Electron transfer in amperometric biosensor

Polohova, V. and Snejdarkova, M. (2008) Electron transfer in amperometric biosensors. Chemicki Listy, 102 (3), 173-182. 

The entrapment of enzymes in conducting materials opens further perspectives in the optimization of direct electron transfer in amperometric biosensors as the virtual electrode surface is increased. Even enzyme molecules immobilized far away from the electrode surface may be connected by the conducting matrix, and thus are able to participate in direct electron-transfer reactions. 

Application of Ferrocenyl-Containing Dendrimers in the Electrochemical Recognition of Anions and as Electron Transfer Mediators in Amperometric Biosensors... 

Silicon-based dendrimers 8 and 9 (Fc = ferrocenyl) also showed oxidative precipitation onto electrodes to give idealized electrochemistry as films.181 Specifically, the peak current was linear with scan rate and the potential difference between the anodic and cathodic waves was small (AE = 10 mV at a scan rate of 100 mV/s).182 This latter observation indicated that the rate of electron transfer was rapid. For molecule 9, the surface coverage was measured as = 2 x 10 10 mol/cm2. These molecules were also explored as mediators in amperometric biosensors.183 In contrast, molecule 10 showed two redox peaks, indicative of interaction between the two ferrocenyl units at each peripheral site. 181 When oxidation of one of the two interacting redox units results in some electron sharing between the two units (Robin-Day class II mixed valence species), the second oxidation is naturally... 

Habermuller K, Mosbach M, Schuhmann W. Electron-transfer mechanisms in amperometric biosensors. Fresenius Journal of Analytical Chemistry 2000, 366,560-568. 

Lotzbeyer T, Schuhmann W, Schmidt H. Electron transfer principles in amperometric biosensors direct electron transfer between enzymes and electrode surface. Sensors and Actuators B 1996, 33, 50-54. 

Amperometric sensors monitor current flow, at a selected, fixed potential, between the working electrode and the reference electrode. In amperometric biosensors, the two-electrode configuration is often employed. However, when operating in media of poor conductivity (hydroalcoholic solutions, organic solvents), a three-electrode system is best (29). The amperometric sensor exhibits a linear response versus the concentration of the substrate. In these enzyme electrodes, either the reactant or the product of the enzymatic reaction must be electroactive (oxidizable or reducible) at the electrode surface. Optimization of amperometric sensors, with regard to stability, low background currents, and fast electron-transfer kinetics, constitutes a complete task. 

Ldtzbeyer, T., Schuhmann, W., and Schmidt, H.L. (1996) Electron transfer principles in amperometric biosensors ... 

Schuhmann, W. (1995) Electron-transfer pathways in amperometric biosensors -ferrocene-modified enzymes entrapped in conducting-polymer layers. Biosensors ef Bioelectronics, 10 (1-2), 181-193. 

The application of direct electrochemistry of small redox proteins is not restricted to cytochrome c. For example, the hydroxylation of aromatic compounds was possible by promoted electron transfer from p-cresol methylhydroxylase (a monooxygenase from Pseudomonas putida) to a modified gold electrode [87] via the blue copper protein azurin. All these results prove that well-oriented non-covalent binding of redox proteins on appropriate electrode surfaces increases the probability of fast electron transfer, a prerequisite for unmediated biosensors. Although direct electron-transfer reactions based on small redox proteins and modified electrode surfaces are not extensively used in amperometric biosensors, the understanding of possible electron-transfer mechanisms is important for systems with proteins bearing catalytic activity. 

Figure 10.4 Modes of electron transfer in a conducting polymer-based (CP) amperometric biosensor (a) enzyme and mediator immobilised on conducting polymer, (b) enzyme linked with conducting polymer through mediator, (c) enzyme directly linked to conducting polymers without any mediator...

To fulfill both the requirement of CFME for the practical applications and the necessity of Au substrate to assemble so-called promoters to construct the third-generation biosensor, Tian el al. have combined the electrochemical deposition of Au nanoparticles (Au-NPs) onto carbon fiber microelectrodes with the self-assembly of a monolayer on these Au-NPs to facilitate the direct electron transfer of SOD at the carbon fiber microelectrode. The strategy enabled a third-generation amperometric 02 biosensor to be readily fabricated on the carbon fiber microelectrode. This CFME-based biosensor is envisaged to have great potential for (he detection of 02" in biological systems [158],... 

The enzyme can be incorporated into an amperometric sensor in a thick gel layer, in which case the depletion region due to the electrochemical reaction is usually confined within this layer. Alternatively, enzyme can be immobilized at the surface of the electrode or even within the electrode material itself, in which case the depletion region extends into the solution in the same way as it would for an unmodified electrode. In the latter case, the enzyme can then be seen as a true electrocatalyst that facilitates the interfacial electron transfer, which would otherwise be too slow. The general principles of the design and operation of these biosensors is illustrated on the example of the most studied enzymatic sensor, the glucose electrode (Fig. 2.14, Section 2.3.1). 

In the above two independent studies, the feasibility of CPMV as a nanobuilding block for chemical conjugation with redox-active compounds was demonstrated. The resulting robust, and monodisperse particles could serve as a multielectron reservoir that might lead to the development of nanoscale electron transfer mediators in redox catalysis, molecular recognition, and amperometric biosensors and to nanoelectronic devices such as molecular batteries or capacitors. 

Amperometric biosensors based on flavin-containing enzymes have been studied for nearly 30 years. These sensors typically undergo several chemical or electrochemical steps which produce a measurable current that is related to the substrate concentration. In the initial step, the substrate converts the oxidized flavin adenine dinucleotide (FAD) center of the enzyme into its reduced form (FADH2). Because these redox centers are essentially electrically insulated within the enzyme molecule, direct electron transfer to the surface of a conventional electrode does not occur to a substantial degree. The classical" methods (1-4) of indirectly measuring the amount of reduced enzyme, and hence the amount of substrate present, rely on the natural enzymatic reaction ... 

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