Ferrocenes diffusion limited

A common experimental situation is the electrolysis of a neutral species to form an ion. The one-electron oxidation of ferrocene to ferricenium is an example of such a process. Since the current arises from the flux of a neutral to the electrode, the diffusion-limited current is unaffected by lowering the electrolyte concentration (Fig. 12.5, left panel). However, evaluation of the diffu-sion/migration equations shows that the charged product of the electrochemical reaction is removed from the diffusion layer by ion migration while inert ions of opposite charge are drawn in by the process of migration, again to maintain electroneutrality [68]. 

The efficiency of electron-transfer reduction of Cgo can be expressed by the selfexchange rates between Coo and the radical anion (Ceo ), which is the most fundamental property of electron-transfer reactions in solution. In fact, an electrochemical study on Ceo has indicated that the electron transfer of Ceo is fast, as one would expect for a large spherical reactant. This conclusion is based on the electroreduction kinetics of Ceo in a benzonitrile solution of tetrabutylammonium perchlorate at ultramicroelectrodes by applying the ac admittance technique [29]. The reported standard rate constant for the electroreduction of Ceo (0.3 cm s ) is comparable with that known for the ferricenium ion (0.2 cm s l) [22], whereas the self-exchange rate constant of ferrocene in acetonitrile is reported as 5.3 x 10 s , far smaller than the diffusion limit [30, 31]. 

Hill and Cass have previously asserted that the important rate of electron transfer between GOx and, in their case ferrocene mediator, can be determined in homogeneous solution phase by the use of electrocatalytic currents obtained from die steady-state GOx-glucose reaction, and working curves derived by hticholson and Shain relating diffusion limited and catalytic currents.(7,S) A priori, this approach... 

One of the first CPs to be studied as a modified electrode was P(Py), at which the electrochemistry of ferrocene (FeCp2) was studied [55]. This FeCp2 system (E° +0.42 V vs. SCE) showed near ideal reversible electrochemical behavior, with proportional to indicating diffusion limited processes, cathodic/anodic peak current ratios of near unity, and cathodic/anodic peak separations close to 70 mV. More recently, poly (bithiophene) modified electrodes have been used to monitor the electrochemistry of FeCp2 as well as / -benzoquinone, and to probe the semiconducting behavior of this CP [56]. Ferrocene appears popular as a near-reversible probe material for CP-modified electrodes, having been studied with poly(thiophene) and other CP modified electrodes as well [57]. 

In organic electrolyte (Figure 3a), a well-defined and stable redox process was observed and assigned to the oxidation and reduction of ferrocene groups immobilized on the surface of MOF particles. The position of the oxidation and reduction peaks was independent of the scan rate, indicative of fast electron transfer. However, the value of ca. 100 mV for A p indicates (Table 1) that at least one of the species involved in the redox reaction is not attached to the surface. The peak current for the oxidation of ferrocene in [Zn40(bdc-NH2)(btb)4/3] was indeed found to be proportional to which is a result in-between the theoretical predictions for diffusion-limited (v ) and surface-confined reaction (v). For ferrocene in Al(OH)(bdc-NH2), the exponent value of 0.56 was found, in agreement with the theoretical value of 1/2 expected for a diffusion-limited process. 

Fig. 51.4. Normalized feedback current-distance curves obtained with a 25 pm Pt UME in ImM ferrocene methanol in 0.1M Na2S04. The substrate potential was varied to control the feedback effect (1) 150 mV, (2) 100 mV, (3) 50 mV, (4) OmV, (5) —50 mV, (6) —100 mV, (7) —150 mV and (8) —200 mV vs. Ag/AgCl reference electrode. (9) and (10) are the limiting curves for conductor and insulator substrate, respectively. The tip was held at 0.4 V where the oxidation was diffusion-controlled.

Besides the substances mentioned so far, functionalized fuUerenes like the simple Bingel adduct can be intercalated into nanotubes as well (Section 2.5.5.2). The formation of peapods has further been described for metallocenes (e.g., ferrocene), porphyrines (e.g., erbium phthalocyanine complex) and small fragments of nanotubes. The most important prerequisite for the feasibility of inclusion is always a suitable proportion of sizes of both the tube and the structure to be embedded. For example, this effect can be observed for the intercalation of different cobaltocene derivatives into SWNT. The endohedral functionalization only takes place at an internal diameter of 0.92nm or above (Figure 3.100). But there is also an upper limit to successful incorporation. When the diameter of the nanotube is too large, the embedded species can easily diffuse away again from the host. Only few molecules are consequently found inside such a wide tube. 

Since the lowest triplet state of nitrostilbenes at room temperature can be probed by laser flash photolysis, quenching rate constants could be measured directly [188,200], Values for kq are close to the diffusion-controlled limit (Table 17). Comparable results have been obtained for naphthyl-[410] and 2-anthrylethylenes [433] and for ADBs [33, 143, 145-147, 232, 411], For substituted 1-NPEs, conclusions about the position of the triplet equilibrium were drawn on the basis of rate constants for quenching by ferrocene and oxygen [441]. 

However, slow leakage of the soluble form of the redox mediator (the ferrici-nium ion) may lead to biosensor deterioration and precludes its use for on-line monitoring. The use of soluble high molecular weight diffusing mediators should in principle limit this phenomenon. For this purpose, the protein BSA was labeled with Fc groups by reductive amination with ferrocene carboxaldehyde 3 in the presence of sodium borohydride (Scheme 6.1) and the labeled protein was shovm to behave as a redox mediator in the electrocatalysis of glucose by GOx 181. 

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