Sensors and Actuators B: Chemical

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

Contents lists available atScienceDirect

Sensors and Actuators B: Chemical

j o u r n a l h o m e p a g e :w w w e l s e v i e r c o m / l o c a t e / s n b

Review

Recent progress in the development of nano-structured conducting

polymers/nanocomposites for sensor applications

Rajesha,∗, Tarushee Ahujab, Devendra Kumarb

aCouncil of Scientific & Industrial Research, 14, Satsang Vihar Marg, Special Institutional Area, Delhi 110067, India

bDepartment of Applied Chemistry, Delhi College of Engineering, University of Delhi, Bawana Road, Delhi 110042, India

a r t i c l e i n f o

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

a b s t r a c t

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 csir@yahoo.com ( 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

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.

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

pre-pared with SWNTs for fabricating electrochemical sensors with

remarkably improved sensitivity towards H2O2 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–SiO2 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␮A mM−1cm−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

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.

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

the improved mechanical and optical properties of nanoparticles

has led to the fabrication of many devices.Fig 2shows 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

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 modified 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 Fe3O4 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 inFig 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

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 Al2O3

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

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

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 nanorconduct-ings 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] Al2O3nanoporous 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]

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

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 been

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 discussed by Alam et al.[117] By using a single-step elec-trodeposition between electrodes in channels created on insulating surfaces conducting polymer nanowires of controlled dimensions and high aspect ratio 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 as active 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

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 used 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, NH3, NO2, CO) has also been used widely Polythiophene

based sensor has shown the detection of ppb of hydrazine gases

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× 10−13mol l−1and the

detection limit is 2.3× 10−14mol 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, leadload-ing 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

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 elecelec-trochemically fabricated polyaniline nanowires and methylene blue for DNA hybridization detection has been presented[137] The sensitivity of the method was very attractive and the detection limit for target sequences reaches 1.0× 10−12mol 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 developconduct-ing nanosensors for the detection of DNA

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−1cm−2and 1.52× 10−6M, 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

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 inFig 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] It has

been reported that a glutamates micro biosensor can be 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× 10−6to 1.5× 10−2M for the detection of sodium nitrite has been observed at the PANI/MWNTs-modified electrode with a sensitivity of 719.0 mA M−1cm−2 and a detection limit of 1.0␮M [153]

A functionalized single wall CNTs/PPy composite served as amperometric glucose biosensors[154] A biosensor for 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 TiO2 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]

Table 1

Characteristics of nano-structured conducting polymer/nanocomposite based sensors/biosensors.

potential)

Reference

Nanowires/nanofibres/nanoparticles

80–180 nm

ethanol, pH sensor

Nanocomposites

Poly (N-acetylaniline)/CNT Hydroquinone 1× 10 −6 –5× 10 −3 mol l−1 8 × 10 −7 mol l−1 [133]

Poly (TTCA) thiophene derivative Bisphenol A 5–40 nm 1–100 ng ml −1 0.3 ng ml −1 [141]

GLOx/nano CP/Pt CP-polythiophene derivative Glutamate 0.2–100␮M 0.1±0.03 ␮M 0.55 V [150]

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,

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 being

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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