Amperometric biosensors electron transfer

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... 

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). 

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. 

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 ... 

The covalent attachment of electron transfer mediators to siloxane or ethylene oxide polymers produces highly efficient relay systems for use in amperometric sensors based on flavin-containing oxidases. It is clear from the response curves that the biosensors can be optimized through systematic changes in the polymeric backbone. The results discussed above, as well as those described previously (25-32), show that the mediating ability of these flexible polymers is quite general and that it is possible to systematically tailor these systems in order to enhance this mediating ability. 

CNTs and other nano-sized carbon structures are promising materials for bioapplications, which was predicted even previous to their discovery. These nanoparticles have been applied in bioimaging and drag delivery, as implant materials and scaffolds for tissue growth, to modulate neuronal development and for lipid bilayer membranes. Considerable research has been done in the field of biosensors. Novel optical properties of CNTs have made them potential quantum dot sensors, as well as light emitters. Electrical conductance of CNTs has been exploited for field transistor based biosensors. CNTs and other nano-sized carbon structures are considered third generation amperometric biosensors, where direct electron transfer between the enzyme active center and the transducer takes place. Nanoparticle functionalization is required to achieve their full potential in many fields, including bio-applications. 

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. 

The direct electron-transfer communication between HRP and electrodes allows the electroreduction of H2O2 in the potential range —0.2 to 0 V (vs. SCE) and its coupling to these oxidases can thus yield amperometric biosensors (Figure 1B). The... 

Enzyme biosensors containing pol3mieric electron transfer systems have been studied for more than a decade. One of the earlier systems was first reported by Degani and Heller [1,2] using electron transfer relays to improve electrochemical assay of substrates. Soon after Okamoto, Skotheim, Hale and co-workers reported various flexible polymeric electron transfer systems appUed to amperometric enz5une biosensors [3-16], Heller and co-workers further developed a concept of wired amperometric enzyme electrodes [17—27] to increase sensor accuracy and stability. 

Tetrathiafulvalene containing polymeric electron transfer systems Tetrathiafulvalene (TTF) is another mediator many researchers have utilized as an electron mediator in constructing amperometric biosensors. Electrodes constructed of TTF efficiently oxidize glucose oxidase [118,119], lactate oxidase [120] and choline oxidase [121]. These sensors like those using viologen containing polymeric electron transfer systems have the... 

Amperometric enzyme biosensors using polymeric electron transfer systems... 

---