Electrodes ferrocene modified

Functionalized conducting monomers can be deposited on electrode surfaces aiming for covalent attachment or entrapment of sensor components. Electrically conductive polymers (qv), eg, polypyrrole, polyaniline [25233-30-17, and polythiophene/23 2JJ-J4-j5y, can be formed at the anode by electrochemical polymerization. For integration of bioselective compounds or redox polymers into conductive polymers, functionalization of conductive polymer films, whether before or after polymerization, is essential. In Figure 7, a schematic representation of an amperomethc biosensor where the enzyme is covalendy bound to a functionalized conductive polymer, eg, P-amino (polypyrrole) or poly[A/-(4-aminophenyl)-2,2 -dithienyl]pyrrole, is shown. Entrapment of ferrocene-modified GOD within polypyrrole is shown in Figure 7. 

By setting the ratio of the oxidized and reduced forms of a redox couple in an electrode coating film to unity, the potential of this electrode in an inert electrolyte is poised at the half-wave potential of the couple. This has indeed been shown for platinum wires coated with polyvinylferrocene or ferrocene modified polypyrrole But the long term stability of these electrodes during cell connection... 

It is noted that the anodic peak current prominently increases with an increase in the molar ratio of ferrocene to glucose oxidase whilst the amount of enzyme self-assembled on the electrode surface is fixed as presented in Figs. 14-16. This indicates that each modified ferrocene may contribute to electron transfer between the enzyme and the electrode in the case of gold-black electrode, the ferrocene-modified enzyme could form multi electron transfer paths on the porous gold-black electrode. 

Substrate concentration dependence of response current of the gold-black electrode was compared with that of gold disk electrode. The ferrocene-modified glucose oxidase which was used in this measurement had 11 ferrocenes per glucose oxidase. The electrode potential was controlled at 0.4 V vs. Ag/AgCl. The response current was recorded when the output reached at a steady state. The response current was enhanced when ferrocene-modified glucose oxidase was self-assembled on a porous gold-black electrode. 

The porous matrix of gold-black electrode has enabled ferrocene-modified glucose oxidase to perform the smooth electron transfer by means of easy access between self-assembled molecules and electrode surface. 

Fig.21 Glucose concentration dependence of response current on ferrocene-modified glucose oxidase self-assembledon the gold black ( ) and plain gold (0) electrodes...

Ferrocenyl-based polymers are established as useful materials for the modification of electrodes, as electrochemical biosensors, and as nonlinear optical systems. The redox behavior of ferrocene can be tuned by substituent effects and novel properties can result for example, permethylation of the cyclopentadienyl rings lowers the oxidation potential, and the chaige transfer salt of decamethylfer-rocene with tetracyanocthylene, [FeCpJ]" (TCNE], is a ferromagnet below = 4.8 K, and electrode surfaces modified with a pentamethylferrocene derivative have been used as sensors for cytochrome c These diverse properties have provided an added impetus to studies on ferrocene dendrimers. 

Perhaps the original hope for these polymers was that they would act simultaneously as immobilisation matrix and mediator, facilitating electron transfer between the enzyme and electrode and eliminating the need for either O2 or an additional redox mediator. This did not appear to be the case for polypyrrole, and in fact while a copolymer of pyrrole and a ferrocene modified pyrrole did achieve the mediation (43), the response suggested that far from enhancing the charge transport, the polypyrrole acted as an inert diffusion barrier. Since these early reports, other mediator doped polypyrroles have been reported (44t45) and curiosity about the actual role of polypyrrole or any other electrochemically deposited polymer, has lead to many studies more concerned with the kinetics of the enzyme linked reactions and the film transport properties, than with the achievement of a real biosensor. 

Figure 4. (left) Steady state response to 31.5 mM glucose of the ferrocene-modified poly(siloxane) / glucose oxidase / carbon paste electrodes at several applied potentials. The relay systems are indicated next to each curve, which is the mean result for four electrodes. 

Figure 8. (right) Glucose calibration curves for the ferrocene-modified poly(ethylene oxide)/glucose oxidase/carbon paste electrodes at E =... 

See also - bifunctional mediator, - biofuel cells, -> catalytic current, - catalytic hydrogen evolution, - dye cell, -> enzyme electrodes, -> ferrocene, - glucose sensor, -> indirect and direct electrolysis, and - surface-modified electrodes. 

An electrochemical OP sensor by the nonenzymatic route was reported based on chemical modification of the surface of a gold electrode with ferrocene derivative (Fc). For this purpose, the gold electrode was modified with dithioFc derivative to form an aminoFc-monolayer-modified electrode (Khan et al, 2007). The principle of operation of the aminoFc-modified electrode for OP sensing is that chloro-or cyano-substitued OP compounds covalently bind to aminoFc moieties, by which the redox potential of the surface-confined Fc can be altered. In fact, ca. 110 and 60 mV shifts in the redox potential were observed, suggesting a possible use of the sensors for detecting OPs from the potential shifts. 

Figure 3. The enantioselective bioelectrocatalyzed oxidation of glucose by glucose oxidase at an electrode modified by a chiral electron-transfer mediator. (A) Organization of the chiral ferrocene monolayer-modified Au electrode and its interaction with soluble GOx. EDC = l-(3-dimethylami-nopropyl)-3-ethylcarbodiimide hydrochloride. (B) Cyclic voltammograms of the ferrocene-modified electrode (curves a and b for (i )-Fc (2) and (5)-Fc (3), respectively) in the presence of 1 x 10 M GOx and 50 mM glucose 0.1 M phosphate buffer, pH 7.0 potential scan rate, 5 mV s electrode area, 0.26 cm. ...

Figure 9. (A) The preparation of a nonordered polymeric layer of glucose oxidase electrically wired by ferrocene groups incorporated in the polymer film. (B) Cyclic voltammograms of the GOx/ferrocene-modified electrode in the absence (a) and presence (b) of glucose, 30 mM. Performed under argon, in phosphate buffer, pH 7 potential scan rate, 10 mV s. Inset calibration curve for the amperometric response to glucose at 0.35 V vs. SCE measured under N2(a) and air (b). Adapted from Ref. [90] with permission.

Figure 10. (A) The stepwise assembly and electrical contacting of a cross-linked organized multilayer array of glucose oxidase (GOx) on an An electrode. (B) Cyclic voltammograms of the GOx/ ferrocene-modified electrode in the presence of glucose (20 mM) in (a) one-, (b) four- and (c) eight-layer configurations. Inset amperometric responses of the four-layer GOx array at 0.4 V vs. SCE as a function of glucose concentration. Recorded in 0.1 M phosphate buffer, pH 7.3, under argon.

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... 

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... 

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. 

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... 

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