DOI: 10.1002/adma.200600028

Carbon-Nanotube-Based Glucose/O2 Biofuel Cells**

By Yiming Yan, Wei Zheng, Lei Su, and Lanqun Mao*

Biofuel cells (BFCs) utilize biocatalysts such as enzymesand microorganisms for the conversion of chemical energyinto electrical energy.[1–4] These BFCs represent a new kind ofenergy-conversion technology that is distinct from conven-tional fuel cells, such as H2/O2 and methanol/O2 fuel cells,mainly in that they can operate under moderate conditions,such as in mild media and at ambient temperatures. More-over, compared with the noble-metal catalysts used in conven-tional fuel cells, the biocatalysts used in the BFCs are more ef-ficient and selective toward the biomass. More remarkably,the biomass consumed by the BFCs, such as glucose and oxy-gen, is generally endogenous to biological systems. As such,BFCs are envisaged to be able to power the bioelectronicsin vivo, finding uses in systems such as implantable biosensorsor pacemakers in the human body.[5–8] These striking proper-ties and the potential applications of BFCs have evoked inten-sive interest in the basic study and development of BFCs inrecent years.[9–14]

It is known that carbon nanotubes (CNTs), a new kind ofcarbon-based nanomaterial, possess unique structural andelectronic properties, and are finding striking applications invarious research and industrial fields,[15–21] including electro-chemistry.[22–24] In addition to their excellent electrochemicalproperties, CNTs have several characteristics that make themvery suitable for the development of enzymatic BFCs. For ex-ample, CNTs bear graphene sidewalls that are chemically in-ert and highly hydrophobic, with a dense p–p stacking. Such aproperty essentially makes CNTs well suited as a support forthe redox mediators[25–27] generally employed for shuttling theelectron transfer of biocatalysts, for example enzymes andproteins, or for the conversion and oxidation of the NADH(nicotinamide adenine dinucleotide with hydrogen) cofactorwhen dehydrogenases are used as the anode biocatalysts.Moreover, as demonstrated recently,[28–30] the use of CNTscould largely facilitate the direct electron transfer of the en-zymes and proteins. On the other hand, CNTs have a goodconductivity (depending on the sort of CNTs used) and a highsurface area to weight ratio (ca. 300 m2 g–1)[31] as well as the

ability to form a 3D matrix that can be used for both enzymeimmobilization and electrode reactions.

In this Communication we demonstrate the first single-walled carbon nanotube (SWNT)-based glucose/O2 biofuelcell with glucose dehydrogenase (GDH) as the anode biocata-lyst for the oxidation of glucose, with NAD+ as the cofactorand laccase (from Trametes versicolor) as the cathode biocata-lyst for O2 reduction (Scheme 1). On the SWNT anode, meth-ylene blue (MB) was adsorbed through the interactions be-tween MB and SWNTs, as described in our earlier work.[25]

Although the formed MB–SWNT adsorptive adduct exhibitsexcellent electrochemical properties and a good stability, its

activity for redox-mediating the oxidation of NADH wasfound to be not as high as expected. This could be simply veri-fied from the potentials for the oxidation of NADH at differ-ent kinds of electrodes as shown in Figure S1 (SupportingInformation). The potential for NADH oxidation at theMB–SWNT adduct (Fig. S1D) is more negative than that forthe other kinds of carbon materials used (glassy carbon (GC,Fig. S1A), graphite (Fig. S1B), and SWNTs alone (Fig. S1C)),but is still more positive than the redox potential of the MB–SWNT adduct itself (Fig. S1D). This phenomenon is consis-tent with previous reports where other kinds of redox dyesare adsorbed onto the CNTs.[32]

To further accelerate the oxidation of NADH and therebylower the overpotential involved in the bioelectrocatalyticoxidation of glucose, the MB–SWNT adduct was electropoly-

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–[*] Prof. L. Mao, Y. Yan, W. Zheng, Dr. L. Su

Center for Molecular ScienceInstitute of Chemistry, Chinese Academy of SciencesBeijing 100080 (P.R. China)E-mail: lqmao@iccas.ac.cn

[**] We thank the financial support by NSF of China (20375043,20435030, and 20575071), Chinese Academy of Sciences (KJCX2-SW-H06), and State Key Laboratory of Electroanalytical Chemistry,Changchun Institute of Applied Chemistry. Supporting Informationis available online from Wiley InterScience or from the author.

E1= glucose dehydrogenase E2= Laccase

Resistance

polyMB

NAD+

NADHGluconolacton

Glucose

e

E1

H2O

e

E2

O2

e e

• •

Scheme 1. Schematic illustration of the working principle of the SWNT-based glucose/O2 biofuel cell. At the GDH/polyMB–SWNT bioanode,glucose is oxidized to gluconolacton by NAD+-dependant GDH support-ed onto the polyMB–SWNT. At the laccase-SWNT biocathode, O2 is re-duced to water under the bioelectrocatalysis of laccase supported ontothe SWNTs.

merized under a constant potential of + 0.80 V to generate apolyMB–SWNT nanocomposite that is anticipated to possessa higher activity for mediating the oxidation of NADH thanthe MB–SWNT adduct. Figure 1A depicts cyclic voltammo-grams (CVs) showing the electropolymerization process ofthe MB–SWNT adduct. The peak currents at – 0.27 V ob-tained for the MB–SWNT adduct were clearly decreased, anda pair of new redox peaks at – 0.10 V appeared, with the peakcurrents increasing with the time employed for the electropo-lymerization, suggesting the formation of the polyMB–SWNTnanocomposite. The nanocomposite formation through elec-tropolymerization was also verified using Fourier transformIR (FTIR) spectroscopy as displayed in Figure 1B. The stableadsorption of MB onto the SWNTs to form the MB–SWNTadduct was evident from the spectra of MB and MB–SWNT(Fig. 1B). After being polarized at + 0.80 V in the phosphatesolution, the formed nanocomposite exhibited a spectrum dis-tinct from that of the MB–SWNT (Fig. 1B). For example, theband at 1710 cm–1 for the ring stretch of the adsorbed MB

molecules disappeared, while a new band appeared at1017 cm–1, which could be assigned to the bending vibrationof N–CH3

[33–35] after the MB–SWNT adduct was polymerizedelectrochemically.

We next studied the oxidation of NADH at the formedpolyMB–SWNT nanocomposite (Fig. S2C) and found thatsuch an oxidation process at the polyMB films appears to bedependent on the substrate employed for confining thepolyMB films. The polyMB films confined onto SWNTs ex-hibit the best activity for redox-mediating the oxidation ofNADH, compared with those confined onto GC and graphite.This was rationalized from the low potential (– 0.10 V) for theoxidation of NADH at the polyMB film confined ontoSWNTs (the polyMB–SWNT nanocomposite), which is morenegative than those at the polyMB films confined onto GC(Fig. S2A) and graphite (Fig. S2B), and is close to the redoxpotential of the polyMB–SWNT nanocomposite. This low-po-tential oxidation of NADH to NAD+ was further exploited todevelop a GDH-immobilized polyMB–SWNT bioanode (de-noted as GDH/polyMB–SWNT) for the oxidation of glucose,as depicted in Figure 2A, with a bioelectrocatalytic mecha-nism as shown in Scheme 1. The oxidation of glucose to gluco-

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−10

0

10

I (µ

A)

E (V) vs. Ag/AgCl

A

500 1000 1500 2000

×5

×10

×20

Wavenumber (cm-1

)

Tra

nsm

itta

nce

polyMB-SWNT

MB-SWNT

SWNT

MB

B

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45

Figure 1. A) CV showing the electropolymerization process of MB sup-ported onto SWNTs. For the electropolymerization, the MB–SWNT ad-duct confined onto a GC electrode was polarized at + 0.80 V in 0.10 M

phosphate solution (pH 6.0). The CVs were recorded after the MB–SWNTadduct was electropolymerized for different times (10, 15, 20, and45 min, as indicated in the Figure). Scan rate: 20 mVs–1. I: current, E: cellpotential. B) Fourier transform IR (FTIR) spectra of MB, SWNT, MB–SWNT, and polyMB–SWNT.

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I (µ

A)

A

−0.2 0 0.20

100

200

300

E (V) vs. Ag/AgCl

j (µ

A/c

m2)

1

2

3 B

Figure 2. A) CVs of the GDH/polyMB–SWNT bioanode in 0.10 M phos-phate solution containing 10 mM NAD+ in the absence (dotted line) andpresence (solid line) of 30 mM glucose. Scan rate: 20 mVs–1. B) Polariza-tion curves of the bioanode. Glucose concentrations: 15 (1), 45 (2), and90 mM (3) in quiescent phosphate solution (0.10 M, pH 6.0, 20 °C) con-taining 10 mM NAD+. Scan rate: 1 mVs–1. j: current density.

nolacton occurs at a low potential of – 0.10 V, which is close tothat for the oxidation of NADH (Fig. S2C). Figure 2B dis-plays polarization curves for the oxidation of glucose of differ-ent concentrations at the GDH/polyMB–SWNT bioanode.The oxidation of glucose was observed at – 0.15 V versusAg/AgCl and reached its 330 lA cm–2 plateau at 0.10 V for90 mM glucose.

Laccase, an enzyme specific for catalyzing a four-electronreduction of O2, has been used previously as the cathode bio-catalyst in enzymatic BFCs in which the electron communica-tion between laccase and the cathode is shuttled in most casesby redox mediators.[36–39] In contrast to these cases, the directelectron transfer of laccase in this study was successfullyachieved by using SWNTs. Figure 3B depicts differentialpulse voltammograms (DPVs) obtained at the laccase-immo-

bilized SWNT (laccase-SWNT) biocathode in the phosphatesolution (pH 6.0) under anaerobic conditions. By comparingthe DPVs obtained at SWNTs without laccase (Fig. 3A) andthe laccase-SWNT biocathode (Fig. 3B), one may find thatthe laccase-SWNT biocathode exhibits a pair of redox peaksascribed to the direct electrochemistry of Trametes versicolorlaccase. The formal potential was 0.58 V versus Ag/AgCl(0.78 V vs. the normal hydrogen electrode (NHE)), which wasidentical to the redox potential of the Cu ion at the type 1 Cusite of laccase (0.78 V vs. NHE).[40,41] As mentioned above,this direct electron communication between the laccase andthe SWNTs is expected to be important for the developmentof the biocathode because of the low overpotential involvedin the O2 reduction. As expected, the O2 reduction at the lac-case-SWNT biocathode in the phosphate solution (pH 6.0)was observed at 0.60 V versus Ag/AgCl (0.80 V vs. NHE;Fig. S3), a value close to the equilibrium value of E′O2/H2O

(0.87 V vs. NHE), suggesting a low overpotential is requiredfor O2 reduction at the prepared laccase-SWNT biocathode.The potential for O2 reduction obtained here is more positivethan that obtained with laccase as the biocatalyst when theelectron transfer of laccase was shuttled by mediators.[42,43] Asseen in Figure 3C, the catalytic reduction of O2 was observedat 0.60 V versus Ag/AgCl, reaching its 18 lA cm–2 plateau atca. 0.48 V under ambient air.

We studied the crossover between the bioanode and bio-cathode in the prepared SWNT-based glucose/O2 BFC andfound that the BFC suffers from a small amount of crossover,as revealed with the experiments on the oxidation of glucoseat the biocathode and on the O2 reduction at the bioanode.This property essentially makes it possible to assemble a com-partmentless glucose/O2 BFC in this study.

The power density of the as-assembled glucose/O2 BFC wasfound to be dependent on the solution pH, as shown in Fig-ure S4. The power density of the BFC is near its maximum be-tween pH 4.5 and pH 6 and is almost independent of pH inthe same range. However, the power decreases sharply whenthe pH is increased, and the cell loses its power completely atpH 8.0 because of loss of activity of laccase and GDH in sucha solution. Figure 4A displays the polarization curve and therelationship between the power density and the current den-sity of the assembled SWNT-based glucose/O2 BFC in phos-phate solution at pH 6.0. The open-circuit voltage of the BFCwas 0.80 V and the power density was 9.5 lW cm–2 at 0.52 V.When the cell operated continuously for one week in quies-cent phosphate solution (0.10 M, pH 6.0, 20 °C) containing10 mM NAD+ and 30 mM glucose under ambient air, it lostca. 20 % of its power in the first 24 h and then ca. 5 % per day(Fig. 4B). The power density of the assembled SWNT-basedglucose/O2 BFC, which is dominated by that of the bio-cathode, is comparable to most kinds of enzymatic BFCs re-ported thus far under the same conditions of solution pHand temperature,[44–48] yet is lower than that reported by Hel-ler and co-workers who used carbon fiber microelec-trodes.[38,49–51] The lower power density of the SWNT-basedBFC demonstrated in this study could be attributed to the

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−20

0

E (V) vs. Ag/AgCl

j (µ

A/c

m2)

1

2

0.2 0.4 0.6 0.8

B

2 µA

0.2 0.4 0.6 0.8

A

2 µA

C

Figure 3. DPV of A) the SWNT and B) the laccase-SWNT in 0.10 M N2-saturated phosphate solution (pH 6.0). Differential pulse voltammetryconditions: amplitude 0.05 V; pulse width 0.05 s; pulse period 0.2 s.C) Polarization curves of the biocathode for O2 reduction under ambientair (2) and saturated O2 (1) in quiescent phosphate solution (0.10 M,pH 6.0, 20 °C). Scan rate: 0.5 mVs–1.

slower mass transport at the planar electrodes used in thepresent case compared with that at the carbon fiber micro-electrodes, and possibly to the slower rate of the direct elec-tron transfer of laccase compared with the transfer shuttledby redox mediators such as electron-conducting redox poly-mers. We are currently working to find a reliable way to im-prove the power output of the SWNT-based enzymatic BFCs.

In summary, we have demonstrated a compartmentless glu-cose/O2 biofuel cell with the use of SWNTs. The use of theSWNTs has been found to offer a straightforward and effec-tive route to enzymatic BFCs with a high voltage. Althoughthe improvement of the power density of the assembledSWNT-based glucose/O2 BFC is still under way, the strategydemonstrated here is believed to be generally applicable forthe development of other kinds of enzymatic BFCs, and couldprove to be a new and facile route to nanostructured enzy-matic BFCs.

Experimental

SWNTs with an average diameter less than 2 nm and a length ofabout 50 lm were purchased from Shenzhen Nanoport (Shenzhen,

China). The SWNTs were purified by refluxing the as-receivedSWNTs in 2.6 M HNO3 for 10 h. GC electrodes (3 mm diameter,Bioanalytical Systems), used as the substrate electrode, were first pol-ished with emery paper (# 2000) and then with 0.3 and 0.05 lm alumi-na slurry on a polishing cloth, cleaned under bath sonication for10 min, and thoroughly rinsed with distilled water. SWNTs were dis-persed into N,N-dimethylformamide to give a 1 mg mL–1 suspension.4 lL of the prepared suspension was coated onto a GC electrode toobtain the SWNT electrode. After being air-dried, the SWNT elec-trode was immersed into 0.1 mM aqueous MB solution for 3 h. Theelectrode (denoted as MB–SWNT electrode) was thoroughly rinsedwith water to remove the nonadsorbed MB. The SWNT–MB elec-trode was then polarized at + 0.80 V versus Ag/AgCl in a 0.10 M phos-phate solution (pH 6.0) for 1 h for the electropolymerization of MB toform a polyMB–SWNT nanocomposite. 1 % bovine serum albumin(Sigma) was mixed with GDH (E.C. 1.1.1.47, initial activity of216 U mg–1, Sigma) with a volume ratio of 1:1, and 8 lL of the result-ing mixture was coated onto the polyMB–SWNT electrode. Theenzyme coated on the electrode was then crosslinked with 4 lL of40 mM glutaraldehyde and rinsed with distilled water. This electrode(GDH/polyMB–SWNT) was used as the bioanode of the glucose/O2

BFC. The biocathode of the BFC was prepared by directly coating2 lL of the purified laccase [29] (E.C. 1.10.3.2, from Trametes versico-lor, initial activity: 23.76 U mg–1) on the SWNT electrode and dryingthe electrode under ambient temperature for about 30 min.

Cyclic voltammetry, differential pulse voltammetry, and cell polar-ization were performed using a computer-controlled electrochemicalanalyzer (CHI 660A, Austin, USA). A platinum spiral wire was usedas the counter electrode and all potentials were referred to a KCl-sat-urated Ag/AgCl electrode. A 0.10 M phosphate solution (pH 6.0) wasused as the supporting electrolyte. For assembling a glucose/O2 BFC,the prepared bioanode and the biocathode were immersed into a1 mL vessel containing 30 mM glucose and 10 mM NAD+ in 0.10 M

phosphate solution (pH 6.0) under ambient air. FTIR (Tensor 27 In-frared Spectrometer, Bruker, Germany) was used for characterizationof the MB–SWNT and the polyMB–SWNT. The sample of theMB–SWNT used for the FTIR measurements was prepared using amethod reported in our earlier work [25]. A polyMB–SWNT samplewas collected from the products of the MB–SWNT confined onto in-dium tin oxide (ITO) after electropolymerization at 1.2 V for 3 h.The samples were pressed into KBr pellets for the FTIR measure-ments.

Received: January 5, 2006Final version: May 19, 2006

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ll (

µW

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jcell (µA/cm2)

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Figure 4. A) Polarization curve (�) of the assembled SWNT-based com-partmentless glucose/O2 biofuel cell (Scheme 1) and the dependence ofthe power output (�) on the current density in quiescent phosphatebuffer (0.10 M, pH 6.0, 20 °C) containing 10 mM NAD+ and 30 mM glu-cose under ambient air. B) Stability of the assembled biofuel cell. The ex-ternal load in the test was 1 MX. P is the power measured as a functionof time, and Pmax is the maximum power value.

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