Biocatalyst electrodes

Ikeda, T., Miyaoka, S., Ozawa, S., Matsushita, F., Kobayashi, D., and Senda, M. (1991) Amperometric biosensors based on biocatalyst electrodes. Mediated and mediatorless enzyme electrodes. Analytical Sciences, 7, 1443-1446. 

Senda, M., Ikeda, T., Hiasa, H., Miki, K., Amperometric Biosensors Based on a Biocatalyst Electrode with Entrapped Mediator. Anal. Sci. 2 (1986) 501-506. 

Electrodes on which a redox enzyme(s) and an electron-transfer mediator(s) are co-immo-bilized are called (mediated) biocatalyst electrodes [1,2]. Figure 12 illustrates the kinetic... 

In the kinetic scheme of a biocatalyst electrode given in Fig. 12, the concentration polarization of the mediator occurs only within a finite thickness of the immobilized layer. Since the solution is usually stirred, the concentration polarization of the substrate will be neglected at the outside of the membrane and then the substrate polarization is restricted within the immobilized layer and the membrane. As a result, the catalytic current reaches a steady state after a certain period of time in any case. The enzymic reaction in the immobilized layer with excess amounts of Mox is expressed by... 

Because of electrode surface area enhancement and their high aspect ratio, CNTs support the production of high current density when used to construct enzyme cathodes and anodes. CNTs are constructed as multiwalled (MWCNT) or single-walled (SWCNT) configurations with various chiral vectors. Engineering the bio-nano interface involves careful selection and application of CNTs with specific properties that enhance DET between the conductive carbon architecture and the biocatalyst. Electrodes composed of (or decorated with) CNTs have a larger surface area for enzyme immobihzation than conventional carbon surfaces the enlarged electrode surface area increases the current density. 

Biocatalyst electrodes, that is, modified electrodes carrying an immobilized enzyme in which the electrode behaves as a substitute for a chemical electron acceptor or donor in the enzyme reaction can be used in such novel applications as biosensors, bioreactors and biofuel cells 2 ... 

The bioelectrocatalytic oxidation or reduction of substrates at biocatalyst electrodes can be accelerated by the presence of a small molecule which functions as an electron transfer mediator between the electrode and the prosthetic group of the immobilized enzyme " . Two types of the enzyme-modified electrode with entrapped mediator have been designed" , a carbon paste electrode and a porous electrode, both modified with enzyme and a reservoir of mediator. In the former type of electrode , where glucose oxidase (GOD) was immobilized on the surface of a carbon paste electrode along with p-benzoquinone (BQ) by coating the enzyme-loaded surface with a semipermeable membrane, BQ was dissolved into the immobilized enzyme layer and retained there effectively to serve as an electron transfer mediator between the carbon paste electrode and the immobilized enzyme. It has been shown " that the kinetics of the bioelectrocatalytic oxidation of D-glucose (Glc) at the membrane-coated GOD-modified carbon paste electrode with mixed-in BQ can be explained by theoretical equations in which the diffusion enzyme reaction of the substrate and mediator in the immobilized-enzyme layer and the diffusion of the substrate (and mediators) in the coating membrane are taken into account - . In this study, a number of quinone derivatives and a few ferrocene derivatives were examined as electron transfer mediators in the GOD electrode, and the mediator activity of the compounds was evaluated on the basis of theoretical equations. 

The kinetics of the electrocatalytic oxidation of D-glucose at film-coated GOD CPEs with mixed-in mediator were investigated and explained by the theory of a biocatalyst electrode with an entrapped mediator. A number... 

Biocatalytic membrane electrodes have an ISE or a gas sensing electrode in contact with a thin layer of biocatalytic material, which can be an immobilized enzyme, bacterial particles or a tissue slice, as shown in Fig. 3 The biocatalyst converts substrate (the analyte) into product, which is measured by the electrode. Electrodes of this type are often referred to as biosensors . 

The use of hydrogen peroxide as an oxidant is not compatible with the operation of a biocatalytic fuel cell in vivo, because of low levels of peroxide available, and the toxicity associated with this reactive oxygen species. In addition peroxide reduction cannot be used in a membraneless system as it could well be oxidized at the anode. Nevertheless, some elegant approaches to biocatalytic fuel cell electrode configuration have been demonstrated using peroxidases as the biocatalyst and will be briefly reviewed here. 

As referred to previously, if the active site of a biocatalyst is close enough to the electrode surface, direct electron transfer to/from an electrode can result. It has been shown in recent years that direct electron transfer from the GOx active site is possible using appropriate electrode preparation procedures. These preparation procedures usually aim to provide nano-structured features on the electrode surface that can penetrate sufficiently the GOx active site to allow for direct electron transfer. The direct electron... 

As dehydrogenases (DH) are widely distributed enzymes, a number of studies have been carried out with these biocatalysts. For example, Willner el al. [20] have used a PQQ-monolayer functionalized gold electrode for the catalytic oxidation of NADH in the presence of Ca2+. In this scheme, the pyrrolo-quinoline quinine co-factor, PQQ, was covalently linked, as before for the GOx system [15, 20, 21], to the Au electrode,... 

A remaining crucial technological milestone to pass for an implanted device remains the stability of the biocatalytic fuel cell, which should be expressed in months or years rather than days or weeks. Recent reports on the use of BOD biocatalytic electrodes in serum have, for example, highlighted instabilities associated with the presence of 02, urate or metal ions [99, 100], and enzyme deactivation in its oxidized state [101]. Strategies to be considered include the use of new biocatalysts with improved thermal properties, or stability towards interferences and inhibitors, the use of nanostructured electrode surfaces and chemical coupling of films to such surfaces, to improve film stability, and the design of redox mediator libraries tailored towards both mediation and immobilization. 

The use of additional membranes, which selectively convert nonionic analytes into ionic species that can be determined via ISEs is another common approach. An abundance of ingenious designs make use of biocatalysts for the development of potentiometric biosensors. Much of the earlier designs have made use of enzymes as the molecular recognition element. The products that are associated with such enzyme-catalyzed reactions can be readily monitored with the potentiometric transducer by coating the traditional electrodes with the enzyme. 

The main idea demonstrated by Willner and coworkers [20] is the ability to construct multilayered nanoparticle electrodes, which are porous. In a related study Patolsky et al. extended this idea further using biocatalysts to detect H2O2 [18]. In this example, the construction of the electrode is similar to the one described above but the redox-active bridging molecule was replaced with microperoxidase-11 (MP-11). 

Enzyme biocatalyst assemblies on electrode surfaces usually do not achieve significant electron-transfer communication between the redox center and the conductive support, mostly because of the electrical insulation of the biocatalytic site by the surrounding protein matrixes. During the past four decades, several methods have been proposed and investigated in the field of bioelectrochemical technology in an effort to establish efficient electrical communication between biocatalysts and electrodes. " In general, electron transfer is classified by two different mechanisms (see Figure 2) ... 

The main purpose of redox mediation is to increase the rate of electron transfer between the active site of enzyme biocatalysts and an electrode by eliminating the need for the enzyme to interact directly with the electrode surface. Depending on the enzyme and... 

An alternative containment scheme is immobilization of active species on a surface" " or within a tethered polymer brush or network. ° Surface immobilization can achieve high surface utilization by locating mediators and biocatalysts within nanometers of conducting surfaces. Immobilization on polymer networks allows for dense packing of enzymes within electrode volumes at the expense of long-distance electron mediation between the enzyme active center and a conductive surface. Such mediation often represents the rate-limiting step in the overall electrode reaction. 

The recent literature in bioelectrochemical technology, covering primarily the electrochemical aspects of enzyme immobilization and mediation, includes few reports describing engineering aspects of enzymatic biofuel cells or related devices. Current engineering efforts address issues of catalytic rate and stability by seeking improved kinetic and thermodynamic properties in modified enzymes or synthesized enzyme mimics. Equally important is the development of materials and electrode structures that fully maximize the reaction rates of known biocatalysts within a stable environment. Ultimately, the performance of biocatalysts can be assessed only by their implementation in practical devices. 

The design of biocatalytic electrodes for activity toward gaseous substrates, such as dioxygen or hydrogen, requires special consideration. An optimal electrode must balance transport in three different phases, namely, the gaseous phase (the source of substrate), the aqueous phase (where the product water is released and ionic transport takes place), and the solid phase (where electronic transport occurs). Whereas the selectivity of biocatalysts facilitates membraneless cells for implementation in biological systems that provide an ambient electrolyte, gas-diffusion biofuel cells require an electro-... 

---