enzyme and an electrode. Degani et al. proposed the intramolecular electron pathway of ferrocene molecules which were covalently bound to glucose oxidase [ 4 ]. However, few fabrication methods have been developed to form a monolayer of mediator-modified enzymes on the electrode surface. We have succeeded in development of a novel preparation of the electron transfer system of mediator-modified enzyme by self-assembly in a porous gold-black electrode as schematically shown in Fig.12 [14]. 

Fig.12 Scheme of self-assembly of mediator-modified enzyme on the gold 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. 

In contrast to the mediator-modified electrodes, Degani et al. modified glucose oxidase itself by means of covalently bound ferrocene [4]. After modifying enzymes with ferrocene carboxylic acid, they observed direct electron transfer from the active site of the enzyme to a gold or platinum... 

Aizawa et al. [14] have immobilized mediator-modified glucose oxidase within micropores of a gold black electrode by self-assembling via the thiol-gold interaction. 

Imamura M, Haruyama T, Kobatake E, Ikariyama Y, Aizawa M. Self-assembly of mediator-modified enzyme in porous gold-black electrode for biosensing. Sensors and Actuators B 1995, 24, 113-116. 

Electrode surface modified by the addition of a polymer containing mediating groups The polymers used are non-conducting, hence, devices suffer from poor charge transport [56]... 

The electroreductions on mediator-modified electrodes can be also used for analytical purposes. Although this aspect is beyond the scope of this section, a sample of the most recent results are presented. 

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 vast amount of literature exists on enzyme-modified metal nanopartides. Crumbliss and co-workers pioneered the use of metal nanopartides for enzymatic sensors for various analytes such as H2O2, glucose, xanthine and hypoxanthine [156-158]. GCE or Pt electrodes are modified with enzyme-capped Au colloids, either by simple evaporation or electrodeposition. The nanopartides act as mediators, transferring electrons between the redox-active site on the immobilized biomolecule and the electrode and thus eliminating the need for external mediators. These sensors are classified as third generation biosensors . 

Figure 6. SEM image of glassy carbon electrode surface modified with gold nanoparticles prepared with the seed-mediated growth approach. Reproduced from [32], copyright 2007, with permission from the Japan Society for Analytical Chemistry.

Mediator Modified Screen Printed Electrode Based AChE Sensors... 

Kashiwagi, Y., Pan, Q.H., Yanagisawa, Y., Sibayama, N., and Osa, T. (1994) The effects of chain-length of ferrocene moiety on electrical communication of mediators-modified and enzyme-modified electrodes. Denki Kagaku, 62 (12), 1240-1245. 

The mediators are bound to amino acids near the prosthetic group. For fixation of the relays the protein has to be unfolded and renatured after the chemical modification procedure. The small distance between the bound mediator molecules (maximum 1 nm) provides a very fast tunneling process. Enzyme electrodes employing glucose oxidase or lactate oxidase modified in this way operate like mediator-modified electrodes, without reagent addition. Owing to their favorable structure, such sensors respond to the analyte in less than... 

Figure 14-9. Cyclic voltammogram of a graphite electrode modified with P-naphthoyl Nile Blue, a) buffer (pH 8.0) b) after addition of NADH (10 mM). The increase in the anodic current is attributed to electrocatalytic oxidation of NADH at the mediator-modified electrode. - SOO to +300 mV vs. SCE 5 mV s" 0.1 M phosphate buffer (pH 8.0) with 0.5 M NaCl graphite disk electrode, 6.4 mm diameter.

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