Osmium redox

FIGURE 12.5 Structure of the osmium redox polymer, OsPVI, formed by coordination of an [Os(2,2 -bipyridine)2Cl]+ complex to polyvinylimidazole in a usually 1 9 ratio. 

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. 

Kato T, Liu JK, Yamamoto K, Osborne PG, Niwa O. 1996. Detection of basal acetylcholine release in the microdialysis of rat frontal cortex by high- performance liquid chromatography using a horseradish peroxidase-osmium redox polymer electrode with pre-enzyme reactor. J Chromatogr B 682 162-166. 

In another report, binding of IgG to anti-IgG immobilized on a Au-coated BK7 chip (sealed by PDMS) was detected by SPR [741]. Lactate was also determined using the SPR method. First, lactate was oxidized by LOX immobilized on an osmium redox polymer to produce H202. This molecule was then reduced by HRP immobilized in the same redox polymer. This surface reaction was detected by SPR [741]. 

A BASJ ion-exchange microbore column (45 cm x 1 mm i.d.) 0.05 Sodium phosphate buffer of pH 8.5 containing 0.1 mM EDTA [60 pL/min]. Electrochemical at horseradish peroxidase osmium redox polymer-modified vitreous C electrodes at 0 mV versus Ag/AgCl. Rat frontal cortex dialysate samples [188]... 

Figure 5.5 Structure proposed for the osmium redox polymers (a) [Os(2,2 -bipyridine)2 (PVI)ioClf and (b) polyvinylpyridine-[Os(/ /,N -dimethyl-2,2 -biimidazole),Cl] ",...

Poly(l-vinylimidazole)i2-[Os-(4,4 -dimethyl-2,2-bipyridyl)2Cl2] and poly(vi-nylpyridine)-[Os-(Ai,Ai -methylated-2,2 -biimidazole)3] were reported for their efficient capability of mediating electrons transfer between bacterial cells to electrodes. With S. oneidensis, the osmium redox polymer modified anode showed a 4-fold increase in current generation and a significant decrease in the start-up time for electrocatalysis. Using an anode modified with electropolymerized polypyrrole, a dramatie improvement in energy output was noticed in the MFCs. MFCs with a polypyrrole/ anthraquinone-2,6-disulfonic disodium salt (PPy/AQDS)-modified anode... 

Figure 5.15 Amperometric responses from a spectrographic graphite electrode modified with only osmium redox polymer (no enzyme) and osmium redox polymer with the molybdenum domain of human SO (HSO-MD) or holo human SO (ASO) (3 pM). After intervals of 1 and 2 min., aliquots of sulfite (400 pM) were repeatedly added. Conditions 20°C 100 mm Tris buffer solution, pH 8.5 E app = -0.10 V vs. Ag/AgCl. Reproduced from ref. 96 with permission from John Wiley and Sons. Copyright 2015 Wiley-VCH Verlag GmbH Co. KGaA, Weinheim.

In this section we presented a discussion of mediated catalysis using one particular type of electroactive polymer system, that based on polyvinylpyridine containing coordinatively attached bisbip3nidine chloro ruthenium or osmium redox centers. We could of course discuss many more classes of polymer systems. Chapter 2 is intended to be a tutorial, so a comprehensive and exhaustive summary of the experimental literature is unnecessary. For further details the reader is referred to reviews by Hillman and Saveant et alS for a very comprehensive discussion of a wide variety of systems. 

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. 

The reduced enzyme is reoxidized by osmium redox couples on the polymer chain close to the active site on the enzyme The electrons from the reaction then propagates to the electrode surface by means of electron hopping between the redox sites in the polymer The polymer is crosslinked by PEGDGE to the electrode surface, where the osmium complex oxidizes and the electrons are transferred to the electrode The electron transfer procedure is shown in figure 1 The current produced is proportional to the rate of glucose conversion, and provides a measure of the substrate concentration... 

The redox polymer has three functions, it communicates with the active site in the enzyme, transfers the electrons to the electrode surface and it forms a matrix in which the enzyme is immobilized [3], This is the reasons which makes the osmium polymer such a successful mediator. A very sensitive and fast biosensor is achieved with the redox polymer bound enzyme. There are no membrane passages that will delay the reaction and the mediator is immobilized in the same matrix and will not diffuse away. An amperometric glucose sensor with the mediator electrostatically bound to a PVP polymer has been studied in a previous paper [4], where the mediator loss seemed to be a problem. The hydrophilicity of the osmium redox polymer also contributes to the rapid response because of the fast transport of water soluble substrates and products. 

By means of a potentiostat (fig 2) the redox state of the polymer-modified FET can be adjusted to any redox state (e g 400 mV vs SCE), which corresponds to a determined concentration ratio of oxidized and reduced osmium redox centres according to the Nm s r equation The gate electrode is used as working electrode in a three electrode arrangement If we apply several pulses of a potentiostatic current to the polymer film, it will be charged stepwise to reach the potentiostatically given potential (e g 400 mV, fig 3a)... 

Kumar, R. and D. Leech. Immobilisation of alkylamine-functionalised osmium redox complex on glassy carbon using electrochemical oxidation. Electrochim. Acta 140, 2014 209-216. 

Figure 3. Dependence of current on potential for a NaI04 oxidized horseradish peroxidase immobilized in a 3-dimensional epoxy hydrogel free of electron relaying osmium redox centers. (A) noH2C (B) 0.1mMH2O2 Conditions aerated pH 7 physiological phosphate buffer solution scan rate 2.5 mV s l 500 RPM.

Figure 5. Dependence of current density on hydrogen peroxide concentration for cathodes based on different peroxidases, (open circles) NaI04 treated horseradish peroxidase (closed circles) native horseradish peroxidase (open squares) lactoperoxidase (closed squares) Aithromyces ramosus. Each electrode contains approximately 10/ g osmium redox polymer, polyethylene glycol diglycidyl ether crosslinker and 1 to 4/

Figure 6. Dependence of normalized current on the weight fraction of Aithromyces ramosus peroxidase (ARP) in the film. The osmium redox polymer and crosslinker amounts were held constant at approximately 10 and l]4g. Conditions aerated pH 7 physiological phosphate buffer solution 1000 RPM.

Park T.M., Iwuoha E.I., Smyth M.R., Freaney R., McShane A.J. Sol-gel based amperometric biosensor incorporating an osmium redox polymer as mediator for detection of 1-lactate. Talanta 1997 44(6) 973-978... 

Electric conduction can also be carried out by a hopping mechanism between adjacent redox sites. This mode of self-exchange charge transfer within silicate composite was exploited by the incorporation of redox polymers such as the osmium redox polymer, [Os(bpy)(2)(PVP)(10)Cl]Cl in the silicate [101], A similar approach based on the creation of naphthoquinone-modified silicate CCE was also demonstrated by Rabinovich et al. [47], Here, the binder itself functions as a charge transfer polymer due to the presence of pendant quinone functionalities. 

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