Anodic bioelectrocatalysis

As already discussed, microorganisms extract a certain share of energy for their living from the maximum theoretically exploitable energetic difference. In case of anodic bioelectrocatalysis, the energy difference is situated between the microbial substrate, that is, the fuel, and the potential of the terminal electron transfer site. Furthermore, and like in conventional electrocatalytic systems [5], several energetic losses at the bioelectrocatalyst-electrode interface occur (Figure 8.2). 

ANODIC BIOELECTROCATALYSIS FROM METABOLIC PATHWAYS TO METABOLONS... 

If the active site of the enzyme is located sufficiently close to the electrode surface electrons can be transferred directly from the enzyme to the electrode as depicted in Figure 5.3a. In the case of an anodic reaction, the electrode replaces the natural co-substrate (such as oxygen) as an electron acceptor. This process is known as direct electron transfer (DFT), often categorized as third-generation enzyme electrodes in the biosensor literature, and is the most elegant and simplest method of bioelectrocatalysis between an enzyme active site and an electrode. 

Cyclic voltammetry is a method frequently used to measure 7s,i ni. Mediated bioelectrocatalysis yields cyclic voltammograms (CVs) of different shapes as illustrated in Fig. 2, depending on the measuring conditions [11]. Curve (a) is the wave for a reversible electrode reaction of an Mox/Mred redox couple. Bioelectrocatalysis mediated with the Mqx/ Mred redox couple produces a sigmoidal catalytic wave as curve (c) under the conditions [Mred] - M and [S] Ks. When [Mred] is increased to higher concentrations, an anodic peak of the diffusion current of Mred rnay be overlapped on the catalytic current as depicted by curve (d) the current, however, becomes steady state after appropriate periods... 

The studies of the kinetics of bioelectrocatalytic transformations show that in some systems (for instance, adsorbed laccase ) the kinetic parameters correspond to the phenomenology of electrochemical kinetics, while in other systems (for instance, lactate oxidation they fit the phenomenology of enzymatic catalysis. In the latter case, we observe a hyperbolic dependence of anode current on the substrate concentration, as expected from the Michaelis-Menten equation. The absence of a general theory of bioelectrocatalysis does not permit us to examine the kinetics of electrochemical reactions in the presence of enzymes under different conditions. At present we can only try to estimate the scope of possible accelerations of electrochemical reactions by making some simple assumptions. 

In conclusion, in the actual state of the art, electrochemical mineralization of organic pollutants with cogeneration of electrical energy is not feasible, due to the lack of active electrocatalytic anode material. Bioelectrocatalysis is a new and active field and can overcome this problem as has been demonstrated recently in the development of biofuel cells. Nevertheless, this technology is yet in its infancy. 

The OCP of a BFC is determined when there is no current flowing across the device C/max = 0) and thus, no bioelectrocatalysis is taking place at either the bioanode or the biocathode. As previously discussed, the use of an electron mediator requires a potential difference between the electron mediator and the enzyme s cofactor. This therefore can result in lower OCPs of BFCs, since the potential at both the bioanode and the biocathode is determined by the onset potential of the anodic electron mediator (F J and cathodic electron mediator (F ) 7). In... 

Fig. 7 Schematic of a biofuel cell utilizing mediated bioelectrocatalysis at both the anode and the cathode,... 

Specific redox characteristics of a catalyst derived from CV scans are also used to confirm an enzyme s ability for bioelectrocatalysis by either direct electron transfer (DET) or mediated electron transfer (MET) to the electrode. DET and MET are two distinct mechanisms of bioelectrocatalysis. MET has the advantage of being compatible with almost all naturally occurring oxidoreductase enzymes and coenzymes, but it requires additional components (either smaU-molecule redox mediators or redox polymers) because the enzymes cannot efficiently transfer electrons to the electrode. These additional components make the system more complex and less stable [8]. The vast majority of oxidoreductase enzymes that require MET to an electrode are nicotinamide adenine dinucleotide (NAD" ) dependent. Two of the most commonly encountered NAD -dependent enzymes in BFC anodes are glucose dehydrogenase (GDH) and alcohol dehydrogenase (ADH). These enzymes have been thoroughly characterized in respect to half-cell electrochemistry and have been demonstrated for operation in BFC. More information about MET can be found in Chapter 9. 

Twelve quinone derivatives and three ferrocene derivatives were examined. Their cyclic voltammograms were measured with a film-coated GOD (0.5 Mg) CPE without mixed-in mediator immersed in the basal solution containing 1 mmol/dm mediator. However, for some mediators, such as tetra-methyl p-benzoquinone, 1,2-naphthoquinone, 1,4-naphthoquinone, ferrocene, and 1,1 -dimethyl-ferrocene a saturated solution (generally, of low solubility) was used. Figures 5 and 6 show the cyclic voltammograms of 2,6-dichoro-p-benzoquinone and BQ. The mid-point potential, Emid, was obtained from the cathodic and anodic peak potentials, Epc and Epa, respectively, by Emid (Epc+Epa)/2. All compounds examined were active as electron transfer mediators for bioelectrocatalysis with the GOD CPEs with mixed-in mediator (usually mn = 5%), except tetramethyl-p-benzoquinone (that is. 

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