Shelley D. Minteer

Extended lifetime biofuel cells

Over the last 40 years, researchers have been studying and improving enzymatic biofuel cells, but until the last five years, the technology was plagued by short active lifetimes (typically 8 hours to 7 days) that prohibited the commercial use of this technology. This tutorial review introduces the topic of enzymatic biofuel cells and discusses the recent work done to stabilize and immobilize enzymes at bioanodes and biocathodes of biofuel cells. This review covers a wide variety of fuel systems from sugar to alcohols and covers both direct electron transfer (DET) systems and mediated electron transfer (MET) systems.

Recent advances in material science for developing enzyme electrodes

The enzyme-modified electrode is the fundamental component of amperometric biosensors and biofuel cells. The selection of appropriate combinations of materials, such as: enzyme, electron transport mediator, binding and encapsulation materials, conductive support matrix and solid support, for construction of enzyme-modified electrodes governs the efficiency of the electrodes in terms of electron transfer kinetics, mass transport, stability, and reproducibility. This review investigates the varieties of materials that can be used for these purposes. Recent innovation in conductive electro-active polymers, functionalized polymers, biocompatible composite materials, composites of transition metal-based complexes and organometallic compounds, sol-gel and hydro-gel materials, nanomaterials, other nano-metal composites, and nano-metal oxides are reviewed and discussed here. In addition, the critical issues related to the construction of enzyme electrodes and their application for biosensor and biofuel cell applications are also highlighted in this article. Effort has been made to cover the recent literature on the advancement of materials sciences to develop enzyme electrodes and their potential applications for the construction of biosensors and biofuel cells.

Nanomaterials for bio-functionalized electrodes

Recent years have faced stimulating developments in the functionalization of electrode surfaces with biological materials, notably due to the significant input of nanosciences and nanotechnology. In this review (over 450 references), we are discussing the interest of both nano-objects (metal nanoparticles and quantum dots, carbon nanotubes and graphene) and nano-engineered and/or nanostructured materials (template-based materials, advanced organic polymers) for the rational design of bio-functionalized electrodes and related (bio)sensing systems. The attractiveness of such nanomaterials relies not only on their ability to act as effective immobilization matrices, which are, e.g., likely to enhance the long-term stability of bioelectrochemical devices, but also on their intrinsic and unique features (large surface areas, electrocatalytic properties, controlled morphology and structure, possible use as labels) that can be advantageously combined with the functioning of biomolecules, thus contributing to improved bioelectrode performance in terms of sensitivity and selectivity (enzymatic biosensors, DNA sensors, immunosensors and cell sensors) or power (biofuel cells).

Enzymatic biofuel cells: 30 years of critical advancements.

Enzymatic biofuel cells are bioelectronic devices that utilize oxidoreductase enzymes to catalyze the conversion of chemical energy into electrical energy. This review details the advancements in the field of enzymatic biofuel cells over the last 30 years. These advancements include strategies for improving operational stability and electrochemical performance, as well as device fabrication for a variety of applications, including implantable biofuel cells and self-powered sensors. It also discusses the current scientific and engineering challenges in the field that will need to be addressed in the future for commercial viability of the technology.

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.

Substrate channelling as an approach to cascade reactions

Millions of years of evolution have produced biological systems capable of efficient one-pot multi-step catalysis. The underlying mechanisms that facilitate these reaction processes are increasingly providing inspiration in synthetic chemistry. Substrate channelling, where intermediates between enzymatic steps are not in equilibrium with the bulk solution, enables increased efficiencies and yields in reaction and diffusion processes. Here, we review different mechanisms of substrate channelling found in nature and provide an overview of the analytical methods used to quantify these effects. The incorporation of substrate channelling into synthetic cascades is a rapidly developing concept, and recent examples of the fabrication of cascades with controlled diffusion and flux of intermediates are presented.

New Materials for Biological Fuel Cells

Major improvements in biological fuel cells over the last ten years have been the result of the development and application of new materials. These new materials include: nanomaterials, such as nanotubes and graphene, that improve the electron transfer between the biocatalyst and electrode surface; materials that provide improved stability and immobilization of biocatalysts; materials that increase the conductivity and surface area of the electrodes; and materials that aid facile mass transport. With a focus on enzymatic biological fuel cell technology, this brief review gives an overview of the latest developments in each of these material science areas and describes how this progress has improved the performance of biological fuel cells to yield a feasible technology.

Citric acid cycle biomimic on a carbon electrode

The citric acid cycle is one of the main metabolic pathways living cells utilize to completely oxidize biofuels to carbon dioxide and water. The overall goal of this research is to mimic the citric acid cycle at the carbon surface of an electrode in order to achieve complete oxidation of ethanol at a bioanode to increase biofuel cell energy density. In order to mimic this process, dehydrogenase enzymes (known to be the electron or energy producing enzymes of the citric acid cycle) are immobilized in cascades at an electrode surface along with non-energy producing enzymes necessary for the cycle to progress. Six enzymatic schemes were investigated each containing an additional dehydrogenase enzyme involved in the complete oxidation of ethanol. An increase in current density is observed along with an increase in power density with each additional dehydrogenase immobilized on an electrode, reflecting increased electron production at the bioanode with deeper oxidation of the ethanol biofuel. By mimicking the complete citric acid cycle on a carbon electrode, power density was increased 8.71-fold compared to a single enzyme (alcohol dehydrogenase)-based ethanol/air biofuel cell.

Enzyme Immobilization in Biotechnology

Enzymes are proteins that catalyze chemical reactions. Unlike more traditional organic and inorganic catalysts, enzymes are large and fragile molecules, so over the years, scientists and engineers have found it more difficult to immobilize enzyme catalysts on easily separateable supports for use and re-use in a variety of technologies. Over the last decade, enzyme immobilization has become more important in industry, medicine, and biotechnology. This review will detail recent patents for techniques for enzyme immobilization, along with patents for chemical and biotechnological processes that can employ immobilized enzymes, which allow for the re-use of the enzymatic catalysts. These techniques include methods varying from physical adsorption and covalent attachment to entrapment in polymers and sol-gels. These techniques have shown value in the development of biosensors, bioprocessing for the chemical industry and the pharmaceutical industry, and bioremediation.

Growth of Phthalocyanine Doped and Undoped Nanotubes Using Mild Synthesis Conditions For Development of Novel Oxygen Reduction Catalysts

Precious metal alloys have been the predominant electrocatalyst used for oxygen reduction in fuel cells since the 1960s. Although performance of these catalysts is high, they do have drawbacks. The two main problems with precious metal alloys are catalyst passivation and cost. This is why new novel catalysts are being developed and employed for oxygen reduction. This paper details the low temperature solvothermal synthesis and characterization of carbon nanotubes that have been doped with both iron and cobalt centered phthalocyanine. The synthesis is a novel low-temperature, supercritical solvent synthesis that reduces halocarbons to form a metal chloride byproduct and carbon nanotubes. Perchlorinated phthalocyanine was added to the nanotube synthesis to incorporate the phthalocyanine structure into the graphene sheets of the nanotubes to produce doped nanotubes that have the catalytic oxygen reduction capabilities of the metallo-phthalocyanine and the advantageous material qualities of carbon nanotubes. The cobalt phthalocyanine doped carbon nanotubes showed a half wave oxygen reduction potential of -0.050 ± 0.005 V vs Hg\HgO, in comparison to platinums half wave oxygen reduction potential of -0.197 ± 0.002 V vs Hg\HgO.