The influence of gold support on the bioelectrocatalytic activity of glucose oxidase and cellobiose dehydrogenase immobilized on self-assembled monolayer-modified high surface area gold electrodes such as rough gold and gold nanoparticles has been studied. The two types of enzyme-modified electrodes showed very high activity towards sugar oxidation. However, it has been shown that the largest part of this electrocatalytic activity comes from the underlying gold surface. These findings are of special importance for bioelectrochemical studies of enzymatic electrodes, where the immobilization support might show electrocatalytic properties toward the substrates of investigated enzymes.

A review of some historical developments made in the field of enzymic fuel cells, discussing important design considerations taken when constructing mediator-, cofactor-, and membrane-less biol. fuel cells. Since the topic is rather extensive, only biol. fuel cells utilizing direct electron transfer reactions on both the anodic and cathodic sides are considered. Moreover, the performance of mostly glucose/oxygen biodevices is analyzed and compared. We also present some unpublished results on mediator-, cofactor-, and membrane-less glucose/oxygen biol. fuel cells recently designed in our group and tested in different human physiol. fluids, such as blood, plasma, saliva, and tears. Finally, further perspectives for biol. fuel cell applications are highlighted.

We report on the fabrication and characterisation of a gold-nanoparticle (AuNP)-basedmediatorlesssugar/oxygen biofuel cell (BFC) operating in neutral sugar-containing buffers and human physiological fluids, such as blood and plasma. First, Corynascus thermophilus cellobiose dehydrogenase (CtCDH) and Myrothecium verrucaria bilirubin oxidase (MvBOx), used as anodic and cathodic bioelements, respectively, were immobilised on goldelectrodesmodified with 20 nm AuNPs. Detailed characterisation and optimisation of a new CDH/AuNP-based bioanode were performed and the following fundamental parameters were obtained: (i) the redox potential of the haem-containing centre of the enzyme was measured to be 75 mV vs. NHE, (ii) the surface coverage of CtCDH was found to be 0.65 pmol cm−2 corresponding to a sub-monolayer coverage of the thiol-modified AuNPs by the enzyme, (iii) a turnover number for CtCDH immobilised on thiol-modified AuNPs was calculated to be ca. 0.5 s−1, and (iv) the maximal current densities as high as 40 μA cm−2 were registered in sugar-containing neutral buffers. Second, both biomodified electrodes, namely the CtCDH/AuNP-based bioanode and the MvBOx/AuNP-based biocathode, were combined into a functional BFC and the designed biodevices were carefully investigated. The following characteristics of the mediator-, separator- and membrane-less, miniature BFC were obtained: in phosphate buffer; an open-circuit voltage of 0.68 V, a maximum power density of 15 μW cm−2 at a cell voltage of 0.52 V and in human blood; an open-circuit voltage of 0.65 V, a maximum power density of 3 μW cm−2 at a cell voltage of 0.45 V, respectively. The estimated half-lives of the biodevices were found to be >12, <8, and <2 h in a sugar-containing buffer, human plasma, and blood, respectively. The basic characteristics of mediatorlesssugar/oxygen BFCs were significantly improved compared with previously designed biodevices, because of the usage of three-dimensional AuNP-modifiedelectrodes.

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.

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.

We present data on operation of a miniature membrane-less, direct electron transfer based enzymatic fuel cell in human sweat and saliva. The enzymatic fuel cell was fabricated following our previous reports on miniature biofuel cells, utilizing gold nanoparticle modified gold microwires with immobilized cellobiose dehydrogenase and bilirubin oxidase. The following average characteristics of miniature glucose/oxygen biodevices operating in human sweat and saliva, respectively, were registered: 580 and 560 mV open-circuit voltage, 0.26 and 0.1 μW cm–2 power density at a cell voltage of 0.5 V, with up to ten times higher power output at 0.2 V. When saliva collected after meal ingestion was used, roughly a two-fold increase in power output was obtained, with a further two-fold increase by addition of 500 μM glucose. Likewise, the power generated in sweat at 0.5 V increased two-fold by addition of 500 μM glucose.

A review. Interdisciplinary research has combined the efforts of many scientists and engineers to gain an understanding of biotic and abiotic electrochem. processes, materials properties, biomedical, and engineering approaches for the development of alternative power-​generating and​/or energy-​harvesting devices, aiming to solve health-​related issues and to improve the quality of human life. This review intends to recapitulate the principles of biofuel cell development and the progress over the years, thanks to the contribution of cross-​disciplinary researchers that have combined knowledge and innovative ideas to the field. The emergence of biofuel cells, as a response to the demand of elec. power devices that can operate under physiol. conditions, are reviewed. Implantable biofuel cells operating inside living organisms have been envisioned for over fifty years, but few reports of implanted devices have existed up until very recently. The very first report of an implanted biofuel cell (implanted in a grape) was published only in 2003 by Adam Heller and his coworkers. This work was a result of earlier scientific efforts of this group to "wire" enzymes to the electrode surface. The last couple of years have, however, seen a multitude of biofuel cells being implanted and operating in different living organisms, including mammals. Herein, the evolution of the biofuel concept, the understanding and employment of catalyst and biocatalyst processes to mimic biol. processes, are explored. These potentially green technol. biodevices are designed to be applied for biomedical applications to power nano- and microelectronic devices, drug delivery systems, biosensors, and many more.

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.