Highly Efficient and Versatile Anodes for Biofuel Cells Based on Cellobiose Dehydrogenase from Myriococcum thermophilum

Abstract

A powerful alternative to glucose oxidase as anode material in implantable biofuel cells is presented: Cellobiose dehydrogenase (CDH) from the ascomycete Myriococcum thermophilum (MtCDH) catalyzes the electrochemical oxidation of glucose, lactose, and cellobiose over a broad pH range. Current densities of more than 1 mA·cm−2 can be reached when MtCDH is wired to an Os redox polymer in the presence of single-walled carbon nanotubes and when lactose is used as a substrate at pH 8. In contrast to CDHs from basidiomycete fungi, which oxidize only β-1,4-linked di- and oligosaccharides efficiently, MtCDH is also able to oxidize glucose and other monosaccharides at relatively high turnover rates. The current density toward oxidation of 5 mM glucose under physiological conditions was about 100 μA·cm−2. Outstanding properties of MtCDH are high-temperature stability; a strong discrimination of oxygen turnover (and therefore no H2O2 production) in the presence of alternative electron acceptors; an ability to oxidize a range of carbohydrates, and a working pH from 3 to 10, which allows for combination with a variety of enzyme-based cathodes for oxygen reduction. The performance and stability of a membraneless glucose biofuel cell consisting of an MtCDH-modified anode and a Pt black cathode working under physiological conditions (PBS buffer, pH 7.4, 37 °C) were investigated over a period of 3 days. A maximum voltage of 500 mV, a maximum current density of almost 700 μA·cm−2, and a maximum power density of 157 μW·cm−2 at an operating voltage of 280 mV (under oxygen purging/nonquiescent conditions) could be obtained with glucose (100 mM) as the substrate. Furthermore, the direct and mediated electron-transfer properties of MtCDH are compared in this work. The electrocatalytic current detected for mediated electron transfer (MET) is much higher and starts at a less positive potential than that for direct electron transfer (DET). The reason is that, in MET, the Os redox polymer is able to collect the electrons from the catalytically active flavin domain, whereas DET requires the oxidation of the heme domain, which has a more positive redox potential. The electrocatalytic current densities for DET and MET are significantly increased in the presence of single-walled carbon nanotubes.