Integrated Enzyme‐Based Biofuel Cells–A Review

Abstract

The electrical contacting of redox enzymes with electrodes is one of the most fundamental processes in bioelectrochemistry. Redox enzymes usually lack direct electron transfer communication between their active redox centres and electrode supports. This barrier for electron transfer (ET) is explained by the Marcus electron transfer theory [1] that states that the electron transfer rate between a donor and an acceptor pair is given by Eq. 1, where d and d o are the actual distance and the Van der Waals distance separating the donor-acceptor pair, respectively, DG o and k are the free energy change and the reorganisation energy accompanying the electron transfer (ET) process, respectively, and b is the electronic coupling coefficient.Realizing that the dimensions (diameters) of redox proteins are in the range of 70-200 Å, and that the redox centres are embedded in the protein matrices, the spatial separation of the biocatalytic redox sites from the electrode prevents the electrical contacting of the enzyme with the electrode [2]. Different methods to establish electrical communication between the redox centres of enzymes and electrodes were developed in the past 25 years. These include, Figure 1, the use of diffusional electron mediators that transport electrons between the redox centres and the electrode, path (A) [3], the tethering of redox-active relay units to the protein (on the periphery as well as inner protein sites) to shorten the electron transfer distances and to mediate ET between the biocatalytic redox centres and the electrode, path (B) [4], and to immobilise the redox enzymes in electroactive polymer matrices, and particularly, redox-active hydrogels, that transport the electrons between the enzyme active sites and the electrodes by means of flexible charge carrying redox-active segments associated with the polymer matrices, path (C) [5]. Also, the reconstitution of apo-enzymes on relay/cofactor units associated with electrodes provided an effective means to electrically contact redox enzymes with electrodes [6]. According to this paradigm, Figure 1, path (d), the native cofactor is extracted from the enzyme, and the reconstitution of the resulting enzyme on the relay-cofactor dyad linked to the electrode yields a AbstractEnzyme-based biofuel cells provide versatile means to generate electrical power from biomass or biofuel substrates, and to use biological fluids as fuel-sources for the electrical activation of implantable electronic medical devices, or prosthetic aids. This review addresses recent advances for assembling biofuel cells based on integrated, electrically contacted thin film-modified enzyme electrodes. Different methods to electrically communicate the enzymes associated with the anodes/cathodes of the biofuel cell elements are presented. These include: (i) The reconstitution of apoenzymes on relay-cofactor monolayers assembled on electrodes, or the crosslinking of cofactor-enzyme affinity complexes assembled on electrodes. (ii) The immobilisation of enzymes in redox-active hydrogels a...

Homogeneous catalytic O 2 reduction to water by a cytochrome c oxidase model with trapping of intermediates and mechanistic insights

“…A detailed mechanistic-kinetic study on the homogeneous catalytic system reveals spectroscopically detectable intermediates and that the rate-determining step changes from the O 2 -binding process at 25°C room temperature (RT) to the O-O bond cleavage of a newly observed Fe III -OOH species at lower temperature (−60°C). At RT, the rate of O 2 -binding to 6 LFeCu is significantly faster than that for 6 LFe, whereas the rates of the O-O bond cleavage of the Fe III -OOH species observed (−60°C) with either the 6 LFeCu or 6 LFe catalyst are nearly the same. Thus, the role of the Cu ion is to assist the heme and lead to faster O 2 -binding at RT.…”

“…To help to examine the role of the Cu in this system, a Cu-free version ( 6 LFe) (15) was also subjected to the same catalytic reaction conditions as 6 LFeCu. Even in the absence of Cu the reaction proceeds through the 4e − process (Fig.…”

“…The goal is to provide insights and elucidate mechanisms of the four-electron reduction of O 2 by coordination complexes including heme-Cu assemblies, as may be relevant to biological systems including CcO but also of technological significance such as in fuel cell chemistry (5)(6)(7).…”

Integrated photosystem II-based photo-bioelectrochemical cells

“…These PBFCs require the immobilization and electrical contacting of the light harnessing components with the electrode supports to the extent that photo-induced electrontransfer processes occur between the photosynthetic RC, thus enabling the generation of photocurrents (or their use for the synthesis of fuel products). The electrical contacting of redox proteins with electrodes attracted substantial research efforts directed to the development of amperometric biosensors [26][27][28][29] or biofuel cell elements [30][31][32][33][34][35] . Different methods to integrate redox proteins with electrodes in electrically contacted configurations were developed, including the modification of the proteins with relay units 36,37 , the reconstitution of the proteins on relay-cofactor units [38][39][40] and the immobilization of the proteins in redox-active polymer matrices 41 .…”

Design of a bioelectrocatalytic electrode interface for oxygen reduction in biofuel cells based on a specifically adapted Os-complex containing redox polymer with entrapped Trametes hirsuta laccase

“…Besides fundamental understanding of how O 2 is bioelectrocatalytically reduced to H 2 O in nature avoiding intermediate formation of reactive oxygen species, applications in biosensors and membrane-less biofuel cells are of increasing importance [10][11][12][13][14][15][16]. Crucial for a membrane-less biofuel cell is that no cross reactivity between the cathode and anode reactions and moreover no reactions between the substrates used as fuels for the anode and cathode side occur.…”