Thin Solid Films 517 (2009) 3883–3887

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Flexible electrochemical biosensors based on O2 plasma functionalized MWCNT

Jun-Yong Lee a, Eun-Jin Park a, Cheol-Jin Lee c, Soo-Won Kim c, James Jungho Pak c, Nam Ki Min b,⁎a Department of Biomicrosystem Technology, Korea University, Seoul, Republic of Koreab Department of Control and Instrumentation Engineering, Korea University, Jochiwon, Republic of Koreac School of Electrical Engineering, Korea University, Seoul, Republic of Korea

⁎ Corresponding author.E-mail address: nkmin@korea.ac.kr (N.K. Min).

0040-6090/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.tsf.2009.01.130

a b s t r a c t

a r t i c l e i n f o

Available online 4 February 2009

Keywords:

A flexible glucose sensor is ffilms on polydimethylsiloxanenzyme immobilization, the

MWCNTsPlasma-treatmentFlexible biosensors

GOD/ MWCNT/Au/PDMS electrode exhibits a sensitivity of 18.15 μA mm−2mM−1

and a detection limit of 0.01 mM (signal to noise ratio was about 3). This high sensitivity may be attributed to alarge enzyme loading and a higher electrocatalytic activity and electron transfer exhibited by O2 plasma-

abricated using O2 plasma-functionalized multiwalled carbon nanotube (MWCNT)e (PDMS) substrates and its performance is electrochemically characterized. After

functionalized CNTs than the pristine CNT, due to some oxygen-contained groups present on the O2 plasma-functionalized CNT surface, which has been verified by XPS spectrum.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Recently, the fabrication of sensors on flexible substrates hasattracted considerable attention due to its potential use in applica-tions such as potable, wearable and implantable sensors [1–2]. Carbonnanotubes (CNTs) are very attractive materials for the development ofelectrochemical biosensors because of their capability to providestrong electrocatalytic activity and minimize surface fouling of thesensors [3]. Carbon nanotubes have been known to mediate fastelectron-transfer kinetics for a wide range of electroactive species,such as hydrogen peroxide or NADH. Moreover, carbon nanotubescombine strength and flexibility, so they are excellent candidates forflexible and wearable sensors. However, the growth of carbonnanotubes typically involves high temperature processes, rangingfrom 600 to 1000 °C. Therefore carbon nanotubes cannot be growndirectly on plastic substrates, because most plastics deform or melt attemperature of 100–200 °C. Recently, several approaches have emergedto address this problem [1,4,5]. While there are several reports onflexible CNT gas sensors, such as organic vapor sensors, NTFET gassensors and chemiresistors, there are very few reports that describeelectrochemical biosensors based on flexible CNT film [6].

On the other hand, gas species and biomaterials adsorb strongly tooxygenated groups (−COOH, −OH, CfO) on the surface of CNTs. Oneof the most widely used chemical approaches to introduce function-alities onto CNTs involves with strong oxidizing agents such as HNO3

and/or H2SO4. Beside, it has been demonstrated that carbon nanotubesidewalls can be covalently fluorinated within the temperature rangebetween 250 and 400 °C or derivatized with certain highly reactive

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chemicals such as dichlorocarbene [7]. The treatment in such harshconditions, however, caused detrimental damages to the ends andsidewalls of CNTs, decreased their stability [8], and even cut them intoshort pieces [9]. A particularly attractive option is the surface modi-fication of carbon nanotubes while retaining their electronic proper-ties and providing a linkage to the biological molecules.

In these regards, we developed a novel flexible glucose biosensorbased on O2 plasma-functionalized multiwalled carbon nanotubes(MWCNTs) films on polydimethylsiloxane (PDMS) substrates. Themain purpose of O2 plasma treatment of the carbon nanotubes is tointroduce oxygen containing functional groups onto their surfacewithout influencing their bulk properties and flexible biosensorstructure. The electrochemical behavior of the modified CNT electrodewas investigated using glucose oxidase as a model enzyme for thedevelopment of a working flexible biosensor.

2. Experimental details

2.1. MWCNT preparation

The CNTs used in this study were chosen to be MWCNTs syn-thesized by the catalytic reaction of C2H4 over Fe/Mo/Al2O3 catalyst at923 K in an Ar/H2 atmosphere. A detailed description of the reactionsystem and the relevant CNT growth conditions has been described inthe previous paper [10]. A mixture of Fe(NO3)3, 9H2O and Mo wasdissolved in ethanol, and this solution was added to aluminumisopropoxide dissolved in ethanol. The mixture was then subjected toevaporation on a water bath at 353 K. The catalyst was calcined at923 K in O2 for 2 h, followed by a mechanical grinding for severalhours. For the production of MWCNTs, C2H4, Ar, and H2 were intro-duced into the quartz tube at flow rates of 300, 500, and 500 sccm,respectively.

Fig. 1. Schematics of a MWCNT/Au/PDMS electrode (top) and SEM images of the fabricated structure (bottom).

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2.2. Fabrication of flexible GOD/MWCNT/Au/PDMS electrodes

The schematics and SEM images of a MWCNT/Au/PDMS electrodeare shown in Fig. 1. MWCNT/Au/PDMS electrodes have been fab-ricated by vacuum filtration and thin film technology. The dispersedMWCNT solutionwas filtered through (PTFE) membrane. After dryingfiltered MWCNTs, Au was evaporated on the surface of MWCNTs usingthermal evaporator. Then, PDMS was poured on top of Au/MWCNTlayers and cured at 100 °C for 30 min. After curing, PTFE filter has beenremoved from MWCNT/Au/PDMS electrode.

The oxygen plasma treatments were carried out in different con-ditions. The typical plasma treatment parameters were 10 sccm O2 flowrate, 25 °C substrate temperature, 20–60W rf power, and 1–3 min treat-ment times.

Fig. 2. SEM images of the surface of the O2 plas

The enzyme was added by dropping an aliquot of Glucose oxidase(GOx)(5000 units/mL) in phosphate buffer onto an MWCNT/Au/PDMS substrate and then the device was dried in air.

2.3. Measurements

Scanning electron microscopy (SEM) was carried out on a Hitachi S-4300 and X-ray photoelectron spectroscopy (XPS) datawere recorded ona Physical Electronics PHI 5800 ESCA System. The electrochemical mea-surements were performed with a Perkin Elmer 263A potentiostat,interfaced toacomputer, using conventional two-electrode configuration.Both bare and GOD-immobilized MWCNT/Au/PDMS electrodes wereused as the working electrode. The working electrode and a Pt counterelectrode were immersed analyte solutions with magnetic stirring.

ma-treated MWCNT/Au/PDMS electrodes.

Fig. 4. Relative concentration of oxygen calculated from XPS spectra of plasma-treatedMWCNTs for different plasma treatment time.

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3. Results and discussion

3.1. Chemical characterization

Fig. 2(a)–(f) show SEM images of the surface of the MWCNT/Au/PDMS electrodes plasma-treated with different parameters. The config-uration of the CNT surface was significantly changed after O2 plasmatreatment on the top of the electrodes. As the plasma treatment time andpower increased, the nanotubes became thinner or bent.

In oxygen plasmas, oxygen molecules are dissociated and oxygenatoms are generated. The dissociation energy of oxygen is 5.11 eV [11].Further more, oxygen atoms are ionized in the plasma. The energeticspecies of oxygen in the plasma is considered to work as the oxidizingregent for CNTs. Thus, Oxygen plasma treatment was reported to giverise to CfO and C\O\C functional groups at the MWCNT surface as aresult of O2 dissociation at vacancies created during the plasma treat-ment [12].

XPS analyses were performed in order to evaluate the chemicalconcentration and the chemical changes at the surface of theMWCNTsas a consequence of the oxygenplasma treatment. Fig. 3 shows the XPSsurvey spectra before and after the oxygen plasma treatment. Thesespectra reveal the presence of carbon and oxygen on the sample.The peak near 283.2 eV, observed on both spectra, is generated byphotoelectrons emitted from the C1s core level and the peak thatappears near 532 eV in the spectrum, recorded on the MWCNTs afterthe oxygen plasma treatment, is generated by photoelectrons emittedfrom the O1s core level. Therefore it can be assumed that the oxygenplasma treatment effectively generated oxygen species on MWCNTs[11].

Fig. 3. XPS spectra of MWCNTs (a) before and (b) after O2 plasma treatment. Plasmapower: 20 W, pressure: 0.1 Torr, treatment time: 60 s.

The relative concentration of oxygen evaluated by XPS was foundto increase with increasing plasma treatment time, as shown in Fig. 4.The oxygenated surface groups are suggested to be the anchoring sitesfor functional groups or gas molecules.

3.2. Cyclic voltammograms for MWCNT/Au/PDMS electrodes

In order to estimate the effect of plasma parameters on electrodeproperties in terms of reversibility and sensitivity, the cyclic

Fig. 5. Cyclic voltammograms of O2 plasma-treated MWCNT/Au/PDMS electrodes in10 mM potassium ferricyanide (K3[Fe(CN)6])+3 M KCl/DI water solution.

Fig. 7. Cyclic voltammograms of O2 plasma-treated MWCNT/Au/PDMS electrodes inresponse to glucose.

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voltammogram was performed in 10 mM potassium ferricyanide(K3[Fe(CN)6]) dissolved in 3 M KCl/DI water solution for MWCNT/Au/PDMS electrodes plasma-treated at different plasma conditions. Theinfluence of the plasma power and treatment time upon the peak-to-peak separation (ΔEp) and peak current (Ip) is shown in Fig. 5. It isapparent that the oxidation/reduction current peaks get biggerwith theincrease inplasmapower. At the same time, thepeak-to-peak separationΔEp reduces with increasing plasma power. It has been recognized thatthe electron transfer capacity of electrodes in electrochemical processesis directly associated to the surface conditions of electrode materials.The decrease in the ΔEp value indicates that the O2 plasma-functiona-lized CNTs have a faster electron transfer capability than the pristineCNTs.

On the other hand, the amount of the plasma-functionalized CNTsin the electrode affects the sensing characteristics of the biosensor.Fig. 6 shows the response current of the electrode as a function of O2

plasma-functionalizedCNTconcentration. As expected, the output cur-rent in the presence of potassium ferricyanide increases linearly withincreasing concentration of functionalized CNTs within a measure-ment range of 20mg. Thus, increasing of the CNTconcentration resultsin increasing sensitivity of MWCNT/Au/PDMS electrodes.

3.3. Electrocatalysitic behavior to glucose

Fig. 7 (a) shows cyclic voltammograms of different GOD/MWCNT/Au/PDMS electrodes in response to glucose. The GOD-immobilizedMWCNT/Au/PDMS electrode gives much higher current than the onewithout glucose because there is addition of faradic current fromreaction of glucose on the electrode. The result suggests that the O2

plasma-functionalized CNTs can play a similar role as the pristine CNTsin enhancing the electron transfer of enzymatic electrodes. The devicesalso showed a glucose response much higher than that from acorresponding pristine CNT electrode. This high sensitivity may be dueto a large enzyme loading and a high electrocatalytic activity exhibitedby O2 plasma-functionalized CNTs. The reason the O2 plasma-treatedCNTs can facilitate the electron transfer of GOD more easily than thepristine CNT may be due to some oxygen contained groups present onthe surface, such as hydroxyl, carbonyl, and carboxyl groups, which hasbeenverifiedbyXPS spectrum(Figs. 3 and4) [13]. Itwas reported [14,15]that CNT can promote the direct electron transfer of redox-activebiomolecules, due to the presence of the oxygen-contained groups onthe CNTsurface or the hydrophilic groups on the oxygenplasma-treatedCNT surface [16].

3.4. Amperometric response of the biosensor

To further study the performance of the biosensor, the ampero-metric response of GOD/MWCNT/Au/PDMS electrodes wasmeasured

Fig. 6. Response current of the MWCNT/Au/PDMS electrode as a function of plasmafunctionalized CNT concentration.

by successively adding the same amount of glucose to the buffersolution. The GOD/MWCNT/Au/PDMS electrodes responded rapidlyto the substrate with a response time (achieving 90% of the steady-state current) of less than 5 s. Such a rapid response can be attributedto the fast diffusion of the substrate in the CNT film.

The GOD electrodes exhibited a well-defined concentration depen-dence over glucose concentration range 0.0–4.0 mM. Fig. 8 shows thetypical calibration curve of the steady-state current vs. glucose concen-tration obtained from amperometric response. As shown, the responsecurrent increased linearly with the increasing glucose concentration.The linear range was 0.05–4 mM with a correlation coefficient (R) of

Fig. 8. Calibration curve of GOD/MWCNT/Au/PDMS electrodes.

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0.992 and a sensitivity of 18.15mm−2mM−1. The detection limit was aslow as 0.01 mM (signal to noise ratio was about 3).

The reproducibility of the GOD/MWCNT/Au/PDMS electrodes wasinvestigated at different concentrations of glucose by cyclic voltam-metric measurements. The variation coefficient of the sensor responsewas within 3% for successive measurements, indicating the biosensorhad good reproducibility. To evaluate the electrode-to-electrode re-producibility, five biosensors were prepared under the same condi-tions independently, and they showed a variation coefficient of 4.23%,indicating an acceptable electrode-to-electrode reproducibility.

4. Conclusions

We have prepared a flexible amperometric biosensor based on O2

plasma-functionalized multiwalled carbon nanotube (MWCNT) filmson polydimethylsiloxane (PDMS) substrates. The plasma-treatedMWCNT/Au/PDMS electrode has a glucose response much higherthan that from a corresponding pristine CNT electrode. This highsensitivity may be due to a large enzyme loading and a highelectrocatalytic activity exhibited by O2 plasma-functionalized CNTs,and shows promise for their potential applications as biosensors. Finallythe flexible biosensor technology presented here is readily extended tothe fabrication of other biosensors based onother enzymes andproteins.

Acknowledgments

This work was supported by grant No. K20601000002-07E0100-00210 from Korea Foundation for International Cooperation of Science& Technology and by a Korea University Grant.

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