Conventional Redox-Polymers

The poly(I)-based transistor is the first illustration of a microelectrochemical transistor based on a combination of a conducting and a conventional redox polymer as the active material. The transistor "turns on" at VG corresponding to oxidation of the polythiophene backbone. The resistivity of poly(I) declines by a factor of 105 upon changing VG from 0.4 V to 0.8 V vs. Ag+/Ag. When Vg is moved close to the one-electron reduction potential of V2+/+, the conventional redox conductivity gives a small degree of "turn on". A sharp Iq-Vq characteristic results, with an Ip(peak) at Vq = E° (V2+/+). Though the microelectrochemical devices based on conventional redox conduction have both slow switching speed and a... 

In this chapter, we describe three different systems with which to construct electro- and photo-functional molecular assemblies on electrode surfaces. The first is the bottom-up fabrication of redox-conducting metal complex oligomers on an electrode surface and their characteristic redox conduction behavior, distinct from conventional redox polymers.11-13 The second is a photoelectric conversion system using a porphyrin and redoxconducting metal complex.14 The third is the use of a cyanobacterial photosystem I with molecular wires for a biophotosensor and photoelectrode.15 16 These systems will be the precursors of new types of molecular devices working in electrolyte solution. 

FIGURE 1. Two types of films with redox complexes on the electrode, (a) Conventional redox polymer film (redox sites (circles) are randomly located), (b) Highly ordered film prepared by the bottom-up method. 

We describe here that the redox oligomer wires fabricated with the stepwise coordination method show characteristic electron transport behavior distinct from conventional redox polymers. Redox polymers are representative electron-conducting substances in which redox species are connected to form a polymer wire.21-25 The electron transport was treated according to the concept of redox conduction, based on the dilfusional motion of collective electron transfer pathways, composed of electron hopping terms and/or physical diffusion.17,18,26-30 In the characterization of redox conduction, the Cottrell equation can be applied to the initial current—time curve after the potential step in potential step chronoamperometry (PSCA), which causes the redox reaction of the redox polymer film ... 

In this chapter, we presented three different systems of molecular assemblies using molecular wires. The first involved the fabrication of the molecular wire system with metal complex oligomer or polymer wires composed of bis(terpyridine)metal complexes using the bottom-up method. This system showed characteristic electron transfer distinct from conventional redox polymers. The second involved the fabrication of a photoelectric conversion system using ITO electrodes modified with porphyrin-terminated bis(terpyr-idine)metal complex wires by the stepwise coordination method, which demonstrated that the electronic nature of the molecular wire is critical to the photoelectron transfer from the porphyrin to ITO. This system proposed a new, facile fabrication method of molecular assemblies effective for photoelectron transfer. The third involved the fabrication of a bioconjugated photonic system composed of molecular wires and photosystem I. The feasibility of the biophotosensor and the biophotoelectrode has been demonstrated. This system proposed that the bioconjugation and the surface bottom-up fabrication of molecular wires are useful approaches in the development of biomo-lecular devices. These three systems of molecular assemblies will provide unprecedented functional molecular devices with desired structures and electron transfer control. 

These are presented by two subclasses of electroactive polymer (i) -conjugated polymers of both organic and inorganic nature [5-15] and (ii) conventional redox polymers [26], and by inorganic ion-insertion (intercalation) compounds [27, 28[ (see the top of Scheme 11.1b). Despite the different nature of their chemical bonds, all of these compounds are mixed, electronic-ionic conductors [29], and hence, their electronic and/or ionic conductivity is expected to change with the applied potential in a predictable, characteristic manner (see Section cl 1.4). 

Classical metals, insulators Ion-radical salts and n-conjugated oligomers Inorganic ir-conjugated polymers and polymerlike carbonaceous materials n-Conjugated polymers Conventional redox-polymers Inorganic ntercatation compounds... 

Conventional redox polymers can also form the basis of electrochemical transistors. Conventional redox polymers have lower maximum conductivity/ and yield devices having lower values of Ijy than conducting polymers or metal oxides. Conventional redox polymers offer an important design advantage, however. Nearly any stable redox active material can be incorporated into a polymeric system to form a conventional redox polymer. This allows the fabrication of devices with a wide range of chemical sensitivities. 

Examples of conventional redox polymer-based devices include those based on viologen, ferrocene, and quinone-based redox polymers. 

There is electron hopping between the redox centers (process 1), as in conventional redox polymers. Electron transfer may also occur through the polymer backbone via a metal-metal electronic interaction (process 2, superexchange pathway) or via polymer-based charge carriers (process 3, polymer mediation). The electronic interactions between the tt-system of the polymer and the metal centers usually enhance the rate of the electron transfer process. Electron transfer via polymer-based charge carriers requires the polymer to be electronically conductive at potentials close to the formal potential of the redox groups. 

A horseradish peroxidase-osmium redox polymer-modified glassy carbon electrode (HRP-GCE) has also been applied to this analysis to improve sensitivity and reduce problems with faradic interference. Kato and colleagues (1996) employed this electrode in measurement of basal ACh in microdialysates using a precolumn enzyme reactor. This system was three to five times more sensitive than a conventional Pt electrode. ACh in rat hippocampus dialysate was quantitated at 9 5 fmol/15 pi (n = 8). ACh was analyzed in PC12 cells in a similar assay by Kim and colleagues (2004). No precolumn enzyme reactor was employed. 

The control of electron transfer is a critical issue in the fabrication of molecular electronic devices from the viewpoint of electronic circuit formation however, electron transfer processes of redox polymer-coated electrodes fabricated using a conventional polymer-coating method usually shows a diffusion-like behavior because the redox sites are randomly distributed in the polymer film (Fig. la) 17-20 consequently, it is difficult to control the electron transfer direction in three dimensions. 

Our approach to the color-variable LEDs presented here has a number of important advantages (1) The two redox polymers modify the charge-inj ection properties of the polymer/metal interfaces, allowing the use of high-workfunction metals as electrodes. This potentially reduces the aging problems associated with conventional polymer LEDs, which must use reactive low-workfunction metals to... 

Conducting polymer-based immobilization or wired enzymes is a global enzyme immobilization method that differs in many respects from those just described. In one example, a redox polymer is formed on the surface by the oxidation of pyrrole molecules to pyrrole radical cations, which then polymerize on the surface to form conductive polypyrrole [60,68]. Other conducting polymers include polyvinylpyridine, polythiophene, polyaniline, and polyindole. If enzymes are present in the solution as polymerization takes place, they are entrapped within the polymer. When these polymers are cross-linked with redox mediators such as [Os(bpy)2Cl]+ 2 the resulting amperometric (or potentiomet-ric) biosensors are referred to as wired enzyme electrodes [5-7]. The distance between the redox centers of the polymer and the FADH2 centers of the reduced enzyme is reduced sufficiently for electrons to be transferred and, therefore, for the mediated electro-oxidation of glucose on conventional electrodes. These electrodes do not require diffusing redox mediators or membranes to contain the enzyme and the redox polymer. 

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... 

As shown above, the typical redox polymer contains an appreciable density of mobile ionic species. Thus, the ionic conductivity of the polymer phase is rather high. An electrical double layer can form at the metal/polymer interface, i.e., the electric potential drops sharply at the polymer side of the interface (see Fig. 20.38). Consequently, the polymer-bound redox sites that contact the metal electrode surface may be oxidized/reduced by a conventional electrochemical mechanism. 

Reid [35] prepared graft polymers from polysaccharides such as cellulose or starch and acrylamide with acrylic acid and a crosslinker such as MBA. The polymerization was conducted as a suspension polymerization in an aromatic hydrocarbon with a minor amount of methanol. Initiation was by conventional redox systems such as persulfate/bisulfite. Patentable features were the use of the suspension type process and the use of a crosslinker. 

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