recent progress in the development of nano - structured conducting

Sensors and Actuators B 136 (2009) 275–286
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical
journal homepage: www.elsevier.com/locate/snb
Review
Recent progress in the development of nano-structured conducting
polymers/nanocomposites for sensor applications
Rajesh
a,∗
, Tarushee Ahuja
b
, Devendra Kumar
b
a
Council of Scientific & Industrial Research, 14, Satsang Vihar Marg, Special Institutional Area, Delhi 110067, India
b
Department of Applied Chemistry, Delhi College of Engineering, University of Delhi, Bawana Road, Delhi 110042, India
article info
Article history:
Received 18 July 2008
Received in revised form 22 August 2008
Accepted 4 September 2008
Available online 20 September 2008
Keywords:
Conducting polymers
Nanowires
Nanotubes
Nanoparticles
Nanocomposites
Nanobiosensors
abstract

Nanomaterials of conjugated polymers are found to have superior performance relative to conventional
materials due to their much larger exposed surface area. The present paper gives an overview of various
recent synthetic approaches involving template free and template oriented techniques suitable for the
growth of nanomaterials of conjugated polymers, their merits and application in making nanodevices. The
characteristics of nano-structured conducting polymers and polymer nanocomposites, their application
in sensors/biosensors and advances made in this field are reviewed.
© 2008 Elsevier B.V. All rights reserved.
Contents
1. Introduction 275
1.1. Nano-structured conducting polymers and polymer nanocomposites 276
2. Growth of conducting polymer nanowires/nanotubes/nanoparticles 277
2.1. Template oriented synthesis of nanowires/nanotubes/nanoparticles 277
2.2. Template free synthesis of nanowires/nanotubes/nanoparticles 280
3. Applications of nano-structured conducting polymers/nanocomposites in sensors/biosensors 280
4. Conclusion 283
Acknowledgements . 283
References 283
Biographies 286
1. Introduction
In the recent years, the development of nanomaterials for the
ultra sensitive detection of biological species has received great
Abbreviations: MNP, metal nanoparticles; CNT, carbon nanotubes; SWNTs,
single-walled nanotubes; MWNTs, multi-walled nanotubes; CP, conducting poly-
mers; CPNWs, conducting polymer nanowires; PPy, polypyrrole; PANI, polyaniline;
PEDOT, poly(ethylenedioxythiophene); DNA, Deoxyribonucleic acid; GOx, glucose
oxidase; NSA, ␤-napthalene sulfonic acid; POAS, poly (o-anisidine).
∗
Corresponding author.
E-mail address: rajesh
( Rajesh).

attention because of their unique optical, electronic, chemical and
mechanical properties. Materials like metal (gold, silver), carbon
and polymers (especially conducting polymers) have been used
to prepare nanomaterials such as nanoparticles [1,2], nanotubes
[3,4] and nanowires [5–7]. These materials are promising for a
variety of applications including optical and electronic nanode-
vices, and chemical and biological sensors [8]. Novel nanomaterials
for use in bioassay applications represent a rapidly advancing
field. Various nano-structures have been investigated to deter-
mine their properties and possible applications in biosensors. Some
of the most promising near term realizations of nanotechnol-
ogy are at the interface of physical and biological system. Uses,
0925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.snb.2008.09.014
276 Rajesh et al. / Sensors and Actuators B 136 (2009) 275–286
properties and fabrication of these materials as nanosensors have
been reported particularly for high-density arrays [9–12]. Hun-
dreds of research articles using nanomaterials for electrochemical
biosensing have been published since then. There are several
reviews available which are based on electrochemical nanobiosen-
sor [13–17]. Recently, Reshetilov and Bezborodov discussed the
fundamental nature of interpenetration of nanotechnology and
biosensor technology [18].
Carbon nanotubes (CNTs) have been of great interest, both
from a fundamental point of view and for potential applica-
tions. Their mechanical and unique electronic properties open
a broad range of applications including nanoelectronic devices,
composites, chemical sensors, biosensors and more [19]. Carbon
nanotubes can be classified as single-walled nanotubes (SWNTs)
and multi-walled nanotubes (MWNTs). SWNTs consist of a cylin-

drical single sheet with a diameter between 1 and 3 nm and a
length of several micrometers. They possess a cylindrical nano-
structure formed by rolling up a single graphite sheet into a
tube. MWNTs consist of a coaxial arrangement of concentric sin-
gle nanotubes like rings of a tree trunk separated from one
another by 0.34 nm. They usually have a diameter of about
2–20 nm. The production of SWNTs or MWNTs is highly depen-
dent on the synthesis process and conditions [20]. CNTs are
promising as an immobilization substance because of their signif-
icant mechanical strength, high surface area, excellent electrical
conductivity and good stability [3,20]. Due to these properties,
CNTs have the ability to promote electron transfer reactions
when used as an electrode. Synthesis, processing and device fab-
rication techniques for nanotubes have greatly improved with
recent intensive research [21]. Various electro analytical prop-
erties and applications of CNTs have appeared in the literature
[22–24]. Platinum nanoparticles with diameter 2–3 nm were pre-
pared with SWNTs for fabricating electrochemical sensors with
remarkably improved sensitivity towards H
2
O
2
where Nafion,
a perflourosulphonated polymer was used to solubilize SWNTs.
The response time and detection limit of this biosensor was
3 s and 0.5 ␮M, respectively [25]. Zho and group developed an
amperometric glucose biosensor based on electrodeposition of
Platinum nanoparticles on MWNTs and immobilizing enzyme with
chitosan–SiO
2

sol–gel. The biosensor exhibits good response to
glucose with linear range of 1 ␮M–23 mM, a low detection limit
1 ␮M, a short response time (within 5 s) and high sensitivity
(58.9 ␮AmM
−1
cm
−2
) [26].
Nanoparticles provide an ideal remedy to the usually contradic-
tory issues applied in the optimization of immobilized enzymes,
i.e. minimum diffusion limitations, maximum surface area per
unit mass and highly effective enzyme loading [27]. Compos-
ite electrodes containing MNP (metal nanoparticles) are used as
chemical sensors [28] or for the construction of MNP based elec-
trochemical biosensors [13,29]. Recently, novel routes to synthesize
polymer stabilized metal nanoparticles (PSMNP) using inert (non-
functionalized) polymers as MNP stabilizing media have been
developed [30].
Since the discovery that conjugated polymers can be made to
conduct electricity through doping [31], a tremendous amount of
research has been carried out in the field of conducting polymers
[32]. As the chemical and physical properties of polymers may be
tailored by the chemist for particular needs; they gained impor-
tance in the construction of sensing devices [33]. These conducting
polymers are of great scientific and technological importance
because of their unique electrical, electronic, magnetic and opti-
cal properties [34–37]. Nanoscale ␲-conjugated organic molecules
and polymers can be used for biosensors, electrochemical devices,
single electron transistors, nanotips of field emission display, etc.
[38–42].

A thin film of conducting polymer having both high conductivity
and fine structure in nanoscale is a suitable component for the fab-
rication of enzyme electrodes and thus can be used in detection of
several analytes [43]. Dai and Mau [44] presented some important
issues concerning the surface and interface control of polymeric
biomaterials and conjugated polymers for biomedical applications.
With the recent development in nanoscience and nan-
otechnology, conducting polymer nano-structures received an
ever-increasing attention and keeping this in view we have
made efforts to present an updated review on synthetic method-
ologies of nano-structured conducting polymers and polymer
composites and their potential applications in the field of nanosen-
sors/biosensors.
1.1. Nano-structured conducting polymers and polymer
nanocomposites
CP nanowires (CPNWs) are an attractive alternative to silicon
nanowires and carbon nanotubes because of their tunable conduc-
tivity, flexibility, chemical diversity, and ease of processing [45].
The conductivity of these materials can be controlled chemically,
making conducting polymer nanowires also a promising sensing
material for ultra sensitive, trace-level biological and chemical
nanosensors [46].
Conducting polymers containing the analyte binding species
are said to be doped conducting polymer materials and nanowires
of this material are called doped conducting polymer nanowires.
These doped conducting polymer nanowires can be made by
incorporating analyte-detecting species into a conducting polymer.
Whenever, there is a contact of these doped nanowires with the
analyte, there are changes in the electrical characteristics. The use
of nanomaterials of CP could greatly improve diffusion since they

have much greater exposed surface area, as well as much greater
penetration depth for gas molecules relative to their bulk counter-
parts [47] as a result of this the basic properties of a biosensor like
detection limit get enhanced. The oriented microstructure and the
high surface area also favors high enzyme loading and has poten-
tial for high sensitivity detection. Moreover, the relative stability
is increased due to efficient bonding of enzyme on the transducer
surface which gives it better reproducibility.
Nanomaterials of polyaniline have received much attention
because of greater surface area that allows fast diffusion of gas
molecules into the structure. There are different routes to pre-
pare nanofibres of various conducting polymers. PANI nanofibres
were prepared by chemical polymerization of aniline [48]. Simi-
larly polypyrrole (PPy) nanofibres were synthesized (60–100 nm in
diameter) in presence of p-hydroxy-azobenzene sulfonic acid as a
functional dopant [49]. In general the fabrication of nanomaterial
based electronic biosensors involves three distinct steps (i) pro-
duction of nanomaterials, (ii) merging nanomaterials into defined
electrodes and (iii) integration of electronic and microfluidic com-
ponents. Nano-dimensional conducting polymers have also been
reported to exhibit unique properties such as greater conductivity
and more rapid electrochemical switching speeds [50].
Polymer–nanoparticle composite materials have also attracted
the interest of a number of researchers, due to their synergistic and
Fig. 1. Formation of nanocomposites.
Rajesh et al. / Sensors and Actuators B 136 (2009) 275–286 277
Fig. 2. Preparation of the polythiophene coated gold nanoparticles from 3-(10-bromodecyl)thiophene (BDT) via thiol 3-(10-mercaptodecyl)thiophene (MDT) [52].
hybrid properties derived from several components [51]. A sim-
ple representation for the formation of nanocomposites is given in
Fig. 1. Ease of processability of an organic polymer combined with

the improved mechanical and optical properties of nanoparticles
has led to the fabrication of many devices. Fig. 2 shows the prepa-
ration of composite nanoparticles from gold core/polythiophene
shell, which can be stably, dispersed in common organic solvents
and thus shows potential applications in electronic devices [52].
A large number of new composite materials with a synergetic
or complementary behavior can be obtained with applications
in electronic or nanoelectronic devices, because of the interac-
tion between electron donor and acceptor. Potential aspects of
conducting polymers/nanocomposites have also been discussed
in the literature [53–56]. CNTs are used as an additive to mod-
ify the properties of polymers [57]. However, their compatibility
has been a serious issue which can be increased by functionaliza-
tion [58,59] or by formation of ultra thin films of composites with
finely dispersed nanomaterials [60]. Recently, conducting poly-
mers/carbon nanotubes composites have attracted considerable
interest not only because the CNTs can improve the electrical and
mechanical properties of polymer, but also because the composites
possessed properties of individual components with a synergistic
effect [55,61].
Stamm and co-workers have recently reported ultra thin
transparent conducting film of polymer modifie d multi-walled
carbon nanotubes [62,63]. The high conductivity of polymer/CNT
nanocomposites has open up new opportunities for chemi-
cal/biosensors [64–68]. PANI/CNTs composites have been prepared
by in situ chemical polymerization of aniline [69]. These approaches
improved the electrical conductivity, electrochemical capacitance
or mechanical strength of the polymer. Single-walled CNT/PANI
composite films with good uniformity and dispersion were pre-
pared by electrochemical methods where aniline is used to

solubilize SWNTs via formation of donor–acceptor complex which
results in enhanced electro activity and conductivity of the com-
posite film [70]. Synthesis of composites of MWCNTs with PPy
has also been reported [71]. Composites of conducting polymers
containing magnetic nanoclusters have also attracted consider-
able attention because of their unique magnetic, electrical and
optical properties. Nano-structures of polyaniline composites con-
taining Fe
3
O
4
nanoparticles were prepared by a template free
method in presence of ␤-napthalene sulfonic acid (NSA) as a dopant
[72]. While nanocomposites comprised of PtRu alloy nanoparticles
and an electronically conducting polymer were prepared for the
anode electrode in direct methanol fuel cell [73]. Two conduct-
ing polymers poly (N vinylcarbazole) and poly (9-(4-vinylphenyl)
carbazole) were used. Several approaches have been developed
to functionalize the CNTs in both molecular and supramolecular
chemistry as illustrated in Fig.3[74].
2. Growth of conducting polymer
nanowires/nanotubes/nanoparticles
Various methods including template synthesis, scanning probe
electrochemical polymerization and electro-spinning have been
devised to prepare nanotubes and nanofibres of conducting poly-
mers. Conductive polymers with nano-structures can be prepared
by template method, non-template ways and seeding approaches.
Inorganic aluminum oxide, zeolite with channels and polymer
membranes with porosity have been commonly used as templates
where as in non-template way, either the polymerization takes

place at interface or surfactant, and polyelectrolytes are added for
structural direction.
Electrochemical polymerization [7] and some physical meth-
ods, such as electro-spinning [75] and mechanical stretching [76]
can produce conducting polymer nanofibres without templates,
but these materials have only been made on a very limited scale.
Several methods are there to obtain materials with promising appli-
cations in electronics, such as polyaniline and polypyrrole fibres
with diameter smaller than 1000 nm [22,77,78].
2.1. Template oriented synthesis of
nanowires/nanotubes/nanoparticles
Mostly, the formation of CP nano-structures relies on the
guidance of templates for example, channels of zeolites [79] or
nanoporous membranes [80] or the self-assembly of functional
molecules such as surfactants [81], polyelectrolytes [82]or complex
278 Rajesh et al. / Sensors and Actuators B 136 (2009) 275–286
Fig. 3. Several functionalization mechanisms for SWNTs: (from Ref. [74] with permission). (A) Defect-group functionalization; (B) covalent sidewall functionalization; (C)
noncovalent exohedral functionalization with surfactants; (D) noncovalent exohedral functionalization with polymers; and (E) endohedral functionalization with C
60
.
organic dopants [83]. Zeolite channels, track-etched polycarbonate,
anodized alumina, etc. are used as hard templates where as surfac-
tants like micelles, liquid crystals, etc. are used as soft templates.
In the template approach, the dimensions and the morphology of
the polymer structures are defined (or limited) by the porous sup-
port. Thus, the synthetic conditions need to be designed carefully
so that we can use them as templates and once the synthesis is
over they can be removed in their pure state. This method uses
pores in a micro porous membrane as a template for microtube for-
mation and thus used to synthesize tubular conducting polymers

[84]. The template synthesized method proposed by Georger et al.
[85] was successfully applied in the synthesis of polyacetylene [86],
poly (3-methylthiophene) [87], polypyrrole [88] and polyaniline
[89] tubes. In template self-assembly, the individual components
interact with each other and an external force or special constraint
[90]. The development of nano-structures in electronic polymers
over multiple length scales triggered by very small amounts of
added nanoscale templates has attracted tremendous interest in
recent years. Template synthesis entails the preparation of variety
of micro- and nanomaterials of a desired morphology and therefore
provides a route for enhancing nano-structured order. Template
is defined as a central structure within which a network forms
in such a way that removal of the template creates a filled cav-
ity with morphological and/or stereochemical features related to
those templates [91].
To date, oriented conducting polymer nano-structures including
oriented polypyrrole or polyaniline nanorods or nanotubes, were
mostly obtained with porous membrane as the template [92]. Car-
bon nanotubes can also be used as the template to deposit a thin
polyaniline/polypyrrole-polymer coating on the surface of the car-
bon nanotubes electrochemically [93,94].
Doped and dedoped nanotubes and nanowires of conduct-
ing polypyrrole, polyaniline and polythiophene were synthesized
by the electrochemical polymerization method, using Al
2
O
3
nanoporous templates [95]. Polypyrrole nanotubules were also
synthesized using AAO (anodic aluminum oxides) membranes as
template by electrochemical ac method [96]. The electrochemical

and chemical template synthesis of polypyrrole within the pores of
polycarbonate membranes has been reported [81,92,97,98]. Con-
ducting PPy nanotubes of varying diameters were prepared having
higher conductivity than PPy thin films, which was attributed
to alignment of polymer chain along the pore axis. Nanoparti-
cles were formed by redox enzyme–glucose oxidase by initiated
polymerization [99]. The self-assembly of Au/PPy and Au/PPy/Au
nanowires into three-dimensional vesicle-like structures has also
been reported [100]. Similarly, gold-capped, protein modified
Fig. 4. Method used for accessible and total protein binding sites in PPy nanowires
(from Ref. [101] with permission).
Rajesh et al. / Sensors and Actuators B 136 (2009) 275–286 279
Fig. 5. Fabrication of polyaniline nanowire immobilized on a Si surface with stretched double-stranded DNA as a guiding template (based on Ref. [46] with permission).
polypyrrole nanowires were grown electrochemically using porous
aluminium oxide as a template. Fig. 4 illustrates two different
methods to quantify the amount of protein binding sites in PPy
nanowires [101]. While a strategy for the fabrication of conducting
polymer nanowires on thermally oxidized Si surfaces by the use of
DNA as templates was also reported (Fig. 5) [46]. Controllable elec-
trical conductivity was granted along individual DNA molecules by
coating a thin layer of conducting polymer, polyaniline, along the
DNA strands immobilized on a silicon chip. Multiple junctions of
DNA wrapped single-walled CNTs in self-doped PANI nanocompos-
ites were used to enhance the sensitivity and stability of biosensors
[56]. Fig. 6 shows the schematic representation of ss-DNA wrapped
SWCNTs.
Fig. 6. An ss-DNA wrapped SWCNT (from Ref. [57] with permission).
A novel concept of fabrication of multilayer network films
on electrodes to form stable anionic monolayers (templates)
on carbon and metals has been developed [102]. In these

hybrid films, the layers of negatively charged polyoxometallate
or polyoxometallate-protected (stabilized) Pt nanoparticles are
linked or electrostatically attracted by ultra thin layers of positively
charged conducting polymers (PANI, PPy, PEDOT). The films are
functionalized and show electrocatalytic properties towards reduc-
tion of nitrite, bromate, hydrogen peroxide and oxygen. Also, a new
method to control both the nucleation and growth of highly porous
polyaniline nanofibre films using porous poly (styrene-block-2-
vinylpyridine) diblock copolymer (PS-b-P2VP) films as templates
was reported [103]. The diameter of the nanofibres was indepen-
dent of the experimental conditions used for the electrochemical
deposition and could be tuned by controlling the pore size, which
is defined by the molecular weight of the block copolymer. Fig. 7
shows the schematic illustration of the process of fabricating this
porous polyaniline nanofibre film. Sol–gel can also be used as tem-
plate for the growth of conducting polymers and thus can be used
as micro or nanoelectrode arrays [104–106].
A large area, highly uniform and ordered polypyrrole nanowires
and nanotube arrays have been fabricated by chemical oxidation
polymerization [107] and electro polymerization [70] with the help
of a porous anodic aluminium oxide template. Similarly, conduct-
ing polymer (PANI) nanowires and nanorings were synthesized
by electrochemical growth on gold electrodes modified with self-
assembled monolayers of well separated thiolated cyclodextrins in
an alkanethiol ‘forest’ (molecular template) [108]. A simple strategy
for the synthesis of wire/ribbon like polypyrrole nano-structures
involves the use of lamellar inorganic/organic mesostructures
as template which was formed during polymerization between
surfactant cations and oxidizing anions which degrade automati-
cally after polymerization [109].Al

2
O
3
nanoporous templates have
been used to fabricate nanotubes, nanowires and double walled
nanotubes of conducting poly (p-phenylenevinylene), poly (3,4
ethylenedioxythiophene) and polypyrrole through electrochemical
polymerization or chemical vapor deposition method [110].
The synthesis and characterization of monodisperse
silica–polyaniline-core-shell nanoparticles, which had less than
30 nm diameters has been reported [111]. The silica cores serve
as templates for adsorption of aniline monomers as well as
280 Rajesh et al. / Sensors and Actuators B 136 (2009) 275–286
Fig. 7. Schematic illustration of the process of fabricating a porous polyaniline nanofibre film (Ref. [103] with permission). (a) Preparation of a PS-b-P2VP monolayer micellar
film on a Au substrate; (b) generation of the cavitations in the PS-b-P2VP monolayer film via treatment with acetic acid followed by removal of the solvent; (c) formation of
the PANI nuclei only in the pores of the block copolymer film at an early stage in the electrochemical deposition; and (d) formation of a highly porous PANI nanofibre film
after the PANI overgrowth and intertwining.
counter ions for doping of the synthesized PANI. Similarly, a
synthesis protocol for stable aqueous colloidal solutions of poly
(4-styrenesulphonate) templated polyaniline was described [112].
A one step electrochemical co-deposition method has b een
used to prepare nanoparticles containing semi conducting poly-
mer inverse opals. Gold and cadmium telluride nanoparticles were
electrodeposited along with pyrrole in the interstitial voids of col-
loidal crystals of polymer spheres and following template removal,
composite inverse opals were obtained [113]. An extremely sim-
ple “nanofibres seeding” method to synthesize bulk quantities of
nanofibres of the electronic polymer polyaniline in one step with-
out the need for large organic dopants, surfactants, and/or large
amount of insoluble templates has been described [114]. Here,

seeding the reaction with very small amount of nanofibres, regard-
less of their chemical nature, results in a precipitate with bulk
fibrillar morphology.
2.2. Template free synthesis of
nanowires/nanotubes/nanoparticles
In non-template self-assembly, the individual components
interact to produce a larger structure without the assistance of
external forces or spatial constraint. Despite the variety of syn-
thetic approaches to CP nano-structures, the need for a method
capable of making pure, uniform, template free CP nano-structures
arises. This is a fabrication strategy, which requires only the mixing
of components to achieve an ordered structure and is appealing
both for its simplicity and its potential efficiency [91]. Template
free method of synthesizing nano-structures has several advan-
tages like simple synthesis, purification with no template removing
steps needed. Also, uniform nanofibres are formed, which are easily
scalable and reproducible. They show superior performance as sen-
sors because the diameter of nanomaterials is at nanoscale and are
water dispersible that facilitates environmental friendly process-
ing and biological application. Syntheses of high quality polyaniline
nanofibres having diameters between 30 and 50 nm,under ambient
conditions uses aqueous/organic interfacial polymerization [115].
The films possess much faster gas phase doping/dedoping times
compared with conventional cast films and therefore have been
used for sensing application. The same group prepared polyaniline
nanofibres by template free chemical synthesis for the detection
of hazardous HCl waste produced in exhaust plumes from solid
rocket motors [47]. Similarly a template free, site-specific electro-
chemical method to precise fabrication of individually addressable
conducting polymer (polyaniline) nanowires on electrode junc-

tions on a parallel oriented array was described [116]. The effects of
electrolyte gating and doping on transistors based on conducting
polymer nanowires electrode junction arrays in buffered aqueous
media was discusse d by Alam et al.[117]. By usinga single-step elec-
trodeposition between electrodes in channels created on insulating
surfaces conducting polymer nanowires of controlled dimensions
and high aspectratio were fabricated [118]. The technique is capable
of producing arrays of individually addressable nanowire sensor,
with site-specific positioning, alignment and chemical composi-
tions.
An assembly of large arrays of oriented nanowire aligned con-
ducting polymer (PANI) has been devised to support the polymer
instead of a porous membrane template. The oriented nanowire
was prepared through controlled nucleation and growth during a
stepwise electrochemical deposition process in which a large num-
ber of nuclei were first deposited on the substrate using a large
current density. This unique conductive polymer nanowire has
potential for chemical and biosensing applications [119].Stamm
and co-workers have evaluated the possibility of using polypyrrole
nanowires asactive elements in sensors [120]. They have developed
a simple chemical route to conductive polypyrrole nanowires by the
grafting of PPy from isolated synthetic polyelectrolyte molecule.
Also, the direct electrochemical synthesis of large arrays of uniform
and oriented nanowires of conducting polymers with a diameter
much smaller than 100 nm, on a variety of substrates (Pt, Si, Au, car-
bon, silica), without using a supporting template has been reported
[7].
3. Applications of nano-structured conducting
polymers/nanocomposites in sensors/biosensors
Use of nanomaterials in biosensors allows the use of many

new signal transduction technologies in their manufacture [121].
In molecular electronics and sensors, CPs has been used as poten-
tial systems for the immobilization of enzymes [24,122–126].In
these systems, there is a direct transfer of electrons to and from
the enzymes. The entrapment of enzymes in CP films provides a
Rajesh et al. / Sensors and Actuators B 136 (2009) 275–286 281
controlled method of localizing biologically active molecules in
defined area on the electrodes. Also the use of conducting poly-
mers in the area of bioanalytical sciences is of great interest since
their biocompatibility opens up the possibility of using them as in
vivo biosensors, for continuous monitoring of drugs or metabolites
in biological fluids [127].
Nanomaterials can be use d in a variety of electrochemical
biosensing schemes thereby enhancing the performance of these
devices and opening new horizons in their applications. Nanoparti-
cles, nanowires and nanotubes have already made an impact on the
field of electrochemical biosensors, ranging from glucose enzyme
electrodes to genoelectronic sensors. As conducting polymer nano-
materials are light weight, have large surface area, adjustable
transport properties, chemical specificities, low cost, easy pro-
cessing and scalable productions, they are used for applications
in nanoelectric devices, chemical and biological sensors [128].
Thin polypyrrole nanofilms doped with sulphate were prepared
chemically by interfacial polymerization which makes insertion of
various functional groups to pyrrole films possible and provided
various applications in developing chemical and biological sensors
[129].
Currently, nanoparticle based protocols are being exploited for
detection of proteins. The property associated with nanowires
and nanotubes, which enable us to modify them with biological

recognition elements, imparts high selectivity to these devices.
Nanomaterials based electrochemical sensors are expected to
create a major impact upon clinical diagnosis, environmental mon-
itoring, security surveillance, or for ensuring our food safety. The
use of biological elements in biosensor construction comes with
a challenge of preserving their biological integrity outside their
natural environment. For this reasons these biological components
of biosensors are generally immobilized onto supports by phys-
ical, covalent or electrochemical methods. Nanoparticles provide
a good solution to the problems associated with optimization of
immobilized enzymes: minimum diffusion limitations, maximum
surface area per unit mass and high effective enzyme loading
[27]. Conducting polymers particularly in the form of thin films
or blends or composite as sensors for air-borne volatiles (alco-
hols, NH
3
,NO
2
, CO) has also been used widely. Polythiophene
based sensor has shown the detection of ppb of hydrazine gases
[127]. Also, polyaniline–SnO
2
/TiO
2
nanocomposite ultra thin films
have been fabricated for CO gas sensing [130]. A novel sensi-
tive electrochemical biosensor based on magnetite nanoparticles
for monitoring DNA hybridization was prepared by using MWNT-
COOH/PPy-modified glassy carbon electrode. The range of the
biosensor was found to be 6.9 × 10

−14
–8.6 × 10
−13
mol l
−1
and the
detection limit is 2.3 × 10
−14
mol l
−1
[131]. Like conducting poly-
mers which have proved to show good sensing performance, the
surface area and good electronic property provided by CNTs is also
an attractive feature in the advancement of a chemical/biosensor.
Mostly, CNTs are used for gas sensing which is accomplished by
measuring change in electrical properties of CNTs induced by the
change transfer with gas molecules or the mass change due to
physical adsorption of gas molecules. Glucose oxidase containing
polypyrrole/aligned carbon nanotube coaxial nanowire electrode
was prepared and used as novel glucose sensor [132]. The 3-D
structure of CNTs provide a good template for a large enzyme load-
ing in an ultra thin polymer layer, leading to a glucose response
of 10–20 times higher than that from a corresponding flat elec-
trode. Kong et al. developed a hydroquinone sensor based on the
synergistic effect of MWCNT and conducting poly (N-acetylaniline)
polymer and the accumulation effect of ␤-cyclodextrin, with a sen-
sitive detection, stability and reproducibility of the electrode [133].
Tu et al. studied the over oxidation of PPy–MWCNT composite
film in neutral and alkaline solutions by electrochemical quartz
crystal impedance and used this OPPy/CNT/NaOH/Au electrode for

sensing dopamine with a limit of detection down to 1.7 nmol l
−1
[134]. The synthesis of polyaniline nanoparticles with dodecyl-
benzylsulphonic acid that was successfully electrodeposited on
the surface of glassy carbon electrodes to form nano-structured
films was reported [135]. This effective biosensor format, exhibits
higher signal to background ratios and shorter response times.
Similar conducting polymer nanojunction sensor for glucose is
potentially useful for in vivo detection [136]. Each junction was
formed by bridging a pair of nanoelectrodes separated by a small
gap (20–60 nm) with PANI/GOx and the sensor developed gives a
fast response of <200 ms. The synthesis of a novel sensitive elec-
trochemical DNA biosensor based on electrochemically fabricated
polyaniline nanowires and methylene blue for D NA hybridization
detection has been presented [137]. The sensitivity of the method
was very attractive and the detection limit for target sequences
reaches 1.0 × 10
−12
mol l
−1
. It has been demonstrated that conduct-
ing micro and nano-containers can be prepared by electrochemical
polymerization of appropriate monomers using soap bubbles as
a soft template [138]. These containers are very attractive for a
wide range of applications, ranging from sensors to controlled
release of drugs. Many such reports produce the use of conduct-
ing polymers for developing nanosensors for the detection of DNA
[139,140]. Recently, an impedimetric immunosensor for the direct
detection of “bisphenol A” was fabricated by immobilizing a poly-
clonal antibody onto nanoparticle comprising conducting polymer

layers through covalent bond formation [141]. Similarly, polyani-
line, nanofibres can also be used for gas sensing application [142].
Thin films of conventional PANI and PANI nanofibres were com-
pared by depositing on interdigitated gold electrode, where PANI
nanofibre films showed an enormous increase in response and sen-
sitivity towards HCl vapors (Fig. 8) [76].
Conducting polymer nanowire biosensors have also been shown
to be attractive for label-free bioaffinity sensing. For example,
the real time monitoring of nanomolar concentrations of biotin
at an avidin-embedded polypyrrole nanowire has been demon-
strated [143]. In another such approach use of polypyrrole nanowire
modified electrodes characterized by their amperometric response
towards nitrate ions is reported [144]. The sensitivity and detec-
tion limit was found to be 336.28 mA M
−1
cm
−2
and 1.52× 10
−6
M,
respectively. Another highly sensitive and selective nitrate sensor
has been demonstrated by using electrochemical doping approach
on PPy nanowires [70]. The feasibility of fabricating single polypyr-
role and polyaniline nanowires and their application as DNA
sensors (1 nm) was also studied [145]. Such an enzyme based
glucose sensor has been fabricated and characterized, based on
co-electrodeposition of redox polymer poly (vinylimidazole) and
glucose oxidase onto a low-noise carbon fibre nanoelectrode [146].
Fig. 8. Schematic diagram showing a typical sensor experiment: gold interdigitated
electrodes (left) are coated with polyaniline film by drop casting (middle), and the

resistance of the film is monitored as the sensor is exposed to vapor (right) (from
Ref. [47] with permission).
282 Rajesh et al. / Sensors and Actuators B 136 (2009) 275–286
Fig. 9. Mechanism underlying the sensor response. (Top) formation of a layer of CdS-ODN nanoparticles on the PPy-ODN film after hybridization. (Bottom) binding of
unlabelled ODN probes to the PPy-ODN film (from Ref. [147]), CdS: cadmium sulfide; ODN: oligonucleotides.
This nanosensor offers a highly sensitive and rapid response to
glucose at an operating potential of 0.22 V, with a linear range of
0.01–15 mM and a detection limit of 0.004 mM.
The use of conducting polymer substrates and the amplifica-
tion afforded by semiconductor nanoparticles can be combined
to construct a novel DNA sensor as illustrated in Fig. 9 [147].A
label-free approach has been used in electrochemical DNA sensor
based on functionalized conducting copolymer [148]. Polyaniline
and mercaptosuccinic-acid-capped gold nanoparticle multilayer
films have also been used for biological applications [149].Ithas
been reported thata glutamates micro biosensor canbe made based
on glutamate oxidase immobilized onto the nano-structured con-
ducting polymer layers for the in vivo measurement of glutamate
release [150].
Conducting polymer can be exploited as an excellent tool for
the preparation of nanocomposites with entrapped nanoscaled
biomolecules, mainly proteins, enzymes and DNA oligomers.
Recently, conducting polymer/CNTs composites have received sig-
nificant interest because the incorporation of CNTs into conducting
polymers can lead to new composite materials possessing the prop-
erties of each component with a synergistic effect that would
be useful in particular application [151]. The subtle electronic
properties suggest that CNT have the ability to promote electron
transfer reaction when used as an electrode [152]. CNT/polymer
composites have been used for immobilization of metalloproteins

and enzymes by either physical adsorption or covalent binding.
Polypyrrole and polyaniline can be used for fabrication of CNT/PPy
and CNT/PANI nanocomposite electrodes due to the ease in the
preparation through copolymerization by a chemical or electro-
chemical approach and the resulting nanocomposites exhibits high
conductivity and stability [68,153]. PANI/CNTs composite modi-
fied electrode fabricated by galvanostatic electro polymerization
of aniline on MWNTs-modified gold electrode, exhibits enhanced
electrolytic behavior to the reduction of nitrite and facilitates the
detection of nitrite at an applied potential of 0.0 V. A linear range
from 5.0 × 10
−6
to 1 .5 × 10
−2
M for the detection of sodium nitrite
has been observed at the PANI/MWNTs-modified electrode with a
sensitivity of 719.0 mA M
−1
cm
−2
and a detection limit of 1.0 ␮M
[153].
A functionalized single wall CNTs/PPy composite served as
amperometricglucose biosensors [154]. A biosensorfor choline was
developed using layer by layer assembled functionalized MWNTs
and PANI multilayer film. By using the conducting polymer PANI,
the biosensor immobilized abundant CNTs stably and achieved the
aim of anti interference, with a rapid response and expanded lin-
ear response range [155]. While multi-walled aligned CNTs are
used to provide a novel electrode platform for inherently con-

ducting polymer based biosensor [156]. Such, an amperometric
glucose biosensor based on immobilization of glucose oxidase in
a composite film of poly (o-aminophenol) and CNT, which are
electrochemically copolymerized at a gold electrode, was devel-
oped. The biosensor has a detection limit of 0.01–10 mM with
a good stability and reproducibility [152]. Nanocomposite mate-
rials of poly (o-anisidine) containing TiO
2
nanoparticles, carbon
black, and MWNT were deposited in thin films to investigate
for impedance characteristics for biosensing application [157].A
nanocomposite of poly (aniline boronic acid) with an ss-DNA
wrapped single-walled CNTs on a gold electrode by in situ electro-
chemical polymerization is reported [56]. Similarly, a novel hybrid
material based on carbon nanotubes–polyaniline–nickel haxa-
cyanoferrate nanocomposite was synthesized by electrodeposition
on glassy carbon electrode [158]. Also two routes to synthesize
surface-aminated polypyrrole-silica nanocomposite particles were
investigated [159].
Rajesh et al. / Sensors and Actuators B 136 (2009) 275–286 283
Table 1
Characteristics of nano-structured conducting polymer/nanocomposite based sensors/biosensors.
Matrix Analyte Diameter/size Range Detection limit Voltage (oxidation
potential)
Reference
Nanowires/nanofibres/nanoparticles
PANI nanowires PPy nanowires NH
3
50–80 nm 1 ppm 0.1 V [45]
80–180 nm

Ox/PPy NP/SiO
2
/Pt Glucose 25 ± 10 nm +500 mV [99]
PANI framework HCl, NH
3
,
ethanol, pH
sensor
40–80 nm 0.68 V [117]
PANI nanofibres/FeHCF H
2
O
2
0–50 ppm 0.1 V [119]
PPy/GOx/CNT nanowires Glucose 50 nm [132]
PPy nanowires on graphite electrode Nitrate ions 1.52 × 10
−6
M [144]
Pd/PPy & PANI nanowires H
2
gas, DNA 75 nm–1 ␮m1nM [145]
Poly (vinyl imadazole)/GOx/CFNE Glucose 0.01–15 mM 0.004 mM 0.22 V [146]
PPy nanowires Nitrate 10 ␮M–1 mM 4.5 ± 1 ␮M [162]
Nanocomposites
Poly (aniline boronic acid)/ss-DNA/SWNT Dopamine 1.3 ± 0.4 nm 1–10 nm 0.04–0.79 V [57]
PSG/PANI/PAA nanoelectrode Glucose 1 × 10
−5
–2 × 10
−3
M5× 10

−6
M [106]
MWNT-COOH/Ppy/GC DNA 6.9 × 10
−14
–8.6 × 10
−13
mol l
−1
2.3 × 10
−4
mol l
−1
[131]
Poly (N-acetylaniline)/CNT Hydroquinone 1× 10
−6
–5 × 10
−3
mol l
−1
8 × 10
−7
mol l
−1
[133]
Ppy–CNT/NaOH Dopamine 4 × 10
−8
–1.4 × 10
−6
mol l
−1

Up to 1.7 nmol l
−1
[134]
Nano PANI/DBSA/GC H
2
O
2
10 nm +700 mV [135]
Micro/nano Ppy/Gox Glucose 0.001–0.02 M [138]
Ppy/MWNT-COOH DNA 1 × 10
−5
–3 × 10
−8
mol l
−1
[139]
Poly (TTCA) thiophene derivative Bisphenol A 5–40 nm 1–100 ng ml
−1
0.3 ng ml
−1
[141]
Ppy/CdS np ODN 3.7–370 nm ∼1nm [147]
GLOx/nano CP/Pt CP-polythiophene derivative Glutamate 0.2–100 ␮M0.1±0.03 ␮M 0.55 V [150]
Au/POAP/CNT/Gox Poly (o-aminophenol) Glucose Up to 5 mM 0.01–10 mM 0.75 V [152]
PANI/CNT composite Nitrite 30–60 nm 5.0 × 10
−6
–1.5 × 10
−2
M1.0␮M 0.0 V [153]
Fc-SWNT/Ppy/Gox/GC Glucose 0.75 V [154]

MWNTs/PANI/ClOx/GC Choline 1 × 10
−6
–2 × 10
−3
M 0.3 ␮M 0.4 V [155]
Ppy/Gox/CNT Glucose Up to 20 mM 1 V [156]
PVC: polyvinyl chloride; PS: polysulfone; PSMNP: polymer stabilized metal nanoparticle; PANI: polyaniline; PAA: poly (acrylic acid); PSG: porous sol–gel; PPy: polypyrrole;
FeHCF: ferrocene hexacyanoferrate; GOx: glucose oxidase; GLOx: glutamate oxidase; PVP: poly (vinylpyridine); POAP: poly (o-aminophenol); DBSA: dodecyl benzene sulfonic
acid; CFNE: carbonfibre nanoelectrode; POAS: poly (o-anisidine); ODN: oligo-nucleotide; and Poly (TTCA): poly (terthiophene carboxylic acid).
In addition to the striking applications in confining the CNTs
onto macro-sized electrodes, the strategy through the use of
conductive films to confine the CNTs may be applicable for prepar-
ing CNT-based microelectrodes, because the procedures for the
preparation of these nanocomposites can be easily conducted on
electrodes through electrochemical polymerization and these elec-
trodes are thus available for electrochemical measurements [22].A
polyaniline composite film was prepared through a chemical oxi-
dation method by adding CNTs as nanofibre seeds and was used to
examine gas response to trimethylamine[160]. Polyaniline/MWNTs
composite films prepared by in situ and ex situ methods show higher
electrical conductivity over neat PANI [161].
Characteristics of sensors/biosensors based on various nano-
structured conducting polymers and polymer nanocomposites
are summarized in Table 1 [45,56,99,106,111,117,119,130–142,
144–147,150,152–156,160,162].
4. Conclusion
The developments in nano-structured conducting polymers
and polymer nanocomposites have large impact on biomedical
research. Significant advances in the fabrications of nanobiosen-
sors/sensors using nano-structured conducting polymers are b eing

persistently made. In this review, we briefly described the meth-
ods, which provide different synthetic routes with advantages
and disadvantages therein to prepare the nano-structured con-
ducting polymers and polymer nanocomposites. The study also
demonstrates the role of nano-structured conducting polymers
in the emerging field of nanosensors/biosensors. A detail analy-
sis has been carried out on the latest research advancement made
in the development of nano-structured conducting polymers and
polymer nanocomposites based sensors/biosensors. As the surface
nano-structure becomes more demanding and complex, more syn-
thetic methods for the construction of nano-structured materials
will be required. These methods will use new nanotechnologi-
cal approaches to conducting polymers and their applications in
biomedical research. Increasing interest in and practical use of
nanotechnology, especially, in conducting polymers and polymer
composites have lead the researchers to the rapid development
of nanosensors/biosensors with improved processability and func-
tionality over previously developed biosensors.
Acknowledgements
Authors are grateful to Director, Delhi College of Engineering for
his kind encouragement and support. One of the authors Tarushee
Ahuja is thankful to the institution for financial assistance.
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Biographies

Rajesh has obtained his doctoral degree in chemistry in 1998 from University of
Delhi, Delhi, India. He has done research in the field synthesis and characterization
of novel biomimetic compounds. He worked in the field of technical development
of cholesterol biosensor at National Physical Laboratory, Delhi, India, as postdoc-
toral fellow from 1998 to 1999. He joined Council of Scientific & Industrial Research,
Government of India, as scientist in 2000. He joined Graduate School of Life Science
& Systems Engineering, Kyushu Institute of Technology, Japan, as JSPS postdoctoral
fellow and worked in the research field of conducting polymer-based biosensors
from 2002 to 2004. At present, he is engaged with research activities in the area of
conducting polymers and their applications in biomedical sciences.
Tarushee Ahuja received her MSc degree in chemistry from Indian Institute of
Technology, Delhi in 2005. She is currently pursuing PhD in Polymer Science & Tech-
nology under the supervision of Dr Rajesh and Dr D. Kumar from University of Delhi,
Delhi, India. Her area of research includes application of conducting polymers in
biosensors.
Devendra Kumar received his doctoral degree in Polymer Science & Technology
from University of Delhi, Delhi in 1998. He is currently working as Assistant Profes-
sor in Department of Applied Chemistry and Polymer Technology, Delhi College of
Engineering, University of Delhi. His current area of research includes conducting
polymers and their applications.