Osmium polymer-based

As with all supramolecular structures, one of the most important issues is whether a direct relationship between the structure of a material and its function or properties can be established. In the following, some examples of polymer systems which show such a correlation will be discussed. The materials addressed will include block copolymers, polyalkylthiophenes and a multilayer system based on the self-assembly of polyelectrolytes. Detailed studies on the electrochemical properties of redox-active polymers, based on poly(vinyl pyridine) modified with pendent osmium polypyridyl moieties, have shown that electrochemical, neutron reflectivity and electrochemical quartz crystal microbalance measurements can yield detailed information about the structural aspects of thin layers of these materials. 

Octahedral ruthenium(II) and osmium(II) polypyridyl complexes combine thermal and chemical stability with very interesting photophysical and electronic properties (see Chapter 2, Section 2.3.2). These considerations have prompted a range of studies that target polymers based on polypyridyl and related complexes [7]. For example, in the mid- and late 1990s, crosslinked films derived from the thermal, electro- or photopolymerization of polyfunctional monomeric complexes 7.1 and... 

Kang HL, Liu RG, Sun HF, Zhen JM, Li QM, Huang Y (2012) Osmium bipyridine-containing redox polymers based on cellulose and their reversible redox activity. J Phys ChemB 116 55-62... 

A general problem of the potentiometric measurement is the low exchange current density of enzymatic substrate redox couples. Therefore additional mediators have to be applied with a high exchange current densitiy and fast electrode kinetics, which can react with the enzymatic products. Most often, these mediators are soluble low molecular weight substances, e.g. hexacyanoferrate. Until now, redox polymers based containing covalently bound osmium complexes [5 ] were only used in combination with oxidases for amperometric sensors. We could show that this redox polymer (E =... 

A Redox polymer based on osmium bis-bipyridine and polyvinylpyridine used to wire glucose oxidase [39,54,55]. 

In 1955, it was demonstrated that redox polymers could be cross-linked into redox hydrogels [62]. Since the advent of the wired GOx anode described in the previous section, BFC electrodes using an osmium redox polymer hydrogel have appeared extensively in the literature [63-66]. Redox polymer mediators based on species other than osmium have also been demonstrated in recent BFC systems. Redox polymers based on ferrocene, for example, are a continuing area of research [56]. Sato et al. reported a glucose dehydrogenase BFC with a redox mediator based on vitamin K (see panel E of Table 9.1) in 2005 [57]. Six years later, Meredith et al. reported that a polyethylenimine polymer modified with 3-(dimethylferrocenyl)propyl redox centers is a durable and efficient mediator to GOx despite being a neutral molecule (see panel F of Table 9.1) [58]. 

Furthermore, the utilization of preformed films of polypyrrole functionalized by suitable monomeric ruthenium complexes allows the circumvention of problems due to the moderate stability of these complexes to aerial oxidation when free in solution. A similar CO/HCOO-selectivity with regards to the substitution of the V-pyrrole-bpy ligand by an electron-with-drawing group is retained in those composite materials.98 The related osmium-based redox-active polymer [Os°(bpy)(CO)2] was prepared, and is also an excellent electrocatalyst for the reduction of C02 in aqueous media.99 However, the selectivity toward CO vs. HCOO- production is lower. 

An alternative strategy for co-immobilization of mediator and GOx is based on adsorption of enzyme, cross-linked, as was described for the laccase-based biocatalytic cathodes [30, 37 42], to an osmium-based redox polymer film, on carbon electrodes [1-3, 54],... 

Phosphine complexes, osmium, 19 642 Phosphine coordination complexes, of uranium, 25 436 Phosphine derivatives, 19 28 Phosphine oxide(s), 11 495-496 19 66 predicted deviations from Raoult s law based on hydrogen-bonding interactions, 8 814t in salicylic acid manufacture, 22 8 Phosphine oxide diols/triols, 11 501 Phosphine selenides, 22 90 Phosphinic acid, 19 20, 54-55 Phosphinic anhydride, 11 499 Phosphinothricin acetyltransferase (PAT) proteins, 13 360 Phosphite esters, 19 20 Phosphites, in VDC polymer stabilization, 25 720... 

Because of the importance of olefin metathesis in the industrial production of olefins and polymers, many different catalysts have been developed. Almost all of these are transition metal-derived, some rare exceptions being EtAlCl2 [758], Me4Sn/Al203 [759], and irradiated silica [760]. The majority of catalytic systems are based on tungsten, molybdenum, and rhenium, but titanium-, tantalum-, ruthenium-, osmium-, and iridium-based catalysts have also proven useful for many applications. 

More recently, osmium-based redox polymers of similar structure have been developed as mediators for enzyme-catalyzed reactions relevant to biofuel cells. In this context, the chief development objectives have been tuning the redox potential for both anodes... 

Figure 19.7—Amperometric detection of glucose. The reaction cycle is shown on the left. A sandwich-type biosensor involving glucose oxidase co-immobilised with an osmium-based redox mediator in a polyvinyl polymer is shown on the right.

A recent development31 is the preparation of metal polymer complexes directly on the electrode via the electrochemically induced polymerization of the metal complex. Ruthenium(II) and osmium(II) complexes with ligands containing aromatic amines, e.g. 3- or 4-aminopyridine or 5-amino-1,10-phenanthroline, are electrochemically polymerized to yield a film of the metal polymer on the electrode surface. The polymerization involves free radicals, which are formed via the initial oxidation of the metal complex to a radical cation and subsequent reaction of the radical cation with a base to yield the free radical. 

Some other natural compounds have been transformed for their use in the synthesis of polymers via olefin metathesis processes. As mentioned in the introduction, furans, which are obtained from carbohydrates, are perfect precursors of monomers for ROMP via simple Diels-Alder cycloadditions (n) (Scheme 25) [26]. In this regard, the first example of the ROMP of 7-oxabicyclo[2.2.1]hept-5-ene derivatives was reported by Novak and Grubbs in 1988 using ruthenium- and osmium-based catalysts [186]. The number of examples of ROMP with monomers with this generic structure is vast, and it is out of the scope of this chapter to cover all of them. However, it is worth mentioning here the great potential of a renewable platform chemical like furan (and derived compounds), which gives access to such a variety of monomers. 

In the first step, the precursor, typically a ruthenium or osmium bis(2,2,-bipyridyl) (bpy) complex, reacts with solvent (S) to produce a solvated complex. When solvents such as dry methanol and ethanol are used, only one chloride is exchanged and the species [Ru(bpy)2(PVP) Cl]+ is obtained as the sole product. The nature of the coordination sphere around the metal center can be determined by UV-visible (UV/Vis) spectroscopy (Xmax, 496 nm) and by its redox potential, (about 0.65 V (vs. SCE), depending on the electrolyte being used). By a systematic variation of the ratio of monomer units to redox-active centers, the loading of the polymer backbone ( n) can be varied systematically. (Here, n stands for the number of monomer units in the polymer per redox-active center, e.g. in a PVP-based, n = 10 polymer, there are 10 pyridine units for every redox center. 

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. 

Mao L, Yamamoto K (2000) Amperometric biosensor for glutathione based on osmium-polyvinylpyridine gel polymer and glutathione sulfhydryl oxidase. Electroanalysis 12 ... 

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