Electrocatalytic Modified Electrodes

3 Electrocatalytic Modified Electrodes Often the desired redox reaction at the bare electrode involves slow electron-transfer kinetics and therefore occurs at an appreciable rate only at potentials substantially higher than its thermodynamic redox potential. Such reactions can be catalyzed by attaching to the surface a suitable electron transfer mediator (45,46). Knowledge of homogeneous solution kinetics is often used to select the surface-bound catalyst. The function of the mediator is to facilitate the charge transfer between the analyte and the electrode. In most cases the mediated reaction sequence (e.g., for a reduction process) can be described by 

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Chen, Q., Wang, J., Rayson, G., Tian, B., and Lin, Y., Sensor array for carbohydrates and amino acids based on electrocatalytic modified electrodes, Anal. Chem., 65, 251, 1993. 

There are two general problems with the use of electrocatalytic modified electrodes in flow systems. In order for the method to have a wide linear dynamic range, the rate of electrolysis must be controlled by mass transport of the analyte from the bulk solution to the electrode surface. Second, the modified electrode must be stable during both experimental operations and storage. 

Wang J, Chen Q, Cepria G (1996) Electrocatalytic modified electrode for remote monitoring of hydrazines. Talanta 43 1387-1391... 

Intensive research on the electrocatalytic properties of polymer-modified electrodes has been going on for many years Until recently, most known coatings were redox polymers. Combining redox polymers with conducting polymers should, in principle, further improve the electrocatalytic activity of such systems, as the conducting polymers are, in addition, electron carriers and reservoirs. One possibility of intercalating electroactive redox centres in the conducting polymer is to incorporate redoxactive anions — which act as dopants — into the polymer. Most research has been done on PPy, doped with inter alia Co 96) RyQ- 297) (--q. and Fe-phthalocyanines 298,299) Co-porphyrines Evidently, in these... 

Figure 17.7 Electrocatalysis of O2 reduction by Pycnoporus cinnabarinus laccase on a 2-aminoanthracene-modified pyrolytic graphite edge (PGE) electrode and an unmodified PGE electrode at 25 °C in sodium citrate buffer (200 mM, pH 4). Red curves were recorded immediately after spotting laccase solution onto the electrode, while black curves were recorded after exchanging the electrochemical cell solution for enzyme-fiiee buffer solution. Insets show the long-term percentage change in limiting current (at 0.44 V vs. SHE) for electrocatalytic O2 reduction by laccase on an unmodified PGE electrode ( ) or a 2-aminoanthracene modified electrode ( ) after storage at 4 °C, and a cartoon representation of the probable route for electron transfer through the anthracene (shown in blue) to the blue Cu center of laccase. Reproduced by permission of The Royal Society of Chemistry fi om Blanford et al., 2007. (See color insert.)...

Special electrochemical sensors that operate on the principle of the voltammetric cell have been developed. The area of chemically modified solid electrodes (CMSEs) is a rapidly growing field, giving rise to the development of new electroanalytical methods with increased selectivity and sensitivity for the determination of a wide variety of analytes [490]. CMSEs are typically used to preconcentrate the electroactive target analyte(s) from the solution. The use of polymer coatings showing electrocatalytic activity to modify electrode surfaces constitutes an interesting approach to fabricate sensing surfaces useful for analytical purposes [491]. 

Re(bpy)(CO)3Cl-modified electrodes has not yet been explained. However, from the cyclic voltammograms of fac-Re(bpy)(CO)3Cl (Fig. 14) and from the intermediate complexes formed by electrolysis in acetonitrile in the presence and absence of C02, two different electrocatalytic pathways (Fig. 15) were suggested144 initial one-electron reduction of the catalyst at ca. -1.5 V versus SCE followed by the reduction of C02 to give CO and C03, and initial two-electron reduction of the catalyst at ca. -1.8 V to give CO with no C03. The electrochemistry of [Re(CO)3(dmbpy)Cl] (dmbpy = 4,4 -dimethyl-2,2 -bipyridine) was investigated145 to obtain mechanistic information on C02 reduction, and the catalytic reac-... 

Electrocatalytic regeneration of NADH has also been performed at polymer-modified electrodes with pendent [(r/s-Cs M cs) R h( bpy )C1]1 55,56 and [Rh(terpy)2]3+ 57 complexes, and coupled to enzymatic reduction with alcohol dehydrogenase.55,57... 

Modified electrodes containing cyclam derivatives have been prepared. The approach utilizing cyclam incorporated in Nafion film on a carbon electrode shows that the catalytic efficiency of the system is much lower than observed when the catalyst is adsorbed on the mercury. With electrodes prepared following the Langmuir Blodgett technique, only the electrode materials that allow the orientation of the monolayer so that the tail points to the substrate were found to be electrocatalytically active.165... 

Techniques for attaching such ruthenium electrocatalysts to the electrode surface, and thereby realizing some of the advantages of the modified electrode devices, have been developed.512-521 The electrocatalytic activity of these films have been evaluated and some preparative scale experiments performed. The modified electrodes are active and selective catalysts for oxidation of alcohols.5 6-521 However, the kinetics of the catalysis is markedly slower with films compared to bulk solution. This is a consequence of the slowness of the access to highest oxidation states of the complex and of the chemical reactions coupled with the electron transfer in films. In compensation, the stability of catalysts is dramatically improved in films, especially with complexes sensitive to bpy ligand loss like [Ru(bpy)2(0)2]2 + 51, 519 521... 

E. Casero, F. Pariente, and E. Lorenzo, Electrocatalytic oxidation of nitric oxide at indium hexacyanof-errate film-modified electrodes. Anal. Bioanal. Chem. 375, 294—299 (2003). 

Electrocatalysis in oxidation has apparently first been shown for ascorbic acid oxidation by Prussian blue [60] and later by nickel hexacyanoferrate [61]. More valuable for analytical applications was the discovery in the early 1990s of the oxidation of sulfite [62] and thiosulfate [18, 63] at nickel [62, 63] and also ferric, indium, and cobalt [18] hexacyanoferrates. More recently electrocatalytic activity in thiosulfate oxidation was shown also for zinc [23] hexacyanoferrate. Prussian blue-modified electrodes allowed sulfite determination in wine products [64], which is important for the wine industry. 

S.-M. Chen, Electrocatalytic oxidation of thiosulfate by metal hexacyanoferrate film modified electrodes. J. Electroanal. Chem. 417, 145-153 (1996). 

A. Eftekhari, Electrochemical behavior and electrocatalytic activity of a zinc hexacyanoferrate film directly modified electrode. J. Electroanal. Chem. 537, 59-66 (2002). 

F. Li and S. Dong, The electrocatalytic oxidation of ascorbic acid on Prussian blue film modified electrodes. Electrochim. Acta 32, 1511—1513 (1987). 

S.F. Wang, M.A. Jiang, and X.Y. Zhou, Electrocatalytic oxidation of ascorbic acid on nickel hexacyanoferrate film modified electrode. Gaodeng Xuexiao Huaxue Xuebao 13, 325-327 (1992). 

P.N. Deepa and S.S. Narayanan, Sol-gel coated Prussian blue modified electrode for electrocatalytic oxidation and amperometric determination of thiols. Bull. Electrochem. 17, 259-264 (2001). 

W.Y. Hou and E. Wang, Flow-injection amperometric detection of hydrazine by electrocatalytic oxidation at a Prussian blue film-modified electrode. Anal. Chim. Acta 257, 275—280 (1992). 

The first CNT-modified electrode was reported by Britto et al. in 1996 to study the oxidation of dopamine [16]. The CNT-composite electrode was constructed with bro-moform as the binder. The cyclic voltammetry showed a high degree of reversibility in the redox reaction of dopamine (see Fig. 15.3). Valentini and Rubianes have reported another type of CNT paste electrode by mixing CNTs with mineral oil. This kind of electrode shows excellent electrocatalytic activity toward many materials such as dopamine, ascorbic acid, uric acid, 3,4-dihydroxyphenylacetic acid [39], hydrogen peroxide, and NADH [7], Wang and Musameh have fabricated the CNT/Teflon composite electrodes with attractive electrochemical performance, based on the dispersion of CNTs within a Teflon binder. It has been demonstrated that the electrocatalytic properties of CNTs are not impaired by their association with the Teflon binder [15]. 

Figure 15.14 illustrates a typical voltammetric result for the determination of dopamine in the presence of ascorbic acid with a CNT-modified electrode. The selective voltammetric detection of uric acid [82] or norepinephrine [83] in the presence of ascorbic acid has been demonstrated with a (3-cyclodextrin-modified electrodes incorporating CNTs. Ye et al. [84] have studied the electrocatalytic oxidation of uric acid and ascorbic acid at a well-aligned CNT electrode, which can be used for the selective determination of uric acid in the presence of ascorbic acid. The simultaneous determination of dopamine and serotonin on a CNT-modified GC electrode has also been described [85],... 

G.C. Zhao, Z.Z. Yin, L. Zhang, and X.W. Wei, Direct electrochemistry of cytochrome c on a multi-walled carbon nanotube modified electrode and its electrocatalytic activity for the reduction of H2O2. Electrochem. Commun. 7, 256-260 (2005). 

A way to circumvent the first problem is to ensure that all of the active material is present at the electrode surface. That is, employ a chemically modified electrode where a precursor to the active electrocatalyst is incorporated. The field of chemically modified electrodes Q) is approaching a more mature state and there are now numerous methodologies for the incorporation of materials that exhibit electrocatalytic activity. Furthermore, some of these synthetic procedures allow for the precise control of the coverage so that electrodes modified with a few monolayers of redox active material can be reproducibly prepared. Q)... 

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