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 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 [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. 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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 Professor in Department of Applied Chemistry and Polymer Technology, Delhi College of Engineering, University of Delhi His current area of research... Sensors and Actuators B 136 (2009) 275–286 [73] J.-H Choi, K.-W Park, H.-K Lee, Y.-M Kim, J.-S Lee, Y.-E Sung, Nano- composite of PtRu alloy electro catalyst and electronically conducting polymer for use as the anode in a direct methanol fuel cell, Electrochim Acta 48 (2003) 2781–2789 [74] A Hirsch, Functionalization of single-walled carbon nanotubes, Angew Chem Int Ed 41 (2002) 1853–1859 [75] A.G MacDiarmid,... 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