Cellobiose Dehydrogenase Aryl Diazonium Modified Single Walled Carbon Nanotubes: Enhanced Direct Electron Transfer through a Positively Charged Surface

One of the challenges in the field of biosensors and biofuel cells is to establish a highly efficient electron transfer rate between the active site of redox enzymes and electrodes to fully access the catalytic potential of the biocatalyst and achieve high current densities. We report on very efficient direct electron transfer (DET) between cellobiose dehydrogenase (CDH) from Phanerochaete sordida (PsCDH) and surface modified single walled carbon nanotubes (SWCNT). Sonicated SWCNTs were adsorbed on the top of glassy carbon electrodes and modified with aryl diazonium salts generated in situ from p-aminobenzoic acid and p-phenylenediamine, thus featuring at acidic pH (3.5 and 4.5) negative or positive surface charges. After adsorption of PsCDH, both electrode types showed excellent long-term stability and very efficient DET. The modified electrode presenting p-aminophenyl groups produced a DET current density of 500,mu A cm(-2) at 200 mV vs normal hydrogen reference electrode (NHE) in a 5 mM lactose solution buffered at pH 3.5. This is the highest reported DET value so far using a CDH modified electrode and comes close to electrodes using mediated electron transfer. Moreover, the onset of the electrocatalytic current for lactose oxidation started at 70 mV vs NHE, a potential which is 50 mV lower compared to when unmodified SWCNTs were used. This effect potentially reduces the interference by oxidizable matrix components in biosensors and increases the open circuit potential in biofuel cells. The stability of the electrode was greatly increased compared with unmodified but cross-linked SWCNTs electrodes and lost only 15% of the initial current after 50 h of constant potential scanning

Investigation of the pH-Dependent Electron Transfer Mechanism of Ascomycetous Class II Cellobiose Dehydrogenases on Electrodes

Cellobiose dehydrogenase (CDH) is capable of direct electron transfer (DET) on various carbon and thiol-modified gold electrodes. As a result, these systems have been utilized as biocatalyst in biosensors and biofuel cell anodes. Class I CDHs, from basidiomycetous fungi, are highly specific to cellulose or lactose, and DET is only observed at pH values below 5.5. To extend the applicability of CDH-based electrodes, the catalytic properties and the behavior on electrode surfaces of ascomycetous class II CDHs from Chaetomium attrobrunneum, Corynascus thermophilus, Dichomera saubinetii, Hypoxylon haematostroma, Neurospora crassa, and Stachybotrys bisbyi were investigated. We found that class II CDHs have diverse properties but generally show a lower substrate specificity than class I CDHs by converting also glucose and maltose. Intramolecular electron transfer (JET) and DET at neutral and alkaline pH were observed and elucidated by steady-state kinetics, pre-steady-state kinetics, and electrochemical measurements. The CDHs ability to interact with the electron acceptor cytochrome c and to communicate with electrode surfaces through DET at various pH conditions was used to classify the investigated enzymes. In combination with stopped-flow measurements, a model for the kinetics of the pH-dependent JET is developed. The efficient glucose turnover at neutral/alkaline pH makes some of these new CDHs potential candidates for glucose biosensors and biofuel cell anodes

Investigation of electron transfer between cellobiose dehydrogenase from Myriococcum Thermophilum and gold electrodes

Cellobiose dehydrogenase (CDH) is a monomeric protein consisting of two subdomains: a larger flavin-associated domain (DHcdh) and a smaller heme-binding domain (CYTcdh), connected via a protease cleavable linker region. In this study, the inter-domain electron transfer, using the CDH from the ascomycete fungus Myriococcum thermophilum and thiol (SAM) modified gold electrodes, was investigated with cyclic voltammetry and UV-VIS spectroelectrochemistry. The effect of the SAM and pH on the formal potential of the heme domain of CDH and on the current generated by the electrocatalytic oxidation of cellobiose and lactose was evaluated with voltammetric techniques. The oxidation-reduction midpoint potentials of the DHcdh, CYTcdh, and whole CDH unit were estimated at different pH values using a long-optical-pathway thin capillary-type spectroelectrochemical cell

Investigation of graphite electrodes modified with cellobiose dehydrogenase from the ascomycete Myriococcum thermophilum

The catalytic properties of cellobiose dehydrogenase (CDH) from the ascomycete fungus Myriococcum thermophilum adsorbed on a graphite electrode were investigated for a large variety of carbohydrate substrates. The effects of applied potential, pH and buffer composition were tested and optimized, and the most suitable conditions were used to evaluate the detection limit, linear range, and sensitivity of the sensor for different carbohydrates in the flow injection mode. Subsequently, the long term stability of the modified electrodes was determined. Additionally, the direct and mediated electron transfer between the active site of the enzyme and the electrode has been investigated by amperometric flow injection measurements in the absence and presence of the mediator 1,4-benzoquinone in the presence of cellobiose or lactose

A third generation glucose biosensor based on cellobiose dehydrogenase from Corynascus thermophilus and single-walled carbon nanotubes.

A third generation glucose biosensor working under physiological conditions with a linear range of 0.1-30 mM, a detection limit of 0.05 mM, and a sensitivity of 222 nA microM(-1) cm(-2) has been developed by co-adsorption of cellobiose dehydrogenase (CDH) from the ascomycete Corynascus thermophilus (CtCDH) and oxidatively shortened single-walled carbon nanotubes (SWCNTs)

Highly efficient and versatile anodes for biofuel cells based on cellobiose dehydrogenase from Myriococcum thermophilum

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 beta-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 mu A . cm(-1). 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 degrees C) were investigated over a period of 3 days. A maximum voltage of 500 mV, a maximum current density of almost 700 mu A.cm(-2), and a maximum power density of 157 mu W.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

Comparison of Direct and Mediated Electron Transfer for Cellobiose Dehydrogenase from Phanerochaete sordida.

Direct and mediated electron transfer (DET and MET) between the enzyme and electrodes were compared for cellobiose dehydrogenase (CDH) from the basidiomycete Phanerochaete sordida (PsCDH). For DET, PsCDH was adsorbed at pyrolytic graphite (PG) electrodes while for MET the enzyme was covalently linked to a low potential Os redox polymer. Both types of electrodes were prepared in the presence of single walled carbon nanotubes (SWCNTs). DET requires the oxidation of the heme domain, while MET occurs partially via the heme and the flavin domain at pH 3.5. At pH 6 MET occurs solely via the flavin domain. Most probably, the interaction of the domains decreases from pH 3.5 to 6.0 due to electrostatic repulsion of deprotonated amino acid residues, covering the surfaces of both domains. MET starts at a lower potential than DET. The midpoint potentials at pH 3.5 for the flavin (40 mV) and the heme domain (170 mV) were determined with spectroelectrochemistry. The electrochemical and spectroelectrochemical measurements presented in this work are in conformity. The pH dependency of DET and MET was investigated for PsCDH. The optimum was observed between pH 4 and 4.5 pH for DET and in the range of pH 5-6 for MET. The current densities obtained by MET are 1 order of magnitude higher than by DET. During multicycle cyclic voltammetry experiments carried out at different pHs, the PsCDH modified electrode working by MET turned out to be very stable. In order to characterize a PsCDH modified anode working by MET with respect to biofuel cell applications, this electrode was combined with a Pt-black cathode as model for a membraneless biofuel cell. In comparison to DET, a 10 times higher maximum current and maximum power density in a biofuel cell application could be achieved by MET. While CDH modified electrodes working by DET are highly qualified for applications in amperometric biosensors, a much better performance as biofuel cell anodes can be obtained by MET. The use of CDH modified electrodes working by MET for biofuel cell applications results in a less positive onset of the electrocatalytic current (which may lead to an increased cell voltage), higher current and power density, and much better long-term stability over a broad range of pH

Direct electron transfer at cellobiose dehydrogenase modified anodes for biofuel cells

Cellobiose dehydrogenases (CDHs, EC 1.1.99.18) contain a larger flavin-associated (dehydrogenase) domain and a smaller heme-binding (cytochrome) domain. CDHs from basidiomycete fungi oxidize at an appreciable level cellobiose, cellodextrins, and lactose, and those from ascomycetes may additionally oxidize some monosaccharides to their corresponding lactones at the flavin domain. CDHs are able to communicate directly with an electrode via their heme domain. In this work, different types of CDHs have been adsorbed on graphite electrodes and studied with respect to their direct electron transfer (DET) properties. Electrochemical studies were performed in the presence and absence of single-walled carbon nanotubes (SWCNTs) using lactose as substrate. In the presence of SWCNTs, the electrocatalytic current for substrate oxidation based on DET between enzyme and electrode was significantly increased. Furthermore, the onset of the electrocatalytic current was at lower potential than in the absence of SWCNTs. The highest electrocatalytic activity toward oxidation of lactose was found for CDH from the basidiomycete Phanerochaete sordida. Based on CDH from Phanerochaete sordida, an anode for biofuel cells was developed. This anode using lactose as substrate was combined with a Pt black cathode for oxygen reduction as a model for a membrane-less biofuel cell in which the processes at both electrodes occur by DET

Cellobiose dehydrogenase modified electrodes: advances by materials science and biochemical engineering

The flavocytochrome cellobiose dehydrogenase (CDH) is a versatile biorecognition element capable of detecting carbohydrates as well as quinones and catecholamines. In addition, it can be used as an anode biocatalyst for enzymatic biofuel cells to power miniaturised sensor-transmitter systems. Various electrode materials and designs have been tested in the past decade to utilize and enhance the direct electron transfer (DET) from the enzyme to the electrode. Additionally, mediated electron transfer (MET) approaches via soluble redox mediators and redox polymers have been pursued. Biosensors for cellobiose, lactose and glucose determination are based on CDH from different fungal producers, which show differences with respect to substrate specificity, pH optima, DET efficiency and surface binding affinity. Biosensors for the detection of quinones and catecholamines can use carbohydrates for analyte regeneration and signal amplification. This review discusses different approaches to enhance the sensitivity and selectivity of CDH-based biosensors, which focus on (1) more efficient DET on chemically modified or nanostructured electrodes, (2) the synthesis of custom-made redox polymers for higher MET currents and (3) the engineering of enzymes and reaction pathways. Combination of these strategies will enable the design of sensitive and selective CDH-based biosensors with reduced electrode size for the detection of analytes in continuous on-site and point-of-care applications