Direct Cell-Surface Electron Transfer

Various mechanisms of electron transfer and linking species have been identified and exploited. Basically there are two major types of linking species (i) soluble compounds (artificial or self-produced mediators), and (ii) compounds bonded to the microbial cell membrane (membrane-bond proteins or nanowires). Accordingly, electron transfer mechanisms from microorganisms to the electrode can be divided into five primary types, which will be discussed in the following section (1) direct cell-surface electron transfer, (2) direct electron transfer via nanowires, (3) electron transfer via exogenous redox mediators, (4) endogenous redox mediators and (5) reduced metabolic products. 

Since the direct electron transfer via outer membrane cytochromes requires the physical contact of the bacterial cell to the fuel cell anode, only bacteria in the first monolayer at the anode surface are electrochemically active and responsible for electricity generation. Consequently, the MFC performance is limited by the maximum cell density in this bacterial monolayer. For example, maximum current densities as low as 0.6 and 6.5 pA cm were achieved for MFCs based on pure Shewanella putrefaciens [13] and Geobacter sulfurreducens [21], respectively. 

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The choice of immobilization strategy obviously depends on the enzyme, electrode surface, and fuel properties, and on whether a mediator is required, and a wide range of strategies have been employed. Some general examples are represented in Fig. 17.4. Key goals are to stabilize the enzyme under fuel cell operating conditions and to optimize both electron transfer and the efficiency of fuel/oxidant mass transport. Here, we highlight a few approaches that have been particularly useful in electrocatalysis directed towards fuel cell applications. 

Figure 17.4 Cartoon representation of strategies for studying and exploiting enzymes on electrodes that have been used in electrocatalysis for fuel cells, (a) Attachment or physisorption of an enzyme on an electrode such that redox centers in the protein are in direct electronic contact with the surface, (b) Specific attachment of an enzyme to an electrode modified with a substrate, cofactor, or analog that contacts the protein close to a redox center. Examples include attachment of the modifier via a conductive linker, (c) Entrapment of an enzyme within a polymer containing redox mediator molecules that transfer electrons to/from centers in the protein, (d) Attachment of an enzyme onto carbon nanotubes prepared on an electrode, giving a large surface area conducting network with direct electron transfer to each enzyme molecule.

While direct electron transfer to laccases may help elucidate the mechanism of action of these enzymes it is unlikely that this process will supply sufficient power for a viable implantable biocatalytic fuel cell, because of difficulties associated with the correct orientation of the laccase and the two-dimensional nature of the biocatalytic layer on the surface. However, a recent attempt to immobilize laccase in a carbon dispersion, to provide electrodes with correctly oriented laccase for direct electron transfer, and a higher density of electrode material shows promise [53],... 

Several important energy-related applications, including hydrogen production, fuel cells, and CO2 reduction, have thrust electrocatalysis into the forefront of catalysis research recently. Electrocatalysis involves several physiochemical environmental dfects, which poses substantial challenges for the theoreticians. First, there is the electric potential which can aifect the thermodynamics of the system and the kinetics of the electron transfer reactions. The electrolyte, which is usually aqueous, contains water and ions that can interact directly with a surface and charged/polar adsorbates, and indirectly with the charge in the electrode to form the electrochemical double layer, which sets up an electric field at the interface that further affects interfacial reactivity. 

There are different directions by which CMEs can benefit analytical applications. These include acceleration of electron transfer reactions, preferential accumulation, or selective membrane permeation. Such steps can impart higher selectivity, sensitivity, or stability on electrochemical devices. These analytical applications and improvements have been extensively reviewed (45-47). Many other important applications, including electrochromic display devices, controlled release of drugs, electrosynthesis, fuel cells, and corrosion protection, should also benefit from the rational design of electrode surfaces. 

The discussion above must be modified if the acceptor molecules adsorb onto the electrode surface. Under these circumstances, the electrons are captured by the acceptor directly rather than through an intermediary solvated electron, and such direct photoassisted electron transfer has been much studied recently with the advent of dye-sensitized solar cells and molecular electronic devices. The normal approach is to use two-photon excitation both to probe the existence of localized states at the surface and to explore their dynamics. In experiments of this nature, which have been mostly carried out on ad-... 

A label-free electrochemical impedance immunosensor for the rapid detection of E. coli 0157 H7 consists of immobilized anti- . coli antibodies on an indium-tin oxide IDA microelectrode [123]. The binding of E. coli cells to the IDA microelectrode surface increases the electron-transfer resistance, which is directly measured with electrochemical impedance spectroscopy in the presence of [Fe(CN)(6)] as a redox probe. The electron-transfer resistance correlates with the concentration of E. coli cells in a range from 4.36 X 10 to 4.36 X 10 cfu ml with a detection Umit of 10 cfu ml . ... 

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