Analytical Techniques for Characterizing Enzymatic Biofuel Cells

Formation of Supported Thylakoid Membrane Bioanodes for Effective Electron Transfer and Stable Photocurrent

The light-dependent reactions of photosynthesis use light energy to generate photoelectrons traveling through the thylakoid membranes (TMs). Extracting the photoelectrons from the TMs to form bioanodes can have various applications. Most studies focus on modifying the electrode materials to increase the collected photocurrent. Seldom studies have investigated how the orientation of the TMs influences photocurrent collection. In addition, the formation of reactive oxygen species (ROS) during photosynthesis is a challenge for stable photocurrent generation. Here, we enhanced the photoelectron transfer from the TMs to electrodes by depositing expanded thylakoids as planar supported membranes onto an electrode. The high contact area between the external electrodes and TMs per unit mass of thylakoid allows the thylakoid to more effectively transfer electrons to the electrodes, thereby reducing the free electrons available for the ROS generation. We expanded the naturally stacked thylakoids into liposomes through osmotic pressure and dropcasted them onto an Au electrode. The electrochemical impedance measurement showed that the supported membrane bioanode formed by the expanded liposomes had a lower photoelectron transfer resistance. Additionally, we observed that the expanded TM bioanode provided a higher photocurrent and was more durable to air/water interfacial tension. These results suggest that the effective contact between the expanded TM and electrodes can lead to more efficient electron transfer and increase the system robustness. The photo fuel cell (PFC) made by the expanded TM bioanode had a higher open-circuit voltage than the one made by the stacked TM bioanode. Interestingly, we found that PFCs made of high-load TM bioanodes had fast photocurrent decay under continuous operation at high cell voltages. The poor contact of large numbers of TMs with the electrodes at the high-load TM bioanodes could cause more ROS accumulation and therefore decreased the operational stability, supporting the importance of effective contact between TMs and the electrodes.

Continuous Determination of Glucose Using a Membraneless, Microfluidic Enzymatic Biofuel Cell

In this article, we describe an enzyme-based, membraneless, microfluidic biofuel cell for the continuous determination of glucose using electrochemical power generation as a transducing signal. Enzymes were immobilized on multi-walled carbon nanotube (MWCNT) electrodes placed parallel to the co-laminar flow in a Y-shaped microchannel. The microchannel was produced with polydimethylsiloxane (PDMS) using soft lithography, while the MWCNT electrodes were replicated via a PDMS stencil on indium tin oxide (ITO) glass. Moreover, the electrodes were modified with glucose oxidase and laccase by direct covalent bonding. The device was studied at different MWCNT deposition amounts and electrolyte flow rates to achieve optimum settings. The experimental results demonstrated that glucose could be determined linearly up to a concentration of 4 mM at a sensitivity of 31 mV∙mM^-1cm^-2.

Graphene based enzymatic bioelectrodes and biofuel cells

The excellent electrical conductivity and ease of functionalization make graphene a promising material for use in enzymatic bioelectrodes and biofuel cells. Enzyme based biofuel cells have attracted substantial interest due to their potential to harvest energy from organic materials. This review provides an overview of the functional properties and applications of graphene in the construction of biofuel cells as alternative power sources. The review covers the current state-of-the-art research in graphene based nanomaterials (physicochemical properties and surface functionalities), the role of these parameters in enhancing electron transfer, the stability and activity of immobilized enzymes, and how enhanced power density can be achieved. Specific examples of enzyme immobilization methods, enzyme loading, stability and function on graphene, functionalized graphene and graphene based nanocomposite materials are discussed along with their advantages and limitations. Finally, a critical evaluation of the performance of graphene based enzymatic biofuel cells, the current status, challenges and future research needs are provided.

Employing FAD-dependent glucose dehydrogenase within a glucose/oxygen enzymatic fuel cell operating in human serum

Flavin adenine dinucleotide-dependent glucose dehydrogenase (FAD-GDH) is emerging as an oxygen-insensitive alternative to glucose oxidase (GOx) as the biocatalyst for bioelectrodes and bioanodes in glucose sensing and glucose enzymatic fuel cells (EFCs). Glucose EFCs, which utilize oxygen as the oxidant and final electron acceptor, have the added benefit of being able to be implanted within living hosts. These can then produce electrical energy from physiological glucose concentrations and power internal or external devices. EFCs were prepared with FAD-GDH and bilirubin oxidase (BOx) to evaluate the suitability of FAD-GDH within an implantable setting. Maximum current and power densities of 186.6±7.1 μA cm(-2) and 39.5±1.3 μW cm(-2) were observed when operating in human serum at 21 °C, which increased to 285.7±31.3 μA cm(-2) and 57.5±5.4 μW cm(-2) at 37 °C. Although good stability was observed with continual near-optimal operation of the EFCs in human serum at 21 °C for 24 h, device failure was observed between 13-14 h when continually operated at 37 °C.

Hydrogen peroxide produced by glucose oxidase affects the performance of laccase cathodes in glucose/oxygen fuel cells: FAD-dependent glucose dehydrogenase as a replacement

Hydrogen peroxide production by glucose oxidase (GOx) and its negative effect on laccase performance have been studied. Simultaneously, FAD-dependent glucose dehydrogenase (FAD-GDH), an O2-insensitive enzyme, has been evaluated as a substitute. Experiments focused on determining the effect of the side reaction of GOx between its natural electron acceptor O2 (consumed) and hydrogen peroxide (produced) in the electrolyte. Firstly, oxygen consumption was investigated by both GOx and FAD-GDH in the presence of substrate. Relatively high electrocatalytic currents were obtained with both enzymes. O2 consumption was observed with immobilized GOx only, whilst O2 concentration remained stable for the FAD-GDH. Dissolved oxygen depletion effects on laccase electrode performances were investigated with both an oxidizing and a reducing electrode immersed in a single compartment. In the presence of glucose, dramatic decreases in cathodic currents were recorded when laccase electrodes were combined with a GOx-based electrode only. Furthermore, it appeared that the major loss of performance of the cathode was due to the increase of H2O2 concentration in the bulk solution induced laccase inhibition. 24 h stability experiments suggest that the use of O2-insensitive FAD-GDH as to obviate in situ peroxide production by GOx is effective. Open-circuit potentials of 0.66 ± 0.03 V and power densities of 122.2 ± 5.8 μW cm(-2) were observed for FAD-GDH/laccase biofuel cells.

Recent advances in application of chitosan in fuel cells

Fuel cells are electrochemical devices which convert chemical energy into electrical energy. Fuel cells have attracted attention due to their potential as a promising alternative to traditional power sources. More recently, efficient and environmentally benign biopolymer “chitosan” have been extensively investigated as a novel material for its application in fuel cells. This biopolymer can be used in both membrane electrolyte and electrode in various fuel cells such as alkaline polymer electrolyte fuel cells, direct methanol fuel cells and biofuel cells. This review provides an overview of main available fuel cells following by application of chitosan as novel biopolymer in fuel cells technology. Recent achievements are included and recommendations are also given for areas of future research.

Biofuel cells: enhanced enzymatic bioelectrocatalysis

Enzymatic biofuel cells represent an emerging technology that can create electrical energy from biologically renewable catalysts and fuels. A wide variety of redox enzymes have been employed to create unique biofuel cells that can be used in applications such as implantable power sources, energy sources for small electronic devices, self-powered sensors, and bioelectrocatalytic logic gates. This review addresses the fundamental concepts necessary to understand the operating principles of biofuel cells, as well as recent advances in mediated electron transfer- and direct electron transfer-based biofuel cells, which have been developed to create bioelectrical devices that can produce significant power and remain stable for long periods.

Methylene green electrodeposited on SWNTs-based "bucky" papers for NADH and l-malate oxidation

This research introduces a cavity anode design based on new single-walled nanotube (SWNTs) papers, "bucky" papers, used for the oxidation (and regeneration) of nicotinamide adenine dinucleotide (NADH) and the oxidation of l-malate. The materials designed are paper-like processed composites containing also additives: BP11 sample contains SWNTs and isopropanol (IPA); the BPMG sample contains SWNTs, IPA, and methylene green (MG). NADH/NAD(+) is the cofactor responsible for the oxidation of l-malate by malate dehydrogenase (MDH), in the Krebs' cycle. Because of the high overpotential of NADH oxidation, poly methylene green (PMG) was utilized as the electrocatalyst to produce NAD(+). The electrocatalyst was deposited on the surface of the "bucky" papers by electropolymerization by means of 10 voltammetric cycles in a range of -0.5 V and +1.3 V (vs Ag/AgCl reference electrode) at a scan rate of 5 mV/s. The catalytic performance of PMG was evaluated by chronoamperometric measurements of NADH oxidation at 0.3 V in phosphate buffer and l-malate oxidation at 0.1 V in the presence of MDH. For both "bucky" papers, the chronoamperometric curves of PMG, current vs NADH concentration, show a linear relationship demonstrating to have a first order Fick's law behavior for concentrations of NADH lower than 6 mM. The chronoamperometric curves in the presence of MDH, current against l-malate concentration, show a Michaelis-Menten behavior where no inhibition or competitive reaction are detected. Additionally, the anodic materials were characterized by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), the polymerization of MG is effectively observed in the form of particles nucleation. The anodes show an excellent electrocatalytic activity toward NADH oxidation. The electrode design is feasible, reproducible, and overall stable.

Increasing the coulombic efficiency of glucose biofuel cell anodes by combination of redox enzymes

A highly efficient anode for glucose biofuel cells has been developed by a combination of pyranose dehydrogenase from Agaricus meleagris (AmPDH) and cellobiose dehydrogenase from Myriococcum thermophilum (MtCDH). These two enzymes differ in how they oxidize glucose. AmPDH oxidizes glucose at the C(2) and C(3) carbon, whereas MtCDH at the C(1) carbon. Both enzymes oxidize efficiently a number of other mono- and disaccharides. They do not react directly with oxygen and produce no H(2)O(2). Electrodes were prepared by embedding (i) only AmPDH (in order to study this enzyme separately) and (ii) a mixture of AmPDH and MtCDH in an Os redox polymer hydrogel. Single-walled carbon nanotubes (SWCNTs) were added in order to enhance the current density. The electrodes were investigated with linear sweep and cyclic voltammetry in the presence of different substrates at physiological conditions. The electrochemical measurements revealed that the product of one enzyme can serve as a substrate for the other. In addition, a kinetic pathway analysis was performed by spectrophotometric measurements leading to the conclusion that up to six electrons can be gained from one glucose molecule through a combination of AmPDH and MtCDH. Hence, the combination of redox enzymes can lead to an enzymatic biofuel cell anode with an increased coulombic efficiency far beyond the usual yields of two electrons per substrate molecule.