Redox osmium complex-modified

Another approach is based on the application of redox polders, e.g. osmium complex-modified poly(vinyl pyridine) (9-11) or ferrocene-modified poly(siloxanes) (12,13X crosslinked together with an enzyme on the top of the electrode. The electron transfer fi-om the active site of the polymer-entrapped enzyme to the electrode surfece occurs to a first polymer-bound mediator which has suflSdently approached the prosthetic group to attain a fast rate constant for the electron-tranrfer reaction. From this first mediator the redox equivalents are transported along the polymer chains by means of electron hopping between adjacent polymer-linked mediator molecules (Fig. 2). Extremely fast amperometric enzyme electrodes have been obtained with si ificantly decreased dependence fi-om the oxygen partial pressure. However, die redox polymer/enzyme/crosslinker mbcture has to applied either manually or by dipcoating procedures onto the electrode surface. 

Figure 2.5 Schematic representation of the Au/MPS/PAH-Os/solution interface modeled in Refs. [118-120] using the molecular theory for modified polyelectrolyte electrodes described in Section 2.5. The red arrows indicate the chemical equilibria considered by the theory. The redox polymer, PAH-Os (see Figure 2.4), is divided into the poly(allyl-amine) backbone (depicted as blue and light blue solid lines) and the pyridine-bipyridine osmium complexes. Each osmium complex is in redox equilibrium with the gold substrate and, dependingon its potential, can be in an oxidized Os(lll) (red spheres) or in a reduced Os(ll) (blue sphere) state. The allyl-amine units can be in a positively charged protonated state (plus signs on the polymer...

The example considered is the redox polymer, [Os(bpy)2(PVP)ioCl]Cl, where PVP is poly(4-vinylpyridine) and 10 signifies the ratio of pyridine monomer units to metal centers. Figure 5.66 illustrates the structure of this metallopolymer. As discussed previously in Chapter 4, thin films of this material on electrode surfaces can be prepared by solvent evaporation or spin-coating. The voltammetric properties of the polymer-modified electrodes made by using this material are well-defined and are consistent with electrochemically reversible processes [90,91]. The redox properties of these polymers are based on the presence of the pendent redox-active groups, typically those associated with the Os(n/m) couple, since the polymer backbone is not redox-active. In sensing applications, the redox-active site, the osmium complex in this present example, acts as a mediator between a redox-active substrate in solution and the electrode. In this way, such redox-active layers can be used as electrocatalysts, thus giving them widespread use in biosensors. 

Subsequent research on osmium-modified polymers has shown how the redox potential can be controlled by altering bipyridine ligands of the immobilized osmium complexes. The ability to reliably modulate redox potential has significantly broadened the range of enzymes with which osmium redox polymer are compatible and allows for mediation of oxidative enzymatic reactions as well as reductive enzymatic reactions. 

A new type of (bio)chemical sensor, the redox-sensitive field-effect transistor is described It consists of a conventional ISFET with a noble metal added on top of the gate insulator The gate electrode is modified with a redox polymer containing osmium complexes The potentiostatic multi-puls method is introduced which allows the adjustment of the redox potential of the gate to a desired value in a stepwise way It is shown that the open circuit potential after switching off the potentiostat is a good measurement of the presence of the redox active species NADH... 

When a metal complex is formed with the nucleic acid base of single-stranded DNA (ssDNA), such ssDNA can be used as an alternative electrochemically-active DNA-binding ligand. For example, Palecek and coworkers reported the formation of a reversible redox-active metal complex by the reaction of osmium tetraoxide-pyridine with the thymine (T) base of ssDNA, and they developed an electrochemical gene detection method based on this modified oligonucleotide as a DNA probe [3]. 

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