Sensors and Actuators B 113 (2006) 617–622

Carbon nanotube-epoxy composites forelectrochemical sensing

Martin Pumera, Arben Merkoc¸i ∗, Salvador AlegretGrup de Sensors i Biosensors, Departament de Quımica, Universitat Autonoma de Barcelona,

E-08193 Bellaterra, Barcelona, Catalonia, Spain

Available online 10 August 2005

Abstract

Rigid and conductive carbon nanotube-epoxy composite (CNTEC) electrodes were constructed from two kinds of multiwall carbon nan-otubes differing in the length (0.5–2 and 0.5–200�m) mixed with epoxy resin. The electrochemical behavior of CNTEC electrodes wascharacterized by using cyclic voltammetry of ferricyanide, NADH and hydrogen peroxide. The behavior of CNTEC electrodes prepared withdifferent percentages of CNT has been compared with that of graphite-epoxy composite (GEC) electrode. It was found that long-carbonnanotube (0.5–200�m) based epoxy composite electrodes show strong electrocatalytic activity towards NADH and hydrogen peroxide whiles e for theb red withG on nanotubec©

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hort-carbon nanotube (0.5–2�m) based epoxy composites show similar oxidation potential as graphite-epoxy composite electrodoth NADH and H2O2. In all cases, CNTEC electrodes provide better reversibility, peak shape, sensitivity and stability compaEC electrode. The obtained experimental results demonstrate remarkable electrochemical and mechanical advantages of carbomposites compared to graphite composites for sensor applications.2005 Published by Elsevier B.V.

eywords: Carbon nanotubes; NADH; Hydrogen peroxide; Composite electrode

. Introduction

Carbon nanotubes (CNT) received considerable attentions a new class of nanomaterials since their discovery in 1991

1,2]. They show unique mechanical, chemical and elec-ronic properties[3,4] which led to a variety of applications,.e. scanning probes[5], nanoelectronic and memory storageevices[6,7] or field emitters[8].

Distinctive properties of CNT, such as a high surface area,bility to accumulate analyte, minimization of surface foulingnd electrocatalytic activity are very attractive for electro-hemical sensing[9,10]. Recent studies demonstrated thatNT exhibits strong electrocatalytic activity for a wide rangef compounds, such as neurotransmitters[11–14], NADH

15–18], hydrogen peroxide[11,16,17,19], ascorbic[11–13]nd uric acid[11], cytochromec [20], hydrazines[21], hydro-en sulfide[22], amino acids[23] and DNA [24]. It was

∗ Corresponding author. Tel.: +34 93581 1976; fax: +34 93581 2379.E-mail address: arben.merkoci@uab.es (A. Merkoc¸i).

suggested that electrocatalytic properties originate fromopen ends of CNT[18].

Most of the CNT based electrodes for electroalytical applications are based on physical adsorpof CNT onto electrode surfaces, usually glassy ca[11–13,15,18–20,22,23]. However, it is important to note thCNT dispersed in mineral oil[17,24] or consolidated intTeflon[16] have been recently used.

Graphite carbon powder is used frequently as theductive phase in various composite electrodes developour group[25]. Many advantages of these compositedeveloping genosensors[26], immunosensors[27], strippingsensors[28] for heavy metals or amperometric sensorsamino acids[29] have been shown. The improved elecchemical properties such as signal to noise ratio andcurrent density compared to a wide variety of carbon etrodes (including glassy carbon and carbon paste electrhave been attributed to the random assembly of graphiteticles behaving as a microelectrode array and also theybeen attributed to the rigid non-conducting epoxy regionthe electrode surface area[25,30].

925-4005/$ – see front matter © 2005 Published by Elsevier B.V.oi:10.1016/j.snb.2005.07.010

618 M. Pumera et al. / Sensors and Actuators B 113 (2006) 617–622

Although nanotube-filled polymer composites are well-known in material science[31], to the best of our knowledgethere is not any report on the use of rigid dispersed CNTcomposites in sensor applications. This paper deals for thefirst time with outstanding properties of CNT incorporatedinto an epoxy polymer, forming an epoxy composite hybridmaterial as a new electrode with improved electrochemicalsensing properties.

This novel CNT-epoxy composite is simple, cheap andresults in interesting electrode material. The preparation andcharacterization of this multifunctional material is discussed,and electrochemical responses to important analytes are stud-ied.

2. Experimental

2.1. Apparatus and procedures

Cyclic voltammetry (CV) and chronoamperometryexperiments were performed using an electrochemicalanalyzer Autolab 20 (Eco Chemie, The Netherlands)connected to a personal computer with GPES software.Electrochemical experiments were carried out in a 20 mlvoltammetric cell, at room temperature (25◦C), using athree-electrode configuration. A platinum electrode serveda encee EC)e rodesw dured t in0 tionw ions.T ientc

d bys 0.

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th,0 nma -n sitionm any)w edb eac,S ep ,U Aa oxyT� rm( genp tainedf from

Merck (Hohenbrunn, Germany). All solutions were preparedin double-distilled water.

2.3. Preparation of CNTEC and GEC electrodes

Epoxy resin (Epotek H77A, Epoxy Technology, MA,USA) and hardener (Epotek H77B) were mixed manuallyin the ratio 20:3 (w/w) using a spatula. CNTEC electrodeshave been produced by loading the epoxy resin, before cur-ing, with different amounts (i.e. 10, 12.5, 15, 17.5 and 20%(w/w)) of carbon nanotubes. Two different types of multi-wall carbon nanotubes (MWCNT), varying in the length anddiameter (CNT-200: length, 0.5–200�m; o.d. 30–50 nm andCNT-2: length, 0.5–2�m; o.d., 20–30 nm), were examined.When the resin and hardener were well mixed, the carbonnanotubes were added at the different ratios and mixed for30 min. The resulting paste was placed into a cylindrical PVCsleeve (6 mm i.d.) as reported earlier[27]. Electrical contactwas completed using copper disk and wire. The GEC elec-trodes were prepared in similar way using graphite powderand epoxy resin/hardener to contain 20% (w/w) of graphitein the resulting composite as optimized in previous works[25]. Conducting composite was cured at 40◦C for 1 week.Prior to use, the surface of the electrode was polished withabrasive paper and then with alumina paper (polishing strips301044-001, Orion, Spain).

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s an auxiliary electrode and an Ag/AgCl as a referlectrode. Carbon nanotube-epoxy composite (CNTlectrodes and graphite-epoxy composite (GEC) electere fabricated in our laboratory according to the proceescribed below. All measurements were carried ou.05 M phosphate buffer (pH 7.4). Amperometric detecas performed under forced-convention batch condithe desired working potential was applied and transurrents were allowed to decay to a steady state value.

Micrographs of the electrode surfaces were obtainecanning electron microscopy (SEM) using Hitachi S-57

.2. Reagents

Multiwall carbon nanotubes (CNT-200: leng.5–200�m; o.d., 30–50 nm; wall thickness, 12–18nd CNT-2: length, 0.5–2�m; o.d., 20–30 nm; wall thickess, 1–2 nm; both produced by chemical vapour depoethod) were purchased from Aldrich (Stenheim, Germith ∼95% purity. Further purification was accomplishy stirring the carbon nanotubes in 2 M nitric acid (PanRpain; http://www.panreac.es) at 25◦C for 24 h. Graphitowder (particle size 2–10�m) was obtained from BDHK (http://www.bdh.com). Epoxy resin Epotek H77nd hardener Epotek H77B were received from Epechnology, Billerica, MA, USA (http://www.epotek.com).-Nicotinamide adenine dinucleotide reduced fo

NADH), potassium ferricyanide, potassium dihydrohosphate and potassium hydrogen phosphate were ob

rom Sigma. Hydrogen peroxide (30%) was purchased

. Results and discussion

.1. CNTEC characterisation by SEM

The dispersion and bonding of the nanotubes to the eesin matrix is the important issue in producing the Cpoxy composite materials[31]. To improve the dispersiond interfacial bonding between the resin and the nanot

he nanotubes must be well-dispersed in the epoxy resinmportant to mix well the carbon nanotube with polymervoid the poor dispersion of nanotubes that would resulecrease in the strength of composites.

SEM microscopy was used to gain insight on theace characteristics of carbon nanotube- and graphite-eomposite electrodes.Fig. 1compares SEM images of lonarbon nanotube-based CNT-200-EC electrode (A), sarbon nanotube-based CNT-2-EC electrode (B) and coional GEC electrode (C). SEM micrographs show goodersion of CNTs in the polymer matrix. Sponge-like topohy of the surface of CNT-200-EC electrode is observedhile the CNT-2-EC electrodes show more uniform sur

opography (B). Graphite-epoxy composite electrode is ccterized by its rough surface formed by irregularly shaicrometer-sized flakes of graphite (C). Different surforphologies are expected for different carbon/epoxy cosite ratios. For SEM images of carbon powder of CNT-NT-2 and graphite (before mixing with epoxy resin),igs. S-1, S-2 and S-3, respectively (Supplementary Da

M. Pumera et al. / Sensors and Actuators B 113 (2006) 617–622 619

Fig. 1. SEM images for long-carbon nanotube-based CNT-200-EC electrode (A), short-carbon nanotube-based CNT-2-EC electrode (B) and conventionalGECelectrode (C). All electrode surfaces have been polished in the same way as explained in the text. The same acceleration voltage (5 kV) and resolution are used.

3.2. CNTEC composition

In order to estimate the best CNT/epoxy composition ratioin terms of reversibility and sensitivity, cyclic voltammetryof 1 mM potassium ferricyanide was performed with scanrate 0.1 V s−1 between potentials 0.8 and−0.5 V (versusAg/AgCl) for CNTEC electrodes containing different con-tents of CNT-200 or CNT-2 nanotubes. The influence ofthe CNT/epoxy resin composition upon the peak-to-peakseparation (�Ep) and peak current (Ip) is shown inFig. 2.As expected, with an increasing presence of the carbonnanotubes in the composite, the peak-to-peak separationdecrease. This reflects the decrease of the resistance of theCNTEC electrode surface. Increasing the CNT content resultsin increasing sensitivity of CNTEC electrodes. It can bealso seen fromFig. 2 that long-carbon nanotube (CNT-200)based electrodes are more sensitive to the conducting epoxycomposition than short-carbon nanotube (CNT-2) based elec-trodes. Composites containing more than 20% of CNT (thecorresponding paste before curing was too dry) had a poormechanical stability. Therefore, in the following work wereused electrodes containing 20% (w/w) CNT.

3.3. Electrochemical reactivity

er-r -EC;Fa -i tento eak-t eakswp cteris-t ely.T poxyc evel-o Anal-

ysis of the ferricyanide faradaic current as a function of thescan rate resulted in a linearIp versusν1/2 relationship over0.01–0.25 V s−1 range indicating that the current is controlledby a semi-infinite linear diffusion (conditions as inFig. 3, notshown).

The behaviour of NADH and hydrogen peroxide atCNTEC electrodes was also investigated.Fig. 4 compares

Fig. 2. The effect of the carbon nanotube content upon (a) the peak potentialseparation (�Ep) and (b) peak current of 1 mM ferricyanide of the (A) CNT-200-EC, (B) CNT-2-EC electrodes. Conditions: 50 mM phosphate buffer,pH 7.4; scan rate 0.1 V s−1.

Fig. 3 compares cyclic voltammograms of 1 mM ficyanide at long-carbon nanotube based (CNT-200ig. 3A), short-carbon nanotube based (CNT-2-EC;Fig. 3B)nd graphite powder based (GEC;Fig. 3C) epoxy compos

te electrodes for their optimal compositions (20% conf carbon nanotube or graphite, w/w). The highest p

o-peak separation together with poorly developed pere obtained for the GEC electrode (�Ep = 0.369 V). Lowereak-to-peak separations, 0.302 and 0.211 V, are chara

ic for CNT-200-EC and CNT-2-EC electrodes, respectivhis favourable behaviour of carbon nanotube-based eomposite electrodes is coupled with well-defined and dped peaks (note the differences in the current scale).

620 M. Pumera et al. / Sensors and Actuators B 113 (2006) 617–622

Fig. 3. Cyclic voltammograms for 1 mM ferricyanide of the (A) CNT-200-EC, (B) CNT-2-EC and (C) GEC electrodes. Conditions: 50 mM phosphate buffer,pH 7.4; scan rate 0.1 V s−1; carbon/epoxy ratios, 20:80 (w/w).

cyclic voltammograms for 1 mM NADH at carbon nanotubeCNT-200-EC (A) and CNT-2-EC (B) electrodes withgraphite-epoxy composite electrode (C). A broad NADHoxidation peak is observed at +0.72 V (versus Ag/AgCl) forGEC electrode. For CNT-2-EC electrode, well-developedoxidation peak is observed at the very similar potential+0.74 V. Substantial shift to +0.45 V in the oxidation poten-tial was observed at CNT-200-EC. It is interesting to notethat the large potential shift to the lower potentials (similarto those observed by Musameh et al.[15]) is observed onlyat carbon nanotube CNT-200-EC electrode, while NADHoxidation potentials at CNT-2-EC and graphite powderepoxy electrodes are similar and they remain relatively high.The difference between electrocatalytical behavior of “long”CNT-200-EC and “short” CNT-2-EC towards NADH is notfully understood in the present stage of research.

Fig. 5displays cyclic voltammograms for 1 mM hydrogenperoxide at carbon nanotube CNT-200-EC (A), CNT-2-EC(B) and GEC (C) electrodes. The long-nanotube CNT-200-EC electrode displays oxidation signal around +0.60 V (A),while the short-nanotube CNT-2-EC and graphite-epoxy

composite electrode (B and C, respectively) shows oxidationaround +0.70 V. CNT-200-EC electrode also shows highersensitivity towards oxidation of hydrogen peroxide (note thedifferent scales).

3.4. Analytical utility

Since the oxidation of NADH at conventional solid elec-trodes is prone to the problems with passivation of the surface[16], the stability of response was evaluated.Fig. 6comparesstability of oxidation of NADH at +0.55 V for CNT-200-EC(a), CNT-2-EC (b) and GEC (c) electrodes. The results indi-cate that both carbon nanotube-epoxy composite electrodesprovide a good stability towards oxidation of NADH. Theyshow 96% (CNT-200-EC;Fig. 6a) and 93% (CNT-2-EC;Fig. 6b) of original response even after 45 minutes, whileresponse of GEC electrode (Fig. 6c) decreases by 33% in thesame timescale. Similar passivation layer as on the surface ofGEC electrode also shows glassy carbon electrode[15,32].

The amperometric response of CNTEC and GEC elec-trodes towards NADH was assessed at +0.55 V (see Fig.

00-EC

Fig. 4. Cyclic voltammograms for 1 mM NADH of the (A) CNT-2 , (B) CNT-2-EC and (C) GEC electrodes. Other conditions as inFig. 3.

M. Pumera et al. / Sensors and Actuators B 113 (2006) 617–622 621

Fig. 5. Cyclic voltammograms for 1 mM hydrogen peroxide of the (A) CNT-200-EC, (B) CNT-2-EC and (C) GEC electrodes. Experimental conditions as inFig. 3.

S-4A, Supplementary Data). Nanotube-based electrodes dis-plays a well-defined concentration dependence over con-centration range 0.0–1.0 mM (0.1 mM increments), withsensitivities 72.1 and 29.7�A mM−1 for CNT-200-EC andCNT-2-EC electrodes, respectively (correlation coefficients0.995 and 0.994), while sensitivity of GEC electrode was5.0�A mM−1 (correlation coefficient 0.997). The calibra-tion curves of hydrogen peroxide (Fig. S-4B, Supplemen-tary Data) were obtained at +0.95 V and these were foundto be linear over concentration range 0.0–2.0 mM (0.2 mMincrements) with sensitivities 18.0, 12.9 and 1.9�A mM−1

for CNT-200-EC, CNT-2-EC and GEC electrodes, respec-tively (correlation coefficients 0.995, 0.994 and 0.997). Thehydrogen peroxide response was not affected by regenerating(polishing) the surface. Series of eight successive measure-ments, each recorded on a newly polished surface, yielded toR.S.D. = 5%. Reproducibility of different electrodes preparedfrom the same CNTEC paste showed similar reproducibility(R.S.D. = 6%,n = 3).

F (b)C utions

4. Conclusions

A novel carbon nanotube-epoxy composite electrodehas been fabricated and characterized. As compared withgraphite-epoxy composite (GEC) electrode prepared usingthe same epoxy resin, the CNTEC electrode showed animproved electrochemistry for ferricyanide, NADH andhydrogen peroxide. Lower peak-to-peak separations coupledwith well-defined peaks for CNTEC have been obtained. TheCNTEC material is more robust in the means of mechanicalstrength compared to carbon nanotube paste or Teflon com-posite reported previously.

The new carbon nanotube composite indicates that it maybecome a new class of smart material with unique propertiesand applications. The resulting CNTEC electrode may offer agreat promise for biosensing by incorporating biomolecules,such as enzymes, antibodies or DNA in the CNT/epoxy com-posite. The research in this direction is currently carried outin our laboratory.

Supplementary Data available

SEM images of long-carbon nanotubes CNT-200, short-carbon nanotubes CNT-2 and graphite powder used to preparet andh ablef

A

ofE 04-0 ivent( fM -

ig. 6. Stability of 1 mM NADH response using the (a) CNT-200-EC,NT-2-EC and (c) GEC electrodes. Operation potential, +0.55 V. Soltirring ca. 400 rpm; 50 mM phosphate buffer, pH 7.4.

he electrodes as well as calibration graphs for NADHydrogen peroxide are available. This material is avail

ree of charge via the Internet.

cknowledgements

This work was financially supported by (1) Ministryducation and Culture (MEC) of Spain (Projects BIO202776, MAT2004-05164 and the grant MEC 2003-022 g

o Dr. M. Pumera,); (2) Spanish foundation Ramon Arecesproject ‘Bionanosensores’); (3) “Ramon y Cajal” program oEC (Spain) that supports Dr. A. Merkoc¸i. Authors acknowl

622 M. Pumera et al. / Sensors and Actuators B 113 (2006) 617–622

edge the efficient technical assistance of Mr. Francesc Bohilsfor SEM images.

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iographies

artin Pumera received his PhD in analytical chemistry from Chaniversity, Prague, The Czech Republic in 2001. Shortly after thaecame postdoctoral researcher at Prof. Joseph Wang’s SensoChipatory at NMSU, USA, where he developed new concepts of electroccal detection on Lab-on-a-Chip devices for space (JPL/NASA) andity/forensic applications (US Navy). Currently, he conducts his reseocused on integration of nanobiotechnology (carbon nanotubes, quots, DNA–gold nanoparticle conjugates) on Lab-on-a-Chip platforensor and Biosensor Group at Autonomous University of Barcelo

rben Merkoci was awarded the PhD in chemistry from the Univity of Tirana, Albania, on 1991 and then did postdoctoral researchreece, Hungary, Italy, Spain and USA. His main interest have been

roanalytical methods for several applications in sensors and bioseurrently, he is “Ramon y Cajal” researcher and professor at the Snd Biosensor Group, Chemistry Department, Autonomous Universarcelona. His main research interests concern the design of compiocomposites and nanobioconjugate materials for enzyme, immunNA based electrochemical sensors.

alvador Alegret was made professor of analytical chemistry fromniversitat Autonoma de Barcelona in 1991. He is head of the Sensoiosensor Group in the Chemistry Department. Currently, he is de

o the development of electrochemical chemo- and biosensors basmperometric. potentiometric and ISFET transducers in chemical.atic. immunological and DNA recognising systems. The resulting sevices are being applied in automated analytical systems based or biomimetic instrumentation concepts for monitoring and process

rol in different fields such as biomedicine environment and chemndustry.