Osmium, immobilization

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],... 

A similar polymer, composed of osmium complexed with bis-dichlorobipyridine, chloride, and PVI in a PVI—poly(acrylamide) copolymer (Table 2, compound 3), demonstrated a lower redox potential, 0.57 V vs SHE, at 37.5 °C in a nitrogen-saturated buffer, pH 5 109,156 adduct of this polymer with bilirubin oxidase, an oxygen-reducing enzyme, was immobilized on a carbon paper RDE and generated a current density exceeding 9 mA/cm at 4000 rpm in an O2-saturated PBS buffer, pH 7, 37.5 °C. Current decayed at a rate of 10% per day for 6 days on an RDE at 300 rpm. The performance characteristics of electrodes made with this polymer are compared to other reported results in Table 2. 

Dihydroxylations and aminohydroxylations of alkenes are important reactions in organic synthesis in order to introduce 1,2-functionalization into simple unsaturated precursors. Since these transformations mostly involve toxic osmium tetrox-ide or valuable chiral hgands, attempts to immobilize those reagents are especially appealing. 

Other functionalized supports that are able to serve in the asymmetric dihydroxylation of alkenes were reported by the groups of Sharpless (catalyst 25) [88], Sal-vadori (catalyst 26) [89-91] and Cmdden (catalyst 27) (Scheme 4.13) [92]. Commonly, the oxidations were carried out using K3Fe(CN)g as secondary oxidant in acetone/water or tert-butyl alcohol/water as solvents. For reasons of comparison, the dihydroxylation of trons-stilbene is depicted in Scheme 4.13. The polymeric catalysts could be reused but had to be regenerated after each experiment by treatment with small amounts of osmium tetroxide. A systematic study on the role of the polymeric support and the influence of the alkoxy or aryloxy group in the C-9 position of the immobilized cinchona alkaloids was conducted by Salvadori and coworkers [89-91]. Co-polymerization of a dihydroquinidine phthalazine derivative with hydroxyethylmethacrylate and ethylene glycol dimethacrylate afforded a functionalized polymer (26) with better swelling properties in polar solvents and hence improved performance in the dihydroxylation process [90]. 

Hexacyanoferrates were immobilized on Au covered with SAM of 3,3 -thiodipropionic acid [86]. It has been found from voltammetric studies that the surface coverage of hexacyanoferrate is close to one monolayer and such an electrode exhibits very good surface redox behavior. Cheng et al. [87] have described the formation of an extremely thin multilayer film of polybasic lanthanide heteropolytungstate-molybdate complex and cationic polymer of quaternary poly(4-vinylpyridine), partially complexed with osmium bis(2,2 -bipyridine) on a gold electrode precoated with a cysteamine SAM. Consequently, adsorption of inorganic species might also be related to the properties of SAMs. This problem will be discussed in detail in a separate section later. 

Further variations on the theme have been achieved54 by anchoring species such as [RuivO-(terpy)(py)]2+ or complexes of osmium.55 Iron complexes have also been studied for example, evaporation of a solution containing [Fe(CN)5(H20)]3 and PVP on to an electrode will immobilize the pentacyanoferrate as a pyridyl complex, one in three available pyridyl groups being used to avoid precipitation prior to evaporation of solvent. 

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]. 

Amperometric detection was achieved on two patches of C films (formed by CVD of 3,4,9,10-perylenetetracarboxylie dianhydride) on a glass chip. The microchannels were formed using a 23- im-thick photoresist as a spacer. Glucose oxidase and lactate oxidase were immobilized with HRP on the C films via a coated film of osmium PVPD polymer. Simultaneous measurements of glucose and lactate in rat brain cerebrospinal fluid (first perfused with 50 mM veratridine) were achieved. These two films were spatially separated in order to avoid interdiffusion of H202 formed from the two separate enzymatic reactions. Moreover, the two films were preceded by a third C film immobilized with ascorbate oxidase in order to remove ascorbic acid interference [759]. 

In a more general sense, these observations show that upon immobilization of photoactive compounds onto a solid substrate a substantial difference is detected between the photophysical processes observed for the heterotriad and the dyad in solution. More importantly, direct injection from those moieties not directly bound to the oxide surface can be efficient - this is always fully realized and such an observation is important for the further development of real devices. As a result of this through-space interaction, no osmium-based emission is observed and injection from both the ruthenium and the osmium centers is faster than the laser pulse. An interesting observation is also that upon irradiation of the heterotriad Ti02-Ru-0s, only one final product, i.e. Ti02(e)-Ru(ll)0s(lll), is obtained. In view of the potential of these modified surfaces as potential molecular devices, this is an important feature. The presence of a rigid structure rather than a flexible one, as observed in the Ru-Rh case, clearly leads to a more uniform behavior. 

Acetylcholineesterase and choline oxidase Immobilization of horseradish peroxidase in the redox polymer poly (4-vinylpyridine-chlorobis-(2,2 -bipyridyl) osmium cross-linked by means of polyoxyethylene 400 diglycidyl ether on polished vitreous carbon electrodes. Response time of 30 s and maintained its sensitivity for 24 h at low substrate concentration. Responses were rectilinear up to 10 mM H202 for 100 pM choline with detection limit of 10 nM and 1 pM. [78]... 

Acetylcholineesterase and choline oxidase A glassy C electrode surface was modified with osmium poly (vinyl-pyridine) redox polymer containing horseradish peroxidase (Os-gel-HRP) and then coated with a co-immobilized layer of AChE and ChO. A 22 pL pre-reactor, in which ChO and catalase were immobilized on beads in series, was used to remove choline. The variation in extracellular concentration of ACh released from rat hippocampal tissue culture by electrical stimulation was observed continuously with the online biosensor combined with a microcapillary sampling probe. Measurement of ACh and Ch was carried out by using a split disc C film dual electrode. 

Keywords Asymmetric dihydroxylation, Osmium tetroxide, Cinchona alkaloid, Ligand-accelerated catalysis, Immobilization... 

Although the AD process has found widespread use on the lab scale, industrial applications are obstructed owing to the toxicity and the high cost of the osmium catalyst and the risk of contamination of the products by toxic osmium residues. To address this issue, several research groups have developed immobilized osmium catalysts for use in the osmium-catalyzed dihydroxylations [30]. 

Recently, Yao showed that osmium tetroxide could be immobilized in an ionic liquid. The recyclability of osmium tetroxide was improved by the addition of DMAP. Both the catalyst and the ionic liquid were reused in six consecutive runs without significant reduction in yield [47]. Dihydroxylations in a solvent mixture of l-butyl-3-methylimidazolium hexafluorophosphate ([bmim]PF6), tert-butanol, and water with 0s04 (2 mol%), DMAP (2.4 mol%), and NMO (1.1 equivalents) as cooxidant afforded diols in good yield (73-99% depending... 

Unfortunately, the immobilization of the alkaloid ligands did not result in the simultaneous immobilization of the osmium catalyst due to the weak binding of cinchonidine to the osmium complexes. In most studies, leaching of the osmium catalyst was reported and supplementation of the osmium catalyst was necessary after recovery of the immobilized chiral ligand. In other studies, the recycled ligands were used without further addition of the osmium catalyst resulting in reduced yields or longer reaction times. 

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