Glucose oxidase anode for biofuel cell based on direct electron transfer

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Electrochemistry Communications 8 (2006) 1204–1210

Glucose oxidase anode for biofuel cell based on direct electron transfer

Dmitri Ivnitski a,*, Brittany Branch a, Plamen Atanassov a, Christopher Apblett b

a Department of Chemical and Nuclear Engineering, University of New Mexico, 209 Farris Engineering Center, Room 150,

Albuquerque, NM 87131-0001, USAb Sandia National Laboratories, Albuquerque, NM 87185, USA

Received 26 April 2006; received in revised form 27 May 2006; accepted 31 May 2006Available online 11 July 2006

Abstract

This paper presents a new design concept of a glucose oxidase (GOx) electrode as an anode for the biofuel cell based on direct electrontransfer (DET) between the active site of an enzyme and the multi-walled carbon nanotube (MWNT)-modified electrode surface. Toray�

carbon paper (TP) with a porous three-dimensional network (78% porosity) was used as a matrix for selectively growing multi-walledcarbon nanotubes. The incorporation of MWCNTs into TP was provided by the chemical vapor deposition technique after an electro-chemical transition of cobalt metal seeds. This approach has the ability to efficiently promote DET reactions. The morphologies andelectrochemical characteristics of the GOx modified electrodes were investigated by scanning electron microscopy, cyclic voltammetry,and potentiometric methods. The combination of poly-cation polyethylenimine (PEI) with negatively charged glucose oxidase providesformation of �100 nm thick films on the TP/MWCNT surface. The tetrabutylammonium bromide salt-treated Nafion� was used as GOx

binder and proton-conducting medium. The TP/MWCNT/PEI/GOx/Nafion� modified electrode operates at 25 �C in 0.02 M phosphatebuffer solution (pH 6.9) containing 0.1 M KCl in the presence of 20 mM glucose. The open circuit potential of GOx anode was between�0.38 V and �0.4 V vs. Ag/AgCl, which is closer to the redox potential of the FAD/FADH2 cofactor in the enzyme itself. The GOx

electrode has a potential to work in vivo by using endogenous substances, such as glucose and oxygen. Such a glucose anode allowsfor the development of a new generation of miniaturized membrane-less biofuel cells.� 2006 Elsevier B.V. All rights reserved.

Keywords: Glucose oxidase; Carbon nanotubes; Biofuel cell; Direct electron transfer

1. Introduction

Advances in nanoscience and nanotechnology have cre-ated a new direction for design and development of nextgeneration electronic micro- and nanoscale materials anddevices for industrial, pharmaceutical, clinical, environ-mental, space exploration, and defense applications [1–6].To maintain operation of miniaturized electronic devicesduring long periods of time under natural conditions,new small power sources are required [5,7,8]. In this con-text, miniature biofuel cells might be an excellent alterna-tive energy supply sources for microelectronic devices

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* Corresponding author. Tel.: +1 505 277 2563; fax: +1 505 277 5433.E-mail address: [email protected] (D. Ivnitski).

[5,7–9]. Biofuel cells have traditionally been classified aseither microbial-based and enzymatic fuel cells accordingto whether the enzymes were located inside of microorgan-isms or outside of living cells. The main benefits of enzyme-based biofuel cells are the capability to produce biofuelcells orders of magnitude smaller than equivalently pow-ered microbial cells and allowing operation to take placecloser to the redox potential of the enzyme itself. The min-iaturized cell may be an essential element of independentpower-on-a-chip or microelectromechanical based systemsalong with drug-delivery and autonomous health monitor-ing systems [5,8,10]. For example, during long-durationspace flights the complexities of biosensor/biofuel healthmonitoring systems need to be imbedded in a blood vesselto monitor the astronaut’s condition.

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D. Ivnitski et al. / Electrochemistry Communications 8 (2006) 1204–1210 1205

For an efficient operation of an enzyme-based biofuelcell a number of conditions must be satisfied. First, theenzyme should have high catalytic activity, stability, andbe inexpensive. Second, the redox potential of the anodemust be as negative as possible to give the maximum poten-tial difference between the anode and cathode. Third, theprocess of bioelectrocatalysis requires a developed media-tion and enzyme immobilization methodology for continu-ous and efficient electron transfer from the enzyme to theelectrode surface. In this respect, glucose oxidase (GOx)may be the best enzyme for the biofuel cells. GOx is stable,highly catalytically active, inexpensive, and glucose as afuel is wide spread in the biological environment [11]. Inaqueous solution at pH 7.0, the redox potential for theactive site of the enzyme FAD/FADH2 is sufficiently nega-tive for anode operation [5,12]. At the same time, currentpower generation and lifetime of glucose-oxidase-basedfuel cells mediated by low molecular weight agents stillremain relatively low. Most of the loss in power comesfrom the slow decomposition and loss of the redox media-tors from the glucose oxidase electrode, as well as the dena-turation of the enzyme itself. In addition, application of theredox mediators shifts the redox potential of the GOx

anode to a more positive potential range than the redoxpotential of the FAD/FADH2 system.

Direct electron transfer (DET) of electrons between theactive site of the enzyme and the electrode surface is attrac-tive due to simplicity. It has an excellent potential for min-iaturization and high power-output [13,14]. The enzymeelectrode based on direct electron-transfer approach canwork in a potential range close to the redox potential ofthe enzyme itself. However, several critical challenges mustbe overcome for the successful application of the DETapproach for the biofuel cells. A major challenge is theFAD/FADH2 redox center of GOx is located deeply inthe apoenzyme [11,15,16]. The depth of the redox centerfor GOx is about 13 A, so the electron-transfer ratebetween the active site of glucose oxidase and the electrodesurface is usually slow. Last year numerous efforts havebeen made to reduce the electron tunneling distance byusing different promoters [3,5,7–10,13–17]. One of thestrategies is to integrate carbon nanotubes inside theenzyme so that the distance from the redox centre isdecreased [12,15,16,18]. Carbon nanotubes, which havesmall size, excellent chemical stability and a range of elec-trical conductivity, have become ideal conducting nano-wires for fast DET between the active site of an enzymeand the electrode surface. Recent studies in the field ofmulti-walled carbon nanotubes (MWNTs) modified elec-trodes demonstrated new perspectives for the nanodeviceconstructions [19–21]. The MWNTs have a fast electron-transfer rate for different redox reactions and better electri-cal conductivity compared to single-walled carbon nano-tubes. The unique properties of MWNTs as an electrodematerial include nanometer scale and well-graphitizedstructure, which can be used to construct three-dimensionalnanoelectrode ensembles [19].

In this report, for the first time, we have exploitedunique properties of multi-walled carbon nanotubes forthe fabrication of a GOx electrode as an anode for biofuelcells based on direct mediatorless electron transfer betweenthe active site of the enzyme and the electrode surface.Toray carbon paper with a highly porous three-dimen-sional network was used as a matrix for growth ofMWNTs and GOx immobilization. The tetrabutylammo-nium bromide salt-treated Nafion� was used as a GOx bin-der and proton-conducting medium.

2. Experimental

2.1. Materials

Glucose oxidase from Asper�gillus niger (EC1.1.3.4, G-7141, Sigma), polyethilenimine (P-3143, Sigma), Toray car-bon paper TGPH-060, from E-TEK, NJ USA. Tetrabuty-lammonium bromide salt-treated Nafion� was obtainedfrom Professor Shelley Minteer from Saint Louis Univer-sity. Other chemicals of analytical grade were obtainedfrom standard sources. All solutions were prepared withdeionized water passed through a purification system.

2.2. The MWCNT electrode preparation

Toray carbon paper (TP) is a synthetic lightweightmatrix with pore spaces on the micro-meter scale. Thepaper is modified with the growth of MWNTs by chemicalvapor deposition (CVD) from an acetylene precursor at anelevated temperature of 700 �C for 1 h. This process is pre-ceded by electrodeposition of cobalt nanoparticles as a cat-alyst for carbon nanotube (CNT) growth in a 5 wt%Co(CH3COO)2 and 2 wt% H3BO3 solution at a depositionpotential of �1.2 V vs. Ag/AgCl reference electrode atroom temperature [21]. Since the MWNTs are grown byusing Co nanoparticles as a template, the cobalt seeds werecompletely blocked between the Toray carbon paper andthe vertically grown aligned multi-walled carbon nano-tubes. The cyclic voltammograms of TP-MWNT electrodeshave shown no evidence of Co oxidation. They demon-strated capacitance current only from �0.6 V to +0.6 Vvs. Ag/AgCl.

2.3. GOx electrode preparation

Glucose oxidase was immobilized on the Toray� carbonpaper modified with MWNTs based on the following tech-nique; the carbon paper with incorporated MWNTs werecut into 4 mm diameter disks. The geometric area of eachelectrode was 0.1 cm2. The deposition of glucose oxidasemolecules (GOx) on the TP/MWNTs surface was carriedout by entrapment of the enzyme within polyethyleneimine(PEI) film. The positive charged PEI was used as a binderbetween the negatively charged GOx enzymes and the TP/MWNTs electrode surface. 2 mg GOx was mixed with

Fig. 1. Toray carbon paper (a) with CVD deposited multi-walled CNT (b)and the final glucose electrode structure after PEI coating and glucoseoxidase immobilization in modified Nafion matrix (c).

1206 D. Ivnitski et al. / Electrochemistry Communications 8 (2006) 1204–1210

0.05 ml of 0.5% PEI solution prepared in 20 mM phos-phate buffer, pH 6.9. A 4 ll aliquot of GOx-PEI solutionwas cast on both sides of the TP/MWNTs surface andthe TP/MWNTs/PEI/GOx disks where then dried at roomtemperature for 4 h. Then, 4 ll of 0.5% Tetrabutylammo-nium bromide salt-treated Nafion solution was cast onTP/MWNTs/PEI/GOx surface and stored at room temper-ature before use. Finally, a TP/MWNTs/PEI/GOx/Naf-ion� disk (B 4 mm) was inserted into a Teflon tube witha sealing platinum wire connector and this was used as aGOx working electrode. The tetrabutylammonium bromidesalt-treated Nafion was used as an ion conductor with aless acidic environment and as an electrochemistry promot-ing polymeric binder [22,23]. The tetrabutylammoniumbromide salt-treated Nafion� film is more biocompatiblewith immobilized GOx and it provides strong and stableadhesion of multi-walled carbon nanotubes and enzymemolecules on the TP surface.

The surface morphology of the TP/MWNTs/PEI/GOx/Nafion� disks was investigated by a Hitachi (S-5200) scan-ning electron microscope equipped with a PCT spectrome-ter. The microscope was operated at 10 kV for imaging.

The surface concentration of GOx was calculated fromthe following equation [24]:

C ¼ Q=nFA;

where C is the surface concentration of GOx, Q is thecharge obtained from integration of the anodic peak, n isthe number of electrons per GOx molecule, F is Faradayconstant, and A is the electrode surface area. The electro-chemical accessible surface area was calculated using thecapacitance of the electrode obtained from cyclic voltam-metry and the specific capacitance for carbonaceous mate-rial to be 20 lF/cm2. The coulometric assay has shown thatthe surface concentration of the glucose oxidase is�0.7 · 10�12 mol/cm2.

2.4. Electrochemical measurements

Electrochemical measurements were performed withPotentiostat/Galvanostat (PAR Model 263A) in a conven-tional three-electrode cell of volume 8 ml, which includedTP/MWNTs/PEI/GOx/Nafion� working, platinum netcounter and Ag/AgCl reference electrodes. At the start ofthe cyclic voltammetry experiments nitrogen was bubbledthrough the solution for 20 min. The cyclic voltammetrywas used to demonstrate direct electron transfer. Mediatedelectron transfer was performed to validate overall cata-lytic activity of the immobilized enzyme. Galvanostaticmeasurements were performed starting at open circuitunder constant stirring. To imitate load, different resis-tances were switched into the external circuit. The workingsolution was 20 mM phosphate buffer containing 0.1 MKCl. All electrochemical experiments were carried out at20 ± 0.5 �C. Data of cyclic voltammogram were used tocalculate the electron transfer rate constant using themethod of Laviron [25].

3. Results and discussion

Fig. 1 shows scanning electron microscopy (SEM)images of the Toray� carbon paper (TP) without (a) andwith multi-walled carbon nanotubes on the surface (b).The length of the individual CNT filaments and the spacingbetween them is at the nanometer scale. The density of thecarbon nanotubes is proportional to the amount of cobaltdeposited and the volume fraction is proportional to thetime of CVD. Control of the cobalt deposition and the time

Fig. 3. Cyclic voltammograms of the TP/MWNTs/PEI, GOx, Nafionelectrode in 20 mM phosphate buffer, pH 6.0, 0.1 M KCl at various scanrates, mV/s: (1) 20; (2) 40; (3) 80; (4) 100. The inset shows the oxidationand reduction peak currents (ipc) versus scan rate.


of carbon vapor deposition allow building surface architec-tures with distinct levels of hierarchical organization ofpore structures. Combination of Toray carbon paper withcarbon nanotubes has resulted in a highly porous three-dimensional network with dramatically increasing elec-trode surface area from the original Toray paper 2 m2/gto two orders of magnitude and provides efficient electro-chemical power generation.

The electrochemical characteristics of the TP/MWNTs/GOx, Nafion-modified electrodes were investigated by cyc-lic voltammetry and potentiometric method. The key issuewas to clarify if glucose oxidase immobilized on TP/MWNTs surface can undergo DET with the electrodeand, at the same time, be able to retain its catalytic proper-ties. First, the DET between the active sites of GOx andelectrode was investigated under conditions where there isno glucose present (no catalytic turnover). The cyclic vol-tammograms of GOx electrodes (Fig. 2, curves 2 and 3)have shown that a pair of well-defined redox peaks (reduc-tion and oxidation) appears when glucose oxidase immobi-lized on the TP electrodes modified with multi-walledcarbon nanotubes. The redox peaks can be considered asa result of the redox reaction of the active site of GOx

immobilized on the surface of the TP/MWCNTs electrode.Controlled experiments show that in the absence of theMWNTs there are no peaks observed with TP/GOx elec-trodes (Fig. 2, curve 1). This means that electrical contactbetween the redox center of GOx and Toray carbon paperis provided through the multi-walled carbon nanotubeslocated on the electrode surface. Fig. 3 shows the effectof different scan rates on the anodic and cathodic peak cur-rents. The peak currents (ipc) versus scan rate are shown inthe inset (Fig. 3). We found that there is linear proportion-ality between the anodic and cathodic peak currents at ascan rate ranging from 40 mV/s to 260 mV/s. This a typical

Fig. 2. Cyclic voltammograms of GOx modified electrodes: (1) TP/GOx;(2) TP/MWNTs/GOx; (3) TP/MWNT/PEI/GOx. Scan rate 10 mV/s.

characteristic of thin layer electrochemical behavior, sug-gesting that the reaction is not a diffusion-controlled pro-cess and the redox peaks are from the surface boundprosthetic group FAD of the GOx. The peak separationsbetween 24 mV and 50 mV at scan rate of 10–80 mV/s indi-cate that the heterogeneous pseudo reversible electron-transfer process was fast. With increasing pH solutionsthe potentials of anodic and cathodic peaks and formalpotential are shifted to the negative direction. The formalpotentials were �372 mV at pH 6.0 and �460 mV vs.Ag/AgCl at pH 7.9. The electron-transfer rate constantwas 2.3 s�1, which is close to that reported [12,26] of nano-tube modified electrodes and significantly greater than themagnitude reported [27] of self-assembled monolayer elec-trodes. Other workers report that an electron-transfer rateconstant above 0.1 s�1 indicates that the distance betweenthe active site of GOx and the electrode surface is signifi-cantly less than 13 A [18,26]. The reason for this mightbe the location of carbon nanotubes close to the active siteof the enzyme. Thus, the redox peaks in Figs. 2 and 3 canbe considered as a result of the redox reaction of the pros-thetic (FAD/FADH2) group bound to the apoenzyme.

The fact that glucose oxidase is able to simultaneouslyundergo DET with the electrode and to retain its catalyticactivity has been confirmed by the study of bioelectrocata-lytic activity of the GOx immobilized on the surface of TP/MWNTs modified electrodes in the absence and in the pres-ence of glucose. As demonstrated in Fig. 4, the TP/MWNTs/PEI/GOx/Nafion� modified electrode shows agood catalytic current toward glucose in 20 mM phosphatebuffer solution (pH 6.9) containing 1 mM hydroquinone asan electron-transfer mediator. A large electrocatalytic ano-dic current is observed in the presence of glucose, whichindicates a biocatalytical activity of glucose oxidase. Thisstatement is also supported by potentiometric data. Fig. 5

Fig. 4. Cyclic voltammograms of TP/MWNTs/PEI, GOx, Nafion-modified electrode in 20 mM phosphate buffer, pH 6.9, 0.1 M KCl containing 2 mMHQ: (1) in the absence of glucose and (2) with 20 mM glucose. Scan rate: 50 mV/s. 20 �C.

Fig. 5. Potentiometric responses of the TP/MWNTs/PEI, GOx, Nafionelectrode to successive additions of 0.3 mM glucose (a) and dependence ofpotentials of: (1) TP/PEI, GOx, Nafion (1) and (2) TP/MWNTs/PEI,GOx, Nafion electrodes as a function of glucose concentrations (b).Potentials were measured vs. Ag/AgCl at pH 6.9 and 20 �C.


shows the dependence of the potential of the TP/MWNT/PEI/GOx/Nafion� electrode as a function of glucoseconcentration. Redox potentials are negatively shiftedimmediately in the presence of glucose in the 20 mM phos-phate solution, pH 6.9, 0.1 M KCl. At concentrations ofglucose 1.5 mM and higher the potential reaches stable neg-

ative magnitude between �380 mV and �400 mV vs. Ag/AgCl. There are no potential changes observed with TP/PEI/GOx electrodes without carbon nanotubes (Fig. 5,curve 1b). Thus, results presented in Figs. 2–5 exhibit thatGOx immobilized on the TP/MWNTs surface demonstratesnot only direct electron transfer but also biocatalytic activ-ity to glucose. Secondly, MWCNTs play an active role con-cerning direct electron transfer between the active site ofGOx and the electrode surface. During immobilization ofglucose oxidase on the TP/MWNT surface, the CNTs areprobably located close to the active site of the enzyme facil-itating DET. Another factor, which might be critical indecreasing the electron-transfer barrier, is partial denatur-ation of GOx molecules (molecules partially unfolded) dur-ing immobilization. Finally, it is important that the TP/MWNTs/PEI/GOx/Nafion� modified electrode can oper-ate with redox potentials close to the FAD/FADH2 redoxsystem at room temperature and pH 7.0.

The application of the TP/MWNTs/PEI/GOx/Nafion�

modified electrode as an anode for the biofuel cell has beendemonstrated during GOx electrode testing in galvano-static regime (Fig. 6). Analysis has shown that the catalyticelectrooxidation current of glucose appears at �380 mVwith a current density of 0.01 mA/cm2 and reaches0.4 mA/cm2 at �120 mV vs. Ag/AgCl. Current densitywas calculated versus geometric electrode area, giving0.1 cm2. The open circuit potential (�400 mV) of the glu-cose oxidase modified electrode is close to the redox poten-tial of the FAD/FADH2 cofactor in the enzyme itself.Thus, new GOx electrodes based on direct mediatorlesselectron transfer between the active site of the enzymeand multi-walled carbon nanotubes offer promising solu-tions for generations of new classes of miniaturizes mem-brane-less biofuel cells.

Fig. 6. Polarization curves of three TP/MWNTs/PEI, GOx, Nafionelectrodes. 20 mM phosphate buffer, pH 6.9, 0.1 M KCl, 20 mM glucose,20 �C. 0.1 cm2 electrodes.


4. Conclusions

This paper introduces a biofuel cell anode design based onDET oxidation of the substrate by GOx. We have demon-strated successful application of a glucose oxidase electrodeas an anode for the biofuel cell based on DET between GOx

and a composite surface architecture comprised of MWCNTgrowth on a porous carbonaceous matrix. The open circuitpotential of GOx anode was between �380 mV and�400 mV vs. Ag/AgCl, which are close to the redox potentialof the FAD/FADH2 cofactor in the enzyme itself. The GOx

electrode was working in 20 mM phosphate buffer at pH 6.9,0.1 M KCl in the presence 20 mM glucose in ambient condi-tions. The GOx electrode based on direct electron transferprovides an excellent opportunity to simplify and miniatur-ize membrane-less biofuel cells. It can work in biologicalmedium by using endogenous substances, such as glucoseand oxygen only. The combination of MWCNTs withGOx has a potential to create reliable, miniaturized, andmediatorless glucose oxidase electrodes with high powerfor the biofuel cells.

For development effective and reliable glucose oxidaseanode for biofuel cell it is important to understand themechanism of direct electron transfer between GOx andelectrode surface. Many papers related to this subject indi-cate that application of nanometer scale and well-graphi-tized structure of CNT particles facilitates DET betweenactive site of GOx and electrode surface. It can be hypoth-esized that the highly porous three-dimensional carbonnanotube network on the microelectrode surface has prob-ably working as ‘‘nanowire’’ around enzyme molecule andpromotes the electron transfer between enzyme and elec-trode surface.

The work now in progress is aimed at detail character-ization of the interfacial electron-transfer rates, analysisof biocatalytic rate constants and understanding the mech-anism of the direct electron transfer between GOx and elec-trode surface. Our research will be also directed at

optimization such parameters as reproducibility and lifetime of the GOx electrode.

Acknowledgement

The authors wish to thank the Defense AdvancedResearch Project Agency for financial support.

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Glucose oxidase anode for biofuel cell based on direct electron transfer

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