Enzyme-based glucose fuel cell using Vitamin K3-immobilized polymer as an electron mediator

Fabrication and characterization of electrically conducting electrochemically synthesized polypyrrole-based enzymatic biofuel cell anode with biocompatible redox mediator vitamin K3

Enzymatic biofuel cells (EBFCs) hold tremendous potential to power biomedical devices, biosensors, and bioelectronics. Unlike conventional toxic batteries, these electrochemical devices are biocompatible, harnessing energy from physiological fluids and producing usable electrical energy. But the commercialization of EBFCs is limited by the low operational stability, limited power output and poor electron transport efficiency of the enzymatic electrodes. In this study, a novel bioanode exhibiting a high electron transfer ability and long-term stability was fabricated. For the preparation of the anode, surfactant-assisted polypyrrole (PPy) was electrochemically co-deposited on a platinum wire with the simultaneous entrapment of vitamin K3 (VK3) and GOx (glucose oxidase) in the PPy matrix. Herein, conducting PPy acts as an electron transfer enhancer and provides appropriate electrical communication between the active site of the enzyme glucose oxidase (GOx) and the electrode surface. Biocompatible redox mediator vitamin K3 was employed as an electron transfer mediator to shuttle electrons between the oxidized fuel glucose and surface of the electrode in the electrochemical cell. The electrical conductivity of PPy was measured using the four-probe technique of conductivity measurement of semiconductors. The morphological characterization of as-synthesized anode (PPy/CTAB/VK3/GOx) was performed by Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The electrochemical characterization was studied by cyclic voltammetry (CV), linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) techniques. It was observed that the room-temperature conductivity of PPy lies in the semiconducting range and it also shows good stability on exposure to laboratory air, making it a promising material to provide electrical contact. The study developed a bioanode producing a modest current density of 6.35 mA cm-2 in 20 mM glucose solution. The stability, current output and ease of manufacturing process of the electrode make it particularly suitable for employment in biofuel cell applications.

Glucose Fuel Cells and Membranes: A Brief Overview and Literature Analysis

Glucose is a ubiquitous source of energy for nearly all living things, and glucose fuel cells (GFCs) are regarded as a sustainable power source because glucose is renewable, easily available, cheap, abundant, non-toxic and easy-to-store. Numerous efforts have been devoted to developing and improving GFC performance; however, there is still no commercially viable devices on the market. Membranes play an essential role in GFCs for the establishment of a suitable local microenvironment, selective ion conducting and prevention of substrate crossover. However, our knowledge on them is still limited, especially on how to achieve comparable efficacy with that of a biological system. This review article provides the first brief overview on these aspects, particularly keeping in sight the research trends, current challenges, and the future prospects. We aim to bring together literature analysis and technological discussion on GFCs and membranes by using bibliometrics, and provide new ideas for researchers in this field to overcome challenges on developing high-performance GFCs.

NAD(P)-dependent glucose dehydrogenase: Applications for biosensors, bioelectrodes, and biofuel cells

This review discusses the physical and chemical properties of nicotinamide redox cofactor dependent glucose dehydrogenase (NAD(P) dependent GDH) and its extensive application in biosensors and bio-fuel cells. GDHs from different organisms show diverse biochemical properties (e.g., activity and stability) and preferences towards cofactors, such as nicotinamide adenine dinucleotide (NAD⁺) and nicotinamide adenine dinucleotide phosphate (NADP⁺). The (NAD(P)⁺) play important roles in biological electron transfer, however, there are some difficulties related to their application in devices that originate from their chemical properties and labile binding to the GDH enzyme. This review discusses the electrode modifications aimed at immobilising NAD⁺ or NADP⁺ cofactors and GDH at electrodes. Binding of the enzyme was achieved by appropriate protein engineering techniques, including polymerisation, hydrophobisation or hydrophilisation processes. Various enzyme-modified electrodes applied in biosensors, enzymatic fuel cells, and biobatteries are compared. Importantly, GDH can operate alone or as part of an enzymatic cascade, which often improves the functional parameters of the biofuel cell or simply allows use of cheaper fuels. Overall, this review explores how NAD(P)-dependent GDH has recently demonstrated high potential for use in various systems to generate electricity from biological sources for applications in implantable biomedical devices, wireless sensors, and portable electronic devices.

Tuning Structural Changes in Glucose Oxidase for Enzyme Fuel Cell Applications

Stabilization and electrical contacting of redox enzymes with electrodes are fundamental requirements for bioelectronics devices, including biosensors and enzyme fuel cells (EFCs). In this study, we show increased glucose oxidase (GOx) stability by immobilization with Nafion. The immobilization process affected GOx conformation but was not detrimental to its activity, which was maintained for more than 120 days. The GOx/Nafion system was interfaced to a carbon cloth electrode and assembled in a prototypal EFC fed with glucose. Polarization and power density curves demonstrated that GOx/Nafion system was able to generate power, exploiting a Nafion-assisted electron transfer process to the electrode. Our findings are consistent with the onset of pH-dependent conformational equilibrium for the enzyme secondary structure and its active site. Significantly, the protective effect exerted by Nafion on the enzyme structure may be tuned by varying parameters such as the pH to fabricate durable EFCs with good electrocatalytic performance.

Deep oxidation of glucose in enzymatic fuel cells through a synthetic enzymatic pathway containing a cascade of two thermostable dehydrogenases

A synthetic enzymatic pathway was designed for the deep oxidation of glucose in enzymatic fuel cells (EFCs). Polyphosphate glucokinase converts glucose to glucose-6-phosphate using low-cost, stable polyphosphate rather than costly ATP. Two NAD-dependent dehydrogenases (glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase) that were immobilized on the bioanode were responsible for generating two NADH per glucose-6-phosphate (i.e., four electrons were generated per glucose via a diaphorase-vitamin K(3) electron shuttle system at the anode). Additionally, to prolong the enzyme lifetime and increase the power output, all of the recombinant enzymes that originated from thermophiles were expressed in Escherichia coli and purified to homogeneity. The maximum power density of the EFC with two dehydrogenases was 0.0203 mW cm(-2) in 10 mM glucose at room temperature, which was 32% higher than that of an EFC with one dehydrogenase, suggesting that the deep oxidation of glucose had occurred. When the temperature was increased to 50°C, the maximum power density increased to 0.322 mW cm(-2), which was approximately eight times higher than that based on mesophilic enzymes at the same temperature. Our results suggest that the deep oxidation of glucose could be achieved by using multiple dehydrogenases in synthetic cascade pathways and that high power output could be achieved by using thermostable enzymes at elevated temperatures.

Automatic, sequential power generation for prolonging the net lifetime of a miniature biofuel cell stack

A sequential power generation system for prolonging the net lifetime of a miniature biofuel cell stack has been developed. The system consists of layered chambers of enzyme fuel cells designed to be exposed sequentially to fuel solution by automatically switched fuel flow. The cell chambers were initially separated by magnetized plastic covers sealed with a degradable glue, poly(lactic-co-glycolic acid) (PLGA). The time that the cover was opened by attraction with an external magnet, thereby activating the following cell, was adjustable from a few hours to a few weeks by controlling the weight ratio of Fe(3)O(4) in the covers and the molecular weight of PLGA. By using sequential power generation in this way, the power output of the system was stable for longer periods, and therefore the net lifetime of the stack has been extended as compared with that of a single biofuel cell.

Immobilization of Hydroquinone through a Spacer to Polymer Grafted on Carbon Black for a High-Surface-Area Biofuel Cell Electrode

We immobilized hydroquinone through a spacer to polymer grafted on carbon black and achieved a high-surface-area biofuel cell electrode. Quinone compounds are well-known to transfer electrons in the respiratory chain and have been considered prospective mediators in biofuel cells because of their relatively negative redox potentials. Evaluation of three different spacer arms tethering hydroquinone to linear polymers revealed that only the hydrophilic and flexible di(ethylene oxide) spacer made it possible for immobilized hydroquinone to transfer electrons from glucose oxidase (GOD) to an electrode; direct immobilization and an alkyl spacer did not. The electrode comprising hydroquinone immobilized through di(ethylene oxide) spacer to polymer grafted on carbon black transferred electrons from GOD to the electrode. The potential at which an anodic current began to increase was more negative by about 0.2 V than that for a vinylferrocene-mediated electrode, while the increase in the anodic current density was of the same order.