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...

Photosynthesis is a sustainable process that converts light energy into chemical energy. substantial research efforts are directed towards the application of the photosynthetic reaction centres, photosystems I and II, as active components for the light-induced generation of electrical power or fuel products. nonetheless, no integrated photo-bioelectrochemical device that produces electrical power, upon irradiation of an aqueous solution that includes two inter-connected electrodes is known. Here we report the assembly of photobiofuel cells that generate electricity upon irradiation of biomaterial-functionalized electrodes in aqueous solutions. The cells are composed of electrically contacted photosystem II-functionalized photoanodes and an electrically wired bilirubin oxidase/carbon nanotubes-modified cathode. Illumination of the photoanodes yields the oxidation of water to o 2 and the transfer of electrons through the external circuit to the cathode, where o 2 is re-reduced to water.

Electrochemical sensors for the analysis of TNT with enhanced sensitivities are described. The enhanced sensitivities are achieved by tailoring pi-donor-acceptor interactions between TNT and pi-donor-modified electrodes or pi-donor-cross-linked Au nanoparticles linked to the electrode. In one configuration a p-aminothiophenolate monolayer-modified electrode leads to the analysis of TNT with a sensitivity corresponding to 17 ppb (74 nM). In the second configuration, the cross-linking of Au NPs by oligothioaniline bridges to the electrode yields a functionalized electrode that detects TNT with a sensitivity that corresponds to 460 ppt (2 nM). Most impressively, the imprinting of molecular TNT recognition sites into the pi-donor oligoaniline-cross-linked Au nanoparticles yields a functionalized electrode with a sensitivity that corresponds to 46 ppt (200 pM). The electrode reveals high selectivity, reusability, and stability.

The hemin/G-quadruplex horseradish peroxidase-mimicking DNAzyme is assembled on Au electrodes. It reveals bioelectrocatalytic properties and electrocatalyzes the reduction of H(2)O(2). The bioelectrocatalytic functions of the hemin/G-quadruplex DNAzyme are used to develop electrochemical sensors that follow the activity of glucose oxidase and biosensors for the detection of DNA or low-molecular-weight substrates (adenosine monophosphate, AMP). Hairpin nucleic structures that include the G-quadruplex sequence in a caged configuration and the nucleic acid sequence complementary to the analyte DNA, or the aptamer sequence for AMP, are immobilized on Au-electrode surfaces. In the presence of the DNA analyte, or AMP, the hairpin structures are opened, and the hemin/G-quadruplex horseradish peroxidase-mimicking DNAzyme structures are generated on the electrode surfaces. The bioelectrocatalytic cathodic currents generated by the functionalized electrodes, upon the electrochemical reduction of H(2)O(2), provide a quantitative measure for the detection of the target analytes. The DNA target was analyzed with a detection limit of 1 x 10(-12) M, while the detection limit for analyzing AMP was 1 x 10(-6) M. Methods to regenerate the sensing surfaces are presented.

A bis-aniline-cross-linked Au nanoparticles (NPs) composite is electropolymerized on Au surfaces. The association of trinitrotoluene, TNT, to the bis-aniline bridging units via pi-donor-acceptor interactions allows the amplified detection of TNT by following the surface plasmon resonance (SPR) reflectance changes as a result of the coupling between the localized plasmon of the AuNPs and the surface plasmon wave associated with the gold surface. The detection limit for analyzing TNT by this method is approximately 10 pM. The electropolymerization of the bis-aniline-cross-linked AuNPs composite in the presence of picric acid results in a molecular-imprinted matrix for the enhanced binding of TNT. The imprinted AuNPs composite enabled the sensing of TNT with a detection limit that corresponded to 10 fM. Analysis of the SPR reflectance changes in the presence of different concentrations of TNT revealed a two-step calibration curve that included the ultrasensitive detection of TNT by the imprinted sites in the composite, KassI. for the association of TNT to the imprinted sites, 6.4 x 10(12) M-1, followed by a less sensitive detection of TNT by the nonimprinted pi-donor bis-aniline sites (KNIass. = 3.9 x 10(9) M-1). The imprinted AuNPs composite reveals impressive selectivity. The structural and functional features of the bis-aniline-cross-linked AuNPs composites were characterized by different methods including ellipsometry, AFM, and electrochemical means. The dielectric properties of the AuNPs composite in the presence of different concentrations of TNT were evaluated by the theoretical fitting of the respective experimental SPR curves. The ultrasensitive detection of the TNT by the AuNPs composite was attributed to the changes of the dielectric properties of the composite, as a result of the formation of the pi-donor-acceptor complexes between TNT and the bis-aniline units. These changes in the dielectric properties lead to a change in the conductivity of the AuNPs matrix.