Ferrocene voltammetric response

Figure 5. Cyclic voltammetric response of Ferrocene - (A) in CH2CI2, (B) with added CN-[P], (C) after addition of Cr(CN-[P])6.

FIGURE 5.25. Avidin-biotin construction of a monolayer glucose oxidase electrode with an attached ferrocenium cosubstrate and cyclic voltammetric response in a phosphate buffer (pH 8) at 25°C and a scan rate of 0.04 V/s. a attached ferrocene alone, h In the presence of 0.5 M glucose, c Variation of the inverse of the plateau current with the inverse of substrate concentration. Adapted from Figure 1 in reference 24, with permission from the American Chemical Society. 

As far as the electrode adsorption phenomena caused by P° y erro cenes are concerned, Figure 26 shows the cyclic voltammetric response o the illustrated fluorenyl-ferrocene polymer (Fw 7000). 

It exhibits a single oxidation process (Ea = + 0.59 V, vs. SCE) affected by some adsorption problems. These adsorption phenomena, which typically affect the electrochemical response of these derivatives, sometimes make it difficult to ascertain by controlled potential coulometry the effective number of electrons involved in the oxidation step. In this case, the (approximate) number of electrons involved per molecule of dendrimer, nd, can be roughly calculated by comparing the cyclic voltammetric responses of the dendrimer with that of the ferrocene monomer using the following empirical equation.27,40... 

In this connection it is useful to look at the voltammetric response of the dendrimer illustrated in Figure 30, which is constituted by a central ferrocene core branched to ten outer ferrocenyl units.54... 

Figure 33 shows the cyclic voltammetric response of the 8-Fc derivative. It gives rise to two separate reversible oxidations (each one involving four electrons) separated by 0.19 V. This means that the overall oxidation proceeds through two separated steps each one involving four alternate ferrocene units. 

Cyclic voltammetric response of a ferrocene molecule (a) initial (b) after cation complexation... 

In order to illustrate the possibly extended redox aptitude of the ferrocene complexes, Figure 7 shows the cyclic voltammetric response of the ferrocenyl-terpyridine-Ru(II) complex [Fc -(C6H4)-terpy-Ru-terpy-(C6H4)-Fc ]2+ [Fc = (fj5-C5Me4H)Fe(i 5-C5Me4)].5... 

Figure 18 Cyclic voltammetric responses at different temperatures of ferrocene in a 16 17 1 chloroethane tetrahydrofuran 2-methyl-tetrahydrofuran solution containing LiBF4 (0.6 mol dm 3) as supporting electrolyte, (a) Platinum electrode (b) Tl 1223 ( Tl0.sPbo.s Sr2Ca2Cu306). Scan rate 0.025 V s. Potential values are referred to a pseudo-reference silver wire...

The intercalation of polycyclic aromatic compounds into duplex DNA structures was used to develop nucleic acid-based electrochemical sensors.66 For example, the bis-ferrocene-tethered naphthalene diimide (16) was used as a redox-active intercalator to probe DNA hybridization.67 The thiolated probe was assembled on a Au electrode, and the formation of the duplex DNA with the complementary analyte nucleic acid was probed by the intercalation of (16) into the double-stranded nucleic acid structure and by following the voltammetric response of the ferrocene units (Fig. 12.17a). The method enabled the analysis of the target DNA with a sensitivity that corresponded to ca. 1 x 10-20mol. 

Figure 12. Voltammetric response of a GOx-SWCNT-modified glassy carbon electrode in the absence (red) and presence (blue) of 0.5 mM ferrocene monocarboxylic acid. The catalytic response (green) after the addition of 50 mM glucose is also shown. From reference 89.

Table 2 shows the results of the cyclic voltammetry experiments carried out with benzoquinone solutions. It is seen that the cyclic voltammetric response is also well defined, before and after gamma irradiation. As soon as the electrode is exposed to air, however, important deformations in the cyclic voltammetric peaks are observed (Figs. 1,2). A very well-defined peak of ferrocene was still observed with this electrode, while the cyclic voltammetric peak of benzoquinone vanishes irreversibly. 

Polythiophenes, 3-substituted by bipyridyl [268], benzoquinone [269], viologen [270], ferrocene [271], and tetrathiafulvalene [272], have been reported. The redox group is generally apparent in the voltammetric response of the polymers obtained, while the response of the polyconjugated chain is not, an indication of degradation of the backbone. Polymers of better quality with ferrocene and viologen pendants were later electrodeposited from bi- and terthiophenes [273]. 

Another approach to confining the volume is to use vials that have picoliter volumes (7). These can be fabricated with lithographic techniques. Electrochanical experiments using a standard reduction-oxidation couple, ferrocene-carboxylic acid, have been performed in volumes as small as 1 pL. Peak-shaped voltanunetry and an increase in the current on the reverse wave of the cycUc voltammogram are observed in the voltammetric response when ultrasmall volumes (16 pL or less) are used. This deviation from bulk microelectrode behavior is observed only at slower scan rates in the smaller microvials. The voltammetric behavior in the small-volume experiments depends on the scan rate, vial size, and analyte concentration. A physical model based on restriction of analyte in these well-defined... 

Figure 6.3.11.4 Voltammetric response of a 10.5-nm radius Pt nanodisk electrode corresponding to the oxidation of ferrocene in acetonitrile. Also shown is the i—Vresponse prior to exposing the Pt disk.

However, there remains a possibility that the appearance of two pairs of voltammetric peaks may arise from particle adsorption onto the electrode surface. As in solid films, the ferrocene moieties might exhibit different energetic states and accessibility to counterions because of spatial effects. This hypothesis is discounted by results from two additional experiments. First, the cathodic and anodic current density of the redox peaks was found to be linearly proportional to the square root of potential scan rates, suggesting that the charge-transfer processes were under diffusion control. Second, after the electrochanical measurements in the Ru=CH-Fc particle solution, the Au electrode was taken out and rinsed with a copious amount of DMF and then immersed into a same electrolyte solution without the nanoparticles. Only featureless voltammetric responses were observed, as shown in Figure 3.13 (long dashed curve). In short, both measurements signify minimal surface adsorption of the particles. 

Delacote, C., J. R Bouillon, and A. Walcarius, 2006. Voltammetric response of ferrocene-grafted mesoporous silica. Electrochim Acta 51 6373-83. 

A very pronounced double-layer effect is also observed in the voltammetry of adsorbed outer-sphere reactants." For instance, the voltammetry of self-assembled monolayers (SAMs) of alkane-thiols containing a terminal redox group (e.g., ferrocene) is strongly influenced by the electric field across the SAM, and this is manifested in peak broadening and a shift in the half-wave potential. Because redox-active SAMs are frequently geometrically very well defined, and their dielectric properties can be measured, the electric field across these layers can be readily computed from electrostatics to obtain the electric potential at the redox center. This in turn can be used to compute the influence of the electric potential distribution on voltammetric response, which can then be quantitatively compared to experiment. 

Because of the peculiar electrochemical behavior (a single cyclic voltammetric wave characterized by remarkable electrochemical and chemical reversibility), dendrimers terminated with ferrocene-type units can be profitable used as exoreceptors, provided that they contain a group able of interacting through noncovalent bonds with the species to be recognized. Furthermore, such a group has to be located near the ferrocene units to sufficiently perturb their electrochemical response as a consequence of the interaction with the guest species. 

After the desired coverage has been achieved (via deposition time), the PVF+-coated electrode can be rinsed with water and transferred to aqueous 0.1 mol dm 3 sodium perchlorate for characterization. Since both oxidation states of the film are insoluble in aqueous media, we are able to investigate the redox switching process of surface-confined material, as schematically illustrated in Scheme 13.4. The polymer coverage (I7mol cm-2, expressed in terms of immobilised ferrocene sites) is obtained coulo-metrically by integration of the current response to a slow voltammetric scan. We now discuss the behaviour of thin (<0.1 jtm) PVF films after... 

Beer. P.D. Sikanyika. H. Biackburn, C. McAleer, J.F. Lithium redox-responsive ferroeene bis-tertiary amide derivatives. J. Organomet. Chem. 1988. 350. C15-C19. Sutcliffe. O.B. Chesney. A. Bryce. M.R. Voltammetric cation sensors based on ferrocene derivatives with oxaline and imine substituents. J. Organomet. Chem. 2001, 637-639, 134-138. 

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