Detailed impedance and voltammetric studies of hexameric octaheme nitrite reductase immobilized on carbon-based nanomaterials, specifically nanotubes and nanoparticles, were performed. Well-pronounced bioelectrocatalytic reduction of nitrite on enzyme-modified electrodes was obtained. Analysis of the impedance data indicated the absence of long-lived intermediates involved in the nitrite reduction. Cyclic voltammograms of biomodified electrodes had a bi-sigmoidal shape, which pointed to the presence of two enzyme orientations on carbon supports. The maximum (limiting) catalytic currents were determined and, by applying the correction by the mixed kinetics equation, the Tafel dependences were plotted for each catalytic wave/each enzyme orientation. Finally, two schemes for the rate-limiting processes during bioelectrocatalysis were proposed, viz. for low- and high-potential orientations.

A review. Being inspired by a very recent review entitled: "Electrocatalysis and bioelectrocatalysis - Distinction without a difference" and following the general approach employed by Prof. Dr. Schuhmann, in the current work we point to the similarities and differences between oxygen electroredn. and bioelectroredn. processes. To addnl. distinguish our paper from the recent review we touch on different bioelements, such as redox proteins and living cells, but we still keep a special emphasis on oxidoreductases, biocatalysts par excellence. Moreover, we also exclusively focus on oxygen electroredn. based on direct electron transfer reactions. On the one hand, we corroborate the previously made conclusion regarding intrinsically high activity of the active sites of biol. catalysts, esp. redox enzymes, which results in mass transfer and heterogeneous electron transfer limited currents from oxygen reducing bioelectrodes. On the other hand, we disagree with the statements regarding the exceptionality of precious metal catalysts, and the notion of a huge trade-​off between high activity and stability of non-​precious metal catalysts and bioelements. We show that the activity and stability of noble metal based cathodes is very far from perfect, esp. when these electrodes operate in complex electrolytes, such as physiol. fluids, e.g. human blood.

Miniature, self-contained biodevices powered by biofuel cells may enable a new generation of implantable, wireless, minimally invasive neural interfaces for neurophysiological in vivo studies and for clinical applications. Here we report on the fabrication of a direct electron transfer based glucose/oxygen enzymatic fuel cell (EFC) from genuinely three-dimensional (3D) nanostructured microscale gold electrodes, modified with suitable biocatalysts. We show that the process underlying the simple fabrication method of 3D nanostructured electrodes is based on an electrochemically driven transformation of physically deposited gold nanoparticles. We experimentally demonstrate that mediator-, cofactor-, and membrane-less EFCs do operate in cerebrospinal fluid and in the brain of a rat, producing amounts of electrical power sufficient to drive a self-contained biodevice, viz. 7 μW cm−2 in vitro and 2 μW cm−2 in vivo at an operating voltage of 0.4 V. Last but not least, we also demonstrate an inductive coupling between 3D nanobioelectrodes and living neurons.

A flexible electrochemical micro(bio)sensor has been designed for determination of several biological compounds, specifically, ascorbate, dopamine, and glucose, in human lachrymal liquid (tears). The microsensor for simultaneous determination of ascorbate and dopamine concentrations was based on a gold microwire modified with the tetrathiafulvalen–7,7,8,8-tetracyanoquinodimethane complex as a catalyst. To monitor glucose concentration in tears, glucose dehydrogenase was immobilized on a gold microwire modified with carbon nanotubes and an osmium redox polymer. A capillary microcell was constructed for sampling tears. The cell had a working volume of 60–100 nL with a sampling deviation of 6.7 %. To check if the microcell was properly filled with buffer or tear sample, a control electrode was introduced into the construction. The electrode was used to measure the electrical resistance of a fully filled nanovolume cell. The mechanical flexibility is one of the most important features of the prototype and allowed direct collection of tears with minimized risk of damage to the eye.

A microscale membrane-​less biofuel cell, capable of generating elec. energy from human lachrymal liq., was developed by using the ascorbate and oxygen naturally present in tears as fuel and oxidant. The biodevice is based on three-​dimensional nanostructured gold electrodes covered with abiotic (conductive org. complex) and biol. (redox enzyme) materials functioning as efficient anodic and cathodic catalysts, resp. Three-​dimensional nanostructured electrodes were fabricated by modifying 100 μm gold wires with 17 nm gold nanoparticles, which were further modified with tetrathiafulvalene-​tetracyanoquinodimethane conducting complex to create the anode and with Myrothecium verrucaria bilirubin oxidase to create the biocathode. When operated in human tears, the biodevice exhibited the following characteristics: an open circuit voltage of 0.54 V, a maximal power d. of 3.1 μW cm-​2 at 0.25 V and 0.72 μW cm-​2 at 0.4 V, with a stable c.d. output of over 0.55 μA cm-​2 at 0.4 V for 6 h of continuous operation. These findings support the authors' proposition that an ascorbate​/oxygen biofuel cell could be a suitable power source for glucose-​sensing contact lenses to be used for continuous health monitoring by diabetes patients.

Here we present unequivocal exptl. proof that microscale cofactor- and membrane-less, direct electron transfer based enzymic fuel cells do produce significant amts. of elec. energy in human lachrymal liq. (tears). 100 μm diam. gold wires, covered with 17 nm gold nanoparticles, were used to fashion three-dimensional nanostructured microelectrodes, which were biomodified with Corynascus thermophilus cellobiose dehydrogenase and Myrothecium verrucaria bilirubin oxidase as anodic and cathodic bioelements, resp. The following characteristics of miniature glucose/oxygen biodevices operating in human tears were registered: 0.57 V open-circuit voltage, about 1 μW cm-2 max. power d. at a cell voltage of 0.5 V, and more than 20 h operational half-life. Theor. calcns. regarding the max. recoverable elec. energy can be extd. from the biofuel and the biooxidant, glucose and mol. oxygen, each readily available in human lachrymal liq., fully support our belief that biofuel cells can be used as elec. power sources for so called smart contact lenses.

The deactivation of a polycrystalline gold electrode is observed during oxygen electroreduction reaction (ORR) in basic medium. At that, the cause of the process is chemical decomposition of the ORR intermediate and blocking of active sites of the electrode surface by hydroxyl radical-like species. The deactivation mechanism is discussed.

Highlights

► Gold autodeactivation during oxygen electroreduction reaction. ► Strong hydroxyl radical-like species adsorption. ► Superoxide radical-like species formation on p-Au. ► Impedance data analysis and model calculations.

The catalytic cycle of multicopper oxidases (MCOs) involves intramol. electron transfer (IET) from the Cu-T1 copper ion, which is the primary site of the one-electron oxidns. of the substrate, to the trinuclear copper cluster (TNC), which is the site of the four-electron redn. of dioxygen to water. In this study we report a detailed characterization of the kinetic and electrochem. properties of bilirubin oxidase (BOx) - a member of the MCO family. The exptl. results strongly indicate that under certain conditions, e.g. in alk. solns., the IET can be the rate-limiting step in the BOx catalytic cycle. The data also suggest that one of the catalytically relevant intermediates (most likely characterized by an intermediate oxidn. state of the TNC) formed during the catalytic cycle of BOx has a redox potential close to 0.4 V, indicating an uphill IET process from the T1 copper site (0.7 V) to the Cu-T23. These suggestions are supported by calcns. of the IET rate, based on the exptl. obsd. Gibbs free energy change and theor. ests. of reorganization energy obtained by combined quantum and mol. mech. (QM/MM) calcns.

To elucidate the mechanism of bilirubin oxidase (BOx)function in order to design efficient and stablebiocathodes working at different conditions, the enzymewas studied thoroughly. BOx is a copper-containing redoxenzyme that catalyzes the oxidation of a variety ofdifferent organic and inorganic compounds withconcomitant reduction of O2 directly to H2O.