Ferrocene-osmium complexes

The electroactive units in the dendrimers that we are going to discuss are the metal-based moieties. An important requirement for any kind of application is the chemical redox reversibility of such moieties. The most common metal complexes able to exhibit a chemically reversible redox behavior are ferrocene and its derivatives and the iron, ruthenium and osmium complexes of polypyridine ligands. Therefore it is not surprising that most of the investigated dendrimers contain such metal-based moieties. In the electrochemical window accessible in the usual solvents (around +2/-2V) ferrocene-type complexes undergo only one redox process, whereas iron, ruthenium and osmium polypyridine complexes undergo a metal-based oxidation process and at least three ligand-based reduction processes. 

In contrast to ferrocenes, osmium and ruthenium complexes are capable of forming coordinative bonds with donor centers of GO including histidine imidazoles. There are therefore two ways of bringing coordinated transition metals onto enzyme surfaces, i.e., via natural and artificial donor sites. Artificial centers are commonly made of functionalized pyridines or imidazoles, which must be covalently attached to GO followed by the complexation of an osmium or... 

Use of mediators e.g. ferrocenes, potassium ferrocyanide osmium complexes [14-16] Monitoring of reduction of oxidised peroxidase [17]... 

Use of mediators e.g. Meldola s Blue, sodium ferrocyanide, ferrocene derivatives, osmium complex-modified conducting polymer, OsObpylsCls, tetrathiafulvalene,... 

The electroactive labels most used in genosensing design are ferrocene and its derivates [24-27] (the reversible oxidation process of ferrocene can be detected by means of several electrochemical techniques), osmium complexes [28], platinum complexes [29], gold complexes [30, 31], and metallic [32-36] or semiconductor nanoparticles [37]. Among the last ones, gold nanoparticles are the most used, their detection can be carried out by means of the measurement of resistance or capacitance changes, usually after an amplification procedure with silver, or by means of the anodic stripping voltammetry of Au(lll) obtained after the nanoparticle oxidation Fig. 9.3. 

Eor ferrocene sites at the end of long alkanethiols self-organized at gold electrodes and diluted with unsubstituted thiols with the redox moiety in contact with the electrolyte (Fig. 4a), Chidsey has reported [34] curved Tafel plots (Fig. 4b), which could be fitted by equations derived from Marcus theory with values of k = 0.85 eV and Z = 6.73 x 10 s"l eV" for a reaction rate of A = 2.5 s at in Fig. 4(b). Similar curvature in Tafel plots has been reported by Faulkner and coworkers [35] for adsorbed osmium complexes at ultramicro-electrodes (UME). The temperature dependence of the rate coefficient could also be fitted from Marcus equation and electron states in the metal and coupling factors given by quantum mechanics. 

As discussed above for ferrocene derivatives, small water-soluble ruthenium and osmium complexes are good candidates for redox enzyme catalysis mediation for their reversible (II/III) behavior and relative stabiKty in the two-oxidation state in water. The alteration of the aromatic rings is a means of tuning of the redox potential/structure characteristics of the complexes, which is important for efficient redox enzyme mediation [75, 76]. Table 1 gives the redox potentials in acetonitrile of a series of neutral osmium(II) dichloride complexes with different substituted ligands [77]. 

Other redox-active polyelectrolyte films were prepared from ferrocene-derivatized polly(allylamine) and poly(vinyl pyridine) as well as an osmium complex of poly(vinyl pyridine) [44-46]. These films were synthesized to mediate electron transfer between the electrode and a charged enzyme that was a constituent of the polyelectrolyte film. In the case of ferrocene-derivatized poly(allylamine) or polyfvinyl pyridine), cyclic voltammetry of the bound ferrocene moiety showed small peak splittings (<50 mV at a scan rate of 50 and 20 mV s respectively) [45, 46). The amount of electroactive material increased with the number of deposited layers, but the first layer contained significantly more electroactive ferrocene than the later layers in the poly(allylamine) system [46]. 

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. 

The redox reaction between the polyelectrolytes in PEM films and specific molecules in solutions has been used to induce responsive film swelhng. To name a few, multilayer films containing a ferrocene-derivatized polyaUylamine hydrochloride (PAH-Fc) [185] and an osmium complex-derivatized polyaUylamine hydrochlorides (PAHOs) [186] can swell by 10% of initial film thickness upon oxidation of the Os(II). Multilayer capsules with layered anionic and cationic polyferrocenylsilanes to form multilayer capsules expand and increase their per-meabihty upon chemical oxidation of the ferrocene units [187]. Additionally, a poly(L-glutamic acid)/PAH multilayer film is reported to take up ferrocyanide ions from solutions and can expand and contract by 5-10% in response to electrochemical oxidation and reduction of the ferrocyanide species [188]. 

So far, it has been reported that redox polymers, such as polysiloxane or polyarrylamide on which ferrocene is introduced as a side chain, are effective mediators [15-25]. Other effective mediators include poly(vinyl pyridine) or poly(vinyl imidazole) on which osmium complex is introduced in the side chain. However, the structure-mediator fimction correlation has been poorly understood. Here, the glucose sensor was used as an example of enzyme sensors, and as shown in Table 1, sensors using enzymes other than glucose oxidase have also been actively studied. It is now possible to detect electrochemically materials in the body using an electrode with an enzyme and a polymer gel. 

Since the first report on the ferrocene mediated oxidation of glucose by GOx [69], extensive solution-phase studies have been undertaken in an attempt to elucidate the factors controlling the mediator-enzyme interaction. Although the use of solution-phase mediators is not compatible with a membraneless biocatalytic fuel cell, such studies can help elucidate the relationship between enzyme structure, mediator size, structure and mobility, and mediation thermodynamics and kinetics. For example, comprehensive studies on ferrocene and its derivatives [70] and polypy-ridyl complexes of ruthenium and osmium [71, 72] as mediators of GOx have been undertaken. Ferrocenes have come to the fore as mediators to GOx, surpassing many others, because of factors such as their mediation efficiency, stability in the reduced form, pH independent redox potentials, ease of synthesis, and substitutional versatility. Ferrocenes are also of sufficiently small size to diffuse easily to the active site of GOx. However, solution phase mediation can only be used if the future biocatalytic fuel cell... 

Carbocations as electron acceptors in aromatic EDA complexes 192 Bis(arene)iron(II) complexes with arene and ferrocene donors 198 Carbonylmetallate anions as electron donors in charge-transfer salts 204 Aromatic EDA complexes with osmium tetroxide 219... 

As extra evidence for the rule good for GO-good for HRP , the enzyme-osmium wiring technology has been successively applied for HRP containing systems (200). This stimulated spectral and electrochemical kinetic studies of the HRP-catalyzed oxidation of osmium(II) and ruthenium(II) complexes. Similar to ferrocenes [Eq. (37)], the oxidation follows Eq. (44). 

Ferrocene was one of the earliest mediators used [10] but is somewhat hydrophobic so derivatives of the molecule are often employed [39-43]. Ferricyanide can also be used, and the use of MWCNT with this mediator was shown to enhance its effectiveness [33]. Other groups have studied a wide diversity of novel mediator systems such as poly(vinylferrocene-co-acrylamide) dispersed within an alumina nanoparticle membrane [34], ruthenium [35] and osmium [36,37] complexes, and the phenazine pigment pyocyanin, which is produced by the bacteria Pseudomonas aeruginosa [38]. 

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