Matthew T. Meredith

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.

High Current Density Ferrocene-Modified Linear Poly(ethylenimine) Bioanodes and Their Use in Biofuel Cells

Linear poly(ethylenimine) (-[CH 2 CH 2 NH] n -, LPEI) was modified by attachment of 3-(dimethylferrocenyl)propyl groups to ca. 17% of its nitrogen atoms (FcMe 2 -C 3 -LPEI) to form a new redox polymer for use as an anodic mediator in glucose/O 2 biofuel cells. The electrochemical properties of this polymer were compared to those of 3-ferrocenylpropyl-modified LPEI (Fc-C 3 -LPEI). When Fc-C 3 -LPEI or FcMe 2 -C 3 -LPEI was mixed with glucose oxidase and cross-linked with ethylene glycol diglycidyl ether to form hydrogels on planar, glassy carbon electrodes, limiting catalytic bioanodic current densities of up to ~2 mA/cm 2 at 37°C were produced. The use of dimethylferrocene moieties in place of ferrocene moieties lowered the E 1/2 of the films by 0.09 V and significantly increased electrochemical and operational stabilities. FcMe 2 -C 3 -LPEI was shown to be the more effective polymer for use in biofuel cells and, when coupled with a stationary O 2 cathode comprised of laccase and cross-linked poly[(vinylpyridine)Os(bipyridyl) 2 Cl 2+/3+ ] as a mediator, produced power densities of up to 56 wW/cm 2 at 37°C. Power density increased to 146 μW/cm 2 when a rotating biocathode was used. The stability of the biofuel cells constructed with FcMe 2 -C 3 -LPEI was higher than that of the cells using Fc-C 3 -LPEI.

Enzymatic biofuel cells utilizing a biomimetic cofactor

The performance of immobilized enzyme systems is often limited by cofactor diffusion and regeneration. Here, we demonstrate an engineered enzyme capable of utilizing the minimal cofactor nicotinamide mononucleotide (NMN(+)) to address these limitations. Significant gains in performance are observed with NMN(+) in immobilized systems, despite a decreased turnover rate with the minimal cofactor.

Inhibition and activation of glucose oxidase bioanodes for use in a self-powered EDTA sensor

Self-powered sensors are able to automatically signal the presence of a specific analyte without the aid of an external power source, making them useful as potential devices for batteryless sensing. Here, we present a self-powered enzymatic ethylenediaminetetraacetic acid (EDTA) sensor based on the inhibition and subsequent activation of glucose oxidase (GOx)-based bioelectrodes within the framework of a biofuel cell. Although EDTA is not redox-active, it is detected by the activation of a Cu(2+)-inhibited GOx bioanode in either a typical amperometric sensor (using a standard three-electrode setup) or in a self-powered sensor where the GOx bioanode is coupled to a platinum cathode. The sensors are able to detect concentrations of EDTA that correspond to the amount of Cu(2+) that is used to inhibit the enzymatic electrode. The self-powered sensor shows a greater than 10-fold increase in power output when it is activated by the presence of EDTA. This represents the first time that a non-redox-active analyte has been detected in a self-powered sensor that turns on in the presence of said analyte.

Hydrophobic Salt-modified Nafion for Enzyme Immobilization and Stabilization

Over the last decade, there has been a wealth of application for immobilized and stabilized enzymes including biocatalysis, biosensors, and biofuel cells. In most bioelectrochemical applications, enzymes or organelles are immobilized onto an electrode surface with the use of some type of polymer matrix. This polymer scaffold should keep the enzymes stable and allow for the facile diffusion of molecules and ions in and out of the matrix. Most polymers used for this type of immobilization are based on polyamines or polyalcohols - polymers that mimic the natural environment of the enzymes that they encapsulate and stabilize the enzyme through hydrogen or ionic bonding. Another method for stabilizing enzymes involves the use of micelles, which contain hydrophobic regions that can encapsulate and stabilize enzymes. In particular, the Minteer group has developed a micellar polymer based on commercially available Nafion. Nafion itself is a micellar polymer that allows for the channel-assisted diffusion of protons and other small cations, but the micelles and channels are extremely small and the polymer is very acidic due to sulfonic acid side chains, which is unfavorable for enzyme immobilization. However, when Nafion is mixed with an excess of hydrophobic alkyl ammonium salts such as tetrabutylammonium bromide (TBAB), the quaternary ammonium cations replace the protons and become the counter ions to the sulfonate groups on the polymer side chains (Figure 1). This results in larger micelles and channels within the polymer that allow for the diffusion of large substrates and ions that are necessary for enzymatic function such as nicotinamide adenine dinucleotide (NAD). This modified Nafion polymer has been used to immobilize many different types of enzymes as well as mitochondria for use in biosensors and biofuel cells. This paper describes a novel procedure for making this micellar polymer enzyme immobilization membrane that can stabilize enzymes. The synthesis of the micellar enzyme immobilization membrane, the procedure for immobilizing enzymes within the membrane, and the assays for studying enzymatic specific activity of the immobilized enzyme are detailed below.