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Magnetic composites

DOI: 10.1002/smll.200701018

Polyaniline-Coated Fe3O4 Nanoparticle–Carbon-Nanotube Composite and its Application inElectrochemical Biosensing**

Zhun Liu, Joseph Wang, Donghai Xie, and Gang Chen*

Polyaniline (PA) is one of the most important conducting

polymers that is used in a wide range of applications owing to

its relatively facile processability, mechanical flexibility, low

cost, electrical conductivity, thermal and chemical stability,

etc.[1] As emeraldine base (EB) and emeraldine salt (ES), PA

can be interchanged by doping and dedoping with acids and

bases.[1] It can release and adsorb negatively charged

substances by electrochemical (EC) dedoping and redoping.[1]

Multifunctional PA nanostructures have been prepared by

blending PA with electrical, optical, and magnetic nanopar-

ticles to form nanocomposites.[1] Fe3O4 nanoparticles have

received a lot of attention because of their promising magnetic

properties and potential applications in color imaging,

electromagnetic shielding, magnetic recording media, soft

magnetic materials, and ferrofluids. Various PA–Fe3O4

composites have been intensively investigated because of

their novel properties. Usually, PA–Fe3O4 nanoparticles form

core–shell or one-dimensional (1D) structures.[2] They can be

prepared by an in situ polymerization of aniline monomer in

an aqueous solution containing Fe3O4 nanoparticles and

ammonium peroxydisulfate (APS).

Carbon nanotubes (CNTs) have attracted much attention

since the pioneering work by Iijima et al. in 1991 because of

their high electrical conductivity, mechanical strength, and

chemical stability.[3] Significant interest has been generated in

recent years in research that mixes CNTs with various

inorganic and organic substances by physical and chemical

approaches to prepare multifunctional composites. A variety

of CNT composites have been prepared and investigated,

including composites of CNT–polysulfone, CNT–Teflon,

CNT–epoxy, CNT–copper, CNT–poly(methyl methacrylate),

etc.[4] Recently, extensive efforts have been made to prepare

functional PA–CNT composites that exhibit enhanced

electrical, thermal or mechanical properties relative to PA

[�] Z. Liu, Dr. D. Xie, Prof. G. Chen

School of Pharmacy, Fudan University

Shanghai 200032 (P. R. China)

Fax: (þ86) 21-6418-7117

E-mail: gangchen@fudan.edu.cn

Prof. Dr. J. Wang

Departments of Chemical & Materials Engineering and Chemistry

Arizona State University

Tempe, AZ 85287 (USA)

[��] This work was financially supported by NSFC (20405002, 20675017),Shanghai Science Committee (051107089), State Education Ministryof China, and the 863 Program of China (2007AA04Z309).

: Supporting Information is available on the WWW under http://www.small-journal.com or from the author.

� 2008 WILEY-VCH Verlag GmbH &

or CNTs alone.[5] PA–CNT nanocomposites have been

prepared in situ by chemical and electrochemical oxidation

of aniline in the presence of CNTs[6] and have found a wide

range of applications in micro- and nanoelectronic compo-

nents, sensors, optoelectronic devices and electrochemical

capacitors.[7] Recently, Pumera et al. have prepared a

PA–polypyrrole composite by the chemical deposition of

polypyrrole in the presence of APS.[8] In addition, magnetic

CNT–Fe3O4 nanoparticle composites for electrochemical

sensing, solid phase extraction, etc. have been prepared by

the coprecipitation of Fe3þ and Fe2þ in alkaline solution.[9]

In this work, a novel PA-coated Fe3O4 nanoparticle–CNT

composite (PA–Fe3O4–CNT) has been prepared by the

coprecipitation of Fe3þ and Fe2þ and the in situ polymeriza-

tion of aniline (Scheme 1). Its characterization and application

in the electrochemical doping of enzyme for sensing glucose

have also been described.

Figure 1A illustrates the transmission electron microscopy

(TEM) image of a nanotube in the PA–Fe3O4–CNT

composite. A layer of PA has been deposited onto CNTs

with Fe3O4 nanoparticles embedded inside. Although the

composite was sonicated in ethanol before TEM measure-

ments, a large amount of Fe3O4 nanoparticles were found on

the surface of the CNTs or in the polymer layer, indicating

Fe3O4 nanoparticles were entrapped in the PA layer on the

CNT. It can be estimated that the size of the Fe3O4

nanoparticles (black particles) is in the range of 10 to

20 nm. The PA coating on CNTs was not uniform because

Fe3O4 nanoparticles were embedded inside. The thickness of

PA coated on CNTs was in the range of 15 to 30 nm with an

average value of approximately 20 nm. The scanning electron

microscopy (SEM) images of multi-walled CNT (MWCNT),

PA–Fe3O4–CNT composite, and Fe3O4 nanoparticle–CNT

composite are shown in the Supporting Information (SI) in

Figure S1 and Figure S2. It can be seen clearly that the

nanotubes in PA–Fe3O4–CNT composites appear thicker and

have rough surface features (Figure S1B) relative to the

smooth bare CNTs (Figure 1A). Figure S2 in the SI indicates

the diameter of the nanotubes in the Fe3O4–CNT composite

increased after being coated with PA. Figure S3A in the SI

shows the TEM images of polyaniline-coated Fe3O4 particles

(PA–Fe3O4) (�1:1, w/w) that tend to aggregate. When CNT

was introduced into PA–Fe3O4 composite as cores, the

aggregation was minimized because both Fe3O4 and PA were

deposited on CNTs.

Because both the coprecipitation of Fe3þ and Fe2þ and the

in situ polymerization of aniline in aqueous solution can occur

quantitatively, the content of each component in the

as-synthesized PA–Fe3O4–CNT composite was kept almost

the same by controlling the amount of Fe3þ, Fe2þ, and aniline

in this study. Figure 1B shows the thermogravimetric analysis

(TGA) and differential TGA (DTGA) curves of

PA–Fe3O4–CNT composite at a heating rate of 10 8C min�1.

1. Obvious weight loss of the composite has been found in the

temperature ranges 220–550 8C and 560–700 8C because of the

decomposition of PA and CNT, respectively. Based on the

TGA curve, the weight fraction of PA, Fe3O4, and CNT in

CNT–PMMA composite was estimated to be approximately 1/

3, which is in agreement with the expected value.

Co. KGaA, Weinheim small 2008, 4, No. 4, 462–466

Scheme 1. Preparation route of PA–Fe3O4–CNT composite and immo-

bilization process of glucose oxidase based on electrochemical (EC)

doping.

Figure 2. XRD curves of CNT, Fe3O4–CNT composite, polyaniline (PA),

and PA–Fe3O4–CNT composite.

Figure 2 shows the X-ray diffraction (XRD) curves of

CNT, Fe3O4–CNT composite, PA, and PA–Fe3O4–CNT

composite. Diffraction peaks assigned to CNT at

2u¼ 26.58[10] can be clearly seen in the XRD curves of

CNT, and Fe3O4–CNT composite, indicating that the CNT

Figure 1. A) TEM image of PA–Fe3O4–CNT and B) the TGA (a) and DTGA

(b) curves of PA–Fe3O4–CNT.

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structure was not destroyed after the successive deposition of

Fe3O4 and PA.As shown in Figure 2, the other twoweak peaks

of CNTs merged with that of Fe3O4. Seven characteristic

peaks of Fe3O4 (2u¼ 18.4, 30.2, 35.6, 43.3, 53.7, 57.3, and

62.88), marked by their indices ((111), (220), (311), (400),

(422), (511), and (440), respectively),[11] were observed in the

XRD curves of both Fe3O4–CNT and PA–Fe3O4–CNT

composites, indicating that the Fe3O4 particles in the

composites were pure Fe3O4 with a spinel structure. The

average crystallite size of Fe3O4 particles in PA–Fe3O4–CNT

composite was estimated to be 17 nm using the Scherrer

formula,[12] based on the peak at 35.68 in its XRD spectrum. It

was in good agreement with the sizes of Fe3O4 particles

(�20 nm) observed in the TEM images. The peaks at 18.8 and

25.18 in the XRD curve of PA indicate that PA is partially

crystalline. The broad diffraction peak of PA in the

PA–Fe3O4–CNT composite is weak, indicating that the

crystallinity of PA is much lower than that of pristine PA

and the interactions among PA, Fe3O4, and CNT restrict the

crystallization of PA.

Fourier-transform infrared (FTIR) spectra (in SI, Figure

S4) of CNT, PA, and PA–Fe3O4–CNT composite have been

measured. Absorption bands of CNT pretreated by concen-

trated HNO3 are observed at 3400, 1709, and 1565 cm�l, which

are attributed to O–H, C––O, and C–C, respectively. The peaks

at 1640, 1200, and 1090 cm�l correspond to the vibration of the

carboxylic acid groups.[13] Both pristine PA and the PA in the

composite were in the form of ES. In the IR spectra of pristine

PA and PA–Fe3O4–CNT composite, the peak at approxi-

mately 810 cm�1 and the band between 800–500 cm�1 were

assigned to the aromatic C–H bending of the 1,4-disubstituted

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Figure 3. A) XPS spectrum of PA–Fe3O4–CNT composite and B) the

hysteresis loops of Fe3O4 nanoparticles and PA–Fe3O4–CNT composite.

Figure 4. Amperometric response to the successive addition of 1 mM

glucose at the potential of �0.1 V (vs. SCE) of the magnetic electrode

loaded with PA–Fe3O4–CNT (a) and PA–Fe3O4 composite (b) with

glucose oxidase immobilized and the bare electrode (c). Also shown (in

the inset) are the plots of amperometric currents versus the concen-

trations of glucose at (a) and (b).

464

aromatic ring[14] and the vibration of C–H bonds in the

benzene rings, respectively.[5e] The peaks at 1500 and 1580

cm�1 can be assigned to the stretching vibration of benzoid

and quinoid rings, respectively, thereby indicating the

oxidation state of PA (ES).[1b] The peaks at approximately

1300 and 1140 cm�1 were attributed to the C–N stretching and

C––N stretching of PA, respectively. The peak at 1240 cm�1

was assigned to the stretching vibration of C–Nþ in phosphoric

acid doped PA.[5e] As reported by Baibarac et al.,[5b] a marked

increase in the intensity ratio between the quinoid and benzoid

ring vibrations was observed for PA–Fe3O4–CNT composite

compared to that of the pristine PA (ES). Absorption bands

near 3430 cm�l in both the PA-related spectra were assigned to

the N–H stretching.

X-ray photoelectron spectrometry (XPS) was employed to

measure the elemental map of PA–Fe3O4–CNT composite

(Figure 3A). As expected, the peaks of carbon (C1s), nitrogen

(N1s), iron (Fe2p3), oxygen (OKLL and O1s), and phosphorus

(P2p3) have been found in the spectrum. Although the

composite was carefully washed with copious water, sulfur

(S2p3) was found, indicating sulfur-containing anions might be

entrapped in the composite.

In order to characterize the magnetic properties, a

vibrating sample magnetometer was used to record hysteresis

loops of Fe3O4 nanoparticles and PA–Fe3O4–CNT composite

(Figure 3B). The magnetization curves of both samples exhibit

superparamagnetic behavior (i.e., no remanence remained

when the applied magnetic field was removed). The super-

paramagnetism of PA–Fe3O4–CNT composite can be attrib-

uted to its Fe3O4 component. The magnetic saturation (MS)

value of Fe3O4 nanoparticles (63.6 emu g�1) is approximately

three time that of PA–Fe3O4–CNT composite (19.8 emu g�1).

www.small-journal.com � 2008 WILEY-VCH Verlag G

It is obvious that the MS value of PA–Fe3O4–CNT composite

mainly depends on its content.

The superparamagnetism of PA–Fe3O4–CNT composite is

critical for its application in biomedical and bioengineering

fields. The superparamagnetic property prevented the mag-

netic material from aggregating and enabled it to redisperse

rapidly when the outside magnetic field was removed. More

importantly, the Fe3O4 nanoparticles on CNTs were warped

by PA; the CNT cores of PA–Fe3O4–CNT composite can

significantly enhance its dispersibility in water. When 20mg of

PA–Fe3O4–CNT composite was dispersed in 10 mL of water,

the composite was able to remain in suspension for 24 h

without visible sediment. Additionally, the dispersed compo-

site could be separated to the wall of the container after only

2min using a magnet of 2000 G (1 G¼ 10�4 T). As shown in

the SI in Figure S5, this redispersion and separation process

could be repeated readily. The high dispersibility and

sensitivity to a magnetic field exhibited by the novel magnetic

nanocomposite indicates great promise for a wide range of

applications.

The PA outside the nanotubes in the PA–Fe3O4–CNT

composite not only wraps the Fe3O4 nanoparticles on CNT but

also prevents the Fe3O4 nanoparticles from being oxidized. It

has been reported that black Fe3O4 nanoparticles tends to be

oxidized to form brown Fe2O3.[2] It was found that the color of

Fe3O4–CNT composite turned from black to brown after it

was kept in water for 4 days at room temperature. In

comparison, a color change was not foundwith the green-black

PA–Fe3O4–CNT composite even after it was stored in water

for 4 months.

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In this study, the PA–Fe3O4–CNT composite was magne-

tically loaded on the surface of a graphite–epoxy disc electrode

(SI, Figure S6) and was dedoped at �0.5 V (versus a saturated

calomel electrode (SCE)). Glucose oxidase (GOx) was

electrochemically doped into the PA–Fe3O4–CNT composite

to form magnetic glucose biosensor. The electrochemical

doping of GOxwas carried atþ0.65 V (vs. SCE) in a phosphate

buffer (pH¼ 5.5) that can keep GOx (isoelectric point (pI)

value of 4.3)[15] negatively charged. Figure 4 shows the typical

amperometric response of the magnetic electrode loaded with

GOx-immobilized PA–Fe3O4–CNT and PA–Fe3O4 composite

and the bare electrode (c) to the successive addition of 1mM

glucose at a potential of �0.1 V (vs. SCE) in 50mM phosphate

buffer (pH¼ 7.4). As expected, the bare electrode was not

responsive to these concentration changes at this low detection

potential. However, well-defined and fast amperometric signals

were observed on the magnetic electrodes loaded with

GOx-immobilized PA–Fe3O4–CNT and PA–Fe3O4 compo-

sites with the regression equations of y¼ 2.3482xþ 0.7898

(R2¼ 0.9987, n¼ 5) and y¼ 0.4305xþ 0.5307 (R2¼ 0.9865),

where y, x, and R are current response (mA), glucose

concentration (mM), and correlation coefficient, respectively.

The PA–Fe3O4–CNT-based glucose sensor shows much higher

sensitivity and linearity than the PA–Fe3O4-based sensor,

indicating that CNTs significantly enhance the performances of

the biosensor. CNTs in PA–Fe3O4–CNT can increase the

conductivity and specific surface area so that more GOx can be

immobilized in the composite. It has been demonstrated

that CNTs on electrodes significantly reduce the over-

potential of the redox reaction of hydrogen peroxide (the

product of the oxidation of glucose in the presence of GOx)

significantly[15] so that the electron-transfer reaction of

hydrogen peroxide is promoted. The reaction occurring at

the PA–Fe3O4–CNT-based biosensor reached a dynamic

equilibrium very quickly upon each addition of glucose

solution, generating a steady-state current signal within 5–6

s. In addition, the reproducibility of the sensor preparation

as well as of the sensor performance was indicated from

a series of seven measurements of 5mM glucose each

recorded on a freshly prepared surface, yielding satisfactory

reproducible signals with relative standard deviation (RSD)

of 5.8%.

In conclusion, well-assembled PA–Fe3O4–CNT compo-

sites have been prepared by a simple two-step deposition

approach. CNTs attached to Fe3O4 nanoparticles were then

coated by a layer of PA to form a novel 1D super-

paramagnetic conductive material. CNTs form the cores of

the novel composite and PA acts as the ‘‘outer clothing’’ that

protects the nanostructures. The three-component composite

has diverse properties because each component brings the

composite different chemical and physical properties. One of

the most promising properties of the composite is that some

substances can be immobilized in it by electrochemical

doping. In this work, this effect has been employed to

immobilize glucose oxidase and the composite was magne-

tically loaded on an electrode with the aid of magnets for

glucose sensing. The magnetic nanocomposite can be

removed upon taking away the magnets inside the electrode,

providing a useful approach to preparing renewable

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CNT-based biosensors. PA–Fe3O4–CNT composite may also

find applications in drug delivery, tumor treatment, enzyme

engineering, batteries, electro-magnetorheological fluids,

electromagnetic shielding, magnetic recording, and so on.

Experimental Section

Multi-walled carbon nanotubes (40–60 nm diameter, 5–15mm

long) were pretreated by stirring in concentrated nitric acid at

60 8C for 12 h. Fe3O4–CNT composite was prepared by suspending

90mg purified CNT in 20 mL of water containing 157mg (0.4

mmol) (NH4)2Fe(SO4)2 �6H2O and 386mg (0.8 mmol) NH4Fe(-

SO4)2 � 12H2O at 50 8C. After the mixture solution was sonicated

(30W, 40 kHz) for 10min, 5 mL of aqueous 5 M NH4OH was added

dropwise while the mixture solution was sonicated. The pH of the

final mixture was in the range of 11–12. The reaction was carried

out at 50 8C for 30min under constant mechanical stirring. The

precipitate was isolated with the aid of a magnet. The supernatant

was separated from the precipitate by decantation. Impurities

(such as sulfate and ammonia) in the Fe3O4–CNT samples were

removed by washing with copious amounts of doubly distilled

water. The content of CNT in the composite was approximately

50 wt%. If no CNT was added to the coprecipitation solution of

Fe2þ and Fe3þ, pure Fe3O4 nanoparticles could be obtained for the

control experiments.

To prepare PA–Fe3O4–CNT composite, the obtained

Fe3O4–CNT nanocomposite was dispersed in 15 mL of water.

Subsequently, 0.1 mL of phosphoric acid (85 wt%) and 0.1mL

(�1mmol) of aniline were successively added. After the solution

was stirred mechanically for 10min, it was mixed with aqueous

solution of ammonium peroxydisulfate (APS) (0.23 g, 1 mmol,

dissolved in 5 mL of water). The polymerization of aniline was

allowed to proceed for 4 h at approximately 4 8C under mechanical

stirring. The resulting precipitate was washed with copious

amounts of water and absolute ethanol (three times with each)

and was dried under vacuum at room temperature for 24 h to

obtain green-black PA–Fe3O4–CNT composite. If the Fe3O4–CNT

was replaced by the pure Fe3O4 nanoparticles mentioned above,

PA-coated Fe3O4 nanoparticles (PA–Fe3O4) could be obtained as a

control. The deposited PA was insoluble and presented in its

emeraldine salt form.

Before loading the magnetic nanomaterials, 15 pieces of

NdFeB permanent magnets were inserted into the glass tube of a

graphite–epoxy composite electrode until the magnets touched

the inner surface of the electrode (2.2mm inner diameter (ID)).

20mg of the magnetic composites was dispersed in 10 mL of

water with the aid of sonication to form the loading solution. Then,

10 mL of this loading solution containing 20 mg CNT was taken out

with a pipette under sonication and was dropped onto the surface

of a piece of clean Plexiglas plate. As shown in the SI (Figure S6),

the carbon disc part of the magnetic electrode was allowed to

touch the top of the water drop. The magnetic composite would

move toward the surface of the electrode and aggregated there to

form a modification layer. The magnets inside the electrode tube

can prevent the magnetic particles from escaping. After the

electrode was reduced in 50mM phosphate buffer (pH¼5.5) at

�0.5 V (vs. SCE) for 20min, it was immersed in 50mM phosphate

buffer (pH¼5.5) containing 5mg mL�1 glucose oxidase (GOx, 158

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466

units per mg) and a constant potential of þ0.65 V (vs. SCE) was

applied to the electrode to dope GOx for 20min. Measurements of

the amperometric response of the electrodes to the glucose

solution were performed at a constant applied potential of �0.1 V

(vs. SCE) while the solution was being stirred continuously at

300 rpm. The supporting electrolyte was 50mM phosphate buffer

(pH¼7.4).

Reagents, fabrication processes of electrode, instrumentation

and operation procedures are available in the Supporting

Information.

Keywords:biosensors . Fe3O4

. glucose . nanocomposites . polyanilines

[1] a) G. Premamoy, K. S. Samir, C. Amit, Eur. Polym. J. 1999, 35, 699;b) H. Zengin, W. Zhou, J. Jin, R. Czerw, D. W. Smith, L. Echegoyen, D.

L. Carroll, S. H. Foulger, J. Ballato, Adv. Mat. 2004, 14, 1480; c) X.Lu, Y. Yu, L. Chen, H. Mao, H. Gao, J. Wang, W. Zhang, Y. Wei,

Nanotechnology 2005, 16, 1660.[2] a) J. G. Deng, C. L. He, Y. X. Peng, J. H. Wang, X. P. Long, P. Li, A. S. C.

Chan, Synth. Met. 2003, 139, 295; b) X. F. Lu, H. Mao, D. M. Chao,

W. J. Zhang, Y. We, J. Solid State Chem. 2006, 179, 2609.[3] a) S. Iijima, Nature 1991, 354, 56; b) R. H. Baughman, A. Zakhidov,

W. A. de Heer, Science 2002, 297, 787; c) P. M. Ajayan, Chem. Rev.

1999, 99, 1787.[4] a) S. Sanchez, M. Pumera, E. Cabruja, E. Fabregas, Analyst, 2007,

132, 142; b) J. Wang, M. Musameh, Anal. Chem. 2003, 75, 2075; c)G. Chen, L. Y. Zhang, W. Wang, Talanta 2004, 64, 1018; d) J. Wang,

G. Chen, M. Wang, M. P. Chatrathi, Analyst 2004, 129, 512; e) M. J.

Esplandiu, V. G. Bittner, K. P. Giapis, C. P. Collier, Nano Lett. 2004,4, 1873.

www.small-journal.com � 2008 WILEY-VCH Verlag G

[5] a) Z. X. Wei, M. X. Wan, T. Lin, L. M. Dai, Adv. Mater. 2003, 15, 136;b) M. Baibarac, I. Baltog, S. Lefrant, J. Y. Mevellec, O. Chauvet,

Chem. Mater. 2003, 15, 4149; c) T. M. Wu, Y. W. Lin, C. S. Liao,

Carbon 2005, 43, 734; d) R. Sainz, A. M. Benito, M. T. Martinez, J. F.

Galindo, J. Sotres, A. M. Bari, B. Coraze, O. Chauvet, W. K. Maser,

Adv. Mater. 2005, 17, 278; e) X. B. Yan, Z. J. Han, Y. Yang, B. K. Tay,J. Phys. Chem. C 2007, 111, 4125.

[6] a) M. Cochet, W. K. Maser, A. M. Benito, M. A. Callejas, M. T.

Martinez, J. M. Benoit, J. Schreiber, O. Chauvet, Chem. Commun.

2001, 1450; b) C. Downs, J. Nugent, P. M. Ajayan, D. J. Duquette, S.

V. Santhanam, Adv. Mater. 1999, 11, 1028.[7] a) M. Gao, S. Huang, L. Dai, G. Wallace, R. Gao, Z. Wang, Angew.

Chem. Int. Ed. 2000, 39, 3664; b) A. Hassanien, M. Gao, M.

Tokumoto, L. Dai, Chem. Phys. Lett. 2001, 342, 479; c) R. Sainz,A. M. Benito, M. T. Martinez, J. F. Galindo, J. Sotres, A. M. Baro, B.

Corraze, O. Chauvet, W. K. Maser, Adv. Mater. 2005, 17, 278.[8] M. Pumera, B. Smid, X. S. Peng, D. Golberg, J. Tang, I. Ichinose,

Chem. Eur. J. 2007, 13, 7644.[9] a) S. Qu, J. Wang, J. L. Kong, P. Y. Yang, G. Chen, Talanta 2007, 71,

1096; b) X. J. Peng, Z. K. Luan, Z. C. Di, Z. G. Zhang, C. L. Zhu,

Carbon 2005, 43, 880.[10] W. Z. Zhu, D. E. Miser, W. G. Chan, M. R. Hajaligol, Mater. Chem.

Phys. 2003, 82, 638.[11] D. H. Chen, M. H. Liao, J. Mol. Catal. B: Enzym. 2002, 16, 283.[12] M. Kryszewski, J. K. Jeszka, Synth. Met. 1998, 94, 99.[13] D. Mawhinney, V. Naumenko, A. Kuznetsova, J. Yates, Jr, J. Liu, R. E.

Smalley, J. Am. Chem. Soc. 2000, 122, 2383.[14] R. Cruz-Silva, J. Romero-Garcia, J. L. Angulo-Sanchez, E. Flores-

Loyola, M. H. Farias, F. F. Castillon, J. A. Diaz, Polymer 2004, 45,4711.

[15] a) S. L. Mu, H. G. Xue, Sens. Actu. B 1996, 31, 155; b) J. Wang,

Electroanalysis 2005, 17, 7.

mbH & Co. KGaA, Weinheim

Received: October 23, 2007Published online: March 31, 2008

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