Preparation of polyaniline/multiwalled carbon nanotube composite by novel electrophoretic route

C A R B O N 4 6 ( 2 0 0 8 ) 1 7 2 7 – 1 7 3 5

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Preparation of polyaniline/multiwalled carbon nanotubecomposite by novel electrophoretic route

Chetna Dhanda,b, Sunil K. Aryaa,b, Surinder Pal Singha, Bhanu Pratap Singha,Monika Dattab, B.D. Malhotraa,*

aBiomolecular Electronics and Conducting Polymer Research Group, National Physical Laboratory, Dr. K.S. Krishnan Marg,

New Delhi 110 012, IndiabDepartment of Chemistry, University of Delhi, Delhi 110 007, India

A R T I C L E I N F O

Article history:

Received 10 March 2008

Accepted 20 July 2008

Available online 31 July 2008

0008-6223/$ - see front matter � 2008 Elsevidoi:10.1016/j.carbon.2008.07.028

* Corresponding author:E-mail address: [email protected]

A B S T R A C T

A nano-structured composite film comprising of emeraldine salt (ES) and carboxyl group

functionalized multiwalled carbon nanotubes (MWCNT-c) has been electrophoretically pre-

pared from their colloidal suspension on an indium–tin–oxide (ITO) coated glass plate. This

nano-structured composite film (ES/MWCNT-c) has been characterized using atomic force

microscopy (AFM), ultraviolet–visible (UV–visible) spectroscopy, Fourier transform infrared

(FT-IR) spectroscopy, cyclic voltammetry (CV), electrochemical impedance spectroscopy

(EIS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

The AFM studies reveal porous morphology with uniformly distributed MWCNT-c in this

composite film. The SEM and TEM investigations reveal wrapping of MWCNT-c with the

chains of ES. UV–visible and FT-IR investigations show the formation of MWCNT-c doped

composite at the molecular level. The results of the CV and EIS studies indicate enhanced

electrochemical and charge transfer behavior of the composite. The application of ES/

MWCNT-c/ITO electrode to biosensor for cholesterol indicates short response time (10 s)

and high sensitivity (6800 nA mM�1).

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Carbon nanotubes (CNT) owing to their unique structural,

mechanical, electronic and thermal properties have recently

triggered a great deal of interest [1,2]. CNT have been shown

to have potential applications in field emitters, probe tips

for scanning tunneling microscopy, nano-electronic switches,

nano-transistors, biosensors and actuators, etc. [3–7]. How-

ever aggregation and agglomeration of CNT, due to high sur-

face free energy and existence of inter-tube van der Waals

forces, is presently a major obstacle for the realization of their

technological potential. To obtain processable CNT, two

important approaches have been reported (i) covalent attach-

ment of CNT with alkyl chains [8] and (ii) polymer/CNT com-

er Ltd. All rights reservedt.in (B.D. Malhotra).

posite preparation [9–14]. Among these, covalent chemical

approach is known to disrupt the extended p-network at

CNT surfaces resulting in their poor mechanical and electrical

properties [15]. On the other hand, polymer/CNT composite

preparation via non-covalent supra-molecular approach [16]

helps to use their unique properties and makes them attrac-

tive building blocks for development of novel nanomaterials

for desired application. This approach involves the wrapping

of CNT with high molecular weight polymers. These high

molecular weight polymers could disrupt the van der Waals

interactions without affecting the p-network of CNT that

would otherwise cause the aggregation of CNT into bundles.

This results in the uniform dispersion of CNT in a desired

polymer matrix [16]. The polymer/CNT composite has been

.

1728 C A R B O N 4 6 ( 2 0 0 8 ) 1 7 2 7 – 1 7 3 5

known to exhibit properties of each of the constituent with

synergic effect [17,18].

CNT blends with polymers have been utilized for the fabri-

cation of photovoltaic devices [19], wherein CNT not only act

as electron acceptors but also provide exciton dissociation

centers. However, difficulties in the fabrication of homoge-

nous polymer/CNT based films have hindered their wide-

scale applications. Recently, polyaniline (PANI)/CNT compos-

ite has gained much attention [20,21]. The presence of small

amount of CNT in PANI nanofibers has been reported to result

in faster charge transport leading to enhanced actuation per-

formance [7]. CNT in PANI matrix has been reported to act as

dopant [22] resulting in improved electrical conductivity of

the PANI matrix both in its reduced and neutral state. It fur-

ther helps in increasing the mechanical and chemical stabil-

ity of a given matrix. The PANI/CNT composite has also been

found to improve electrocatalytic activation of the redox en-

zymes via enhancing electrochemical transduction of the bio-

chemical process [23,24]. The uniformly distributed CNT in

PANI matrix may lead to improved properties of composite

for biosensor and microelectronics devices. High mechanical

strength, high conductivity and large surface area associated

with CNT are particularly relevant for the fabrication of an

amperometric biosensor, as these properties may result in

high enzyme loading and improved signal response.

For the preparation of PANI/CNT composite, CNT suspen-

sion in neutral aqueous solution of aniline has been used

for electro-deposition [25,26]. However, poor solubility of ani-

line in aqueous solution and poor quality of the films pro-

duced has limited the use of this method. This difficulty of

dissolving aniline in neutral solution has been overcome by

adding CNT directly in pure aniline to form stable charge

transfer complex [25,26]. For composite formation, deposition

of PANI films onto preformed CNT layer whiskers has also

been tried [27]. However, this results in the non-uniform dis-

tribution of CNT in the given matrix and poor adherence of

the composite film onto the desired substrate. In these meth-

ods for PANI/CNT composite formation [25–27], CNT simply

provide porous structure and a network for enhanced elec-

tron conduction. However, the composite still requires addi-

tional anions for doping. In other words, the inclusion of

CNT in the PANI matrix does not result in any significant

improvement of its electrochemical behavior. To overcome

these problems, there is a considerable need to find new

methods for the PANI/CNT composite formation.

Electrophoretic deposition [28] has recently gained much

attention for the deposition of uniform, dense and electro-

chemically reversible films of conducting polymer from col-

loidal suspension of preformed polymer for desired

applications [29–31]. In addition, this technique provides a

unique strategy to prepare nano-structured conjugated poly-

mer films [32,33] with nanoporous and densely packed struc-

tures with controlled thickness and morphology.

In this work, attempts have been made for the electropho-

retic deposition of the composite comprising of emeraldine

salt (ES) and carboxyl group functionalized multiwalled car-

bon nanotubes (MWCNT-c), i.e., ES/MWCNT-c onto indium–

tin–oxide (ITO) coated glass plates. This ES/MWCNT-c film

has been investigated as immobilization matrix for the

enzyme (cholesterol oxidase, ChOx) to reveal its application

to biosensor for cholesterol.

2. Experimental

Multiwalled carbon nanotubes (MWCNTs) (90%) were syn-

thesized by catalytic chemical vapor deposition using a mix-

ture of ferrocene and toluene as a source of catalyst and

hydrocarbon, respectively [34]. These MWCNTs were then

purified and functionalized using boiling acid treatment

[35]. Polyaniline in its ES form was synthesized by oxidative

chemical polymerization of aniline (ANI) using ammonium

persulphate (APS) at a molar ratio of 1:1 [36]. For polyaniline

synthesis, solution of APS (11.4 g) in precooled 1 M HCl

(50 mL) and ANI (4.6 mL) again in precooled 1 M HCl

(50 mL) was prepared. The ANI solution was kept in ice bath

(0–5 �C) with continuous stirring. To this ANI solution, APS

was added dropwise over a period of 30 min with constant

stirring to ensure thorough mixing. The polymerization pro-

cess was carried out at 0–5 �C for 4 h with stirring after

which it was left overnight. Finally the product was filtered

and washing was done first with 1 M HCl and then with

THF until the filtrate became colourless. Emeraldine base

(EB) was prepared by de-protonating the wet ES in 10% aque-

ous ammonia solution. The prepared EB was then dried in a

vacuum oven and stored at room temperature.

Electrophoretic deposition of ES/MWCNT-c composite was

carried out with a DC battery (Bio Rad, Model 200/2.0) using

the set-up shown in Fig. 1. The colloidal suspension of desired

concentration (10 mg L�1) required for electrophoretic deposi-

tion was prepared by mixing 100 lL of PANI/formic acid stock

solution (1 mg mL�1) with 100 lL of MWCNT-c dispersion

(1 mg mL�1 in acetonitrile) in 9.8 mL of acetonitrile. In the col-

loidal suspension, both PANI and MWCNT-c dissociate into

ions due to high dielectric medium offered by acetonitrile

resulting in the formation of positively charged PANI coated

MWCNT-c colloidal spheres. The interactions that are likely

to facilitate wrapping of MWCNT-c with positively charged

PANI molecules are shown in Fig. 1. These colloidal spheres

are stabilized by symmetrically distributed negatively

charged formate ions around them.

During electrophoretic film deposition, positively charged

colloid spheres of composite in acidic colloidal suspension

start dragging towards negatively biased ITO electrode. In

the standardization of voltage and time, the DC potential of

120 V and 30 s are found to be optimum to obtain uniform

homogenous and thin composite film (200 nm) via grain-by-

grain deposition of colloid particles on the electrode surface.

No deposition is observed on the positively biased platinum

electrode, indicating that all the MWCNT-c perhaps get com-

pletely wrapped by positively charged ES chains in colloidal

suspension. For the deposition of homogenous composite

film, gentle shaking or brief sonication of the colloidal solu-

tion for a few seconds is found to be necessary to achieve

the desired dispersion of CNT.

For biosensor studies, cholesterol oxidase (ChOx) has been

covalently immobilized onto ES/MWCNT-c/ITO electrode

using N 0-(3-dimethylaminopropyl carbodiimide) and N-

hydroxysuccinimide chemistry [29].

Colloidal suspension(MWCNT-c/ES)

ITO-coated glass

1cm

Platinum electrode

(+)(-)

Colloidal suspension(MWCNT-c/ES)

ITO-coated glass

1cm

Platinum electrode

(+)(-)

OO-

O-

O

OO

-

O-

O

N+

NH

NH

N+

..

H

H

n

N+

NH

NH

N+.

.

H

H n

NH

N+

N+

NH

.

.

H

H

H

N+

N+

NH NH ..H H

(a)

(b)O

O

O

O- O

-

OH

OH

O

O-

(c)

n

n

MWCNT-c wrapping by ES

+

MWCNT-c

Emeraldine salt (ES)

ES/MWCNT-c composite

(i)

(ii)

Fig. 1 – (i) Pictorial view of electrophoretic set-up used for composite film formation and (ii) interactions responsible for

ES/MWCNT-c composite formation: (a) p-stacking (b) electrostatic interactions and (c) hydrogen bonding.

C A R B O N 4 6 ( 2 0 0 8 ) 1 7 2 7 – 1 7 3 5 1729

The morphology and microstructures of ES/MWCNT-c film

have been investigated using Vecco DICP2 atomic force micro-

scope loaded with SPM-LAB analysis software. The investiga-

tion of wrapping of MWCNT-c with ES chains has been carried

out using scanning electron microscope (JEOL, JSM-35) and

transmission electron microscope (JEOL, JEM-2100). The

molecular analysis of all the samples has been done using

Fourier transform infrared (FT-IR) and ultraviolet–visible

(UV–visible) spectroscopies. The FT-IR spectra in the region

of 700–1800 cm�1 have been recorded using Perkin–Elmer

FT-IR model ‘‘Spectrum BX’’ equipped with the specular

reflectance accessory. The UV–visible studies have been car-

ried out using UV–visible spectrophotometer (Model

2200DPCV, Phoenix). Electrochemical investigations of both

ES and ES/MWCNT-c films have been carried out using Auto-

lab Potentiostat/Galvanostat (Eco Chemie, Netherlands) in a

conventional three-electrode electrochemical cell consisting

of Ag/AgCl as reference electrode and platinum foil as the

counter electrode. Cyclic voltammetry (CV) studies have been

carried out in phosphate buffer saline (PBS) solution (50 mM,

pH 7.0, 0.9% NaCl). Electrochemical impedance spectroscopy

(EIS) studies have been performed in the frequency range,

100 kHz–10 mHz with amplitude of 5 mV in PBS containing

mixture of 5 mM FeðCNÞ4�6 (ferrocyanide) and 5 mM of

FeðCNÞ3�6 (ferricyanide), i.e., 5 mM FeðCNÞ3�=4�6 as a redox

probe. All chemical reagents used in these experiments have

been procured from Sigma–Aldrich (USA) and have been used

as received. The response studies of ChOx/ES/MWCNT-c/ITO

bioelectrode have been done using linear sweep voltammetry

(LSV).

3. Results and discussion

3.1. FT-IR studies

Fig. 2 shows the FT-IR spectra of (i) MWCNT-c, (ii) ES/MWCNT-

c composite and (iii) ES film in the region of 1800–700 cm�1. In

the FT-IR spectra of MWCNT-c (Fig. 2, curve (i)), the band at

1080 cm�1 is assigned to O–H deformation vibrations, peak

at 1166 cm�1 is assigned to C–O stretching vibrations. The

peak at 1728 cm�1 (Fig. 2, curve (i)) is attributed to –C@O

stretching vibrations in the carboxyl group. This indicates

the –COOH group functionalization of MWCNT [37]. Further,

FT-IR spectra shows a characteristic peak of MWCNT around

1600 cm�1 corresponding to –C@C– stretching. The FT-IR spec-

tra of ES (Fig. 2, curve (iii)) and the electrophoretically depos-

ited composite (Fig. 2, curve (ii)) show the presence of quinoid

and benzenoid ring vibrations at 1589 cm�1 and 1485 cm�1,

respectively [38]. The higher intensity of quinoid band com-

pared to that of the benzenoid band (Fig. 2, curve (iii)) is attrib-

uted to the presence of the PANI in its ES form.

The FT-IR spectrum of the ES/MWCNT-c composite (Fig. 2,

curve (ii)) film exhibits an increased quinoid (Q) to benzenoid

(B) band (1589/1486 cm�1) intensity ratio (4.73) compared to

that of the ES (2.85) without CNT. The observed increased ra-

tio reveals the richness of the composite with quinoid units

1800 1600 1400 1200 1000 800

(ii)

(iii)

(i)

1296

1123

11481303

1485

1589

1591

1600

1080

1166

1728

Tra

nsm

itan

ce (

%T

)

Wavenumber (cm-1)

17.22 a.u. 3.63 a.u.

28.9 a.u. 10.15 a.u.

1400150016001700

(ii)

(iii)

(i)

1296

1123

11481303

1485

1589

1591

1600

1080

1166

1728

Wavenumber (cm-1)

17.22 a.u. 3.63 a.u.

28.9 a.u. 10.15 a.u.

1400150016001700

17.22 a.u. 3.63 a.u.

28.9 a.u. 10.15 a.u.

1400150016001700

Fig. 2 – FT-IR spectra of MWCNT-c (i), ES/MWCNT-c composite (ii) and ES (iii).

200 300 400 500 600 700 800 900

(iii)

(ii)

(i)

ES

ES/MWCNT-c

Abs

orba

nce

(Abs

)

Wavelength (λ)

Fig. 3 – UV–visible spectra of the electrophoretically

deposited ES and ES/MWCNT-c composite film.

1730 C A R B O N 4 6 ( 2 0 0 8 ) 1 7 2 7 – 1 7 3 5

that can be assigned to the strong p-stacking (Fig. 1) interac-

tions between the two aromatic components of the compos-

ite. These interactions are likely to promote the stabilization

of the quinoid ring structure in the ES. Further, the shift in

peaks at 1303 cm�1 (C@N (Q) stretching vibration) and

1148 cm�1 (B–N+@Q stretching) towards lower wave number

indicates increased electron delocalization in the composite

compared to that of pure ES. This observed downshift of –

C@N (Q) and B–N+@Q stretching vibrations indicates bond

elongation and is attributed to strong electrostatic interac-

tions of –COO� species of the MWCNT-c with these groups.

3.2. UV–visible studies

UV–visible spectra of the pure ES and ES/MWCNT-c com-

posite film (Fig. 3) have been recorded to further corroborate

the composite formation.

It can be seen (Fig. 3) that the UV–visible spectra of ES

show three sharp peaks with the maxima at 320 nm (peak

i), 395 nm (peak ii) and 656 nm (peak iii). The peaks at

320 nm and 656 nm correspond to p–p* transitions centered

on the benzenoid and quinoid units, respectively. The

395 nm peak arises due to the formation of a doping level ow-

ing to the ‘exciton’ transition, caused by inter-band charge

transfer from benzenoid to quinoid moieties of the proton-

ated ES. Interestingly, in the UV–visible spectra of ES/

MWCNT-c composite film, the peak (i) retains its position

whereas a slight blue shift in the peak (ii) and considerably

large red shift in peak (iii) is observed. This red shift in peak

(iii) and the blue shift observed in peak (ii) may be assigned

to the site selective interactions [39] between MWCNT-c and

quinoid ring of ES, facilitating charge transfer from quinoid

unit of ES to MWCNT-c via highly reactive imine groups.

The generation of a new peak at 275 nm in the UV–visible

spectra of composite is attributed to p–p* transition in

MWCNT-c. This peak is in agreement with the results re-

ported in literature for EB/MWCNT [40] and ES/MWCNT [41].

The peak around 520 nm for ES/MWCNT-c is expected to arise

from the doping induced electronic state by the interactions

between imine sites of PANI and carboxyl groups in

MWCNT-c. The presence of the peaks at 275 nm and 520 nm

along with the peaks of ES indicates the formation of

composite.

3.3. AFM studies

To investigate the surface properties, AFM studies have

been carried out for electrophoretically deposited ES/

MWCNT-c composite film. The 2D micrograph of the compos-

ite film (Fig. 4i) reveals uniform distribution of MWCNT-c in

Height (nm)

(i) (ii)100 % 10 μm

10 μm

5 μm

5 μm

0 μm

0 μm

75 %

50 %

25 %

0 % 0.00 295.5473.89 147.77 221.66

Fig. 4 – AFM analysis of ES/MWCNT-c film: (i) two-dimensional (2D) non-contact mode micrograph and (ii) bearing ratio

plot of a 10 · 10 lm2 AFM scan on composite film.

C A R B O N 4 6 ( 2 0 0 8 ) 1 7 2 7 – 1 7 3 5 1731

the ES matrix without any agglomeration. The value of the

roughness (estimated as root mean square, rms) of the com-

posite film obtained using height distribution analysis has

been found to be 33 nm. It may be noted that this value of

the roughness is higher than that of pure ES film (8.6 nm)

(data not shown) and indicates increased available surface.

The high value of roughness observed indicates porous mor-

phology of the film. Further, the bearing ratio plot (an integral

of the height histogram from the top surface, i.e., plot of the

percentage of data points at or above a given height) has been

utilized to evaluate the surface topography of the ES/MWCNT-

c composite film (Fig. 4ii). The observed single slope in the

curve of bearing ratio plot (10 · 10 lm2 AFM scan) reveals

homogenous morphology in the composite film [42].

3.4. SEM and TEM analyses

SEM and TEM analyses have been done to investigate the

wrapping of MWCNT-c with ES. SEM micrographs (Fig. 5i

and ii) show the surface morphology of MWCNT-c and ES/

MWCNT-c composite after the film fabrication on the ITO

plates. These micrographs clearly reveal the ES wrapping

and significant increase in the diameter of MWCNT-c after

the composite formation [43]. Moreover, the change in the

surface characteristics and blurred edges of MWCNT-c visible

in the TEM micrograph of ES/MWCNT-c composite (Fig. 4iv) as

compared to the sharp and well-defined edges in MWCNT-c

(Fig. 4iii) are also indicative of ES wrapping onto MWCNT-c.

3.5. CV studies

To study the electrochemical behavior, CV studies of ES

film and ES/MWCNT-c composite film (Fig. 6) have been inves-

tigated in PBS in the range of �0.8 to 0.8 V at the scan rate of

50 mV s�1. The CV of ES shows redox behavior with the ano-

dic peak (0.115 V) and cathodic peak (�0.290 V) corresponding

to the transition of PANI backbone from the fully reduced leu-

coemeraldine state to the partially oxidized ES state and vice

versa.

In the CV of ES/MWCNT-c composite film, the presence of

MWCNT-c leads to �4.6 folds enhancement in the oxidation

peak and�2.5 folds in the reduction peak current of ES. This in-

crease in thevalue of current indicates fasterelectron transport

within the bulk-film and charge transport in the parallel inter-

face of solution and ES/MWCNT-c composite film. This result

can be attributed to the increase in the bulk concentration of

the redox species (C�0) from 1.28 · 10�6 mol cm�2 in the ES to

3.18 · 10�6 mol cm�2 in the composite. Thevalue of C�0 has been

estimated using the following equation [44].

ip ¼ 0:227nFAC�0k0 exp�anaF

RTEp � E00� �� �

ð1Þ

where ip is the anodic peak current, n is the number of elec-

trons transferred (2), F is the Faraday constant

(96485.34 C mol�1), R is the gas constant (8.314 J mol�1 K�1),

A is the area of electrode surface (cm2), C�0 is the surface con-

centration of the ionic species in the film (mol cm�2), Ep is the

peak potential and E00 is the formal potential. �anaF/RT and k0

(rate constant) correspond to the slope and intercept of ln(ip)

versus Ep � E00 curve at different scan rates.

This increase in the redox species supports strong interac-

tions between the aromatic structures of MWCNT-c and ES,

that help in the uncoiling of the otherwise folded PANI chains.

In the open conformation, it is likely that more number of ex-

posed moieties are available for oxidation leading to higher

faradic current. Further it may be noted that the anodic and

cathodic currents in the CV of ES/MWCNT-c composite (Fig. 6)

are almost equal (difference in the anodic and cathodic cur-

rents, DI = 0.05 mA) as compared to pure ES (DI = 0.25 mA),

which is one of the conditions required for a system to be elec-

trochemically reversible. We do not understand exactly the rea-

son of observed phenomenon. However, it can be due to the fact

that in pure ES film the oxidation and reduction occurs with

simultaneous doping and un-doping by the ions present in

Fig. 5 – SEM (i and ii) and TEM (iii and iv) micrograph of MWNT-c and ES/MWCNT-c composite.

-0.9 -0.6 -0.3 0.0 0.3 0.6 0. 9

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5ES/MWCNT-cES

Cur

rent

(mA

)

Potential (V)

Fig. 6 – Cyclic voltammograms comparing the

electrochemical hysteresis of ES film to that of ES/MWCNT-c

composite film in phosphate buffer saline (PBS) solution

(50 mM, pH 7.0, 0.9% NaCl).

1732 C A R B O N 4 6 ( 2 0 0 8 ) 1 7 2 7 – 1 7 3 5

the electrolyte medium. And such kind of doping and un-dop-

ing is not fully reversible and results in unequal peak current. In

ES/MWCNT-c composite, presence of MWCNT-c in the matrix

acts as the dopant for polyaniline and stabilizes the system

during electrochemical measurements. This leads to the stea-

dy and reversible behavior of the ES/MWCNT-c composite.

Moreover, the introduction of MWCNT-c into the ES matrix re-

sults in shift of the peaks towards the higher potential. This

may be due to strong interactions between the ES chains and

MWCNT-c that force the system to oxidize and reduce at com-

paratively higher potential.

3.6. EIS studies

EIS studies have been carried out to further support the en-

hanced electrochemical response of the ES/MWCNT-c compos-

ite film. Fig. 7 shows that, in spite of similar shapes of the two

Nyquist plots, there is remarkable decrease in the diameter of

the semicircle (indicative of charge transfer resistance) for

the composite film. The smaller semicircle indicates that the

charge transfer resistance (210 X) for ES/MWCNT-c composite

film is much lower than that of the ES film (1.12 kX). This en-

hanced charge transfer is likely to be due to grain-to-grain wir-

ing of the PANI chains with the well-dispersed and uniformly

distributed MWCNT-c in the film. Besides this, presence of

MWCNT-c provides ‘conducting bridges’ between ES conduct-

ing domains and function as charge hopping centers facilitat-

ing faster charge transfer via electrons from one polymer

chain to the other and finally to the electrode (ITO) surface.

3.7. Response studies of ChOx/ES/MWCNT-c/ITObioelectrode

To show the potential application of observed higher sur-

face area and enhanced electrochemical properties of the

0 200 400 600 800 1000 1200 1400 1600 1800

0

100

200

300

400

PANI-MWNT-c

PANI

-Zim

(oh

ms)

Zre(ohms)

Fig. 7 – Impedance spectra of electrophoretically deposited

ES and ES/MWCNT-c composite film.

C A R B O N 4 6 ( 2 0 0 8 ) 1 7 2 7 – 1 7 3 5 1733

composite (ES/MWCNT-c), amperometric response studies of

fabricated ChOx/ES/MWCNT-c/ITO bioelectrode have been

carried out. LSV studies in PBS in the range of �0.8 to 0.8 V

have been used to investigate the enzymatic activity of the

bioelectrode prepared by covalent immobilization of biomole-

cule (ChOx). The variation in the current measured at a fixed

voltage of 0.28 V in LSV scans as a function of cholesterol con-

centration (1.3–13 mM) is shown in Fig. 8. The measured cur-

rent has been found to increase with increase in cholesterol

concentration. The bioelectrode is found to exhibit short re-

sponse time (minimum time required for significant re-

sponse) of 10 s and high sensitivity of 6800 nA mM�1 as

compared to other cholesterol sensing bioelectrodes reported

in literature [45–47]. This enhanced sensitivity is attributed to

the incorporation of the MWCNT-c in the matrix and to the

intimate association between these two aromatic structures.

Both CV and EIS studies reveal drastic improvement in the

electrochemical response of the polyaniline matrix after the

incorporation of MWCNT-c. This clearly indicates grain-to-

0 2 4 6 8 10 12 140.40

0.41

0.42

0.43

0.44

0.45

0.46

0.47

0.48

Cur

rent

[mA

]

Cholesterol Concentration [mM]

Fig. 8 – Variation in the amperometric current measured

at 0.28 V as a function of cholesterol concentration.

grain wiring of the polyaniline chains with well-dispersed

and well aligned MWCNT-c in the film. These conducting

bridges of MWCNT-c function as electron tunneling centers

resulting in improved response time and sensitivity by pro-

viding better electronic communication.

4. Conclusions

It has been demonstrated that electrophoretic deposition

using colloidal solution can be employed for the preparation

of uniform ES/MWCNT-c composite film (thickness

�200 nm) on ITO surface with enhanced electrochemical

and charge transfer properties. The AFM studies indicating

homogenous topology and enhanced roughness (33 nm) in

this composite film reveal increased available surface for bio-

molecule immobilization. The SEM and TEM studies have re-

vealed wrapping of MWCNT-c with ES chains. The

amperometric response studies of ChOx modified composite

bioelectrode (ChOx/ES/MWCNT-c/ITO) for fabrication of cho-

lesterol biosensor reveal that the films prepared by electro-

phoretic deposition provide superior matrix for biomolecule

immobilization. Detailed studies relating to the performance

of this cholesterol sensing electrode will soon be communi-

cated for publication. And it would be interesting to utilize

this composite for development of other biosensors for esti-

mation of total cholesterol, lipoproteins and triglycerides, etc.

Acknowledgements

We thank Dr. Vikram Kumar, Director, National Physical Lab-

oratory, New Delhi, for providing facilities. Chetna Dhand is

thankful to the Council of Scientific and Industrial Research

(CSIR), India, for the award of a Senior Research Fellowship.

We also thank Dr. Ch. Mohan Rao (CCMB, Hyderabad) and

Dr. K. N. Sood (NPL, India) for TEM and SEM measurements.

We thank Dr. Vinay Gupta, Department of Physics, University

of Delhi for AFM and film thickness measurements. We

acknowledge the financial support received from the Depart-

ment of Science and Technology (DST), Government of India,

India–Japan Project (DST/INT/JAP/P-21/07 and Department of

Biotechnology Government of India (DBT/GAP070832).

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