Electron transfer in amperometric

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. 

Borgmann, S., Hartwich, G., Schulte, A., and Schuhmann, W. (2006) Amperometric enzyme sensors based on direct and mediated electron transfer, in Electrochemistry of Nucleic Acids and Proteins. Towards Electrochemical Sensors for Genomics and Proteomics (eds E. Palecek, F. Scheller, and J. Wang), Elsevier, Amsterdam, pp. 599-655. 

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...

In some instances, the design of an amperometric immunosensor may be such that the enzyme is located some distance away from the electrode surface, or the presence of interfering substances in biological samples may require using an alternative electron transfer pathway. This usually involves a redox-active species with a small molecular... 

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],... 

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... 

It would appear certain that the most important need in LCEC is the development of improved electrode materials. It may be possible in the near future to design an electrode that will give superior performance for certain classes of compounds. Modifying electrode surfaces by covalent attachment of various ligands or electron-transfer catalysts (including enzymes) can provide the key to better amperometric devices for all sorts of analytical purposes. Research in the area of chemically modified electrodes (CMEs) has been reviewed (see Chap. 13) [6,11]. Those interested in improving the performance of electrochemical detectors would do well to study these developments in detail. 

In general, traditional electrode materials are substituted by electrode superstructures designed to facilitate a specific task. Thus, various modifiers have been attached to the electrode that lower the overall activation energy of the electron transfer for specific species, increase or decrease the mass transport, or selectively accumulate the analyte. These approaches are the key issues in the design of chemical selectivity of amperometric sensors. The long-term chemical and functional stability of the electrode, although important for chemical sensors as well, is typically focused on the use of modified electrodes in energy conversion devices. Examples of electroactive modifiers are shown in Table 7.2. 

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). 

First examples of the amperometric detection of H202 accomplished in such a range were based on the use of an enzyme, namely horseradish peroxidase (HRP), a prototypical hemeprotein peroxidase, which catalyses the reduction of H202 and due to its peculiar structure, allows direct electron transfer between its active site and the electrode surface at low applied potential [14 17]. This approach, although it shows good sensitivity and accuracy, suffers from some important shortcomings such as low stability and the limited binding of HRP to solid surfaces. 

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