Ferrocene-modified siloxane polymer

Siloxane Polymers. The synthesis of the ferrocene-modified siloxane polymers (A - E) has been described previously (25,27,32). Briefly, the methyl(2-ferrocenylethyl)-siloxane polymers were prepared by the hydrosilylation of vinylferrocene with the methylhydrosiloxane homopolymer or the methylhydrosiloxane-dimethylsiloxane copolymers (m n ratios of 1 1, 1 2, and 1 7.5 see Figure 1) in the presence of chloroplatinic acid as a catalyst. The methyl(9-ferrocenylnonyl)siloxane-dimethylsiloxane (1 2) copolymer was prepared via hydrosilylation of 9-ferrocenyl-l-nonene with the methylhydrosiloxane-dimethylsiloxane (1 2) copolymer. The molecular weight range of these ferrocene-modified siloxane polymers is approximately 5,000-10,000. Purification of the polymers was achieved by reprecipitation from chloroform solution, via dropwise... 

In contrast, Boguslavsky and co-workers used a flexible polymer chain to put electron relays [15]. Their polymers provide communication between redox centres in glucose oxidase (GOD) and the electrode. No mediation occurred when ferrocene was attached to a non silicone backbone. Their ferrocene-modified siloxane polymers are stable and non diffusing. Therefore, biosensors based on these redox polymers give good response and stability. 

The ratio of ferrocene-modified siloxane subunits to unsubstituted siloxane subunits rrv.n ratio) was varied as was the length (a ) of the alkyl side chain onto which the ferrocene moiety was attached as shown in Fig. 3.3. The electrode containing co-polymer with m n ratio of 1 1 or 1 2 was the more efficient electron relay systems. The ferrocene-modified homopolymer on the other hand loses flexibility due to steric hindrance caused by the side chain substitution by ferrocene, preventing efficient electron transfer from the enzyme to the electrode. The length of the alkyl side chain onto which the ferrocene moiety is attached was also found to influence the electron transfer efficiency of the electron relay system. Maximal current density was measured... 

These ferrocene modified polysiloxane polymers were also used to construct glycolate [6,7], lactate [7], acetylcholine [12,81], glutamate [12] and cholesterol [81] sensors. All these electrodes showed that ferrocene containing siloxane polymers efficiently shuttled electrons between redox center(s) of enzyme and the electrode surface. 

A ferrocene modified siloxane redox polymeric electron transfer system in carbon paste electrodes for aldose biosensors using PQQ-dependent aldose dehydrogenase was reported by Smolander et al. [86]. Polymethyl(ll-ferrocenyl-4,7,10-trioxa-undecanyl)methyl(12-amino-4,7,10-trioxa-dodecyl)-siloxane (1 1 random co-polymer) (Fig. 3.6) was found to be an efficient electron transfer system yieldii better electrode operational stabiUty than those constructed with dimethylferrocene fi ee mediator. The hydrophilic nature of the pendant chain and side chain on dimethyl siloxane units favorably interact with enzjnne causing efficient electron transfer from coenzyme PQQ of aldose dehydrogenase to the electrode surface. 

Ferrocene modified flexible polymeric electron transfer systems Ferrocene and its derivatives are readily available and commonly used organometalUc redox mediators, so it is quite natural that they were selected first to synthesize mediator modified polymeric electron transfer systems. Siloxane pol5uners are flexible but aqueous insoluble pol3nmers. As previously indicated, a flexible polymer backbone allows close contact between the redox center(s) of the enzyme and the mediator, and the water insoluble property of the polymer prevents not only redox polymer from leaching into bulk media but also prevents enzyme diffusion away fi-om the electrode surface by entrapping it in the polymer/carbon paste matrix. Therefore, ferrocene and... 

Karan et al. [10] reported glucose sensors using quinone modified poly-siloxane (Fig. 3.8a-A) and acrylonitrile-ethylene (Fig. 3.8a-B) co-polymers and glucose oxidase. Sensors constructed with glucose oxidase and quinone modified polysiloxane were considerably more efficient than those using acrylonitrile-ethylene system to transfer electrons from reduced glucose oxidase to a conventional carbon paste electrode. Their results coincide with those described previously for the ferrocene-modified polysiloxane system. The excellent flexibility of poly(siloxane) allows it to function as an efficient... 

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. 

Electrochemical doping of insulating polymers has been attempted for polyacetylene, polypyrrole, poly-A/-vinyl carbazole and phthalocyaninato-poly-siloxane. Significantly, Shirota et al. [91] claim to have achieved the first synthesis of electrically conducting poly(vinyl ferrocene) by the method of electrochemical deposition (ECD) [91]. This is based on the insolubilization of doped polymers from a solution of neutral polymers. A typical procedure applied [91] for polyvinyl ferrocene is to dissolve the polymer in dichlorometh-ane and oxidize it anodically with Ag/Ag+ reference electrode under selective conditions. The modified polymer [91] (Fig. 28) is a partially oxidized mixed valence salt containing ferrocene and ferrocenium ion pendant groups with C104 as the counter anion. 

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