Enzymes electrocatalysts

Enzyme catalysis in the 1970s became a subject of interest to electrochemists since the enzymes, unlike most other metal and nonmetal electrocatalysts, have a number of important special features ... 

They have an exceedingly high specific activity per active site the turnover number y is as high as 10 to 10 s in certain enzyme reactions, while at ordinary electrocatalysts having a number of reaction sites on the order of 10 cm , yhas a value of about 1 s at a current density of lOmA/cm. Thus, the specific catalytic activity of tfie active sites of enzymes is many orders of magnitude fiigher tfian tfiat of all other known catalysts for electrochemical (and also chemical) processes. 

Motivations for exploring enzymes in fuel cell catalysis are both intellectual and applied. Using enzymes as electrocatalysts, there is scope for creation of fuel cells... 

As we demonstrate in this chapter, enzymes can be extremely active electrocatalysts at ambient temperatures and mild pH, and have significantly higher reaction selectivity than precious metals. The main disadvantage in applying redox enzymes for electrocatalysis arises from their large size, which means that the catalytic active site density is low. Enzymes also have a relatively short hfetime (usually not more than a few months), making them more suited to disposable applications. 

The alcohol tolerance of O2 reduction by bilirubin oxidase means that membraneless designs should be possible provided that the enzymes and mediators (if required) are immoblized at the electrodes. Minteer and co-workers have made use of NAD -dependent alcohol dehydrogenase enzymes trapped within a tetraaUcylammonium ion-exchanged Nafion film incorporating NAD+/NADH for oxidation of methanol or ethanol [Akers et al., 2005 Topcagic and Minteer, 2006]. The polymer is coated onto an electrode modified with polymethylene green, which acts as an electrocatalyst... 

Nonenzymatic regeneration of NAD(P)H requires the regioselective transfer of two electrons and a proton to NAD(P)+. Various rhodium(III) complexes are effective electrocatalysts capable of mimicking hydrogenase enzymes.48-54... 

The electrochemical rate constants for hydrogen peroxide reduction have been found to be dependent on the amount of Prussian blue deposited, confirming that H202 penetrates the films, and the inner layers of the polycrystal take part in the catalysis. For 4-6 nmol cm 2 of Prussian blue the electrochemical rate constant exceeds 0.01cm s-1 [12], which corresponds to the bi-molecular rate constant of kcat = 3 X 103 L mol 1s 1 [114], The rate constant of hydrogen peroxide reduction by ferrocyanide catalyzed by enzyme peroxidase was 2 X 104 L mol 1 s 1 [116]. Thus, the activity of the natural enzyme peroxidase is of a similar order of magnitude as the catalytic activity of our Prussian blue-based electrocatalyst. Due to the high catalytic activity and selectivity, which are comparable with biocatalysis, we were able to denote the specially deposited Prussian blue as an artificial peroxidase [114, 117]. 

Catalysis is known as the science of accelerating chemical transformations. In general, various starting materials are converted to more complex molecules with versatile applications. Traditionally, catalysts are divided into homogeneous and heterogeneous catalysts, biocatalysts (enzymes), photocatalysts, and electrocatalysts, which are mainly used... 

Investigations of enzyme-catalyzed direct electron transfer introduce the basis for a future generation of electrocatalysts based on enzyme mimics. This avenue could offer new methods of synthesis for nonprecious metal electrocatalysts, based on nano-structured (for example, sol—gel-derived) molecular imprints from a biological catalyst (enzyme) with pronounced and, in some cases, unique electrocatalytic properties. Computational approaches to the study of transition state stabilization by biocatalysts has led to the concept of theozymes . " ... 

Reactions in which the nature of the substrate is vital (e.g., as in electrocatalysis, corrosion, electrodeposition) do not offer opportunities for application of a technique in which the substrate is regarded essentially as an electron source or sink, rather than as an electrocatalyst. The very large field of bioelectrochemistiy (which involves concepts such as enzymes as electrodes and even offers electrochemical mechanisms for metabolism) would offer difficulties for potential sweep applications because of the very high resistance of the substrate.21... 

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). 

Table 13.2 summarises the different approaches used to construct enzyme electrochemical biosensors for application to food analysis based on the different types of enzymes available. Generally, the main problems of many of the proposed amperometric devices have been poor selectivity due to high potential values required to monitor the enzyme reaction, and poor sensitivity. Typical interferences in food samples are reducing compounds, such as ascorbic acid, uric acid, bilirubin and acetaminophen. Electrocatalysts, redox mediators or a second enzyme coupled reaction have been used to overcome these problems (see Table 13.2), in order to achieve the required specifications in terms of selectivity and sensitivity. 

A typical approach is to utilise a substrate which when hydrolysed by the enzyme gives rise to a product which can be easily detected elect-rochemically. Thiocholine can be easily detected using screen-printed carbon electrodes doped with cobalt phthalocyanine (CoPC) [18,19], which acts as an electrocatalyst for the oxidation of thiocholine at a lowered working potential of approximately +100 mV (vs. Ag/AgCl) [18,19], thereby minimising interference from other electroactive compounds ... 

An alternative biosensor system has been developed by Hart et al. [44] which involves the use of the NAD+-dependent GDH enzyme. The first step of the reaction scheme involves the enzymatic reduction of NAD+ to NADH, which is bought about by the action of GDH on glucose. The analytical signal arises from the electrocatalytic oxidation of NADH back to NAD+ in the presence of the electrocatalyst Meldola s Blue (MB), at a potential of only 0Y. Biosensors utilising this mediator have been reviewed elsewhere [1,17]. Razumiene et al. [45] employed a similar system using both GDH and alcohol dehydrogenase with the cofactor pyrroloquinoline quinone (PQQ), the oxidation of which was mediated by a ferrocene derivative. 

The 1980s were spent in discovering what not to do if one wished to carry enzymes on electrodes and use their powers as electrocatalysts ... 

There are only two ways to obtain a successful, adsorbed enzyme acting as an electrocatalyst (Rusling, 1997) ... 

It has long been established that Pt is the most efficient singlemetal electrode for the catalysis of both reactions (1) and (2). In the case of ddiydrogen activation, no metal electrocatalyst performs better than platinum. However, aside from the fact that platinum is a precious metal, a major drawback is that commercial (fossil-based) hydrogen contains residual amounts of impurities (e g., carbon monoxide) that only serve to poison the catalyst surface." To address this particular problem, present research has focused on the employment of metal additives (e.g., Ru) or of molecular catalysts that mimic the impressive activity of biological materials (e g., hydrogenase enzymes) " the use of molecular catalysts appears to be the more attractive option since such com-... 

Enzymes have been considered in bio fuel cells as anode electrocatalysts since their use avoids the problem of poisoning the anode with carbon monoxide present in reforming gas, allowing the use of cheap hydrogen-containing fuels such as methanol. Even though enzymatic fuel cells have been reported to have power output and stability limitations, some of them are currently being used to produce electricity to power small electrical devices with power demands in the order of micro- and milh- Watts as power output limitations are overcome. 

Enzymatic fuel cells (EFCs) have recently emerged as a potential source for power generation. An EFC typically employs one or more redox active enzymes as the electrocatalyst iimnobihzed on a condnctive electrode snbstrate. EFCs are desired over... 

For molecular electrocatalysts otherwise, and especially transition metal macrocycles, the electrocatalytic activity is often modified by subtle structural and electronic factors spanning the entire mechanistic spectrum, that is, from strict four-electron reduction, as for the much publicized cofacial di-cobalt porphyrin, in which the distance between the Co centers was set at about 4 A [12], to strict two-electron reduction, as in the monomeric (single ring) Co(II) 4,4, 4",4" -tetrasulfophthalo-cyanine (CoTsPc) [20] and Co(II) 5,10,15,20-tetraphenyl porphyrin (CoTPP) [21]. Not surprisingly, nature has evolved highly specific enzymes for oxygen transport, oxygen reduction to water, superoxide dismutation and peroxide decomposition. 

Cracknell, J.A., Vincent, K.A., and Armstrong, F.A. (2008) Enzymes as working or inspirational electrocatalysts for fuel cells and electrolysis. Chemical Reviews, 108 (7), 2439-2461. 

This method was applied to assemble integrated electrically-contacted NAD(P)-dcpcndcnt enzyme electrodes. The direct electrochemical reduction of NAD(l ) cofactors or the electrochemical oxidation of NAD(P)H cofactors are kineticaUy unfavored. Different diffusional redox mediators such as quinones, phenazine, phenoxazine, ferrocene or Os-complexes were employed as electrocatalysts for the oxidation of NAD(P)H cofactors An effective electrocatalyst for the oxidation of the NAD(P)H is pyrroloquinoline quinone, PQQ, (7), and its immobilization on electrode surfaces led to efficient electrocatalytic interfaces (particularly in the presence of Ca ions) for the oxidation of the NAD(P)H cofactors. This observation led to the organization of integrated electrically contacted enzyme-electrodes as depicted in Fig. 3-20 for the organization of a lactate dehydrogenase electrode. 

Electrochemical regeneration of NAD(P)H represents another interesting method 134 361. The system involves electron transfer from the electrode to the electron mediator such as methyl viologen or acetophenone etc., then to the NAD(P)+ (which is catalyzed by an electrocatalyst such as ferredoxin-NADP reductase or alcohol dehydrogenase, etc.) [34l Other methods involve the direct reduction of NAD on the electrode[35). Both one-enzyme systems and two-enzyme systems have been reported. 

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