Diffusional mediators

Not surprisingly, ferrocenyl (Fc) derivatives have been widely used either as diffusional or non-diffusional charge transfer mediators because of their almost ideal electrochemical properties, i.e. fast ET rate, low redox potentials and high chemical stability of both the oxidized and the reduced forms. Examples of protein labeling with ferrocene redox probes in the field of amperometric biosensors are reviewed below. 

This work led to the first mass-marketed single-use glucose enzyme biosensor device called the ExacTECH pen (Medisense, Abington, UK) [7]. 

This redox protein was co-adsorbed with the enzyme fructose deshydrogenase on an electrode and an amperometric biosensor for fructose was elaborated [9]. 

This conjugate displayed reversible electrochemistry but appeared unable to electro-catalyze the oxidation of glucose by glucose oxidase at a glassy carbon electrode. This behavior was explained by the large molecular size of the conjugate [10]. 

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Z. Gao, F. Xie, M. Shariff, M. Arshad and J.Y. Ying, A disposable glucose biosensor based on diffusional mediator dispersed in nanoparticulate membrane on screen-printed carbon electrode, Sens. Actuators B Chem., 111-112 (2005) 339-346. 

The power-law relationship in Eq. 6.1 also has implications for measurements of floccule size and dimension during the flocculation process itself. If the principal contributor to floccule growth is collisional encounters between particle units of comparable size, the increase in N per encounter will be equal approximately to N itself. Moreover, if diffusionally mediated collisions are the cause of these encounters, the kinetics of collision will be described by a second-order rate coefficient 9... 

Diffusionally mediated collisions between two floccules of equal size can be described by a second-order rate coefficient KD = 8irRD, where R is the radius and D is the diffusion coefficient of a floccule. Upon invoking the Stokes-Einstein relation, D = kBT/67ri7R, one derives Eq. 6.2. For an introductory discussion of the second-order rate law for particle collisions, see, for example, Chap. 11 in P. C. Hiemenz, Principles of Colloid and Surface Chemistry, Marcel Dekker, New York, 1986. 

In some cases the polymer incorporates the mediator, producing a redox macromolecule, e.g., poly[(ferrocenyl)amidopropyl]pyrrole, ferrocene-modified poly(ethylene oxide), " and highly flexible ferrocene-modified silox-ane. These redox polymers have short chains and low molecular weights (< 20 kDa) they function as a special kind of diffusional mediators and do not alleviate the loss of a mediator to the bulk solution during amperometric detection. 

Polymer- or Inorganic Matrix-immobilized Enzymes Activated by Diffusional Mediators... 

For the efficient electrooxidation of NAD(P)H, mediated electrocatalysis is necessary [22, 170, 171], and a wide range of diffusional mediators has been studied [188-193], Organic compounds that undergo two-electron reduction-oxidation processes and also function as proton acceptors-donors upon their redox transformations (such as ortho- and para-derivatives of quinones, phenylenedi-amines and aminophenols) have been found to be ideal for the mediation of NAD(P)H oxidation, although single-electron-transfer mediators (e.g. ferrocene derivatives) are also capable of oxidizing NAD(P)H [190, 191], Some compounds demonstrate very high rates for the mediated oxidation of NAD(P)H in aqueous solutions [188,189,194,195],... 

Immobilized low potential electron-transfer mediators (e.g. viologens) are more promising than diffusional mediators for the practical regeneration of NAD(P)H coupled with further biocat-alytic reactions. The immobiKzation of viologens usually results in significant positive potential shift of their redox potential [269-272], which however, badly affects their efficiency. The potential shift... 

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