Electroactive surface mediator

As an alternative to DET, small, artificial substrate/co-substrate electroactive molecules (mediators) can be used to shuttle electrons between the enzyme and the electrode (Figure 5.3b). This involves a process in which the enzyme takes part in the first redox reaction with the substrate and is re-oxidized or reduced by the mediator which in turn is regenerated, through a combination of physical diffusion and self-exchange, at the electrode surface. The mediator circulates continuously between the enzyme and the electrode, cycled between its oxidized and reduced forms, producing current. This process is known as mediated electron transfer (MET). 

In an ideal case the electroactive mediator is attached in a monolayer coverage to a flat surface. The immobilized redox couple shows a significantly different electrochemical behaviour in comparison with that transported to the electrode by diffusion from the electrolyte. For instance, the reversible charge transfer reaction of an immobilized mediator is characterized by a symmetrical cyclic voltammogram ( pc - Epa = 0 jpa = —jpc= /p ) depicted in Fig. 5.31. The peak current density, p, is directly proportional to the potential sweep rate, v ... 

Considerable potential exists to design surface modified electrodes which can mimic the behaviour of electronic components. For example, a rectifying interface can be produced by using two-layer polymer films on electrodes. The electroactive species in the layers have different redox potentials. Thus electron transfer between the electrode (e.g. platinum) and the outer electroactive layer is forced to occur catalytically by electron transfer mediation through the inner electroactive layer. 

In indirect oxidation, organic pollutants do not exchange electrons directly with the anode surface but they exchange through the mediation of some electroactive species regenerated there, which act/acts as an intermediary for shuttling electrons between the electrode and the organics. Indirect electrolysis can be a reversible or an irreversible process. 

This paper describes a process for activating polyimide surfaces for electroless metal plating. A thin surface region of a polyimide film can be electro-chemically reduced when contacted with certain reducing agent solutions. The electroactivity of polyimides is used to mediate electron transfer for depositing catalytic metal (e.g., Pd, Pt, Ni, Cu) seeds onto the polymer surface. The proposed metal deposition mechanism presented is based on results obtained from cyclic voltammetric, UV-visible, and Rutherford backscattering analysis of reduced and metallized polyimide films. This process allows blanket and full-additive metallization of polymeric materials for electronic device fabrication. 

It has been shown that electroactive polymer films on electrodes can mediate electron transfer for metal deposition (9-11). Haushalter and Krause (5) have described the treatment of PMDA-ODA films with highly reactive Zintl complexes (e.g., Sn9 4, SnTe4 4) to yield an intercalated material able to reduce ions of platinum, palladium and silver at the film surface. Mazur et al., (12) reported the deposition of conductive Ag, Cu, and Au metal interlayers within a PMDA-ODA film by electrochemical reduction. 

This paper describes a new seeding process for electroless metallization of polyimides and other electroactive polymers. Polyimide films can be reduced electrochemically at an electrode surface or by contact with an appropriate reducing agent in an electrolyte solution. In the latter case, only the outer surface of the film undergoes reduction. Once the polyimide surface is reduced it then can mediate electron transfer to metal ions or metal complexes in solution causing metal to be deposited at the surface with concurrent reoxidation of the polyimide. 

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