In the last decade, several studies from different fields have contributed significantly to the development of the enzyme biofuel cell (BFC). Moreover, the design of biocatalyst‐modified electrodes is a model for the development of synthetic and biomimetic electrocatalysts. The revival of the interest in the old BFC idea developed in the early 20th century is related to the development of new electrode materials as well as to the potential application of enzyme BFC miniaturisation in implantable bioelectronics. Furthermore, fundamental studies on half‐cells and electron transfer involving protein film electrodes performed in the last few decades have established new types of electrodes for BFC applications. In terms of applicability, some research groups have sought to obtain unit cells with different properties and characteristics such as membrane‐less BFCs by using flexible and low‐costing materials; in addition, micrometric BFCs, which can be implanted in different body parts, have also been developed to supply electrical power to bioelectronics devices. Thus, in this review, we focus on introducing the main concepts and challenges in enzyme BFC research. Some important topics in kinetics and thermodynamics applied to BFCs are also discussed. Wherever possible, an extensive review of works undertaken in recent years is performed. The topics are organised into three major themes: thermodynamics, kinetics and challenges in applicability.

Direct electron transfer (DET) between redox enzymes and electrode surfaces is of growing interest and an important strategy in the development of biofuel cells and biosensors. Among the nanomaterials utilized at electrode/enzyme interfaces to enhance the electronic communication, graphene oxide (GO) has been identified as a highly promising candidate. It is postulated that GO layers decrease the distance between the flavin cofactor (FAD/FADH2) of the glucose oxidase enzyme (GOx) and the electrode surface, though experimental evidence concerning the distance dependence of the rate constant for heterogeneous electron-transfer (k(het)) has not yet been observed. In this work, we report the experimentally observed DET of the GOx enzyme adsorbed on flexible carbon fiber (FCF) electrodes modified with GO (FCF-GO), where the k(het) between GO and electroactive GOx has been measured at a structurally well-defined interface. The curves obtained from the Marcus theory were used to obtain k(het), by using the model proposed by Chidsey. In agreement with experimental data, this model proved to be useful to systematically probe the dependence of electron transfer rates on distance, in order to provide an empirical basis to understand the origin of interfacial DET between GO and GOx. We also demonstrate that the presence of GO at the enzyme/electrode interface diminishes the activation energy by decreasing the distance between the electrode surface and FAD/FADH2.

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Copper tungstate (CuWO) crystals were synthesized by the sonochemistry (SC) method, and then, heat treated in a conventional furnace at different temperatures for 1h. The structural evolution, growth mechanism and photoluminescence (PL) properties of these crystals were thoroughly investigated. X-ray diffraction patterns, micro-Raman spectra and Fourier transformed infrared spectra indicated that crystals heat treated and 100°C and 200°C have water molecules in their lattice (copper tungstate dihydrate (CuWO·2HO) with monoclinic structure), when the crystals are calcinated at 300°C have the presence of two phase (CuWO·2HO and CuWO), while the others heat treated at 400°C and 500°C have a single CuWO triclinic structure. Field emission scanning electron microscopy revealed a change in the morphological features of these crystals with the increase of the heat treatment temperature. Transmission electron microscopy (TEM), high resolution-TEM images and selected area electron diffraction were employed to examine the shape, size and structure of these crystals. Ultraviolet-Visible spectra evidenced a decrease of band gap values with the increase of the temperature, which were correlated with the reduction of intermediary energy levels within the band gap. The intense photoluminescence (PL) emission was detected for the sample heat treat at 300°C for 1h, which have a mixture of CuWO·2HO and CuWO phases. Therefore, there is a synergic effect between the intermediary energy levels arising from these two phases during the electronic transitions responsible for PL emissions.

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The presence of gold nanoparticles (AuNPs) at the protein/electrode interface has a significant impact on the electrodic microenvironment, and allows the optimization of the activity catalysis as well as electrochemical properties. Here, we report a novel and accurate methodology to observe AuNP mediated electron transfer mechanism from Cytochrome c (Cyt c) to a polycrystalline gold surface. Poly(allylamine hydrochloride) molecules (PAH) were used as spacers between Cyt c and the electrode surface, and the electron rate constant within the PAH layer was measured in the presence and absence of AuNPs. Based on cyclic voltammetric experiments and Marcus Theory, a four-fold increase in the electron rate constant was observed in the presence of AuNPs, and the reorganization energy was estimated to be 0.49 eV. Furthermore, AuNPs decreased the effective distance between the redox center of Cyt c and the electrode surface by 20%. These results suggest that the electron transfer properties of Cyt c based protein electrodes are significantly enhanced in the presence of the AuNPs.

Supramolecular self-assembly has been demonstrated to be a useful approach to developing new functional nanomaterials. In this work, we used a cobalt Prussian blue analogue (PBA, Co3[Co(CN)6]2) compound and a β-cyclodextrin (CD) macrocycle to develop a novel host-guest PBA-CD nanomaterial. The preparation of the functional magnetic material involved the self-assembly of CD molecules onto a PBA surface by a co-precipitation method. According to transmission electronic microscopy results, PBA-CD exhibited a polydisperse structure composed of 3D nanocubes with a mean edge length of 85 nm, which became shorter after CD incorporation. The supramolecular arrangement and structural, crystalline and thermal properties of the hybrid material were studied in detail by vibrational and electronic spectroscopies and X-ray diffraction. The cyclic voltammogram of the hybrid material in a 0.1 mol·L−1 NaCl supporting electrolyte exhibited a quasi-reversible redox process, attributed to Co2+/Co3+ conversion, with an E1/2 value of 0.46 V (vs. SCE), with higher reversibility observed for the system in the presence of CD. The standard rate constants for PBA and PBA-CD were determined to be 0.07 and 0.13 s−1, respectively, which suggests that the interaction between the nanocubes and CD at the supramolecular level improves electron transfer. We expect that the properties observed for the hybrid material make it a potential candidate for (bio)sensing designs with a desirable capability for drug delivery.

ABTRACT:We developed and experimentally verified an analytical model to describe diffusion of oligonucleotides from stable hydrogel beads. The synthesized alginate beads are